Resource 6: Background information on Yeast
Background information / subject knowledge for teacher
Yeast and baking
Background information for bakery visit
There are many types of yeast. The species of yeast used in baking is known as Saccharomyces cerevisiae.
Yeast cells are globose to elongate in shape. They are found in soils and on plant surfaces and are especially abundant in sugary media such as flower nectar and fruits.
Yeasts are saprophytes and feed mostly on sugars in the medium around them. Saprophytic organisms feed by secreting digestive enzymes on dead organic material and absorbing the products of digestion.
Yeasts reproduce by budding. During this process a small bump, the bud, protrudes from the parent cell. This enlarges and matures. The bud eventually breaks away from the mother cell to form a separate daughter cell. Some of the yeast cells in the photograph above can be seen with buds on them.
The production of alcohol by yeast is exploited in wine and beer making. Saccharomyces cerevisiae is most commonly used, but other species are important for some types of beer.
Several yeasts have been used in scientific research into genetics [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] and cell biology. In 1996 S. cerevisiae was the first eukaryote to have its DNA fully sequenced as part of the Genome project.
Other species of yeast, such as Candida albicans, are pathogens and can cause infections in humans.
Notes on constituents of bread making
Amylase enzymes in the moistened flour convert the starch in flour to glucose, which the yeast cells use as their respiratory substrate.
Flour protein, called gluten, helps make the dough stretchy (elastic and plastic). This helps to ensure that the carbon dioxide remains trapped as it enlarges the bubbles within the dough. Kneading the dough changes the structure of the proteins in the flour, making them more elastic so the bubbles of gas are trapped. This makes the bread light and chewy.
Types of flour: strong flours contain plenty of gluten, but very little α-amylase enzyme. Wholemeal flour is rich in α-amylase.
Ascorbic acid (vitamin C) may also be added as a flour improver. It makes the dough more elastic and better at trapping gases and as a result reduces the time required for leavening. This is an important consideration in commercial bread production.
Potassium bromate is sometimes used as a flour improver.
Salt is often added in the bread making process. It inhibits the action of proteases (protein digesting enzymes) and so prevents gluten from being weakened into a sticky mass that cannot retain carbon dioxide.
Excess salt forms strong ionic bonds with side chains of protein molecules. This makes them less stretchy and leads to tough bread. Excess salt also inhibits yeast growth.
Questions about yeast.
Q1 Where do you find yeasts naturally?
A Yeasts are found worldwide in soils, on plant surfaces and in the atmosphere. Yeasts are especially abundant in sugary mediums such as flower nectar and fruits.
Q2 When did people first learn about yeast and how it could be used in bread-making?
A Yeast is probably one of the earliest domesticated organisms. People have used yeast for fermentation and baking throughout history. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4000-year-old bakeries and breweries. In 1680, the Dutch scientist Anton van Leeuwenhoek first observed yeast cells under the microscope. At the time he did not consider them to be living organisms. In 1857, the French scientist Louis Pasteur proved that alcoholic fermentation was caused by living yeast cells.
Q3 How is yeast produced commercially now?
A Yeast is grown in a medium of sugar beet or cane molasses in large batch culture vats (50 000-200 000 litre capacity). Yeasts can grow in the presence or absence of oxygen.
The commercial production of yeast occurs in aerobic conditions. These conditions allow maximum multiplication of yeast cells. If multiplication is too rapid, oxygen levels fall and respiration becomes anaerobic. Hence oxygen levels must be monitored.
(N.B. When baking bread, the yeast in dough must be allowed to respire anaerobically. There is very little cell multiplication. Instead, the sugar is used mainly to produce alcohol and carbon dioxide).
After removal from the vats, the yeast is centrifuged and washed several times, then chilled to 2–4˚C.
Water is removed by dehydrators and the yeast packaged.
From http://en.wikipedia.org/wiki/Baker's_yeast (Accessed 2008)
Q4 What special conditions are required for yeast growth and multiplication?
A Yeast must be grown under aerobic conditions in order for the cells to multiply. Hence oxygen must be supplied and levels must be monitored carefully.
Ammonium sulphate, (NH4)2 SO4, is often added as a source of nitrogen for the yeast cells.
The pH must be kept in the range 4.0–5.5. As the yeast cells use up ammonium ions from the ammonium sulphate in order to get nitrogen, this tends to create acid conditions. This must be adjusted by periodic addition of alkali to keep the pH in the correct range.
Other substances may be added to aid growth, e.g. biotin or pantothenic acid.
The temperature must be kept at 30–35˚C to ensure reactions work most efficiently. Enzymes in the yeast cells are essential for respiration to happen. Enzymes are very sensitive to temperature.
Q5 How big is a yeast cell?
A The typical yeast cell is approximately equal in size to a human red blood cell and is spherical to ellipsoidal in shape. Because of its small size, it takes about 30 billion yeast cells to make up to 1 g of compressed baker’s yeast.
A yeast cell is around 5–10 micrometres (μm) by 4–8 micrometres (μm).
There are 1000 micrometres in one millimetre.
The very sharp point on a pin is about 100 micrometres (μm) across.
Q6 What is the most important difference in the growth conditions for the production of yeast cells and the use of yeast cells in baking?
A The production of yeast cells occurs in aerobic conditions, so oxygen must be supplied. For baking, no oxygen should be supplied and the yeast must be allowed to respire anaerobically.
Q7 What is the meaning of the scientific name of bakers yeast, Saccharomyces cerevisiae?
A Saccharomyces means sugar loving. The species name, cerevisiae, comes from the name Ceres, the Roman goddess of agriculture.
Resource 5: Making bread
Section 4 : Nutrition, conservation and ecology