Effect of different binders and encapsulation techniques in the structure and functional properties of microdiets for fish larvae

Abstract

The improvement of micro diets, for fish larvae, has been a major challenge in past years; however some problems remain without a resolution. Besides its tremendous growth rates, at early developmental stages, fish larvae present an immature digestive system and a progressive metamorphic process that suggest special nutritional requirements. It is therefore essential to provide fish larvae with easy and/or quickly metabolized protein sources, like protein hydrolisates, high bioavailable forms of vitamins and trace minerals. The low molecular weight of such compounds, in association to the high surface area of the micro-particles, makes them highly soluble and prone to water leaching losses. These nutrient losses, not only makes these nutrients unavailable to the larvae, but originate an important increase of organic matter in the rearing tanks, leading to detrimental water quality criteria. The development of high-performing micro-particulate diets for fish larvae requires a delicate balance between an adequate quantitative and qualitative nutritional supply and good water stability of the feeds, without compromising the palatability and the digestive breakdown. A series of binders, protein-based (e.g. fish gelatin, wheat gluten, spray-dried plasma) and carbohydrate-based (gums, alginates, pectins, starches, dextrin, polyols) were incorporated at graded levels (1, 2.5 and 5%) in a standard formulation for seabream larvae. Diets were manufactured by a proprietary low-shear extrusion process and afterwards processed to obtain a 200-400 micron fraction. All diets were analyzed for the following criteria: water leaching of nitrogenous compounds, turbidimetry and particle water stability. An additionally approach relied on the use of various encapsulation techniques (spray-drying, fluid-bed drying and vacuum coating) to incorporate trace minerals (Zn and Mn) and amino acids (taurine, lysine and methionine). Technological results show that the beneficial effects towards nutrient leaching and physical stability of the micro-pellets after immersion in water are highly dependent on binder type and less markedly on binder dose. Through the use of selected binders (e.g. guar gum and gelatin) in association to a spray-drying encapsulation process, we were able to significantly alter the release profile of Zn, Mn and taurine. Some of these prototypes were also tested in vivo with gilthead seabream larvae and clearly suggest that microencapsulation technologies that allow a controlled release profile of nutrients result in beneficial effects to larval growth and quality criteria. The results of several specific scenarios will be presented.