Fermentation is the process which turns our musts (prepared mixtures) into wine. Without fermentation, they would remain more or less as we prepared them, for a short time at least. If we did not add suitable yeast, but merely left the prepared mixture to itself, fermentation of sorts would commence. And undoubtedly this would be caused by a mixture of undesirable yeasts and bacteria which were on the fruits when we prepared the must or which came into contact with it later.
An enormous amount of study and research has been and still is being done on the process of fermentation. A century ago it was barely understood; indeed, yeast was not generally accepted as the medium which brought about fermentation. Today We could not hope to make wines without special yeast. By this I do not mean expensive yeast but selected yeast. There are countless varieties of yeast as we see under ‘Spoilage’, but the yeasts we use to make wines must be pure. They must not be contaminated with wild, or undesirable yeast. Every possible care is taken to ensure that the yeasts we buy are suitable for our purpose and as pure as modern processes will allow.
Yeast, it may surprise many, is a living organism. When obtained it is inactive, or dormant.
To produce wines from the musts we prepare we add yeast of a special type and give it the means not only to become active, but also to live and reproduce itself. In return, it produces a number of by-products, the main one being alcohol. In addition, glycerine, acetaldehyde and other substances are produced in quite minute quantities. Yet these are important to the fullness or completeness of fermentation.
It has been found that enzymes secreted by the yeast cells bring about the all-important changes necessary for full and complete fermentation. It seems that yeast, given suitable conditions, will produce the enzymes that it needs to take itself through all the necessary stages of fermentation. As one enzyme completes its work, it stops (becomes inactive) to allow another to take \ over. If this were not the case, too much of certain substances, and not enough of others, would be produced. But this does not happen.
All in all, yeast and the enzymes it produces form a complex system complete in itself, relying only on the means of sustaining itself. And this it does on the sugar we put into the must. But if we give it too much sugar to begin with its reaction is slow and erratic, or perhaps it will not react at all. Hence the general desirability of starting our ferments at a specific gravity of not more than 1.110 as measured by the The Hydrometer. Hence also the need to produce in the must a certain amount of acid, tannin and other desirable matter.
As already mentioned, yeast is dormant when purchased. If we add it to the must in this state, we may have to wait several days for it to become active. So we make what we call a nucleus ferment. In doing this we produce a colony of active yeasts which, when put into a prepared must, will start the must into active fermentation almost immediately. The overriding importance of this is that the prepared must is not left open for a day or so to attacks by wild yeasts and bacteria. Even when covered as directed in the methods, a must where there is no fermentation going on is open to attack because there is not a constant stream of carbon dioxide gas (CO2) escaping through the puckers of the tied-down covering to keep out the air.
This brings me to the importance of what we call fermenting under anaerobic conditions — fermenting in the absence of atmospheric oxygen. Years ago, at
the age of five, I was introduced to home-made wines by falling head first into a tub of fermenting dandelions in my grandfather’s outhouse. And, alas, until recently open fermentation was still practised. Yeast creates anaerobic conditions for itself to a certain degree, but it cannot prevent the escape of the gas it produces. By covering fermenting wines with tied-down sheet polythene, we can control the release of the gas and leave the wine fermenting with a protective cloud of carbon dioxide gas above it.
Later we put the fermenting wines under fermentation locks, usually at the stage where the fermentation has slowed down and where the gas being produced would no longer be sufficient to form a protective cloud.
From all this it will be seen at once that yeast produces not only all the essentials for a full and complete fermentation, but also its own protection from wild yeasts and bacteria so far as it can. When the yeast leaves off, we take over in order to safeguard it and the wine we are preparing.
But whatever happens during fermentation, most amateurs are concerned mainly with the production of alcohol, flavour and bouquet.
When using English wild and garden fruits we cannot use enough to produce in a must the amount of sugar needed to make the amount of alcohol we want, simply because these fruits contain so little sugar and so much acid and tannin that the wines would be undrinkable owing to the high acidity and astringency. For example, to put into a must enough natural fruit sugar we might need as much as 35 kg (80 lb) of blackberries, plums or whatever we might be using — and nobody would be able to drink it if that amount of fruit were used to make 5 ltr (1 gall) of wine.
