(from Emile Peynaud’s Connaissance et Travail du Vin)
Oenology is the science of microbiology. Winemaking is essentially applied microbiology.
Micro-organisms make wine: yeast and bacteria (and in some conditions certain strains of each can destroy it).
The microorganisms responsible for wine have not only a huge impact on its composition, but also its taste and smell.
Vinification and conservation of wine are both dominated by microbiological problems, and making a good wine depends upon understanding both how to control and manage both yeasts and lactic bacteria. Ageing wine, and keeping it stable for long periods, is equally a fight against microorganisms that can potentially alter its makeup.
The word ferment comes from Latin fervere, to boil. To put it extremely simply, crushing grapes releases their juice, the juice becomes cloudy, heats up, bubbles of carbon dioxide are formed, the liquid loses its sugary taste and becomes 'vineux' (wine).
About 90% of the sugar in the original juice becomes alcohol, the rest becomes glycerol, succinic acid, acetic acid, lactic acid, acetaldehyde, pyruvic acid, superior alcohols etc.
Pasteur didn't discover yeasts, but he was the first to understand the link between these living micro-organisms and the transformation of sugar.
Yeasts are unicellulaires - single cell organisms. They are microscopic single cell mushrooms that decompose sugar into alcohol and carbon dioxide.
Generally, the cells find the energy they need to survive by two types of catabolism; that is to say two types of breaking down of organic matter.
1) respiration, which needs oxygen
2) fermentation, which takes place in the absence of oxygen.
Respiration releases lots of energy, but fermentation is a bad use of energy because it's a slow process, it doesn't fully break down the organic matter and so needs lots of sugar to fulfil its energy requirements (and by consuming the sugar alcohol is produced as a byproduct).
There are around 30 successive chemical transformations during fermentation, each using enzymes as their catalyst. Think of enzymes as the tools of yeasts (transporters), each adapted to a different stage of vinification. Each transformation needs a different tool, a different enzyme. The secondary products (glycerol etc) are byproducts of each of these reactions.
Yeasts are the agents of fermentation. It's possible to grow yeasts like microscopic vegetables. They reproduce making mother and daughter cells asexually or sexually.
There are many species of yeast each with their own specificities, morphology and reproduction methods.
Yeasts are classified by a double-barrelled Latin name - first refers to genus, second to species.
Eg Saccharomyces (genus, meaning sugar mushroom) ellipsoideus (species, meaning with an elliptical form) - otherwise known as sacharomyces cerevisiae or vini.
Yeasts that are present during vinification can be elliptical, apiculee (like a lemon), round or allongee (stretched out like a sausage).
Most wine yeasts come into existence in one of two ways - vegetative reproduction through forming buds, or through producing spores that then germinate to produce yeast.
Either way (& usually second way initially, when in vineyard) as soon as a wine yeast finds itself in a nutrient rich environment (sugary must), one yeast cell will produce a bud/daughter cell, and as soon as they are the same size, the two cells separate and the process repeats itself. In optimum conditions it takes two hours for the number of yeast cells to double.
As soon as the environment becomes hostile (for example the yeast cells have exhausted the available sugar supply) they stop producing the buds, and some will close their spores. Their 'life cycle' slows down and they are able to resist conditions that should in theory kill them (eg sulphur dioxide addition or v high alcohol in the environment). This is an exceptional characteristic of wine yeasts, so yeast cells in lees are 'dead', but in the right conditions the spores could germinate again, and create new yeast cells.
In one drop of fermenting must, there can be 5 million yeast cells.
Yeasts live on the skin cells of ripe grapes at harvest and are brought into the winery with the grapes themselves. During winter, they live mainly on the top layer of the soil, and in the summer insects are the main transportation system for getting them to the grapes (along with other microorganisms). There are very few yeasts found on the skins of unripe grapes - they start to appear after veraison, once insects start taking an interest in the grapes. It's chance in many ways how the yeasts end up being distributed, which explains why there are not really specific strains of yeast for specific grape varieties (although climate has some influence on modification of character).
Besides good yeasts, there are also acetic bacteria & lactic bacteria on the grapes - so potential for spoilage also.
