Alginates in Foods - Practical Food Rheology - page 118

Alginate interior

Ca 2+ diffusion

Alginate surface

Ca 2+ source (e.g. CaCl 2 )

Fig. 6.5 Schematic diagram of external gelation.

The simplest route to immobilisation is to drop an alginate solution
containing the encapsulant through a syringe into a bath of calcium
chloride, a method first developed by Kierstan and Bucke (1977). A
disadvantage of this technique is that bead size is restricted by the
aperture of the syringe and the viscosity of the alginate, although this
can be overcome by techniques such as applying electrostatic pulses
(Hommel et al. , 1988), vibrating needles (Hulst et al. , 1985), atomisation
(Matsumoto et al. , 1986) or more recently novel microfluidics (Sugiura
et al. , 2005). The main drawback, however, is that this technique is
not easily scaled up (Poncelet et al. , 1992). To achieve a smaller bead
size with a more practical scale up procedure, an emulsion templating
technique was developed (Wan et al. , 1992), whereby a water in oil
emulsion is formed using alginate and the encapsulant in the water phase.
Once a stable emulsion is formed, CaCl 2 can be added, which cross-
links the alginate droplets, giving a narrow particle size distribution. The
particle size, as with most emulsions, is controlled by the selection of
material for the oil phase and shear rate used in forming the emulsion,
in addition to the rate of addition of cross-linker.
If a larger bulk gel is required, the alginate solution can be added
to a dialysis membrane, which is then submerged into a solution of
CaCl 2 . The gelation time using this method can be several hours. The
concentration of the CaCl 2 and the time the alginate is exposed regulate
the gel strength and homogeneity.
Because of the complex nature of food systems, it is important to
understand interactions of alginate gels with other molecules in the sys-
tem. In particular, phosphates and citrates which are regularly used in
food can complex calcium, causing the dissolution of alginate gels. In
addition, as alginate is a polyanion, there is potential for electrostatic
interactions with other macromolecules, for example proteins (Zhao
et al. , 2009) or other polysaccharides (Toft et al. , 1986) within food,