Citric Acid as a Facilitator for the Integration of Okara in Soymilk Gels

Abstract

This study aims to explore the effects of citric acid (CA) in the development of soymilk okara (MO) gels with improved integration of okara in soymilk protein matrix. A two-step approach was adopted. The first step focused on changes in the preparation of ingredients, soybeans, soymilk (M) and okara (O), and their effects on gel texture. The addition of CA at 0.14%, 1.25% and 2.5% (w/w) to M induced protein coagulation, significantly enhancing the mechanical and viscoelastic properties of thermally prepared soymilk gels. The particle size profile of O was manipulated by drying duration producing uniform MO gels with shorter drying time. The surface morphology of CA-treated okara (OCA), visualized with SEM, indicated that the structure of O loosened in OCA0.14 and degraded in OCA2.5. The second step focused on the preparation of soymilk okara gels with CA at two concentrations, 0.14% and 2.5% (w/w). The role of CA on the structure of okara and the coagulation of soymilk was examined by comparing three different formulations: (i) CA treatment of okara prior to its addition to soymilk (M+OCA), (ii) CA treatment of soymilk prior to okara addition (M+CA+O), and (iii) CA addition to a mixture of soymilk and okara (MO+CA). The CA soymilk okara gels were obtained by heat treatment and their physicochemical and texture attributes were examined. The concentration and order of CA addition had different effects on the gels. FTIR, TGA and SEM analyses of CA-treated O (OCA) revealed structural modification in OCA2.5, including decomposition and release of pectic substances, which were not detected in OCA0.14. The CA0.14 soymilk okara gels exhibited similar failure stress, failure strain and Young’s modulus, while viscoelasticity was influenced by the sequence of CA addition, in comparison to MO. Elasticity (Go’) of M+OCA and MO+CA increased threefold and viscosity (Go”) doubled, whereas M+CA+O exhibited a 4.5-fold increase in Go’ and a 3.5-fold increase in Go”. At CA2.5, the failure stress of the CA soymilk okara gels was similar to MO but with a lower failure strain and higher Young’s Modulus which indicated less cohesive, stiffer gels. The Go’ and Go” of M+OCA increased threefold and twofold, while nearly 4.5-fold in Go’ and 4-fold in Go” increases were observed for M+CA+O and MO+CA. The gel microstructure visualized with CLSM revealed changes in the protein network from strand-like in MO to particulate in the presence of CA, with CA2.5 soymilk okara gels having a denser protein network than CA0.14 soymilk okara gels. However, CLSM provided limited insights on the effect of the sequence of CA addition on gel microstructure. In conclusion, the properties of soymilk okara gels explored in this study indicate that the gel texture can be modulated by altering CA concentration or CA sequence of addition. The CA-modification of O was not essential as similar or superior improvement in texture were achieved by pre-aggregating soymilk proteins with CA in the M+CA+O and MO+CA gel formulations.