Yeast Starters..... all you ever wanted to know

All you ever needed to know about yeast starters

Extract from Progressive Winemaking
Peter Duncan and Bryan Acton


Yeast starters are very easy to prepare and no difficulties should be encountered if the following directions are observed. A wine bottle is first sterilised with the stock (10%) sulphite solution mentioned in an earlier chapter and then thoroughly rinsed out with boiled water (cooled) to remove any residual sulphite. About a half a pint of must or an alternative basic starter medium to which about a tablespoonful (3 teaspoons) of sugar, a teaspoon of citric acid and a little yeast nutrient have been added is next sterilised by boiling it for a few minutes, after which it is cooled to room temperature under the tap . The cooled sterile solution is then poured into the wine bottle, the yeast culture added and the bottle stoppered tightly with a plug of cotton wool. This starter is finally stood in a warm place at a temperature of about 75 to 80 deg F for a few days until active fermentation begins when it is ready for use.
Many media are suitable for the preparation of yeast starters, but it is often advisable to use a bottle of must as the basis of the starter so as the yeast becomes acclimatised from the outset to living under the conditions it will experience during the fermentation of the must. If a small quantity of must cannot be obtained then pure strained fruit juices, particularly orange juice, or a solution containing a tablespoonful (3 teaspoons) of malt extract dissolved in about half a pint of water can be used as an alternative basic starter media. Several proprietary starter media are also available and give good results, most of these having a malt basis
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Since yeast starters are often the key to a successful fermentation, the salient features of their preparation are worth a more detailed discussion to ensure that the winemaker has a clear understanding of the principles involved. The importance of sterilisation in the preparation of starters must first be emphasised. Since the starter medium is a solution rich in nutrients which can equally well support the growth of many moulds and bacteria other than yeast, the latter may become contaminated with undesirable spoilage organisms if steps are not taken to prevent their growth. The starter medium and its container must therefore be sterilised before introducing the yeast culture to avoid mishaps of this nature. The container, usually a wine bottle, is most conveniently sterilised with a strong sulphite solution. Sulphiting the actual starter medium is less satisfactory because the normal maximum dose of sulphite will kill only a proportion of the micro-organisms which may be present. The rest will merely be temporarily inhibited from further growth and may cause trouble later. Sulphite may also retard the development of the yeast during this early stage in the formation of the colony. Boiling is thus a much better method of sterilising the starter medium because it sterilises the solution more efficiently than sulphite because no substances inhibitory to the yeast subsequently remain in it.

The growth of yeast is also favoured by adding acid to the starter, for many micro-organisms do not grow well under strongly acid conditions which are readily tolerated by yeast. Citric acid is therefore an essential ingredient of yeast starters for this reason. Tartaric acid may also be used, but it is less satisfactory because its inclusion somehow seems to prolong the induction period normally expected before the first visible signs of fermentation are observed. The acid content of the starter should not of course, be overlooked if the acidity of the must is checked prior to its addition

It has already been mentioned that active growth and reproduction of yeast is only observed when it has a plentiful supply of oxygen. The effect of temperature upon the metabolism of the yeast has also been noted. Since a starter is prepared solely for the purpose of transforming the original dormant yeast culture into an actively fermenting nucleus colony before it is added to the must, warm aerobic conditions must be established in the starter medium to permit rapid propagation of the yeast. The starter bottle is therefore plugged with cotton wool which allows air to enter freely to replace that used up by the yeast but which at the same time excludes undesirable moulds and bacteria, and is stored in a warm place at a temperature of 75 to 80 deg F. The bottle may also be shaken occasionally to promote aeration...

The final important feature of yeast starters concerns the addition of nutrients and sugar. The basic starter medium normally contains a fair amount of nutrients, but it is as well to augment this supply with a pinch each of ammonium and potassium phosphate and a vitamin B1 tablet or a little marmite, since yeast growth would seriously be retarded by a shortage of these essential nutrients. In areas where the water is soft, a few crystals of magnesium sulphate may also be included if the starter medium is based on a malt extract. A small amount of sugar must also be added to act as food reserve for the yeast, but in view of the fact that yeast grows better in dilute sugar solutions about 1 tablespoonful of sugar is adequate. If the starter begins to ferment before the must is ready, it can always be kept active by the addition of a little more sugar.

Although wine yeasts are rather more expensive than bread yeast, it is often overlooked that the starter prepared from a single culture can be made to ferment many gallons of wine. Only about three quarters of the starter needs to be added to any one must. If the remaining solution is topped up with more freshly sterilised starter medium (and a fresh plug of cotton wool inserted in the bottle), further yeast growth will take place and the starter will be in full fermentation a few days later. This procedure can be repeated ad infinitum provided the starter remains free from bacterial or fungal infection, but for this reason it is advisable to renew the culture at least once a year. Starters can also be kept in a viable but quiescent state for several weeks by storing the starter bottle in a cool place at about 35 to 40 deg F once the initial short fermentation has ceased. They can later be re-activated simply by adding a little sugar syrup and standing the starter bottle in warm surroundings.

In conclusion, it is worth noting that in an emergency the deposit removed at the second or later rackings can be used to start another fermentation, since this sediment is comprised mainly of viable yeast cells which have been inhibited from further activity either through lack of sugar ort by the high concentration of alcohol in the wine. The lees from the first racking should not be used for this purpose, because it is heavily contaminated with dead yeast cells and other unwanted debris. At subsequent rackings the deposit will contain a much higher proportion of yeast and will therefore give much more reliable results