Stabilization of whey protein isolate (WPI) through interactions with sugar beet pectin (SBP) induced by controlled dry-heating

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It has been shown previously that reacting whey protein isolate (WPI) with sugar beet pectin (SBP) through a Maillard type reaction in a dry-state led to noticeable improvement in the physical, chemical and functional properties of the end products. Studies on the changes in the molecular structure of whey protein as a result of the reaction were carried out in this work to provide a molecular understanding of these observed improvements. The effect of interacting and conjugating with SBP on the apparent secondary structure of WPI was examined by far-UV circular dichroism (CD) spectroscopy and followed by spectral deconvolution. Results showed that the structure of WPI, especially the antiparallel β-sheet element, was preserved by reacting with SBP at the weight ratios of WPI:SBP = 3:1, 2:1, and 1:1. The analysis of difference UV-VIS spectra indicated significant changes occurred in the molecular electronic states of both WPI and the feruloyl moieties of SBP. Initial Maillard reaction products were detected in the form of the FRET complexes with WPI as the donor, and SBP, the acceptor by the studies of the steady-state fluorescence and the lifetime of the excited states. The tertiary structural contacts in WPI, probed by the intrinsic Trp fluorescence, were disrupted significantly upon reacting with SBP at an equal ratio (1:1). The thermal stability of WPI, however, at both secondary and tertiary structural levels, was greatly improved by the dry-heating induced reaction with SBP at all levels. The thermal stability of the feruloyl groups of SBP was also enhanced by reacting with WPI. Furthermore, the fluorescence lifetime studies definitively identified the distinctive advanced Maillard reaction end products (AMREPs) formed between WPI and SBP at varying ratios.