Search

Freezing can adversely affect the quality of liquid egg through complex inter-particle interactions of egg micro-particles. The effect of pectin, PG-Alginate (PGA), Arabic Gum, maltodextrin and defrost temperature on the rheological properties, microstructure and color of frozen egg was studied. The fresh sample exhibited a light yellow color with high values of L and positive values of a and b. Freezing resulted in a reduced L value with slight changes in a and b. The additives could enhance the lightness with the closest values of L to that of the fresh sample detected for PGA and pectin. Fresh egg with a low viscosity exhibited near-Newtonian flow behavior. However, the freezing-thawing significantly increased the viscosity. Accordingly, the flow behavior index of the frozen samples was decreased significantly along with a sharp increase in the consistency index, revealing a pseudo-plastic behavior. All frozen samples exhibited higher viscosities than the fresh sample. Fresh samples contained evenly distributed micro-particles ranging from 0.05 to 5.50 µm centering at 1.27 µm. Freezing resulted in aggregated particles with significantly larger sizes. Maltodextrin significantly reduced the particle size. Further reduction was achieved by the addition of pectin, PGA, and Arabic gum. Smallest particle size distributions were achieved at pectin and PGA concentrations of 0.25 and 0.1%, respectively. The results of this study can be employed for the development of new products based on frozen egg with no added sugar or salt, while maintaining the physical and functional properties of the final product. [ABSTRACT FROM AUTHOR]

Abstract: Doogh is a traditional Iranian drink prepared by fragmentation and dilution of yogurt, with addition of salt and flavouring. In the present work, we have used viscometry, microscopy, particle-size analysis and measurements of serum separation to explore the effect of very low concentrations of gellan gum (0.01, 0.03 and 0.05wt %), alone or in combination with 0.25wt % high-methoxy pectin (HMP), on its structure and stability. HMP is known to prevent association of casein particles in acidic milk drinks by steric stabilisation, forming a protective layer bound electrostatically to the surface of the particles. Doogh incorporating 0.25wt % HMP alone showed satisfactory stability on storage for ∼10 days at 5°C, but after 15 days there was obvious separation into a dense sediment and a much clearer upper layer that occupied more than 80% of the total volume, which we attribute to progressive sedimentation of individual sterically-stabilised particles. Samples incorporating gellan (with or without HMP), by contrast, showed rapid development of a clear serum phase, with little further separation at longer times (up to ∼1 month), suggesting expression of fluid by contraction of a gel network (i.e. syneresis rather than sedimentation). Particle size increased dramatically (more than 10-fold) with increasing concentration of gellan, and at the highest concentration studied (0.05wt %) a continuous network of casein-rich strands was observed by phase-contrast microscopy. The concentration of NaCl used in the doogh samples (0.5wt %≈85mM) is known to be sufficient to maintain gellan in its ordered (double-helix) conformation, which has higher charge density than individual molecules of HMP. We suggest that network structure is formed by electrostatic attachment of gellan to fragments of acid-casein gel, thus increasing particle size and inhibiting surface-coverage by HMP, with weaker associations between gellan helices allowing the samples to flow. Observed decrease in serum volume with increasing concentration of gellan is attributed to formation of progressively stronger coupled networks with greater resistance to syneresis. Stabilisation of doogh with 0.05wt % gellan in combination with 0.25wt % HMP had no adverse effect on organoleptic acceptability, and reduced serum volume on protracted storage to ∼10% (in comparison with over 80% for the same concentration of HMP alone), suggesting that gellan could be of practical value in extending the shelf-life of doogh (and related acidic milk drinks). [Copyright &y& Elsevier]