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Supercritical fluid extraction as a potential mitigation strategy for the reduction of acrylamide level in coffee
Journal of Food Engineering 115 (2013) 292–297
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Supercritical fluid extraction as a potential mitigation strategy for the reduction of acrylamide level in coffee Mauro Banchero a,⇑, Gloria Pellegrino b, Luigi Manna a a b
Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, Italy Innovation Center, Lavazza S.p.A., Strada Settimo 410, Torino, Italy
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 6 September 2012 Accepted 27 October 2012 Available online 5 November 2012 Keywords: Supercritical fluid extraction Carbon dioxide Cosolvent Acrylamide Coffee Mitigation Roasting
a b s t r a c t The removal of acrylamide from coffee through supercritical CO2 extraction has been investigated for the first time. Two steps were performed: a pre-roasting treatment and a supercritical extraction process. The main aim of this first work was to investigate the feasibility of the process more than the impact of the treatment on the organoleptic properties of coffee. The efficiency of acrylamide removal was checked by measuring its content in the coffee beans before and after the supercritical treatment. The role of temperature, pressure, extraction time and the addition of a modifier (ethanol) was examined. The supercritical treatment did not affect the caffeine content of coffee and a maximum extraction efficiency of 79% was found for acrylamide. While a pressure variation did not significantly affect the results, temperature affected the extraction process at the highest extent. The addition of ethanol resulted in a significant increase in the extraction performance due to the change in polarity of the supercritical solvent mixture. The best working conditions in the experimental range here investigated were 100 °C, 200 bar and 9.5% w/w ethanol. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Acrylamide is a chemical compound that has been labeled as potentially carcinogenic to humans by the International Agency for Research on Cancer since 1994 (Parzefall, 2008). In the last few years, a great deal of concern has arisen over its presence in food. Relevant amounts of acrylamide, in fact, can be found in several carbohydrate-rich foods such as potatoes, coffee and cereals when they are cooked at high temperatures (Capuano and Fogliano, 2011). It has been clearly established that the major pathway for acrylamide formation in foods is the Maillard reaction, initiated by the condensation of aspargine, an aminoacid, and reducing carbohydrates, such as fructose or glucose, or, alternatively, reactive carbonyls (Guenther et al., 2007; Zhang et al., 2009). As far as coffee is concerned, more than one pathway for acrylamide formation can be expected since the coffee beans are roasted in the 220–250 °C range, which is higher with respect to other foods. These include several thermally driven reactions that involve other intermediates such as acrolein, acrylic acid and 3aminopropionamide. Furthermore, the acrylamide content in roasted coffee is determined by concomitant formation and elimination reactions occurring during the roasting process (Guenther et al., 2007). It was observed that acrylamide is formed at the beginning of the roasting step and, after reaching the maximum le⇑ Corresponding author. Tel.: +39 011 0904703; fax: +39 011 0904648. E-mail address: [email protected] (M. Banchero). 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.10.045
vel, it decreases shortly. It was also found that light roasted coffees may contain relatively higher amounts of acrylamide than the dark roasted ones (Lantz et al., 2006). The hypothesis is that the acrylamide concentration in coffee decreases with increasing thermal input (darker roasting) and that the reactions leading to the depletion of acrylamide predominate towards the end of roasting. These reactions are probably connected to the polymerization of acrylamide or its reaction with food components (FoodDrinkEurope Toolbox, 2011). However, both a darker roasting and an extension of the roasting time are not applicable as options to reduce the acrylamide level in coffee since they could generate undesirable compounds with a negative impact on the taste or aroma of the final product (Stadler, 2005). The acrylamide content in roasted coffee beans is extremely variable for the reasons mentioned in the previous discussion and the levels usually reported in the literature are within the range 0.035–0.54 mg/kg of coffee (Alves et al., 2010). The acrylamide content also depends on the coffee species. A comparison between Robusta and Arabica coffee, which are the two coffee species of higher economical impact, showed that the first one produces a higher acrylamide level. This is probably due to a slightly higher content of aspargine in the green beans of the Robusta species (Lantz et al., 2006). A lot of mitigation strategies have been proposed to reduce the acrylamide content in potato and cereal-based products while very limited process options are available for coffee since they can affect the final product quality (Capuano and Fogliano, 2011). One of the
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most promising ways to reduce the acrylamide content in foods is the addition of the enzyme asparginase, which is able to catalyze the hydrolysis of aspargine thus lowering the content of the precursor aspargine. While this enzyme has been successfully applied at lab scale both to potato and cereal-based products, only preliminary results are available for coffee (Capuano and Fogliano, 2011). They showed only a 10–45% reduction of acrylamide after roasting probably due to the fact that aspargine is not the only acrylamide precursor in coffee beans. Furthermore, the taste of coffee is significantly and negatively influenced by the treatment process (FoodDrinkEurope Toolbox, 2011). Roasting green coffee under a steam/pressure atmosphere was proposed as an alternative roasting process and resulted in longer roasting times and in a reduction potential of only 10% in the acrylamide content (Guenther et al., 2007). Another study involved the addition of glycine, an amminoacid, which would inhibit the Maillard reaction between aspargine and carbonyl compounds but the practical applicability of the method or the impact on the final product has still to be established (Seal et al., 2008). Finally, predrying or decaffeination of green coffee beans did not show any significant impact on the acrylamide level in the roasted product (FoodDrinkEurope Toolbox, 2011). Supercritical CO2 extraction is a clean and efficient method for the processing of solid food matrices. It can be used both to extract valuable bioactive compounds such as flavors, colorants and other biomolecules and to remove undesirable compounds such as organic pollutants, toxins and pesticides (Pereira and Meireles, 2010; Sovová and Stateva, 2011). Carbon dioxide is the most used supercritical solvent because it is cheap, non-flammable, non-toxic and its critical point (Tc = 31.1 °C Pc = 7.37 MPa) is lower than that of many other fluids. In the supercritical state it approaches gaseous-like viscosity and high diffusivity while density is near that of a liquid with a solvent strength that can be easily tuned by simply varying temperature and pressure. A simple expansion step at common environmental pressure values allows the solvent to be easily removed from the food matrix and the extract to be recovered (Brunner, 2005). One drawback of supercritical CO2 is its low polarity. This problem can be overcome employing small percentages of polar modifiers or co-solvents, such as methanol, ethanol and water, to change the polarity of the solvent (Pereira and Meireles, 2010). This results in an improvement of the