Effects of acidity, fat and solid non fat content of milk on destruction of Listeria monocytogenes biotype 4b by microwave radiation

ABSTRACT

The efficacy of microwave radiation in destruction of Listeria monocytogenes Shiraz bio-Iran.type 4b in milk with various concentrations of solid non fat [SNF], fat and acidity were studied. Experimental study. Bacterial suspensions [about 1.5x10[7] CPU ml[+1] in milk containing various concentrations of SNF [7.5 and 8.5percent],fat [2.5,3.0and3.5percent] andacidity [1.6,1.7 and 1.8 g L' lactic acid] were prepared in volumes of 50 ml. Then the samples were exposed to microwave radiation in triplicate for 0,10,20,23,26,29,32,35 and 40 seconds and the viable cell count was performed with standard plate count method. Repeated Measures ANOVA and Duncan's tests were employed to determine the differences in the rates of bacterial destruction in the milk samples after irradiating. Under different durations of radiation, a useful effect of bacterial killing by microwave irradiation was observed. Reduction in viable cell count was significantly faster when the SNF and/or fat concentration of milk increased. The speed of decline in cell populations was significantly slower when the acidity increased. Contrary to the conventional heating process, the present study showed that the destructive effect of microwave heating increased by increasing the SNF and fat concentration of milk. Although, pH of some foods and culture media within the range of 5.5-7.0 had no significant effect on destruction properties of microwave, milk acidity affected it significantly. It seems that increased acidity causes increased ionic content of milk, which in turn results in lower penetration of microwave into milk