Processing of Horticultural Crops (1+2)

Packaging and Storage of frozen foods

Packaging of frozen foods
There are three types of packaging used for frozen foods: primary, secondary, and tertiary. The primary package is in direct contact with the food and the food is kept inside the package up to the time of use. Secondary packaging is a form of multiple packaging used to handle packages together for sale. Tertiary packaging is used for bulk transportation of products.

• Packaging materials should be moisture-vapour-proof (e.g. glass and rigid plastic) to prevent evaporation, thus retaining the highest quality in frozen foods.
• Oxygen should also be completely evacuated from the package using a vacuum or gas-flush system to prevent migration of moisture and oxygen.
• Most bags, wrapping materials and waxed cartons used in freezing packaging are moisture-vapour-resistant.
• The containers should be leakage free while easy to seal.
• Durability of the material is another important factor to consider, since the packaging material must not become brittle at low temperatures and crack.
• Glass, plastic, tin and heavily waxed cardboard materials are rigid containers used for packaging of liquid food products.
• Non-rigid containers include bags and sheets made of moisture-vapour-resistant heavy aluminium foil, polyethylene or laminated papers.
• Bags are the most commonly used packaging materials for frozen fruits and vegetables due to their flexibility during processing and handling. They can be used with or without outer cardboard cartons to protect against tearing.

Storage of frozen foods
Frozen products should be kept at the lowest possible temperature during frozen storage, transport, and distribution in achieving a high-quality product and to check deteriorative changes. The storage life of frozen foods at -18^oC is given in Table-13.2. The lower the product temperature is, the slower the speed of reaction is leading to loss of quality. The temperatures of supply chains in freezing applications from the factory to the retail cabinet should be carefully monitored. The temperature regime covering the freezing process, the cold-store temperatures (-18°C), distribution temperatures (-15°C) and retail (-12°C) are generally recommended.

Quality changes in freeze foods: The major effect of freezing on quality of food is the damage caused to cells by ice crystal growth. In fruits and vegetables, more rigid cell structure may be damaged by ice crystals. Extent of damage depends on size of crystals, and hence rate of heat transfer. During slow freezing, ice crystals grow in intercellular spaces and deform and rupture adjacent cell walls. Cells become dehydrated and permanently damaged by increased solute concentration. In case of fast freezing, smaller ice crystals form within both cells and intercellular spaces. Texture of food is thus retained in fast freezing compared to slow freezing.

Table 13.2: Storage life of frozen food at -18^oC.

Storage time	Type of food	Products
3 months or less	Meat	Liver, heart, brain, precooked bacon, precooked sausages
	Bakery products	Sponge cake, spice cake
3-6 months	Meat	Porks, beef, lamb kidney, tongue etc
	Poultry	Precooked turkey
6-12 months	Meats	Beef, veal, lamb, pork, smoked and roasted meat, processed pies
	Poultry	Raw and fried chicken
	Fish	Fatty fish, raw and cooked
	Vegetables	Asparagus, beans, Brussels sprout, corn
More than 12 months	Meat	Beef, lamb roasted
	Eggs	Whole, white yolk
	Fruits	Raspberries, strawberries, peach, apricot etc
	Vegetables	Lima bean, broccoli, cauliflower, green peas, spinach

Freezing causes negligible changes to pigments, flavours, nutritionally important components, although these may be lost in preparation procedures or deteriorate later during frozen storage. Main changes to frozen foods during storage are degradation of pigments, loss of water soluble vitamins at sub freezing temperatures, residual enzyme activity by polyphenoloxidase, lipoxygenase and off flavour developed due to oxidation of lipids.

1. Recrystallization: It is a physical change where small ice crystals combine to form large crystals.
2. Sublimation: It is a condition when water goes from solid to gaseous state without passing through liquid phase as opposed to way ice would normally melt if placed in glass of water is called sublimation.
3. Freezer burn: One of the most common forms of quality degradation due to moisture migration (sublimation) in frozen foods is freezer burn, a condition defined as the glassy appearance in some frozen products produced by ice crystals evaporating on the surface area of a product. This quality defect can be prevented by using heavyweight, moisture proof packaging during the freezing process.
4. Chemical changes: Maillard and enzymatic browning, flavour deterioration, protein insoluble and degradation of chlorophyll and vitamins. Chemical changes like insolubilization or gelation of proteins, lipid oxidation and degradation of vitamins occurs at fairly slow rate at 0⁰F (-18⁰C) but at fast rate as temperature is increased.

