Microwave-assisted Thermal Sterilization Research Articles

Storage and thermal stability of selected vegetable purees processed with microwave-assisted thermal sterilization

Storage and thermal stability of selected vegetable purees processed with microwave-assisted thermal sterilization

Development and validation of engineering charts: Heating time and optimal salt content prediction for microwave assisted thermal sterilization

Through decades of research and development efforts, Microwave Assisted Thermal Sterilization (MATS) systems started commercial operation for production of shelf-stable meals in 2019. It is now highly desirable to develop user-friendly tools to facilitate the development of commercial MATS processes. The aim of this research was to develop engineering charts by applying Maxwell's and heat transfer equations to illustrate the relationships among dielectric properties, temperature, salt content, product thickness and heating rate for microwave heating in a MATS system. Three G-T (Gezahegn-Tang) charts were developed for mashed potato, peas, and rice products with MATLAB software. The charts can be used to determine the heating temperature, time and optimum salt content for a maximum heating rate at the center of a product. Experimental validation tests using mashed potato samples confirmed that the temperature profiles measured at the cold spots of the samples in the microwave heating section of the MATS system were positively correlated with predicted temperatures from the chart (R2 = 0.94–0.99). Furthermore, the predicted optimal salt contents for 23 mm thick mashed potato (0.1%), 18 mm thick pea (0.5%) and 25 mm thick rice (1%) samples agreed with the experimental heating pattern test results. The optimal salt content is inversely proportional with a product temperature and thickness, which increases the microwave power decay before reaching the center. With the help of these charts, the food industry practitioners can save significant time and resources during the development of food products and schedules for the MATS processes.

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

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

Effect of Hurdle Approaches Using Conventional and Moderate Thermal Processing Technologies for Microbial Inactivation in Fruit and Vegetable Products.

Thermal processing of packaged fruit and vegetable products is targeted at eliminating microbial contaminants (related to spoilage or pathogenicity) and extending shelf life using microbial inactivation or/and by reducing enzymatic activity in the food. The conventional process of thermal processing involves sterilization (canning and retorting) and pasteurization. The parameters used to design the thermal processing regime depend on the time (minutes) required to eliminate a known population of bacteria in a given food matrix under specified conditions. However, due to the effect of thermal exposure on the sensitive nutrients such as vitamins or bioactive compounds present in fruits and vegetables, alternative technologies and their combinations are required to minimize nutrient loss. The novel moderate thermal regimes aim to eliminate bacterial contaminants while retaining nutritional quality. This review focuses on the “thermal” processing regimes for fruit and vegetable products, including conventional sterilization and pasteurization as well as mild to moderate thermal techniques such as pressure-assisted thermal sterilization (PATS), microwave-assisted thermal sterilization (MATS) and pulsed electric field (PEF) in combination with thermal treatment as a hurdle approach or a combined regime.

Creating foods for older adults: Emotional responses and liking of microwave-assisted thermal sterilization processed meals.

The purpose of this study was to examine emotional and liking responses to foods designed for older adults and made using microwave-assisted thermal sterilization technology (MATS). Six chicken pasta meals (three each with and without herbs) were formulated with three concentrations of salt at 100%, 75%, and 50%. Seventy-six community-dwelling older adults conducted sensory and emotional evaluations. Sensory testing involved measuring liking of various sensory attributes using a 9-point hedonic scale, Just-about-right scales to measure appropriateness of the intensity of the attributes, and check-all-that-apply questions to identify perceived flavor and texture attributes. EsSense25 methodology was used for capturing food-evoked emotional responses. Significant differences existed in all measured sensory attributes and in 14 out of the 25 tested emotions across the six meals. Liking scores for all pastas with herbs and high salt pasta with no herbs were not significantly different for all tested attributes and fell between neither like nor dislike and like slightly on the 9-point hedonic scale. These samples were also associated with positive emotions related to energy and activation. Low-salt pastas with no herbs were consistently the least liked samples and evoked negative emotions. Results show that sodium content can be reduced by up to half when herbs are added to microwave-processed pasta meals without compromising liking. PRACTICAL APPLICATION: Chicken pasta meals manufactured using microwave-assisted thermal sterilization technology are acceptable to community living individuals 60 years and older. Emotional responses to the meal are positive. When formulating these meals, herbs can be added to lower sodium content formulations to improve liking and increase the number of positive emotions associated with the meal. Microwave-assisted thermal sterilization, sodium reduction, emotional responses, consumer liking, older adults.

