Yeast is a microorganism which grow by feeding on sugars. In food manufacture, yeast is used in fermentation and leavening. The fungi feed on sugars, producing alcohol (ethanol) and carbon dioxide. The carbon dioxide is used in beer and wine manufacture. In sparkling wines and beer some of the carbon dioxide is retained in the finished beverage. The fermentation of wine is initiated by naturally occurring yeasts present in the grapes.

One yeast cell can ferment approximately its own weight of glucose per hour. Yeast is also used in making bread. The yeast respires with oxygen by feeding on sugars, breaking them down into carbon dioxide, water and alcohol (Ethanol). The carbon dioxide makes the dough rise, the bread is then baked in an oven for 30 minutes. This kills the yeast and evaporates the alcohol. This is how bread is made.


For thousands of years, the process of fermentation has been used to make bread, beer and wine. Today fermentation is used to make foods such as bread and yogurt, alcoholic drinks such as wine, drugs such as penicillin and chemicals such as methanol and citric acid. Fermentation is a chemical process. Tiny organisms called microbes grow by converting the sugars in food, such as fruits and grain, into alcohol and carbon dioxide. Microbes can live almost anywhere. It is likely that fermentation was discovered by accident when fruits or grain were stored in containers. One safe and commonly used microbe is yeast. Not all microbes are safe to eat – many are harmful and poisonous.

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Industrial fermentation processes begin with suitable microorganisms and specified conditions; e.g., careful adjustment of nutrient concentration. There are many products of yeast fermentation such as alcohol, glycerol, and carbon dioxide.

In industrial fermentation microbes like yeast and bacteria can be used to make useful chemicals on a large scale. The fermenter is filled with nutrients and a small amount of a microbe, such as yeast. It is important to make sure that the microbe has the best possible conditions for growth. These include:

* Sterile conditions. The fermenter vessel is made out of stainless steel. Super-heated steam is pumped through it to kill any unwanted microbes.

* Oxygen. Some types of microbes need oxygen to grow. Sterile air is bubbled in from the bottom of the fermenter. Motorised paddles make sure the oxygen is mixed in.

* Heat. The reactions inside the generator release a lot of heat. Too much heat would kill the microbes. Water circulates in a cooling jacket to lower the temperature.

The process of alcoholic fermentation requires careful control for the production of high quality wines. Requirements include suppression of the growth of undesirable microorganisms, the presence of adequate numbers of desirable yeasts, proper nutrition for yeast growth, temperature control for prevention of excessive heat and the prevention of oxidation. Enzymes in the yeast act like catalysts, they speed up the reaction of fermentation.

These enzymes are specially designed to speed up fermentation and will not work in other reactions. Temperature control during alcoholic fermentation is necessary to (1) facilitate yeast growth, (2) extract flavours and colours from the grape skins, (3) permit accumulation of desirable by-products, and (4) prevent undue rise in temperature, denaturing the yeast cells. Wine making and bread making are two examples of biotechnology, this means using microbes, plant cells or animal cells to make substances that are useful to us. Many useful chemicals have been produced using fermentation techniques.

There are many experiments to test that

Yeast + water + glucose ? CO2 + Ethanol. One of these is to use two test tubes one filled with yeast suspension plus diazine green and liquid paraffin on top. This is to prevent oxygen reaching the yeast. The other is filled with hydrogen carbonate indicator solution and the test tubes are connected with a delivery tube. This test is to see whether yeast can respire anaerobically (Without air). If the hydrogen carbonate indicator solution turns yellow it means that no oxygen was present when the yeast respires. However, if the hydrogen carbonate indicator stays pinkish colour oxygen was present when the yeast fermented. Another previous test has proved that oxygen is not necessary for respiration.

The method for this test is to put a yeast and glucose solution into a boiling tube along with two drops of diazine green. This is to indicate when all the oxygen has been removed because in the presence of oxygen it is blue and when no oxygen is present it turns pink. Then liquid paraffin is added to the top of the solution creating a seal so no more oxygen can get in. When the diazine green turns pink indicating that the yeast has used up all the oxygen, a bung on top of the boiling tube with a delivery tube is placed in the mouth of a test tube filled with hydrocarbonate indicator solution. If the yeast has respired the cabon dioxide from the solution will turn the hydrocarbonate indicator yellow.


One of the factors that will speed up the fermentation of yeast is the temperature at which the yeast respires. My prediction is that yeast can ferment faster at a temperature of 380c than it will at room temperature (240c). I believe this because when an object or organism is given heat energy the object will speed up whatever it is doing. This is because the particles of the object are given energy so they speed up, therefore increasing the speed of the reaction. However this does not happen with enzymes as they are living and work best at 380c which is body temperature. I think this will work with yeast cells as long as the temperature does not exceed 500c. If the temperature does go higher than this the enzymes in the yeast cells will be de-natured and die.

To test this prediction I will measure the rate at which carbon dioxide is produced from the fermentation of yeast which has been heated to a temperature of 380c. I will then compare this to the rate at which carbon dioxide is produced from yeast kept at room temperature (approximately 240c). If the yeast in the heated boiling tube produces 1ml of CO2 faster than the yeast at room temperature then this will show that temperature does have an effect on the speed at which yeast ferments.


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