Title: Process-structure-function relations of pectin in plant-based foods: the use of anti-homogalacturonan antibodies in an integrated methodological approach.
Other Titles: Proces-structuur-functie relaties van pectine in plantgebaseerde levensmiddelen: het gebruik van anti-homogalacturonan antilichamen in een geïntegreerde methodologische aanpak.
Authors: Christiaens, Stefanie
Issue Date: 3-May-2012
Abstract: Pectin has been identified as a critical structural component of plant cell walls contributing to tissue integrity and rigidity. As a result, this polysaccharide strongly determines the textural/rheological quality of fruit- and vegetable-based products. PectinÂ’s functional properties depend on its molecular fine structure, which can be altered enzymatically as well as non-enzymatically. Several modifications in pectin structure occur during food processing, rendering pectin a major point of interest for food technologists seeking to tailor the textural/rheological properties of plant-based food products. So far, the process-structure-function relation of pectin in food matrices has predominantly been investigated via the physicochemical analysis of isolated cell-wall material with the consequent loss of most spatial information. Today, monoclonal anti-pectin antibodies, originally generated in the context of plant cell wall biology, provide a new range of opportunities as they allow the precise localisation of defined structural pectic domains in intact cell walls in plant tissues. Antibodies binding to the homogalacturonan (HG) domain of pectin are of particular interest for food technologists as HG in general and its degree of methyl-esterification (DM) in particular strongly determine the functionality of pectin in food matrices. The current study aimed at the implementation of these anti-HG antibodies in an integrated methodological approach in order to unravel the structure-function relation of pectin during processing of plant-based foods.In a first step, the potential of the anti-HG antibodies to investigate the effect of processing on pectin in fruits and vegetables was explored. Specifically, the binding specificity of these probes towards pectins and methoxylated polygalacturonic acids with defined degree and pattern of methyl-esterification was determined via immuno-dot assays and, moreover, a microscopic method to use these monoclonal antibodies in situ in plant-based food systems was developed and implemented. Subsequently, four different case studies (on broccoli tissue, broccoli purée, carrot purée and tomato suspensions) were performed in which the traditional physicochemical analysis of isolated cell-wall material was combined with the ex situ and in situ application of anti-HG antibodies to acquire in-depth insight into the process-structure-function relations of pectin. Broccoli, carrot and tomato were subjected to various thermal, high-pressure and mechanical treatments changing pectinÂ’s structure and, hence, its functional properties. Thermal processing of broccoli and carrot was demonstrated to result in a thermosolubilisation and ß-eliminative depolymerisation of pectin. The induced weakening of the intercellular adhesion could be related to texture softening of broccoli tissue, whereas syneresis phenomena were limited in broccoli and carrot purées. In situ visualisation of pectin with anti-HG antibodies JIM7 and LM20 showed that, after thermal processing, highly methyl-esterified, water-soluble pectic polymers became detached and loosely bound to the cell wall. Conversely, pretreatments stimulating the endogenous pectin methylesterase (PME) activity were shown to cause a decrease in the DM of pectin and, correspondingly, an increase in Ca2+-cross-linked pectin at the expense of loosely bound water-soluble pectin. The resulting stronger intercellular adhesion could be linked to a reduced texture degradation during subsequent thermal processing of broccoli tissue and a higher resistance of broccoli and carrot tissue to physical disintegration during purée preparation. For carrot, this alteration in pectin structure was reflected in a purée with a high consistency and a large degree of separation between serum and pulp. Anti-HG antibody PAM1 revealed that process-induced pectin demethoxylation by endogenous PME mainly took place at the tricellular junctions of adjacent cells in broccoli and at discrete regions of the inner face of the cell wall adjacent to the plasma membrane in carrot. In tomato, process-induced pectin changes did not only depend on the activity of PME, but also on the activity of the depolymerising enzyme polygalacturonase (PG). In case PG was selectively inactivated, i.e. in high-pressure pretreated tomatoes, de-esterification of pectin by PME also resulted in a high level of Ca2+-cross-linked pectin and a stronger intercellular adhesion, similar as in broccoli and carrot. Immunolabeling showed that in intact tomato fruit, pectin de-esterification was endogenously regulated by the physical restriction of PME activity in the cell wall matrix, whereas in disintegrated tomato tissue intensive demethoxylation of pectin could occur throughout the entire cell wall. Finally, high-pressure homogenisation, an intense shear treatment used for tissue disruption, affected pectin solubility depending on the plant matrix. This thesis shows that anti-HG antibodies are valuable tools in elucidating the process-structure-function relations of pectin in food matrices by providing the opportunity to explore local changes in pectin structure in situ. The obtained results form a sound basis to control the textural/rheological properties of plant-based food products and to identify targets for food-structure engineering.
ISBN: 978-90-8826-237-1
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Food and Microbial Technology

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