Reversed-Flow Gas Chromatography as a Tool for Studying the

Article pubs.acs.org/JAFC

Cite This: J. Agric. Food Chem. 2018, 66, 12111−12121

Reversed-Flow Gas Chromatography as a Tool for Studying the Interaction between Aroma Compounds and Starch Lambros Farmakis,† Athanasia Koliadima,‡ George Karaiskakis,‡ and John Kapolos*,† †

Food Technology Department, Technological and Educational Institute of Peloponnese, 24100 Kalamata, Greece Chemistry Department, University of Patras, 26504 Patras, Greece

J. Agric. Food Chem. 2018.66:12111-12121. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 01/15/19. For personal use only.

‡

ABSTRACT: The versatile technique of reversed-flow gas chromatography was introduced to calculate physicochemical quantities for the interaction between aroma compounds and starch. Adsorption, adsorption/desorption, and surface reaction rate constants as well as surface diffusion coefficients for the vapors of aroma compounds over the different starch surfaces were calculated in the temperature range of 303.15−333.15 K. Enthalpies of adsorption between −45.5 and −109.0 kJ mol−1 and enthalpies of physicochemical interaction between 6.8 and 47.4 kJ mol−1 were also calculated for all the systems studied. From the obtained results, it is concluded that the interaction forces between aroma compounds and starch correspond to weak energy bonds such as hydrogen bonds and dipole−dipole interactions. For all the systems studied, except for the system heptanal/potato, physical sorption of aroma compounds on starch granules was indicated according to the calculated activation energies. KEYWORDS: aroma compounds, reversed flow gas chromatography, interaction enthalpies, physicochemical quantities, starch

1. INTRODUCTION Starch is a complex of two different polysaccharide molecules, the linear and helical amylose and the branched amylopectin, the basic building block of both polymers being glucose. Both amylose and amylopectin are composed of α-glucosidic units connected to each other through a 1,4-oxygen atom. In addition to this, amylopectin contains 1,6 branch points. Linear amylose has the ability to form inclusion complexes with suitable guest molecules (alcohols, aldehydes, terpenes, lactones, etc.), while amylopectin has a limited ability to form complexes because only the long external branches are able to form helices.1 To develop the desired flavor and odor, manufacturers add to foods a variety of aroma compounds in addition to those that are present in foods by nature. The added aroma compounds interact with food ingredients, and the strength of this interaction depends on the physicochemical characteristics of volatile aroma compounds and nonvolatile food components (lipids, proteins, and polysaccharides) as well as on the bonds that are formed.2−9 The interactions between volatile aroma substances and nonvolatile compounds are of two types: (a) attractive (fixation of volatile compounds on nonvolatile substrates) or (b) repulsive (release of the volatile compounds). The nature of these interactions depends on the physicochemical properties of the compounds and, in particular, on the binding that may occur between them.10 Additionally, the interactions between aroma compounds and carbohydrates generally have weak energy and depend on numerous factors such as nature and amount of aroma compounds and carbohydrates.11 The mechanisms that govern the interactions between aroma compounds and food ingredients are (a) the distribution of aroma compounds molecules between different phases of food, (b) the diffusion of aroma compounds through the bulk of food, and (c) the linking of aroma compound molecules with © 2018 American Chemical Society

food components. These mechanisms depend on the type of compound encapsulated, the morphological and energetic properties of the surface, and the composition of the starch granules and especially the percentage of amylose and amylopectin, which is in correlation with the origin of the starch sample.12,13 Many techniques such as nuclear magnetic resonance spectroscopy, X-ray diffraction, scanning electron microscopy, differential scanning calorimetry, and static or dynamic headspace gas chromatography have been used to study these interactions.14−20 However, while with these techniques both the amount and degree of retention of aroma compounds as well as the capacity of starch can be determined, it is not possible (except of static or dynamic headspace gas chromatography) to calculate other physicochemical (adsorption and desorption constants, partition coefficients, thermodynamic quantities, etc.) and kinetic parameters (reaction rate constants) to investigate the mechanism which determines the containment or release of aroma compounds from starch. Moreover, with the above techniques, it is not possible to calculate the surface energy or determine the distribution of active sites of the starch surface. So, the above techniques cannot answer thoroughly why some foods do not have a stable aroma and why some are more effective in containing aroma compounds than others. The technique of inverse gas chromatography, IGC, has been successfully applied to study volatile compound−material interactions as well as to calculate physicochemical parameters for the interaction between starch and aroma compounds.21−27 The major advantage of IGC is that starch is used as a stationary phase, and the aroma compounds are injected over the Received: Revised: Accepted: Published: 12111

August 12, 2018 October 23, 2018 October 25, 2018 October 25, 2018 DOI: 10.1021/acs.jafc.8b04360 J. Agric. Food Chem. 2018, 66, 12111−12121

Article

Journal of Agricultural and Food Chemistry Table 1. Physicochemical Properties of Aroma Compounds

diacetyl DL-limonene

heptanal 1-hexanol

chemical formula

Mw (g mol−1)

d (g mL−1)b

bp (°C)c

odor descriptor

molar volume (cm3 mol−1)

log P (25 °C)

vapor pressure (hPa)a

water solubility (g/L)b

C4H6O2 C10H16 C7H14O C6H14O

86.09 136.24 114.19 102.18

0.990 0.841 0.818 0.814

88 176 152.8 157

buttery lemon fruity green grass

88.8 163.3 141.4 125.1

−1.33 4.45 2.50 1.94

52.0 2.1 1.0 3.0

200.0 insoluble insoluble 5.9

At 20 °C. bAt 25 °C. cAt 760 mmHg.

