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  • 1.
    Funebo, Tomas
    et al.
    SIK – Institutet för livsmedel och bioteknik.
    Ohlsson, Thomas
    SIK – Institutet för livsmedel och bioteknik.
    Dielectric properties of fruits and vegetables as a function of temperature and moisture content1999In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823, Vol. 34, no 1, p. 42-54Article in journal (Refereed)
    Abstract [en]

    The relative permittivity ??, and relative loss factor, ??, of apple, chervil, mushroom, parsley, and strawberry were measured at 2800 MHz, with a cylindrical TM012 perturbation cavity. The measurements were made over a range of moistures and temperatures. The results show that the relative loss factor has a maximum value at an intermediate moisture content for all the fruits and vegetables investigated. The relative permittivity decreases with increasing temperature at a high moisture content, but at intermediate moisture contents ?? increases with temperature. Equations were developed for interpolation of the relative permittivity and the loss factor.

  • 2.
    Raaholt, Birgitta
    et al.
    RISE - Research Institutes of Sweden, Bioscience and Materials, Agrifood and Bioscience.
    Hamberg, Lars
    RISE - Research Institutes of Sweden, Bioscience and Materials, Agrifood and Bioscience.
    Isaksson, Sven
    RISE - Research Institutes of Sweden, Bioscience and Materials, Agrifood and Bioscience.
    Continuous tubular microwave heating of particulate foods at high temperatures2017In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823Article in journal (Refereed)
    Abstract [en]

    A pilot-scale process for continuous in-flow microwave processing of particulate pumpable foods, designed and implemented at RISE Agrifood and Bioscience, was evaluated for heat treatment of a particulate, viscous model food at high-temperature conditions at 2450 MHz. The microwave system has three consecutive cavities, one excited by the TM020 microwave mode that heats primarily in the centre of the tube, and two cavities fed by TM120 modes that heat primarily in the tube periphery. In this paper, combined TM020 and TM120 tubular microwave heating is evaluated as an alternative to high-temperature short-time (HTST) processing for a high-concentrated particulate model product. Rapidness in heating of the product was evaluated after tubular microwave heating for different time-temperature conditions, corresponding to the required microbiological inactivation for a model product intended for storage at ambient conditions. Moreover, the effects on product quality of the microwave heated model soup were investigated in terms of texture, piece integrity and colour. Microstructural analysis was used to gain an understanding of the effects of heating at a microscopic scale. It was found that the microwave-assisted HTST system results in large process flexibility. Additionally, it offers advantages in product quality in terms of piece integrity and texture.

  • 3.
    Raaholt, Birgitta
    et al.
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Food and Bioscience.
    Isaksson, Sven
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Food and Bioscience.
    Hamberg, Lars
    RISE, SP – Sveriges Tekniska Forskningsinstitut, SP Food and Bioscience.
    Fhager, Andreas
    Chalmers University of Technology, Sweden.
    Hamnerius, Yngve
    Chalmers University of Technology, Sweden.
    Continuous tubular microwave heating of homogeneous foods: evaluation of heating uniformity2016In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823, Vol. 50, no 1, p. 43-65Article in journal (Refereed)
    Abstract [en]

    A pilot-scale process for continuous in-flow microwave processing of foods, designed and implemented at SP Food and Bioscience, was evaluated for heat treatment of a homogeneous model food for high-temperature short-time (HTST) conditions, at constant total input microwave power, at 2450 MHz. The microwave system has three consecutive cavities, one excited by the TM020 microwave mode that heats primarily in the tube centre, and two TM120 mode cavities that heat primarily in the tube periphery. The temperature uniformity of the homogeneous model food after microwave heating is here evaluated in terms of spatial distribution, for different set-ups of input microwave power in each cavity and for different order of the placement of the cavities, while maintaining the total input microwave power. The microwave heating uniformity is evaluated, based on measured and calculated radial temperature profiles. Combined TM020 and TM120 heating was found to result in more uniform heating by means of spatial temperature uniformity over the tube cross section. Furthermore, appropriately selected microwave power distribution between the centre and periphery heating cavities results in a stable heating profile in the studied food, that differs only about 10 °C or less between highest and lowest average values directly after microwave heating.

