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Workshop TargetsContinuous Multiphase

Aseptic Processingof Foods

CONTENTS:Continuous Multiphase Aseptic Processing of Foods •.43

John W. Larkin

Measuring Residence Time and Modellingthe System 44

Sudhir K. Sastry

Biological Validation 48Joseph E. Marcy

Statistical Design and Analysis ••.•....•.•.•.•.•.•...•.•.•••.•...•..52Michel Digeronimo, Wallace Garthright, and

John Larkin

Issues Involved in Producing a MultiphaseFood Product '56

Dominick Damiano

Reprinted from Food Technology 51 (10) 43';62@Institute of Food Technologists

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5 PEe A L 5 E C ~.T o N

Cream of potato soup istypical of the productsthis workshop addressed.

bution (the procedureused to determine theparticle receiving theminimum thermal treat-ment), and validation ofthe model used to estab-lish the process.

What Pushed theIndustry Forward?

The continued inter-est in these types of pro-cesses, the limited num-ber of filings, and thedifficulty of document-ing the lethal treatmentdelivered to each portionof the food product re-sulted in uncertainty inthe food processingcommunity as to whatwas needed to establish a process (Dal Porto,1993). In 1995, in response to these concerns, theNational Center for Food Safety and Technology(NCFST) and the Center for Aseptic Processingand Packaging Studies (CAPPS) organized aworkshop for experts in aseptic processing offoods. The objective of the workshop was to es-tablish a dialogue and a mechanism for exchangeon issues surrounding the application and imple-mentation of aseptic processing of multiphasefoods, and to come to a consensus on these issues.

Representatives of 11 universities, 5 govern-ment agencies, 22 food companies, and NFPApar-ticipated in the workshop, which was held on No-vember 14-15, 1995, and March 12-13, 1996.Theorganizers wished to address the immediate con-

. . cerns that needed to be resolved to develop anaseptic multiphase food process. Th'e topic areasdiscussed by working groups were particle resi-

.. dence time, mathematical modeling, physicalproperty measurement procedures, biological val-'idation, and statistical design and analysis of data.

The workshop also included a discussion of acase study developed by members of NFPA,

For many years, food processors have

tried to develop an aseptic process for

food products containing particles. The develop-

ment of such a process has been hindered by the

requirement to demonstrate an adequate

thermal treatment for every portion of the

product.

During the past 13 years, the Food and DrugAdministration received only a few filings foraseptically processed low-acid canned foods con-taining particles. Two filings received in the mid-to-late 1980s generated many questions withinFDAconcerning process establishment methodol-ogy, test results, and the identification and controlof certain critical factors. FDA had several discus-sions with each firm about its process and how itwas established. For reasons unknown to FDA,both firms decided to discontinue their involve-ment in aseptic multi phase food processingprojects and withdrew the filings from furtherconsideration. In 1989, FDA identified several is-sues that it expected a food processor to addresswhen developing a filing (Dignan et aI., 1989).What has proved to be the most difficult is the re-quirement to biologically validate the thermaltreatment delivered by the system.

The National Food Processors Association(NFPA) made a concerted effort to develop proto-cols for the establishment of an aseptic mul-tiphase food process (Chandarana and Unver-ferth, 1996). In the early 1990s, a food industryconsortium formed by NFPA commissioned tworesearch projects to develop procedures that couldbe used to collect the necessary information todocument an adequately scheduled process.Lengthy discussions between FDA and NFPA tookplace during the project. At the project's end, FDAwas still unable to resolve several fundamental is-sues (Anonymous, 1995). In particular, questions.or concerns remained regarding biological valida-tion (number of particles used and how this in-formation would be used), residence time distri- .

JOHN W. L.ARKIN

foods containing

The author. a Professional

Member of 1FT,is Food Process

Hazard Analysis Branch Chief,

NaTIonal Center for Fcod Safety

and TechnolOgy. Food and Drug

AdministraTIon, 6502 S. Archer

Rd, Summit-Argo, IL 60501 ,

aseptic processes for

particulates

discuss concerns

that needed to be

resolved to develop

Workshop Targets 'Contiriuous l1ultiphase.

AseptIc ProcessmgWorking groups 0f Foods

VOL. 51, NO.10 -OCTOBER 1997 f'OODTECHNOLOGY 43

'If

S PEe A L. 5 E C T o N

Measuring Residence Timell,

and Modeling the SystemSUDHIR K. SASTRY ,i

The author, a Professional Member of 1FT,is Professor, (Dept of Food, Agricultural and BiologicalII,

Engineering, Ohio State University, 590 Woody Hayes q~.,Col~mbus, OH 43210.

AseRticWorkshopGo N TIN U E 0 j'

I:CAPPS, and FMC Corp. The case s udywas developed to try to bring the discus-sions of the workshop "down to eahh."The fictitious food product used in!lthecase study was cream of potato soupconcentrate. The case study illustratedthe types of issues that a processor bayneed to consider when developing ~scheduled thermal process for asepticmultiphase food products. A copy Clf thecase study can be obtained from eitherNCFST or CAPPS. ,II

Not all of the issues surroundirigaseptic processing of multiphase foodswere discussed at the workshop, but amajor step forward was made in un~er-standing the important ones. The partic-.ipants recognized that each process :rillbe unique and that the results of the

II'

The National Center tor Food Safety

and Technology and the Center for "I

Aseptic Processing and Packaging :i I

Studies sponsored a workshop in

November 1995 and March 1996 to:reach a consensus on concerns that:needed to be resolved to develop ani,

aseptic muItiphase food process. '11

This article presents the results of theworking groups on (1) measuremen~ ofresidence time distribution (RTD) a~d(2) mathematical modeling and meaLsurement of physical properties. The:~scope covered aseptic processing usirtgconventional heating (surface, rather~than internal generation). The RTD dis-cussion is likely independent of mode ofheating, but conclusions regarding sys-tem thermal behavior apply only to cbn-ventional aseptic processing. II,

44 FOODTECHNOLOGY

workshop are not the official viewpointsor recommendations of FDA, the UnitedStates Army, NFPA, or any of the partici-pating university or industrial organiza-tions. They also recognized that it is theprocessor's responsibility to demonstratethe ability of his process to commerciallysterilize every portion of the food prod-uct produced.

What Happened Next?The progress made at the workshop

was significant in helping to establish adirection for a food processor to pursuewhen establishing an aseptic multiphasefood process. The following four articlesin this issue of Food Technology presentthe results of several of the workinggroup discussions.

After the workshop, Tetra Pak, Inc.,Buffalo Grove, Ill., used the case study asa guide and, with several new analyticalprocedures, developed a commercial fil-ing for cream of potato soup. The filingwas accepted by FDA in May 1997 (Pala-niappan and Sizer, 1997). Future success-

Measurement of RTDReasons for measurement of RTD in-

clude determination of the fastest-mov-ing particle, which is necessary for (a)designing a process via mathematicalmodeling to ensure commercial sterility,and (b) biological validation of a model.The biological test must contain sampleparticles that represent the fastest-mov-ing particle.

RTD measurement is needed becauseof the difficulty in noninvasive measure-ment of particle internal temperaturesduring continuous flow. If the cold-spottemperature of the slowest-heating par-ticle could be measured at the end of theheater and the hold tube, RTD measure-ment would be unnecessary. Severalmethods for measurement of RTD areavailable:

Optical Methods, such as particletracking velocimetry or PTV (Zitoun,1996) and other visualization tech-

ful fIlings will be dependent on the pro-cessor's use of scientifically sound prin-ciples, as outlined by the workshop par-ticipants, and documentation of the le-thality delivered to every portion of theproduct processed.

REFERENCESAnonymous. 1995. Food Chem. News 37(18): 26-27.Chandarana, DJ and Unverferth, JA 1996. Residence

time distribution of particulate foods at aseptic process-ing temperatures. J. Food Eng. 28: 349-360.

Dal Porto, A 1993. Aseptic advances tangled in bureau.cratic red tape. Food Proc. 54(10): 75-76.

Dignan, D.M., Berry, M.R, Pflug, !.J., and Gardine, lD.1989. Safety considerations in establishing asepticprocesses for low-acid foods containing particulates.Food Techno!. 43(3): 118-121, 131.

Palaniappan, S. and Sizer, GE. 1997. Aseptic processvalidation for food containing particulates. Food Tech-no!. 51 (8): 60-62, 64, 66, 68.

Updated August 1997 from a paper presented during theforum, "NCFST and CAPPS Workshop on AsepticProcessing of Multiphase Foods, " at the Annual Meetingof the Institute of Food Technologists, New Orleans, La.,June 22-26, 1996 .

