Packaging

Packaging is a standard requirement for Brassica crops in transit to the retail consumer. Effective packaging will enhance the attractiveness of the product and retain quality characters for longer periods. Defective packaging accelerates deterioration and destroys quality. Deterioration is especially rapid where packaging allows the accumulation of toxic compounds, such as ethylene, that contribute towards accelerated deterioration.

The formation of ethylene speeds up the processes of senescence and, in Brassica products such as cauliflower, broccoli (calabrese) and sprouting broccoli, leads to yellowing. Changing the concentrations of oxygen and carbon dioxide in small packs of Brassica vegetables retains their fresh quality and extends shelf life. This is an application of controlled atmosphere storage technology as used for bulk volumes of top fruit applied to small produce packs for the supermarket shelves. Lowering oxygen and increasing carbon dioxide concentrations around the produce blocks the synthesis of ethylene, thus slowing the ripening processes. The use of MAP started in the late 1940s when plastic film polymers became widely available for civilian use. It is defined as 'an alteration in the composition of the gases surrounding fresh produce when such commodities are sealed in plastic films'.

Plastic film technology and packaging techniques have advanced substantially in the intervening 50 years, and MAP is now a major marketing tool. Substantial problems remain, such as the development of off-flavours and fermentation in broccoli due to the innately high rates of respiration. Modified atmosphere packaging is now of great importance, however, for retaining freshness and quality. Few consumers realize that opening a pack of baby leaves releases a minute burst of carbon dioxide or nitrogen. The changed atmosphere is generated either by natural respiration or, more recently, by artificially adding nitrogen gas as the produce is sealed.

Reducing the oxygen content of atmospheres surrounding produce diminishes respiration and ethylene production and represses microbial and enzymic degradation. These processes start automatically with all fruit and vegetables from the instant they are harvested. Storage at around 5°C further reduces deterioration. The fresh turgid appearance of produce is helped by increasing the relative humidity in the pack and results from reducing the temperature. The danger is that excessive moisture from fruit and vegetables accumulates inside the pack, especially when temperatures fluctuate. The higher humidity inside the pack condenses into water droplets when the film surface is cooler than the air inside the pack. This can be overcome by using anti-fogging chemicals. A careful balance is required since vegetables such as broccoli (calabrese), cauliflower and Chinese cabbage need a moist atmosphere in order to avoid desiccation.

A key factor in MAP is finding films with a permeability that prevents an accumulation of damaging concentrations of either water vapour or carbon dioxide, or excessive depletion of oxygen. As an approximation, films are 4-6 times more permeable to carbon dioxide than they are to oxygen. Transmission of water vapour is made independently of gas diffusion by using multilayered plastics. One of the favoured newer products is 'linear low density polyethylene'. This has a better structure compared with simpler films, making it tougher and more suited to heat sealing with greater impact resistance and tear strength. Levels of carbon dioxide or oxygen take time to adjust inside the pack. Active adjustment by gas flushing speeds this up and uses mixtures of carbon dioxide, oxygen and nitrogen to purge the packages.

Alternatively, the package may be evacuated and refilled naturally by respiratory gases coming from the produce. Some plastic films are perforated with 'microperforations' that allow a greater degree of control of water vapour content but have lesser effect on gases. Plastic film manufacturers are now marketing products that are claimed to extend the life expectancy of fruit and vegetables. Some films have permeable windows made of silicon or polyvinyl polymers that control the diffusion process more efficiently. Early experiments in the USA with winter green cabbage showed that carbon dioxide and oxygen concentrations were lower than expected where plastic films incorporating silicon windows were used. The cabbage heads showed better colour retention, fresher appearance, firmer texture and smaller losses in weight.

The use of microperforated plastics and an ethylene retardant, such as MCP, reduced senescence-related yellowing in broccoli. A new generation of packaging materials uses environmentally responsive high permeability films equipped with molecular temperature switches. These could help reduce the fermentation risks in MAP packs. This approach can be extended further with packaging that has a built-in ethanol sensor that identifies the early stages of deterioration.

Perforation-mediated MAP relies on the use of perforations (tubes) of different sizes to control a package's gas content for both whole and freshly cut fruit and vegetables. Designing efficient packages demands knowledge of gas exchange rates through the perforations. In addition, circulating cooled air round the packages helps control the rate of gas exchange. A research aim for plastic manufacturers is to develop polymeric films with a selective barrier capable of matching the respiration of the produce; alternatives for such tasks are polypropylene, polyvinyl chloride and polyethylene films. For broccoli, packing in polypropylene film increased storage life. Broccoli stored in polyvinyl chloride deteriorated faster than when it was packaged in the other two materials. Broccoli has a high respiration rate, is very sensitive to ethylene and loses water rapidly, and consequently, is a difficult product to package. Using polypropylene extended the shelf life to 3-4 weeks in air at 0°C compared with only 2-3 days at 20°C. The standard storage recommendations for broccoli are 0°C with a relative humidity of 95%, 1-2% carbon dioxide and 5-10% oxygen.

New Brassica vegetable products are emerging suited to MAP packages, for example shredded Galega kale, which is very popular in Portugal where it is used as a companion to their traditional delicacy of salted, dried fish. Studies demonstrated that MAP with an atmosphere of 1-2% (v/v) oxygen and 15-20% (v/v) carbon dioxide will extend the shelf life of shredded Galega kale at 20°C to 4 days as compared with 2 days with ambient air storage.

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