General unit operations commonly applied to all fruits and vegetables are given in the previous section, however specific preparations are required for each kind. Therefore, food prototypes based on fruits and vegetables are used to demonstrate the freezing application on specific food products.
One of the most common commercially frozen fruit products is mixed frozen berries as is shown in Figure 16. The mixtures typically contain combinations of raspberries, blackberries, blueberries, and strawberries. There is a wide range of mixtures available in the market for frozen berry combinations. Raspberries and blackberries for example, which are known to freeze well and retain their wholeness and shape, dependent on the structure of the fruit, are strongly associated with their cultivar. The processing requirements for different varieties of berries do not change significantly. Therefore, a mixture of raspberries and blackberries is chosen in this case as a fruit formulation to simplify the freezing process.
Figure 16. Raspberries and blackberries.
Procedure for processing mixed berries
Full-flavoured, ripe berries of like size preferably with tender skins are selected.
Berries are sorted, washed, and drained.
Berries are packed into containers and covered with cold 40 percent sugar syrup, with proper headspace.
Polyethylene freezer bags are sealed and frozen.
Frozen mixed vegetables constitute a large portion of the frozen vegetable market and are now available in an ever-increasing variety of mixtures as is shown in Figure 17. The mixtures include three or more types of vegetables, properly prepared and blanched. The USDA standards for frozen mixed vegetables describe this item as a mixture containing three or more of the basic vegetables - beans, carrots, corn, and peas. When three vegetables are used, none of the vegetables should be more than 40 percent of the total weight; the individual percent decreases with increased number of vegetable types (Hui et al., 2001).
Figure 17. Mixed vegetables
In a mixed frozen vegetable product, vegetables of different sizes are present in the mixture. Therefore, during pre-freeze treatments, especially blanching, care must be taken to be sure all vegetables are blanched properly.
Procedure for processing mixed vegetables
A mixture of four vegetables in which none of the vegetables is less than 8 percent by weight nor more than 35 percent by weight of all the frozen mixed vegetables are selected.
The vegetables are sorted, washed and peeled.
They are cut into uniform size and blanched in hot water for 5 minutes, and immediately cooled after blanching.
Packed and frozen.
Figure 18. Flow diagram of freezing process for fruit-based product.
Figure 19. Flow diagram of freezing process of vegetable-based product.
Initial selection of the processing site should be based on significant considerations. The design of the general processing plant has great importance in assuring quality of the final product and the effectiveness of the process (Mayes and Telling, 1993). Some recommendations related to the location, and the general plant layout, are summarized in the following section (Herron, 1968). Also Figures 18 and 19 show a general flow diagram recommendable for freezing of fruits and vegetables respectively.
Areas free from objectionable odours, smoke, flies, ash, and dust or other sources of contamination shall be considered in choosing the location for the food processing plant.
Frozen food preparation plants shall be completely separate from areas used as living or sleeping quarters by solid partitions with no connecting openings.
Product preparation and processing (including freezing) departments shall be of sufficient size to permit the installation of all necessary equipment with proper space for plant operations.
The proper flow of the product shall be arranged in the plant, without backtracking, from the time raw materials are received until the frozen, packaged article is shipped from the plant.
Raw material storage rooms and areas where pre-freezing operations (e.g., washing and peeling of fruits and vegetables, and preparation of meats) are carried out shall be separated from rooms or areas where frozen food is formulated, processed, and packaged.
Doors connecting various rooms or openings to the outside shall be tight-fitted, solid, and kept in a closed position by self-closing devices.
Facilities for efficient quick-freezing of the product shall be provided and conveniently located near the food processing and packaging departments. Proper freezer storage shall be provided with convenient access to the quick freezing facilities.
A separate room for storing inedible materials such as fruit and vegetable peels, pending removal from the plant, shall be provided in a location convenient to the various preparation and processing areas. This waste storage room shall be of sufficient size to permit proper storage.
The discharge from the exhaust system, if used, shall be located far away from fresh air inlets into the plant. Packaging and labeling material shall be stored in a separately enclosed space convenient to the packaging department.
Employees shall not be permitted to eat in food processing or packaging areas. Well located, properly ventilated dressing rooms and toilet rooms of ample size with self-closing doors shall be provided for employees (Herron, 1968).