Effect of Elevated Temperature Storage on the Digestible Reactive Lysine Content of Unhydrolyzed- and Hydrolyzed-Lactose Milk-Based Products

The study aimed to evaluate the effects of storage at elevated temperatures on reactive Lys content and true ileal reactive Lys digestibility in a skim milk powder and a hydrolyzed-lactose skim milk powder. A validated bioassay based on guanidination of food and digesta samples was applied. Semisynthetic diets containing the milk powders as the sole sources of protein were formulated and fed to growing rats. Chromic oxide was included in each diet as an indigestible marker. Digesta were collected posteuthanasia and analyzed along with the diets for reactive Lys (homoarginine), and true ileal reactive Lys digestibility was calculated after correction for endogenous Lys loss. For the skim milk powder, there was no decrease in reactive Lys digestibility (Lys availability) when the powder was stored at 30 and 35°C for 18 mo. In contrast, when stored at 40°C for 12 mo, a small but statistically significant (6%) decrease was observed. For the hydrolyzed-lactose product, a 22% decrease in Lys availability was observed after storage at 35°C for 18 mo, and a 17% decrease was observed when stored at 40°C for only 6 mo. Digestible reactive (available) Lys content decreased by more than 20% for the skim milk powder stored at 30 and 35°C for 18 mo and 40% when stored at 40°C for 12 mo. Furthermore, available Lys decreased in the hydrolyzed-lactose skim milk powder by 41% when stored at 30°C for only 18 mo and 34 and 65% when stored at 35 and 40°C, respectively, for 6 mo. Elevated temperatures and prolonged storage periods negatively influenced the available Lys contents of both milk powders. The decrease in available Lys content and Lys availability was greater for the hydrolyzed-lactose skim milk powder compared with the normal skim milk powder, after storage at elevated temperatures.