Dielectric properties of water relevant to microwave assisted thermal pasteurization and sterilization of packaged foods

Microwave assisted thermal sterilization and pasteurization systems, as emerging thermal processing technologies, use circulation water to reduce edge heating of the food packages in the microwave heating sections. Yet, the influence of the circulation water on microwave heating efficiency for food is unknown. This research studied the dielectric properties of purified water from a reverse osmosis unit and tap water over microwave frequencies of 150–2500 MHz at 23–120 °C and evaluated microwave loses in the circulation water. The values of the dielectric constant of purified and tap water decreased (P < 0.05) with increasing temperature and slightly decreased with increasing frequency. The value of the loss factor of purified water also decreased with temperature (P < 0.05) but increased with frequency. The trend was more complicated for the loss factor of tap water. The penetration depth of 915 MHz microwave power in purified water was 399 and 457 mm at 90 and 120 °C, respectively, it was reduced to 161 and 136 mm in tap water. The microwave power loss in the 18–30 mm deep purified water, depending on the thickness of the food packages, was 4–7%, compared to 11–20% in tap water. Thus, using purified circulation water in microwave assisted thermal sterilization (MATS) and pasteurization systems (MAPS) can significantly improve the efficiency of microwave heating of packaged foods. Industrial relevanceThe information from this research are highly relevant to industrial operations of microwave-assisted thermal sterilization and pasteurization systems of pre-packaged foods. The dielectric property data will contribute to the literature related to the physical properties of water in microwave heating.

Applications of novel processing technologies to enhance the safety and bioactivity of milk.

Bioactive compounds in food can have high impacts on human health, such as antioxidant, antithrombotic, antitumor, and anti-inflammatory activities. However, many of them are sensitive to thermal treatments incurred during processing, which can reduce their availability and activity. Milk, including ovine, caprine, bovine, and human is a rich source of bioactive compounds, including immunoglobulins, vitamins, and amino acids. However, processing by various novel thermal and non-thermal technologies has different levels of impacts on these compounds, according to the studies reported in the literature, predominantly in the last 10 years. The reported effect of these technologies either covers microbial inactivation or the bioactive composition; however, there is a lack of comprehensive compilation of studies that compare the effect of these technologies on bioactive compounds in milk (especially, caprine and ovine) to microbial inactivation at similar settings. This research gap makes it challenging to conclude on the specific processing parameters that could be optimized to achieve targets of microbial safety and nutritional quality at the same time. This review covers the effect of a wide range of thermal and non-thermal processing technologies including high-pressure processing, pressure-assisted thermal sterilization, pulsed-electric field treatment, cold plasma, microwave-assisted thermal sterilization, ultra-high-pressure homogenization, ultrasonication, irradiation on the bioactive compounds as well as on microbial inactivation in milk. Although a combination of more than one technology could improve the reduction of bacterial contaminants to meet the required food safety standards and retain bioactive compounds, there is still scope for research on these hurdle approaches to simultaneously achieve food safety and bioactivity targets.

Effect of Electric Field Distribution on the Heating Uniformity of a Model Ready-to-Eat Meal in Microwave-Assisted Thermal Sterilization Using the FDTD Method.

Microwave assisted thermal sterilization (MATS) is a novel microwave technology currently used in the commercial production of ready-to-eat meals. It combines surface heating of high-temperature circulation water with internal microwave heating in cavities. The heating pattern inside the food packages in a MATS process depends heavily on the electric field distribution formed by microwaves from the top and bottom windows of the microwave heating cavities. The purpose of this research was to study the effect of the electric field on 922 MHz microwave heating of ready-to-eat meals as they moved through the microwave chamber of a pilot-scale MATS system using the finite-difference time-domain (FDTD) method. A three-dimensional numerical simulation model was developed as a digital twin of the MATS process of food moving through the microwave chamber. The simulation showed that the electric field intensity of the MATS microwave cavity was greatest on the surface and side edge of the cavity and of the food. There was a strong similarity of the experimental heating pattern with that of the electric field distribution simulated by a computer model. The digital twin modeling approach can be used to design options for improving the heating uniformity and throughput of ready-to-eat meals in MATS industrial systems.

