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Primary Fermentation of Wine
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During the primary fermentation of wine, the two grape sugars, glucose and fructose are converted to alcohol (ethanol) by the action yeast. The by-products of primary fermentation are aromas and flavours, the gas carbon dioxide, and heat. The production of heat during fermentation (i.e. it is an exothermic process) means that during fermentation the temperature of the fermentation vessel will rise, and will require action on the part of the winemaker to cool it down. White fermentation is usually conducted in the range of 8-19OC, and red wine fermentions typically are allowed to run at between 25 and 32oC. At temperatures higher than this, there can be a loss of desirable aroma and flavour compounds, and unattractive aroma characters in the spectrum of caramel, burnt or cooked characters can be produced. There are many types of yeast, but two closely related types known as Saccharomyces cerevisiae and Saccharomyces bayanus are the ones that are responsible for fermentation.
These species of yeast are encouraged to conduct the fermentation because they:
In white winemaking the juice is usually inoculated with yeast following its clarification. The most common type of inoculation is that using an active dried yeast culture. Yeasts are freeze dried and stored in vacuum packed tins by yeast supply companies ready for use. The winemaker then re-hydrates the yeast in warm water bringing them out of their state of suspended animation. The hydrated yeast is then added to the juice.
- Are alcohol tolerant. That is, they can continue to ferment sugars to alcohol even during the latter stages of fermentation when the sugar is low but the alcohol content is high.
- Can establish a viable population in an environment of high sugar (190-270 grams per litre) and high acidity.
- Are strong and consistent fermenters even at cold temperatures.
- They ferment quickly and only stop when all the grape sugars have been utilised. Otherwise we would be buying sweet low alcohol wines.
- Are more tolerant to sulfur dioxide than other yeasts and bacteria.
- They produce wine like aroma and flavour characters. A free list of wine fermentation characters can be found here, or can be purchased as a beaut full colour laminated wine tasting wheel here.
Another less interventionist approach is to let nature take its course. The grapes have a bloom that contains active cells of Saccharomyces cerevisiae. After juicing the grapes these grape yeast cells and other yeast cells picked up from the winery equipment then start to convert the grape sugars to alcohol. This type of fermentation is variously called natural, indigenous or spontaneous fermentation.
The former approach is common in New World winemaking whilst the latter is more often practiced in Europe. So what are the advantages and disadvantages of each approach to inoculation?
The advantages of winemaker inoculation are:
The advantages of spontaneous inoculation are:
- Choice. Many different strains of Saccharomyces cerevisiae have been isolated and have different properties. Some produce very fruity (estery) aromas, others produce more neutral characters. Some ferment better at colder temperatures than others.
- Consistency. All strains are selected for their ability to meet the general necessary criteria given above. This maximises the chances of clean and complete fermentations occurring.
The disadvantages of spontaneous fermentation are:
- Better Mouthfeel. Many commentators' report that they feel that spontaneous fermentation promotes better mouthfeel in wines. That is, the wines are thought to be softer and creamier than those made using single strain starter cultures. This suggestion however has not been conclusively demonstrated scientifically. The rational for the suggestion is that the natural yeast flora on the grape is genetically heterogeneous, i.e. consists of multiple strains and that this is the reason for the improved mouthfeel.
- Cost. Spontaneous fermentation costs nothing to initiate.
The white juice may either be entirely fermented in barrel, or in tank, and when partially completed, transferred to barrel. With red wines, practicalities demand that they be fermented in tank for most of duration of the ferment (so as to ensure appropriate extraction of colour and tannin from the skins), before the coloured partly fermented juice is drained into barrel to complete the fermentation. Temperature control in barrel ferments is pretty basic. The barrels are placed in a cool room and the wine is left to ferment. The smaller volume and relatively high surface area of the barrel aids in temperature control. Barrel fermentation is generally carried out for premium full bodied styles and used with varieties such as Chardonnay, Semillon and occasionally Sauvignon Blanc. Any full bodied red wine can benefit from partial barrel fermentation.
- Lag time. Yeast cells increase in number by division. One becomes two, two become four, four become eight etc. The initial numbers of yeast cells in an un-inoculated juice are by nature low. Dividing from a low initial base of cells means that it takes longer for fermentation to become active. A wait of 3-4 days is typical. In large commercial wineries where the vintage is planned to work like clockwork, with tanks becoming free just when purchased fruit is to arrive at the weighbridge, the uncertainties of spontaneous fermentation present excessive risk. When inoculating, the winemaker adds about 5 billion cells per litre. Quite a head start over spontaneous fermentation.
- Higher probability of spoilage. In theory this should be a problem but in practice it rarely is. Sometimes other spoilage yeasts and bacteria take advantage of the ecological void caused by the shortage of active yeast cells in the early stages of spontaneous ferments. In practice this can be avoided by ensuring that the acidity of the juice is sufficiently high. Saccharamyces cerevisiae is more tolerant to acidity and sulfur dioxide than most other organisms, and winemakers use these facts and engineer the juice environment to favour its growth over the undesirable microbes.
White wines are often left in contact with the dead yeast cells (lees) that fall to the bottom of the barrel following the completion of fermentation. This contact is usually for a period of six to nine months, and the lees can either be left or stirred (battonage) at regular intervals. Barrel fermentation and lees contact increases the aroma and flavour complexity, imparting smokey, toasty and cheesy flavours to white wines. Some winemakers also feel that barrel ferment results in better integration of fruit and oak, and that imparts a creamy texture to the mouth-feel. However if it is overdone, the wine can take on overt doughy and vegemite characters.
Fermentation is completed when all the fermentable sugars have been converted to alcohol. This end point is measured chemically. Once the wine is deemed free of fermentable sugar, i.e. dry it is cooled to 4oC and the dead yeast cells (known as gross lees) are allowed to settle. The relatively clear wine is then racked to remove the gross lees before it it is stabilised and filtered ready for bottling.