Living Organisation


                                                                                    Animal                               Vegetable           

                                                                                                                                     |                                               |

                                                                                                                               Flowering                        Flowerless 


                                              Ferns----------------Fungi----------------- Mosses--------------------Seaweed                                      



 YEAST   is a living organism belonging to the vegetable kingdom of the class Fungi of the order Saccharomyces Cerevisiae. (Saccharo meaning sugar, myces meaning envelop or skin)  This name is given to Bakers Yeast as it must be remembered that there are other types of yeast. This name applies to a large series of unicellular fungi which have the power of producing the gas Carbon Dioxide (CO2) and alcohol when sown in a substrate of sugar which is a yeast food.

A single yeast cell measures 0.001mm (1/4000 in) and can only be see under a microscope.

28g (1oz) of fresh yeast contains 400,000,000,000 cells.

It comprises of a Cell wall.. Nucleus...Cell Membrane...Vacuole... Cytoplasm.

Contains 71% Moisture...14% Protein...10% Carbohydrates...3% Mineral matter 2% Fat


single yeast cell

Yeast is available in three forms 

  • Compressed (fresh bakers yeast)
  • Dried.
  • Liquid.

Effects of Temperature on Yeast Growth

The best temperature for the growth of yeast is between 25C (76F) to 32C (90F) although the majority of doughs should be made between 25C (76F) to 29C( 84F).

Yeast will not grow normally below 10C(50F). It will be seen from this that it is necessary at all times that yeast should be carefully handled when making a dough. When the yeast grows in the dough it forms CO2 and a small quantity. The CO2 aerates the bread making it palatable and easier to digest.

Effects of Ingredients on Yeast. 

  • SALT.  Salt slows down the growth of yeast. It controls and stabilises the rate of fermentation within the dough when used at the correct amounts.Too much salt will retard or destroy the yeast. Yeast must never come into direct contact with salt.
  • SUGAR. Yeast requires sugar for food and works best in solutions  that contain no more than 13%. Doughs that contain sugar above this level will be retarded, that is, a reduced rate of fermentation.High levels of sugar can destroy the yeast. As with salt, never let yeast come in direct contact with the yeast.
  • FATS. High levels of fat have a retarding effect on yeast activity.
  • SPICES. High levels of spice have a retarding effect on yeast growth.

The yeast cell behaves in two entirely different ways according to the condition it finds itself.

 If supplied with a source of energy such as sugar together with other foods such as nitrogen and phosphate and vigorously aerated, the yeast cell concentrates on growing and reproducing itself. Each cell produces a bud which increases in size until a completely new cell is formed, the cells then divide and the process is repeated. One cell becomes 2 2 cells become 4 4 cells become 8 and so on. Under the right conditions this process of reproduction will take place once every three hours and forms the basis of commercial Bakers yeast production.

If on the other hand yeast has a supply of sugar and other nutrients but without an adequate supply of air it ferments, that is it utilizes the sugar to produce mainly eythyl alcohol and carbon dioxide with small but important quantities of other by products. The absence of air permits very little energy to be devoted to growth. These are the conditions that exist in a normal bread making dough.

Sporulation. When faced with unfavourable conditions such as 

(A) insufficient food.

(B) lack of moisture           

(C) unsuitable temperature.

 the nucleus will use up all the food that is present in the cytoplasm. The nucleus then divides into four and the cell wall thickens to give greater protection to the yeast spores. The yeast spores are able to survive in this condition for very long periods of time until the favourable conditions return when it will revert to reproduction by budding.

Yeast, by the process of fermentation and its enzymic action upon the other ingredients in the dough, produces a delicious flavour which is unique as well as a gas to give a light aerated loaf and also produces bread with an attractive shape and colour.


Fresh yeast should be stored at O - 4C and generally has a shelf life of 1 month.

Dried yeast should be kept in an air tight container and stored in a cool dry place or as per manufacturers instructions.