FRUITS AND VEGETABLE PACKAGING

Fruits and vegetables are living organisms even after harvesting; they can remain fresh only as long as normal metabolism continues. Metabolism involves absorption of oxygen which breaks down the carbohydrates in the product to water and carbon dioxide. If the availability of oxygen is restricted, the chemical reaction changes and small quantities of alcohol are produced. This results in off-odours and flavours and breakdown of plant cells.

This series of events is called anaerobic decay and can spoil fruits or vegetables within a few hours. Fruits and vegetables have very high moisture content, ranging from 75-95%. Their equilibrium relative humidites are as high as 98%. Under any normal atmospheric condition, they will dry rapidly. This causes wilting and shriveling due to loss of rigidity and shrinkage of the cells.

Proper packaging can prolong the storage life of fresh fruits and vegetables by preventing moisture loss and thereby wilting. The rate of moisture loss varies with the product and water vapour permeability of the packaging film. The use of small perforations for oxygen permeation has an insignificant effect upon moisture loss.

A type of spoilage much prevalent in fruits and vegetables is that caused by microorganisms such as yeasts, molds, and bacteria. These organisms can cause destruction by growing on the exterior of the product or they may invade the interior through a surface bruise or cut and cause internal decay. Therefore careful handling and packaging are very important in preservation of freshness and quality.

Normal ripening of fruits and vegetables causes alterations in colour, texture, odour and flavour. At some point, for each type, ideal ripeness is achieved. Beyond that point, the product becomes overripe and quality deteriorates. The primary goal of fresh produce merchandising is to deliver the product to the consumer at such a point in the ripening scale, that it will achieve perfect ripeness at time of eating. This of course is extremely difficult to do. In practice, the produce is delivered somewhat underripe at the time of purchase and the consumer delays consumption until ripening is completed.

Since all these processes are highly sensitive to temperature, they can be slowed down by storing the produce under refrigeration. Each fruit or vegetable has an ideal temperature for storage. If this temperature is not used, deleterious result occurs—for example, tomatoes will not ripen if chilled below 4°C, bananas will turn black below 11°C and potatoes develop a sweet flavor below 4°C.

An additional reason for the necessity of refrigeration for fresh produce is the heat generated due to metabolism or respiration. For example, green beans, sweet corn, broccoli, green peas, spinach, and strawberries generate from 15,000 to 50,000 Btu of energy per ton per 24 hours of storage at 15°C. Even when chilled to 0^o – 4^oC, they still evolve 2,500-17,000 Btu per ton per 24 hours. This heat must be taken into consideration in the design of refrigeration equipment.

Some types of fresh produce give off volatile compounds during ripening which will impart unacceptable odour and flavour if not allowed to escape, or they may prematurely ripen the fruit.

APPLICATIONS OF PACKAGING

The particular type of package used depends upon the shape and perishability of the product. There are five main classifications— soft fruit, hard fruit; stem products; root vegetables; and green vegetables. Soft fruits are highly perishable and easily subject to anaerobic spoilage. They bruise and crush easily which leads to rotting. They are packaged in semi-rigid containers with a cover of cellophane, cellulose acetate, polystyrene or other suitable film cover.

Sometimes, polyethylene bags with ventilation holes are used. Adequate ventilation is a must to avoid fogging. Handling must be gentle and avoided as much as possible. Shelf-life is limited due to individual damage and decay. Some berries under ideal conditions only retain top quality for 2 or 3 days. Typical soft fruits are cherries, grapes, blueberries, strawberries, raspberries and plums.

Hard fruits are better able to resist damage from handling. They are also less perishable and have lower respiratory rates. Shelf-life is weeks rather than days. The most common package is an open tray, a plastic film overwrap or sleeve. Hard fruits may also be bagged in perforated polyethylene film or in nets. Examples of hard fruits are apples, bananas, citrus fruits, peaches, pears and tomatoes.

Stem products are highly perishable as they rapidly lose moisture. They should be bagged or wrapped in moisture proof cellophane or polyethylene with ventilation, or they should be banded or sleeved with shrink film. Typical stem products are celery, rhubarb, and asparagus. Root vegetables are not highly perishable.

They can be stored for long periods; however, it is desirable to protect them against moisture losses. They are washed, graded and sized prior to packaging, which is usually in durable polyethylene bags with perforations. Typical root vegetables include carrots, turnips, radishes, onions, beets, yams and potatoes.

TYPE OF PACKAGING

Packaging can be classified in a number of ways; the most important one is by stages of distribution system for which it is primarily intended.

• Consumer or unit packaging,
• Transport packaging;
• Unit load packaging.

