Knowledge Base

Yeast

 

Yeast is a living organism and is a plant of the fungi group. It produces the gas carbon dioxide when fermented. This occurs when it is given food in the form of sugar, moisture and warmth. The prime temperatures of consistent fermentation are from 25-29° C (77-84° F).

To quicken the fermentation process, ascorbic acid (Vitamin C) can be added to the liquid. There are commercial yeast additives now readily available on the market which can halve the process time.

Types of yeast
Compressed
Is the most widely used. It is a very pure form of yeast packed and sold in cakes. This yeast crumbles easily and has a fresh smell. It will keep in a cold place for 2-3 days.
Dried
Can be stored indefinitely if dry and well sealed. It takes longer to cream and is more concentrated.
Fresh
Has a pleasant characteristic smell and is a grey clay-ish colour, it also crumbles easily and will cream readily.

 

More about fresh yeast
Fresh compressed yeast should feel tough and springy, and when broken should show a clean fracture and break with a snappy sound. It should smell slightly acid, similar to apple, and be practically tasteless. Whilst heat over 49°C will kill yeast, it can be frozen, but it must be used quickly once thawed.

When yeast is kept in a warm dry place, it becomes white and crumbly and quite hot in the centre. This is caused by chemical action going on in the yeast which changes its composition, causing it to lose its strength as a ferment very quickly and when it is used, must be used in larger quantities. If it acquires an old or cheesy smell it is best thrown away. If it is kept in a warm, moist place, it becomes brown and soft, and ultimately acquiring a bad smell; of course it is then not fit to use.

 

More about dried yeast
Dried yeast can be stored for up to 6 months in an air-tight container and is reconstituted by sprinkling the required amount of yeast on to some slightly sweetened water (temperature 43°C) and leave for a period of 10 – 20 minutes until frothy.

 

Fermentation
Yeast is a single celled micro-organism classified as members of the fungus family. Exact measurements should be adhered to in order to create consistent, successful recipes.This organism when introduced to warmth and moisture, ferments which produces carbon dioxide gas and ethyl alcohol, whilst at the same time reproducing itself. We introduce yeast into our goods and provide it with food in the form of sugars, moisture and the correct temperature so that it ferments under ideal conditions. This carbon dioxide that is produced is required for goods to become aerated. Wheat flour contains 2.5% of sugar which is sufficient for fermentation. dough without any additional sugar being added. Once the sugar content reaches approximately 12% it has a retarding effect. If a dough is very sweet, then extra yeast should be added to compensate for this effect.

TEMPERATURES
Temperatures are of the utmost importance, yeast at 0°C is dormant but its activity increases as the temperature rises until it reaches the temperature of 49°C where it is killed. The ideal working temperatures are between 21°C – 29°C. By using the following formula the ideal dough temperature can be kept consistent by adjusting the water temperature. As 25°C gives good results, to obtain the water temperature, double the temperature of the finished dough.

SALT
Although it enhances the flavour, it also strengthens the gluten content in the flour, extends shelf life and can produce a whiter crumb.

However, if the salt comes into content with the yeast or there is excess salt, the fermentation can become retarded or kill the yeast.

Conditions for fermentation.
Yeast requires food, warmth and moisture. It is destroyed at temperatures higher than 29° C, and its activity is retarded at lower temperatures. The yeast can also be destroyed during the mixing or rising processes if it is put in an overly hot place.

The group of enzymes present in the yeast changes the simple sugars – glucose and fructose – to carbon dioxide and alcohol, this carbon dioxide is what causes the rise in dough.

Fats and spices also have a retarding effect, but the addition of extra yeast can compensate for this retarding effect.

 

Making of dough
Before actually using the dough, it is given a “Bulk Fermentation”, not only for causing the production of carbon dioxide and alcohol, but also encouraging the protein gluten in the dough to become softer and rubberier.This enables the gluten to stretch even further and retain more gas. The dough is then divided into the required sizes called “scaling”.

The pieces are then shaped, called “moulding”. The moulded pieces are then placed in a cabinet containing steam called a “prover”, of a temperature not exceeding 49° until approximately doubled in size. This process is called “proving”.

The baking temperature required is between 205° and 240°. Bread and rolls require the higher temperature, whereas goods containing a high amount of sugar require the lower temperature, such as brioche dough.

 

 

Cooking temperatures
Always have the correct baking temperature. If the oven is too hot, goods will be insufficiently cooked in the centre or overcooked on the exterior. If the oven is not hot enough, goods continue proving whilst in the oven, dry out and not take on a beautiful dark brown colour.

 

Related Terms
Fermentation-The production of gases bought about by yeast developing.

Retarding-This is a delay in action of the yeast working. Salt and sugar has this effect, if used in direct contact with yeast.

Kneading-Stretching the dough to develop gluten.

Gluten-Elastic-like substance found in the protein of flour when liquid is added. Gluten keeps the stable elasticity in a finished bread product.

Improver/Format-A chemically engineered product used to encourage the development of yeast.

Proving-The term used when the dough is left to double in size, through the fermentation process.

Knocking Back-The dough is pressed down back to its original size, after the proving process to produce a more consistent structure.

Scaling-This is the process of sizing dough pieces into specific weights.

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