Influence of ultra-high barrier packaging on the shelf-life of microwave-assisted thermally sterilized chicken pasta

This accelerated shelf life study of a ready-to-eat meal processed with a microwave-assisted thermal sterilization (MATS) system aimed to evaluate the performance of double metal oxide coated polyethylene terephthalate (PET) layers-based ultra-high barrier packaging along with ethylene vinyl alcohol (EVOH)-based pouches, and aluminum (AL) foil-based pouches (served as control). First, we evaluated barrier changes in the two types of pouches (double-layered PET and EVOH) caused by MATS processing. Next, we determined the influence of barrier properties on the physical, chemical and sensory quality of MATS-processed chicken pasta meals at accelerated storage conditions at 49 °C. Sensory evaluation was also done at lower storage temperature of 38 °C. Results demonstrated that MATS processing did not affect the barrier properties of double metal oxide coated PET pouches, however, it significantly influenced the barrier properties of EVOH-based pouches. After storage at 49 °C for 60 days, there was a small total color change (ΔE = 3.5) in chicken pasta meals in the double coated film pouches. This result was similar to that obtained for aluminum foil pouches (p > 0.05), but significantly lower than that for the EVOH pouches (ΔE = 5.5). The double metal oxide coated PET packaging provided an excellent barrier against lipid oxidation similar to that of aluminum (AL) foil-based pouches. A trained sensory panel scored the recipe between 6.0 and 6.45 immediately after MATS processing on a labelled effective magnitude (LAM) scale from 1 to 9 (worst to best quality). Results indicate that the newly developed double-layered PET pouches offer an excellent alternative to higher density metal foil-based packaging.

Designing thinner wall ethylene-vinyl alcohol copolymer and polypropylene-based semi-rigid trays for microwave-assisted thermal sterilization and pasteurization processes

Abstract The goal of this research was to design a thinner wall tray for microwave-assisted thermal processing technologies. Three-layer (PP//EVOH//PP) sheets of different EVOH- and PP-layer thicknesses were coextruded. The sheets were then thermoformed into 8-oz trays with dimensions of 150 × 100 × 50 mm (L × W×D). Trays were filled with model foods and subjected to conventional and microwave-assisted pasteurization and sterilization processes. Barrier and mechanical properties were assessed before and after processing. Oxygen ingress, weight loss, and color kinetics were measured over a storage period of 3–6 months at 7, 23, and 37 °C. The newly-designed trays showed comparable or better performance in terms of O2 ingress and weight loss, with a 24 % reduction in wall thickness. Therefore, thinner EVOH and PP-based trays hold promise for use in microwave-assisted sterilization and pasteurization processes. This will result in a reduced carbon footprint and provide more economical alternatives to other forms of packaging that are currently available.

Optimization of microwave heating on orange sacs products

Various applications of microwave have been put into effect in the field of food processing. The microwave assisted thermal sterilization (MATS) was of great significance because of its less time consuming and higher efficient. In this paper, response surface methodology was used to optimize the process. The sterilization of orange sacs was potimiazed using by response surface methodology (RSM). The optimal processing parameters of MATS on orange sacs are as follow: microwave power, 6 kW, microwave sterilization time, 350 s, amount of orange sacs, 4500 g. Sterilized under this condition by MATS, orange sacs only maintained 3.7 CFU lethal magnitude which was consistent with the standards requirement. The key factor in the response surface using microwave sterilization and the interaction between them were also determined and discussed which could make it evident that MATS should be an idea candidate in the sterilization of some fruit or vegetable products as ready to eat. From these results, the best conditions of microwave sterilization are power 6kw, microwave sterilization time 350s and material quantity 4500g.

