INTERACTIONS AMONG PROTEINS AND CARBOHYDRATES UNDER THERMAL PROCESSING CONDITIONS AND THEIR EFFECTS ON DAIRY FOULING

2020-06-23T19:16:15Z (GMT) by Yizhe Zhang

In dairy processing, dairy ingredients need to be thermally treated to ensure product quality and safety for an extended shelf life. During thermal processes, milk protein denatures and interacts with other dairy ingredients to form a layer of deposit on heated surfaces, known as fouling which can deteriorate process efficiency and product safety. Milk is a complex mixture of proteins, fats, carbohydrates, minerals and vitamins. The heat-sensitive B-lactoglobulin (B-lg) is known to be a key component in fouling formation (constituting 50% of type A fouling deposits) during milk pasteurization, as B-lg unfolds when heated and exposes the reactive sulfhydryl groups that can interact with other proteins and ingredients to form deposits. Although casein (80% of milk proteins) is known to interact with denatured B-lg, no fouling studies have been performed with particular focus on the effect of casein on whey protein fouling.

Carbohydrates are an ingredient widely added in various dairy products as sweetener, stabilizer, texturizer, and fat replacer. Simple sugars have a protective effect on whey protein denaturation, but their effect on dairy fouling is not known. Polysaccharides can interact with milk proteins through electrostatic and hydrophobic interactions, as well as hydrogen bonding. The addition of polysaccharide (carrageenan) has been reported to cause opposite effects on protein deposition, however, no conclusive mechanism has been proposed to elucidate how protein-polysaccharide interaction at pasteurization temperatures affects the fouling behavior of dairy products.

In this dissertation, different model dairy solutions and real dairy products were used to study the effect of composition, including protein distribution and additions of simple sugars and polysaccharides, on dairy fouling. Fouling deposits were formed and analyzed using a bench-top spinning disc apparatus operating under well-controlled temperatures and shear stresses characterized by computational fluid dynamics simulations. By studying the fouling behavior of camel milk and comparing with bovine milk, milk without B-lg was found to still foul and form deposits containing casein, α-lactalbumin, serum albumin with a reduced thermal resistance due to a more porous structure. Results also showed that the addition of 10 wt% sugar reduced whey protein fouling by more than 30% and affected the structure and adhesion strength of deposits. Furthermore, the presence of carrageenan in dairy solutions can promote the denaturation of B-lg when heated and form a more compact deposit, resulting in more severe fouling. Overall, this dissertation provides a fundamental understanding of the fouling characteristics of complex dairy products. The knowledge gained is expected to help the dairy industry select suitable ingredients to mitigate or prevent the fouling problem.