| Selenium?Se?is an essential micronutrient to human metabolism,which could protect body from oxidative stress,modulates growth and development,helps maintain defense against infections and anti-viral activity.Selenium deficiency can cause serious health issues like poor immune response,cognitive decline,Kashine Beck disease and keshan disease.Supplementation of Se has been done with Se-enriched yeast and inorganic Se compounds.Lactobacillus probiotic can convert inorganic selenium into organic selenium.Organic selenium is safer and easier to absorb than inorganic selenium.Supplementing selenium-enriched probiotic Lactobacillus can supplement organic selenium and can also play its probiotic function,However,there are few reports about the preparation of selenium-enriched probiotics.In this study,23 strains of probiotic Lactobacillus preserved in the laboratory were used as test strains,and strains with high tolerance and high conversion rate to inorganic selenium were obtained through preliminary screening and rescreening,and the bacteria species were identified,and then the medium composition and culture conditions of Se-enriched probiotic Lactobacillus were optimized by single factor and responsesurfacemethodology.Thentheeffectofdifferent cryoprotectants on the freeze-drying of this strain was studied,and Se-enriched probiotic powder was obtained.The Se-enriched probiotic goat milk powder and goat milk tablets were developed with the use of the Se-enriched probiotic powder.Finally,the acceleration test was used toexplorethestabilityofSe-enrichedprobioticpowder,selenium-enriched probiotic goat milk powder and tablets.The results obtained are as follows:?1?5 strains of L20,L22,L49,L69,and 8014 were obtained from 23probiotic Lactobacillus by using color of selenium-enriched medium,amount of bacterial sediment,and selenium conversion rate as response.By rescreening of simulating gastrointestinal fluid tolerance,the best Lactobacillus L20 was obtained and identified as Lactobacillus rhamnosus by 16S rDNA.The maximum selenium tolerance of the strain was 7 mg/mL,and the optimal concentration of sodium selenite was 20?g/mL,the conversion rate to inorganic selenium was 88.83%in MRS broth,and the viable counts was 2.34×108 CFU/mL.?2?The growth medium of L.rhamnosus L20 was optimized by single factor and response surface methodology as follows:beef extract powder 2.13%,glucose 4.24%,urea 0.45%,sodium acetate 0.5%,oligo galactose 0.5%and soybean peptone 1.75%,the viable counts,cell dry weight and conversion rate of L.rhamnosus L20 in the optimized medium were 2.19×108 CFU/mL,2.37 g/L and 82.12%,respectively,which were 35.23%,19.40%and 18.01%higher than the control.There was no significant difference between the true and predicted values,indicating that it is feasible to use response surface methodology to optimize the medium composition.?3?Based on the optimized culture medium,the single factor and Central Composite Design were used to optimize the culture conditions of L.rhamnosus L20.The optimal culture conditions were as follows:initial pH 6.5,inoculation volume 2.18%,and culture at 37°C for 16 h.The viable counts and selenium conversion rate of L.rhamnosus L20 were2.43×108 CFU/mL and 93.27%,respectively.They were 12.79%and13.82%higher than the control.?4?The effects of antifreeze factors and 7 compound cryoprotectants on freeze-drying of L.rhamnosus L20 were studied,and the stability of selenium-enriched and non-selenium-enriched bacteria powders was compared.It was found that antifreeze factors can not help improve the freeze-dried survival rate of L.rhamnosus L20 and the viable counts of bacteria powder.The best cryoprotectants was C4,which is composed of5.5%glycine,0.8%sodium bicarbonate,7.0%xylo-oligosaccharides,4.5%arginine,25%skim milk,and 25%phosphate buffer,the freeze-dried viability,the viable counts of freeze-dried powder and the yield of the obtained selenium-enriched L.rhamnosus powder were70.66%,7.29×10100 CFU/g and 5.26 g/L,respectively.The inactivation rate constants of selenium-enriched and non-selenium-enriched L. rhamnosus powders were 2.62×10-6 and 2.24×10-6,respectively,indicating that the stability of selenium-enriched bacteria powder was lower than that of selenium-free bacteria powder.Selenium-enriched L.rhamnosus powder frozen?-18??for 10 years,the viable counts was 4.3×1010CFU/g,indicating that its stability was better.?5?The Se-enriched L.rhamnosus L20 goat milk powder and goat milk tablets were developed by using the prepared bacteria powder,and the stability was studied by the accelerated test.The results showed that the inactivation rate constants of L.rhamnosus L20 in goat milk powder and goat milk tablets were 2.4×10-44 and 1.9×10-4,respectively,namely that the stability of L.rhamnosus L20 in goat milk tablets was higher than that of goat milk powder.At 25°C,the predicted shelf life was 285 and406 days,respectively,with good stability.This study screened and obtained L.rhamnosus L20 with high Se-enriching capacity,prepared active probiotic powder,developed Se-enriched probiotic goat milk powder and tablets,and predicted the shelf life,which could be provide bacterial powder raw materials for the development of Se-enriched probiotic products,and also provide technical support for the subsequent industrialization of Se-enriched probiotic goat milk powder and tablets. |
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