Postharvest handling of leafy Asian Vegetables

Graeme Thomson, IHD Knoxfield, (03) 9210 9222

Good postharvest management delivers horticultural produce to the end consumer in a condition which is virtually unchanged from harvest time. Handling should avoid mechanical injuries, preserve the product's food reserves and storage life, restrict water loss and avoid proliferation of microorganisms.

As soon as a vegetable is harvested it starts to deteriorate. Its water source is removed and assimilation of carbohydrates becomes restricted.

Leafy vegetables are particularly susceptible to leaf damage which is not only aesthetically unappealing but also allows accelerated water loss at cut surfaces. To avoid leaf damage do not expose the vegetables to drops, knocks and collisions. Ensure there is no exposure to sharp edges, nails etc. during bin-filling, grading and packing.

Intact, healthy leaves of produce like Chinese cabbage and Chinese broccoli (kailaan) provide a mechanism for water loss and consequent wilting because of their large surface area. The solution is to use packaging to maintain high humidity around the harvested vegetables, but not so much that produce is in contact with free water. Various types of carton liners are available made from polyethylene or polypropylene. Once filled, the bag's opening should be loosely folded.

Plastic packaging should only be used with low temperatures. If vegetables with field heat are packed into plastic, or if packed vegetables are allowed to warm, then they will deteriorate quickly. Produce should be cooled to appropriate storage temperatures before packing (e.g. 1°C for Chinese flowering cabbage (choy sum)). Once packed they should be transported and stored at the correct temperatures.

Fresh plants use oxygen and their food reserves during respiration to stay alive. Temperature is one of the most important factors affecting rate of respiration. Lower storage temperatures reduce respiration and lower consumption of food reserves, thereby preserving the product's life. Cooling vegetables also reduces their water loss.

As a general rule, storage at temperatures above but close to zero is best for many crops. However, some are sensitive to low temperatures and can develop severe chilling injuries. Never allow any fresh produce to freeze. Care should be taken to avoid chilling of vegetables grown in tropical conditions. Chinese broccoli, Chinese cabbage, Chinese chard (pak choy), Chinese chives, hot mint and Chinese flowering cabbage (choy sum) store best between 0°C and 1°C. However, crops like Thai basil should be stored at around 12°C.

There are different methods to remove field heat from vegetables. The best require specialised equipment. For example, 'vacuum cooling' is suited to leafy crops; 'forced air cooling' is suitable for a vented container (carton or bin). The simplest method of cooling is by 'contact icing' but it can physically damage some delicate plant parts.

Another way to reduce respiration rates of vegetables is to modify the gaseous atmosphere around the harvested produce. If oxygen levels are lowered and carbon dioxide increased, the rate of respiration can be reduced.

Modified atmosphere packaging (MAP) uses the product's natural production of CO2 and consumption of O2 to help create a gaseous environment where the respiration rate is lowered. This must be carried out in combination with low temperatures and high humidity. MAP has been demonstrated to improve storage of crops such as broccoli, pears and lettuce.

Recent work on RIRDC co-funded project DAV-128A 'Diversifying Asian Vegetable Markets' has begun development of MA (modified atmosphere) packages for selected Asian vegetables. Early indications suggest that product held in MA generally has superior appearance at out-turn, is greener and less affected by storage rots. Storage times could be increased by as much as a third with MA thereby increasing flexibility in marketing and distribution, and potentially opening export opportunities. However, remember that correct cooling and use of modified atmospheres will help maintain quality but not improve it - initial quality at harvest must be high.

For further information on storage temperatures,
please contact Graeme Thomson on (03) 9210 9222.

Sponge Luffa (Luffa cylindrica)

Sponge luffa also known as "sze kwa" in Chinese and "muop khia" in Vietnamese is a climbing annual growing to 4.5 m. Leaves are five-lobed, 10 cm long and 12 cm wide. Flowers are yellow, 6-8 cm in diameter. Male and female flowers are separate. The fruits are green, usually lightly striped, 30-45 cm long, cylindrical and smooth. They are much heavier than the fruits of angled luffa (see previous month's newsletter) and tend to be larger at the lower end giving a bat-shaped description.

Sponge luffa is grown in warm and tropical climates (optimum 35°C) and needs a long growing season (80-120 days). Seeds should be soaked in water for 24 hours before sowing. Space plants 1m apart, and provide support. For use as a vegetable, pick luffas 10-15 cm long, just before use as they do not store well.

Luffa sponges - allow fruits to hang on the vine until the skin hardens and the stems turn yellow. Upon harvesting, continue the drying process for a few weeks until the skin is brown. Peel off the skin, remove the pulp by rinsing in running water. It may be necessary to soak them for several days to complete the job. Luffas can be bleached in weak household bleach.

Preparation
Select unblemished fruits (the smaller and younger the better). Wash and slice.

Cooking
Chinese
The Cantonese people tend to use this in soups. Simmer it for hours with meat or pork bones or toss into quick vegetable soups (with carrots, spring onions, ginger and/or preserved mustard).

Western
Steam, boil, or sauté as you would squash. Cook it well and season with something aromatic (pepper, ginger, onions).

Websites
Agriculture Victoria, IHD Knoxfield: Asian Vegetables Thesaurus

Pick from the RIRDC book shelf

Market for Processed Food and Beverage Products in Urban China
Mr Suku Bhaskaran, Australian Food Marketing Centre
Professor Nicholas Samuel
Department of Agricultural Business, University of Adelaide
RIRDC Research Paper 94/11
$30 ($6 p&h) 76 pages

Surveys 5000 shoppers in four major Chinese cities to analyse consumer characteristics and expenditure patterns, market size, and market growth prospects. Reveals the dramatically changing dietary habits of urban Chinese consumers - away from traditional home preparations to western convenience foods.

The book can be purchased from
Rural Industries Research and Development Corporation (RIRDC)
P.O. Box 4776, Kingston, ACT 2600, Phone: (02) 6272 4819, Fax: (02) 6272 5877
Webpage: http://www.rirdc.gov.au/

For more RIRDC publications see our first 'Asian Foods' Newsletter.