Functionality of ultra-high barrier metal oxide-coated polymer films for in-package, thermally sterilized food products

Abstract Defects in metal oxide (MO)-coated polymer films increase their oxygen and water vapor transmission rates (OTRs, WVTRs), lowering the shelf-life of packaged food. In this study, we investigated the effects of thermal processing on six multilayered MO-coated films: MO-A (MO-coated PET(12 μm)//Oriented-Nylon 6 (15 μm)//Cast-Polypropylene (50 μm)), MO-B (SiOx-coated PET(12 μm)//Oriented-Nylon 6 (15 μm)//Cast-Polypropylene (60 μm)), MO-C (Overlayer/AlOx-organic-coated PET(13 μm)//Oriented-Nylon 6 (15 μm)//Cast-Polypropylene (50 μm)), MO-D (Overlayer/SiOx-coated PET(12 μm)//Oriented-Nylon 6 (15 μm)//Cast-Polypropylene (60 μm)), MO-E (AlOx-coated PET(12 μm)//AlOx-coated PET(12 μm)//Oriented-Nylon 6 (15 μm)//Cast-Polypropylene (70 μm)) and MO-F (AlOx-coated PET(12 μm)//AlOx-coated PET(12 μm)//AlOx-coated PET(12 μm)//Oriented-Nylon 6 (15 μm)//Cast-Polypropylene (70 μm)). Water filled film pouches were processed with a pilot-scale retort (30 and 40 min at 121 °C; F0 = 6–9 min) and microwave-assisted thermal sterilization unit (25-kW, single-mode, 915 MHz MATS system; F0 = 10.9 min). Subsequently, the pouches were emptied, filled with a color-changing oxygen sensor and stored at 23 ± 2 °C for 180-days. MO-F with three AlOx-coated PET layers, showed no oxygen permeation before and after retort and MATS processes. Compared to other films, MO-F exhibited low WVTRs after processing. Oxygen sensor in MO-F changed color at limited locations, indicating fewer defects. Confocal laser scanning microscope and scanning electron microscope images also showed less deterioration in MO-F. The dielectric loss factor (er) for MO-F was significantly less than other films (P

The potential for microwave technology and the ideal profile method to aid in salt reduction

This study was the first to evaluate the influence of the combination strategies of flavor addition and microwave-assisted thermal sterilization (MATS) processing for salt reduction implications. In freshly prepared mashed potatoes, a 30% and 50% salt reduction (w/w) in comparison to a 100% salt sample with three flavor variations (no additional flavor, garlic, and pepper) were investigated. Also, using the ideal profile method (IPM), the influence of MATS versus retort processing, in comparison to a freshly prepared sample, and flavor addition on mashed potato sensory properties and acceptance was investigated. Chemical characterization using the electronic tongue for nonvolatile compounds and headspace solid-phase microextraction (HS-SPME)/gas chromatography-mass spectrometry (GC-MS) for volatile analysis was completed. IPM revealed the ideal data were consistent at both the panel and consumer levels from a sensory and hedonic perspective. Results demonstrated the ideal mashed potato product would remain low in bitterness but have more intense pepper and potato aromas and flavors than the current samples evaluated. The salt level could be reduced by 50% while still maintaining flavor and overall acceptance in freshly prepared samples, but this was accompanied by a loss in saltiness intensity perception. The saltiness intensity was not different from the freshly prepared samples when processed via MATS but was different when processed by the retort. For chemical characterization, the electronic tongue showed a high discrimination index (>89%) and correlated highly (>0.8) with many sensory attributes. As salt concentration in the mashed potatoes decreased, the recovery of volatile compounds decreased. The present work contributes to the understanding of product reformulation for the purpose of salt reduction. PRACTICAL APPLICATION: Product developers need strategies to bring salt down to target levels while maintaining consumer acceptance. The combination strategies of flavor addition and MATS processing may allow for a new strategy to assist product developers in reaching salt reduction targets. Furthermore, developers should bear in mind that noticeable intensity differences may not alter the preference for the product. Thus, intensity differences that result in changes in acceptance should be the focus of quality insurance rather than utilizing just noticeable differences.

