Yeasts in Food

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Normal yeasts require a minimum water activity of 0. Another group which can tolerate high salt concentrations and low water activity is Zygosaccharomyces rouxii , which is associated with fermentations in which salting is an integral part of the process. Alcohol and acids are two primary products of fermentation, both used to good effect in the preservation of foods. Several alcohol-fermented foods are preceded by an acid fermentation and in the presence of oxygen and acetobacter , alcohol can be fermented to produce acetic acid.

Most food spoilage organisms cannot survive in either alcoholic or acidic environments. Therefore, the production of both these end products can prevent a food from undergoing spoilage and extend its shelf life. Primitive wines and beers have been produced, with the aid of yeasts, for thousands of years, although it was not until about four hundred years ago that micro-organisms associated with the fermentation were observed and identified. Since then, the knowledge of yeasts and the conditions necessary for fermentation of wine and beer has increased to the point where pure culture fermentations are now used to ensure consistent product quality.

Originally, alcoholic fermentations would have been spontaneous events that resulted from the activity of micro-organisms naturally present. These non-scientific methods are still used today for the home preparation of many of the worlds traditional beers and wines. Alcoholic drinks fall into two broad categories: wines and beers. Wines are made from the juice of fruits and beers from cereal grains. The principal carbohydrates in fruit juices are soluble sugars; the principal carbohydrate in grains is starch, an insoluble polysaccharide.

The yeasts that bring about alcoholic fermentation can attack soluble sugars but do not produce starch-splitting enzymes. Wines can therefore be made by the direct fermentation of the raw material, while the production of beer requires the hydrolysis of starch to yield sugars fermentable by yeast, as a preliminary step Stanier, Dourdoff and Adelberg, Raw fruit juice is usually a strongly acidic solution, containing from 10 to 25 percent soluble sugars.

Yeasts in Food

Its acidity and high sugar concentration make it an unfavourable medium for the growth of bacteria but highly suitable for yeasts and moulds. Raw fruit juice naturally contains many yeasts, moulds, and bacteria, derived from the surface of the fruit. Normally the yeast used in alcoholic fermentation is a strain of the species Saccharomyces cerevisiae Adams, The fermentation may be allowed to proceed spontaneously, or can be "started" by inoculation with a must that has been previously successfully fermented by S.

Many modern wineries eliminate the original microbial population of the must by pasteurisation or by treatment with sulphur dioxide. The must is then inoculated with a starter culture derived from a pure culture of a suitable strain of wine yeast. This procedure eliminates many of the uncertainties and difficulties of older methods. At the start of the fermentation, the must is aerated slightly to build up a large and vigorous yeast population; once fermentation sets in, the rapid production of carbon dioxide maintains anaerobic conditions, which prevent the growth of undesirable aerobic organisms, such as bacteria and moulds.

The temperature of fermentation is usually from 25 to 30 o C, and the duration of the fermentation process may extend from a few days to two weeks. As soon as the desired degree of sugar disappearance and alcohol production has been attained, the microbiological phase of wine making is over. Thereafter, the quality and stability of the wine depend very largely on preventing further microbial activity, both during the "aging" in wooden casks and after bottling Stanier et al , At all stages during its manufacture, fruit juice alcohol is subject to spoilage by undesirable microorganisms.

Pasteur, whose descriptions of the organisms responsible and recommendations for overcoming them are still valid today, first scientifically explored the problem of the "diseases" of wines. The most serious aerobic spoilage processes are brought about by film-forming yeasts and acetic acid bacteria, both of which grow at the expense of the alcohol, converting it to acetic acid or to carbon dioxide and water. The chief danger from these organisms arises when access of air is not carefully regulated during aging. Much more serious are the diseases caused by fermentative bacteria, particularly rod-shaped lactic acid bacteria, which utilise any residual sugar and impart a mousy taste to the wine.

Such wines are known as turned wines. Since oxygen is unnecessary for the growth of lactic acid bacteria, wine spoilage of this kind can occur even after bottling. These risks of spoilage can be minimised by pasteurisation after bottling Stanier et al , Grape wine is perhaps the most common fruit juice alcohol.

Because of the commercialisation of the product for industry, the process has received most research attention and is documented in detail. The production of grape wine involves the following basic steps: crushing the grapes to extract the juice; alcoholic fermentation; maltolactic fermentation if desired; bulk storage and maturation of the wine in a cellar; clarification and packaging.