Approx 70 species of yeast, 15 genus.
One thing to note - yeasts on the grapes are not exactly the same as yeasts in must during fermentation. And yeasts species almost always vary from beginning to end of the fermentation process.
It's only after crushing that the yeasts start to really multiply, the crusher-destemmer machine act as turbocharger (breeding ground) of the yeast cells.
Certain anti-rot treatments inhibit the development of 'the right' yeast cells, and the microflora of rotten grapes is not the same as that of healthy grapes.
Saccharomyces oviformis (also called saccharomyces bayanus) is capable of resisting high alcohol.
Torulopsis stellata is specific to noble rot grapes (and also rotten grapes) - often starts off a fermentation but is very susceptible to sulphur.
But most common remains saccharomyces cerivisiae - although often doesn't establish itself until around 3-4% alcohol, until by mid-fermentation it is fully established. It does well not particularly because of a high resistance to alcohol but because it converts sugar more quickly than other yeast species.
If the must is very high in sugar, saccharomyces oviformis often takes over at the end of fermentation. Some can go as high as 20%abv, and are very useful for finishing off stuck fermentations of high alcohol must.
Difficult yeasts - levures nuisible/levures d'alteration
Often a different genus from a vinification yeast, and they can rest undetected for months. Several yeasts can cause clouding of wine, affecting the limpidity. If there is any residual sugar, these yeasts can cause refermentation. Can happen equally in vat, barrel or bottle.
This is one of the reasons that filtration is important. Ideally a finished stable wine has no more than 3000 yeast cells per cm3. A young wine (still cloudy so before fining) might have 200,000 per cm3. Good filtration can go to 1500, while centrifugal machines can get remaining yeasts down to 200-1000 per cm3.
The problem is that these remaining yeast are often the most resistant ones, so the levures d'alteration, that can cause harm. These are not the yeasts of early fermentation but ones that are able to survive the presence of alcohol and free sulphur. They are often just one species, and can be seen as equally dangerous as certain bacteria.
In wines with high alcohol, it's saccharomyces oviformis - this is the yeast that starts refermentation in sweet wines, that forms the 'voile' layer of flore in certain sherries etc and that starts the second fermentation in champagne.
In low alcohol, saccharomyces bailii is v resistant to free sulphur
And brettanomyces can also develop, often at the wine's surface, with the barnyard aroma.
Yeasts capable of infection can exist on floors of vats, in old barrels, in the bottling machines (especially because there can be injections of air at this point that can cause yeast infections).
Fermentation is not a simple process - different yeasts are working at different stages, and lactic bacteria can start fermenting malic acid at the same time, which is often helpful. Trying to ferment under entirely sterile conditions with no interference from anything is not desirable. Adding selected yeasts can also work sometimes, but it is not necessary, and not always desirable. It's hard to get rid of the indigenous yeasts to allow the cultivated ones to establish themselves (easier in white then red must) - a good debourbage or settling after addition of sulphur helps this (sometimes to the point that addition of yeast is essential).
To avoid uneven starts of fermentation and uneven length (so to ensure a nice even and speedy fermentation), many winemakers practise the traditional method of 'pied de cuve'. This is when take a pre-selected yeast from specific grapes that you put in a freshly harvested vat of grapes and which is used to encourage a swift start to fermentation. Often find that the first vat is slow to start and then each subsequent vat gets faster and faster. Sometimes the last vat finishes before the first.
The way to do an effecive pied de cuve is to cut a few of the ripest and healthiest grapes around 8 days before harvest, crush them, sulphite them then start off the fermentation with whatever amount of yeast is needed. Once this is in full fermentation, you use it to start off the first real fermentation tank.
Modern oenology often uses powdered yeasts. Especially useful in cold years when natural fermentation is tough to start. Or when pesticide residue on grapes, or after highly successful settling of white grape must. Or if grapes have rot so the natural yeast isn't good. Can be hugely useful and good for quality if used in right circumstances. Don’t add it direct to must but first into tepid water for 20-30 minutes to regain their vitality, then into the must. Or to restart a stuck fermentation, need to make it be actively fermenting before introducing it.
