2- preparación para hacer yogurt
In its broadest sense, fermentation refers to any process by which large organic molecules are broken down to simpler molecules as the result of the action of microorganisms (organisms so small they can be seen only with the aid of a microscope). The most familiar type of fermentation is the process by which sugars and starches are converted to alcohol by enzymes in yeasts. (Enzymes are chemicals that act as catalysts, which spark reactions.) To distinguish this reaction from other kinds of fermentation, the process is sometimes known as alcoholic or ethanolic fermentation.
Ethanolic fermentation was one of the first chemical reactions observed by humans. In nature, various types of food "go bad" as a result of bacterial action. Early in history, humans discovered that this kind of change could result in the formation of products that were actually enjoyable to consume. The "spoilage" (fermentation) of fruit juices, for example, resulted in the formation of primitive forms of wine.
The mechanism by which fermentation occurs was the subject of extensive debate in the early 1800s. It was a key issue among those arguing over the concept of vitalism, the notion that living organisms are in some way essentially different from nonliving objects. One aspect in this debate centered on the role of so-called "ferments" in the conversion of sugars and starches to alcohol. Vitalists argued that ferments (what we now know as enzymes) are linked to a living cell. Destroy a cell, they said, and ferments can no longer cause fermentation.
A crucial experiment on this issue was carried out in 1896 by the German chemist Eduard Buchner (1860–1917). Buchner ground up a group of cells with sand until they were totally destroyed. He then extracted the liquid that remained and added it to a sugar solution. His assumption was that fermentation could no longer occur since the cells that had held the ferments were dead. Thus, they no longer carried the "life-force" needed to bring about fermentation. He was amazed to discover that the cell-free liquid did indeed cause fermentation. It was obvious that the ferments themselves, distinct from any living organism, could cause fermentation. Words to Know
Enzyme: An organic compound that speeds up the rate of chemical reactions in living organisms.
Ferment: An early term used to describe the substances we now know as enzymes.
Gasohol: A synthetic fuel consisting of a mixture of about 90 percent gasoline and 10 percent alcohol.
Vitalism: The concept that compounds found within living organisms are somehow essentially different from those found in nonliving objects.
Wastewater: Water that carries away the waste products of personal, municipal, and industrial operations.
Wild yeast: A naturally occurring yeast. Theory
The chemical reaction that occurs in fermentation can be described quite easily. Starch is converted to simple sugars such as sucrose and glucose. Those sugars are then converted to alcohol (ethyl alcohol) and carbon dioxide:
This description does not really provide an idea as to how complex the fermentation process really is. During the 1930s, two German biochemists, Gustav Embden (1874–1933) and Otto Meyerhof (1884–1951), worked out the sequence of reactions by which glucose ferments. Embden and Meyerhof found that it required a sequence of 12 reactions in order to accomplish the "simple" change from glucose to ethyl alcohol and carbon dioxide. A number of enzymes are needed to carry out this sequence of reactions, the most important of which is zymase, found in yeast cells. These enzymes are sensitive to environmental conditions in which they live. When the concentration of alcohol in a liquid reaches about 14 percent, they are inactivated. For this reason, no fermentation product (such as wine) can have an alcoholic concentration of more than about 14 percent. Uses
The alcoholic beverages that can be produced by fermentation vary widely, depending primarily on two factors, the plant that is fermented and the enzymes used for fermentation. Human societies use, of course, the materials that are available to them. Thus, various peoples have used grapes, berries, corn, rice, wheat, honey, potatoes, barley, hops, cactus juice, cassava roots, and other plant materials for fermentation. The products of such reactions are various forms of beer, wine, or distilled liquors, which may be given specific names depending on the source from which they come. In Japan, for example, rice wine is known as sake. Wine prepared from honey is known as mead. Beer is the fermentation product of barley, hops, and/or malt sugar.
