Sanitising efficacy of lactic acid combined with low-concentration sodium hypochlorite on Listeria innocua in organic broccoli sprouts

Abstract

The sanitising effects of combined lactic acid (LA, 2%, v/v) and low-concentration sodium hypochlorite (SH, 4 mg/L) on Listeria innocua were investigated. The disinfectant kinetics and percentage of injured cells showed that the combined treatment resulted in dramatic inactivation of L. innocua after 60 s. The Weibull model with a high R² (1.00) and low root mean squared error (RMSE) (0.19) and Akaike Information Criterion (AIC) (-7.58) was verified as the most superior model to describe effects of the combination treatment. Protein leakage and the fluorescence intensity ratio revealed that LA might disrupt the cytoplasmic membrane rapidly and that SH could synergistically promote the sanitising effect. Atomic force microscopy (AFM) images showed notable morphological changes in the combination treated cells, which presented lower width (0.73 μm) and height (0.29 μm), and increased roughness (10.37 nm). Moreover, massive leakage of intracellular components indicated severe cell collapse. The antimicrobial effects on organic broccoli sprouts showed that the combined treatment resulted in reductions of 0.82, 1.51, and 1.77 log colony forming units (CFU)/g fresh weight (FW) for aerobic bacteria, yeasts and moulds, and inoculated L. innocua at day 0. In addition, there were no significant differences of sensory quality results between treatment and control groups and they were all greater than the acceptance limit. Thus, the disinfection treatments had no negative effects on the storage quality of the organic broccoli sprouts. The combined treatment could be a potential sanitising strategy for organic sprouts.

Keywords: Atomic force microscopy; Modelling; Organic acid; Organic produce; Pathogen; Sanitisation.