Evaluating protein digestibility of biorefined forage crops in growing pigs: effects of plant species and precipitation methods

Biorefined forage proteins show potential as protein alternatives for monogastric animals. Achieving high protein quality is central for the effective integration of alternative protein sources in food production systems. However, there is a limited understanding of how plant species and processing parameters influence the quality of the extracted proteins. The aim of the study was to evaluate the ileal digestibility of CP and amino acids (AA) in biorefined red clover, festulolium and lucerne precipitated at 85 °C. For lucerne, the study additionally aimed to determine the effect of alternative precipitation methods (60 °C, a two-step heating method (55 + 85 °C), fermentation, and centrifugation prior to 85 °C heat treatment). It was hypothesised that biorefined lucerne would have superior protein digestibility compared to red clover and festulolium. Furthermore, while CP content could be increased through further processing, such enhancement would reduce digestibility. Protein was extracted from the freshly harvested biomass through biorefining. The standardised ileal digestibility (SID) of CP and AA was evaluated in an incomplete Latin square design with eight male castrated pigs (initial weight 30.7 kg) fitted with a T-cannula in the terminal ileum. Endogenous losses of CP and AA were estimated by feeding an N-free diet. Results demonstrated that protein-rich concentrates with CP contents ranging between 488 and 628 g/kg DM could be extracted from the forage crops, with the composition significantly influenced by the choice of plant species and the applied production methods. Standardised ileal digestibility of CP was significantly lower in protein extracted from red clover (69.3%) compared to lucerne (79.6%) and festulolium (77.0%, P = 0.0013). Lucerne protein precipitated at 85 °C exhibited the numerically highest CP SID and alternative precipitation did not improve the SID of CP and AA. Protein extracted from lucerne and festulolium using precipitation at 85 °C had a high content of digestible essential AA, despite lower overall CP SID in the latter. In conclusion, high-protein concentrates can be produced through biorefining of forage crops, particularly from lucerne and festulolium. The protein quality and composition are significantly affected by the chosen precipitation method, with 85 °C heat precipitation showing the best results in terms of digestibility. Festulolium and lucerne emerge as promising protein alternatives due to their high digestible AA content.