Fermented Milk Products

I. History

Fermentation of milk dates back thousands of years to locations all over the world. This process was used as a means of preserving a highly perishable product (http://www.accessexcellence.org/LC/SS/ferm_background.html) and to produce new flavors for an old food staple. In the early years of milk fermentation, milk was simply allowed to be fermented by its normal microbiota, but the actual process was not completely understood. Cultures could be maintained by inoculating fresh milk with fermented milk. (Kerr and McHale 2001). Today, lactic acid-producing microorganisms are added to milk to decrease the pH of the milk and produce many different fermented milk products.

II. Pathway

The general pathway for fermentation of milk involves the production of lactic acid from lactose in the milk. This lowers the pH and results in a variety of products. In this diagram, glucose can be replaced by many different sugars, including lactose, to produce the same product.

http://scidiv.bcc.ctc.edu/rkr/Biology201/lectures/Respiration/Respiration.html

The protein in milk that is responsible for curdling is casein. With the pH drop, casein molecules coagulate in a structure similar to that shown below.

http://www.public.iastate.edu/~cfford/101Milk.htm

III. Products

http://members.aol.com/ccitsa/cci05.htm

Yogurt

Text Box: http://www.public.iastate.edu/~cfford/101Milk.htm Yogurt is produced from whole or skim milk. It is inoculated with a starter culture, which usually contains Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria ferment the lactose in the milk to lactic acid, causing the milk to curdle and form yogurt. If the product is not pasteurized, the result is yogurt with “active cultures.” http://www.jic.bbsrc.ac.uk/exhibitions/bio-future/tradeg.htm

Sour Cream

Sour cream is produced in much the same way as yogurt. However, light cream is used instead of whole or skim milk. Streptococcus spp. and Lactobacillus spp. are used as starter cultures to form lactic acid from lactose. The lowered pH causes the cream to coagulate, resulting in sour cream. http://www.public.iastate.edu/~cfford/101Milk.htm

Gram stain of Streptococcus lactus in buttermilk, showing casein

Kefir

Kefir is a fermented milk product that utilizes a wide variety of microorganisms to produce a wide variety of products in addition to lactic acid including ethanol, free fatty acids, and acetaldehyde (Wszolec et al 2001). At the base of kefir production are kefir grains. These grains, which are not actually what are traditionally known as grains, contain many microorganisms including bacteria such as Lactobacillus spp. and non- pathogenic Streptococcus spp., yeasts such as Saccharomyces delbrucki and S. cerevisiae, as well as many other organisms that have yet to be classified. When milk is inoculated with these grains, it is allowed to ferment for 18 to 24 hours, during which the variety of products mentioned above are produced.

Cheese

Cheese is yet another product of milk fermentation that requires factors in addition to the traditional production of lactic acid. The coagulation of casein due to lactic acid production and the subsequent drop in pH and the addition of rennet, an enzyme derived from the lining of the stomachs of calves, form the curd of cheese Without rennet, a soft cheese such as cottage cheese or cream cheese would result. http://www.public.iastate.edu/~cfford/101Milk.htm Yeasts, molds, and bacteria are all involved in the processes that produce different cheeses, and their locations on or in the curd are what results in many cheese types. The table below shows the bacteria involved in different cheeses:

Species	Major known function	Product
Propionibacterium shermanii	Flavor & eye formation	Swiss cheese family
Lactobacillus bugaricus Lactobacillus lactis Lactobacillus helveticus	Acid and flavor	Bulgarian buttermilk, yogurt, kefir, koumiss, Swiss, Emmental, and Italian cheeses
Lactobacillus acidophilus	Acid	Acidophilus, buttermilk
Streptococcus thermophilus	Acid	Emmental, Cheddar, and Italian cheeses, and yogurt
Streptococcus diacetilactis	Acid	Sour cream, ripe cream, butter, cheese, buttermilk and starter cultures.
Streptococcus lactis Streptococcus cremoris	Acid	Cultured buttermilk, sour cream, cottage cheese, all types of foreign and domestic cheeses, and starter cultures.
Streptococcus durans Streptococcus faecalis	Acid and flavor	Soft Italian, cheddar, and some Swiss cheeses.
Leuconostoc citrovorum Leuconostoc dextranicum	Flavor	Cultured buttermilk, sour cream, cottage cheese, ripened cream butter, and starter cultures.

http://www.science-projects.com/dairybacteria.htm

Streptococcus lactis

http://www.public.iastate.edu/~cfford/101Milk.htm

IV. Links for your fermentation needs

Dom’s Kefir In-site

Cheese Making Illustrated

Making Yogurt at Home

V. References

Kerr, TJ and BB McHale. Applications in General Microbiology: A Laboratory Manual. Winston-Salem: Hunter Textbooks, 2001.

Wszolek, M, AY Tamimi, DD Muir, and MNI Barclay (2001). “Properties of kefir made in Scotland and Poland using bovine, caprine, and ovine milk with different starter cultures.” Lebensmittel-Wissenschaft und Technologie. 34: 251-261.