Chemistry Reference
In-Depth Information
Incorporation of lipids or lipophilic compounds has typically been achieved
by encapsulation with a wall material of complex coacervates.
2,3
The long-term
stability of such oily encapsulates is excellent (4 weeks or months) for
compounds like carotenoids,
4
which have an extremely high partition coe-
cient of K
O/W
B
10
18
. Similarly, compounds with an extremely low partition
coefficient can, in principle, be retained by a hydrophobic wall material in an
aqueous environment. The most water-soluble compounds that could be of
interest to encapsulate in foods are compounds like the vitamins B and C,
which have partition coefficients around 10
2
.
An example of a technique for encapsulating water-soluble compounds is the
use of duplex or water-in-oil-in-water (W/O/W) emulsions. This technique has
shown its potential in controlled release of drugs.
5,6
But the experience with
duplex emulsions is that even the most extremely water-soluble compounds leak
out within a few minutes (or even faster). For instance, Rayner et al.
7
reported
the encapsulation of dextran blue (a hydrophilic colourant) in a W/O/W
emulsion, made with sunflower oil, polyglycerol polyricinoleate (PGPR), and
caseinate. They succeeded in entrapping dextran blue for 16 s in the droplets in
an aqueous environment; but, due to osmotic bursting, the droplets exploded
and released the dye. Instead of using liquid wall materials, one could use solid
fat instead. However, this increases the complexity of the preparation consid-
erably, and it also increases the chances of crack formation triggered by osmotic
differences. We therefore conclude that existing encapsulation techniques can-
not be used for the purpose of long-term retention of water-soluble compounds
in aqueous applications like foods. This calls for the use of a drastically different
type of wall material.
Waxes are often mentioned as excellent moisture barriers. They are esters of
long-chain alcohols and fatty acids, and are found naturally in fruits, seeds, and
petroleum. The most commonly used wax in foods is produced by insects -
namely beeswax.
8
The most abundant ester in beeswax is C
30
-C
16
(Figure 1).
Beeswax is used as glazing agent on candy or as coating on fruits.
Waxes are known for their hydrophobicity. At room temperature a wax is
ductile without giving cracks
9
; generally waxes are softer if their chains are of
unequal length. Also there are indications that the plate-like crystals are efficient
in hampering the diffusion of small compounds.
10,11
Bodmeier et al.
12
explored
the use of wax for encapsulation of a water-soluble compound using the W/O/W
technique. Unfortunately, they found that the drug release (in this case pseu-
doephedrine) was much faster than from traditional polymeric microspheres,
possibly because wall thicknesses were not large enough. In some recent appli-
cations,
13,14
it has also been found that for small wax microcapsules (
20 mm
size) containing hydrophilic compounds, approximately 25-50% of the com-
pounds leak out within half an hour.
In theory larger capsules should give better retention. Indeed it has been
shown
14
that large particles (from 50 mm to 10 mm) can give excellent stability
compared to similar particles prepared from solid fat. Unfortunately, even the
smallest have been shown to be too large for application in foods because they
gave a sandy texture.
16,17
Table 1, taken from Heith and Prinz,
17
can be used as
B
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