Dielectric Properties of Cloudy Apple Juices Relevant to Microwave Pasteurization

• Published in: Food and bioprocess technology 2016-08, Vol.9 (8), p.1345-1357
• Main Authors: Siguemoto, Érica Sayuri, Gut, Jorge Andrey Wilhelms
• Format: Article
• Language: English
• Subjects: Agriculture; Apples; Biotechnology; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Continuous flow; Dielectric loss; Dielectric properties; Dissipation factor; Electrical conductivity; Electrical properties; Electrical resistivity; Food Science; Fruit juices; Fruits; Industrial plants; Juices; Microwaves; New technology; Original Paper; Pasteurization; Penetration depth; Permittivity; Temperature effects; Thermal runaway; Tubes
• ISSN: 1935-5130; 1935-5149
• DOI: 10.1007/s11947-016-1723-0

Continuous flow microwave heating is an emerging technology that brings advantages to the pasteurization of food products such as fruit juices. In order to correctly design the microwave cavity and applicator tube, knowing the dielectric properties of the product is important. The relative electrical permittivity describes the capacity to store electrical energy, while the dielectric loss factor describes the ability to dissipate energy as heat. The penetration depth of microwaves depends on these properties and on the field frequency. In this work, the dielectric properties and electrical conductivity of cloudy apple juices, obtained from different varieties of apples (Gala, Fuji, Granny Smith, and Red Delicious) and from an industrial plant (Gala), were determined from 500 to 3000 MHz and temperatures between 10 and 90 °C. Penetration depth was also calculated, and all results were correlated with temperature for commercial frequencies of 915 and 2450 MHz. At these frequencies, apple variety showed little influence and the permittivity decreased almost linearly with temperature. The loss factor curves showed a minimum around 40 °C at 915 MHz and were slightly smaller for juices with lower conductivity. At 2450 MHz, the loss factor varied between 7 and 18 with a negative temperature effect, which indicates that thermal runaway should not be a problem. The penetration depth ranged from 33 to 58 mm at 915 MHz and from 9 to 23 mm at 2450 MHz, depending on temperature and variety, which is useful information for the design of applicator tubes.