Biomedical Engineering Reference
In-Depth Information
ascorbyl moiety is the same as that of ascorbic acid, the degree of dissociation of the ascorbyl
moiety increases at the higher pH. The exponential increase in the CMC with an increase in
pH would be explained by the increase in the degree of dissociation [57]. The
a
value was
almost constant at any pH, and the value was the same as those obtained at different
temperatures for decanoyl and lauroyl ascorbates.
The emulsifier properties of saturated acyl ascorbates with even carbon numbers of acyl
chains from 8 to 14 for preparation of O/W emulsions using soybean oil as an oily phase were
examined [30]. An appropriate amount of an acyl ascorbate, which was specified to give a
final concentration of 5 x 10
-7
to 5 x 10
-3
mol/kg-solution (water plus soybean oil), was
weighed into an amber glass vial, and 19 g of distilled water and 1 g of soybean oil were
added into the vial. The buffer solutions used were 0.1 or 0.01 mol/L sodium citrate-HCl for
pH 2, 3 and 4, 0.1 or 0.01 mol/L sodium citrate for pH 5, and 0.1 or 0.01 mol/L sodium
citrate-NaOH for pH 5.5 and 6. The mixture was emulsified to produce an O/W emulsion
with a rotor/stator homogenizer for 1 min at a power control setting of 70. The emulsion was
further homogenized to produce a fine O/W emulsion with small oil droplets using a high-
pressure homogenizer at a pressure of
ca
. 98 MPa. The particle size distribution of the oil
droplets was measured using a laser diffraction particle size analyzer after appropriate
dilution with distilled water. The stability of the emulsion was estimated by a turbidity
method [58]. The emulsion was stored at 30
o
C with gentle stirring. At appropriate intervals,
aliquots of the emulsion were removed and diluted 50 and 400 times for the emulsions with
0.1% (w/v) sodium dodecyl sulfate. The absorbance at 500 nm of the diluted emulsion was
then measured using a spectrophotometer. Figure 15 shows the median diameters of oil
droplets in the O/W emulsions prepared at various concentrations of acyl ascorbates with
different acyl chain lengths. Except for palmitoyl ascorbate, the median diameter decreased
with the increasing emulsifier concentration. The inner figure of Figure 15 shows the
relationship between the median diameter,
d
p
, observed at an emulsifier concentration of 0.5
mmol/kg and the carbon number of the acyl chain of the emulsifier,
n
. The emulsifiers with
acyl chains shorter than or equal to 14 gave fine emulsions with median diameters of
ca.
1
μ
m. O/W emulsions consisting of distilled water and soybean oil were prepared at various
concentrations of acyl ascorbates with different acyl chain lengths, and their stabilities were
measured at 30
o
C by the turbidity method (Figure 16). The emulsions prepared using
octanoyl, myristoyl and palmitoyl ascorbates as the emulsifiers were unstable at the
concentrations tested. The stabilities of the emulsions prepared with decanoyl and lauroyl
ascorbates depended on the emulsifier concentrations and increased with higher
concentrations. The hydrophilic-lipophilic balance (HLB) numbers of octanoyl, decanoyl,
lauroyl, myristoyl and palmitoyl ascorbates, which were evaluated according to the Davies'
equation [59], are 11.8, 10.9, 9.90, 8.95 and 8.00, respectively. McClements (1999)
mentioned that the maximum stability of O/W emulsions is obtained using surfactants with an
HLB number around 10 to 12 [60]. This criterion was almost applicable for acyl ascorbates,
and decanoyl and lauroyl ascorbates produced fine and stable O/W emulsions.
The pH and
ionic strength of the aqueous phase of O/W emulsion are factors affecting the stability. Their
effects on the stability were examined for the emulsions prepared using lauroyl ascorbate at a
concentration of 0.5 mmol/kg. When the buffer concentrations of 0.1 mol/L were high, and
the emulsions were very unstable at any pH value. The instability would be ascribed to the
short Debye length surrounding the oil droplets. When the buffer concentrations were 0.01
