From its humble beginnings as a backyard crop, carambola is now one of Malaysia’s leading exports. Also known as starfruit for its star-shaped appearance when sliced, the commercial cultivation of carambola started in the 1970s. By 1988, exports to Hong Kong and Singapore markets were reported to reach 13,000 tonnes, worth USD 4.9 million.
Realizing the economic potential of carambola and tropical fruits, the Fruit Industry Development Program (1986-2000) was developed by the government with the following key strategies:
• Expansion of local and export market through a strategic marketing plan;
• Intensify crop management research to reduce labor and production costs; and
• Develop a working group to coordinate relevant agencies, producers, and exporter.
Research and development by the Malaysian Agriculture Research and Development Institute (MARDI)
1. Breeding for quality improvement
Initiated in the 90s, the breeding programme was implemented to improve fruit quality, skin color, brix levels, and vitamin C content. Two commercial cultivars (B10 and B17) and two pollinator cultivars (B2 and B11) were used as parents in a diallel cross designed to yield hybrid seeds. Three hybrids (B1711, B1002, and B0217) were then selected after the fruits underwent sensory evaluation against commercial cultivars (B10 and B17). These hybrids have the potential to be exported at full maturity (‘Golden carambola’) for fresh consumption and are currently undergoing location verification trials.
2. Floral biology and clonal compatibility
At 2.79-2.98%, the natural fruit set of carambola is low. Flowers are heterodistylous, meaning flowers have two different distinct lengths of pistil styles. Studies showed that to improve fruit set, cross pollination between flowers with short styles and long styles are required.
It was recommended to plant pollinator clones with long styles such (B2 or B11) in plot of commercial clones with short styles (B10) to improve fruit set and increase yield.
3. Mineral nutrition of carambola
From a field experiment, evidence suggested that carambola production is inhibited by high application of nitrogen fertilizer. A combination of a low level of N and a high level of K was found beneficial for growth and yield. For example, 0.8 kg N and 4.8 kg K2O/tree/year is recommended for commercial application.
Upon further investigation, applying heavy doses of fertilizer exposed the plants to intermittent shocks that manifested in leaf water potential, stomatal conductance and photosynthetic rate.
4. Fruit fly management
Bactrocera fruit flies (Dacus dorsalis complex) are the most threatening insect pest of carambola. Without proper control, they can damage all fruits and can lead to a total loss of crops.
Together with the Australian Centre for International Agricultural Research (ACIAR), MARDI developed PROMAR, a protein bait for fruit flies. A mixture of one part of PROMAR and two parts water/insecticide is recommended to be spot sprayed on a small portion of the foliage, reducing fruit fly damage to less than 2%.
5. Quality production under netted structures
Traditionally, carambola is planted in an open field. Fruits have to be bagged individually to protect them from pests, which is very labor-intensive. Planting carambola inside a netted structure reduces labor costs and fulfills the stringent requirements of importing countries to comply with global good agricultural practices requirements. This also aims to adhere with the Fruit Fly Free Place of Production (FFFPP) Protocol for exporting to countries such as China and the United States.
Yield
Aside from pests, heavy rains can reduce fruit set. Netted structures can reduce the impact of rain on the fruit. This is crucial for October-December, where heavy rain is common. Under netted structures, the total fruit weight per tree was 75 kg, estimated at 33.7 t/ha. Meanwhile, the fruit weight for trees in the open field is only about 6 kg per tree, yielding 2.7 t/ha. After grading, the exportable fruit from trees in netted structures (23.8 t/ha) was higher than those in the open field (2.2 tons/ha).
Fruit quality
Fruits that are exposed to direct sunlight look bleached and lacked luster and firmness. While bagging under the netted structure is not a requirement, wrapped fruits were observed to be crunchier. Fruits under netted structures also have lower pesticide residue.
Preharvest Calcium application
Carambola treated with calcium was shown to have thicker and denser cell walls, which lead to firmer fruit texture and wing tips that can reduce bruising damage from handling.
6. Post harvest handling
Since 1989, carambola exports are have been transported by sea. Fruit handling crucial in long voyages to ensure that the fruits arrive in excellent condition. MARDI developed an index that determined the proper maturity to harvest fruits for specific markets. The recommended packinghouse procedures for sorting, cleaning, grading, and packaging were also determined.
Carambola as a superfruit
Studies show that carambola is rich in apigenins and procyanidins, which are polyphenols known for health benefiting properties. This fruit also contains phenolic acids such as conjugates of ferulic and sinapic acid.
Three types of procynadin are relatively abundant: procynadin dimer, procynadin trimer and a conjugate of procynadin. Procyanidins belong to the class of flavon-3-ols, which are present in green tea. Procynadins are found to be effective scavengers of free radicals and possibly have chemopreventive properties. The phytochemical profile of carambola is unique for the relative abundance of apigenin sugar conjugates. Apigenins are flavones commonly found in celery and sweet red peppers.
The synergistic effects of the complex mixture of these compounds may have additive health benefits. With the rich source of different phytonutrients, carambola can be considered as a superfruit t hat can be processed as a functional drink or ingredient.
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This article is adapted from a presentation by Zabedah Mahmood, Malaysian Agriculture Research and Development Institute (MARDI), at the International Symposium on Superfruits: Myth or Truth, Vietnam, 1-3 July 2013. All photos are derived from the powerpoint presentation.