extraction efficiency by increasing the solubility of the solute (Herrero et al., 2010) or the swelling of the solid matrix that facilitates the solute–solvent contact (Pereira and Meireles, 2010). Many applications of supercritical fluid extraction can be found in the literature (Herrero et al., 2010). In particular, the employment of supercritical CO2 for coffee decaffeination was successfully developed on an industrial scale since the 1970s and mainly consists in the extraction of caffeine from moistened green coffee beans through water saturated supercritical CO2 (Ramalakshmi and Raghavan, 1999). Moreover, both the caffeine and the other compounds that are extracted from coffee beans or husks can be employed in the pharmaceutical, cosmetic or food industry (de Azevedo et al., 2008; Tello et al., 2011). In this work the possibility of removing acrylamide from coffee beans through supercritical CO2 extraction has been investigated for the first time. The feasibility of the process is the preliminary step to candidate this technique as a potential mitigation strategy to reduce the acrylamide content in coffee. This could be quite appealing to the coffee industries not only because it is an environmentally friendly technique, but also because they are already acquainted with this high-pressure technology thanks to the increasing diffusion of the supercritical coffee decaffeination process. In the first part of the work, pre-roasting experiments were conducted in a lab-scale drum roaster to maximize the acrylamide amount in the coffee beans to be treated with the supercritical sol-
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vent. The supercritical extraction tests were, then, performed in a continuous laboratory apparatus and the efficiency of acrylamide removal was checked by the measurement of the acrylamide content in the coffee beans before and after the supercritical treatment. The role of temperature, pressure, extraction time and the addition of a modifier (ethanol) was investigated to find out how these parameters affect the extraction yield.
2. Materials and methods The experiments were performed on Robusta coffee beans supplied by Lavazza S.p.A. (Italy). The first part of the work consisted in performing a pre-roasting treatment of the green coffee beans to produce a significant amount of acrylamide in the samples to be treated with the supercritical solvent. The pre-roasting treatment was conducted in a labscale drum roaster equipped with electrical heating and with a capacity of 1 kg/batch. Pre-roasting consisted in maintaining the coffee beans at constant temperature for a fixed period of time. In a typical experiment, the roasting chamber was pre-heated at a constant value before introducing the green coffee beans. After a transient period of 15 min, the temperature of the coffee reached the target value that was kept constant for the desired treatment time. Different experiments were conducted varying the temperature and time of the pre-roasting treatment to maximize the acrylamide amount in the samples. Two different batches of pre-roasted coffee were selected and subjected to supercritical extraction. The supercritical extraction process was carried out in the apparatus shown in Fig. 1. It was a continuous apparatus where the supercritical solvent flowed through a 50 ml extraction vessel that contained approximately 30 g of the pre-roasted coffee beans and was discharged through a heated back pressure regulator, which provided the depressurization of the system and allowed the precipitated solute to be collected in a proper solvent trap. Further details about the supercritical apparatus can be found in previous works (Ferri et al., 2004, 2006; Banchero et al., 2009). The CO2 was purchased from SIAD S.p.A. (Italy) and was contained in a gas cylinder provided with a dip tube. Pump 1 is equipped with a cooling system to avoid the cavitation of the solvent. When the three-way mixing valve was closed, the extracting supercritical solvent was pure CO2, whereas when the mixing valve was open, a cosolvent (ethanol) could be introduced through Pump 2. Ethanol (purity P 99.8%) was also contained in the solvent trap and was supplied by Sigma–Aldrich. The supercritical solvent flowed through a heating coil before entering the extraction vessel. Both the heating coil and the extraction vessel were positioned inside an oven. The apparatus was also equipped with a pressure indicator (PI/1) and a Coriolis mass flow meter (FI/2). In a typical experiment, the extraction vessel was initially bypassed and the apparatus was run until the required conditions of temperature, pressure and mass flow rate were reached. Then, the extraction vessel was pressurized with the supercritical solvent and the extraction process could begin. Preliminary experiments showed that an overall mass flow rate of 1 g/min could guarantee an appropriate residence time of the supercritical solvent mixture in the extraction vessel. Extraction experiments were performed in duplicate. The solvent in the trap was periodically replaced, weighted and assayed through a Perkin Elmer UNICAM UV2-300 spectrophotometer in the UV range. Three different peaks could be detected at 204, 230 and 272 nm but none of them could be directly related to the concentration of a specific chemical compound due to the overlap of the UV absorbance peaks of the extracted components. An HPLC analysis of the solvent in the trap showed that the acrylamide concentration was too low to guarantee an accurate quanti-
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Fig. 1. Experimental apparatus.
tative measurement of the extraction efficiency. Anyway, the knowledge of the mass of the solvent in the trap and the absorbance at 272 nm, which was the sharpest peak, were used to estimate the cumulative amount of the total extracted material. This allowed the trend of the extraction process to be monitored versus time. The extraction efficiency of acrylamide (e%) was evaluated from the analysis of the coffee beans according to the following equation:
e% ¼
AAi AAf 100 AAi
ð1Þ
where AAi is the initial acrylamide content of pre-roasted coffee beans and AAf is the acrylamide content after the supercritical treatment. The acrylamide analyses of coffee beans were performed by Eurofins Scientific (Chemical Control s.r.l., Cuneo, Italy) using HPLC/MS with deuterium-labeled acrylamide as an internal standard. The limit of quantification certified by the lab is equal to 0.15 mg/kg. The measurement uncertainty, with a level of confidence of 95%, varied in the 8–35% range depending on the acrylamide concentration. Further details about the analytical methods adopted by Eurofins laboratories to detect acrylamide in different foodstuffs can be found elsewhere (Hoenicke et al., 2004). The Eurofins laboratory also performed some analyses to check if the caffeine content of the coffee beans was affected by the supercritical treatment. The caffeine content before and after the supercritical treatment was always equal to 2.1 ± 0.2% w/w. 3. Results and discussion 3.1. The pre-roasting treatment The pre-roasting experiments were conducted at temperatures and treatment-times lower than those generally adopted during the conventional roasting process. This procedure was chosen in order to achieve high amounts of acrylamide in the samples to be treated with the supercritical solvent. Furthermore, the temperatures of the pre-roasting treatment were selected in order to avoid or limit the formation of those chemical compounds that are responsible for taste and aroma of the final coffee brew and that could be removed by a supercritical treatment. Even though the investigation of the impact of the extraction process on the organoleptic properties of coffee was not the aim of this preliminary work, this was considered more appropriate to prevent a strong modification of the quality of the product.
