Isolation, characterisation and functional properties of pectin from gold kiwifruit (Actinidia chinensis cv. Hort16A) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
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Date
2011
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Massey University
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Abstract
This research was concerned with the isolation, physicochemical characterisation and
functional properties of pectin from gold kiwifruit cv. Hort16A. The process of extracting
pectin from gold kiwifruit was developed by evaluating three different techniques (acid,
water and enzyme), four different conditions (time, temperature, puree to solvent ratio and
enzyme concentration) and fruit of two different maturities (early-season and main-season
fruit). The effects of the extraction techniques and conditions on the physicochemical
properties and functional properties of pectin were studied in detail. The effects of the
extraction techniques and the fruit maturity on the functional properties of pectin were also
investigated. The chemical compositions, physical features and rheological behaviours of
the extracted pectins were determined, underlining the importance of these features to the
functional properties of gold kiwifruit pectin.
The total non-starch polysaccharide composition, sugar composition, protein and ash
contents, degree of esterification (DE) and molecular properties (weight-average molecular
weight (Mw), polydispersity index and root mean square radius) of pectin are influenced by
the extraction techniques, the extraction conditions and the degree of maturity. Pectin from
early-harvested fruit (EHF) (less mature fruit) is more difficult to extract than pectin from
main-harvested fruit (MHF) (more mature fruit) when using extraction methods that have
been developed for MHF. This is probably because the cell wall network of less mature
fruit is more compact/dense and the pectin could still be insoluble, whereas the cell wall
network of more mature fruit is less tightly bound because of physiological changes during
maturation. Purified pectin from EHF is characterised by a lower yield (1.52 versus 3.64%
w/w), a higher DE (90 versus 84%), a lower galacturonic acid (GalA) content (40.21 versus
55.50% w/w), more branching chains (a side chain every 47–57 GalA residues versus every
50-97 GalA residues), a higher protein content (25.94 versus 13.82% w/w), a lower Mw
(9.7 x 105 versus 2.52 x 106 g/mol) and a higher viscosity than purified pectin from MHF.
Overall, the physicochemical properties of gold kiwifruit pectin are consistent with its
rheological behaviour.
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Pectins extracted from fruit of different maturities and using different methods have
different physicochemical properties. MHF pectin extracted using a water method is
characterised by an Mw of 3.75 x 106 g/mol, a GalA content of 51.87% w/w, a degree of
branching of 50 and a DE of 84%. MHF pectin extracted using an enzymatic method has a
lower Mw (1.65 x 106 g/mol), similar to that of enzyme-extracted EHF pectin (0.21 x 106
g/mol). In contrast, EHF pectin extracted using a water method has low Mw (1.03 x 106
g/mol) and GalA content (42.88%), suggesting that gold kiwifruit pectin with high Mw is
recovered from more mature fruit.
The rheological properties of the extracted pectin are affected by its Mw and DE. Purified
pectin extracted by enzymatic treatment from both EHF and MHF exhibits the lowest
viscosity and Mw. Purified EHF pectin obtained by water treatment exhibits a higher
viscosity even though the pectin is characterised by a lower Mw (1.03 x 106 g/mol) with a
higher DE (90%) compared to acid-extracted EHF pectin (a higher Mw: 1.66 x 106 g/mol
and lower DE: 88%). This is because of low electrostatic repulsion of high DE pectin,
which reduces intra- and/or intermolecular distances, resulting in greater molecular
association, and the greater degree of branching, which leads to more chain-chain
associations, therefore exhibits higher viscosity. A similar trend is observed for MHF
pectin. Water-extracted MHF pectin, with a high Mw (3.75 x 106 g/mol) and a low DE
(84%), exhibits a lower viscosity than EHF pectin (a low Mw but a high DE, i.e. 90%). The
viscoelastic properties of the pectin are consistent with its physical properties.
Gold kiwifruit pomace, a by-product from gold kiwifruit juice manufacture, may be a new
source of pectin. The physicochemical properties of pomace pectin are quite different from
those of whole fruit pectin. Purified pomace pectin is characterised by a higher GalA
content (64–68% w/w) but a lower Mw (6.7–8.4 x 105 g/mol) and exhibits a lower viscosity.
In the work on the functional properties of gold kiwifruit pectin, its gelation was
investigated and was compared with that of other commercial pectins (apple and citrus).
Gold kiwifruit pectin, which is classified as a high methoxyl pectin (HMP), has “weak gel”
properties and gels at high temperature. Because of low electrostatic repulsion among the
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pectin chains, the gelation properties are not markedly affected by varying the gelation
conditions such as pH and sugar concentration. However, increasing the concentration of
pectin leads to an increase in the gel strength. The viscoelasticity of the gel is strongly
influenced by the DE; the gel strength increases as the DE decreases, because high charge
density (or low DE) pectins have numerous active sites with high probability for hydrogen
bonding, and sufficient hydrophobic interactions, thus resulting in more stabilised molecular
networks.
A common defect in acidified milk drinks (AMDs) is sedimentation caused by protein
aggregation. HMP is commonly added to prevent this separation. In this part of the work,
the influence of gold kiwifruit pectins (DE 84, 85 and 90%) on the stability of AMDs (10%
w/w low heat skim milk powder) was evaluated and was compared with that of pectins from
other sources (apple and citrus). The stabilities, in terms of serum separation, amount of
adsorbed and non-adsorbed pectin, viscosity and particle size, were compared at different
pectin concentrations (0.1–1.0% w/w). High Mw gold kiwifruit pectin (DE 84%) stabilises
an AMD at lower concentrations (0.3% w/w) than other commercial HMPs. In addition,
HMPs with very low charge density and very low Mw do not have the ability to stabilise
AMDs at the pectin concentrations used in this study. These results suggest that Mw has a
very important role in stabilising an AMD. In addition, the viscosity effect created by high
Mw apparently supports the dispersion of casein–pectin complexes in the drink.
Description
Keywords
Kiwifruit pectin, Pectin extraction, Acidified milk drinks