Edited by Neil H. Mermelstein,senior Associate Editor.

niques.Magnetic Methods, which involve

introduction of tagged particles contain-ing small magnets (Chandarana and Un-verferth 1996), the passage of which isdetected by a voltage generated withincoils at selected locations of the processequipment.

Magnetic Resonance Imaging, usedfor flow visualization in food systems(Manavel et aI., 1993).

History Methods, such as chemicalmarkers and thermal memory cells,. which involve determining the effect of aprocess on a chemical reaction or diffu-sion process and back calculating pro-cessing parameters (Kim and Taub, 1993;Swartzel et al., 1991).

Ultrasonic Methods, which involvedetection of RTD by the Doppler scatter-ing of ultrasound waves by the movingparticles.

Salt Tracer, a well-established ap-proach used in the chemical engineeringliterature for pure fluids, in which RTDis detected by electrical conductivitymeasurements.

The group opinion was that any ofthese methods could be used, withintheir respective applicable ranges.

OCTOBER 1997 • VOL. 51, NO.1 0

..

~I

Fig. l-"Blunting"effect on fluid velocityprofile when solids areadded to a carrier fluid.(a) velocity profile ofpure fluid; (b) velocityprofile when solids andsolutes are added

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Theoretical Velocity Profilesfor Pure fluidsWhile the solid-liquid.flow problem

is too complex to be solved in detail the-oretically, limiting cases may be under-stood for pure liquids. Practically allmodifications of product and equipment"blunt" the velocity profile. For example,addition of solids decreases maximumvelocity (Fig. 1); addition of macromole-cules makes fluids pseudoplastic-theonly reported dilatancy appears to in-volve ungelatinized starch suspensions(Dail and Steffe, 1990)-but this situa-tion can be controlled by precooking thesuspension; and bends and curvature intubes create secondary flows that narrowRTD.Thus, the fastest velocity of a pure:fluid is expected to be greater than orequal to the,velocity of the fastest parti-cle in a solid-liquid mixture.Therefore, if a velocity profile can be

theoretically determined for a homoge-neous carrier fluid, the fastest fluid ve-locity can be used as the fastest particlevelocity in lieu of experimental measure-ments. For example, in holding tubes, itis sufficient to assume that the fastestparticle moves at twice the mean fluidvelocity, corresponding to the theoreticalsolution to steady, fully developed tubeflow of a Newtonian fluid. Since the ad-dition of ingredients and solids onlyblunts the velocity profile, the Newto-nian case is considered conservative.Situations may exist where particles .

may move faster than the local fluid. Forexample, a particle may be accelerated asit flows through a venturi and continueto flow faster than the local fluid down-stream of the venturi as a result of iner-tia (Fig. 2). This phenomenon is not ex-pected in steady, fully developed holdingtube flows.

Processing ofMultiple Particle TypesCommercial products may'contain

more than one particle type, and it isnecessary to assess which has the lowestresidence time. Thus, RTD must be de-termined for samples for each particletype. This process may be expedited if itis determined which particle type bestrepresented the characteristics of the sys-tem. Results may be product specific,and research in this area is needed.

RTD Informationfrom Separate ShiftsOne concern is the duration of the

residence time experiments. If the mag-

VOL. 51, NO.1 0 • OCTOBER 1997'

.'

netic particle approach is used, where theparticles must be introduced one at atime and a large number (299) of parti-cles need to be tested, oneshift may not suffice foran experiment. However,if a system is properlycontrolled, succeedingshifts would be expectedto have similar velocityprofiles and RTDs. Thus,the use of data accumu-lated over several shifts

(a)

C?_____[2(b)_8FOODTECHNOLOGY 45

IiS PEe A L 5 E C T o N

countered during this process, and atime scale that would cause the particleto "experience" the fluid temperatureconditions at a faster rate. Worst-casescenario simulations are needed for holdtube sizing.

Minimum AcceptableFeatures Of a Model

The following are the minimum de-sirable features of a mathematical modelfor aseptic multiphase processing.

Heater Section. For thermally mixedsystems (e.g., pure fluids in turbulentflow and solid-liquid mixtures in swept-surface heat exchangers, where radialtemperature variations are eliminated bymixing):

For fluid temperature simulations,(1) an energy balance must be conductedon the fluid, combined with temperature

measurement at the inlet and outlet ofthe heater; (2) the value of the fluid-par-ticle convective heat transfer coefficienth that yields the best agreement withrrieasured inlet and outlet fluid tempera-tures must be determined by trial-and-error or by some other numerical proce-dure; and (3) a "reality check" must beconducted to ensure that the estimate ofhfpis realistic (e.g., Balasubramaniamand Sastry 1994a, b, 1996a, b; Cacace etal., 1994;Alhamdan, 1995; Gadonna etaI., 1996; and others). For these fluidtemperature simulations, the assumptionof a worst-case hrp(e.g., corresp~nding toa stagnant fluid) 1S not conservative, be-cause such particles will not be a signifi-cant thermal "drag" on the fluid (Fig. 3).

For worst-case simulations, the par-ticle is considered to experience the fluidtemperatures on a time scale corre-sponding to the fastest-moving particle.h for these simulations must be conser-fp,vatlve.

When the heater system is not ther-mally mixed (e.g., tubular heat exchang-ers), other considerations may apply. Thefollowing recommendations are made:

(1) Although fluid temperatures ap-proximate an exponential curve connect-

•2-1l1ustration of

eleration of a particlethrough a venturi,followed by inertial effect,whereby the particle ismoving faster than the

behavior to the model. Also, the modelassists in process control by identifyingoperating parameter adjustments neededto correct for process variations.

Important process parameters can beselected for monitoring, and their mea-surement procedures and instrumenta-tion decided using the model.

Steps in ModelingIt is desirable to mathematically pre-

dict all particle and fluid velocities, par-ticle positions, orientations, and fluidand particle temperature distributions.However, this is not possible because oflimited understanding of the basic phys-ics, lack of precise knowledge of all ini-tial conditions, and limitations in com-putational abilities. Thus, simplifying as-sumptions are necessary to make theproblem tractable. It is only necessary toaddress the worst-case scenarios in ar-riving at an accept-able model.

Most mathe-matical models Go+- 0---(e.g., Sastry, 1986;Chandarana andGavin, 1989), in-volve the followingsteps:

1.Prediction of Fluid Temperature.Most models use an energy balance onthe fluid phase to account for heat enter-ing or leaving via system walls and heatexchange with particles to predict thefluid temperature.

2. Prediction of Representative Par-ticle Temperature. The heat conductionequation is solved for a particle to beconsidered "representative" (defined be-low) of the system.

3. Iteration. Since the fluid tempera-ture in step 1 is dependent on the parti-cle temperature predicted in step 2 andvice versa, steps 1 and 2 are iterated untilthe temperature converges.

This approach simulates the fluidtemperature 4istribution throughout theheater. It is then possible to conduct aseparate set of simulations to identify theworst-case scenario by solving the heatconduction equation for the slowest-heating particle. For this purpose, thepreviously determined fluid temperatureis used in a time-dependent convective ,boundary condition (Sastry, 1986; Chan-darana and Gavin, 1989), along with thephysical properties, characteristics, andtime scales of the slowest-heating parti-cle, e.g., a particle of the largest size en-

(under such controlled conditions),would be an acceptable procedure. It is

IIassumed here that the processor under-stands and can control its shift-to-shiftvariations.

IIReuse of Sample Particles

If a sample of transducer particlesreasonably represent the actual productvariations, they can be used for meJsure-

IIment purposes. RTD data need to be col-lected from independent samples fromthe population. If the same transduter

d 'I.particle is used repeatedly to eterl1}IneRTD, it is not clear how it might repre-sent variations in the sample population.An understanding of the impact of a

th . d 'Itracer's repeated use on e In epen~dence of a sample would be needed.: Inparticular, an insignificant effect of re-peated tracer use on the sample RT~would need to be shown.

Test Conditions ,al .'1Data are needed from actu pro~ess• !lconditions, since the objective 1S pre-

sumed to be filing of a commercial pro-cess with the Food and Drug AdmiuJ.s-tration. II

Reasons for MathematicalModeling II

There are various reasons for model-ing a process:

Since particle temperatures cannptbe measured during continuous flow, themlJdel permits sizing the hold tube ~b-quired for commercial sterility. Withoutmodeling, hold tube sizing becomes atrial-and-error process, with potenti~llydisastrous consequences if the trials donot adequately simulate commercialpractice.

