Kinetics of molecular encapsulation of 1-methylcyclopropene into α-cyclodextrin

Article abstract of DOI:10.1021/jf072357t

1-Methylcyclopropene (1-MCP), an ethylene inhibiting regulator, is commercially available in the form of an inclusion complex with α-cyclodextrin (α-CD). In this study, molecular encapsulation of gaseous 1-MCP into aqueous α-CD was investigated in a closed, agitated vessel with a flat gas-liquid interface. Molecular encapsulation of gaseous 1-MCP by α-CD is a simultaneous two-step reaction which involves the aqueous dissolution of gaseous 1-MCP and the encapsulation of the dissolved molecules by α-CD. The kinetics and mechanism of molecular encapsulation were analyzed based on the depletion rate of 1-MCP in the headspace of the vessel. The encapsulation rates could be explained quantitatively by the gas absorption theory with a pseudo-first-order reaction between 1-MCP and α-CD. The negative value of the calculated apparent activation energy of encapsulation (-24.4 kJ/mol) implied the significant effect of exothermic aqueous dissolution of 1-MCP. An encapsulation temperature of 15°C was optimal; at this temperature, the highest 1-MCP yield and best inclusion ratio of inclusion complex were obtained. Changes in the X-ray diffraction pattern suggested that the crystal lattice structure of α-CD was altered upon inclusion of 1-MCP.

Full text of DOI:10.1021/jf072357t

11020 J. Agric. Food Chem. 2007, 55, 11020–11026
Kinetics of Molecular Encapsulation of
-Methylcyclopropene into r-Cyclodextrin
1
TZE LOON NEOH, KOUSUKE YAMAUCHI, HIDEFUMI YOSHII,* AND
TAKESHI FURUTA
Department of Biotechnology, Tottori University, 4-101 Koyamacho-minami, Tottori 680-8552, Japan
1-Methylcyclopropene (1-MCP), an ethylene inhibiting regulator, is commercially available in the form
of an inclusion complex with R-cyclodextrin (R-CD). In this study, molecular encapsulation of gaseous
-MCP into aqueous R-CD was investigated in a closed, agitated vessel with a flat gas–liquid interface.
1
Molecular encapsulation of gaseous 1-MCP by R-CD is a simultaneous two-step reaction which
involves the aqueous dissolution of gaseous 1-MCP and the encapsulation of the dissolved molecules
by R-CD. The kinetics and mechanism of molecular encapsulation were analyzed based on the
depletion rate of 1-MCP in the headspace of the vessel. The encapsulation rates could be explained
quantitatively by the gas absorption theory with a pseudo-first-order reaction between 1-MCP and
R-CD. The negative value of the calculated apparent activation energy of encapsulation (-24.4 kJ/
mol) implied the significant effect of exothermic aqueous dissolution of 1-MCP. An encapsulation
temperature of 15 °C was optimal; at this temperature, the highest 1-MCP yield and best inclusion
ratio of inclusion complex were obtained. Changes in the X-ray diffraction pattern suggested that the
crystal lattice structure of R-CD was altered upon inclusion of 1-MCP.
KEYWORDS: 1-Methylcyclopropene; r-cyclodextrin; gas encapsulation; inclusion complex; ethylene
inhibitor
INTRODUCTION
Molecular encapsulation of 1-MCP by CD transforms the
gaseous 1-MCP into powder form for easy and safe handling,
storage, transportation, and application to target plant materials.
Being encapsulated, 1-MCP does not exhibit a very high vapor
pressure and is therefore protected from oxidation and other
chemical degradation reactions (5). 1-MCP is released as a gas
from a formulated CD powder through aqueous dissolution of
the CD and release of the encapsulated gas (1).
1-Methylcyclopropene (1-MCP) is a four-carbon cyclic olefin
that has a three-membered ring to which a methyl group is
attached at the C1 position. 1-MCP is an ethylene inhibiting
plant growth regulator that works at parts-per-billion levels (1).
Its nontoxic mode of action and negligible residue make it an
extremely favorable agrochemical in regards to humans, animals,
and the environment (2). The mechanism of action of 1-MCP
in inhibiting ethylene is presumed to be through tight binding
to ethylene receptors of target plant materials, thereby blocking
ethylene action (3). Under normal environmental conditions,
Cyclodextrins (CDs) are natural starch derivatives resulted
from enzymatic degradation by cyclodextrin glycosyltransferase
(
CGTase). They are a family of cyclic oligosaccharides with
truncated molecular structure which are hydrophilic on the
exterior and relatively hydrophobic in the cavity. The unique
molecular structure of CDs imparts to them the ability to
encapsulate either the whole or part of appropriately sized
molecules which are less polar than water within their cavities.
