Yeast

Yeast is a single celled, nucleated type of fungus. It is typically only a few micrometers in diameter, and so not visible to the human eye. However there are at least 1,500 different types of yeast and they vary in size, with some as large as 40 micrometers.

If you are baking bread and adding yeast you will see grains of yeast. This is dried yeast. These grains are formed from as many as 1 million live yeast cells in a growth medium encapsulated by a layer of dry dead yeast cells. Live yeasts can reproduce themselves by budding to produce smaller versions of themselves. Yeasts can survive the temperature in your freezer as well as temperatures up to 60C (140F) which means that they can survive the brewing process but not the high temperatures used to bake bread.

Perhaps the best known yeast, is that used in baking which is called saccharomyces cerevisiae. This little yeast has the distinction of being the first non-bacterial creature to have its genome sequenced.

Uses

Baking

Yeast, normally saccharomyces cerevisiae, or baker's yeast, is used to make dough rise. It converts a sugar in the dough, called maltose, into alcohol and carbon dioxide. The gas, carbon dioxide is what makes the bread rise. The rising process can last from 1-2 hours in a warm environment. However the time it takes will depend on the temperature. At 37C (98F), yeast activity will be maximal. Nevertheless, at other temperatures rising is perfectly adequate, it just takes longer.

After rising the dough is baked and can typically reach temperatures of over 230C (446F), which kills off all the yeast cells.  The alcohol is boiled off during baking, as it is more volatile than water. 

A yeast called Candida milleri can also be used, although this is mostly with sourdough. However sourdough can be produced without yeast as its sour taste actually comes from bacteria, namely lactobacilli. These are available in yoghurt, and so a simple home made sourdough can be produced from flour, water, and yoghurt with baking soda being used to make it rise.

Brewing

Beer is produced by the fermentation of a cereal grain, typically barley which creates alcohol and carbon dioxide. The type of yeast used is typically saccharomyces cerevisiae for real ales, and saccharomyces pastorianus (formerly s. carbergensis), for lagers. Hops are used to flavour the product.

Draught beer is any beer dispensed from a large container such as cask or keg, rather than a bottle or can. Real Ales, normally dispensed from casks (barrel shaped) normally contain live yeast in a sediment at the bottom of the cask. The other type of draught beer, keg beer, is normally dispensed to the customer under pressure. The beer in this case is pasteurized and so all the yeast will have been destroyed. 

When it comes to bottles and cans, bottles sometimes contain live yeast and are called bottle conditioned beers if this is so. Canned beers rarely contain yeast. As a general rule if your beer is carbonated then it is unlikely to contain yeast, as yeast also produces bubbles, but rather less fizzy ones.

Marmite

You either love it or hate it, but what is it? Marmite or similar products such as Vegemite and Vitam-R are produced by mixing plenty of salt with a suspension of yeast cells. The solution surrounding the yeast cell becomes very salty. If you remember your school science, the water inside the yeast cells then moves across the semi-permeable membrane (cell wall) by osmosis from the weaker to the stronger solution. This results in the yeast cells losing all their water, shrivelling up and splitting their cell walls.

The dying yeast cells are then heated to complete the process, with the cell wall remnants being then removed. The enzymes within the yeast then break down the remaining proteins into amino acids and peptides. This is autolysed yeast extract. It is full of flavour as it contains a lot of glutamate, which is associated with the delicious savoury taste known as umami(1).

Monosodium Glutamate (MSG)

Yeast extract is used as a flavour enhancer in many foods. Some of its glutamate may be converted into monosodium glutamate, especially if it is a yeast used for baking. These extracts are used by some manufacturers of processed foods to hide the fact that their foods contain MSG. Yeast extract can be called a natural product, and the MSG in it need not appear on the label unless it comprises over 78% of the product.

Winemaking

Wine is produced from must (red wine) or juice (white wine) which is fermented with yeast for between 1-2 weeks, converting the grape sugars into alcohol and carbon dioxide. As with baking, saccharomyces cerevisiae and other types of saccharomyces are the most commonly used types of yeast. While yeast may impart some flavour into wines, its effect decreases as a wine ages.

Health benefits

Vegan diets

Yeast and yeast extracts are often sold on the basis that they are full of valuable vitamins. In particular B vitamins such as thiamin, niacin, folic acid and vitamin B12, as well as minerals such as chromium and selenium. As such they could be useful in vegan diets, which can easily be poor in vitamin B12.

Wound healing

Just like honey(2), there is some evidence that brewer's yeast may help speed wound recovery(3). Extracts of Baker's yeast have also been used successfully to treat ulcers(4).

Inflammatory Bowel disease - IBD

IBDs such as Colitis and Crohn's disease have responded to treatment with yeast strains, specifically saccharomyces boulardii. This is likely due to yeast's ability to reduce inflammation. The use of yeast to treat colitis has been tested on mice(5), where it alleviated some symptoms including weight loss. In a human clinical setting, 17 out of 25 patients with mild to moderate flare ups achieved remission after one month of treatment(6). In the case of Crohn's disease a clinical test using s. boulardii on 32 patients reduced relapses from 37.5% to 6.25%(7).

Allergy

Yeast contains histamine, a substance that can provoke immune reactions. If you consume too much yeast, then your body's capacity to dampen down the reaction may be overcome temporarily. Also a small number of people have genuine allergic reactions to yeast, which can be detected with skin prick tests.

DrDobbin says:

There do appear to be some some health benefits resulting from either using or consuming yeast to speed up wound recovery and alleviate IBDs. While some may think that yeast is problematic for many people, I have not seen much evidence of this. For most people yeast containing foods are not a problem and may improve health, especially within the gut where they have probiotic properties.

References

1) http://www.drdobbin.co.uk/umami

2) http://www.drdobbin.co.uk/honey-it-better-sugar-solution

3) http://www.ncbi.nlm.nih.gov/pubmed/1691911

4) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3430226/

5) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2864839/#B9

6) http://www.ncbi.nlm.nih.gov/pubmed/12840682

7) http://www.ncbi.nlm.nih.gov/pubmed/10961730