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Dielectric Properties of Cloudy Apple Juices Relevant to Microwave Pasteurization

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Abstract

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.

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Abbreviations

a i :

Polynomial coefficient of order i

a w :

Water activity (–)

c :

Speed of light in free space (2.9979 × 108 m/s)

C ″ σ :

Relative contribution of ionic conduction to the loss factor (%)

C ″d :

Relative contribution of dipole rotation to the loss factor (%)

d p :

Penetration depth (m)

e :

Euler number (2.7183)

f :

Field frequency (Hz)

R 2 :

Coefficient of determination

T :

Temperature (K)

TSS:

Total soluble solids (°Brix, %)

TTA:

Titratable acidity (g malic acid/100 mL)

ε :

Complex relative permittivity (–)

ε s :

Relative permittivity at zero frequency or DC circuit (–)

ε o :

Relative electrical permittivity of free space (8.854 × 10−12 F/m)

ε ∞ :

Relative permittivity at high frequencies (–)

ε′:

Relative electrical permittivity (–)

ε″:

Dielectric loss factor (–)

ε ″d :

Dielectric loss factor due to dipole rotation (–)

ε ″ σ :

Dielectric loss factor due to ionic conduction (–)

π :

Pi number (3.1416)

σ :

Electrical conductivity, DC circuit (S/m)

σ est :

Standard error of estimate

τ :

Relaxation time (s)

ω :

Angular frequency (rad/s)

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Acknowledgments

The authors acknowledge the financial support from the São Paulo Research Foundation (FAPESP) under grants 2012/04073-0, 2013/07914-8 and 2014/06026-4.

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Authors and Affiliations

  1. Department of Chemical Engineering, Escola Politécnica, University of São Paulo, Av. Prof. Luciano Gualberto, trav.3, n.380, São Paulo, SP, 05508-010, Brazil

    Érica Sayuri Siguemoto & Jorge Andrey Wilhelms Gut

  2. Food Research Center (FoRC), University of São Paulo, São Paulo, SP, Brazil

    Jorge Andrey Wilhelms Gut

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  1. Érica Sayuri Siguemoto
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Correspondence to Jorge Andrey Wilhelms Gut.

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Siguemoto, É.S., Gut, J.A.W. Dielectric Properties of Cloudy Apple Juices Relevant to Microwave Pasteurization. Food Bioprocess Technol 9, 1345–1357 (2016). https://doi.org/10.1007/s11947-016-1723-0

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