The possibility of using the vacuum impregnation as a pretreatment before drying apple cubes was investigated. The impregnation was carried out for 4.5 min with the vacuum pressure of 0.01 MPa. The sucrose solution or a mixture of sucrose and the citric acid were used as infiltration liquids. Apple cubes were dried by convective drying (CD) (at temperature of 50 and 70 °C) or microwave-vacuum drying (MVD) (at power of microwaves 120 and 480 W). The drying kinetics was determined and quality factors (anti-oxidant activity, mechanical–rheological properties and colour changes) were analysed. Studies proved that the vacuum impregnation causes increasing of the drying dynamics (12.50–28.57 % during CD and 12.50–18.48 % during MVD). All impregnated apples were darker, yellower and susceptible for deformation or cutting than non-impregnated. The highest level of anti-oxidant activity was observed when two-component impregnant before MVD by power of 480 W was used.
Mechanical and rheological properties of zucchini dried with spouted microwave - assisted heating was analysed. During spouted drying two levels of microwave power were applied: 100W and 250W. Tests were carried out for the raw material blanched in the temperature of 60ºC for 10 minutes and not subjected to a pretreatment. Compression resistance, cutting resistance and stresses relaxation ratios a and b were determined. A significant impact of drying conditions on the product quality within the scope of mechanical and rheological properties was reported. Raw zucchini has 5 to 7 times higher compression resistance, several times lowered cutting resistance and almost two times higher elasticity in comparison to dried material. Low-temperature blanching significantly affected the values of all investigated properties of dried zucchini. Change in microwave powers during spouted drying significantly influenced the increase of the compression work value.
The objective of the research was to indicate the impact of sirup type used for osmotic dehydration and the microwaves power for microwave and vacuum drying on the selected mechanical and rheological properties of dried pumpkin. Pumpkin cultivar Rugosa was osmotically dehydrated in three concentrates: apple, aronia and cherry. Initially prepared material was dried with microwave method in the reduced pressure conditions which was within 6-4 kPa at the microwave power which was 240 and 480 W. Dried material was subjected to compression tests and stresses relaxation tests. Compression work values (Pś), cutting work (Pp) and indicators describing material elasticity (a and b) were computed. An initial treatment in the form of osmotic dehydration carried out before microwave drying in the reduced pressure conditions allows obtaining dried pumpkin with a raised resistance to compression and cutting and the decreased elasticity in comparison to dried material obtained from a pumpkin which was not subjected to the initial treatment. Microwaves power influences the values of compression and cutting of dried pumpkin.
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