Yifen Wang

Dielectric properties of foods relevant to RF and microwave pasteurization and sterilization

A custom-built temperature-controlled test cell and an Agilent 4291B impedance analyzer were used to determine the dielectric properties of a whey protein gel, a liquid whey protein mixture, and a macaroni and cheese product and their constituents. Dielectric constants, loss factors, and penetration depths for each sample over a temperature range from 20 to 121.1 °C, at frequencies of 27, 40, 915, and 1800 MHz are reported. As temperature increased, dielectric constants of whey protein products increased at 27 and 40 MHz, but decreased at 915 and 1800 MHz. Dielectric loss factors of whey protein products increased sharply with increasing temperatures at 27 and 40 MHz, but increased mildly at 915 and 1800 MHz. Similar results were observed with macaroni and cheese. The penetration depths of electromagnetic energy at 27 and 40 MHz were about four times as great as those at the microwave frequencies 915 and 1800 MHz in all tested samples.

Dielectric properties of fruits and insect pests as related to radio frequency and microwave treatments.

Information on dielectric properties of commodities and insect pests is needed in developing thermal treatments for postharvest insect control based on radio frequency (RF) and microwave energy. Dielectric properties of six commodities along with four associated insect pests were measured between 1 and 1800 MHz using an open-ended coaxial-line probe technique and at temperatures between 20 and 608C. The dielectric loss factor of fresh fruits and insects decreased with increasing frequency at constant temperatures. The loss factor of fresh fruits and insects increased almost linearly with increasing temperature at 27 MHz radio frequency, but remained nearly constant at 915 MHz microwave frequency. Both dielectric constant and loss factor of nuts were very low compared to those of fresh fruits and insects. The temperature effect on dielectric properties of nuts was not significant at 27 MHz. The large difference in the loss factor between insects and nuts at 27 MHz suggests possible differential heating of insects in nuts when treated at the same time in a RF system.

Effects of Antimicrobial Coating from Catfish Skin Gelatin on Quality and Shelf Life of Fresh White Shrimp (Penaeus vannamei)

UNLABELLED The present study was conducted to evaluate the effect of fish skin gelatin-based antimicrobial coating on the shelf life of fresh white shrimp (Penaeus vannamei). Antimicrobial coating was prepared by incorporating potassium sorbate, sodium tripolyphosphate, or both ingredients in coating solution made from catfish skin gelatin. Shrimp samples were untreated, rinsed with water, or coated with the antimicrobial films, and then they were stored in ice under aerobic conditions. Samples were taken periodically and evaluated for total aerobic plate count, psychrotrophic bacteria count, near-infrared spectrum (NIR), springiness, pH value, and color measurement. The results showed that the antimicrobial coating could retard microbial growth and prolong shelf life for up to 10 d. Partial least square analysis produced an acceptable model fitting between the NIR and the changes of microbial count in shrimp samples. The springiness and pH value of shrimp muscle were not affected by the coating treatments. Color parameter a* value increased linearly for all the treatments during the whole ice-storage period. PRACTICAL APPLICATION   One of the most important technical and economic problems facing the shrimp industry is the relatively short shelf life of fresh shrimp products. Catfish skin gelatin-based antibacterial edible films/coatings provide an ideal technique to solve this problem. The shrimp industry will be the immediate beneficiary of the study, and the information provided by this research will be expended to other seafood.

Chemical Treatments for Reducing the Yellow Discoloration of Channel Catfish (Ictalurus punctatus) Fillets

The effect of chemical pretreatments on the color and carotenoid content of yellow discolored channel catfish fillets was studied. The color and carotenoid content of the fillets were analyzed by the L*a*b* color system and high-performance liquid chromatography, respectively. Untreated fillets turned more yellow and darker after 12-d storage. Sodium bicarbonate had a beneficial effect on reducing the degree of yellowness, but the fillets still turned darker after storage. Sodium bisulfite gave the best results with fillets becoming less yellow and brighter after storage. Ascorbic acid, butylated hydroxyanisole, citric acid, and sodium metabisulfite were not successful at reducing the discoloration. The sum of carotenoid contents of untreated fillets decreased significantly (P < 0.05) during storage as compared to the fresh fillets. However, the sum of the carotenoid contents of fillets treated by various chemicals was not significantly (P > 0.05) different from the fresh or untreated fillets. The appearance of catfish fillets may be improved during storage by pretreating with sodium bisulfite.

