Decimal reduction times of acid-adapted and non-adapted Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes in young Cocos nucifera Linn. liquid endosperm

Investigation of the potential direct and cross protection effects of sublethal injured Salmonella Typhimurium induced by radio frequency heating stress

Many studies demonstrated that radio frequency (RF) was an effective pasteurization method for low-moisture foods (LMFs), and our previous study confirmed RF heating stress generated sublethal injured cells (SICs) of Salmonella enterica serovar Typhimurium (S. Typhimurium) in red pepper powder with initial a_w ≥ 0.53. So this study investigated the potential direct protection and cross protection effects of the SICs of S. Typhimurium to multiple stresses, and analyzed fatty acid composition and cell morphology. Results showed that the SICs were repaired after incubating for 5 h, and there were no obvious direct and cross protection effects by exposing to different external stresses (heat, 15% ethanol, pH 3.0 acid buffer solution, 10% salt). According to the fatty acid composition analysis, no significant difference (p > 0.05) between the ratio of unsaturated to saturated fatty acids (UFA/SFA) was observed for SICs of S. Typhimurium and control cells, indicating the same membrane fluidity which can support the experimental results. This study investigated and confirmed there are no direct and cross protection effects for the SICs of S. Typhimurium induced by RF heating stress, and it would be helpful for deeply understand the response of pathogens under RF heating stress.

Strain Variability in Growth and Thermal Inactivation Characteristics of Listeria monocytogenes Strains after Acid Adaptation

ABSTRACT

Given the importance of strain variability to predictive microbiology and risk assessment, this study aimed to quantify the magnitude of strain variability in growth and thermal inactivation kinetics behaviors after acid adaptation. Thirty-three Listeria monocytogenes strains were exposed to acid-adapted tryptic soy broth supplemented with yeast extract (TSBYE; pH 5.5) and non-acid-adapted TSBYE (pH 7.0) for 20 h. Next, the growth parameters of these adapted and nonadapted strains that grew in nonbuffered TSBYE at 25°C were estimated. The tested strains were inactivated at 60°C in nonbuffered broth to obtain the heat resistance parameters. The results revealed that strain variability was present in the growth and thermal inactivation characteristics. The maximum specific growth rate ranged from 0.21 to 0.44 h-1 and from 0.20 to 0.45 h-1 after acid and nonacid adaptation, respectively. The lag times were from 0.69 to 2.56 h and from 0.24 to 3.36 h for acid-adapted and non-acid-adapted cells, respectively. The apparent D-values at 60°C of the pathogen ranged between 0.56 and 3.93 min and between 0.52 and 3.63 min for the presence and absence of acid adaptation condition, respectively. Acid adaptation significantly (P < 0.05) increased the magnitude of strain variability in the thermal inactivation characteristics of L. monocytogenes, with the coefficient of variation increasing to 0.17, whereas acid adaptation did not significantly (P ≥ 0.05) influence the variabilities in the growth parameters of the tested strains. Furthermore, the subsequent growth behaviors of all strains did not exhibit significant (P > 0.05) changes after exposure to acidic broth. However, the thermal resistance of most (25 of 33) of the tested strains increased (P < 0.05) after growing in acid-adapted broth. The relevant data generated in the present study can be used to describe the strain variability in predictive microbiology and to deeply understand the behavioral responses of different strains to acid adaptation.

HIGHLIGHTS

Microbial shelf life of coconut water subjected to various inoculation levels of Listeria monocytogenes and storage conditions

The study determined the growth kinetic parameters of a cocktail of Listeria monocytogenes 1/2c and 4b strains in coconut water (pH 4.76, 5.0°Brix, 0.09% malic acid, a_w 0.998) subjected to low (~2.0 log CFU/mL) and high (~4.0 log CFU/mL) contamination levels, and exposed to different storage temperatures (4 °C, 17 °C, 30 °C, and 35 °C). The pathogen proliferated in all tested conditions except in that with low contamination stored at 4 °C. Despite not growing at 4 °C, the pathogen was detectable throughout the storage period, which lasted for almost 400 h. In conditions where the pathogens proliferated, growth lag (t_lag) ranged from 0.0 to 68.3 h. The growth rates (K_G) ranged from 0.05 to 0.48 log CFU/h, while the final populations ranged from 6.3 to 8.7 log CFU/mL. Both storage temperature and contamination level significantly (P < 0.05) affected the growth parameters. Sanitary risk times (SRT) were determined with the microbiological shelf life (SL) of coconut water. In some of the conditions tested, SRT took place before SL (SRT < SL), emphasizing the importance of having good hygienic and manufacturing practices in place for such a vulnerable commodity.

