Influence of Water Activity on Thermal Resistance of Microorganisms in Low-Moisture Foods: A Review

ABSTRACTSalmonellaspp. exhibit prolonged survivability and high tolerance to heat in low-moisture foods. The reported thermal resistance parameters ofSalmonellaspp. 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 ofSalmonella entericaserovar Enteritidis PT 30 and its potential surrogateEnterococcus faeciumNRRL 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°Cvalues 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°Cvalues of bacterial cells exponentially increased theD80°C(the time needed to achieve a 1-log reduction in a bacterial population at 80°C) values forS. Enteritidis andE. faeciumon silicon dioxide. The log-linear relationship between theD80°Cvalues for each strain in silicon dioxide and its aw,80°Cvalues was also verified for organic wheat flour.E. faeciumshowed consistently higherD80°Cvalues thanS. Enteritidis over the aw,80°Crange tested. The estimated zaw(the change in aw,80°Cneeded to changeD80°Cby 1 log) values ofS. Enteritidis andE. faeciumwere 0.31 and 0.28, respectively. This study provides insight into the interpretation ofSalmonellathermal resistance that could guide the development and validation of thermal processing of low-moisture foods.IMPORTANCEIn this paper, we established that the thermal resistance of the pathogenS. Enteritidis and its surrogateEnterococcus faecium, as reflected byDvalues at 80°C, increases sharply with decreasing relative humidity in the environment. The log-linear relationship between theD80°Cvalues of each strain in silicon dioxide and its aw,80°Cvalues 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.

Download Full-text

Effect of Talc as a Dry-Inoculation Carrier on Thermal Resistance of Enterococcus faecium NRRL B-2354 in Almond Meal

Journal of Food Protection ◽

10.4315/0362-028x.jfp-18-543 ◽

2019 ◽

Vol 82 (7) ◽

pp. 1110-1115 ◽

Cited By ~ 3

Author(s):

NURUL HAWA AHMAD ◽

CEMRE ÖZTABAK ◽

BRADLEY P. MARKS ◽

ELLIOT T. RYSER

Keyword(s):

Thermal Resistance ◽

Water Activity ◽

Enterococcus Faecium ◽

Thermal Inactivation ◽

Experimental Conditions ◽

Time Points ◽

Test Cells ◽

Low Moisture Foods ◽

Differential Medium ◽

Resistance To Heat

ABSTRACTDry inoculation (DI) methods using a dry carrier have gained considerable interest for assessing thermal inactivation of Salmonella and other microorganisms in low-moisture foods. However, the effect of carrier residues on microbial resistance to heat remains largely unknown. This study aimed to determine the effect of talc powder on thermal resistance of Enterococcus faecium NRRL-B2354 (a Salmonella surrogate) in almond meal at 0.45 water activity (aw). Whole almonds were either immersed in an E. faecium suspension for wet inoculation (WI) or mixed with inoculated talc powder for DI. Two additional experimental conditions, inoculation of WI almond meal with added uninoculated talc (WT) and inoculated talc powder alone, were conducted. After WI, DI, and WT, the almonds were equilibrated to 0.45 aw, ground into a meal, and reequilibrated to 0.45 aw. Isothermal treatments were performed by heating almond meal (about 1 g) in aluminum test cells in a water bath at 80°C, with samples collected at more than five sequential time points from triplicate isothermal runs. E. faecium was enumerated by immediately cooling, diluting, and plating the samples on a nonselective or differential medium. E. faecium was more thermally resistant in DI (D80°C: 63.5 ± 1.9 min) compared with WI almond meal (D80°C: 40.5 ± 1.0 min; P < 0.05), but the resistance in WT almond meal (46.9 ± 0.9 min) was between and different from (P < 0.05) both DI and WI. E. faecium was less resistant in talc powder alone (20.6 ± 1.1 min) compared with all other almond meal samples. Overall, residual talc affected the thermal resistance of E. faecium. Therefore, when determining thermal resistance or validating commercial processes, carriers such as talc should not be used for inoculation of low-moisture foods without first knowing their impact on the target organism.HIGHLIGHTS

Download Full-text

Improving design of thermal water activity cell to study thermal resistance of Salmonella in low-moisture foods

LWT ◽

10.1016/j.lwt.2018.02.046 ◽

2018 ◽

Vol 92 ◽

pp. 371-379 ◽

Cited By ~ 7

Author(s):

Ravi Kiran Tadapaneni ◽

Jie Xu ◽

Ren Yang ◽

Juming Tang

Keyword(s):

Thermal Resistance ◽

Water Activity ◽

Thermal Water ◽

Low Moisture Foods

Download Full-text

Effects of Inoculation Procedures on Variability and Repeatability of Salmonella Thermal Resistance in Wheat Flour

Journal of Food Protection ◽

10.4315/0362-028x.jfp-16-057 ◽

2016 ◽

Vol 79 (11) ◽

pp. 1833-1839 ◽

Cited By ~ 31

Author(s):

IAN M. HILDEBRANDT ◽

BRADLEY P. MARKS ◽

ELLIOT T. RYSER ◽

ROSSANA VILLA-ROJAS ◽

JUMING TANG ◽

...

