Effect of processing and microbial enzyme treatment. on the nutritive value of canola meal for rainbow trout. (Onchorynchus mykiss).

Abstract

Salmonids (trout, salmon and charr) require high dietary concentrations of protein, and they utilize protein and lipids as their principle sources of dietary energy. In culture conditions the protein is mainly supplied by fish meal and other products from the capture fishery. However, the high cost, and variability in fish meal quality due to processing and storage conditions, dissimilar raw material quality, as well as fluctuations in the supply of fish meal are major concerns. Feed accounts for 40% to 60% of the operating costs of salmon fanning. Much of this cost is associated with the inclusion of fish meal as the predominant protein source. Replacement of at least some of this fish meal by cheaper plant protein products is one way of reducing these costs. This study was undertaken to assess the chemical composition and potential nutritive value (digestibility) of canola protein products developed by processing canola meal by substituting fish meal in salmonid diets, with rainbow trout (<i>Oncorhynchus mykiss</i>) in fresh water as the test animal. Sieving, methanol/ammonia treatment and the use of three types of enzyme (Phytase, SP-249, Alpha-Gal) treatments, singly or in various combinations, were employed in this study. In addition, a commercially-produced canola protein isolate (CPI) was also evaluated for potential nutritive value. It was hoped that one of these products would be potentially suitable as a replacement for fish meal. The results indicated that sieving canola meal did not have any significant influence on its chemical composition. Treatment of canola meal with methanol/ammonia however, decreased the amount of total glucosinolates by over 80%, and lowered the levels of phenolic compounds but this protocol increased the concentration of phytate. Application of carbohydrases did not result in any appreciable improvement in the fibre composition of the meals, although it did lead to increases in the protein content and levels of amino acids, and to decreases in the levels of glucosinolates and phytate. Low levels of glucosinolates were detected in all the canola protein products that received the enzyme treatments either singly or in combination. The processing methods used in the production of canola protein isolate (CPI) significantly improved the nutrient composition of the resultant product. Levels of total glucosinolates, protein, gross energy and phytate of the canola protein isolate (CPI) were 1.95 μmol/g, 90.8% , 24.4 MJ/kg and 4.3 μmol/g, respectively compared to 8.95 μmol/g, 36.3%, 19.6 MJ/kg and 45.5 umol/g, of commercial canola meal. In a three-week digestibility experiment, the effect of processing canola meal on the digestibility of dry matter, protein and energy by 74 g rainbow trout was assessed using a modified "Guelph system" of faecal collection. The canola products were included at 30% of the diets (70 % reference: 30% test canola protein product). Each of the test diet treatment was assigned to triplicate groups of fish using a completely randomized block design with chromic oxide (0.5%) as an indigestible marker. The fish were hand fed to satiation twice daily in special designed 150 L fibre glass tanks supplied with fresh running water (9.9°C to 11.0°C) at a flow rate of 4-5 L/min. Processing methods had variable effects on the apparent nutrient (dry matter, protein and energy) digestibility coefficients. All the laboratory processing protocols employed with a view to enhance the nutritional value of canola meal had significant negative effects on dry matter digestibility coefficients and generally negligible effects on protein digestibility. There were however, significant reductions in protein digestibility coefficients in the meals treated with SP- 249 (77.4%) alone or in combination with Alpha-Gal (79.5%) relative to untreated canola meal (88.1%). Energy digestibility coefficients and the digestible energy contents of the test canola protein products were the least affected by the processing methods selected for this study. However, treatment of canola meal with either methanol/ammonia or combination of carbohydrate degrading enzymes had negative effects on the energy digestibility coefficients and digestible energy contents. For instance, these two parameters were lowered by about 14 percentage units and 3 Ml/kg in the ammoniated canola meal and by about 17 percentage units and 2.7 MJ/kg for the meal treated with the two carbohydrate-degrading enzymes. Digestibility coefficients for dry matter, protein and energy for the canola protein isolate (CPI) were 77.1%, 97.6%, and 84.7 %, respectively, and it is noteworthy that all these values were much higher than those values found for the untreated and treated canola meals Fish growth performance and feed utilization was not significantly affected by either the type of processing treatment employed or the kind of canola product included in the diet. However, fish fed the diets containing canola protein isolate or those fed the diet containing the alpha galactosidase-treated canola meal showed non-significant improvements in weight gain, specific growth rate and relative growth. Mean daily dry feed intakes per fish (DFI) were higher for fish receiving the diets containing the laboratory processed canola meals than for those consuming diets containing the canola protein isolate. The effect of processing treatment tended to (decrease utilization) increase the feed to gain ratio (FGR) of rainbow trout fed diets containing the test canola protein products. Fish consuming the diet containing the canola protein isolate, however, exhibited improved FGR value (1.22) relative to those fed the herring meal control diet (1.45). In general, the canola protein isolate (CPI) had a higher content of available (digestible) protein and energy for rainbow trout than fish meal and the laboratory processed canola meals. This advantage was also translated into improvement in fish performance. While the processing methods employed resulted in some improvements in the chemical composition and nutrient digestibility of canola protein products, this did not bring about any enhancement of growth and feed utilization of rainbow trout. However, it must be emphasized that the diets employed in the digestibility study were not balanced with respect to nutrient and energy content. Hence, valid conclusions regarding fish performance (growth) can not be drawn from these results. On the basis of the digestibility assessment conducted in the present study, it is concluded that some canola protein sources (canola protein isolate) may have potential as substitutes for fish meal in salmonid diets. Also it can be concluded that careful processing of canola meals or seed with a view to reducing or eliminating the antinutritional factors present in the meal or seeds can result in protein products that can be used to potentially replace fish meal in salmonid diets without compromising fish performance. It was also found that although laboratory processing improved the chemical composition of canola meal, it did not have any significant effect in nutrient (dry matter, protein and energy) utilization or fish performance.