Effect of okra extract on properties of wheat, corn and rice starches.

摘要

Seedless okra pods were extracted with 0.05 M NaOH. The extract was centrifuged and the supernatant and the precipitate were freeze-dried. Size exclusion high pressure chromatography showed that the supernatant contained peptides covalently bound to the sugars. Wheat, corn and rice starch blends were prepared by replacing 0, 5, 10 and 15% of the starch with dried supernatant (DSP) or dried precipitate (DPP). The effect of DSP and DPP on starch properties was determined by rapid visco-analyser (RVA), gel texture analysis and dynamic rheology. The presence of DSP significantly (p<=0.05) reduced the peak temperature of starch compared to 100% starch. The DPP containing samples exhibited significantly higher peak viscosity due to the presence of more cellulose. Similar effect was observed on the setback of all starches. Starch-gel firmness was significantly reduced by DSP. Although corn starch showed the highest gel firmness value, it had the highest drop in firmness, whereas wheat starch showed a linear reduction in firmness compared to corn starch. The reduction in gel firmness is due to two factors, the replacement of starch and the action of the DSP. The reduction in gel firmness was found to be 39% for 15% wheat starch replacement. All test frequencies showed that starches exhibited G' higher than G". The elastic moduli (G') of wheat starch reached a plateau about 1360 Pa, while the phase shifts were in the range of 5 degrees -17 degrees . Corn starch showed slightly weaker viscoelastic solid properties compared to wheat starch with a G'-plateau around 1120 Pa, and phase shifts range between 5 degrees and 16 degrees , while rice starch was the weakest. These variations are due to the starch granule structure and the amylose/amylopectin ratio.