A flow cytometric approach to monitor the effects of gentle preservation techniques in the postharvest chain

Abstract

Decontamination of fresh produce requires mild treatment techniques that reduces microbial load without affecting the product quality. Chemical disinfection (peracetic acid and ozone treatment) is applied to fresh produce with varying success. Non-thermal plasma treatment is of grown interest in food microbiology but its potential regarding mild food surface decontamination is not fully investigated yet. In this study, the potential of flow cytometry for short-time monitoring of inactivation effects was investigated. It was shown that the test bacteria (E. coli, L. innocua, P. carotovorum) lose their culturability due to the applied inactivation treatments but physiological activities were still detectable using flow cytometry. The remaining physiological activities may still result in product spoilage and may still cause human diseases. E. coli cells were completely depolarized (loss of membrane potential) after treatment (15 s) with 0.25 % PAA at 10 °C, and after treatment (10 s) with 2.8 mg l O3 at ~ 10°C. The membrane potential of plasma treated cells remained almost constant at an operating power of 20 W over a time period of 3 min, and subsequently decreased within 30 s of further treatment. Complete membrane permeabilisation was observed after 10 s O3 treatment, but treatment with PAA and plasma did not completely permeabilise the cells within 2 min and 4 min, respectively. Similar results were obtained for esterase activity. O3 inactivates cellular esterase but esterase activity was detected after 4 min plasma treatment and 2 min PAA treatment. The degree of bacterial damage due to the inactivation processes is highly dependent on treatment parameters as well as on treated bacteria. This study clearly indicates the potential of flow cytometry to monitor the efficiency of inactivation processes within a short time. This enables the definition of critical process parameters.