AY Honors/Baking/Answer Key

1. Explain the difference in food value between whole wheat flour and white flour.

The two biggest differences between white bread and whole wheat are the processing and the nutritional value.

Flour is made from wheat berries. The wheat berry is made up of the bran, the germ and the endosperm. All parts are filled with nutrients and are used in whole wheat flour.

White bread on the other hand, uses only the endosperm - the starchy inner layer. There is a total of 30 nutrients missing in white bread. The nutritional difference is immense and has measurable impact on our health.

WHAT IS THE IMPACT OF SWITCHING?

The fiber content of whole wheat bread has several health benefits.

Fiber helps the digestive system. It also creates a 'full' sensation and thus can help with weight control. Research has been conducted by Harvard and other organizations that shows men and women who eat high-fiber foods have less heart attacks and strokes than those who don't.

There is also an increased risk of diabetes in children who eat refined white flour - a risk that has been proven by the increase in cases of childhood diabetes.

2. Describe the effects of yeast in bread making.

See the article on Wikipedia: http://en.wikipedia.org/wiki/Yeast

Yeast interacts chemically with sugar in warm water to provide leavening. This allows the bread to "rise".

In more detail, yeasts constitute a group of 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. 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.

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 sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is bottled, while in baking the carbon dioxide raises the bread, and the ethanol evaporates.

An example with glucose as the substrate is

C6H12O6 (glucose) →2C2H5OH + 2CO2

Yeasts 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.

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 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 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.

Yeast fermentations comprise the oldest and largest application of microbial 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.

Saccharomyces cerevisiae is extensively used as a model organism by biologists studying genetics and molecular biology, as well as by protein engineers who use Saccharomyces cerevisiae as a host for yeast display.