Processing of Horticultural Crops (1+2)

Exercise

Practical 34 - Freezing of Vegetables

Aim: To conduct practical on freezing of vegetables

Theory: Freezing is often considered the simplest and most natural way of preservation for vegetables. Frozen vegetables and potatoes form a significant proportion of the market in terms of frozen food consumption. The quality of frozen vegetables depends on the quality of fresh products, since freezing does not improve product quality. Pre-process handling, from the time vegetables are picked until ready to eat, is the important factor affecting quality of finished product. The process variables for freezing of different vegetables and flow chart are shown in Table 34.1 and Fig 34.1. Generally fruits after preliminary treatments are packed in sugar syrup and frozen in freezer.

Raw material, ingredients and utensils required

1. Vegetables like beans, peas, carrot, cauliflower etc. are most commonly frozen.
2. Stainless steel knives, peelers, blanchers, heating equipment, Freezer, utensils, salt, sugar, citric acid, ascorbic acid etc.

Procedure for freezing

1. Selection of raw material: Vegetables at peak flavour and texture are used for freezing. Post harvest delays in handling vegetables are known to produce deterioration in flavour, texture, colour, and nutrients. Therefore, the delays between harvest and processing should be reduced to retain fresh quality prior to freezing.
2. Follow different steps for preparation of vegetables for freezing (Table 34.1)
3. Blanching: Blanching of vegetables is carried out to inactivate enzymes. The vegetables are kept in boiling water or under steam to pre-determined period followed by immediate cooling.

Table 34.1: Process variables for preparation of vegetables for freezing

Vegetable	Preparation	Blanch time followed by Freezing
Asparagus	Wash and sort by size. Remove tough ends. Cut stalks into 5-cm lengths.	Water blanch	2 min.
		Steam blanch	3 min.
Beans	Wash and trim the ends. Cut to desired size pieces.	Water blanch	3 min. (whole), 2min. (cut)
		Steam blanch	4 min. (whole), 3min. (cut)
Beets	Wash and remove the tops leaving 2.5 cm of stem and root.	Cook for 25-30 min. until tender. Cool promptly, peel, trim. Cut into slices or cubes and pack.
Broccoli	Wash and cut to desired size pieces.	Water blanch	3 min.
		Steam blanch	3 min.
Cabbage	Wash and cut into wedges	Water blanch	3 min.
		Steam blanch	4 min.
Carrots	Wash, peel and trim. Cut to desired size.	Water blanch	5 min.
Cauliflower	Discard leaves, stem and wash. Break into florets	Water blanch	5 min. (whole)
		Steam blanch	7 min. (whole)
Corn	Remove husks, trim ends and wash	Water blanch	5 min.
		Steam blanch	7 min.
Herbs	Washing and cut the roots	No heat treatment is needed.
Mushrooms	Wipe mushrooms with paper towel Sort, trim and cut large sized mushrooms	Frozen without heat treatment
Peas	Depodding/shelling of peas	Water blanch	1-1/2 min.
		Steam blanch	1-1/2 min.
Potatoes	Peel, cut or grate as desired	Water blanch	5 min. (Whole), 2-3 min. (pieces)

Cooling: Cooling vegetables by cold water, air blasting or ice will often reduce the rate of post-harvest losses sufficiently, providing extra hours of high quality retention for transporting raw material to considerable distances from the field to the processing plant.

Freezing: Carry out freezing either in chest freezer (-20^oC to -30^oC), air blast freezer (-18⁰C to -40⁰C) or in tunnel freezer. The temperature regime covering the freezing process, the cold-store temperatures (-18°C), distribution temperatures (-15°C) and retail (-12°C) are generally recommended.

Packaging: Packaging of frozen vegetables is done to exclude air from the fruit tissue. Replacement of oxygen with brine or use of vacuum and oxygen-impermeable films is used for packaging frozen vegetables. Plastic bags, paper bags and cans (with or without oxygen removal) are common packages.

Storage: Store the frozen products in a cool and dry place (in refrigerator).