Study On The Generation And Control Of Carbonylated Proteins During The Processing Of Goat Dairy Products

Raw milk contains unsaturated fatty acids,endogenous oxidative enzymes,metal catalysts and other components that are susceptible to the generation of free radicals and reactive oxidation products during processing.The protein backbone is vulnerable to oxidation by these free radicals,which ultimately affects the organoleptic and nutritional quality of dairy products.Malondialdehyde(MDA)is one of the most important reactive carbonyl compounds in food.The accumulation of carbonylated proteins can lead to changes in the nutritional quality of protein-based foods,particularly in their digestive properties.Sheep milk and its related products are of increasing interest due to their easy digestibility and absorption and low allergenicity.However,the formation of carbonyl compounds during the processing of sheep milk and their relationship with the digestive properties of proteins are not well understood.Therefore,in this study,the relationship between the production of carbonylated proteins and digestive properties was investigated using an in vitro simulated digestion model in a simulated MDA oxidation system,based on the study that there are significant changes in the oxidation and digestive properties of proteins during the processing of sheep dairy products.On this basis,the effect of yam with antioxidant activity as a functional thickening agent on the quality and carbonyl compound production of fermented goat milk was evaluated,thus providing a reference for further improving the safety and nutritional quality of goat milk products.The main studies and results are as follows:(1)The carbonyl content,sulfhydryl content,secondary structure,surface hydrophobicity,particle size,ζ-potential,digestibility,pepsin digestion process and other indicators were used to study the formation of carbonyl compounds and the change pattern of protein digestion characteristics during the production of goat milk powder.The results showed that both homogenisation and sterilisation treatments increased the carbonyl content of sheep milk protein from 12.71 nmol/mg to 14.89 nmol/mg and decreased the sulfhydryl content from 2.09 nmol/mg to 1.71 nmol/mg.The malondialdehyde content was 0.08-0.12mmol/L.These treatments also had significant effects on the digestive properties of sheep milk protein.The digestibility of the concentrated sheep milk protein was 93.08% with a longest half-life of 16.10 min,while the highest digestibility of 98.78% and shortest half-life of 9.05 min were obtained after spray drying.(2)In order to clarify the relationship between the production of carbonyl compounds and the digestive characteristics of milk proteins,a simulated system of malondialdehyde generation by TMP was used to characterize the production of carbonyl compounds and the changes of protein structure by indicators such as carbonyl content,sulfhydryl content,surface hydrophobicity,ζ-potential,particle size,secondary structure content,digestibility and pepsin digestion process.The Pearson correlation analysis was carried out to characterize the amount of carbonyl compounds produced and the changes in protein structure.The results showed that when the concentration of malondialdehyde increased from 0 mmol/L to 6 mmol/L,the carbonyl content of sheep whey protein increased from24.71 nmol/mg to 207.90 nmol/mg;the sulfhydryl content decreased from 2.88 nmol/mg to1.46 nmol/mg,and the digestibility of sheep whey protein decreased from 91.38% to 75.66%.The correlation coefficient between the amount of carbonyl compounds produced and the protein digestibility was-0.967.(3)In order to control the amount of carbonylated protein production during heat treatment and to improve the poor coagulation of fermented goat milk,yams containing antioxidant components were used as oxidants and thickeners to investigate their effects on the carbonyl compound content and quality of fermented goat milk during processing.The results showed that the oxidation of proteins during processing reduced the texture and water retention of fermented goat’s milk.Phenolic substances contained in yams were effective in reducing the carbonyl content during heat treatment and fermentation in a dose-response relationship.The quality of the fermented goat milk was assessed using p H,titratable acidity,texture,lactic acid bacteria viable count and GC/MS.3% of yam was added to significantly improve the texture and lactic acid bacteria viable count of the fermented goat milk and to reduce the p H of the fermented goat milk.The above results indicate that the addition of 3%yam prior to fermentation is an effective method for producing functional fermented goat’s milk,not only to reduce the production of carbonyl compounds,but also to improve the texture of coagulated fermented goat’s milk and to promote the production of characteristic flavour components.