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Pectin is a heteropolysaccharide abundant in the cell wall of plants and is obtained mainly from fruit (citrus and apple), thus its properties are particularly prone to changes occurring during ripening process. Properties of pectin depend on the string‐like structure (conformation, stiffness) of the molecules that determines their mutual interaction and with the surrounding environment. Therefore, in this review the primary, secondary, and structures of higher levels of pectin chains are discussed in relation to external factors including crosslinking mechanisms. The review shows that the primary structure of pectin is relatively well known, however, we still know little about the conformation and properties of the more realistic systems of higher orders involving side chains, functional groups, and complexes of pectin domains. In particular, there is lack of knowledge on the influence of postharvest changes and extraction method on the primary and secondary structure of pectin that would affect conformation in a given environment and assembly to higher structural levels. Exploring the above‐mentioned issues will allow to improve our understanding of pectin functionality and will help to tailor new functionalities for the food industry based on natural but often biologically variable source. The review also demonstrates that atomic force microscopy is a very convenient and adequate tool for the evaluation of pectin conformation since it allows for the relatively straightforward stretching of the pectin molecule in order to measure the force‐extension curve which is directly related to its stiffness or flexibility.

Main conclusionDuring on-tree ripening, the pectin distribution changed from polydispersed in cell wall to cumulated in cell wall corners. During apple storage, the pectin distribution returned to evenly dispersed along the cell wall.The plant cell wall influences the texture properties of fruit tissue for example apples become softer during ripening and postharvest storage. This softening process is believed to be mainly connected with changes in the cell wall composition due to polysaccharides undergoing an enzymatic degradation. These changes in polysaccharides are currently mainly investigated via chemical analysis or monoclonal labeling. Here, we propose the application of Raman microscopy for evaluating the changes in the polysaccharide distribution in the cell wall of apples during both ripening and postharvest storage. The apples were harvested 1 month and 2 weeks before optimal harvest date as well as at the optimal harvest date. The apples harvested at optimal harvest date were stored for 3 months. The Raman maps, as well as the chemical analysis were obtained for each harvest date and after 1, 2 and 3 months of storage, respectively. The analysis of the Raman maps showed that the pectins in the middle lamella and primary cell wall undergo a degradation. The changes in cellulose and hemicellulose were less pronounced. These findings were confirmed by the chemical analysis results. During development changes of pectins from a polydispersed form in the cell walls to a cumulated form in cell wall corners could be observed. In contrast after 3 months of apple storage we could observe an substantial pectin decrease. The obtained results demonstrate that Raman chemical imaging might be a very useful tool for a first identification of compositional changes in plant tissue during their development. The great advantage Raman microspectroscopy offers is the simultaneous localization and identification of polysaccharides within the cell wall and plant tissue.Electronic supplementary materialThe online version of this article (doi:10.1007/s00425-015-2456-4) contains supplementary material, which is available to authorized users.

The nanostructure of polysaccharides is supposed to determine properties such as stiffness or diffusivity of cell walls and their functionality for various tailored properties of food. However, at present, a relation of these nano-properties with sensory texture and firmness remains to some degree unknown. In this work, water (WSP), calcium chelator (CSP) and sodium carbonate (DASP) soluble pectins, hemicellulose and cellulose, extracted from cell walls of two pear cultivars 'Xenia' and 'Conference' at their harvest times, were studied. An atomic force microscope and image analysis were used to evaluate diameter and branching of the molecules. Sensory texture of 'Xenia' was considered as better and its firmness (87 N) was higher than 'Conference' (76 N). WSP molecules were present as short molecules with a height of about 0.5 nm for both cultivars. A chain-like and branched CSP fraction had diameter of about 0.3-0.4 nm for both cultivars with a pronounced contribution of molecules with diameter of about 1 nm for 'Xenia', which had also higher branching index. DASP revealed similar regular structures for both cultivars however the network was much denser for 'Xenia'. A rod-like hemicellulose molecules had length of about 20-400 nm and diameter of 1 nm for 'Xenia' and 1-4 nm for 'Conference'. Cellulose diameter for both cultivars was about 23 nm. This study showed that less degraded, thicker and more branched pectin molecules were associated with higher firmness and more favourable texture. Hemicellulose provided a positive contribution to texture when they were thinner and more flexible.

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