So when we use, say, 1.5-2.5 kg of fruit to 5 ltr (about 4-6 lb per gall) in order to obtain as near as possible the strength of flavour we need (not forgetting roughly the amount of acid and tannin we need as well), there is a grave shortage of sugar. Obviously, with so little fruit being used, the amount of fruit sugar in a must will be very small, enough perhaps to make about 1 or 2% of alcohol. And since good wines require alcohol in the region of 15% by volume, we must obviously add sugar to produce the type of wine we want and the amount of alcohol we want it to contain.
Here it is important to understand the meaning of alcohol tolerance of yeasts. In general, we use the word ‘tolerate’ to mean the limit of our endurance or ‘the amount we can put up with’. So we can call the maximum tolerance of yeast ‘the amount of alcohol it can put up with’. And this is in the region of 15% by volume. When this level is reached the yeast is destroyed by the alcohol, no more fermentation takes place and no more alcohol is made.
When yeast is added to a must, its first action is to convert the household sugar we add to ‘invert sugar’. This is brought about by an enzyme in the yeast known as invertase. This change is necessary because yeast cannot ferment cane sugar in the form in which it is usually added. Having done this, the yeast almost at once begins to reproduce itself. Each yeast cell — so tiny that something like one thousand of them could queue across a penny — sends out a bud which in turn sends out another bud, and so on. At certain stages the cell looks something like a deformed potato.
During this reproduction process the yeast feeds upon the sugar, turning approximately half into alcohol and half into carbon dioxide gas. It is in the early stages of yeast reproduction that fermentation is very vigorous and the production of alcohol is quite high. The colony of yeast continues to reproduce millions of new cells as others die. Initially the production of new yeasts exceeds the death of others, so that we have an increasing number of yeast cells hour by hour.
After a while the rate slows down, and it is at this stage that those who evolve methods and recipes, as I do myself, recommend putting the wine under fermentation locks. We judge as nearly as possible when this is likely to be necessary and include the instruction in the’ method in the hope that the individual operator’s must will react as we expect it to. Sometimes it does not, and we receive indignant letters from readers whose wines are climbing out of the jars through the fermentation lock. Fortunately these happenings are rare. The slowing-down of fermentation is increased by using a fermentation lock because we cut off the oxygen supply. This causes the yeast to turn to the sugar for its oxygen, thus using up more sugar and producing a little more alcohol than if it obtained its oxygen from outside.
After a fermentation lock has been fitted, we usually regard the wine as having stepped into the secondary fermentation stage, although there is no clear distinction between the primary and secondary stages, because the two overlap. By secondary, we mean, in general, the less vigorous fermentation that goes on for several weeks, according to the composition of the must, temperature and other factors which have bearing on the length of fermentation.
Composition of the Must
A reasonably well-balanced must made up from a good recipe will contain all the essentials for full and complete fermentation. Until a few years ago, hardly any amateur had heard of yeast nutrients which we now add in order to help the yeast to make the maximum alcohol.
I have said that yeast and the enzymes it produces form a complex system capable of producing its own needs to take it through fermentation. This is true
as regards the enzymes needed for this purpose. But yeast needs more than that , to give it the basis from which to work. The yeast can reproduce-itself on sugar alone, but if it is to continue to do this and survive the strength of alcohol it produces it must have other things as well. If it does not get them, fermentation is likely to stop early on so that perhaps only 8% or 9% of alcohol by volume or less is made. English wild and garden fruits undoubtedly contain most of the essential needs of the yeast. But bear in mind that we dilute these juices to such an extent that these essentials are reduced practically to insignificance, in the same way as the sugar they contain. To make up for this dilution we add the essential substances in the form of chemical nutrients, just as we add sugar.