Early in human history, people used naturally occurring yeasts for fermentation. The products of such reactions depended on whatever enzymes might occur in those "wild" yeasts. Today, wine-makers are able to select from a variety of specially cultured (grown) yeasts that control the precise direction that fermentation will take.
Ethyl alcohol is not the only useful product of fermentation. The carbon dioxide generated during fermentation is also an important component of many baked goods. When the batter for bread is mixed, for example, a small amount of sugar and yeast are added. During the rising period, sugar is fermented by enzymes in the yeast, with the formation of carbon dioxide gas. The carbon dioxide gives the batter bulkiness and texture that would be lacking without the fermentation process.
Fermentation has a number of commercial applications beyond those described thus far. Many occur in the food preparation and processing industry. A variety of bacteria are used in the production of olives, cucumber pickles, and sauerkraut from raw olives, cucumbers, and cabbage, respectively. The selection of exactly the right bacteria and the right conditions (for example, acidity and salt concentration) is an art in producing food products with exactly the desired flavors. An interesting line of research in the food sciences is aimed at the production of edible food products by the fermentation of petroleum.
In some cases, antibiotics and other drugs can be prepared by fermentation if no other commercially efficient method is available. For example, the important drug cortisone can be prepared by the fermentation of a plant steroid known as diosgenin. The enzymes used in the reaction are provided by the mold Rhizopus nigricans.
One of the most successful commercial applications of fermentation has been the production of ethyl alcohol for use in gasohol. Gasohol is a mixture of about 90 percent gasoline and 10 percent alcohol. The alcohol needed for this product can be obtained from the fermentation of agricultural and municipal wastes. The use of gasohol provides a promising method for using renewable resources (plant material) to extend the availability of a nonrenewable resource (gasoline).
Another application of the fermentation process is in the treatment of wastewater. In the activated sludge process, aerobic bacteria (bacteria that can live without oxygen) are used to ferment organic material in wastewater. Solid wastes are converted to carbon dioxide, water, and mineral salts.
[ See also Alcohol ; Bacteria ; Brewing ; Carbon dioxide ; Enzyme ; Yeast ]
List of Reagents and Instruments
Beakers Heat source Incubator, 43ºC Thermometer
Milk Starter culture (lactobacilus culture ? )or plain yogurt from local stores
1- Heat 1 liter (approximately 1 quart) of milk in a beaker slowly to 85 ºC and maintain at that temperature for 2 minutes. This step kills undesirable contaminant microorganisms. It also denaturizes inhibitory enzymes that retard the subsequent yogurt fermentation. If you are attempting this procedure at home with a sauce pan, use caution so as not to allow the milk to boil over and make a mess on your kitchen stove. See Note 1.
2- Cool milk in a cold water bath to 42-44 ºC. The cooling process should take about 15 minutes.
3- Add 5 g of starter culture to the cooled milk and mix with a glass rod. See Note 2.
4- Cover the container to minimize the possibility of contamination. Incubate at 42ºC for 3 to 6 hours undisturbed until the desired custard consistency is reached. Yogurt is set when the mixture stops flowing as the container is tipped slowly. Fluid yogurt results if the mixture is stirred as the coagulum is being formed. See Note 3.
5-The fresh made yogurt is ready for consumption when it is set. However, you may want to refrigerate it first if you are not accustomed to warm yogurt. Refrigeration also stops the growth of the lactic acid culture, which is thermophilic. (Thermophilic cultures grow best at high temperatures.) See Note 4.
6- Use of Lactobacillus acidophilus: Grind 4 yogurt tablets (about 1 g) into fine powder. Repeat Steps 3-5.
7- For entrepreneurs or simply hungry/thrifty students: You can recycle a small part of the finished product as the starter culture for the next batch. Theoretically, you can multiply or maintain your supply of yogurt indefinitely. However, in actuality, extended recycling is not recommended because the composition of the mixed culture will gradually deviate from the ideal one, and hence the flavor.