CDs are comprised of D-glucopyranose units linked through
R-1,4 glycosidic bonds. Native CDs are named according to
their numbers of D-glucopyranose units. Those made up of six,
seven, and eight D-glucopyranose units are called R-, ꢀ-, and
γ-CD, respectively. The cavity diameter of native CDs ascends
with the number of D-glucopyranose units from 5.7 to 7.8 to
9.5 Å. The molecular encapsulation of carbon dioxide with
R-CD has been utilized for the isolation of R-CD through the
precipitation of a gas inclusion complex (8). Encapsulation and
release of carbon dioxide into and from R-CD have also been
reported previously (9).
1
-MCP is present in the gaseous state. It is chemically unstable
and begins to self-react immediately. Furthermore, it also
presents an explosive hazard when compressed. On account of
the aforesaid difficulties, 1-MCP is never isolated in the
manufacturing process for safety reasons. Instead, 1-MCP is
produced as an inclusion complex with R-cyclodextrin (R-CD),
by which its explosiveness is greatly diminished (4). Procedures
for molecular encapsulation of 1-MCP with R-CD in both
batch (5, 6) and continuous manners (7) have been patented.
Despite the fact that the study of the kinetics of the encapsulation
reaction is essential for improvement of the encapsulation
process, data on kinetic analysis of the encapsulation reaction
are currently not available.
*
Corresponding author. Telephone: +81-857-31-5272. Fax: +81-
8
57-31-0881. E-mail: foodeng.yoshii@bio.tottori-u.ac.jp.
1
0.1021/jf072357t CCC: $37.00  2007 American Chemical Society
Published on Web 12/04/2007

Molecular Encapsulation of 1-Methylcyclopropene
J. Agric. Food Chem., Vol. 55, No. 26, 2007 11021
The physiological effects of 1-MCP have been widely studied
on a wide range of fruits, vegetables, and ornamental crops
including apple, banana, pear, broccoli, lettuce, pea, carnation,
and rose. 1-MCP has also been employed as a new tool to
elucidate the fundamental processes involved in various physi-
ological changes in fruits and vegetables (10). Besides, a
controlled release system of 1-MCP has been either pat-
ented (11, 12) or studied academically (13) to cater for different
methods of application. To date, in spite of a considerable
amount of research on this comparatively newly found organic
compound, the kinetics of encapsulation of 1-MCP with R-CD
still remains unexplored.
Our research objective was to characterize the 1-MCP/R-CD
inclusion complex and the mechanism of molecular encapsula-
tion of gaseous 1-MCP in aqueous R-CD, and to determine the
parameters that influence the encapsulation reaction. The
parameters studied include R-CD concentration, initial 1-MCP
headspace concentration in the encapsulation system, encapsula-
tion temperature, and agitation rate. This information is par-
ticularly essential in optimizing the encapsulation process. The
encapsulation reaction was allowed to occur in a closed, agitated
vessel with a flat gas–liquid interface.
Figure 1. Experimental setup during transfer of 1-MCP from the reaction
vessel to the encapsulation vessel. Nitrogen was injected into the reaction
vessel to create atmospheric pressure in order to promote 1-MCP transfer.
¨
pump V-700 connected to the vacuum controller V-850 (BUCHI
MATERIALS AND METHODS
Labotechnik AG, Flawil, Switzerland). This bottle was designated as
the reaction vessel. Roughly 1, 2, or 3 g of previously thawed
suspension liquid of lithium salt of 1-MCP was injected into the reaction
vessel using a 20-mL syringe with a 18G needle through a rubber
septum (Shimadzu Corp., Kyoto, Japan) fitted on the screw cap.
Agitation was performed to promote complete aqueous neutralization
for about 15 min. Upon its formation, the 1-MCP gas evaporated and
accumulated in the head space of the reaction vessel.
Materials. R-CD of 99% minimum purity was purchased from
Ensuiko Sugar Refining Co., Ltd. (Tokyo, Japan). The R-CD powder
was dried in-Vacuo at 90 °C for 24 h before use. All the chemicals
used were of reagent grade unless otherwise indicated. 3-Chloro-2-
methylpropene (98%) and lithium diisopropylamide (30 wt % suspen-
sion in mineral oil) were purchased from Sigma-Aldrich Japan K. K.
