THE ROLE OF THE PLANT CELL WALL ON THE BIOACCESSIBILITY AND BIOAVAILABILITY OF ANTHOCYANINS AND PHENOLIC ACIDS RELEASED FROM FRUITS AND VEGETABLES.

Anneline Padayachee (2011). THE ROLE OF THE PLANT CELL WALL ON THE BIOACCESSIBILITY AND BIOAVAILABILITY OF ANTHOCYANINS AND PHENOLIC ACIDS RELEASED FROM FRUITS AND VEGETABLES. PhD Thesis, School of Agriculture and Food Sciences, The University of Queensland.

       
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Author Anneline Padayachee
Thesis Title THE ROLE OF THE PLANT CELL WALL ON THE BIOACCESSIBILITY AND BIOAVAILABILITY OF ANTHOCYANINS AND PHENOLIC ACIDS RELEASED FROM FRUITS AND VEGETABLES.
School, Centre or Institute School of Agriculture and Food Sciences
Institution The University of Queensland
Publication date 2011-12
Thesis type PhD Thesis
Supervisor Prof. Mike J. Gidley
Dr. Deirdre Mikkelsen
Dr. Li Day
Total pages 119
Total colour pages 17
Total black and white pages 102
Language eng
Subjects 090805 Food Processing
090803 Food Nutritional Balance
090801 Food Chemistry and Molecular Gastronomy (excl. Wine)
Abstract/Summary Fruits and vegetables are well known sources of vitamins, minerals and fibre as well as polyphenolic compounds and other phytonutrients. Extensive research into the chemoprotective benefits of plant-based foods has found that such compounds and their metabolites have antioxidant, anti-inflammatory and/or anti-carcinogenic properties. Whilst the health benefits associated with high dietary intake of fruit and vegetable polyphenols is a well researched field, the role of the plant cell wall on the bioaccessibility of these compounds is not. Polyphenolic and other nutrient compounds are located within the vacuole of plant cells surrounded by a lipid membrane and thereby separated from the plant cell wall which encapsulates the cell. In order for polyphenols and other nutrients to be bioavailable (i.e. absorbed during digestion through the gastrointestinal tract), they must first become bioaccessible (i.e. released from the plant cell ready to be absorbed by the body). In order for nutrients to be bioaccessible, the plant cell wall needs to be breached with the consequent possibility of binding between nutrients and plant cell walls. This thesis explores the nature of interactions between plant cell walls and two important classes of polyphenolic phytonutrients, anthocyanins and phenolic acids. The main structural constituent of the fruit and vegetable cell wall is cellulose, which is surrounded by a matrix of pectin and hemicelluloses, and at times lignin. Lignin directly affects the textural and palatability qualities of plant foods, and is generally not present in large amounts in fruits and vegetables. Cellulose fibres are tough and insoluble, and hence able to resist substantial force and provide support to the plant cells, whilst pectin increases the flexibility through formation of a co-extensive network with the rigid cellulose fibre network. The degree of methyl esterification (DE) of carboxyl groups is commonly used to differentiate pectins and to control network formation. The role of cell walls in polyphenol bioaccessibility has not been researched extensively. Rupture of the cell wall during mastication or processing (e.g. pureeing or juicing) results in polyphenols being released from the cell vacuole with consequent contact with cell walls for the first time, with the potential for binding interactions to occur. The human upper gastrointestinal tract (stomach and small intestine (S.I.) is unable to digest plant cell walls. Therefore if polyphenols bound to plant cell walls at the point of ingestion are not released from the cell wall material during transit through the gastric and small intestinal digestive phases, they will be carried by the cell walls to the large intestine where they may be released by bacterial fermentation of cell wall polymers and potentially further metabolised by bacteria before being either absorbed or excreted. This research fills several major gaps in current knowledge by: 1) identifying potential mechanisms for polyphenol-plant cell wall interactions; 2) assessing the bioaccessibility of polyphenols in a real vegetable system; and 3) examining the release of plant cell wall-bound polyphenols during gastric and S.I. digestion and the potential delivery of polyphenols to the colon. The extent of anthocyanin and phenolic acid interaction with cell wall components was investigated by monitoring the rate of polyphenol depletion from diluted purple carrot juice