F-value Theory In Pulse Electric Field And Its Application On Watermelon Juice Freezing Concentration

Pulse electric field (PEF) is one of the mostly studied technologies in the field of non-thermal food processing. At present, the unified standard for sterilization effect dose not appeared, and it hasn't been reported that two non-thermal processing technologies of pulsed electric field and freeze concentration are improved and integrated together, being applied to the concentration of fruit juice. This paper would focus on the effect of temperature, conductivity and pulse width on Escherichia coli sterilization which induced by PEF. Based on thermal death time (F-value) theory,electric field strength death time (F-value) theory was introduced to establish a unified standard for sterilization effect with Escherichia coli O157:H7 as the target microorganism. Moreover it was applied to freeze concentration-treated watermelon juice. The change of indicators and the sensory evaluation were investigated respectively for freeze concentration-treated watermelon juice, heat-treated watermelon juice and untreated watermelon juice during -18℃storage; The experimental results was as follow:1. With temperature increasing, the killing effect of PEF was improved, but there is no significance. The effect of conductivity was not significant, too; and high conductivity might raise the liquid temperature and increase ineffective energy cost. With studying on the effect of conductivity on sterilization, the paper also researched influence of temperature on conductivity. The milk conductivity could reach 0.48ms/cm with reducing the temperature, but the effect of consistency on milk conductivity was not significant at low temperature. The influence of temperature on conductivity of concentration-treated juice was remarkable. Under low temperature, conductivity of concentration-treated juice was less than 1ms/cm. The state of concentration-treated juice was suitable for PEF processing by changing its conductivity through controlling the temperature. The experiment showed that pulse width had no significant effect on Escherichia coli sterilization at 30 mL/min. 2. The electric field strength death rate E ERZLR = 10 ^(E-E_R)/Z was introduced for the first time. Electric field strength death time (F-value) theory was established, and the lethality was described by the area under the lethal rate curve. Taking 26.7 kV/cm electric field strength as a reference, D26.7 = 136μs, Z (electric field strength resistance constant) = 22.4 kV/cm, the theory death time was calculated under the certain electric field strength. Compared with the actual death time, the theoretic time was close to the experimental one which was longer and according with the actual production.3. With the application of electric field strength death time theory on freeze concentration-trated watermelon juice, the technology of integration of freeze concentration and PEF was applied on watermelon juice at low temperature, and microbial population in the treated juice was conformed to the national standard. The voltage of concentration-treated juice was higher than that of untreated juice, and the current of concentration-treated juice was far lower than that of untreated one when accepted PEF treated. Considering temperature rise, freeze concentration-treated juice was better than untreated juice, which had lower temperature rise, reduced energy cost and achieved better economic benefit. The change of pH, soluble solids were a little for different treated watermelon juice, total sugar content keeps invariant, and VC of all juice was decreased during storage. From the result of sensory evaluation, concentration and PEF-treated juice was closed to untreated juice, heat-treated juice was worst.