Hydrothermal extraction of antioxidant compounds from green coffee

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Hydrothermal extraction of antioxidant compounds from green coffee beans and decomposition kinetics of 3-o-caffeoylquinic acid Takafumi Sato, Takuya Takahata, Tetsuo Honma, Masaru Watanabe, masayoshi wagatsuma, Shiho Matsuda, Richard Lee Smith, and Naotsugu Itoh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00821 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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Industrial & Engineering Chemistry Research

Hydrothermal extraction of antioxidant compounds from green coffee beans and decomposition kinetics of 3-o-caffeoylquinic acid

Takafumi Sato,a,* Takuya Takahata,a Tetsuo Honma,b Masaru Watanabe,c,d Masayoshi Wagatsuma,d Shiho Matsuda,d Richard Lee Smith, Jr.,c,d Naotsugu Itoha

a

Department of Material and Environmental Chemistry, Utsunomiya University, 7-1-2, Yoto, Utsunomiya, 321-8585, Japan

b

Department of Industrial System Engineering, National Institute of Technology,

Hachinohe College, 16-1, Uwanotai, Tamonoki-Aza, Hachinohe 039-1192, Japan

c

Research Center of Supercritical Fluid Technology, Graduate School of Engineering, Tohoku University, 6-6-11, Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan

1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

d

Graduate School of Environmental Studies, Tohoku University, 6-6-11, Aoba, Aramaki, Aoba-ku, Sendai, 980-8579, Japan

*Corresponding author: Takafumi Sato,

E-mail: [email protected]

Tel. & Fax: +81-28-689-6159

Keywords

reaction kinetics; chlorogenic acids; separation; subcritical water; bioresource

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Abstract

Separation of antioxidant compounds (caffeoylquinic acids (CQAs), phenolics, melanoidin, caffeine) from green coffee beans with hydrothermal extraction and decomposition kinetics of 3-o-caffeoylquinic acid (3-CQA) are reported.

Antioxidant

capacity (AOC) of the extracts increased as extraction temperature was increased up to 410 K and then it decreased up to extraction temperatures of 500 K. was further increased above 500 K, AOC remarkably increased.

As extraction temperature The decomposition rate

of 3-CQA in water was determined from 433 to 513 K. The increase and decrease in AOC with extraction temperature can be attributed to hydrolysis of oligomeric structures (glycosides) in the coffee beans that yield CQAs, the decomposition of the CQAs and to the formation of melanoidins that had a characteristic brown color.

Hydrothermal extraction

provides an effective method for separation of antioxidant compounds from green coffee beans and the effluent extracts may be suitable for food products.

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1. Introduction Coffee is a popular beverage and has many beneficial health effects as documented in a review by Poole et al.1

Coffee beans contain phenolic compounds2,3 that act as

antioxidants, and chlorogenic acids that have antioxidant properties and prevent neurodegenerative diseases and stroke.2

However, in commonly used ways to process

coffee, many antioxidants inherent to green coffee beans are lost due to removal of its outer skin (cherry), fermentation treatment, drying and roasting that can cause an estimated 40-fold decrease in the amount of chlorogenic acids in the bean.4

Thus, new methods

such as puffing4 and extraction with supercritical CO25 have been explored as methods to recover or preserve valuable antioxidants in coffee beans.

In this work, an efficient

method for isolating antioxidant compounds from green coffee beans is studied that is denoted as hydrothermal extraction. Hydrothermal extraction contacts the green coffee beans with a flowing liquid stream of hot water to leach target compounds and possibly decompose some constituents in either the substrate or in products depending on conditions.

4


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The term, hot water, is commonly used to refer to the condition of water in its liquid state at temperatures of (40 to 100) oC.

When liquid water is at a temperature that is higher

than its normal boiling point and at pressures higher than its saturation pressure, its condition is referred to as being hydrothermal.

Hydrothermal extraction has many

applications in food that are reviewed by Castro-Puyana et al.6

The physical properties of

water in its hydrothermal condition, such as dielectric constant and self-ionization constant change greatly as temperature is increased about 373 K and rapid and selective extractions are possible.6

Hydrothermal extraction requires only water as a solvent and thus the

method is environmentally-friendly compared with other techniques that use organic solvents. Hydrothermal extraction of green coffee beans can be expected to provide antioxidant compounds such as chlorogenic acids because hot water can dissolve many phenolic compounds.7

The antioxidant capacity, chlorogenic acid and total phenolic content of the

extracts depend on temperature and pressure of the hydrothermal extraction conditions. Getachew and Chun8 applied hydrothermal extraction to five types of green coffee beans and reported that the amount of chlorogenic acids, antioxidant capacity and total phenolic 5



content at 453 K were lower than those at 493 K.

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Getachew and Chun9 confirmed the

trends of antioxidant capacity with extraction temperature in other experiments. Mayanga-Torres et al.10 evaluated the total phenol content obtained from hydrothermal extraction of green coffee powder at temperatures of (423 to 573) K and revealed that the total phenol content slightly increased with increasing temperature.

In these studies8-10,

hydrothermal extractions were performed at temperatures of (400 to 500) K, however, the dependence of the antioxidant capacity, chlorogenic acid content and total phenolic content with temperature and pressure were not reported in detail. In applying hydrothermal extraction to coffee beans, the decomposition kinetics of chlorogenic acids are important for understanding the conditions, because they are some of the key compounds that govern antioxidant capacity.2

The compound, 5-o-caffeoylquinic

acid decomposes under reflux11 and under hydrothermal conditions.12

In the literature, 13

the decomposition rate of chlorogenic acids at hydrothermal conditions has been studied at (373 to 473) K but the trends have not been measured at higher temperatures. Conditions of a hydrothermal extraction are important for realizing industrial application. Hydrothermal extraction can be used to convert spent coffee grounds into a wide range of 6


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functional materials while spent coffee grounds were used as catalysts14 or adsorbents.15 The antioxidant capacity and total phenol content have been studied for spent coffee grounds according to hydrothermal extraction temperature, liquid/solid ratio and extraction time with statistical methods16-18 to understand interactions between variables identified in extraction experiments.