(
Tokyo, Japan). Potassium bromide (KBr) was obtained from Wako
Pure Chemical Industries, Ltd. (Osaka, Japan). Nitrogen and isobutylene
standard gas (100 µL/L) were purchased from Sumitomo Seika
Chemicals Co., Ltd. (Osaka, Japan) in gas cylinders. Distilled water
was used throughout the entire experiment.
Meanwhile, vacuum was also pulled to about 0.27 kPa in the
encapsulation vessel containing temperature-equilibrated R-CD solution.
The reaction and encapsulation vessel were connected to each other
with a Teflon tube of 6 mm i.d. through the BVLM 20-0808 bulkhead
union elbows (Pisco USA, Inc., Bensenville, IL) installed on the screw
caps of each vessel (Figure 1). Circulation of the head space in both
vessels was enhanced with a mixing fan (25 mm × 25 mm × 10 mm)
1-MCP Synthesis. 1-MCP was synthesized according to the method
reported by Sisler and Serek (14) with some modification. Ap-
proximately 2.4 mL of 3-chloro-2-methylpropene (98%) was withdrawn
with a glass syringe and then injected into a screw-capped amber GC
vial containing 21.42 g of lithium diisopropylamide (30 wt %
suspension in mineral oil) through the butyl rubber stopper. Injection
was carried out over 1 h while the chemicals were gently mixed with
a magnetic stirrer. Mixing was continued after injection for another 30
min to ensure complete reaction. 1-MCP was formed as a lithium salt
suspended in mineral oil. After reaction, vacuum was pulled to about
(
ICFAN, Shicoh Engineering Co., Ltd., Kanagawa, Japan). Transfer
of 1-MCP from the reaction vessel to the encapsulation vessel, which
was driven by a pressure gradient, was carried out by opening the valves
of the elbows. The valves were then closed, and nitrogen was filled
into the reaction vessel to create atmospheric pressure. The valves were
reopened, and 1-MCP was transferred by the equalization of pressure
in both vessels. This procedure was repeated until the pressure of both
vessels reached the atmospheric value. Encapsulation was carried out
at temperatures of 15, 20, 25, 27, and 30 °C. Encapsulation was
promoted by agitation of the R-CD solution at agitation rates varied
from 0, 50, 100, 200, to 300 rpm. The head space mixing fan was kept
running throughout the experiment. During the encapsulation process,
the depletion of 1-MCP in the head space of the encapsulation vessel
was monitored over time by gas chromatography. Gas chromatography
analysis of 1-MCP was performed as described in the later subsection.
The depletion rate of 1-MCP during the initial stage of encapsulation
0.1 kPa on the suspension liquid to eliminate volatile impurities (mainly
the remaining 3-chloro-2-methylpropene), and then the product was
stored at -25 °C until use. 1-MCP gas could be produced by aqueous
neutralization of the suspension liquid by adding it into distilled
water.
Molecular Encapsulation of 1-MCP. Molecular encapsulation of
1
-MCP into R-CD was carried out in a closed, agitated vessel with a
flat gas–liquid interface. A 500-mL SCHOTT DURAN laboratory glass
bottle (DURAN Produktions GmbH and Co. KG, Mainz, Germany)
with a modified screw cap was used as the encapsulation vessel. R-CD
solutions of 30, 50, and 87.3 mM at 20 °C were prepared in the
encapsulation vessel, based on 100 g of distilled water by dissolving
(
0–180 min) is postulated to reflect the encapsulation rate of 1-MCP
into the R-CD cavity. The encapsulation time studied was 9 h. For
each particular treatment, the encapsulation process was performed in
duplicate. At the end of every encapsulation process, the R-CD solution
containing the precipitate of the inclusion complex was centrifuged at
3
, 5, and 9 g of R-CD, respectively. Saturation of R-CD was reached
at a concentration of 87.3 mM at 20 °C. The encapsulation vessel was
first placed in an 80 °C water bath for about 15 min to fully dissolve
the R-CD crystal. Subsequently, the vessel was immersed in a water
bath at corresponding temperatures for temperature equilibration before
the encapsulation process.
3
000 rpm for 15 min. The supernatant was decanted, leaving the wet
precipitate, which was then dried in-Vacuo for 24 h before further
analyses.
