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YEAST AND YEAST NUTRIENT

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Yeast is a single-celled fungus-like organism (scientifically called Saccharomyces, meaning "sugar fungus") which ferments sugar to alcohol under deoxygenated, or anaerobic conditions. There are several types of yeast, some of which are much more suitable for winemaking than others.  

To start with, brewer's or baker's yeast (Sacch. cerevisiae) should be avoided. In its fresh form, this type has an unpleasant flavor which can be imparted to a wine. In its dried form it works reasonably well during fermentation, but it consists of much smaller cells than the genuine wine yeast (Saccharomyces cerevisiae ellipsoideus) and so does not readily form a sediment when fermentation is complete. Clearing the wine can then be a problem. Genuine wine yeasts are derived from the yeasts which occur naturally on the skin of grapes as bloom. There are both general-purpose and specific varieties of wine yeast available in dried granulated form. 

GENERAL PURPOSE YEASTS

These yeasts are a mixture of strains selected to give the best all-round performance. This means, really, that they have four desirable characteristics: 

1) They ferment rapidly and vigorously. No one wants to wait months for the finished product! However, it is worth remembering that rapid fermentation also depends on a good supply of yeast nutrient and vitamins (see below), and a stable, consistent temperature at the correct level. 

2) They have a tolerance to high levels of alcohol. Although alcohol is exactly what the winemaker wants in his wine, to the yeast alcohol is nothing more than a poisonous waste product of its own metabolism. And at a certain level of alcohol, which varies between different types of yeast, the yeast is simply inhibited or killed. General-purpose yeasts can ferment up to about 16% or 17% alcohol by volume - more than enough for table wines. 

3) They rapidly form a firm sediment when fermentation is complete. As I mentioned, one of the problems with baker's yeast is that the cells are small and do not drop rapidly out of suspension when the ferment has finished. This problem shouldn't really arise with wine yeasts, although the "dropping out" of the yeast can speeded up by moving the wine into a cool place. These yeasts form a firm sediment  which remains undisturbed when the wine is siphoned off (racked). 

4) They do not impart off-flavors to wine. Once, winemakers had to siphon the wine off the yeast sediment as soon possible when fermentation had finished - this in case the yeast began to decay and imparted a bad smell or taste to the wine. Although one should still rack as soon as possible, the of a good general-purpose yeast tainting a wine during or after fermentation seems quite small. 

All in all, therefore, the requirements of the average wine are fulfilled by the general-purpose yeasts; the two most well known are C.W.E.'s Formula 67 and Unican's Super Yeast.  However, any good-quality brand will successfully carry out any task asked of it - except dry sherry and sparkling wine production. (See later.)

Specific Yeast Varieties

As well as the general-purpose yeasts, you can get a range of specific yeasts labeled Bordeaux, Burgundy, Port, Graves, Champagne and so on. These are cultures of different strains of S. cerevisiae. You may wonder why so many different types of wine yeast are available if the general-purpose wine yeasts are so satisfactory. This is an interesting question. 

Forts of all, the use of a specific wine yeast does not produce a wine of the corresponding type. In fact the idea underlying the use of different strains of yeasts is that different musts produce an environment which is more suitable to certain strains of yeasts than to others. Thus, for example, a strong peach-based must is said to favour a Sauternes yeast, and so on. The reasoning is that the composition of the must resembles the composition of the corresponding grape and hence the type of wine from which the yeast was originally derived. In addition, it is claimed that each type of yeast has a slightly different metabolism and therefore produces a slightly different flavor and bouquet in the finished wine. But would anyone except an expert notice these differences? 

One way of finding out for yourself would be to ferment two similar musts with different yeasts and to compare the finished wines. My own opinion is that a correctly balanced and constituted must is far more important than the variety of yeast used. If you have this, then using specific varieties of yeast is nothing more than fine-tuning. However, two cases where it is essential that specific varieties of yeast are used is in the making of dry sherry and sparkling wine. This is explained later. 

Perhaps the only other time when the amateur need use a specific type of yeast is in the production of strong sweet dessert wines: Sauternes and Port yeasts are best for white and red wines, respectively, of this type. 

Note that if you wanted to produce a wine of the highest possible alcoholic strength, for example to make a mock port or sherry without having to add any extra alcohol, you would have to use a Madeira, Tokay or Sherry yeast, since those have the highest alcohol tolerance of all. To recap, then, I would suggest the following scheme: 

Type of wine                                 Type of yeast

White dessert wines                   Sauternes
Red dessert wines                      Port
Sparkling wines                           Champagne
Sherry                                             Sherry flor
Port                                                  Madeira 
High alcohol                                  Madeira or Tokay
All others                                        General purpose 

YEAST NUTRIENT 

Just as plants need fertilizer, so yeast needs certain chemicals in particular, nitrogen (N), phosphorus (P) and sulfur (S) - for vigorous growth and effective fermentation. 

Some ingredients provide all these elements, but most country wine musts are deficient in one or more. To overcome this, you can add a yeast nutrient preparation when preparing the must. The best nutrient is a mixture of ammonium phosphate and ammonium sulfate crystals. This is included in some wine yeast preparations; if not, simply dissolve one level teaspoon of the mixture in the cool must or yeast starter (see below). The nutrient compound is destroyed by hot water. 