Legal standards for frozen foods: Several agencies exist that establish regulatory standards for frozen fruits and vegetables based on import-export regulations of countries around the world. Some of the general regulations for consideration in the freezing of fruits and vegetables are summarized by the Canadian Food Inspection Agency, Liaison.

A) Frozen fruit products

a. Shall be prepared from fresh or previously frozen fruit that is preserved by freezing.
b. Shall be packed

• with or without a sweetening ingredient in dry form
  
• in a packing medium consisting of water, with/without a sweetening agent
• one or more fruit juices, concentrated and reconstituted fruit juices, fruit purees or fruit nectars, with or without a sweetening agent.
• May contain citric acid or ascorbic acid, in accordance with good manufacturing practice (GMP’s).
• May contain any other substance the addition of which to frozen fruit is in accordance with GMP’s and is generally recognized as safe.

B) Frozen vegetable products

• Shall be prepared from fresh vegetables or a mixture of frozen vegetables.
• May contain salt.
• May contain any other substance the addition of which to frozen vegetables is in accordance with GMP’s and is generally recognized as safe.

Labelling of frozen foods
Most regulatory agencies depending on the location of production require some specific information to be included on the label of the frozen food package. Several basic requirements are recommended for labelling of frozen products is:

1. The common or usual name of the ingredients.
2. The form/style of vegetables or fruits like whole, slices or halves. If the form is visible through the package, it needs not to be mention.
3. The variety or colour in case of some fruits and vegetables.
4. Total contents (net weight) must be stated in grams for containers holding 1 kilogram or less.

Other information required on the label, although not on the front panel

a) Ingredients like spices, flavouring, colouring or special sweetener, if used
b) Any special type of treatment
c) The packer/distributor name and place of business
d) Nutritional information
e) Expiration date
f) Storage requirements
g) Heating instructions

Labels may also give the quality or grade, count, size and maturity of the vegetables, cooking directions and recipes or serving ideas. If the label lists the number of servings per container, the law requires that the size of the serving be given in common measures such as ounces or cups.

I. Freezing of fruits
The method for freezing of fruits depends upon the intended use. If the fruit is to be eaten without any further processing after thawing, texture characteristics are more important. In general, conventional methods of freezing tend to destroy the turgidity of living cells in fruit tissue. Different from vegetables, fruits do not have a fibrous structure that can resist this destructive effect. Additionally, fruits to be frozen are harvested in a fully ripe state and are soft in texture. On the contrary, most vegetables are frozen in an immature state. Fruits have delicate flavours that are easily damaged or changed by heat, indicating they are best eaten when raw and decrease in quality with processing.
Further, attractive colour is important for frozen fruits. Chemical treatments or additives are often used to inactivate the deteriorative enzymes in fruits. Therefore, proper processing is essential for all steps involved from harvesting to packaging and distribution. A process variable for freezing of different fruits given in Table 13.3 and flow chart for freezing process discussed in Fig 13.12.

Harvesting: The characteristics of raw materials for freezing include ability to withstand rough handling, uniformity in ripening, free from diseases, insect attack and bruises. The use of mechanical harvesting generally causes bruising of fruits and results in a wide range of maturity levels for fruits. In contrast, hand-picking provides gentler handling and sorting of fruits. For assessment of optimum maturity a combination of colour and pressure tests are generally used.

