| Ginkgo kernel is the seed of Ginkgo biloba L.It is a complex matrix that contains starch,crud protein,terpene lactones and ginkgolic acids.Because ginkgo kernel has some toxic side effects,it is now facing unsalable problem.Lactic acid fermentation can not only improve the flavor and nutritional value of food,but also eliminate the anti-nutritional factors and prolong the shelf life.Study on the effect of lactic acid fermentation on nutritional quality and food safety of ginkgo kernel juice is important to explore new ginkgo kernel processing ways.In recent years,the impact of ultrasound on microbial fermentation has attracted great attention.The results showed low-frequency ultrasound treatment could promote the growth of microorganisms and the yield of target fermentation products,thus improving the fermentation efficiency.Therefore,the fermentation characteristics of lactic acid bacteria in ginkgo kernel juice,and the effects of lactic acid fermentation on the bioactive components and physiological activities of ginkgo kernel juice were studied.Then,the impact of lowfrequency and low-intensity ultrasound treatment on metabolism of lactic acid bacteria during ginkgo kernel juice fermentation was investigated.Moreover,a lactic acid fermented ginkgo kernel beverage was developed.The results are shown as follows:1.The growth characteristics of Lactobacillus acidophilus,Lactobacillus plantarum and Lactobacillus casei in ginkgo kernel juice and their metabolism of organic acids,sugars,soluble proteins,amino acids and volatile flavor compounds were studied.The results showed that three lactic acid bacteria grew well in ginkgo kernel juice,and the maximum viable cell counts could reach 8.67±0.04,8.71±0.06 and 8.58±0.08 Log CFU/mL,respectively.All stains can make full use of sugars to yield acids,so that the pH value of juice decreased significantly(p<0.05).Glucose was the major carbon source used most by lactic acid bacteria and its utilization ratio was about 90%.Together with the continuous increase in lactic acid content,the contents of other organic acids were in a dynamic change.The total amino acids content decreased by 32%at the end of fermentation.The types of volatile flavor compounds were enriched after lactic acid fermentation.The relative content of alcohols,acids,esters,ketones and hydrocarbons increased after fermentation,whereas the relative content of aldehydes decreased.Among three lactic acid bacteria,L.plantarum showed higher growth and metabolic activity.2.The effect of lactic acid fermentation on the main bioactive components,including ginkgolic acids,ginkgolides and phenolics,as well as antioxidant and antimicrobial activities of ginkgo kernel juice was studied.The results showed that four kinds of ginkgolic acid contents decreased significantly(p<0.05),while three kinds of ginkgolides contents increased gradually during lactic acid fermentation.After 48 h of fermentation,the total ginkgolic acids content in ginkgo kernel juice fermented by L.acidophilus,L.plantarum and L.casei decreased from 3.12±0.07 to 0.72±0.05,0.76±0.08 and 0.691±0.06 mg/L,respectively.While the total ginkgolides content was 1.66,1.60 and 1.64-fold higher than that in unfermented juice,respectively.Under the action of lactic acid bacteria,the phenolic compounds were degraded,synthesized and transformed.The total phenolic content was increased by 9.03%?8.27%and 8.10%after 48 h of fermentation by L.acidophilus,L.plantarum and L.casei,respectively.In addition,the antioxidant and antimicrobial activities of ginkgo kernel juice were both enhanced through lactic acid fermentation.Totally,ginkgo kernel juice fermented by lactic acid bacteria has higher nutritional quality and food safety.3.According to the aforementioned results,L.plantarum was selected as the fermentation strain due to its higher growth and metabolism activity in ginkgo kernel juice.The effect of low frequency(20 kHz)ultrasound at different intensities(58.9,98.6 and 120.8 W/L,respectively)on the metabolism of L.plantarum during ginkgo kernel juice fermentation was studied.The results showed that three intensities of ultrasound treatment could promote the growth of L.plantarum,and significantly(p<0.05)increased the viable cell counts in the stationary period,thus delaying the time of entering the declining period.In the early stage of fermentation,low-frequency ultrasound treatment promoted L.plantarum to utilize sugar,increase lactic acid yield and metabolize amino acids.However,in the later stage,the effect of ultrasound was weakened.In addition,low-frequency and low-intensity ultrasound treatment played a positive role in the degradation of ginkgolic acids and the enrichment of ginkgolides and phenolic compounds by L.plantarum.In the end of fermentation,the total ginkgolic acids content in 58.9,98.6 and 120.8 W/L ultrasound groups was 9.01%,8.10%and 15.18%lower than that in the control groups.While the total ginkgolides content was 0.10%,4.01%and 2.67%higher than that in the control groups,and the total phenolic content was 0.54%?0.27%and 0.68%higher than that in the control groups.After 48 h of fermentation,the ABTS free radical scavenging capacities of 58.9,98.6 and 120.8 W/L ultrasound groups were 0.82%,0.70%and 1.87%higher than that of the control groups.But there was no significant difference(p>0.05)in ferric reducing antioxidant power between the ultrasound groups and the control groups.Totally,low-frequency ultrasound treatment has some positive impacts on the metabolism of L.plantarum during ginkgo kernel juice fermentation.4.A lactic acid fermented ginkgo kernel beverage mixed with seabuckthorn juice,pear juice,sweetener mixture and stabilizer was developed by single factor and orthogonal test.The quality changes during storage were studied.The optimum formula of lactic acid fermented ginkgo kernel beverage was listed as follows:fermented ginkgo kernel juice 40%,seabuckthorn juice and pear juice at a ratio of 1:1,sweetener mixture 10%.The optimum addition of the stabilizer blends was CMC-Na 0.10%,guar gum 0.08%,xanthan gum 0.03%.The beverage mixture prepared according to the above formula had bright color,soft taste,harmonious odor and uniform stability.The pH value and soluble solid content of the beverage decreased during storage,but the overall changes were not significant.The content of vitamin C decreased by 70%.The total phenolic content and antioxidant activity first decreased and then increased.The juice was microbio logically safe during storage.But in the later period of storage,the beverage showed slight browning and precipitation,resulting in the decline of sensory quality. |
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