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Kissing a cactus: mechanical harvesting of prickly pears.

Worldwide, Mexico is the largest producer and consumer of prickly pear cactus fruit (called tuna in Spanish). Around 20.000 small growers raise prickly pear cactus on 56,000 ha (138,000 acres) in the low-rainfall regions of Mexico. Average yield of tuna ranges from seven to eight tons per ha, with an annual production of 394,000 tons. Around 99 percent of this production goes for the fresh market, both domestic and international, and the remainder is used as raw feedstock for small food industries. Yields of 25 tons per ha (2.5 acre) or higher could be obtained through good crop management and mechanization.


Mechanization makes the difference

Introduction of mechanization has already started to modify crop management. Planting density has changed from a crowded 2,000 plants per ha four decades ago to a more manageable 625 plants per ha today and the quality of the produce has improved. This planting density does not interfere with mechanized operations. In fact, the plant spacing, both between plants and between rows, varies among growers. Several other factors have contributed to the improvement in production: climate, environment, new varieties, and better soil science, coupled with research on cultivation management, fertilizing, pest control, crop handling, and storage. As well, there has been government support to increase planted land irrigation, and marketing in order to achieve profitability for this important agro-industry.


Both domestic and international markets demand high tonnage production and high-quality produce. In order to meet this increasing demand, production of tuna is steadily growing in South America and North Africa, as well as in Israel, Italy, South Africa, and the United States. Despite many efforts, this industry has not reached its full potential. The most pressing issue is the development of technology that can bridge the gaps in the production-to-consumer chain.

Skilled maneuvering is key to picking

Marketable tunas are typically 20 to 70 mm (1 to 3 in.) in diameter, and each plant can produce 30 to 40 kg (66 to 88 lb) per year. The fruit can be round, pear- or fig-shaped, and color ranges from yellowish green to dark purple. Through pruning, the maximum height of the prickly pear cactus plant is kept at 1.8 m (6 ft), and the overall diameter is kept within at 2.0 m (6.5 ft). The cladodes (the cactus leaves or pads) are covered with stiff thorns, while the tuna has detachable microscopic hairs, called glochids, that can easily penetrate bare skin. This painful experience scares off untrained workers. Even for experienced workers, hand harvesting is dangerous because the picker has to maneuver around the stiff thorns to reach the ripe fruit. Pickers typically do not wear any protection on their hands, arms, and faces. At most, they may wear rubber or leather gloves to protect their hands from thorns and glochids. As a result, prickly pear harvesting is slow and expensive because skilled labor is required and the field conditions are arduous and unsafe.


Pickers prefer to start harvesting tunas in the early morning, as soon as daylight allows them to locate the tunas and safely maneuver around the cactus thorns, because windless and misty conditions reduce the chances of the glochids separating from the fruit. The harvesting season extends from June to October, so the extra seasonal demand for skilled labor, and subsequent labor shortages, increase production costs. On already established cactus plantations, harvesting costs represent 16 to 25 percent of total tuna production costs. Inexpert fruit handling can cause crop losses of up to 10 percent, which reduces the shelf-life, vigor during transport, and visual appearance of the fruit, as well as the farmers' income.

Take your pick: hand or knife?

There are two hand-harvesting techniques. In the first method, the picker grabs the ripe tuna and detaches it from the cactus pad by twisting it off gently, taking care not to damage the connection point to the cladode. In the second method, the picker holds the fruit, bends it slightly at the connection point, and then slides a knife through the connection. Once the fruit is free of the cactus, it is dropped into a bucket that the picker carries from plant to plant. Filled buckets are unloaded into boxes for transport to the selection, packaging, and storage facility. Harvesting costs vary according by region. On low-yield plantations, a worker can be paid as little as $0.90 USD per 25 kg (55 lb) box.

Separating the fruit from the cactus with a knife is a clean operation, and fruit that has been harvested by cutting suffers much less injury than fruit that has been twisted off the cactus. In general, maintaining fruit quality depends on avoiding damage to the skin and base of the fruit during the harvest operation and avoiding impact damage when the fruit is dropped, the bucket is emptied, and later during removal of the spines and glochids.

