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Thermal Drying of Biomaterials with Porous CarriersC.Z. Stmmilto a , I. Zbicinski a & X. D. Liu ba Faculty of Process and Environmental Engineering. Technical University of Loódzź. ul.Woólczanska , Loódzź, 175.90–924, Loódzźb Depanment of Mechanical Engineering, Tianjin Institute of Light Industry. 1038 Dagu NanluRoad , Tianjin, P.R., 300222, ChinaPublished online: 19 Oct 2007.

To cite this article: C.Z. Stmmilto , I. Zbicinski & X. D. Liu (1995) Thermal Drying of Biomaterials with Porous Carriers, DryingTechnology: An International Journal, 13:5-7, 1447-1462, DOI: 10.1080/07373939508917032

To link to this article: http://dx.doi.org/10.1080/07373939508917032

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DRYING TECHNOLOGY. 13(5-7). 1447-1462 (1995)

THERMAL DRYING OF BIOMATERIALS WITH POROUS CARRIERS

Cz. Strumillot. I. Zbicinskil and X. D. LiuZ 1. Faculty of Process and Environmental Engineering. Technical University

of t o d i , ul. Wolczariska 175. 90-924. L o d i Poland 2. Department of Mechanical Engineering. Tianjin Institute of Light Industry.

1038 Dagu Nanlu Road. 300222. Tianjin, P.R. China

Key Words and Phrases: dehumidified air; drying with carrier; quality retention; thermosensitive material; yeast

ABSTRACT

Thermal drylng of bioproducts often leads to a significant deterioration of product quality. One of the simplest methods for bener quality retention of thermosensitive materials is drying of the materials in a mixture with porous carriers. In order to determine the effect of carriers presence on the mechanism of dehydration process a set of experiments was carried out in a laboratory vibrofluidized bed dryer. The experiments were performed for three different carriers (wheat bran, ground rape and flour). Baker's yeast was taken as an example of a sensitive material. The effect of following parameters on degradation processes was analysed: concentration and kind of a carrier. temperarure and flowrate of inlet air and frequency of a vibrated bed. The analysis of the results proves that temperature of inlet air and kind and concentration of a carrier are the most important factors affecting final product quality. The results show that the main effect of a carrier's presence on quality retention comprises changes of a structure of labile material. Sorptional properties of the carrier are less imponant. In second part of the work dehumidified air was applied as a drying agent to enhance quality retention of biomaterials. Results obtained are encouraging, however this method requires further research.

Copyright 0 1995 by Marsel Dckkcr. Ins

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INTRODUCTION

During thermal drying of biosynthesis products, such as bakets and brewets

yeast. antibiotics, vitamins, enzymes, protein product etc., different types of

degradation processes may occur L1). The degradation processes not only depends

on drying process parameters (temperature. air flow etc.) but also on physical

characteristics of biosynthesis materials such as particle size, porosity,

hygroscopicity etc.

To obtain good product quality a proper choice of drying equipment and

operating parameters of the process is decisive. There are also other possibilities to

enhance final quality of labile materials. One of the simple methods is drying of the

materials in a mixture with porous carriers (2. 3. 4). However, this method can be

applied if the carrier and the labile material do not have to be separated after drying

vrocess.

Carriers mixed with biomaterials decrease biomass' moisture content (by

absorption of water). change their porosity and surface/volume ratio a. These

effects improve drying characteristics and lead to better retention of product's

quality after thermal drying. Some biomaterials, such as most of bacteria and some

paste like materials can hardly be formed into granules and dried in directly

fluidized bed or drum dryers. However, if they are mixed with porous carriers or sprayed onto a fluidized bed of a carrier; fluidized bed dryers or rotary dryers can

be applied. . .

Another possibility to improve drying quality indexes for labile materials

might be using dehumidified air as a drying agent. In the atmosphere of dry air

driving force is higher and wet-bulb ;emperatwe of the material is lower than in the

atmosphere of ambient air. The latter effect may be particularly important to

maintain high quality of thermosensitive products.

No research in this field has been reported in the literature. The main aim of

this work was to determine a drying mechanism in the presence of porous carriers

and to evaluate the application of dry air as a drying agent for sensitive materials.

