Effect of alkali treatment on waxy wheat starch
L. COPELAND (1), S. Al-Dhaher (1). (1) Univ of Sydney, Sydney, NSW, Australia
Alkali treatment is used for starch modification and alkali-washing step is often used in the isolation of starch granules. Alkali treated starch granules appear unchanged morphologically, but have reduced amylose content and changed functionality. The mechanism and time frame of these changes are not understood, nor are the effects of alkali on waxy starches. In this study, starch from four waxy genotypes derived from an Australian soft wheat variety (QAL2000) was exposed to 0.1 M NaOH at 35oC for 1, 5 and 15 days. The recoveries of starch after the incubations were 91 to 95%. The amylose content of QAL2000 starch (27%) decreased to 23%, 19% and 18% after 1, 5 and 15 days, respectively, whereas the waxy genotypes did not change from the original value of 1-2%. Relative starch crystallinity did not change, but the DSC onset and peak endothermic transition temperatures increased by 3-5oC for all of the starches. There was a sharp decline after 1 day in swelling power (15.1 to 11.8) and peak and final pasting viscosities (PV, 452 to 110 cP; FV, 498 to 134 cP) for QAL2000 starch, but no further changes occurred with longer exposure to alkali. Smaller decreases were noted initially for the waxy starches in swelling power (from 18-20 to 17) and pasting viscosities (typically, PV, 1696 to 561; FV, 873 to 272) but, unlike QAL2000, after 5 and 15 days the swelling power increased to close to the original values and there was a 60-80% recovery in pasting viscosities. When starch was isolated with an alkali-wash step, QAL2000 granules had lower peak viscosity than with a buffer wash (452 cP vs 627 cP), but waxy granules had higher peak viscosity (1696 vs 1315 cP). The effects of alkali on QAL2000 are likely to be due to the loss of amylose molecules involved in granular organization, whereas the effects on waxy starch may be similar to annealing.
View Presentation |