A good nutrient will contain a balanced consortium of all the chemicals the yeast is likely to need and certainly those in which our greatly diluted musts are deficient. Several important chemicals will be found in sufficient quantity even in greatly diluted musts. But generally we must add ammonium sulphate, magnesium sulphate (Epsom salts) and pottassium phosphate. All of these, with certain additives, are contained in a good nutrient.
Temperature For Fermentation
This is another important factor affecting fermentation. And although some writers on the subject recommend a figure as high as 26°C (80°F), I have found that a reasonably constant temperature in the region of 18° to 21°C (65° to 70° F) is perfectly satisfactory.
Making wines all the year round is quite simple these days. But it used to be almost impossible in the old days. Wines made in late summer or early autumn almost always stopped fermenting on the first cold night of late autumn or early winter. This resulted in the wine-maker thinking that his wine had ceased altogether. And so he bunged it down to clear and, when ready, bottled it as a finished product. When a warm spell, in February or perhaps a bit later, penetrated to the yeast, it became active again and began to make the amount of alcohol it would have made if the cold had not prevented it.
This meant that corks or bungs flew out, and the wine clouded as yeast was brought into suspension by the agitation of renewed fermentation. Many wine-makers thought their wines had gone wrong and threw them away. Others did go wrong because the blown bungs were not noticed and the wines were left uncovered and therefore open to attack by wild yeasts and bacteria.
Today all these troubles are a thing of the past because we can use thermostatically controlled heaters to keep our wines at the correct temperature. They can be installed quite cheaply and are inexpensive to run. This enables fermentation to go on unhindered throughout the coldest weather and ensures trouble-free wine-making with the maximum alcohol being made without undue problems. Trouble-free continuous fermentation is of the utmost importance for top-quality results; flavour, bouquet and all-round quality depends on this.
As mentioned, we rarely add yeast in its dormant state because of the delay before it becomes active. In preparing a nucleus in the following manner, we are able to have a small batch of yeast already fermenting to add to a prepared must. I am aware, of course, that a great many people use yeast successfully without preparing a nucleus, but there will always be those who confound the critics.
Making a Nucleus
Many proprietory yeasts are supplied with directions for making a nucleus, but there are no hard and fast rules except that it should be reasonably balanced. By this, I do not mean that each little part of it must be measured. What I mean is that we should use a fruit juice, which, for all intents and purposes, may be regarded as balanced when diluted with a little water and sugar. Lemon, orange or any other fruit juice, or diluted fruit syrups such as Ribena are all perfectly satisfactory. Even diluted malt extract is suitable. But if we use this, we must add a little acid and tannin as well.
You can make a good nucleus with .3 dl (1 fl oz) of any fruit juice and 1.5 dl (5 fl oz) of water in which 50 g (2 oz) of sugar have been boiled. If the juice is lemon or orange, add a tablespoon of strong tea as well to make up the deficiency of tannin in the citrus fruits. Add the yeast to the liquid and pour it into a small narrow-necked bottle. Plug the neck with a firm knob of cotton wool to protect it against the ingress of wild yeast and bacteria.
The ideal type of bottle to use for your nucleus is the sort of baby-feed bottle that can be boiled. Having watched my grandchildren being fed, I decided that if the hole in the teat were made from inside with a needle, the teat itself filled with cotton wool and then fitted to the bottle with its screw cap, the nucleus would be adequately protected; the gas can escape through that tiny hole and the cotton wool plug will prevent the ingress of spoilage organisms until the yeast begins to ferment and so form its own protective barrier of carbon dioxide gas.
If you do not want to use this sort of bottle some other sort will do, but the neck must be closed with a knob of cotton wool. In this case you should run a lighted match round the cotton wool before removing it to destroy any wild yeast or bacteria that might be trapped in it. Shake the bottle before adding the nucleus to the must.
Adding a colony of active yeast is a great benefit. It means that the prepared must is in active fermentation in a matter of hours, whereas in many cases, depending on the type of yeast used, it may be three or four days before fermentation gets under way, thus leaving the must exposed, even though it may be covered, to attacks by wild yeasts and bacteria. Not all yeasts require to be made into a nucleus. Many added in their dry state will have the must in
active fermentation in an hour or so. It is all a matter of choosing the best yeast for your purpose, although I have found that those requiring to be made into a nucleus usually produce the better-quality wines.
