Water activity change at elevated temperatures and thermal resistance of Salmonella in all purpose wheat flour and peanut butter

Abstract

Water activity (a w) is a major factor affecting pathogen heat resistance in low-moisture foods. However, there is a lack of data for a w at elevated temperatures that occur during actual thermal processing conditions, and its influence on thermal tolerance of pathogens. The objective of this study was to gain an in-depth understanding of the relationship between temperature-induced changes in a w and thermal resistance of Salmonella in all purpose flour and peanut butter at elevated temperatures. Equilibrium water sorption isotherms (water content vs. water activity) for all purpose flour and peanut butter over the range of 20 to 80ºC were generated using a vapor sorption analyzer and a newly developed thermal cell. The thermal resistance (D 80-values) of Salmonella in all purpose flour and peanut butter with initial a w of 0.45 (measured at room temperature, ~20 o C) was determined via isothermal treatment of small (< 1 g) samples. When increasing sample temperature from 20 to 80ºC in sealed cells, the a w of all purpose flour increased from 0.45 to 0.80, but the a w of peanut butter decreased from 0.45 to 0.04. The corresponding estimated D 80-values of Salmonella in all purpose flour and peanut butter with room temperature a w of 0.45 were 6.9 ± 0.7 min and 17.0 ± 0.9 min, respectively. The significantly (P < 0.05) higher D 80-value of Salmonella in peanut butter than in all purpose flour may be partially attributed to the reduced a w in peanut butter in comparison to the increased a w in all purpose flour at 80 o C. The improved understanding of the temperature-induced changes in a w of low-moisture products of different composition provides a new insight into seemly unpredictable results, when using heat treatments to control Salmonella in such food systems.

Comparative influence of salinity and temperature on cassava flour product by Lactobacillus plantarum and Lactobacillus acidophilus during single culture fermentation

“…It also was lower than on wheat flour (10.74%) and arrowroot flour (11.37%) (USDA 2016).The maximum moisture content of modified cassava flour according to SNI 7622-2011 is 13%. The low water content means a good keeping in quality of the flour products (Ojokoh et al 2014;Syamaladevi et al 2016).…”

Water Diffusion from a Bacterial Cell in Low‐Moisture Foods

“…We selected a 10% increment change in relative humidity to achieve water activity intervals of 0.1 for the water sorption curve. The equilibrium water content values were recorded at each water activity level (Syamaladevi and others ). Average values of at least 2 replicates of water contents and water activities were determined.…”

“…To generate water sorption isotherms of bacterial cells at 80 °C, we used a newly developed sealed thermal cell containing a commercial relative humidity sensor (HX15‐W, Omega Engineering, Inc.) (Syamaladevi and others ). We selected 80 °C for the isotherm testing, as this was used as a treatment temperature in several reported studies on pathogen control in low‐moisture foods (Ha and others ; Syamaladevi and others ). The high temperature relative humidity sensor measured the relative humidity of the air headspace above the bacterial cells at 80 °C at equilibrium.…”

“…We calibrated this instrumental set‐up using the reported values of water activities of lithium chloride (13.41 M) and sodium chloride (6 M) solutions at different temperatures, obtained from their osmotic coefficients (Gibbard and Scatchard ; Gibbard and others ). The water sorption isotherm data of bacterial cells were fitted with Guggenheim, Anderson, and de Boer (GAB) model (Syamaladevi and others ). The GAB model is expressed as: where X is the dry basis water content of the material, X m is the monolayer water content (dry basis), C and K are parameters which have physical meaning based on the multilayer adsorption of water.…”

“…At a fixed water content, water activity of a material changes with temperature. The degree of such change depends on material composition (Syamaladevi and others ). Thus, when a low‐moisture food contaminated with bacterial cells is heated to a high temperature, the water activity of the food may differ from that of the bacterial cells.…”