cap.20_alcohol txtbook 4th (book).pdf

8/13/2019 CAP.20_Alcohol Txtbook 4th (book).pdf

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Practical management of yeast: conversion of sugars to ethanol 121

Chapter 10

Practical management of yeast: conversion of sugars to ethanol

DAVE R. KELSALL AND T. PEARSE LYONS

Alltech Inc., Nicholasville, Kentucky, USA

Introduction

Fermentation is the critical step in a distillery. Itis here that yeast converts sugar to ethanol; andit is here that contaminating microorganisms findan opportunity to divert the process to lactic acid,

glycerol, acetic acid, etc. The fermentation step(and to a much lesser degree the cookingprocess) is also where yeast must be coachedto produce maximum levels of ethanol,sometimes as high as 23%. Fermentation willalso have a direct bearing on downstreamprocessing. If sugar levels remaining in beer aretoo high, evaporative capacity may be impairedor syrup contents increased. Both can lead todistillers grains being more difficult to dry andaltered in color as Maillard reactions occurbetween proteins and sugars. Truly thefermentation step is the heart of the distillery.

When reflecting on the overall flow diagramof the distillery (Figure 1) the central role of yeastis obvious. The process of converting sugars toethanol takes between 10 and 60 hrs duringwhich heat is generated. The various factorsaffecting the efficiency of that process need tobe considered. In the chapters by Russell andIngledew, the biochemistry of ethanol productionby yeast is covered in detail. In this chapter wewill discuss the factors involved in taking

liquefied mash from cooking (1-2% glucose) toensure that pre-formed sugar and other boundsugars (starch and other polysaccharides) arereleased in a timely fashion for maximum

conversion to ethanol by yeast.

Choosing a yeast strain

One of the most important decisions a distillermust make is the selection of yeast strain. Thebest characterized yeast is Saccharomycescerevisiae , but within this species there arethousands of unique strains. Which one to use?Several decisions must be made (Table 1). Thefirst decision is whether or not to grow the strainin-house. In fact, yeast strain maintenance andgrowth is a job best left to the experts. For thevery small cost per gallon it represents (less than0.5 cents), there is just too much at stake.

Table 1. Choices among yeast strains.

Grow in-house?

Active dried or pressed?

High alcohol tolerance needed? Thermosacc

Lower alcohol in fermentor: Superstart


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122 D.R. Kelsall and T.P. Lyons

ACTIVE DRY OR PRESSED YEAST?

The choice between active dried (95% solids)

or pressed yeast (35% solids) very often comesdown to two points: adequate cold storage(pressed yeast must be stored at 5C and has ashelf life of 1-2 months) and whether the yeastmust start immediately. Yeast requires time toadapt to any new environment and activate itsmetabolism, but first must be rehydrated. Thisis a period of zero growth but intensebiochemical activity. If we are conditioning theyeast, the advantage of a pressed yeast is thatthe lag phase is minimized because the yeast isalready hydrated. Cell numbers are typically 7-10 billion cells/g. Active or instant dry yeast

(ADY) are vacuum packed, often undernitrogen, and are easy to store with minimallosses in activity (5% over 6 months at 5C).ADY requires wetting, and the lag phase tendsto be 1-2 hrs. Cell numbers 15-20 billion cells/g.

IS HIGH TEMPERATURE AND/OR HIGH

ALCOHOL TOLERANCE NEEDED?

Yeast is sensitive to temperature, high or low,and responds by activating stress mechanisms

1 bushel of

corn

(56 lbs)

Fermentable

sugar

(36 lbs)

Ethanol

(17.6 lbs)

CO2(18.4 lbs)

Heat

(7450 BTU)

DDGS

(17 lbs)

+

+

+

Starch

(32 lbs)

Milling

Liquefaction

Saccharification

Fermentation

Recovery

Dryhouse

Figure 1. Distillery product flow.

including elevation of intracellular trehalose.Additionally, certain proteins, called heat shockproteins, are synthesized. Some strains are better

able to withstand temperature stress than others.Such yeast are also usually able to tolerate higheralcohol levels. In the 1980s it was customary torun fermenters at 8-10% ethanol but now 16-17% is the norm. One yeast, Thermosacc , isa good example of a stress resistant strain.Smaller in size (5 m instead of 8-10 m, Figure2), it has a temperature tolerance of >40C andan ethanol tolerance in excess of 20%. Anunusual feature is its ability to tolerate high aceticand lactic acid levels. This may explainobservations of cleaner fermentations asinfecting microorganisms such as Lactobacillus

have less impact. In beverage alcoholproduction, little, if any, change in congenershas been reported following its use.

