engleski

Application of modern pre-treatments in plum drying

The technology of plum drying is mainly based on the implementation of high temperatures between 85 and 90 °C with a lower final water content in fruit, between 20 and 26 %. Recent research studies of plum drying includes implementation of temperatures lower than 50 °C, with a higher final water content of about 35 %. The plum drying process is slow and energy-demanding due to a specific waxy layer that covers the fruit skin. In order to remove the waxy coating and allow free movement of water the use of pre-treatment before drying is necessary. The use of pretreatment allows the formation of the microscopic cracks increasing the water permeability. The usual, most commonly used pre-treatments in technology of plum drying are based on the use of different thermal and chemical treatments. However, the growing demand for more environmentally- friendly products has led to the development of abrasive pre-treatments which use various inert materials to remove wax coating from the surface of the fruit skin. Therefore, the aim of this study was to determine the influence of abrasive pre-treatment on drying time compared to the conventional ones. For the research purposes, plum fruits of cv. ‘Bistrica’ from experimental orchard from Croatian Agency for Agriculture and Food in Donja Zelina were used. In addition to remove the waxy lawyer, before drying various pre-treatments were used primarily divided on the mechanical, chemical and thermal. Mechanical pre-treatment included the abrasion at three different cylinder rotation times: 5, 10 and 15 min. Chemical pre- treatment included immersion of plums in alkaline solution (KOH) of different concentrations of 0.5, 1 and 1.5 % (v / v) at 60 °C for one minute. Thermal treatment included immersion of fruits in distilled water temperature of 60 °C in duration of one minute. As a control sample, not treated, respectively fruits from which waxy lawyer was not removed, was used. Also, before drying, fruit stone was removed by mechanical procedure, while the drying process was conducted by convection in chamber dryer at flow rate of 2.0 m s-1 and constant temperature of 42 °C. According to the obtained results drying time was significantly reduced compared to the control sample (97 h) ; in samples treated with distilled water (thermal treatment) for 21 h, using a KOH solution of 1.5 % for up to 68 h. In samples pre-treated by abrasion for 15 min drying time was also reduced for 68 h compared to the control samples proving the exceptional efficiency of abrasive pre-treatment in plum drying. Abrasive pretreatments so far have not been sufficiently explored and further research about effects of abrasive pre-treatment on drying process is necessary.

abrasion, chemical pre-treatment, drying time, convection, plum

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