In both China and Australia the factor most limiting expansion of the vegetable industry is the short shelf-life of the products. Many leafy vegetables, such as pak choy and broccoli, tend to perish quickly after harvest. Commodities such as Chinese cabbage and oriental bunching onions are storable but losses during storage could be further reduced. The problem is important because these commodities account for at least 30% of China's total fruit and vegetable industry.
This project examined current handling and storage methods in China for pak choy, oriental bunching onions, Chinese cabbage and broccoli. Scientists explored what could be done to extend shelf life, and also identified inherent physiological factors that currently limited the postharvest life of pak choy and Chinese cabbage.
This project aimed to reduce the after-harvest wastage of leafy vegetables by improving methods to prolong their shelf-life.
The team measured losses for the four commodities in China by sampling at different points in the handling chain and at different times of the year. Questionnaires gathered information on the storage methods and handling procedures in use.
In the laboratory scientists kept produce under a range of temperature regimes to observe shelf life and loss rates. They measured respiration rates and the production of the ripening hormone ethylene, and tested the effects on shelf life from treatments such as removing ethylene, storing crops in modified atmospheres, and heat shock.
The researchers also investigated what before-harvest factors could affect the postharvest life of pak choy and Chinese cabbage and whether time of harvest had any effect..
When this project on extending the shelf-life of leafy vegetables was conceived, the profound changes in marketing systems for vegetables in China were only just beginning. This project was therefore timely, and contributed to both technology development and process mapping to identify gaps and needs in the chain between producer and consumer.
Project studies involved assessment of handling systems and finding ways to optimise the environments for handling vegetables, also better knowledge of physiological factors limiting shelf-life of pak choy and Chinese cabbage. It was found that longer-term storage of Chinese cabbage had become less of an issue because it was now available year-round from southern China. Problems now arose from mechanical injury due to poor handling and rot from lack of adequate temperature control. Film wrapping Chinese cabbage was shown to reduce moisture loss of outer leaves, but the scientists then needed to address the problem of rots developing in heads.
Storage of oriental bunching onions was still necessary, as southern areas apparently cannot produce or supply this crop during winter. Their losses arose mainly from inadequate temperature control. Refrigeration, humidity control and modified atmosphere packaging of the onions reduced losses and increased shelf-life, but cost may be prohibitive. Cheaper means of avoiding temperature abuse and moisture are still needed.
Pak choy, a low-value product, was limited by wilting, yellowing and mechanical injury in various circumstances, but overcoming some of the losses may be relatively inexpensive. Preliminary studies indicated that plastic overwraps would have a beneficial impact on moisture loss, be cheap and probably reusable. This simple technology could be transferred to other crops where moisture loss is a problem between harvest and sale.
By contrast, high-value broccoli destined for export suffered very little loss, although cooling of produce was inefficient and could benefit from the introduction of forced air cooling. This demonstrated that if a product was of sufficient value, wastage could be reduced to a minimum. Domestic broccoli and other lower value products, however, were still exhibiting losses. Use of heat-shock and MCP (an anti-ethylene fumigant) fumigation were both found to have prospects as low-cost means of maintaining quality of broccoli. Moisture loss of broccoli during ambient marketing was still a problem.
Studies of means to control temperature revealed that the practice of inserting bamboo tubes throughout loads is partially effective in China. Insulated covers combined with pre-cooling or cooling during transit also reduced losses.
In Australia, modified atmosphere packaging was found to be extremely effective in retarding yellowing of not only pak choy, but a range of other leafy brassicas used in the fresh-cut industry. Use of MCP helped protect pak choy against the adverse effects (yellowing) of exogenous ethylene. Initiation of yellowing in pak choy was linked to endogenous sugar levels in the leaves. Shelf-life was therefore directly related to the effect of a handling system on the rate of sugar usage.
Chinese cabbage had low rates of respiration, ethylene production, and water loss during storage, especially at low temperatures, which would explain its relatively long storage life. 'Patchy Papery Necrosis' was identified as a postharvest chilling injury, rather than a nutritional or pathogen effect, so this will need to be considered when transporting or storing Chinese cabbages from both China and Australia.
The project enhanced the capabilities of the Beijing Vegetable Research Center (BVRC, a Beijing Provincial Government agency) and provided training for farmers, traders and supermarket personnel. BVRC is now undertaking key projects to improve the Beijing vegetable distribution system, plan the supply of vegetables for the 2008 Olympics and develop export markets for fresh vegetables. A Beijing Vegetable Exporters' Association has been formed, and BVRC is using project results to guide members exporting Chinese cabbage, bunching onions and broccoli.
BVRC was also involved in the development of handling and cooling systems for suppliers of lettuce to fast food chain, McDonalds, Beijing (that included construction of forced-air cooling facility adjacent to the production area). Such was the improvement in lettuce quality that McDonalds Shanghai is now also sourcing product from Beijing.
Results of the project have been incorporated into the BVRC International Training Course on Vegetable Science and Technologies funded in part by the Chinese Government. In 2003 project scientist Mr Zhang Shufang was a course lecturer.
Links:
[1] http://www.aciar.gov.au/country/China
[2] http://www.aciar.gov.au/programarea/Horticulture