Separation and characterization of A- and B-type starch granules in wheat endosperm

Mature wheat (Triticum aestivum L.) endosperm contains two types of starch granules: large A-type and small B-type. Two methods, microsieving or centrifugal sedimentation through aqueous solutions of sucrose, maltose, or Percoll were used to separate A- and B-type starch granules. Microsieving could not completely separate the two types of starch granules, while centrifuging through maltose and sucrose solutions gave a homogenous population for B-type starch granules only. Centrifuging through two Percoll solutions (70 and 100%, v/v) produced purified populations of both the A- and B-type starch granules. Analysis of starch granule size distribution in the purified A- and B-type granule populations and in the whole-starch granule population obtained directly from wheat endosperm confirmed that the purified A- and B-type starch granule populations represented their counterparts in mature wheat endosperm. Centrifugations through two Percoll solutions were used to purify A- and B-type starch granule populations from six wheat cultivars. The amylose concentrations and gelatinization properties of these populations were analyzed. All of the A-type starch granules contained higher amylose concentrations and had higher gelatinization enthalpies than did B-type starch granules. Although A- and B-type starch granules started to gelatinize at a similar temperature, B-type starch granules had higher gelatinization peak and completion temperatures than did A-type starch granules.