Packaging and Storage of frozen foods
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.
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 -18oC 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 -18oC.
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.
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
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:
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
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 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
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:
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.
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.