Improving radio frequency heating uniformity using a novel rotator for microorganism control and its effect on physiochemical properties of raisins

Raisin Production by Convective-Microwave Dryer and Optimization of Ochratoxin A Content, Physicochemical, Sensory Characteristics, and Fungal Load of Raisin

Raisins are so popular in the human diet as a nutritional and sweet snack. The quality of this foodstuff depends on drying conditions. To minimize ochratoxin A (OTA) content and yeast and mold content (YMC) in raisins with favorable physicochemical and sensory properties, the response surface methodology (RSM) and the face-centered central composite design (FCCD) were utilized. The independent variables were studied over a range of microwave power (0-220 W) and temperature air (35°C-55°C). The microwave power had a significant effect on the OTA value and YMC of dried samples. The use of microwaves in drying contributed to a higher decline in OTA content and YMC than in conventionally dried samples. The regression models detected for OTA content, acidity, pH, rehydration ratio (RR), water holding capacity (WHC), shrinkage, appearance, texture, acceptability, and YMC were significant and reliable. The desirability function was applied to recognize an optimized situation. The result of the RSM analysis showed the optimum conditions for obtaining healthy raisins with desired physicochemical and sensory attributes by microwave-convective dryer at 220 W microwave power and 38.68°C air temperature.

Application of radio frequency energy in processing of fruit and vegetable products

Thermal processing is commonly employed to ensure the quality and extend the shelf-life of fruits and vegetables. Radio frequency (RF) heating has been used as a promising alternative treatment to replace conventional thermal processing methods with advantages of rapid, volumetric, and deep penetration heating characteristics. This article provides comprehensive information regarding RF heating uniformity and applications in processing of fruit and vegetable products, including disinfestation, blanching, drying, and pasteurization. The dielectric properties of fruits and vegetables and their products have also been summarized. In addition, recommendations for future research on RF heating are proposed to enhance practical applications for fruits and vegetables processing in future.

A High-Efficiency Radio-Frequency-Assisted Hot-Air Drying Method for the Production of Restructured Bitter Melon and Apple Chips

Nowadays, consumers are increasingly demanding processed food products with high levels of beneficial components. Bitter melon and apple are both nutritious foods rich in bioactive compounds. In this study, restructured bitter melon and apple chips were processed using four drying techniques: hot-air drying with/without exhaust air recirculation (EAR), and radio-frequency-assisted hot-air drying (RFHAD) with/without EAR. The drying characteristics, effective moisture diffusivity (Deff), specific energy consumption (SEC), total energy consumption (TEC), and some selected quality characteristics of the dehydrated chips were evaluated. The experimental results show that the application of radio frequency (RF) energy significantly facilitates water evaporation in the drying material, resulting in a significant (p < 0.05) reduction of drying duration by 31~39% over the experimental test parameters. The higher Deff values obtained from RFHAD and RFHAD + EAR were 6.062 × 10-9 to 6.889 × 10-9 m2/s, while lower SEC values ranged from 301.57 to 328.79 kW·h/kg. Furthermore, the dried products possessed better or fairly good quality (such as a lower color difference of 5.41~6.52, a lower shrinkage ratio of 18.24~19.13%, better antioxidant capacity, higher chlorophyll, total flavonoid, and total phenolic content, a lower polyphenol oxidase activity of 49.82~52.04 U·min-1g-1, smaller diameter and thickness changes, and a lower hardness of 27.75~30.48 N) compared to those of hot-air-dried chips. The combination of RF-assisted air drying and partial recirculating of dryer exhaust air achieved the highest saving in TEC of about 12.4%, along with a lower moisture absorption capacity and no deterioration of product quality attributes. This drying concept is therefore recommended for the industrial drying of several food materials.

