Lesson- 18 Selection criteria of packaging material for processed foods

18.0 Introduction

Foods are materials, raw, processed, or formulated, that are consumed orally by humans or animals for growth, health, satisfaction, pleasure, and satisfying social needs. Generally, there is no limitation on the amount of food that may be consumed (as there is for a drug in the form of dosage). This does not mean that we can eat any food item as much as we want. Excessive amounts could be lethal, for example, salt, fat, and sugar. Chemically, foods are mainly composed of water, lipids, fat, and carbohydrate with small proportions of minerals and organic compounds. Minerals include salts and organic substances include vitamins, emulsifiers, acids, antioxidants, pigments, polyphenols, and flavor-producing compounds. The different classes of foods are perishable, nonperishable, harvested, fresh, minimally processed, preserved, manufactured, formulated, primary, secondary derivatives, synthetic, functional, and medical foods. The preservation method is mainly based on the types of food that need to be prepared or formulated.

Fresh fruit and vegetables have a short shelf life under ambient conditions of temperature and humidity due to their highly perishable nature. They soon lose their freshness and become subjected to mould and bacterial attack, and consequently decay and become useless as articles of human diet. Fruits and vegetables are an important supplement to the human balanced diet as they provide the essential minerals, vitamins and dietary fiber (roughage) for maintaining the tear and wear of the over body. Fresh fruits and vegetables are valued for their quick sources of available energy. Fresh fruits have high water content (70-96%), varying amount of carbohydrate (3-27%) and fiber (0.2-3.1%) and a low content of protein, fat and minerals. Fruits are important source of Pro-vitamin A and vitamin C. The loss of moisture causes vegetables to wilt and become limp. In addition, vegetables also supply fair amount of carbohydrates, protein and energy and add colour, flavor and aroma to human diet. India is one of the largest producers of fruits and vegetables after china. Ideal climatic conditions ensure round the year availability of broad range of fruit and vegetables in large quantities.

Packaging provides quality and quantity assurance besides creating hygienic environment for food product. It offers security through tamper proof designs and contributes to the product image through structural and graphical design. Food safety is permanent importance as package products against biological, chemical and distribution damages. The primary objective or packaging is to protect the contents during storage, transportation and distribution against deterioration. It may be physical, chemical or biological. According to Robertson (1992), packaging as the enclosure of the products, items or packages in a wrapped pouch, bag, box, cup, tray, can, tube, bottle or other containers to perform the various functions, i.e. containment, protection, information, promotion, etc.

Packaging of food serves many purposes such as providing effective protection to package foods against external contamination from environment; preserving the quality of food. It is one of the most important parameters that sale the product and also a communication device to provide detailed information about the product like, contents, ingredients, notional values, cooking instruction, packaging and expiry dates, etc. generally packaging material s are lighter in weight, easier to open, reseal and store, ensure safe transport and distribution, protect the product from adverse effects such as heat, cold, moisture, etc.

18.1. Selection criteria of packaging material for the raw and processed foods

18.1.1 Mechanical Damage

Fresh, processed and manufactured foods are susceptible to mechanical damage. The bruising of soft fruits, the break-up of heat processed vegetables and the cracking of biscuits are examples. Such damage may result from sudden impacts or shocks during handling and transport, vibration during transport by road, rail and air and compression loads imposed when packages are stacked in warehouses or large transport vehicles. Appropriate packaging can reduce the incidence and extent of such mechanical damage. Packaging alone is not the whole answer. Good handling and transport procedures and equipment are also necessary. The selection of a packaging material of sufficient strength and rigidity can reduce damage due to compression loads. Metal, glass and rigid plastic materials may be used for primary or consumer packages. Fiberboard and timber materials are used for secondary or outer packages. The incorporation of cushioning materials into the packaging can protect against impacts, shock and vibration. Corrugated papers and boards, pulp board and foamed plastics are examples of such cushioning materials. Restricting movement of the product within the package may also reduce damage. This may be achieved by tight-wrapping or shrink-wrapping. Inserts in boxes or cases or thermoformed trays may be used to provide compartments for individual items such as eggs and fruits.