concentrate in the presence of pure cellulose or cellulose-pectin composites containing either low DE or high DE pectins, produced by the bacterium Gluconacetobacter xylinus. Binding of anthocyanins to plant cell wall components seems to be a 2-stage process with initially (1 hour) a limited amount (13-18%) of anthocyanins bound to the surface of the cellulose or cellulose-pectin composite. Whilst anthocyanins bound to both cellulose and pectin, more were bound to pectin. With prolonged exposure (7 days) to cell wall material, a gradual increase in anthocyanin binding occurred with approximately 35% binding to cellulose and the high DE composite and up to 80% binding to the low DE composite. This may be due to anthocyanins stacking on top of a base layer. Support for localised deposition of anthocyanins was found from confocal microscopy which showed apparent local high concentrations of anthocyanins alongside regions with much less, if any, bound anthocyanins. There does not seem to be molecular selectivity as depletion from solution of individual anthocyanin molecules is similar throughout the binding process. Phenolic acid depletion from solution in the presence of model cell walls also occurred. However unlike anthocyanins, more phenolic acids were bound to the pure cellulose initially than to the cellulose-pectin composites. Phenolic acid depletion was rapid, with approximately 20% phenolic acids binding to cell wall components within the first hour. This gradually increased to approximately 35% (low DE composite) and 50% (high DE composite and pure cellulose) over 14 days contact time. Additionally phenolic acid depletion from different concentrations of diluted purple carrot juice concentrate was similar indicating that cell walls may have a limited saturation level for binding phenolic acids. Conversely, doubling the concentration of free anthocyanins available led to at least twice as much binding with cell wall components. This indicates that whilst anthocyanin binding is slow, it may not be limited by available binding sites. Extrapolation of data from these model systems to carrot puree suggests that significant amounts of anthocyanins and phenolic acids could bind to the plant cell wall, potentially restricting their bioavailability in the S.I. In order to ascertain the potential extent of polyphenol delivery to the large intestine via plant fibre in a real food system, polyphenols bound to cell wall material in a black carrot puree were subjected to simulated gastric and small intestinal digestion. It was found that the majority of available hydrophilic polyphenols derived from black carrots bound to the PCW matter with only ~ 36 % of the total phenolic acids (~ 1.2 mg/g puree) and 30 % of anthocyanins (1.3 mg/g puree) in the black carrot puree being released into the liquid phase. Approximately 30 % of the bound phenolic acids and anthocyanins could be extracted with acidified methanol. However, only <5 % of anthocyanins and phenolic acids were released during simulated gastric and small intestinal digestion. This is in agreement with results from the model cell wall system where the majority of bound polyphenols remained bound after simulated gastric and small intestinal digestion indicating that the binding mechanism between polyphenols and cell wall components is sufficiently strong to resist pH changes during the digestive process. This may be due to a 3-phase mechanism with initial random binding of anthocyanins and phenolic acids to cellulose fibres followed by further deposition of anthocyanins and phenolic acids, finally resulting in penetration into the internal cavity of the fibre of at least some polyphenols. As almost all polyphenols bound to cell wall components remain bound during simulated gastric and small intestinal digestion, they would be expected to be delivered to the large intestine where fermentation and metabolism by gut bacteria can occur. The extent of release and uptake of polyphenols bound to cell walls by gut bacteria fermentation and the effect on large bowel health should be addressed in future studies.
Keyword Anthocyanins
Polyphenols
Phenolic Acids
confocal microscopy
Plant cell wall
cellulose
Pectin
Simulated digestion
Additional Notes Colour pages: 32, 35, 38, 40, 42, 56, 64, 65, 67, 68, 69, 77, 78, 80, 87, 97, 101. Landscape pages: 91, 96, 116, 117, 118, 119.

 
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Created: Thu, 28 Jun 2012, 15:11:34 EST by Ms Anneline Padayachee on behalf of Library - Information Access Service