Automated Chamber for Prickle Pear Cauterization

Mexico is the world largest producer of cactus pear ( Opuntia spp .) and is searching for new processes that allow to export them. The fruit is rich in nutrients but is highly perishable having ambient shelf life of 9 days. Cauterization and cryo-cauterization techniques have increased shelf life to 2 and 3 months without refrigeration. An automatic system was developed to flip the fruit towards the dry-ice wall inside a chamber. A horizontal actuator pushed the container to cauterize the fruit. The entire cauterizing process for each fruit lasted 23 seconds. By exchanging the dry-ice wall every 500 fruits, a 100% cauterizer efficiency was achieved.


Introduction
Cactus pear (Opuntia spp.) is a fruit produced in arid and semi-arid regions around the world [1]. Mexico is the world's largest producer of cactus pear (Opuntia spp.) with 79.4% of world production, and with 49,165 ha under cultivation [2]. Italy is now the world's largest exporter of cactus pear to the European Union (EU), producing about 87 thousand tons annually in Sicily, which corresponds to 96% of the total Italian harvest [3]. Cactus pear is generally consumed fresh, but is highly perishable, showing high incidence of spots and rotting after 9 days. After 20 days at ambient conditions, almost 70% of the fruit was visibly damaged [4]. By cooling prickle pears to 10 °C, shelf life increased to 6 weeks [5].
Fresh cactus pear shelf life can be increased to 32 days in modified atmosphere packaging with less than 20 kPa CO2 [6]. A cauterizer cut and seal 120 pieces of fruit per hour [7]; cuts were made close to the cactus cladode at 150 °C during harvest, destroying thorns.
After 2 months of storage, 78% of the pears were unspoiled.
The application of uniform heat treatments has been effective in controlling postharvest diseases, but can damage the treated fruit tissue if not applied carefully. Excessive heating period may damage the fruit, while insufficient heating may leave non-sterilized surface segments [8]. Further developments of cauterizer machines by Hahn [9][10][11][12] applied heat to sliced pears. After applying a constant pressure of 100 kPa at 200 °C during 30 s to harvested cactus pears increased shelf life to 2 months, controlling effectively postharvest diseases [9]. However, heat application is expensive when many fruits are cauterized. A cryo-cauterization process used a pneumatic robotic gripper to press a cactus pear against a dry-ice wall within a thermally isolated chamber. Fruit cryocauterizing at 150kPa for 15s increased shelf life to 90 days, keeping 86% of marketable fruit [11].
Automation of agriculture tasks have improved pre-harvest, harvest and post-harvest stages. Machine vision sorting of fruits presents advantages of high accuracy, precision and processing speed [13]. Non-contact detection makes grading and sorting free of mankind diseases. Fruits and vegetables produced in farms are sorted according to quality and maturity levels and decisions taken of the market it can be sent on the basis of transportation delay [14]. Post-harvesting operations require quality detection [15] and skin defects [16]. A pepper robotic harvester system [17] avoiding stem and fruit damage would be highly successful. Although food processing methods extend the shelf life of fruit and vegetable products, fresh-cut produce may lead to flavor loss, discoloration, rapid softening, and increased rate of vitamin loss [18]. Emerging smart packaging reduce losses, maintain quality, add value and extend shelf-life of agricultural produce [19]. It alerts the consumer from contamination of pathogens, pesticide residues or food degradation in food packaging products [20]. Intelligent packaging with nano sensors senses and informs the condition of the product to provide information about quality during transport, distribution, and storage [21]. This technology may also be used to detect adverse reactions in consumers after taking food such as gluten, peanuts and tree nuts [22]. Automation for the application of these sensors is considered a future close innovation. In this paper, an automatic system used for prickle pear cryo cauterization within a very small chamber was developed. The chamber size obeys to the rapid melting of dry-ice caused by environmental conditions. A minimum number of operations is required and the system efficiency evaluated to cauterize 120 pears per hour.

Mechanism for Pear Cauterization
The top-surface sliced prickly pear is moved within a container into the cryo-cauterized chamber ( Figure 1) by means of a conveyor belt. This band transports the fruits in a direction parallel to the dry-ice wall where cauterization takes place. A trapezoidal metal container resembles a porcelain coffee mug and holds the prickly pear ( Figure 2) whose flat base stops at the bottom. The container base is 3mm thick ( Figure 2) and has force sensor in between the base and the prickle pear. The system mechanism has to perform the following functions:

1.
Detect when the prickle pear is present within the metal container.

3.
Displace it against the dry ice wall.

4.
Return it and deposit it to the original band.

Electrical Automation
Once implemented the chamber with its mechanisms, the sensors for automation were obtained. The process begins when the conveyor belt is turned on, which will enter the cauterization chamber and an inductive presence sensor (SI1) will detect when it is in the desired position. This sensor will serve to turn off the band and at the same time it will feed the actuator (A1) so that its rod moves upward, initiating the rotation of the container. The rod or plunger will come out from the container base and an inductive sensor (SI2) provides a signal to limit its travel. The container flips unto a pair of rails and a capacitive sensor (SC3) detects its presence, and its signal activates the horizontal actuator (A2).
Its plunger with the electromagnet at its end pushes the fruit container towards the dry-ice wall. The force sensor (SF) starts measuring when the prickle pear touches the wall. The contact is maintained for 15 seconds and at the same time a negative voltage is applied for 5 seconds to the vertical actuator so that it returns to its initial position. Force sensor resistors (FSR) provide a resistance that is inversely proportional to the applied force and can measure grabbing and pressing. Flex Force sensors enable non-intrusive static and dynamic force measurements presenting better force linearity, hysteresis and temperature sensitivity than other thinfilm force sensors [23][24][25]. Voltage processing was done through a Wheatstone bridge having the variable flex force sensor in one arm as in the data glove [25].

Embedded System
The control system employs only digital inputs and outputs and all the outputs were connected to port 1 of the ATM8951 microcontroller ( Figure 5). Three output pins of port-1 were

Experimental Fruit Measurements
The container diameter was fixed based on the maximum Corrales and Hernandez using the same varieties [26]. Considering these values, the container was designed to be 5.5cm long with a diameter of 5.7cm.

Chamber and Mechanism Evaluation
The total weight of the container plus the prickly pear was less than 500 grams considering that the deposit weighs 285 grams.
Although smaller pistons can be employed, it was decided to work with ones already proven for use in hospitals. These actuators have motors that work at 12 or 24 VDC and handle loads of up to 750N.
The current obtained at 12 VDC was 200 mA, being half that when operating at 24 VDC. Linear rod speed was the main parameter to study since it could affect container rotation and fruit stability.
An experiment was carried out to determine the best operation at three different speeds (25mms -1 , 35mms -1 and 45mms -1 ). Fifty fruits within containers were rotated for each speed after lifting the container base with the actuator. The two quicker speeds used a spindle pitch of 4mm, while the low speed worked with a 2mm pitch. Twenty-one prickly pears (42%) felt in between the rails when the rod moved at the highest speed. As speed was reduced to 35mms -1 only 6 (12%) prickly pears came out from the container. prickly pears was of 8%, so 4 fruits were not properly cauterized.
The shape of these prickle pears did not allow them to contact the flex sensor at the container end. Therefore, no reading was obtained and the cauterization timing provided by the microcontroller was null; after a minute the fruit was still inside the chamber contacting the dry-ice wall. Only prickle pears with equatorial diameters smaller than 55mm were used in the first experiment with 1000 fruits. The processing of the first 1000 fruits took a little more than 500 minutes (4 hours and 20 minutes