Method for determining the physical stability of shrimp feeds in water

Three methods for measuring the water stability and leaching characteristics of shrimp pelleted feeds were developed and tested: static water method, horizontal shaking method and vertical shaking method. Each method was tested using a commercial and experimental pelleted shrimp feed. Both feeds had a pellet size of 2.4 mm x 5.0 mm with similar proximate composition (35% protein and 9% fat). Each test run consisted of leaching c. 2 g of feed at six intervals of immersion time (0, 30, 60, 120, 240, 360 min), two concentrations of salinity (0, 34 ppt), and three different water temperatures (15, 25, 35 degrees C). A refrigerated circulating water bath was used to maintain the desired level of water temperature. For the static water and horizontal shaking methods, a Buchner filtration apparatus with Whatman filter paper no. 3 (5 mu) was used to separate the remaining pellets from leached water. The vertical shaking method (VanKel) utilized a perforated stainless-steel basket (381 mu) for leaching and serving as a filter to separate pellets from leached water. For all three methods, the leached feed and original feed samples were dried in a convection oven at 105 degrees C for 24 h and then cooled in a desiccator. Dried feed samples were weighed and analysed for dry matter retention. Pellet stability was calculated as the ratio of dry matter retention after leaching and dry matter of original samples expressed as a percentage. Results indicated that any method for pellet stability analysis should take into account water temperature and salinity in addition to actual pellet agitation rate to obtain accurate and consistent measurement of pellet disintegration and nutrient leaching. Dry matter retention was different for each shrimp feed with experimental feed (72.8%) showing lower retention than commercial feed (88%) after 6 h of VanKel leaching under normal shrimp culture conditions. The three methods provided reproducible results with both horizontal and vertical shaking methods recommended for routine laboratory analysis because they could provide different degrees of pellet agitation that simulate actual indoor and outdoor shrimp culture conditions.