Table 13.3: 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. Drain prior to use.	Pack in 30-40% syrup containing 0.02% ascorbic acid
Apricots	Wash, halve and remove pit. Peel and slice if desired. If peeling is not possible, heat apricots 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.	Add 0.05% ascorbic acid to puree.
Berries	Select firm, fully ripe berries. Sort, wash and drain.	Use 30% syrup, dry unsweetened pack, dry sugar pack or tray pack.
Cherries (sour/ sweet)	Select healthy, well ripened and coloured cherries. Sort, wash thoroughly and remove pit.	Pack in 30-40% syrup containing 0.02% ascorbic acid. For pies, pack in dry sugar.
Citrus fruits, (sections or slices)	Select firm fruit, free of soft spots. Wash and peel.	Pack in 40% syrup or in fruit juice. Use 0.02% ascorbic acid in syrup or juice.
Grapes	Select firm and ripe grapes. Wash and remove stems. Cut grapes with seeds in half and remove seeds.	Pack in 20% syrup or pack without sugar. Use dry pack for halved and tray pack for whole grapes.
Melons (cantaloupe, watermelon)	Select firm-fleshed, well-coloured, ripe melons. Wash rinds well. Slice or cut into chunks.	Pack in 30% syrup or pack dry using no sugar. Pulp also may be crushed (except watermelon), adding 0.5% sugar. Freeze in recipe-size containers.

Pre-process handling (peeling, slicing/cutting)
Quality of the raw materials prior to freezing is the major consideration for successful freezing. Washing and cutting generally results in losses when applied after thawing. Thus, fruits should be prepared prior to the freezing process in terms of peeling, slicing or cutting. Peeling is done by scalding the fruit in hot water, steam or hot lye solutions in case of fruits that require peeling. Banana, tomato, mango and kiwi fruits are cut into smaller cubes or slices prior to freezing.

Blanching: The objective of blanching is to inactivate the enzymes causing detrimental changes in colour, odour, flavour and nutritive value, but heat treatment causes loss of such characteristics in fruits. Therefore, only a few types of fruits are blanched for inactivation of enzymes prior to freezing. The loss of water-soluble minerals and vitamins during blanching should also be minimized by keeping optimum blanching temperature and time.

Addition of sugar syrup: Addition of sugar is an important pre-treatment for fruits prior to freezing since the treatment has the effect of excluding oxygen from the fruit, which helps to retaining colour and appearance. Sugars when dissolved in solutions act by withdrawing water from cells by osmosis, resulting in very concentrated solutions inside the cells. The high concentration of solutes depresses the freezing point and therefore reduces the freezing within the cells, which inhibits excessive structural damage. Sugar syrups (30-60%) are commonly used to cover the fruit completely. It acts as a barrier to oxygen transmission and browning. Addition of syrup has a protective effect on flavour, odour, colour and nutritive value during freezing, especially for frozen berries.

Packaging
Packaging of frozen fruits is done to exclude air from the fruit tissues. Replacement of oxygen with sugar solution or inert gas consuming the oxygen by glucose-oxidase and/or the use of vacuum and oxygen-impermeable films is used for packaging frozen fruits. Plastic bags, plastic pots, paper bags and cans are some of the most commonly used packaging materials (with or without oxygen removal) selected on basis of penetration properties and thickness. There are several types of fruit packs suitable for freezing: syrup pack, sugar pack, unsweetened pack, tray pack and sugar replacement pack. The type of pack is usually selected according to the intended use for the fruit.
1. Syrup pack: For most fruits, 40 percent sugar syrup is recommended. Lighter syrups are lower in calories and mostly desirable for mild-flavoured fruits to prevent masking the flavour, while heavier syrups may be used for very sour fruits. Cooled syrup is used to cover the prepared fruits. Pectin can also be used in the syrup to reduce sugar content in syrups when freezing berries, cherries and peaches.

2. Sugar packs: In preparing a sugar pack, sugar is first sprinkled over the fruit and the container is agitated gently until the juice is drawn out and the sugar is dissolved. This type of pack is generally used for soft sliced fruits such as peaches, strawberries, plums and cherries to cover the fruit with syrup. Some whole fruits may also be coated with sugar prior to freezing.

3. Unsweetened packs: Unsweetened packs can be prepared in several ways, either dry-packed, covered with water containing ascorbic acid, or packed in unsweetened juice. When water or juice is used in syrup and sugar packs, fruit is submerged by using a small piece of crumpled water-resistant material. Generally, unsweetened packs yield a lower quality product when compared with sugar packs, with the exception that some fruits such as raspberries, blueberries, scalded apples, gooseberries, currants, and cranberries maintain good quality without sugar.