18.1.2 Permeability Characteristics

The rate of permeation of water vapor, gases (O2, CO2, N2, and ethylene) and volatile odor compounds into or out of the package is an important consideration, in the case of packaging films, laminates and coated papers. Foods with relatively high moisture contents tend to lose water to the atmosphere. This results in a loss of weight and deterioration in appearance and texture. Meat and cheese are typical examples of such foods. Products with relatively low moisture contents will tend to pick up moisture, particularly when exposed to a high humidity atmosphere. Dry powders such as cake mixes and custard powders may cake and lose their free flowing characteristics. Biscuits and snack foods may lose their crispness. If the water activity of a dehydrated product is allowed to rise above a certain critical level, microbiological spoilage may occur. In such cases a packaging material with a low permeability to water vapor, effectively sealed, is required. In contrast, fresh fruit and vegetables continue to respire after harvesting. They use up oxygen and produce water vapor, carbon dioxide and ethylene. As a result, the humidity inside the package increases. If a high humidity develops, condensation may occur within the package when the temperature fluctuates. In such cases, it is necessary to allow for the passage of water vapor out of the package. A packaging material which is semipermeable to water vapor is required in this case.

The shelf life of many foods may be extended by creating an atmosphere in the package which is low in oxygen. This can be achieved by vacuum packaging or by replacing the air in the package with carbon dioxide and/or nitrogen. Cheese, cooked and cured meat products, dried meats, egg and coffee powders are examples of such foods. In such cases, the packaging material should have a low permeability to gases and be effectively sealed. If a respiring food is sealed in a gastight container, the oxygen will be used up and replaced with carbon dioxide. The rate at which this occurs depends on the rate of respiration of the food, the amount in the package and the temperature. Over a period of time, an anaerobic atmosphere will develop inside the container. If the oxygen content falls below 2%, anaerobic respiration will set in and the food will spoil rapidly. The influence of the level of carbon dioxide in the package varies from product to product. Some fruits and vegetables can tolerate, and may even benefit from, high levels of carbon dioxide while others do not. In such cases, it is necessary to select a packaging material which permits the movement of oxygen into and carbon dioxide out of the package, at a rate which is optimum for the contents. Ethylene is produced by respiring fruits. Even when present in low concentrations, this can accelerate the ripening of the fruit. The packaging material must have an adequate permeability to ethylene to avoid this problem.

To retain the pleasant odor associated with many foods, such as coffee, it is necessary to select a packaging material that is a good barrier to the volatile compounds which contribute to that odor. Such materials may also prevent the contents from developing taints due to the absorption of foreign odors. It is worth noting here that films that are good barriers to water vapor may be permeable to volatiles. In those cases where the movement of gases and vapours is to be minimized, metal and glass containers, suitably sealed, may be used. Many flexible film materials, particularly if used in laminates, are also good barriers to vapours and gases. Where some movement of vapours and/or gases is desirable, films that are semi-permeable to them may be used. For products with high respiration rates the packaging material may be perforated. A range of micro-perforated films is available for such applications. In the case of an intact polymer film, the rate at which vapours and gases pass through it is specified by its ‘permeability’ or ‘permeability constant’, P, defined by the following relationship:

\[{\bf{P}} = \frac{{{\bf{q}}{\bf{l}}}}{{{\bf{A}}({{\bf{P}}_1} - {{\bf{P}}_2})}}\]

where q is the quantity of vapor or gas passing through A, an area of the film in unit time, l is the thickness of the film and p1, p2 are the partial pressures of the vapor or gas in equilibrium with the film at its two faces. The permeability of a film to water vapor is usually expressed as x g m–2 day–1 (i.e. per 24 h) and is also known as the water vapor transfer rate (WVTR). Highly permeable films have values of WVTR in the range from 200 g m–2 day–1 to >800 g m–2 day–1, while those with low permeability have values of 10 g m–2 day–1 or below. The permeability of a film to gases is usually expressed as x cm3 m–2 day–1. Highly permeable films have P values from 1000 cm3 m–2 day–1 to >25 000 cm3 m–2 day– 1, while those with low permeability have values of 10 cm3 m–2 day–1 or below. When stating the P value of a film, the thickness of the film and the conditions under which it was measured, mainly the temperature and (p1, p2), must be given.