a

of alcoholic fermentation,33,34 rate constants for catalytic reactions,35−40 thermodynamic study of polymer solutions,41 etc. In the present work, RF-GC was used to study the interaction between four aroma compounds (1-hexanol, heptanal, diacetyl, and DL-limonene) and starch granules from a different origin (corn, wheat, rice, and potato). Physicochemical quantities such as adsorption rate constants, k1, adsorption/desorption rate constants, kR, surface reaction rate constants, k2, diffusion coefficients, Dy, as well as enthalpies of adsorption, ΔHa, for the four aroma compounds on each starch can be calculated directly from experimental data, and from that, the influence of aroma compounds as well as of starch origin on the aroma/starch interaction can be clarified. The innovative part of this work consists of: (i) applying the RF-GC technique to study the interaction between aroma compounds and starch granules, which is of great importance in food science, as starch is the most common carbohydrate in the human diet, is contained in many foods, and used as an additive for food processing; (ii) determining in a single experiment four kinetic (adsorption rate constant, k1, adsorption/ desorption rate constant, kR, surface reaction rate constant, k2, and diffusion coefficient, Dy) and two thermodynamic (enthalpy of adsorption, ΔHa, and enthalpy for the physicochemical interaction, ΔHint) parameters for the interaction between various aroma compounds and starch granules from different origins; and (iii) investigating the mechanism for the interaction between aroma compounds and starch by studying the influence of the nature of the aroma compounds and of the starch origin on the aroma/starch interaction.

Table 2. Specific Surface Area (SSA) and Glass Transition Temperature (Tg) of Starches from Different Origins starch origin

SSA (m2 gr−1)

Tg (°C)

rice potato wheat corn

1.075 0.277 0.239 0.450

65.3 71.2 72.8 69.1

surface of starch. Thus, the interaction between starch and aroma compounds can be studied while the humidity of starch can be controlled. The disadvantages of this technique are that it cannot be used to determine kinetic parameters, and the aroma compounds have to be at infinite dilution. In the present work, a versatile gas chromatographic technique called reversed-flow gas chromatography (RF-GC) is introduced with the aim of developing a new technique for studying the interactions between aroma compounds and starch. An experimental setup was developed, and mathematical analysis was applied to experimental data to calculate physicochemical parameters (rate constants as well as equilibrium constants) referring to the aroma/starch interactions. This technique, which is a subtechnique of inverse gas chromatography (IGC), was developed in 1980 by Katsanos et al. at the University of Patras.28 RF-GC is a time-honored method which involves the change of the carrier gas flow direction at various time intervals. It has a vast number of applications in a wide scientific field such as the determination of diffusion and mass transfer coefficients,29−31 adsorption isotherms,32 the kinetic study

Figure 1. SEM micrographs for starch granules from different origins: (a) wheat, (b) corn, (c) potato, and (d) rice. 12112

DOI: 10.1021/acs.jafc.8b04360 J. Agric. Food Chem. 2018, 66, 12111−12121

Article

Journal of Agricultural and Food Chemistry

with a flow rate (corrected at column temperature) of approximately 0.35 × 10−6 m3 s−1. 2.2. Apparatus for Reversed-Flow Gas Chromatography. RF-GC is a flow perturbation technique based on reversing the carrier gas flow direction from time to time. The application of RF-GC method requires a commercial gas chromatograph, Shimadzu 14B in the case of our experiments, with any kind of detector (flame ionization detector, FID, in our case), capable of detecting the solute(s) contained in the carrier gas. A T-shape cell (sampling cell) constructed from glass or stainless steel chromatographic tube of any inner diameter was accommodated inside the oven of the chromatograph. The T-shape cell was divided into three parts. The first is called sampling column, the second the diffusion column, and the third the glass vessel. The sampling column was made of a stainless-steel chromatographic tube with 4 mm i.d. and was separated into two equal branches l = l′ = 0.60 m. The diffusion column, also made of stainless-steel chromatographic tube with the same diameter, was connected perpendicularly to the sampling column at its middle point and had a length L1 = 0.54 m. Finally, the vessel L2 containing the starch (0.5 g) was constructed from glass and had 17.5 × 10−3 m i.d. It was connected to the column L1 by means of a 6.35 × 10−3 m Swagelok union. The sampling cell was connected to the carrier gas inlet and the detector in such a way that the direction of the carrier gas flow through the sampling column (it is stagnant in the diffusion column as well as in the glass vessel) can be reversed at any time desired. This can be done by using a six-port valve to connect the ends D1 and D2 of the sampling column to the carrier gas supply and the detector, as shown schematically in Figure 2. With the valve in the position indicated by the solid lines, the carrier gas (dried and regulated by a gasflow controller) enters the column at D2 and leaves it from D1 toward the detector. By switching the valve to the other position (dashed lines), the direction of the carrier gas flow is reversed and enters the column at D1. To avoid condensation of solutes inside the valve and in the connection tubes, the entire valve body as well as the connecting tubes were heated. 2.3. Method. If pure carrier gas was passed through the sampling column, no signal was detected and nothing happened on reversing the flow. But after injecting 1 μL of aroma compound into the glass vessel containing the starch, a continuous concentration−time curve appeared. After the appearance of the continuously rising concentration−time curve, the reversing procedure for the helium carrier gas flow started by means of an electronic valve supplied by VICI (Valco Instrument Co.). Each reversal always lasted 6 s, a time which is smaller than the dead time of the carrier gas in the sampling column. The flow reversal recorded the concentration of the aroma compound vapors at the junction x = l′ (cf Figure 2) at the moment of the reversal. This concentration recording had the form of extra chromatographic peaks, which we call sample peaks (cf Figure 3), superimposed on the otherwise continuous detector signal. By repeating this sampling procedure at various times, a series of these sample peaks was produced. Each experiment lasted approximately 500 min. The time interval between flow reversals was 5 min for the first 200 min of the experiment, while for the next 300 min, the flow reversal of the carrier gas was performed every 10 min. The total number of collected peaks was approximately 70. The area under the curve or the height H from the continuous signal of the sample peaks, measured as a function of the time t when the flow reversal is made, is proportional to the concentration of the substance under study at the junction x = l′ of the sampling cell at time t. Measuring the height H experimentally as a function of t, one can construct the diffusion band, the shape and the distortion of which leads to the determination of physicochemical parameters referring to the interaction between aroma compounds and starch granules. To calculate the diffusion coefficients of the vapors of each aroma compound into the carrier gas, exactly the same experimental setup and procedure were followed. The only difference was that the glass vessel was void of any solid material.

Figure 2. Outline of the experimental arrangement used to study the interaction between aroma compounds and starch from different origins.

