Effects Of Ohmic Heating On Urease Activity And Metal Ion Release From Electrodes

Heating of food materials which directly relates to the final quality of products, energyconsuming during processing and production cost, is an important operation in foodprocessing. Ohmic heating of food materials is a new technology in food engineering. Inohmic heating process, foods serve as a part of an electric circuit through which alternatingcurrent flows, causing heat to be generated within the foods due to the electrical resistance ofthe foods. Ohmic heating technique research began relatively early in abroad, and continuousohmic heating apparatus are being used successfully in USA, UK, and Japan. Up to now, theresearch of ohmic heating technique in our country is a beginning state, and there are noohmic heating devices introduced. And the effect of ohmic heating on food quality is notstudied. Therefore, it is significant to study the effects of ohmic heating on food anti-nutritionand metal release from the electrodes for the better development of ohmic heating.Two aspects are studied in this paper. The first is to study the effects of ohmic heating onthe soymilk urease activity using self-made ohmic heating setup. The second is to study theeffects of ohmic heating on the metal release from stainless steel electrodes.(1) The quantitative determination conditions of urease activity was optimised. Theresults showed that the determination results were stable and repeatable when the detectionwavelength was379nm, reaction temperature40°C, reaction time20min, system pH7,reaction terminator10%sodium tungstate and5%sulfuric acid and substrate4%urea.(2) The urease activity of conventional heated soymilk was quantitative detected usingthe optimized conditions and qualitative detected using the method of QBT2132-2008respectively. The results showed that the urease activity was decreased with extend of holdingtime and increase of ending temperature. The urease was negative when the activity value waslower than16.79IU.(3) The urease activity of ohmic heated soymilk was quantitative detected using theoptimized conditions. The results showed that ohmic heating could also inactivate the ureaseand the urease activity value of ohmic heated soymilk was lower than that of conventional heated soymilk at the same ending temperature and holding time. The urease activity valuewas the lowest when the ending temperature was100℃，holding time15min，heatingvoltage220V，heating frequency5kHz.(4) Fe, Cr, Mn, Ni contents of ohmic heated soymilk were measured using ICP. Theresults showed that the Cr was not released from stainless steel electrodes because itselectrolytic electromotive force was not reached when the heating voltage was160-220V，heating frequency50-10000Hz, ending temperature85-100℃，holding time0-15min. ButFe, Mn, Ni were released from the stainless steel electrodes to soymilk. And their migrationincreased with the increase of heating voltage and ending temperature, decreased with theincrease of frequency and extend of holding time.(5) The best combinations of making the metals release lowest were obtained through thesample range analysis of orthogonal experimental results. And the best conditions, whichwere100℃of ending temperature，holding for15min，heating voltage220V，and heatingfrequency5kHz, were obtained combining with the effects of ohmic heating on ureaseactivity.