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Effect of threonine and glycine on acetaldehyde formation in goats' milk yogurt

Summary The effect of threonine on the biochemical activity of yogurt starter was investigated by the addition of L-threonine to cows' and goats' milk in two concentrations (5 and 10mg/100g). A separate experiment was conducted to investigate the effect of free glycine on the formation of acetaldehyde from yogurt starter. Increasing amounts of glycine (1, 2, 3, 4 and 5 mg/100 g) were added to samples of cows' milk. Addition of threonine to milk samples did not influence acid production, total bacterial growth, or the balance between cocci and rods in the yogurts. Raised levels of threonine resulted in increased production of acetaldehyde. This increase was more pronounced in goats' milk than in cows' milk. The production of acetaldehyde in cows' milk without added threonine was significantly greater than in goats' milk without addition of threonine. Addition of threonine did not influence the formation of diacetyl and α-acetolactic acid, whereas production of acetoin in both milks was reduced. No marked difference in the formation of C02 was observed in samples with or without addition of threonine. Addition of free glycine to cows' milk had a marked negative effect on the production of acetaldehyde by yogurt starter. Addition of 3 mg glycine/100 g cows' milk gave approximately the same amount of acetaldehyde as obtained in goats' milk yogurt. It is suggested that the relatively low amount of acetaldehyde usually observed in goats' milk yogurt is caused by a feed-back inhibition of threonine aldolase produced by the relatively high amount of free glycine present in goats' milk.