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

Oregano Oil, Epsilon-Polylysine and Citric Acid Assisted Inactivation of Salmonella in Two Kinds of Tahini during Thermal Treatment and Storage

Tahini and tahini-based products are popular with consumers due to their special flavor and high nutritional values, but often have been linked to Salmonella outbreaks. The objective of this study was to compare effects of different kinds of natural antimicrobials on Salmonella inactivation in undiluted and diluted tahini during thermal treatment and storage. Results showed that the Weibull model was more suitable to describe the thermal inactivation behavior of S. montevideo CICC21588 in two kinds of tahini than the first-order model. Inactivation curves were concave-upward in undiluted tahini but concave-downward in diluted tahini. During storage of undiluted tahini, 3% oregano oil caused extra 1.44 or 0.80 log CFU/g reductions after 7 days at 25 °C or 4 °C compared to the control and 0.5% citric acid caused an extra reduction of 0.75 log CFU/g after 7 d at 4 °C. For diluted tahini, 2–3% oregano oil and 0.4–0.5% ε-polylysine reduced more populations compared to undiluted tahini. These antimicrobials all inhibited the growth of S. montevideo during 24 h at 25 °C and ε-polylysine had the best effect. Furthermore, these antimicrobials enhanced the Salmonella inactivation in diluted tahini during thermal treatment, and there was less of a synergistic effect of thermal and antimicrobials in undiluted tahini due to less sublethal injured cells caused by heat. This study may provide useful information for Salmonella inactivation in tahini.

A Comparison of Three Methods for Determining Thermal Inactivation Kinetics: A Case Study on Salmonella enterica in Whole Milk Powder

ABSTRACT

Different methods for determining the thermal inactivation kinetics of microorganisms can result in discrepancies in thermal resistance values. In this study, thermal resistance of Salmonella in whole milk powder was determined with three methods: thermal death time (TDT) disk in water bath, pouches in water bath, and the TDT Sandwich system. Samples from three production lots of whole milk powder were inoculated with a five-strain Salmonella cocktail and equilibrated to a water activity of 0.20. The samples were then subjected to three isothermal treatments at 75, 80, or 85°C. Samples were removed at six time points and cultures were enumerated for survivors. The inactivation data were fitted to two consolidated models: two primary models (log linear and Weibull) and one secondary model (Bigelow). Normality testing indicated that all the model parameters were normally distributed. None of the model parameters for both consolidated models were significantly different (α = 0.05). The amount of inactivation during the come-up time was also not significantly different among the methods (α = 0.05). However, the TDT Sandwich resulted in less inactivation during the come-up time and overall less variation in model parameters. The survivor data from all three methods were combined and fitted to both consolidated models. The Weibull had a lower root mean square error and a better fit, according to the corrected Akaike's information criterion. The three thermal treatment methods produced results that were not significantly different; thus, the methods are interchangeable, at least for Salmonella in whole milk powder. Comparisons with more methods, other microorganisms, and larger varieties of food products using the same framework presented in this study could provide guidance for standardizing thermal inactivation kinetics studies for microorganisms in foods.

HIGHLIGHTS

Effect of Physical Structures of Food Matrices on Heat Resistance of Enterococcus faecium NRRL-2356 in Wheat Kernels, Flour and Dough

Nonpathogenic surrogate microorganisms, with a similar or slightly higher thermal resistance of the target pathogens, are usually recommended for validating practical pasteurization processes. The aim of this study was to explore a surrogate microorganism in wheat products by comparing the thermal resistance of three common bacteria in wheat kernels and flour. The most heat-resistant Enterococcus faecium NRRL-2356 rather than Salmonella cocktail and Escherichia coli ATCC 25922 was determined when heating at different temperature-time combinations at a fixed heating rate of 5 °C/min in a heating block system. The most heat-resistant pathogen was selected to investigate the influences of physical structures of food matrices. The results indicated that the heat resistance of E. faecium was influenced by physical structures of food matrices and reduced at wheat kernel structural conditions. The inactivation of E. faecium was better fitted in the Weibull distribution model for wheat dough structural conditions while in first-order kinetics for wheat kernel and flour structural conditions due to the changes of physical structures during heating. A better pasteurization effect could be achieved in wheat kernel structure in this study, which may provide technical support for thermal inactivation of pathogens in wheat-based food processing.

TDT Sandwich: An open source dry heat system for characterizing the thermal resistance of microorganisms

The determination of the thermal death kinetics of microorganisms has traditionally been performed with liquid baths which have some disadvantages such as liquid spillage and liquid infiltration into samples. The TDT Sandwich was developed as a free, open source alternative that utilizes dry heat. The system is capable of heating samples up to 140 °C and maintaining it within 0.2 °C of the target temperature. Other features of the TDT Sandwich include adjustable heating rates up to approximately 100 °C/min, real-time display and recording of temperature readings at a nominal rate of 5 Hz, an optional thermocouple for acquiring temperature of samples, built-in heating timer, and customizable operating parameters. The modular nature of the TDT Sandwich allows multiple units to be connected to a computer or laptop. Operation of the TDT Sandwich is done through a computer program which, along with the build instructions and microcontroller program, are open source and are available for free to the public at https://doi.org/10.17605/OSF.IO/5Q3Y7.

A comprehensive review on recent developments of radio frequency treatment for pasteurizing agricultural products

Recent pathogen incidents have forced food industry to seek for alternative processes in postharvest pasteurization of agricultural commodities. Radio frequency (RF) heating has been used as one alternative treatment to replace chemical fumigation and other conventional thermal methods since it is relatively easy to apply and leaves no chemical residues. RF technology transfers electromagnetic energy into large bulk volume of the products to provide a fast and volumetric heating. There are two types of RF technology commonly applied in lab and industry to generate the heat energy: free running oscillator and 50-Ω systems. Several reviews have been published to introduce the application of RF heating in food processing. However, few reviews have a comprehensive summary of RF treatment for pasteurizing agricultural products. The objective of this review was to introduce the developments in the RF pasteurization of agricultural commodities and to present future directions of the RF heating applications. While the recent developments in the RF pasteurization were presented, thermal death kinetics of targeted pathogens as influenced by water activity, pathogen species and heating rates, non-thermal effects of RF heating, combining RF heating with other technologies for pasteurization, RF heating uniformity improvements using computer simulation and development of practical RF pasteurization processes were also focused. This review is expected to provide a comprehensive understanding of RF pasteurization for agricultural products and promote the industrial-scale applications of RF technology with possible process protocol optimization purposes.