Pasteurization of Salmonella spp. in black fungus (Auricularia auricula) powder by radio frequency heating

Radio frequency (RF) heating has been studied to inactivate bacteria in some powder foods. In this study, a 6 kW, 27.12 MHz RF system was used to pasteurize Salmonella in black fungus (Auricularia auricula) powder. The effects of different conditions (initial aw, electrodes gaps, particle sizes) on RF heating rate and uniformity were investigated. The results showed that RF heating rate was significantly (p < 0.05) improved with decreasing electrodes gap and increasing initial aw, and the heating rate was the slowest when the particle size was 120-160 mesh. However, these factors had no significant (p > 0.05) influence on heating uniformity. RF pasteurization of Salmonella in black fungus powder was also studied. The results showed that, to inactivate Salmonella for 5 log reductions in the cold spot (the center of surface layer), the time needed and bacteria heat resistance at designated temperature (65, 75, 85 °C) decreased with increasing aw, and the first order kinetics and Weibull model could be used to fit inactivation curves of Salmonella with well goodness. Quality analysis results showed that although RF pasteurization had no significant (p > 0.05) effect on Auricularia auricula polysaccharide (AAP) and total polyphenols, obvious changes were found on color. Results suggested that RF pasteurization can be considered as an effective pasteurization method for black fungus powder.

Evaluation of Pilot-Scale Radio Frequency Heating Uniformity for Beef Sausage Pasteurization Process

Radio frequency (RF) heating has the advantages of a much faster heating rate as well as the great potential for sterilization of food compared to traditional thermal sterilization. A new kettle was designed for sterilization experiments applying RF energy (27.12 MHz, 6 kW). In this research, beef sausages were pasteurized by RF heating alone, the dielectric properties (DPs) of which were determined, and heating uniformity and heating rate were evaluated under different conditions. The results indicate that the DPs of samples were significantly influenced (p < 0.01) by the temperature and frequency. The electrode gap, sample height and NaCl content had significant effects (p < 0.01) on the heating uniformity when using RF energy alone. The best heating uniformity was obtained under an electrode gap of 180 mm, a sample height of 80 mm and NaCl content of 3%. The cold points and hot spots were located at the edge of the upper section and geometric center of the sample, respectively. This study reveals the great potential in solid food for pasteurization using RF energy alone. Future studies should focus on sterilization applying RF energy and SW simultaneously using the newly designed kettle.

Radio frequency heating of green peas (Pisum sativum L.): The improvement of heating uniformity and its dry blanching effect

A radio frequency (RF) heating system was used to process green peas and a rotating system was introduced to improve heating rate and heating uniformity. Results revealed that rotation treatment accelerated heating rate effectively and improved heating uniformity index significantly (p < 0.05) from 0.175 (0 rpm, 55°C) to 0.029 (60 rpm, 55°C). After being treated with RF and traditional hot water blanching at 85°C, the residual lipoxygenase activities of green peas were 1.90 ± 0.71% and 35.51 ± 5.25%, respectively, confirming RF possessed better blanching efficiency. Meanwhile, weight loss, electrolyte leakage rate, color, and texture of green peas all had significant changes (p < 0.05) through RF heating. PRACTICAL APPLICATION: A rotation device can effectively improve heating uniformity of RF electromagnetic heating. The rotation device could be expanded or further developed into continuous feeding conveying device for industrial production.

Recent developments in low-moisture foods: microbial validation studies of thermal pasteurization processes

Outbreaks associated with low-moisture foods (e.g., wheat flour, nuts, and cereals) have urged the development of novel technologies and re-validation of legacy pasteurization process. For various thermal pasteurization processes, they share same scientific facts (e.g., bacterial heat resistance increased at reduced water activity) and guidelines. However, they also face specific challenges because of their different heat transfer mechanisms, processing conditions, or associated low-moisture foods' formulations. In this article, we first introduced the general structural for validating a thermal process and the shared basic information that would support our understanding of the key elements of each thermal process. Then, we reviewed the current progress of validation studies of 7 individual heating technologies (drying roasting, radiofrequency-assisted pasteurization, superheated steam, etc.) and the combined treatments (e.g., infrared and hot air). Last, we discussed knowledge gaps that require more scientific data in the future studies. We aimed to provide a process-centric view point of thermal pasteurization studies of low-moisture foods. The information could provide detailed protocol for process developers, operators, and managers to enhance low-moisture foods safety.