Quantitative Analysis of 1-MCP Included in r-CD. Gas chro-
matography was employed for quantification of the included amount
of 1-MCP in R-CD as described in the following subsection. The
inclusion ratio of 1-MCP is presented in a dimensionless term defined
as the molar ratio of the included 1-MCP to R-CD. Procedurewise,
approximately 10 mg of inclusion complex was weighed into a 60-mL
Encapsulation was carried out at three different initial 1-MCP head
space concentrations of approximately 40,000, 80,000, and 100,000
µL/L. First, vacuum was pulled to about 0.27 kPa in a 500-mL
SCHOTT DURAN laboratory glass bottle with a modified screw cap
containing 100 g of distilled water using the PTFE diaphragm vacuum

11022 J. Agric. Food Chem., Vol. 55, No. 26, 2007
Neoh et al.
amber GC vial, which was then closed with a butyl rubber stopper and
screw-capped. Roughly 3 g of distilled water was injected into the vial,
and vortex mixing was performed for 5 min to ensure thorough
dissolution of the inclusion complex. Subsequently, 0.1 mL of the head
space was sampled with a gas tight syringe for gas chromatography.
Calculation of the inclusion ratio was performed on a dry basis where
the moisture contents of the inclusion complexes were determined by
thermogravimetry (TG) as described in the later subsection.
1
-MCP Analysis by Gas Chromatography. A 0.1 mL head space
sample was separated with the GC-2010 gas chromatograph (Shimadzu
Corp., Kyoto, Japan) fitted with the ULBON HR-1 capillary column (30
mL × 0.53 mm i.d. × 5 µm film) (Shinwa Chemical Industries, Ltd.,
Kyoto, Japan) and a flame ionization detector. The injection port and
detector temperatures were set at 110 and 210 °C, respectively. The initial
column temperature was 40 °C with a hold time of 5 min, followed by
heating to 100 at 20 °C/min, with a final hold of 12 min. Quantification of
Figure 2. Depletion of 1-MCP in the head space of the encapsulation
system containing R-CD solutions of concentrations at (a) 30 mM, (b) 50
mM, and (c) 87.3 mM. The solid lines represent data correlation using
Avrami’s equation. The initial 1-MCP head space concentration was tested
at ≈80,000 µL/L (closed symbols) and ≈100,000 µL/L (open symbols).
An initial 1-MCP head space concentration of 40,000 µL/L (9) was tested
only for a 87.3 mM R-CD solution. Other parameters were set constant:
encapsulation temperature, 20 °C; encapsulation time, 9 h; agitation rate,
1-MCP was accomplished using an external standard protocol (15). The
isobutylene gas standard of 100 µL/L concentration was used. It was
presumed to have a response factor similar to that for 1-MCP.
Powder X-ray Diffractometry (PXRD). Powder X-ray diffraction
profiles of both uncomplexed R-CD and the 1-MCP/R-CD complex
were recorded on the M03XHF X-ray diffractometer (MAC Science
Co., Ltd., Tokyo, Japan) with X-ray Powder Research Software System
200 rpm.
(
XPRESS). Measurements were performed using Cu KR radiation (λ
1.5406 Å) operating at 40 kV and 20 mA. Samples were scanned at
a scanning speed of 4°/min over a diffraction angular range of 3° <
)
1
00,000 µL/L (data not shown) ruled out the possibility that
2
θ < 30°. The divergence slit, receiving slit, and time constant were
its depletion might be attributed mainly to self-polymerization
and decomposition. The 24-h stability of 10 µL/L 1-MCP
has also been reported by Nanthachai et al. (16). Besides, a
set at 1°, 0.15 mm, and 2 s, respectively.
Thermal Analysis of the Inclusion Complex. Thermogravimetric
(
TG) curves were recorded on the EXSTAR 6000 TG/DTA (TG/DTA
5
-h 1-MCP stability test has also been carried out in an
6200, SII Nano Technology Inc., Tokyo, Japan) equipped with the Muse
Measurement software, version 3.7 (SII Nano Technology Inc.).
Samples of 10 ( 0.5 mg were weighed into aluminum pans for analysis
at a heating rate of 5 °C/min from 30 to 260 °C in a nitrogen
atmosphere. The data were analyzed using the Muse Measurement
software, version 3.7.
Correlation Analysis of 1-MCP Depletion Data. Data on 1-MCP
depletion in the head space of the encapsulation vessel were empirically
fitted to exponential decay-type curves using scientific graphing and
analysis software (Origin7, OriginLab Corp., Northampton, MA).
