Antibacterial activity of cranberry juice concentrate on freshness and sensory quality of ready to eat (RTE) foods

Ready-to-eat (RTE) foods are not further treated before consumption in such a way that may significantly reduce the microbial load, therefore the risk of foodborne disease must be considered. In this regard, the use of natural antimicrobial compounds is an interesting method to be considered. On this topic, the antibacterial activity of cranberry juice concentrate (CJC) have been evaluated in vitro and in situ against 3 foodborne pathogenic bacteria. Results showed a high antimicrobial effect with a noticeable inhibition capacity against Escherichia coli O157:H7, Listeria monocytogenes, Salmonella typhimurium. Acid sensitivity studies of bacteria indicated that at the same pH level (pH = 2.4) in presence of organic acid solution (citric and quinic acids), cranberry juice concentrate showed greater antibacterial effects than the acids due to their phenolic compounds. In situ studies showed 2.5, 1.8 and 5 log reduction of E. coli, L. monocytogenes and S. typhimurium, respectively in presence of cranberry juice concentrate, on pre-cut red peppers after 7 days of storage at 4 °C. A total inhibition of L. monocytogenes on fresh cranberry fruits in primary day of storage, was observed. Cranberries treated with CJC also showed a 3 log reduction of S. typhimurium after 4 days of storage at 4 °C. The results suggest that CJC can be an effective preservation, source of natural antibacterial, to protect the RTE foods from foodborne pathogens contamination without effecting on sensorial properties of treated samples and allow to maintain the freshness, sensory and the nutritional quality of RTE foods.     

Effects of sanitizing treatments with atmospheric cold plasma,SDS and lactic acid on verotoxin-producing Escherichia coli and Listeria monocytogenes in red chicory (radicchio)

The main objective of this study was to evaluate the synergistic effect of atmospheric cold plasma (ACP), Sodium Dodecyl Sulphate (SDS) and lactic acid (LA) on L. monocytogenes and verotoxin-producing E. coli in red chicory. Experimentally inoculated samples were pre-treated with either SDS, or SDS + LA for 5, 10 or 15 min. Pre-treated samples were then submerged in deionized water and either exposed to ACP generated by dielectric barrier discharge device (DBD: fixed parameters: 19.15 V and 3.15 A) for 15 min or left untreated. All combinations of treatments were evaluated for sensory effects. Viable counts of verotoxin-producing E. coli on red chicory decreased by more than 4 logs (4.78 Log CFU/cm² vs control) after a treatment with LA + SDS for 5 min and ACP for 15 min and often dropped below the limit of quantification. L. monocytogenes showed a higher tolerance to this sanitizing treatment and the level of inactivation was higher than 3 logs (3.77 Log CFU/cm² vs control) only by increasing the duration of the washing step in LA + SDS to 15 min. The different treatments had no detrimental effects on colour, freshness and texture of red chicory, but odour and overall acceptability of the samples treated by ACP slightly decreased during storage. Further optimization of treatment parameters for maintaining fresh characteristics are needed, but the effectiveness of combinations of sanitizers and ACP on other pathogens and fresh produce worth to be investigated.     

Development of a real-time PCR procedure for quantification of viable Escherichia coli in populations of E. coli exposed to lactic acid,and the acid tolerance of verotoxigenic E. coli (VTEC) from cattle hides

The aims of this study were to develop a real-time PCR procedure for determining the effects on Escherichia coli of treatment for decontaminating beef carcasses with lactic acid solution, and determining if there were differences in the acid tolerance of E. coli generally and verotoxigenic E. coli (VTEC). Suspensions of E. coli were incubated with 4% lactic acid at pH 3.6. The numbers of surviving E. coli at different incubation times were determined from plate counts and from quantification by real-time PCR of the uidA gene in DNA preparations. The numbers of viable E. coli progressively declined, by about 4 log units during incubation for 6 h. The mean cycle threshold (Ct) values for uidA in DNA from samples collected at different times and treated or not treated with propidium monoazide (PMA) before DNA extraction were similar. Treatment with 1% sodium deoxycholate (SD) before PMA treatment resulted in an increase of >6 Ct when the reduction in viable cell number was around 1 log. When E. coli incubated with 4% lactic acid solutions of pH 2.4, 2.8, 3.2 or 3.6 were resuscitated in half strength brain heart infusion (BHI) for 2 h before treatments with SD and PMA, the slope of the plot relating Ct values to the numbers of viable E. coli was 1.85 Ct log cfu⁻¹ with the correlation coefficient (R²) being 0.80. The findings indicate that while the membranes of E. coli inactivated by 4% lactic acid were largely impermeable to PMA, the membranes of both dead and injured cells were rendered permeable to PMA by treatment with 1% SD. Resuscitation in BHI restored the membrane barrier properties of the injured cells. Treatment with lactic acid resulted in increases in Ct values of 4.1, 3.7, 2.5 and 1.5 for the uidA, stx₁, stx₂ and eae genes, respectively; and the increases in Ct values for the latter two genes were significantly different (p < 0.05) from that for the uidA gene. This indicates that VTEC carrying stx₂ and/or eae were more acid resistant than other E. coli. Thus, caution should be exercised when using generic E. coli as an indicator for VTEC for assessment of the antimicrobial efficacy of organic acid decontaminating treatments at abattoirs.     

Application of antimicrobial nanocomposite film packaging on the postharvest quality and specific spoilage organisms of mushrooms (Russula virescens)

The use of biodegradable packaging films for post-harvest preservation of wild edible mushrooms is an eco-friendly method. In this study, the nanocomposite films were prepared based on poly (lactic acid) and mesoporous silica (2, 4 and 6 wt% based on polylactide) loaded with citral. The effects of nanocomposite film on the postharvest quality and specific spoilage bacteria of Russula virescens wild mushroom were evaluated. The results showed that firmness, weight loss, color, electrolyte leakage for Russula virescens treated with mesoporous silica loaded with citral based on poly (lactic acid) films were maintained compared to the control group. In particular, the 4% mesoporous silica loaded with citral group prolonged the freshness of mushroom by 6 days. Acinetobacter, Lactococcus, Weissella, Serratia, unclassified Enterobacteriaceae, and unclassified Gammaproteobacteria were identified as the main dominant bacteria. After 12 d of storage, the relative abundance of these bacterial for mushrooms packed by 4 wt% mesoporous silica loaded with citral based on poly (lactic acid) film was reduced, except Lactococcus. This revealed that the poly (lactic acid) film prepared by 4 wt% mesoporous silica loaded with citral could be used to extend the shelf life of Russula virescens.