Yeast Notes

8/4/2019 Yeast Notes

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THE BASIC MORPHOLOGY OF WINE YEASTS

Saccharomyces Cerevisiaecells observed under the microscope

AND THEIR ROLE IN FERMENTATION

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Contents

Yeasts

2. Naming conventions

3. The process of alcoholic fermentation

4. The yeast fermentation equation

5. Reproduction and Morphology

Budding

Spores

Morphology

6. Using yeasts in winemaking

7. Commercial yeasts

ermentations

8. Indigenous yeastsSpoilage yeastsPractical aspects of indigenous fermentations

9. Conditions for development of yeasts

ermentation

Limit of fermentation and temperature

Critical temperatures for yeast

Influence of aeration

Nutritional needs of yeasts

10. References

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1. YEASTS

Yeasts are unicellular, microscopic fungi that reproduce by budding and binary fission. Theydecompose sugar into alcohol and carbon dioxide. The size of yeast cells varies according to

species. Diameters can range from 2 to 10 m. 1 m = 106

meters.

In general, cells find the energy they need in two ways, both of which involve the dissipation oforganic matter.

Breathing. Breathing takes oxygen from the air, produces advanced molecular fission, andreleases a lot of energy.

Fermentation. Fermentation takes place in the absence of oxygen. It is inefficient because thedissipation of matter that it promotes is incomplete. Yeasts have to transform a great deal ofsugar into alcohol to satisfy their energy needs.

2. NAMING CONVENTIONS

There exists a considerable number of yeast species differentiated by theirshape, theirproperties,theirmode of reproduction, and by the way they transform sugarto alcohol.

Wine yeasts belong to a dozen genera each divided into species. They are designated by a doublename in Latin, the first corresponding to the genus and the second to the species.

For example, the yeast named Saccharomyces ellipsoideus belongs to the genus Saccharomyces(literally the sugar fungus, which transforms sugar) and the species ellipsoideus (elliptical in shape).

In modern winemaking, the elliptic shaped Saccharomyces cerevisiae is the preferred yeast species.Within the Saccharomyces cerevisiae species, there are many strains that show very minor geneticdifferences. Winemakers may prefer a specific commercial strain of yeast for its known attributes

over another strain.

3. THE PROCESS OF ALCOHOLIC FERMENTATION

Fermentation of crushed grapes and juice is a common occurrence and has been happeningnaturally and spontaneously for thousands of years.

Damaged or split berries release their juice and indigenous yeast present on the grape skin will beginto use the juice as a food source. Spontaneous fermentation will commence.

Today, winemakers typically use commercially selected strains of yeast, which they add to juice and

must (juice, skins and seeds), as well as indigenous yeasts. Once fermentation is underway, sugar istransformed into alcohol and carbon dioxide gas. The must goes cloudy and gets hot. The

temperature may rise to as much as 35C unless refrigeration is used. Bubbles of carbon dioxide gas

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rise causing the must to seethe. As the sugar is fermented, the liquid loses its sweet taste andbecomes vinous. Wine is made.

The chemical mechanism of sugar fermentation is extremely complex. The most importanttransformations comprise no less than 30 or so successive reactions, bringing into play a great manyenzymes. The enzymes are the tools of the yeasts adapted to one stage of the transformation. Eachreaction necessitates a different tool, a different enzyme. The by-products that have already beenmentioned are like the remnants of these multiple reactions. The by-products are important to the

final product, the finished wine.

Yeasts initially, preferentially use the glucose portion of the grape sugars, building up cell biomasswith little alcohol produced. As fermentation proceeds, ethanol is produced by utilization of both theglucose and fructose portion of the grape sugars. A small percentage of the fructose portion of thegrape sugars is usually consumed last. This occurs because yeasts are required to provide activetransport through the cell membrane for fructose whereas yeasts do not have the same requirement

for glucose. Fructose is twice as sweet as glucose.

4. THE YEAST FERMENTATION EQUATION

The following is a simple but for our purposes, perfectly adequate, balanced chemical equation torepresent the fermentation process.

