For the wine microbiology exam with the professor Patrizia Romano I wrote a paper on starter cultures for winemaking that I'd like to share with you. Given that I started from the must, from what composes it and from its treatments, I analyzed the characteristics of wine yeasts to reach an interesting conclusion. THE wine yeasts, better called wine yeasts, are responsible for fermentation, a very complex process that depends both on the chemical composition of the must and on the simultaneous intervention of different microorganisms both from a chemical and biological point of view. Consequently, the progress of this process and its result depends on:
- Composition of the must;
- Enzymatic kit of yeasts;
- Environmental context in which yeasts work.
It follows that fermentation can be spontaneous, or take place with the yeasts present on the grapes or in the winemaking environment, or take place with selected yeasts.
Wine yeasts / 1: Fermentation with indigenous yeasts
The concentration of yeasts on the skin of the berries increases with increasing maturation. At an early stage of ripening on the berries there are yeasts of the soil, bark and leaves that almost always do not contribute to fermentation. In an advanced stage of maturation there are Hanseniaspora and Metschnikowia yeasts (between 50% and 75% of the total) and other minor strains, but Saccharomyces cerevisiae are present in very low concentrations (sometimes even absent), i.e. the main ones responsible for fermentation .
Therefore the non-Saccharomyces yeasts with their large populations in the first phase of fermentation influence both the growth kinetics and the metabolism of Saccharomyces yeasts, then in a second phase of fermentation the environmental conditions become unsuitable (due to the presence of alcohol) and therefore the Saccharomyces take over thanks to their greater alcoholic power.
From this we can understand how the fermentation process, from an ecological point of view, provides for the sequential development of some species of yeasts - but also of different strains within the same species - which are gradually replaced according to the changing conditions (i.e. increase in alcohol content). This yeast evolution continues until the optimal conditions are created for the development of Saccharomyces cerevisiae at the end of the process.
Saccharomyces cerevisiae have a fermentative power - that is the maximum amount of alcohol that the strain can form - higher than 14% vol.
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Wine yeasts / 2: Alcoholic fermentation
2.1 Alcoholic fermentation in white winemaking
Fermentation without maceration is carried out at a temperature around 18-20 ° C to ensure the best qualities of finesse of the aromas and flavors of white wine, which is more subject to oxidation by nature. Double-walled fermenters with the flow of a refrigerant solution cause the yeasts to slowly transform the sugars.
To obtain white wines with greater personality and destined for evolution, white vinification can include a rapid skin maceration of the juice in contact with the skins, to extract pigments and odorous substances. To avoid any damage to the quality of the must, it is cooled to around 8-10 ° C. Cryomaceration can be performed by cooling down to 0-2 ° C. The cold maceration of the whole berries improves the extraction of the substances during the subsequent pressing, with a consequent intensification of the aromas.
2.2 Alcoholic fermentation in red vinification
By managing the maceration time, it is possible to intervene in a different way on the extraction, safeguarding the elegance of the final product. In the first days of contact, the anthocyanins are extracted which in a few days give very intense colors with marked shades of ruby red, purple or dark blue. In the following days, part of the pigments are reabsorbed by the skins and the color weakens a little to the advantage of a better extraction of the total polyphenols (influenced by: contact time of the must with the skins, temperature, pumping over, punching down, presence of sulfur dioxide and enzymes) and therefore of a greater structure and taste of the wine. The fermentation of some red wines can end in barrique, where the wine that is being formed is enriched with the aromatic components of the wood. Many red wines are produced with a maceration of 10-15 days, for the lighter ones and to be consumed young it is reduced to 4-5 days, while only a few musts are left to macerate for up to 4 weeks (Langhe, Barolo). The high fermentation temperature is another factor that increases the dissolution of the substances present in the skins but does not always maintain the quality of the wine. The compromise is reached around 25 ° C for wines to be drunk young in which it is important to preserve the fruity aromas, 30 ° C for wines for aging, in order not to damage the yeasts and not to alter the organoleptic characteristics of the wine. During fermentation, the pomace forms a compact layer on the surface called the cap which prevents good contact with the liquid part. To overcome this drawback and facilitate extraction, the must is remixed with fulling or pumping over. At the middle or at the end of maceration, all the liquid part of the must is removed from the tank, leaving only the solid part inside. Subsequently, the liquid is reinserted from above or below and completely breaks the cap formed by the skins. In this way, the twofold objective of oxygenating the must very well and of mixing the substances that may have stratified, favoring the fermentation and extraction processes, is achieved. While the must continues to ferment the yeasts transform the sugars into ethyl alcohol and carbon dioxide, form secondary substances, the color becomes more intense and many components are extracted and pass from the skins to the liquid part. Since carbon dioxide causes the wort to boil, this step is called tumultuous fermentation.
Wine yeasts / 3: Technological characteristics of wine yeasts
They are those commonly evaluated in musts and consequently in wines and are the basis for the choice of starter cultures for winemaking.