2. MATERIALS AND METHODS 2.1. Materials. In the present work, four aroma compounds, (814546), diacetyl (803528), 1-hexanol (804393), and heptanal (806918), of 99% purity from Merck A.G. were used. Their chemical formulas, molecular weights, and physicochemical characteristics are presented in Table 1. Starch granules from corn (S4180), wheat (S2760), potato (S2004), and rice (S7260) were obtained from Sigma-Aldrich. The specific surface areas of all starches, which were measured by Micrometrics, Gemini 2375 instrument, as well as their glass transition temperatures, which were determined using TG 209 F3, Tarsus, Netzsch instrument, are compiled in Table 2. Finally, by using scanning electron microscopy (FEI, Quanta FEG650) the size distribution and the morphology of all starch granules were obtained. In Figure 1 SEM micrographs for all used starches are illustrated. The carrier gas was helium of 99.999% purity from Air Liquide (Athens, Greece), dried by being passed through a gas purifier No. 452 of Matheson Gas Products (East Rutherford, NJ, United States) DL-limonene

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Figure 3. Part of the chromatogram for the system

DL-limonene

and starch from rice at 323.15 K. 2 −1 Table 3. Experimental Diffusion Coefficients (Dexp z , cm s ) of Aroma Compounds’ Vapors into the Carrier Gas (He) Determined by the RF-GC Method with the Theoretical Values (Dth z ) Calculated Using the FSG Equation at Various Temperatures

In all experiments, the pressure drop along the cell was negligible, and the solid bed was under a pressure of 1 atm. The working temperatures were 303.15, 313.15, 323.15, and 333.15 K.

3. THEORY 3.1. Determination of Diffusion Coefficients of Aroma Compounds’ Vapors into the Carrier Gas. By plotting ln H against time, t, when empty vessel was used, a diffusion band was produced, and the diffusion coefficients, Dz, of the vapors of aroma compounds into the carrier gas were calculated from the slope of the linear part of the diffusion band after the maximum according to the equation:29 slope = −

Dz (cm2 s−1) binary system 1-hexanol/He

heptanal/He

π 2Dz L12

(1) diacetyl/He

3.2. Determination of Physicochemical Parameters for the Interaction between Aroma Compounds and Starch. It is known42 that the calculation of physicochemical parameters by RF-GC is based on a theoretical analysis of the diffusion band obtained by plotting H1/M or (1/M) ln H against t in (min), where M (dimensionless) is the response factor for the detector (1 for the FID) and t the time when the respective flow reversal is made. The height H (in arbitrary units, for instance cm) is proportional to the concentration c (l′, t), H1/M = gc(l′, t), where c(l′, t) is the concentration of the aroma compound, mol cm−3, measured at the junction of the sampling column with the diffusion column (Figure 2) and g is the calibration factor of the detector, cm per mol cm−3. The equation that describes the concentration of aroma compounds as a function of time is

DL-limonene/He

a

% accuracy:

|Dzexp − Dzth| Dzth

T/K

Dexp z

Dth z

% accuracya

303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15

0.354 0.378 0.392 0.413 0.324 0.345 0.378 0.404 0.411 0.438 0.464 0.496 0.281 0.304 0.313 0.343

0.380 0.402 0.425 0.448 0.359 0.380 0.401 0.423 0.439 0.465 0.491 0.518 0.308 0.327 0.345 0.364

6.8 6.0 7.8 7.8 9.7 9.2 5.5 4.5 6.4 5.8 5.5 4.2 8.7 7.0 9.3 5.8

× 100 .

Y = (a 2 + a1 + a 2Q )(kR + k 2) + a1a 2 + k1kR = B1B2 + B1B3 + B1B4 + B2 B3 + B2 B4 + B3B4

(4)

Z = a1a 2(kR + k 2) + a1k1kR + k1kRk 2 + a 2QkRk1 = −(B1B2 B3 + B1B2 B4 + B1B3B4 + B2 B3B4 )

(5)

4

H1/ M = gc(l′,t ) =

∑ Aiexp(Bi t ) i=1

W = (a 2Q + a1)k1kRk 2 = B1B2 B3B4

(2)

(6)

where

where the exponential coefficients of time are given from the equations:

a1 =

X = a1 + a 2 + a 2Q + kR + k 2 = −(B1 + B2 + B3 + B4 )

2Dz L12

; a2 =

2Dy L 22

; Q = 2ayL 2 /azL1

(7)

and X, Y, Z, and W are auxiliary parameters.

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Table 4. Mean Values with Their Standard Deviations for Kinetic Parameters Referring to the Interaction between 1-Hexanol and Starch Granules from Different Origins and for Diffusion Coefficients of 1-Hexanol on Starch Granules at Various Temperatures starch origin rice

potato

wheat

corn

T (K)

k1 (103 s−1)

303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15

0.59 0.74 0.95 1.68 0.54 0.84 1.30 1.73 0.42 0.59 0.68 0.90 0.15 0.28 0.35 1.01

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

k2 (104 s−1)

0.07 0.05 0.05 0.04 0.06 0.05 0.04 0.03 0.07 0.06 0.03 0.04 0.04 0.07 0.04 0.03

0.62 0.80 0.86 0.91 1.46 1.63 1.97 2.18 1.06 1.54 2.23 2.75 0.39 0.54 0.87 1.64

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

kR (103 s−1)

0.07 0.06 0.03 0.03 0.07 0.07 0.09 0.1 0.04 0.06 0.09 0.11 0.03 0.04 0.06 0.01

2.06 2.29 2.55 2.45 1.01 1.11 1.32 1.99 1.35 1.21 0.60 3.64 3.90 1.26 1.13 0.70

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Dy (105 cm2 s−1)

0.08 0.07 0.07 0.06 0.05 0.06 0.06 0.08 0.05 0.04 0.05 0.05 0.08 0.06 0.09 0.03

0.41 1.13 2.93 5.59 0.73 0.90 2.52 6.60 0.11 3.22 5.96 8.26 0.48 1.43 2.01 2.43

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.02 0.07 0.17 0.33 0.07 0.08 0.23 0.10 0.01 0.02 0.04 0.06 0.02 0.07 0.10 0.12