C6H12O6 2C2H5OH + 2CO2 + heat

grape sugars yeast ethyl alcohol carbon dioxideprimarily glucose (ethanol) gasand fructose

180 grams 92 grams 88 grams

The weights given are based on molecular weights of the products and reactants. In fact the amountsof alcohol and carbon dioxide yielded by the fermentation process are somewhat less than shown,say 90-92%, because there are some other fermentation products produced.

Some other fermentation products include

Glycerol can contribute to smooth mouthfeel, viscosity and some sweetness

Succinic acid. Its bitter-salty taste limits it use in wine acidification

Butylene glycol (sweet tasting, Austrians know all about this)

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Acetic acid, CH3COOH, around 0.5 g/L can be produced during fermentation. Higher

amounts may be formed by bacterial or oxidative yeast activity during and after fermentation.

Ethyl acetate (also called acetone) is produced concurrently with acetic acid, usually in theratio 1 part ethyl acetate to 5 parts acetic acid. Ethyl acetate is more readily detectedorganoleptically (aeroplane glue, nail varnish remover) than is acetic acid (vinegary). EthylAcetic acid bacteria can directly synthesize ethyl acetate. As well ethyl acetate can be formedby acetic acid reacting with ethanol.

Lactic acid (yeast will degrade a small amount of naturally occurring malic acid duringfermentation, especially Schizosaccharomyces pombe, which may degrade all the malic acidbut also produces other off-characters)

Higher alcohols (fusel oils) tend to be hot and medicinal

Acetaldehyde, CH3CHO, is the major aldehyde found in wine. Aldehydes are carbonyl

compounds distinguished by the terminal location of the carbonyl (C=O) functional group onthe molecule. Combined with other oxidized compounds it contributes to the fragrance ofsherry and other oxidized wines. Acetaldehyde is one of the early metabolic by-products offermentation. As fermentation approaches completion, acetaldehyde is transported back intoyeast cells and reduced to ethanol.

Pyruvic acid

Acetoin (buttery taste produced by yeast)

And other compounds

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5. REPRODUCTION AND MORPHOLOGY

Yeasts are found in three classes of fungi, characterized by their reproduction mode:

Sac fungi (Ascomycetes)

Club fungi (Basidiomycetes)

Imperfect fungi (Deuteromycetes)

The yeasts found on the surface of the grape and in wine belong to

Ascomycetes (Capable of reproducing sexually as well as by budding. Important in fermentation) Deuteromycetes (Only able to reproduce by budding. Some species of Candida are important as

spoilage yeasts.)

The majority of wine yeasts show two possible methods ofreproduction according to prevailingconditions.

1. Vegetative or asexual reproduction by budding

Bi-polare.g. Kloeckera apiculata, an indigenous, spoilage yeast that extrudes daughter cells

at opposite poles of their cell creating a lemon shape over successive generations. Multilateral e.g. Saccharomyces cerevisiae, a yeast that produces buds (daughter cells) fromany part of the external surface of the cell.

Bud fission e.g. Schizosaccharomyces pombe (a mixture of budding and binary fission).

2. Sexual reproduction by the formation ofascospores, which after germination give rise to moreyeasts.

Budding

As soon as a yeast cell is added to grape juice or must, a swelling can soon be seen appearing on itscircumference that gradually gets bigger at the same time that the shape of the small daughter cellbegins to be defined.

When the two cells are approximately the same size, they separate and budding will continueinvolving both the new daughter cell and the original mother cell.

Under ideal conditions, only two hours are needed to double the yeast population with up to sixhours needed as yeast nutrients become limiting.

Spores

When sporiferous yeast diploid cells are in a hostile nutritive medium (for example, depleted offermenting sugar, poor in nitrogen, and very aerated) they stop multiplying. Some transform into akind of sac with a thick cell wall. These sacs are called asci. (Refer to figures 5 and 6). Asci arerobust and are able to survive under harsh conditions, for example dehydration, contact withchemical agents such as sulphur dioxide, and heat, that normally would be fatal to yeasts. Each ascicontains four haploid ascospores issued from meiotic division of the nucleus. Grape must and wineare not propitious to yeast sporulation. Wine yeasts, both indigenous and selected, do not sporulateeasily and when they do they often produce non-viable cells.