Keyword(s):

Thermal Resistance ◽

Water Activity ◽

Wheat Flour ◽

Thermal Inactivation ◽

Culture Method ◽

Isothermal Treatment ◽

Inactivation Kinetics ◽

Inoculation Methods ◽

Low Moisture Foods ◽

D Values

ABSTRACT Limited prior research has shown that inoculation methods affect thermal resistance of Salmonella in low-moisture foods; however, these effects and their repeatability have not been systematically quantified. Consequently, method variability across studies limits utility of individual data sets and cross-study comparisons. Therefore, the objective was to evaluate the effects of inoculation methodologies on stability and thermal resistance of Salmonella in a low-moisture food (wheat flour), and the repeatability of those results, based on data generated by two independent laboratories. The experimental design consisted of a cross-laboratory comparison, both conducting isothermal Salmonella inactivation studies in wheat flour (~0.45 water activity, 80°C), utilizing five different inoculation methods: (i) broth-based liquid inoculum, (ii) lawn-based liquid inoculum, (iii) lawn-based pelletized inoculum, (iv) direct harvest of lawn culture with wheat flour, and (v) fomite transfer of a lawn culture. Inoculated wheat flour was equilibrated ~5 days to ~0.45 water activity and then was subjected to isothermal treatment (80°C) in aluminum test cells. Results indicated that inoculation method impacted repeatability, population stability, and inactivation kinetics (α = 0.05), regardless of laboratory. Salmonella inoculated with the broth-based liquid inoculum method and the fomite transfer of a lawn culture method exhibited instability during equilibration. Lawn-based cultures resulted in stable populations prior to thermal treatment; however, the method using direct harvest of lawn culture with wheat flour yielded different D-values across the laboratories (α = 0.05), which was attributed to larger potential impact of operator variability. The lawn-based liquid inoculum and the lawn-based pelletized inoculum methods yielded stable inoculation levels and repeatable D-values (~250 and ~285 s, respectively). Also, inoculation level (3 to 8 log CFU/g) did not affect D-values (using the lawn-based liquid inoculum method). Overall, the results demonstrate that inoculation methods significantly affect Salmonella population kinetics and subsequent interpretation of thermal inactivation data for low-moisture foods.

Download Full-text

Influence of water activity on the heat resistance of Salmonella enterica in selected low-moisture foods

International Journal of Food Microbiology ◽

10.1016/j.ijfoodmicro.2020.108813 ◽

2020 ◽

Vol 334 ◽

pp. 108813

Author(s):

Bina Gautam ◽

Byju N. Govindan ◽

Michael Gӓnzle ◽

M.S. Roopesh

Keyword(s):

Heat Resistance ◽

Water Activity ◽

Salmonella Enterica ◽

Influence Of Water ◽

Low Moisture Foods

Download Full-text

Development of a Dry Inoculation Method for Thermal Challenge Studies in Low-Moisture Foods by Using Talc as a Carrier for Salmonella and a Surrogate (Enterococcus faecium)

Journal of Food Protection ◽

10.4315/0362-028x.jfp-14-396 ◽

2015 ◽

Vol 78 (6) ◽

pp. 1106-1112 ◽

Cited By ~ 27

Author(s):

ELENA ENACHE ◽

AI KATAOKA ◽

D. GLENN BLACK ◽

CARLA D. NAPIER ◽

RICHARD PODOLAK ◽

...

Keyword(s):

Thermal Resistance ◽

Heat Resistance ◽

Water Activity ◽

Enterococcus Faecium ◽

Thermal Inactivation ◽

Death Time ◽

Significant Difference ◽

Growth Method ◽

Low Moisture Foods ◽

Inoculum Preparation

The objective of this study was to obtain dry inocula of Salmonella Tennessee and Enterococcus faecium, a surrogate for thermal inactivation of Salmonella in low-moisture foods, and to compare their thermal resistance and stability over time in terms of survival. Two methods of cell growth were compared: cells harvested from a lawn on tryptic soy agar (TSA-cells) and from tryptic soy broth (TSB-cells). Concentrated cultures of each organism were inoculated onto talc powder, incubated at 35°C for 24 h, and dried for additional 24 h at room temperature (23 ± 2°C) to achieve a final water activity of ≤0.55 before sieving. Cell reductions of Salmonella and E. faecium during the drying process were between 0.14 and 0.96 log CFU/g, depending on growth method used. There was no difference between microbial counts at days 1 and 30. Heat resistance of the dry inoculum on talc inoculated into a model peanut paste (50% fat and 0.6 water activity) was determined after 1 and 30 days of preparation, using thermal death time tests conducted at 85°C. For Salmonella, there was no significant difference between the thermal resistance (D85°C) for the TSB-cells and TSA-cells (e.g. day 1 cells D85°C = 1.05 and 1.07 min, respectively), and there was no significant difference in D85°C between dry inocula on talc used either 1 or 30 days after preparation (P > 0.05). However, the use the dry inocula of E. faecium yielded different results: the TSB-grown cells had a significantly (P < 0.05) greater heat resistance than TSA-grown cells (e.g. D85°C for TSB-cells = 3.42 min versus 2.60 min for TSA-cells). E. faecium had significantly (P < 0.05) greater heat resistance than Salmonella Tennessee regardless what cell type was used for dry inoculum preparation; therefore, it proved to be a conservative but appropriate surrogate for thermal inactivation of Salmonella in low-moisture food matrices under the tested conditions.

Download Full-text