In this work, hydrothermal extraction temperature, pressure and

extraction time are considered as key variables based on literature reports. In this study, hydrothermal extraction was applied to green coffee beans with a semi-batch system, with the objective being to determine the antioxidant capacity, the amount of chlorogenic acids and the total phenolic content of extract samples.

The

decomposition of a specific chlorogenic acid in water under hydrothermal conditions was determined to elucidate hydrolysis kinetics.

Finally, conditions for obtaining high

antioxidant extracts are estimated considering the reaction kinetics.

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2. Materials and methods Brazil No. 2 (Arabica) green coffee beans were obtained from Coffee Tonya (Utsunomiya, Japan).

The elliptical-shaped beans had a major axis of about 10 mm and a

minor axis of about 7 mm. The beans were stored enclosed at room temperature and used without further treatment.

Chlorogenic acid hydrate (>98.0 %, dry basis), caffeine

(>98.0 %) and 1,1-diphenyl-2-picrylhydrazyl free radical (>97.0 %) were purchased from Tokyo

Chemical

Industry

Co.,

Ltd.

The

compound,

(±)-6-Hydroxy-2,5,7,8-

tetramethylchromane-2-carboxylic acid (Trolox) (>98.0 %) used in the food analyses and 3-o-caffeoylquinic acid (CQA) (>98.0 %) were purchased from Wako Pure Chemical Industries Ltd. The 0.1 N Folin-Ciocalteu reagent was purchased from Merck.

Distilled

water used in the hydrothermal extractions was obtained from a water purifier (WG-222, Yamato Co.). Figure 1 shows the experimental apparatus for hydrothermal extraction of coffee beans. The extractor was made of stainless steel (length = 105 mm, i.d. = 8.5 mm, o.d. =12.7 mm) and Swagelok unions (model SS-810-6-1ZV). cm3.

The internal volume of the extractor was 6

At first, the coffee beans were loaded into the extractor. 8


The typical amount of

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coffee beans used was about 0.8 g except for experiments that were used to evaluate the effect of particle size and temperature for which the loadings used were 0.5 g and 0.1 g, respectively.

Temperature was controlled by a forced convection oven (MTS-CH202030,

Tokyo Motoyama Syokai Co. Ltd.) that was set to a temperature between 323 K and 573 K. After loading the extractor with sample, the extractor and preheater were connected to the tubing and then water was supplied with an HPLC pump (Model 576, GL Science Inc.) at a flow rate of 1.0 g/min. After all lines were filled with water, the preheater and extractor were inserted into the oven through the hole at top of the oven and the extraction time was considered to start. The water flowed from the pump into a preheater that was made of a stainless-steel 1/16 inch coil and then into the extractor.

The effluent from the extractor

flowed into tubing that was cooled in a water bath through a filter (model SS-2TF-05, Swagelok), and the stream was depressurized with a back-pressure regulator (26-1700 series, Tescom) and the contents were recovered in a trap. performed for time periods of (20 to 60) min.

Hydrothermal extraction was

After each experiment, extract solutions

were diluted with water to make 100 cm3 of total liquid volume for analysis.

9



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Initially, water filled in the apparatus was heated and expanded by heating in the preheater and extractor, and then the water was pushed out to the back-pressure regulator side especially in the low pressure region so that it took several minutes for temperature and pressure of system to stabilize.

Extraction data contain initial portions of extractions

at temperatures from room temperature to the setpoint temperature. The rate of hydrolysis of chlorogenic acids defined as a single compound, 3-o-caffeoylquinic acid (3-CQA) was evaluated at 433 K with a 10 cm3-glass batch reactor and with a 6 cm3-batch reactor made of 316-stainless steel (SS316) at (473 to 513) K. Samples of 2.0 g of 3-CQA aqueous solution (1 wt% of 3-CQA concentration) were loaded into the reactor and sealed after displacement of air with 1 MPa of nitrogen gas.

The glass

reactor was heated in a microwave heating apparatus19 at 433 K for (15 to 45) min.

The

stainless-steel reactor was heated in a molten salt bath (50 % NaNO3–50 % KNO3) at (473 to 513) K for (5 to 45) min.

Heating time for achieving the reaction temperature in the

microwave heating was 30 s and that in the molten salt bath was 90 s.

For the stainless

steel reactor, after a given time period, the reactor was rapidly quenched by immersion into

10


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water at room temperature. The contents of the reactor were recovered by rinsing with water to make up a total of 30 g sample for analysis. The caffeine and caffeoylquinic acids in the extraction experiments were analyzed with an HPLC-UV system.

The column was an Inertsil ODS-4 (GL Science Inc.) and the

detector wavelength was set to 254 nm for caffeine analysis and to 325 nm for caffeoylquinic

acid

analysis.

In

the

analyses,

the

3-o-caffeoylquinic

acid,

4-o-caffeoylquinic acid and 5-o-caffeoylquinic acid peaks could not be resolved so that the amount of caffeoylquinic acids was evaluated by considering the sum of the peak areas of 3-o-caffeoylquinic,

4-o-caffeoylquinic

and

5-o-caffeoylquinic

acids.

