The highest loading
Figure 4–DSC curves of microcapsules containing thyme oil with different
emulsifiers; (A) Sodium lauryl sulfate, (B) Tween 80, (C) Pluronic F-127.
efficiency of thyme oil was 77.5% in microcapsules using 2% SLS.
There was a significant difference of loading efficiency between
different kinds of emulsifiers.
Emulsifiers are amphiphilic compounds, which have hydrophilic
head part and hydrophobic tail part. Emulsifier molecules interact
with interfaces of oil and water as a result of forming a globular
shape. This interaction reduces interfacial tension and contact surface.
Thus, the droplets are surrounded by emulsifier molecules
and cause an increase of the stability. The hydrophilic/lipophilic
balance (HLB) can affect the loading efficiency and release rate
from microcapsules (Ko and others 2012). Martin and others
(2009) showed that the high HLB value can favor the protection
of nonpolar compounds. In our work, P127 (HLB = 18 to 23),
T80 (HLB = 15), and SLS (HLB = 40) were used as emulsifiers.
SLS, which has the highest HLB value, was the best emulsifier for
the encapsulation of thyme oil due to their high loading efficiency
(77.5%). The lowest loading efficiency appeared in microcapsules
using P127, even if HLB value of P127 is higher than that of T80.
It was caused by a thermal sensitive property of P127. Below gelation
temperature, P127 appear weak hydrophobic properties and as
increasing temperature, hydrophobic properties gradually increase
(Bercea and others 2011). Hong and others (2009) reported that
the gelation temperature of P127 is less than 37 ◦C and it depends
on the concentration (the gelation temperature of P127 decreased
with increasing concentration of P127). The low concentration of
P127 was used (2%) in this study and consequently the gelation
temperature would be high. It means that hydrophobic properties
of P127 decreased because temperature in emulsification process
was lower than gelling temperature. Therefore, it was difficult to
encapsulate thyme oil sufficiently.