Structural and physicochemical properties of sesame cake protein isolates obtained by different extraction methods

Two different methods were employed to isolate proteins from defatted sesame cake, i.e., isoelectric point precipitation (SCIP) and protein micellization (SCM). The protein content of both isolates was similar (∼90%), whereas the SCIP appeared darker than the SCM. The lowest protein solubility was observed at pH 5.0–6.0 (isoelectric point region), with the SCM exhibiting higher solubility over the range of pH 2.0–10.0, compared to its isoelectric precipitation counterpart. Foaming capacity and stability improved with increasing NaCl (0.0–1.0 M) concentration and were higher for the SCM preparation. The emulsification properties were also pH dependent. The SCM retained its native structure, as evidenced by calorimetric, confocal microscopy and FTIR measurements. Fat absorption capacity appeared similar for both isolates, whereas the SCIP exhibited higher water retention capacity and in vitro protein digestibility, but lower levels of total phenolics than SCM. Upon heating, the paste-like rheological behavior of SCIP dispersions, which concurred with a more densely packed microstructure of smaller size protein aggregates, was maintained, while the liquid-like SCM dispersions gave stronger gel-like structures due to protein unfolding. Under reducing and non-reducing conditions, both isolates showed similar SDS-PAGE patterns, although some subtle compositional variations between the two isolates were noted for the low-abundance bands at higher loadings. Overall, the functional properties of sesame cake protein isolates were largely dependent on the preparation method; the SCM isolate displayed superior solubility, phenolic content, foaming properties and gelling capacity upon thermal processing, while SCIP preparation showed enhanced water retention and viscosity in aqueous dispersions and digestibility.