YEAST STARTER

Wine yeast preparations of any type, whether they are dried granulated forms in packets, compressed tablets, or liquid cultures, are all dormant when they are purchased and need to be reactivated before they can be added to a must.  I recommend that you buy the dried granulated form in sealed foil bags. These have a shelf life of three or more years. It's not advisable to add any yeast - even the granulated sort - directly to a wine must. If this is done, there is a considerable delay before a strong colony can build up, and infection with spoilage bacteria then becomes a real possibility. 

This can be avoided by activating the yeast in a "starter" bottle and adding the starter to the must. This ensures that the yeast begins to ferment the sugar in the must almost at once, and the carbon dioxide which is produced then acts as a barrier over the surface and helps to exclude bacteria. A yeast starter is best made up in a sterile screw-top glass jar or bottle - a litre fruit-juice bottle is ideal. Here is the method.

1 Boil up 0.25 pint (300 ml) of water containing: 2 oz (60 g) sugar, 1/4 teaspoon citric acid and 1/4 pint (150 ml) of the must; 12 oz (60 g) sugar and 1/4 pint (150 ml) of orange juice; 12 oz (60 g) sugar, 1/4 teaspoon citric acid and 1 tablespoon liquid malt extract. Note: these quantities of sugar and acid should be deducted from the amounts specified in the recipes. (1/4 as written here means a quarter or fourth.)

2 Allow to cool to about 20 - 25 degrees C (68 - 77 degrees F) with the jar covered. 

3 Disperse yeast culture into the liquid, adding a nutrient preparation if one is not included with the yeast.

4 Place the lid loosely on the jar and leave it at room temperature for at least twelve hours. This will produce a good, strong yeast colony ready to ferment the must. 

5 Disperse the yeast into the must. Fermentation will start shortly. Using the quantities and preparation times indicated above is a great way to get your wine fermenting. 

VITAMIN SUPPLEMENTS

The growth of a yeast colony is helped by the addition of yeast nutrient. It is also extremely helpful to add a Vitamin B supplement to the must in order to assist the yeast. This can be done by dissolving two or three vitamin B tablets in a wine must, or by using a vitaminized yeast nutrient such as Tronozymol. Vitamin B tablets are available from winemaking shops or chemists, who sell them under various trade names or under the chemical name of thiamine hydrochloride. Do not use any tablets which have a strong color and smell - this may be apparent in the finished wine.

Finally, some winemakers think they can save money by re-using the yeast sediment left in the fermentation vessel when a wine has been racked. This is a misguided action. There's a risk of off-flavors from dead yeast and a chance that infection will spread from one wine to another if the must has been accidentally contaminated at any stage. 

THE PROCESS OF FERMENTATION

Under well oxygenated conditions, yeast reproduces rapidly and uses available sugar to produce energy by a process of aerobic metabolism. The by-products of this metabolism are carbon dioxide and water: 

C6 H12 O6 + 6 02  => 6 C02 + 6 H20

Sugar + Oxgyen => Carbon dioxide + Water

The process begins in the starter bottle, and continues during pulp fermentation. It tails off completely when the wine is put under air-lock in the demijohn. At the same time, as the carbon dioxide gas builds up, the yeast gradually switches to anaerobic metabolism, the by-products of which are carbon dioxide and alcohol: 

C6 H12 06 => 2 C2 H5 OH + 2 C02 

Sugar => Alcohol  + Carbon dioxide 

This is is the source of the alcohol in the wine. The production of a small amount of alcohol during the pulp fermentation leaches out the flavor and color of the winemaking ingredients. But the major part of the conversion of sugar to alcohol occurs in the demijohn where air is excluded. 

Like all living organisms, yeast has an optimum temperature range. It is killed at temperatures over 35 degrees C (95 F) and works very slowly below 15 degrees C (59 F). The exact temperature during fermentation is not critical; an ideal range seems to be 20 - 25 degrees C (68 -77 F). In fact it is probably more important to avoid fluctuations in temperature than to keep exactly to those figures. Fluctuations have a very disruptive effect on the smooth progress of a fermentation. 

Therefore in cold weather it may be useful to build some kind of fermentation cabinet, well insulated and heated by a low-power electric light bulb. Alternatively, the floor or a shelf near a radiator and out of cold draughts, or an airing cupboard, can provide a suitable and fairly constant temperature. If you want to be really sophisticated, three types of demijohn heaters are commercially available: (a) an electrically heated mat on which the fermentation vessel stands; (b) an electrically heated belt which wraps around the vessel; and (c) a thermostatically controlled immersion heater which fits through the demijohn bung. Fermentation generally lasts from one to five months after the wine is transferred to a demijohn. This wide range of time depends on many factors: the availability of nutrient to the yeast, the amount of sugar in the wine and the temperature being the most obvious. In an extremely hot, dry summer, fermentation may proceed to completion in as little as two weeks. (Don't mistake this situation for a "stuck" fermentation, which is one where the yeast has stopped working for some reason while there is only a little alcohol and a lot of sugar in the wine. This is described in more detail in the section "Faults and remedies.")


Next: concentrated grape juice and sugar

 

 

 


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Original posting date
01/01/2008
01 January 2008

Original posting date
01/01/2008
01 January 2008