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O R I G I N A L P A P E R
Effect of Ascorbic Acid or Acyl Ascorbate on the Stabilityof
Catechin in Oil-In-Water Emulsion
Yoshiyuki Watanabe Takahiro Suzuki
Hirofumi Nakanishi Ayumi Sakuragochi
Shuji Adachi
Received: 7 December 2010 / Revised: 2 July 2011 / Accepted: 19
July 2011 / Published online: 3 August 2011
AOCS 2011
Abstract The degradation of (?)-catechin in an oil-in-
water emulsion using methyl dodecanate as an oil phasewith or
without ascorbic acid or acyl ascorbate was kinet-
ically examined at 40 C. The rate constant, k, of the first-
order kinetics for the degradation with ascorbic acid or
octanoyl ascorbate depended on the added amount, whereas
the kvalue with hexadecanoyl ascorbate was independent
of the amount. The k value for a smaller oil droplet with
each ascorbate was lower than that for a larger oil droplet.
Catechin did not partition well into the methyl dodecanate
phase, but did adsorb slightly onto the interface between
the
methyl dodecanate and water. The suppressive effect of
acyl ascorbate on the catechin degradation in the emulsion
was lower than that of hydrophilic ascorbic acid at the low
concentration, but the peroxidative ability also was lower.
Most of the catechin molecules in the emulsion degraded in
the water phase. The catechin degradation in the emulsion
with small oil droplets depended on the acyl chain length of
the ascorbates more than in large oil droplets.
Keywords Acyl ascorbate Catechin Degradation
kinetics First-order equation Oil-in-water emulsion
Introduction
L-Ascorbic acid, recognized as vitamin C, is a water-soluble
vitamin and widely used as an additive in foods and cos-
metics because of its strong reducing ability. The enzymatic
synthesis of acyl ascorbate through the condensation of
ascorbic and fatty acids using a lipase in an organic
solvent
has been studied [13]. When compared to a chemical
method, the enzymatic synthesis has some advantages, such
as the simplicities of the reaction and purification
processes
and its high regioselectivity. Acyl ascorbate is an amphi-
philic antioxidant, because it consists of ascorbic and
fatty
acids as a hydrophilic antioxidant and lipophilic group,
respectively. Acyl ascorbate would be expected to be a
useful food additive as it has an antitumor activity and
metastasis-inhibitory effect [4,5]. Many foods are emulsi-
fied materials, for example, milk, mayonnaise, coffee
creamers, salad dressings, butter, and baby foods [6], and
an
improvement of the lipid oxidation in emulsions is crucial
for the production and storage of food products. Acyl
ascorbate has a good emulsifying ability [7], and exhibits
an
antioxidative activity against microencapsulated lipids
prepared by emulsification in a food polymer solution and
spray-drying [8]. Many studies on the lipid oxidative sta-
bility in emulsions and the effects of the emulsifiers and
antioxidants on it have been reported [9, 10]. However,
there
has been no report about the effect of the amphiphilic
antioxidant, such as acyl ascorbate, against a water-soluble
and unstable compound in the oil-in-water type emulsion. A
better understanding of its behavior in the emulsion would
be required for its effective use in many emulsified
products.
