All plants respire (breathe), that is various organic substances - and especially sugars - are broken down to obtain energy for other life processes in the cells. This process requires oxygen, which is always present in the air (21%). When oxygen is present, the respiration is called aerobic.
The speed of respiration varies greatly from product to product. Cut vegetables also respire at a much faster rate than whole ones. Sliced carrot, for example, respires five times faster than whole, peeled carrot.
The speed of respiration may be changed by changing temperature, access to oxygen and access to carbon dioxide.
Lowering the O2 level of fresh vegetables
For most vegetables the O2 level must be lowered to below 8% to achieve an effect. But there are limits to how low you should go, depending on the type of vegetable. If the oxygen content becomes lower than about 2% a change may occur from aerobic to anaerobic (without oxygen) respiration. This means that the plant cells go over to another means of breaking down the organic substances to obtain energy when oxygen is in short supply.
During respiration carbon dioxide is created, which has a beneficial effect on the speed of respiration. CO2 has a more or less inhibiting effect on micro-organisms. Many micro-organisms are inhibited by CO2 concentrations higher than 10%. But too high a CO2 content can give odour problems.
Avoiding limp vegetables
Ethylene is an active plant hormone that advances ageing both in vegetables that produce the gas themselves and in vegetables that are stored together with them. The gas does not affect the respiration speed of leaf vegetables, but for potatoes, carrots, swedes and parsley root respiration speed increases when ethylene is present. Substances like acetaldehyde and ethanol are also developed. These give the vegetables an unpleasant taste and smell, while the tissue weakens over time and breaks down into a soft structure. MAP inhibits somewhat the production of ethylene and also reduces sensitivity to ethylene.
The packaging used for vegetables in modified atmospheres must fulfil many requirements. Nofima Mat is working on understanding how polymer type, additives in packaging materials, production parameters and storage conditions (temperature and humidity) affect the transmission of both carbon dioxide and oxygen (i.e. permselectivity). This knowledge is important for developing packaging materials with gas transmission properties suitable for the needs and requirements of the different products.