Certain product and process param-eters have critical influences on the s~fetyof the processed product, and their iden-tification can be greatly simplified b~iuseof a model. Byvarying product and s,ys-tern parameters in simulation, it is pqssi-ble to determine critical control poin:~s.

Once we have a verified mathem~ti-cal model, we can understand how a ,II,

process behaves. Thus, any deviations orunusual behavior during processing maybe detected by comparing real physical

II'

ResidenceTime80 N TIN U E D

46 FOODTECHNOLOGY OCTOBER 1997 • VOL. 51, NO.1 0

I=-"""""---~"".

S PEe I' A L SEC T o Ning inlet and outlet temperatures, the ex-act character of the "thermal trajectory"needs to be characterized. Thus, fluidtemperature predictions along the tubeaxis need to be verified by checks ofmeasured fluid temperatures at interme-diate points along the heater.

(2) It is desirable to quantify the ex-tent of radial temperature variation.Monitoring at multiple points furthercomplicates an already complex process.Monitoring of fluid inlet and outlet tem-peratures and their variations is neces-sary. Monitoring of at least one internalfluid temperature is considered desirableto establish the fluid thermal trajectory.

(3) Fouling may alter the expectedthermal trajectory away from the above(usually exponential) estimate. However,significant deviations due to foulingwould likely be accompanied by changesin pressure drop and heating-mediumtemperature. to compensate for the foul-ing-hence, any such deviations shouldbe detectable on-line. If fouling is part ofthe worst-case scenario, it should be test-ed.

Hold Tube. The approach should besimilar to that for'a tubular heater.

Cooler. The approach should be sim-ilar to that for a tubular heater. However,for fluid temperature simulations, a lowvalue of hfp is conservative. This assump-tion causes the fluid temperature to droprapidly, with a low "thermal drag" fromthe particles. For worst-case simulations,however, a high value ofhfp is conserva-tive, because the particle surface is con-sidered to cool rapidly in response to thecold carrier fluid. Also, since the worst-case assumption that the particleachieves the carrier medium tempera-ture is conservative, br~akup is notconsidered a major issue in modeling.

Determination of htpTo determine hfp' SUItable combi-

nations of available methods could beused, including the following:

1. The assumption of Nu = 2.0based on classical solutions corre-sponding to heat conduction from aninfinite medium to a sphere.

2. Stationary particle methods(Chandarana et aI., 1990; Chang andToledo, 1989), where a stationary par-ticle is held in a fluid stream, which isadjusted to a desired flow rate.

3. Moving particle method, wherea particle is moved through atube atthe same velocity as an unattachedparticle, while having a thermocouple

VOL. 51, NO.1 0 • OCTOBER 1997

attached to it (Sastry et aI., 1990).4. Liquid crystal methods, where a

transducer particle is coated with liquidcrystal, which changes color with tem-perature. The particle is videotaped dur-ing its passage through the process sys-tem, and the colors (P!ecalibratedagainst temperature) are analyzed to de-termine hrp (Stoforos et aI., 1989; Bala-subramamam and Sastry, 1994b, 1996a).

5. Temperature pill, a temperaturesensor which transmits temperature in-formation to an external antenna (Bala-subramaniam and Sastry, 1996b; Bhami-dipati and Singh, 1995).

6. History indicators, such as chemicalmarker and thermal memory cell, detectan endpoint thermal conversion, and canbe used to back calculate heat transferover an entire heat-hold-eool process.

7. Melting-point indicator (Mwangiet al., 1993), where the melting of a poly-'mer indicator produces a color change.

8. Fluid temperature disturbance,where a large volume of cold particles isintroduced into a flowing fluid and thefluid thermal disturbance determinedand used for calculating hfp (Alhamdan,1995).

9. Temperature monitoring by mag-netic resonance imaging to determinethe temperature proflle of an object(Schrader et aI., 1992).

10. Off-line sampling, which involvessampling particles at various points inthe system, measuring internal tempera-ture, and calculating hfp'Thermophysical Properties

In aseptic processing of foods, ther-mal conductivity k increases slightly anddensity decreases with temperature. The

Distance along heater. .

A 3-1l1ustration of the effect of the.umed htpon fluid temperature predic-

tion via energy balances in a heater section.Low hfpvalues do not yield conservativefluid temperature predictions

apparent specific heat C may either in-crease slightly or chang:' depending onphase transitions. For modeling purpos-es, the use of constant values is accept-able, if their variation is understood andaccounted for by selection of the mostconservative real value.

The overall heat transfer coefficientwas considered easy to measure and not.a major issue.

An acceptable choice as a representaC

tive particle is a particle of size and shapethat yield reasonable fluid temperatures(with the same reality checks as for fluidtemperature simulations).

Periodic Verificationby Monitoring

Periodic model verification is possi-ble if flow rates, pressure drops, and inletand outlet temperatures are monitoredsimultaneously and operational shiftscould be detected. For example, an outletfluid temperature which responds tochanging inlet temperature in ways pre-dicted by the model could be one mea-sure of verification.

Software DocumentationChanges made in process modeling

software should be properly document-ed, using procedures that are standard inthe software industry.

Critical Control PointsThe following are possible critical

control points:1. Product composition, including

particle preparation steps (if appropri-ate), maximum particle size, particleloading, carrier formulation, preparation

steps, viscosity, and product quality (asit affects heating rate).

2. Initial temperature, includingpreparation steps, agitation, and de- ;-'greeofmix.

3. Flow rate.4. Process temperatures, including

fluid temperature at the end of theheater, and fluid temperature at theend of the hold tube.

5. Heating-medium temperature.6. Product pressure drop.7. System pressure.8. Swept-surface heat exchanger

dasher speed (if appropriate);' '.

Guidelines for Process FilingThe above procedures are intended

to serve as guidelines for processorsinterested in filing aseptic processesfor multiphase foods. While the list is .

-FOODTECHNOLOGY 47

sip:I~Residence

TimeGo N TIN U E a :lnot comprehensive, it represents a con-sensus among the academic, industrial,and governmental groups on the apJproach to be used in process filing. ~IREFERENCESAlhamdan, AM. 1995. Particle residence time distiibu-tion and bulk heat transfer coefficients of two-phaseflow in scraped surface heat exchanger and holdingtubes. Ph.D. dissertation, Ohio State Univ., Columbus.

Balasubramaniam,V.M. and Sastry, S.K. 1994a. Liquid-to-particle heat transfer in continuous flow through ahorizontal scraped surface heat exchanger. FoodiBio-prod. Proc.;Trans.IChemE, PartC, 72: 189-196.

Balasubramaniam,V.M. and Sastry, S.K. 1994b. Convec-tive heat transfer at particle-liquid interface in con,tinu-ous tube flow at elevated fluid temperatures. J FoodSci. 58: 675-681'1

Balasubramaniam,V.M., and Sastry, S.K.1996a. Liquid-to-particle heat transfer in continuous tube flow: 80m-parison between experimental techniques. Inti. J. 'FoodSci. TechnoL 31: 177-187. .

Balasubramaniam,V.M. and Sastry, SK 1996b. Nonin-vasive estimation of convective heat transfer beMeenfluid and particle in continuous flow using a remoletemperature sensor. J. Food Proc. Eng. 19: 223-240.

Bhamidipati, S. and Singh, R.K. 1995. Determination of

E C ALSECT

fluid-particle convective heat transfer coefficient. Trans.ASAE 38: 857 -862.

Cacace, D., Palmieri, L.: Pirone, G., Dipollina, G., andMasi, P. 1994. Biological validation of mathematicalmodeling of the thermal processing of particulatefoods: The influence of heat transfer coefficient deter-mination. J. Food Eng. 23: 51-68.

Chandarana, 0.1. and Gavin,A ilL 1989. Establishingthermal processes for foods to be processed aseptical-ly: A theoretical comparison of process developmentmethods. J. Food Sci. 54: 198-204.

Chandarana, D.L, Gavin,A III, and Wheaton, F.w. 1990.Particle/fluid interface heat transfer under UHT condi-tions at low particle/fluid relative velocities. J FoodProc. Eng. 13: 191-206.

Chandarana, 0.1. and Unverferth, JA 1996. Residencetime distribution of particulate foods at aseptic process-ing temperatures. J. Food Eng. 28: 349:360.

Chang, S.Y.and Toledo, R.T. 1989. Heat transfer andsimulated sterilization of particulate solids in a continu-ously flowing system. J. Food Sci. 54: 1017-1023.

Dail, RV and Steffe, J.F. 1990. Rheological characteriza-tion of crosslinked waxy maize starch solutions underlow acid aseptic processing conditions using tube vis-cometry techniques. J. Food Sci. 55: 1660-1665.

