Considerable loss in viability of probiotics in room temperature could be attributed to increasing cell metabolism and death at higher temperatures (compared to refrigerated storage) as well as to the enhanced antagonistic impact of yogurt bacteria (especially L. delbrueckii ssp. bulgaricus) on probiotic bacteria. Yogurt bacteria can suppress probiotics during yogurt storage via ‘post-acidification’ process (28) which is noticeably intensified in temperatures of more than 5ºC. Within aforementioned process, not only increasing titratable acidity and decreasing pH but also formation of some metabolites such as hydrogen peroxide, short-chain fatty acids and bacteriocins are highly detrimental to probiotic cells (5). In all treatments exposed to room temperature, an increase in titratable acidity to 120.2°D during storage is the main evidence for post-acidification. Mortazavian et al. (29) reported that storage of ABY-type fermented milks at temperatures of more than 5ºC (8°C) led to a domination of L. delbrueckii ssp. bulgaricus and excessive post-acidification by this organism.