Agriculture Reference
In-Depth Information
de-esterifi cation, normally using acid, but enzyme treatments are also available.
Addition of ammonia during this process produces a third type of pectin, called
amidated low methoxyl pectin (LMA). The differing structure of these pectin
types determines their functionality and application.
Chemically, pectin is a linear polysaccharide of galacturonic acid molecules
(Moorhouse 2004). Along the chain, some galacturonic acid units are naturally
esterifi ed with a methyl group (methoxyl), while the rest are unsubstituted. Pectin
is classifi ed according to the degree of esterifi cation (DE), also known as the
degree of methoxylation (DM). If the DE of the pectin is greater than 50%, it is
classifi ed as HM pectin. If the DE is less than 50% then it is classifi ed as LM
pectin. Pectin is a source of soluble fi bre.
The key property of pectin is its ability to form stable gels. Pectin is soluble in
cold or hot water. In contrast to most other hydrocolloids, pectin solutions are most
stable in acid conditions, even at high temperatures. However, they will degrade
under alkaline conditions, even at room temperature. Slow degradation is seen at
pH 5.5-7.0 in combination with heat, so careful control of the pH is important.
HM pectin has several key factors required for gelation: low pH (<3.5), high
soluble solids, usually sugar (>55%), and the pectin solution needs heating and
cooling to form a gel. The solution must be heated to above the setting temperature
of the gel, which varies depending on the DE. For very high DE pectins (75-80%
DE), the gelling temperature is 85-95°C; this type also has a faster set time and so
is classed as a rapid set pectin. As the DE decreases, the gelling temperature
decreases, until at 50% DE, the gelling temperature is as low as 60°C, and the set
time is slower, giving a slow set pectin. The gel setting time is very important, as
it determines the time available for depositing the product before it gels. For
smaller, batch operations, a rapid set pectin is used, as this ensures that the product
cools quickly in the jars, evenly distributing the fruit through the product. For a
large industrial-scale production fi lling into one tonne containers, a rapid set
pectin is unsuitable as the high fi lling temperature required would mean that the
product took far too long too cool, and any fruit particles would sediment. Instead,
a slow set pectin is used, allowing the product to be fi lled at a lower temperature.
HM pectin gels are usually solid, cuttable gels that do not remelt on heating -
this makes them bake stable. The gel breaks on shearing but does not reform,
making it prone to syneresis. HM pectin will not gel above pH3.5, but does give
some viscosity, used for example in some fruit beverages.
LM pectin gels by a different mechanism, which is less restrictive in terms of
pH and soluble solids levels, though both of these affect the gelation process.
Instead, calcium is required for the production of LM pectin gels. The calcium
cross links non-esterifi ed carboxyl groups, forming divalent ion bridges (Fig. 8.2),
using a mechanism known as the egg box model. As the DE of the LM pectin
decreases, the gel setting time also decreases, as the pectin becomes more reactive
to the calcium. Sequestrants such as citrate or phosphate are used to control
calcium availability, and therefore the pectin set, to prevent pre-gelling. LM
pectin is less heat and acid tolerant than HM pectin. LM pectin gels are typically
soft and spreadable, and bake stable (gels do not remelt).
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