4. Tray packs: Unsweetened packs are generally prepared by using tray packs in which a single layer of prepared fruit is spread on shallow trays, frozen and packaged in freezer bags promptly.

5. Sugar replacement packs: Artificial sweeteners can be used instead of sugar in the form of sugar substitutes. The sweet taste of sugar can be replaced by using these kinds of sweeteners, however the beneficial effects of sugar like colour protection and thick syrup cannot be replaced.

II. Freezing of vegetables
Freezing is often considered the simplest and most natural way of preservation used for vegetables. Frozen vegetables 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 picking of vegetables until its ultimate use, is the important factor affecting quality of finished product. A process variable for freezing of different vegetables given in Table 13.4 and flow chart for freezing process discussed in Fig 13.13.

Table 13.4: Process variables for preparation of vegetables for freezing

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

1. Selection of raw material: Selection of a suitable cultivar and harvesting at optimum maturity are the two most important factors affecting raw material quality. Raw material characteristics are usually related to the vegetable cultivar, crop production, crop maturity, harvesting practices, crop storage, transport, and factory reception. Important characteristics for selection vegetables for freezing are as follows:

• Suitability for mechanical harvesting
• Uniform maturity
• Good flavour, uniform colour and desirable texture
• Resistance to diseases
• High yield
• Harvesting

At optimum maturity, physiological changes in several vegetables take place very rapidly. Thus, the determination of optimum harvesting time is critical. Some vegetables such as green peas and sweet corn only have a short period during which they are of prime quality. If harvesting is delayed beyond this point, quality deteriorates and the crop may quickly become unacceptable.

2. Pre-process handling: 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 delay between harvest and processing should be reduced to retain fresh quality prior to freezing. 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. Vegetables are sorted and graded to discard any diseased, bruised and non uniform ripened part.

3. Blanching: Blanching is the exposure of the vegetables to boiling water or steam for a brief period of time to inactivate enzymes. All vegetables (except herbs and green peppers) need to be blanched and promptly cooled prior to freezing, since heating slows or stops the enzyme action. After maturation, however the enzymes can cause loss in quality, flavour, colour, texture and nutrients. If vegetables are not heated sufficiently, the enzymes will continue to be active during frozen storage and may cause the vegetables to toughen or develop off-flavours and colours. Blanching is usually carried out between 75 and 95°C for 1 to 10 minutes, depending on the size of individual vegetable pieces. Blanched vegetables should be promptly cooled down to control and minimize the degradation of soluble and heat-labile nutrients. Vegetables can be blanched in hot water, steam, and in the microwave.

• Hot water blanching is the most common way of processing vegetables. For water blanching, vegetables are put in a basket and then placed in a kettle of boiling water covered with a lid.
• Steam blanching takes longer than the water blanching method, but helps retain water-soluble nutrients such as water-soluble vitamins. For steam blanching, a single layer of vegetables is placed on a rack or in a basket at 3-5 cm above water boiling in a kettle. A tightly fitted lid is placed on the kettle and timing is started.
• Microwave blanching is usually recommended for small quantities of vegetables prior to freezing.

4. Packaging: There are several factors to consider in packaging frozen vegetables, which include protection from atmospheric oxygen, prevention of moisture loss, retention of flavour and rate of heat transfer through the package. Vegetables for freezing are packed either as dry pack or tray pack.

Dry-pack method: In the dry pack method, the blanched and drained vegetables are put into meal-sized freezer bags and packed tightly to cut down on the amount of air in the package. Proper headspace (approximately 2 cm) is left at the top of rigid containers before closing. For freezer bags, the headspace is larger. However, provision for headspace is not necessary for vegetables such as broccoli, asparagus and brussels sprouts as they do not pack tightly in containers.

Tray-pack method: In the tray pack method, the chilled, well-drained vegetables are placed in a single layer on shallow trays or pans. Trays are placed in a freezer until the vegetables become firm and then removed.