Stages of Fermentation
Fermentation is seen as a frothing on the surface of the must, accompanied by a gentle hissing or fizzing which is quite audible. There is also a pleasant smell which is often quite different from the smell one would expect the fruits being – fermented to give off.
During the first few days, fermentation is usually very vigorous, the wine becoming warmed by energy produced by the yeast. Furthermore, where fruits are being fermented, the pips and skins rise to form a cake upon the surface. For this reason it is wise to allow several inches of headroom in the fermenting vessel. When juices only are fermented, a heavy frothing will appear.
The secondary fermentation stage is often regarded as imminent when the cake of skins levels off to flatness and when the froth of a juice ferment peters out, leaving the surface reasonably free from froth. When the heavier part of the cake of skins has sunk and the frothing of a juice ferment has completely stopped, it is reasonable to assume that the secondary fermentation has begun.
It is at this stage, regardless of directions to the contrary in methods, that the •fruits should be pressed free of juice and the wine strained into ajar to be put under a fermentation lock. Juice ferments are treated similarly but need no straining. During the stage under fermentation locks fermentation continues slowly but surely and the amount of alcohol increases accordingly.
Do not be worried by prolonged fermentation. Indeed a long, steady fermentation is of great importance in producing good-quality wines.
It is safe to say that all the time the solution on the lock is pushed up to the out-going side, a little fermentation is still going on, regardless of whether bubbles are passing through or not. It must be borne in mind that the longer fermentation continues, the more alcohol is produced and this alcohol has a weakening effect upon the yeast. This effect continues until the maximum amount of alcohol the yeast can tolerate is finally reached. At this stage the whole complex yeast system is destroyed, and, as we have seen, fermentation ceases. The last of the minute solids forming slight hazes settle out now that there is no longer any disturbance caused by yeast action to keep them in suspension.
Many people tend to become worried by a slow fermentation and either increase the temperature or lower it in the hope of creating more favourable conditions of the yeast. Both these courses are unwise. First, overheating can destroy the yeast and harm the must. Secondly, lowering the temperature to any great extent can cause the yeast to go dormant.
In both cases, restarting the yeast may be very difficult and attempts are often quite futile. Even adding.new yeast or some of the original nucleus may fail simply because the percentage of alcohol present comes as too great a shock to the yeast. Bear in mind that there is no alcohol present when the yeast is normally added to a must and that, during the course of fermentation, the yeast becomes accustomed to an ever-increasing amount of alcohol. But putting yeast into a must already containing say, 10% to 11% of alcohol, is a very different matter.
The Fermentation Lock
There is no doubt that this little piece of equipment is one of the most important in the wine-maker’s armoury, and it is a pity that certain writers on this subject tend to lead people to think otherwise.
When the wine is racked for the first time and then put into jars, fermentation locks are fitted as a matter of course. Sterilizing solution is poured in to the level shown and fermentation is allowed to proceed under anaerobic conditions. This state is, as already explained, created by the yeast itself by the production of a heavier-than-air cloud of carbon dioxide gas above the wine. A thin tube of glass stuffed with cotton wool would allow the excess gas to e’scape and would prevent the ingress of bacteria, insects and moulds which cause the wines to spoil.
If this were all that is needed, then fermentation locks could be dispensed with. But this is not so. Bear in mind that fermentation locks are used during what we call the secondary fermentation stage and, as we have seen, this is the stage where fermentation gradually slows down. It is during the slowing down that some sort of trap is necessary to retain sufficient gas and at the same time release the excess. This function is performed admirably by the sterilizing solution in the fermentation lock.
Water would suffice for this purpose were it not for some special considerations. First, water cannot remain pure indefinitely, and any moulds or bacteria that might come into contact with it would automatically contaminate it. Indeed, I have seen wine flies actually drown themselves in the water in the lock in their desperate efforts to reach the wine under its protection.
