Storage stability of vitamin C fortified purple mashed potatoes processed with microwave-assisted thermal sterilization system

Quality changes in ready-to-eat, shelf-stable foods, during storage can be influenced by many factors, such as processing, storage conditions, and the barrier properties of the packaging. This research investigated retention of vitamin C and anthocyanin in purple mashed potatoes as influenced by packaging barrier properties and encapsulation during storage after microwave assisted thermal sterilization. Purple mashed potatoes fortified with encapsulated (EVC) or non-encapsulated vitamin C (NVC) were packaged in two high-barrier polymer pouches (TLMO and PAA), processed with a pilot-scale microwave assisted thermal sterilization (MATS) system (F0 = 10.7 min), and stored at 37.8 °C for 7 months. MATS processing caused a significant increase (P < 0.05) in the oxygen transmission rates (OTRs) of PAA pouches but did not affect the barrier properties of TLMO pouches. PAA film also had a significantly higher (P < 0.05) water vapor transmission rate (WVTRs) than TLMO films, which resulted in a significantly higher (P < 0.05) weight loss in the samples packaged in PAA pouches than TLMO pouches. Purple mashed potatoes containing encapsulated vitamin C in both TLMO and PAA pouches showed the highest retention over 2 months of storage at 37.8 °C than non-encapsulated vitamin C. Additionally, purple mashed potatoes exposed to 700 lumens light showed a significantly higher (P < 0.05) deterioration in the anthocyanin, total phenolic content, color, and vitamin C. Overall, MATS processed purple mashed potatoes in high barrier polymeric packaging can minimize the quality changes when stored in dark conditions during storage and have an extended shelf life

Dielectric properties of salmon (Oncorhynchus keta) and sturgeon (Acipenser transmontanus) caviar at radio frequency (RF) and microwave (MW) pasteurization frequencies

Abstract Radio frequency (RF) and microwave (MW) heating provide an important advantage of more rapid heat penetration in pasteurization processes for heat labile high value foods, which to date, have only been pasteurized by conductive heating. The objectives of this work were to determine the dielectric constant, loss factor and power penetration depth for salmon (0.8% and 2.3% total salt) and sturgeon (0.20 and 3.3% salt) caviars at RF frequency of 27 MHz and MW frequency of 915 MHz (20-80°C). The dielectric constant (e 0 ) and dielectric loss factor (e 00 ) for salmon and sturgeon caviar increased markedly with increasing temperature at 27 MHz but not at 915 MHz. Power penetration depth was higher at 27 MHz compared to 915 MHz, and in unsalted compared to salted roe. Power penetration depth tended to decrease as temperature increased

Unlocking Potentials of Microwaves for Food Safety and Quality

Microwave is an effective means to deliver energy to food through polymeric package materials, offering potential for developing short-time in-package sterilization and pasteurization processes. The complex physics related to microwave propagation and microwave heating require special attention to the design of process systems and development of thermal processes in compliance with regulatory requirements for food safety. This article describes the basic microwave properties relevant to heating uniformity and system design, and provides a historical overview on the development of microwave-assisted thermal sterilization (MATS) and pasteurization systems in research laboratories and used in food plants. It presents recent activities on the development of 915 MHz single-mode MATS technology, the procedures leading to regulatory acceptance, and sensory results of the processed products. The article discusses needs for further efforts to bridge remaining knowledge gaps and facilitate transfer of academic research to industrial implementation

Computer simulation of radio frequency heating of model fruit immersed in water

The present study investigated the effect of different factors on temperature distribution within a spherical object in a parallel plate radio frequency (RF) heating system. A finite element-computer simulation program - FEMLAB was used to solve the electromagnetic and Navier-Stoke equations for developing a model to study the effect of dielectric properties of a model fruit and its surrounding medium. The model fruit was prepared from 1% gellan gel for experimental validation of the simulation results. The results showed that spherically-shaped samples surrounded with air between RF electrodes and placed in the proximity of electrodes would not heat uniformly. Immersing the model fruit in water helped to reduce uneven heating within the model fruit, but created a new problem because the model fruits were found to heat unevenly at different horizontal positions. Horizontal and vertical model fruit positions with respect to electrodes significantly influenced the heating patterns inside the model fruit. The study suggested that movement and rotation of the spherical object is the only plausible solution for improving heating uniformity. The developed computer model can be further used for prediction of the heating pattern of fresh fruit as influenced by dielectric properties, size, shape, and surrounding media to design thermal treatments of specific commodities

