Importance of size and charge of carbohydrate chains in the preparation of functional glycoproteins with excellent emulsifying properties from tofu whey

The importance of size and charge of carbohydrate moiety in tofu whey–carbohydrate conjugates, formed through Maillard-type glycosylation, was investigated. Tofu whey (TW) ultrafiltered fractions (UFTW > 3 kDa and UFTW < 3 kDa), acid-precipitated protein (APP), β-conglycinin (7S) or glycinin (11S) were covalently conjugated with carbohydrates (galactomannan (GM), okara polysaccharides, xyloglucan (XG), chitin and chitosan oligo), through naturally-occurring Maillard-type glycosylation at controlled temperature (60 °C), relative humidity (65%) and pH (7.0) for 7-days. The UFTW > 3 kDa fraction conjugated to polysaccharides or oligosaccharides showed improved emulsifying activity index (EAI) and emulsion stability (ES) (p < 0.05). The UFTW < 3 kDa, APP, 11S and 7S–oligosaccharide conjugates showed almost 3.0 times improvement in ES but no significant difference in EAI. In addition, UFTW < 3 kDa, APP, 7S, and 11S–chitosan oligo conjugates showed 5.0 times lower EAI and ES than those conjugates with XG or chitin oligo, due to electrostatic interaction between the negatively-charged groups of soy protein and positively-charged groups of chitosan oligo. Tofu whey fraction–carbohydrate glycoproteins showed both high and low water or oil-binding capacities, implying that different carbohydrates utilised in the preparation of glycoproteins possess different properties and mechanism in water or oil-binding capacity.