On planet Earth yeast is a prolific organism. It is not a plant, but a fungus. While it needs moisture and food to thrive and reproduce itself while feeding, in the wild it still has the ability to reproduce by creating spores when circumstances change and it dries out. These airborne spores are everywhere, and botanists who study yeast will tell you that there are probably a million spores in your every breath. Being such a tiny micro-organism, scientists did not really get to grips with yeast until the microscope was sufficiently developed, and the famous Louis Pasteur led the way.
Within decades yeast factories had sprung up so that brewers, bakers, and distillers could be supplied with consistent industrial yeast. The strain that is manufactured is brewers yeast, saccharomyces cerevisiae, and for bakers that entailed no change because the strain bakers had been using for centuries was brewers yeast – obtained by visiting the brewer and taking either froth or sediment away to make the “barm” that would remain active for some time in the bakery to leaven the bread.
However, not all bakers over the centuries would be dependent on the brewer for yeast. There were those who harnessed wild yeasts from nature and made their bread by the sourdough method. Their work involves capturing a wild yeast that would cluster on the husk of the grain in the field or on the skin of cultivated fruit. Having established a colony of yeast the baker would feed it carefully to build up its numbers and vigour, and the medium of dough in which it was stored was called the “leaven”. Continually pampered and fed it would leaven a bread dough whenever required.
Students who attend the course called The Yeast Trio will have the three different types of yeast displayed and explained: first there is wild yeast that gives us sourdough bread. (At PANARY students get to work with both wheat and rye leavens). The second type is called fresh yeast, and it is moist. When it is packed in blocks ranging in size from a match box to a one kilogram block, its full title is “compressed fresh yeast”. Nowadays it is also available in liquid form, delivered by road to large bakeries in a tanker lorry. The third type is dried yeast.
In the factory, fresh yeast is grown in huge tanks of molasses with oxygen bubbling through. As it devours the molasses and reproduces, in time the whole tank of molasses becomes a tank of yeast. The sludge at the bottom is the basis for marmite and vegemite. Old-school bakers always mix the fresh yeast in some of the dough water before beginning to mix and knead. It must be kept permanently in the refrigerator, with its life shortened if it is allowed to get warmer than 4 – 5 degrees C. Stored properly it lasts for over a month, even two.
In the same factory, both types of dried yeast are made. From the fresh yeast is made the old-fashioned type of dried yeast, called Active Dried Yeast, recognisable because it is formed during the drying into small granules that resemble coriander seed. It has no chemical additives, and hence is favoured by the Real Bread Campaign. You are advised to whisk it into water, adding some sugar or honey to encourage it to froth is optional, and actually unnecessary. Stored completely air-tight it can last for months.
The modern type of dried yeast, called Instant Dried Yeast, is made from a sub-strain of saccharomyces cerevisiae, developed in the second half of the 20th century, and is a more vigorous feeder on grain, hence its name “instant”. Its appearance would remind you of iron filings. It contains an emulsifier to enable it, a very dry product, to be more effectively incorporated into moist dough. Those who distrust industrially made food worry that it must contain undeclared enzymes to give it such fermentation speed, but the manufacturer says it does not contain added enzymes, but simply performs faster because it is a different strain. For dough mixing you are advised to mix the instant dried yeast into the flour before starting to knead.
In home baking, when you have a recipe for fresh yeast, and wish to use dried yeast – halve the amount. In commercial baking, with bigger quantities of dough, it is found that instant dried yeast need only be used at 35-40% of the fresh yeast weight.
Here is some help with measuring when you are setting a sponge and need only a tiny amount of yeast:
- one gram of fresh yeast = the volume of a chick pea
- 1 gm fresh = half a gram of instant dried yeast = ⅛ of a teaspoon