Leaching is the uptake of water by salmonid feed pellets leading to their physical breakdown. The result is a dilution of nutrients on a dry matter basis. Early diets for salmon were often moist, containing 70% dry matter & 30% water. These diets were highly palatable, but prone to great nutrient loss. The manufacture of pelleted diets for fish was a response to the need to effectively deliver nutrient to the fish by improving pellet stability. Steam-pelleted diets, and later extruded diets, increased the dry matter content of the diets to roughly 90%. Extruded diets are principally used today and can be manufactured to float or sink including varying sink rates.
Floating feed is another form of extruded pellet but undergoes a different process. Think of making floating feed as a ‘popcorn effect’. The mash (diet ingredients) is cooked inside the extruder (machine) and forced through the die play at a higher pressure causing rapid expansion on the other side of the due plate causing it to expand creating air bubbles inside the pellet and lowering its density below that of the water and Archimedes principle takes over. Pelagic feeders such as salmonid fish usually consume food soon after it’s been delivered to the water. Demersal diet feeders, such as sturgeon or halibut, are much more prone to consuming leached feeds because of their slower, more deliberate feed response.
Experiment 1: Comparison of pellet stability for steam, extruded and moist diets
Preparation of pellets- The ingredients were the same for all test pellets, however the moist diet contained an increased ratio of moisture. Steam pellets were processed by forcing the mash through a conditioning chamber mixed with steam, the pellets are then forced through set size holes to determine the pellet diameter and cut based on desired pellet length. Extruded pellets were processed under high heat and steam pressure where the starches are gelatinized. As the pellets exit the barrel, trapped steam was released resulting in the expansion of the pellets.
- Three replicates of 2-5g of feed for each treatment is added to a nylon bag
- Samples are immersed 20.1°C fresh water at the same
- At 0, 5, 20, 40, & 60 minutes, samples are removed and placed in the drying oven (60˚C) for 24- hours to obtain a dry weight
- Percent water stability was calculated:
% Water Stability = [Dry Matter Loss / Initial Weight] * 100
where Dry Matter Loss = Initial Weight – Dry Residue Weight
Experiment 2: Comparison of nitrogen loss in floating versus sinking pellets
Preparation of pellets- Floating and sinking pellets both underwent the extrusion process. The Guaranteed Analysis of Crude Protein= 43%
- 1.5g of Atlantic salmon (A) floating and (B) sinking pellets were placed into separate containers with 1L of 20.1 °C freshwater
- At time 0, 10, 20, 30 and 60 minutes, a sample is drawn to measure Total Ammonia Nitrogen
- The colourmetric reaction of sodium salicylate method (LaMotte kit) was used to determine Total Ammonia Nitrogen (mg/L).