Table 5. Mean Values with Their Standard Deviations for Kinetic Parameters Referring to the Interaction between Heptanal and Starch Granules from Different Origins and for Diffusion Coefficients of Heptanal on Starch Granules at Various Temperatures starch origin rice

potato

wheat

corn

T (K)

k1 (103 s−1)

303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15

1.23 1.61 1.98 2.41 1.50 1.81 2.77 3.09 2.15 2.43 2.60 2.91 1.69 1.94 2.75 5.75

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

k2 (104 s−1)

0.08 0.07 0.04 0.02 0.06 0.08 0.03 0.03 0.05 0.02 0.03 0.04 0.04 0.06 0.03 0.05

0.54 0.62 0.68 0.85 0.75 1.06 3.16 5.69 0.63 0.70 0.76 0.82 0.47 0.52 0.63 0.80

ay ns δ(y − L 2) + k1 as as

∫0

t

cy(τ )dτ

∂cy

(8)

∂t

where c*s is the local adsorbed equilibrium concentration of aromacompound at time t (mol g−1), ns is the initially adsorbed concentration of aroma compound (mol), as is the number of starch granules per unit length of column bed (g cm−1), δ (y-L2) is the Dirac’s delta function for the initial condition of the bed when the aroma compound is introduced as an instantaneous pulse at the point y = L2 (cm−1), y is the length coordinate along section L2, (cm), ay is the free cross sectional area in the solid bed in region y (cm2), cy is the gaseous concentration of aroma compound in region y (mol cm−3), and τ is the dummy variable for time (s). (ii) The mass balance equation for aroma compounds in the gaseous region z of the diffusion column: ∂cz ∂ 2c = Dz 2z ∂t ∂z

0.06 0.05 0.04 0.05 0.05 0.07 0.02 0.03 0.03 0.03 0.04 0.04 0.04 0.04 0.05 0.07

2.19 1.47 1.26 1.09 1.94 1.88 1.04 1.32 2.70 1.34 1.46 1.46 1.68 0.81 1.04 1.32

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Dy (105 cm2 s−1)

0.05 0.04 0.04 0.03 0.08 0.04 0.05 0.07 0.02 0.04 0.04 0.03 0.07 0.04 0.05 0.05

7.87 15.69 20.26 39.16 0.71 1.53 12.93 35.28 3.43 15.26 22.50 28.57 1.56 12.31 28.38 33.11

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.54 1.07 1.38 2.66 0.04 0.09 0.75 2.05 0.30 1.34 1.98 2.51 0.09 0.68 1.56 1.82

where cz is the gaseous concentration of aroma compound in the region z (mol cm−3). (iii) The mass balance equation for aroma compounds in the glass vessel containing starch from different origins:

To produce the eq 2, a system of four differential equations has to be solved. These equations are (i−iv): (i) The local adsorption isotherm of aroma compounds: cs* =

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

kR (103 s−1)

= Dy

∂ 2cy ∂y 2

− kR

as (cs* − cs) ay

(10)

(iv) The rate of change of the adsorbed concentration: ∂cs = kR (cs* − cs) − k 2cs ∂t

(11)

where cs is the concentration of aroma compound adsorbed on the solid (mol g−1). By taking into account the initial conditions (cf eq 126 of ref 43) and the boundary conditions (cf eqs 16, 18, and 19 of ref 44), the system of the above differential eqs 8−11 was solved, leading to the eq 2. To calculate the physicochemical parameters mentioned above, another mathematical approach for solving the above system of differential equations has to be adopted.45 It has been mentioned that they cannot be calculated only from the four eqs 6−9, and

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Table 6. Mean Values with Their Standard Deviation for Kinetic Parameters Referring to the Interaction between Diacetyl and Starch Granules from Different Origins and for Diffusion Coefficients of Diacetyl on Starch Granules at Various Temperatures starch origin rice

potato

wheat

corn

T (K)

k1 (103 s−1)

303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15

2.03 2.38 2.85 3.65 4.14 5.15 5.91 8.76 0.79 1.14 2.31 2.94 2.35 2.49 2.77 2.95

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

k2 (104 s−1)

0.08 0.09 0.05 0.03 0.05 0.04 0.04 0.02 0.07 0.05 0.03 0.04 0.08 0.09 0.01 0.02

0.86 1.05 1.38 1.53 1.10 1.18 1.24 1.43 1.07 1.19 1.37 1.52 0.92 0.96 1.00 1.04

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.1 0.06 0.04 0.05 0.05 0.06 0.04 0.01 0.07 0.08 0.09 0.1 0.05 0.05 0.05 0.06

kR (103 s−1) 1.73 1.23 1.27 1.24 1.39 1.44 1.50 1.38 1.37 1.56 1.36 0.98 1.04 1.09 0.98 1.32

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.1 0.08 0.06 0.05 0.05 0.04 0.05 0.03 0.06 0.07 0.05 0.07 0.06 0.07 0.04 0.05

Dy (105 cm2 s−1) 20.49 26.22 38.28 61.01 4.67 10.59 13.01 15.59 29.21 30.48 55.04 59.73 9.99 17.89 20.14 23.95

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.00 1.28 1.88 2.99 0.32 0.72 0.89 1.06 2.13 2.23 4.02 4.36 0.78 1.41 1.99 1.89

Table 7. Mean Values with Their Standard Deviation for Kinetic Parameters Referring to the Interaction between DL-Limonene and Starch Granules from Different Origins and for Diffusion Coefficients of DL-Limonene on Starch Granules at Various Temperatures starch origin rice

potato

wheat

corn

T (K)

k1 (103 s−1)

303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15 303.15 313.15 323.15 333.15

1.81 2.18 3.49 5.70 1.07 1.38 1.75 2.64 1.67 1.84 2.12 2.55 1.23 2.07 4.08 6.05

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

k2 (104 s−1)

0.06 0.03 0.01 0.03 0.07 0.08 0.01 0.02 0.09 0.01 0.02 0.05 0.04 0.06 0.04 0.06

0.89 0.98 1.10 1.21 1.17 1.32 1.41 1.91 0.59 0.72 0.80 0.95 0.71 0.86 0.94 1.08

another mathematical approach had to be adopted.45 This led to the same eq 5 with i = 5−7. Instead of eqs 3−6 above, the following relations are valid: a 2a1 X1 = + kR + k 2 = −(B5 + B6 + B7 ) a1 + a 2 + a 2Q

