Viable starter culture, beta-galactosidase activity, and lactose in duodenum after yogurt ingestion in lactase-deficient humans.

Ten lactose malabsorbers were intubated and given fresh or heated yogurt to which polyethylene-glycol (PEG) and spores of Bacillus stearothermophilus (SBS) had been added as internal standards. In duodenal samples taken after fresh yogurt ingestion, viable starter culture was detected for 60 min in 6 of 7 subjects and the ratio of microbial beta-galactosidase activity to SBS remained similar during this period to its value in the preingested yogurt. In the two groups ingesting fresh and heated yogurt respectively, ratios of lactose to PEG remained similar to preingested values for 90 min and duodenal pH remained less than 5.1. In vitro, at pH 5.0, beta-galactosidase activity in yogurt dropped by 80%. These data clearly show that after fresh yogurt ingestion, viable starter culture reaches the duodenum and contains beta-galactosidase activity. However, the buffering capacity of the yogurt that protects bacteria from acidic gastric secretion also prevents microbial beta-galactosidase from hydrolyzing lactose in the duodenum.