The fresh vegetables in storage will breathe in oxygen and release carbon dioxide and ethylene meanwhile the water contained inside the vegetables will be decreased. That’s why those vegetables become not fresh. The best way to keep the vegetables fresh is to store them in an environment where the concentration of oxygen and ethylene is low, while the concentration of carbon dioxide is high. Meanwhile the ambient temperature should be low and the environmental humidity should be high so that the vegetables can remain fresh and the shelf life can be extended.

The most commonly used fresh-keeping packaging is modified atmospheric packaging (MAP), which can restrain the deterioration of fresh vegetables by controlling the gas composition inside the package. Generally, carbon dioxide, nitrogen and oxygen are injected into the MAP in certain proportion to replace the air. The carbon dioxide can inhibit the growth of a majority of putrefactive bacteria and mildew. The oxygen can inhibit the growth of anaerobic bacteria and maintain the respiratory metabolism of fresh vegetables. The nitrogen, as one kind of inert gas, is used to maintain the compact shape of the package and meanwhile it will not react with the vegetables.

It can be easily understood that the MAP used for fresh-keeping packaging should be made of packaging materials with ultra-high oxygen barrier property so that the oxygen in the external environment will not enter the package and influence the gas composition inside the package. This article provides a simplified method for oxygen permeability test of packaging film of MAP for fresh-keeping packaging used for fresh vegetables.

The testing instrument used is Labthink’s OX2/230 Oxygen Transmission Rate Test System. This instrument uses a coulometric detector to determine the oxygen transmission rate. The specimen is mounted as a sealed semi-barrier between two chambers at ambient atmospheric pressure. One chamber is slowly purged by a stream of nitrogen at a given temperature and relative humidity and the other chamber is purged by a stream of oxygen at the same temperature as the nitrogen stream but may have a different relative humidity than the nitrogen stream. In this case the environment would more closely simulate actual shelf conditions. As oxygen gas permeates through the film into the nitrogen carrier gas, it is transported to the coulometric detector where it produces an electrical current, the magnitude of which is proportional to the amount of oxygen flowing into the detector per unit time. Note: For the package specimens, nitrogen flows inside the package while oxygen flow outside the package.

Figure 1. MAP Fresh-keeping Packaging

The test specimens are composite film taken from fresh-keeping package for fresh potatoes. The tests can be performed in the following procedures.

1)      Condition the composite films in an desiccant container at 23℃ for 48 hours. Then remove the films from the desiccant container and cut 3 pieces 108x108mm specimens.

2)      Apply a thin layer of vacuum grease on the edge of the flange of test chamber. Do not apply the vacuum grease on the central area.

3)      Place the 3 specimens on the vacuum grease and press them gently. Then clamp the specimens tightly.

4)      Add distilled water which is used to adjust the humidity of O₂ and N₂. Turn on the gas supply and adjust the pressure as required.

5)      Switch on the instrument and run the software.

6)      Set test temperature and other test parameters.

7)      Click Start Test to start the test. The system will automatically record the test data in testing process and provide test results when the tests are completed.

8)      Turn off the gas supply and switch off the instrument.

For more details about OX2/230 Oxygen Transmission Rate Test System, please visit www.labthink.com

About Labthink Instruments Co., Ltd:

Labthink Instruments Co., Ltd is one leading supplier of packaging testing instruments, which is headquartered in Jinan, China and operated in Boston, U.S.A.