Before actually starting the fermentation of a must it is highly desirable to know something of the principles and processes that are involved. It is possible to control the alcoholic strength of the wine quite precisely by controlling the specific gravity of the must. When fruit juice and sugar are dissolved in water a syrupy must is produced. One gallon of this must is heavier in weight than one gallon of plain water, which is in turn heavier than one gallon of water and alcohol mixed, because alcohol is lighter in weight than plain water.
This variation in the weight or gravity of the must in relation to the weight of the water is called the 'specific gravity'. The gravity of the must is specific to the gravity of the, water and not to any other liquid. Water is taken as the standard for this purpose, because of its universal availability. The gravity of water is known as 1.000 because a cubic foot of water weighs approximately 1,000 oz.
The specific gravity of the must can very easily be obtained by the use of a hydrometer, and this instrument, which has been known since 1786, is without doubt the most valuable and useful piece of equipment that a serious winemaker can possess. A hydrometer is a thin glass tube with graduation fermentation marks on it, weighted in a slightly bulbous end, which is inserted into the liquid to be tested. The tube will float upright and a reading can be taken from the depth of the hydrometer in the liquid.
With the aid of the following suitable conversion tables, from the third edition of The Winemakers' Companion, published by Messrs. Mills & Boon, you can calculate fairly accurately how much alcohol will be made by the fermentation of the must you have prepared:
CELLSPACING="1" CELLPADDING="7" WIDTH="470">S.G.
Potential % Amount of alcohol by sugar in volume
Amount of sugar in the gallon
Amount of sugar added the gallon
Volume of one gallon with sugar added
Ib.
oz.
Ib.
oz.
Gal.
Fl. oz.
1.010
0·9
2
2 1/2
1
1
1.015
1·6
4
5
1
3
1.020
2·3
7
8
1
5
1.025
3·0
9
10
1
7
1.030
3·7
12
13
1
8
1.035
4·4
15
1
0
1
10
1.040
5·1
1
1
1
2
1
11
1.045
5·8
1
3
1
4
1
13
1.050
6·5
1
5
1
7
1
14
1.055
7·2
1
7
1
9
1
16
1.060
7·8
1
9
1
11
1
17
1.065
8·6
1
11
1
14
1
19
1.070
9·2
1
13
2
1
1
20
1.075
9·9
1
15
2
4
1
22
1.080
10·6
2
1
2
6
1
23
1.085
11·3
2
4
2
9
1
25
1.090
12·0
2
6
2
12
1
27
1.095
12·7
2
8
2
15
1
28
1.100
13·4
2
10
3
2
1
30
1.105
14·1
2
12
3
5
1
32
1.110
14·9
2
14
3
8
1
33
1.115
15·6
3
0
3
11
1
35
1.120
16·3
3
2
3
14
1
37
1.125
17·0
3
4
4
1
1
38
1.130
17·7
3
6
4
4
1
40
1.135
18·4
3
8
4
7
1
42
If necessary you can now vary the must as you wish by adding more sugar to increase the specific gravity or more water to reduce it. In the commercial making of wine the specific gravity of the must used obviously alters slightly from year to year depending entirely upon the amount of sunshine and rain that the vine has received at different times. This is one of the several factors which vary the quality of commercial wines from year to year and which causes vintages to be great or otherwise. When testing a must with a hydrometer it is important that the temperature should be round about 60"F. Substantial variations from this temperature can obviously alter the specific gravity and in case you are ever so concerned a set of correction table now follows:
CELLSPACING="1" CELLPADDING="7" WIDTH="593">Temperature in degrees
Correction of the last figure of the specific gravity reading
Centigrade
Fahrenheit
10
50
Subtract 0·6
15
59
No correction necessary
20
68
Add 0·9
25
77
Add 2·0
30
86
Add 3.4
35
95
Add 5·0
40
104
Add 6·8
As a general guide, table wines should be started at a specific gravity of about 1.080, but clearly it doesn't matter if it is 1.078 or 1.083. A starting figure of 1.080 gives a finished alcohol content of about 10% by volume, which for all ordinary purposes is perfectly adequate. Dessert wines, however, need to be somewhat stronger and these should be started at about 1.100 and up to half, as much sugar again should be added, in small doses, during fermentation. When making dessert wines, hydrometer readings should be taken at regular intervals and you will note the steady decrease in the specific gravity and therefore the increase in the quality of alcohol being formed. When the reading gets down to 1.010 a supply of sugar can be added to bring it up to no more than 1.020. This is usually the equivalent of some 4 oz. of sugar in a gallon of must. If you can do this five or six times and the wine stops working at 1.020 or below then you will have achieved a very strong wine, which is also as sweet as a port wine. When adding sugar to a fermenting wine it is always important to take some wine out of the jar, dissolve the sugar in the wine carefully and pour it back slowly. If dry sugar grains are added the must will froth and virtually boil over, so that some wine is in fact lost. The sugar naturally takes up some volume in the jar and when starting a fermentation that you intend to prolong some space should be left for the additional sugar.