Controlling yeast stress factors that affect

alcohol yield

TEMPERATURE: THE FIRST STRESS FACTOR

Inability to precisely control the temperature of


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the microorganism that competes with yeast forglucose. Heat also increases the impact of otherstress factors such as mycotoxins and salt levels.

Combining enzyme activities to control sugar

delivery to yeast

How can we best control fermentationtemperature? The easiest way to reducefermentor temperature would be to reduce thesugar level going in and therefore reduce yeastgrowth; however, less alcohol would beproduced. Since the objective is more, not lessalcohol, this is not an option. However, sugarlevel, or more importantly sugar type, is

important.Yeast must be maintained in a growth

(budding) phase, since their ability to producealcohol is more than 30 times greater duringgrowth than in non-growth mode. Yeast howevercan respond to high glucose levels in two ways.Cell growth is either inhibited by high glucoselevels or yeast may grow rapidly and then stop.In either case there is an over-dosing effect onfermentation efficiency and possibly a spike intemperature. Glucose must be spoon-fed to theyeast; and for this reason it is recommended thatglucoamylases be used both sparingly and in

conjunction with an enzyme produced in surfaceculture called Rhizozyme. The combinationprovides ideally balanced sugar delivery to

maintain good yeast growth and maximumalcohol productivity while avoiding hightemperature peaks. Glucoamylase (AllcoholaseII L400) provides glucose at the start offermentation while the Rhizozyme, with pHand temperature optimums more closely alignedto fermentation conditions, continues to workduring fermentation. The net result is a steady,slow release of glucose and a steady formationof alcohol by yeast.

RhizozymeTMalso affects ethanol yield as it iscapable of hydrolyzing starch that isunconverted in the cook process and alsoconverts some of the cellulose in the corn toglucose.

The key is to consider cooking andfermentation together. Cooking must bedesigned to release as much starch, both boundand unbound, as possible without overloadingthe yeast with glucose. It must also be recognizedthat corn has, in addition to starch, otherpotentially useful carbohydrates includingcellulose and hemicellulose. Conventionalglucoamylases do not degrade either, whereasglucoamylase produced by surface culturefermentation (Koji) also contains a range ofcarbohydrase enzymes as side activities thatdegrade some of the cellulose thereby releasing

more sugar, which boosts yield (Table 2,Figure 4).

Glucoamylase

Ethanol(%

v/v)

Rhizozyme

Figure 4.Comparison of alcohol yields from high solids corn mash with traditional and surface culture glucoamylases.


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Table 2. Comparison of surface culture and

conventional glucoamylase.

Production Surface culture Conventional

(Koji) fermentation deep tank

liquid

Activities

Glucoamylase 40 400

Cellulase 400

Hemicellulase 350

-amylase 5000

Phytase 300

pH profile Broad Narrow

Temperature

optimum, C 30-40 0-60

Activity ongelatinized starch Rapid Slow

Activity on raw

starch Reasonable Slow

Yeast stimulation Significant Poor

Modeling provides a prediction of the increasein yield to be expected. Predicted yields of 3.1gallons per bushel, 17-18% higher than thestandard 2.65, have been calculated based onincreased sugar release. This is 18 gallons (>65liters) more per tonne of grain. Therecommendation is to use a 50/50 blend ofRhizozyme Koji and conventionalglucoamylase to maximize yield. Where distillerscan reliably measure yield, the results in termsof costs per gallon reveal optimum returns.

Equipment for heat control

Despite the sequential feeding of glucose, heatis generated during fermentation and must beremoved. Four types of heat exchangers aretypically used; and these are illustrated later inthis chapter when fermentation design isdiscussed.

a) Internal cooling coils. These are the leastdesirable option because they are practicallyimpossible to clean and sterilize.

b) Internal cooling panels. Like cooling coils,panels are suspended in the fermentationvessel. Also like cooling coils, cooling isadequate but sterilization may be difficult.

c) External cooling jacket. Cooling jackets,often dimpled for added contact with thefermentation, can cover either part of the

vessel or the entire vessel. Recirculating

coolant through the jacket allowsfermentation temperatures to be monitored.If jackets are spaced out over the fermentorsides, then cooling can commence as thefermentor is filled.

d) Heat exchanger with recirculation. Contentsof the fermentor are pumped through a heatexchanger (typically spiral or plate-and-frame) and returned to the fermentor. Thishas the added advantage of acting as a meansof keeping the fermentor agitated; andnutrients (sterol donors, peptides, oxygen,etc.) can be introduced at critical times. It is

suggested that optimum control oftemperature can be obtained by placing theheat exchangers in such a position thatrecirculation can start at the beginning of thefill when the yeast starts producing heat.

