Effects of Surface Concentration on Drying Behavior of Carbohydrate Solutions

ABSTRACT

As the surface properties of the drying materials are very important not only for the drying rate but also for the quality change during drying, the effects of surface concentration on the drying behavior of liquid foods (sugar solutions) were investigated by isothermal drying experiments and by numerical calculation experiments. The isothermal drying experiments with gelled sugar solution systems (sucrose and maltodextrin) were carried out at various relative humidity (RH) values (RH = 0 to 84%). Separate experiments were carried out for determination of the desorption isotherms.

The isothermal drying curves of sugar solutions at RH = 0 to 51% were very similar. Numerical simulations also showed that the drying curves of these sugars at the surface concentration = 0 and 0.1 are almost the same, although the concentration distributions are different.

When a small amount of gelatin was added to sugar solutions, the drying rate decreased remarkably as the gelatin might form a thin film (skin) near the surface, and consequently the retention of ethanol increased.

Keywords:

• Water activity
• Desorption isotherm
• Water diffusion coefficient
• Equilibrium water content
• Drying rate
• Surface concentration

ACKNOWLEDGMENT

This research was partly supported by the MAFF project of bioenergy conversion.

Notes

¹Although the symbol for the water content in this journal is usually denoted as X, we use the symbol u in this paper. This is because since the first regular regime paper by Schoeber,^{[ 3 ]} the symbol u has been used in the Dutch and Japanese research groups