CONSUMER PACKAGING

The package in which consumer receives the produce is called consumer packaging. The term prepackaging of produce refers to consumer units prior to its presentation to the final consumer. Prepackaging may be undertaken at any stage throughout the distribution chain from the field to the retailer's premises, depending upon the need of produce for protection, expected transport and storage time, required shelf life, packaging material costs and costs of packaging and sorting at different points, transport and storage cost and latest knowledge of the market requirements.

Types of Consumer Package

Bags

Bags are the most common and favoured retail packs because of their low material and packaging cost. In terms of cost to strength ratio, 25-40 mm low density. Grapes in ventilated pouch polyethylene or 12.5 mm high density polyethylene bags are most suited. Net bags are used to provide desired ventilation and allow free air movement for the produce such as citrus fruits, onions, potatoes, etc. The bags can be made of paper, perforated polyethylene or polypropylene film, plastic or cotton nets.

Tray

Tray packs made of foamed polystyrene or PVC or PP are overwrapped with heat shrinkable or stretch films. A tight wrap immobilizes the fruits and keeps them apart. Trays of moulded pulp, card board, thermoformed plastic or expanded polystyrene are also used.

Sleeve Packs

These combine the low cost of bags and protective qualities and sales appeal of tray packs. Wraps of plastic film such as polyethylene or PVC, in the form of shrink-wrap, stretch film or cling film and regular net stocking or expanded plastic netting can also be used. The traditional fruits and vegetable retail trader packs the produce in the presence of consumers in the qualities and quantities required by them. The package normally used is a simple wrap of paper or a paper or polyethylene bag. Sleeve packs can be fabricated to contain from one to as many as ten fruits. The main advantage in sleeve packs is that they immobilize the produce at a fraction of cost of tray packs and the produce can be seen from all sides without damage to the fruit.

Transport packaging

Transport packaging for fresh produce may be divided into two size groups:

1. The predominant size group, suitable for carrying by man, is in the range of 15 to 25 kg.
2. The other group, recently becoming increasingly popular in 200-500 kg range suitable for fork lift handling is referred as pallet container.

Wooden Boxes

Includes natural wood and industrially manufactured wood-based sheet materials. Timber used must be inexpensive and easily worked. All wood that is used for the production of the packaging should be well dried in order to prevent cracks and mould growth later. Manufactured wood based sheet materials include ply wood, hard board and particle board. Plywood is usually made from birch. It is rigid and strong, though perhaps somewhat less resistant to splintering than poplar, but is smoother and flatter to be suitable for direct printing. Hard board is dark in colour but its appearance can be improved with decorative printing, but deforms after long storage in high relative humidity. Particle board is thicker and rigid but relatively brittle.

Corrugated Fibreboard Boxes

Corrugated fibreboard (CFB) boxes are the most commonly used shipping containers where cartons, glass, cans and pouches are the unit containers. The popularity of CFB box as a container in food industry as well as in other industrial packaging is for the following reasons:

• Low cost to strength and weight ratio.
• Smooth and non-abrasive surface.
• Good cushioning characteristics.
• Excellent printability.
• Easy to set up and collapsible for storage,
• Reusable and recyclable market.

Plastic Corrugated Boxes

The most commonly used material for plastic corrugated box is polypropylene and HDPE. Its advantage over CFB is low weight to strength ratio and its reusability. The printability is also excellent when compared to CFB boxes. But CFB box has an edge over plastic fibreboard boxes when cushioning properties are taken into consideration. The disadvantages are ultraviolet degradation and temperature resistant, which can be taken care by use of additives.

Plastic Crates

Plastic crates, usually made up of HDPE or polypropylene by injection moulding have been replacing wooden and wire crates. These crates must have good resistant properties to ultraviolet degradation and shock damages.

Sacks

These are flexible shipping containers which are generally used in food industries to transport raw materials viz. fruits and vegetables from the field. If the weight of content is more than 10 kg then it is called sack otherwise bag. The commonly used materials for sacks are cotton, jute, flan, woven plastics (HDPE, Polypropylene). These sacks are advantageous to use as they cost less, have high strength, reusable and require little space for the empties. Disadvantage of plastic woven sack is poor stack ability due to low coefficient of friction, which can be overcome by making antis lip bags.

Palletization

Pallets have been standardized keeping in view of the standard package sizes and sea containers. The sizes of the pallets are of strategic importance since they correspond directly to the sizes of various types of containers, ship cargo compartments, trucks, fork trucks, etc. Most commonly used pallet sizes are 120x80 cm (Euro pallet) and 120x100 cm (Sea pallet). Reusable plastic pallets are in use. They are made up of HDPE or polypropylene. They are easy to clean and light in weight as compared to wooden pallets. Palletized loads are used in order to reduce handling costs by allowing substitution of mechanical handling for manual methods with the following advantages:

• A decrease in sorting operations.
• Reduced labelling requirement.
• Better utilization of storage space.
• A reduction in mechanical strains and damages.
• A reduction in total distribution time.
• Better maintenance of product quality.
• Two principles are used in the assembly of pallet loads.
• The modular principle, in which all packages are oriented in the same direction.
• The two-way principle, in which the packages in each tier form a pattern such that some packages are oriented lengthwise and others cross-wise on the pallet.

