Re: how does milk ferment?

Greetings

The following link will give you the procedure in fermenting milk, followed by
the History of fermentation.
http://clnet.dyndns.org/microbio/l-ferment.html
Enjoy



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
 
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 bugaricusLactobacillus lactisLactobacillus 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 lactisStreptococcus cremoris 	Acid 	Cultured buttermilk, 
sour cream, cottage cheese, all types of foreign and domestic cheeses, and 
starter cultures. 
Streptococcus duransStreptococcus faecalis 	Acid and flavor 	Soft 
Italian, cheddar, and some Swiss cheeses. 
Leuconostoc citrovorumLeuconostoc 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

Thanks for taking the time to send in a question to the MadSci network

June Wingert
Associate Scientist
Biotechnology Firm
Texas