Changes in molecular structure and stability of beta-lactoglobulin induced by heating with sugar beet pectin in the dry-state

Authors: Qi, Phoebe; Wickham, Edward

Submitted to: Food Hydrocolloids
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2020
Publication Date: 2/28/2020
Citation: Qi, P.X., Wickham, E.D. 2020. Changes in molecular structure and stability of beta-lactoglobulin induced by heating with sugar beet pectin in the dry-state. Food Hydrocolloids. 105:1-11. https://doi.org/10.1016/j.foodhyd.2020.105809.
DOI: https://doi.org/10.1016/j.foodhyd.2020.105809

Interpretive Summary: Whey protein isolate (WPI), a mixture of several whey proteins that includes beta-lactoglobulin (beta-LG, about 65% of the total whey protein) and alpha-lactalbumin (alpha-LA, about 25% of the total whey protein), is used in dietary supplements and appears as the main ingredient in various food and beverage formulations. It is soluble in water but can become insoluble and unstable when subjected to treatments such as ultra-pasteurization. Sugar beet pectin (SBP), found in sugar beet pulp and a by-product of sugar manufacturing, is one of the most versatile yet underused dietary fibers. In order to prevent WPI from becoming unstable in water, we combined and heated it with SBP, but the combination became unstable when doing the same for beta-LG with SBP, which was unexpected. Studies at the molecular level showed that the structure of beta-LG was transformed by SBP and that other proteins from WPI must contribute to its stability. This study will guide us in our efforts to develop new interacting systems containing whey proteins and SBP and perhaps other dietary fibers to improve the quality of food and beverage formulations.

Technical Abstract: We have shown previously that dry-heating whey protein isolate (WPI) and sugar beet pectin (SBP) led to a noticeable improvement in the physical, chemical, and emulsion stability of WPI. To discern the role that beta-lactoglobulin (beta-LG), the major protein of WPI, played at the molecular level in the observed functional properties, we investigated, in detail, the changes that occurred in the molecular structure and thermal stability of beta-LG as a result of heating with SBP in the dry state. Far-UV CD spectroscopy analysis revealed that nearly all the secondary structural elements of beta-LG were affected slightly by dry-heating alone. The presence of varying mass ratios of SBP, beta-LG:SBP = 1:0, 5:1, 3:1, 2:1, 1:1, and 0:1 did not significantly cause an adverse effect on the structural integrity and thermal stability of beta-LG. The tertiary structure of beta-LG, however, experienced considerable disruption as probed by near-UV CD and Trp intrinsic fluorescence spectroscopy. Despite the non-radiative nature of the FRET complexes formed between beta-LG and SBP, a donor-acceptor relationship was established by the detection of fluorescence lifetimes corresponding to their distinctive excited states. Unlike the case of dry-heated WPI and SBP, the protection of the tertiary structure of beta-LG by reacting with SBP was mostly ineffective. Such deficiency suggested that the existence and collaboration of other proteins in WPI are essential in developing the interacting complexes and conjugates with the remarkable stability as determined.