Quantification of the electrostatic and covalent interaction between whey proteins and low methoxyl pectin using PFG‐NMR diffusometry

Abstract

As interaction between proteins and polysaccharides is expected to change the average diffusion coefficient and the distribution width, it is believed that NMR diffusometry can be a useful tool to study and quantify the interaction between whey protein isolate (WPI) and low methoxyl pectin (LMP). For this purpose, the influence of pH, dry‐heat‐treatment duration, and WPI to LMP ratio on WPI and LMP interaction was evaluated using pulsed field gradient NMR. At pH 5.0 and 5.5, approximately 40–50% of the WPI present in the mixture interacted with LMP through electrostatic interaction. Hence, it is important to measure the degree of covalent interaction at a pH where electrostatic interaction between WPI and LMP is limited. The diffusion coefficient of WPI gradually decreased as the mixtures of WPI and LMP were incubated for 1, 2, 8, and 16 days, which was accompanied by an increase in the distribution width. With regard to the LMP concentration, the higher the LMP concentration, the higher the amount of WPI bound to LMP and the smaller the diffusion coefficient of WPI. Overall, the results indicate that NMR diffusometry enables the quantification of the interaction in biopolymer mixtures. Whereas the results showed a similar trend as that obtained via chemical analysis, pulsed field gradient NMR does not require any sample pretreatment.

Whey protein-polysaccharide conjugates obtained via dry heat treatment to improve the heat stability of whey protein stabilized emulsions

“…Previously, the formation of WPI-LMP conjugates was found to be rapid during the first two days of dry heat treatment and continuously increased at a slower rate upon extending the dry heat treatment to 16 days at 60 o C (Setiowati et al, 2018;Setiowati et al, 2016). In a recent study, 3 days of dry heat treatment was found to be the optimum duration for conjugation of α-lactalbumin and βlactoglobulin with polysaccharides (Mulcahy et al, 2016).…”

“…Similarly, emulsions stabilized by WPI-pectin conjugates were reported to be able to withstand heating at 80 and 120 o C in the presence and absence of 30-50 mM NaCl (Mulcahy et al, 2016;Setiowati et al, 2017). Furthermore, despite of the fact that WPI-pectin conjugates which were dry heat treated for only 1 day had a lower yield than those dry heat treated for 16 days, the heat stability of these two conjugates was comparable (Setiowati et al, 2017;Setiowati et al, 2018). Thus, the heat stabilizing activity of the conjugates is not influenced by the yield or degree of interaction to a large extent.…”

“…This indicated that the emulsifying activity of the WPI-LMP conjugates might be related to the yield of the conjugation reaction, as longer dry heat treatment and higher pectin concentration led to a higher conjugation yield (Setiowati et al, 2018;Wefers et al, 2018). Conjugates with a higher yield of conjugation (higher degree of interaction) resulted in a higher emulsifying activity.…”

“…Dry heat treatment of the dry heat treatment (Wefers et al, 2018). As conjugation largely affected the diffusion behavior of the proteins, but hardly changed the diffusivity of the pectin, it follows that complexes typically involve only 1 pectin molecule, chemically bound to either 1 or more protein molecules (Setiowati et al, 2018). No additional chemicals are needed, making this method safe for food applications (Damodaran, 2005;Kato, 2002).…”

“…WPI and pectin induces the formation of complexes, as can be observed from the reduced amount of free amino groups, the emergence of high molecular weight components in SDS-PAGE and the decreased mobility of the whey proteins as measured by diffusion-NMR (Jiménez-Castaño,Setiowati et al, 2018;Setiowati, Vermeir, Martins, De Meulenaer, & Van der Meeren, 2016;Wefers, Bindereif, Karbstein, & van der Schaaf, 2018). These conjugates are formed by covalent attachment of the polysaccharides to WPI through Maillard type reactions during…”