Starch molecules generally is deposited as discrete semi-crystalline aggregates known as starch granule in the plant. Starch granules are the most vital energy reserve in plants. They are mainly composed of amylopectin with a minor fraction of amylose. The structure of starch Granule is proposed to consist of concentric shells, semi-crystalline, and soft amorphous layers. The shape, volume and structure of starch granule are the most important factors contributing to starch quality. The starch granule number, volume and starch composition are distinct in different starch species.
The granule size distribution of starch is a very important characteristic that strongly affects its physicochemical properties and chemical composition which in turn may affect the functionality. It directly affects the ratio of amylopectin to amylose, the gelatinization properties, and the properties of the starch. Starch granule size distribution is an important factor that influences the quality of many processed products.
Figure 1. Light microscopic image of purified starch granules
Each plant species has its own unique starch granule size. It is generally accepted that wheat starch granules can be divided into large A and small B types according to their size and shape. The large A type starch granules are discoid or lenticular in shape with an average diameter that ranges from 10 to 35 µm, and it accounts for 3% of the total number of wheat starch granules. The small B-type starch granules are usually spherical or polygonal in shape with diameter less than 10 µm, and it contributes to more than 90% of the total number of starch granules. Wheat starch granules in some literatures also can be categorized into three types with different size ranges: A type granules with diameter larger than 15 μm, B type granules with diameter ranges from 5 to 15 µm, and C type granules diameter smaller than 5 μm. The granule size of maize starch usually ranges from 1 to 7 μm for small granules and 15 to 20 μm for large granules. Rice starch granules generally have diameters that ranges from 3 to 5 μm in size. The large A type and the small B type starch granules have different physicochemical and functional properties.
Figure 2. Example of starch granule size distribution from developing wheat endosperm
Numerous studies have been conducted on granule size distribution of starch. Digital image analysis coupled with light microscopy can be used to record physical parameters for each individual particle and to distinguish among individual granules. However, the image analysis is limited to a small sample size, and acquiring data and analysis can be time-consuming. Laser diffraction sizing of particles significantly reduces analysis time to minutes per sample with results including volume, number, and surface area percent. Currently the laser diffraction particle size analyzer is commonly used for fast and accurate analysis of granule size distributions for both wet and dry dispersions.
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- Wenyang Li, Suhui Yan, Xiaqing Shi, Congyu Zhang, Qingqin Shao, Feng Xu, Jianfei Wang (2015) Starch granule size distribution from twelve wheat cultivars in east China's Huaibei region. Canadian Journal of Plant Science, 96 (2), 176-182.
- Renuka N. Waduge, Danusha N. Kalinga, Eric Bertoft, and Koushik Seetharaman (2014) Molecular Structure and Organization of Starch Granules from Developing Wheat Endosperm. Cereal Chem., 91(6):578–586.