Harvest help

To help tuna pickers safely perform their work and increase their harvest earnings, we have designed and tested a simple, sturdy harvesting aid that is both more efficient and less damaging to the fruit. The main criteria for the prototype were:

* The worker had to be able to separate the tuna from the cactus without touching the fruit by hand.

* The harvesting aid had to increase harvesting capacity compared to hand harvesting.

* Damage to the fruit had to be less than or equal to the damage that occurs during hand harvesting.

* The harvesting aid had to have low weight, be simple to use, and be safe to operate.


* The harvesting aid had to be moveable, and it had to allow unrestrained movement by the pickers.

* The harvesting aid had to have a simple design for ease of manufacturing. In addition, it had to be reliable, simple to repair. and have low maintenance costs.

* As additional criteria, the design had to enable the possibility of adding mechanical features, of complete automation, or even for developing a harvesting robot in the future.

After a technological analysis involving economics, engineering options, and the harvest aid's required performance in field conditions, a pneumatic system was chosen to be the central component. A pneumatic approach would significantly reduce damage to the fruit, and it could be the basis for both a mechanized harvesting aid and a fully automated harvester in the future. In addition, a pneumatic machine would be easy to adjust, operate, and maintain in field conditions.


Soft as a kiss, nuzzling with a nozzle

The key to the harvesting aid is nicknamed the "kissing nozzle." Two main functions are performed by this device. The first function is making contact with the fruit. To protect the fruit, the connection has to be as delicate as possible, just like a kiss. This is achieved with a soft ring of flexible rubber. In the second function, the nozzle grips the fruit firmly, using suction. The fruit can then be bent aside slightly, allowing the picker to slice the connection between the fruit and the cactus. Because of the kissing nozzle's pneumatic grip, the position of the nozzle on the fruit has no effect on the cutting operation, allowing the picker to approach a fruit from whichever direction is most convenient. After the fruit has been separated from the cactus, the nozzle is held over a collection tray and the fruit is released by changing the airflow with a valve.

The prototype harvester was installed on a wheeled trailer, with two extended arms to carry the nozzle's pneumatic hoses high enough to pass over the cactus plants. The arms can be folded for road transport. The additional cost of using a farm tractor or other vehicle to pull the tuna harvester between cactus rows is relatively low. For example, the harvester could be pulled by a pickup truck, a common vehicle in rural Mexico. Because the harvester has its own power source, it could even be pulled by draft animals.

The assembly on the trailer consists of a gasoline engine, an air compressor, an accumulator for pressurized air, an air filter, pressure gauges, one-way pass and release valves, fruit collecting trays, as well as couplings for the pneumatic hoses and kissing nozzles. The trailer assembly also includes a container for harvested fruit so pickers are spared the burden of carrying heavy buckets.

The design allows coupling of up to four nozzles without compromising the air pressure, allowing four pickers to work simultaneously and independently of each other. With this configuration, each picker has a working range of up to 2 m (6 ft) from the trailer. Since the trailer is pulled between rows, workers can be positioned along both sides of two cactus rows. This increases the productivity by up to four times that of hand harvesting.


The results of this project have demonstrated the advantages of mechanized harvesting of prickly pear cactus fruit, as well as providing a safer work environment for the pickers, increasing productivity and profitability, and ensuring that consistently high-quality fruit is delivered to the market.

ASABE member Hipolito Ortiz-Laurel is an agricultural and biological engineering adjunct professor and mechanized systems management specialist, Colegio de Postgraduados, Campus Cordoba, Veracruz, Mexico;

Dietmar Rossel-Kipping is a mechanical engineering adjunct professor and agricultural machinery design specialist, Colegio de Postgraduados, Campus San Luis Potosi, San Luis Potosi, Mexico;
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Author:Ortiz-Laurel, Hipolito; Rossel-Kipping, Dietmar
Publication:Resource: Engineering & Technology for a Sustainable World
Geographic Code:1MEX
Date:May 1, 2011
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