Catechins are constituents of green tea with an antiox-
idative activity [11] and have a suppressive ability against
adipocyte differentiation [12]. Most tea drinks containing
catechins are kept warm in the vending system during the
Y. Watanabe (&) T. Suzuki H. Nakanishi A.
SakuragochiDepartment of Biotechnology and Chemistry,
Faculty of Engineering, Kinki University,
1 Takayaumenobe, Higashihiroshima 739-2116, Japan
e-mail: [email protected]
S. Adachi
Division of Food Science and Biotechnology,
Graduate School of Agriculture, Kyoto University,
Sakyo-ku, Kyoto 606-8502, Japan
1 3
J Am Oil Chem Soc (2012) 89:269274
DOI 10.1007/s11746-011-1913-x
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cold season, and heat processing at high temperature is
needed to sterilize the spores of the thermophilic anaer-
obes. Therefore, the stability of tea catechins in an
aqueous
solution has been investigated under various conditions
[13,14]. Furthermore, the degradation has been kinetically
examined using first-order kinetics [15, 16]. We have
studied the influence of ascorbic acid or octanoyl
ascorbate,
which is one of the most soluble acyl ascorbates in water,on the
degradation of (?)-catechin in an aqueous solution,
and that of the coexistence of octanoyl ascorbate and citric
acid on the catechin stability [17,18]. However, the effect
of acyl ascorbate on the stability of catechins in the
emulsion, which is a more complex system, is not clear.
In this study, the degradation of (?)-catechin in an oil-in-
water emulsionwith or withoutascorbic acid or acylascorbate
was kinetically examined. The degradation was expressed by
first-order kinetics, and the kinetic parameters were
evaluated.
In addition, emulsions with the different droplet size
distri-
butions were prepared, and the effect of the diameter of
theoil
droplets in the emulsion in the presence of acyl ascorbate onthe
degradation kinetics was also investigated.
Experimental Procedures
Materials
(?)-Catechin (purity[ 98%) was purchased from Sigma,
St. Louis, MO, USA. The immobilized lipase fromCandida
antarctica was obtained from Roche Molecular Biochemi-
cals, Mannheim, Germany. L(?)-Ascorbic acid was pur-
chased from NacalaiTesque, Kyoto,Japan. Theoctanoic and
hexadecanoic acids, and methyl dodecanate were purchased
from Wako Pure Chemical Industries, Osaka, Japan. The
hydrophilic surfactant, SY-Glyster ML-750 (decaglycerol
monododecanate), was supplied by Sakamoto Yakuhin
Kogyo, Osaka. Caffeine, which was used as the internal
standard for the determination of catechin by HPLC, was
purchased from Kishida Chemical, Osaka. All other chem-
icals of analytical grade were purchased from either Wako or
Yoneyama Chemical, Osaka.
6-O-Octanoyl and hexadecanoyl L-ascorbate were syn-
thesized by the condensation of ascorbic and octanoic or
hexadecanoic acids in acetone using the immobilized lipase
and then purified according to previous methods [3].
Preparation of Oil-In-Water Emulsion
Five milliliters of a 1.0 mmol/L catechin aqueous solution
with 10 g/L SY-Glyster ML-750 as the water phase and
the same volume of methyl dodecanate as the oil phase
were added to a test tube. Ascorbic acid or acyl ascorbate
was added to the water phase at the initial concentration of
0.001, 0.01, 0.1, 1.3, 2.5 or 3.8 mmol/L. The mixture was
emulsified using a rotor/stator homogenizer (Ultra-Turrax
T25, IKA Japan, Nara, Japan) for 5 min at 1 9 104 rpm
in the test tube immersed in ice water to produce the coarse
oil-in-water emulsion. After the pre-emulsification, the
coarse emulsion was circulated through a membrane filter
with a pore size of 0.8 or 3.0 lm using a peristaltic pump
at
2.0 mL/min for 30 min to reduce and monodisperse thediameter of
the oil droplets. The prepared emulsion was put
into an amber glass vial with a screw-cap and stored in the
dark at 40 C with magnetic stirring at 100 rpm.