Gadonna, J.P., Pain, JP, and Barigou, M 1996. Deter-mination of the convective heat transfer coefficient be-tween a free particle and a conveying fluid in a hori-zontal pipe. Food Bioprod. Proc.; Trans. IChemE. PartC, 74: 27 -39.

Kim, H-J. and Taub, I.A. 1993. Intrinsic chemical markersfor aseptic processing of particulate foods. FoodTech-noL 47(1): 91-97, 99.

Manavel, J. E., Powell, R.L., McCarthy, M.J., and McCar-thy, K.L. 1993. Magnetic resonance imaging of mul-tiphase systems. In "Particulate Two-Phase Flow," ed.

M. Roco, pp. 127.140. Butterworth-Heinemann, Bos-ton. -

Mwangi, J.M., Rizvi, S.S.H., and Datta, A.K. 1993. Heattransfer to a particle in shear flow: Application to asep-tic processing. J Food Eng. 19: 55-74.

Sastry, S.K. 1986. Mathematical evaluation of processschedules for aseptic processing of low-acid foodscontaining discrete particulates. J. Food Sci. 51:1323-1328.

Sastry, S.K., Lima. M, Brunn,T., Brim, J, and Heskitt,BF 1990. Liquid-to-particle heat transfer during con-tinuous tube flow: Influence of flow rate and particle totube diameter ratio. J. Food Proc. Eng. 13: 239-253.

Schrader, GW, Litchfield. J.B., and Schmidt, S.J. 1992.Magnetic resonance imaging applications in the foodindustry. FoodTechno!. 46(12): 77-83.

Stoforos, NG, Park, KL., and Merson. R.L. 1989. Heattransfer in particulate foods during aseptic processing.Presented at Ann. Mtg., Inst. of Food Technologists,Chicago, June 25-29, 1989.

Swartzel, K.R.. Ganesan, S.G, Kuehn, R.T., Hamaker,R.W., and Sadeghi, F. 1991. Thermal memory cell andthermal system evaluation U.S.patent 5,021 ,981.

Zitoun, KB 1996. Continous flow of solid-liquid foodmixtures during ohmic heating: Fluid interstitial veloci-ties, solid area fraction, orientation and rotation. Ph.D.dissertation. Ohio State Univ.. Columbus.

Updated August 1997 from a paper presented during theforum. "NCFST and CAPPS Workshop on AsepticProcessing of Multiphase Foods, " at the Annual Meetingof the Institute of Food Technologists, New Orleans, La.,June 22-26, 1996.

Edited by Neil H.Mennelstein,senior Associate Editor.

'Iriir ••••••••••••••

~~2!~~cal1alidation. .

Theauthor,a ProfessionalMemberof 1FT isAssociateProfessor,Dept.of FoodScienceandTech-nology,VirginiaPolytechnicInstituteand StateUniversity,,Blacksburg,VA,24061.

The Food and Drug Administratiolhas Prior Attempts at ValidationBiological verification of an aseptic

suggested that the establishment of; process for low-acid multiphase (partic-each aseptic process for multiphase~ ulate) foods requires some form of bio-food products should consist of fou~ logical indicator that can be processed

through the aseptic system, retrieved in-elements: identifying and selecting a tact, and tested quantitatively for sterili-sterilizing value F0 for the product; :: ty. A variety of bioindicators using bacte-d i . . d Ith 1f rial spores have been developed and usedeve opmg a conservatIve rno e a"t . va' th mal processesIn nous er .reliably predicts the total lethality of A biological thermocouple system inthe heat process; quantitatively which suspended spores are encapsulat~

'I ed in a carrier and do not come intoverifying the lethality delivered by II contact with the product has been .usedmeans of a bioindicator; and listing'llie successfully with canned products (Pflugcritical factors of each aseptic proce!sls and Smith, 1977; Pflug et al., 1980; Jones

'I et al., 1980). Similarly, Hersom andand the procedures to be used for II Shore (1981) suspended spores in glasscontrolling these factors (Dignan et al., beads which were then embedded within1989). vegetable particles. Hunter (1972) em-

bedded spores in polymethacrylate beads

48 FOODTECHNOLOGY

and conveyed them through the thermalprocess, but embedding the spores istime consuming, recovery requires ace-tone which may be deleterious to theheat-injured spores, and the heat appliedis dry rather than wet.

Dallyn et al. (1977) suspended sporesin alginate beads. This approach provid-ed easy preparation of the particles, easyrecovery of the spores, and wet ratherthan dry heating. This type ofbioindica-tor was also used by Bean et al. (1979)and Heppel (1985). The particle sizesused in these studies were quite small,however, and thus not completely appli-cable to larger particulate foods.

A larger particle sizewas achieved byBrown et al. (1984), who embeddedspores in cubes formed from alginatemixed with pureed potatoes, peas, ormeat. However, particle shrinkage appar-

OCTOBER 1997 • VOL. 51, NO.1 0

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5 PEe I A L 5 E C T' o N

Biologic.alValidationeo N TIN U E 0

ently resulted in significant differencesbetween the calccl.ated and predicted le-thality of the particles, and the particleswere not shelf stable.

Sastry et a1.(1988).also developed abioindicator with a larger particle size.They suspended spores in an alginate so-lution, infused the spore suspension intomushrooms, and immobilized the sporesby gelation of the alginate. These parti-cles could then be freeze dried for shelfstability and rehydrated before use. Amajor disadvantage of this particle is thelarge inherent spore population presentin mushrooms, which makes accuratequantitation of the number of infusedspores difficult to determine.

Biological methods are needed tovalidate mathematical models which areused to determine acceptable thermalprocesses for aseptically produced foods.Various biological methods are currentlybeing used by food processing compa-nies to validate models and to confirmthe effectiveness of a specific asepticthermal process, in much the same waythat an inoculated pack is used to con-firm lethality of thermal processes forcanned foods (Segner et aI., 1989).

An inoculated pack test assumes thatthe food will have the inoculum evenlydistributed throughout the food. The testcans are placed at the slowest-heatingposition in the retort. This combinationof factors guarantees that the inoculatedpack will be measuring the worst-caseprocess at the coldest point in the slow-est-heating container.

The conditions achieved in inoculat-ed test packs cannot be replicated in bio-logical challenge studies with multiphaseaseptic foods. In a multiphase asepticprocess, each particle of the food receivesa different thermal process, with oneparticle receiving the least thermal pro-cess.Because it is not possible to identifywhich particle will re<:eivethe least ther-mal process, a statistically significantnumber of particles need to be tested.

Developing a ProtocolAworking group of scientists from

industry, trade associations, regulatoryagencies, and academia was convened bythe National Center for Food Safety and

50 FOODTECHNOLOGY

Technology (NCFST) and the Center forAseptic Processing and Packaging Stud-ies (CAPPS) in late 1995 and early 1996to assess the status of biological valida-tion of aseptic multiphase foods andproduce a standard operating protocolfor validation of aseptic processes. Theworking group addressed the followingproblem areas: .

Biological validation test require-ments. The working group agreed thatmicrobiological challenge testing meth-ods could be used (1) to validate a math-ematical model developed to predict thelethality occurring in an aseptic rilul- -tiphase food and (2) to serve as part of a~process filing to confirm lethality of aspecific operational system, .

All microbiological testing for bio-logical validation of a thermal processshould be sufficient to achieve commer-cial sterility. Biological validation ofmultiphase aseptic foods is to demon-strate the effect of thermal processing onthe particle which would receive theminimum thermal treatment. This'would be determined after consideringwhich particle was the slowest heatingand the fastest moving through the hold --tube.

Selection of test microorganisms.The test microorganism should have sta-ble thermal characteristics during anaseptic processing procedure and be ap-propriate for the intended use. Also, itmust have appropriate thermal resistance(D value). If the D value is too high,"skips" (improbable results) occur incount reduction. If the D value is toolow, large numbers of microorganismsare needed to ensure survivors. The Dvalue and z value should be determinedinthe same ~edia in which the incuba-tion is done; this is often the food beingprocessed. The D and z values should bedetermined experimentally for the testmicroorganisms at the temperature con-ditions of the aseptic process. Replica-tions of the D and z value determina-tions are required to assess the variabilityof the microorganism's thermal resis-tance.

Viability of the test microorganismmust be established in the incubationmedia. Care should be taken to includethe oxygen conditions (aerobic or anaer-obic) which will exist in the package dur-ing incubation. The test microorganismmay be either a mesophile or a thermo-phile, depending on the intended appli-cation. Because positive identification ofthe test microorganism is often needed, a

test microorganism with a unique identi-ty is desirable.