Typical

spectrograms are given in supporting information (Fig. S1). The existence of the caffeoylquinic acids were measured with a UPLC/Q-TOF analyzer (ACQUITY UPLC/Xevo G2-s Q-TOF, Waters) that gave an elemental composition of C16H18O9 which corresponded to 3- or 4- or 5- o-caffeoylquinic acid by comparison with masses between measured precursor ions and theoretical monoisotopic masses. For the decomposition study of 3-CQA in the batch reactors, an HPLC-RI system was employed to detect 3-CQA. The column was RSpak KC-811 (Showa Denko K. K.) and 11



the refractive index detector was RI 1530 (Jasco Co.).

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All isomers of CQA were assumed

to have a decomposition rate of 3-CQA that was evaluated through the disappearance of the 3-CQA peak in the HPLC chromatogram.

The experimental variation of the

decomposition rates for a given condition was within 3 %. The antioxidant capacity of the extract was evaluated by DPPH radical-scavenging.20

A

3 mL of 0.175 mM DPPH ethanol solution was added to 0.5 mL of each sample, the mixture was shaken and left for 25 min in the dark, and then the absorbance at 517 nm was measured with a UV-vis spectrophotometer (UV-1600, Shimadzu Corp.). capacity was evaluated in terms of the amount of Trolox. evaluated with the Folin–Ciocalteu method.

The antioxidant

The total phenolic content was

At first, 0.2 mL of 0.1 N Folin–Ciocalteu

reagent was added to 1.0 mL of sample and the solution was left for five minutes in the dark.

Subsequently, 0.8 mL of 7.5 wt% sodium carbonate aqueous solution was added to

the solution that was left for 60 min in the dark before measuring the absorbance at 765 nm of the solution with a UV-vis spectrophotometer (UV-1600, Shimadzu Corp.). phenolic content was evaluated on the basis of the amount of gallic acid.

The total

The melanoidin

level was defined as the absorbance of 440 nm of the sample according to the literature.21,22 12


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The amount of caffeine, amount of caffeoylquinic acids, the antioxidant capacity and the total phenolic content were defined by Eqs. (1)-(4). [ ∕ − ] =

ℎ ℎ [] ℎ [] (1)

[ ∕ − ] =

ℎ ℎ [] ℎ [] (2)

[ ! ∕ − ] =

" ℎ [] ℎ [] (3)

ℎ [ #! ∕ − ] =

ℎ ℎ ℎ [] ℎ [] (4)

In this study, the coefficients of variation determined from the experimental measurements were 1.8 %, 9.1 %, 6.9 % and 4.1 %, for the amount of caffeine, the amount of CQAs, the antioxidant capacity and the total phenolic content, respectively. The conversion of 3-CQA (XCQA) was evaluated with Eq. (5): 13



$%&' [%] = )1 −

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+ , [] . × 100 - [] (5)

3. Results and discussion Table 1 shows the effect of the coffee bean particle size as starting material on the antioxidant capacity, the amount of CQAs and total phenolic content obtained for a single set of conditions.

The antioxidant capacity, the amount of CQAs and total phenolic

content for the different sizes were similar, which means that the mass transfer of the targeted compounds into the solvent phase could be considered to be similar. Fig. 2 shows the effect of temperature on the hydrothermal extraction of caffeine at constant pressure.

In Fig. 2, the amount of caffeine extracted increased with increasing

temperature up to about 410 K, and then it slightly decreased until 490 K.

At

temperatures greater than 500 K (Fig. 2), the amount of caffeine extracted greatly increased compared with the lower temperatures and then it decreased at the highest temperature studied, 573 K.

In the hydrothermal extraction of coffee silverskin that contains 1 wt% of 14


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caffeine,23 the amount of caffeine was found to be independent of temperature probably due to the thinness of the material.24

On the other hand, Shalmashi et al.25 found that caffeine

yield from tea leaves obtained with hydrothermal extraction increased with increasing temperatures up to 453 K, and then it decreased at temperatures of 473 K.

The trends for

caffeine according to the hydrothermal extraction temperature for coffee beans, silverskin and tea leaves show that caffeine contained in the solid matrix of each material is different. Caffeine contained in coffee silverskin has high mass transfer, because the caffeine can be readily extracted in high yields at ambient temperature (298 K), while caffeine in the cell walls of the coffee beans or in tea leaves are closely bound with chlorogenic acids and other components to form oligomeric structures that are insoluble in water at ambient temperature.

Thus, hydrothermal extraction of caffeine from the coffee beans and tea

leaves is enhanced with increasing temperature.24,25

The nature of the oligomeric

structures will be discussed in a later section. Caffeine in its free- and dissolved- forms can also react during hydrothermal extraction25 and thus the decrease in the amount of caffeine observed during the measurements may be due to decomposition, especially for extraction temperatures greater than 550 K. 15


The


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increased transport of the caffeine into the aqueous phase with increasing extraction temperature is probably due to two reasons: (1) enhancement of solubility of caffeine caused by possible liquefaction due to its melting point being 511 K and (2) hydrolysis of cellulosic cell walls of the bean material that is favored at around 523 K.

The dissolution

of cellulose in pure water occurs over the temperature ranges studied in this work.26 Further, as hydrothermal extraction proceeds, the presence of various acids will be present in the aqueous phase that may act as a homogeneous catalyst that can promote cellulose hydrolysis.27

The degradation of cellulosic cell walls by conditions of the hydrothermal

extraction and the physical change of caffeine from solid state to liquid state help to explain the sharp increase in the amount of caffeine at around 500 K. Fig. 3 shows typical photos of the effluent solutions obtained for hydrothermal extraction of green coffee beans.

The color of the extract solution was transparent at low

temperatures and became light brown with increasing temperature and clearly changed to brown around 500 K.

The color changes observed (Fig. 3) can be attributed to the

formation of melanoidin28 as described later.

