Water sorption isotherms and heat of sorption of osmotically pretreated and oven-dried papaya varieties

Author: K. O. Falade ^* and P. I. Uzo-Peters

Received 6 August 2004, accepted 18 October 2004.

Abstract

Adsorption isotherms of osmotically pretreated and subsequently oven-dried papaya (Carica papaya) varieties (Sunrise Solo, Homestead Green, Homestead Purple) were investigated using gravimetric static method. Ripe papaya varieties were cut into 10 mm x 10 mm x 50 mm slabs and immersed into 52, 60 and 68°B sucrose solutions maintained at 25°C in water bath. A fruit:solution ratio of 1:20 was maintained for 12 hours. Osmosed papaya slabs were subsequently oven-dried at 60°C for 72 hours. Adsorption isotherms of osmotically pretreated and oven-dried papaya varieties followed the type III (J-shaped) isotherms. Depression of water activity occurred in preosmosed oven-dried papaya slabs pretreated at higher sucrose solution concentrations. Guggenheim-Anderson-deBoer (GAB) model was used to fit the experimental data and to calculate monolayer moisture (M_m) content. M_m increased with an increase in pretreatment sucrose solution concentration. However, M_m decreased with increased equilibrium temperature. Isosteric heat of sorption increased with decreased moisture contents and pretreatment sucrose solution concentration. Heat of sorption of preosmosed oven-dried papaya slices was in the following order: ‘Homestead Purple’ > ‘Homestead Green’ > ‘Sunrise Solo’.