Inoculation procedures. The proce-dures for inoculation of particles foraseptic processing could be consideredappropriate for either the count reduc-tion method or the inoculated packmethod. While there are distinct differ-ences between the two procedures, bothhave appropriate uses.

In the count reduction method, aknown number of microorganisms areimplanted into the appropriate location(usually the center) of a food particle,

Simulated potato cubes made of epoxy andembedded with magnets, as well as chicken-alginate cubes, have been used to biologi-callyvalidate a multiphase aseptic process.

and the particle is passed through theaseptic processing system, then recov-ered. The number of surviving microor-ganisms is determined, and the lethalityof the aseptic process is established. Thismethod is much more rapid than an in-oculated pack method, which requireslengthy incubation periods. However, itis very costly because of the additionallabor required to collect the inoculatedparticles and enumerate the survivors.Only a limited number of particles canbe processed because of the difficulty ofmanufacturing and collecting the inocu-latedparticles.

Count reduction methods can be ac-complished by inoculating particles witha series of microorganism levels such as10" lOS, 106, etc., and recovering the in-oculated particles for determination ofsurvivors; this is usually difficult to ac-complish. A more common approach isto inoculate all particles to the same levelof microorganisms and then processthem through the continuous-flow ster-ilization system at a series of appropriateprocess temperatures. Varying the lengthof the hold tube and varying the flow

OCTOBER 1997 • VOL. 51, NO.1 0

,

rate of the product are not acceptablJprocedures, since they easily could IIchange the residence time pattern of theparticle in the hold tube. II

The inoculated pack method re- II~quires a relatively large number of parti-cles to ensure that an inoculated particlewill be placed in the containers to be in-cubated (Digeronimo et aI., 1997). Tllephysical characteristics of the critical:!heating food particle is determined a~dused to determine the characteristics Ibfthe inoculated particles (Sastry, 1997tOne proposed method of producing ~n-oculated particles is to use dehydrate~particles and to rehydrate them in anl!in-oculum suspension. The ability of theparticle to retain !;heinoculum shoul~ beverified. This procedure can produce I:

hundreds or even thousands of particleswhich can be used with virtually any IIIfood product. Because a large number ofinoculated particles are being used, is'~sues such as clumping during commer-cial operations need to be addressed. II,

Sample preparation. The inoculatedfood particle should have known and: re-producible physical and compositionalcharacteristics. If a dehydrated food i~IIbeing used, care should be taken to en-sure that the inoculated particle is fullyhydrated. There is also a concern that: thephysical characteristics of the food nbthave been altered in the dehydration4re-hydration operation. The lethality rateshould be no greater in the inoculatedparticle than in the food it represents!The inoculum level in the food partiJleshould be appropriate for the test beingconducted. Inoculum levels should beverified and the degree ofleaching deter-mined. Placement of the inoculum i~ theparticle should reflect the assumptionsof the model being tested.

Impact of thermal processing onlin-oculated samples. Biological validati0nof an aseptic process requires that thefood particle being used for testing be-have in a manner identical to that of thefood intended for processing. The tes~particle should maintain its physical in-tegrity throughout the aseptic process.The physical properties of the test parti-cle should be known before and afterl

'IIthermal processing to ensure that no:

MICHEL DIGERONIMO: WALLACE GARTHRIGHT. AND JOHN W. LARKINAuthor Digeronimo is President, Dover Brook Associates, P,O, Box 177, Chester, NY 1091,8, Author Garthright is Mathematical Statis-

tician, Food,and Drug Administration, 200 CSt., SW, Washington, DC 20204, And author Larkin, a Professional Member of 1FT, isFood Process Hazard Analysis Branch Chief, National Center tor Food Sately and Technology, Food and Drug Administration, 6502 S,

Archer Rd" Summit-Argo, IL 60501, Send reprint requests to author Digeronimo,

..........................

Statistical Designand Analysis

. . . . . .. . .

Although several variables have to beconsidered when establishing a thermalprocess, one of the more critical ones isthe particle's minimum residence time.Determining the length of time a particleis in the heating sections of the asepticsystem, and especially the hold tube, isessential, because process lethality isbased on both temperature and time. In-troduction of particles into the fluid sig-nificantly complicates the flow condi-tions and consequently can affect resi-dence time. The relevance and trustwor-

REFERENCESBean, PG, Dallyn, H, and Ranjith, H,MP, 1979, The useof alginate spore beads in the investigation of ultra. hightemperature processing, In "Food Microbiology andTechnology," ed, B, Jarvis, J.H,B, Christian, and H,D,Michener, pp, 281-294, Medicina Viva Seruizio Con.gressi S,r.l., Parma,

Brown, K.L" Ayres, C.A., Gaze, J,E" and Newman, M,E,1984, Thermal destruction of bacterial spores immobi.lized in food/alginate particles, Food Micro, 1: 187-198,

Dallyn, H, Falloon,WC., and Bean, P,G, 1977. Methodfor immobilization of bacterial spores in alginate gel.Lab, Pract. 26: 773-775,

Digeronimo, M" Garthright, W, and Larkin, J,W 1997,Statistical design and analysis, Food Technol, 51 (10):52-56,

Dignan, D,M" Berry, M,R" Pflug, I.J" and Gardine, T.D,1989, Safety considerations in establishing asepticprocesses for low. acid foods containing particulates,Food Technol. 43(3): 118-121,

Heppel, N,J. 1985, Measurements 'of the liquid.solid heattransfer coefficient during continuous sterilization offood stuffs containing particles, Presented at 4th IntI.

has been recovered, identification of thesurviving microorganism is needed.

Analysis of data. Count reductiondata should be analyzed, based on the as-sumptions of the model used. If the le-thality model is based on cold-point le-thality, care should be taken not to useintegrated sterilization values. Inoculatedpack data need to be analyzed by themethods outlined by the' Statistical De-sign and Analysis Working Group (Di-geronimo et aI., 1997).

o NA L 5 E C T

Aseptic manufacturing of low-acid,

single-phase foods such as milk and

milk-based products, cheese sauces,

and puddings is well established as a

reliable and effective means of heatpreservation. However, establishing ascheduled process to ensure commer-cial sterility of an aseptically manufac-

tured food containing particulates is

more complex.

changes have been made. Test particlesshould be evaluated to determine if theinoculum has been retained throughoutthe aseptic process.

Many models for aseptic systems donot include the lethal effects of the cool-ing operation. Care should be taken toaccount for these effects, eitherby elimi-nating the heating effect during coolingor by deducting calculated lethality.

Collection of samples and recoveryof survivors. Collection of inoculatedparticles can be done in three ways: inoc-ulated pack (growth/no growth), countreduction (number of survivors), and di-rect particle recovery (growth/nogrowth). In the inoculated pack method,the assumption is that any spoilage iscaused by a single particle in the con-tainer, even if multiple particles could bepresent. Apparently sterile inoculatedpacks must have the absence of viable or-ganisms confirmed by recovering a rep-resentative number of inoculated parti-cles in an appropriate growth medium.

Count reduction should be donewith an appropriate growth medium andgrowth conditions. Since count reduc-tion requires identification of the inocu-lated particle, a permanent method ofdistinguishing the inoculated particle isrequired. Once the inoculated particle

E C5 'lip

.~,

BiologicalValidationGo N TIN U E D

52' FOODTECHNOLOGY OCTOBER1997 ~ VOL. 51,NO. 10

•

I

Congo on Engineering and Foods, Edmonton, Alberta,Canada, July 7-10.

Hersom, A.C. and Shore, OJ. 1981. Aseptic processingof foods COinprising sauce and solids. Food Technol.35(5): 53-62.

,Hunter, G.M. 1972. Continuous sterilization of li'Quid me-dia containing suspended particles. Food Technol. Aus-tr. 24(4)158-159, 162, 164-165 ,

Jones, A.T, Pflug, I.J" and Blanchett, R. 1980. Perfor-mance of bacterial spores in a carrier system in mea-suring the F0 value delivered to cans of food heated ina Steritort J. Food Sci. 45: 940-945,

Pflug, I.J. and Smith, G,M. 1977. The use of biologicalindicators for monitoring wet-heat sterilization process-es. In "Sterilization of Medical Products," ed. E.R.L.Gaughran and K. Kereluk, p, 193-222 Multiscience,Montreal.