Stability of Catechin
At appropriate intervals, 200lL of the emulsion was
removed from the vial and mixed with 400 lL of a mixture
of chloroform and methanol (2:1 by vol.). The mixture was
centrifuged for 5 min at 1.5 9 104 rpm. Fifty microliters of
the water phase was mixed with 50lL of 1.3 mmol/L
caffeine solution and 900 lL of the eluent, which was amixture
of methanol, water and phosphoric acid (20:80:0.5
by vol.), for the HPLC analysis. Twenty microliters of the
mixture was applied to a Chemcosorb300-5C18 column
(4.6 mm/ 9 250 mm; Chemco Scientific, Osaka) and
eluted at 1.0 mL/min using a Shimadzu LC-10AT pump
(Kyoto). The elution profiles of the catechin and caffeine
were monitored using a Shimadzu SPD-10A UV detector at
280 nm. The amount of remaining catechin, which was
recovered in the water phase, in the emulsion was measured
in duplicate, and the mean value was calculated. As below
mentioned, the partition coefficient of catechin between
methyl dodecanate and water was very low or zero.
Therefore, the remaining catechin in the emulsion was
judged to be in the aqueous phase after centrifugation.
Oil-Droplet Size Distribution in the Oil-In-Water
Emulsion
Twenty microliters of the emulsion were periodically
removed from the vial to measure the oil-droplet size
distribution using a centrifugal particle size analyzer
(SA-CP3L, Shimadzu, Kyoto). The sample was diluted
100400 times with distilled water, followed by the droplet
size analysis. The droplet size distribution in the emulsion
was measured in triplicate, and the mean value was cal-
culated from the median diameter, d, of the oil droplet as
an index of the emulsion stability.
Partition Coefficient of Catechin Between Methyl
Dodecanate and Water
The partition coefficient, P, of catechin between methyl
dodecanate and distilled water was measured, according to
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a previous report [17]. One mililiter of each 0.1, 1, or
10 mmol/L catechin solution,Vaq, and each 5, 10 or 20 mL
of methyl dodecanate, Vorg, were mixed in an amber vial.
The vial was immersed in a water bath at 40 C. After
shaking at 100 revolutions per h for 1 h, the vial was left
for about 15 min for phase separation. The concentration of
catechin in the water phase at the partition equilibrium,
Caq, was measured by the above-mentioned HPLC analy-sis,
proceeded by sampling 500 lL of the water phase. The
measurement of the partition coefficient was done in trip-
licate, and the mean value was estimated.
Statistical Analysis
The effect of the concentration of the ascorbates on the
rate
constant for the degradation of catechin in the emulsion
was determined by ANOVA. Significant differences were
determined byttests atP\ 0.05. For the average values of
the median diameters of the oil droplets of 5.3lm, the
degree of freedom was 4 for no ascorbates. The degreeswith
ascorbic acid were 2 at 0.001 mmol/L, 4 at
0.01 mmol/L, 8 at 0.1 mmol/L, 6 at 1.3 mmol/L, 5 at
2.5 mmol/L, and 4 at 3.8 mmol/L, respectively. The
degrees with octanoyl ascorbate were 3 at 0.01 mmol/L, 5
at 0.1 mmol/L, 4 at 1.3 mmol/L, 8 at 2.5 mmol/L, and 4 at
3.8 mmol/L, respectively. The degrees with hexadecanoyl
ascorbate were 4 at 0.001 mmol/L, 5 at 0.01 mmol/L, 5 at
0.1 mmol/L, and 5 at 1.3 mmol/L, respectively. At 9.9 lm
of the average median diameters, the degrees were 4 for no
ascorbates, 5 for 0.01 mmol/L ascorbic acid, 4 for
0.01 mmol/L octanoyl ascorbate, and 4 for 0.01 mmol/L
hexadecanoyl ascorbate, respectively.
