Introduction to Yogurt Preparation:

Microbial fermentations are used to produce and preserve a wide variety of foods.  You already have experience with the fermentation involved in the production of wines.  In this type of fermentation, yeast converts sugars such as glucose and fructose to ethyl alcohol and carbon dioxide.  In dairy fermentations (ex., production of cheeses, buttermilk, sour cream, and yogurt), lactic acid bacteria (LAB) ferment the milk sugar lactose to lactic acid.  The lactic acid lowers the pH of the milk and inhibits or kills acid-intolerant microbes.  Thus, fermentation effectively preserves milk and provides a variety of tastes, textures, and aromas.

Milk is highly nutritive and quite susceptible to spoilage.  Thousands of years ago, people inhabiting many parts of the earth learned to preserve milk.  These "sour" milks have varied from country to country depending on the source of milk, conditions of culture, and microbial "starter culture" used.  Milk from donkeys to zebras has been used.  A few of the more unusual examples include the Russian fermentation product kumiss, which is made from raw horse milk and contains 2% alcohol, and the Swedish product surmjölk, which is made from reindeer milk.  Although people did not know it at the time, milk was fermented by resident lactic acid bacteria that were normal flora on the tools and vessels used for manipulation and storage.  If a particularly tasty fermented milk product was made, often a portion of the previous batch was used as an inoculum for the next batch.    

Today, two fermented cow milk products, buttermilk and yogurt, are widely used.  The starter culture used in commercial yogurt production is typically a 1:1 ratio of Streptococcus thermophilus and Lactobacillus bulgaricus.  S. thermophilus produces lactic acid during fermentation of lactose.  Under acidic conditions the casein (milk protein) coagulates to form a semi-solid curd characteristic of yogurts.  The fermentation products of L. bulgaricus, such as diacetyl, contribute to the flavors and aroma of yogurt.

In today's lab, each group will produce a batch of yogurt from milk by using an inoculum from commercial yogurt.  Gram stained slides will be made from the finished product to determine the types of microbes and relative numbers of microbes present.

Rachel Watson, M.S.
AG 5010
766-3524
Cell: 307-760-2942
rwatson@uwyo.edu