Z1 =

a1 + a 2Q k1kR k 2 = −(B5B6 B7 ) a1 + a 2 + a 2Q

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.05 ± 0.07 1.57 ± 0.10 9.00 ± 0.57 66.86 ± 4.21 0.05 ± 0.00 0.10 ± 0.00 0.716 ± 0.07 6.28 ± 0.07 10.31 ± 0.69 19.43 ± 1.30 25.36 ± 1.70 35.28 ± 2.36 2.72 ± 0.17 5.37 ± 0.39 11.81 ± 0.74 18.20 ± 1.15

0.84 1.57 1.97 3.77 2.61 2.83 4.87 6.41 0.93 1.15 1.28 1.73 1.18 1.75 1.99 2.54

0.07 0.05 0.05 0.07 0.08 0.07 0.04 0.06 0.06 0.07 0.04 0.06 0.06 0.06 0.05 0.06

4. RESULTS AND DISCUSSION From the experimentally determined values of, Dexp z at various temperatures, which are given in Table 3, with the theoretical ones, Dth z , calculated according to the Fuller−Schettler− Giddings equation,46 the following conclusions can be drawn: (i) in all systems, the Dexp z values increase with increasing temperature, as the theory predicts; and (ii) the accuracy of the reversed-flow gas chromatography technique in determining diffusion coefficients of the aroma compounds’ vapors into the carrier gas helium, which was calculated by the formula listed at the end of Table 4, varies between 4.5 and 9.7%, showing that RF-GC is a reliable technique for studying diffusion phenomena. The results for the parameters k1, k2, kR, and Dy obtained in the present work by the RF-GC technique are listed in Tables 4−7. The given values are mean values after three replications at each temperature for all the binary systems (aroma

α1a 2(kR + k 2) + (a1 + a 2Q )k1kR a1 + a 2 + a 2Q

= B5B6 + B5B7 + B6 B7

0.08 0.08 0.06 0.02 0.1 0.1 0.1 0.2 0.02 0.03 0.03 0.03 0.05 0.06 0.07 0.08

Dy (105 cm2 s−1)

and starch from different origins, a nonlinear regression analysis PC program in GW-BASIC44 has to be applied.

(12)

Y1 =

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

kR (103 s−1)

(13)

(14)

The physicochemical parameters previously defined, k1, kR, k2, and Dy, are hidden under the exponential coefficients of time B1, B2, B3, B4, B5, B6, and B7, while the pre-exponential factors have not been used in the calculations of the physicochemical parameters. To calculate the physicochemical parameters referring to the interaction between aroma compounds 12116


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Table 8. Enthalpies of Adsorption (ΔHa), Enthalpies for the Physicochemical Interactions (ΔHi) between Aroma Compounds and Starch Granules from Different Origins, and Enthalpies of Condensation (ΔHc) of Aroma Compounds starch origin ΔHc (kJ mol−1)

rice ΔHa kJ/mol

potato

ΔHi = |ΔHa| − |ΔHc| kJ/mol

ΔHa kJ/mol

wheat


corn

ΔHa kJ/mol


ΔHa kJ/mol


−61.6 (1-hexanol)

−89.3

27.7

−95.0

33.4

−82.2

20.6

−109.0

47.4

−48.0 (heptanal)

−66.7

18.7

−70.1

22.1

−55.7

7.7

−81.8

33.8

−38.7 (diacetyl)

−55.1

16.4

−58.2

19.5

−78.7

40.0

−45.5

6.8

−44.0 (DL-limonene)

−77.5

33.5

−68.6

24.6

−56.6

12.6

−89.6

45.6

Figure 4. Variation of k1 with the temperature for the interaction between aroma compounds and starch from (a) rice, (b) corn, (c) potato, and (d) wheat.

compound/starch). The obtained results for the mean values of the physicochemical parameters with their corresponding standard deviations were used to estimate the precision of the method for the calculation for each physicochemical quantity. The precision for each quantity is computed from the relation: precision (%) = 100 − 100 ×

standard deviation mean value

well as to the small molecule of diacetyl. These results are plotted in Figure 4. (ii) In all cases, the values of the parameters k1 and k2 increase with increasing temperature, as the theory predicts. (iii) Except for the systems with the DL-limonene in which the rate constant kR increases with increasing temperature, in the rest of the systems, an abnormal behavior for the variation of kR with temperature is observed. It could be attributed to the fact that kR is a complex rate constant pertaining to adsorption and desorption of aroma compounds on starch. (iv) In all cases, the Dy values increase with increasing temperature, as the theory predicts. (v) The Dy values for the system DL-limonene/potato in all temperatures are much smaller than the corresponding values

(15)

From the listed results in Tables 5−8 the following conclusions can be drawn (i−viii): (i) The values of k1 for all binary systems are of the same order of magnitude, and the larger values are observed for the interaction between diacetyl and potato. It could be attributed to the size of potato starch as 12117


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Figure 5. Variation of Dy with the temperature for the interaction between aroma compounds and starch from (a) rice, (b) corn, (c) potato, and (d) wheat.

