Development of caseinate glycoconjugates as functional food ingredients

Abstract

The value of New Zealand's agricultural products can be increased if they can be sold as specialty ingredients with desired functional properties rather than as commodity products. In an approach to improve the functional properties of milk proteins, caseinate was 'dry'-heated under controlled conditions in the presence of reducing sugars and reducing sugar mixtures. No added chemicals, enzymes, or extraneous materials were used. The successful reaction between the casein molecules and reducing sugars afforded the protein with a dramatic increase in viscosity. Caseinate glycoconjugates prepared with the monosaccharide ribose, glucose or fructose, or with the disaccharide, lactose, showed a significant increase in viscosity that was l0- to 28-fold that of unmodified caseinate. The most reactive sugar was ribose followed by glucose > fructose > lactose. Less sugar and shorter reaction times were required for sugar moieties showing higher reactivity. The reaction mechanism underlying the viscosity increase was thought to involve extensive protein aggregation and the cross-linking of polypeptide chains via dicarbonyl species, generated from the Maillard reaction. Although ribose and glucose showed high reactivity they also caused severe browning. The extent of reaction causing viscosity increase and the reaction causing browning development were not related, and depended on the formula composition. Careful manipulation of the precise concentrations and reaction conditions produced conjugates showing specified viscosity and colour. Reaction of caseinate/fructose (1:5 reactive amino:carbonyl mole ratio) at 60°C, 67% RH (relative humidity) for 48 h, increased the viscosity of caseinate 24-fold. At 80 % RH the reaction occurred rapidly proceeding to form insoluble gels within 24 h. In contrast, reaction of caseinate/inulin/fructose (1:2.5:5 reactive amino:carbonyl mole ratio) at 60°C, 80 % RH for 48 h produced viscous solutions having l5-fold the viscosity of the unmodified caseinate. Thus, incorporation of inulin prevented rapid formation of insoluble gels. Caseinate/inulin/fructose glycoconjugates produced less browning than caseinate/monosaccharide formulations and could be affiliated with the nutritional benefits and technological properties of inulin. Solid-state 13C-NMR. (nuclear magnetic resonance spectroscopy) provided strong evidence that Maillard-t1pe interactions had occurred between caseinate/fructose and caseinate/inulin/fructose during the 'dry'-heat process. Electron microscopy techniques revealed that the caseinate/fructose conjugates were extensively aggregated, forming large, interconnected macroscopic networks. Using gel electrophoresis the aggregation process was shown to increase with prolonged heating. In contrast to caseinate/fructose, caseinate/inulin/fructose conjugates revealed limited aggregation forming less compact networks. Possible mechanisms by which inulin modified the reaction were postulated. The caseinate/reducing sugar glycoconjugates are expected to have application as functional food ingredients.