Vibrational spectroscopic investigation of heat-induced changes in functional groups related to protein structural conformation in camelina seeds and their relationship to digestion in dairy cows

Abstract

The objective of this study was to use Fourier transform/infrared-attenuated total reflectance (FT/IR-ATR) molecular spectroscopy to quantify the heat-induced changes in feed protein molecular structures in relation to protein digestion in dairy cows. Camelina seeds were evaluated in this study as a model for feed protein. The seeds were either heated in air-draft oven (dry heating) or in autoclave (moist heating) at 120°C for 60 min or kept as raw (control). The parameters evaluated were Cornell net protein and carbohydrate system (CNCPS) subfractions, in situ ruminal degradation kinetics, intestinal digestibility of rumen undegraded protein (RUP) and protein molecular structures. Moist heating decreased (P < 0.05) the content of total rumen degradable (RDP) crude protein (CP) subfractions and increased the content of total RUP subfractions compared with raw seeds, indicating a significant shift at the site of protein digestion from rumen to post-ruminal tract. The decrease in RDP was mainly related to the marked decrease in rapidly solubilised (PA) and degradable (PB1) fractions, whereas the moderately degradable (PB2) and slowly degradable (PB3) fractions increased, suggesting a decrease in degradation rate of RDP. The in situ rumen incubation study revealed that moist heating decreased (P < 0.05) RDP and increased (P < 0.05) RUP and its intestinal digestibility. The molecular spectroscopy study revealed that moist heating altered protein molecular structures. Except PA and lag time, dry heating did not significantly alter any of the CNCPS CP subfraction, in situ ruminal CP degradation parameters, intestinal digestibility of RUP, and protein molecular structures. The correlation analysis showed that the heat-induced changes in protein secondary structures, α-helix-to-β-sheet ratio, were positively correlated (P < 0.05) with the contents PA (r = 0.90), PB1 (r = 0.89), RDP (r = 0.72) and intestinal digestibility (r = 0.91) of RUP, and negatively correlated (P < 0.05) with PB2 (r = –0.90), PB3 (r = –0.85) and RUP (–0.87). These results showed that compared with dry heating, moist heating significantly changed protein subfractions, rumen degradability and intestinal digestibility, and these changes were strongly associated with changes in protein molecular structures.