Drying of inshell walnuts and walnut kernels under different convection conditions
2019
Kic, P., Czech Univ. of Life Sciences, Prague (Czech Republic)
Walnuts are among foods with a significant health-preventive effect. Walnuts significantly improve cardiovascular activity, they are a rich source of minerals, vitamins and proteins, and their consumption helps reduce cholesterol and have a number of other health-related preventive effects. The high season of walnut harvest is in late September and early October. At this time, however, rainy weather and moisture often make worse the quality of nuts, so it is necessary to know the appropriate drying methods, specifically to prevent mould formation. This research is focused on drying inshell walnuts and walnut kernels under different convection conditions. A special device for convection drying with air flow passing through the material from the bottom through supporting trays with a sieve by constant temperature (average 24.5 ºC) and relative humidity (41 %) was used for drying when the air velocity was 1.2 m•sE−1. The results were compared with natural convection drying by the same temperature and humidity, but with zero air velocity. The dry matter content was measured gravimetrically after drying in a hot air dryer at 105 ºC. The evaluation of the measurement results was focused on the progress and course of the drying rate, changes in the water content and changes of moisture depending on the time and their mutual relations. The increased air velocity for convection influenced the drying process positively. Drying air velocity 1.2 m•sE−1 increased the drying rate of walnuts (inshell as well kernels) approximately 1.7 times. Drying of walnut kernels allowed a reduction of moisture by 3 %, the final moisture of inshell walnuts was about 6.8 %. The drying time of kernels was about five times shorter than that of the inshell walnuts. The experimental data create the background for calculation of the main parameters useful for description and modelling of the drying process, which can be helpful, e.g., for decision of the optimum drying time.
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