抄録
The heat sterilization procedures are widely used in food industrial field. Although these procedures are convenient and effective for the sterilization of microorganisms, they sometimes spoiled products by heating or denature food materials. To solve these problems, on the other hand, the sterilization effects applied cavitation is paid attention. Such effects ascribe to the impulsive pressures due to the collapsing of the cavitation bubbles, i.e., vaporous bubble or gaseous bubble. However, it remains unclear whether such effects highly depend on the types of bubbles. In this paper, therefore, we study about the sterilization effects on the Esherichia cvii in the physiological salt solutions under the two kinds of cavitators, which consist of a nozzle of 1mm in diameter and a exponential horn vibrating at the frequency of 20kHz. Viable count of E. coli was measured by plate counting method. After treatment of cell suspension, a part of samples were withdrawn, diluted by physiological saline solution, and spread onto agar plates. After incubation for 24h at 30℃, colonies of E. coli formed are observed as shown in Fig. 1. Viabilities of E. coli were calculated from number of colonies formed, volume of cell suspension applied onto agar plates and the dilution rate. Survival rates S of E. coli were designated rates of viability after the treatment for t min versus viability at 0 time. The viability curves against the incubation time were expressed as the Arrhenius equation, where N t, k, and Dwere designated as number of viable cells, incubation time, death rate equation, and decimal reduction time (D value) which is estimated from time required for the reduction of viabilities to reduced to one tenth. Figure 2 shows the survival curves for two kinds of cavitation. The sterilization effects are clearly visible in cavitating flows and the corresponding D value remains about 900. While, for the ultrasonic cavitation (Vibratory cavi.) under the atmospheric pressure, D value decreases to around 50, suggesting remarkable sterilization effects. It is found that (1) the sterilization effect is clearly visible in the flow cavitation; (2) the sterilization intensity deeply depends upon the types of bubbles, and the vaporous bubble plays an larger role in such sterilization intensity; (3) for the ultrasonic cavitation under atmospheric pressure, Dvalue is comparable to that for heat sterilization procedures; (4) from SEM photos shown in Fig. 3, the impulsive dents and the destructive damages are clearly observed on the cell membrane of E. coli.
