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The Effect of Different Sucrose Concentrations on the Growth of Yeast

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I will be looking at yeast and seeing if it is effected by different concentrations of sucrose. Yeast is a single celled organism called fungi. It is a very useful and has been used by humans for thousands of years. It is used to ferment drinks such as fruit juices into alcoholic drinks and make bread rise. The anaerobic respiration yeast carries out is often called fermentation because it results in the production of alcohol. Baker’s yeast, not the same as winemaker’s or brewer’s yeast, can be bought fresh or dried.

Fresh yeast can be stored for 2-3 weeks in a plastic container in the refrigerator or for several months in the freezer. Dried yeast granules can be kept several months. 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.

For making wine 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. Brewers’ yeast, itself a by-product of the brewing industry, is prepared for commercial distribution in dried or powdered form by means of a spray-drier. Such yeast can only be used in making alcoholic drinks, and is unsuitable for baking bread or cakes. I will do this by using different concentrations of sucrose. I will be using distilled water, 2%, 4%, 6%, 8% and 10%. I will put the same amount of yeast into each sucrose solution and leave in an incubator for one week.

After one week I will take a sample of each, add methylene blue to it to colour the yeast cells and make them easier to see and use a haemocytometer to see how much yeast has grown. I will work this out by counting five out of the twenty-five squares and then multiplying that by twenty-five. I will repeat this five times using a different sample each time to rule out any strange results. For my experiment I will need one microscope, one haemocytometer slide, distilled water, five different concentrations of sucrose solution, methylene blue, a capillarity pipette, yeast and thirty test tubes.

My hypothesis is that the yeast growth will increase with the increased sucrose concentration. My null hypothesis is that there will be no difference in the yeast growth in each of the sucrose solutions and if there is it is only by chance. My independent variable is the different concentrations of sucrose and my dependent variable is the growth of yeast. I will be keeping certain things the same such as the amount of yeast I use, and then length of time I leave it for and the volume of solution I use.

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