The many styles of wine fermentation

Microbial population dynamics can be influenced by seemingly inconsequential processing decisions and some microbial characters may be desired in some wines but considered spoilage in others. Fermentation management strategies must take into consideration the style of wine being made, the inherent risks of different processing decisions and seasonal or vintage variation in composition of both fruit and microbial biota.

A brief overview of the wine production process

The production of wine is a multistep yet simple process (Figure 1). The composition of the wine is influenced by the processing decisions made along the way to the bottle. The first important decision is the time of harvest. The timing of harvest not only impacts the composition of the grape influencing relative levels of sugars, amino acid content and types and amounts of flavour precursors, but also affects the relative numbers and species of microbiota that will be present on the grape and therefore present in the juice and ultimately in the wine itself. Different vineyards and sites within a vineyard will support differing populations of microbiota¹ as will the presence of damage to the clusters or berries, the presence and types of insect vectors and physical factors such as temperature and exposure of the grape berries to UV light. These factors can be modified to an extent by vineyard management practices but some will be inherent to the site and type of grape varieties planted².

Following harvest, grapes are crushed and the resulting must (juice, seeds and skins) can be used directly for fermentation as in red wine processing or the must pressed to release the juice from the skins and seeds which is used in white and blush wine production³. Grape skins contain the red pigments or anthocyanins that give red wines their characteristic colour and the fermentation on the skins releases these important pigmented compounds into the juice so that they will be present in the wine. Grapes are not washed post-harvest nor is there any heat or pasteurisation treatment except in special cases so materials other than grapes, such as soil, leaves and insects may also be a source of microorganisms³.

In red wines the must, and in white or blush wines the juice, is then transferred to a tank for fermentation. Fermentations may be inoculated with commercial preparations of microorganisms or left to ferment by the organisms coming from the vineyard or present in the winery. Both vineyards and wineries develop distinctive populations of microorganisms as has been recently revealed by next generation sequencing of vineyard, grape and winery surfaces^4,5 and these differences can persist throughout the fermentation³. The primary or alcoholic fermentation is considered to be complete once sugar levels drop to a low to undetectable level, generally less than 2g/L from a starting concentration of 240g/L, an equimolar mixture of glucose and fructose³. The juice must be fermented in wood, stainless steel, plastic bins or concrete vats with or without temperature control. In red wine production, the carbon dioxide produced as a consequence of sugar catabolism via glycolysis becomes entrapped in the skins and as a consequence, the skins form a floating cap on the surface of the wine. The cap must be periodically bathed with fermenting wine or submerged and broken up into the wine to enable efficient extraction and to avoid the build-up of microbial populations on the upper surface of the cap. Cap management becomes important not only for colour extraction but for management of microbial populations³. Thus multiple phases present in red wine fermentations: the liquid phase of the juice, the solid phase of the grape skins and seeds, the surfaces of the containers that can house development of biofilms, and the exposure to air on the surface of the tank. Fermentations of red wines are conducted at temperatures ranging from 18 to 32°C, depending upon the variety of grape and style of wine desired³. Once fermentation is complete, the red wine is pressed off of the skins and seeds and subjected to the aging program of the winery.

White wine fermentations are restricted to the juice extracted from the grapes immediately upon crushing and pressing. White juices often undergo the process of ‘cold settling’ in which the juice is held at a low temperature (4 to 10°C) for 24 to 48 hours to allow the solids from the grape berries to settle to the bottom of the tank³. The juice is then separated from the solids and fermented at a warmer temperature, typically 12 to 16°C. The white fermentations are conducted at temperatures lower than those of red fermentations in order to retain volatile flavour components. In some situations, red musts are also held at low temperatures in a process referred to as a ‘cold soak’ to extract the readily soluble anthocyanins and other components⁶. This low temperature incubation of juices and musts allow the development of cyrotolerant microbial species early in the production of wine^7-9. Changes in processing temperature impose a strong selection on resident microbial populations.

Following fermentation, wines are then aged prior to bottling. This aging period allows for important chemical transformations of the wine to occur that leads to flavour modification and integration. A second microbial fermentation may occur at this time, characterised by the conversion of the grape acid malate to lactate¹⁰. Members of the lactic acid bacteria conduct this conversion that is colloquially called the malolactic fermentation. As with the primary or alcoholic fermentation, the malolactic fermentation may be induced by addition of commercial preparations of bacteria, by using wine already undergoing active malolactic fermentation as an inoculum, or by simply relying on the lactic acid bacteria indigenous to the winery to conduct the fermentation. In some cases, if conditions are favourable, the malolactic fermentation may initiate prior to the initiation of the yeast fermentation.

It is important to emphasise that even today wine production is not a sterile process. It takes advantage of the ability of Saccharomyces to dominate mixed-culture fermentation and produce sufficient ethanol to inhibit other microbes. Similarly, the malolactic fermentation generally relies on the activity of a single lactic acid bacterium, Oenococcus oeni, for the evolution of desired bacterial flavours and for the consumption of nutrients that would otherwise be available for spoilage organisms^10,11.

Fermentation processes

With respect to the primary fermentation, there are two classes of processes: inoculated and autochthonous. Autochthonous or native fermentations are conducted by the microbiota present on the grapes and in the winery. In either case, the fermentation is eventually dominated by the yeast Saccharomyces. Two species of Saccharomyces, S. cerevisiae and S. bayanus, are commonly found in wine fermentations. Many commercial strains and native isolates are hybrids of these and other species of Saccharomyces^12-18.

