Pre-treatment of fruit

Fruits of approximately equal size are used for the production of dried fruit. Differences in fruit size give rise to differences in drying times. Sorting by size is essential, therefore, if a product that is uniform in appearance and degree of drying is to be obtained. 

Depending on taste, the fruit can be previously peeled or crushed. The stones should always he removed. In the case of apples and pears, the core is cut out before crushing. Plums and apricots are cut in half. These fruit halves can he joined together again, but they are usually dried separately, with the cut surface upwards. 

Thermal pre-treatment can be carried out before the fruits enter the dryer. This reduces both the initial microbial count of the fruit and the degree of enzymatic browning. The cells in the pieces of fruit that are heated break up, thus accelerating the subsequent drying process. 

Plums have on their surface a layer of wax which slows down the evaporation of water. Heating destroys this layer, so a higher drying rate is achieved. 

The most common method of thermal pre-treatment is blanching. In this process, wire baskets are used to immerse fruits or pieces of fruit into boiling water. Depending on the species and size of the fruits, the time required for blanching ranges from 10 s to a few minutes (or up to 10 min in the case of quinces). 

---

Torreggiani, D., Lucas, T., and Raoult-Wack, A.L. 2000. The pre-treatment of fruits and vegetables. In Managing Frozen Foods, ed. C. Kennedy, pp. 54—80. Boca Raton, EL Woodhead Publishing Limited. 

---