MILK FERMENTERS page 33

Milk is extremely perishable and many means have been developed to preserve it. The earliest one which has been used for many thousands of years is fermentation. By inoculating or introducing the appropriate bacteria, the lactose in the milk is fermented to lactic acid. Lactic acid preserves the milk by lowering the pH, preventing the growth of putrefactive or pathogenic bacteria which do not grow well in acid conditions.

At the same time, casein, the predominant protein in milk, is insoluble in acid, and it precipitates which thickens the product. Numerous strains of bacteria are capable of converting lactose to lactic acid, and we will look at several fermented milk products to study the morphology and staining characteristics of these bacteria.

1. Make a thin smear of each milk product well spaced on the same slide, labeling with a wax pencil Y, B and S. (see protocol Smear and Staining of Bacterial Specimens)

2. Stain them according to the procedure for the Gram stain (see related protocol Gram Stain Protocol)

3. View the stained smear at 400x to determine the characteristic features, select a field which is well spread and typically stained. Then switch to 1000x.

4. Illustrate typical fields for each milk product showing all observed morphologies of bacteria. Label the morphologies and their probable identities according to the following type of bacteria expected in these fermented milk products:

YOGURT: Yogurt is produced by a mixed culture of two types of bacteria. It contains Streptococcus thermophiles and either Lactobacillus acidophilus or L. bulgaricus.

BUTTERMILK: Fermentation of this product is performed by a culture of Streptococcus lactis which produces lactic acid plus Leuconostoc citrovorum which converts lactic acid to aldehydes and ketones which gives it its flavor and aroma.

SOUR CREAM: Is produced by the same bacteria as buttermilk, but the starting milk product is pasteurized light cream. Bacteria are much less numerous than in buttermilk.

For more information on cheese making and other uses of fermentation in milk, see: http://Biology.clc.uc.edu/Fankhauser/Cheese/MLK_FERM99.HTM