Ensuring a successful alcoholic fermentation
The development and reproduction of yeast cells is essentially all that fermentation is - the transformation of sugar into alcohol will only happen as a result of the yeast cells multiplying, and if fermentation stops, it's a sign that the yeast cells are stopping to grow and multiply.
Like all living beings, yeasts need specific conditions to thrive - they are very sensitive to temperatures, they need oxygen, they need sugar, minerals, azote. And the higher the alcohol, the more difficult it becomes.
Temperature needs to be approx more than 13 degrees (or either won’t start or will be dangerously slow) and below 35 degrees (most yeasts start to die at 31-32 degrees). As a general rule, fermentation speeds up as temperature rises up to a certain point, approx 35 degrees when it slows and yeasts dies.
But - the higher the temperature, the quicker fermentation starts and the quicker it stops, and the lower the alcoholic degree that is reached.
This is because the maximum population of yeasts is lower at high temperatures (because less of them can survive. They seem to almost tire themselves out through rapid multiplication, and are less able to assimilate the azote). To keep temperature more ambient (and therefore alcohol higher), it's better to use a barrel for fermentation.
Temperature is the key way to control fermentation. Ideal 26-30 for reds, 18-20 for whites. But temperature at which fermentation slows or stops varies according to aeration, the composition of the must (its richness in sugar, how dense it is in azote and other nutrients), the species of yeast used and other factors.
But key is not to wait until things become problematic before lowering the temperature.
Influence of oxygen
Yeast cells need oxygen to multiply, and in a fully anaerobic environment, they continue to multiply only for several 'generations’ of mother-daughters then stop. They just need a touch of oxygen to restart. Pasteur called fermentation 'life without air' because yeasts find the energy to survive without oxygen but for a long ferment needs some controlled traces of oxygen.
Yeast need oxygen to synthesise sterols (organic substances, sources of vitamins and hormones) and assimilate fatty acids that they need.
When grapes are first picked they get oxygen through crushing, destemming and (in white grapes) pressing, all of which helps ensure a rapid start to the fermentation process. In a fermenting barrel, you'll find the yeast population is higher near the top of the barrel, where the oxygen is greater.
Remontage, pumping over, can be an effective way to aerate in an otherwise closed vat (as exposure to oxygen during fermentation can cause other problems such as acetic bacteria). Better to do more pumping overs rather than to lengthen each one. Best to do more frequently at the start of fermentation, to ensure the yeast can benefit from exposure to oxygen at the moment it most needs it. But not too early - need to aerate the yeast cells that need it, not simply the must. So day two is perhaps most efficient time to begin aerating.
The other beneficial effect is that different zones of the vat are homogenised in terms of sugar content and temperature. And ensures the yeast cells are evenly distributed. If left without pumping over, the majority of yeast cells are found in the marc/cap (because this is comprised of grape skins). This is also the hottest part.
One other key effect is the dispersal if the phenolic elements in the skins to the must - ie pumping over ensures colour, tannins, anthocyanins and aroma compounds are dispersed through the must.
1) as soon as vat is full, carry out a 'homogenisation' pumping over to ensure everything is more evenly distributed. This can be with or without oxygen.
2) when fermentation is getting established, do an aeration pumping over to ensure the yeast has the nutrients it needs.
3) then as necessary, but do one towards the end to again ensure homogenisation.
(if fermentation does stop before you want, most likely have to run off the wine, not just add more yeast to the vat).
Yeasts consume assimilable azote during fermentation, so need to ensure they have enough. Certain terroirs lack azote naturally, and there can also be insufficient amounts in hot years when grapes are over ripe also, or rainy years if grapes are affected by rot. In these cases, it is necessary to add azotes (usually by ammonium salts). Best to be added before the start of fermentation. Need also vitamin B1 (thiamine). Healthy grapes have this naturally, but if lots of sulphur used (eg rotted grapes) may need to add very small quantities.
Level of acidity doesn't hugely affect the development of yeasts in itself, but low acidity can mean dangerous spoilage bacteria can develop if fermentation stops before complete.