| Soybean paste is a kind of condiment with unique flavor,which is usually made of soybean,grain and other raw materials and fermented by fungi in the air or Aspergillus oryzae or Aspergillus sojae,yeast and Lactobacillus.Pollution in soybean paste production is the main source of hidden dangers in soybean paste.Pathogenic Escherichia coli can be parasitic in the intestines of humans and animals,which has strong resistance to acid,low temperature and other adverse environment.Pichia membranaefaciens is salt-tolerant and can degrade organic matter in food tissues and decompose nutrients such as lactic acid and other esters,leading to product quality degradation.Pasteurization is commonly used to extend the shelf life of soybean paste,but high temperatures can destroy the color and nutritional quality of the soybean paste.At present,the combination of ultrasound and low-temperature heating(thermosonication,TS)has become an alternative technology for pasteurization due to its simple operation and low equipment requirements,which can minimize the physical and chemical quality of food and changes in sensory characteristics caused by thermal processing.In this paper,the lethal law and mechanism of E.coli and P.membranaefaciens in soybean paste under thermosonication were studied,and the effects of different thermosonication conditions on the quality of soybean paste were discussed.The effect of thermosonication on the storage quality of soybean paste at different storage temperatures was further researched,which provides a reference for the application of thermosonication to the soybean paste industry.The main findings are as follows,(1)The lethal law of E.coli and P.membranaefaciens in soybean paste under thermosonication treatment: Thermosonication(TS,600 W,20+60 kHz,10/5 s,25,40,45,50,55,60 ?)for 0 ? 40 min was employed to inactivate E.coli in soybean paste and the inactivation date of E.coli after TS treatment had a high goodness of fit with Weibull model.Results showed a 5-log reduction was achieved in 35.09 min by T60?,33.57 min by TS50?,12.02 min by TS55? and 5 min by TS60?.Thermosonication(TS,600 W,20+40 kHz,10/3 s,25,40,50,55,60 ?)for 0 ? 20 min was employed to inactivate P.membranaefaciens in soybean paste and the inactivation date of P.membranaefaciens after TS treatment was fitted by the Weibull model.The results showed that 5-log reduction was achieved in 18.61 min by T65?,15.41 min by TS50?,7.49 min by TS55? and 2.27 min by TS60?.(2)Inactivation mechanism of E.coli and P.membranaefaciens by thermosonication: After observation by scanning electron microscopy and transmission electron microscopy,it was found that the cell morphology was changed and the cell structure was destroyed.After TS treament,it was concluded that the protein leakage of E.coli and P.membranaefaciens increased to 61.48 ?g/mL and 34.72 ?g/mL;DRA decreased by 61.62% and 75.77%;the average particle sizes were reduced from 1348.09 nm and 6.522 ?m to 865.01 nm and 4.491 ?m,respectively;the fluorescence intensity of FDA and Rh123 had the largest decrease,resulting in a decrease in cell membrane voltage and an increase in cell membrane permeability.It was further explained that it was due to the physical destruction of the cell structure,the leakage of cell contents and the inhibition of cell respiration eventually led to the death of microorganisms.(3)Effects of thermosonication on the physicochemical and sensory qualities of soybean paste: Compared with thermal treatment,TS had little effect on the color,texture,rheology,pH,total acid,amino acid nitrogen,reducing sugar and flavor substances of soybean paste as a whole and would not significantly change the physical and chemical characteristics of food.Especially,after 50 ? and 55 ? coupled ultrasonic treatment(600 W,20+60 kHz,10/5 s)for 16 min,the content of reducing sugar in soybean paste increased to 5.383% and 5.443%.Nine major aromatic active compounds were detected from soybean paste,including acetic acid,benzaldehyde,benzene acetaldehyde,benzoic acid,ethyl ester,linoleic acid ethyl ester,benzeneacetic acid,ethyl ester,phenethyl alcohol,and 2-methoxy-4-vinylphenol and hexadecanoic acid,ethyl ester.When the ultrasonic conditions were 600 W,20 + 60 kHz,10/5 s,the total amount of ester compounds after TS50?,TS55?,TS60? treatment for 16 min increased from 19.09% to 23.22%,22.38% and 19.79% and the total amount of aldehyde compounds dropped from 22.53% to 21.34%,21.93% and 21.92%,respectively.After heat treatment,although the types of aromatic compounds increased,the total amount decreased.However,after the low-temperature(50 ?)coupled ultrasonic treatment(600 W,20+40 kHz,10/3 s)for 3 min,the types and contents of aromatic compounds were increased,of which the total amount increased from 86.69% to 90.76%.It was showed that TS treatment could minimize the loss of volatile substances in the food and maintain the original aroma components of the food,and even promote the aroma of soybean paste.(4)Effect of thermosonication on storage properties of soybean paste: It was found that the temperature had a significant effect on the storage quality of soybean paste.The higher the temperature,the faster the decline.Combined with low temperature storage(4 ?)after thermosonication,the physicochemical properties and sensory qualities of soybean paste(color,pH,total acid,reducing sugar,amino acid nitrogen,aroma)could be well maintained.Compared with thermal treatment,thermosonication was more effective in maintaining the overall quality of soybean paste at the storage temperature of 4 ? and 25 ?. |
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