PROTEIN-LIPID INTERACTIONS OF REDUCTIVELY METHYLATED C(14) SOY PROTEIN FRACTIONS IN A MODEL SOY-BASED INFANT FORMULA SYSTEM

Protein-lipid interactions in a model soy-based infant formula have been investigated using reductively methylated ('14)C soy protein fractions. A low level of modification was used to label the soy protein fractions and no significant change in the physico-chemical nature of the proteins was apparent using a number of methods. These ('14)C soy proteins were then used with ('3)H lipid to determine the extent of lipid association in a model soy-based infant formula.;Preparation of reductively methylated ('14)C soy protein fractions was accomplished using ('14)C formaldehyde and NaCNBH(,3). The use of this reducing agent have approximately 2x the rate of encorporation of label compared to NaBH(,4). After reductively methylating a total soy protein extract, (('14)C) 7S and 11S soy proteins could be fractionally precipitated from this mixture. Concentration of the protein fractions in this manner significantly reduced the amount of material that needed to be dialized.;Functional differences were shown for the (('14)C) 7S and 11S soy protein fractions in a model infant formula system. The primary method of investigating these functional differences was to determine the amount of lipid associated protein in preparative ultracentrifuge cream plugs. The following conclusions were reached: (1) The type of protein associated with the lipid in an emulsion system is probably more important in determining properties of the emulsion than is the quantity of protein. (2) (('14)C) 11S soy protein fraction was associated with the lipid to a greater extent in almost all cases compared to the (('14)C) 7S soy protein fraction. (3) The association of the (('14)C) 11S fraction with lipid is very pH sensitive especially when approaching the pKa. (('14)C) 7S fraction was not affected appreciably by pH in the pH range that was studied (6.4-7.3). (4) (('14)C) 11S soy protein fraction is more sensitive to heat effects than is (('14)C) 7S and may cause severe gelation, thickening or clustering of the fat globules depending on the pH of the system.;A number of physico-chemical methods were used to characterize prepared soy protein fractions. Each of the methods provided information on the soy proteins, but it was determined that no one method gave complete characterization and that more than one system should be used to characterize soy protein systems. It was determined that soy protein fractions could be prepared by adjusting the pH, temperature or ionic strength of soy protein extracts. Purity of the fractions, however, could not be maximized simultaneously with yield. Characterization of commercial soy protein isolates showed vast differences between the nature of the various isolates and heat seemed to be the most prevalent type of processing treatment after regular isoelectric precipitation.