A rapid analytical and quantitative evaluation of formaldehyde in squid based on Tri-step IR and partial least squares (PLS)

•Tri-step IR was developed for qualitative and quantitative profiling of squids.•Formaldehyde concentration in squid soaked water play a role in formaldehyde in squid.•Spectral curve-fitting was applied to further disclose the variation rule of protein secondary structure.•Tri-step IR combined with PLS can accurately predict formaldehyde concentration in squids.

Formaldehyde abuse for retaining squid freshness is detrimental to public health. The aim is to establish a rapid and quantitative detection method of formaldehyde in squid for screening massive samples. A linear relationship between formaldehyde concentration in squid and formaldehyde concentration in squid soaked water was observed using HPLC. Chemical profile variances of squids were rapidly analyzed by Tri-step infrared spectroscopy. Specifically, with increasing formaldehyde concentration, peak intensities of 2927 cm−1 (vas(CH2)), 1081 cm−1 (glycogen), 1631 cm−1 (β-sheet proteins) decreased while 1655 cm−1 (α-helix proteins) increased. Spectral curve-fitting results further disclosed that β-sheet proteins were transformed to α-helix and β-turn conformations. Furthermore, a quantitative prediction model based on IR spectra was established by PLS (R2, 0.9787; RMSEC, 5.51). The developed method was applicable for rapid (c.a. 5 min) and quantitative analysis of formaldehyde in squids with LOD of 15 mg/kg.