Impact of the concentration method and starter culture on the sensory defects of nonfat Greek-style yogurts

Abstract

High protein nonfat yogurts can exhibit chalkiness, graininess, and astringency. The aim of this work was to study the effect of different concentration methods and starter cultures on the sensory properties of nonfat Greek-style yogurt. Concentration methods done at pilot-scale included traditional straining (S), ultrafiltration after fermentation (UF), mechanical separation (M), and milk fortification with MPC85 (F) before culturing. The target protein level was 9% and non-concentrated yogurts (NC) were used as controls. These treatments were carried out in triplicate using two commercial starter cultures (YoFlex Mild 2.0 (M) and Premium 11 (P), Novonesis) which provide different textures. Yogurts were analyzed for their composition, starter culture numbers, rheological properties, water holding capacity, particle size, microstructure, peptide profiles, exopolysaccharide (EPS) content, and sensory properties. Two-way ANOVA was applied considering two factors, concentration method and starter culture type. Overall, the concentration method had the greatest effect on sensory attributes. The high protein yogurts exhibited increased chalkiness (p< 0.05) compared to control, while graininess was associated with fortified and UF treatments (p< 0.05). Pearson correlation between graininess and chalkiness was significant (r= 0.73), and graininess was positively correlated with EPS levels (r= 0.65). Astringency depended on both the concentration method and starter culture, and it was significantly higher in the yogurts concentrated after culturing (UF, M and S). In addition, the yogurts made with M were more astringent than those made with P (p< 0.05). When astringency levels were plotted against D×90 particle size (size at which 90% of particles are smaller), the data was described by a power-law relationship (R2= 0.76) with lower D×90 correlated with higher astringency. Hence, astringency might be affected by cluster size (gel microstructure). These findings highlight the complexity of chalky and astringent defects and provide insights to help limit their impact in high protein yogurt.