Of the four types, the cooling jacket is best forminimizing infection, but the heat exchanger isthe most effective at heat control. As the distillermoves towards very high levels of ethanol (over20% by volume), heat recovery and maintenanceof low temperatures will become even moreimportant as temperature rises of 6-8C will

become the norm. A word of caution: somedistilleries, particularly in continuous operations,share external heat exchangers betweenfermentors in order to economize. Based on theexperience of many existing distilleries using thissharing concept, infection can easily betransferred among fermentors as well as havinginadequate cooling.

INFECTION: THE SECOND STRESS FACTOR

The various types of infection are described in

detail elsewhere in this book, however severalpoints are relevant in this context. Lactobacilliconsume glucose to produce lactic acid andacetic acid, which is the second major factoraffecting the yield of alcohol in fermentation,which in turn has a major impact on distilleryeconomics (Table 3). Yeast do produce someorganic acids during fermentation, butconcentrations are relatively low compared tothose produced by lactobacilli and othercontaminating bacteria. As a general rule of


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resistance to both penicillin and virginiamycinis increasingly common.

One of the strategies to avoid this risk is totake advantage of the synergy that occurs whenantimicrobial substances are combined, whichlowers the amounts used. Lactoside, acombination of antimicrobials withoutvirginiamycin, has proven very effective for thisreason. A routine maintenance dose of 1-2 ppmis practical as a precautionary measure. Whenthere is infection the dosage is increased to 2-5ppm. There is no evidence of carryover of theseantibiotics into spent grains and solubles.

ALCOHOL LEVELS: A THIRD STRESS FACTOR

High alcohol beers have a tendency to stopfermenting. Understanding how alcohol levelsact to stress yeast requires a working knowledgeof yeast metabolism and growth. It has beenshown that when yeast are in the reproductivephase they produce alcohol over 30 times fasterthan when not reproducing. Figure 5 showstypical yeast growth patterns or phases duringfermentation. The first phase is the so-called lagphase, during which the yeast adapt to thefermentor environment. During this period there

is little or no yeast growth and consequently littleor no alcohol production. This period can lastfrom 4 to 12 hrs. Ways of managing andreducing the length of the lag phase are shownin Figure 6 in reference to yeast conditioning.

Time (hours)

Cellpopulation(m

illionsperml)

Figure 5. Typical yeast growth curve in distillery mash.

Good agitation

Good cooling

Can be sterilized

Figure 6. Yeast conditioning tanks: A key factor in

maximizing yeast cell numbers and improving alcohol levels.

The second phase is the exponential growthphase. This is the most important phase wherenearly all of the alcohol is produced. There is alimited time in which the yeast stay in this phase;and this is the factor limiting the quantity ofalcohol produced during a given fermentation.The length of time during which the yeast canremain reproducing depends on the nutritionavailable in the fermentor.

The three major factors that significantly affectalcohol yield are therefore temperature, acidityof the mash and the alcohol level produced

during fermentation. Yeast is tremendouslyresilient and usually produces efficiently despiteall the negative conditions imposed. Yeastfermentation will yield well if the temperature isa few degrees high or if the acidity is a littlehigh indicating a mild infection. Beerscontaining up to 20% alcohol v/v can beproduced without slowing fermentation.However, when changes in two or more of thesefactors happen simultaneously, serious losses inyield usually occur. For example, in a mash at37C contaminated with lactobacilli at 10 millionCFU/ml, it is likely that fermentation would stop

at 8% alcohol, a 33% loss in yield.

MAXIMIZE YEAST PERFORMANCE BY

CONDITIONING

Yeast is the powerhouse of any distillery; andwithout healthy yeast, alcohol percentages in thefermentor and alcohol yield will drop.Conditioning tanks are a way to ensure goodcell numbers in the fermentor (Table 5). At thestart of batch fermentation there should be a


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minimum of 50 million cells/ml, which shouldincrease to 150-200 million cells/ml at the heightof fermentation and drop to 100 million cells/ml at the end. Continuous fermentors shouldmaintain cell numbers of 150 million from a highof 250-300 million cells/ml. Yeast viability asmeasured by methylene blue stain should be 98-99% in the conditioning tank and 90% at thestart of fermentation. There will be a drop toaround 50% viability at the end of fermentation.In a cascade continuous fermentation system,viability was seen to drop to 30% in the lastfermentor.