In favorable conditions, i.e. nutritive sugar-enriched media, the haploid ascospores germinate,breaking the cell wall of the ascus, and begin to multiply. In S. cerevisiae, the ascospores have twomating types. Sexual coupling occurs between two cells of different mating type. The vegetativediploid yeast cell resulting from cellular and nuclear fusion of the ascospores multiplies by budding.Morphology

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Yeasts found in vinification will typically present one of the four following shapes: elliptical or ovoid,elongated (in the shape of a sausage), spherical, and apiculate.

In modern winemaking, where inoculations of known yeast strains are preferred, the yeast cell shapeviewed under the microscope will commonly be elliptical, the shape ofSaccharomyces cerevisiaecells.

Figure 1. Various shapes of wine yeasts. From left to right: Elliptic yeast of the genus Saccharomyces

Sporulated elliptic yeast

Round yeast, previously known as Torula

Tiny elongated yeast, Torulopsis stellata

Apiculate yeast, Hanseniaspora

Large apiculate yeast, Saccharomycodes ludwigii(Peynaud, 1984)

Figure 2. Observation of two enological yeasts species having an apiculated form

a) Hanseniaspora uvarumb) Saccharomycodes ludwigii(Ribreau-Gayon et al, 2000)

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Figure 3. Binary fission Schizosaccharomyces. Formation of a cell wall occurs between mother anddaughter followed by separation.(Ribreau-Gayon et al, 2000)

Figure 4. Multilateral budding characteristic ofSaccharomyces spp. occurs on the shoulder area ofthe yeast. Each bud arises at a location separate from others. Upon separation of daughter cells, themother is left with bud scars. Each budding cycle depletes the mother cells membrane byapproximately one half. Cell membrane synthesis occurs under aerobic conditions. Thus, after threeor four budding cycles, the fermentation must complete with a stationary-phase population. Older

cells of multilateral budding Brettanomyces and Dekkera spp. may exhibit a cell shape suggestive ofrestrictive polar budding. Cells are appear boat-shaped and described as ogival.(Fugelsang, 1996)

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Figure 5. Stages of yeast development. From left to right: budding vegetative cells, ascosporedevelopment, spore release from ascus.(Jackson, 2000)

Figure 6.Saccharomyces cerevisiae cells placed on a sugar-agar medium for several weeks. Ascicontaining ascospores can be observed.

(Ribreau-Gayon et al, 2000)

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6. USING YEASTS IN WINEMAKING

Fermentation of grape juice and must is initiated by either

inoculating the juice or must with a commercial yeast, or by

allowing a spontaneous fermentation to occur using indigenous yeasts present on thegrape skins.

Yeast populations in fully fermenting grape juice and must are extremely dense, in the region of 80

million to 120 million, i.e. around 108

yeasts per milliliter. In one drop of fermenting grape juice

there may be 5 million yeasts. Yeast populations will need to be this high if the fermentation is togo to complete dryness, i.e. if most of the grape sugars are to be completely fermented.

7. COMMERCIAL YEASTS

Most commercial yeasts are strains ofSaccharomyces cerevisiae that have been developed andselected for particular characteristics. Winemakers will choose one commercial strain over another

because of its known attributes.

It is common practice in New World winemaking countries such as New Zealand, Australia, USA andCanada, to inoculate grape juice or must with commercial yeasts in an attempt to ensure

consistency of wine style

known performance attributes

protection of varietal characters

greater control of fermentation

Commercial yeasts are sold as dried powders and several American and European brands havebeen on the market for up to forty years. They are prepared by vacuum dehydrating yeasts in hot air

with a protective support present. They are not freeze-dried preparations. Yeasts are easily keptrefrigerated as dried powders, and are available all year round. They can be used in sparkling wineproduction, fermentation of stored musts or juices, and to re-inoculate stuck ferments (where afermentation has stopped prematurely with unwanted residual sugar still remaining).

