Composition of sweet and bitter lupin seed hulls with observations on the apparent digestibility of sweet lupin seed hulls by young rats

Seed hulls from both sweet and bitter varieties of lupins (Lupinus angustifolius) have been analysed for constituent polysaccharides. The hulls contained 15 to 32% oxalate‐soluble pectic polysaccharide, excluding pentose, 12 to 14% hemi‐cellulose, 44 to 51% cellulose, 0.3 to 0.4% lignin, 2.7 to 3.8% protein and 2% ash. “Crude fibre” was 54.9%. In agreement with the low lignin values all of the hemicellulose was extractable with alkali without prior delignification and 70 to 80% of the polysaccharide in depectinated hulls was hydrolysed by cellulase plus hemicellulase, likewise without delignification. Sweet lupin seed hulls, added to a complete basal diet up to 40%, did not depress growth of young rats significantly; compensatory increase in food consumption occurred. With 60% hulls in the diet growth and food efficiency were reduced. Ground hulls did depress food and protein digestibility in direct proportion to their amount in the diet, possibly because of physical entrapment in the excessive bulk in the gut. The digestibility of the “crude fibre” of the hulls was 33% when the diet contained 20% hulls; this decreased to 22% when the content of hulls in the diet was increased to 60%.