Hydrocolloid Hydration a
Hydrocolloids [227] are hydrophilicb polymers, of vegetable, animal, microbial or synthetic origin, that
generally contain many hydroxyl groups and may be polyelectrolytes.
They are naturally present or added to control the functional properties
of aqueous foodstuffs. Most important amongst these properties are viscosity (including thickening and gelling)
and water binding but also significant are many others including
emulsion stabilization, prevention of ice recrystallization and
organoleptic properties. The degree with which the hydrocolloid solutions mix with saliva, determined by their degree of chain entanglement, determines flavour perception [1050].
Other more specialist applications include
adhesion, suspension, flocculation, foam stabilization and film
formation. Foodstuffs are very complex materials and this together
with the multifactorial functionality of the hydrocolloids has resulted
in several different hydrocolloids being required; the most important
of which are listed below.
Each of these hydrocolloids consists of mixtures of similar, but
not identical, molecules and different sources, methods of preparation,
thermal processing and foodstuff environment (for example, salt
content, pH and temperature) all affect the physical properties
they exhibit. Descriptions of hydrocolloids often present idealized
structures but it should be remembered that they are natural products
(or derivatives) with structures determined by stochastic enzymic
action, not laid down exactly by the genetic code. They are made
up of mixtures of molecules with different molecular weights and
no one molecule is likely to be conformationally identical or even
structurally identical (cellulose excepted)
to any other.
Mixtures of hydrocolloids show such a complexity of non-additive
properties that it is only recently that these can be interpreted
as a science rather than an art. There is enormous potential in
combining the structure-function knowledge of polysaccharides with
that of the structuring of water. The particular parameters of each
application must be examined carefully, noting the effects required
(for example, texture, flow, bite, water content, stability, stickiness,
cohesiveness, resilience, springiness, extensibility, processing
time, process tolerance) and taking due regard of the type, source,
grade and structural heterogeneity of the hydrocolloid(s).
Mixed hydrocolloids, at low concentrations (a),
influence large volumes of water within their radius
of gyration. As the concentrations increase different
scenarios are possible. Individual molecules may fold up to
form globular structures (b). This entropy-driven
process allows the retention of rotational freedom. Further
concentration (c) may cause phase separation
(d) due to the different influences on the
water structuring.
More commonly, hydrocolloids retain their extended structures.
This may give rise to mixed entanglement (f;
rarer) or phase-separated entanglement (e,
commoner). Mixed entanglement may give rise to gels (g)
at higher concentrations. Phase separations may be entropy-driven
as they may allow greater freedom of movement due to the similarity
in the molecular shapes (h).
Sometimes junction-zones arise, forming gels. |
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All hydrocolloids interact with water,
reducing its diffusion and stabilizing its presence. Generally neutral
hydrocolloids are less soluble whereas polyelectrolytes are more
soluble but hydration kinetics depends on many factors; xanthan,
guar and carboxymethylcellulose are soluble in cold water but carrageenan,
locust bean gum and many alginates require hot water for complete
hydration. Such water may be held specifically through direct hydrogen
bonding or the structuring of water or within extensive but contained
inter- and intra-molecular voids. Interactions between hydrocolloids
and water depend on hydrogen bonding and
therefore on temperature and pressure in the same way as water
cluster formation. Similarly, there is a reversible balance
between entropy loss and enthalpy gain but the process may be kinetically
limited and optimum networks may never be achieved. Hydrocolloids
may exhibit a wide range of conformations in solution as the links
along the polymeric chains can rotate relatively freely within valleys
in the potential energy landscapes.
Large, conformationally stiff hydrocolloids present essentially
static surfaces encouraging extensive structuring in the surrounding
water. Water binding affects texture and processing characteristics,
prevents syneresis and may have substantial economical
benefit. In particular, hydrocolloids can provide water for increasing
the flexibility (plasticizing) of other food components. They can
also effect ice crystal formation and growth so exerting a particular
influence on the texture of frozen foods. Some hydrocolloids, such
as locust bean gum and xanthan
gum, may form stronger gels on freeze-thaw due to kinetically
irreversible changes consequent upon forced association as water
is removed (as ice) on freezing [468]. 
As hydrocolloids can dramatically affect the flow behavior of many
times their own weight of water, most hydrocolloids are used to
increase viscosity (see rheology page),
which is used to stabilize foodstuffs by preventing settling, phase
separation, foam collapse and crystallization. Viscosity generally
changes with concentration, temperature [236]
and shear strain rate in a complex manner
dependent on the hydrocolloid(s) and other materials present; mixtures
of hydrocolloids may act synergically to increase viscosity or antagonistically
to reduce it. These changes may generally be fitted to equations
such as:
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where η is the viscosity, a is pre-exponential
factor and C* is the concentration
in units specific for the circumstances; often about 1%
wt/vol. |
where η is the viscosity, A is pre-exponential
factor and Ea is a
constant (known as 'activation energy' from the similarity
with the Arrhenius equation), R is the gas constant and T is the
absolute temperature. |
Unfortunately these equations cannot be combined
as the specific concentration units change with temperature.
Their non-linear nature means that extreme care should
be taken when investigating possible synergistic relationships.