of Dy in the other systems. It could be attributed to the higher molar volume of DL-limonene compared to the other aroma compounds as well as to the fact that potato has one of the smaller specific surface areas (cf Table 2). (vi) In all cases, 1-hexanol diffuses more difficultly, while diacetyl diffuses more easily. The above can be attributed to the smaller molecule of diacetyl compared to the other compounds. Also, the smaller values of Dy were observed for the system potato starch/aroma compound. The Dy parameter for the diacetyl probe in the four starches used decreases with the order: potato < corn < rice < wheat. It can be attributed to the lack of channels in potato starch compared to the other starches as well as to the smooth surface of potato starch.20 In potato starch, the number of chains per branched molecule is zero, while the same number for the rest of the starches is between 11.9 and 19.2 for the wheat, between 4.7 and 8.7 for the rice, and 4.4 for the corn starch.47 The latter numbers explain the variation of the Dy parameter of the diacetyl probe with the origin of starch found in the present work. The same behavior was also observed with the DL-limonene molecule, as there is no any possibility of bonding between the probe molecule and the substrate starch. Concerning the probe of 1-hexanol, due to the presence of −OH group, there is a possibility of interaction between 1-hexanol and starch and the variation of the Dy parameter with the origin of starch not following the order mentioned above for diacetyl

and DL-limonene. The same happens for the heptanal molecule due to the presence of the carbonyl group, which can be attributed to the possibility of a reaction between carbonyl group and surface molecules of starch granules. The variation of Dy with temperature is shown in Figure 5. (vii) The variation of k1 parameter with the origin of starch for the system diacetyl/starch follows, as it was expected, the opposite order from that observed for the Dy parameter: wheat < rice < corn < potato. In some cases, the order rice < corn reverses, as the number of chains per branched molecule for the two starches is very close. An anomalous behavior was observed for the variation of k1 with the origin of starch for the systems 1-hexanol/starch, heptanal/starch, and DL-limonene. It could be attributed to the presence of hydroxyl group of 1-hexanol, carbonyl group of heptanal, and probably to the bigger molecular size of DL-limonene. One general conclusion is that the k1 parameter for all studied systems and temperatures is a lower value with the probe 1-hexanol. (viii) The rate constant k2 parameter is the highest value with the probe diacetyl and the lowest value with the probe heptanal. On the other hand, the k2 parameter has the highest value in 8 from 16 cases with the potato starch, in 3 from 16 cases with the wheat starch, in 3 from 16 cases with the rice corn, and in 2 from 16 cases with the corn starch. The k2 values for all binary systems are plotted in Figure 6. 12118


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Figure 6. Variation of k2 with the temperature for the interaction between aroma compounds and starch from (a) rice, (b) corn, (c) potato, and (d) wheat.

By plotting ln k1 against 1/T, the enthalpy for the interaction between aroma compounds and starch from different origins can be calculated according to ref 22. First, from the slopes of each plot, the enthalpy of adsorption, ΔHa, for the four aroma compounds on each starch can be calculated according to the equation: slope = −ΔHa/R. Subsequently and taking into account the enthalpy of condensation of the aroma compounds ΔHc (found in tables),48 the enthalpy for the physicochemical interactions, ΔHi, between aroma compounds and starch can be calculated, as the difference ΔHi = |ΔHa| − |ΔHc|.22 The results are given in Table 8. From the results listed in Table 8, the following conclusions can be drawn (i−vi): (i) ΔHa values ranged from −45.5 to −109.0 kJ mol−1, which are in accordance with those given for the interaction of aroma compounds and high amylose corn starch22 for the adsorption of heptane and octane on cellulose49 and for alcohols (C1−C4) on ethylene vinyl alcohol copolymer.50 (ii) The values for the enthalpy of the physicochemical interactions between aroma compounds and starch ranged from 6.8 to 47.4 kJ mol−1, which correspond to weak energy bonds such as hydrogen bonds and dipole−dipole interactions. Hydrogen bonds and dipole−dipole interactions are likely to be involved for the more polar compounds of 1-hexanol and heptanal, as hydrogen bonds can be formed between the

hydroxyl groups of starch and the hydroxyl group of 1-hexanol or the carbonyl group of heptanal. 1-Hexanal is more likely to interact via hydrogen bonds, explaining why 1-hexanol shows an enthalpy of interaction stronger than that of heptanal in all starches used (27.7 kJ mol−1 for 1-hexanol and 18.7 kJ mol−1 for heptanal in the rice starch, 33.4 kJ mol−1 for 1-hexanol and 22.1 kJ mol−1 heptanal in the potato starch, 20.6 kJ mol−1 for 1-hexanol and 7.7 kJ mol−1 for heptanal in the wheat starch, and 47.4 kJ mol−1 for 1-hexanol and 33.8 kJ mol−1 for heptanal in the corn starch). (iii) Although hydrogen bonds are not involved in the diacetyl and DL-limonene molecules, the corresponding enthalpies of interaction are quite important, especially in the corn starch for DL-limonene and in the wheat starch for diacetyl. It could be attributed, at least for the DL-limonene molecule, to a possible interaction between DL-limonene and noncomplexed lipids present in the high amylose corn starch. (iv) The absolute values of enthalpies of adsorption, ΔHa, for all studied systems appear as a maximum value with the probe 1-hexanol as the parameter k1, which is inversely proportional to ΔHa, which appears as the lowest value with the probe 1-hexanol. The lower value of ΔHa was observed with the probe diacetyl for the rice, potato, and corn starches and with the probe heptanal for the wheat starch. An analogous behavior was also observed for the variation of the k1 parameter with the 12119


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Journal of Agricultural and Food Chemistry nature of the probe molecule in all studied starches. The |ΔHa| value for the probes 1-hexanol, heptanal, and DL-limonene varies with the starch origin as follows: corn > potato > rice > wheat, while for the probe diacetyl as follows: wheat > potato > rice > corn. (v) The enthalpies of interaction between aroma compounds and starch granules, ΔHi = |ΔHa| − |ΔHc|, vary with the starch origin in the same way as the |ΔHa| parameter in all probes used. For the probes 1-hexanol, heptanal, and DL-limonene, this variation is as follows: corn > potato > rice > wheat, while for the probe diacetyl the variation follows the order: wheat > potato > rice > corn. The lowest values of ΔHi were found with the probe diacetyl for the rice, potato, and corn starches and with the probe heptanal for the wheat starch, in accordance with the variation of |ΔHa| with the starch origin. The lowest value of ΔHi corresponds to the highest value of the k2 parameter, as the surface reaction rate constant, k2, is inversely proportional to the enthalpy of interaction between aroma compounds and starch granules. By plotting ln k2 against 1/T, the activation energies, Ea, for the interactions between aroma compounds and starch from different origin can be calculated. The calculated values of Ea for all systems are below 40 kJ mol−1, which indicate a physical sorption of aroma compounds over starch granules, except for the system heptanal/potato, which was calculated equal to 60.5 kJ mol−1, a value that indicated chemisorption. An activation procedure was observed, and the influence of aroma compound structure as well as of starch origin on the possible reaction between aroma compounds and starch was clarified. According to eq 15, the precision values for each physicochemical parameter for all systems can be calculated, and from the results, it is obvious that although the values of physicochemical parameters were obtained through complex algebraic equations, which means that accuracy or uncertainties could not be calculated, the method of RF-GC seems to have a good precision (89−99%). Finally, from the above results, RF-GC proved to be a useful technique for calculating physicochemical quantities, referring to the interaction between aroma compounds and starch granules from different origins, and can be used for calculating thermodynamic as well as kinetic parameters with a single experiment.

