| Soy sauce is easy to be contaminated by harmful microorganisms during the production process.Pichia membranaefaciens,a salt and heat resistant film-forming yeast,degrades organic matter in soy sauce and decomposes nutrients,leading to a decrease in product quality.Thermal treatment is the main way to sterilize soy sauce currently,but the high temperature will cause the soy sauce to darken in color,produce a burnt taste,lose amino acids and other nutrients,and form carcinogenic substances such as ethyl carbamate.Therefore,it is important to look for new ways to sterilize soy sauce for the development of the soy sauce industry.Currently,ultrasound is proposed as one of the emerging food preservation technologies.Ultrasound can well maintain the color,aroma and taste of food and minimize the loss of nutrients.The combination of ultrasound and heat(thermoultrasound)can not only improve the inactivation efficacy of ultrasound alone,but also reduce the impact of high temperature on food sensory and physicochemical properties.Potassium sorbate is an antibacterial agent that can extend the shelf life of soy sauce.The inactivation effect and mechanism of thermoultrasound combined with potassium sorbate on P.membranaefaciens in soy sauce were investigated and the quality and storage quality of soy sauce after treatment were also evaluated in this paper.,which provided theoretical and scientific basis for the application of this technology in the soy sauce industry.The main findings of the paper are as follows:(1)When P.membranaefaciens was treated by thermoultrasound(TS)combined with potassium sorbate(PS)for 20 min at 300 W,10/3 s,20+40 k Hz,60°C and 0.1 mg/m L of potassium sorbate,the largest reduction of 5.34 log was achieved.The P.membranaefaciens was treated by heat combined with potassium sorbate(T+PS)at 50,55,60 and 65°C and thermoultrasound combined with potassium sorbate(TS+PS)at 25,40,50,55 and 60°C,respectively,for 0～20 min.The inactivation kinetic curves were fitted by models,and Dose-Response model fitted the curves better than First Order and Weibull models,indicating Dose-Response model could be used to describe the inactivation effect of T+PS and TS+PS on P.membranaefaciens in soy sauce.(2)SEM and TEM observation results showed that the cell surface of P.membranaefaciens wrinkled,cell structure collapsed,cell membrane damaged intracellular contents outflowed and cytoplasm and electron density unevenly distributed after TS+PS treatment.When P.membranaefaciens was treated by TS+PS for 20 min,the OD value of DNA leakage increased from 0.013±0.002 to 0.196±0.002,protein leakage increased from 0.30±0.07 ug/m L to24.09±0.47 ug/m L.TTC-dehydrogenase activity decreased by 57.53%,the mean particle size of Soxhlet decreased from 11939.17 nm to 6221.68 nm,the cell membrane permeability increased significantly,the membrane potential was severely depolarized and the lipid and the protein content of the membrane components decreased.The expression of heat shock protein Hsp82 and Hsp70 genes was down regulated after TS+PS treatment,indicating that the cells were severely damaged and could not maintain their homeostasis.(3)TS+PS treatment had little effect on p H and total acid of soy sauce.Amino acid nitrogen,reducing sugar decreased by 16.4%and 2.4%after TS+PS treatment,while decreased by 20.0%and 4.0%after T90 treatment.The brightness of soy sauce was improved by TS+PS treatment.Protease activity(acidic,neutral and alkaline)decreased after TS+PS treatment,but the lowest protease activity was observed after T90 treatment.T90 and TS+PS treatments resulted in the highest increments in the mid-products and end-products of the Maillard reaction,respectively.The viscosity of the soy sauce increased by 11.57%after TS+PS treatment,while decreased by13.46%after T90 treatment.The types of volatile flavor compounds in the soy sauce changed little before and after TS+PS treatment,but the content of flavor compounds slightly decreased,with the decreases by 28.22%and 14.12%after T90 and TS+PS treatments,respectively.Compared with heat treatment at high temperature,TS+PS treatment could reduce the loss of volatile flavor compounds and maintain the flavor of soy sauce.Meanwhile,a total of 29 major aroma active compounds were identified based on the OAV values of volatile flavor compounds.After treatment,soy sauce was stored at 4℃,25℃and 37℃respectively.With the increasing of storage time,the p H value,amino acid nitrogen and reducing sugar content decreased,and the total acid content of soy sauce increased,and the higher the temperature,the more change.The L~*,a~*and b~*values of soy sauce were significantly affected by the storage temperature,and the higher the temperature,the faster the decrease.However,the storage quality of soy sauce after TS+PS treatment was generally better than that of heat treatment at high temperature. |
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