Two set of experiments were conducted: the first one at lower temperatures and shorter treatment-times, the second at higher temperatures and longer treatment-times. In the first set of experiments, the temperature and time of the pre-roasting treatment were varied in the 107–134 °C range and 5–10 min range, respectively. The pre-heating temperature of the roasting chamber was varied between 140 and 170 °C. A maximum acrylamide content of 0.37 ± 0.05 mg/kg was found after treating the coffee beans at 132 °C for 5 min. These working conditions were selected to prepare a first batch of samples (denoted as batch 1) for the subsequent supercritical treatment. In the second set of experiments, the treatment-temperature ranged from 125 to 151 °C while the treatment-times were equal to 10, 15 or 20 min. The pre-heating temperature of the roasting chamber was varied between 190 and 210 °C. The maximum acrylamide content of 0.90 ± 0.05 mg/kg was obtained after treating the coffee beans at 151 °C for 20 min, which were selected as the optimal working conditions to prepare the second batch of samples (denoted as batch 2) for the supercritical treatment.
3.2. The supercritical extraction process Table 1 reports both the operating conditions and the acrylamide extraction efficiencies (e%) of the experimental tests. The first set of experiments was conducted on the coffee beans from batch 1 without any addition of ethanol in order to investigate the role of the temperature of the pure supercritical solvent. The following experiments were performed on the coffee beans both from batch 1 and batch 2 in the presence of ethanol as a cosolvent: tests were conducted at different temperature, pressure, cosolvent concentration and duration of the extraction process. The table reports also the ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel (mSF/ mcoffee). The periodical spectrophotometric analysis of the solvent in the trap allowed us to monitor the cumulative amount of the total extracted material versus time in each test, as it was reported in Section 2. If the absorbance of the reference peak is assumed to be roughly proportional to the extracted amount of acrylamide, this information can be used to observe the temporal evolution of the acrylamide extraction by normalizing each curve with respect to the total amount of acrylamide extracted at the end of each test. Fig. 2 reports, for example, the trend of the acrylamide extraction process versus time for a single experiment.
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M. Banchero et al. / Journal of Food Engineering 115 (2013) 292–297 Table 1 Operating conditions and results of the acrylamide extraction tests. Batch of pre-roasted coffee (mg/kg) 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 a b
T (°C)
P (bar)
Ethanol (% w/w)
Extraction time (min)
mSF/mcoffeea (g/g)
e%b
25 40 60 80 100 120 80 80 80 80 80 80 100 100 100 100 80 80 100 100
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 300 300 300 300
0 0 0 0 0 0 9.5 5.0 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5
345 345 345 345 345 345 345 345 345 525 925 1305 345 525 925 1305 345 525 345 525
12.1 ± 0.2 12.5 ± 0.1 12.3 ± 0.3 12.5 ± 0.7 12.3 ± 0.3 12.7 ± 0.1 11.8 ± 0.2 11.7 ± 0.3 12.4 ± 0.2 18.9 ± 0.3 34.1 ± 0.6 47.5 ± 0.2 12.2 ± 0.0 18.6 ± 0.1 34.6 ± 0.5 47.1 ± 0.3 12.2 ± 0.3 18.6 ± 0.5 12.3 ± 0.2 18.8 ± 0.4
10.7 ± 1.2% 8.3 ± 0.7% 7.1 ± 1.3% 12.5 ± 0.9% 23.1 ± 1.3% 34.4 ± 0.7% 20.3 ± 1.4% 14.9 ± 0.7% 22.7 ± 0.4% 28.4 ± 2.3% 49.1 ± 2.7% 63.8 ± 1.1% 38.4 ± 1.4% 41.8 ± 1.8% 66.3 ± 1.4% 78.9 ± 1.9% 25.1 ± 0.5% 29.8 ± 2.0% 42.3 ± 2.1% 45.0 ± 2.8%
Ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel. Extraction efficiency of acrylamide (defined in Eq. (1)).
60
60
Extraction efficiency, %
Extraction efficiency, %
50
40
30
20 batch 2, 100 ˚C, 300 bar, Ethanol 9.5%
80 ˚C, 200 bar 80 ˚C, 300 bar 100 ˚C, 300 bar
50
40
30
20
10 10
0 0
100
200
300
400
500
600
Extraction time (min)
Fig. 2. Typical trend of the acrylamide extraction process versus time evaluated by UV measurement at 272 nm.
3.2.1. The influence of temperature and pressure The efficiency of the extraction process depends on temperature and pressure. Fig. 3 reports a selection of the experimental tests in order to point out the combined effect of the temperature and pressure on the extraction efficiency of acrylamide. It can be observed that while a temperature increase of 20 °C strongly affects the extraction efficiency, a pressure increase of 100 bar is almost negligible. As a general rule, at constant temperature, an increase in pressure would increase the solvent density and its solvation power. However, the pressure of the supercritical solvent does not seem to significantly affect the solvation power towards acrylamide in the experimental range considered in this work. This is probably connected to the fact that pressure values are so high that the compressibility of the supercritical fluid is minimum in the thermal range here considered. Since the extraction process is mainly affected by temperature a deeper investigation of this variable on the extraction efficiency was carried out. Fig. 4 reports a plot of the different results obtained on the coffee beans from batch 1 without the addition of cosolvent. The figure clearly shows that at low temperatures (25–50 °C) the extraction efficiency is scarcely affected by temperature while it increases when the temperature is raised above
10
15
20
25
30
35
mSF/mcoffee (g/g)
Fig. 3. Combined effect of temperature and pressure on the efficiency of the acrylamide extraction process plotted versus the ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel (batch 2, n = 2, 9.5% w/w ethanol).
