NANO STRUCTURE OF NATIVE PECTIN SUGAR ACID GELS VISUALIZED BY ATOMIC FORCE MICROSCOPY

Authors: Fishman, Marshall; Cooke, Peter; Coffin, David

Submitted to: Biomacro Molecules
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2004
Publication Date: 1/21/2004
Citation: Fishman, M.L., Cooke, P.H., Coffin, D.R. Nano structure of native pectin sugar acid gels visualized by atomic force microscopy. Biomacromolecules.2004. 5(2), 334-341.

Interpretive Summary: The need to increase utilization of low valued co-products derived from the processing of fruit has prompted us to investigate the structure of pectin gels. Pectin is a polysaccharide found in orange peels. In this work we show that high quality food grade pectin can be extracted rapidly from a portion of the peel, by microwave heating under pressure in a dilute acid solution. The quality of the orange pectin is demonstrated by microscope images which show that the molecular structure of orange pectin gel is comparable to that of food gels made from commercial pectin. This research should be of help to fruit growers and processors by increasing the demand and value of their by-products without increasing the cost of the basic commodity to the consumer.

Technical Abstract: Height and phase shift images of high methoxyl sugar acid gels (HMSAG) of pectin were obtained by atomic force microscopy in the Tapping ModeTM. Images revealed those pores in these gels were fluid and flattened out when allowed to stand. These images revealed for the first time the structure of adsorbed sugar on pectin in the hydrated native gels and how the pectin framework is organized within these gels. Segmentation of images revealed that the underlying pectin framework contained combinations of rods, segmented rods, and kinked rods connected end to end and laterally. The open network of strands was similar to pectin aggregates imaged earlier by electron microscopy from 5 mM NaCl solution (Fishman et al., Archives of Biochem. Biophys. 294, 253, 1992). Area measurements revealed that the ratio of bound sugar to pectin was in excess of 100 to one (w/w). Furthermore, images indicated relatively small differences in the organization of native Commercial Citrus Pectin, Orange Albedo Pectin and Lime Albedo Pectin gels at optimal pH. The findings are consistent with earlier gel strength measurements of these gels. In addition, values of gel strength were consistent with values of molar mass and viscosity of the constituent pectins in that they increased in the same order. Finally, we demonstrated the advantage of simultaneous visualization of height and phase shift images for observing and quantitating the nano structure of relatively soft gels which are fully hydrated with a buffer.