Department of Chemical Engineering, Monash University, Clayton, Victoria, Australia.
Colloids Surf B Biointerfaces. 2011 Oct 15;87(2):255-66. doi: 10.1016/j.colsurfb.2011.05.026. Epub 2011 May 24.
Material segregation has been reported for industrial spray-dried milk powders, which indicates potential material migration during drying process. The relevant colloidal transport phenomenon and the underlying mechanism are still under debate. This study extended the glass-filament single droplet drying technique to observe not only the drying behaviour but also the dissolution behaviour of the correspondingly dried single particle. At progressively longer drying stage, a solvent droplet (water or ethanol) was attached to the semi-dried milk particle and the interaction between the solvent and the particle was video-recorded. Based on the different dissolution and wetting behaviours observed, material migration during milk drying was studied. Fresh skim milk and fresh whole milk were investigated using water and ethanol as solvents. Fat started to accumulate on the surface as soon as drying was started. At the initial stage of drying, the fat layer remained thin and the solubility of the semi-dried milk particle was much affected by lactose and protein present underneath the fat layer. Fat kept accumulating at the surface as drying progressed and the accumulation was completed by the middle stage of drying. The results from drying of model milk materials (pure sodium caseinate solution and lactose/sodium caseinate mixed solution) supported the colloidal transport phenomena observed for the milk drying. When mixed with lactose, sodium caseinate did not form an apparent solvent-resistant protein shell during drying. The extended technique of glass-filament single droplet approach provides a powerful tool in examining the solubility of individual particle after drying.
已有研究报道工业喷雾干燥乳粉存在物料分离现象，这表明在干燥过程中可能存在物质迁移。相关胶体输运现象及其潜在机理仍存在争议。本研究将玻璃纤维单液滴干燥技术拓展应用，不仅观察相应干燥单颗粒的干燥行为，还观察其溶解行为。在逐渐延长的干燥阶段，将溶剂液滴（水或乙醇）附着于半干燥乳颗粒上，并对溶剂与颗粒之间的相互作用进行视频记录。基于观察到的不同溶解和润湿行为，研究了乳粉干燥过程中的物质迁移现象。分别以水和乙醇作为溶剂，研究新鲜脱脂乳和新鲜全脂乳。干燥一开始，脂肪就开始在表面聚集。在干燥的初始阶段，脂肪层仍然很薄，半干燥乳颗粒的溶解度受脂肪层下方乳糖和蛋白质的影响很大。随着干燥的进行，脂肪不断在表面聚集，在干燥中期完成聚集。模型乳材料（纯酪蛋白酸钠溶液和乳糖/酪蛋白酸钠混合溶液）干燥的结果支持了对乳粉干燥过程中观察到的胶体输运现象的解释。当与乳糖混合时，酪蛋白酸钠在干燥过程中并未形成明显耐溶剂的蛋白质外壳。玻璃纤维单液滴延伸技术为研究干燥后单个颗粒的溶解度提供了有力工具。
