Abstract
Hot air drying (HAD) at temperatures below the spontaneous evaporation temperature could be combined with microwave (MW) radiation as a thermal energy source in order to reduce the drying time. A photon flux in the microwave range interacts with dipolar molecules (water) through orientation and induction, producing electrical energy storage and thermal energy accumulation and generating an increase in the internal energy of food. The different mechanisms involved in water transport could change when the microwave penetration depth exceeds the sample characteristic dimension of mass transport. The aim of this paper is to determine the effect of MW in the combined HAD-MW drying of raw potato in order to obtain the real driving forces and mechanisms involved in the water transport, with the purpose of optimizing the MW power used. For this purpose, combined drying was carried out on potato samples (0, 4 and 6 W/g). The sample surface temperature was monitored by infrared thermography, and the sample mass was measured continuously through a precision balance. In parallel with continuous drying, another drying treatment was performed at different times (20, 40, 60, 90, 120, 180, 420 min) and conditions (0, 4 and 6 W/g) to analyze the dielectric properties, mass, moisture, volume and water activity. The results show that it is possible to monitor combined drying by infrared thermography, and it can be concluded that the convection heating is mostly transformed into surface water evaporation, with negligible thermal conduction from the surface, and microwave radiation is mostly transformed into an increase in the potato’s internal energy.
Highlights
During hot air drying (HAD) at temperatures below the spontaneous evaporation temperature, the main force that drives water removal from the product is the gradient of water chemical potential generated between the air, with a very low relative humidity, and the product, with high water activity, causing a water flux from the product to the air [1]
The aim of this paper is to determine the effect of MW in the combined HAD-MW drying of raw potato at a drying air temperature below the spontaneous evaporation temperature, with the potato having a smaller characteristic dimension than the MW penetration depth, in order to obtain the real driving forces and the mechanisms involved in the water transport, with the purpose of optimizing the MW power used
In a combined drying process of hot air and microwaves at temperatures lower than the spontaneous evaporation temperature and in samples with a characteristic dimension less than the microwave penetration depth, it is possible to conclude that the convection heating is mostly transformed into surface water evaporation, and microwave heating is mostly tra1n0so-f 12 formed into an increase in the internal energy of the food, that is, an increase in the internal temperature
Summary
During hot air drying (HAD) at temperatures below the spontaneous evaporation temperature, the main force that drives water removal from the product is the gradient of water chemical potential generated between the air, with a very low relative humidity, and the product, with high water activity, causing a water flux from the product to the air [1]. The diversity of water molecule interactions that occur inside the potato tissue [4], as well as the different ways that water molecules have to reach the surface, make the internal water transport fast at the beginning of drying, as the water molecules with high mobility are mobilized through the fastest route (apoplastic pathways). In this drying period, surface evaporation is the limiting factor of the overall transport. The internal transport becomes the limiting factor [5]
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