You open the plastic package and inside are four species of living, breathing lettuce leaves. No it's not a sci-fi horror but the prelude to your Caesar Salad as you rush to prepare dinner for four in one hour.
Many uninformed consumers believe the pre-packed dry coleslaw mix, baby leaf lettuce and ready-to-use vegetables increasingly appearing on supermarket shelves come in plastic bags of air.
They are, in fact, modified atmosphere packages, fine-tuned to the respiratory rate of the produce they contain. These packages can extend shelf life with a gas mixture which has less oxygen and more carbon dioxide than normally found in air.
"The vegetables are breathing or respiring. They are consuming oxygen and producing carbon dioxide and modifying the atmosphere in the pack," explained Prof David O'Beirne of the department of life sciences at the University of Limerick. For 10 years, he and teams of research students have been studying the effect modified atmosphere packaging has on vegetables and fruit, and on the pathogens that may contaminate them. The packaging is a follow-on from "control atmosphere stores", first used in the US to extend the storage life of apples in the early part of the last century. Packaging, usually made of oriented polypropylene of varying thickness and sometimes with micro-perforations, began to be used in France in the 1970s and in recent years can be found in most supermarkets. It is designed to prevent the browning of cut surfaces of fresh produce, giving them a shelf life of about five days in a refrigerated unit.
"If you cut vegetables, even if you put them in the fridge, they are going to go brown at the cut edges," Prof O'Beirne pointed out. "But the gas barrier properties of the packages have to be right. Different products respire at different rates genetically and if you cut them, they respire faster and all of this has to be taken into account in choosing the right packaging. "If the packaging is too impermeable, you are going to lose all the oxygen. When you get the packaging right, it holds the oxygen at about 3 per cent and lets the carbon dioxide go up to about 5 per cent," Prof O'Beirne said. The normal oxygen level in air is 21 per cent, and 0.03 per cent for carbon dioxide. One aspect of the Limerick research has been whether the gas atmospheres increase the risk of food poisoning. If a producer is not on top of the technology and uses an inappropriate packaging material, an inappropriate atmosphere can result.
Prof O'Beirne has advised industry on the compatibility between the packaging and the product. At warm temperatures, the localised environment can enhance the growth of listeria or E.coli if the product is contaminated with these. "When you temperature abuse the product, the hazard issue comes into focus. People need to know that they should not leave it at the back of a car in summer. It should be held at four degrees Celsius. If you hold it at eight degrees Celsius, you can get significant growth of food poisoning organisms in worse case scenarios," Prof O'Beirne stated. The effect of argon gas on certain products is another area of the university's research. "Essentially, argon slows down enzyme activity right across the whole spectrum, including browning. We are trying to understand how it works. "Some of the gas companies are selling this type of technology in the pharmaceutical industry. We feel the technology is incompletely understood. It needs more understanding to effectively apply it and make sure there is no downside to these novel gases," Prof O'Beirne noted.
The researchers have also examined the effect of the gases on the product and the extent to which its vitamins are destroyed. This loss is not significant, as it happens.
"It is redefining freshness in a way. But it certainly is convenient and, in many ways, it is of high quality," he said. "If the atmosphere is right, it will retain most of its vitamins." Prof O'Beirne is hoping to do research next on the effects of the technology on retention of nutrients using lettuce and carrots. "If we can get it to work for those, we can extend it to other products reasonably successfully."