A.P. Gunning

Using AFM to image the internal structure of starch granules

Atomic force microscopy (AFM) has been used to observe the ultra-structure of starch granules for starches from different botanical sources (maize and potato). Starch granules were embedded, sectioned and imaged in dc constant force topographic, error signal and force modulation modes. AFM images of the starches embedded in Nanoplast®, a resin previously used for AFM and electron microscopy studies, failed to reveal the growth ring structure within the granules consistently. Furthermore, ultra-structural features could not unequivocally be attributed to the ‘blocklet’ structure of the granule because of the underlying granular texture of the resin itself. In contrast the use of rapid setting Araldite, a non-penetrating resin, allowed the ultra-structure of the granules to be viewed without the necessity of pre-treatment (lintnerisation or enzymatic degradation) steps. The images demonstrated clearly the importance of the choice of embedding material, and showed that under the correct conditions ‘growth rings’ and blocklet structures can be observed in ‘near native’ granules.

Visualization of plant cell walls by atomic force microscopy.

Atomic force microscopy has been used to visualize the ultrastructure of hydrated plant cell wall material from prepared apple (Malus pumila MILL; Cox orange pippin), water chestnut (Eleocharis dulcis L.), potato (Solanum tuberosum L.; Bintje), and carrot (Daucus carota L.; Amsterdamse bak) parenchyma. Samples of cell wall material in aqueous suspension were deposited onto freshly cleaved mica. Excess water was blotted away and the moist samples were imaged in air at ambient temperature and humidity. The three-dimensional images obtained highlighted the layered structure of the plant cell walls and revealed features interpreted as individual cellulose microfibrils and plasmodesmata.

Gelation of gelatin observation in the bulk and at the air-water interface

Gelation of gelatin under various conditions has been followed by atomic force microscopy (AFM) with the objective of understanding more fully the structure formed during the gelation process. AFM images were obtained of the structures formed from both the bulk sol and in surface films during the onset of gelation. While gelation occurred in the bulk sol, the extent of helix formation was monitored by measurements of optical rotation, and the molecular aggregation was imaged by AFM. Interfacial gelatin films formed at the air-water interface were also studied. Measurements of surface tension and surface rheology were made periodically and Langmuir-Blodgett films were drawn from the interface to allow AFM imaging of the structure of the interfacial layer as a function of time. Structural studies reveal that at low levels of helical content the gelatin molecules assemble into aggregates containing short segments of dimensions comparable to those expected for gelatin triple helices. With time larger fibrous structures appear whose dimensions suggest that they are bundles of triple helices. As gelation proceeds, the number density of fibers increases at the expense of the smaller aggregates, eventually assembling into a fibrous network. The gel structure appears to be sensitive to the thermal history, and this is particularly important in determining the structure and properties of the interfacial films.

The effect of physiological conditions on the surface structure of proteins: Setting the scene for human digestion of emulsions

Understanding and manipulating the interfacial mechanisms that control human digestion of food emulsions is a crucial step towards improved control of dietary intake. This article reports initial studies on the effects of the physiological conditions within the stomach on the properties of the film formed by the milk protein (

Atomic force microscopy in food research: A new technique comes of age

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Imaging bacterial polysaccharides by AFM

-lactoglobulin) at the air-water interface. Atomic force microscopy (AFM), surface tension and surface rheology techniques were used to visualize and examine the effect of gastric conditions on the network structure. The effects of changes in temperature, pH and ionic strength on a pre-formed interfacial structure were characterized in order to simulate the actual digestion process. Changes in ionic strength had little effect on the surface properties. In isolation, acidification reduced both the dilatational and the surface shear modulus, mainly due to strong repulsive electrostatic interactions within the surface layer and raising the temperature to body temperature accelerated the rearrangements within the surface layer, resulting in a decrease of the dilatational response and an increase of surface pressure. Together pH and temperature display an unexpected synergism, independent of the ionic strength. Thus, exposure of a pre-formed interfacial

Characterising semi-refined iota-carrageenan networks by atomic force microscopy

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Characterisation of the polysaccharide produced by Acetobacter xylinum strain CR1/4 by light scattering and atomic force microscopy

-lactoglobulin film to simulated gastric conditions reduced the surface dilatational modulus and surface shear moduli. This is attributed to a weakening of the surface network in which the surface rearrangements of the protein prior to exposure to gastric conditions might play a crucial role.

Watching molecular processes with the atomic force microscope: dynamics of polymer adsorption and desorption at the single molecule level

Probe microscopes image surfaces by touching them with a sharp probe or stylus. The resolution of such microscopes is determined by the size and shape of the probe, and the accuracy with which it can be positioned relative to the sample surface. Under suitable conditions, these microscopes are capable of imaging atoms and molecules. This article focuses on the potential of such techniques for studying complex food materials.

Galectin 3-β-galactobiose interactions.

SummaryAtomic force microscopy has been used to image the bacterial polysaccharides xanthan, acetan and gellan. Images were obtained under constant force conditions in a liquid cell. Drops of dilute solutions of the polysaccharides were deposited onto freshly cleaved surfaces of mica and allowed to dry in air. The deposits were then imaged under butanol. Xanthan and acetan form entangled networks upon deposition. Individual molecules can be identified. Under suitable circumstances it has been possible to image the helical structure of acetan. Gellan forms gels upon concentration during drying and images have been obtained of the gel network.