DAIRY AND SOY PROTEINS: ACID AND HEAT INDUCED INTERACTIONS
Based on the high consumer demand for high protein foods, there is some interest and potential for the creation of soy protein fortified dairy products. However, studies that evaluated combinations of milk and soy proteins highlighted the incompatibility between these classes of proteins. The differences in structure and properties between milk and soy proteins can potentially lead to significant inhomogeneities in mixed systems, which is highly undesirable for the manufacture of high quality food products. The few studies that exist to date on milk protein-soy protein mixtures were conducted in low to moderate concentration liquid systems, and were focused primarily on the effect of temperature on the protein interactions. Very little research is available on solid milk-soy protein systems. In the first part of this thesis work, a study was conducted to evaluate the effect of pH and temperature, two major factors that affect protein conformation, on the structure and texture of a milk-soy protein product. Mixtures of skim milk and soy protein isolate, at a total protein concentration of 4.7% and a 3:1 (w/w) milk protein to soy protein ratio, were used to prepare a cheese-like product using a hybrid cheese making and tofu making process. Four pH levels (4.6, 4.9, 5.2, 5.5) and four temperatures (65 °C, 75 °C, 85 °C and 95°C) were used. The structure, texture, color and moisture content of the resulting products were evaluated. For all samples, the structure appeared as a network of aggregated proteins. Large protein aggregates and large spacing between aggregates were observed in the pH 4.6 samples, while the pH 5.5 samples had a more homogenous structure. Samples treated at 95 °C had denser aggregates with smaller spacing than the samples in the 65 °C groups. Hardness and elasticity of the product significantly increased as pH decreased and processing temperature increased (p<0.05). Additionally, a significant synergistic effect of pH and temperature on the structure and mechanical properties of the mixed soy protein-milk protein systems was observed. Significant darkening occurred for the products with higher pH and higher temperature (p<0.05). The findings of this study can be used as a basis for developing a high concentration milk-soy protein network with uniform structure, based on known properties of these proteins. The second part of this work focuses on the development of a shelf stable string cheese product. Conventionally, cheese products (except process cheese) are stored at refrigeration temperature to ensure food safety and quality. To develop a shelf stable string cheese product, two main parameters of product needed to be achieved and maintained under an anaerobic environment: low water activity (aw <0.93) and stringiness. The work was conducted in two phases: I) development and testing of prototypes at bench-top scale; II) formulation optimization at pilot scale and shelf life study. The microbiological and physical quality of the cheese product was monitored during storage, both under refrigeration and room temperature conditions. Ultimately, a successful prototype was developed at pilot scale, and is ready for adoption at commercial scale.
Dairy; pH; Soy protein; cheese; Temperature; Agriculture
Moraru, Carmen I.
Schmit, Todd Michael
Food Science and Technology
M.S., Food Science and Technology
Master of Science
dissertation or thesis