Effect of concentration and incubation temperature on the acid induced aggregation of soymilk

The molecular details of the aggregation of soy protein particles in soymilk during acid-induced gelation using glucono-delta-lactone (GDL) were investigated. Soymilk samples were prepared from different water-to-bean ratios and contained approximately 4% and 7% protein. The effects of protein concentration and incubation temperature (30 or 7 °C) on soymilk gelation were observed using rheology and diffusing wave spectroscopy at two different GDL concentrations. During acidification, there was a decrease in electrostatic repulsion between particles which well correlated with the pH of aggregation determined by dynamic light scattering. Gelation of soymilk occurred at about pH 5.8, and neither the rate of acidification nor the protein concentration affected the gelation pH. Gel stiffness was affected by protein concentration. A detailed study of the soluble fraction during the preceding stages of aggregation clearly demonstrated that glycinin components were the first to destabilize during acidification, followed by the β-conglycinin subunits. Decreasing the incubation temperature from 30 °C to 7 °C increased the pH of gelation and the gel modulus (G′) measured by rheology. It was concluded that short range interactions play a major role in the formation of the protein network in soymilk curd.

Soymilk was acidified with glucono-δ-lactone, and structure formation was followed by measuring the gel modulus using rheology (filled symbols) and the apparent particle radius by diffusing wave spectroscopy (empty symbols). The results clearly demonstrate that gelation of soymilk particles occurs because of charge neutralization and short range interactions with hydrogen bonds playing a major role in the formation of the protein network as soymilk samples incubated at 7 °C (blue) aggregate at an earlier pH and show a higher gel modulus than soymilk gels obtained at 30 °C (red).Figure optionsDownload as PowerPoint slideHighlights
► The gelation pH of soymilk during acidification with glucono delta lactone is 5.8.
► The protein concentration and amount of glucono delta lactone do not affect the pH of gelation.
► A decrease in electrostatic repulsion causes colloidal destabilization of the soymilk particles.
► Glycinin protein destabilizes first during acidification.
► It is demonstrated for the first time that hydrogen bridges play a major role in the formation of the network.