Pflug, I.J., Smith, G" Holcomb, R., and Blanchett, R,1980. Measuring sterilizing values in containers of foodusing thermocouples and biological indicator units. J.Food Protect: 43(2): 119-123,

Sastry, S.K. 1997. Measuring residence time and modelingsystem. Food Technol. 51 (1 0) 52-56

Sastry, S,K., Li, SJ., Patel, P., Konanayakam, M" Bafna,P., Ooores, S., and Bulman, R.B. 1988. A bioindicatorfor verification of thermal processes for particulatefoods. J. Food Sci.'53(5) 1528-1531,

Segner, WP, Ragusa, T.J., Marcus, G.L" and Soutter, EA1989, Biological evaluation of a heat transfer simulauonfor sterilizing low-acid large paruculate foods for asepticpackaging. J. Food Proc, Preserv. 13: 257-274,

Updated August 1997 from a paper presented during theNCFST and CAPPS Workshop on Aseptic Processing ofMultiphase Foods at the Annual Meeting of the Institute ofFood Technologists, New Orleans, La" June 22-26,1996.

Edited by Neil H. Mermelstein,Senior Associate Editor.

thiness of residence time data depend onnot only the experimental design butalso the statistical technique used to ana-lyze the results.

Selection of the statistical method iscrucial-incorrect use of a technique canresult in an incorrect estimate of the ster-ilizing capacity of the heat process. Mostof the time, statistical misuse is due toinsufficient understanding of the con-cepts of the mathematicaltechniques in-volved and of the heating and flow char-acteristics of the product in the asepticsystem. A firm comprehension of the un-derlying concepts of the statistical meth-od and of the thermal process allows aproper evaluation of the legitimacy ofthe selected minimum process lethality(the Fo value of the scheduled process).Avoidance of a self-inflicted careless sta-

lVOL.51,NO.10 • ,OCTOBER 1997

tistical analysis is essential.The National Center for Food Safety

and Technology (NCFST) and the Cen-ter for Aseptic Processing and PackagingStudies (CAPPS) held a workshop onaseptic processing of multiphase foods atthe 1996Annual Meeting of the Instituteof Food Technologists. A working groupon statistical design and analysis met todiscuss and develop appropriate statisti-cal procedures for use in the establish-ment of a multiphase aseptic food pro-

cess. This article presents the group'sfindings.

Particle Residence TimesSeveral researchers have measured

particle residence times through the 'heater(s), hold tube, or cooler(s) and at-tempted to fit the data to familiar statis-tical distributions. These have included adistribution-free method (Berry, 1989); alog-normal distribution model (Duttaand Sastry, 1990); aseries of superposi-

. FOODTECHNOLOGY 53

I5 PEe A L 5 E C T o N

StatisticalDesignGo N TIN U E D

I

. did' 'b' . h JtlOne norma lstn utlOns WIt tuenormal distribution describing the fastestparticles used to estimate the statisticallyfastest particle (Palmieri et al., 1992D; anautocatalytic logistic growth modell(Ab-delrahim and Ramaswamy, 1993); thefirst, second, and third central momentsof the normal distribution (Baptista etal., 1995); and the gamma and log-nor-mal distribution functions (ChandJranaand Unverferth, 1996).

After careful analysis of these ap-proaches, the working group agreed thata distribution-free method was the bostappropriate method to determine r~li-ably the characteristic fastest particle ofthe system. Because the small tail of oneside of the curve (i.e., the fastest pCU;lticle)is the area of the residence time distribu-tion of primary interest and the topl per-centile of the function's tail is difficult toaccurately establish, this top percentilerepresents a characteristic fastest reSi-dence time for all of the product thlt willever be processed in the system.

A cautious approach to estimation ofthe top percentile was taken. The groupwanted a 95% confidence (C) that thefastest time measured would be indudedin the fastest percentile (P) of all resi-dence times. This value of C = 0.95 waschosen because it represented the likeli-hood of collecting a characteristic parti-

'!cle within the fastest percentile in 19 outof 20 tests, a reasonable criterion forthese types of tests. The fastest percentilewas chosen as 1% of the particle's resi-dence time population, which was Jon-sidered consistent with the traditiorialuse of 2-3 standard deviations of themean to describe normal distributedvariations in retort applications.

The mathematical solution to tH.eprobability of obtaining the fastest parti-cle from a population of residence timesis very straightforward. If we measure theresidence times ofN particles, the ptoba-,~,'

bility that none is a member of the fastestpercentile is (1- P)I'. Therefore the prob-ability that at least one of the residencetimes is from the fastest percentile i~C =1- (1- P)N.Choosing a value ofNkuchthat C > 0.95 will ensure that at leatt oneparticle residence tim~ will be in the topP percentile of the population of resi-

54 f'OODTECHNOLOGY

dence times. The smallest such N is 299(i.e., 0.95 < 1 - 0.99299).

The collection of intact processedparticles is necessary for an accurate as-sessment of the system. A fractured par-ticle most likely flows through the sys-tem differently and is not characteristicof the population of particles that arebeing measured by the test.

Statistics ofBiological Validation

A number of biological validationprocedures have been developed formultiphase aseptic food processes, in-cluding an alginate-gel-bead procedure(Dallyn et al., 1977) and a knotted-string-enumeration method (Segner etal., 1989). Both groups of researchersmistakenly assumed that the statisticalanalysis procedures that had been usedfor a number of years in the canning in-dustry (Halvorson and Ziegler, 1933)were appropriate for continuous mul-tiphase aseptic food processing systems.Halvorson and Ziegler's underlying as-sumption of equal treatments for eachparticle cannot be satisfied for continu-ous multi phase aseptic systems becausethere is no way to gauge the residencetimes of the specific inoculated particles.

The working group recommended in-oculating a sufficient number of particlesto ensure ,that at least 299 are recoveredintact and enumerated successfully.Thisis a departure from traditional biologicaltesting procedures. Each particle shouldbe inoculated with the concentration ofmicrobes that one wishes to demonstratecan be killed by the process. Because eachparticle may turn out to be in the fastestpercentile, each one must get a sufficientkill. Because it is rarely desirable to enu-merate separately the concentration ofsurvivors for each particle, in practice thismeans that each particle must fail to showgrowth after processing. When 299 intactparticles are recovered and show nogrowth, this gives a 95% confidence that aparticle in the fastest 1% received a ther-mal treatment sufficient to destroy at leastthe concentration of microbes used to in-oculate the particles.

The processor should inoculate andtreat enough particles to allow for parti-cle breakup and for testing mishaps. Thebest practice is to inoculate and treat 299+ b particles to allow for at least 299 + fto be recovered intact and tested for ste-rility (or enumeration). A finding of sur-viving microbes is a process failure, not atesting failure. A testing failure is an oc-

currence that prevents any reading of theresult.

Finally, all readable results from the299 + f particles should be used as thebasis for the validation. A processor can-not pick and choose which of the pro-cessed 299 + f particles it will use todemonstrate sterility. For example, ifparticle breakup is expected to be lessthan 10% and testing failures less than1%, the test might be run with l.Ux 299= 332 inoculated particles. If 330 surviveintact and there are no testing failures,then all 330 must test satisfactorily, i.e.,must show no growth of the target or-ganisms.

Because a processor cannot tellwhich particular particle is in the fastestpercentile, every particle tested must beviewed as belonging to the fastest per-centile and be processed to a total com-mercially sterile endpoint. This require-ment to show adequate kill in every par-ticle tested must not be confused withthe combined overall kill measured bythe quantal method. In the quantalmethod, the thermal kill of the process isdetermined from both the number of or-ganisms used to inoculate each unit andthe number of processed units that stilldemonstrate growth after the process(Halvorson and Ziegler, 1933). For ex-ample, if 100 units are inoculated with1,000 microbes (3 logs), for a total of100,000 organisms, and 2 of the 100 pro-cessed units show growth, an estimate ofabout 2-5 surviving organisms of the to-tal 100,000 is made. This results in aclaim of more than a 4-log reduction inthe concentration of the inoculated or-ganisms.

This sort of calculation is not appli-cable for multiphase aseptic food pro-cessing, since each of 100 inoculated par-ticles receives a differentthermal treat-ment. The quantal approach requiresidentical treatments. The residence timeand thus the heating effects are too var-ied among the particles for them to beoverlooked. Therefore, one either mustshow that all particles have been rid ofviable test microbes or must recover the299+ particles and enumerate the survi-vors for each particle. In the latter case,the log reduction for the process is deter-mined on the particle demonstrating theleast log reduction.