Combination of mild heat and plant essential oil constituents to inactivate resistant variants of Escherichia coli in buffer and in coconut water

The growing demand for minimally processed foods with clean labels has stimulated research into mild processing methods and natural antimicrobials to replace intensive heating and conventional preservatives, respectively. However, we have previously demonstrated that repetitive exposure of some bacteria to mild heat or subinhibitory concentrations of essential oil constituents (EOCs) may induce the emergence of mutants with increased resistance to these treatments. Since the combination of mild heat with some EOCs has a synergistic effect on microbial inactivation, we evaluated the potential of such combinations against our resistant E. coli mutants. While citral, carvacrol and t-cinnamaldehyde synergistically increased heat inactivation (53.0 °C, 10 min) of the wild-type MG1655 suspended in buffer, only the combination with carvacrol (200 μl/l) was able to mitigate the increased resistance of all the mutants. Moreover, the combination of heat and carvacrol acted synergistically inactivating heat-resistant variants of E. coli O157:H7 (ATCC 43888). This combined treatment could synergistically achieve more than 5 log₁₀ reductions of the most resistant mutants in coconut water, although the temperature had to be raised to 57.0 °C. Therefore, the combination of mild heat with carvacrol appears to hold promise for mild processing, and it is expected to counteract the development of heat resistance.

Decimal reduction energies of UV-C-irradiated spoilage yeasts in coconut liquid endosperm

The ultraviolet-C (UV-C) decimal reduction energy (D_UV-C) values of 17 spoilage yeasts and their composited inoculum were determined in coconut liquid endosperm (pH 5.26, 5.8 °Brix, 0.04% malic acid, 0.17% w/v insoluble solids). Growth kinetic parameters of all the test yeast strains were first established to standardize the growth stage of the cells prior to inactivation studies. Approximately 4.0 to 5.0 log CFU/mL cells in the mid-stationary growth phase (30.3 to 39.9 h, 25 °C) were suspended in 4 mL turbulent flowing juice and subjected to UV-C irradiation at a surface irradiance range of 3.42 to 4.99 mW/cm². Survivor populations after exposure to predetermined UV-C energy were enumerated, and were used to derive the D_UV-C values using the linear regression and Baranyi and Roberts (1994) model fitting. Results show that the yeast strains exhibited either log-linear or biphasic inactivation behavior with inactivation lag. The most UV-C resistant spoilage yeast was found to be Cryptococcus albidus (LJY1) with D_UV-C values of 122.72 and 214.89 mJ/cm² determined from linear regression and model-fitting, respectively. The least UV-C resistant was Torulaspora delbrueckii (LYJ5) with a D_UV-C of 17.34 (linear regression) and 17.35 mJ/cm² (model-fitting). The D_UV-C values determined from the model fitting were generally greater than those calculated from linear regression, although only those determined for C. albidus were significantly different. To the investigators' knowledge, this is the first report of the UV-C inactivation kinetic parameters of Kluyveromyces marxianus, Trichosporon cutaneum, Pichia anomala, and Meyerozyma guilliermondii and C. albidus in coconut liquid endosperm. The results of this study can be used in the establishment and validation of UV-C process schedules for coconut liquid endosperm and other similar commodities.

Evaluation of sodium benzoate and licorice (Glycyrrhiza glabra) root extract as heat-sensitizing additives against Escherichia coli O157:H7 in mildly heated young coconut liquid endosperm

This study evaluated the use of sodium benzoate (SB) and licorice root extract (LRE) as heat-sensitizing additives against Escherichia coli O157:H7 in mildly heated young coconut liquid endosperm. Consumer acceptance scoring showed that maximum permissible supplementation (MPS) levels for SB and LRE were at 300 and 250 ppm, respectively. The MPS values were considered in the generation of a 2-factor rotatable central composite design for the tested SB and LRE concentration combinations. Liquid endosperm with various SB and LRE supplementation combinations was inoculated with E. coli O157:H7 and heated to 55°C. The susceptibility of the cells towards heating was expressed in terms of the decimal reduction time (D55 ). Response surface analysis showed that only the individual linear effect of benzoate significantly influenced D55 value, where increasing supplementation level resulted in increasing susceptibility. The results reported could serve as baseline information in further investigating other additives that could be used as heat-sensitizing agents against pathogens in heat-labile food systems.

SIGNIFICANCE AND IMPACT OF THE STUDY

Fruit juice products have been linked to outbreaks of microbial infection, where unpasteurized products were proven vectors of diseases. Processors often opt not to apply heat process to juice products as the preservation technique often compromises the sensorial quality. This work evaluated two common additives for their heat-sensitizing effects against E. coli O157:H7 in coconut liquid endosperm, the results of which may serve as baseline information to small- and medium-scale processors, and researchers in the establishment of mild heat process schedule for the test commodity and other similar products.