60 °C. This can be explained by the effect of temperature on the solubility of acrylamide in the supercritical fluid, which is more complex than that of pressure. The effect of temperature on the solubility of a solute in a supercritical fluid results from the combination of two factors: an increase in temperature decreases the solvent density and its solvation power, but it also raises the vapor pressure of the solute enhancing its solubility in the supercritical fluid (Pereira and Meireles, 2010). This can explain the trend observed in Fig. 4: the two opposite factors are roughly balanced at low temperatures while the vapor-pressure-increase overcomes the density-decrease phenomenon when the temperature is raised above 60 °C. To better explain the above discussion the vapor pressure of acrylamide was estimated at different temperatures through DIADEMÒ 2004 (companion software available for the DIPPR 801 database); the calculation error is reported to be lower than 25%. The vapor pressure trend versus temperature is reported in Fig. 5 and shows a steep increase above 60 °C. The melting point of acrylamide is 84.5 °C (Carpenter and Davis, 1957). It is well known that the melting point of a pure solid can be depressed significantly under
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M. Banchero et al. / Journal of Food Engineering 115 (2013) 292–297 25
Extraction efficiency, %
Extraction efficiency, %
40
30
20
10
20
15
10
5
0
0 0
20
40
60
80
100
120
0%
140
Temperature (˚C)
700
Vapor pressure (Pa)
600 500 liquid acrylamide 400 300 melting point
200 solid acrylamide 100 0 30
40
50
60
70
80
90
100
110
120
Temperature (˚C)
Fig. 5. Vapor pressure of acrylamide versus temperature – calculated through DIADEMÒ 2004.
the influence of high pressure CO2 (Lucien and Foster, 2000). This implies that, during the supercritical extraction process, acrylamide may reasonably approach the liquid state at 60–70 °C, which would correspond to a huge increase in the vapor pressure trend. The visual observation of the coffee beans after the supercritical treatment showed a not-negligible color-change when they were processed at 120 °C probably due to the occurrence of some chemical reactions related to coffee roasting. This could be a drawback since it could involve the removal of the newly formed aroma and taste compounds by means of the supercritical solvent. For this reason, a maximum temperature of 100 °C was chosen to conduct the following experimental tests. 3.2.2. The influence of the cosolvent addition and duration of the extraction process Ethanol (5–9.5% w/w) was added to increase the polarity of the supercritical fluid without affecting the single-phase characteristics of the solvent mixture (Secuianu et al., 2008; Blanch-Ojea et al., 2012). In Fig. 6 the extraction efficiency obtained without the addition of ethanol is compared with those obtained with different ethanol content: all the experiments were conducted on the same batch of coffee beans (batch 1) and at the same working conditions (80 °C, 200 bar, 345 min of extraction time). The figure clearly shows that adding 9.5% w/w of cosolvent involves a 60% increase in the extrac-
Fig. 6. The effect of the content of ethanol on the efficiency of the acrylamide extraction process plotted versus time (batch 1, n = 2, 80 °C, 200 bar, 345 min of extraction time).
tion efficiency of acrylamide, which is raised from 12.5 ± 0.9% to 20.3 ± 1.4%. Experiments performed at 80 °C and 200 bar with a 5% w/w of ethanol showed intermediate values of extraction efficiency after 345 min of extraction times (14.9 ± 0.7%). The high extraction efficiency connected to the ethanol addition was confirmed when the experiments were conducted with the coffee beans from batch 2, which had a higher initial acrylamide content. Experiments performed at 80 °C, 200 bar and 9.5% w/w ethanol showed an extraction efficiency of 22.7 ± 0.4% after 345 min, which raised up to 38.4 ± 1.4% when temperature was increased up to 100 °C. Eventually, a set of experiments were conducted to investigate how the duration of the process affected the extraction efficiency. Longer extraction times, in fact, correspond to higher mSF/mcoffee ratios. The experiments were conducted with the coffee beans from batch 2 at the maximum ethanol content (9.5% w/w) and the results are reported in Fig. 7 that shows the effect of the mSF/ mcoffee ratio on the extraction efficiency at two different temperatures (80 and 100 °C). It can be observed that an increase in the mSF/mcoffee ratio involves a considerable increase in the extraction efficiency, as it was expected. In the experimental range here investigated, a maximum extraction efficiency of 63.8 ± 1.1% was observed when the system was operated at 80 °C and a maximum value of 78.9 ± 1.9% when it was run at 100 °C. This last value is the highest extraction efficiency that was found in this work. 90 80 Extraction efficiency, %
Fig. 4. Efficiency of the acrylamide extraction process versus temperature without any ethanol addition (batch 1, n = 2, 200 bar, 345 min of extraction time).
5% 9.5 % Ethanol content (% w/w)
70 60 50 40 30 80 ˚C, 200 bar 100 ˚C, 200 bar
20 10 0 0
10
20
30
40
50
mSF/mcoffee (g/g)
Fig. 7. The effect of the ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel on the efficiency of the acrylamide extraction process at different temperatures (batch 2, n = 2, 9.5% w/ w ethanol).
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3.3. The impact on the organoleptic properties of the coffee brew – preliminary considerations
properties of coffee in order to meet the aroma and taste standards suitable to Lavazza coffee consumers.
The impact of the supercritical treatment on the organoleptic properties of coffee was not the aim of this preliminary work. Anyway, this aspect is very important in perspective of a future industrial application of this technology. As it was explained in Section 1 conventional roasting is generally carried out at 220–250 °C (Guenther et al., 2007) while the pre-roasting treatment was conducted at much lower temperatures (107–151 °C). This resulted in the production of a significant amount of acylamide to be extracted but also in minimum formation of the aroma and taste constituents of coffee. This last point is quite advantageous from an organoleptic point of view. In this way, in fact, the majority of aroma and taste compounds has still to be formed and cannot be removed by the supercritical treatment. This was confirmed by exploratory degustation tests, which were conducted by coffee testing experts on the coffee brews prepared with some samples of the coffee beans that had been previously subjected to the supercritical treatment. These degustation tests, which were conducted according to the standards generally adopted by Lavazza S.p.A., also pointed out that the supercritical removal of acrylamide must be followed by a cleaning step with pure supercritical CO2 in order to remove the ethanol residues. It can be preliminary concluded, then, that the supercriticalacrylamide-mitigation strategy is expected to only slightly modify the sensory properties of coffee. Future research will be conducted to optimize the supercritical treatment in order to match an efficient removal of acrylamide with the aroma and taste standards suitable to Lavazza coffee consumers.