The working group acknowledgedthat replications of a test only change thelevelsof C and P used to determine thestatistical reliability of the results, and thatonly one test of 299 + f intact particles is

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StatisticalDesign80 N TIN U E D

necessary to establish an understanding ofthe lethal treatment delivered by the pro-cess.HO'Y'ever,the group agreed that itwas critical for the processor to knowahead of time the type of trial-to-trialfluctuations that may occur for the pro-cess.The test run of 299 + f particlesneeds to be conducted on a system wherethe process is known to deliver the mini-mum thermally treated particle.

Demonstration ofa lack of signifi-cant trial-to-trial fluctuations should in-clude a sufficient number of tests so thatday-to-day variations, startup, fouling,personnel, and other factors are takeninto account. A processor may want tobiologically validate the system, at thescheduled process temperature, severaltimes so as to include runs at the begin-ning and end of one process on one dayand another on a different d~y.

No Magic Formula .The working group strongly stipulat-

ed that just performing an experimentwith 299+ particles is not going to be themagic solution to the establishment of amultiphase aseptic food process. A pro-cessor needs to make sure that all of thecritical factors are defined and that ap-propriate measurement techniques-as

indicated by the workshop's residence-time, modeling, and biological-valida-tion working groups-,-are employed.

Every precaution' needs to be takento ensure that the process developer hasconsulted the technical literature, avail-able proprietary data, food engineeringtheory, and comments from the otherworking groups when developing a con-tinuous multiphase aseptic food process.During development of the processingsystem, the developer will in alllikeli-hood need to conduct a sufficient num-ber of experiments to determine andmeasure the critical factors that are im-portant to the process before actually at-tempting to measure the characteristicfastest particle or biologically validatethe processing system. With this inmind, the working group also agreedthat for design and optimization studies,a smaller number of particles than 299can be used to determine failures ortrends.

It is the responsibility of the proces-sor and the process authority to estab-lish a multiphase aseptic food processthat is adequate and verifiable. Particularcare should be taken to ensure that allthe requirements and assumptions asso-ciated with a procedure used to measurean aspect of the process are satisfied.The statistical procedures outlined bythe working group will ,onlylend a mea-sure of I;onfidence to a procedure; they

, will not ~ake a bad proc~dure good.

REFERENCESAbdelrahim, KA and Ramaswamy, H,S, 1993, Mathe-matical characterization of residence time distributioncurves of carrot cubes in a pilot scale aseptic process-ing system, Lebensm, U.-TechnoI26: 498.504,

Baptista, P,N, Oliveira, FAR" Cunha, L,M, ,and Oliveira,JC, 1995, Influence of large solid spherical particleson the residence time distribution of the fluid in two-phase tubular flow, Inti J, Food Sci. Technol. 30: 625-637

Berry, M.R. 1989, Predicting fastest particle residencetime, In ':Proceedings of the First International Congresson Aseptic Processing Technologies," ed, J,V, Cham-bers, pp, 6-17, Food Science Dept., Purdue University,West Lafayette, Ind,

Chandarana, 0.1. and Unverferth, JA 1996, Residencetime distribution of particulate foods at aseptic process-ing temperatures J Food Eng, 28: 349-360,

Dallyn, H, Falloon, w.e., and Bean, PG,1977. Methodfor the immobilization of bacterial spores in alginategel. Lab, Pract. 26: 773-775,

Dutta, B, and Sastry, S,K, 1990, Velocity distributions offood particle suspensions in holding tube flow: Distribu-tion characteristics and fastest-particle velocities, J,Food Sci. 55: 1703-1710,

Halvorson, H,O, and Ziegler, N,R, 1933, Application of, statistics to problems in bacteriology, I.A means of de-

termining bacterial population by the dilution method, J,Bacterial 25(2): 101-121,

- Palmieri, L, Cacace, D" Dipollian, G" and Dall'Aglio, G,1992, Residence time distribution of food suspensionscontaining large particles when flowing in tubular ,sys-tems, J, Food Eng, 17: 225.239

Segner, W.P,Ragusa, TJ" Marcus, e.L" and Soutter,EA 1989, BiologicaLevaluation of a heat transfer sim-ulation for sterilizing low-acid large particulate foods foraseptic packaging, J Food Proc, Eng, 13: 257 -27 4,

Updated August 1997 from a paper presented during theforum, "NCFST and CAPPS Workshop on AsepticProcessing of Multiphase Foods," at the Annual Meetingof the Institute of Food Technologists, New Orleans, La"June 22-26, 1996,

Edited by Neil H. Mermelstein,Senior Associate Editor.

Issues Involved in Producing'a Multiphase Food ProductDOMINICK DAMIANO "The author is Senior ResearchScientist,NestleResearchand Development,Inc" 201 HousatonicAve" NewMilford,CT06776,

Thermal processing of multiphase food, products has received considerable

attention in recent years. While there

are continuous processes for low-acidparticulate foods elsewhere in the

world, there have been none in theUnited States.

Many have attributed this to the dif-ficulty and/or uncertainty in getting a

56 FOODTECHNOLOGY

particulate low-acid continuous thermalprocess filing accepted by the appropri-ate regulatory agency. The Food andDrug Administration has, through scien-tific meetings and publications in jour-nals,"stated its position over the past 10yea~sor so as to what would be requiredin a successful filing (Dignan et al.,1989). This position has evolved in lightof scientific publications over the sametime span, but there still has been no ap~

parent commercial activity by industryin the U.S.

To address this situation, a workshop'on aseptic processing of multiphasefoods was held in late 1995 and early1996 (Larkin, 1997). The workshopbrought together representatives of gov-ernment, industry, and academia to ad-dress regulatory and other technical is-sues. The workshop went a long way inaddressing these issues, but did not ad-

OCTOBER1997 • VOL.51,NO. 10

'A L 5 E C T; o N

IssuesInvolvedGo N TIN U E D IIdress other technical issues, as weIllasnontechnical and business issues. 11hisarticle will broadly discuss these aJdother issues to provide a greater under-standing of what is required for an\ac-ceptable regulatory filing. I

Working Group IssuesIssues covered by the working iroups

(Sastry, 1977;Marcy, 1997;Digerortimoet aI., 1997) are briefly described b~low:

Critical Particle. The following,"theme issue" was discussed at the ,wI' ork-

- I

shop: If there was a way to measure thetemperature of the "critical part" of the"critical particle" continuously, then onlya hold tube calculation would be rJ~qui red, no different from what is neededfor liquids. The "critical particle" is theone particle of all particles processJdwhich contains the discreet elementwhich receives the least thermal tre~t.ment (or other treatment that contl:'ib-utes to lethality). This can be becaJbe it isthe fastest or it contains the discreet ele-ment which is the coolest, or a cOnl~ina-tion of both. The key is to "prove" thatthe "critical" part of the critical pa~ticlereceived the target thermal treatment.

Statistical Design and Analysis! Thefollowing issues were generally agrJed onas being resolved: (1) There is genefaldissatisfaction with existing models forpredicting particle residence time d'istri-bution (RTD). These models are es~en-tially empirical and use all of the re~i-dence time data (fast, slow, and average),leading to possible errors in the preaic-tion of consequence-that of the fJstestparticle(s). This issue was addressed bydeciding that the fastest particle determi-nation would require physical meaJure-ment. (2) For RTD measurements or forbiological validation, a minimum samplesize of299 different, intact, represeata-tive particles is required to concludb thata significantly fastest particle was collect-ed (distribution free analysis). Addl'ess-ing this issue has helped to put to r~st

l'fears that very large numbers of pa~ticleswould be required to collect a "fastest"particle. (3) More work is needed bfforemost probable number (MPN) tecH-niques are useful (i.e., the Halvorsoh-Zeigler equation is inappropriate fo'r es-

"

II

60 FOODTECHNOlOGY

timating lethal treatment in aseptic sys-tems). (4) A non-distribution-based ex-perimental design should be used to as-sure that the fastest particle is collected.

However, the following question re-mains: For a well-designed process (us-ing nonparametric statistics), are repeti-tions of runs of 299 (replicate confirma-tions) particles necessary?A consensuson this point was not reached, so eachprocessor will need to address this issueindividually. This issue can have a sub-stantial impact on time and cost of con-ducting RTD measurements and biologi-cal validation determinations.

Mathematical Modeling arid Physi-cal Properties. The following issues weregenerally agreed on as being resolved: (l)Biological validation should be used toverify a model, and the model used todetermine the process. For a model, thefluid-to-particle surface heat transfer co-efficient hfpmust be determined. Onecan claim a Nusselt number Nu = 2.0,which is the theoretical case of conduc-tion through a film of stagnant liquidaround a spherical particle. However,this represents the lowest theoretical val-ue for hfpand is ultraconservative. (2)Methods to measure and determinephysical property data for use in modelsare needed. (3) As an aid 'in model devel-opment, the fluid temperature can aleways be directly measured anywhere inthe system or conservatively estimated.(4) If credit is to be given for lethalityduring cooling, it must be demonstratedthat particle breakup does not occur (tu-bular only) or that the particle nevergoes below the surrounding cooling liq-uid temperature.

