Fig. 1.

a FTIR spectra, and b second derivative of the spectra of the collagen from chicken MSM residue extracted using a 2² factorial design with a central point. T1: 30 min of pre-treatment with ultrasound and 24 h of enzymatic extraction; T2: 30 min of pre-treatment with ultrasound and 48 h of enzymatic extraction; T3: 48 h of enzymatic extraction; T4: 24 h of enzymatic extraction; PC: 15 min of pre-treatment with ultrasound and 36 h of enzymatic extraction