Moisture sorption isotherms and estimation of isosteric heat of sorption of donkey milk powder

The moisture sorption isotherms of donkey milk powder were studied at 20, 25 and 30°C for 0.22 to 0.93 aw through gravimetric method. GAB, Peleg, Smith and Oswin mathematical models were fitted to experimental data. Equilibrium moisture content of the sample increased from 0.0499 to 0.3255, 0.0447 to 0.2904, 0.0426 to 0.2618 kg.kg−1 solid at 20, 25 and 30°C with increasing aw from 0.22 to 0.93. Isotherm curves showed type II behavior. Monolayer moisture content decreased from 0.0373 to 0.0301 g.100 g−1 solids at 20 to 30°C. The net isosteric heat of sorption decreased from 98.55 to 0.74 kJ.mol−1 with increased moisture content from 2 to 20% (d.b.). The adsorption isotherm curves suggest that the sample requires more attention when handled in RH higher than 24, 33 and 42% at 20, 25 and 30°C. Novelty impact statement Donkey milk powder was stable below 42% RH at 30°C. The best-fitting Oswin model equation provides a solid foundation for future research on drying and storage of donkey milk powder. Isosteric heat of sorption of donkey milk powder decreases with increase in moisture content.