DETERMINATION OF THE POLYMORPHIC COMPOSITION OF SMOOTH PEA STARCH

A method for estimating the proportions of 'A' and 'B' polymorphs comp rising a sample of 'C' type starch is proposed which uses established experimental techniques with commercially available spreadsheet and X- ray analysis software. Waxy maize, potato and smooth pea starches were used to provide X-ray diffraction patterns characteristic of the 'A', 'B' and 'C' starch polymorphs. Samples of amorphous starches were also prepared. The method initially involved subtraction of the amorphous phase and instrumental background from the X-ray diffraction patterns of each starch sample using the spreadsheet program, Lotus 1-2-3. The remainder of the pattern, representing the crystalline portion of the starch sample, was then analysed by profile fitting to elucidate the p ositions and areas of individual diffraction peaks. The ratio of the t otal peak area to the areas under peaks characteristic of 'A' and 'B' type starches, respectively, were used to calculate the relative propo rtions of these polymorphs in smooth pea starch. These proportions wer e found to be 56+/-3% 'A' polymorph to 44+/-3% 'B' polymorph. A 'C' ty pe pattern was constructed by using Lotus 1-2-3 to combine diffraction patterns from the crystalline portions of 'A' and 'B' type starches i n the proportions given above. Polymorph patterns were obtained by man ipulation of the diffraction patterns from the crystalline portions of starches using Lotus 1-2-3. An 'A' type pattern was obtained by subtr action of a 'B' type pattern from that of a 'C' type. Similarly, a 'B' type pattern was obtained by subtraction of an 'A' type pattern from that of a 'C' type. (C) 1997 Elsevier Science Ltd.