Yeast

Yeasts are single-celled (unicellular) fungi, a few species of which are commonly used to leaven bread, ferment alcoholic beverages, and even drive experimental fuel cells. Most yeasts belong to the division Ascomycota, though some are Basidiomycota. A few yeasts, such as Candida albicans, can cause infection in humans (Candidiasis). More than one thousand species of yeasts have been described. The most commonly used yeast is Saccharomyces cerevisiae, which was domesticated for wine, bread, and beer production thousands of years ago.

Physiology

Yeast species can have either obligately aerobic or facultatively anaerobic physiology. There is no known obligately anaerobic yeast. In the absence of oxygen, fermentative yeasts produce their energy by converting carbohydrates into carbon dioxide and ethanol (alcohol) or lactic acid. In brewing, the ethanol is bottled, while in baking the carbon dioxide raises the bread, and most of the ethanol evaporates.

An example with glucose as the substrate is that

C6H12O6 (glucose) →2C2H5OH + 2CO2

Use in biotechnology

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. Baker's yeast is used for bread production, brewer's yeast is used for beer fermentation, and yeast is also used for wine fermentation. Yeast are also one of the most widely used model organisms for genetics and cell biology.

Reproduction

Yeast can reproduce asexually through budding or sexually through the formation of ascospores. During asexual reproduction, a new bud grows out of the parent yeast when the condition is right, then, after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions yeasts that are capable of sexual reproduction will form ascospores. Yeasts that are not capable of going through the full sexual cycle are classified in the genus Candida.

Growth environment

Many yeasts can be isolated from sugar-rich environmental samples. Some good examples include fruits and berries (such as grapes, apples or peaches), exudates from plants (such as plant saps or cacti). Some yeasts are found in association with soil and insects.

A common medium used for the cultivation of yeasts is called potato dextrose agar (PDA) or potato dextrose broth. Potato extract is made by autoclaving (i.e. pressure-cooking) cut-up potatoes with water for 5 to 10 minutes and then decanting off the broth. Dextrose (glucose) is then added (10 g/L) and the medium is sterilized by autoclaving.

Challah and Bread Baking

Ingredients

• Yeast is a living organism needing warmth and water to grow. It makes the dough rise by giving off gas that expands the cells held together by the gluten. Sugar speeds up this process. Salt slows it down.
• Salt inhibits yeast. Too much salt slows it action, too little salt allows the yeast to expand before the flavor has developed. Salt also give bread a good flavor and acts as a preservative.
• Sweeteners, such as sugar or honey, are food for the yeast. A little bit of sweetener feeds the yeast, while a little more gives a sweet flavor without harming the growth of the yeast. More sugar will make the yeast grow faster, but too much will slow it down. Sweet breads generally require more yeast to compensate for the extra sugar. Sugar will also give a browner crust.
• Flour is the body of the bread. The amount of flow needed for a particular recipes will vary because the amount of water the flour absorbs is not constant. It will vary according to the weather on the day you are baking, the variety of wheat used, and when the wheat was harvested.
• Gluten, found in the innermost part of the wheat, holds everything together. The yeast gives off gas that expands, the cells. Gluten then holds them in the expanded condition. It is developed by mixing, kneading and rising. Underdeveloped gluten will give a heavier loaf. The amount of gluten in different types of flour is in descending order as follows: gluten four, bread flour, all purpose white flour, whole-flour, rye flour.
• Eggs and Oil, add richness to the Challah or bread and act as a preservative. They also shorten the gluten strands, giving a more tender texture.

When opening eggs examine each one before combing with the recipe to make sure there are no blood spots. If a blood spot is found, discard the egg, and rinse out the cup or dish with cold water.