Two Bifidobacterium strains contained in two different fermented milks behave very differently when exposed to an in vitro simulated gastric environment. One strain survives very well during at least 90 min (greater than 10(7)/g), but the second strain studied is much less resistant. These in vitro results, with slight differences, were confirmed by an in vivo study in humans. The assessment of the gastric emptying rate of these products allows an estimation of the amount of Bifidobacterium that may pass into the small intestine.

Lactose in yogurt with live bacteria is better tolerated than lactose in other dairy foods, partly because of the activity of microbial beta-galactosidase (beta-gal), which digests lactose in vivo. To evaluate the ability of different strains and species of lactic acid bacteria to digest lactose in vivo, yogurts (containing mixtures of strains of Streptococcus salivarius subsp thermophilus and Lactobacillus delbrueckii subsp bulgaricus) and fermented milks (containing individual species of S thermophilus, L bulgaricus, L acidophilus, or Bifidobacterium bifidus) that varied in microbial beta-gal activity were produced. Selected products were fed to healthy people who cannot digest lactose, and breath hydrogen production was monitored. All yogurts dramatically and similarly improved lactose digestion, regardless of their total or specific beta-gal activity. The response to fermented milks varied from marginal improvement with B bifidus milk to nearly complete lactose digestion with L bulgaricus milk. The results suggest that total beta-gal was not the limiting factor in promoting lactose digestion, perhaps because of a limited rate of intracellular substrate transport.