Water Sorption Isotherm

Results and discussions

Water sorption isotherms

The water sorption isotherm (Fig. 2a & b) was a convex curve indicating type III isotherm as classified by the IUPAC (International Union of Pure Applied Chemistry). This type of isotherm describes the macroporous and hydrophobic nature of the pumpkin powder and the weak interaction between the adsorbate and adsorbent at 25 °C (Ng and Mintova 2008). Perhaps, that was because of the domination of carbohydrate and fiber in the structure. The adsorption of water started from the moisture content of 13–15 % at 0.1 water activity and ended at 98–105 % at 0.9 aw. Since the pumpkin structure has 0.26 % of proteins (amino acid) and fat which can eventually oxidize upon absorbing moisture at water activity 0.4 (Labuza et al,. 1972), the relative humidity must be below 20 % according to Fig. 2. The freeze–dried pumpkin powder showed a similar adsorption–desorption trend to other fruits and vegetables such as raspberry (Syamaladevi et al,. 2009), date (Alhamdan and Hassan 1999), fresh pumpkin parenchyma, and seeds (Mayor et al,. 2005). Due to the structural diversity, the pumpkin powder absorbed more than its original weight on average like apples, pear, and lemon peel (Mrad et al,. 2013, García-Pérez et al,. 2008).

The best–fitted model was GAB, Fig. 2a, as

4), and moisture content decrease. The increase in moisture content resulted in a decrease in the food stability at a defined temperature. One possible theory is that after the glass transition occurred the pumpkin components start to have a segmental mobility within the structure (e.g., free volume theory). The thermodynamic theory suggests that the change in Tg is plasticizing effect at the molecular level (Roos 2010). Therefore, the unfreezable water content (no melting point) was acting as plasticizer agent in the pumpkin structure (Sablani et al,. 2010). Unfreezable water was explored further