Unitization

Corner posts made of plastic or wood or moulded paper boards are generally used as columns for unitization. The boxes are held together by means of strapping or stretch wrapping around the boxes. The strapping is of polypropylene.

NEW TRENDS

Several new technologies offer the packer opportunities to modify the atmosphere inside the shipping package during distribution even though such control traditionally finishes with the sealing of the package. Packaging is termed as “active” when it performs some desired role other than to provide an inert barrier to the external environment.

The goal of developing such packaging is the achievement of a more ideal match of the properties of the package to the requirements of the food. Hence, it addresses one or more specific needs of the food without necessarily having any impact on other food properties.

SOME TECHNOLOGICAL INNOVATIONS

Most applications of active packaging involve the use of polymers. The role played by the polymer may be that of a conventional packaging material as in the fabrication of sachets that are used commercially to scavenge oxygen or carbon dioxide, or to release ethanol or carbon dioxide. When the active agent is dispersed in a packaging film or sheet, as in OXYGUARD™ thermoformed trays (Toyo Seikan Kaisha, Japan), the polymer acts as a somewhat permeable carrier.

The polymer can play an even more active role when it interacts physically or chemically with the components of the headspace of the package. Such interactions include absorption as in the case of humidity buffering linings for produce cartons, or reactions as in the case of oxygen scavenging plastics.

Ethylene Scavengers

Ethylene (C2H4) acts as a plant hormone that has different physiological effects on fresh fruit and vegetables. It accelerates respiration, leading to maturity and senescence, and also softening and ripening of many kinds of fruits. Furthermore, ethylene accumulation can cause yellowing of green vegetables and may be responsible for a number of specific postharvest disorders in fresh fruits and vegetables.

Although some effects of ethylene are positive such as degreening of citrus fruit, ethylene is often detrimental to the quality and shelf life of fruits and vegetables. To prolong shelf life and maintain an acceptable visual and organoleptic quality, accumulation of ethylene in the packaging should be avoided. Most of these ethylene absorbers are supplied as sachets or integrated into films.

Potassium permanganate (KMnO4), which oxidizes ethylene to acetate and ethanol and in this process, colour changes from purple to brown indicating the remaining C2H4 scavenging capacity. Products based on KMnO4 cannot be integrated into food contact materials, but are only supplied in the form of sachets because KMnO4 is toxic and has a purple colour.

The silica adsorbs the ethylene and the permanganate oxidizes it to acetate and ethanol. Another type of C2H4 scavenging concept is based on the adsorption and subsequent breakdown of ethylene on activated carbon. Charcoal containing PdCl as a metal catalyst was effective at 200C in preventing the accumulation of ethylene, in reducing the rate of softening in mini-mally processed kiwifruits and bananas and in reducing chlorophyll loss in spinach leaves but not in broccoli.

Other C2H4 adsorbing technologies are based on inclusion of finely dispersed minerals such as zeolites, clays and Japanese oya into packaging films. Most of these packaging films, however, are opaque and not capable of adsorbing C2H4 sufficiently. Although the incorporated minerals may adsorb ethylene, they also alter the permeability of the films: C2H4 and CO2 will diffuse much more rapidly and O2 will enter more readily than through pure PE.

These effects can improve shelf life and reduce headspace C2H4 concentrations independently of any C2H4 adsorption. In fact, any powdered material can be used to reach such effects.

Active packaging employs a packaging material that interacts with the internal gas environment to extend the shelf life of a food. Such new technologies continuously modify the gas environment (and may interact with the surface of the food) by removing gases from or adding gases to the headspace inside a package.

Recent technological innovations for control of specific gases within a package involve the use of chemical scavengers to absorb a gas or alternatively other chemicals that may release a specific gas as required. The table below sets out some areas of atmosphere control in which active packaging is being successfully used.

Active Packaging System	Application
Oxygen scavenging	Most food classes
Carbon dioxide production	Most food affected by moulds
Water vapour removal	Dried and meld-sensitive foods
Ethylene removal	Horticultural produce
Ethanol release	Baked foods (where permitted)

Ethylene Scavenging

A chemical reagent, incorporated into the packaging film, traps the ethylene produced by ripening fruit or vegetables. The reaction is irreversible and only small quantities of the scavenger are required to remove ethylene at the concentrations at which it is produced. A feature of this system is its pink colour, which can be used as an indicator of the extent of reaction and shows when the scavenger is used up.

