Freezing of vegetables
Enzymes in vegetables are inactivated by blanching. Blanching is the exposure of the vegetable to boiling water or steam for a brief period of time. The vegetable must then be rapidly cooled in ice water to prevent cooking. Blanching also helps to destroy microorganisms on the surface of the vegetables. It makes vegetables, such as broccoli and spinach, more compact, so they take up less room in the freezer. Follow the time recommended for blanching each vegetable carefully. Over-blanching results in a cooked product and a loss of flavor, color and nutrients. Under-blanching stimulates enzyme activity and is worse than no blanching at all.
Here we present the general steps (Figure 7.5) for a freezing process for fruits and vegetables. Depending on the product and conditions, these steps may vary. In fact, all kinds of produce require different processing. Pretreatment methods affect the quality of the final product. Different varieties of fruits and vegetables require different pretreatment methods in order to attain good quality frozen products. Optimum storage conditions must be provided in tennis of temperature and flow velocity of air.
Freezing has very little effect on the nutrient content of foods. Some fruits and vegetables are blanched (immersed in boiling water for a short period) before freezing to inactivate enzymes and yeasts that would continue to cause food spoilage, even in the freezer. This process can cause some of the vitamin C (15 to 20%) to be lost. In spite of these losses, vegetables and fruits are frozen in peak condition soon after harvesting and are often higher in nutrients than their ‘fresh’ counterparts. Harvested produce can sometimes take many days to be sorted, transported and distributed to stores. During this time, vitamins and minerals can be slowly lost from the food. Fresh soft fruits and green vegetables can lose as much as 15% of their vitamin C content daily when kept at room temperature.