Effects of antimicrobials in the formulation and post-packaging thermal treatment to control Listeria monocytogenes in post-processing inoculated frankfurters

Abstract

Listeria monocytogenes presents a major food safety concern due to its ability to survive or grow at refrigeration temperatures (4°C) and in other adverse conditions (salt, heat, low pH, and large temperature range). Since L. monocytogenes transmission, in the case of human outbreaks, is generally associated with post-processing contamination of ready-to-eat foods, actions to eliminate or reduce the pathogen's occurrence in foods should be applied during processing and post-processing. The objectives of these studies were to determine the effectiveness of certain antimicrobials (i.e., sodium lactate at 3% or 6%, sodium diacetate at 0.25% or 0.50%, sodium acetate at 0.25% or 0.50%, nisin as Nisaplin® at 0.15%, lysozyme at 0.01 %, monolaurin at 0.01 %, glucono-delta-lactone at 0.25%, and allyl isothiocyanate at 0.02%, in the first study-Chapter III) or their combinations (i.e., sodium lactate at 3% with sodium acetate at 0.25%, sodium lactate at 3% with sodium diacetate at 0.25%, or sodium lactate at 3% with glucono-delta-lactone at 0.25%, in the second study-Chapter IV) included in the formulation of frankfurters against L. monocytogenes inoculated on their surface after peeling and before vacuum packaging, as well as the antimicrobial effect of thermal treatment (i.e., dipping in hot water of 7 5-80°C for 30-90 sec; in the second study-Chapter IV). Samples were stored (for 50 or 120 days) at 4°C and periodically analyzed for microbial growth (TSA YE and PALCAM agar), pH, moisture, fat, and water activity. In the first experiment of the first study (Chapter III), growth of L. mo11ocytogenes reached I 08 cfu/cm2 in the frankfurters containing no antimicrobials during storage. Sodium lactate at 6% and sodium diacetate at 0.50% incorporated in the formulation of frankfurters singly, inhibited growth of L. monocytogenes stored at 4°C for 120 days. Sodium lactate at 3% inhibited growth of the pathogen for 50 days, while nisin at 0.15% added in the formulation of the frankfurters, in the second experiment, inhibited growth of the organism for 10 days. Under the conditions of the second experiment in the first study (Chapter III), other antimicrobials used singly in the product did not affect growth of L. monocytogenes during storage. The water activity of products containing sodium acetate (0.25 or 0.50%) and sodium diacetate (0.25 or 0.50%) was slightly below that of the control (0.972) while sodium lactate decreased remarkably the water activity of the product as its concentration increased from 3% to 6% reaching, respectively, 0.946 and 0.933. Nisin (0.15%), allyl isothiocyanate (0.02)%, lysozyme (0.01)%, monolaurin (0.01)%, and glucono-delta-lactone (0.25%) did not affect the water activity of the product. None of the individual antimicrobials added in the formulation affected the cooking yield, or the moisture and fat contents of the products. On day 0, pH of the products containing sodium diacetate (0.25 or 0.50%) decreased to 6.03 and 5.87, respectively, compared to that of the inoculated control (6.31). However, pH of the inoculated control decreased dramatically to reach 5.42 during storage as growth of the pathogen reached higher levels (107-108 cfu/cm2). Combinations of sodium lactate (3%) with sodium acetate (0.25%), sodium diacetate (0.25%), or glucono-delta-lactone (0.25%) inhibited growth of L. monocytogenes for 120 days on frankfurters stored at 4°C, while growth of L. monocytogenes on the inoculated control frankfurters reached l 07 cfu/cm2 in 35 days. When thermal treatment and antimicrobial combinations were applied to the frankfurters (Chapter IV), inhibition of L. monocytogenes was enhanced. Thermal treatment used alone on the vacuum packaged frankfurters inhibited growth of the pathogen for 35 days when one frankfurter per bag was dipped in hot (75 or 80°C) water for 90 sec. While the water activity and the cooking yield were not affected by the combination of antimicrobials in the products, sodium lactate (3%) combined with sodium diacetate (0.25%) decreased slightly the moisture while the fat content increased by approximatively 3%. The combination of sodium lactate (3%) with sodium diacetate (0.25%) or glucono-delta-lactone (0.25%) decreased the pH of the frankfurters. Moreover, the pH in the inoculated control decreased dramatically after 35 days of storage at 4°C and reached, at the end of the storage, 5.30 for the non-dipped frankfurters and 5.58 when they were dipped in hot water (80°C). The pH of frankfurters with no antimicrobials but dipped in hot (75-80°C) water dropped remarkably after 35 days in all treatments as a consequence of the high level of microbial growth on the product. In addition, the microbial counts on TSA YE agar were higher than those on P ALCAM agar in all treatments during storage. These results indicate that use of combinations of permissible levels of antimicrobials in the formulation exhibited an antimicrobial activity in cooked meat products stored at 4°C for 120 days. In addition, thermal treatment may enhance the inhibitory activity of the antimicrobials during storage.