Xiaoxi Kou

Dielectric properties of almond kernels associated with radio frequency and microwave pasteurization

To develop advanced pasteurization treatments based on radio frequency (RF) or microwave (MW) energy, dielectric properties of almond kernels were measured by using an open-ended coaxial-line probe and impedance analyzer at frequencies between 10 and 3000 MHz, moisture contents between 4.2% to 19.6% w.b. and temperatures between 20 and 90 °C. The results showed that both dielectric constant and loss factor of the almond kernels decreased sharply with increasing frequency over the RF range (10–300 MHz), but gradually over the measured MW range (300–3000 MHz). Both dielectric constant and loss factor of almond kernels increased with increasing temperature and moisture content, and largely enhanced at higher temperature and moisture levels. Quadratic polynomial equations were developed to best fit the relationship between dielectric constant or loss factor at 27, 40, 915 or 2450 MHz and sample temperature/moisture content with R2 greater than 0.967. Penetration depth of electromagnetic wave into samples decreased with increasing frequency (27–2450 MHz), moisture content (4.2–19.6% w.b.) and temperature (20–90 °C). The temperature profiles of RF heated almond kernels under three moisture levels were made using experiment and computer simulation based on measured dielectric properties. Based on the result of this study, RF treatment has potential to be practically used for pasteurization of almond kernels with acceptable heating uniformity.

Performance of a Heating Block System Designed for Studying the Heat Resistance of Bacteria in Foods

Knowledge of bacteria’s heat resistance is essential for developing effective thermal treatments. Choosing an appropriate test method is important to accurately determine bacteria’s heat resistances. Although being a major factor to influence the thermo-tolerance of bacteria, the heating rate in samples cannot be controlled in water or oil bath methods due to main dependence on sample’s thermal properties. A heating block system (HBS) was designed to regulate the heating rates in liquid, semi-solid and solid foods using a temperature controller. Distilled water, apple juice, mashed potato, almond powder and beef were selected to evaluate the HBS’s performance by experiment and computer simulation. The results showed that the heating rates of 1, 5 and 10 °C/min with final set-point temperatures and holding times could be easily and precisely achieved in five selected food materials. A good agreement in sample central temperature profiles was obtained under various heating rates between experiment and simulation. The experimental and simulated results showed that the HBS could provide a sufficiently uniform heating environment in food samples. The effect of heating rate on bacterial thermal resistance was evaluated with the HBS. The system may hold potential applications for rapid and accurate assessments of bacteria’s thermo-tolerances.

Identifying possible non-thermal effects of radio frequency energy on inactivating food microorganisms

Radio frequency (RF) heating has been successfully used for inactivating microorganisms in agricultural and food products. Athermal (non-thermal) effects of RF energy on microorganisms have been frequently proposed in the literature, resulting in difficulties for developing effective thermal treatment protocols. The purpose of this study was to identify if the athermal inactivation of microorganisms existed during RF treatments. Escherichia coli and Staphylococcus aureus in apple juice and mashed potato were exposed to both RF and conventional thermal energies to compare their inactivation populations. A thermal death time (TDT) heating block system was used as conventional thermal energy source to simulate the same heating treatment conditions, involving heating temperature, heating rate and uniformity, of a RF treatment at a frequency of 27.12 MHz. Results showed that a similar and uniform temperature distribution in tested samples was achieved in both heating systems, so that the central sample temperature could be used as representative one for evaluating thermal inactivation of microorganisms. The survival patterns of two target microorganisms in two food samples were similar both for RF and heating block treatments since their absolute difference of survival populations was <1 log CFU/ml. The statistical analysis indicated no significant difference (P > 0.05) in inactivating bacteria between the RF and the heating block treatments at each set of temperatures. The solid temperature and microbial inactivation data demonstrated that only thermal effect of RF energy at 27.12 MHz was observed on inactivating microorganisms in foods.

Effect of water activity and heating rate on Staphylococcus aureus heat resistance in walnut shells

Water activity (aw) and heating rate have shown important effects on the thermo-tolerance of pathogens in low moisture foods during thermal treatments. In this study, three strains were selected to compare the heat resistance in walnut shell powder and finally the most heat resistant S. aureus ATCC 25923 was chosen to investigate the influence of aw and heating rate using a heating block system (HBS). The results showed that S. aureus ATCC 25923 became more thermo-tolerant at lower aw. The D-values of S. aureus ATCC 25923 increased with decreasing water activity and heating rates (<1°C/min). A significant increase in heat resistance of S. aureus ATCC 25923 in walnut shell powder was observed only for the heating rates of 0.2 and 0.5°C/min but not at 1, 5 and 10°C/min. There was a rapid reduction of S. aureus ATCC 25923 at elevated temperatures from 26 to 56°C at a heating rate of 0.1°C/min. The inactivation under non-isothermal conditions was better fitted by Weibull distribution (R2=0.97 to 0.99) than first-order kinetics (R2=0.88 to 0.98). These results suggest that an appropriate increase in moisture content of in-shell walnuts and heating rate during thermal process can improve the inactivation efficiency of pathogens in low moisture foods.

Inactivation kinetics of food-borne pathogens subjected to thermal treatments: a review

Abstract Thermal processing technologies are safe and easy to control methods without leaving residues, and could be used to inactivate food-borne pathogens, ensure food quality and provide the food with sufficient stability during storage. Establishing inactivation kinetics of food-borne pathogens is essential in developing effective pasteurisation protocols without damaging food quality. This study presents a comprehensive review of recent progresses in inactivation kinetics of food-borne pathogens. It covers theoretical bases and experimental methods for developing thermal inactivation kinetics of food-borne pathogens and making comparisons and applications of the common thermal death kinetic models. Finally, it proposes possible recommendations on the future research directions of establishing inactivation kinetic models for food-borne pathogens in thermal processing.

Thermal inactivation of Aspergillus flavus in peanut kernels as influenced by temperature, water activity and heating rate

Infection of Aspergillus flavus, which can produce aflatoxin, is a major problem for peanut safe storage. Thermal inactivation kinetics of Aspergillus flavus is essential to design an effective heat treatment process. In this study, thermal inactivation kinetics of Aspergillus flavus in peanut kernel flour at four water activity (aw) levels (0.720, 0.783, 0.846, and 0.921) with three temperatures for each aw was studied using a thermal-death-time heating block system and fitted with first-order kinetic and Weibull models. The influence of heating rates on thermotolerance of Aspergillus flavus was also investigated. The results showed that the Weibull distribution model had better coefficient of determination from 0.954 to 0.996, as compared to that (from 0.866 to 0.980) of the first-order kinetic model. An upward concavity was found with the inactivation curve, indicating a tailing effect. Model parameters (D, δ, and p) were estimated with the modified Bigelow equations to predict survival curves of Aspergillus flavus at any temperature and aw. The reduced heat resistance of Aspergillus flavus at high heating rates above 1 °C/min suggests that developing fast thermal processes is preferred for pasteurizing peanuts in food industry. A case study was presented for applying the cumulated lethal time model to design the industrial heating process based on the thermal kinetics of Aspergillus flavus.