ABSTRACT
Porphyran (Por) prepared from dried nori was applied as a functional modifier of a soybean protein isolate (SPI) to conjugate with SPI from defatted soybean by the Maillard reaction (79% relative humidity and 60 degrees C for 7 days). Two kinds of partially denatured conjugate (Conj 45 and Conj 63) were obtained from the reaction product by sequential extraction at pH 4.5 and pH 6.3, and the respective yield and weight ratios of the SPI and Por moieties were 8.4% and 1:1 for Conj 45 and 11.7% and 1:0.16 for Conj 63. Conj 63 demonstrated improved solubility between pH 5.0 and pH 8.0, while Conj 45 exhibited substantially complete solubility over the pH range of 2.0-8.0. Conj 63 showed more tolerance against digestion with pancreatin than SPI, whereas this was lost after denaturation. Conj 63 and Conj 45 both showed a markedly higher emulsion activity index and emulsion stability than SPI, even at pH 3.0; in particular, Conj 45 exhibited outstanding emulsifying ability. Conj 63 had about a two-fold higher calcium-binding ability than SPI, and Conj 63 and Conj 45 did not aggregate with added Ca2+ and Mg2+. It is believed that Por could be a valuable functional modifier of SPI for providing soybean protein-based liquid foods such as beverages by conjugation through the Maillard reaction.
Subject(s)
Maillard Reaction , Sepharose/analogs & derivatives , Soybean Proteins/chemistry , Chemical Phenomena , Chemistry, Physical , Emulsifying Agents , Pancreatin/metabolism , Protein Denaturation , Rhodophyta/chemistry , Sepharose/chemistry , Sepharose/metabolism , Solubility , Soybean Proteins/metabolismABSTRACT
A soybean protein isolate (SPI), and its beta-conglycinin and glycinin componets were obtained from defatted soybean flour by applying dissolution and precipitation based on the difference in their solubility depending on each isoelectric point. The purity evaluated by SDS-PAGE of the beta-conglycinin and glycinin preparations was about 84% and 80%, respectively, resulting in a clear difference in the pH dependence on solubility. A BET plot derived from the water sorption isotherm at 25 degrees C showed that the amount of the monolayer adsorption of these preparations was about 6-9%, the value for the beta-conglycinin preparation being about 1.5 times higher than that for the glycinin preparation. The beta-conglycinin and glycinin preparations were respectively denatured at around 75 degrees C and 86 degrees C in the presence of excess water, whereas the denaturation temperature of both preparations was markedly increased by decreasing sorbed water content below 40%, corresponding well with the unfrozen water content.
