BSI Education

Integrated Assignment 01

Looking after yeast

The quality of good beer depends on the quality and activity of the yeast that is added to the wort during the brewing process. Many breweries keep the yeast that is skimmed off at the end of one fermentation to add to the next fermentation. Yeast is a living organism and needs to be kept in the right conditions to keep it alive and active. Breweries choose fermentation and storage conditions that suit their own variety of yeast. To do this they need ways of checking how viable a batch of yeast is. Viability can be expressed as either:

  • the proportion of live and active yeast cells in a batch; or
  • the number of cells present that will multiply and form colonies.

Yeasts are also used in breadmaking and to make flavourings, food supplements, animal foods and important industrial enzymes. All these processes depend on being able to keep active samples of the right strains of yeast, ready to use in the production processes.

Your assignment is to develop two separate standard procedures to check the viability of samples of yeast. You will then use these procedures to decide on the best storage conditions for the yeast.

What you have to do

Microscopic examination

  1. Prepare a suspension of yeast by mixing about 1 g of fresh baking yeast (or 0.5 g dried yeast) into 500 cm3 of water at 20°C. Stir and allow time for the yeast to disperse evenly through the water.
  2. Place one drop of the yeast suspension on a microscope slide. Add one drop of methylene blue stain. Leave for 1 minute. Cover with a cover slip.
  3. Examine the slide under the microscope. If individual cells can be seen clearly, count the total number of cells in a field of view and record how many of them were stained blue. These are the dead yeast cells. If there is a thick layer of cells, repeat the staining after diluting the sample of yeast.
  4. Prepare a standard procedure for the measurement of the percentage of viable yeast cells in a suspension of yeast.

Plate count

  1. Use a similar procedure to SP 0003:2005 to count the number of viable yeast cells in a suspension of yeast. Use sterile water for the serial dilutions. Use malt extract agar in the Petri dish.
  2. Prepare a standard procedure for the measurement of the number of viable yeast cells in a batch of yeast.

Yeast viability tests

  1. Use one of the procedures to find out three important facts about the viability of the yeast sample:
    • How does freezing for short periods affect yeast viability?
    • How does storage for several days or weeks in a refrigerator affect yeast viability?
    • How does the viability of yeast change over a period of time, while the yeast is kept at 40°C? (This is called its 'viability' profile)

What you need to think about

  • Methylene blue stains dead cells blue. Yeast cells that are alive convert the methylene blue to a colourless chemical.
  • The percentage viability can be calculated from the formula:
    % viable cells =

    (total number of cells counted – number of blue cells) x 100

    total number of cells counted

  • Consider how many cells should be counted to give valid results. What is the best method of counting cells through a microscope?
  • What other factors might affect the yeast viability? Are these going to affect your procedure?
  • It is important to use aseptic techniques when preparing dilutions and setting up the cultures. Make sure that you know exactly what you are going to do before you start.
  • The appearance as well as the number of colonies after incubation can be a useful source of information about the quality of the yeast.
  • Consider doing several identical tests on the same sample. How consistent are the results?
  • re the methylene blue test and the plate count measuring exactly the same characteristic of the yeast?
  • Which procedure would be best to use to look at the viability of yeast samples under different conditions?
  • To obtain a viability profile you need to measure the yeast’s viability at intervals while it is kept at a constant temperature. Use a water bath or well insulated beaker of water. Allow a test tube containing 10 cm3 water to reach the test temperature. Add 1 cm3 of a freshly prepared yeast suspension and mix well. Sample the mix now and at 2 minute intervals up to 12 minutes, transferring the samples to an ice bath. Determine the percentage viability of each sample.
  • Decide the best way to present your results to show how viability:
    • varies with changing conditions;
    • varies over time at a particular temperature (the viability profile).

Presenting your conclusions

Present the two standard procedures you drew up for testing yeast viability, together with a report describing how one of these standard procedures was used to test the viability of a sample of yeast under various conditions. Present your results clearly and give your conclusions about the viability of the yeast sample under different conditions.

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