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Methods previously developed for the atomic force microscopy (AFM) imaging of individual polysaccharides (Kirby, A. R.; Gunning, A. P.; Morris, V. J. Biopolymers 1996, 38, 355−366) have been extended in order to image networks and gels formed by the bacterial polysaccharide gellan gum. Networks were formed by air-drying solutions of K+ gellan onto freshly cleaved mica. AFM images were obtained in the direct current contact mode at constant force under butanol. Network formation can be inhibited by diluting the stock gellan solution or by converting K+ gellan into the tetramethylammonium salt form. Inhibition of network formation led to AFM images of heterogeneous populations of gellan aggregates (gel precursors). Attempts have been made to image the surface of bulk aqueous gellan gels under butanol. The quality of the images obtained improved with increasing gel modulus. For rigid acid-set gellan gels, molecular resolution was achieved, revealing a bifurcated branched fibrous network.

In this review, the main features of the pectin disassembly process during fruit ripening are first discussed, and then the nanostructural characterization of fruit pectins by AFM and its relationship with texture and postharvest fruit shelf life is reviewed. In general, fruit pectins are visualized under AFM as linear chains, a few of which show long branches, and aggregates. Number- and weight-average values obtained from these images are in good agreement with chromatographic analyses. Most AFM studies indicate reductions in the length of individual pectin chains and the frequency of aggregates as the fruits ripen. Pectins extracted with sodium carbonate, supposedly located within the primary cell wall, are the most affected.

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.

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Techniques have been developed for the routine reliable imaging of polysaccharides by atomic force microscopy (AFM). The polysaccharides are deposited from aqueous solution onto the surface of freshly cleaved mica, air dried, and then imaged under alcohols. The rationale behind the development of the methodology is described and data is presented for the bacterial polysaccharides xanthan, acetan, and the plant polysaccharides 1‐carrageenan and pectin. Studies on uncoated polysaccharides have demonstrated the improved resolution achievable when compared to more traditional metal‐coated samples or replicas. For acetan the present methodology has permitted imaging of the helical structure. Finally, in addition to data obtained on individual polysaccharides, AFM images have also been obtained of the network structures formed by κ‐carrageenan and gellan gum. © 1996 John Wiley & Sons, Inc.