Frequency Distribution in Domestic Microwave Ovens and Its Influence on Heating Pattern

In this study, snapshots of operating frequency profiles of domestic microwave ovens were collected to reveal the extent of microwave frequency variations under different operation conditions. A computer simulation model was developed based on the finite difference time domain method to analyze the influence of the shifting frequency on heating patterns of foods in a microwave oven. The results showed that the operating frequencies of empty and loaded domestic microwave ovens varied widely even among ovens of the same model purchased on the same date. Each microwave oven had its unique characteristic operating frequencies, which were also affected by the location and shape of the load. The simulated heating patterns of a gellan gel model food when heated on a rotary plate agreed well with the experimental results, which supported the reliability of the developed simulation model. Simulation indicated that the heating patterns of a stationary model food load changed with the varying operating frequency. However, the heating pattern of a rotary model food load was not sensitive to microwave frequencies due to the severe edge heating overshadowing the effects of the frequency variations.

Radio-Frequency Applications for Food Processing and Safety

Radio-frequency (RF) heating, as a thermal-processing technology, has been extending its applications in the food industry. Although RF has shown some unique advantages over conventional methods in industrial drying and frozen food thawing, more research is needed to make it applicable for food safety applications because of its complex heating mechanism. This review provides comprehensive information regarding RF-heating history, mechanism, fundamentals, and applications that have already been fully developed or are still under research. The application of mathematical modeling as a useful tool in RF food processing is also reviewed in detail. At the end of the review, we summarize the active research groups in the RF food thermal-processing field, and address the current problems that still need to be overcome.

Validation of Xanthophyll Levels in Catfish Fillets Using Digital Photography

Catfish fillets with yellow coloration have become a prevalent problem for the catfish industry. This problem is due to the unacceptability in the market because the shift in fillet color is considered of lower quality by the consumers. To help the catfish industry better understand the yellow coloration of catfish fillet, a digital photography measurement method was developed to evaluate the yellowness. Sixty catfish fillets with a range of degrees of visible yellowness were taken directly from the processing line. The fillets were photographed in a light box with a digital camera. The photos were calibrated with the X-Rite ColorChecker standardized color target. CIELAB readings of the fillet photos were recorded, and the b* value was used to indicate the yellowness of catfish fillets. The xanthophyll levels of fillets were analyzed with high performance liquid chromatography. The actual xanthophyll level in catfish fillet was calculated as the sum of lutein, zeaxanthin, and alloxanthin. A linear correlation was found between the LAB b* values and xanthophyll levels of the 60 catfish fillets.

Dielectric Properties and Radio Frequency Heating of Heterogeneous Foods

Dielectric properties of major food ingredients (seasoned cooked ground beef, mozzarella cheese, noodles and sauce) in meat lasagna were determined using an open-ended coaxial line probe method at a range of 201 discrete frequencies between 1 and 1800 MHz and a range of temperature between 20 °C to 121 °C. Dielectric constants and loss factors increased as temperature increased at radio frequencies (RF) of 27 and 40 MHz, while these properties were relatively constant at microwave frequency of 915 MHz. Power penetration depths calculated from dielectric properties at 27 MHz for seasoned cooked ground beef (SCG beef), noodles and mozzarella cheese varied between 30 and 33 mm at 121 °C. A slightly shorter penetration depth in sauce (21 mm) was found at 27 MHz and 121 °C. Relatively uniform heating based on RF energy at 27 MHz was observed when SCG beef, mozzarella cheese and sauce were uniformly distributed between layers of noodles inside a 6-lb capacity combat group-ration polymeric tray. The greatest temperature differences when the product reached 121 °C among different ingredients inside a tray at approximately the same location were within the range of 2 to 5 °C. RF heating can reduce heatup time to one-third of that of retort heating. This study suggests that RF heating has a potential to produce pre-packaged heterogeneous food if major ingredients (SCG beef and noodles) have dielectric loss factors within the same range (1600-1800 at 27 MHz and 121 °C).