Reduction of Salmonella Embedded in Lymph Nodes During Grinding Using Lactic Acid and Peracetic Acid Treatments on Beef Trim, Combined with Acidified Sodium Chlorite
Inestroza, Brenda Sofia
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Salmonella within lymph nodes can be a source of contamination for beef trim and ground beef products. Organic acids and acidified sodium chlorite are commonly used as post-harvest interventions to reduce microbial contamination on the surface of the carcass or trim. However, the use of these interventions during grinding to reduce pathogens protected within lymph nodes has not been previously evaluated. The objectives of this study were as follows: 1) Determine the effectiveness of lactic acid (LA, 4.5%) and peracetic acid (PAA, 220 ppm) treatments combined with acidified sodium chlorite (ASC, 1000 ppm) to reduce Salmonella at high (4.8 log CFU/g) and low (2.31 log CFU/g) concentrations embedded in lymph nodes in beef trim during grinding. 2) Conduct sensory analysis to evaluate the impact of the chemical interventions on organoleptic properties of ground beef. 3) Evaluate the induction of a possible stress response on Salmonella Montevideo to the chemical intervention. We hypothesized the chemical intervention with organic acid combined with acidified sodium chlorite as secondary intervention during grinding process will reduce Salmonella introduced through contaminated lymph nodes on ground beef products. In addition, differences in flavor, color, and texture among treated samples are not expected. Finally, differences in expression of housekeeping genes could provide insight into how Salmonella Montevideo adapt and survive in the presence of chemical intervention tested. Subiliac lymph nodes were inoculated with a cocktail containing rifampicin resistant derivatives of 6 Salmonella strains recognized as the most frequent of clinical significance or isolated from cattle lymph nodes. Organic acids were applied to the trim/node mixture using a six-nozzle sanitizing CHAD spray cabinet. After the first application, the sample was coarsely ground using a meat grinder. The product was then fine ground and an ASC solution was applied through a spray application. Samples were stored at 2-4°C in the dark, and three subsamples for each treatment combination were obtained at 1, 24, and 72 h after grinding. Resultant Salmonella population were enumerated on selective medium supplemented with rifampicin with a thin layer of a non-selective medium to allow for recovery of injured cells. All treatments significantly reduced Salmonella introduced through contaminated lymph nodes on ground beef products (P< 0.05). When Salmonella was present at high concentrations (4.8 log10 CFU/g), LA + ASC, PAA + ASC, and ASC alone reduced rifampicin-resistant non-typhoidal Salmonella significantly (P < 0.05) by 0.59, 0.47, 0.50 log cycles after 1 hour and by 0.67, 0.51, 0.59 log cycles after 72 hours, respectively. No significant difference between ASC and peracetic acid + ASC was observed (P=0.23). In the ground beef containing low concentrations of Salmonella, the same treatments showed similar reductions of 0.66, 0.42, 0.52 log cycles and 0.76, 0.51 and 0.64 log cycles after 1 and 72 hours, respectively. No significant difference between LA and PAA + ASC was observed (P = 0.90). To conduct the sensory analysis, ground beef was obtained in the same manner described in the chemical treatment methodology with the exception that subiliac lymph nodes were not included, hence trims were not inoculated with Salmonella. The Triangle test indicated that panelists detected a significant difference (P < 0.05) in beef patties between the control and the LA + ASC, PAA 220 ppm, and ASC1000 ppm within treatment after 24 and 72 hours. Panelists were unable to identify a significant difference (P > 0.05) between control, LA 4.5%, and PAA + ASC beef patties samples at 24 and 72 hours. The instrumental color results indicated that the five individual chemical treatments compared with the control were significant different in L*, a*, and b* values. There were statistically significant differences (P < 0.05) for L* (luminance) values between the control and lactic acid 4.5%, peracetic acid (220 ppm), peracetic acid + acidified sodium chlorite and ASC alone (1000 ppm) at the three sampling points (1, 24 and 72 hours). Regardless to the analysis of a* (redness) values indicated a statistically significant difference (P < 0.05) between control beef patties and LA, LA + ASC, PAA, and PAA + ASC. All the treated samples and the control became less red through the time. The b* (yellowness) values indicated no statistically significant differences when comparing the control beef patties with LA + ASC (P = 0.18), PAA (P = 0.70), PAA + ASC (P = 0.65) and ASC (P = 0.65). Beef patties treated with lactic acid had a slightly significant difference with control beef patties treated with lactic acid (P = 0.045). A difference was observe in the expression level of the housekeeping genes rpoS and dnaK when S. Montevideo was exposed to stress condition by the acid shock of LA, PAA and ASC for 5, 15 and 30 min. The highest expression of rpoS (6.374-fold) was induced by the lactid acid at 30 min. There was not significant difference (P > 0.05) among the expression level of rpoS in the presence of ASC. When the dnaK gene expression was evaluated, the maximun level reached was 3.310-fold after the 30 min. of exposure in the lactic acid. The result of this study can be used by the meat industry to improve the safety of ground beef that may be contaminated with Salmonella within lymph nodes incorporated during process.