Physicochemical, functional and structural properties of native Malawian cocoyam and sweetpotato starches

[摘要] English: Starches isolated from Malawian sweetpotato, cocoyam and cassava were comparatively studied and evaluated for physicochemical, functional and structural properties in order to unveil their characteristic properties and unravel their potential for industrial application. Further, sweetpotato and cocoyam starches were modified by physical and chemical means, and changes in their functional properties were determined. The results showed that cocoyam, sweetpotato and cassava starch exhibited properties that were unique to botanical source. The pH of the starches ranged within the 4.7 to 7.0 range required for use in low acid foods while moisture content was below the maximum limit required for prolonged storage of the starches. Ash and amylose content of the starches ranged from 0.03 to 0.28%, and 10.6 to 34.4%, respectively while protein and fat levels ranged from 0.35 to 0.84%, and 0.085 to 0.160%, respectively. All starches exhibited high levels of potassium, followed by calcium, phosphorus, magnesium and sodium. Iron, manganese and zinc were present in very low levels compared to the other elements. Cocoyam starch granules exhibited mostly round/spherical shapes similar to that of cassava starch, while sweetpotato starch granules were mostly polygonal in shape. Granule size ranged from 4.0 to 18.7 um, 4.0 to 48.0 um and 5.3 to 22.7 um for cocoyam, sweetpotato and cassava starches, respectively. Cocoyam starch had a larger fraction of smaller sized (0-10 um) granules than sweetpotato and cassava starch. Sweetpotato and cassava starches had mostly medium sized (10-25 um) granules. All starches exhibited B-type crystallinity with distinctive major diffraction peaks at 5.4-5.6° 20 angles, however relatively strong peaks at 7.6-7.8° 20 angles suggesting the presence of single helical conformations. Water absorption capacity, swelling power and solubility of all starches increased with increasing temperature. Cassava starches exhibited higher water absorption capacity, swelling power, paste clarity, resistance to retrogradation and viscosity than cocoyam and sweetpotato starches. Cocoyam starches exhibited higher gelatinization temperatures than sweetpotato and cassava starches. Cocoyam and cassava starches gave similar gelatinization enthalpies which were higher than those of sweetpotato starches. Cocoyam starches displayed higher retrogradation tendencies than sweetpotato and cassava starches. Cocoyam starches displayed higher blue values and wavelength of maximum absorption but lower reducing capacity values than sweetpotato and cassava starches. There was similarity in the packing and orientation of chains in the amorphous regions of sweetpotato and cocoyam starches as evidenced by similar values of the extent of acid hydrolysis of the starches for 12 days. Average molecular weight of amylopectin molecules of the starches studied by HPSEC ranged from 1.42 x 106 to 1.67 x 106 Da. Cocoyam starch contained amylose molecules of lower molecular weight than sweetpotato and cassava starches. Analysis of isoamylase debranched starches showed that sweetpotato starch had the largest fraction of long-chain amylopectins (47%) and the lowest fraction of short-chain amylopectins (45%), while cocoyam starch exhibited the largest fraction of short chain amylopectins (62%) and smallest proportion of large chain amylopectins (32%). Acetylation enhanced water absorption capacity, swelling power and solubility of the starches, but reduced gelatinization temperatures, gelatinization enthalpy and retrogradation tendencies of the starches. Acid hydrolysis reduced the water absorption capacity and swelling power but improved the solubility and paste clarity of starch, and increased gelatinization temperatures and enthalpy. Annealing reduced water absorption capacity, swelling power, solubility and pastes clarity but increased gelatinization temperatures and enthalpy. Due to their differences in physicochemical, functional and structural properties, starches from Malawian cocoyam, sweetpotato and cassava can play different roles in the Malawian industry.

[发布机构] University of the Free State