Results and Discussion
Effect of the Amount of Ascorbic Acid or Acyl
Ascorbate on the Stability of Catechin
in the Oil-In-Water Emulsion
Figure1 shows the degradation processes of catechin in the
oil-in-water emulsions at 40 C with no ascorbates,ascorbic acid,
octanoyl ascorbate, and hexadecanoyl
ascorbate. The average values of the median diameters of
the oil droplets were 5.3 and 9.9 lm, and the standard
deviations were 0.23 and 0.95. C/C0represents the fraction
of remaining catechin in the emulsion, whereCand C0 are
the catechin concentrations at any time t and t= 0,
respectively. The transient changes in the median diame-
ters of the oil droplets in the oil-in-water emulsion con-
taining catechin and no ascorbates at 40 C are present in
Fig.2. The data indicates that the emulsions were stable
during the measurements for the catechin degradation
regardless of initial mean diameter. The degradationkinetics of
catechin was assumed to be expressed by first-
order kinetics [18]:
C
C0expkt 1
wherekis the rate constant. The rate constant of the first-
order kinetics was calculated to best-fit the experimental
results by the regression function of Microsoft Excel 2007
as shown by the solid curves in Fig.1. From previous
results, each residual area per molecule, a, evaluated from
the surface tension for 6-O-hexanoyl, octanoyl, decanoyl
and dodecanoyl ascorbates was ca. 0.30 nm2 [19]. Using
0.8
1.0 (a) (b) (c) (d)
0.4
0.6
C/C0
0.2
Time [h]
0 100 200 0 100 200 0 100 200 0 100 200
Fig. 1 Degradation processes of catechin in the oil-in-water
emul-
sion at 40 C with a no ascorbates, b ascorbic acid, c
octanoyl
ascorbate anddhexadecanoyl ascorbate at the concentrations of
0.001
(open circles),0.01 (open diamonds), 0.1 (open
triangles),1.3
(inverted open triangles),2.5 (right pointed open triangles) and
3.8
(left pointed open triangles) mmol/L. C and C0 are the
catechin
concentrations at timetand initial one, respectively. The
squaresand
othersymbolsrepresent the average values of the median diameters
of
the oil droplet in the emulsion of 9.9 and 5.3 lm, respectively.
The
closed symbolsrepresent the emulsion without ascorbic acid and
acyl
ascorbates. The solid curves were calculated using the estimated
rate
constant of first-order kinetics
J Am Oil Chem Soc (2012) 89:269274 271
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the a value, the total number of oil droplets, n, and
interfacial area per oil droplet,A, the maximum number of
adsorbable ascorbate molecules on the overall interface,m,
was estimated:
nV
v
6V
pd3 2
A pd2 3
mnA
a
6V
da
4
where V is the total volume of oil, v is the volume per oil
droplet, anddis the median diameter of an oil droplet. The
relationship between the kvalue for the emulsion with the
mean diameter of 5.3 lm and the ratio of added ascorbate
molecules,mA, tom is shown in Fig.3. Ascorbic acid sup-
pressed the degradation of catechin at the low concentra-
tions. Mochizuki et al. [20] suggested that the first step
for
the autoxidation of catechin in an aqueous solution was the
one-electron oxidation of the B-ring of the catechins by
molecular oxygen. Superoxide anion and a semiquinone
radical resulted from one-electron oxidation which in turn
catalyze the autoxidation through radical chain reactions.The
function of ascorbic acid due to the enediol-lactone
resonant system in the molecule would contribute to sup-
pressing the radical chain reactions. However, the suppres-
sive effect gradually decreased with increase in ascorbic
acid
molecules, and thekvalue became higher than that with no
ascorbate at all. Ascorbic acid is known to exhibit a perox-
idative action [21, 22]. Octanoyl ascorbate also showed a
similar behavior, but the anti/peroxidative effect was
weaker
than that of ascorbic acid. This would be attributed to its
amphiphilic property. It is suggested that molecules such as
octanoyl ascorbate were difficult to disperse in the water
phase in the presence of catechin, in comparison with
ascorbic acid, due to sorption onto the interface, the
partic-
ipation in the emulsifying, or partition into the oil phase.
At
mA/m[ 1, thekvalue rapidly increased in the presence of
octanoyl ascorbate. A similar tendency was observed for the
non-amphiphilic ascorbic acid. On the other hand, the
kvalue for the degradation of catechin in the emulsion
withhexadecanoyl ascorbate was independent of the concentra-
tion of the ascorbate and almost equal to the control. Most
of
hexadecanoyl ascorbate molecules, with high lipophilicity,
appear to be in the oil phase and not involved with the
catechin degradation that occurred in the water phase.
