Use of extruded products from the exotic species Plecostomus punctatus. Part 2: Extrusion conditions

Abstract

In Part 1, it was established that to solve the problem that is presently going on at the Lic. Adolfo López-Mateos dam, known as “El Infiernillo”, in Michoacán, Mexico, created by the involuntary introduction of Plecostomus punctatus, known in the area as “Devil Fish”, that has proliferated and dominated on the existing fisheries, this research suggests its use as protein source for extruded feedstuffs for other species with a higher commercial value. The objective was to obtain an extruded balanced feedstuff based on Plecostomus punctatus meal. A laboratory scale extruder made in Mexico and an imported laboratory scale extruder were started and put into operation to obtain the desired feedstuffs considering as protein source a commercial fish meal to evaluate its performance. Once the best processing conditions were established, Plecostomus punctatus meal was introduced in the selected diet suggested in the literature for tilapia (Oreochromis spp.) cultivated in aquaculture. A modification of the diet composition was done introducing other carbohydrates (wheat flour instead of wheat “afrechillo” and wheat bran instead of rice bran). Extruders operating conditions were modified to obtain low density pellets, that could float in a water column, just as the tilapia commercial feedstuffs used as controls. A 2³ experimental design for the three main variables (temperature, initial moisture content, and extruder screw velocity) was set, considering the best results obtained in preliminary experiments, assigning a negative value (-) to the lowest range and a positive one (+) to the highest range. For these experiments, feeding screw velocity was considered constant. Conclusions derived from this research were: An acceptable fillet contents was found in big specimens. However, the effort to extract it from the animals is important, and might affect the economic and technical feasibility of the process. If the fish is dried, its manual separation is much easier. There is an initiative to exploit “Devil Fish” fillets for human consumption, and considering that this project were viable, its residues would be adequate as part of the proposed diet, since its protein contents is 27%. The nutritional quality of the dehydrated fillet as well as the meal given by the methodology used make its unit operations satisfactory, since very low mass losses were found. Dried fillets might be used as “surimi” or to be directly sold as dried fish, such as Norway cod but to a much lower price. Particle size of the different meals, fish and carbohydrates, play a very important role in the feedstuffs physical characteristics before and after extrusion. Thus, all raw materials were previously pretreated to comply with particle size specifications. Grinding and sieving unit operations are very important for industrial scaling-up and should be optimized to reduce costs. Based on bromatological analyses, “Devil Fish” integral meal has the necessary protein contents for tilapia extruded feedstuffs. Carbohydrates needed in the feed mixture to obtain good physical characteristics (hydrostability, matter losses, and expansion percentage) were 28%, according to this research. It also complies with its protein requirements. Low cost Mexican extruder as it was originally built, even with some modifications carried out during these experiments has not the desirable characteristics for low density feedstuffs production required for tilapia. If used for shrimp or langoustine feedstuffs that should be denser is ideal. Results obtained in the Wenger X-5 extruder for the experimental design 2³ indicate that temperature and initial moisture contents should be around 130°C and 40%, respectively. To obtain this temperature, steam at 600 kPa (6.0 kg_f/cm²) is required. The best operating conditions found with Plecostomus punctatus meal to get the same hydrostability as the commercial feedstuff and a very good value for pellets expansion index, 2.1, at the extruder die exit, with the minimum matter losses, were 130°C at the extrusion zone, 40% initial moisture contents, 850 rpm for the extruder screw, and 25 rpm for the feeding screw. For future tests, as well as for scaling-up, the following recommendations were drawn: Steam should have a minimum pressure of 588.40 kPa (6 kg_f/cm²) entering the Wenger extruder. If lower pressure steam, for example, below 539.37 kPa (5.5 kg_f/cm²) enters, temperature drops and hydrostability characteristics (floatation) are also altered. As the two Wenger X-5 extruders used in these experiments were located in different institutions this variable could not be controlled (increase of boiler pressure). As “Devil Fish” meal is the main raw material in the process, it is necessary to optimize the drying or dehydration unit operation. It is recommended to open a research line to design and operate equipments for this process, taking advantage of climatic conditions of the zone (dry-tropic), with solar-gas aerothermal driers, for example. As wheat and soy are not products grown in the “El Infiernillo” dam agricultural area, new substitutes for the tilapia diet should be tested considering byproducts of the region, such as sorghum. Once the desired pellets^[1] are obtained new economical analysis should be carried out to minimize costs. Finally, grinding and sieving operations should be optimized with the new materials available in the zone, such as sorghum, to minimize production costs.

[1] Derived from Latin “pila”, soft matter ball, such as snow, clay, etc., easily malleable