MadSci Network: Microbiology

Re: why does yogurt have a longer shelf life

Date: Mon Mar 1 09:51:57 1999
Posted By: Glynis Kolling, Grad student, Food Science, Rutgers Univeristy
Area of science: Microbiology
ID: 920094855.Mi

   The normal shelf life of yogurt is 20 - 40 days from processing.  Yogurt 
has a longer shelf life than milk for two major reasons: pH and numbers of 
beneficial bacteria present at the end of production. 
   In general, the pH of plain yogurt ranges from 4.4 - 4.2.  This pH range 
is not favorable for growth of many bacteria; however, there are bacteria 
that can tolerate acidic conditions.  Yogurt production uses two main 
bacterial cultures:  Lactobacillus bulgaricus and Streptococcus 
thermophilus.  Both of these bacteria are lactic acid bacteria (LAB); that 
is, they produce lactic acid during normal growth.  
   During fermentation, lactic acid produced will result in a decrease of 
pH of the yogurt.  LAB can tolerate acidic conditions generated; however, 
growth of other bacteria may be inhibited.  In addition, at low pH, lactic 
acid will dissociate (RCOOH --> RCOO- and H+).  In general, the dissociated 
form of a weak organic acid acts as an antimicrobial agent.  
   The other factor playing a major role in the shelf stability of yogurt 
are the high numbers of live LAB (as many as one billion live LAB/ml may be 
present at the end of yogurt production!). The large numbers of viable LAB 
will thus act as competitive microflora against other microflora (i.e. 
spoilage and pathogenic bacteria) not normally present in yogurt.  

   The production of plain yogurt from milk begins with homogenization and 
pasteurization.  Before pasteurization, stabilizers (such as gelatin or 
modified food starched) may also be added (usually, stabilizers are added 
for production of Swiss and Sundae style and flavored yogurts).  The milk 
is then cooled to 46.7C and inoculated at 1.25% each of Lb. bulgaricus and 
S. thermophilus.  After inoculation, the mix is poured into yogurt cups and 
incubated at 46.7C for 3-5 hours.  During this time, the pH will decrease 
due to lactic acid production.  Other characteristic compounds (i.e. acetic 
acie, diacetyl and acetaldehyde) of yogurt will also be formed during 
fermentation.  After incubation, yogurt is then stored at refrigeration 
   In addition to Lb. bulgaricus and S. thermophilus, other "probiotic" 
(beneficial) bacterial cultures may be added.  These include Lb. 
acidophilus, and some strains of Bifidobacteria.  Research targeting the 
therapeutic benefits of these bacteria is a current "hot topic".  
   There are two types of yogurts: Swiss (also known as Continental or 
stirred-style) and Sundae style.  Swiss style is flavored yogurt (either 
flavors or fruit purees).  If flavorings are used, they are added prior to 
incubation.  If fruit purees are used, they are added and blended into 
plain yogurt and packed.  Sundae style yogurt has fruit purees on the 
bottom of plain or vanilla yogurt.  

Kosikowski, F.V. 1982. Cheese and Fermented Foods. 2nd Ed. Brookrondale, 

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