Fresh and processed blueberries are essential to the human diet. The inestimable value of these berries is that they are a major source of biologically active phenolic substances. To preserve the positive properties of the berries, it is necessary to preserve them. There are various methods of preservation: drying, preservation by heat (pasteurization or sterilization of products) or sugar, chemical preservation, quick freezing, etc.
Cold preservation is one of the best and most reliable ways of protecting blueberries from spoilage. Cold storage preserves the nutritional value and quality of the berries and effectively inhibits the effects of micro-organisms, chemical, and biochemical processes, oxygen in the air, heat, and light. Frozen products are in demand in Western and Northern European countries.
Freezing of berries can also be done at home, under domestic conditions. However, this must be done quickly. Slow freezing causes large ice crystals to form inside the berries, damaging and tearing the cells. Fast freezing causes rapid water crystallization and the formation of small ice crystals which are less damaging to the berries. Home freezing of berries can therefore be done in freezers with a maximum temperature of -18 °C. To freeze berries as quickly as possible (half an hour is very desirable, but in a domestic fridge such a short time is difficult to achieve), the freezer compartment must be spacious. Before freezing, the blueberries should be sorted (discarding those that are spoiled, bruised, and unripe), sorted, and discarded if they are too small or too large. Once washed and dried (either by blowing air on them or simply allowing them to drain and dry), they should be placed on a tray in a single layer and placed in the freezing chamber. Once frozen, the loose berries are placed in freezer jars or bags and stacked in the freezer as high as they will go.
Berries that have been washed but drained must not be placed in freezer containers or bags and stacked in the freezer. In this case, the berries will freeze to pieces and will be difficult to remove and thaw intact.
Frozen berries can be stored at -18 °C even until the next harvest.
Berries can also be frozen with liquid nitrogen, but they must still be stored in the freezer at a temperature of at least -18 °C. This method is suitable for industrial refrigerators and is not suitable for domestic use.
Healthy frozen berries are more suitable for confectionery. For dessert, it is better to freeze berries that have been grated with sugar. In this case, the freezing speed is also irrelevant. The washed berries are chopped or grated, mixed with sugar, placed in freezer containers, covered, and stacked in the chamber. The sugar should be added according to taste, but tasting results show that most people find the berries tastiest with around 35 % sugar added.
Rapid freezing preserves the quality
The blueberries are frozen in a special freezing tunnel. The loose berries are first washed, then moved upwards on a conveyor until they are dry, and then on a conveyor to the tunnel where a jet of very cold air is blown in. Within three minutes, their temperature drops to -22 °C. From below, turbines blow a powerful jet of air to keep the berries from freezing and rolling over each other. The rapid freezing preserves all the vitamins and other valuable substances in the berries, while the thawing keeps them intact. The frozen berries are then sorted into three types using a calibrator.
The entire freezing and storage process is monitored and recorded by a computer system so that the quality of the stored produce is guaranteed. The storage chambers are kept at a temperature of -8 to 20 °C. The berries are stored in paper bags of 25 kg each.