Implication of Sodium Hypochlorite as a Sanitizer in Ready-to-Eat Salad Processing and Advantages of the Use of Alternative Rapid Bacterial Detection Methods

Abstract

The use of disinfection agents in the washing processing of ready-to-eat (RTE) vegetables, especially sodium hypochlorite, is a common industrial practice performed to enhance microbiological quality. However, some studies have reported a restart of bacterial growth and a substantial increase in bacterial load during early storage associated with the use of disinfection agents, which might represent a risk for consumers. We evaluated the effect of sodium hypochlorite on bacterial growth trends during the shelf-life in Lactuca sativa, simulating the industrial procedures for RTE vegetable packaging. Immediately after sodium hypochlorite treatment, an effective abatement of the bacterial load was observed, followed by a restart of growth throughout storage. After 5 days, the bacterial load was close to that reached by the control samples, indicating that the net increase in bacterial load was significantly higher in the treated samples. This might be ascribed to the reduction in competitive microflora and/or to the induction of adaptive responses by resting bacteria, which might select disinfectant-resistant bacteria. These findings elicit some concerns about the actual duration of the shelf-life; products might decrease their microbiological quality earlier during storage, pointing out the need to better clarify the impact of sodium hypochlorite as a sanitizer to closer consider its use in RTE vegetable processing. Furthermore, due to the importance of the rapid estimation of bacterial load and the early detection of foodborne pathogens throughout the food chain, the accuracy of the rapid bacteria detection method, the Micro Biological Survey (MBS), and its effectiveness for microbiological analyses of RTE vegetables were evaluated.

Keywords: food safety; food sanitizers; microbiological quality; rapid microbiological methods; ready-to-eat vegetables; shelf-life; sodium hypochlorite.

Figure 1

L. sativa average contamination levels under different experimental conditions evaluated using the plate count method: (a) TAMC and (b) Enterobacteriaceae. CTRL: Unprocessed; 1: samples rinsed under flowing water in absence of sodium hypochlorite (treatment 1); 2: samples immersed in water bath with sodium hypochlorite (0.22 g/L) followed by rinsing under flowing water (treatment 2) (SD < 10%). * Significant difference compared to “CTRL” (p < 0.05); ** Significant difference compared to “CTRL” and compared to “treatment 1” (p < 0.05); n = 30.

Figure 2

Average contamination levels of L. sativa samples at different storage times, evaluated using the plate count method: (a) TAMC and (b) Enterobacteriaceae (SD < 10%). * Significant difference (p < 0.05); n = 40. Control samples were rinsed under flowing water, and the treated samples were immersed in a 0.22 g/L aqueous solution of sodium hypochlorite (NaClO) and then rinsed under flowing water. The analyses were performed immediately before packaging (t = 0) and after 2 and 5 days (t = 2; t = 5) of storage.

Figure 3

Correlation lines between the results obtained with the MBS method and reference methods for TAMC (a) and Enterobacteriaceae (b). The quantitative results obtained with the reference method were plotted against the results obtained with the MBS method. Regression parameters of TAMC (slope = 1.00, R² = 0.8, n = 50) Enterobacteriaceae (slope = 1.00, R² = 0.82, n = 50). Dashed lines represent 90% prediction intervals.