The effect of refrigeration and room temperature storage conditions on the physico-chemical characteristics of hybrid and freeze-dried blueberries
Tamás Antal
Abstract
In order to preserve the nutritional properties of fruits after dehydration, it is necessary to use appropriate storage methods. The traditional freeze-dried and combination-dried i. e. vacuum pre- and freeze post-dried, freeze pre- and vacuum post-dried, mid-infrared-freeze-dried, and freeze-mid-infrared dried blueberry samples were stored in vacuum packs for 6 months. The reference drying method, i.e. freeze drying at a pressure of 80–90 Pa and a temperature range between −25 and 20 °C, with a drying time of 22 h, was used. The blueberries were mid-infrared dried at 60 °C for 5 min before and after freeze-drying, and the experimental materials were also vacuum dried at 60 °C for 4 h at 7 kPa vacuum pressure before and after freeze-drying. Drying operation times were ranked from the shortest to the longest as follows: freeze-mid-infrared drying, mid-infrared freeze-drying, freeze pre- and vacuum post-drying, and vacuum pre- and freeze post-drying and conventional lyophilization. The aim of this study was to investigate the effects of different storage conditions, namely refrigeration (5 °C in a refrigerator) and room temperature (23 °C on a shelf in a dark place) on the moisture content, water activity, firmness, rehydration ratio, polyphenol- and flavonoid content, as well as antioxidant capacity of dried blueberries. The dried and vacuum-packaged blueberries stored in the refrigerator and at room temperature can be characterized by the stability of their physical and chemical properties throughout the storage period. It was found that a traditional freeze-drying (FD), freeze-drying and vacuum-drying combination of 4 h at 60 °C (FD-VD4h60 °C) together with vacuum packaging at ambient temperature, is sufficient to ensure a shelf-stable whole blueberry product.