FTIR fingerprinting of structural changes of milk proteins induced by heat treatment, deamidation and dephosphorylation

Changes in protein structure and interactions during processing and storage of UHT milk affect its shelf-life. In our previous study, we demonstrated that changes in FTIR spectra correlate well with the development of a sediment (a storage instability) during storage. However, due to a number of possible changes during heating and storage, conformational changes specific to a particular interaction among milk proteins remain largely unknown. The aim of the current study was to evaluate the possibility of fingerprinting two selected changes, i.e., deamidation and dephosphorylation, using FTIR. Enzymatic deamidation and dephosphorylation were carried out prior to heat treatment. Principal component analysis revealed that heat treatment induced different changes in their secondary structure of control, deamidated and dephosphorylated milk samples. In contrast to a significant (P < 0.05) decrease in β-sheet (1624 cm⁻¹) with rise in β-turns (1674 cm⁻¹) in heated control samples, both deamidation and dephosphorylation of skim milk before heat treatment created more ordered secondary structure, significant (P < 0.05) increase in α-helix (1650-52 cm⁻¹) and β-sheet in milk proteins at expense of 3₁₀-helix (1661 cm⁻¹), random (1645-46 cm⁻¹) and β-turn (1674 cm⁻¹). The only difference between heated deamidated and dephosphorylated samples, when compared to unheated samples, was decrease in large loops (1656 cm⁻¹) in the latter opposed to the increase in the former. The outcomes from this study can be applied for different interactions and help to arrive to a complete understanding of the conformational changes affecting the storage stability of UHT milk.