Fourier transform infrared (FTIR) spectra were measured using
a spectrometer (Bruker FT-IR Vertex 70, USA) and analyzed using
PE-GRAMS/32 1600 software, as described by Liao et al. [31].
Briefly, 0.5 mg of the dry commercial bromelain and purified, cryoprotected
bromelain were mixed with 300 mg ground potassium
bromide and compressed to get a pellet. The spectra were
smoothed with a 15-point Savitsky–Golay function to remove
any possible white-noise. The baseline of the spectrum in the
amide I region was leveled and zeroed, then the spectrum of the sample was normalized for area in the region and the intensity
of a-helical and b-sheet bands was recorded.