Use of oscillating electromagnetic fields in the processing of viscous vegetable homogenates
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ENG- Industrial thermal treatment of liquid foods is often carried out by tunnel pasteurization, or continuous-flow pasteurization by putting the food material in indirect contact with hot water or steam in a way that typically requires high amounts of space, water, and energy. Furthermore, this energy is often generated in boilers powered by fossil fuels, which increases the carbon footprint of the process and limits the sustainability of the food sector.
Radiofrequency (RF) technology is a dielectric heating method that causes heat generation volumetrically within the food material by placing it in contact with an oscillating electromagnetic field at a range of 10 to 300 MHz. This method has the potential to make industrial thermal treatments substantially less resource-intensive and to improve the nutritional and sensory properties of the resulting products.
Within the food sector, vegetable homogenates are particularly difficult to pasteurize using conventional methods due to their high viscosity, and the thermolabile fresh-like properties consumers seek in them. The present doctoral dissertation evaluated the application of RF heating on a tomato-based viscous homogenate using a RF staggered-through field tunnel, as well as a continuous-flow pasteurization unit.
A systematic review was conducted to identify the gaps in the knowledge regarding dielectric heating of liquid foods. It was observed that the mechanisms behind the heating behavior of packaged fluid foods during RF heating had not been approached in the literature.
As a first step, the dielectric properties of a tomato-based homogenate were measured using the coaxial probe method. The effect of temperature, salt content and oil content were evaluated at different frequencies from 10 to 30000 MHz. A predictive model for in-package RF heating on a staggered-through field applicator using the finite-element method was developed. The model was validated with heating experiments using two nominal voltages (3000 V and 5000 V) and two electrode gaps (8.14 and 8.64 cm). The mechanisms behind the temperature distribution and the heating rates of the samples under this heating process were elucidated and the effects of electrode gap, voltage, and formulation were studied. Finally, a method for obtaining an equivalent voltage was described in order to apply one-dimensional equations instead of the calculation-intensive finite-element method.
To study continuous-flow RF pasteurization, a tomato-based homogenate was prepared, salmorejo, using tomato, salt, oil, and other ingredients. Pasteurizations of this product were carried out using conventional continuous-flow pasteurization and continuous-flow RF pasteurization using two different approaches: integrating a STE system for the heat generation or using heat exclusively from an electric boiler. Both RF and CP technologies were compared regarding their energy savings when incorporating STE at different seasons. It was observed that pasteurizations could be carried out almost exclusively with STE in high radiation months. It was also observed that the use of RF pasteurization did not cause higher energy consumptions, moreover, it had the potential to increase the fraction of energy coming from the STE system by reducing the set-point temperature of the buffer tank
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