Performance Evaluation of Aluminum Test Cell Designed for Determining the Heat Resistance of Bacterial Spores in Foods

Thermal inactivation kinetic studies are necessary to determine heat resistances of spores in the development of new thermal processes for low-acid shelf-stable products. Most currently available sample holders used in the kinetic studies take long time to reach the target sample temperature, hence fail to provide isothermal condition. In this research, novel aluminum test cells were developed to facilitate easy loading and unloading solid and liquid food samples in a hermetically sealed 1 ml cavity to evaluate the heat resistance of bacterial spores when heated at temperatures above 100 °C. Design of the test cell was governed by minimum come-up time. A finite element model based on the commercial software ‘FEMLAB’ was used to simulate transient heat transfer and finalize the test cell dimensions. Performance of the new test cell was evaluated against capillary and aluminum thermal death time tube methods in characterizing the heat resistance of Clostridium sporogenes PA 3679 spores in a phosphate buffer and mashed potato at 121 °C. D121 values of PA 3679 spores in both the phosphate buffer and mashed potato using the new test cells were not significantly different (P \u3e 0.05) from those by the capillary tube method. The results indicated that the new test cell is appropriate for studying the inactivation kinetics of bacterial spores in microbial validation of conventional and novel thermal processes for low acid shelf-stable foods

Characterization of Radio Frequency Heating of Fresh Fruits Influenced by Dielectric Properties

Because of its fast and volumetric nature, radio frequency (RF) heating has been looked upon as a way to overcome the problems associated with conventional heating methods used for disinfestation of fruits. But non-uniform heating within fruits is a major obstacle in adaptation of this technology. In this study, RF heating patterns influenced by dielectric properties (DPs) of fruits were investigated both experimentally and mathematically. A computer simulation model was developed using FEMLAB 3.4, a commercial software for solving Maxwell’s electromagnetic and Fourier’s heat transfer equations. Orange, apple, grapefruit, peach, and avocado fruits, selected for these studies were subjected to RF heating in a water filled container equipped with a mechanism to keep fruits rotating and moving during RF heating in a 27.12 MHz, 12 kW parallel plate RF unit. DPs of constitutional parts of the selected fruits were measured by open-ended coaxial probe method. The study showed that dissimilarity in peel and pulp DPs greatly influenced the RF heating behavior of the fruits. Core heating was prominent in apple, peeled orange and grapefruit; whereas subsurface/peripheral heating in whole oranges and grapefruit, and avocado. The computer model was an effective tool in characterizing and explaining the heating patterns in the fruits based on DPs. The study helped in better understanding the complex RF heating characteristics of fruits, which may be useful in assessing the design feasibility of product specific RF energy based treatment protocol

iii STUDY OF ELECTROMAGNETIC FIELD UNIFORMITY IN RADIO FREQUENCY HEATING APPLICATOR ABSTRACT

First of all, I thank my major advisor, Dr. Juming Tang, for his guidance, motivation and encouragement throughout the course of my study and research at Washington State University. I also express my sincere thanks to my doctoral committe

Water Diffusion from a Bacterial Cell in Low-Moisture Foods

We used a Fick's unsteady state diffusion equation to estimate the time required for a single spherical shaped bacterium (assuming Enterococcus faecium as the target microorganism) in low-moisture foods to equilibrate with the environment. We generated water sorption isotherms of freeze-dried E. faecium. The water activity of bacterial cells at given water content increased considerably as temperature increased from 20 to 80 °C, as observed in the sorption isotherms of bacterial cells. When the water vapor diffusion coefficient was assumed as between 10(-12) and 10(-10) m(2) /s for bacterial cells, the predicted equilibration times (teq ) ranged from 8.24×10(-4) to 8.24×10(-2) s. Considering a cell membrane barrier with a lower water diffusion coefficient (10(-15) m(2) /s) around the bacterial cell with a water diffusion coefficient of 10(-12) m(2) /s, the teq predicted using COMSOL Multiphysics program was 3.8×10(-1) s. This result suggests that a single bacterium equilibrates rapidly (within seconds) with change in environmental humidity and temperature

ABSTRACT

I am grateful to my advisor Dr. Juming Tang for his support and guidance throughout the course of this work. I am also grateful to my committee members Dr. Barbara Rasco, Dr. Barry Swanson, and Dr. Cornelius Ivory for their encouragement and helpful suggestions