Responses of acid-stressed Salmonella Typhimurium in broth and chicken patties to subsequent antimicrobial stress with ɛ-polylysine and combined potassium lactate and sodium diacetate

Effects of conjugates of ε-polylysine-dextran created through Maillard reaction on quality and storage stability of the chicken gel

The present study mainly focused on the effects of the conjugates of PL-dextran produced through the Maillard reaction on the quality and storage stability of chicken gel for 5 days at 4 ℃. According to the results of the texture profile, water retention capacity (WRC), low-field nuclear magnetic resonance (LF NMR), aerobic plate count (APC), and total volatile basic nitrogen (TVBN), ε-polylysine (PL) could improve chicken gel storage stability while decreasing the quality of protein gels (p < 0.05). Additionally, adding dextran with high or low molecular weight could significantly increase the quality of gel during storage (p < 0.05), whereas decreased storage stability could be obtained (p < 0.05). In general, conjugates formed by PL and dextran with high molecular weight were beneficial for quality maintenance. In comparison, the polymers produced from the low molecular weight of dextran could modify the storage stability of gels. Adding conjugates of dextran and PL benefited the structure formation of protein gel, while PL would retain part of antibacterial activity when crosslinked with dextran. Therefore, it could be concluded that the quality improvement effect of PL-dextran addition on gel quality was greater than its antibacterial effect, which would impact the formulation design of novel emulsion-type meat products.

Predictive Modeling of the Effect of ε-Polylysine Hydrochloride on Growth and Thermal Inactivation of Listeria monocytogenes in Fish Balls

This study aimed to evaluate the effect of ε-polylysine hydrochloride (ε-PLH) on the growth and thermal inactivation kinetics of Listeria monocytogenes in fish balls. Samples, supplemented with ε-PLH (0, 150, or 300 ppm, w/w), were inoculated with a three-strain cocktail of L. monocytogenes and incubated at constant temperatures of 3.4, 8, 12, or 16 °C for growth studies, or heated at 60, 62.5, 65, or 67.5 °C for thermal inactivation tests. The growth curves were fitted to the Huang primary model, and the Huang and Ratkowsky square-root models (SRM) were used as the secondary models to evaluate the effect of temperature on bacterial growth. The survival during heating was analyzed with a log-linear model. The results showed that, while the lag time of L. monocytogenes was affected by both ε-PLH concentration and temperature, the specific growth rate was unaffected by ε-PLH. Under the same temperature, a 10-time in increase of the lag time would be expected for every 565 ppm in the increase of ε-PLH concentration. Using the Ratkowsky SRM, the estimated nominal minimum growth temperature was -2.04 °C, while the minimum growth temperature was 0.29 °C when estimated with the Huang SRM. Validation at 10 °C showed that the Huang primary model, in combination with either the Huang or Ratkowsky SRM, could accurately predict the growth of L. monocytogenes. On the other hand, the thermal resistance of the pathogen was significantly reduced by increase in temperature or ε-PLH. The thermal z value of L. monocytogenes was 5.78 °C, and the ε-PLH z value was 1642 ppm. The results of this study showed that the combined application of ε-PLH and temperature can be used to control L. monocytogenes in fish balls and to improve food safety and reduce risks to public health.

Effect of Product Dimensions and Surface Browning Method on Salmonella Contamination in Frozen, Surface-Browned, Breaded Chicken Products Treated with Antimicrobials

Not-ready-to-eat breaded chicken products formulated with antimicrobial ingredients were tested for the effect of sample dimensions, surface browning method and final internal sample temperature on inoculated Salmonella populations. Fresh chicken breast meat portions (5 × 5 × 5 cm), inoculated with Salmonella (7-strain mixture; 5 log CFU/g), were mixed with (5% v/w total moisture enhancement) (i) distilled water (control), (ii) caprylic acid (CAA; 0.0625%) and carvacrol (CAR; 0.075%), (iii) CAA (0.25%) and ε-polylysine (POL; 0.5%), (iv) CAR (0.15%) and POL (0.5%), or (v) CAA (0.0625%), CAR (0.075%) and POL (0.5%). Sodium chloride (1.2%) and sodium tripolyphosphate (0.3%) were added to all treatments. The mixtures were then ground and formed into 9 × 5 × 3 cm (150 g) or 9 × 2.5 × 2 cm (50 g) portions. The products were breaded, browned in (i) an oven (208 °C, 15 min) or (ii) deep fryer (190 °C, 15 s), packaged, and stored at -20 °C (8 d). Overall, maximum internal temperatures of 62.4 ± 4.0 °C (9 × 2.5 × 2 cm) and 46.0 ± 3.0 °C (9 × 5 × 3 cm) were reached in oven-browned samples, and 35.0 ± 1.1 °C (9 × 2.5 × 2 cm) and 31.7 ± 2.6 °C (9 × 5 × 3 cm) in fryer-browned samples. Irrespective of formulation treatment, total (after frozen storage) reductions of Salmonella were greater (P < 0.05) for 9 × 2.5 × 2 cm oven-browned samples (3.8 to at least 4.6 log CFU/g) than for 9 × 5 × 3 cm oven-browned samples (0.7 to 2.5 log CFU/g). Product dimensions did not (P ≥ 0.05) affect Salmonella reductions (0.6 to 2.8 log CFU/g) in fryer-browned samples. All antimicrobial treatments reduced Salmonella to undetectable levels (<0.3 log CFU/g) in oven-browned 9 × 2.5 × 2 cm samples. Overall, the data may be useful for the selection of antimicrobials, product dimensions, and surface browning methods for reducing Salmonella contamination.

