Submitted to: Carbohydrate Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2006
Publication Date: 1/2/2007
Citation: Li, L., Blanco, M.H., Jane, J. 2007. Physicochemical properties of pericarp and endosperm starches during maize development. Carbohydrate Polymers. 67:630-639. Interpretive Summary: Corn is a major source of starch, and provides up to 95% of all starch manufactured in the US. Starch comprises up to 78% of kernel weight and is important for grain yield, total energy, and numerous specialty products including bio-fuels such as ethanol. Starch is stored in granules in the kernel, and present in the endosperm (internal tissue excluding the embryo), and pericarp (external outer tissue). The molecular structure of starch includes two major polymers- amylose (linear molecule of repeating glucose units), and amylopectin (branched chain glucose units). The functional properties of starch have important economic value and are determined by factors such as the ratio of amylose to amylopectin, the degree of molecular branching, and starch granule size, number, and shape. More information is needed about the synthesis of starch during kernel development, the molecular structure of amylopectin molecules, and the interrelationship with functional properties such as gelatinization, and retrogradation. This study used a “model” corn inbred, B73 (important historical and commercial interest, and model for molecular biology) for starch isolation, and molecular observations beginning at 5 days after pollination (DAP), and repeated observations at 12, 14, 30, and 45 DAP. Both endosperm, and pericarp tissue was studied. In the endosperm, total starch content, granule size, and amylose increased during kernel development. Branch chain length distribution of endosperm amylopectins was measured by degree of polymerization (DP), and indicated that shorter chain lengths (DP 23.6) were present at 10 DAP, and increased to the maximum (DP 26.7) at 14 DAP, and then gradually decreased beginning at 30 DAP (DP25.4), and further decrease (DP24.9) at 45 DAP. These molecular changes were accompanied by changes in thermal properties such as increasing onset of gelatinization temperatures between 8 DAP (61.3°C) to 14 DAP (69.0°C), and gradually decreasing at 30 DAP (67.4°C), to 45 DAP (62.8°C). In contrast to endosperm tissue, there were no significant changes in granule size, amylose content, starch content, or thermal properties of pericarp starch during kernel development. This study suggested that the mechanism of starch synthesis in the endosperm and in the pericarp may be different. The research is of interest to a broad range of stakeholders such as food chemists, plant physiologists, geneticists, end users in the starch industry, and ultimately the consumer. The impact of this research will improve long term economic value of corn nutritional quality by enhancing germplasm resources for starch traits. This includes improved starch digestibility for livestock and human nutrition, and the development of resistant starch for human dietary research in obesity, and diabetes.
Technical Abstract: Endosperm starch and pericarp starch were isolated from maize (B73) kernels at different developmental stages. Starch granules, with small size (2-4 'm diameter), were first observed in the endosperm on 5 days after pollination (DAP). The size of endosperm-starch granules remained similar until 12DAP, but the number increased extensively. A substantial increase in granule size was observed from 14DAP (diameter 4-7 'm) to 30DAP (diameter10-23 'm). The size of starch granules on 30DAP is similar to that of the mature and dried endosperm-starch granules harvested on 45DAP. The starch content of the endosperm was little before 12DAP (less than 2%) and increased rapidly from 10.7% on 14DAP to 88.9% on 30DAP. The amylose content of the endosperm starch increased from 9.2% on 14DAP to 24.2% on 30DAP and 24.4% on 45DAP (mature and dried). The average amylopectin branch chain-length of the endosperm amylopectin increased from DP23.6 on 10DAP to DP26.9 on14DAP and then decreased to DP25.4 on 30DAP and DP24.9 on 45DAP. The onset gelatinization temperature of the endosperm starch increased from 61.3'C on 8DAP to 69.0'C on 14DAP and then decreased to 62.8'C on 45DAP. The results indicated that the structure of endosperm starch was not synthesized consistently through the maturation of kernel. The pericarp starch, however, showed similar granule size, starch content, amylose content, amylopectin structure and thermal properties at different developmental stages of the kernel.