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(3) Langourieux, S.; Crouzet, J. In Food Macromolecules and Colloids; Dickinson, E., Lorient, D., Eds.; The Royal Society of Chemistry: Cambridge, UK, 1995; pp 123−133. (4) Bakker, J. In Ingredient Interactions: Effect of Food Quality; Gaonkar, A. G., Ed.; Marcel Dekker Inc.: New York, 1995; pp 411− 439. (5) Land, D. G. In Flavor−Food Interactions; Mc Gorrin, R. J., Leland, J. V., Eds.; American Chemical Society: Washington, DC, 1996; pp 2−11. (6) Harvey, B. A.; Druaux, C.; Voilley, A. In Food Macromolecule and Colloids; Dickinson, E., Lorient, D., Eds.; The Royal Society of Chemistry: Cambridge, UK, 1995; pp 154−163. (7) Druaux, C.; Voilley, A. Effect of food composition and microstructure on volatile flavour release. Trends Food Sci. Technol. 1997, 8 (11), 364−368. (8) Taylor, A. J. In Current Topics in Flavours and Fragrances; Swift, K. A. D., Ed.; Kluwer Academic Publishers: Dordrecht, 1999; pp 123−138. (9) Zafeiropoulou, T.; Evageliou, V.; Gardeli, C.; Yanniotis, S.; Komaitis, M. Retention of selected aroma compounds by gelatine matrices. Food Hydrocolloids 2012, 28 (1), 105−109. (10) Le Thanh, M.; Thibeaudeau, P.; Thibaut, M. A.; Voilley, A. Interactions between volatile and non-volatile compounds in the presence of water. Food Chem. 1992, 43, 129−135. (11) Solms, J. In Interactions of Foods Components; Birch, G. C.; Lindley, M. G., Eds.; Elsevier Applied Science: London, 1986; pp 189−210. (12) Arvisenet, G.; Voilley, A.; Cayot, N. Retention of aroma compounds in starch matrices: Competitions between aroma compounds toward amylose and amylopectin. J. Agric. Food Chem. 2002, 50 (25), 7345−7349. (13) Li, B. G.; Zhang, L. M. Inclusion Complexation of Amylose. Prog. Chem. 2010, 22 (6), 1161−1168. (14) Stephan, A.; Bucking, M.; Steinhart, H. Novel analytical tools for food flavours. Food Res. Int. 2000, 33 (3−4), 199−209. (15) Jung, D. M.; de Ropp, J. S.; Ebeler, S. E. Application of pulsed field gradient NMR techniques for investigating binding of flavor compounds to macromolecules. J. Agric. Food Chem. 2002, 50 (15), 4262−4269. (16) Tavel, L.; Guichard, E.; Moreau, C. Contribution of NMR Spectroscopy to Flavour Release and Perception. Annu. Rep. NMR Spectrosc. 2008, 64, 173−188. (17) Poinot, P.; Arvisenet, G.; Ledauphin, J.; Gaillard, J. L.; Prost, C. How can aroma-related cross-modal interactions be analysed? A review of current methodologies. Food Qual. Prefer. 2013, 28, 304− 316. (18) Lafarge, C.; Cayot, N.; Hory, C.; Goncalves, L.; Chassemont, C.; Le Bail, P. Effect of konjac glucomannan addition on aroma release in gels containing potato starch. Food Res. Int. 2014, 64, 412− 419. (19) Ayed, C.; Lubbers, S.; Andriot, I.; Merabtine, Y.; Guichard, E.; Tromelin, A. Impact of structural features of odorant molecules on their retention/release behaviours in dairy and pectin gels. Food Res. Int. 2014, 62, 846−859. (20) Jorgensen, A. D.; Jensen, S. L.; Ziegler, G.; Pandeya, A.; Buleon, A.; Svensson, B.; Blennow, A. Structural and physical effects of aroma compound binding to native starch granules. Starch/Sträke. 2012, 64 (6), 461−469. (21) Delarue, J.; Giampaoli, P. Study of interaction phenomena between aroma compounds and carbohydrate matrixes by inverse gas chromatography. J. Agric. Food Chem. 2000, 48, 2372−2375. (22) Boutboul, A.; Lenfant, F.; Giampaoli, P.; Feigenbaum, A.; Ducruet, V. Use of inverse gas chromatography to determine thermodynamic parameters of aroma−starch interactions. J. Chromatogr. A 2002, 969, 9−16. (23) Niederer, B.; Le, A.; Cantergiani, E. Thermodynamic study of two different chewing-gum bases by inverse gas chromatography. J. Chromatogr. A 2003, 996 (1−2), 189−194.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +302721045103. ORCID

John Kapolos: 0000-0003-3337-073X Funding

This research has been cofinanced by the European Union (European Social Fund, ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) Research Funding Program: ARCHIMEDES III. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Tietz, M.; Buettner, A.; Conde-Petit, B. Changes in structure and aroma release from starch-aroma systems upon alpha-amylase addition. Eur. Food Res. Technol. 2008, 227 (5), 1439−1446. (2) Overbosch, P.; Afterof, W. G. M.; Haring, P. G. M. Flavor Release in the Mouth. Food Rev. Int. 1991, 7, 137−184. 12120