Acknowledgment
4. Conclusion The use of supercritical extraction to reduce the acrylamide content in coffee has been investigated in this work for the first time. The work consisted into two steps. In the first step a preroasting treatment was set-up with the aim of obtaining high amounts of acrylamide in the coffee beans to be treated with the supercritical fluid. The experiments resulted in the preparation of two different batches: one with a medium (batch 1, 0.37 mg/kg) and one with a high acrylamide content (batch 2, 0.90 mg/kg). The supercritical extraction was the second step. The efficiency of acrylamide removal ranged from 8% to 45% when the duration of the extraction was below 525 min, which corresponds to a consumption of supercritical solvent equal to approximately 19 g/g of coffee beans. When the extraction time was raised up to 1305 min, a maximum extraction efficiency of 79% was found, which corresponds to a consumption of supercritical solvent of 47 g/g of coffee beans. While a pressure variation in the 200– 300 bar range did not significantly affect the results, temperature was the variable that affected the extraction process at the highest extent since the extraction efficiency was highly increased above 60–80 °C. The addition of ethanol (up to 9.5% w/w) resulted in a significant increase in the extraction performance due to the change in polarity of the supercritical solvent mixture. The best working conditions were found to be equal to 100 °C, 200 bar and 9.5% w/w ethanol. These results show that supercritical extraction can be a valid tool to remove acrylamide from coffee. Furthermore, the caffeine content in the coffee beans was not significantly affected by the supercritical treatment. Anyway, this is a first important result and the goal to propose supercritical extraction as a powerful strategy to mitigate the acrylamide content in coffee requires further steps. Future research will match the optimization of the acrylamide-extraction process with the investigation of the organoleptic
The authors acknowledge financial support from Regione Piemonte, Project ITACA, food platform (POR FESR 2007-2013). References Alves, R.C., Soares, C., Casal, S., Fernandes, J.O., Beatriz, M., Oliveira, P.P., 2010. Acrylamide in espresso coffee: influence of species, roast degree and brew length. Food Chemistry 119 (3), 929–934. Banchero, M., Ferri, A., Manna, L., 2009. The phase partition of disperse dyes in the dyeing of polyethylene terephthalate with a supercritical CO2/methanol mixture. The Journal of Supercritical Fluids 48 (1), 72–78. Blanch-Ojea, R., Tiggelaar, R.M., Pallares, J., Grau, F.X., Gardeniers, J.G.E., 2012. Flow of CO2–ethanol and of CO2–methanol in a non-adiabatic microfluidic T-junction at high pressures. Microfluidics and Nanofluidics 12 (6), 927–940. Brunner, G., 2005. Supercritical fluids: technology and application to food processing. Journal of Food Engineering 67 (1–2), 21–33. Capuano, E., Fogliano, V., 2011. Acrylamide and 5-hydroxymethylfurfural (HMF): a review on metabolism, toxicity, occurrence in food and mitigation strategies. LWT – Food Science and Technology 44 (4), 793–810. Carpenter, E.L., Davis, H.S., 1957. Acrylamide. Its preparation and properties. Journal of Applied Chemistry 7 (12), 671–676. de Azevedo, A.B.A., Mazzafera, P., Mohamed, R.S., Vieira de Melo, S.A.B., Kieckbusch, T.G., 2008. Extraction of caffeine, chlorogenic acids and lipids from green coffee beans using supercritical carbon dioxide and co-solvents. Brazilian Journal of Chemical Engineering 25 (3), 543–552. Ferri, A., Banchero, M., Manna, L., Sicardi, S., 2004. An experimental technique for measuring high solubilities of dyes in supercritical carbon dioxide. The Journal of Supercritical Fluids 30 (1), 41–49. Ferri, A., Banchero, M., Manna, L., Sicardi, S., 2006. Dye uptake and partition ratio of disperse dyes between a PET yarn and supercritical carbon dioxide. The Journal of Supercritical Fluids 37 (1), 107–114. FoodDrinkEurope, 2011. Acrylamide Toolbox Document, September 2011. (Retrieved 21.05.12). Guenther, H., Anklam, E., Wenzl, T., Stadler, R.H., 2007. Acrylamide in coffee: review of progress in analysis, formation and level reduction. Food Additives & Contaminants 24 (Suppl. 1), 60–70. Herrero, M., Mandiola, J.A., Cifuentes, A., Ibáñez, E., 2010. Supercritical fluid extraction: recent advances and applications. Journal of Chromatography A 1217 (16), 2495–2511. Hoenicke, K., Gatermann, R., Harder, W., Harting, L., 2004. Analysis of acrylamide in different foodstuffs using liquid chromatography–tandem mass spectrometry and gas chromatography–tandem mass spectrometry. Analytica Chimica Acta 520 (1–2), 207–215. Lantz, I., Ternité, R., Wilkens, J., Hoenicke, K., Guenther, H., van der Stegen, G.H.D., 2006. Studies on acrylamide levels in roasting, storage and brewing of coffee. Molecular Nutrition and Food Research 50 (11), 1039–1046. Lucien, F.P., Foster, N.R., 2000. Solubilities of solid mixtures in supercritical carbon dioxide: a review. The Journal of Supercritical Fluids 17 (2), 111–134. Parzefall, W., 2008. Minireview on the toxicity of dietary acrylamide. Food and Chemical Toxicology 46 (4), 1360–1364. Pereira, C.G., Meireles, M.A.A., 2010. Supercritical fluid extraction of bioactive compounds: fundamentals, applications and economic perspectives. Food and Bioprocess Technology 3 (3), 340–372. Ramalakshmi, K., Raghavan, B., 1999. Caffeine in coffee: its removal. Why and how? Critical Reviews in Food Science Nutrition 39 (5), 441–456. Seal, C.J., de Mul, A., Eisenbrand, G., Haverkort, A.J., Franke, K., Lalljie, S.P.D., Mykkänen, H., Reimerdes, E., Scholz, G., Somoza, V., Tuijtelaars, S., van Boekel, M., van Klaveren, J., Wilcockson, S.J., Wilms, L., 2008. Risk-benefit considerations of mitigation measures on acrylamide content of foods – a case study on potatoes, cereals and coffee. British Journal of Nutrition 99 (Suppl. 2), S1–S46. Secuianu, C., Feroiu, V., Geana˘, D., 2008. Phase behavior for carbon dioxide + ethanol system: experimental measurements and modeling with a cubic equation of state. The Journal of Supercritical Fluids 47 (2), 109–116. Sovová, H., Stateva, R.P., 2011. Supercritical fluid extraction from vegetables materials. Reviews in Chemical Engineering 27 (3–4), 79–156. Stadler, R.H., 2005. Acrylamide formation in different foods and potential strategies for reduction. Advances in Experimental Medicine and Biology 561, 157–169. Tello, J., Viguera, M., Calvo, L., 2011. Extraction of caffeine from Robusta coffee (Coffea canephora var. Robusta) husks using supercritical carbon dioxide. The Journal of Supercritical Fluids 59, 53–60. Zhang, Y., Ren, Y., Zhang, Y., 2009. New research developments on acrylamide: analytical chemistry, formation mechanism, and mitigation recipes. Chemical Reviews 109 (9), 4375–4397.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Supercritical fluid extraction as a potential mitigation strategy for the reduction of acrylamide level in coffee Mauro Banchero a,⇑, Gloria Pellegrino b, Luigi Manna a a b
Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, Italy Innovation Center, Lavazza S.p.A., Strada Settimo 410, Torino, Italy
a r t i c l e
i n f o
Article history: Received 1 June 2012 Received in revised form 6 September 2012 Accepted 27 October 2012 Available online 5 November 2012 Keywords: Supercritical fluid extraction Carbon dioxide Cosolvent Acrylamide Coffee Mitigation Roasting
a b s t r a c t The removal of acrylamide from coffee through supercritical CO2 extraction has been investigated for the first time. Two steps were performed: a pre-roasting treatment and a supercritical extraction process. The main aim of this first work was to investigate the feasibility of the process more than the impact of the treatment on the organoleptic properties of coffee. The efficiency of acrylamide removal was checked by measuring its content in the coffee beans before and after the supercritical treatment. The role of temperature, pressure, extraction time and the addition of a modifier (ethanol) was examined. The supercritical treatment did not affect the caffeine content of coffee and a maximum extraction efficiency of 79% was found for acrylamide. While a pressure variation did not significantly affect the results, temperature affected the extraction process at the highest extent. The addition of ethanol resulted in a significant increase in the extraction performance due to the change in polarity of the supercritical solvent mixture. The best working conditions in the experimental range here investigated were 100 °C, 200 bar and 9.5% w/w ethanol. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Acrylamide is a chemical compound that has been labeled as potentially carcinogenic to humans by the International Agency for Research on Cancer since 1994 (Parzefall, 2008). In the last few years, a great deal of concern has arisen over its presence in food. Relevant amounts of acrylamide, in fact, can be found in several carbohydrate-rich foods such as potatoes, coffee and cereals when they are cooked at high temperatures (Capuano and Fogliano, 2011). It has been clearly established that the major pathway for acrylamide formation in foods is the Maillard reaction, initiated by the condensation of aspargine, an aminoacid, and reducing carbohydrates, such as fructose or glucose, or, alternatively, reactive carbonyls (Guenther et al., 2007; Zhang et al., 2009). As far as coffee is concerned, more than one pathway for acrylamide formation can be expected since the coffee beans are roasted in the 220–250 °C range, which is higher with respect to other foods. These include several thermally driven reactions that involve other intermediates such as acrolein, acrylic acid and 3aminopropionamide. Furthermore, the acrylamide content in roasted coffee is determined by concomitant formation and elimination reactions occurring during the roasting process (Guenther et al., 2007). It was observed that acrylamide is formed at the beginning of the roasting step and, after reaching the maximum le⇑ Corresponding author. Tel.: +39 011 0904703; fax: +39 011 0904648. E-mail address: [email protected] (M. Banchero). 0260-8774/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jfoodeng.2012.10.045
vel, it decreases shortly. It was also found that light roasted coffees may contain relatively higher amounts of acrylamide than the dark roasted ones (Lantz et al., 2006). The hypothesis is that the acrylamide concentration in coffee decreases with increasing thermal input (darker roasting) and that the reactions leading to the depletion of acrylamide predominate towards the end of roasting. These reactions are probably connected to the polymerization of acrylamide or its reaction with food components (FoodDrinkEurope Toolbox, 2011). However, both a darker roasting and an extension of the roasting time are not applicable as options to reduce the acrylamide level in coffee since they could generate undesirable compounds with a negative impact on the taste or aroma of the final product (Stadler, 2005). The acrylamide content in roasted coffee beans is extremely variable for the reasons mentioned in the previous discussion and the levels usually reported in the literature are within the range 0.035–0.54 mg/kg of coffee (Alves et al., 2010). The acrylamide content also depends on the coffee species. A comparison between Robusta and Arabica coffee, which are the two coffee species of higher economical impact, showed that the first one produces a higher acrylamide level. This is probably due to a slightly higher content of aspargine in the green beans of the Robusta species (Lantz et al., 2006). A lot of mitigation strategies have been proposed to reduce the acrylamide content in potato and cereal-based products while very limited process options are available for coffee since they can affect the final product quality (Capuano and Fogliano, 2011). One of the
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most promising ways to reduce the acrylamide content in foods is the addition of the enzyme asparginase, which is able to catalyze the hydrolysis of aspargine thus lowering the content of the precursor aspargine. While this enzyme has been successfully applied at lab scale both to potato and cereal-based products, only preliminary results are available for coffee (Capuano and Fogliano, 2011). They showed only a 10–45% reduction of acrylamide after roasting probably due to the fact that aspargine is not the only acrylamide precursor in coffee beans. Furthermore, the taste of coffee is significantly and negatively influenced by the treatment process (FoodDrinkEurope Toolbox, 2011). Roasting green coffee under a steam/pressure atmosphere was proposed as an alternative roasting process and resulted in longer roasting times and in a reduction potential of only 10% in the acrylamide content (Guenther et al., 2007). Another study involved the addition of glycine, an amminoacid, which would inhibit the Maillard reaction between aspargine and carbonyl compounds but the practical applicability of the method or the impact on the final product has still to be established (Seal et al., 2008). Finally, predrying or decaffeination of green coffee beans did not show any significant impact on the acrylamide level in the roasted product (FoodDrinkEurope Toolbox, 2011). Supercritical CO2 extraction is a clean and efficient method for the processing of solid food matrices. It