Effects Of Ohmic Heating On The Sterilization And Quality Of Soymilk

Soymilk is widely accepted in Asian countries as a traditional protein beverage with bountiful nutrition.The quality of soymilk is greatly affected by boiling,which is an essential and important link in the process of conventional soybean products manufacturing.Conventional boiling takes high-temperature steam as heat source which is directly or indirectly injected into raw soymilk.However,in the direct injection method,large numbers of bubbles and local overheating are two major reasons which could induce the decrease of soymilk quality.The utilization of heat energy is also affected significantly by the adhesion on heat-transfer surface in the indirect injection method.With the improvement of people’s living standards,much more attentions are paid to the quality of soymilk.Conventional boiling technics cannot meet people’s requirements for the high-quality soymilk gradually due to its disadvantages.Ohmic heating,defined as a novel volumetric heating technology of food processing,has many advantages such as homogeneous,none heat-transfer surface,easy to be controlled and environmentally friendly.It also been considered as one of the most immense potential of heat treatment technologies in the future.Researches on ohmic heating started early in aboard,especially in the U.S.,UK and Japan.The continuous ohmic heating apparatus have been commercialized and popularized in food processing industry already.However,researches on ohmic heating in domestic are still in initial stage,and there are no more mature commercial applications yet.The application of ohmic heating in food processing is also needs further researches.The purpose of this research is to provide theoretical guidance for the application of ohmic heating in soymilk processing industry.With soymilk as the main raw material,this paper mainly studied the application of ohmic heating in the soymilk processing composed of three parts.The effect of ohmic heating on the flavor compounds of soymilk,the contents of phytic acid and the microbial community in the soymilk were studied in the first,second and last part respectively.The main conclusions of this research were as follows:1.To investigate the effect of ohmic heating on the flavor compounds of soymilk,the headspace solid-phase micro-extraction（HS-SPME）coupled with gas chromatography-mass spectrometric（GC-MS）was employed to analysis the volatile substances.The condition of HS-SPME on volatile substances of soymilk was also optimized.It was found that the optimum extraction fiber of HS-SPME was 65μm PDMS/DVB,compared to the 100μm PDMS and 50/30μm DVB/CAR/PDMS.35 kinds of flavor compounds were detected in soybean milk,mainly including aldehydes,alcohols,ketones,esters and heterocyclic hydrocarbons,among which there were 16 kinds of aldehydes,5 kinds of alcohols,3 kinds of ketones,1 kind of esters and 10 kinds of heterocyclic hydrocarbons.Hexanal,（E）-2-heptenal,1-octen-3-ol,2-pentyl-furan,（E）-2-octenal,nonanal,（E,E）-2,4-nonadienal,（E）-2-decenal,2,4-decadienal,（E）-2-dodecenal,etc.The beany flavors could be reduced significantly when soymilk was heated in ohmic heating method.2.The extracting technology of phytic acid from soymilk was optimized.Factors of pH,extraction temperature and extraction time which could affect the extracting ratio of phytic acid were evaluated firstly in this research.Based on the single-factor experiments,optimum conditions for the extraction of phytic acid from soymilk were obtained through Box-Behnken center-united experiment design and response surface methodology.The results showed that the optimum conditions for the extraction of phytic acid from soymilk were as follows:taking HCl-Na₂SO₄ solution as extracted fluid,the pH extracted fluid was 2.7,the extraction time temperature was 31.5°C and extraction time was 120.5 min.The extracting ratio of phytic acid from soymilk was 1.32%±0.02%in this condition.Verification test showed that the relative error between predicted value and experimental value was 0.804%<1%.The analysis results fit well with the actual situation.3.The effect of ohmic heating on the content of phytic acid in soymilk was investigated in this research.The results showed that the content of phytic acid in soymilk was a dynamic changing process along with the changing of temperature.It seems that the existing form of phytic acid in soymilk was probably not in free state but in combination with protein（especially the 7S and 11S globulin）.In addition,the electric field might have promotion impact to the denaturation of soybean protein according to the changes of phytic acid content in soymilk at different temperature spots.4.The effect of ohmic heating on the microbial community in soymilk was discussed at last.Factors of heating temperature,voltage and electric field frequency which could affect the bactericidal rate of microbial community in soymilk were evaluated firstly in this paper.Based on the single-factor experiments,optimum electric conditions for the bactericidal rate of microbial community in soymilk were obtained through Box-Behnken center-united experiment design and RSM.The results showed that the optimum electric conditions for bactericidal rate of microbial community in soymilk were as follows:heating temperature of82°C,voltage of 200 V and electric field frequency of 190 Hz.The bactericidal rate of microbial community in soymilk was 100%in this condition.Verification test showed that the relative error between predicted value and experimental value was 0.044%<1%.The analysis results fit well with the actual situation.