THE EFFECT OF PH ON THE RELATIONSHIP BETWEEN HYDROPHILIC LIPOPHILIC CHARACTERISTICS AND EMULSIFICATION PROPERTIES OF SOY PROTEINS/

Hydrophilic characteristics of four soybean protein isolates, determin ed through measurement of spontaneous water absorption capacity (WAC), were greatly affected by pH in the range of 2 to 7. While minimum WAC was apparent near the isoelectric region and increased away from the pI (isoelectric point) for most isolates, a partially hydrolysed soy p rotein isolate behaved the opposite way. Lipophilic characteristics, d etermined as the spontaneous oil absorption capacity (OAC), varied up to 120% as pH changed. The balance between hydrophile and lipophile (W OAI), calculated as the ratio between WAC and OAC, showed a response t o pH similar to WAC, with a minimum value near the pI, except for the hydrolysed isolate. Emulsifying capacity (EC) was maximum near the pI. However, the hydrolysed isolate showed a greater EC, reaching a maxim um at pH 3, which could be attributed to peptides of different size an d net charge with no single pI. As the hydrophilic/lipophilic balance reached lower values as a result of the effect of pH, EC increased, su ggesting that a relatively high hydrophobicity is needed to enhance th e EC of soy proteins. The effect of pH on emulsion stability was oppos ite to that shown on EC, except for the hydrolysed isolate, as maximum instability was apparent near the pI, where electrostatic and steric repulsions between oil droplets are highly decreased. The effect of pH on emulsion stability was closely related to WAC and indicated that a high hydration of the interfacial film is necessary for increasing st ability against creaming and flocculation. (C) 1996 Academic Press Lim ited