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ONION DEHYDRATION: A MODEL TO CALCULATE DRYING RATE
Hence the typical sample piece is a body with four lateral faces corresponding to the cutting sections, and two horizontal ones, corresponding to the foil boundary surfaces.
Those surfaces are covered by a membrane fairly impervious to water migration, so drying takes place mainly through the lateral, porous faces.
Thus, the smaller the piece size, the larger the area ratio of the permeable faces to the other two, and the larger the drying rate.
Reciprocally, the product quality is reduced if the pieces are too small.
Most of the chemicals giving the typical taste and aroma to onions, are result of reactions between enzymes and substracts, normally kept separate within the tissue, named precursors. When the structure is broken, as it happens during cutting, precursors react and the products are lost later.
In summary, the smaller the pieces, the larger the amount of chemicals preserved from reacting, keeping the onion hedonic characteristics until consumption.
It is apparent that drying rate and quality are opposite functions of the pieces size, and consequently it is expected that must be an optimal size.
The model presented allowes the calculation of the drying time as a function of the sample shape and size, and the air temperature, moisture content and velocity for rotary driers, providing criteria for the selection of the optimum size.