Dielectric properties, effect of geometry, and quality changes of whole, nonfat milk powder and their mixtures associated with radio frequency heating

Abstract

Radio frequency (RF) heating is a promising pasteurization technique providing a fast and more uniform heat generation throughout a sample. In this paper, the effect of composition, frequency (1–30 MHz) and temperature (20–90 °C) on the dielectric properties of whole, nonfat milk powder and their mixtures were studied to develop an effective RF pasteurization. Moreover, the effect of the shape and dimension of the container on RF heating were evaluated. Milk powders were loaded into rectangular and cylindrical boxes with the same volume and different heights (3, 6 and 9 cm), and heated in a 6 kW, 27.12 MHz pilot scale free-running oscillator RF oven. The heating rate, temperature profile, penetration depth, and electric field strength were determined. The quality of the RF heated milk powders was evaluated by measuring moisture content, water activity, solubility, and color. Overall, heating milk powder in the 6 cm height cylindrical box showed a relatively more uniform temperature distribution than 6 cm height rectangular box, with moderate heating rate as well as improved quality in the finished product with less changes in moisture content, water activity, solubility and color. Electric field strength increased with the increase in the height of the container.