Confocal Raman and AFM imaging of individual granules of octenyl succinate modified and natural waxy maize starch

This is the first known report of applying confocal Raman and AFM imaging to single modified and native starch granules. Chemical modification of starch is used to replace OH groups on glucose units with a hydrophobic group (e.g., octenyl succinate). The esterification of waxy maize starch involves a two phase reaction mixture. The uniformity of distribution of the modifier is of concern for the product. The modifier load can be determined by other means, however vibrational microspectroscopy is useful to assess the distribution of the modifying groups. We have previously employed FT-IR microspectroscopy to individual flattened starch granules to determine which granules were esterified in the processing step. The intergranular census showed remarkable agreement with the calculated formulation. Synchrotron infrared confocal microspectroscopic mapping of 10 μm × 10 μm areas of flattened granules indicated intragranular localization of carbonyl groups within the limits of spatial resolution. Specimens from the same lot of octenyl succinated waxy maize starch were subjected to Raman confocal microspectroscopic imaging and AFM topography to compare modified and native single starch granules. Without the necessity of thin specimen preparation Raman microspectroscopic imaging revealed modification of the outer surface of the essentially spherical granule in comparison to the native granule was achieved with a nominal spatial resolution of 200 nm (the diffraction limit). Also, individual flattened granules were imaged. The confocal configuration resulted in 3-D images, depth profiles, and spectroscopic evidence of molecular orientation changes likely resulting from the flattening process. The AFM topography provided subnanometer vertical resolution and 1 μm lateral precision with a simultaneously recorded phase image to reveal highly localized, differing functional characteristics.