Table 5. Keys to a good yeast conditioning tank (pre-

fermentor).

Keep Brix to 10-20, preferably ~14Brix

Provide good agitation

Use peptide based yeast food

Achieve minimum of 200 ppm FAN (free amino nitrogen)

Add Rhizozyme to spoon-feed yeast glucose

(0.05% of mash)

Hold for 4-8 hrs with agitation

Achieve 300-500 million cells/ml before transfer to main

fermentor

Pump to fermentor

Clean and sterilize

Repeat

A typical charge for a continuous yeastconditioning tank is shown in Table 6. The mashwas sterilized and cooled to 90F. Yeast wasadded with good agitation along with yeast foodand saccharifying enzyme (Rhizozyme). Themash was held for 8 hrs at which stage a cellcount in excess of 500 million was achieved.The mash was transferred to the fermentor andthe tank cleaned and sterilized. In this waydistillers ensure good yeast growth withminimum lag phase periods.

Table 6. Typical charge for yeast conditioning tank for a

500,000 gallon fermentor.

Prefermentor size 20,000 gallons

Mash Direct from jet cooker: dilute to

14Brix

Active yeast 200 lb ThermosaccTM

Peptide yeast food 20 lbs AYF

Rhizozyme l lb

Yeast cell numbers

expected 400-500 million

Time 8 hrs

MYCOTOXINS: A STRESS FACTOR

In 1985 the Food and Agricultural Organization(FAO) of the United Nations estimated that at least25% of the worlds grain supply is contaminatedwith mycotoxins. The type and degree ofcontamination depend on many factors includingconditions during growth and harvest andgeographic area. Toxins such as zearalenone andvomitoxin (deoxynivalenol) produced by growthof Fusarium molds in stored grain tend to be themycotoxins of concern in the more temperateclimates of North America and Europe whileaflatoxin, produced by various species ofAspergillus, predominates in tropical climates and

can be a serious factor in parts of the US. Howevera wide range of toxins and mixtures of toxins areusually present and all are of concern in the animalfeed and human food chain. To a distiller, the mainconcern with mycotoxins is whether they passthrough into the DDGS. The FDA have setdefinitive limits on aflatoxin levels for interstategrain shipments; and the increasing knowledge ofthe variety of mycotoxins and extent ofcontamination possible have heightened concernin all of the animal feed industry.

Of more recent concern to the distiller has beenthe finding that mycotoxins can also affect yeast

growth (Table 7). Perhaps presence of these toxinsmay provide at least part of the reason behindunexplained unfinished fermentations. Since notall mycotoxins are destroyed by heating, it isunlikely that the cooking step would remove orinactivate them. In animals many of themycotoxins damage protein metabolism causingreduced growth and increased diseasesusceptibility. Research into ways of nutritionallycompensating for mycotoxin presence in animalsfeeds led to discovery that yeast cell wallglucomannans have characteristics that make themvery specific adsorbents for certain mycotoxins.

For example, about 80% of the zearalenone insolution can be bound and inactivated by addingyeast glucomannans. These products are alsobeginning to be used successfully in thefermentation industry.

Table 7. Effects of mycotoxins on yeast growth.

Mycotoxin Level required to inhibit

yeast growth (ppm)

Zearalenone 50

Vomitoxin 100

Fumonisin 10


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of other feedstock-related stress factors such asmycotoxins or phytin content will also becomemore important as we move toward higherethanol production.

Fermentation systems used in distilleries

Before considering the specific managementtools available to the distiller, it is necessary toconsider the differences among the fermentationsystems in use. Over the last 30 years thebrewing and distilling industries have developednew fermenting systems; and the distilling

industry has largely converted to rapid batchfermentation with cylindro-conical or slopingbottom fermentors, or to cascade continuousfermentation using cylindro-conical fermentors.

BATCH FERMENTATION

A typical cylindro-conical fermentor with skirtsupport is illustrated in Figure 8. Thesefermentors are usually designed to ferment from30,000 gallons to 750,000 gallons. Fermentorsneed to be fabricated on site if the diameter andlength exceed a size that can be safelytransported by road.