3.1 Fermentation activity
The fermentation activity is the essential requirement to have an excellent fermentation and consequently a quality wine. In practice it is the ability of the wine yeast to prevail quickly and finish the fermentation.
3.2 Production of sulfur compounds
Sulfur dioxide (SO2) and hydrogen sulphide (H2S) derive directly from the reduction of sulphates present in the must by the action of yeasts.
3.3 Resistance to antimicrobial compounds
- Natural products of yeast metabolic activity: Ethanol limits or stops alcoholic fermentation as it acts directly on yeast and inhibits the transport of glucose. The more alcohol-tolerant yeasts protect themselves by decreasing saturated fatty acids and increasing unsaturated fatty acids and trehalose (natural sugar that regulates osmotic pressure and stabilizes proteins). Low temperatures (10-20 ° C) inhibit the action of ethanol favoring non-Saccharomyces yeasts which can even become dominant with a change in the flavor of the wine. Since ethanol tolerance is closely linked to a strain's ability to dominate during fermentation, alcohol resistance must be carefully assessed for all yeasts that are selected.
- Chemical additives: The yeasts must maintain the fermentation vigor unaltered in the presence of normally added sulfur dioxide. SO2 is used as an antimicrobial agent for the control of yeasts present on grapes as it exerts a selective action on indigenous species. 30% of Saccharomyces cerevisiae strains resist SO2 and ferment up to a SO2 concentration of 150 ppm (to freeze at 200 ppm).
- Il copper which is used as a pesticide is residual in the must and depending on the quantity present it can have a selective or toxic action on the yeasts, to the point of stopping fermentation and negatively affecting the organoleptic qualities. Some yeasts resist copper well and accumulate it in the cell removing it from the wine. To make organic wine, the variability of the S. cerevisiae species can be used for the selection of starter crops.
- Yeasts react differently to fitopharmaceuticals and the wilder strains are naturally more resilient, to the point where they could override starter crops in their presence.
- In case of prolonged and systemic use of the fungicides there are episodes of resistance that have canceled the effihunt. These no longer have effThese inhibitors on alcoholic fermentation, on the contrary, stimulate the yeast to the point of making it produce a greater quantity of alcohol.
3.4 The killer factor
Killer yeasts produce protein or glycoprotein toxins that are lethal to sensitive yeasts. Similarly, killer yeasts are immune to their toxin, but can be sensitive to the toxin emitted by other killer yeasts. There are also non-killer yeast strains that no longer produce the toxin but are immune to it. This activity is comparable to a competitive advantage. Sometimes wild killer yeasts can prevail over starter cultures in wine processesfiindustrial cation. During fermentation the effThe effect of killer toxins depends on the initial ratio between killer strains and sensitive strains, the possible presence of protective or neutral yeasts, the level of inoculation and the availability of nitrogen. The killer yeasts delay or block fermentation by damaging the organoleptic qualities of the wine due to the metabolites (acetic acid ...) that are obtained.
Wine yeasts / 4: Qualitative characteristics of wine yeasts
These are the numerous transformation and / or demolition activities of the components in the must. Their identification allows to choose the most suitable yeast strain for the fermentation of that speciesfico must by virtue of the type of wine to be obtained. The yeast therefore has a huge impact on the result final as it determines most of the positive or negative olfactory and taste-olfactory qualities.
4.1 Acetic acid (Volatile acidity)
It is produced by yeasts during fermentation and therefore, in an amount that varies between 0,3 and 0,6 g / l, it is always present in wine. An amount greater than 0,6 g / l is negative for the olfactory characteristics.
Apiculate yeasts produce a lot of acetic acid and can be contaminants, but there is a lot of variability between strain and strain. For example, the Hanseniaspora species produce a quantity of acetic acid that varies between 0,5 and 5 g / l for pure fermentations.
S. cerevisiae has a low production of acetic acid ranging between 150 and 2000 mg / l and l variability also depends on the place of origin.
Torulaspora delbrueckii is the species with the lowest acetic acid production.
It is one of the most important secondary compounds produced during alcoholic fermentation.
It contributes significantlyficative to the viscosity and roundness of the wine with an effpositive effect on the olfactory and taste-olfactory characteristics offinendo the flavour of wine.
The glycerol concentration, which ranges from 1 to 12 g / l, is directly proportional to the quality of the wine.
La glycerol concentration depends from: must composition, initial sugar content, fermentation temperature, pH, oxygen, solfitation, acidity e especially from the yeasts that participated in the alcoholic fermentation. In particular S. cerevisiae, S'codes ludwigii and Z. bailii produce high levels of glycerol.
The concentration of glycerol is directly proportional to the fermentation power: as the production of ethanol increases, the production of glycerol increases.
4.3 Aromatic compounds
The most important secondary compounds produced during alcoholic fermentation are the higher alcohols (n-propanol, isobutanol, active amyl alcohols, isoamyl alcohol and 2-phenylethanol…) which derive catabolically from the corresponding amino acids in the must and from glucose without involvement of the precursor amino acids.
