Temperature-Dependent Dielectric Properties of Raw Cow’s and Goat’s Milk from 10 to 4,500 MHz Relevant to Radio-frequency and Microwave Pasteurization Process

Radio frequency processing and recent advances on thawing and tempering of frozen food products

During radio frequency (RF) thawing-tempering (defrosting) of frozen food products, some regions, mostly along the corners and edges, heat-thaw first due to the strong interaction of electric field and evolved heating leading to temperature increase. Resulting higher power absorption along these regions, compared to the rest of the volume, is the major cause of this problem. Besides, increase in temperature with phase change results in a significant increase of dielectric properties. This situation leads to runaway heating, which triggers the non-uniform temperature distribution in an accelerated manner. All these power absorption and temperature non-uniformity-based changes lead to significant quality changes, drip losses, and microbial growth. Based on this background, the objective of this review was to provide a comprehensive background regarding the most relevant and novel defrosting application studies using RF process, dielectric property data for frozen foods in the RF band, and novel mathematical modeling based computer simulation approaches to achieve a uniform process. Experimental and modeling studies were related with electrode position, sample geometry and size, electrode gap of the applied RF process, and the potential of charged electrode. Applying translational and rotational movement of the food product and the charged electrode vertical movement during the process to adjust the electric field and use of two-cavity systems and curved electrodes were also explained in detail. The data presented in this review is expected to give an insight information for further development of innovative RF thawing/tempering systems.

Dielectric Properties of Infant Formulae, Human Milk and Whole and Low-Fat Cow Milk Relevant for Microwave Heating

The dielectric properties of three different reconstituted infant formulae, human milk, as well as, whole and low-fat pasteurized cow milk were measured at temperature interval from (5 to 70) °C and at frequency of 2450 MHz. The dependency of the dielectric properties with temperature is presented. The values of penetration depth were determined and can be used to estimate the ideal bottle-feeding or container diameter to use in a domestic microwave oven or specific microwave equipment to pasteurize human milk in Human Milk Banks (HMB).

Dielectric characterization of raw and packed soy milks from 0.5 to 20 GHz at temperatures from 20 to 70 ºC

For microwave heating pasteurization processes, knowledge of the dielectric characteristics of foods are very important. In this paper, we present the dielectric properties of raw soy milk and commercial packed soy milk of four different flavors (light, natural, chocolate and pecan) from 500 MHz to 20 GHz, covering most of the assigned frequencies by the Federal Communications Commission for heating purposes. Experiments were performed using an open-ended-coaxial probe and a vector network analyzer. This characterization was carried out for temperatures ranging from 20 to 70 °C in steps of 10 °C. The dielectric constant of soy milks decreased with increasing frequency, while increasing temperature resulted in decreasing of the values. The dielectric loss factor presents a U shape behavior, where the loss started decreasing from 500 MHz to about 3 GHz and then, it increased again up to 20 GHz. In addition, higher temperatures decreased the dielectric loss. Applying the higher order Debye´s equation, two relaxation times were calculated for the soymilks, with good agreement with the measured properties. Deeper penetration of microwaves were obtained for raw soy milk at 915 MHz, making it suitable for microwave pasteurization.