16


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Fig. 4 shows the temperature dependence of extract characteristics for hydrothermal extractions made at 5 MPa.

The antioxidant capacity increased with extraction

temperature up to around 410 K, then decreased and greatly increased as temperature exceeded 500 K. This trend was in accordance with previous hydrothermal extraction studies made on green coffee beans in that the antioxidant capacity initially increases with extraction temperature,9 but in this work, it was found that the antioxidant capacity changes with temperature according to several factors.

In the experiments of this work (Fig. 4), the

amount of CQAs increased with increasing temperature up to about 410 K, and then decreased.

The total phenolic content (Fig. 4) increased with increasing temperature, and

slightly decreased around 410 K and then seemed to reach a plateau.

The trend of the total

phenolic content with temperature for extracts obtained by hydrothermal extraction in this work were similar to those previously reported for hydrothermal extractions of green coffee beans and coffee silverskin.8-10,24

The melanoidin index increased with increasing

temperature and significantly increased around 500 K, which probably lead to the color change of the extract solutions.

The high melanoidin indices for sample extracts obtained

at high-temperature with hydrothermal extraction is in accordance with a previous report.8 17



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The trend of the antioxidant capacity with temperature seems to correspond to both the increase and decrease in the amount of CQAs, total phenolic content and caffeine, and the monotonic increase in the melanoidin index and large increase in the amount of caffeine. Considering that CQAs, phenolics, melanoidin and caffeine all have antioxidant properties,2,28-31 the trend of antioxidant capacity with extraction temperature was most likely affected by all compounds including caffeine. The trends shown in Fig. 4 for the amount of CQAs extracted with extraction temperature can be discussed in more detail.

Since the solubility of CQA (3-CQA) is known for some

conditions in hot water,12 the amount of CQAs should increase proportionally with increasing temperature of the hydrothermal extraction.

As shown in Fig. 4, the amount of

CQAs obtained was enhanced for extraction temperatures up to 400 K.

Further, in

addition to the solubility of the compounds in water at the given conditions, their physical state (solid, liquid) is also an important factor for their extraction, as discussed for the trends observed for caffeine with extraction temperature.

The melting point of CQA

(3-CQA) is around 483 K and if the CQA compounds were in a liquid-state, then larger amounts of CQAs could be expected in the extract. Another factor in the trends observed 18


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for CQA extraction with temperature is related to hydrolysis.

To recover water-soluble

polyphenolic molecules from a natural plant with hydrothermal extraction, hydrolysis of oligomeric structures plays an important role.31 In a previous study, it was reported that the recovery yield of an aglycone of glycoside in a natural plant remarkably increased over 400 K and hydrolysis of glycosides was revealed as one of the important mechanisms through kinetic analysis, that is, enthalpy-entropy compensation was confirmed for ether hydrolysis at hydrothermal conditions such that ether hydrolysis occurs via the same mechanism.32

The structural distribution of CQAs in natural plants is typically oligomeric

structures like glycosides33 so that structural relationships affected by hydrolysis probably also affects the extraction trends.

Although the structural distribution of CQAs in green

coffee beans is unknown, some parts of CQAs that were recovered in the hydrothermal extraction should be proportional to the decomposition of oligomeric structures via hydrolysis.

This discussion also applies to the hydrothermal extraction of caffeine,

because enhancement of caffeine yield was observed at around 500 K (Fig. 2) and the phenomena seems to occur for lignocellulosic materials such as Japanese cedar.34

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One factor in the trends observed for the amount of CQAs obtained with hydrothermal extraction (Fig. 4) is related to CQA decomposition at the given conditions.

To test the

hypothesis of CQA decomposition at given extraction temperatures, the hydrolysis rate of 3-CQA was measured with batch experiments.

Fig. 5 shows the experimental results of

3-CQA conversion (XCQA) in water under hydrothermal conditions at (433 to 513) K for reaction times of (5 to 45) min.

As shown in Fig. 5, CQA decomposed proportionally to

reaction time and the decomposition rate of CQA steadily increased with increasing reaction temperature.

From the experimental results (Fig. 5), the rate constant of

decomposition of CQA was evaluated from an Arrhenius plot of the conversion by assuming pseudo first-order kinetics for 3-CQA.

An Arrhenius plot for 3-CQA is shown

in Fig. S2 (supporting information). The activation energy and pre-exponential factor of 3-CQA decomposition in hydrothermal water were determined to be 94.3 kJ mol-1 and 108.9 min-1, respectively. (Fig. 5).

The calculated results were in accordance with the experimental data

In a previous study, the hydrothermal decomposition rate of CQA at (373 to 473)

K was determined with a batch type reactor.

The rate constants of CQA decomposition at

473 K were not largely different from those in this study (3.4×10-2 min-1) and those in the 20


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literature (4.2×10-2 min-1),13 but the activation energy and pre-exponential factors were 28.4 kJ mol-1 and 101.8 min-1, respectively13 and were different from those in this study.

In

the literature, enthalpy-entropy compensation has been determined to be valid for ester hydrolysis34 as well as for ether hydrolysis.31 For ester hydrolysis, a linear relationship between the pre-exponential factor, log (A, s-1), and activation energy, E (kJ mol-1) was confirmed as E = 10.43 logA + 31.8 by estimation using values from the literature.35

In

this study, the logarithm of the pre-exponential factor, log (A, s-1), was 7.1 and the activation energy can be calculated to be 105 kJ mol-1 based on the above equation.

