Improving the nutritional quality of microwave-vacuum dried strawberries:: A preliminary study

Microwave-vacuum drying is an up-to-date technique for the conservation of fruits and vegetables, resulting in products with improved texture and colour. In order to evaluate the effects on the content of ascorbic acid, anthocyanins and phenolic compounds, and antioxidant capacity, three strawberry varieties were subjected to microwave-vacuum (MV) drying combined with convective pre- and post-drying, convective drying, and freeze-drying. The MV drying procedure comprised immersion in a pre-treatment bath, pre-drying in a belt dryer to a moisture load of approx 45g/100g dry matter, and microwave treatment at 4kPa for 8min, leading to products with < 5g/100g moisture. During post-drying, the moisture content was further reduced to approx 2g/100g. Convective drying and MV drying decreased the content of ascorbic acid to approx 40% of the initial value, phenolic compounds to approx 35%, and the antioxidative capacity to approx 60%. No reduction was observed in freeze-dried strawberries. Bypassing the pre-treatment bath, extending residence time in the pre-dryer to reduce temperature peaks, and reducing MV treatment time increased the recovery of ascorbic acid to approx 65%. Phenolic compounds remained stable, and the reduction of the antioxidative capacity was limited to 10%-25%. Except ascorbic acid, the residual concentration of nutritionally relevant compounds after improving the processing conditions was close to the values as observed for freeze-dried products.