Correlation analysis was performed on 1-MCP retention R, with
encapsulation time t for each treatment using the Avrami’s equation:
encapsulation system comprising instead distilled water,
revealing a negligible 1-MCP loss upon aqueous dissolution
(
data not shown). The finding is supported by the low Henry’s
-8
law constant of 1-MCP (1.27 × 10 mol/L · Pa) (4), which
further ruled out the possibility that 1-MCP depletion was
attributable mostly to dissolution into the liquid phase. Both
findings, therefore, strengthened the presumption that 1-MCP
depletion in the encapsulation system containing the R-CD
solution happened primarily due to its encapsulation by
aqueous R-CD.
n
R ) exp(-[kt] )
(1)
The effects of the initial 1-MCP head space concentration
were studied by varying the initial concentration of 1-MCP at
where R (unitless) is the 1-MCP retention, defined as the ratio of the
-MCP head space concentration at time t (s) to its initial value, k
1/s) is the 1-MCP depletion rate constant, and n (unitless) is the
1
(
4
0,000, 80,000, and 100,000 µL/L. R-CD solutions at concen-
trations of 30, 50, and 87.3 mM were tested, and the data were
presented in parts a, b, and c, respectively, of Figure 2. The
rest of the parameters, namely encapsulation temperature,
encapsulation time, and agitation rate, were set at 20 °C, 9 h,
and 200 rpm, respectively. As shown in Figure 2, after
normalization of the 1-MCP head space concentration data
against their initial values, in those treatments with identical
R-CD concentration, the effect of initial 1-MCP head space
concentration was negligible, where no significant difference
was found between treatments that varied in initial 1-MCP head
space concentration. The results implied that the dependency
of encapsulation rate on the initial 1-MCP head space concen-
tration could be described by a first-order relation. The formation
of an inclusion complex between 1-MCP and R-CD was
assumed to be on a 1:1 basis, and the first-order dependency
on 1-MCP concentration could be described by the following
reaction scheme:
depletion mechanism parameter. As the initial rate of 1-MCP
depletion is postulated to reflect the encapsulation rate of 1-MCP,
the k value is presumed to be equivalent to the apparent encapsula-
tion rate constant and n is presumed to be equivalent to the
encapsulation mechanism parameter. The apparent encapsulation rate
constant k during the first 180 min of the encapsulation process was
determined from eq 1 for comparison of the treatments. The
encapsulation mechanism parameter, n, was fixed at 0.65 for all the
treatments, as it allowed the most satisfactory fitting to the empirical
data.
Statistical Analysis. All statistical analyses were carried out using
Origin7. The apparent encapsulation rate constant determined by
correlation of 1-MCP depletion data for each treatment during the first
1
80 min of the encapsulation process was subjected to analysis of
variance (ANOVA) on a significant level of P ) 0.05. Data were
analyzed by parameters, namely R-CD concentration, initial 1-MCP
head space concentration in the encapsulation system, encapsulation
temperature, and agitation rate.
RESULTS AND DISCUSSION
1
-MCP + R-CD h complex h precipitate
(2)
Effect of Initial 1-MCP Head Space Concentration. The
2
4-h stability of 1-MCP in the head space of an empty
At the early stage of encapsulation, the reaction could be
approximated as an irreversible reaction where the rate of
encapsulation vessel at a concentration of approximately

Molecular Encapsulation of 1-Methylcyclopropene
J. Agric. Food Chem., Vol. 55, No. 26, 2007 11023
Figure 3. Linear correlation between the apparent encapsulation rate
constant k and the R-CD concentration. The coefficient of determination
R was 0.99.
2
Figure 4. Depletion of 1-MCP in the head space of the encapsulation
system under various agitation rates of 0 rpm (O), 50 rpm (4), 100 rpm
(0), 200 rpm (3), and 300 rpm ()). Other parameters were set constant:
the encapsulation reaction could be described as
follows:
initial 1-MCP head space concentration, ≈80,000 µL/L; R-CD concentra-
tion, 50 mM; encapsulation temperature, 20 °C; encapsulation time, 9 h.
r1-MCP ) rR-CD ) k CR-CDC1-MCP
(3)
2
Based on film theory, for the encapsulation reaction of
gaseous 1-MCP in aqueous R-CD, which approximates the
absorption with an irreversible pseudo-first-order reaction,
the 1-MCP absorption rate, N1-MCP, could be described by the
following equation (17):
where r1-MCP is the encapsulation reaction rate with respect to
-MCP, rR-CD is the encapsulation reaction rate with respect to
1
R-CD, k2 is the second-order encapsulation reaction rate
constant, and CR-CD is the concentration of the R-CD solution.