The

activation energy determined by the experiments in this study was 94.3 kJ mol-1 and the difference between the estimated and experimental value is 11 % (=(105-94.3)/94.3×100). On the other hand, the estimated activation energy from the above equation with the pre-exponential factor (A = 101.8 min-1= 10-0.007 s-1)13 was 31.8 kJ mol-1 and the difference between the experimental value (28.4 kJ mol-1) and the estimated value (31.8 kJ mol-1) is 13 % (=(31.8-28.4)/28.4×100). Several mechanisms have been proposed for esters undergoing hydrolysis under hydrothermal conditions.36,37

In those works, the intermediate and activated complex 21



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structures of hydrolyzed ethyl acetate in supercritical water are different from those in subcritical water, because the apparent activation energy, which is the intrinsic activation energy of transformation of an intermediate into activated complex plus heat of formation of the intermediate of the hydrolysis at supercritical conditions, is generally higher (ca. 343 kJ mol-1 at 30 MPa) than that in subcritical water (113 kJ mol-1 at 30 MPa).36

With

increasing temperature, the density of water decreases and its ion product increases at hydrothermal conditions such that the solvation structure (hydration) and protonation of CQAs might be more sensitive to temperature than protonation of ethyl acetate because of the complexity of the CQA structure.

Therefore, the hydration structure of the water

molecule around the CQA molecule to form an intermediate followed by formation of the activated complex is affected by temperature.

By using an enthalpy-entropy

compensation diagram for CQA hydrolysis, both kinetic parameters can be laid onto a single line and thus the hydrolysis occurs via the same reaction mechanism, which is closely related to water molecule hydration structures that form the reaction intermediate and complex. Thus, it is considered that the experimental data for CQA hydrolysis in hydrothermal water in this study and the literature13 are in accordance and the intermediate 22


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or activated complex structure of the CQA hydrolysis is probably different below and above 473 K. To confirm this point in detail, further experimental and theoretical studies are needed that quantify the hydration structures around CQAs under hydrothermal conditions.

At temperatures higher than 473 K, the data in this study are solely reported

and are used in the following discussion. To estimate the decomposition of CQAs during the extraction, the dependence of conversion of 3-CQA on reaction temperature at 10 min, 30 min and 60 min of reaction time was estimated and simulated (Fig. 6).

As shown by simulation results in Fig. 6,

CQAs should be relatively stable at temperatures below 420 K, however, at temperatures greater than 420 K, CQAs can be expected to undergo hydrolysis and decompose and thus the yield of CQA should decrease when the sample is at temperatures greater than 400 K for long periods of time. Caffeine is probably the most well-known compound contained in coffee beans and is a powerful stimulant. For producing solutions of antioxidants that have health benefits, selective extraction of CQAs to produce solutions that do not contain caffeine would be desirable.

In hydrothermal extraction, CQAs should be relatively more stable at lower 23



temperatures than at higher temperatures.

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From the analyses made in this work,

hydrothermal extraction of green coffee beans at around 400 K should be suitable for producing solutions of CQAs that contain low amounts of caffeine.

In Figure 4, the

largest amount of CQAs was 59 mg/g-sample with 13 mg/g-sample of caffeine at 398 K and 5 MPa.

Getachew and Chun8 extracted green coffee beans of Brazilian origin with

water under hydrothermal conditions and obtained a maximum of 67 mg/g-sample CQAs with 14 mg/g-sample of caffeine at 453 K and 3 MPa, and 12 mg/g-sample of caffeine at 493 K and 6 MPa.

The extractions in this work gave smaller, but comparable amounts of

CQAs for similar amounts of caffeine as results obtained by Getachew and Chun8 that can be attributed to differences in the raw materials and conditions used in the extractions. Hydrothermal extractions of green coffee beans at moderate temperatures (< 420 K) should give the highest CQA-to-caffeine selectivities. Fig. 7 shows the pressure dependence of the amount of caffeine in the extract at 473 K and 30 min contact time. According to the experimental procedure used, larger variation in the data occurs in lower pressure region.

The conditions producing steam may have

24


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extracted some caffeine or caused changes in the substrate whereas caffeine extraction proceeded with little variation in pressure when the state of water was liquid. Fig. 8 shows the pressure dependence of the extract characteristics for hydrothermal extractions of green coffee beans at 473 K.

The antioxidant capacity, amount of CQAs

and total phenolic content increased with increasing pressure below about 2 MPa and did not vary much with pressure above 2 MPa.

The antioxidant capacity was influenced by

the amount of CQAs and phenolics. The maximal values for antioxidant capacity, amount of CQAs and total phenolic content can be determined by balancing system conditions between extraction and decomposition of the target compounds according to their reaction kinetics. Several researchers have reported on statistical experimental design methods for hydrothermal extraction of spent coffee ground.16-18

A series of experiments were run

using hydrothermal extraction temperature, pressure and time with statistical experimental design to determine possible interactions with antioxidant capacity, amount of CQAs and total phenolic content with the objective to maximize antioxidant capacity and minimize caffeine extraction.

Table 2 shows the experimental data with variables used in the 25



analyses in which the temperature range of the experiments was chosen to suppress the extraction of caffeine.

The antioxidant capacity, the amount of CQAs and total phenolic

content of experimental data were correlated with second order polynomial functions of X1, X2 and X3 to determine the coefficient of each term. X1, X2 and X3 were corresponded to temperature, pressure and extraction time, respectively.

The regressed equations were

determined as follows: [ ! ⁄ − ] = 5.00$4 + 1.85$7 − 8.80$8 −17.38$47 − 14.97$77 − 6.43$87 + 8.98$4 $7 − 14.34$4 $8 + 7.13$7 $8 + 69.36 (6) [mg⁄g − sample] = 29.89$4 − 19.40$7 + 5.60$8 −20.59$47 − 32.97$77 − 2.98$87 + 39.14$4 $7 − 0.42$4 $8 − 5.14$7 $8 + 40.50 (7) ℎ [#! ⁄ − ] = −0.32$4 + 0.94$7 + 0.39$8 −1.08$47 − 1.23$77 − 0.23$87 + 0.19$4 $7 − 0.39$4 $8 + 0.19$7 $8 + 6.24 (8) Parity plots for antioxidant capacity, amount of CQAs and total phenolic content for all conditions are shown in Fig. S3 (Supporting Information).

