Moisture Adsorption Characteristics of Ginger (Zingiber officinale) Powders

Abstract

The adsorption equilibrium moisture content of the powders of peeled ginger (PGP) and unpeeled ginger (UGP) at 20, 30, 40 and 50 °C were investigated using static gravimetric method. The data were fitted to various sorption models. The thermodynamics of water vapour adsorption of the powders was also investigated at moisture contents in the range of 8 to 20 g H₂O/100 g solid. Both powders exhibited the type II isotherms. The equilibrium moisture content of the powder decreased with increasing temperature and increased with water activity. The monolayer moisture content also decreased with an increase in temperature. The powder of peeled ginger was less hygroscopic than that of unpeeled ginger and is therefore more shelf- stable. The GAB, Henderson and Oswin equations accurately fitted experimental sorption data (RMS < 10 %); however, the GAB model gave the best fit. The net isosteric heats (ΔH_st) of sorption generally decreased with increasing moisture content. The ΔH_st of UGP was generally higher than that of PGP. The isosteric heat of the first molecule (ΔH_o) of UGP was also higher (72.042 kJ/mol) than that of the PGP (59.695 kJ/mol). On the other hand, the characteristic moisture content (M_ch) of UGP was lower (4.058 g H₂O/100g solid) than its equivalent for PGP (4.155 g H₂O/100 g solid). The values of ΔH_o and M_ch indicated the relative low moisture affinity of UGP compared to PGP. The enthalpies of sorption for PGP were lower than UGP. Conversely, the entropy of sorption of UGP was higher (6.233 to 158.180 J/mol-K) than that of PGP (2.496 to 125.721 J/mol-K).