Improving the quality of vegetable foodstuffs by microwave inactivation

Abstract

With the aim of improving the loss of quality in retorted vegetables, experiments on pretreatment inactivation using microwaves were carried out to allow the heating intensity to be reduced during retorting. Microwave heating reduced the bacteria level by 10³ CFU/g, and was a more effective method considering the short processing time of 3 min and the required energy being 70-80% of that when using steam. The inactivation effect was due to dielectric heat generation by the high-frequency microwaves. The inactivation effect for heat-resistant Bacillus amyloliquefaciens was indicated by a reduction of 10² CFU/g after 3 min of microwave heating. The total bacteria counts for peeled potato and spicy sauce with vegetables decreased by 3-4 log CFU/g after 3 min using microwaves, and heat-resistant microorganisms were reduced by 2 log CFU/g. Combining microwave heating and mild retorting is expected to produce higher quality vegetable foodstuffs compared to conventional retorting.

Keywords: Dielectric; Microwave heating; Pretreatment inactivation; Retort; Vegetable foodstuffs.

Fig. 1

Comparisons of the number of viable cells (A) and heating energy (B) for peeled potatoes when using steam or microwave heating. ST: no treatment, CS: conventional steam, SHS: superheated steam, MW: microwave heating, TC: total cell count, HR: heat-resistant microorganism. Bar data are mean and standard-deviation values; those marked with different letters differ significantly by ANOVA with Duncan’s multiple-range test at p < 0.05

Fig. 2

Temporal changes in viable cell counts and microwave energy for heat-resistant B. amyloliquefaciens during microwave heating. “filled square”: heating energy. Bar data are mean and standard-deviation values; those marked with different letters differ significantly by ANOVA with Duncan’s multiple-range test at p < 0.05

Fig. 3

Comparison of total number of viable cells (A) and number of heat-resistant viable cells (B) for potatoes and sauce with vegetables by microwave heating. ST: no treatment, TEST: microwave heating. Data are mean and standard-deviation values; those marked with different letters differ significantly by ANOVA with Duncan’s multiple-range test at p < 0.05