Higher alcohols are very important for wine aromas and their production is due to yeasts, the quantity varies between 100 and 500 mg / l depending on the composition of the must, the initial quantity of sugar, oxygen and nitrogen. Small amounts of higher alcohols inflthey work positively in wine, while high quantities inflthey work negatively. For example, isoamyl alcohol in high quantities gives it a burning smell.
The only higher alcohol that is positive even in high amounts is the 2-phenylethanol (derives from the amino acid phenylanine) which gives a delicious rose scent. Its concentration depends on the composition of the musts (= grape variety) and on the yeast that dominated during fermentation. In particular, cryotolerant yeasts and S. bayanus produce the largest amount of 2-phenylethanol on average 240 mg / l.
THEacetaldehyde it is the most important carbonyl compound (it represents 90% of aldehydes), precursor of acetate, acetoin and ethanol. The content of acetaldehyde it varies according to the yeast species from 10 to 300 mg / l and the degree of oxidation of the wine depends on this quantity. A low level of acetaldehyde gives a pleasant fruit aroma. A high level of acetaldehyde, above 100 mg / l, gives the wine a pungent and irritating smell. Above 500 mg / l the wine is no longer marketable.
- - esteri derive from condensation of acetic acid and lactic acid with ethanol and other wine alcohols. In particular, ethyl acetate gives an unpleasant hint of vinegar. Quantity ranging between 50 and 80 mg / l: they give the wine positive organoleptic characteristics. Quantity ranging between 150 and 180 mg / l: they give the wine negative organoleptic characteristics.
- THEvinegar it is an organic compound naturally produced by yeasts of the genus Saccharomyces during alcoholic fermentation and is present in wine in an amount ranging from 2 to 32 mg / l. Influence the aromas of wine. non-Saccharomyces: they make a lot of acetoin (from 80 to 250 mg / l); Saccharomyces: they make little acetoin (from 25 to 100 mg / l).
Yeasts produce different amounts of ethyl acetate depending on the species and the strain.
- non-Saccharomyces: they produce a lot of ethyl acetate (from 30-120 to 300 mg / l for S'codes ludwigii);
- Saccharomyces: S. cerevisiae produce little ethyl acetate (from 1 to 70-100 mg / l) while for the other Saccharomyces yeasts there is a lot of variety of strain.
How does wine affect the wine yeast species / strain?
The products obtained with fermentation are basically the same regardless of the yeast species / strain, but the quantities of product inside the wine change so much.
- Compounds that VARY LITTLE within the species: they can be considered as characteristic / distinctive of that species.
- Compounds that VARY SO MUCH within the species: they can be considered as characteristic / distinctive of a certain strain, but not of the species to which the strain belongs.
How do yeasts interact with grape varieties?
Since the quality of the wine also depends on the pattern of yeasts that led the alcoholic fermentation, it is essential to use selected yeasts chosen on the basis of the needs of the must and of the wine to be obtained and to have better taste-olfactory characteristics.
However use the same starters to make different wines in different geographic areasfithat different, although correct, is not the best choice as there is a potential risk of sensory flattening of the wines produced. For this reason we tend to select the S. cerevisiae strain according to the grape variety, the pedoclimatic environment and the cultivation and wine techniquesfication.
Almost paradoxically, musts that come from noble vines do not express their qualitative potential without pairing with a specific starter.fico. For selected yeast specifico we mean a yeast capable - through its metabolic activity - of enhancing the aromas and typical characteristics of the grape. Within the S. cerevisiae species there are numerous strains which, despite having the same genes that you codefiwith their characters, they are able to give the same starting must very different qualitative and sensorial characteristics. For this reason, the choice of yeast must not only be made according to the guarantee of the complete and regular performance of the alcoholic fermentation, but also with the aim of enhancing the characteristics of the vine and avoiding the dreaded gustatory flattening. The production of secondary compounds thus becomes the character of selection to identify the starter capable of enhancing the vine and its terroir.
Wine yeasts / 5: Health characteristics
We often forget that wine is first of all a food and as such it must be treated, regardless of the fact that the nutritional principles are present only as a trace rather than as an appreciable quantity. In wine there can be positive substances (eg polyphenols) for human health, or negative ones such as biogenic amines. THE yeasts must also be selected by virtue of their ability to produce positive substances and reduce the presence of any negative substances produced.
The goal is to use the selection of indigenous speci wine yeastsfithis for the vine is twofold: on the one hand to avoid the flattening of the taste, on the other hand to enhance the olfactory and taste-olfactory characteristics of the vine and of the terroir in order to produce a wine of higher quality.
<br>• P. Romano, Technological and quality properties of wine yeast species, In: M. Vincenzini, P. Romano, GA Farris, Microbiology of wine, Ambrosiana Publishing House, 2005
<br>• Angela Capece, Rocchina Pietrafesa, Gabriella Siesto, Rossana Romaniello, Patrizia Romano, The strength of the indigenous yeast selected to face the new challenges of the market, OICCE TIMES Issue 83, Year XXI, Summer 2020
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