Yeast is a one-cell, plant micro-organism: essentially, a fungus.
The main variety that we use is "Saccharomyces cerevisiae." In the food world, we also call this "Baker's Yeast." In the alcohol world, they call this "Ale Yeast", as it floats on top the beer to make ales. To make lager beer, brewers use a different strain of yeast altogether, which lies and ferments on the bottom of the beer (useful if you are making beer in the winter months, as yeast at the bottom is less susceptible to cold drafts than it would be floating at the top.)
To produce yeast commercially, one lively looking yeast cell is chosen (using a microscope) from the desired type of Yeast. The lucky cell is transferred to a sterile tube or dish with nutrients, where it reproduces itself by merrily budding away to make more yeast cells. Then, the cell and his new family are divided up, and transferred to flasks with more nutrients, and told to go forth and multiply some more. This new population is then used to populate large tanks, and the new population of those large tanks is then transferred to even larger tanks -- sometimes holding up to 60,000 gallons. At this stage, the yeast will have multiplied 5 to 8 times, and tons of yeast will have come from 1 cell.
Centrifugal force is then used to separate the yeasties from the nutrient liquid, and the yeast is washed. What's left is a froth of yeast, or "Yeast cream", as it is called in the industry, some of which is sold in tankers as is to large industrial bakers, some of which is dried and packaged in various forms, and some of which is compressed in cakes to be sold as live yeast.
The thing to remember is, yeast isn't a chemical. It's a living thing. Laurel Robertson, of Laurel's Kitchen fame, called them her "yeast beasties." For Yeast to grow, it needs heat between 70 and 90 F (21 to 32 C.) It is killed when temperatures reach 132 F (55 C), and de-activated when temperatures drop to 40 F (4.5 C.) It needs food, in the form of sugar and/or starches, which it converts to carbon dioxide. And it needs water, or moisture to drink (as it were.)
When dehydrated, yeast remains in a state of suspended animation. It re-animates the minute it comes into contact with any moisture or water; so you need to make sure that the minute it comes alive, that it has food, so that it doesn't immediately die as well.
In small amounts, sugar provides the yeast cells with additional food. In larger amounts, it slows down their progress because it upsets the water balance within the yeast cells. Sugar competes with flour proteins for water which the flour proteins use to develop gluten. For this reason, high-sugar dessert breads take longer to rise. However, because sugar molecules like to absorb water, it means the finished bread will stay moister longer. Sugar also causes browning reactions in the crust.
Salt in small amounts inhibits the activity of protein-digesting enzymes in the yeast, which would want to eat the gluten that is being developed. However, too much salt will also dramatically inhibit the activity of the yeast. Bread risen without salt will have a much coarser texture. If you have to watch your salt intake, look for some Tuscan bread recipes, many of which are traditionally made without any salt at all.
We use yeast to kick off fermentation in bread and alcohol. In bread, the carbon dioxide gas bubbles get trapped among the gluten and cause the dough to rise. For the most part, yeast is only of use in doughs where there is gluten. Yeast produces carbon dioxide very slowly; the dough has to be elastic and strong enough to hold it for a period of time. This elasticity comes from gluten. Weaker doughs, whose structure isn't able for long to fight against air bubbles trying to escape, needs a faster source of gas: for them, this comes from chemical leaveners such as baking soda and powder.
You may be wondering why yeast produces alcohol in -- well, alcohol -- while it produces carbon dioxide in bread. Well the fact of the matter is that yeast produces both carbon dioxide and ethanol in both beer and bread. In beer the carbon dioxide helps to carbonate the beer and make it bubbly (nothing worse than a flat beer), and in bread, the small amounts of ethanol alcohol produced are burned off during baking. Plus, in bread, Yeast only gets a few hours while rising to produce a bit of ethanol. In brewing, however, Yeast gets up to a week to make alcohol. The yeast continues making alcohol up until the brew is 10% alcohol, at which point there is too much alcohol in the environment for the yeast, and the yeast dies.
As there have apparently been some (as in a few) reported cases of yeast infection in women reputedly caused by working with dough, it is recommended to wash your hands thoroughly after working with yeast-risen dough.
1 teaspoon dry yeast = 5 g compressed (aka fresh) yeast
The Romans preferred yeast-raised breads, thinking them more nutritious. They made yeast by either mixing millet or wheat bran with grape juice and allowing it to ferment in the air and then drying it in the sun, or by putting barley cakes in clay pots until they became sour and yeast developed on them. It wasn't until the microscope was invented in 1676, however, that man was able to get a close look at yeast. In 1859, Louis Pasteur discovered how yeast actually works.
Before the advent of commercial yeast cakes in 1870 in America, everyone had to grow their own yeast starters at home (or borrow from a neighbour). Dried yeast was invented during the Second World War.
A medieval English term for yeast was "godisgoode."
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