Determining Shelf Life of Ready-to-Eat Macaroni and Cheese in High Barrier and Oxygen Scavenger Packaging Sterilized via Microwave-Assisted Thermal Sterilization

Ready-to-eat macaroni and cheese filled in novel oxygen scavenger and metal oxide–coated high-barrier polymer packages were processed in pilot scale 915-MHz microwave-assisted thermal sterilization system (MATS). Also, aluminum foil packages were processed in Allpax retort system to compare packaging performance. Physicochemical and sensory attributes of macaroni and cheese packaged in different oxygen and water vapor transmission rates were evaluated and stored for 6 months at 37.8 °C. Findings showed oxygen transmission rate (OTR) increase by 2–7 times and water vapor transmission rate (WVTR) increase by 2.5–24 times after MATS processing. OTR of polymeric packaging had no significant effect on vitamin A and vitamin E, shear force, and food color. Comparable results between polymeric and aluminum foil packaging were observed throughout the shelf life. This indicates that oxygen scavenger and high-barrier packaging with OTRs ~ 0.03–0.34 cc/m2 day and WVTRs ~ 0.62–7.19 g/m2 day can be used for ready-to-eat meals with extended shelf life for soldiers and astronauts.

Color, vitamin C, β-carotene and sensory quality retention in microwave-assisted thermally sterilized sweet potato puree: Effects of polymeric package gas barrier during storage

In this study, we investigated the effects of package barrier properties on shelf-life of vitamin C-fortified sweet potato puree (SPP) processed with microwave-assisted thermal sterilization (MATS). Results show a change in SPP color during both processing and storage. The vitamin C in SPP decreased from 201.7 ± 4.7 to 185.8 ± 15.6 mg/100g during MATS and further decreased to as low as 13.6 ± 4.1 and 10.0 ± 0.3 mg/100g the end of 9 or 18 month, depending on the oxygen barrier and temperature. The total β-carotene content after processing and storage was slightly higher than before processing. Pouches with oxygen transmission rates (OTRs) of less than 0.3 cc/m2⋅day showed higher retention of color, vitamin C, and predicted shelf life than other pouches. However, this did not affect the consumer acceptance. Most panelists considered MATS processed SPP as a desirable baby food throughout the 18 months. Efforts are still needed to stabilize vitamin C content for over 3 years for military and NASA missions.

Effect of changes in salt content and food thickness on electromagnetic heating of rice, mashed potatoes and peas in 915 MHz single mode microwave cavity

A mathematical model for predicting electromagnetic power dissipation within a rectangular dielectric slab heated by equal intensity 915 MHz plane waves from top and bottom was developed. A dimensionless parameter (J-T number) which is a combination of the loss factor (εr″), dielectric constant (εr′) and food thickness (L) was proposed. This unique number provided direct insight into the relationship between food dielectric properties, thickness, product temperature, and thermal lethality. For the validation tests, mashed potatoes, peas and rice samples with 0–2% salt content were processed in a pilot scale microwave assisted thermal sterilization (MATS) system. In each food, the combination of dielectric properties and thickness which gave J-T number of 1.8–2.2 at 100–121°C, provided the highest lethalities. MATS is a novel commercial technology being adapted in the food industry. A qualitative assessment of the combined effect of food properties on lethalities using this model will be helpful in process development for MATS systems.

Computational evaluation of food carrier designs to improve heating uniformity in microwave assisted thermal pasteurization

Abstract Microwave assisted thermal pasteurization system (MAPS) is a novel food safety technology that employs carriers made from stainless steel to move pre-packaged foods inside 915 MHz single mode microwave cavities to eliminate bacterial and viral pathogens. This paper studied the performance of metal carriers for 16 oz and 10 oz food packages in the MAPS. A simulation model built with Quick-wave software was developed to analyze the electromagnetic field distribution inside a MAP system as affected by the presence of the metal food carriers. Computer simulations were validated using a mashed potato model food processed in a pilot scale MAP system; heating patterns of the samples were detected by a chemical-marker based computer vision method. Results showed that different designs of the food carriers could be used to modify electric field distribution to obtain relatively uniform heating patterns within the microwave cavities. Simulation results also illustrated that magnetron frequency variations between 900 MHz and 920 MHz do not affect the heating patterns of food packages processed using carriers containing metal parts. The results demonstrated that the MAPS with moving metal carriers has stable and predictable heating patterns. Industrial relevance Microwave transparent materials such as plastics have been used to make transport carriers for food packages in microwave assisted thermal sterilization (MATS) (Tang, 2015). However, polymers may have a short life in the high-temperature processing conditions and thus may not be desirable in an industrial setting. The simulations and experiments conducted in this work showed that the novel concept of metal tray carrier is an effective mechanism for transporting pre-packaged foods in microwave heating systems. The validated computer simulation model presented in this work will be a time efficient, economical and convenient tool to evaluate tray carrier designs for efficient and uniform microwave heating in MAPS.

