Effects of Prior Freezing Conditions on the Quality of Blueberries in a Freeze-Drying Process
Abstract. Freeze-drying has played an increasingly important role in the production of dehydrated foods. This article discusses the operating conditions of the prior freezing process of blueberry fruit to maintain the fruit aroma during the sublimation process. The properties of frozen fruit, such as ice crystal size, seem to depend on the freezing speed, leading to some aroma loss during the sublimation process. The temperature of the deep freezer was set at -20°C, -40°C, -60°C, or -80°C to determine the effects of changing the freezing speed of blueberries during prior freezing. Northern highbush blueberry cultivars Dixi and Elliott harvested in a university orchard (Tokyo, Japan) were used as the test material. The sublimation process of freeze-drying was done using a transparent vacuum desiccator connected to a vacuum pump through a vapor cooling trap under heating conditions provided by a far-infrared heater. Trapped vapors were analyzed with GC-MS to identify and quantify the aroma compounds of the blueberries, such as benzaldehyde. The change in the appearance of the freeze-dried blueberries following prior freezing at various speeds is also described. The amounts of typical volatile compounds, such as acetic acid, 2-hexanol, and 3-hexanol, decreased as the freezing speed increased. Most volatile aroma compounds could be preserved when blueberries were frozen rapidly in a deep freezer.