Iowa is the largest soybean and egg producer in the U.S., producing over 500 million bushels of soybeans and more than 14 billion eggs per year (USDA 2009; Iowa Egg Council 2008). There is, therefore, a unique opportunity to contribute to the development of Iowa economy by identifying ways to improve value of these products. The objective of this research was to assess the effects of high-pressure processing (HPP) on soymilk and its potential for tofu production, and on the egg protein phosvitin. In the first study, soymilk pH was adjusted to 6.0 or 6.5 and a thermal or pressure pre-treatment was applied. Tofu was produced under pressure or by thermal treatment. Quality parameters including particle size and stability of soymilk, tofu yield, moisture and crude protein content, and water holding capacity and texture of the different tofu were measured. Hydrolysis of glucono-δ-lactone (GDL) in water and soymilk was performed and rates of hydrolysis under thermal and pressure treatments were determined. Finally, microstructures of the tofu were compared in order to characterize effect of the different soymilk pre-treatment and method of tofu production.

Change of soymilk particle size was dependent upon the pre-treatment applied. After pressure (400 MPa, 5 min) or thermal treatment (85 yC, 30 min), the particle size of the pH 6.5 soymilk decreased by 12-fold when compared to the control, while after pressure treatment at 600 MPa the soymilk particle size increased 2-fold. All combination of pH, pre-treatment, and method of production resulted in formation of a tofu, except the pH 6.0 soymilk that was submitted to a thermal pre-treatment prior to pressure treatment to generate tofu production. Pressure-produced tofu, regardless of pressure level (400 and 600 MPa), produced higher yields than thermal produced tofu. Several advantages of using HPP to produce tofu exist, including a shorter processing time and the elimination of a pressing step.

In the second study, native phosvitin was pressurized in combination with a high temperature (65 yC) with the pH adjusted to 2.3, 7.0 and 11.0. Circular dichroism and size exclusion chromatography were performed to determine structural changes due to pressure, temperature and pH changes. Phosphatase treatment for up to 18 h was used to dephosphorylate phosvitin prior to in vitro digestion that simulated gastrointestinal digestion. SDS-PAGE was used to characterize phosvitin peptides formation. Extent of dephosphorylation and changes in angiotensin converting enzyme inhibitory activity and antioxidant activity were monitored after HPP and enzymatic treatments.

Structure of phosvitin was maintained after treatment at 600 MPa, initial temperature of 65 yC, processing temperature 83 yC, for 2 or 30 min, illustrating high stability of the protein in these conditions. The percent of helices and β sheets of the phosvitin increased by 8.1 and 22.6%, respectively, when the pH was adjusted from neutral (7.0) to acidic (2.3). The percent of phosphate released increased as the phosphatase treatment time increased, reaching 62.8% after 18 h. Changes in the phosvitin peptide profile were observed after phosphatase treatment (18 h) followed by protease treatment (pepsin and pancreatin, 3 h each), with the formation of peptides of 29, 27 and 21 kDa. Antioxidant activity of these peptides increased by 71.0% compared to native digested phosvitin, suggesting that not only is the amount of phosphate important, but the amino acid sequence and the peptides obtained as well.