 18.1.3. Grease proofness

In the case of fatty foods, it is necessary to prevent egress of grease or oil to the outside of the package, where it would spoil its appearance and possibly interfere with the printing and decoration. Greaseproof and parchment papers may give adequate protection to dry fatty foods, such as chocolateand milk powder, while hydrophilic films or laminates are used with wet foods, such as meat or fish.


18.1.4. Temperature

A package must be able to withstand the changes in temperature which it is likely to encounter, without any reduction in performance or undesirable change in appearance. This is of particular importance when foods are heated or cooled in the package. For many decades’ metal and glass containers were used for foods which were retorted in the package. It is only in relatively recent times that heat resistant laminates were developed for this purpose. Some packaging films become brittle when exposed to low temperatures and are not suitable for packaging frozen foods. The rate of change of temperature may be important. For example, glass containers have to be heated and cooled slowly to avoid breakage. The method of heating may influence the choice of packaging. Many new packaging materials have been developed for foods which are to be processed or heated by microwaves.

18.1.5. Light

Many food components are sensitive to light, particularly at the blue and ultraviolet end of the spectrum. Vitamins may be destroyed, colors may fade and fats may develop rancidity when exposed to such light waves. The use of packaging materials which are opaque to light will prevent these changes. If it is desirable that the contents be visible, for example to check the clarity of a liquid, colored materials which filter out short wavelength light may be used. Amber glass bottles, commonly used for beer in the UK, perform this function. Pigmented plastic bottles are used for some health drinks.

 18.2. Chemical Compatibility of the Packaging Material and the Contents of the Package

 It is essential in food packaging that no health hazard to the consumer should arise as a result of toxic substances, present in the packaging material, leaching into the contents. In the case of flexible packaging films, such substances may be residual monomers from the polymerization process or additives such as stabilizers, plasticizers, colouring materials etc. To establish the safety of such packaging materials two questions need to be answered: (a) are there any toxic substances present in the packaging material and (b) will they leach into the product? Toxicological testing of just one chemical compound is lengthy, complicated and expensive, usually involving extensive animal feeding trials and requiring expert interpretation of the results.

 18.3. Protection against Microbial Contamination

Another role of the package may be to prevent or limit the contamination of the contents by microorganisms from sources outside the package. This is most important in the case of foods that are heat-sterilized in the package, where it is essential that post-process contamination does not occur. The metal can has dominated this field for decades and still does. The reliability of the double seam in preventing contamination is one reason for this dominance. Some closures for glass containers are also effective barriers to contamination. It is only in relatively recent times that plastic containers have been developed, which not only withstand the rigors of heat processing, but also whose heat seals are effective in preventing post process contamination. Effective seals are also necessary on cartons, cups and other containers which are aseptically filled with UHT products. The sealing requirements for containers for pasteurized products and foods preserved by drying, freezing, curing, etc. are not so rigorous. However, they should still provide a high level of protection against microbial contamination.

 18.4. In-Package Microflora

The permeability of the packaging material to gases and the packaging procedure employed can influence the type of microorganisms that grow within the package. Packaging foods in materials that are highly permeable to gases is not likely to bring about any significant change in the microflora, compared to unpackaged foods. However, when a fresh or mildly processed food is packaged in a material that has a low permeability to gases and when an anaerobic atmosphere is created within the package, as a result of respiration of the product or because of vacuum or gas packaging, the type of microorganisms that grow inside the package are likely to be different to those that would grow in the unpackaged food. There is a danger that pathogenic microorganisms could flourish under these conditions and result in food poisoning. Such packaging procedures should not be used without a detailed study of the microbiological implications, taking into account the type of food, the treatment it receives before packaging, the hygienic conditions under which it is packaged and the temperature at which the packaged product is to be stored, transported, displayed in the retail outlet and kept in the home of the consumer.

Last modified: Wednesday, 3 July 2013, 9:09 AM