MICROBES AND FOOD
Yeast is a single celled fungus. It is a very useful organism and has been used by humans for thousands of years. It is used to ferment fruit juices into alcoholic drinks and make bread rise.
FermentationIf there was no other way of respiring without oxygen, then all activity would come to a sudden stop. It is possible to respire without oxygen. This is called anaerobic respiration. Oxygen is needed to full break down the glucose and release all the energy stored in it. In anaerobic respiration glucose cannot be fully broken down and only a small amount of energy is released.
Plants and fungi can respire anaerobically and they produce ethanol (alcohol) and carbon dioxide.
Some organisms like yeast respire this way all the time. The anaerobic respiration yeast carries out is often called fermentation because it results in the production of alcohol.
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The yeast and fruit juices are put inside a sterile jar called a demijohn. As the yeast respires anaerobically it does not need a supply of oxygen. As it respires ethanol is made as well as carbon dioxide gas. The bubble trap contains water. It allows gases to leave the bottle but prevents things like bacteria getting in and spoiling the wine. The bottle needs to be kept warm but not hot. In warm conditions the yeast will respire quite quickly, but if it is too hot the yeast will die. |
When yeast respires it makes carbon dioxide gas. When the gas is made in the dough, it gets trapped and so tiny bubbles form inside the dough. This makes the bread rise. 'Knocking back' gets rid of large bubbles which can develop. When the dough is baked the bubbles expand. The yeast dies in the baking process and ethanol is evaporated.
Production of yoghurt
This process relies on the growth of bacteria - usually mixed cultures including Lactobacillus species - on milk, and the conversion is due to anaerobic respiration by the bacteria. The milk is usually skimmed milk - "low fat"- often concentrated with extra "milk solids".
As the bacteria respire anaerobically, using the lactose (milk sugar), they produce lactic acid:
lactose = lactic acid + energy
(milk sugar)
In fact, it is said that Lactobacillus breaks down milk protein to chains of amino acids, which are then used by Streptococcus to make an acid which Lactobacillus uses to convert lactose into lactic acid.
The acidic conditions (pH 3.7-4.3) cause milk proteins to be coagulated, which provides the thickened texture of yoghurt. Other chemical products from the bacterial growth, e.g. ethanol (acetaldehyde) contribute to the flavour.
Keeping qualities
As the yoghurt is cooled, the bacterial growth rate is reduced and the product can keep for about 10 days at 5 °C. The normal numbers of bacteria are about 108 (100,000,000) per gram. After some time, acid slowly released by the bacteria gradually kills them and causes the proteins to separate into curds and whey.
Yeasts and moulds can cause problems if they contaminate yoghurt - causing pots to become "blown", due to the production of carbon dioxide.
How Stuff Works - Bread Making
Biotech Bytes Box - Yeast and Fermentation
Brewing - how yeast works
FOOD PRESERVATION
The basic idea behind all forms of food preservation is either:
- To slow down the activity of disease-causing bacteria
- To kill the bacteria altogether
In certain cases, a preservation technique may also destroy enzymes naturally found in a food that cause it to spoil or discolour quickly. An enzyme is a special protein that acts as a catalyst for a chemical reaction, and enzymes are fairly fragile. By increasing the temperature of food to about 150 degrees Fahrenheit (66 degrees Celsius), enzymes are destroyed.
A food that is sterile contains no bacteria. Unless sterilized and sealed, all food contains bacteria. For example, bacteria naturally living in milk will spoil the milk in two or three hours if the milk is left out on the kitchen counter at room temperature. By putting the milk in the refrigerator you don't eliminate the bacteria already there, but you do slow down the bacteria enough that the milk will stay fresh for a week or two.
Refrigeration and FreezingRefrigeration and freezing are probably the most popular forms of food preservation in use today. In the case of refrigeration, the idea is to slow bacterial action to a crawl so that it takes food much longer (perhaps a week or two, rather than half a day) to spoil. In the case of freezing, the idea is to stop bacterial action altogether. Frozen bacteria are completely inactive.
Freezing has no effect on the taste or texture of most meats, has minimal effects on vegetables, but often completely changes fruits (which become mushy).
CanningSince 1825 or so, canning has provided a way for people to store foods for extremely long periods of time. In canning, you boil the food in the can to kill all the bacteria and seal the can (either before or while the food is boiling) to prevent any new bacteria from getting in. Since the food in the can is completely sterile, it does not spoil. Once you open the can, bacteria enter and begin attacking the food, so you have to "refrigerate the contents after opening".
DehydrationMany foods are dehydrated to preserve them. Since most bacteria die or become completely inactive when dried, dried foods kept in air-tight containers can last quite a long time.
Freeze-DryingFreeze-drying is a special form of drying that removes all moisture and tends to have less of an effect on a food's taste than normal dehydration does.
In freeze-drying, food is frozen and placed in a strong vacuum. The water in the food then sublimates -- that is, it turns straight from ice into vapour. Freeze-drying is most commonly used to make instant coffee, but also works extremely well on fruits such as apples.
SaltingSalting, especially of meat, is an ancient preservation technique. The salt draws out moisture and creates an environment inhospitable to bacteria. If salted in cold weather (so that the meat does not spoil while the salt has time to take effect), salted meat can last for years.
Pickling
Pickling was widely used to preserve meats, fruits and vegetables in the past, but today is used almost exclusively to produce "pickles," or pickled cucumbers. Pickling uses the preservative qualities of salt (see above) combined with the preservative qualities of acid, such as acetic acid (vinegar). Acid environments inhibit bacteria.
Pasteurizing
Pasteurization is a compromise. If you boil a food you can kill all bacteria and make the food sterile, but you often significantly affect the taste and nutritional value of the food. When you pasteurize a food (almost always a liquid), what you are doing is heating it to a high enough temperature to kill certain (but not all) bacteria and to disable certain enzymes, and in return you are minimizing the effects on taste as much as you can.
Ultra high temperature (UHT) pasteurization completely sterilizes the product. It is used to created "boxes of milk" that you see on the shelf at the grocery store. In UHT pasteurization, the temperature of the milk is raised to about 285 degrees F (141 degrees C) for one or two seconds, sterilizing the milk.
Fermenting
Fermentation uses yeast to produce alcohol. Alcohol is a good preservative because it kills bacteria. When you ferment grape juice you create wine, which will last quite a long time (decades if necessary) without refrigeration. Normal grape juice would mould in days.
Nuclear radiation is able to kill bacteria without significantly changing the food containing the bacteria. So if you seal food in plastic and then radiate it, the food will become sterile and can be stored on a shelf without refrigeration. Unlike canning, however, you do not significantly change the taste or texture of the food when you irradiate it.