Study On Ultrasound-assisted Preparation Of Fermented Milk Containing Peptides By Lactobacillus Paracasei

Lactobacillus paracasei（L.paracasei）is a common probiotic with high safety and good physiological effects.Recent years,L.paracasei has been widely used in fermentative production as a fermentative strain of dairy products.Traditional lactobacillus fermented milk is mainly fermented to obtain probiotic-rich product,the functional component is too single,therefore its health promotion effect is limited.In addition,the growth and reproduction cycle of the microorganism are long,the material exchange rate between the fermentation substrate and the microorganism is low,resulting in a low yield and low activity of the fermentation.In recent years,a new kind of functional fermented milk with active physiologically peptides have shown a good market prospect.Meanwhile,some researchers at home and abroad have found that applying appropriate ultrasonic stimulation during the fermentation could increase the fermentation efficiency effectively,speed up the fermentation process and improve the fermentation yield and functional properties.In this study,the skim milk powder is fermented through L.paracasei to obtain a new peptide-containing fermented milk.The fermented milk is stimulated with ultrasound to solve the problems of single fuctional component and low fermentation efficiency of traditional fermentation.The main research contents and results are as follows:（1）The fermentation parameters were optimized based on pH,acidity,peptide contents,viable cell count and 1,1-diphenyl-2-picrylhydrazyl（DPPH）radical scavenging activity of fermented milk.A shaking speed of 180 r/min,the fermentation temperature of 37℃,a fermentation time of 24 h,a skim milk powder concentration of 12%,a glucose concentration of 6%,100 mL distilled water and an inoculum size of 5%（v/v）were found to be the optimal conditions to obtain the highest peptide concentration of 3.77 mg/mL,highest viable cell count of 8.5?10⁷ CFU/mL and highest DPPH radical scavenging activity of 63.12%.（2）The sonication conditions on the L.paracasei seed culture and on the fermentation process were respectively studied.The seed culture of L.paracasei was sonicated during prelogarithmic phase.The optimum ultrasonic treatment conditions were determined through the single factor experiment.The results revealed that the biomass increased by 119.23%compared with control group under the conditions such as frequency 28 kHz,power 100 W/L,ultrasonic pulsed model of on-time 100 s and off-time 10 s,and ultrasonic time 1 hour.In the study of ultrasonic treatment during the fermentation process,single-factor tests and response surface tests were used to optimize and verify the conditions of the culture medium and ultrasonic treatment.The optimal ultrasonic treatment period was at 9 hour’s fermentation of the inoculation,the fermented milk was treated with sonication under the condition of 28kHz,100 W/L,35 min,the skim milk powder concentration was 12%and the glucose concentration of 6%into 100 mL distilled water.After fermentation,the peptide content in fermented milk was 5.90 mg/mL.The ultrasonic peptide conversion rate was 64.23%higher than that of the control group without ultrasound.（3）The effect of ultrasonic treatment on the content of organic acids in fermented milk was studied.When fermented milk was fermented at 9 h,the sonication was performed under the frequency of 28 kHz,ultrasonic pulse duty cycle of on time 100 s,off time 10 s,ultrasonic power of 100 W/L and the ultrasonic time of35 min.The fermentation was continued until 24 h.The content of malic acid,α-ketoglutarate,citric acid and succinic acid in the ultrasonic group was significantly higher than that in the non-ultrasonic group.The increment ofα-ketoglutarate was the highest of 61.65%.The increment of malic acid and acetic acid were 18.48%and16.21%respectively.Two kinds of new flavor substances were detected by GC-MS analysis after sonication.They were 2-methylpropionate and 3,5-ditert-butylphenol.With sonication,the fermented milk could be more aromatic.（4）The mathematical models were employed to study the fermentation kinetics of L.paracasei with and without ultrasonic treatment,including the cell growth kinetic model,the substrate consumption kinetic model and the product formation kinetic model.The growth of the bacteria in both fermentation processes was consistent with Logistic model.The maximum specific growth rate(X_max)in the ultrasonic group was 14.20%higher than that in the non-ultrasonic group.The product production of both processes met the dynamic model proposed by Luedeking and Piret,in which the product constant（β）associated with cell growth in the ultrasonic group model increased by 99.62%compared of the non-ultrasonic group.The substrate consumption of both processes was in accordance with the Luedeking-Piret model.The yield of the substrate cell growth constant(Y_x/s)in the ultrasonic group model was increased by 14.78%compared with the non-ultrasonic group.The research results show that ultrasonic treatment could accelerate cell growth,promote the fermentation reaction and increase the fermentation efficiency.