Intermittent Drying of Walnuts: Evaluations of Warm Air Consumption on a Thin Layer

Walnuts trade standards require a maximum humidity of the 8% (w/w) to protect the commercialized fruits during their shelf-life. To achieve the target humidity, walnuts must be dried. During drying, the water migrates slowly from the inner to the outer parts of the fruits before its transfer to the warm air. However, the water migration is slow and the efficiency in the use of the drying heat is poor. The intermittent drying alternates periods when warm air is driven to fruits, with tempering periods allowing the water to diffuse on fruit surface. The aim of the work is to test if the intermittency in walnuts drying could provide advantages in the use of warm air and quantify them. Two trials were carried out al laboratory scale to test the effect of the intermittent drying at different frequency and on shelled and in shell walnuts. In the walnuts continuously dried were always the faster to complete the process. However, intermittent drying allowed to decrease the use of heated air roughly of the 30% at 30 min−1 and of the 25% at 60 min−1. Considering shelled walnuts intermittent drying saved the 36% of time using heat air. In conclusion, intermittency in walnuts drying could decrease the use of heat air of the process of roughly the 30–35%, and consequently it could improve process sustainability. Further improvement in the process sustainability could be achieved through the optimization of the intermittency conditions.