Desirable characteristics of wine yeasts include

1. The efficient conversion of grape sugar to alcohol2. The ability to effect a quick initiation of fermentation, say within 48 hours3. Sulphur dioxide tolerance4. The ability to carry out even fermentation5. The ability to ferment at low temperatures, important if you want to retain fruity characters6. The ability to ferment to dryness which requires that they are alcohol tolerant and can ferment

in the presence of relatively high alcohol concentration7. Good flocculation ability following fermentation to aid clarification8. The production of a desirable fermentation bouquet9. Low foaming

10. Low H2S or mercaptan formation

11.Relatively high glycerol production to aid the sensory qualities of the wine12.Relatively low alcohol production to avoid hotness

13.Low acetic acid formation

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To be really efficient, to would be better if indigenous yeasts on the grapes and in the winery, wereeliminated before juice or must was inoculated with commercial yeasts. Theoretically at least, it ispossible that indigenous yeasts, better adapted, could outclass the yeasts introduced. However it isnot possible to sterilize vintage. Elimination or containment of indigenous yeasts is more easilyachieved in white wine making, where the must is homogeneous and allows for better mixing, than inred winemaking

Inoculation using commercial yeasts is particularly recommended if fermentation is slow to start. It

can be useful in cold years when grapes arrive at the winery at a temperature below 15o

C.Commercial yeasts may better handle fungicide residues. They are generally more tolerant ofsulphur. Inoculation will be required if white musts are drastically sedimented, especially whenclarified by centrifugation or some other means. With inoculation, defective wild microflora, such asyeasts producing ethyl acetate, hydrogen sulfide, or sulfur dioxide, or having other undesirablecharacteristics, can be supplanted.

Practical aspects of inoculated fermentations

Inoculation is carried out with the juice at 15C or higher.Keeping the juice warm for say 24 hours after inoculation may be helpful to ensure a good start to

fermentation. Yeast and supplements are prepared and added to juice or must as follows.

Yeast is typically added at approximately 0.2g/L for whites and 0.3g/L for reds. Lower amounts mightbe used if the yeast strain was very vigorous. More could be used if the strain was less vigorous.

Generally yeast is re-hydrated in warm water at 40C. It is sprinkled (without stirring) onto a volumeof water, equal (in millilitres) to 10 times the weight of yeast (in grams). After 20 minutes or so, asmall amount of juice may be added to the re-hydrating yeast to help acclimatise the yeast to thesugar levels that will be encountered in the juice or must, and to reduce the temperature. Ideally there-hydrated yeast should be added to the juice or must when it has pretty much reached the sametemperature of the juice or must it is being added to.

DAP or di-ammonium phosphate is a nitrogen supplement for yeasts. It is mixed up in warm water orjuice and added to the juice or must. DAP is added at a rate of around 0.4g/L. However it ispreferable not to add all in one hit. Typically we would add 0.2 g/L at the time of yeast inoculation, 0.1g/L day later and a further 0.1 g/L a day after that. This is to ensure that yeasts also take up aminoacids. Yeasts can use nitrogen in the form of ammonia, or the ammonium ion, which are supplied byDAP and which are also contained in grapes. Yeasts also need amino acids. The amino N content ina grape must is typically 2 3 times higher than ammonia N. If a large amount of DAP is initially

added, this balance is shifted and can cause metabolic disturbances in yeast cells, H2S production.

Amino acids will not be taken up in a pattern that is beneficial to the health of the fermentation.

Yeasts need their amino acids. Amino acids are also important flavour and aroma pre-cursors in awines sensory profile. Dont let yeasts spoil their dinner by gorging on ammonia first.

Superfoodis a proprietary commercial product sold as a complete yeast nutrient blend. It is usuallyadded at a rate of between 0.1 g/L and 0.2 g/L at the time of yeast inoculation. Superfoodcomes inthe form of a powder and is mixed into a slurry with water or juice before being added to the juice ormust. Superfood contains yeast hulls, yeast extract, DAP (around 33%), minerals and vitamins.

