Protein Changes And Bioavailability Of UHT Milk During Storage

Ultra-high temperature sterilized(UHT)milk has the characteristics of long shelf life and does not need cold chain logistics,but its product quality is still affected by the number of microbial cells in raw milk during storage.Centrifugal presterilization technology is a kind of technology that first uses high-speed centrifugation to remove 90 % of microorganisms and somatic cells in raw milk and then carries out UHT sterilization.This technology has a positive effect on improving the quality of UHT milk during shelf life and nutritional quality.In this study,the changes of physicochemical properties,composition and structure of milk proteins of normal UHT(N-UHT)milk and centrifugal pre-sterilized UHT(C-UHT)milk during storage were investigated.The digestive characteristics of milk protein,the composition and peptide mapping of digestive products were studied by dynamic human gastric digestive system and simulated intestinal fluid.The effects of centrifugal sterilization technology on the properties and protein bioavailability of UHT milk were explored.Through the experiment,the following conclusions are drawn during storage,the color of UHT milk became darker,particle size increased,absolute value of Zeta potential decreased,viscosity and acidity increased,and fibrinolytic protein activity increased.The stability of latex colloid decreased,physical and chemical stability of C-UHT milk were better than those of N-UHT milk.Heat treatment increased the content of free a mino acids,but the total content of free a mino acids did not change significantly in UHT milk during storage.After storage for 6 months,the non-protein nitrogen of N-UHT and C-UHT increased from 4.2 % to 8.7 % and 7.9 %,respectively.Thermal treatment resulted in the degradation of all milk proteins and depolymerization of casein into whey phase,and the degradation and depolymerization of casein continued during storage.After storage for 6 months,total protein in N-UHT and C-UHT milk was degraded from 3.92g/L to 29.57g/L and 31.42g/L,respectively.The distribution of casein in micellar phase decreased from 100 % to 53.4 % and 58.2 %,respectively.And The degradation rates of casein were 28.6 % and 22.6 %,respectively.The degradation rates of whey protein were 8.5 % and 11.3 %,respectively.The degree of protein degradation and phase transfer of CUHT was lower than that of N-UHT.In the dynamic digestive system of human stomach,milk proteins were hydrolyzed into polypeptides and a small amount of a mino acids.The degree of hydrolysis of milk proteins was: UHT-6m>UHT-3m>UHT-D0>raw milk.The UHT milk was completely digested in the simulated intestinal fluid for 30 min.After 120 min of intestinal digestion,a mino acids(? 30 %)and peptides(?70 %)were obtained.And there was no significant difference between N-UHT and C-UHT milk in the content of digestive products in gastrointestinal tract.Peptides in the gastrointestinal tract were mainly derived from casein.In the simulated gastric digestive fluid,4554 and 5321 peptides were detected in N-UHT and C-UHT milk,respectively,of which 3537 were identical and 53 and 59 were bioactive peptides,respectively;In the simulated intestinal digestive fluid,the total peptide number of N-UHT and C-UHT milk were 3897 and 3968,respectively,including 3006 identical peptides and 41 and 36 bioactive peptides,respectively.The bioactive peptide abundance of C-UHT was the highest in the simulated gastric fluid.In addition,the bioactive peptide abundance of N-UHT was the highest in simulated intestinal fluid.