Location: Dairy and Functional Foods ResearchTitle: Effect of heat and homogenization on in vitro digestion of milk Author
|Ren, Daxi - Zhejiang University|
|Van Hekken, Diane|
Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2016
Publication Date: 6/1/2016
Publication URL: http://handle.nal.usda.gov/10113/62625
Citation: Tunick, M.H., Ren, D.X., Van Hekken, D.L., Bonnaillie, L., Paul, M., Kwoczak, R., Tomasula, P.M. 2016. Effect of heat and homogenization on in vitro digestion of milk. Journal of Dairy Science. 99(6):4124-4139. DOI: 10.3168/jds.2015-10474.
Interpretive Summary: Ingestion of milk results in the breakdown of proteins and fats in the stomach and in the small intestine. The majority of milk consumed in the US is processed using heat (high temperature short time pasteurization (HTST) or ultrahigh temperature pasteurization (UHT)) to ensure the safety of milk and is preceded by homogenization to break the milk fat down into smaller droplets of fat to prevent creaming. Processing induces changes in the proteins and fats but there is little information if these changes affect milk digestibility. In this study, processed whole and skim milk samples were compared to raw milk after simulated gastric and intestinal digestion. Contrary to what most consumers would expect, UHT- homogenized whole milk was found to be more digestible than raw whole milk. UHT and HTST processed skim milk samples were also more digestible than raw skim milk. Several casein phosphopeptides resulting from the gastric and intestinal digestions were also identified but their potential bioactivity in humans has yet to be determined.
Technical Abstract: Central to commercial fluid milk processing is the use of high temperature, short time (HTST) pasteurization to ensure the safety and quality of milk, and homogenization to prevent creaming of fat-containing milk. UHT processed homogenized milk is also available commercially and is typically used to extend the shelf-life of refrigerated specialty milk products or to provide shelf-stable milk. The structures of the milk proteins and lipids are affected by processing but little information is available on the effects of the individual processes or sequences of processes on digestibility. In this study, raw whole milk was subjected to homogenization, HTST pasteurization, and homogenization followed by HTST or UHT processing. Raw skim milk was subjected to the same heating regimens. In vitro gastrointestinal digestion using a fasting model was then used to detect the processing-induced changes in the proteins and lipids. All milk samples showed disappearance of casein, alpha-LA, and minor whey protein bands, persistence of beta-LG, and the appearance of smaller casein and whey peptides with gastric pepsin digestion, with UHT skim the most digestible. The protein band for beta–LG was eliminated within the first 15 min of intestinal pancreatin digestion. The remaining proteins and peptides of raw, HTST, and UHT skim samples were digested rapidly within the first 15 min of intestinal digestion, but intestinal digestion of raw and HTST pasteurized whole milk showed some persistence of the peptides throughout digestion. The availability of more lipid droplets upon homogenization, with greater surface area available for interaction with the peptides, led to persistence of the smaller peptide bands and thus slower intestinal digestion when followed by HTST pasteurization but not by UHT processing, in which the denatured proteins may be more accessible to the digestive enzymes. Homogenization and heat processing also affected the zeta-potential and free fatty acid release during intestinal digestion. Stearic and oleic acids were broken down faster than other fatty acids due to their positions on the outside of the triglyceride molecule. Five different casein phosphopeptide sequences were observed after gastric digestion and 35 sequences were found after intestinal digestion, with activities yet to be explored. Processing affects milk structure and thus digestion and is an important factor to consider in design of foods that impact health and nutrition.