Yeasts become evident in the must in a variety of ways, each one causing the wine to have different characteristics. They can be inoculated in with cultured yeast cells using commercially produced products. Alternatively, winemakers can rely on wild yeasts. The former has only been in use in more recent times.
Wild yeasts or sometimes called natural yeasts are wild in that they naturally live in the environment where the grapes are grown in the air, on vegetation or on equipment and can be quite specific to a particular location (indigenous yeasts). However, there is no such thing as artificial yeast as all yeasts are natural and not synthetic. Any wild yeast species can end up in the must whether it is wanted by the winemaker or not and start to turn wine juice into alcohol during the fermentation process. Wine making is therefore made from fermentation using either wild or inoculated yeast.
Generally speaking, wild or indigenous yeasts have a low resistance to alcohol and when the alcohol reaches a certain level it can cause stuck fermentation. This causes a real headache for a wine producer - flabby low alcohol wines. Wild yeasts also exist on grapes and vines in much smaller numbers than inoculated yeast and it can take longer for wild yeast to multiply (up to a week in larger batches) leaving the grapes vulnerable to infection from bacteria and other organisms as well as oxidation. Grapes picked with a low acidity or pH naturally inhibit spoilage caused by other organisms during the lag phase it takes for spontaneous fermentation to start.
Feral yeast is a yeast that exists specifically in the winery on equipment or on barrels and its origin is often unknown. It could be wild yeast that blew in from outside or came in on winemakers cars, bodies or clothing.
Born to be wild?
Winemakers either love or loathe wild yeast fermentation. The big problem is that unlike inoculated yeasts, wild varieties can be unpredictable. Inoculated yeasts are much more reliable. For thousands of years wine was fermented using wild yeasts found naturally in the vineyard but nowadays most wineries inoculate in order to produce a consistent end product, barrel after barrel and year after year. There are thousands of different types of wild yeast but unfortunately many do not make good wine. Wild yeasts themselves can be broken down into two categories: wine yeast and spoilage yeast - the latter definitely aren't good for wine production.
Specific cultured yeast strains might be able to produce certain characteristics such a particular aroma or have an ability to ferment in certain temperatures. Others can produce sediment that settles quickly in the tank.
The type of grape fermented will determine the strain of yeast chosen but the species of yeast considered most desirable for total alcoholic fermentation is Saccharomyces cerevisiae.
Major characteristics for an ideal yeast for wine making:
- Fermentation speed
- Alcohol tolerance
- So2 tolerance
- Cold tolerance : fermentation below 14ᵒc
- Low foaming activity
- Efficient conversion of sugar into alcohol
- Production of desirable metabolites
- Low production of undesirable metabolites: Such as acetaldehyde, acetic acid, sulphur dioxide and hydrogen sulphide
- Resistance to “Killer” yeasts
- Flocculation: An ideal yeast strain should flocculate at the end of the fermentation, leaving the wine clear and requiring less rigorous filtration
When grapes are harvested they are covered with lots of biological organisms, including bacteria and fungi as well as wild yeasts and winemakers use sulfur dioxide (SO2) kill wild yeasts and inhibit the growth of spoilage organisms. When inoculated yeasts are used the the dose is large enough to start fermentation quickly and for that strain to dominate if other wild strains are present. If rain fell just before harvest, wild yeasts may be washed off by the water and levels will be well down, insufficient to cause fermentation without further intervention. The moisture and the lower levels of yeast can cause bacteria to multiply in the must or it can mean no Saccharomyces cerevisiae in the must at all.
During the lag phase before fermentation starts, the must is vulnerable to bacterial growth and VA (volatile acidity) from acetobacter and lactobacillus. These bacteria can turn the previous wine into vinegar and produce terrible aromas such as rotten eggs.
When making wine with wild yeast, no Sulfur is added to the must and a process called spontaneous fermentation is allowed. The wild yeast just does its own thing at its own pace.
Some wineries have been fermenting on wild yeast for many, many years with successful results. For other winemakers spontaneous fermentation is a trendy new approach, a little on the edge, almost a gamble in the quest for better wines. Wild yeasts can help the grapes to produce specific chemical compounds e.g. esters that you just can't get with commercial products.
Because wild yeasts often take longer to work it allows more skin contact time on grapes potentially giving more body, complexity and colour to the finished wine. Wild yeasts can often impart unusual aromas and flavours they can help product distinctive, interesting wines. No two batches of spontaneous fermentation taste the same, even when grapes are from the same vineyard, fermented together in the same room.
The result can be a wonderful wine with fruit, amazing aroma and flavour, and with good alcohol. Though it is often much more of a gamble than using inoculation, for some the end result if worth the risk and this is the main reason why spontaneous fermentation is used in winemaking. Some winemakers are adamant that wild yeast is the only way as cultured yeasts produce boring, standardised wines with no character.
There is often a half way house though. Winemakers allow spontaneous fermentation to begin using wild yeasts and take regular readings of the amount of sugar or specific gravity of the must, to determine the current alcohol. After alcohol has reached a certain threshold they may sometimes choose to inoculate with commercial S. cerevisiae to ensure complete fermentation. This achieves the desired quality but still allows the complexity of wild yeast.
Old world winemakers use a process called Pied de Cuve to ensure that fermentation begins. A small amount of their grapes are picked a week before the harvest and water may be added to overcome excess acid. The must is stirred to stimulate the wild yeast and start spontaenous fermentation. After several days, these grapes will be nicely fermenting and this starter culture is used to inoculate the harvested grapes when they come in after picking some days later. So instead of inoculating with bought in yeasts, inoculation occurs with the indigenous yeasts.