Milk powder: a model matrix for studying food digestion?
Résumé
Foods provide nutrients that are essential for human to allow normal growth and development. Among those, milk is a unique source of minerals but also provides proteins with high nutritional values. More and more evidences have shown that, not only the composition, but also the structure of foods plays a key role on nutrient bioavailability. Milk has the great advantage to allow the manufacture of a wide variety of foods from the same starting material. In the present study, we have used an ultra-low-heat skim milk powder as a starting material to elaborate dairy products with different structures and have investigated how their structure at different scales (molecular to macroscopic) affects protein digestion. Six skim milk powders were manufactured by spray-drying after having submitted concentrated milk with 25% or 35% dry matter to 3 types of heat-treatments traditionally used in industry (80°C/20s, 85°C/180s, 105°C/60s). Those six samples and a reference unheated one were rehydrated at 13% (w/v) and submitted to an in vitro digestion model mimicking the gastrointestinal tract of an infant. Heat-treatment significantly increased casein resistance to digestion. Hydrophobic casein domains and area carrying post-translational modifications were shown to be highly resistant to digestion. In a second experiment, ultra-low heat milk powder was used as a starting material to manufacture dairy products with similar composition but different structures. Liquids with or without heat-treatment and gels (acid, rennet) were submitted in vivo digestion on 6 mini-pigs, effluents (from the duodenum and mid-jejunum) and plasma were collected during 7 h after the meal intake. Results showed that both the micro and macrostructure have a strong impact on the kinetics of milk protein digestion and the release of dietary amino acids in blood. Ultra-low-heat skim milk powder is therefore an excellent starting material to manufacture dairy products and study their nutritional properties.