Application of vacuum impregnation to washing process for enhancement of antimicrobial effect

Abstract

To enhance antimicrobial effect of sanitizer washing, vacuum impregnation was applied to sanitizer washing to increase the penetration activity of sanitizer into protected site where microbes could escape from contacting with sanitizer. This study was undertaken to evaluate the effect of VI applied to the washing process for removal of pathogens from fresh produce surfaces. At first, its antimicrobial effect was verified by applying to inoculated broccoli. Broccoli was inoculated with Salmonella Typhimuruim and Listeria monocytogenes and treated with simple dipping washing or with VI in 2 % malic acid for 5, 10, 20, or 30 min. There were two methods of VI: continuous and intermittent. When 2 % malic acid alone was applied to inoculated broccoli, mean log reductions of 1.5 and 1.3 log10 CFU/g were observed for S. Typhimurium and L. monocytogenes, respectively, after 5 min. However, there were no further reductions even though treatment times extended from 5 min to 30 min. When VI was applied, there were significant increases (P < 0.05) of reduction of pathogens with increasing vacuum for both continuous and intermittent treatment. In continuous VI treatment, there were no significant (P ≥ 0.05) additional antimicrobial effect with increasing treatment time. In intermittent VI treatment, however, there were significant (P < 0.05) additional antimicrobial effect with increasing treatment time. Scanning electron photomicrographs showed that bacteria tend to attach to or become entrapped in protective sites after simple wash processing (dipping). However, most bacteria were washed out of protective sites after intermittent treatment. Direct treatment of cell suspensions with VI showed that it had no inactivation capacity in itself since there were no significant differences (P ≥ 0.05) between the reduction rates of non- and VI treatment. These results demonstrate that the increased antimicrobial effect of VI can be attributed to increased accessibility of sanitizer and an enhanced washing effect in protected sites on produce. Color, texture and titratable acidity values of broccoli treated with intermittent VI in 2 % malic acid for 30 min were not significantly (P ≥ 0.05) different from those of untreated samples even though a storage interval was needed for titratable acidity values to be reduced to levels comparable to those of untreated controls. On the basis of this result that VI has possibility to be applied to washing process to improve antimicrobial effect, this technique was extended to various samples. VI was applied to organic acid washing against Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes on paprika fruit, carrots, king oyster mushrooms and muskmelons. The samples were treated with intermittent VI with 21.3 kPa and compared with dipping washing in 2 % malic acid for 3, 5, 10, 15 or 20 min. For simple dipping, the reduction rate of paprika was highest among the samples tested and followed by carrots, king oyster mushrooms and muskmelons. For VI treatment, a significant ( P < 0.05) enhanced antimicrobial effect occurred with paprika and carrots. However, there were no significant (P ≥ 0.05) differences in pathogen reductions between dipping and VI treatment for both king oyster mushrooms and muskmelons. This might be due to surface roughness. King oyster mushrooms (Ra = 6.02 ± 1.65) and muskmelons (Ra = 11.43 ± 1.68) had relatively large roughness values compared to those of paprika (Ra = 0.60 ± 0.10) and carrots (Ra = 2.51 ± 0.50). Adequate space for microbes to escape contact with sanitizer could be possible during VI treatment due to the relatively coarse surface roughness present in king oyster mushrooms and muskmelons. This view was based on scanning electron photomicrographs that showed many deep protected sites in king oyster mushrooms and muskmelons with many microbes located deep in these sites following VI treatment. Color, texture and titratable acidity values of paprika and carrots subjected to VI washing treatment with 2 % malic acid for 5 and 20 min were not significantly (P ≥ 0.05) different from those of untreated control samples during 7 day storage.