C1-MCP is the concentration of 1-MCP dissolved in the R-CD
solution, which could be described by HP1-MCP, where H is the
Henry’s Law constant and P1-MCP is the partial pressure of
N1-MCP ) C1-MCP k2CR-CDD
1-MCP
(4)
√
where D1-MCP is the diffusion coefficient of 1-MCP in the
aqueous phase.
In view of the fact that the correlation analysis on 1-MCP
depletion was satisfactory using a fixed n value of 0.65, the
apparent encapsulation rate could also be deduced as varying
proportionally with CR-CD. This result was in rather good
agreement with the film theory which states that the 1-MCP
1
-MCP in the head space of the encapsulation system.
At the early stage of the encapsulation process, since the
concentrations of the R-CD solutions, CR-CD, were incomparably
greater than the concentration of dissolved 1-MCP,
C1-MCP, CR-CD could be regarded as constant. Thus, the com-
plexation reaction could be approximated to be a pseudo-first-
order reaction with respect to the 1-MCP concentration.
absorption rate, N1-MCP, is proportionally dependent on CR-CD
.
Effect of r-CD Concentration. Three different concentra-
tions of R-CD solutions, namely 30, 50, and 87.3 mM, were
studied to examine the effect of R-CD concentration on the
encapsulation rate of 1-MCP (Figure 2a, b, and c). The solid
lines were the fitting results performed using eq 1, which also
reflected the encapsulation of 1-MCP. The fitting was satisfac-
tory, with a fixed n value of 0.65. The 1-MCP head space
concentration dropped at a rather high rate at the beginning of
the encapsulation process, and the depletion rate decreased
gradually until the 1-MCP head space concentrations eventually
leveled off to some constant values which depended on the
R-CD concentration. Correlation analysis was performed until
the first 180 min of encapsulation. The apparent encapsulation
rate constant k was found to be significantly different among
treatments of different R-CD concentrations. The k values
increased with the increase of R-CD concentration. When plotted
as a function of R-CD concentration, the k values were well
described by a linear equation which has an intercept at the
origin, with a coefficient of determination of 0.99 (Figure 3).
The strong linear correlation substantiated that the apparent
encapsulation rate constant k varied proportionally with CR-CD.
From the above findings it can be inferred that the rate-
limiting step of 1-MCP gas encapsulation into aqueous R-CD
is gas absorption with a pseudo-first-order reaction between
1-MCP gas and aqueous R-CD. The apparent encapsulation rate
is approximately proportional to the square root of the R-CD
concentration.
Effect of Agitation. Agitation of the R-CD solution during
encapsulation significantly affected the encapsulation rate. The
effects of agitation were studied by varying the agitation rate
from 0, 50, 100, 200, to 300 rpm. All other parameters were
set constant: initial 1-MCP head space concentration, ≈80,000
µL/L; R-CD concentration, 50 mM; encapsulation temperature,
20 °C; encapsulation time, 9 h.
The 1-MCP head space concentration depleted at decreasing
rates until the first 3 h and almost stopped diminishing thereafter
(Figure 4). Encapsulation was accelerated by faster agitation.
In the treatment without agitation (0 rpm), the 1-MCP head
space concentration diminished only around 20% at the end of
the experiments, whereas, at 300 rpm agitation, the 1-MCP
headspace concentration stabilized only after depletion of nearly
85%.

11024 J. Agric. Food Chem., Vol. 55, No. 26, 2007
Neoh et al.
Figure 6. Inclusion ratio of inclusion complexes (open symbols) and
1-MCP yield (closed symbols) obtained from the encapsulation reaction
performed at encapsulation temperatures between 15 and 30 °C.
Figure 5. Linear correlation between the apparent encapsulation rate
constant k and the agitation rate during the encapsulation reaction. The
2
coefficient of determination R was 0.99.
The apparent encapsulation rate constant k was plotted against
the agitation rate as shown in Figure 5. The data was well fitted
by a linear equation that intercepts the origin, with a coefficient
of determination of 0.99. The strong correlation between the
encapsulation rate constant and the agitation rate is persuasive
in postulating the effect of agitation on encapsulation of 1-MCP
in aqueous R-CD.