Article

Journal of Agricultural and Food Chemistry (24) Murrieta-Pazos, I.; Gaiani, C.; Galet, L.; Calvet, R.; Cuq, B.; Scher, J. Food powders: Surface and form characterization revisited. J. Food Eng. 2012, 112 (1−2), 1−21. (25) Dimopoulou, M.; Tsivintzelis, I.; Ritzoulis, C.; Panayiotou, C. Thermodynamics of a food macromolecular assembly: the case of okra mucilage. RSC Adv. 2016, 6, 20916−20925. (26) Dury-Brun, C.; Hirata, Y.; Guillard, V.; Ducruet, V.; Chalier, P.; Voilley, A. Ethyl hexanoate transfer in paper and plastic food packaging by sorption and permeation experiments. J. Food Eng. 2008, 89 (2), 217−226. (27) Bayati, F.; Boluk, Y.; Choi, P. Inverse Gas Chromatography Study of the Permeability of Aroma through Hydroxypropyl Xylan Films. ACS Sustainable Chem. Eng. 2015, 3 (12), 3114−3122. (28) Katsanos, N. A.; Georgiadou, I. Reversed-flow gas Chromatography for studying heterogeneous catalysis. J. Chem. Soc., Chem. Commun. 1980, 5, 242−243. (29) Katsanos, N. A.; Kapolos, J. Diffusion coefficients of gases in liquids and partition coefficients in gas-liquid interphases by reversedflow gas chromatography. Anal. Chem. 1989, 61, 2231−2237. (30) Katsanos, N. A.; Bakaoukas, N.; Koliadima, A.; Karaiskakis, G.; Jannussis, A. Diffusion and adsorption measurements in porous solids by inverse gas chromatography. J. Phys. Chem. B 2005, 109 (22), 11240−11246. (31) Mc Givern, W. S.; Manion, J. A. Extending reversed-flow chromatographic methods for the measurement of diffusion coefficients to higher temperatures. J. Chromatogr. A 2011, 1218 (46), 8432−8442. (32) Roubani-Kalantzopoulou, F. Determination of isotherms by gas-solid chromatography. Applications. J. Chromatogr. A 2004, 1037 (1−2), 191−221. (33) Lainioti, Ch. G.; Kapolos, J.; Koliadima, A.; Karaiskakis, G. New separation methodologies for the distinction of the growth phase of Sacharomyces cerevisiae cell cycle. J. Chromatogr. A 2010, 1217 (11), 1813−1820. (34) Lainioti, Ch. G.; Kapolos, J.; Farmakis, L.; Karaiskakis, G.; Koliadima, A. Kinetic study of the alcoholic fermentation process in the presence of free and immobilized Saccharomyces Cerevisiae cells, at different initial glucose concentrations by Reversed Flow GC. Chromatographia 2010, 72 (11−12), 1149−1156. (35) Gavril, D.; Georgaka, A.; Karaiskakis, G. Kinetic Study of Oxygen Adsorption over Nanosized Au/gamma-Al2O3 Supported Catalysts under Selective CO Oxidation Conditions. Molecules 2012, 17 (5), 4878−4895. (36) Gavril, D.; Georgaka, A.; Loukopoulos, V.; Karaiskakis, G.; Nieuwenbuys, B. On the mechanism of selective CO oxidation on nanosized Au/γ-Al2O3 catalysts. Gold Bulletin. 2006, 39, 192−199. (37) Gavril, D.; Katsanos, N. A.; Karaiskakis, G. Gas chromatographic kinetic study of carbon monoxide oxidation over platinumrhodium alloy catalysts. J. Chromatogr. A 1999, 852 (2), 507−523. (38) Kotinopoulos, M.; Karaiskakis, G.; Katsanos, N. A. Catalytic determination by Reversed-Flow gas chromatography. J. Chem. Soc., Faraday Trans. 1 1982, 78, 3379−3382. (39) Kapolos, J.; Katsanos, N. A.; Niotis, A. Physicochemical quantities in catalytic reactions measured simultaneously by gas chromatography. Chromatographia 1989, 27 (7−8), 333−339. (40) Stolyarov, B. V.; Katsanos, N. A.; Agathonos, P.; Kapolos, J. Homogeneous catalysis by Reversed-Flow gas chromatography. J. Chromatogr. 1991, 550, 181−192. (41) Agathonos, P.; Karaiskakis, G. Thermodynamic study of polymer−solvent systems by reversed-flow gas chromatography. J. Appl. Polym. Sci. 1989, 37, 2237−2250. (42) Katsanos, N. A. Flow Perturbation Gas Chromatography; Marcel Dekker: New York, 1988. (43) Katsanos, N. A.; Thede, R.; Kalantzopoulou, F. Diffusion, adsorption and catalytic studies by gas chromatography. J. Chromatogr. A 1998, 795 (2), 133−184. (44) Abatzoglou, C.; Iliopoulou, E.; Katsanos, N. A.; RoubaniKalantzopoulou, F.; Kalantzopoulos, A. Deposition parameters of air pollutants on solid surfaces, measured in the presence of surface and

gaseous reactions, with a simultaneous determination of the experimental isotherms. J. Chromatogr. A 1997, 775 (1−2), 211−224. (45) Katsanos, N. A.; Gavril, D.; Karaiskakis, G. Time-resolved determination of surface diffusion coefficients for physically adsorbed or chemisorbed species on heterogeneous surfaces, by inverse gas chromatography. J. Chromatogr. A 2003, 983, 177−193. (46) Fuller, E. N.; Schettler, P. D.; Giddings, J. C. A new method for prediction of binary gas-phase diffusion coefficients. Ind. Eng. Chem. 1966, 58, 18−27. (47) Jane, J. Structural Features of Starch Granules II. In Starch: Chemistry and Technology; BeMiller, J.; Whistler, R., Eds.; Academic Press: New York, 2009; pp193−236. (48) NIST. Chemistry WebBook; National Institute of Standards and Technology: Gaithersburg, MD, 2001. (49) Gauthier, H.; Coupas, A. C.; Villemagne, P.; Gauthier, R. Physicochemical modification of partially esterified cellulose evidence by inverse gas chromatography. J. Appl. Polym. Sci. 1998, 69 (11), 2195−2203. (50) Gavara, R.; Catala, R.; Aucejo, S.; Cabedo, D.; Hernandez, R. Solubility of alcohols in ethylene-vinyl alcohol copolymers by inverse gas chromatography. J. Polym. Sci., Part B: Polym. Phys. 1996, 34 (11), 1907−1915.

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