Standard deviation between

experimental data and calculated values for the antioxidant capacity, the amount of CQAs

26


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and total phenolic content and were 6.1 µmol TE/g-sample, 4.2 mg/g-sample and 0.81 mg GAE/g-sample, respectively. Table 3 shows estimated hydrothermal extraction conditions required for obtaining the maximum values of extract characteristics of green coffee beans as determined from eqs. (6)-(8). The optimal extraction time was close to 60 min, which is the longest extraction time studied.

In general, the amount of solute extracted from a material increases with

extraction time in semi-batch systems as long as the solid-liquid mass transfer is higher than intraparticle diffusion and the solid has ample solute for mass transfer. Fig. 9 shows extract characteristics at each optimal extraction time for a range of pressures and temperatures for hydrothermal extraction of green coffee beans.

Although

the extraction times are relatively long (ca. 60 min), the times can be probably be reduced through the use of sequential hydrothermal methods that promote rapid heating or by microwave methods.38-39 The extract characteristics (Fig. 9) each showed a maximum in pressure below 3 MPa, which is similar to the trends shown in Fig. 8.

The maximum

value with pressure infers competing factors between compound solubilization including hydrolysis of cell membrane and decomposition of antioxidants such as hydrolysis of 27



Page 28 of 54

CQAs and phenolics that govern the antioxidant capacity related to the amount of CQAs and phenolics.

Furthermore, the antioxidant capacity and total phenolic content all have

maxima, which means that the decomposition of components becomes dominant at high-temperatures.

5. Conclusions Green coffee beans were extracted with hot water at temperatures of (343 to 573) K, pressures of (0.1 to 10) MPa and for contact times of up to 60 min.

For experiments run at

473 K, 5 MPa and 30 min contact time, antioxidant capacity, the amount of CQAs and total phenolic content for these beans were similar and did not depend on particle size that ranged from sizes of (0.7 to 1.0) mm to crushed sizes of (840 to 1680) µm or larger.

For

hydrothermal extraction conditions at temperatures below about 500 K, extraction of caffeine could be suppressed. The amount of CQAs and total amount of phenol increased with increasing temperature up to about 400 K, and then decreased, which can be attributed to hydrolysis of oligomeric structures in the coffee beans form the CQAs and the 28


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decomposition of the CQAs. temperature at around 500 K.

The melanoidin index significantly increased with Simulated decomposition hydrolysis conditions based on

decomposition kinetics of 3-CQA with batch experiments showed that 3-CQA should be relatively stable below 420 K for contact times of up to 60 min.

For processing of green

coffee beans in this research, extract characteristics (antioxidant capacity, amount of CQAs, total phenolic content) or hydrothermal extraction conditions can be estimated from simple correlations.

Hydrothermal extraction of green coffee beans is an environmentally-safe

method for obtaining aqueous solutions of antioxidant compounds that may possibly be used to fortify health and sports drinks.

Acknowledgments This work was partially supported by JSPS KAKENHI Grant number 41003079.

Supporting information 29



Typical spectrograms of CQAs at 5 MPa in a semi-batch apparatus, Arrhenius plot for 3-CQA hydrolysis and parity plot of the calculations for Eqs. (6)-(8).

30


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38


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Figure captions

Fig. 1

Schematic of semi-batch hydrothermal extraction apparatus.

Fig. 2

Caffeine in effluent as a function of temperature obtained with hydrothermal

extraction. Conditions of semi-batch apparatus: 5 MPa, 1 g/min water flow rate for 30 min, 0.1 g of green coffee beans in extractor with particle sizes from 840 to 1680 µm.

Fig. 3

Photos of effluent solutions from the hydrothermal extraction of green coffee beans

at temperatures: (a) 348 K, (b) 373 K, (c) 398 K, (d) 423 K, (e) 448 K, (f) 473 K, (g) 498 K, (h) 523 K, (i) 548 K, (j) 573 K. Conditions of semi-batch apparatus: 5 MPa, 1 g/min water flow rate for 30 min, 0.1 g of green coffee beans in extractor with particle sizes from 840 to 1680 µm.

Fig. 4

Antioxidant capacity of effluent solutions versus temperature and amount of

caffeoylquinic acids (CQAs), total phenolic content and melanoidin index for hydrothermal extraction of green coffee beans. Conditions of semi-batch apparatus: 5 MPa, 1 g/min water flow rate for 30 min, 0.1 g of green coffee beans in extractor with particle sizes from 840 to

39



1680 µm. Symbols, ○: Antioxidant capacity (AOC), ∆: Amount of CQAs, □: Total phenolic content (TPC) , ◊:Melanoidin index (M. I.) × 103.

Fig. 5

Conversion of chlorogenic acids as 3-o-caffeoylquinic acid (3-CQA) versus

reaction time. Conditions of batch reactor: 2 g of 1 wt% of 3-CQA. Lines: calculation results assuming first-order kinetics for 3-CQA. Symbols, ◊:433 K, ∆: 473 K, □: 493 K, ○: 513 K.

Fig. 6

Simulated yields for decomposition (hydrolysis) of chlorogenic acids as

3-o-caffeoylquinic acid (3-CQA) as a function of temperature at 10 min (— — —), 30 min (- - -), 60 min (——) reaction time assuming pseudo first-order kinetics. Basis of simulation conditions: 2 g of 1 wt% of 3-CQA in batch reactor.