can be used both to extract valuable bioactive compounds such as flavors, colorants and other biomolecules and to remove undesirable compounds such as organic pollutants, toxins and pesticides (Pereira and Meireles, 2010; Sovová and Stateva, 2011). Carbon dioxide is the most used supercritical solvent because it is cheap, non-flammable, non-toxic and its critical point (Tc = 31.1 °C Pc = 7.37 MPa) is lower than that of many other fluids. In the supercritical state it approaches gaseous-like viscosity and high diffusivity while density is near that of a liquid with a solvent strength that can be easily tuned by simply varying temperature and pressure. A simple expansion step at common environmental pressure values allows the solvent to be easily removed from the food matrix and the extract to be recovered (Brunner, 2005). One drawback of supercritical CO2 is its low polarity. This problem can be overcome employing small percentages of polar modifiers or co-solvents, such as methanol, ethanol and water, to change the polarity of the solvent (Pereira and Meireles, 2010). This results in an improvement of the extraction efficiency by increasing the solubility of the solute (Herrero et al., 2010) or the swelling of the solid matrix that facilitates the solute–solvent contact (Pereira and Meireles, 2010). Many applications of supercritical fluid extraction can be found in the literature (Herrero et al., 2010). In particular, the employment of supercritical CO2 for coffee decaffeination was successfully developed on an industrial scale since the 1970s and mainly consists in the extraction of caffeine from moistened green coffee beans through water saturated supercritical CO2 (Ramalakshmi and Raghavan, 1999). Moreover, both the caffeine and the other compounds that are extracted from coffee beans or husks can be employed in the pharmaceutical, cosmetic or food industry (de Azevedo et al., 2008; Tello et al., 2011). In this work the possibility of removing acrylamide from coffee beans through supercritical CO2 extraction has been investigated for the first time. The feasibility of the process is the preliminary step to candidate this technique as a potential mitigation strategy to reduce the acrylamide content in coffee. This could be quite appealing to the coffee industries not only because it is an environmentally friendly technique, but also because they are already acquainted with this high-pressure technology thanks to the increasing diffusion of the supercritical coffee decaffeination process. In the first part of the work, pre-roasting experiments were conducted in a lab-scale drum roaster to maximize the acrylamide amount in the coffee beans to be treated with the supercritical sol-
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vent. The supercritical extraction tests were, then, performed in a continuous laboratory apparatus and the efficiency of acrylamide removal was checked by the measurement of the acrylamide content in the coffee beans before and after the supercritical treatment. The role of temperature, pressure, extraction time and the addition of a modifier (ethanol) was investigated to find out how these parameters affect the extraction yield.
2. Materials and methods The experiments were performed on Robusta coffee beans supplied by Lavazza S.p.A. (Italy). The first part of the work consisted in performing a pre-roasting treatment of the green coffee beans to produce a significant amount of acrylamide in the samples to be treated with the supercritical solvent. The pre-roasting treatment was conducted in a labscale drum roaster equipped with electrical heating and with a capacity of 1 kg/batch. Pre-roasting consisted in maintaining the coffee beans at constant temperature for a fixed period of time. In a typical experiment, the roasting chamber was pre-heated at a constant value before introducing the green coffee beans. After a transient period of 15 min, the temperature of the coffee reached the target value that was kept constant for the desired treatment time. Different experiments were conducted varying the temperature and time of the pre-roasting treatment to maximize the acrylamide amount in the samples. Two different batches of pre-roasted coffee were selected and subjected to supercritical extraction. The supercritical extraction process was carried out in the apparatus shown in Fig. 1. It was a continuous apparatus where the supercritical solvent flowed through a 50 ml extraction vessel that contained approximately 30 g of the pre-roasted coffee beans and was discharged through a heated back pressure regulator, which provided the depressurization of the system and allowed the precipitated solute to be collected in a proper solvent trap. Further details about the supercritical apparatus can be found in previous works (Ferri et al., 2004, 2006; Banchero et al., 2009). The CO2 was purchased from SIAD S.p.A. (Italy) and was contained in a gas cylinder provided with a dip tube. Pump 1 is equipped with a cooling system to avoid the cavitation of the solvent. When the three-way mixing valve was closed, the extracting supercritical solvent was pure CO2, whereas when the mixing valve was open, a cosolvent (ethanol) could be introduced through Pump 2. Ethanol (purity P 99.8%) was also contained in the solvent trap and was supplied by Sigma–Aldrich. The supercritical solvent flowed through a heating coil before entering the extraction vessel. Both the heating coil and the extraction vessel were positioned inside an oven. The apparatus was also equipped with a pressure indicator (PI/1) and a Coriolis mass flow meter (FI/2). In a typical experiment, the extraction vessel was initially bypassed and the apparatus was run until the required conditions of temperature, pressure and mass flow rate were reached. Then, the extraction vessel was pressurized with the supercritical solvent and the extraction process could begin. Preliminary experiments showed that an overall mass flow rate of 1 g/min could guarantee an appropriate residence time of the supercritical solvent mixture in the extraction vessel. Extraction experiments were performed in duplicate. The solvent in the trap was periodically replaced, weighted and assayed through a Perkin Elmer UNICAM UV2-300 spectrophotometer in the UV range. Three different peaks could be detected at 204, 230 and 272 nm but none of them could be directly related to the concentration of a specific chemical compound due to the overlap of the UV absorbance peaks of the extracted components. An HPLC analysis of the solvent in the trap showed that the acrylamide concentration was too low to guarantee an accurate quanti-
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Fig. 1. Experimental apparatus.