Effect of Acyl Chain Length of Acyl Ascorbate
on the Stability of Catechin in the Oil-In-Water
Emulsion with the Different Median Diameters
of the Oil Droplets
Emulsions with smaller oil droplets show a higher rate of
lipid oxidation due to the higher interfacial area [23]. In
this study, the degradation of catechin, which was a water-
soluble compound, in the oil-in-water emulsion was
examined. The stability of catechin in the emulsion with
the oil-droplet diameter of 5.3 lm was higher than those
with a 9.9 lm diameter, as shown in Fig.1a for the
emulsion in the absence of the ascorbates. Decaglycerol
monododecanate, used as an emulsifier, on the interface
might provide a partial barrier to the radical chain
reactions
10
15
5
d[m]
00 50 100 150 200
Time [h]
Fig. 2 Transient changes in the median diameters of the oil
droplets
in the oil-in-water emulsion with catechin and no ascorbates at
40 C.
Thecirclesandsquaresrepresent the mean values of the diameters
of
5.3 and 9.9 lm, respectively. The bars show the standard
deviation(n = 3)
10-2
k
[h-1]
mA/m
10-4 10-3 10-2 10-1 100 1010-3
Fig. 3 Dependence of the rate constants,k, of the first-order
kinetics
for the degradation of catechin in the oil-in-water emulsion
with the
mean diameter of 5.3 lm at 40 C on the amount of ascorbic
acid
(open circles), octanoyl (open squares) and hexadecanoyl
(open
triangles) ascorbate. The bars represent 95% CI. The solid
curves
were empirically drawn. The broken curve represents the kvalue
for
the degradation without any ascorbates
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for the degradation of catechin. Figure4shows the effect
of the acyl chain length of the acyl ascorbate on the rate
constant for the degradation of catechin in the oil-in-water
emulsion with the two median diameters of the oil droplets.
Each concentration of ascorbic acid or acyl ascorbate in the
emulsion was 0.01 mmol/L. For each ascorbate at this
concentration, the kvalue for the smaller oil droplets was
lower than that for the larger ones. Furthermore, both
k values for the two different droplet diameters were
slightly higher for the longer acyl chain length. This
wouldindicate that the influence of the oil droplet size on the
stability of the catechin was stronger than that of the
ascorbate type, and that the ascorbate with a shorter acyl
chain length was more efficient.
Location of Catechin in Methyl Dodecanate and Water
System
The P values of catechin between the methyl dodecanate
and water, which were estimated assuming no adsorption
onto the interface, increased with the increasing initial
catechin concentration and also depended on the volumeratio of
the organic phase to the water phase. Generally, the
P value should be independent of the initial concentration
and the volume ratio of the two phases. Therefore, we
postulated that catechin molecules are adsorbed onto the
interface between the methyl dodecanate and water at
the surface concentration ofCint. Based on this assumption,
the mass balance for catechin is given by the following
equation:
Caq;0Vaq CaqVaqCorgVorg CintA 5
whereCaq,0, is the initial concentration in the water phase,
Corg, is the concentration in the organic phase at the
partition equilibrium,A is the interfacial area, and Vaqand
Vorg are the volumes of the water and organic phases,
respectively. As theP value is defined asCorg/Caq, Eq.5is
converted to Eq.6
Caq;0 CaqCaqA
PVorg
VaqA
Cint
CaqVaq: 6
The inset in Fig. 5 shows the relationship between
(Caq,0 - Caq)/CaqA and Vorg/VaqA. The plots at any initial
catechin concentration lay on a horizontal line, indicating
that the P value is very low or practically zero. Assuming
that P = 0, Cint was estimated from the intercept of the
line. Figure5 shows the dependence of Cint on Caq. For
each volume ratio, Cint proportionally increased with the
change inCaq. This would indicate that catechin molecules
do not partition well into the methyl dodecanate phase and
a slight adsorption of catechin occurred on the interface.
