Yeasts consist of one cell, and belong to the taxonomic group called fungi, which also contains moulds.
There are many species of yeasts. The most common yeast known is Saccharomyces cerevisiae, which is used in the baking- and brewing industry. Yeasts also play an important role in the production of wine, kefir and some other products. Most yeasts used in the food industry are round and divide themselves through budding. This budding is a characteristic used to recognize them through a microscope. During budding the cells appear in 8-shaped forms.
Yeasts need sugar to grow. They produce alcohol and carbon dioxide from sugar. This reaction makes yeast so important for the food industry. Yeasts also produce pleasant aroma components. These aroma compounds play a very important role for the flavour of the end product. In beer the yeast is needed to produce the alcohol and the carbon dioxide for the brim. In the bread industry, both alcohol and carbon dioxide are formed; the alcohol evaporates during baking.
Yeasts can be found everywhere in nature, especially on plants and fruits. After fruits fall off the tree, fruits become rotten through the activity of moulds, which form alcohol and carbon dioxide from the sugars in it. Sometimes drunk animals appear in the news because they have eaten these spoiled fruits.
The yeast cell is oval or round and has a thin membrane. Under ideal conditions of moisture, temperature, and food supply, it reproduces asexually, by budding. When a yeast cell reaches full growth, a budlike swelling forms on its surface. Part of the parent cell’s nucleus goes into this bud, and a wall is formed between the parent cell and the bud, which then becomes a separate cell. This new cell may break off when it is full grown. It may, however, remain attached as it produces another bud. In this way, chains or clusters of cells are formed. Budding is a rapid process, requiring about 20 minutes to produce a new organism.
Yeasts are grown in the industry in big tanks with sugary water in the presence of oxygen. When the desired amount of yeast is reached the liquid is pumped out, and the yeast is then dried. Nothing else is added in the production of yeast.
Baker’s yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol. Baker’s yeast is of the species Saccharomyces cerevisiae, which is the same species (but a different strain) commonly used in alcoholic fermentation which is called brewer’s yeast. Baker’s yeast is also a single-celled microorganism found on and around the human body.
The use of steamed or boiled potatoes, water from potato boiling, or sugar in a bread dough provides food for the growth of yeasts; however, too much sugar will dehydrate them. Yeast growth is inhibited by both salt and sugar, but more so with salt than sugar. Fats such as butter or eggs slow down yeast growth; however, others say the effect of fat on dough remains unclear, presenting evidence that small amounts of fat are beneficial for baked bread volume.
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, the main differences being the moisture contents. 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. Dry yeast forms are good choices for longer-term storage, often lasting several months at room temperatures without significant loss of viability. 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 a soft solid, beige in colour, and arguably best known in the consumer form as small, foil-wrapped cubes of cake yeast. It is also available in larger-block 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 superseded 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
Active dy yeast is the form of yeast most commonly available to noncommercial bakers. 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 first be proofed or rehydrated. It can be stored at room temperature for a year, or frozen for more than a decade, which means that it has better keeping qualities than other forms, but it 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 per comparable unit volumes. 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.
Rapid-rise yeast is a variety of dried yeast (usually a form of instant yeast) that is of a smaller granular size, thus it dissolves faster in dough, and it provides greater carbon dioxide output to allow faster rising.
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); however, yeast for home use is often packaged in pre-measured doses, either small squares for compressed yeast or sealed packets for dry or instant. For active dry and instant yeast, a single dose (reckoned for the average bread recipe of between 500 g and 1000 g of dough) is generally about 7 g , though comparatively lesser amounts are used when the yeast is used in a pre-ferment. A yeast flavour in the baked bread is generally not noticeable when the bakers’ percent of added yeast is less than 2.5.
Yeast grows best at 26°C, and ferments best at 30 – 35°C. At lower temperatures yeast slows down both processes and becomes “dormant”. At higher temperatures, yeast enzymes do not work well.
The useful physiological properties of yeast have led to their use in the field of biotechnology. Fermentation of sugars by yeast is the oldest and largest application of this technology. Many types of yeasts are used for making many foods: baker’s yeast in bread production; brewer’s yeast in beer fermentation; yeast in wine fermentation, and for xylitol production. So-called red rice yeast is actually a mould, Monascus purpureus. Yeasts include some of the most widely used model organisms for genetics and cell biology.
Shipment / Storage / Risk factors
Used in the brewing and bakery trades and shipped in boxes or drums. Yeast is susceptible to damage arising from carriage temperature variations where damage can be irreparable and mould is present. A maximum carriage temperature normally applies which should be supplied by the shipper and it is also carried under refrigeration both chilled or hard frozen depending on the variety.