Mei-Jun Zhu

Exponentially Increased Thermal Resistance of Salmonella spp. and Enterococcus faecium at Reduced Water Activity

ABSTRACT Salmonella spp. exhibit prolonged survivability and high tolerance to heat in low-moisture foods. The reported thermal resistance parameters of Salmonella spp. in low-moisture foods appear to be unpredictable due to various unknown factors. We report here that temperature-dependent water activity (aw, treatment temperature) plays an important role in the sharply increased thermal resistance of Salmonella enterica serovar Enteritidis PT 30 and its potential surrogate Enterococcus faecium NRRL B-2354. In our study, silicon dioxide granules, as carriers, were separately inoculated with these two microorganisms and were heated at 80°C with controlled relative humidity between 18 and 72% (resulting in corresponding aw,80°C values for bacteria between 0.18 and 0.72) in custom-designed test cells. The inactivation kinetics of both microorganisms fitted a log-linear model (R2, 0.83 to 0.97). Reductions in the aw,80°C values of bacterial cells exponentially increased the D80°C (the time needed to achieve a 1-log reduction in a bacterial population at 80°C) values for S. Enteritidis and E. faecium on silicon dioxide. The log-linear relationship between the D80°C values for each strain in silicon dioxide and its aw,80°C values was also verified for organic wheat flour. E. faecium showed consistently higher D80°C values than S. Enteritidis over the aw,80°C range tested. The estimated zaw (the change in aw,80°C needed to change D80°C by 1 log) values of S. Enteritidis and E. faecium were 0.31 and 0.28, respectively. This study provides insight into the interpretation of Salmonella thermal resistance that could guide the development and validation of thermal processing of low-moisture foods. IMPORTANCE In this paper, we established that the thermal resistance of the pathogen S. Enteritidis and its surrogate Enterococcus faecium, as reflected by D values at 80°C, increases sharply with decreasing relative humidity in the environment. The log-linear relationship between the D80°C values of each strain in silicon dioxide and its aw,80°C values was also verified for organic wheat flour. The results provide new quantitative insight into the way in which the thermal resistance of microorganisms changes in low-moisture systems, and they should aid in the development of effective thermal treatment strategies for pathogen control in low-moisture foods.

Enterococcus faecium as a Salmonella surrogate in the thermal processing of wheat flour: Influence of water activity at high temperatures

This study investigated the influence of temperature-dependent water activity (aw) on thermal resistances of Enterococcus faecium NRRL B-2354 (E.xa0faecium) and Salmonella Enteritidis PT 30 (S. Enteritidis) in wheat flour. The aw for wheat flour samples at 20, 40, and 60u202f°C was determined by a vapor sorption analyzer and at 75, 80 and 85u202f°C using custom-built thermal cells with high temperature humidity sensors. Full-factorial isothermal inactivation studies of both strains in sealed aluminum-test-cells included three temperatures (75, 80, and 85u202f°C) and three aw,25°C levels (0.30, 0.45 and 0.60 within ±0.02 range, prior to the thermal treatments). Isotherm results of wheat flour demonstrate a significant increase (Pu202f<u202f0.05) of aw as temperature rises (e.g. aw,25°Cu202f=u202f0.45u202f±u202f0.02 became aw,80°Cu202f=u202f0.71u202f±u202f0.02 in a closed system). Inactivation kinetics of both microorganisms fitted a log-linear model, the yielded D-values varied from 2.7u202f±u202f0.2u202fmin (D85°C of S. Enteritidis at aw,25°C 0.60u202f±u202f0.02) to 65.8u202f±u202f2.5u202fmin (D75°C of E.xa0faecium at aw,25°C 0.30u202f±u202f0.02). The zT of E.xa0faecium and S. Enteritidis decreased from 16.4 and 16.9u202f°C, respectively, to 10.2u202f°C with increased moisture content (dry basis) from 10 to 14%. Under all tested conditions, E.xa0faecium exhibited equal or higher (1.0-3.1 times) D- and zT-values than those of Salmonella. Overall, E.xa0faecium should be a conservative surrogate for Salmonella in thermal processing of wheat flour for control of Salmonella over a moisture content of 10-14% and treatment temperatures between 75 and 85u202f°C.

Survival of Listeria innocua on Fuji apples under commercial cold storage with or without low dose continuous ozone gaseous

This study evaluated the fate of Listeria innocua, a non-pathogenic species closely related to Listeria monocytogenes, on Fuji apple fruit surfaces during commercial cold storage with and without continuous low doses of gaseous ozone. Unwaxed Fuji apples of commercially acceptable maturity were inoculated with 6.0-7.0 Log10u202fCFU L.xa0innocua/apple, and subjected to refrigerated air (RA, 33u202f°F), controlled atmosphere (CA, 33u202f°F, 2% O2, 1% CO2), or CA with low doses of ozone gas (50.0u202f-87.0u202fppbu202f) storage in a commercial facility for 30 weeks. A set of uninoculated apples was simultaneously subjected to the above storage conditions for total plate count and yeasts and molds enumeration. L.xa0innocua survival under RA and CA storage was similar, which led to 2.5-3.0 Log10u202fCFU/apple reduction during storage. Continuous gaseous ozone application decreased L.xa0innocua population on Fuji apples to ∼1.0 Log10u202fCFU/apple after 30-week storage, and suppressed apple native flora. CA storage delayed apple fruit ripening through reduction of apple firmness and titratable acidity loss, and low dose gaseous ozone application had no negative influence on apple visual quality, including both external and internal disorders. In summary, L.xa0innocua decreased on Fuji apple surfaces during commercial long-term RA and CA storage. Ozone gas has the potential to be used as a supplemental intervention method to control Listeria spp. and to ensure fresh apple safety.