Figure 8. Alcohol fermentor with sloping bottom (Alltech/Bishopric, 1981).

Inspection light

Vent

From CIP module

Mash in

Yeast/enzymes in

20 in. Manway/

sampling port

Coolant

Insulation

(outdoor tanks

in cold climates)

Slope

Concrete pad

Alternative cooling methods:

A. External cooling jacket

B. External heat exchanger with recirculation

C. Internal cooling coils

D. Internal cooling panels

CIP spray head

A

15 in. x 20 in.

access Manway

Recirculation pump

Outlet

B

CAgitator

(if required)

D

Coolant


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Different cooling systems are available. Manydistilleries use chilled water to cool thefermentors. From a microbiological standpointthe most desirable cooling system by far usesexternal cooling jackets. The least desirableoption employs internal cooling coils becausethey are practically impossible to clean andsterilize from the top-mounted CIP (clean-in-place) spray nozzles. The external recirculationheat exchanger is frequently used in distilleriesand is sometimes shared with other fermentors.This is a poor option as there are times whenmore than one fermentor needs to be cooled.The main disadvantage of this system whenshared among fermentors is bacterial cross-contamination. With such a system, themicrobiological condition of all fermentors willbe determined by the fermentor with the mostcontamination. Even when these recirculatingheat exchangers are not shared, they are difficultto clean. Throughput of at least 7 ft3/sec mustbe obtained to ensure turbulent flow through thetubes.

The agitator is required particularly at the startand at the end of fermentation. Frequently theagitators are badly designed and provide poormixing. A folding action is required to ensureproper mixing of the mash solids and to ensure

an even temperature throughout the fermentor.Carbon dioxide (CO2) is removed through the

vent. Fermentors must be fitted with pressurerelief valves and vacuum breakers to avoidserious accidents. The CO2 is frequentlycollected and sold. In any case, CO2 should bescrubbed to remove alcohol, which is returnedto the beer well. The CO2header manifold canbe a source of contamination as infected mashfrom one fermentor can migrate in the CO2 intoanother non-contaminated fermentor. The CO2manifold should be designed to be cleanable.

The lag phase is a great opportunity for

lactobacilli to become established. Bacteriaunder optimum growth conditions can reproduceevery 20-30 minutes; however yeast, which aremuch larger microorganisms, can onlyreproduce every 3 hrs. A single bacteriumreproducing every 20 minutes would produce apopulation of 256 in 3 hrs. Some yeast strainshave lag phases as long as 12 hrs, during whichtime the lactobacilli can become heavilyestablished. Therefore, it is very important tochoose a yeast strain with a very short lag phase

and to use a conditioning tank so that the mainfermentation starts immediately after the yeastis added.

There is a choice of materials to use in themanufacture of fermentors. Stainless steel isgenerally the best choice as it is easier to cleanand sterilize. It also lasts much longer than mildsteel or lined vessels; and when considered overthe expected lifetime of a fermentor it is by farthe most economical option. If chloride levelsare high, then special stainless steels should beconsidered to avoid stress corrosion.

It is important that the slope of the bottom besufficient for the mash to run out whendischarging. This type of fermentor is muchbetter suited for use with clear or semi-clearmashes than with whole cereal mashes.

Fermentor cleaning is accomplished withclean-in-place (CIP) equipment. CIP sprayheadsare high-pressure devices (100-120 psi), whichusually have automated cleaning cycles. Atypical cleaning cycle would be (minimum):

Pre-rinse with water 10 minutesDetergent circulation 20 minutesPost-rinse with water 10 minutesSterilization 10 minutes

The detergent used would be based on causticsoda, normally with added wetting agent,antifoam and de-scaling agent. The causticstrength should normally be in the 3-5% range.Ideally, the detergent should be hot (80-90C).The detergent and rinse waters should becontinually drawn off from the fermentor toprevent accumulation of liquid during the cycle.The detergent and post-rinse should at least berecirculated and continually made up to strength.Chlorine dioxide and iodophors make idealsterilizing agents, but many distillers still use

steam to sterilize the fermentors. This is time-consuming and is probably not as effective aschemical sterilization.

