Vegetables 'sleep' well with plastics

When thinking about ways to extend the shelf life of fresh fruit and vegetables, at home or in commercial storage rooms, look for tools that put these living organisms to 'sleep.'

While sleeping, humans burn fewer calories and loose less water than when we are awake doing any activity, even if we are just sitting around watching television.

For comparison, let's say that two healthy people decide not to eat and drink for one week (don't try this at home), and one sleeps five of the seven days or just watches the clock, whereas the other individual sleeps only three days of the seven and decides to jog every day.

If no one passes away, the one who spent more time sleeping and doing low-key activities will likely be in better shape at the end of the week.

To prolong shelf life of produce, we basically use the same approach - we want our vegetables to be 'stress' free. This can be reached by different ways, but I would like to highlight the importance of plastics as a tool for extending the shelf life of fruit and vegetables.

With the exception of refrigeration, no other technology is more used than modified atmosphere packaging (MAP) to extend the shelf life of fresh commodities. MAP is achieved by altering the common com- position of the atmosphere surrounding the produce.

MAP came on board as a tool for the vegetable industry, thanks to the development of more refined plastics. For the fresh-cut industry in particular, late improvements in plastics have been vital for the revolutionary expansion of ready-to-eat produce in the food market.

After harvest, the metabolism of fruits and vegetables functions very much like that of humans.

Once cut from the mother plant, the tissue from fruit, flowers, leaves, roots and stems will no longer have access to nutrients from the plant. Thus, the energy to keep them alive can only be taken from their own tissue, from what has been stored up to that moment.

Respiration provides the energy-using oxygen, which in combination with carbohydrates releases carbon dioxide and water, just like in mammals. If a vegetable is stored in dry conditions, it will accelerate transpiration just like we do, and eventually will show symptoms of dehydration.

The objective of using MAP after the harvest of vegetables is to slow down the aging process. This can be achieved by forcing the vegetable to hibernate or to sleep.

MAP reduces the respiration by reducing the concentration of oxygen in the produce's atmosphere. Similarly, as carbon dioxide increases, a suppression of respiration is also achieved.

In general, reduced oxygen and elevated carbon dioxide together reduce respiration more than either alone. MAP keeps high relative humidity, allowing the product to retain water for a longer time.

In an ideal scenario, oxygen and carbon dioxide reach the levels that best minimize respiration in less than 48 hours after the product is bagged, wrapped or packaged.

Air contains about 21 percent oxygen and 0.03 percent carbon dioxide. In many cases, the most efficient levels are below 5 percent oxygen and above 10 percent carbon dioxide.

As with many things in life, the excess can be detrimental. You want to put the vegetables to sleep, not to kill them! Plastics that have low permeability to gases - normally the thicker the plastic the lower the permeability to gases - will result in fermentation, off flavors, condensation and accelerated deterioration of the product.

Fortunately, the plastics industry is now launching packaging films that can further extend the quality of products. Control of respiration and dehydration are no longer the only factors targeted.

New plastics are tailored to meet specific needs of respiration and humidity for most fruits and vegetables. In addition, new plastics for the produce industry may contain natural antiethylene (ripening hormone), antifogging (condensation) and antimicrobial agents.

If you want to put vegetables and fruits to sleep at home with common plastics found in the grocery store, consider the following:

Bag or wrap only produce that will be stored in the refrigerator.

Do not bag wet produce.

Remove as much air from the bags as possible.

Bag or wrap produce that is already cold.

Make holes or open the bags often, especially in products that are highly susceptible to wet deterioration.

Use rigid plastic such as lidded trays for fresh-cut fruits - do not fill the container more than 2/3 the volume for long-term storage.

Jorge Fonseca is a vegetable specialist with the University of Arizona at the Yuma Valley Station. He can be reached at 782-3836 or jfonseca@ag.arizona.edu.