The emulsifying properties of Cruciferin-rich and Napin-rich protein isolates from Brassica napus L.

Abstract

The influence of pH (3.0, 5.0 and 7.0) and ionic strength (0, 50 and 100 mM NaCl) on the physicochemical and emulsifying properties of cruciferin-rich (CPI) and napin-rich (NPI) protein isolates were examined. Specifically, the surface characteristics (charge and hydrophobicity), solubility, interfacial tension and emulsifying activity (EAI) and stability (ESI) indices were measured. In the case of the cruciferin-rich protein isolate, surface charge was found to be negatively and positively charged at pHs above and below its isoelectric point (~4.6-4.8), respectively, ranging in potential from -33 mV at pH 8.0 to +33 mV at pH 3.0. In the presence of NaCl, the overall magnitude of charge became reduced at all pHs. In contrast, hydrophobicity, solubility and the ability for CPI to reduce interfacial tension all were found to be dependent upon both pH and NaCl concentration. Solubility was found to be lowest at pH 5.0 (~11%) and 7.0 (16%) for CPI without salt, but was significantly improved with the addition of NaCl (>80%). Interfacial tension was found to be lowest (10-11 mN/m) for pH 5.0 – 0 mM NaCl and pH 7.0 – 50/100 mM NaCl. Overall, the presence of salt reduced EAI with increasing levels of NaCl at pH 5.0 and 7.0, but not at pH 3.0. In contrast, ESI became reduced with the addition of NaCl (regardless of the concentration) from ~15.7 min at 0 mM NaCl to ~12 min with 50/100 mM NaCl, from ~14.7 min at 0 mM NaCl to ~11.5 min with 50/100 mM NaCl and from 15.1 min at 0 mM NaCl to ~11.7 min with 50/100 mM NaCl for pH 3.0, 5.0 and 7.0, respectively. ESI also was found to be unaffected by pH. In the case of a napin-rich protein isolate, surface charge for the NPI in the absence of NaCl ranged between ~ +10 mV to ~ -5 mV depending on the pH, becoming electrically neutral at pH 6.6. The addition of NaCl acted to reduce the surface charge on the NPI and caused a shift in its isoelectric point to pH 3.5 and 3.9 for the 50 and 100 mM NaCl levels, respectively. Overall, surface hydrophobicity for the NPI was reduced as the pH increased, whereas as NaCl levels were raised the hydrophobicity declined. In contrast, NPI solubility was found to be high (~93-100%) regardless of the solvent conditions. The ability of NPI to reduce interfacial tension was enhanced at higher pHs, however the effect of NaCl was pH dependent. Overall, EAI values were similar in magnitude at pH 3.0 and 5.0, and lower at pH 7.0. The effect of NaCl on EAI was similar at pH 3.0 and 7.0, where EAI at the 0 mM and 100 mM NaCl levels were similar in magnitude, but increased significantly at 50 mM NaCl. However, the EAI values at pH 5.0 were reduced as the level of NaCl increased. Overall, the stability of NPI-stabilized emulsions degraded rapidly and the addition of salt induced faster emulsion instability. In summary, CPI and NPI were very different in terms of their physicochemical properties. However, the emulsifying properties were similar in magnitude indicating that they had similar emulsifying potential under the solvent conditions examined.