Although the process is fairly simple, quality control demands that the fermentation is carried out under controlled conditions to ensure a high quality product. The distinctive flavour of grape wine originates from the grapes as raw material and subsequent processing operations. The grapes contribute trace elements of many volatile substances mainly terpenes which give the final product the distinctive fruity character. In addition, they contribute non-volatile compounds tartaric and malic acids which impact on flavour and tannins which give bitterness and astringency. The latter are more prominent in red wines as the tannin components are located in the grape skins.

Although yeasts are the principal organisms involved, filamentous fungi, lactic acid bacteria, acetic acid bacteria and other bacterial groups all play a role in the production of alcoholic fruit products Fleet, Normal grapes harbour a diverse micro-flora, of which the principal yeasts Saccharomyces cerevisiae involved in desirable fermentation are in the minority.

Yeasts (Production - Microbial cultures) - EFFCA

Lactic acid bacteria and acetic acid bacteria are also present. The proportions of each and total numbers present are dependent upon a number of external environmental factors including the temperature, humidity, stage of maturity, damage at harvest and application of fungicides. It is essential to ensure proliferation of the desired species at the expense of the non-desired ones. This is achieved through ensuring fermentation conditions are such to encourage Saccharomyces species.

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This approach produces a wine of generally expected taste and quality. If the fermentation is allowed to proceed naturally, utilising the yeasts present on the surface of the fruits, the end result is less controllable, but produces wines with a range of flavour characteristics. It is likely that natural fermentations are practiced widely around the world, especially for home production of wine. During alcoholic fermentation, yeasts are the prominent species.

Yeasts, Molds, Fungi in Food

In natural fermentations, there is a progressive pattern of yeast growth. Several species of yeast, including Kloeckera, Hanseniaspora, Candida and Metschnikowia , are active for the first two to three days of fermentation. The build up of end products ethanol is toxic to these yeasts and they die off, leaving Saccharomyces cerevisiae to continue the fermentation to the end.

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Because of its tolerance of alcohol, S. Traditionally, fermentation was carried out in large wooden barrels or concrete tanks. Modern wineries now use stainless steel tanks as these are more hygienic and provide better temperature control. The low temperature and slow fermentation favours the retention of volatile compounds. This higher temperature is necessary to extract the pigment from the grape skins Fleet, There are several variables which can affect the fermentation process and final quality of wine.

Factors which are most important to control are:. The key when using nutritional yeast is that it is dry and flaky, so you need a bit of liquid to go with it — olive oil or fairly moist food works well.

What is yeast ? What yeasts are found in the food industry ?

What foods go well with nutritional yeast? I'm addicted to the stuff and like to mix it in with all kinds of dishes. Vegan mac and cheese gets a burst of healthy with the addition of nutritional yeast. I love mixing nutritional yeast in with rice and pasta dishes of almost any sort. There is one exception that I can think of, however. I don't like to mix it in with something containing tomato sauce. The tomato sauce seems to overpower it, so I don't see the point. That said, nutritional yeast does go well with actual tomatoes. Whether as toast or as part of a sandwich, I love to sprinkle a little nutritional yeast on top of olive oil. When I'm in the mood for rice cakes rather than bread, I do the same thing on rice cakes. Well, I like nutritional yeast on various types of beans, and especially in rice or pasta dishes that include beans, but I also really love the simple dish of garbanzo beans plus nutritional yeast plus a little salt.

Mixing nutritional yeast in place of cream in soup can help thicken it. My wife noted that she likes to use it in some soups and sauces in place of cream to thicken it. Note that we aren't vegan, but simply prefer soups and sauces made like that sometimes.

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A little bit of nutritional yeast, maybe olive oil, and salt on some yellow or green beans is delicious. For some extra spunk, adding in a somewhat spicy mix as well is extra tasty we have some curry mixes we tend to use in this way. Adding nutritional yeast to scrambled tofu makes it especially tasty. I tend to forget this, but I really love some scrambled tofu, tomatoes and potatoes for breakfast or even just the scrambled tofu.

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Mixing in some nutritional yeast makes it especially good. With its high calcium content, tofu is a great match for nutritional yeast. Tofu actually has about twice as much calcium per grams as milk and it doesn't turn around and leach that calcium from your bones. It's quite popular to sprinkle nutritional yeast on popcorn. It makes for a tasty and somewhat more nutritious snack. This is another simple dish but one I absolutely love.