Foam Formation and Starch Gelatinization with Alpha-Crystalline Emulsifiers
Sammanfattning: In the baking industry, emulsifiers in the alpha-gel state are used for foam formation. This thesis focuses on some parts of the effects of the emulsifier: how the foam formation and foam stability are influenced by the physical state of the emulsifier, how emulsifiers change the gelatinization and gel formation of starch, and the impact that these factors may have in a true product such as a sponge cake. The emulsifiers studied were either alpha-gels with a varying amount of vesicle structures, beta-crystals, or micelles. In addition to the study of the effects in a sponge cake batter, a sucrose foam was used as a model system for the foam stability studies, and a starch paste was used for the gelatinization studies. The foams were whipped, either with a mixer or in a pressurized vessel, and the foam formation and stability were measured with density measurements, TEM (transmission electron microscopy), CLSM (confocal laser scanning microscopy), and oscillatory rheological measurements. The starch pastes were heated in a Brabender viscometer, smeared onto a slide, stained with iodine and examined with light microscopy. Cooled gels were also embedded and studied with TEM. The main conclusion was that the alpha-gel has specific properties that influence the foam formation, the foam stability, and the starch gelatinization. In a foam, the main advantage of the alpha-gel was that it seemed to have an attractive bridging effect, which stabilized the structure. This led to better dispersion, with smaller bubbles, and a more stable foam, since drainage and creaming decreased. During starch gelatinization, the alpha-gel interacted with the starch granules at low temperatures and restricted the granule swelling below 50°C. Solid beta-crystals did not have this effect on the starch before they melted. Although both the alpha-gel and the beta-crystals transformed into other states above a temperature of about 60°C, this difference in swelling during the early heating was shown to give other rheological properties of the paste at 95-97°C. This thesis also showed that emulsifiers were able to hinder granule swelling, amylose leakage and granule rupture up to 90°C. The rupture temperature did not depend on the emulsifier concentration. When the temperature became too high, the granules disrupted completely without initial leakage of the soluble molecules, which resulted in a different distribution of the components. In combination with sucrose, the granules could be kept intact even longer. It was also shown that emulsifiers changed the microstructure of the amylose gel formed during cooling. At moderate emulsifier concentrations, the amylose aggregated into thicker and more rigid strands than normal, and at high concentrations the amylose gathered into spherical aggregates without any network connecting them. These changes were further pronounced in gels with whole starch granules.
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