Tuesday, December 11, 2018

Acid gelation of whey protein microbeads of different sizes (2016)

By Robi Andoyo, Fanny Guyomarc’h & Marie-Hélène Famelart

Abstract:

In acidified dairy products, the size of the whey protein particles could play a key role in the final structure of the gel. In the present study, small (SM; 2.5 ± 1.2 μm), medium (MM; 4.2 ± 2.2 μm), and large (LM; 18.4 ± 7.2 μm) whey protein microbeads were produced by mixing a 150 g.kg−1 whey protein isolate (WPI) solution and n-dodecane in the presence of polyglycerolpolyricinoleate (PGPR) surfactant at different shear rates and were then stabilized through heat gelation. The microbeads were then washed by centrifugation, dispersed at 70 or 90 g.kg−1 in milk ultrafiltrate, and acidification was performed at 35 °C by adding glucono-δ-lactone to achieve the final pH of ~4.5 in 6 h. Acid gelation was monitored using small deformation rheology, while the gel microstructure was investigated microscopically. The results showed that smaller size of microbeads promoted gels with a higher stiffness and a smaller pore size distribution. The effects were particularly significant at SM microbeads as the number of particles in this system was higher than in LM or MM, hence more connectivity between particles.

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Spatial arrangement of casein micelles and whey protein aggregate in acid gels: Insight on mechanisms (2015)

By Robi Andoyo, Fanny Guyomarc'h, Agnes Burel & Marie-Hélène Famelart

Abstract:

Skim milk used for the yoghurt manufacture contains 2 main colloidal particles, the native micellar casein (NMC) and the heat-induced whey protein aggregates (WPA). The aim of this study was to understand how these 2 colloids organize in space to form binary acid gels. While acid milk gels were considered homogeneous for scales typically above ∼10 μm, shorter length scales were examined to investigate the early stages of particle aggregation. The NMC and WPA were dispersed in milk permeate at different protein concentrations, separately or in an 80/20 w/w mixture (MIX). Acidification was performed at 35 °C with glucono-δ-lactone to achieve a final pH at ∼4.5 in 6 h. Acid gelation was studied by rheology, while the microstructure of the gels at pH 4.5 was studied by confocal scanning laser microscopy and transmission electron microscopy (TEM). Using Shih et al.’s (1990) model on the rheological data, it seemed that aggregation in the NMC and MIX mixtures was driven by the casein micelles and therefore differed from that of the WPA suspension. The different organizations were confirmed using image analysis of confocal or TEM images. The differences in gel formation were discussed in terms of the different interactive properties of the surface of these 2 colloids, together with their different internal structure.

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Model mixtures evidence the respective roles of whey protein particles and casein micelles during acid gelation (2014)

By Robi Andoyo, Fanny Guyomarc'h, Chantal Cauty & Marie Hélène Famelart

Abstract:

In acidified milk, heat-induced whey protein aggregates and casein micelle particles assemble to form a soft gel. The present study was set to evaluate the respective roles of whey protein aggregates (WPIA) and native casein micelles (NMC) during acid gelation by means of changing their ratio in model systems. NMC and WPIA were dispersed in milk permeate at different weight ratios ranging from 0% to 100% NMC for a total protein concentration of ∼45 g kg−1. Acidification was performed at 35 °C by addition of glucono-δ-lactone to achieve the same final pH of 4.5 in 6 h. Acid-induced gelation of these systems was followed using small deformation rheology followed by large deformation test and whey retention measurement at pH 4.5, while their microstructure was investigated microscopically. The results showed that higher content in WPIA promoted faster gelation and led to more elastic gels with smaller pore size and increased whey retention. The effects were particularly dramatic up to ∼10% w/w WPIA, where the aggregates were about equimolar to the casein micelles and covered ∼8% of the micellar surface. The results were discussed in terms of the physical interactions between two populations of colloids of different abilities for acid gelation. It seemed likely that a preferred interaction exists between the casein micelles and the aggregates, and directs the structural and mechanical properties of the acid gel.

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