Hydrocolloid composites – their contribution to physical and oral perception of dairy products

Abstract

The risks of chronic diseases related to high fat intake lead to an increased demand for low-fat food products. However, reducing the fat content incorporates textural and sensory defect that significantly impacts consumer acceptability, especially for dairy products. Therefore, hydrocolloids are used as stabilisers and thickeners in food processing since they can be partial or total fat substitutes in foods that confer desirable characteristics normally provided by fats. This project firstly investigated physicochemical characteristics of pure hydrocolloids solutions (gelatin, k-carrageenan, low methoxyl (LM) pectin and curdlan) under conditions used in dairy products, then the influence of hydrocolloids addition on the stability, flow behaviour, microstructure and lubrication of milk protein solution within varying casein to whey protein ratios was studied. Further, the effect of hydrocolloids and casein to whey protein ratios on the physical property and oral perception of real dairy products (low-fat chocolate milk and yoghurt) was evaluated.The flow and tribological behaviors of hydrocolloids solution/suspension depended on the pH, addition of salts and dosage of hydrocolloids. Near its isoelectric point, gelatin solution had the lowest viscosity, gel strength, lubrication property and poor visual aspect of formed gel. k-Carrageenan solution had the highest viscosity and best lubrication property at neutral pH. The strength of formed gels from k-carrageenan solution was low in acid condition, and the formed gels had unchanged visual aspect. LM pectin solution showed the highest viscosity and best lubrication property at pH 4.0, and decreased with the raise of pH value. LM pectin solution only started to form a stable gel when the concentration reached 1.0% with moderate amount (g 0.025%) of CaCl2 existed. For curdlan suspension, the viscosity was stable at neutral or acidic condition, but increased under alkali condition, and the curdlan suspension showed the least lubrication property at pH 4.0. Adding salts improved the viscosity and tribology property of gelatin, k-carrageenan solution, while the viscosity of LM pectin solution and curdlan suspension decreased with the addition of KCl, and 2 or 3% CaCl2 addition decreased the lubrication property of curdlan suspension. To some extent, the addition of salts depressed gelation of curdlan suspension at pH 5.0, and the addition of CaCl2 (g 0.8%) increased formation of curdlan gel. However, for other hydrocolloids, salts addition increased the gel strength although lower gel strength was obtained when the addition of CaCl2 was beyond 0.8% for k-carrageenan formed gel.For milk protein solution with fixed protein content (3.4%), replacing casein with whey protein isolate improved the protein stability, increased the viscosity and lubrication property of protein solution, whilst the milky white colour of the solution reduced. Gelling was observed with independent addition of 0.05% of k-carrageenan and 0.25% of LM pectin to the protein solution with high casein to whey protein ratio (g 50/50). Addition of hydrocolloids caused significant increase in viscosity and improved the friction coefficient of protein solutions, and protein solution showed the best lubrication property with the addition of 0.25% curdlan. The solutions were more homogeneous when casein/whey protein ratio was 0/100, and adding hydrocolloids caused phase separation, especially for protein solutions with high casein fraction. While when the percentage of casein was fixed to 2.4%, the addition of whey protein isolate did not have any obvious influence on the appearances of protein solutions, although it increased the stability, viscosity and lubrication property of protein solution. Besides, it was further discovery that the protein solution containing curdlan showed the best lubrication property both at 15 and 37 dC.Finally, the influence of hydrocolloids and casein to whey protein ratios on the physical and sensory properties of chocolate milk and yoghurt were investigated. The dynamic sensory tool, Temporal Dominance of Sensations (TDS), was used to monitor the changes in textural perception as perceived during oral processing. Decreasing casein to whey protein isolate ratio (from 80/20 to 20/80) significantly decreased the sedimentation percentage (from 4% to 3%) and improved the lubrication property of chocolate milk, although the resulting chocolate milk samples were darker. Hydrocolloid addition improved the viscosity and lubrication property of chocolate milk, however a significant increase in the particle size and an obvious sedimentation (g 6%) happened with a high proportion of k-carrageenan, LM pectin and curdlan. The TDS data obtained showed that powderiness, thickness, and creaminess was significant dominant attribute (dominance rate of 30%) at the start of consumption for chocolate milk, while residual coating and astringency became dominant later during oral processing. For yoghurt, with the replacement of casein by whey protein or hydrocolloids addition, the texture and viscosity of yoghurt was significantly increased, although too much hydrocolloid caused increased syneresis. The TDS data obtained showed that graininess, thickness, and cohesiveness were the most perceptible attributes at the start of consumption for yoghurt, while the dominant attributes turned to creaminess and mouth coating in the later period of consumption (after 60% onwards).