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close this book Food Chain No. 13 - November 1994
View the document Greetings
View the document The ancient art of biotechnology
View the document Equipment
View the document Booklines
View the document Fermented foods in Sudan
View the document Mursik - fermented miIk in Kenya
View the document The production of Hate de coco
View the document Resource page
View the document Bakers' yeast
View the document Recipe page
View the document Acknowledgments

Bakers' yeast

Moulds, yeast and bacteria are often only thought of as causing spoilage of foods and therefore they should be prevented wherever possible. However some types of these micro-organisms can also be very useful, both directly as food or as part of food processes known as fermentations. In a series of articles, over the next three issues of Food Chain, one example of a yeast (bakers' yeast), a mould (musbrooms), and a bacterium (lactic acid bacteria) are described to show how they can each form the basis of a viable small business.

Bakers' yeast (saccharomyces cerevisiae), as the name implies, is the micro-organism that causes bread dough to rise. It is probably the most widely used and well known microorganism worldwide. Different strains (members of the same 'family') are also used for brewing beers and wines. This article focuses on the micro-organisms used in baking.

Bakers' yeast is produced in four forms - fresh yeast, pressed blocks of yeast, dried activated yeast and dried yeast powder. The pressed yeast has a shelf life of a few weeks if kept refrigerated, whereas the dried forms can keep for several months if kept cool and dry. Pressed yeast and dried actuated yeast can be mixed with sugary water (5-10 per cent sugar) and used almost immediately. Dried powdered yeast has to be 'regenerated' by growing the cells in sugary water for two to four hours before use.


Bakers' yeast becomes active when mixed into a dough which then starts to ferment sugars in the dough to form alcohol and the gas, carbon dioxide (CO2). This is the same fermentation that takes place in brewing but the useful by-product of fermentation is different in each case. In brewing it is the alcohol that is kept and the CO2 is allowed to escape, whereas in baking it is the CO2 that is trapped inside the dough to make it rise. The alcohol is boiled out of the dough and escapes during baking

In many developing countries the quality of bakers' yeast causes problems for bakers. It may be too old or it may have been stored incorrectly so that when bakers try to use the yeast most of the cells are dead. This means that they have to add more yeast than is recommended in order to make the dough rise. This in turn increases their production costs and also gives the bread a pronounced 'yeasty' flavour which many customers do not like.



Table 1. Growth conditions and nutrients needed for yeast growth.








Acidity (ph)



Substrate concentration (%)








Sugar (%)


Molasses, sugar syrup suffi




cient contained in molasses



trace amounts

corn syrup fruit juices

Almost all bakers' yeast is grown on a large scale and distributed through retailers in metal tins, plastic bags or in the case of pressed yeast, in paper wraps. Depending on the source of production and method of distribution, the yeast can be many months old by the time it is bought by remote rural bakers.

In an attempt to solve this problem, IT investigated the possibility of producing bakers' yeast at a small-scale in rural areas. The initial results of the investigation indicate that it may be possible technically, but further work is needed to establish the financial viability of the process. We would be grateful to hear from readers who have experience of yeast production or from those who try out this method under field conditions.


Yeast needs the correct food (nutrients) and growth conditions before the cells will grow and multiply in number. These conditions are different to those that cause it to produce sugar and alcohol and the control of growth conditions is therefore critical to the success of yeast production. The most important condition is the supply of air to the growing cells. If this is insufficient the yeast will go back to alcohol and CO2 production and the yield of cells will be very small.

Other conditions that must be controlled are the acidity of the growing liquid (or 'substrate') and the amount and type of nutrients added.

Table 1 shows the growth conditions and nutrients needed for yeast growth.




The main problem with yeast production to supply an adequate amount of sterile to prevent contamination. In experiment cotton wool filter, sterilized by heating in air at 100°C for 30 minutes, was placed i, sealed container. This works adequately provided the cotton wool remains dry. alternative could be to bubble the through battery acid in a sealed bottle (Fig. 1).

An air pump and diffuser block used aerate fish tanks were found to supply sufficient air for four one-gallon demi-johns substrate.


Cells were grown at 25-30°C for 48 hours. The air was switched off and the cells allowed to settle out for four to five hours. The liquid was decanted off and filtered through a paper filter while the remain) yeast slurry was pressed in a cloth bag form a block of pressed yeast. the yield cells is 40-50 per cent of the weight of sugar in the substrate.

Bakers' yeast grown on sugar, moles: or fruit juice was used to prepare bread a the activity of the yeast was compared commercial pressed yeast.

There was no difference in loaf volume between the different yeast samples though the yeast grown on fruit juice gay. noticeable fruity flavour to the bread, indicating that a washing stage should included after pressing and the yeast shot then be re-pressed.

The yield of yeast from four one-gall. demijohns was approximately 300g.