We presume that encapsulation occurs predominantly at the
gas–liquid interface. Owing to the low aqueous solubility of
the 1-MCP/R-CD inclusion complex, the inclusion complex
molecules crystallize on the gas–liquid interface, apparently
upon formation. As inclusion complex formation and crystal-
lization continues, a thin film made up by crystallized inclusion
complex is formed, covering the gas–liquid interface. This thin
layer of crystals acts as a barrier which obstructs the diffusion of
1-MCP into the R-CD solution. At low agitation rates (0 and 50
rpm), the agitation force was too weak to destroy and sink the
layer of inclusion complex crystal, thus hindering the encapsulation
reaction from progressing further. A thin layer of the above-
described crystallized inclusion complex was vividly observed at
the end of the encapsulation processes carried out at low agitation
rates. The mechanism of action of encapsulation hindrance,
however, is still not fully understood.
Figure 7. Depletion of 1-MCP in the head space of the encapsulation
system studied at encapsulation temperatures of 15 °C ()), 20 °C (3),
25 °C (0), and 27 °C (4). Other parameters were set constant: initial
1-MCP head space concentration, ≈80,000 µL/L; R-CD concentration,
50 mM; encapsulation time, 9 h; agitation rate, 200 rpm.
Effect of Encapsulation Temperature. The effect of en-
capsulation temperature was studied within the range 15-30
was observed. Meanwhile, the 1-MCP yield decreased
exponentially from 100% at 15 °C to almost 0% at 30 °C. It
is conceivable that the phenomenon resulted from the increase
of solubility of the inclusion complex and also the increase
of the tendency to self-polymerize between the dissolved
1-MCP. The optimal encapsulation temperature was thus
determined at 15 °C. The highest inclusion ratio achieved of
approximately 0.95 mol 1-MCP/mol R-CD suggested a 1:1
ratio of 1-MCP/R-CD complexation.
The increase in encapsulation temperature decreased the encap-
sulation rate and thus the apparent encapsulation rate constant, k
(Figure 7). Nonetheless, the effect of encapsulation temperature
on the encapsulation reaction was insignificant. Referring to the
results of the 1-MCP yield and the inclusion ratio, the 1-MCP
depletion at 30 °C might be perceived as resulting from a number
of reactions that contributed to 1-MCP loss. The 1-MCP depletion
at 30 °C is thus not presented in Figure 7.
°
C. All other parameters, namely initial 1-MCP head space
concentration, R-CD concentration, encapsulation time, and
agitation rate, were set at ≈80,000 µL/L, 50 mM, 9 h, and 200
rpm, respectively. Figure 6 shows the inclusion ratio of
inclusion complexes and the 1-MCP yield at different encap-
sulation temperatures. The 1-MCP yield is defined as the ratio
of the total amount of 1-MCP recovered in the inclusion
complex to the total amount of 1-MCP depleted in the head
space of the encapsulation vessel, while the inclusion ratio is
defined as the molar ratio of 1-MCP to R-CD in the collected
inclusion complex. The influence of encapsulation temper-
ature on the inclusion ratios of inclusion complexes prepared
at temperatures of 15-27 °C was not significant, probably
due to the fact that only the 1-MCP/R-CD inclusion complex
precipitated during the encapsulation process. At 30 °C,
however, an acute drop of the inclusion ratio to almost zero

Molecular Encapsulation of 1-Methylcyclopropene
J. Agric. Food Chem., Vol. 55, No. 26, 2007 11025
Figure 8. Arrhenius plot of the apparent encapsulation rate constant.
The open symbol represents the logarithmized k value obtained at the
encapsulation temperature 27 °C. Closed symbols represent values
obtained within the temperature range 15-25 °C.
Figure 9. Powder X-ray diffraction patterns of uncomplexed R-CD and
the 1-MCP/R-CD inclusion complex.
EE + E
D
Figure 8 is an Arrhenius plot where the logarithmized k values
are plotted as a function of the reciprocal temperature. A linear
function could be well fitted to the logarithmized k values between
∆
H +
S
(
)
2
N1-MCP ) H0P1-MCP k0CR-CDD exp
(
-
)
√
0
RT
15 and 25 °C. When the encapsulation temperature went just
(6)
slightly up, to 27 °C, the k value decreased drastically, where the
particular linear function could no longer fit the data. These results
imply a different mechanism of encapsulation at 27 °C from that
at temperatures between 15 and 25 °C. The slope of the straight
line fitted between 15 and 25 °C leads to the apparent activation
energy of encapsulation E of -24.4 kJ/mol with a pre-
The term of ∆H +
(
EE + E
D
/
2
)
would be equivalent to the
apparent activation energy of encapsulation E. As E is the sum
of ∆H and the average value of EE and ED, ∆H is shown to
S
S
S
be the dominating component in E.
