Efeito de pré-tratamentos químicos na taxa de secagem de acerola em monocamada.

Abstract

The West Indian cherry (Malpighia Emarginata DC) also called "acerola" is a fruit widely cultivated in the Northeast region of Brazil. It has a moisture content in turn of 90% (wet basis). It is easily perishable, making necessary a process to diminish the water activity in order to avoid the damage by the action of fungi and other microorganisms allowing the storage for a longer time. The drying of fruits with skin, such us West Indian cherry, grapes, cherries, takes a long time due to the wax layer at the external part of the membrane, that works as a barrier, impeding the water loss. Chemical pre-treatments with dilute solutions or emulsions have been used in order to diminish the drying time, by modifying the physical structure of the external membrane of the fruits. In this work, thin layer drying curves of pre-treated West Indian cherries were obtained in a fixed bed dryer at 70°C and 80°C with air flow rate of 1.5m/s and 1.84m/s. Several pretreatments were used varying the temperature and immersion time to verify their effect on the reduction of the drying time. It was observed that the pre-treatment with saccharose (67%) and NaCI (1%) using an immersion time of 4 hours, as well as NaOH solutions with concentrations varying between 1% to 3% and immersion time from 1 to 10 minutes weren't effective in reducing the drying time. All the above pre-treatments were done at 25°C. However, for the West Indian cherries pre-treated at higher temperatures, the drying time reduction was evident. This fact was observed in the pre-treatments with 94.78% water + 4.74% K2C03 + 0.48% olive oil, during 3min at 35°C or 2s at 90°C, as well as NaOH (0.3%) during 5s at 90°C, obtaining a reduction of time even more than 80min at the higher immersion temperatures. The shrinkage of the fruits during the drying was measured in function of the moisture content. In spite of the different reduction in drying time with the several pre-treatments used in this work, no significant difference on the shrinkage behavior of the West Indian cherries was observed, neither with the untreated fruits. In terms of the drying kinetics, it was observed that the time to get the maximum drying rate, in about of 20min in all the drying curves coincides approximately with the rapid evolution of the inner temperature of the fruit, getting after this period almost isothermal conditions.