Ingredient: Yeast
Category: Baking-
Season: All
Baker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and related products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol.
The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough accelerates the growth of yeasts.
Salt and fats such as butter slow down yeast growth.
The majority of the yeast used in baking is Saccharomyces cerevisiae, the same species commonly used in alcoholic fermentation.
Additionally, Saccharomyces exiguus (also known as S. minor) is a wild yeast found on plants, fruits, and grains that is occasionally used for baking; it is not, however, generally used in a pure form, but comes from being propagated in a sourdough starter.
Baker's yeast:
Is available in a number of different forms. Though each version has certain advantages over the others, the choice of which form to use is largely a question of the requirements of the recipe at hand and the training of the cook preparing it.
With occasional allowances for liquid content and temperature, the different forms of commercial yeast are generally considered interchangeable.
Cream yeast:
Is the closest form to the yeast slurries of the 19th century, being essentially a suspension of yeast cells in liquid, siphoned off from the growth medium.
Its primary use is in industrial bakeries with special high-volume dispensing and mixing equipment, and it is not readily available to small bakeries or home cooks.
Compressed yeast:
Is essentially cream yeast with most of the liquid removed.
It is best known in the form of cake yeast, which is essentially a soft solid, beige in colour, but is also available in crumbled form for bulk usage.
It is highly perishable; though formerly widely available for the consumer market, it has become less common in supermarkets in some countries due to its poor keeping properties, having been obsoleted in some such markets by active dry and instant yeast.
It is still widely available for commercial use, and is somewhat more tolerant of low temperatures than other forms of commercial yeast; however, even there, instant yeast has made significant market inroads.
Active dry yeast:
Is the form of yeast most commonly available to non-commercial bakers, as well as the yeast of choice for situations where long travel or uncontrolled storage conditions are likely.
It consists of coarse oblong granules of yeast, with live yeast cells encapsulated in a thick jacket of dry, dead cells with some growth medium.
Under most conditions, active dry yeast must be proofed or rehydrated first and, despite its better keeping qualities than other forms, is generally considered more sensitive than other forms to thermal shock when actually used in recipes.
Instant yeast:
Appears similar to active dry yeast, but has smaller granules with substantially higher percentages of live cells.
It is more perishable than active dry yeast, but also does not require rehydration, and can usually be added directly to all but the driest doughs.
Instant yeast generally has a small amount of ascorbic acid added as a preservative.
Some producers provide two or more forms of instant yeast in their product portfolio; for example, LeSaffre's "SAF Instant Gold" is designed specifically for doughs with high sugar contents.
Rapid-rise yeast:
Is a variety of yeast (usually a form of instant yeast) designed to provide greater carbon dioxide output, to allow faster rising at the expense of shortened fermentation times.
There is considerable debate as to the value of such a product; while most baking experts believe it reduces the flavour potential of the finished product, Cook's Illustrated magazine, among others, feels that at least for direct-rise recipes, it makes little difference.
Rapid-rise yeast is often marketed specifically for use in bread machines.
Flake yeast:
Is dead yeast, sold primarily as a nutritional supplement.
It has little to no leavening power.
For most commercial uses, yeast of any form is packaged in bulk (blocks or freezer bags for fresh yeast; vacuum-packed brick bags for dry or instant).
Yeast for home use is often packaged in pre-measured doses, either small squares for compressed yeast or sealed packets for dry or instant.
A single dose (reckoned for the average bread recipe of between 500g and 1000g of dough) is generally about 2.5 tsp or about 7g, though comparatively lesser amounts are used when the yeast is used in a pre-ferment.
Baking:
Yeast, most commonly Saccharomyces cerevisiae, is used in baking as a leavening agent, where it converts the fermentable sugars present in the dough into carbon dioxide.
This causes the dough to expand or rise as the carbon dioxide forms pockets or bubbles.
When the dough is baked it "sets" and the pockets remain, giving the baked product a soft and spongy texture.
The use of potatoes, water from potato boiling, eggs, or sugar in a bread dough, accelerates the growth of yeasts.
Salt and fats such as butter slow down yeast growth.
When yeast is used for making bread, it is mixed with flour, salt, and warm water (or milk).
The dough is kneaded until it is smooth, and then left to rise, sometimes until it has doubled in size.
Some bread doughs are knocked back after one rising and left to rise again.
A longer rising time gives a better flavour, but the yeast can fail to raise the bread in the final stages if it is left for too long initially.
The dough is then shaped into loaves, left to rise until it is the correct size, and then baked.
Dried yeast is always used for bread made in a bread machine |