Laser de-thorning cacti

Without causing damage, a laser process de-thorns cacti and can be used in processing other food surfaces

L. Ponce, M. Arronte, E. de Posada, T. Flores, B. Lambert, and J. Cabrera

Click here to enlarge image

In recent years, exciting applications have been developed to process materials with lasers. In many instances, industry has incorporated cutting, marking, welding, cleaning, and other applications into its processes. However, in the food industry laser processing has had a minor impact, although there are reports about food product marking,1,2 potato cutting,3 cheese cutting,4 and peanut cleaning.5 Recently, a new laser application has been developed to resolve a pressing issue: de-thorning of Opunctia cacti.


FIGURE 1 Opunctia Ficus indica
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The Opunctia cactus, a staple consumed in Mexico, dates back to antiquity and its production and marketing has reached many countries due to its great nutritional, organoleptic, and medicinal properties.6 Its grayish-green oval-shaped flat cladodes (see FIGURE 1) are covered by areoles, places where thorns up to 3 cm-long, surrounded by glochid, are located. They produce a red-colored, sweet-flavored fruit called prickly pear that is also studded by thorns.

Its consumption requires a de-thorning process, where an operator either manually or mechanically de-thorns the cladode’s surface by means of a blade. This process results in product damage, with volume losses up to 30% and a shorter shelf life that prevents storing and marketing the de-thorned product.

CICATA-IPN (Mexico), in collaboration with Havana University, has developed a laser technology that de-thorns Opunctia cacti and allows thorn removal by laser ablation. This approach results in a de-thorning process without product damage, thus eliminating losses and increasing shelf life.

Laser process

FIGURE 2 is a schematic diagram of the de-thorning laser machine. The cactus is arranged linearly and moved on the conveyor belt in order to pass through the laser scanning area. As shown, two Nd:YAG lasers are used, which results in scanning both sides of the cladode simultaneously. Beams are directed to the cactus surface by means of moving mirrors, in such a way that the product surface is scanned linearly in a perpendicular direction with respect to its path.


FIGURE 2. Laser de-thorning process diagram
Click here to enlarge image

A key factor in achieving an automated de-thorning process is to have a method to detect, on a real-time basis, both the presence of thorns and their removal. To this end, the operating mode of the machine sets the predetermined sequence; initially the laser works on a low energy pulse (around 300 mJ) tracking mode. This operating mode is maintained until the laser pulse strikes a thorn; at that time a characteristic acoustic signal is produced due to strong beam absorption. When the signal is produced, the acoustic detector signals the system that the laser energy must be increased to completely remove the thorn.


FIGURE 3. Acoustic signal intensity versus number of pulses
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Next, high energy pulses are applied (about 1 J), until the thorn disappears, and then the acoustic signal drops. The detector then alerts the system to continue running on a tracking mode basis. This whole process occurs while the cacti are being moved sequentially along the conveyor belt.


FIGURE 4. De-thorning process: (top) prior to irradiating, (center) after 3 pulses,
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FIGURE 3 charts the acoustic signal intensity versus the number of pulses applied to a thorn. During the early pulses there is a rapid increase in the intensity of the acoustic signal until the maximum is reached, followed by a drop to a constant level when complete thorn removal is achieved. The first two pulses produce a dark coloration at the base of the thorn due to the burning. At this stage, the acoustic signal is still relatively weak because there is no intense ablation of material yet. Later the combustion darkened areoles increases the absorbance inducing a greater ablation rate and a more intense acoustic signal. Next, as the presence of the absorbing materials decreases, the intensity of the acoustic signal decreases as well. This process goes on until the absorption fully disappears, and consequently both the acoustic signal and the plasma disappear too.

Advantages

FIGURE 4 shows a typical de-thorning sequence. The irradiated area has one or more thorns (in FIGURE 4a there are three), surrounded by a dense network of glochids (the tiny cluster of thorns that surround the larger thorns). FIGURE 4b shows that after three pulses only one thorn and some glochid clusters still remain. There are also clean areas where the glochids have been removed. Finally, FIGURE 4c shows that the whole area is free of thorns, glochids, and the remains thereof, leaving a perfectly cleaned and sealed-off crater-shaped area after irradiating with six pulses.

The above sequence is repeated for each of the 40 to 80 thorns that normally stud the cactus pad. In this manner, if we take into account that each thorn must be irradiated no more than ten pulses, an average of 600 pulses are required in order to de-thorn each pad. This is how a laser system with pulse energy of 1 J and a repetition rate of 100 Hz may de-thorn a pad every six seconds.

In summary, laser de-thorning is a process that demonstrates huge potential for the food industry. The production rate reached at this time is of several tens of kg/h, contingent upon the cactus variety, which fully justifies the cost of the laser machine with an average power of around 100 W. The photoacoustic detection allows for real-time monitoring of the thorn ablation, thus guaranteeing the process automation and quality. Most certainly, this new approach, patented by the authors, will be used in the processing of food surfaces. Proof of this is the work that has been performed in Prickly Pear with similar results. 7

References

  1. Il laser nel segmento agro-food”, Salvadeo P., Applicazioni Laser, 3, pp 32-35, 2005.
  2. “Laser coding turns food packages into miniature billboards”, Kincade K., Laser Focus World, 10, pp 85-91, 2004.
  3. “Laser operated seed potato cutter,” Zelinski W.J., Tallackson Th.K., US Patent 6321484, 2001.
  4. “Lasers turn cheese into art”, Graydon O., http://optics.org/articles/news/9/12/(2001).
  5. “Method for shelling of nuts with a laser beam”, Patel Ch.K.N., US Patent 358467, 1980.
  6. “Estudio quimico de nopales (Opuntia)”, Villarreal F., Rojas P., Arellano V., Moreno J., Ciencia Mexicana, V. 22, pp 59–65, 1963.
  7. “Laser induced breakdown spectroscopy of prickly pears spines and glochids: A qualitative analisys”, T. Flores, L. Ponce, G. Bilmes, A. Arronte and F. Alvira, AIP Conf. Proc. 992, 1274, 2008.

L. Ponce (lponce@ipn.mx), M. Arronte, E. de Posada, and T. Flores are with CICATA-IPN, Altamira, Tamps, Mexico. B. Lambert, and J. Cabrera are with IMRE-Havana University, Havana, Cuba.

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