Processing of Horticultural Crops (1+2)

Exercise

Practical 33 - Freezing of fruits

Aim: To conduct practical on freezing of fruits.

Theory: Freezing is a method of preservation in which the food temperature is reduced below freezing point and a proportion of water changes in to ice-crystals. Immobilisation of water to ice and the resulting concentration of dissolved solutes in unfrozen water cause lowering of water activity in the food. Thus, reduction in water activity and use of low temperature coupled with some pre-treatments is the basis for food preservation by freezing. The method for freezing of fruits depends upon the intended use. The process variables for freezing of different fruits and flow chart are shown in Table 33.1 and Fig 33.1. Generally fruits after preliminary treatments are packed in sugar syrup and frozen in freezer. The fruits are frozen to an internal temperature of -18^oC or lower and kept at -18^oC or lower throughout transport and storage.

Raw material, ingredients and utensils required

1. Fruits like pineapple, mango, guava, orange segments, peaches, strawberries, and cherries etc.
2. Stainless steel knives, peelers, blanchers, heating equipment, Freezer, utensils, packages, sugar, citric acid, ascorbic acid etc.

Procedure for freezing of fruits
Pre-process handling

1. Follow different steps for preparation of fruits for freezing (Table 33.1)
2. Blanching: Blanching of fruits is carried out to inactivate enzymes. The prepared fruits are kept in boiling water or under steam to pre-determined period followed by immediate cooling.
3. Addition of sugar syrup (syrup pack, sugar pack, sugar replacement and unsweetened pack).

a) Syrup pack: Use 40 percent sugar syrup for most fruits for freezing. For mild flavoured fruits, use lighter syrup to prevent masking of flavour while for sour fruits use heavier syrup.

Table 33.1: Process variables for freezing of different fruits

Fruit	Preparation	Type of Pack followed by freezing
Apples	Wash, peel, slice and immerse in solution containing citric acid/salt/ ascorbic acid to check browning.	Pack in 30-40% syrup containing 0.02% ascorbic acid.
Apricots	Wash, halve, remove pit, peel and slice if desired. If apricots are not peeled, heat in boiling water for half minute, cool and drain.	Pack in 40% syrup containing 0.02% ascorbic acid.
Avocados	Peel soft and ripe avocados. Cut in half, remove pit and mash pulp (puree).	Add 0.05% ascorbic acid to puree. Package in recipe-size amounts.
Berries	Select firm, fully ripe berries. Sort, wash/ and drain.	Use 30% syrup or dry unsweetened pack, dry sugar pack or tray pack.
Cherries (sour or sweet)	Select well-coloured, tree-ripened cherries. Sort and wash thoroughly.	Pack in 30-40% syrup or in dry sugar.
Citrus fruits	Select firm fruit, free of soft spots. Wash and peel, use segments.	Pack in 40% syrup or in fruit juice. Use 0.02% ascorbic acid in syrup.
Grapes	Select firm, ripe grapes. Wash and remove stems. Slice or use whole.	Pack in 20% syrup or pack without sugar. Use dry pack for halved grapes and tray pack for whole grapes.
Melons (cantaloupe, watermelon)	Select firm-fleshed, well-coloured, ripe melons. Wash rind well. Slice or cut into chunks.	Pack in 30% syrup or pack dry using no sugar. Freeze in recipe-size containers.

b) Sugar packs: Sprinkle sugar over the fruits or fruit slices and gently agitate the container to allow drying out the juice and dissolve the sugar. This sugar pack is generally used for soft sliced fruits such as peaches, strawberries, plums, and cherries, by using sufficient syrup to cover the fruit. Some whole fruits may also be coated with sugar prior to freezing.

c) Tray packs: Unsweetened packs are generally prepared by using tray packs. Spread prepared fruits in single layer on shallow trays and freeze promptly in freezer bags. In tray packs, the fruit sections remain loose without clumping together, which offers the advantage of using frozen fruit piece by piece.

d) Sugar replacement packs: Use artificial sweeteners like saccharine, sorbitol instead of sugar in the form of sugar substitutes. In sugar replacement packs, the sweet taste of sugar is replaced by using artificial sweeteners. Fruits frozen with sugar substitutes will freeze harder and thaw more slowly than fruits preserved with sugar.

Freezing: Carry out freezing of fruits either in chest freezer (-20^oC to -30^oC), air blast freezer (-18⁰C to -40⁰C) or in tunnel freezer.

Packaging: Packaging of frozen fruits is done to exclude air from the fruit tissue. Replacement of oxygen with sugar solution or inert gas or use of vacuum and oxygen-impermeable films is used for packaging frozen fruits. Plastic bags, plastic pots, paper bags and cans (with or without oxygen removal) are common packages. As most foods expand on freezing upto 10% of their volume, the package in which food is frozen should be strong and flexible.

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