Avatar | 7th July 2018

What is Yeast?

Yeast is a single-celled microorganism that is a member of the kingdom Fungi along with moulds and mushrooms. Yeast requires suitable conditions to thrive. These conditions include moisture, oxygen, food, and a suitable temperature. When these conditions are met yeast will be activated resulting in alcohol fermentation and reproduction.  Alcoholic fermentation is the transformation by yeast of sugars into alcohol and carbon dioxide. It is this attribute of yeast that makes it useful to bakers.

For thousands of years bakers used wild yeast to make leavened bread. In the 19th century bakers obtained yeast from beer brewers which they used to create sweet fermented breads which by contrast were much different to the sour flavour of the bread made with wild yeast. 

There are hundreds of yeast species that ferment sugar into carbon-dioxide and alcohol. Saccharomyces cerevisiae is the strain of yeast used in today’s bread baking.

Commercial yeast is available in several forms; cream yeast, compressed yeast, and, Dry yeast. Cream yeast comprises of yeast cells siphoned from the growth medium. Compressed yeast is the same as cream yeast with most of the liquid removed which is available in block form. It is found at speciality food outlets. Finally, Dry yeast is the type most commonly available in supermarkets. 

The common types of yeast.

What is Yeast Yeast is a single-celled

Fresh Yeast

Fresh yeast, also known as compressed, cake, and wet yeast, has a moisture content of around 70 percent. It is a soft solid which has a pale beige colour. Fresh yeast is sold in some supermarkets, now more often sold in smaller specialist food outlets. Being highly perishable it should be stored in the refrigerator. Fresh yeast has been superseded in supermarkets with Active Dry yeast and Instant yeast.

What is Yeast instant yeast

Instant Yeast

Instant Yeast is a type of Dry yeast that was introduced in the 1970’s. Instant yeast is produced using a similar process to active Dry yeast, the process is slightly different which dries the yeast at a faster rate during manufacturing. The milling process produces fine particles, this helps it dissolve faster and activates it quicker. Unlike Active Dry yeast it doesn’t have to be proofed first, it can be mixed with the ingredients and the liquid activates it.


Active Dry Yeast was developed into a granulated yeast during World War II and does not require refrigeration. Active Dry yeast has a longer shelf life than Fresh yeast and causes the dough to rise twice as fast. In most cases Active Dry yeast needs to be hydrated with warm water and proofed prior to it being added to the main ingredients.

Requirements of Yeast.

Moisture; when water is added to yeast the transformation activity of the yeast commences. Oxygen and nutrients are absorbed through the cell membrane and other substances and enzymes disperse into the environment. Yeast requires water to absorb nutrients and this happens in a dissolved state. Yeast can only absorb simple sugars in its absorbed state through the cell membrane. Yeast releases enzymes to break down nutrients in the dough. Salt retards the action of yeast’s fermentation due to water passing through a semipermeable membrane that it applies to yeast’s cells. Salt draws water out of the porous yeast cell reducing what would be available to the yeast, this causes a reduction in fermentation.

Oxygen; this is absorbed into the dough by mixing which enables the yeast to digest nutrients and reproduce. The rise from this process is entirely due to gas production during the fermentation.  It takes several hours for the yeast to begin its absorption cycle. There is insufficient time between mixing and baking for absorption to begin. The available oxygen is used up within in minutes of mixing the dough and fermentation becomes active in this environment.

Food; this is required for the yeast and is produced during the fermentation process by converting starches into sugar. Yeast requires the amylase enzymes which are naturally present in flour to convert starches into fermented sugars. The carbon dioxide that is a by-product of the fermentation process is trapped by the gluten network which provides the dough’s volume. The alcohol that is produced during fermentation evaporates during the cooking stage and anything left adds to the distinctive flavour and aroma of the freshly baked bread.

Temperature; correct dough temperature is key for yeast’s activity. Yeast’s activity is reduced between 32 and 50F (0 and 10C) and between 115 and 131F (46 and 55C). Yeast reaches its thermal end point in the range of 138 and 140F (58 and 60C) .  If Fresh yeast is placed into the freezer the cells begin to die in a matter of days. The yeast will slowly lose its ability to ferment over a matter of weeks when stored below -4F. Dry yeast is already dehydrated and can be stored in the freezer for several months before it begins to deteriorate.

Which type of yeast should be used?

Each type of yeast reacts differently with the dough, with subtle changes to the results. The use of different yeast can impact the bread’s aroma, sweetness, flavour, and the dough’s rise.

Both Dry Active and Instant yeast can be interchanged with no change to the volume used. If a recipe uses Instant yeast and you choose Dry Active yeast instead, add an extra 15 minutes to the rise time of the dough.

Recipes can be converted to accommodate different types of yeast. Below are the formulas required to convert Fresh Yeast (FY) weights into Active Dry Yeast (ADY) and Instant Yeast (IY) and vice versa.

To convert Instant Yeast into Fresh Yeast, you must divide the amount of Instant Yeast by 0.33. For example, if the recipe requires 5g of Instant Yeast you would use the formula below to work out the weight required of Fresh Yeast.

FY = IY / 0.33

IY = 5 / 0.33

FY = 15g

To convert Fresh Yeast into Instant Yeast, you must multiply the amount of Fresh Yeast by 0.33. For example, to convert 15g of Fresh Yeast into Instant Yeast you would use the formula below. Due to the rounding of the decimal place to 0.33 there will be a slight discrepancy in the figures when reversing this formula. For a more precise measurement you can opt to use 0.33 recurring (0.33333).

IY = FY x 0.33

FY = 15 x 0.33

IY = 4.95g

To convert Active Dry Yeast into Fresh Yeast, you must divide the amount of Active Dry Yeast by 0.4. To find out the required weight of Fresh Yeast when the recipe calls for 5g of Active Dry Yeast use the following formula.

FY = ADY / 0.4

ADY = 5 / 0.4

FY = 12.5

To convert Fresh Yeast into Active Dry Yeast, you must multiply the amount of Fresh Yeast by 0.4. For example, to convert 12.5g of Fresh Yeast into Active Dry Yeast you would follow the formula below.

FY = ADY x 0.4

FY = 12.5 x 0.4

ADY = 5g

Below are a few links to Amazon where you can buy the various types of yeast for your next bake.

Saf Active Dry Yeast, 1 pound pouch

Saf Instant Yeast, 1 pound pouch

Red Star Fresh Yeast cake, 2 ounce pack (pack of 8)

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