Yeast hulls (yeast cell walls, cell membranes, envelopes cellularies) are yeast cell membranes leftbehind when yeast extract is prepared. They help yeasts protect their cell membranes against alcoholtoxicity and heat and cold shock. Yeast hulls absorb pesticide residues, toxic short chain fatty acids

produced by yeasts, and other inhibitory compounds. Yeast hulls are normally added at a rate ofbetween 0.1g/L and 0.25 g/L at the time of yeast inoculation. Yeast hulls come in the form of apowder and are mixed into a slurry with water or juice before being added to the juice or must.

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8. INDIGENOUS YEASTS

Rather than inoculate a juice or must with commercial yeasts, a winemaker may prefer to rely onindigenous yeasts present in the juice or must and allow fermentation to occur spontaneously with aview to obtaining increased complexity from a succession of different yeasts involved.

Indigenous yeasts refer to yeasts that occur naturally in the vineyard or in the winery. In the vineyardindigenous yeasts are found on ripe grapes from veraison onwards, particularly around the time of

harvest. They are brought into winery, onto presses, tanks and other equipment, with the grapes anddevelop and proliferate there with great speed. Saccharomyces species are particularly important inun-inoculated fermentations.

Yeasts overwinter in the soil. Few exist on green grapes. In the vineyard in summer, midges andother insects pick up and spread yeasts and other organisms. As well as good yeasts, a number ofother microorganisms are found on grapes including mycodermic yeasts or flower ferment, molds,lactic acid bacteria, and acetic acid bacteria.

The yeasts most commonly found in combination in musts and juices are

Saccharomyces cerevisiae, the common elliptical yeast,

Kloeckera apiculata, the tiny apiculate yeast and Hanseniaspora uvarum, also apiculate

These three species alone represent at least 90% of the strains responsible for fermentation.

Principal species found in vinification

Brettanomyces intermedius Spoilage yeast, not really a fermentation yeast, cancontaminate barrels

Candida sake

Hanseniaspora uvarum Spoilage yeastHansenula

Kloeckera apiculata Spoilage yeast

Kluyveromyces veronae Spoilage yeast

Pichia membranaefaciens Spoilage yeast

Saccharomyces aceti, bailii, bayanus,cerevisiae, oviformis

S. bayanus is of interestS. cerevisiae produces clean ferments with desirablecharacters and low volatile acidityS. oviformis has high-alcohol generating powers

Schizosaccharomyces pombe Able to degrade malic acid, de-acidifying the must, but canalso produce off-characters

Torulopsis stellata Specific to grapes attacked by noble rot

Spoilage Yeasts

A number of indigenous yeasts are regarded as spoilage yeasts and are undesirable in winemaking.They produce off-flavors and taints and other problems. They contaminate containers and equipment.They are typically resistant to alcohol, sulfur dioxide, and to anaerobic conditions, and can remainalive in wine in a latent state for months. The presence of these yeasts in stored wine can affect theclarity and result in the formation of deposits. When the wines contain reducing sugars, a genuinefermentation may be set off and the wine will go gaseous. These disturbances can occur not onlyduring tank-life, but in the barrel, cask or bottle too.

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Practical aspects of indigenous fermentations

Grape skins host very few Saccharomyces yeasts but many non-Saccharomyces yeast species. Themajority are Kloeckera apiculata (and related Hanseniaspora uvarum), as well as other species.

Most un-inoculated ferments, and even many inoculated ones in which the added yeasts die becauseof poor viability or mishandling, begin with non-Saccharomyces vineyard yeasts. Kloeckera tolerates

at least 70 mg/L SO2, and is naturally cold tolerant. Saccharomyces is not. Reds soaking at less than

10C, or white juice warming up slowly after racking are prime candidates forKloeckera growth.

Kloeckera can make ethyl acetate (smells like nail polish remover) just before fermentation, but if astrong, clean fermentation ensues quickly there may be little harm done.

