The effect of microbial β-galactosidase (EC 3.2.1.23) activity on intestinal lactose digestion was estimated directly by following post-prandial venous plasma galactose concentrations. To avoid superimposing effects of free galactose, as with yogurt, fresh or heat-treated suspensions of mechanically disintegrated kefir grains in kefir, containing lactose but no free galactose, were fed to ten Göttingen minipigs. Each meal contained 101·1 (SEM 0·1) mmol lactose in kefir supplemented by either native or heat-treated kefir grains corresponding to a mean β-galactosidase activity of either 72 (SEM 8) U or zero. Feeding kefir with β-galactosidase activity resulted in a 30% enhancement of the mean post-prandial plasma galactose peak concentration from 33 (SEM 7) to 43 (SEM 12) μmol/l (n 10), as well as in 23% greater mean areas under the galactose-response curves (8·1 (SEM 1·5) v. 6·6 (SEM 1·2) mmol/min per 1) if compared with kefir with heat-treated grains. Both differences were significant (P < 0·05; paired Wilcoxon test by ranks). There was no induction of intestinal β-galactosidase (EC 3.2.1.108) activity or intestinal lactose-hydrolysing bacteria by lactose feeding. These results give direct evidence of an enhanced lactose digestion and absorption in native fermented milk products due to the microbial β-galactosidase activity.

Breath hydrogen excretion was measured in eight lactase (EC 3.2.1. 108)-deficient volunteers ingesting 18 g lactose in the form of milk, yoghurt and heated yoghurt. Total excess hydrogen excretion (area under curve) was significantly lower after yoghurt and heated yoghurt, than after milk: 103 (SE 29), 191 (SE 32), and 439 (SE 69) respectively (P < 0.001). The oro-caecal transit time of fermentable components from yoghurt and heated yoghurt (mainly lactose) was longer than that from milk: 165 (SE 17), 206 (SE 19), v. 103 (SE 19) min (P < 0.01). An intestinal perfusion technique was used in the same subjects after ingestion on two consecutive days of 18 g lactose in yoghurt and heated yoghurt. Significantly less lactose was recovered from the terminal ileum after yoghurt than after heated yoghurt meals: 1740 (SE 260) v. 2825 (SE 461) mg (P < 0.05), and approximately one-fifth of the lactase activity contained in yoghurt reached the terminal ileum. These findings indicate that more than 90% of the lactose in yoghurt is digested in the small intestine of lactase-deficient subjects and suggest that both the lactase activity contained in the viable starter culture and a slow oro–caecal transit time are responsible for this excellent absorption.