Any kind of sugar can be used, since chemically all sugars are the same. However, white sugars should preferably be used with white wines or else the wines will take on the fawn colour of the brown sugar. Brown sugar may be used if you wish with red fruits, but it is not recommended that all brown sugar should be used, since this imparts a caramel flavour to the wine, which is quite distinctive and not always entirely pleasant.
You may have heard of invert sugar or seen it recommended. Ordinary sugar known as sucrose consists of a compound of two sugars called glucose and fructose molecularly combined. Yeast secretes an enzyme invertase which splits the two sugars apart, so that each one can the more easily be converted by the apo-zymase complex of enzymes into alcohol and carbon dioxide. Invert sugar is simply sucrose split by boiling it in water with a small amount of acid. The argument for its use is that the yeast can the more quickly start fermenting the separated sugars without the delay of splitting them first. There seems to be little merit for this practice in winemaking, although brewers always add invert sugar to their wort instead of ordinary sucrose. Experience shows that as far as beer is concerned the use of invert sugar adds advantages in head retention when beer is poured out.
Table wines should be finished at a specific gravity of 1.000 or just below because experience shows that very few wines are really palatable when they go as low as 0.990.
Before adding any sugar to a must it is always desirable to take a reading of the natural fruit juice and water extract, since this often contains quite a high proportion of sugar. Apple juice and water, for example, often has a specific gravity of 1.035 and clearly you will not need as much sugar to bring this must up to 1.080 as you will gooseberry, for example, which might have an initial specific gravity of as low as 1.005. Although the recipes at the end give a guidance on sugar, quite clearly the amount of sugar used must be related to the natural sweetness of the fruit used. Acid fruits containing almost no sugar will clearly need more sugar than sweet fruits. When sugar has been added it is time to add the yeast, but before doing so it is as well to know something of this wonderful little cell, which belongs to the botanical world.
There are many hundreds of different strains of yeasts, not all of which ferment the same kinds of sugars. The three most common ones, however, and the ones with which we are mostly concerned, are as follows:
When we buy wine yeast it is in the form either of a tablet of pressed dried yeast or a liquid which contains a solution of yeast. A yeast culture can also be bought which contains pure yeast of a specific variety grown on agar jelly. The Vierka yeast from Germany comes on tiny scraps of dried rose hips. When a must is ready for fermentation the liquid yeast may be added direct, but the other varieties are best prepared beforehand. Some 4 oz. of fruit juice, containing some acid such as lemon juice, two teaspoonfuls of sugar and a little yeast nutrient, are put into a small sterile bottle and yeast is added, the neck of the bottle is plugged with cotton wool and the bottle is then placed in a warm spot for a day or so. Usually within forty-eight hours bubbles can be seen rising and a slight froth appears, indicating that the yeast has become active and is already fermenting the small quantity of sugar present. As the yeast cells reproduce themselves every three hours, it will be seen that quite shortly there will be a substantial number of active living yeast cells present, which can then be added to the must, to set it working without delay.
