ID-10-P-5867 Visualization of internal structure of banana starch granule using AFM and SEM

Introduction: Atomic force microscopy (AFM) is a high resolution technique for studying the external and internal structures of starch granules, without complex sample preparation. Banana starch granule surface and its degradation has been studied in the last few years using microscopic techniques and, recently, it became important to study and visualize the internal structure of starch granule. The technique described by Ridout et al. (2004), where granules were embedded in a resin and sectioned allowing to visualize the inner structure of isolated granules, was used to examine banana starch. Material and Methods: Starch granules were isolated from green and mature bananas (cv. Nanicão), embedded in a non-penetrating resin (Araldite Instant Clear Syringe-90 sec), sectioned using a microtome until a flat and shiny surface is obtained. To achieve contrast, the face of the cut blocks were wetted in steam (20 sec.) and imaged in air using an AFM JPK Nano Wizard II and Alpha300 AR model. The images were obtained in an intermittent contact mode and force modulation imaging. For SEM images, the blocks were coated (10 nm thick platinum) and visualized in a FEI Quanta 600 FEG Microscope. Results: Topographic images of green banana starch (1A, C, G, K) showed large variation of height across the granule (approx. 1 µm), mainly due the absorption of water and swelling of amorphous regions. It seemed that the dark center of the granule was structurally different, with reduced absorption of water compared to the other regions. At higher resolution, the growth rings were visible close to hilum (1D, lateral deflection image and 1F, error signal image) and a different phase contrast was observed in the central region (1H). In another level of structure, the growth rings were composed of globular and uniform structures known as blocklets, hard objects embedded in an amorphous matrix, ranging from 30 to 150 nm, depending on the localization inside the granule (1B, J, N). Otherwise, images of starches from ripe bananas, showed a different level of organization, possibly due to the presence of a degradation mechanism. Granules with different sizes and elongated shapes (2A), absence of growth rings around the hilum (2I, L-M) and dark regions (2F) never reported before indicate holes created by a randomly enzymatic process. More detail is revealed by lateral deflection images (2E, G), which highlighted a dark region in 2F that was harder than the bright region. The more deformable the sample was the greater the tip-contact area becomes, hence the increase in frictional component in soft regions. Phase images (2C, K) confirmed that starches of ripe bananas had different viscoelastic properties and showed larger size of blocklets. Reference. Biomacromol, 5, 1519-1527, 2004.