| The increasing interest of consumers in functional foods has promoted the research and development of probiotic products.The dry production of probiotic can obtain stable and long shelf life,reduce transportation costs and promote trade,and freeze drying and spray drying are the two main ways to dry probiotics.At present,freeze-drying is considered to be the preferred technology for probiotics drying due to mild conditions and high bacterial survival rate,but it is time-consuming and expensive.Spray-drying is low energy costs and high productivity,however high temperature is still a great challenge for the retention of probiotic activity.In order to reduce the loss of probiotic activity during the drying process,researchers are committed to optimizing strains,improving drying conditions and product formulations.In this study,the Maillard reaction products(MRPs)prepared from whey protein isolate(WPI)-xylooligosaccharides(XOS)were used as the protective carrier,and Lactobacillus rhamnosus CICC 22152 as the research strain to explore the protective effect and mechanism of the carrier on bacteria during drying,storage and digestion.L.rhamnosus was cultured with whey protein hydrolysate(WPH)and determined the growth of bacteria in different media,then probiotic microcapsules were prepared to explore the effect of whey protein hydrolysate as a medium and protective carrier on bacterial cells during drying,storage,in vitro digestion.The research results are as follows:(1)In this work,the effect of ratio(1:1,1:2,1:3,1:4,w/w)on Maillard reaction between XOS-WPI under wet heating conditions and some properties of MRPs were investigated.According to the UV-Vis absorbance value,pH and particle size and infrared spectrum test results,MPIs of WPI and XOS were successfully prepared under the selected conditions.The degree of Maillard reaction was the highest when the mixing ratio was 1:2.Compared to a system that has not undergone Maillard reaction,the average particle size of MRPs was reduced,and the largest value was at the ratio of 1:2.The Maillard reaction improved the emulsification activity and emulsion stability of WPI,the MRPs showed the best emulsification activity(38.63±1.16 m2/g)and emulsion stability(65.23±4.51%)at a ratio of 1:2.Rheological tests showed that Maillard reaction modification enhanced the gelation of WPI,and the storage modulus of MRPs at a ratio of 1:3 increased the most in which G' reached a value of about 97,000 Pa(about seven times the value that obtained in the initial WPI gel).At the same shear rate,the apparent viscosity of MRPs solution increased compared to WPI,while the effect of WPI addition on the viscosity of the system predominates.The above results showed that the rheological properties and emulsifying properties of WPI could be modified upon the Maillard reaction.(2)The MRPs of WPI and XOS were prepared by the moist heat method were used as the dry protection carriers and encapsulated Z.rhamnosus by spray drying,to explore the protective effects of MRPs on bacterial during drying,storage,and in vitro simulated digestion.MRPs have a good thermal protection effect on bacteria,among them,the cell survival rate of microcapsule that the wall material is 1:2 XOS-WPI MRPs was as high as 99.83±8.44%,which is about 2 times the WPI and 1.5 times the XOS-WPI mixture,and there was no significant difference in the growth curve between the L.rhamnosus bacterial after drying and fresh.After 10 weeks of storage at 4?,the bacterial activity in each microcapsule did not decrease by more than 1 log cfu/g,and the microcapsules made of MRPs had better storage stability.At the same time,it was found that XOS in the protection carrier as a carbon source nutrient provided continuous energy for the growth and metabolism of L.rhamnosus cells during storage,thereby probiotics were proliferated.The results of rehydration and in vitro digestion showed that the solubility of MRPs was reduced,and the release of bacteria in gastric juice digestion was reduced and delayed,therefore improved the stability of MRPs microcapsules in the digestive tract.After 4 hours of simulated gastrointestinal digestion,the number of viable bacteria in 1:2 XOS-WPI MRPs microcapsules were still as high as 4.09±0.59×109 cfu/g.(3)For L.rhamnosus,the culture effect of the medium(H/MWHP-m)produced by partially replacing peptone with high/moderate whey protein hydrolysate(H/MWPH)were similar to that of MRS medium,while the dual-use medium(H/MWHP-s)is better than MRS medium,and the bacterial concentration is about twice that of MRS medium.WPH-lactose(Lac)can protect L.rhamnosus when only used as drying carrier,and the cell survival rate after spray drying reached 85.56±7.40%.When H/MWPH-s was double-used as medium and protective carrier,the cell survival rate after freeze-drying was as high as 97.13±9.16%,and the cell survival rate after spray-drying was as low as 0.20±0.018%,it's indicated that H/MWPH-s can be used as cryoprotection agent but has poor thermal protection.The single-droplet drying results showed that the temperature at large number of deaths of L.rhamnosus in H/MWPH-s was below 65?,which may be due to the increase of bacterial heat sensitivity or the reduction of H/MWPH-s protection by cell metabolism.And the study found that the storage stability of MWPH microcapsules was higher than that of HWPH microcapsules,but the decline of bacterial activity in all microcapsules were less than 1 log cfu/g after storage at 4? for 12 weeks.In vitro digestion and rehydration results show that microcapsules were quickly disintegrated and released bacteria during gastrointestinal digestion due to the excellent solubility of wall material,leaded to a large number of bacterial deaths.However,according to the intestinal fluid digestion results,H/MWPH-m can improve the tolerance of L.rhamnosus in the intestinal fluid environment,and the MWPH performs better. |
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