Freezing as a pre-treatment in air drying of plums

According to its composition, primarily for its high phenolic content and high antioxidant capacity, prune is among highly valuable nutritional foods. Because of using whole fresh fruit, drying of plums is considered a slow and time-consuming process. Thus, various pre-treatments are applied in order to reduce drying time with no effect on declining nutritional value. The most simple and commonly used pre-treatment is dipping – immersing fruit into boiling water before drying. Since the plum drying season is timely restricted, we had to come up with the idea to extend it by previously freezing the fruit intended for drying and then, after a few months of storage at -20°C, use them for drying. Considering the physical changes of fruit during freezing that could affect drying speed, freezing can be regarded as pre-treatment in the production of prune. Drying of fruit was carried out in an experimental dryer at an air temperature of 90°C until reaching 75% of the total dry matter. Plum cultivars created at the Fruit Research Institute recommended for drying were used (‘Nada’, ‘Čačanska Rodna’ and ‘Valjevka’) and ‘Stanley’, which is the most frequently used for drying in Serbia. The aim of this work was to study the effect of pre-treatment (dipping and freezing) on quality of prune in comparison with non-treated dried fruit. The following quality parameters of prune: contents of sugars, acids, sugar/acid ratio, total phenols and antioxidant capacity as well as organoleptic properties (appearance, taste, aroma, consistence), were tested. Contents of total phenols, antioxidant capacity as well as total organoleptic evaluations in dried fruit obtained by pre-treatments (dipping and freezing) do not differ compared to non-treated fruit (control) in all tested cultivars.