Evaluation of the digestible value of lupin and soybean protein concentrates and isolates when fed to rainbow trout, Oncorhynchus mykiss, using either stripping or settlement faecal collection methods

A series of prototype protein concentrate and isolate products were prepared from the kernel meals of Lupinus angustifolius (cv. Gungarru) and Lupinus luteus (cv. Wodjil). The apparent digestible value of these value-added meals, the original kernel meals and a range of similar soybean based products were compared when fed to rainbow trout. Both faecal stripping and settlement collection methods were used to allow a comparison of the effects of these collection methods on the determination of digestible energy and nutrient values of the component ingredients being tested. Significant improvements in most digestible parameters were observed with increasing levels of processing of both lupin varieties and soybean meal. The largest relative increase in digestibilities of organic matter, energy and protein were seen between the kernel meals and protein concentrates. Improvements with further protein isolation, from concentrate products to isolate products were limited. Phosphorus digestibilities of all lupin products were very high and in contrast to the other nutrient digestibilities diminished with increasing levels of lupin processing. Significant effects on faecal integrity were also noted among the grain products. Significant differences were observed on the digestibility of component ingredients between the two faecal collection methods with the faecal stripping collection method being the more conservative of the two assessments. These differences appear to be significantly related to the effects of ingredient and therefore diet, carbohydrate content on the diet digestion and faecal collection processes.