Inoculated fermentations

Fermentations may be inoculated in one of two ways. The most common is to use a commercial active dry preparation of a yeast strain. There are over 100 commercial yeast strains available that differ in key oenological properties or that are targeted for specific fermentation conditions such as high ethanol tolerance or low nitrogen or micronutrient requirements. These strains also differ in aromatic profiles, fermentation efficiency, tolerances to temperature, and in ability to dominate the microbial profile of the wine. Some of the commercial yeasts have been selected for specific varieties of grapes or of style and produce more or less of the enzymes that impact the conversion of grape precursors to wine aromatic compounds.

Fermentations may also be inoculated using an existing fermentation. This process is also referred to as pitching and levels of inoculation are generally one to two per cent of the tank volume. In both types of inoculated fermentations, sulphur dioxide may be used to inhibit more sensitive microbes enabling a swifter domination of the fermentation by the inoculant. The nitrogen levels of the must are routinely monitored and adjusted to assure a complete fermentation and to avoid a slow or arrested fermentation¹⁹.

Autochthonous fermentations

In contrast, autochthonous or native fermentations are conducted by the organisms originally present in the juice or that gain access to the juice by passage through winery surfaces. In some cases, these yeasts may originate in the vineyard but in other cases the fermentations are conducted by winery residents². There are different production styles for native fermentations. A true autochthonous fermentation receives no addition of microbes and no other adjustments (nutrients, pH or sulphur dioxide) so that there is minimal manipulation of the existing population dynamics. These types of fermentations can be at high risk for the development of off-odours or for slow or incomplete fermentations depending upon the microbes present and the innate nutrient composition of the fruit³. Manipulated autochthonous fermentations are those in which the conditions are adjusted to favour desired populations and to assure fermentation completion. The pH may be adjusted to reduce the activity of the bacterial species present. Most of the lactic acid bacteria are unable to grow below pH 3.5 and wine pH can vary from as low as pH 3.2 to as high as pH 4.2. Adjusting a juice to a pH below 3.5 greatly restricts the numbers of bacteria able to proliferate³. Similarly bacteria and non-Saccharomyces yeasts are more intolerant of sulphur dioxide than Saccharomyces is with the bacteria being most sensitive. Additions of sulphur dioxide can therefore also be used to adjust the microbial populations of the must. Nutrient additions can encourage the growth of the major populations present so can be timed to benefit a desired population by holding off the addition until Saccharomyces has become dominant.

Some autochthonous fermentations are more correctly termed partial native fermentations. In this case there is an inoculation with Saccharomyces at some point in the fermentation after the non-Saccharomyces populations have been able to impact wine aroma. Partial native fermentations are generally employed when there is a greater risk of arrest of fermentation or of the production of off-characters. The microbial populations present during autochthonous fermentations will be influenced by the vintage, winery sanitation practices, the variety of grape, and the adjacencies of the vineyard to other types of commercial operations.

Microbial contribution to wine aroma

The flavour and aroma compounds of a wine can derive from the grape itself, from microbial activity, from oxygen-dependent and independent chemical reactions that occur during aging or from extraction of cooperage such as oak during fermentation or aging. There are several classes of volatile compounds found in wine: alcohols, aliphatics, benzene derivatives, carbonyls, esters, organic acids, S-containing compounds, shikimic acid derivatives, polyols, isoprenoid-derived terpenes and C13 nor-isoprenoids, methoxypyrazines, vinyl and volatile phenols^20-35. There are multiple ways in which the biological activity of Saccharomyces can impact wine flavour and aroma. Yeast may synthesise aroma-active compounds de novo or convert grape components into volatile odour-active forms. Yeast cellular components such as breakdown products of cell wall mannoproteins can modify wine flavour^36,37. Metabolites produced by yeast can partake in the chemical reactions that occur in wine during aging^38-41. Such modification may either create novel flavour-active components or lead to the loss of flavours derived from other sources⁴⁷.

Saccharomyces is capable of producing a wide array of aromatic compounds that have been detected in wine^{22,32-34,37,42-76}.The major classes of compounds formed are acids, alcohols, carbonyl compounds, esters and S-containing volatiles (Table 1). These compounds are often found in levels above the threshold of detection⁴⁷. Table 1 also includes the common descriptive terms given to these compounds. Many are classified simply as ‘fruity’, meaning there is a perception of fruit character but not of a distinctive type of fruit. The fruity yeast esters serve an important role in amplifying the perception of the fruit identifier compounds of the specific wine variety. Non-Saccharomyces yeast can also contribute aroma compounds to the wine either via direct synthesis or due to modification of grape components.

Conclusion

The production of wine with a desired aroma and flavour profile hinges on the ability of the winemaker to manage the presence and metabolic activities of the organisms present during fermentation and aging of the wine. Yeast metabolites can enhance, modify or diminish wine aroma. Although Saccharomyces is the principle architect of wine production, numerous other microorganisms contribute to the final aromatic and flavour profile of the wine²⁷. The goal of fermentation management strategies is to guide the process of microbial activity towards that of flawlessness, in short to act as maestro to achieve the desired masterpiece of flavour and aroma.

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About the author

Linda Bisson received her BA and MA degrees in Biology and Microbiology respectively from San Francisco State University. She obtained her PhD from UC Berkeley and conducted postdoctoral research at Harvard Medical School. In 1985 she joined the faculty in the Department of Viticulture and Enology at UC Davis.