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Basic Energy Concepts
Types of Catabolism
Catabolism of Fats
Catabolism of Proteins
Summary of Catabolism
Fermentations of Importance to Humans
©2000 Timothy Paustian, University of Wisconsin-Madison
For millenia humans have taken advantage of the fermentations microbes perform. Over the years we have learned to control and optimize theses fermentations through trial and error. It was only in the last 100 years that the biochemistry behind fermentation has become clear.
In this section we take a little time off from the serious study of metabolism to look at the production of products we're all familiar with.
Several of the fermented products we consume depend upon yeast fermentation of glucose to ethanol as we just discussed in the section of fermentation.
Beer is a fermentation of barley and hops by yeast. The starch in barley is broken down into glucose and then fermented to ethanol by yeast. The finish product is aged and then packaged for distribution and consumption. It is an involved six stage process, beginning with the formation of malt from barley.
The beer is then put in containers and distributed to customers. Beer normally has a shelf life of about 6 months and after that starts to take on undesirable flavors.
Bread is a simple fermentation of sugar to CO2 and alcohol. The baker first combines flour, sugar, milk and other ingredients with a microorganism, usually a bread yeast such as Saccharomyces cerevisiae, but not always. The ingredients are mixed and then allowed to incubate at 27°C for a few hours. During this time the yeast convert the sugar present to ethanol and CO2. Most incubations are for less than 4 hours not leaving enough time for the yeast to increase in number. The CO2 produced causes the bread to rise (leaven) and become porous. The success of leavening is dependent upon the rate of gas production. This can be increased by adding more yeast, more sugar, or dough conditioners (various salts that the yeast need). Tweaking a recipe by manipulating these factors can speed CO2 production, within reasonable limits. Adding too much of anything can either kill the yeast or cause the bread to rise too quickly. The temperature of incubation is another critical consideration. Saccharomyces grows best at 26 to 28°C and deviations from that temperature will usually result in slow or complete lack of leavening. Failure as a baker can normally be attributed to either not adding the exact amounts of ingredients or inappropriate incubation temperatures during leavening.
Yogurt is a product of fermented milk. Lactic acid bacteria are the major microbes in many milk based fermented products. These bacteria are finicky having many growth requirements all of which can fortunately be satisfied by a milk mixture. Lactose in milk is fermented to lactic acid either via the homofermentative or heterofermentative pathway.
Production of yogurt starts by conditioning the milk. The water content of milk is first lowered 25% by vacuum evaporation and 5% milk solids are added. As a final conditioning step, the milk is heated to 86 to 93°C for 30-60 minutes. This causes some breakdown of proteins and other molecules and kills contaminating microbes that may compete with the starter culture. After cooling to 45°C a 1:1 mixture of Streptococcus thermophilus and Lactobacillus bulgaricus is added. Fermentation is at 45°C until the desired degree of acidity is reached. This usually occurs in 3-5 hours. The finished product may have other ingredients added (such as mold inhibitors or dye) and is packages with fruit. Yogurt is stored at 0-4°C until consumed to prevent spoilage.
There is some evidence that consumption of products containing active cultures of lactic acid bacteria can be beneficial. However, the health claims by proponents of this idea have yet to withstand serious scientific scrutiny. It is reasonable to assume that ingesting lactic acid bacteria may help deter other more sever pathogens such as E. coli or Salmonella typhimurium
Cheese is also a milk fermentation, but its production is more complex. Different bacteria come into play and production periods are much longer than yogurt. Despite there being 20 classes and hundreds of varieties of cheeses the initial manufacturing process is surprisingly similar.
The finished product is sold either as a complete mold (a wheel of cheese) or cut into smaller pieces. Most cheeses are stored at refrigerator temperatures.
We have only touched on a few foods that are made with the help of microbes, there are many more. To give you an idea of the array of products, here is a list of fermented foods and the microbes that are involved in their formation.
Fermentation of organic substrates is one way to make a living, but it is a poor choice. Energy yields are low and microbes have to ferment large amounts of substrate to get enough energy for cellular processes. Anaerobes were probably the first microbes to evolve billions of years ago, but once oxygen became prevalent in the atmosphere, a better method of catabolism evolved, respiration.
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