BSI Education

Integrated Assignment 01

Looking after yeast

This assignment looks at how to measure yeast viability. Active and viable yeast cells are essential for good fermentations. The methylene blue stain distinguishes living from dead cells but does not measure the ability of the cells to multiply fast, an important feature of good brewing yeast. Results from the plate count method take much longer, but it does give an indication of the ability of the cells to multiply. The appearance of the colonies might also show if there is contamination of the sample.

Students should already have experience of following some of the standard procedures listed. In particular, they should have the skills necessary to prepare and examine a microscope slide, and also have some previous experience using aseptic techniques and handling micro-organisms.

The students need to adapt methods outlined in SP 0001:2005, SP 0002:2005 and SP 0003:2005 and develop two new standard procedures. They then use one of these to look at the viability of yeast at different temperatures. They could also look at the tolerance of the yeast to acidity and alkalinity.

Another extension activity could be to prepare spread plates from yeast samples to look at the purity of the sample. The appearance of the growing colonies should be examined without opening the plates.

Practical tips

  • Yeast: fresh baking yeast can be obtained from supermarkets with an in-store bakery, sachets of dried baking and brewing yeasts are widely available.
  • Malt extract agar: dissolve 15 g malt extract and 18 g bacteriological agar in 1 litre of distilled water. Dispense into bottles and sterilise by autoclaving. It is also available from school science suppliers.
  • Methylene blue stain: dissolve 0.01 g methylene blue (CAUTION: HARMFUL) in sodium acetate buffer (0.1 mol dm-3, pH 4.0).

Health and safety note

Methylene blue is harmful if swallowed. It stains skin and clothing.

Apparatus and reagents

  • Standard Procedures:
  • SP 0001:2005 Methods for setting up and adjusting a light microscope
  • SP 0002:2005 Methods of preparing slides for microscopy
  • SP 0003:2005 A method for counting micro-organisms in dairy products

For microscopic examination:

  • 1 litre beaker
  • stirring rod
  • dropper
  • microscope
  • light source (lamp or good daylight)
  • microscope slide and cover slip
  • fresh or dried yeast
  • methylene blue stain

For plate count:

  • sterile apparatus
    • sample bottle
    • rack of 5 test tubes plugged with cotton wool
    • 10 cm3 graduated pipette and filler, or syringe
    • 1 cm3 graduated pipette, or syringe
    • fine dropper (to deliver about 50 drops per cm3)
  • Petri dish of malt extract agar
  • marker pen to write on glass
  • 5 cm3 or 10 cm3 measuring cylinder
  • adhesive tape
  • incubator set at 30 ± 1oC

For viability tests:

  • facilities for storing samples in a fridge, a freezer and at 40oC

Further information

Related links