If there is bumpy, slimy scum, or stringy floating clumps, it is more serious. Within hours, Kloeckeradepletes the must of vitamins, particularly thiamine, leaving Saccharomyces without this importantvitamin unless more is added at the right time. Kloeckera also produces acetic acid, which can actsynergistically with alcohol to cause stuck ferments.

Yeast supplements can be added when Saccharomyces can be seen under a microscope, and notbefore. Alternately some nutrients can be added after an initial 1 - 3Brix drop and the rest of thenutrients in portions until mid-fermentation. (PROS, 2004)

In general we would not add sulphur if we wished to encourage an indigenous ferment.We would include some light lees or solids in a white ferment.

We would quickly heat juice up, after cold settling, to encourage an indigenous fermentation.Kloeckera will kick in at lower temperatures and fall over around 3% alcohol. We dont want thisparticular yeast species to become too dominant, as among other things it tends to produce high VA.

Notwithstanding it is one of a number of species that can contribute positively to complexity.Saccharomyces tend to kick in around 12C. Thus it is helpful if the must is quicklyheated to thistemperature so Kloeckera has a limited influence.

Also oxygenate for a couple of hours to encourage indigenous fermentation.

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9. CONDITIONS FOR DEVELOPMENT OF YEASTS

Yeasts have their precise needs in terms of nutrition and environment. They are very sensitive totemperature. They need oxygen. They need to be appropriately fed with sugars, mineral elements,nitrogenous substances, and vitamins.

Temperature

Yeasts only develop properly in the temperature range 15 - 35C. Below 13 or 14o

C, the start offermentation is practically impossible or else it is so slow that there is a danger of a spontaneousindigenous fermentation starting (usually Kloeckera apiculata). Fermentation cannot be conductedproperly over 35oC.

If the temperature is reached gradually, the yeasts stop working and die, sometimes at temperaturesas low as 30 or 32oC. On the other hand, exceptional circumstances have been seen where thefermentation of an entire vintage, warmed throughout, started off at very high temperature (40 -45oC), with the help of special yeasts, then finished very quickly as it cooled down. Above thesetemperatures and in wine in the presence of alcohol, yeasts are killed in a few minutes. This issterilization by heating, or pasteurization.

Temperature and the speed of fermentation

Sugar transformation speeds up with a rise in temperature(at least up to a point). Fermentation ismuch faster at 30oC than at 25oC and at 25oC than at 20oC and its speed doubles for each 10oCvariation. For each extra degree (centigrade) of temperature, yeasts transform 10% more sugar inthe same period of time. Above 35oC, if at first fermentation is faster, it stops earlier by a kind ofwearing out of the yeasts.

Limit of fermentation and temperature

The amount of sugar yeasts can transform, or the alcoholic strength they can attain, depends on thetemperature. The higher the temperature, the quicker fermentation starts, but the sooner it stops andthe lower the alcoholic strength attained(due to volatilization of alcohol). The maximum yeastpopulation is lower at higher temperatures. When a high alcoholic strength is desired, thefermentation temperature must be kept fairly low.

Everything happens as if the yeasts get tired all the more quickly the faster they work, at highertemperatures. In these conditions, they do not withstand alcohol easily, they are less able toassimilate the nitrogenous substances, they lose sterols more quickly, their reproduction falters andthe fermentation comes to a halt.

Critical temperatures for yeast

The ideal temperature for making red wine is between 26 and 30oC. It is a compromise between theneed to have a sufficiently rapid fermentation with thorough maceration and at the same time avoidfermentation stoppage through overheating. For making white and rose wines, the temperature to bepreferred is lower, around 18 to 20oC.

The notion of critical temperatures for fermentation is an important factor to consider in winemaking.Critical temperature is the temperature above which yeasts no longer reproduce, and die, makingfermentation slow down, then stop. It is difficult to define a precise limit and better therefore to speak

of a risk zone. This may vary in fact according to the aeration, richness of the must, nutritive factorsfor the yeasts, and the actual nature of these. In temperate regions the critical temperature isgenerally set above 30 - 32oC and in hot regions a little beyond this.

