Which Sugars Can Yeast Ferment?

Objective: Yeasts are unicellular microbes with which many people are familiar. In particular, the species Saccharomyces cerevisiae has served the human kind for thousands of years in food and beverage production. The trick of S. cerevisiae is its ability to ferment sugars, resulting in the production of alcochol, hence their use in brewing, and the release of carbon dioxide, hence their use in leavening. However, which sugars are the yeast able to and which of them are yeast unable to ferment? This is the question that I aim to address in the present experiment.

Rationale and Background: To determine the substrates of the yeast, I plan to exploit the pattern of their growth. In an enclosed environment, a yeast population normally goes through four phases, which, in temporal order, are lag phase, log phase, stationary phase and decline phase. Lag phase refers to the period when yeast cells are adapting themselves to the new environment. They take in water, synthesise ribosomes and produce enzymes. Although the cells are active, there is little increase in their number. Once acclimated, the population will leap into the next phase, the log phase, provided that the conditions are favourable. An environment with optimal temperature and pH as well as plenty of nutrients is conducive to exponential growth of the population. However, as nutrients are depleted by increasing number of cells and waste products build up, the size of the population plateaus and eventually shrinks, corresponding to the stationary and decline phase respectively.

Given such behaviour of the yeast population, if a sugar of interest is supplied as the sole nutrient, increase in the number of cells would indicate that yeast can feed on this sugar, whereas a standstill or decrease in cell number would mean the opposite.

Materials and Methods: The experiment can be broken down into the following steps: 1. Prepare a yeast suspension containing an excessive amount of the sugar of interest. Materials and Appartus: Sugar (either Glucose, Fructose, Arabinose, Maltose, Lactose or Sucrose), Dried Yeast, Distilled Water, Balance, 2L Volumetric Flask, Magnetic Stirrer. 2. Measure the intial cell concentration in the suspension. Materials and Appartus: Alcohol, Measuring Cylinder, Conical Flask, Cotton Wool, Haemmocytometer, Capillary Tube, Microscope. 3. Incubate the suspension under optimum conditions. Materials and Appartus: Water Bath. 4. Sample the suspension at premeditated time points and measure the cell concentration in the sample. Materials and Appartus: Same as step 2. 5. Perform ststistical tests. A Student`s t-test will be performed to determine the significance of the findings.