Water state diagram and thermal properties of fructans powders

Abstract

The thermal analyses and the water state diagrams of agavin and inulin were investigated. The thermogravimetric analysis and differential scanning calorimetry (DSC) were used to know the difference between fructans. Linear inulin (220.0 °C) showed a higher thermal stability than the branched agavins (206.7 °C). The samples displayed different physical states and lightness depending on the water activity of storage (a w). The agavin showed different physical states: powder (a w < 0.33), soft lump (a w ≈ 0.43), sticky (0.55 < a w < 0.77), and liquid (a w > 0.85), while inulin showed two physical states: an amorphous powder at a w < 0.55 and a semicrystalline solid at a w > 0.69. These physical states decreased the lightness of fructans. The GAB equation and the Gordon–Taylor equation showed a good fit to the experimental data (R 2 > 0.95). The K constant of GAB equation predicted a lower water affinity of inulin powder, while the agavin powder presented a higher water affinity. The DSC showed a glass transition temperature of 118.1 and 135.3 °C for agavin and inulin anhydrous powders, respectively. An increase in water content in agavin showed a higher depression in the glass transition temperature than in inulin. Also, the inulin exhibited a melting point while agavin not. The water state diagrams showed the critical water activity at which the agavin (0.50) and inulin (0.59) powders were stable to physical changes. This knowledge is used to understand the physical and structural behaviors of agavin and inulin at the different water activities.