Fig. 7

Amount of caffeine in effluent obtained with hydrothermal extraction as a function

of pressure. Conditions of semi-batch apparatus: 473 K, 1 g/min water flow rate for 30 min, 0.8 g of green coffee beans in extractor.

Fig. 8

Dependence of antioxidant capacity, amount of CQAs and total phenolic content

on pressure. Conditions of semi-batch apparatus: 473 K, 1 g/min water flow rate for 30 min, 40


Page 40 of 54

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0.8 g of green coffee beans in extractor. Symbols, ○: Antioxidant capacity (AOC), ∆: Amount of caffeoylquinic acids (CQAs), □: Total phenolic content (TPC).

Fig. 9

Estimated trends of products from hydrothermal extraction calculated from Eqs.

(4)-(6): (a) antioxidant capacity (AOC) for 59.1 min, (b) amount of caffeoylquinic acids (CQAs) for 55.0 min and (c) total phenolic content (TPC) for 59.6 min. Temperatures, i: 393 K, ii: 413 K, iii: 433 K, iv: 453 K, v: 473 K, vi: 493 K. Basis of calculations: semi-batch apparatus, 1 g/min water flow rate, 0.8 g green coffee beans in extractor.

41



Table 1

Page 42 of 54

Variation of extract characteristics, antioxidant capacity (AOC), caffeoylquinic

acids (CQAs), and total phenolic content (TPC) in extract, for hydrothermal extraction of green coffee beans for several material forms.

Conditions: 473 K, 5.0 MPa, water flow

rate of 1.0 g/min with 0.5 g of coffee beans for 30 min contact time.

Material form of green coffee beans As-supplied d = (0.7 to 1.0) mm Coarsely-crushed d > 1680 µm Finely-crushed d = (840 to 1680) µm

AOC

CQAs

TPC

[µmol TE/g-sample]

[mg/g-sample]

[mg GAE/g-sample]

207.9± 14.3

73.1± 6.7

15.5± 0.6

209.9± 14.5

94.9± 8.6

16.2± 0.7

207.2± 14.3

93.5± 8.5

16.3± 0.7

42


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Table 2

Experimental conditions, variable coding and extract characteristics, antioxidant

capacity (AOC), caffeoylquinic acids (CQAs), and total phenolic content (TPC) in extract, for hydrothermal extraction of green coffee beans with a semi-batch system. initial loading, 0.8 g green coffee beans; water flow rate, 1 g/min.

Conditions:

Statistical central

values: X1 (443 K), X2 (1.7 MPa), X3 (40 min). Run

T

P

t

X1

X2

X3

No.

[K]

[MPa]

[min]

(T)

(P)

(t)

AOC

CQAs

TPC

[µmol TE/

[mg/

[mg GAE/

g-sample]

g-sample]

g-sample]

1

393

0.2

20

-1.00

-1.00

-1.00

5.7 ± 0.4

0.6 ± 0.1

1.33 ± 0.05

2

393

0.6

20

-1.00

-0.73

-1.00

21.8 ± 1.5

6.6 ± 0.6

4.02 ± 0.16

3

393

0.4

40

-1.00

-0.87

0.00

52.7 ± 3.6

18.0 ± 1.6

4.30 ± 0.18

4

393

0.6

60

-1.00

-0.73

1.00

51.5 ± 3.6

22.5 ± 2.0

4.19 ± 0.17

5

393

0.2

60

-1.00

-1.00

1.00

52.2 ± 3.6

16.7 ± 1.5

4.69 ± 0.19

6

413

0.4

20

-0.60

-0.87

-1.00

33.7 ± 2.3

11.9 ± 1.1

4.05 ± 0.17

7

413

0.2

40

-0.60

-1.00

0.00

47.5 ± 3.3

22.4 ± 2.0

4.10 ± 0.17

8

413

0.4

40

-0.60

-0.87

0.00

53.9 ± 3.7

29.7 ± 2.7

4.39 ± 0.18

9

413

0.4

40

-0.60

-0.87

0.00

52.2 ± 3.6

29.6 ± 2.7

4.39 ± 0.18

10

413

0.6

40

-0.60

-0.73

0.00

53.5 ± 3.7

26.2 ± 2.4

4.44 ± 0.18

11

413

0.4

60

-0.60

-0.87

1.00

54.4 ± 3.8

41.4 ± 3.8

4.49 ± 0.18

12

433

0.2

20

-0.20

-1.00

-1.00

43.7 ± 3.0

16.5 ± 1.5

3.58 ± 0.15

13

433

0.6

20

-0.20

-0.73

-1.00

49.6 ± 3.4

15.2 ± 1.4

4.86 ± 0.20

14

433

0.4

40

-0.20

-0.87

0.00

60.0 ± 4.1

37.8 ± 3.4

4.94 ± 0.20

15

433

0.2

60

-0.20

-1.00

1.00

52.3 ± 3.6

40.2 ± 3.7

4.30 ± 0.18

16

433

0.6

60

-0.20

-0.73

1.00

51.2 ± 3.5

36.6 ± 3.3

4.14 ± 0.17

17

493

1.2

20

1.00

-0.33

-1.00

54.6 ± 3.8

33.4 ± 3.0

4.12 ± 0.17

18

493

2.2

40

1.00

0.33

0.00

44.4 ± 3.1

46.5 ± 4.2

4.56 ± 0.19

19

493

3.2

20

1.00

1.00

-1.00

48.1 ± 3.3

34.9 ± 3.2

4.35 ± 0.18

20

493

1.2

60

1.00

-0.33

1.00

51.1 ± 3.5

47.9 ± 4.4

4.66 ± 0.19

21

493

3.2

60

1.00

1.00

1.00

49.6 ± 3.4

34.3 ± 3.1

4.87 ± 0.20

22

493

0.2

60

1.00

-1.00

1.00

6.4 ± 0.4

0.7 ±0.1

1.82 ±0.07

43



Table 3

Page 44 of 54

Estimated conditions for processing green coffee beans with hydrothermal

extraction (temperature, T; pressure, P; time, t) obtained by correlation of data (Table 2) with polynomial functions (eqs. (6)-(8)) to obtain maximum values of antioxidant capacity (AOC), caffeoylquinic acids (CQAs), or total phenolic content (TPC) in extract.

