Brady Carter

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

A number of recent outbreaks related to pathogens in low-moisture foods have created urgency for studies to understand the possible causes and identify potential treatments to improve low-moisture food safety. Thermal processing holds the potential to eliminate pathogens such as Salmonella in low-moisture foods. Water activity (aw ) has been recognized as one of the primary factors influencing the thermal resistance of pathogens in low-moisture foods. But most of the reported studies relate thermal resistance of pathogens to aw of low-moisture foods at room temperature. Water activity is a thermodynamic property that varies significantly with temperature and the direction of variation is dependent on the product component. Accurate methods to determine aw at elevated temperatures are needed in related research activities and industrial operations. Adequate design of commercial thermal treatments to control target pathogens in low-moisture products requires knowledge on how aw values change in different foods at elevated temperatures. This paper presents an overview of the factors influencing the thermal resistance of pathogens in low-moisture foods. This review focuses on understanding the influence of water activity and its variation at thermal processing temperature on thermal resistance of pathogens in different low-moisture matrices. It also discusses the research needs to relate thermal resistance of foodborne pathogens to aw value in those foods at elevated temperatures.

A new method to determine the water activity and the net isosteric heats of sorption for low moisture foods at elevated temperatures

In recent years, research studies have shown that the thermal resistance of foodborne pathogens in the low moisture foods is greatly influenced by the water activity (aw) at temperatures relevant to thermal treatments for pathogen control. Yet, there has been a lack of an effective method for accurate measurement of aw at those temperatures. Thus, the main aim of this study was to evaluate a new method for measuring aw of food samples at elevated temperatures. An improved thermal cell with a relative humidity and temperature sensor was used to measure the aw of the three different food samples, namely, organic wheat flour, almond flour, and non-fat milk powder, over the temperature range between 20 and 80°C. For a constant moisture content, the aw data was used to estimate the net isosteric heat of sorption (qst). The qst values were then used in the Clausius Clapeyron equation (CCE) equation to estimate the moisture sorption isotherm for all test food samples at different temperatures. For all the tested samples of any fixed moisture content, aw value generally increased with the temperature. The energy for sorption decreased with increasing moisture content. With the experimentally determined qst value, CCE describes well about the changes in aw of the food samples between 20 and 80°C. This study presents a method to obtain aw of a food sample for a specific moisture content at different temperatures which could be extended to obtain qst values for different moisture contents and hence, the moisture sorption isotherm of a food sample at different temperatures.

Using Dielectric Relaxation Spectroscopy to Characterize the Glass Transition Time of Polydextrose.

Dielectric relaxation spectroscopy was used to characterize the glass transition time, tg , of polydextrose, where the glass transition temperature, Tg , and water activity, aw (relative humidity), were held constant during polydextrose relaxation. The tg was determined from a shift in the peak frequency of the imaginary capacitance spectrum with time. It was found that when the peak frequency reaches 30 mHz, polydextrose undergoes glass transition. Glass transition time, tg , is the time for polydextrose to undergo glass transition at a specific Tg and aw . Results lead to a modified state diagram, where Tg is depressed with increasing aw . This curve forms a boundary: (a) below the boundary, polydextrose does not undergo glass transition and (b) above the boundary, polydextrose rapidly undergoes glass transition. As the boundary curve is specified by a tg value, it can assist in the selection of storage conditions. An important point on the boundary curve is at aw = 0, where Tg0 = 115 °C. The methodology can also be used to calculate the stress-relaxation viscosity of polydextrose as a function of Tg and aw , which is important when characterizing the flow properties of polydextrose initially in powder form.

The critical water activity from dynamic dewpoint isotherms as an indicator of crispness in low moisture cookies

Low moisture cookie snacks are expected to possess a crisp texture when consumed. If this crispness is lost, the product is deemed unacceptable to the consumer. The most important factors influencing the crispness of low moisture cookies are moisture and temperature. Studies have shown that there exists a critical water activity where desirable crispness will be lost. Typically, this critical water activity would be obtained through an extensive texture study. However, high resolution dynamic isotherm curves have recently been shown to identify critical water activity values (RHc) by sharp inflections in the adsorption curve. The purpose of this study was to determine if the dynamic isotherm curves for low water activity snack cookies could similarly be used to identify an RHc and if this RHc could be used as a stability indicator. Dynamic isotherms, developed for two low moisture cookies at three different temperatures, were used to determine the RHc. Then, samples preconditioned to various water activity values and temperatures were analyzed for crispness. The effect of water activity on crispness was more important than temperature. Cookie samples at water activity values less than RHc maintained their crispness, but suffered an abrupt loss in texture at water activity values higher than RHc, yielding a sigmoidal shaped response. Fermi’s equation for sigmoidal response was used to model the response of crispness to water activity and estimate a critical water activity for texture loss. Both the RHc and Fermi’s critical water activity were found in the range of texture loss, but the RHc corresponded with the initial loss in texture, while Fermi’s critical water activity was at the midpoint of the texture loss. Since the RHc can be obtained with much less labor and time than a texture study and provides an effective indication of the loss of crispness, it was concluded that the RHc provides a viable alternative for determining the critical water activity for crispness.

The critical water activity from dynamic dewpoint isotherms as an indicator of pre-mix powder stability

Premix instant powders offer quick, easy alternatives for making common food products such as gravy, muffins, and beverages. To remain viable, premix instant powders must be free of caking while maintaining their wettability, solubility, and dissolution properties. The main extrinsic factors that influence the stability of powders are temperature and water activity. The critical water activity from dynamic isotherms has been demonstrated to be an effective moisture related specification to maintain the stability of glassy powders. Premix instant powders are often primarily wheat flour, which has a high glass transition, so it is questionable if a critical water activity associated with stability can be found. The purpose of this study was to determine if a critical water activity can be determined for premix instant powders using dynamic isotherms and if this critical water activity is associated with product stability. Instant oatmeal muffin powder and instant chicken gravy powder were found to have a critical water activity at approximately 0.70 aw. Samples of the premix powders were then equilibrated to water activities above and below the critical water activity and examined for changes in caking strength. The mean caking strengths at water activities less than the critical water activity were significantly higher than at higher water activities. The premix samples also experienced color and other appearance changes above the critical water activity. The changes in the premix powders’ matrices that occurred at the critical water activity resulted in degradation of the powder, indicating that the critical water activity provides an effective specification to prevent product loss without requiring an extensive stability study.

The Case for Water Activity as a Specification for Wheat Tempering and Flour Production

Moisture plays an important role in processing wheat grain into flour, from proper grain tempering to flour stability. The moisture properties of dry grain, tempered grain, and finished flour are currently tracked using moisture content. However, stability factors such as microbial growth and chemical stability are better correlated to water activity rather than moisture content. To determine whether water activity could be utilized as a more effective specification for wheat flour production, the water activity and moisture sorption properties of dry grain, tempered grain, farina, and flour were determined. Results showed the water activity of dry grain was sufficiently low for long-term stability. Dynamic isotherms of dry grain indicated critical water activity in which the seed coat lost its resistance to water penetration, which corresponded to the water activity of tempered wheat. Finally, the water activity of finished flour and farina typically was below the critical level for mold growth. Based on...