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B1OMATERlAL.S WITH POROUS CARRIERS 1449

SIGNIFICANT TEST FOR QUALITY RETENTION OF BAKER'S YEAST

Bake<s yeast (S. cerevisae) with average initial moisture content 2.3 k g k g

was mixed with three types o f carriers, wheat bran, extracted ground rape and

flour, in different concentrations and then the mixtures were exuuded into pellets o f

about 1.0 mm in diameter.

The experiments were performed in a laboratory scale, batch-type

Mbrofluidized bed dryer 150 mm in diameter. The range o f operating parameters is

shown in Table I

To minimise the number of experiments, they were designed by the method

o f the right intersection (i) and the total number o f experiments was 27.

As quality criteria the relative viability. RVy, and saccharolytic activity.

RAY, of yeast were applied. The viability is defined as a ratio o f the number o f

living cells to the total number of cells:

v y = Number of Living Cells

Total Number of Cells

The relative viability is expressed as a ratio o f the actual to the initial content o f

living cells in the sample:

To evaluate the relative activity o f yeast. oxygen-fiee activity o f mass transfer (CO,

production) was determined on the basis o f manometric measurements with Jalecki

apparatus.

Activity o f yeast is defined by equation (3):

Ay = Volume of CO, Production Unit Weight of Yeast. Unit of Time

(3)

Relative activity ofyeast is described by equation (4):

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TABLE I. Range of overatinn parameters

I T, i. I v I f I 13 1 c I - ... ... .- . -

60 1 40 1 5.00 1 rape I 0 I I 90 I 50 I 6.67 I bran I 25

120 60 8.33 flour 50

A detailed description of the methods used to determine RVy and RAy was given in

(1).

Results of the significant test for relative viability and activity are displayed

on figures I and 2, respectively. The values of F ,,,,, F,,,, and F ,,,, are the critical values of F distribution in the figures.

5 Figure I and 2 show that among all the parameters considered, the

temperature of inlet air. T,,j,, concentration of a carrier. c, and kind of a carrier. p, are significant for both viability and activity during thermal dlying of baker's yeast. On the basis of the above mentioned results two effects were investigated

thoroughly; the effect of a carrier concentration and the effect of a kind of a carrier

on the final quality of bakefs yeast.

EFFECT OF A CARRIER CONCENTRATION ON THE FINAL QUALITY OF BAKER'S YEAST

In the first step to analyse this effect, basic properties of baker yeast and

mixtures of bakeh yeast and carriers were determined by the benzene soaking method (6). Table ? presents initial moisture contents, X, and bed voidages, 6 (%).

for different mixtures.

Table 2 shows that the most porous structures of a mixture yeast-carriers contain wheat bran, less porous ones contain flour.

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BIOMATERIALS WITH POROUS CARRIERS

Figure 1 Significant test for the relative viability

Figure 2 Significant test for the relative act iv i ty

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STRUMIUO. ZBICI~~SKI, AND LIU

TABLE 2 Pro~enies ofbaker's veast. carriers and their mixtures

Figure 3 reports the results o f vibrofluidized bed drying and activity

degrada!ion o f baker's yeast mixed with wheat bran at different concentrations.

Mixtures containing more carrier show different degradation hnetics and better

final quality retention than mixtures with less carrier. The retention o f final activity

for the mixture with 50% o f bran is around 74% while yeast without a carrier

degrades to about 45% of the initial activity.

In the mixture, the carrier introduces non-themal-sorptional mechanism o f

moisture removal from yeast. Presence of a carrier decreases the initial moisture

content of the mixture (Table 2). For bran concentration of 25 %, the average

moisture content o f yeast decreases from 2.36 kgkg to 1.41 kgkg.

This amount o f moisture was removed by sorption to the carrier. The rest of

the moisture (from 1.41 kgkg to the desire moisture content) is removed thermally

by dewatering during the drying process.

Similar conclusions can be drawn from the analysis of the viability

deterioration rate for different carrier concentrations.

