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Abstract
N-ethylmaleimide (NEM) was used to verify that no new disulfide crosslinks were formed during the fascinating rheology of the alkali cold-gelation of whey proteins, which show Sol-Gel-Sol transitions with time at pH > 11.5. These dynamic transitions involve the formation and subsequent destruction of non-covalent interactions between soluble whey aggregates. Therefore, incubation of aggregates with NEM was expected not to affect much the rheology. Experiments show that very little additions of NEM, such as 0.5 mol per mol of protein, delayed and significantly strengthened the metastable gels formed. Interactions between whey protein aggregates were surprisingly enhanced during incubation with NEM as inferred from oscillatory rheometry at different protein concentrations, dynamic swelling, Trp fluorescence and SDS-PAGE measurements.
Highlights
Whey protein solutions can form stable soluble aggregates [1], for example by heating at low protein concentration [2,3], which can be used to thicken or gel systems at low temperatures after changing one or more parameters, for example after adjusting the pH close to the pI [4]
NEM at different molar ratios to that of protein, considering the molecular weight of the major whey protein (β-lactoglobulin, 18.4 kDa), was incubated at 4°C overnight (~12 h) at pH ~7. 8-Anilino-1-naphthalenesulfonate (ANS), NaOH, urea and other chemical reagents used were purchased from Sinopharm (China) of analytical grade and were used as received
Oscillatory rheology is used to follow the Sol-Gel-Sol transitions that occur within 20 min
Summary
Whey protein solutions can form stable soluble aggregates [1], for example by heating at low protein concentration [2,3], which can be used to thicken or gel systems at low temperatures after changing one or more parameters, for example after adjusting the pH close to the pI [4]. We consider the opposite scenario, whereby we induce the final gelation by increasing the pH to alkaline values. Gelation at acidic pH has been extensively studied compared to alkaline pH. Our interest from alkaline conditions comes from the dissolution of protein hydrogels, used to model dairy fouling, which are industrially cleaned with alkali based solutions [5]. The key mechanistic step(s) that limits the alkaline dissolution at pH ~11.5–13.5 of typical heat-induced whey protein gels is not known. The study of the formation or destruction of non-covalent crosslinks in highly
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