Spray Drying Functional Whey Protein Soluble Aggregates Made at Neutral pH

LU, HUIYING. Spray Drying Functional Whey Protein Soluble Aggregates Made at Neutral pH. (Under the direction of Dr. E. Allen Foegeding.) Whey-derived protein ingredients are widely used in various food applications. There are ongoing efforts to tailor and extend the functionalities of whey protein ingredients through protein modification. Heat treatment of whey protein solutions at protein concentrations below the critical gelation concentration generates soluble protein aggregates. Soluble whey protein aggregates made at neutral pH was found to deliver improved functionalities in emulsifying, foaming, thickening and film formation, and provide new applications of “cold gelation (Nicolai et al. 2011). However these soluble aggregates tend to loss dispersibility upon dehydration. This investigation focused the cause and prevention of solubility loss in spray dried soluble whey protein aggregates. The results revealed critical determinants for the solubility loss of spray dried aggregates, such as initial protein concentration (Chapter 2), influences of storage (Chapter 3); content of co-solutes (Chapter 4), size of aggregates (Chapter 5), and length of the heating process used to form the aggregates (Chapter 6). The heated whey protein isolate solutions (0.7% 7% w/v) contained roughly two size groups of aggregates – the large aggregates with molecular weight above 600 kDa, and the small aggregates and denatured monomers & oligomers (SMO) that are smaller than 150 kDa. No aggregates with intermediate sizes were found. Spray drying (Tout = 65 ± 5 C) caused various degrees of solubility loss depending on the weight ratio of large aggregates and SMO. Solubility loss was caused by spray drying-induced secondary aggregation of the protein aggregates, mediated mainly through non-covalent interactions. Furthermore, protein aggregation also occurred in protein powders during storage, and resulted in decreased dispersibility. Non-covalent interactions played the major role in causing protein aggregation during storage, although the formation of covalent bonds was also detected. Soluble aggregates made from heated 0.78% w/v WPI solutions produced aggregate powders that remained fully dispersible over 15-month storage. Correlation was established between the extent of “secondary aggregation” during spray drying and the content of SMO. Increased solubility was seen when the large aggregates was supplemented with SMO. Fully dispersible aggregates powders were produced from aggregate suspensions that contained 30% w/w SMO or more. Secondary aggregation during spray drying was completely prevented when the aggregate suspension contained 49% w/w or more SMO. The mechanism appears to be associated with the interfacial activity of SMO. Alternatively, when large aggregates were co-spray dried with disaccharides (sucrose, lactose, trehalose), their solubility increased. Formulation with disaccharides produced fully soluble aggregates powders but required significant amount of disaccharides (40% w/w), thereby reduced the protein content to < 60% in the final dry mix. Secondary aggregation was largely prevented in the presence of 40% w/w disaccharides regardless of the disaccharide type. The extent of spray drying-induced solubility loss was related with the time of heat treatment. This correlation depends on the initial protein concentration during aggregate formation. Soluble aggregates formed at low protein concentration (0.78% WPI w/v) remained soluble regardless of heating time after being spray dried. For soluble aggregates made at higher concentration (7% WPI w/v), the solubility displayed a parabolic trend with heating time, with lowest solubility seen at a 6-hr heat treatment. Heating up to 30 min generated soluble aggregates that retained 100% solubility; while further heating generated aggregates that had up to 30% solubility loss after being spray dried. The trend of solubility can be explained by the influences of heat time on the changes of 1) the content of small aggregates in the soluble aggregate dispersion; 2) intermolecular electrostatic repulsion and 3) availability of sulfhydryl groups.