Frozen vegetables are commercially packaged vegetables that are sold in the frozen section of the store usually packaged in either rectangular boxes or plastic bags.
The production process of frozen vegetables includes harvesting (picking), sorting, washing, cutting, blanching, packing and freezing.
1. The chemical and physical reactions which take place during the freezing process;
2. Scientific knowledge of the effect of freezing on the tissues of fruits and vegetables; and
3. Food microbiology.
1. The Chemical and Physical Reaction
Enzymes in vegetables are inactivated by the blanching process. Blanching is the exposure of the vegetables to boiling water or steam for a short period of time. The vegetable must then be rapidly cooled in ice water to prevent it from cooking, in most cases blanching is absolutely essential for producing quality frozen vegetables. Blanching also helps to destroy microorganisms on the surface of the vegetable and to make some vegetables, such as broccoli and spinach, more compact.
2. Effect of Freezing on Tissue
Vegetables should be frozen at 0° F or lower for 24 hour before distribution. Water makes up over 90 percent of the weight of most fruits and vegetables. This water and other chemical substances are held within the fairly rigid cell walls which give support structure, and texture to the fruit or vegetable. Freezing fruits and vegetables actually consists of freezing the water contained in the plant cells.
When the water freezes, it expands and the ice crystals cause the cell walls to rupture. Consequently, the texture of the produce, when thawed, will be much softer than it was when raw. This textural difference is especially noticeable in products which are usually consumed raw. For example, when a frozen tomato is thawed, it becomes mushy and watery. This explains why celery, lettuce, and tomatoes are not usually frozen and is the reason for the suggestion that frozen fruits, usually consumed raw, be served before they have completely thawed. In the partially thawed state, the effect of freezing on the vegetable tissue is less noticeable. The extent of cell wall rupture can be controlled by freezing produce as quickly as possible. In rapid freezing the technology currently employed is the IQF (individually quick frozen ) by chilled blast of air .
3. The Microbiology
The freezing process does not actually destroy the microorganisms which may be present on fruits and vegetables. While blanching destroys some microorganisms and there is a gradual decline in the number of these microorganisms during freezer storage, sufficient populations are still present to multiply in numbers and cause spoilage of the product when it thaws. For this reason it is necessary to carefully inspect any frozen products which have accidentally thawed by the freezer going off or the freezer door being left open.
There are two basic methods for packing vegetables for freezing, the tray pack and the dry pack.
Dry pack—This is the method used to describe the packing of blanched and drained vegetables into containers or freezer bags. Pack the vegetables tightly to cut down on the amount of air in the container. If the vegetables are packed in freezer bags, press air out of the unfilled part of the bag. When packing broccoli, alternate the heads and stems.
Tray Pack—this is the method of freezing individual pieces of blanched and drained vegetables on a tray or shallow pan, then packing the frozen pieces into a freezer bag or container. This method produces a product similar to commercially frozen plastic bags of individual vegetable pieces and is particularly good for peas, corn, and beans.
In this method it is most important to pack the individually frozen pieces into a bag or container as soon as they are frozen.
The technology to employed does not have any negative environmental impact.