Probiotic microorganisms are common to be ingested through dairy products, mainly fermented milk products (5, 6). Bifidobacterium spp. and Lactobacillus acidophilus are by far the most important probiotics regularly added to the fermented milks (18). Among the dairy-fermented products, yogurt is the most popular one, and in Europe, the highest consumption of probiotic products is associated with probiotic yogurt (19).
Probiotics should be alive to an adequate number in order to exert their positive effects on the health of the host. This attribute is known as ‘viability’, namely the adequate number of live probiotic cells in a food product at the time of consumption (5). No general agreement has been reached on the recommended levels and the suggested levels ranged from 106 cfu mL-1 (20) to over 107 and 108 cfu mL-1 (5). However, it is generally recommended that the probiotic culture must be present in the product at minimum numbers of 107 cfu mL-1 (21). These suggestions have been made to compensate for the possible decline in the concentration of the probiotic organisms during processing and storage of a probiotic product as well as passage through the upper and lower parts of the gastrointestinal tract. Numerous studies have demonstrated that probiotic strains grow poorly in milk, resulting in low final concentrations in yogurt and even the loss of the viability during prolonged and/or inappropriate storage conditions. Survival of these bacteria during shelf life and until consumption is therefore an important issue.