The relative viscosities of the more important hydrocolloids
are influenced by their molecular weight distribution
and other structural properties, dependent on their
source and preparatory methodology, but are approximately
as shown opposite; A carboxymethylcellulose,
B Guar, C locust
bean gum, D κ-carrageenan,
E λ-carrageenan,
F ι-carrageenan,
G xanthan, H gum
arabic, I pectin, J microcrystalline
cellulose, K methylcellulose,
L konjac mannan. The
viscous (η') and elastic
(η'') components
of the (complex) viscosity (η*)
of the gums, at 0.05% w/v and 0.5% w/v, are compared
at 4°C and 50 s-1 [using data from 694]
where η* = η' + iη''. |
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Many hydrocolloids also gel, so controlling many textural properties.
Gels are liquid-water-containing networks showing solid-like behavior
with characteristic strength, dependent on their concentration,
and hardness and brittleness dependent on the structure of the hydrocolloid(s)
present. Hydrocolloids display both elastic and viscous behavior
where the elasticity occurs when the entangled polymers are unable
to disentangle in time to allow flow. Mixtures of hydrocolloids
may act synergistically, associating to precipitate, gel or form
incompatible biphasic systems; such
phase confinement affecting both viscosity and elasticity. Hydrocolloids
are extremely versatile and they are used for many other purposes
including (a) production of pseudoplasticity (that is, fluidity under shear) at high temperatures to ease
mixing and processing followed by thickening on cooling, (b) liquefaction
on heating followed by gelling on cooling (for example, cold set
gels like agarose, gellan and carrageenan), (c) gelling on
heating to hold the structure together (for example, thermogelling
heat set gels such as curdlan and methyl
cellulose), (d) production and stabilization of multiphase systems
including films.
These properties of hydrocolloids must be due, both singly and
in concert, to their structural characteristics and the way they
interact with water. For example:
- Hydrocolloids gel when intra- or inter-molecular hydrogen bonding
(and sometimes salt formation) is favored over hydrogen bonding
(and sometimes ionic interactions) to water to a sufficient extent
to overcome the entropic cost. Often the hydrocolloids exhibit
a delicate balance between hydrophobicityb and hydrophilicity. Extended hydrocolloids tend to tangle at higher
concentrations and similar molecules may be able to wrap around
each (forming helical junction zones) other without loss of hydrogen
bonding but reducing conformational heterogeneity and minimizing
hydrophobic surface contact with water so releasing it for more
energetically favorable use elsewhere. Under such circumstances
a minimum number of links may need to be formed (that is, a junction zone which, if helical, generally requires a complete
helix) to overcome the entropy effect and form a stable link. Where junction zones grow slowly with time,
the interactions eliminate water and syneresis may occur (as in
some jam and jelly).
- Polysaccharide hydrocolloids stabilize emulsions primarily by
increasing the viscosity (slowing the thermodynamically favored
breakdown of the emulsion) but may also act as emulsifiers, where
their emulsification ability is reported as mainly being due to
accompanying (contaminating or intrinsic) protein moieties [309].
In particular, electrostatic interaction between ionic hydrocolloids
and proteins may give rise to marked emulsification ability with
considerable stability so long as the appropriate pH and ionic
strength regime is continued. Denaturation of the protein is likely
to lead to improved emulsification ability and stability.
- Mixtures of hydrocolloids may avoid self-aggregation at high
concentration due to structural heterogeneity, which discourages
crystallization but encourages solubility. Hydrocolloids may interact
with other food components such as aiding the emulsification of
fats, stabilizing milk protein micelles or affecting the stickiness
of gluten.
- The particle size of hydrocolloids and its distribution are
important parameters concerning the rate of hydration and emulsification
ability.
- Negatively charged hydrocolloids change their structural characteristics
with counter-ion type and concentration (including pH and ionic
strength effects); for example, at high acidity the charges disappear
and the molecules become less extended.
- Physical characteristics may be controlled by thermodynamics
or kinetics (and hence processing history and environment) dependent
on concentration. In particular these may change with time in
a monotonic or oscillatory manner.
- Different hydrocolloids prefer low-density
or higher density water and other hydrocolloids show compatibility
with both. As more intra-molecular hydrogen bonds form so the
hydrocolloids become more hydrophobic and this may change the
local structuring of the water. Mixed hydrocolloids preferring
different environments produce 'excluded volume' effects on each
other's effective concentration and hence rheology.
- In the glassy state, conformational changes are severely inhibited,
but the water held by hydrocolloids may act as plasticizer (allowing
molecular motion) greatly reducing the glass transition temperature
by breaking inter-molecular hydrogen bonding.
Atomic force microscopy has been used to clarify the network structures on gelling, and gives useful results so long as care is taken that the structures do not change during preparation [1242].
Hydrocolloids, together with other dietary fiber, are increasingly
being seen as contributing to a healthy diet, having a number of
positive health benefits. Although this
site concentrates on food aspects, hydrocolloids also have many
other major economic uses such as in the chemicals, oil and cosmetic
industries.
Footnotes
a An introduction
to rheology and an introduction to polysaccharides are given on other pages [Back]
b Hydrophilic solutes
(that is, solutes or structures possessing hydrophilicity)
interact with water with greater or comparable strength to water-water
interactions whereas hydrophobic solutes
(that is, solutes or structures possessing hydrophobicity)
only weakly interact with water with strength far less than water-water
interactions. [Back, 2]
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