ISBN 978-3-8439-2844-1

978-3-8439-2844-1, Reihe Lebensmitteltechnologie

Stefan Nöbel
Dairy microgel suspensions: principles, phenomena and models at different structural levels

216 Seiten, Dissertation Universität Hohenheim Stuttgart-Hohenheim (2016), Softcover, A5

Zusammenfassung / Abstract

Microgels are particles composed of a liquid-filled gel network with a diameter from 0.1 to 100 μm. The amount of solvent bound depends on the temperature, pH and ionic strength. In dairy suspensions, the smallest building block at the nanometer scale, namely the native casein micelle, as well as particle clusters from acidification at the micrometer scale exhibit microgel characteristics. This thesis aims to transfer general microgel principles of formation, characterization and tailored structural properties to dairy products, such as native casein micelles, stirred yogurt and fresh cheese. All processing steps required, i.e., heat-treatment, membrane separation, fermentation and mechanical post-processing, were carried out in-house. The flow properties and viscoelastic response were determined by rotational and small amplitude oscillatory shear rheology respectively. Dynamic and static light scattering, and image analysis was used to study the size of the microgels at different length scales. Some products were sensorially assessed to correlate the instrumentally determined particle size with the in-mouth and visual graininess. The effective water binding of native casein micelles was determined as a function of temperature from 5 to 70°C. A mathematical model was established for casein micelles and particle clusters in fermented dairy products in order to discriminate between their contribution to the viscoelastic response. The microgel size was increased by consciously applying sonication within the pH range 5.4-5.1 during acidification. The sonication-induced particles were perceived as grainy in stirred yogurt. Another method to generate particles in fermented dairy products namely by means of heat-treatment, mechanical processing and subsequent blending was also studied. The viscosity and gel firmness of blended fresh cheese samples was 2.0 and 2.6 times higher than for the standard. Adjusting the multimodal particle size distribution seems to be a promising approach for tailored rheological properties of concentrated dairy suspensions to maintain a smooth and creamy perception.