The relative deterioration rate can be described by K,, defined by an empirical

first-order kinetic model:

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BIOMATERIALS WITH POROUS CARRIERS

Figure 3 The activity degradotion of boker's yeost with wheat bran in different concentrotions

Ta, ,n-600~. h-S.5crn

Where Q is the quality criterion of bakeh yeast. that is, relative activity or

viability. The tint-order rate constant K, has the form :

w r . 0

0.6

0.4

KO = (k,+ k, X+k, T,+k, X T,) exp[(k,+k, X)l(273.1 S+T,,)I (6 )

- G-.--l I 0 2- - [ 0 - pure ysoel - . - ycost with 25% bran - yeast wi th 50% bmn -

where X and T, are the moisture content and temperature of dryed material.

respectively. and k, . k, . ... . k, are the constants determined experimentally.

Figures 4. 5 and 6 show that curves describing relative degradation rate differ

significantly for different concentrations of a carrier. For yeast without a carrier the

degradation rate is high. Unprotected yeast cells are subjected to high ambient

temperature which promotes a high killing rate. The presence of carriers protects

panicles of thermosensitive materials from a direct contact with a drying agent which

makes the degradation curve flat. The more water is removed on contact-sorption

way the better final quality can be achieved.

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Figure 4 Relative deterioration rote of viability fo r the mixture with bran (50%)

Figure 5 Relative deter iorat ion rate o f viabil i ty f o r the mixture with bran (25%)

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BIOMATERIALS WITH WROUS CARRIERS

Figure 6 Relative deterioration rote of viability for the baker's yeast without carrier

EFFECT OF A KIND OF CARRIER ON THE FINAL QUALITY OF BAKER'S YEAST

Each of the three carries used in the experiments (wheat bran, extracted

ground rape and flour) is characterised by different porosity (see Table 2) and

hygroscopicity (1).

The most hygroscopic material is flour the less is bran (2). Let us analyse results of experiments for drying at a relatively high temperature of air (90 OC) and

for identical content of a carrier (15% mass) in a mixture [initial moisture content of

the mixture was similar (Table 2)).

Figure 7 represents deterioration of relative activity of bakefs yeast as a

function of moisture content. Figure 7 shows that relative activity for each mixture differs significantly. The best results were obtained for a mixture with bran where the final retention of the quality was equal to 42'70, the wont for flour as a canier (retention of the activity was only 26%). Similar analysis can be carried out on the

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Figure 7 Degradation of relative activity of baker's yeast with different carriers

basis of the relative deterioration rate of yeast activity. Figures 8. 9 and 10 present

changes of the degradation rate constant as a function of moisture content as well as temperature of the material.

A typical shape of the degradation function was obtained for each carrier 0. A sharp increase of the degradation rate from the beginning of the process to a

certain value of moisture content of the material and then decrease of the degradation rate can be observed. The biggest increase of the deterioration rate was observed for the mixture with flour, the smallest one for wheat bran. The analysis of results proves

that a carrier which "produces" a more porous mixture with biomaterial improves the

drying characteristics and promotes. better quality retention. Hygroscopicity of a carrier plays less important role in this case. Changes in. the structure of the dried

material and developing of heat and mass transfer area are the key factors promoting

quality retention after drying.

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BIOMATERIALS WITH POROUS CARRIERS

Figure 8 Relative deteriorotion rote of activity for the moisture with bron (25%)

Figure 9 Relative deterioration rote of octivity for the mixture with rope (25%)

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STRUMIUO. ZBICI~~SKI, AND LIU

Figure 10 Rclotive detcr iorot ion r o t e of oct iv i ty for the mix tu re w i t h f lour (25%)

DRYMG OF BAKER'S YEAST BY DEHUMIDIFIED AIR

A set of preliminary experiments on drying of bakefs yeast by dehumidified

air as a drying agent was performed in a fluidized bed dryer. The dehumidified drying

agent allows to increase heat and mass transfer driving force without rising the

temperature. Simultaneously, wet bulb temperature o f the material during the drying

process is smaller than for humid air. This effect may be particularly important for

dlying o f labile materials.

Ambient air was d r i ~ d in a dehumidifier (MUNTERS-600, Germany) to about

1.5% o f relative humidity (with temperature rise to about 35-40 OC). Air leaving the

dehumidifier was heated up to desired temperature in a heater.

