Accelerating Vinegar Ageing By Combination Of Ultrasonic Field And Freeze

Newly brewed vinegar has an harsh taste and incongruous smell and needs to be aged to promote maturation.However,the traditional vinegar natural ageing process has a long production cycle,occupies many containers and requires a lot of labour input,resulting in low production efficiency and high labour cost,which seriously restricts the expansion of production scale.Therefore,there is an urgent need to develop a new ageing technology to solve this technical bottleneck.In this study,the raw material of freshly made Zhenjiang vinegar was used to optimize the ultrasonic ageing process by using the characteristic aroma components as the response value,and the free radical related indexes were measured to investigate the mechanism of ageing vinegar by free radical reaction;the freeze concentration process was optimized by simulating the"ice harvest"process of Shanxi vinegar,and the ageing effect of ultrasonic,freeze concentration and their synergistic treatment was compared by the changes of volatile components,physicochemical properties,organic acids,colour and particle size distribution;the quality of co-treated vinegar and vintage vinegar were compared,and the ageing effect of synergistic treatment was comprehensively evaluated based on PCA analysis.The main conclusions of this paper are as follows:（1）A superior ultrasonic ageing process was obtained and the mechanism of ultrasonic ageing was preliminarily analysed.The optimum ultrasonic conditions were:ultrasonic time 60 min,ultrasonic temperature 50°C and ultrasonic power density 15W/100 m L.Under these conditions,the cumulative OAV value of the characteristic aroma increased from 478.64 to 532.56.The different ultrasonic conditions had a greater effect on the cavitation yield,hydroxyl radicals and superoxide anion radicals,and a smaller effect on the degree of hydrogen bonding.The cavitation yield,hydroxyl radicals and superoxide radicals were significantly correlated with the content of characteristic aroma components（p<0.05/0.01）,suggesting that free radicals are responsible for accelerating the ageing of vinegar.A comparison of the aroma components of the four model vinegar of diacetyl-NH₄⁺,acetoin NH₄⁺,acetic acid-phenylethanol and acetic acid-ethanol,both naturally placed and ultrasonicated,revealed that ultrasonic increased the rate of chemical reactions in the model vinegar,promoting the occurrence of the Maillard and esterification reactions to achieve the effect of natural placement for a period of time.（2）A superior freeze condition was obtained and the ageing effects of ultrasonic,freeze and synergistic treatment were compared.The freezing loss rate,physical and chemical properties and volatile components were determined and the optimum freeze conditions were:freeze temperature-45°C,freeze time 30 min and freeze volume100 m L.Under this condition,the loss rate of vinegar was small,the total acid,total ester,reducing sugar and soluble solids content increased by 16.64%,10.38%,10.93%and 6.49%respectively compared with the fresh vinegar,and the cumulative value of aroma activity increased from 478.64 to 516.60.A comparison of the volatile components,physicochemical properties,organic acids,colour and particle size distribution of the fresh,ultrasonicated,frozen and co-treated vinegar showed that the synergistic treatment was more effective than the ultrasonicated and frozen treatments alone in terms of ageing.The total characteristic aroma activity values of the co-treated vinegar were higher than those of the ultrasonicated and frozen vinegar,with an increase of 8.12%compared to the fresh.The electronic nose results showed that the response values of W1C,W3C,W1S and W2S sensors were higher after synergistic treatment than ultrasonic and freeze concentration alone,and the aromatic compound content increased.Total esters,soluble solids and viscosity were higher in the co-treated group than in the ultrasonicated and frozen alone treatments,with increases of 21.23%,7.95%and 29.29%respectively compared to the fresh vinegar.Total acid,reducing sugars,amino acid nitrogen,total phenols and total flavonoids were between the ultrasonicated and frozen treatments,with increases of 9.11%,5.05%,6.34%,10.89%and 11.22%,respectively,compared to the fresh vinegar.Oxalic,lactic,α-ketoglutaric,citric and succinic acid contents were higher in the synergistic treatment group than in the single treatment group,with oxalic,tartaric,malic,pyruvic,lactic,acetic,α-ketoglutaric,citric and succinic acid contents increasing by 30.79%,12.33%,11.19%,12.16%,8.70%,6.99%,73.42%,104.81%and 47.22%.After the synergistic treatment,the L*and a*values of the vinegar samples decreased,the colour darkened slightly,the particle size of the vinegar bodies decreased and became more concentrated,and the quality of the vinegar was improved.（3）The variation of the quality in vintage vinegar during ageing was analysed,and the quality of the vinegar was comprehensively evaluated by PCA analysis for ultrasonic synergistic freezing.The GC-MS results showed that the content of volatile acids,alcohols,esters,aldehydes,ketones and heterocyclic substances increased and then decreased with increasing ageing year,with the most significant changes in the type and content of heterocyclic substances.The results of the electronic nose combined with PCA showed that the volatile composition of the co-treated vinegar was close to that of vinegar aged naturally for 2 years.The physicochemical properties combined with PCA results showed that the physicochemical quality of the co-treated vinegar gradually approached that of the naturally aged vinegar,with the total acid,soluble solids,amino acid nitrogen,total phenols and total flavonoids content between fresh vinegar and 2-year-old vinegar,the total ester content and viscosity slightly higher than that of 2-year-old vinegar and the reducing sugar content slightly higher than that of fresh vinegar.The total organic acid content of the co-treated vinegars was 9.78%higher than that of the fresh vinegar,and most of the organic acid content changed in the same pattern as that of the natural ageing.The organic acids combined with the PCA results showed that the organic acids in the co-treated vinegar gradually moved closer to the naturally aged vinegars,with oxalic,lactic,α-ketoglutaric and citric acid contents between 2 and 4 years old,and tartaric,malic,pyruvic and acetic acid contents between fresh and 2 years old.The colour of the vinegar deepened after the synergistic treatment,the L*values were close to those of naturally aged 2-year-old vinegar,the particle diameter of the vinegar decreased and the vinegar system became homogeneous and stable,with a particle size distribution close to that of a vinegar aged for many years.The results of the sensory evaluation showed that the ultrasonic co-chilling treatment significantly improved the sensory quality of the vinegar and the sensory score was second only to that of the 4year old vinegar.