Alginate is a natural polysaccharide derived from a marine plant,
and its basic structure consists of linear unbranched polymers
containing -(1→4)-linked d-mannuronic acid (M) and -(1→4)-
linked l-guluronic acid (G) residues. In this study, the maximum
encapsulation efficiency before drying that can be attained in
Ca-alginate beads was over 90%, with alginate concentrations
higher than 15 g/L and oil loadings up to 40 vol%. Lowering the
alginate concentration below 15 g/L, or increasing the oil loading
higher than 40 vol%, decreased the encapsulation efficiency. A
logical explanation for both cases is that the encapsulation effi-
ciency depends on the degree of cross-linking at the surface of the
extruded emulsion droplet. If there was a lower concentration or
volume fraction of alginate, once the emulsion droplet dropped into
the gelling bath, there was insufficient cross-linking between the
alginate and calcium ions at the droplet surface, which resulted in
formation of loose Ca-alginate hydrogel wall barriers. These results
are in good agreement with those of Chang and Dobashi (2003) who
reported an encapsulation efficiency of about 90% at an oil loading
of 20 vol%.
This explanation can be further supported when the results
obtained using different alginate types (i.e., high G and high M
alginates) are compared (Fig. 2b). Since the alginates have similar
molecular weights, the physical properties of the emulsion,
such as viscosity and density, can be assumed to be relatively
constant when the same concentration of alginate is used. The difference
between the alginates was in the gelling density because it
is well accepted that most of the calcium cations cross-link to the
guluronic sequences to form hydrogel networks. In other words,
a higher gelling density can be achieved at the emulsion droplet
surface with the high G alginate, thus yielding a higher encapsulation
efficiency. Interestingly, the encapsulation efficiency before
drying by using this method seems to reach a limit at about 90–93%,
because the wet beads are always ‘coated’ by a layer of surface oil
(at about 7–10% to the total oil used) irrespective of the process
parameters. The reason might be due to the oil leakage from the
bead surface before the alginate phase in the emulsion could form
a continuous surface barrier.