The dissolution of short-chain hydrocarbons is often an
exothermic reaction. For example, the heat of aqueous dissolu-
tion of ethylene was reported at about -19 kJ/mol (18), whereas,
by calculation from the data reported by Huq and Wood (19),
the activation energy of ethylene diffusion in water is about 17
-
9
-1
exponential factor of 6.85 × 10
s .
The encapsulation reaction was conceived to be comprised
of a series of reactions that occurred almost simultaneously,
namely the dissolution of gaseous 1-MCP into the aqueous
phase, the diffusion of dissolved 1-MCP in the aqueous phase
before being encapsulated, and the inclusion of the dissolved
kJ/mol. By the assumption that 1-MCP also has ∆H and ED
S
values of similar magnitudes as those for ethylene, EE would
then be predicted as -28 kJ/mol. The predicted exothermic
value shows that 1-MCP encapsulation by R-CD happens rather
readily in the aqueous phase. This could be explained by the
ability of R-CD cavities to provide hydrophobic microenviron-
ments that are capable of stabilizing hydrophobic substances
in the aqueous phase. The readiness of CDs to encapsulate
organic compounds such as Triflumizole has also been reported
with exothermic ∆G values (20).
Powder X-ray Diffractometry (PXRD). Figure 9 presents
the PXRD patterns of uncomplexed R-CD and the 1-MCP/R-
CD inclusion complex. The inclusion complex was prepared
from a 30 mM R-CD solution at 20 °C. The initial 1-MCP head
space concentration was 100,000 µL/L, the agitation rate was
200 rpm, and the encapsulation time was 9 h. Sharp peaks in
the X-ray diffractograms suggested crystallinity of both samples.
The characteristic peaks of uncomplexed R-CD at 2θ ) 4.8°,
1
-MCP into R-CD.
The temperature dependence of the second-order encapsula-
tion reaction rate constant, k2, the diffusion coefficient of 1-MCP
in the aqueous phase, D1-MCP, and the Henry’s law constant of
1
-MCP, H, in eq 4 could be described respectively by the
Arrhenius equation, and the whole equation could be rewritten
as follows:
∆
HS
N1-MCP ) H exp -
)×
0
(
RT
EE
ED
RT
P1-MCP k exp -
CR-CDD exp -
(5)
0
(
)
0
(
)
ꢀ
RT
2
where H0 (M/Pa), k0 (1/s), and D0 (m /s) are the pre-exponential
factors, ∆H (kJ/mol) is the dissolution heat of 1-MCP, E (kJ/
1
1.8°, 14.1°, and 21.6° decreased in intensity or even dissipated
with encapsulation, while the characteristic peaks of the
S
E
mol) is the activation energy of encapsulation, E (kJ/mol) is
D
inclusion complex were found at 2θ ) 7.2°, 12.3°, 13.9°, 14.5°,
the activation energy of 1-MCP diffusion in aqueous phase, R
is the universal gas constant (8.314 J/K·mol), and T (K) is
absolute temperature. Rearranging eq 5 gives the following:
1
4.7°, 20.1°, 20.6°, and 21.8°. The four peaks at 2θ ) 9.5°,
18.6°, 25.5°, and 27.2° were just slightly shifted or stayed
unchanged after encapsulation of 1-MCP. Comparison of the

11026 J. Agric. Food Chem., Vol. 55, No. 26, 2007
Neoh et al.
PXRD patterns of the uncomplexed R-CD and the inclusion
complex substantiated that the changes resulted from encapsula-
tion. Based on these results, it could be deduced that inclusion
of 1-MCP in the R-CD cavity changed the crystal lattice
structure of R-CD.
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(
(
(
In conclusion, the probable ratio of complexation between
1-MCP and R-CD is 1:1. The rate-limiting step of encapsulation
is the gas absorption, with a pseudo-first-order reaction between
the host and guest molecules. The apparent activation energy
of encapsulation was calculated as -24.4 kJ/mol, suggesting a
strong domination by the activation energy of 1-MCP dissolution
over the activation energies of both encapsulation and diffusion.
Under the tested conditions, the optimal encapsulation temper-
ature was determined at 15 °C.
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Received for review August 6, 2007. Revised manuscript received
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JF072357T