During the quite vigorous ferment that usually goes on for a few days after the lock is fitted, the see-saw action of the water could mean that a tiny amount might be thrown far back and find its way down the stem of the lock and into the jar. And if this water were already contaminated, the wine might well be affected. So using sterilizing solution is the best plan, for any bacteria or moulds are destroyed by it.
Further, it sometimes happens that the pressure in the jar falls owing to sudden cold contracting the wine, and air is drawn into the jar. The air is purified because it has to pass through the sterilizing solution. The lock is in fact working in reverse. This occurrence is nothing to worry about, but it does tend to puzzle newcomers.
Apart from the advantages already described, the fermentation lock is a very good guide to the rate of fermentation. This is indicated by the rate at which bubbles are seen passing through. Early on, they may form an almost continuous stream, but later fermentation slows down and bubbles are observed only at long intervals. Later still, there may be no evidence that fermentation is continuing except for the solution being pushed up on the out-going side of the lock. A gas bubble might pass through every one or two hours, but because no one can watch it for this long there appears to be no activity at all. It is therefore sound practice to leave the wine under the lock all the time the solution is pushed up.
Later it will be observed that after a slow fermentation lasting several weeks, the solution is drawn up the wrong side. The lock should be examined daily after this in order to note when the solution has returned to normal, that is, when all the solution remains at the bottom of the U-bend. When this state is observed, it is safe to say that fermentation has ceased.
Users of the hydrometer will know for sure by taking the reading to ascertain whether or not the maximum alcohol for the type of wine being made has been reached. Those not using the hydrometer are advised to leave the wine under locks and in a warm place for three or four weeks before regarding it safe to rack and set aside for storing. This is advised for the very simple reason that, although the lock indicates that fermentation has ceased, there can still be a few yeast spores not yet killed that can carry on the fermentation for a few days longer.
When using the type of fermentation lock illustrated it is advisable to insert a small plug of cotton wool in the open end in order to give double protection to the wine. The solution, which will slowly lose its strength, should be replenished from time to time. There is no need to remove the lock for this, merely add a few drops with an eye dropper or pipette.
Glass locks are often difficult to insert and remove from bungs. To ease this situation, merely moisten the stem with water or sterilizing solution. Rubber bungs take a mighty grip on the stem of locks that have been in jars for months on end. Twisting to remove the locks will break the fragile glass. To break the grip, immerse lock and bung in water for a few minutes. The bung will then slide off easily.
When we say that fermentation has stuck we mean that it has stopped prematurely. In other words, it has stopped before the yeast has made all the alcohol that it can. There are a number of reasons for this.
But first, do not pay too much attention to the fact that fermentation ceases or appears to have ceased just after the fermenting wine is put into a fresh container — for example, when it is poured from the fermentation pail into the jar to be put under a fermentation lock. I say this because there is quite often a break in fermentation at this stage, though I will admit that there should not be. Either way, if this happens it will be found that fermentation is on the go again within a few hours as a rule.
Our main concern must be with ferments when they stop without apparent cause, resulting in a sweet wine where a dry was required or an over-sweet wine when a normally sweet wine was the aim.
Now let me say that once fermentation has begun it should continue unhindered until the maximum alcohol the yeast can make is actually made. So if it stops we must try to find the cause.
Years ago the main cause was the first cold night of autumn. Wines made during late summer and early autumn would ferment vigorously enough until that first cold night when it stopped. The inexperienced hobbyist, noting that the wine had finished, bunged it down and put it away as a finished product, doubtless looking forward to the time when he could proudly bottle it and offer it to friends. But when a mild spell during winter or the first real warm days of spring turned tqi the yeast, stirred by the warmth, recommenced fermenting where it left off. I often wonder how many thousands of gallons of good wines were poured down the sink through this, for the same inexperienced hobbyist, seeing his once clear wine heavily clouded and not tasting very nice, thought that calamity had struck. All that in fact needs to be done if this happens is to bring the wine into the warm, fit a fermentation lock and leave it until all fermentation has ceased again. Actually, although this sort of thing can be a confounded nuisance, it is a blessing in disguise for the simple reason that a far better wine is the outcome. The simplest means of preventing cold stopping fermentation is to make a fermentation cupboard.