  • 4.
    Risman, Per O.
    et al.
    SIK – Svenska livmedelsinstitutet.
    Ohlsson, Thomas
    SIK – Svenska livmedelsinstitutet.
    Wass, B.
    SIK – Svenska livmedelsinstitutet.
    PRINCIPLES AND MODELS OF POWER DENSITY DISTRIBUTION IN MICROWAVE OVEN LOADS.1987In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823, Vol. 22, no 4, p. 193-198Article in journal (Refereed)
    Abstract [en]

    Multi-mode cavity resonances are generally considered the most important phenomenon in transferring energy to the load in microwave ovens. There are, however, other important phenomena, caused by scattering and near-fields. This paper deals with the multi-mode resonant and some other important field types. Some main areas where sufficient knowledge is not available are described and main problems in applying simplified models are discussed.

  • 5. Ryynanen, S.
    et al.
    Risman, P.O.
    Ohlsson, Tomas
    SIK – Institutet för livsmedelsforskning.
    The dielectric properties of native starch solutions: A research note1996In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823, Vol. 31, no 1, p. 50-53Article in journal (Refereed)
    Abstract [en]

    The dielectric properties of four native starch solutions, both gelatinized and ungelatinized with concentrations from 5 to 30% over the temperature range from 3°C to 95°C, were measured by a cavity perturbation technique at 2.75 GHz. The dielectric properties of the starch types did not differ from each other; the most important factor affecting the dielectric properties was the water content. Both the real permittivity (??) and the loss factor (??) decreased with increasing temperature and increasing starch concentration. The differences between gelatinized and nongelatinized samples were small, the ?? of nongelatinized starch being 2-3% higher and the ?? up to 5% lower than gelatinized starch.

  • 6.
    Sundberg, Magnus
    et al.
    SIK – Institutet för livsmedel och bioteknik.
    Kildal, P.
    Ohlsson, Tomas
    SIK – Institutet för livsmedel och bioteknik.
    Moment method analysis of a microwave tunnel oven1998In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823, Vol. 33, no 1, p. 36-48Article in journal (Refereed)
    Abstract [en]

    Industrial microwave tunnel ovens have previously been analyzed using the finite difference time domain (FDTD) method. However, it has been observed that since the computation time on normal computers often becomes very long, it is desirable to develop more efficient methods. In this paper an approach based on the moment method (MM) is used to calculate the electromagnetic field in a tunnel oven. The oven consists of a rectangular cavity mounted on a flat tunnel, containing one or more dielectric layers. The layers correspond to e.g. the food load and the conveyor belt. It is shown that the results from the MM calculations agree well with the FDTD method and that the computation time, compared to FDTD, is significantly reduced. For some problems, the computations are sped-up by a factor of 100. By combining the fast MM computations with the simulated annealing method for optimization, together with suitably chosen performance criteria, it is shown how the design process can be drastically shortened. By varying the dimensions of the cavity and the tunnel, the power distribution in the loads can be optimized for a wide range of applications,and, since the basic design of the ovens is kept constant, the development cost for new equipment can be reduced.

  • 7. Sundberg, Magnus
    et al.
    Risman, P.O.
    Kildal, P.-S.
    Ohlsson, Tomas
    SIK – Institutet för livsmedel och bioteknik.
    Analysis and design of industrial microwave ovens using the finite difference time domain method1996In: The Journal of microwave power and electromagnetic energy, ISSN 0832-7823, Vol. 31, no 3, p. 142-157Article in journal (Refereed)
    Abstract [en]

    The Finite Difference Time Domain (FDTD) method is applied to industrial microwave ovens used in the food industry. Theoretical and practical aspects of FDTD analysis for this purpose are dealt with. Two different open-ended applicator types are described and analyzed numerically and experimentally and their relative performance is discussed. One of these applicators is a novel design based on low impedance TM modes which reduces edge-overheating and load reflections. It is shown that the common belief that the heating pattern from open-ended microwave applicators can be predicted from their radiation fields alone is often in error, and that resonances in the whole system of applicators and food loads are of major importance.

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