Effect of Vacuum Drying Temperature on Drying Kinetics, Effective Moisture Diffusivity and Quality of Peeled Litchi (Litchi chinensis Sonn.)

Abstract

This study aimed at investigating the effect of vacuum drying temperature on drying kinetics, drying rates, effective moisture diffusivity and quality parameters of litchi fruits. Peeled litchi quarters were dried at 50-70 8C at an absolute pressure of 8 kPa. Results show that the vacuum drying occurred in the falling rate period. Drying rates increased with drying temperature. The Page model provided the best fit to the drying kinetics. Effective moisture diffusivity ranged between 1.997 3 10 29 and 5.012 3 10 29 m 2 /s. The temperature dependence of the effective moisture diffusivity was well described by an Arrhenius-type equation. The activation energy for moisture diffusion was 41.81 kJ/mol. Drying temperature did not significantly affect product sensory acceptance, density, shrinkage and rehydration. The products dried at 50 and 70 8C presented slightly superior texture and flavor, while the product dried at 60 8C presented significantly higher lightness and lower water activity. PRACTICAL APPLICATIONSThis work provides valuable information on the vacuum drying of litchi fruits, which are appreciated in many countries due to their pleasant sensory characteristics. Vacuum drying is not a costly drying method. At the same time, vacuum drying produces dried foods of relatively good quality. This study could be used as a reference by small and medium scale food processors interested in producing dried litchi of good quality with relatively low investment in equipment. The understanding of the influence of drying temperature on quality parameters is useful for deciding which temperature is more appropriate depending on the desired quality aspects, while the calculation of engineering parameters is important for designing the drying process.Lychee or litchi (Litchi chinensis Sonn.) is a plant native to the area between southern China, northern Viet Nam and the Malay Peninsula, where it has been grown for more than two thousand years. Litchi fruits present a pink-red, bright red or purple-red skin and a translucent white, sweet, juicy and aromatic flesh. Over the last four hundred years, this plant has been taken to several tropical and subtropical countries (FAO 1983). Litchi has been increasingly grown in Paran a state, Brazil, where the production raised from 176 tons in 2005 to more than 3,000 tons in 2014 (Parana State Agriculture Secretariat 2015).Like most fruits, litchi presents high water content and short shelf-life. One of the options for increasing fruit shelflife is drying. Vacuum drying presents higher costs when compared to atmospheric drying. Among the advantages that justify the use of vacuum drying instead of atmospheric drying is the superior quality of the product, since it is processed at mild temperature and in the absence of oxygen, besides higher drying rates (Reis 2014).A study on the vacuum drying of onion reported an increase in drying rate with a rise in drying temperature from 50 to 70 8C (Mitra et al. 2011). Additionally, they found that the Page model was the most suitable for fittin...

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