| Donkey milk have received increasing attentions since the milk is regarded as a natural high-quality milk source.Donkey milk not only has a chemical composition closer to that of human milk than cow milk,but also has physiological activities such as antioxidation,antitumor,antiinflammatory,antibacterial and immunomodulatory.In order to meet consumers' pursuit of nutritional health,the development of donkey milk and its dairy products has a positive effect.At present,a little information about donkey milk in china was available.Studies in the literature mainly focused on the basic chemical composition and nutrient composition of donkey milk.Little information on the bioavailability of the donkey milk and the feasibility of developing the characteristic dairy products as well as the study on digestive properties of fermented donkey milk during the gastrointestinal digestion was available.In this paper,the processing stability of fresh donkey milk was studied with heating and ultrasonic treatment.Then,the in vitro dynamic digestion model was applied to evaluate the proteins degradation,peptides production,free amino acids release and biological activity of the donkey milk during the gastrointestinal digestion.The feasibility of fermented donkey milk by using three Lactobacillus strains was evaluated in order to develop characteristic dairy products.Finally,the bioavailability and biological activity of fermented donkey milk during the digestion process were further studied with the application of the in vitro dynamic digestion model.The research results are as follows:1.Study on processing stability of fresh donkey milkThe processing stability of fresh donkey milk was studied with heating and ultrasonic treatment respectively.SDS-PAGE,centrifugal precipitation rate,particle size,zeta potential and surface hydrophobicity were determined to study the processing characteristic of donkey milk with heating and ultrasonic treatment.Fresh donkey milk maintained a high stability while lower heating treatments(<100?)were applied.It was concluded that the main protein bands in donkey milk including casein,lactoferrin,lysozyme,a-lactalbumin,P-lactoglobulin were unstable significantly when the heating temperature was higher than 85?.Meanwhile,the centrifugal precipitation rate,particle size and surface hydrophobicity of donkey milk were increased significantly,and the absolute value of zeta potential for donkey milk was reduced significantly(P<0.05).Compared with the heating treatment,no significant efects on the centrifugal precipitation rate,particle size and zeta potential value of donkey milk were observed after ultrasonic treatment(P>0.05).The whey proteins of donkey milk were insensitive to ultrasonic treatment,and the casein bands disappeared completely from the supernatant when the ultrasonic power was higher than 200 W.2.Study on digestive characteristic of donkey milk during the in vitro dynamic bionic gastrointestinal systemThe bioavailability and antioxidant activity of the donkey milk during the gastrointestinal digestion were studied through the in vitro dynamic digestion model which mimicked the in vivo digestion behavior of the food truly.The protein degradation behaviors were comprehensively analyzed from three levels:proteins,peptides and amino acids.At the same time,the cow milk and goat milk in a higher consumption were used for analysis and comparison.In general,the digestion of donkey milk occured mainly during the duodenal phase.There was a limitation on digestion of proteins during gastric digestion.From the SDS-PAGE profiles,the proteolysis of whey proteins was lower than caseins which was degraded significantly at 90 min during the gastric digestion.The whey protein in donkey milk and P-lactoglobulin in cow milk kept resistance to pepsin.On the contrary,the whey proteins of goat milk were more digestible during gastric phase than the other two milks.No protein bands were observed during the intestinal digestion phase,indicating that the proteins degraded completely.Although the peptide release and amino acid production were limited at the gastric level,a significant improvement of peptides and free amino acids contents occurred with the addition of pancreatin and bile.In addition,the highest concentration of free amino acids was detected in donkey milk before and after in vitro digestion.Antioxidant activity of the donkey milk was improved significantly through digestion compared to undigested samples.3.Study on protein proteolysis and rheological characteristic of fermented donkey milkIn order to develop characteristic dairy products,improve flavor,extend the shelf life,three Lactobacillus strains isolated from the laboratory were used to prepare the fermented donkey milk to study the feasibility of fermented donkey milk through proteolysis behaviors and rheological properties during the fermentation process.The cow and goat milk were used for comparison.A higher concentration of peptides content was obtained in donkey milk before or after fermentation compared with cow and goat milk.The changes of peptides before and after fermentation were further analyzed by RP-HPLC.The results showed that there were more peptide peaks in donkey milk in which peptides content was further increased through fermentation.However,it is worth noting that the gel of the fermented donkey milk was very poor with serious precipitation of whey proteins.A series concentrations of whey proteins were added to improve the donkey milk meduim in order to solve the instability of fermented donkey milk.The water holding capacity,acidity,apparent viscisity,elastic modulus G' and viscous modulus G"of fermented donkey milk increased gradually with the addition of whey proteins.The water holding capacity of fermented donkey milk reached 80%when the amount of whey proteins was 3%(m/v).Considering that the excessive whey protein addition reduced the brightness of the products and affected the sensory acceptance,3%(m/v)of the whey proteins was used to prepare for the fermented donkey milk.4.Study on digestive characteristic of fermented donkey milk during the in vitro dynamic bionic gastrointestinal systemFermented donkey milk was prepared by using three Lactobacillus strains with the 3%(m/v)of whey proteins added to improved the donkey milk.The in vitro dynamic digestion model was employed again to evaluate the bioavailability and antioxidant activity of the fermented donkey milk during the gastrointestinal digestion.From the results of SDS-PAGE profiles,it was known that fermentation contributed to the digestion of milk proteins during digestion process.The propeolysis behavior of whey proteins performed in a time dependant manner and the caseins degraded significantly in the early stage of gastric digestion(30 min).In agreement with donkey milk,the peptides and amino acid release of fermented donkey milk were not sensitive to pepsin,however,a significant increasement of peptides and free amino acids content was observed after intestinal digestion.Although a certain decrease of viable bacteria was detected after gastric digestion,the number of viable bacteria was higher than 6 log CFU/mL.A further decrease of viable bacteria about 2-3 log CFU/mL was occurred compared with undigested samples.There were no correlations between different antioxidant capacities.Compared to undigested samples,the antioxidant activity of fermented donkey milk improved at the gastric level,a further improvement of antioxidant activity was observed after the duodenal digestion. |
PDF Full Text Request