Understanding Yeast

Understanding: Yeast

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Yeast comes from the Sanskrit word ‘yas’ which means ‘to seethe’.  It is neither plant nor animal but a single cell, microscopic fungus.  Yeast is a leaven because it created gas bubbles in dough which causes the dough to rise.

Since before civilisation, trace amounts of wild yeasts have blown freely in the air.  Yeast comes from fermenting matter in nature.  When it lands on a suitable host and is provided with sugar, oxygen and warmth it begins to grow and reproduce.  As it grows, it produces carbon dioxide, water and alcohol – a process called fermentation from the Latin ‘ferveo’ meaning ‘to boil’ or ‘seethe’.  The fermenting action is destroyed by temperatures above 43°C and the alcohol and water are evaporated during the baking process.

Archaeologists have found that Triticum boeoticum, the wild wheat ancestor of all other wheat, first grew in the fertile valley above the Persian Gulf (Ancient Mesopotamia, now the areas of Kurdistan and Iran) prior to man.  Because wild yeast is so easy to obtain, one can readily picture Mesopotamians 9000 years ago leaving mounded dough lying in the sunshine while heating an oven or attending to other chores …only to find it had become larger when seeing it again!  They had put nothing living in it but it had grown so it must be alive!  The esteem they received for producing the ‘staff of life’ must have increased with their ability to achieve a ‘miracle’.

As time progressed, ancient bakers found that adding a little honey to dough would increase the activity of the wild yeast.  More rising was produced, making an even lighter bread.

This custom spread from Ancient Mesopotamia, west of the Danube river valley, then to the lakeside dwellings in Switzerland and south into Egypt where Archaeologists have found, in Egyptian tombs, honey bread in the shapes of men and animals much as gingerbread men are made today.  From Egypt, honey bread was carried to Greece where it was improved by use of finer flour due to better milling.  Greek sailors and merchants carried it to ancient Rome and from there legions took it to the boundaries of their empire.

Sourdough bread originated with the discovery that a piece of dough kept from the previous day’s baking would become sour, and could be added to fresh dough as a leaven.  This was called a starter.

Use of a sourdough starter to make a wheat or barley dough was common in Ancient Egypt and the resulting bread, called ta, and constituted the main diet of the Egyptian labourer along with barley beer and onions.

Australian pioneers used sourdough leaven on ships and in the gold rush camps where it could be produced easily and kept active by small additions of fresh dough.

1849 gold miners in the U. S., the ‘Forty-niners’ were also called Sourdoughs.

After centuries of using sourdough starters, Egyptians found that adding froth from beer to dough created a product even lighter than sourdough.

It was this bread along with sourdough bread plus beer and onions which ‘built the pyramids’.  Ancient historians such as Pliny reported that barbarian countries like Spain and France which drank beer had bread ‘lighter than that made elsewhere’.

There are many strains of yeast.  Brewer’s yeast is used for beer making.  Torula yeast, favoured usually as a health drink, is made from wood cellulose or from sugar.  It is high in protein plus almost all of the B complex vitamins.  Yeast used for baking, baker’s yeast, is saccharomyces cerevisiae.

Modern Yeast Production

In 1859, the famous, French scientist, Louis Pasteur, discovered that yeast was the organism which caused fermentation.  In Denmark in the late 1800’s methods were developed for isolating and culturing pure strains of yeast for use in the brewing industry. These techniques were soon applied to the selection and propagation of baker’s yeast.  In 1921, a Danish scientist, Soren Sak, developed a new method of yeast production called Differential Fermentation.  This technique provided the basis of the process by which much of today’s yeast is produced.

Today’s baker’s yeast is made in five main steps:

  1. Molasses is sterilised with steam before being passed through clarifiers for removal of sludge.  The resulting fluid, called wort, is stored in tanks under sterile conditions.
  2. Inoculum, the pure culture used to start the production of commercial yeast, is inoculated into a seed fermenting tank which contains the sterilised wort plus other nutrients.  When the desired number of yeast cells have been produced, the contents of the seed fermenting tank’s contents are transferred to the main fermenting tank.
  3. The seed yeast in the main fermenter continues to be fed wort and other nutrients and at the same time, high volumes of sterilised air are pumped into the fermenter to provide the necessary oxygen to assist in the rapid growth of the cell population.  The yeast cells duplicate themselves every 90 minutes.  Two hundred gram seed yeast can grow to 150 tonnes in five days.  This is enough yeast to make 10 million loaves of bread.
  4. Separation occurs at the end of the fermentation process when the many tonnes of yeast in the main fermenter need to be harvested from the brew.  This is accomplished with large, centrifugal separators.  The yeast cells are then washed with water several times to remove all waste products and produce light, creamy coloured suspension of yeast called cream yeast.

Cream yeast is stored in refrigerated tanks to ensure the high activity is maintained.  It is then transported in insulated, stainless steel tankers to large, commercial bakeries and kept in refrigerated holding tanks at 3°C while being constantly agitated. 

Forms of Yeast

One of the first recorded instances of dried yeast was the Romans’ preparation of it during grape harvests.  Millet or wheat bran was mixed with grape juice, allowed to become contaminated by the air and then dried in the sun.  The resulting cakes were soaked in water when needed.  In a similar process, early New Zealand settlers in the U.S.A. made a wild yeast preparation of hops, rye, Indian corn and water into dough which was allowed to ferment then sun-dried and cut into cakes for future use.

The type of dried yeast used today is credited to the Hungarian, Max Fleischmann.  Unlike the unstable and uncertain wild yeast, Fleischmann’s yeast was a reliable product made of brewer’s yeast.

Today, dried yeast is made from cream yeast.  Cream yeast is ‘dewatered’ over a rotary vacuum filter drum and then fed into a drying process which result in the production of granules or High Activity Dry Yeast.  This is then packed into 500 g and 10 kg foil vacuum packs.  It is ideal for bakers who do not have refrigerated storage space readily available or are located in more remote areas and require a highly active, reliable product which is convenient to use.  Two metric teaspoons of dried yeast equal 30 g of fresh compressed yeast.  Like compressed yeast, it is also packaged in small quantities (7 g sachets) for baking bread at home.  It should be stored in a dry area preferably below 20°C.  Most dried yeast available in Australia had been packed under vacuum after being flushed with nitrogen and should maintain its quality for at least a year provided the seal is not broken or punctured.

In recent years, two types of dried yeast have been developed specifically for the baking industry.  Active dry yeast (A.D.Y.) works more quickly than traditional dried yeast.  It requires no refrigeration, has a long life, and must have water added to it before use.  It comes in 500 g vacuum packs which once opened, should be refrigerated and used within a few days.

Instant active dry yeast (I.A.D.Y.) works more quickly than active dry yeast, does not require refrigeration, has a long shelf life and should be added directly to the dough without adding water first.  It is packed in the same manner as (A.D.Y.) and, after opening, should be refrigerated and used within a few days.