It should be noted in passing that the yeast cell itself does not actually ferment the sugar but that enzymes secreted in the yeast cell, and known as the apo-zymase group, act as a catalyst and by their very presence the sugar which consists of carbon, hydrogen and oxygen is slowly transformed into other substances and eventually only alcohol and carbon dioxide are left. The carbon dioxide floats to the surface in the form of tiny bubbles and the alcohol remains in solution in the wine. A small quantity of glycerine and various acids are also formed, including succinic acid, which helps to give the winey flavour and smell. The yeast eventually dies and decomposes, leaving a residue on the bottom of the jar. It is not killed by the alcohol but the enzymes are inhibited from further catalystic action by the concentration of alcohol. This can easily be proved by starting a fresh must with the residue from a former fermentation.
During fermentation it is important always to use a fermentation lock. This is simply a device to permit the carbon dioxide to escape without allowing any outside air to get in. In the first two days of fermentation there is plenty of air in the must to enable the yeast to reproduce itself rapidly. The yeast is then said to be living in an aerobic state. When the air has been used up the yeast turns more urgently to the sugar and continues to live in an anaerobic condition, i.e. without air. The yeast reproduces itself more slowly in this state, but its enzymes produce more alcohol. Clearly, then, to obtain a satisfactory fermentation of all the sugar it is essential to exclude the air and an efficient trap must be fitted.
Fermentation is best carried out at a fairly constant temperature of 70 F. Any warm place around the house will do, although some winemakers use a fermentation cupboard with a thermometer control. Wide variations of temperature should obviously be avoided. Indeed, it sometimes happens that fermentation will unexpectedly stop because the must has got too cold and the yeast has become inhibited. The remedy is to move the wine to a much warmer place. On the other hand, if the must gets too warm the yeast can also become inhibited or even die. In these circumstances move the must to a somewhat cooler place and give it a good stir so that some air can be admitted to Iiven up the yeast.
During fermentation the temperature of the must increases naturally by about 10"F. due to the energy released by the changes taking place.
Occasionally fermentation will stick because there is insufficient nutrient present to feed the yeast or because you forgot to include any acid! Such a ferment can be difficult to start again, but make good the omission if it is due to acid deficiency and add a teaspoonful of Tronozymol if it is due to nutrient deficiency. In both cases thoroughly aerate the wine to stimulate the yeast and keep in a warm place. Another cause for a fermentation to cease prematurely is the failure to add sufficient sugar. Check the specific gravity of the wine and if it is very low check to make sure that you added enough to begin with and if needs be add some more, dissolved in some of the wine.
Very occasionally a stuck ferment is due to inhibition by carbon dioxide. Sometimes a fermentation trap, especially if it is of an unorthodox variety, can get blocked so that the gas cannot escape. The remedy here is thoroughly to aerate the wine by pouring it vigorously into a fresh jar.
Finally, of course, fermentation will cease because the alcohol tolerance of the yeast has been reached. Some yeasts can tolerate a greater amount of alcohol than others can. The average seems to be about 15% by volume, but occasionally you can get a yeast to ferment up to 17% or perhaps even 18% alcohol, but this is quite exceptional and higher figures should be regarded with scepticism. If your must was too sweet to begin with, say 1.150, it might very well stick at 1.050 when 139% alcohol had been formed.
It is never wise to start a fermentation with too high a gravity in the must. In the first instance the fermentation may be difficult to start because the weight of the sugar overlays the yeast and tends to inhibit it by preserving it in the sugar. And, secondly, if you do get the yeast to start moving the must, the probability is that you will end with an over sweet wine.
It is much better to start low and build up the sugar gradually so that the yeast can develop with the alcohol and form a stronger tolerance to it. In this way you can achieve a very strong wine indeed and you can control the sweetness by never allowing the S.G. to exceed 1.020 in the later stages of the ferment.
When a fermentation has stuck and fails to respond to any of the remedies mentioned there is one last resource. Make up a fresh yeast starter bottle and, when it is fermenting well, add it to half the wine, and when this is going add the remainder. Alternatively the wine can be used for blending and you may be sure that it will start another fermentation.
When a wine has finally finished fermenting, and begins to clear, both the living yeast cells and the decomposing dead cells fall to the bottom of the jar. If the wine is left too long on these lees it sometimes acquires off-flavours from them. This is why it is important to rack the wine from its lees so that the new wine can start its long process of maturation.