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Survival of probiotic bacteria in South African commercial bio-yogurt : research letter

To gain any therapeutic benefit from eating bio-yogurt, it needs to 
meet the criterion of a minimum of 106 viable probiotic cells per ml 
during storage until the expiry date, which is normally 31 days after 
manufacture. Bio-yogurt from three different sources was obtained 
directly after production, and the samples stored at 4 o C and 10 o C. At 
three-day intervals during a storage period of 31 days, enumeration 
of viable probiotic cultures, Lactobacillus acidophilusl/Ig and Bifidobacterium bifidum , and of conventional yogurt starter 
cultures, Streptococcus thermophilusl/Ig and Lactobacillus bulgaricus , 
was conducted. Yogurt pH was also measured on every sampling 
occasion. Viable S. thermophilusl/Ig counts were generally the 
highest in all three products, ranging from initial values of more 
than 108 colony-forming units per ml and decreasing between 1 and 
2 log cycles during the storage period. Initial levels of L. bulgaricusl/Ig
in most samples were higher than 107 cfu/ml and final levels more 
than 106 cfu/ml. In all of the products, L. acidophilusl/Ig complied with 
the criterion of 106 cells per ml yogurt on day 1, but only half of the 
samples contained more than 106 cfu/ml on the expiry date. The 
initial counts of B. bifiduml/Ig in all three products were lower than 
106 cfu/ml and therefore never met the proposed criterion of a 
'therapeutic minimum', suggesting either a low inoculation level or 
that these organisms did not attain the required levels during manufacture. 
Indeed, the inclusion of probiotic cultures in South African 
yogurt seems to be little more than a marketing tool. Storage at 
10 o C had little effect on the viability of all organisms despite 
changes in pH. Initial pH values ranged from 3.45 to 4.32 and 
declined to between 3.28 and 4.19 after 31 days. pH played a major 
role in the survival of the bacteria.