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The winemaker should not wait for the temperature in the tank to reach the risk zone before thinkingabout cooling. Cooling ought to intervene well before. What is important is to avoid crossing into thiszone. The winemaker should avoid reaching this risk zone in the first place and maintain a favorabletemperature, below critical temperature, to avoid destroying the yeasts.

Influence of aeration

Yeasts need oxygen to multiply. In a total absence of air, they only create a few generations and their

growth stops. Then all it needs is to give them a little air again for budding to restart. If the state ofasphyxia is prolonged, the majority of cells die. One yeast cell deprived of oxygen, finds the energy itneeds in the transformation of sugar. But to get prolonged fermentation and obtain fermentedproducts rich in alcohol, new generations of yeasts must constantly be forming and traces of oxygenbecome indispensable.

This need for oxygen is, in a way, indirect. Yeasts require oxygen to synthesize the sterols andassimilate the fatty acids, which they need for cell growth. The sterols are the organic substanceswith several cycles of carbon atoms and with an alcohol function. They are the source of severalhormones and vitamins whose biological importance is considerable. At the beginning offermentation, the first generations of yeasts benefit from the reserves of sterols from the mother

cells, then sterols from the natural environment. If fermentation is carried on in a shortage of air,the sterols are used up and are not renewed. Oxygen is indispensable to sterol synthesis and thus tothe continuation of fermentation.

All steps in which grapes are handled (crushing, de-stemming, pumping through, or again, for whitegrapes, draining off and pressing) make sure of a first aeration, helpful in getting fermentation started.Following this, the more oxygen the yeasts manage to get, the faster the yeasts multiply.

Nutritional needs of yeasts

Yeasts have absolute requirements for certain nutrients in must and juice where they develop. Theirneeds in terms of sugar and mineral substances are easily satisfied. Their needs in respect ofassimilable nitrogenous matter are less easily satisfied.

Yeasts consist of 25-60% nitrogenous substances. To form their cells and reproduce, they need tofind in their environment sufficient nitrogen in a form easy to use.

Nitrogen in the form of ammonia or the ammonium cation, NH4+

is the primary nitrogenous food

consumed by elliptic yeasts, followed by certain free amino acids, such as glutamic acid. In 36 hoursof fermentation, yeasts drain the must of its assimilable nitrogen and of many other nutritive

elements. The rest of the fermentation goes on with starved yeasts in a state of deficiency. Towardthe end of fermentation, they give the wine its amino acids back by excretion.

In certain years, juice and must will be naturally low in YAN (yeast assimilable nitrogen. This may beexacerbated by soil conditions, very ripe grapes, or by fruit being infected with botrytis. DAP can beused to help supplement the supply of ammoniacal nitrogen, however, EC regulations restrict the useof DAP to a maximum of 0.3 g/L.

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10. REFERENCES

Boulton, R.B., Singleton, V.L., Bisson, L.F., Kunkee, R.E. (1995). Principles and Practices ofWinemaking. New York: Chapman and Hall.

Fugelsang, K.C. (1996). Wine Microbiology. New York: Chapman and Hall

Jackson, R.S. (2000). Wine Science: Principles, Practice Perception. London: Academic Press

Nelson Marlborough Institute of Technology. (2004). Science One Laboratory Manual. Blenheim:NMIT.

Pacific Rim Oenolgy Services. (2004). Harvest 2004 Products and Services. Marlborough, NZ: PROS

Peynaud, E. (1984). Knowing and Making Wine. New York: Wiley-Interscience,

Phaff, H.J., Miller, M.W., Mrak, E.M. (1978). The Life of Yeasts: Their Nature, Activity, Ecology andRelation to Mankind. Cambridge, Massachusetts: Harvard University Press

Ribreau-Gayon, P., Dubourdieu, D., Donche, B., Lonvaud, A. (2000). Handbook of EnologyVolume 1, The Microbiology of Wine and Vinifications. London: John Wiley & Sons Ltd

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Yeast Notes