In
this study, a 1 mmol/L catechin aqueous solution was
consistently used for the measurements of the degradation
processes. The P value at the initial concentration of
1 mmol/L was very low or zero. Therefore, the partition of
catechin to the oil phase in the emulsion would not occur
well, though the composition in the emulsion containing
decaglycerol monododecanate as an emulsifier was differ-
ent from that of the mixture for the partition experiment.
On the other hand, the slight adsorption onto the interface
k[h-1]
0 4 8 12 16
Acyl chain length
10-3
10-2
Fig. 4 Relationship between the rate constants, k, of the
first-order
kinetics for the degradation of catechin in the oil-in-water
emulsion
with 0.01 mmol/L ascorbates at 40 C and acyl chain length of
the
ascorbates. Thesymbolsare the same as in Fig. 2.
Thebarsrepresent95% CI. The solid curves were empirically drawn
10-1
10-2
2]
10-3
10-4
Cint[mol/m
aq,0
C
aq
)/C
aq
A[m-2]
08000 12000 16000
100
200
300
Caq[mol/L]
10-5
10-110-210-310-410-5 100 10
(Vorg/VaqA [m
-2]
C
Fig. 5 Effect of the concentration,Caq, of catechin in the water
phase
at the volume ratio of 5 (open circles), 10 (open squares) and
20
(open triangles) of the organic phase to water phase on the
adsorbed
amount, Cint, of catechin onto the interface between methyl
dodec-
anate and water at 40 C. The solid curves were empirically
drawn.The inset shows the relationship between (Caq,0 - Caq)/CaqA
and
Vorg/VaqA at the initial concentration, Caq,0, of catechin in
the water
phase of 0.1 (filled circles), 1 (filled squares) and 10 (filled
triangles)
mmol/L. The broken curves in the inset indicate Cint/CaqA at
the
partition coefficient of zero, P, of catechin between two
phases
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between the oil and water phases could occur. In the
emulsion with the small oil-droplets, catechin molecules
adsorbed onto the interface would increase due to the large
interfacial area. This may contribute to the high stability
of
catechin in the small oil-droplets emulsion, as shown in
Fig.1a. In the presence of acyl ascorbate, some catechin on
the interface might be replaced by the adsorbed ascorbate.
Catechin liberated by the replacement is prone to degra-dation
in the water phase. This is affected by the added
amount of the ascorbate and the droplet size in the emul-
sion, that is, the interfacial area. This is only a
hypothesis,
and more research needs to be done to confirm this
speculation.
Conclusions
Most of the catechin molecules in the emulsion must exist
and degrade in the water phase. Hydrophilic ascorbic acid
effectively suppressed the oxidative degradation ofcatechin.
Amphiphilic octanoyl ascorbate also exhibited
an antioxidative ability against the catechin degradation
in the emulsion, but the effectiveness was low due to the
adsorption onto the interface between the water and oil
phases. Hexadecanoyl ascorbate, with a high lipophilicity,
did not act as an antioxidant against catechin in the
emulsion indicating that the ascorbate was not in the water
phase. It would support these suggestions that the depen-
dence of the degradation in the emulsion with small oil
droplets on the acyl chain length of the ascorbates was
stronger than that with the large oil droplets. As these
ascorbates act not only as an antioxidant, but also as
aproxidant, their consumed quantity should be focused on.
The appropriate quantity is different among acyl ascorbates
and the applied processing food systems. Thus, further
studies are needed for effective application.
Acknowledgments This work was supported by a Grant-in-Aid
for
Young Scientists (B) 19780106 from the Ministry of
Education,
Culture, Sports, Science and Technology (MEXT), Japan.
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