It is expected that the film will be produced and used as a valuable means of extending the export life of fruit, vegetables and flowers. These usually involve the inclusion in the package of a small sachet, which contains an appropriate scavenger. The sachet material itself is highly permeable to ethylene and diffusion through the sachet is not a serious limitation. The reacting chemical for ethylene is usually potassium permanganate, which oxidizes and inactivates it.

Oxygen Scavenging

The presence of oxygen in food packages accelerates the spoilage of many foods. Oxygen can cause off-flavour development, colour change, nutrient loss and microbial attack. Several different systems are being investigated to scavenge oxygen at appropriate rates for the requirements of different foods.

One of the most promising applications of oxygen scavenging systems in food packages is to control meld growth. Most moulds require oxygen to grow and in standard packages it is frequently meld growth, which limits the shelf life of packaged baked goods such as cakes and crumpets and of packaged cheese. Laboratory trials have shown that meld growth on some baked products can be stopped for at least 30 days with active packaging and significant improvements in the free-free life of packaged cheese have also been obtained.

Another promising application is the use of active packaging to delay oxidation of and therefore rancidity development in vegetable oils. Again the use of discrete sachets containing oxygen absorbents has already found commercial application. In this instance the scavenging material is usually finely divided iron oxide. These sachets have been used in some countries to protect the colour of packaged cured meats from oxygen in the headspace and to slow down staling and meld growth on baked products, e.g. pizza crusts.

This approach of inserting a sachet into the package is effective but meets with resistance among food packers. The active ingredients in most systems consist of a non-toxic brown/black powder or aggregate which is visually unappealing if the sachet is broken. A much more attractive approach would be the use of a transparent packaging plastic as the scavenging medium.

Humidity Control

Condensation or 'sweating' is a problem in many kinds of packaged fruit and vegetables. It is of particular concern in cartons of fresh flowers for which there is important export trade.

Unless the relative humidity around flowers is kept at about 98 per cent, water will be lost from the bunches. Such high humidity levels mean there is a very real risk of condensation occurring during transport as the temperature of the flowers may fluctuate by several degrees. When one part of the package becomes cooler than another, water is likely to condense in the cooler areas.

If the water can be kept away from the produce there may be little harm. However when the condensation wets the produce, nutrients leak into the water encouraging rapid meld growth.

When the condensation inside packages is controlled, the food remains dry without drying out the product itself. Therefore sensitive products such as flowers and table grapes are protected from contact with water. This helps to reduce growth of meld.

Carbon Dioxide Release

High carbon dioxide levels are desirable in some food packages because they inhibit surface growth of micro organisms. Fresh meat, poultry, fish, cheeses and strawberries are foods, which can benefit from packaging in a high carbon dioxide atmosphere.

However with the introduction of modified atmosphere packaging there is a need to generate varying concentrations of carbon dioxide to suit specific food requirements. Since carbon dioxide is more permeable through plastic films than is oxygen, carbon dioxide will need to be actively produced in some applications to maintain the desired atmosphere in the package.

Release of Microbial Inhibitors

Ethanol

Antimicrobial activity of ethanol (or common alcohol) is well known and it is used in medical and pharmaceutical applications. Ethanol has been shown to increase the shelf life of bread and other baked products when sprayed onto product surfaces prior to packaging.

A novel method of generating ethanol vapour, is through the use of an ethanol releasing system enclosed in a small sachet which is included in a food package. Food grade ethanol is absorbed onto a fine inert powder which is enclosed in a sachet that is permeable to water vapour. Moisture is absorbed from the food by the inert powder and ethanol vapour is released and permeates the sachet into the food package headspace

Sulfur Dioxide

Sulfur dioxide is primarily used to control meld growth in some fruits. Serious loss of table grapes can occur unless precautions are taken against meld growth. It is necessary to refrigerate grapes in combination with fumigation using low levels of sulfur dioxide.

Fumigation can be conducted in the fruit cool stores as well as in the cartons. Carton fumigation consists of a combination of quick release and slow release systems, which emit small amounts of sulfur dioxide.

When the temperature of the packed grapes rises due to inadequate temperature control, the slow release system fails releasing all its sulfur dioxide quickly. This can lead to illegal residues in the grapes and unsightly bleaching of the fruit.

Considerable amount of work is done to develop systems, which gradually release sulfur dioxide and are less sensitive to high temperature and moisture than those presently used. These systems have potential use for fresh grapes and processed foods permitted to contain sulfur dioxide such as dried tree fruits and wine.

Other Developments

Other systems of active packaging which are either already available or could soon be seen in the market place include:

• Sachets containing iron powder and calcium hydroxide, which scavenge both oxygen and carbon dioxide. These sachets are used to extend the shelf life of ground coffee.
• Film containing microbial inhibitors other than those noted above. Other inhibitors being investigated include metal ions and salts of propionic acid.
• Specially fabricated films to absorb flavours and doors or, conversely, to release them into the package.