The following issues were generallyagreed on as being unresolved: (1) Accu-racy is needed in estimation ofhfp' (2)Heat exchanger fouling may lead to liq-uid temperatures different from thoseused in the model. (3) Developing amodel was clearly outside the scope ofthe workshop. Fortunately, useful mod-els do exist and are available from vari-ous sources. These models are quitecomplex, and any model used wouldneed to be part of the regulatory filing.

Residence Time Distribution. Thefollowing issues were generally agreed onas being resolved: (1) For situationswhere the fastest particles must beknown, the RTD must be measured. Be-cause a model is required to establish thethermal process and biological valida-tion is required to verify the model, the, RTD data need to be as accurate as possi-

ble. (2) As paft of a flling, a processorcan claim, for tubular flow, that no parti-cle is faster than 2.0 times the mass aver-age velocity or faster than the theoreticalfastest liquid velocity proflle. However,except in unusual circumstances, theseclaims will probably not be used in anyfilings. (3) Various options exist for di-rect measurement of fastest particles;magnetic techniques seemed to be theeasiest to apply in a commercial system.(4) If a product or process changes, theRTD needs to be reestablished;

The following issues were generally. agreed on as being unresolved: (1) Whenusing magnetic measurement tech-niques, what effect do particle densitydifferences due to the magnets have onthe data? (2) What is the cost of runninga test, or series of tests, of 299 intact par-ticles? For magnetic RTD measurement,the workshop estimated a particle read-ing every 2-3 minutes. With a margin ofsafety added, a minimum of 299 intactparticles are required. On an industrialline, this would require 360 x 2-3 min-utes, about 12-18 hours of running timewith real product under real condi-tions-in addition to making the parti-cles!

Biological Validation. The followingissues were generally agreed on as beingresolved: (l) Guidelines for choosing andusing the rest organism, and for prepar-ing the inoculated particles were devel-oped. (2) Guidelines for the collection ofsamples were given. One must measureand capture the "critical" particle. Inoc-ulated pack was a favored method. (3)Methods for analysis of data and biologi-cal enumeration were presented. MPNtechniques should be avoided. Data anal-ysis is easier if spores are at the center,but this is more difficult to carry outproperly. (4) Validation is not a singletest, but a series of experiments wherebythere are some survivors at certain exper-imental conditions (easiest is to varytemperature). (5) Particles can be col-lected aseptically into a growth medium.

The following issues were generallyagreed on as being unresolved: (l) As-sessing the value of center point vs inte-grated sterilization value, and analysis ofdata when skips or tailing occurs. (2)The efforts required for maintenance ofheat-resistant spore crops, and ensuringthat sublethal spores grow in the biologi-cal validation tests. (3) Inoculum levelsin particles must be verified, but how doyou prove there is no leaching of spores?(4) How close can "inoculated particles"

OCTOBER1997 • VOL.51,NO.10

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5 P £ C ).L 5 £ C T o Nbe produced to the "actual" food prod-uct, considering physical properties? (5)How should the issue oflethality duringcooling be addressed? Even if credit isnot to be taken for this lethality as partof the filing, it still must be estimated asaccurately as possible as part of any bio-logical validation data analysis. (6) As forthe RTD tests, what potentially are thetime and cost of biological challengetests?

Other IssuesThere are other issues, technical and

business:Technical. (1) The whole workshop

centered on particles. The next questionthat should be addressed is,What is a"particle?" Clearly,it should not be de-fined as a dimension, but in terms whichaddress heating rates. No doubt, a filing ofa particulate-containing product that ishandled as "nonparticulate" is much sim-pler. (2) Products and processes will al-waysbe changing for marketing and otherbusiness reasons. Under what circum-stances will another filingbe necessary?(3) Software and associated models mustbe clearly understood by the user and thethermal process authority. (4) Real resultsof RTD tests and heating characteristics ofactual particles vs formulated particles forbiological validation must be taken intoaccount. How easy will it be to make mag-netic particles?

Business. (1) Is there a business? Whatare the business risks?If a processor in-stalls a new line for an aseptic particulateprocess and after some period of time theproduct doesn't sell,what will the proces-sor do with the process line? How trans-ferable is the equipment? Is the line orequipment so unique that it can't be usedfor anything else?In many firms there ismuch in the way of existing investmentsthat are working fine and making money.They life not just going to be thrownaway.The same goes for knowledge. (2)What is the justification for the process/product? It has to be one or a combina-tion of increased quality, innovative orless-expensive packaging, package size,and, most important, overall cost.

62 FOODTECHNOLOGY

Case StudyA separate working group was

formed of participants from industry,CAPPS, NCFST;and the National FoodProcessors Association (NFPA) to com-plete a case study filing for an FDA-regu-lated product from a fictitious processor(Larkin, 1997). Much of the informationand data in the case study were obtainedfrom studies conducted by NFPA, butmany unresolved issues from the work-ing groups were still unresolved in thecase study. Since this case study wasbased on a fictitious situation, contribu-tors to the case study took liberties to ad-dress some problems. Things may not beso simple in real life.

The following are issues I feel werenot adequately resolved in the casestudy: (l) It presented data on alginateparticles, but what are the differences inheating rate and RTD compared to thereal product? Will it be possible to makeparticles with the same characteristicsas the target particles? (2) The casestudy spore kinetics (z value) were as-sumed constant above 260°F.We knowthat in reality the kinetics are not linear,but this is true for liquids as well. (3)How accurately is the contribution oflethality during cooling accounted for?(4) The cost and time of determiningphysical properties, h~, RTD tests, mod-el simulations and vahdation, and bio~challenge are still concerns. Althoughthis can be a major undertaking, wenow know essentially what is required,and processors are in a better situationto estimate the cost of a particulate fil-ing.

Using the caSestudy as a model, anindustrial equipment supplier, Tetra RexPackaging Systems, Buffalo Grove, Ill.,recently had a filing for a particulateaseptic process accepted by FDA (Pala-niappan and Sizer, 1997). Certainly, forthis filing to be accepted, many of the is-sues discussed above had to be resolvedand/or dealt with to FDA'ssatisfaction.

Is It Worth Doing?Many but not all of the issues that

have been considered hurdles from a

regulatory standpoint have been re-solved. A greater understanding hasbeen achieved of the necessary factorsto be addressed for a successful regula-tory filing. Every situation is unique,and there will probably never be a"cookbook" instruction to follow whena filing is prepared for particulates. Thecase study was compiled as an exampleonly. It should not be implied that if aprocessor follows the example, it willresult in an acceptance. The most im-portant thing is to prove, using goodscience, that a process is safe. The work-shop participants used the term "rea-sonably conservative" to describe howsituations were to be treated.

FDA'sacceptance of the recent asep-tic particulate filing shows that it can bedone. The cost and time of a filing maybe a hurdle, but processors can nowmake better estimates of what the regu-latory filing aspects of a project will costin time and money. The case is no longer"How do we do it?" but "Is it worth do-. ?"mg.

ReferencesDigeronimo, M, Garthrighl, w., and Larkin, J. W. 1997.

Statistical design and analysis.Food Technol. 51 (10):52-56.

Dignan, D. M., Berry, M. R, Pflug, I. J and Gardine,T. D.1989. Safety considerations in establishing asepticprocesses for low-acid foods containing particulates.Food Technol. 43(3): 118-121, 131.

Larkin, J. W.1997. Workshop targets continuous multi-phase aseptic processing of foods. Food Technol.51(10) 43-44

Marcy, J. E., 1997. Biological validation. Food Technol.51 (10): 48-53

Palaniappan,S. and Sizer, C.E 1997. Aseptic processvalidated for foods containing particulates. Food Tech-nol.51 (8): 60-62, 64, 66, 68

Sastry, S. K. 1997. Measuring the residence time andmodeling the system. Food Technol. 51 (10): 44-48

Updated August 1997 from a paper presented during theNCFST and CAPPS Workshop on Aseptic Processing ofMultiphase Foods at the Annual Meeting of the Institute ofFood Technologists, New Orleans, La., June 22-26,1996. The author thanks Nestle Research andDevelopment, Connecticut, for the support given him inparticipating in the workshop. Thanks also goes to allworkshOp participants, especially those who participatedin the case study

Edited by Neil H. Mennelstein,SeniorAssociate Editor.

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