Cell envelope disruption of E. coli exposed to ϵ-polylysine by FESEM and TEM technology

To investigate the action mechanism of ϵ-polylysine (ϵ-PL) against Escherichia coli (E. coli), a new field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) technology has been developed. The log phase E. coli cells were first incubated with ϵ-PL for 8 min, then the samples were directly added onto the silicon platelet and the copper grid, followed by a simple in situ fixation and freezing dehydration. FESEM and TEM were used to examine the ultrastructure changes in the bacterial envelope which was affected by ϵ-PL. various damages of E. coli cell envelope by ϵ-PL have demonstrated the detachment of outer membrane, the swelling of inner membrane, the apical burst of cells and the leakage of cytosol at a minimal inhibitory concentration (MIC) concentration. It also exhibited whole cell lysis at double MIC concentration. In summary, the new FESEM and TEM technology and appropriate sample preparation protocols have been found to be useful for investigating the biocidal activity of ϵ-PL against E. coli.

Antimicrobials for reduction of Salmonella contamination in uncooked, surface-browned breaded chicken products

Surface-browned but uncooked frozen breaded chicken products have been associated with salmonellosis outbreaks due to inadequate or no cooking of the products before consumption. This study was conducted to evaluate the effect of three antimicrobials against Salmonella during manufacture of a surface-browned, uncooked frozen breaded chicken meat product. Fresh chicken breast meat portions (5 by 5 by 5 cm) were inoculated (4 to 5 log CFU/g) with Salmonella and mixed with caprylic acid (CAA; 0.5 and 1.0%), carvacrol (CAR; 0.3 and 0.5%), ε-polylysine (POL; 0.125 and 0.25%), or distilled water (control). Sodium chloride (1.2%) and sodium tripolyphosphate (0.3%) were added to all treatments, and the mixtures were ground (5% total moisture enhancement level) and formed into portions (9 by 5 by 3 cm). The products were breaded and surface browned by baking in an oven (208°C for 15 min) or deep frying in vegetable oil (190°C for 15 s), packaged in polyethylene bags, and stored at -20°C for 7 days. Total reductions of inoculated Salmonella in untreated control oven- or fryer-browned products after frozen storage were 1.2 and 0.8 log CFU/g, respectively. In comparison, treatment with CAA, CAR, or POL reduced initial pathogen counts by 3.3 to >4.5, 4.1 to >4.7, and 1.1 to 1.6 log CFU/g, respectively, regardless of the antimicrobial concentration and browning method. Treatment with 1.0% CAA (oven browned) or 0.5% CAR (oven or fryer browned) reduced Salmonella to nondetectable levels (<0.3 log CFU/g) in stored frozen products. These data may be useful for development of suitable antimicrobial treatments to reduce the risk of Salmonella contamination in surface-browned, uncooked frozen breaded chicken products.

A combined physiological and proteomic approach to reveal lactic-acid-induced alterations in Lactobacillus casei Zhang and its mutant with enhanced lactic acid tolerance

Lactobacillus casei has traditionally been recognized as a probiotic and frequently used as an adjunct culture in fermented dairy products, where acid stress is an environmental condition commonly encountered. In the present study, we carried out a comparative physiological and proteomic study to investigate lactic-acid-induced alterations in Lactobacillus casei Zhang (WT) and its acid-resistant mutant. Analysis of the physiological data showed that the mutant exhibited 33.8% higher glucose phosphoenolpyruvate:sugar phosphotransferase system activity and lower glycolytic pH compared with the WT under acidic conditions. In addition, significant differences were detected in both cells during acid stress between intracellular physiological state, including intracellular pH, H(+)-ATPase activity, and intracellular ATP pool. Comparison of the proteomic data based on 2D-DIGE and i-TRAQ indicated that acid stress invoked a global change in both strains. The mutant protected the cells against acid damage by regulating the expression of key proteins involved in cellular metabolism, DNA replication, RNA synthesis, translation, and some chaperones. Proteome results were validated by Lactobacillus casei displaying higher intracellular aspartate and arginine levels, and the survival at pH 3.3 was improved 1.36- and 2.10-fold by the addition of 50-mM aspartate and arginine, respectively. To our knowledge, this is the first demonstration that aspartate may be involved in acid tolerance in Lactobacillus casei. Results presented here may help us understand acid resistance mechanisms and help formulate new strategies to enhance the industrial applications of this species.

Changes in ultrastructure and Fourier transform infrared spectrum of Salmonella enterica serovar typhimurium cells after exposure to stress conditions

The effect of exposure to acid (pH 2.5), alkaline (pH 11.0), heat (55°C), and oxidative (40 mM H₂O₂) lethal conditions on the ultrastructure and global chemical composition of Salmonella enterica serovar Typhimurium CECT 443 cells was studied using transmission electron microscopy and Fourier transform infrared spectroscopy (FT-IR) combined with multivariate statistical methods (hierarchical cluster analysis and factor analysis). Infrared spectra exhibited marked differences in the five spectral regions for all conditions tested compared to those of nontreated control cells, which suggests the existence of a complex bacterial stress response in which modifications in a wide variety of cellular compounds are involved. The visible spectral changes observed in all of the spectral regions, together with ultrastructural changes observed by transmission electron microscopy and data obtained from membrane integrity tests, indicate the existence of membrane damage or alterations in membrane composition after heat, acid, alkaline, and oxidative treatments. Results obtained in this study indicate the potential of FT-IR spectroscopy to discriminate between intact and injured bacterial cells and between treatment technologies, and they show the adequacy of this technique to study the molecular aspects of bacterial stress response.