Yeast

Yeast has been used in the preparation of food and drink for as long as there have been leavened bread and beer, but it was only in the 19th century, thanks to the work of pasteur, that its nature was understood.

Even now, yeast can understandably be treated as a sort of miracle. Elizabeth David (1977), in a fine account of the history of yeast in English baking, remarks that:

In Chaucer’s England one of the names for yeast or barm was god-disgoode ‘bicause it cometh of the grete grace of God’. These words imply a blessing. To me that is just what it is. It is also mysterious, magical. No matter how familiar its action may become nor how successful the attempts to explain it in terms of chemistry and to manufacture it by the ton, yeast still to a certain extent retains its mystery.

Yeast is a single-celled fungus, of which hundreds of species have been identified. Those of the genera Saccharomyces and Candida are the most useful. The single cells are very small: hundreds of millions to a teaspoonful. Instead of feeding by photosynthesis, as green plants do, they feed on carbohydrates (and other nutrients—their dietary needs are remarkably extensive, considering how small they are) and excrete alcohol. They breathe air and exhale carbon dioxide.

The same species of yeast, Saccharomyces cerevisiae, constitutes both baker’s and brewer’s yeast, and the connection between brewing and baking used to be intimate. A model of a combined brewhouse and bakehouse found in an Egyptian tomb of c.2000 bc is to be seen in the British Museum. Beer leaven, known as ‘barm’, was used for bread-making until quite recent times. The making of beer, black bread, and the alcoholic drink kvass were traditionally linked in Russia. There are still some who believe that bread made with barm had an especially good flavour.

It was perhaps the ubiquity of brewing in N. European countries that made several of them, England for one, rely on yeast as a leavening. The Somersetshire parson William Holland noted in his diary for 1814, ‘The frost being so severe we cannot get barm for baking … People do not care to brew this frosty weather which causes this scarcity of barm.’ In France, where wine was the drink of choice, yeast was viewed at first with suspicion. In Paris in the mid-17th century there was a trend towards a softer crumbed, richer, yeast-raised bread called the Queen’s bread. This was objected to by those who preferred the customary leaven bread and the matter was thus put to a panel of experts who, in 1670, ruled that bread might not be made with yeast although yeast-risen rolls were acceptable.

However, it is no longer true that the same yeast is used for brewing and baking. Many different strains of S. cerevisiae have evolved or been selected, and those which are used for brewing are different from those which best suit the bakers, and are cultured on a different substrate, usually molasses. The various species and strains also differ in a way which has important consequences for flavour. The complex chemical reactions which the enzymes secreted by yeast bring about in the process of fermentation include the formation of minute quantities of organic acids, minor alcohols, and esters. These affect flavour, and they vary from strain to strain. The phenomenon is particularly noticeable in wine-making, where the yeast used is generally a strain of S. ellipsoideus.

Yeasts may grow and work with other micro-organisms. In sourdough bread the yeast, often of the genus Candida, is the partner of lactic acid-producing bacteria. Yeasts also work with bacteria to ferment kefir and koumiss. The manufacture of soy sauce, and of the Japanese drink saké, requires the presence of moulds which produce sugar for the yeast to consume. Many surface-ripened cheeses are worked on by a mixed culture which includes yeast, although always in a subordinate role to moulds (as on brie) or bacteria (as on limburger). Yeasts, moulds, and bacteria all co-operate in the fermentation of cocoa beans, to develop the flavour of chocolate.

There are three types of commercial yeasts for baking to be had.

• Fresh compressed yeast, best bought directly from a bakery, comes in moist cakes which are creamy or grey. The life of yeast in this form is limited, but may be prolonged by refrigeration or freezing.

• Dried yeast, called ‘active dry (baking) yeast’ in the USA, is in the form of dried granules, and keeps for a year or more, all the better if refrigerated. Weight for weight, it has twice the potency of compressed yeast.

• Instant yeast is a type of dried yeast formed into much smaller particles. It is intended to be mixed directly with flour rather than dissolved in water as usual. The strain of S. cerevisiae selected for this yeast is especially fast-acting. The small size of the grains gives it a large surface area, so that it can spring into action as soon as the flour is wetted.

In addition, yeasts themselves may be used as food. They contain much protein and all but one of the B vitamins (B12), and some of them will grow on the most unpromising material. They are consequently used to provide dietary supplements for countries whose diets are deficient in protein. This is a new development, but the consumption of yeast in the form of brewer’s yeast pills and yeast extract dates back to the 19th century and these products still have a reputation as health foods. The best known in Europe, Marmite, was first marketed in 1902 in the English brewing town of Burton upon Trent.

Yeast extract is not, strictly speaking, an extract. The yeast is allowed to ‘digest’ itself with its own enzymes, breaking down its own proteins by hydrolysis. It is then concentrated, and sometimes flavoured. If brewer’s yeast is the starting point, it is treated to remove its unduly bitter taste. The product does not supply complete protein, as meat extract does, but is nutritious.

Wild yeasts are everywhere and are significant features of leaven breads. They may intrude where they are not wanted. If a dish of sugary food, or a fruit juice, starts to taste alcoholic, they are to blame; primitive wines were (and sometimes still are) fermented by the yeasts naturally present on the surface of grapes. Wild yeasts can also induce bloating and softening in home-made pickles. But they cause less trouble than the moulds to which they are related; and the same kinds of hygienic precautions are effective against both.