T

P

t

Maximum

[K]

[MPa]

[min]

value

AOC [µmol TE/g-sample]

433.4

2.1

59.1

73.7

CQAs [mg/g-sample]

485.5

2.0

55.0

54.0

TPC [mg GAE/g-sample]

430.2

2.3

59.6

6.75

Extract characteristic

44


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Table of Contents (TOC)

45



Fig. 1 Schematic of semi-batch hydrothermal extraction apparatus.


Page 46 of 54

Page 47 of 54

80

Caffeine [mg/g-sample]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

40

20

0 300

400 500 Temperature [K]

600

Fig. 2 Caffeine in effluent as a function of temperature obtained with hydrothermal extraction. Conditions of semi-batch apparatus: 5 MPa, 1 g/min water flow rate for 30 min, 0.1 g of green coffee beans in extractor with particle sizes from 840 to 1680 m.



(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Page 48 of 54

(j)

Fig. 3 Photos of effluent solutions from the hydrothermal extraction of green coffee beans at temperatures: (a) 348 K, (b) 373 K, (c) 398 K, (d) 423 K, (e) 448 K, (f) 473 K, (g) 498 K, (h) 523 K, (i) 548 K, (j) 573 K. Conditions of semi-batch apparatus: 5 MPa, 1 g/min water flow rate for 30 min, 0.1 g of green coffee beans in extractor with particle sizes from 840 to 1680 m.


600

AOC [mol TE/g-sample]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

500 400

100

300 200

50

100 0 300


CQAs [mg/g-sample] TPC [mg GAE/g-sample] M. I.  103[-]

Page 49 of 54

0 600

Fig. 4 Antioxidant capacity of effluent solutions versus temperature and amount of CQAs, total phenolic content and melanoidin index for hydrothermal extraction of green coffee beans. Conditions of semi-batch apparatus: 5 MPa, 1 g/min water flow rate for 30 min, 0.1 g of green coffee beans in extractor with particle sizes from 840 to 1680 m. Symbols, ○: Antioxidant capacity (AOC), ∆: Amount of CQAs, □: Total phenolic content (TPC), :Melanoidin index (M. I.)  103.



100 80

XCQA [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60 40 20 0 0

10

20

30

40

50

Reaction time [min]

Fig. 5 Conversion of chlorogenic acids as 3-o-caffeoylquinic acid (3-CQA) versus reaction time. Conditions of batch reactor: 2 g of 1 wt% of 3-CQA concentration. Lines: calculation results assuming first-order kinetics for 3-CQA. Symbols, :433 K, ∆: 473 K, □: 493 K, ○: 513 K.


Page 51 of 54

100 80 XCQA [%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0 300


600

Fig. 6 Simulated yields for decomposition (hydrolysis) of chlorogenic acid as 3-o-caffeoylquinic acid (3-CQA) as a function of temperature at 10 min (— — —), 30 min (- - -), 60 min (——) reaction time assuming pseudo first-order kinetics. Basis of simulation conditions: 2 g of 1 wt% aqueous solution of 3-CQA in batch reactor.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Caffeine [mg/g-sample]


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20

10

0 0

2

4 6 8 Pressure [MPa]

10

Fig. 7 Amount of caffeine in effluent obtained with hydrothermal extraction as a function of pressure. Conditions of semi-batch apparatus: 473 K, 1 g/min water flow rate for 30 min, 0.8 g of green coffee beans in extractor.


100

AOC [mol TE/g-sample] CQAs [mg/g-sample]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

80 10 60 40 5 20 0 0

5 10 Pressure [MPa]

TPC [mg GAE/g-sample]

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0

Fig. 8 Dependence of antioxidant capacity, amount of CQAs and total phenolic content on pressure. Conditions of semi-batch apparatus: 473 K, 1 g/min water flow rate for 30 min, 0.8 g of green coffee beans in extractor. Symbols, ○: Antioxidant capacity (AOC), ∆: Amount of caffeoylquinic acids (CQAs), □: Total phenolic content (TPC).



AOC [mol TE/g-sample]

80 iii

ii

(a)

60 i 40 iv v 20 vi 0

(b)

CQAs [mg/g-sample]

60

40 vi 20

v i

0 TPC [mg GAE/g-sample]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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iii

ii

iii

i

iv

(c) ii

6

iv 4 v 2

vi

0 0

1 2 3 Pressure [MPa]

Fig. 9 Estimated trends of products from hydrothermal extraction calculated from Eqs. (4)-(6): (a) antioxidant capacity (AOC) for 59.1 min, (b) amount of caffeoylquinic acids (CQAs) for 55.0 min and (c) total phenolic content (TPC) for 59.6 min. Temperatures, i:393 K, ii: 413 K, iii: 433 K, iv: 453 K, v: 473 K, vi: 493 K. Basis of calculations: using semi-batch apparatus, 1 g/min water flow rate, 0.8 g of green coffee beans in extractor.


Hydrothermal extraction of antioxidant compounds from green coffee

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