Now, a well-balanced must — that is, one containing all the essentials for a full and complete fermentation — will not stop fermenting prematurely unless interfered with unduly. So many people rack (transfer their wines from one container to another) every couple of weeks or so, when the rule should be to leave it alone. In my methods the first stage of fermentation is carried on in the fermenting pail where many of the fruit or vegetable particles are strained out after a short time. The second short period in the pail is to allow the lighter solids and much of the yeast to settle during a somewhat slower ferment. When the wine is put into ajar with the instruction in the method to leave as much of the muddy deposit in the pail as you can, you ltave behind what would normally build up as sediment in the jar, so there is no need for further racking until fermentation has ceased. In this way we avoid undue disturbance and the risk of stopping fermentation too soon. It is a fact that if fermentation stops at around 12% of alcohol you are not likely to get it going again. New yeast is likely to be killed by the amount of alcohol already present so there is no point in adding any. The result is that what should have been a dry wine is sweet and what should have been a nicely sweet wine is quite syrupy. And you will have to put up with it or blend it. One means of avoiding trouble of this sort is to make sure you obtain a good yeast, and your local wine supply shop will tell you which is the most popular.
Setting the wine near a boiler where the temperature fluctuates from day to day, or in the airing cupboard, is asking for trouble because there can be no control of temperature. Overheating is just as troublesome as a cold night. Where central heating is installed there is no problem and the spare bedroom is often pressed into use for fermentation purposes. The fact is that a constant temperature of around 18°C (65°F) is ideal. So it all boils down to using a reliable recipe that puts into the must all the essentials of a full and complete fermentation with a reliable method, a good yeast and nutrient, keeping the wine warm and leaving it undisturbed until the maximum alcohol is reached.
Many hobbyists use the airing cupboard to keep their fermenting wines warm and provided it does not become overheated, as it very easily can, it often proves satisfactory up to a point. But there are bound to be variations of temperature and at odd times when the fire dies down or goes out, or the electricity supply is switched off, the temperature will fall quite rapidly. This can only result in an erratic fermentation, rapid one day, slow the next; anyone looking at the fermentation lock daily to see how fermentation is progressing might well be puzzled because bubbles will be few one day and many the next and so on. So the airing cupboard is not the best place after all. Overheating will stop fermentation just as surely as a sudden drop in temperature.
Many people use special fermentation pails fitted with a lid containing two holes, one for the wire of an immersion heater of the sort used in tropical fish tanks and another for a fermentation lock. These may well work admirably for some people, but the experience I have had with them was not encouraging. Special trays with an electric element completely encased within it can prove useful. The pail or the jars of wine may be stood in this and the warmth from the tray warms the wine from the base upwards.
Fermentation is often stopped during cool to cold weather if the wine is poured from the fermenting pail (as in the methods) when the jar is cold. This can chill the wine and therefore the yeast. To prevent this during cold weather, fill the jar with hot water — but not hot enough to crack the jar — and let it stand for a few minutes so that the glass absorbs the heat. Then empty and sterilize quickly before filling with wine.
We use nutrients which are included in the recipes and methods and these do assist fermentation, but no more should be added if fermentation sticks. However the addition of a vitamin B tablet will often assist in getting fermentation on the go again. One thing I have not mentioned that will stop fermentation is unnecessary racking — transferring wine to another vessel without good reason. Many people do this each month. I never rack more than once and that is when the wine is put into the jar and the deposit is left in the pail. From then on, the wine is left to finish fermenting undisturbed. If there is a thick deposit of about 10 mm (3/8 in), I rack into another jar before storing. &it there should not be more than 3-6 mm (1/8-¼ in) of deposit in the jar when fermentation ceases and this can be left until the wine is brilliantly clear which in the normal way is only a week or two.