As an example o f the results, relative viability and activity degradation as well

as drying kinetics both for "humid" and dehumidified air are shown in figures I I and

12.

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B I O M A T E W WITH POROUS CARRIERS

Figure 11 Relative viability ond activity degradations

during drying by ombient and dehumidified air

Amblont air: p-34%. T-21.c Oohumidifiod olr: v-1.51. T-3f.c

RVy R*y T -60% h-5.5sm. V-50m /h

Figure 12 Drylng k inet ics of baker ' s y e a s t by a m b i e n t o n d dehurnidl f iad o i r

1.00

0.95

0.00

0.85

0.60 0 5 10 15 20

t

- 1.0

- 0.6 -

- 0.6 -

- 0.4

- 0.2

- o - RVy. amb. oir - - RVy. dohu. 01, . - m y . amb. .ir - - RAY. dahu. air

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In every trial the results obtained were encouraging. As a result of a shorter

drying time to the desired final moisture content and a lower temperature of the

material during the process (Figure. 12) better quality retention was observed for dry

air as a drying agent. The most promising results were obtained for low inlet air

temperature. The final quality retention was about 70% which can be compared with

freeze drying quality. However. further studies in this field are necessary.

CONCLUSIONS

I. The most important parameters affecting the activity and viability of baker's

yeast mixed with carriers during thermal drying are temperature of the inlet air, kinds

of a carrier and concentration of a carrier.

2. During the drying of biomaterial with a carrier. a contact-sorption

mechanism of moisture removal occurs. A porous carrier changes the structure of the biomaterial to more porous and develops heat and mass transfer area of the mixture which intensifies moisture removal from the material, decreases the drying time and

improves the final quality.

3. The application of dehumidified air as a drying agent to enhance quality

retention gives encouraging results. Further research in this field is necessary.

ACKNOWLEDGEMENT

The work was carried out as part of the research project No. 1392/3/91

sponsored by the State Cornminee for Scientific Research in the year 1993.

NOTATION

AY activity, mJ k g ' s ' c concentration of a carrier, % f frequency of vibrofluidized bed. Hz F value of F-test. -

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BIOMATERIALS WITH POROUS CARRIERS

h static bed height. cm KD deterioration constant. min-I k,, kl. ... k6 constants in equation (2). - Q quality criteria for baket's yeast. - RAY relative activity of bakeh yeast. - RVY relative viability of baket's yeast. - V flowrate of inlet air. m3lh VY viability. % X moisture content, k!&g

Greek symbols rp relative humidity of air. % P type of a carrier. - E bed voidage. %

Subscripts 4 a ~ r air c concentration of a carriel

error frequency of vibrofluidized bed

in inlet m material o initial v flowrate of air P type of a carrier

REFERENCES

1. Kaminski. W., Adamiec, 1.. Grabowski, S., Zbicinski. I.. Strumitlo. C. and Mujumdar. A. S. 1992. Application of Degradation Kinetics to Optimization of Drying Process for Yeast's. Drying of Solids. 1992 h n S. Mujumdar. pp. 250-266.

2. Taeymans, D. and Thursfield. 1. 1988. Fluidized Bed-Drying of immobilized Yeast's. in Drying'87, A. S. Mujumdar, pp. 160-165.

3. Tutova E.G., Strumitlo C., Zbicbiski 1. 1993, Intensification of heat and mass transfer during of labile materials. Drying Working Party Meeting. Utrecht, Holland.

4. Zimmermann. K. and Bauer, W. 1989. Fluidized Bed Drying of Microorganisms on Carrier Material. Proc. 5th Conf. on Engineering and Food. London. Vol. 2. pp. 666-678.

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5 . Doway. S. and Wearden, S. 1983. Statistics for Research. John Willey and Sons. Inc., New York.

6. Evem. D. H. 1989. Pore System and Their Characteristics. Elsevier Science Publishers 8. V.. Amsterdam. pp. 1-21.

7. Mishkin, M. Saguy. I . and Karel. M. 1984. Optimization of Nutrient Retention During Processing: Ascorbic Acid in Potato Dehydration. Journal of Food Science. Vol. 9. pp. 1262-1270.

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