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Rural Industries Research & Development Corporation
Leafy Asian Vegetables -
Extending their Shelf Life: Part 2by T.J. O’Hare and L.S. Wong
January 2002
RIRDC Publication No 02/006 RIRDC Project No DAQ-239A
The present project DAQ239A flows on from an earlier project of the same name (DAQ213A) in which it was shown that modified atmosphere packaging had potential to almost double storage life of many fresh-processed leafy Asian vegetables. In that project, optimum oxygen and carbon dioxide atmospheres were identified, cool-chain issues were highlighted, and the role of physiological leaf-age influence the rate of leaf yellowing was introduced for the first time.
Although it was shown that atmosphere control was potentially a powerful tool for the industry, the next step of using this information to produce modified atmosphere packaging (MAP) was yet to be accomplished. The present project took this further step, along with testing other potential methodologies (eg. 1-MCP fumigation) which, if successful, could make modified atmosphere technology redundant.
The practical development of MAP for Asian vegetable leaves proved to be more complex than simply ordering plastic film bags of the appropriate permeability. It was soon realised that other factors came into importance, including aesthetic appearance of the package, film availability, packaging costs, and the unfortunate closure of our industry partner during the project. In the retail market, aesthetic appearance means sales and consequently our science was tempered by consumer demand for a package of high clarity. At this stage, such a bag with optimum permeance characteristics for Asian leafy vegetables does not exist. Polypropylene-based films provide excellent clarity, but the permeance ratio of carbon dioxide to oxygen is not high enough. This results in either one or other of the gases being too high to benefit the leaves. Polyethylene on the other hand has an excellent permeance ratio, but the film clarity is not ideal and looks second-rate to polypropylene. The choice is therefore by necessity a compromise: high clarity with shorter shelflife, or reduced clarity with optimum shelf-life.
Packaging availability also had to be taken into account in the real world of commerce. Due to the small size of the industry, the development of specialised films for Asian leafy vegetables simply is not viable in Australia. Plastic film companies require a certain throughput to maintain lines, so developing a film and maintaining its commercial availability is, at the very least, extremely difficult.
We had first-hand experience of this when our commercial partner who was using an almost ideal polyethylene packaging film was told that manufacture of the film was to cease.
Cost was another issue. Although the fresh-processing industry is a value-adding industry, packaging costs still have to be reasonable if a profit is to be made. This means that fancy packaging may price the product out of the market. This is especially the case as competition in the industry has continued to increase, pulling down the price of the retail (and wholesale) product. An example of this is where we developed a wholesale bag using Landec technology (US), but competition had become so fierce at one stage that wholesalers were no longer using anything more than (secondhand) cardboard boxes to package their leaves. Ultimately, competition (and restaurants failing to pay bills) forced our industry partner out of business.
A major objective of this project was also to trial the anti-ethylene fumigant 1-MCP (methyl cyclopropene) as a means of inhibiting chlorophyll degradation, and hence delay the onset of leaf yellowing which limits the shelf-life of so many Asian leafy vegetables. It was hypothesised that ethylene may be integrally linked to leaf yellowing, and blocking its action with 1-MCP could maintain leaves in a marketable state, irrespective of package atmosphere or the cool-chain. In short however, this was found not to be the case and ethylene was found to be only generated in very small amounts, consequently only having a minor role in leaf yellowing. 1-MCP therefore had little significant effect on the inherent shelf-life of leaves. But having said this, 1-MCP was found to be very effective at protecting leaves from ethylene emanating from other ethylene-rich sources, such as ripening fruit on the same shelf. 1-MCP therefore has potential as a protective agent, but its use may be limited by its cost and the time and facilities required for fumigation.
The idea of differential temperature regimes stemmed from the hypothesis that ethylene was integrally linked to leaf yellowing. It followed therefore that low temperatures may inhibit the production or action of ethylene, and that ethylene production and subsequent leaf-yellowing would not occur until temperatures were raised during retail. However, ethylene was not found to have an integral link to leaf yellowing and this hypothesis was not pursued. Instead, it was soundly established that leaf sugar reserve was the main basis for establishing shelf-life. As sugar was the main respiratory substrate for leaves, it was not until it reached a critical level that chloroplasts (containing chlorophyll) were metabolised to provide additional energy reserve. Sugar hence explained the mode of action of MAP and refrigeration to extend shelf-life in Asian leafy vegetables.
This is fundamentally different to lettuce, where MAP controls the rate of browning through controlling the action of enzymes PPO and PAL. Furthermore, sugar level also explained the differences in shelf-life observed between leaves of different physiological age in the previous project.
Finally, this project capitalised on an earlier project (DAV153A, Diversifying Asian Vegetable Markets) which had developed a series of single-page information leaflets on a range of Asian culinary herbs. We extended this series to include the Asian leafy vegetables that were under study in our project and encourage future projects to continue to expand this series.
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