Design and experiment of crab legs' shell and meat separation mechanism

Abstract: River crab is a kind of conventional tasty aquatic product with high value. Now in China there is still a high demand and price for fresh and live crab with bigger size in market, while a low demand and price for the ones with smaller size. Therefore, deep processing of small size crab and producing meat into shelf-stable crab powder as middle food material are considered. In this way not only the utilization of low valued crab is promoted, but also the overall economic return of crab farming is greatly upgraded. In this process, separating the meat and shell of river crab is one of the most important steps. However, in previous studies of mechanical peeling method, the separation result of shell and meat of crab leg was not satisfactory. In allusion to the problems of high labor intensity of shell and meat separation for river crab legs, low efficiency and easy meat contamination, a shell meat separation device of roller extrusion type is designed in the study. The device includes workbench transmission device, feeder and roller extrusion device. The main principle of the device is carrying crab legs by the slanting slide to the tip of a pair of rollers, which have the same diameter and rotating speed but in opposite directions, and then separating crab leg's shell and meat through squeezing action. Transmission device consists of variable frequency motor, reduction gearbox, and so on. Feeder included slide for carrying, studdle and vibrator. The front end of slide for carrying heads inclines to a pair of rollers and the down end is connected with the vibrator which is supported by flexible studdle. Roller extrusion device consists of a pair of rollers and the plastic scrape above. Before operation, the pretreated crab legs are put into downward slant slide, and then crab legs are moving forward along the transportation way under the vibration of the machine into the tip of rollers to receive the backward extrusion. Treated by the upper roller, meat of crab legs are squeezed out and adhesive to the upper roller, and scraped down by plastic scraper blade. Legs' shell falls down along plastic sliding plate. Thus the separation of crab legs and meat is done. To determine the optimum working parameters of the separation device, the separation efficiency, separation rate and meat quality are chosen as the performance indices, the feeding angle, roller clearance and rotating speed are chosen as the influence factors, and orthogonal test of prototype are carried out, in which the feeding angle is 30°-50°, the roller clearance is 0.8-1.2 mm, and the rotating speed is at 25-35 r/min. The experimental subject is healthy and strong crabs with a body weight of about 100 g. The washed crabs are put into the steamer for steaming. All crabs are teamed for 10 min and taken out from the steamer, and then the crab legs are taken down for separation experiment. With the above optimum parameters of the crab legs' shell and meat separation device, the separation efficiency is over 16.9 kg/h, the separation rate (meat gaining rate) is between 92% and 99%, and the corresponding meat quality grade is 7.5-8.8, which show that the device has achieved a better separation efficiency, and realized the mechanical separation of crab legs' shell and meat. The above experimental result also shows that the rotating speed dominates the separation efficiency while the feeding angle and roller clearance affect little; the roller clearance is the very factor affecting the separation rate, followed by the factor of feeding angle, and the rotating speed affects little; the roller clearance is the very significant factor affecting the meat quality while the feeding angle and rotating speed affect little. The experiments present the following result: the optimum operation parameters are the 40° feeding angle, the 35 r/min rotating speed and the 1.2 mm roller clearance. The study provides a reference to the further device design and the perfection of crab legs' shell and meat separation mechanism.