It is important to control the build up ofbeerstone (calcium magnesium phosphate andcalcium oxalate). Bacteria can penetratebeerstone, which is insoluble in straight alkalinesolutions. Beerstone thus protects the bacteriafrom detergent and sterilant. EDTA-basedchelating agents added to the detergent helpdissolve beerstone and will control


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accumulation. The level of chelating agentneeded can be calculated from the calcium levelin the mash. A problem recently encountered issodium contamination of process condensatewater. During CIP the caustic based detergentcan be neutralized by the CO2 in the fermentorproducing large quantities of soluble sodiumcarbonate that can pollute the processcondensate. Very high sodium levels can weakenthe yeast during fermentation and care must betaken to reduce these sodium levels.

CONTINUOUS FERMENTATION

The most successful continuous fermentationsystem used in distilling is the cascade system(Figure 9). The system illustrated in Figure 9 hasat least two fermentors and a beer well wherethe yeast are recycled. Yeast can only be recycledwhen clear mashes are used. The yeast can thenbe centrifuged, washed and reused. Typically,yeast can be washed using phosphoric acid at apH of 2.2-2.4 and a holding time of 90 minutes.These conditions are sufficient to kill the bacteriawithout doing permanent damage to the yeast.Chlorine dioxide at 40-50 ppm is an alternativeto acid washing, and seems to be gentler on the

yeast.The system in Figure 9 has two fermentors,

whereas most cascade plants have five

fermentors and a pre-fermentor. The majority ofthese plants in the US use whole mash fed intothe first two fermentors. Both are aeratedcontinuously with sterile air and are cooled withexternal coolers. The pre-fermentor feeds yeastequally into both. The mash has been previouslysaccharified and enters the fermentors at a pHof 4.0 or slightly less. Usually these systems, ifyeast stress factors are taken into account,produce 13-15% beers in 24-30 hrs althoughclear mash systems operate in 10-14 hrs. Eachfermentor is individually cooled and agitatedeither with air or CO2 or mechanically. The clearmash systems are capable of maintaining a fixedyeast count although they have the potential torecycle.

Typically yeast cell counts in these fermentorsare slightly higher than in batch systems, withcounts in the range of 180-220 million cells/mlfor the first two fermentors and slightly less asthe mash proceeds through the system. Theviability and percentage of budding cellsdecrease from one fermentor to the next.Typically, the beer entering the beer well couldbe down to 120 million cells/ml with a viabilityof around 30%. Alcohol levels could be 10-11%in Fermentors 1 and 2, 12-13% in Fermentor 3and up to 15% in Fermentors 4 and 5.

The main advantages of continuousfermentation (and they are very importantadvantages) are rapid throughput and the fact

Figure 9.Twin vessel continuous stirred fermentor.

Vessel

No. 2

VesselNo. 1

Agitator drive

Oxygen

column

Oxygen in

Sterilizer

Mash in

Pump

Agitator driveFoam breaker

Beer

outletYeast

outlet

Cooling

Coil

CO2 outlet


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that these fermentors can be run for very longperiods without stoppage. Cleaning costs aretherefore practically eliminated; and vesselutilization is superior to any batch system. Thesecontinuous systems are frequently used for 12months without stopping and are only cleanedwhen the plant has its annual shutdown.

The most serious problem encountered withthe cascade system is infection. The fermentorscan be infected through non-sterile air injectedinto the first two fermentors. In this case, aceticacid bacteria, which convert the ethanol to aceticacid, can be introduced. This can be detectedby a vinegar odor, or more accurately by HPLCanalysis. There is also a possibility of infectionby lactobacilli that usually propagate incontaminated mash coolers or saccharificationtanks.

The new distillers, at least those in the US, haveelected not to use continuous fermentation andare instead using the batch process. Batchsystems are more easily controlled, more flexibleand capable of higher ethanol levels.

Conclusions

Understanding the stress factors that affect yeastand the fermentation equipment will help usmove to the theoretical 23% ethanol that yeastsuch as Thermosacc are capable of producing.It is critical, however, that we push the positives:spoon-feed sugar to yeast, provide yeast withpeptides to ensure high cell numbers in earlystages of fermentation, control temperature andovercome the negatives. These negatives,infection, lactic acid, acetic acid, phytic acid andmycotoxins all prevent yeast, and therefore thedistillery, from achieving potential yields.

Today distilleries must achieve yields of 2.9

gallons per bushel. Tomorrows distilleries willneed over 3.2 gallons per bushel. To do this,yeast must remain the central focus of all efforts.Doing so will allow 23% ethanol to soon becomethe norm.


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