Developing model food systems with rice based products for microwave assisted thermal sterilization

Model foods are effective tools to evaluate heating patterns and determine hot and cold spot locations in food packages during microwave-assisted thermally sterilization (MATS) processes. Previous research on model food development has focused on high-moisture foods, with limited information on medium-moisture foods (0.2–0.6 g water/g food). This research aimed to develop rice model foods to simulate medium-moisture food during MATS processing. The optimal composition of a rice flour gel (RFG) model food was 0.3 g/g rice flour, 0.135 g/g tapioca starch, 0.001 g/g xanthan gum, 0.005 g/g d-ribose, and 0.559 g/g water, and a rice to water ratio of 1:1.2 g/g with 0.005 g/g d-ribose for the rice grain (RG) model. The temperature sensitivities of the models' color parameters could be applicable for safety and quality attribute modeling; the RFG model had a larger range of z-values (11–31 °C) than the RG model (18–27 °C). Validation results showed that the RG model food received more thermal energy than the RFG model, with thermal treatment equivalents at 121 °C of 60.3 and 6.5 min, respectively. The heating pattern in RFG medium-moisture model food was consistent with high-moisture models in MATS processing. Model foods developed in this research could be helpful tools for microwave process development for medium-moisture foods.

Dielectric properties of rice model food systems relevant to microwave sterilization process

Abstract Model foods as chemical marker carriers are needed to evaluate the heating patterns and location of hot and cold spots in developing microwave assisted thermal sterilization (MATS) processes. Previous research on model food development has been conducted for high moisture foods, with limited data on medium moisture foods (20–60% water). This research aimed to determine the dielectric properties (dielectric constants and loss factors) and penetration depths of rice grain (RG) and rice flour gel (RFG) model foods with moisture contents between 50 to 60% (wet basis) and of cooked rice (CR) over a frequency range of 300–3000 MHz at temperatures from 20 to 121 °C. The dielectric properties of rice models with 0% salt and 0.5% d -ribose closely matched the properties of CR; this indicated that rice model foods could be used to emulate CR for heating pattern and cold/hot spot detection in the development of MATS processes at 915 MHz and 2450 MHz. Therefore, rice model foods developed in this research could be useful in the future for the food companies, in order to visualizing the heating pattern and determining the location of cold and hot spots during MATS process of medium moisture foods, such as rice, pasta or macaroni used as main ingredient in ready-to-eat meal products.

Linking morphology changes to barrier properties of polymeric packaging for microwave‐assisted thermal sterilized food

ABSTRACTIt is a priority to develop polymeric packaging that can withstand microwave‐assisted thermal sterilization (MATS) and maintain the quality of low‐acid foods during long‐term storage. In this study, we explored changes in the morphology of pouch films with two multi‐layer structures. The films are based on barrier layers of metal oxide‐coated poly(ethylene terephthalate) (PET) (film A) and ethylene vinyl alcohol (EVOH) (film B). A 8‐oz model food in pouches was processed with MATS (F0 = 9.0 min) and stored at 23, 35 and 45 °C for up to 12 months. Findings reveal that the oxygen barrier of film A was influenced by the coating and crystallinity of PET. The oxygen barrier of film B was primarily affected by the moisture content of the EVOH polymer. Results also show that changes in barrier properties depended on storage temperature. Recrystallization in polymer might be an important morphological change that occurs during storages. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45481.