The protein value of differently processed rapeseed solvent meal and cake assessed by in vitro methods and in tests with rats

The composition, solubility of protein in sodium borate and in 0.5% KOH, protein dispersibility index (PDI), and protein value for rats were determined in eight different rapeseed meals (RSM) and six press cakes. The rapeseed meals were toasted in an industrial process for 49 or 55 min at a pressure of 0.20, 0.23, 0.26, and 0.33 MPa steam; the press cakes were unheated or heated in a laboratory oven for 45 min at 90, 110, 120, 130 or 140 °C. Prolongation of toasting the meals resulted in a small increase of neutral detergent fibre (NDF), acid detergent fibre (ADF) and NDF-bound protein contents, decrease of all protein solubility measurements, and decrease of biological value but not protein digestibility. The effects of increasing steam pressure and resulting small rise of temperature in the toasting–desolventizing unit were negligible. Increasing the heating temperature of cakes from 90 to 140 °C decreased protein solubility, protein digestibility and biological value. The correlation between in vitro and in vivo measurements was closer in cakes than in meals. Meal protein solubility of 55–60% in 0.5% KOH is proposed as indicative of properly processed meals of a high nutritional value, 45–55% of mildly overprocessed meals, and below 45% of overprocessed meals with a low nutritional value for monogastric animals.