tative measurement of the extraction efficiency. Anyway, the knowledge of the mass of the solvent in the trap and the absorbance at 272 nm, which was the sharpest peak, were used to estimate the cumulative amount of the total extracted material. This allowed the trend of the extraction process to be monitored versus time. The extraction efficiency of acrylamide (e%) was evaluated from the analysis of the coffee beans according to the following equation:
e% ¼
AAi AAf 100 AAi
ð1Þ
where AAi is the initial acrylamide content of pre-roasted coffee beans and AAf is the acrylamide content after the supercritical treatment. The acrylamide analyses of coffee beans were performed by Eurofins Scientific (Chemical Control s.r.l., Cuneo, Italy) using HPLC/MS with deuterium-labeled acrylamide as an internal standard. The limit of quantification certified by the lab is equal to 0.15 mg/kg. The measurement uncertainty, with a level of confidence of 95%, varied in the 8–35% range depending on the acrylamide concentration. Further details about the analytical methods adopted by Eurofins laboratories to detect acrylamide in different foodstuffs can be found elsewhere (Hoenicke et al., 2004). The Eurofins laboratory also performed some analyses to check if the caffeine content of the coffee beans was affected by the supercritical treatment. The caffeine content before and after the supercritical treatment was always equal to 2.1 ± 0.2% w/w. 3. Results and discussion 3.1. The pre-roasting treatment The pre-roasting experiments were conducted at temperatures and treatment-times lower than those generally adopted during the conventional roasting process. This procedure was chosen in order to achieve high amounts of acrylamide in the samples to be treated with the supercritical solvent. Furthermore, the temperatures of the pre-roasting treatment were selected in order to avoid or limit the formation of those chemical compounds that are responsible for taste and aroma of the final coffee brew and that could be removed by a supercritical treatment. Even though the investigation of the impact of the extraction process on the organoleptic properties of coffee was not the aim of this preliminary work, this was considered more appropriate to prevent a strong modification of the quality of the product.
Two set of experiments were conducted: the first one at lower temperatures and shorter treatment-times, the second at higher temperatures and longer treatment-times. In the first set of experiments, the temperature and time of the pre-roasting treatment were varied in the 107–134 °C range and 5–10 min range, respectively. The pre-heating temperature of the roasting chamber was varied between 140 and 170 °C. A maximum acrylamide content of 0.37 ± 0.05 mg/kg was found after treating the coffee beans at 132 °C for 5 min. These working conditions were selected to prepare a first batch of samples (denoted as batch 1) for the subsequent supercritical treatment. In the second set of experiments, the treatment-temperature ranged from 125 to 151 °C while the treatment-times were equal to 10, 15 or 20 min. The pre-heating temperature of the roasting chamber was varied between 190 and 210 °C. The maximum acrylamide content of 0.90 ± 0.05 mg/kg was obtained after treating the coffee beans at 151 °C for 20 min, which were selected as the optimal working conditions to prepare the second batch of samples (denoted as batch 2) for the supercritical treatment.
3.2. The supercritical extraction process Table 1 reports both the operating conditions and the acrylamide extraction efficiencies (e%) of the experimental tests. The first set of experiments was conducted on the coffee beans from batch 1 without any addition of ethanol in order to investigate the role of the temperature of the pure supercritical solvent. The following experiments were performed on the coffee beans both from batch 1 and batch 2 in the presence of ethanol as a cosolvent: tests were conducted at different temperature, pressure, cosolvent concentration and duration of the extraction process. The table reports also the ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel (mSF/ mcoffee). The periodical spectrophotometric analysis of the solvent in the trap allowed us to monitor the cumulative amount of the total extracted material versus time in each test, as it was reported in Section 2. If the absorbance of the reference peak is assumed to be roughly proportional to the extracted amount of acrylamide, this information can be used to observe the temporal evolution of the acrylamide extraction by normalizing each curve with respect to the total amount of acrylamide extracted at the end of each test. Fig. 2 reports, for example, the trend of the acrylamide extraction process versus time for a single experiment.
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M. Banchero et al. / Journal of Food Engineering 115 (2013) 292–297 Table 1 Operating conditions and results of the acrylamide extraction tests. Batch of pre-roasted coffee (mg/kg) 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.37 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 0.90 ± 0.05 a b
T (°C)
P (bar)
Ethanol (% w/w)
Extraction time (min)
mSF/mcoffeea (g/g)
e%b
25 40 60 80 100 120 80 80 80 80 80 80 100 100 100 100 80 80 100 100
200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 200 300 300 300 300
0 0 0 0 0 0 9.5 5.0 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5 9.5
345 345 345 345 345 345 345 345 345 525 925 1305 345 525 925 1305 345 525 345 525
12.1 ± 0.2 12.5 ± 0.1 12.3 ± 0.3 12.5 ± 0.7 12.3 ± 0.3 12.7 ± 0.1 11.8 ± 0.2 11.7 ± 0.3 12.4 ± 0.2 18.9 ± 0.3 34.1 ± 0.6 47.5 ± 0.2 12.2 ± 0.0 18.6 ± 0.1 34.6 ± 0.5 47.1 ± 0.3 12.2 ± 0.3 18.6 ± 0.5 12.3 ± 0.2 18.8 ± 0.4
10.7 ± 1.2% 8.3 ± 0.7% 7.1 ± 1.3% 12.5 ± 0.9% 23.1 ± 1.3% 34.4 ± 0.7% 20.3 ± 1.4% 14.9 ± 0.7% 22.7 ± 0.4% 28.4 ± 2.3% 49.1 ± 2.7% 63.8 ± 1.1% 38.4 ± 1.4% 41.8 ± 1.8% 66.3 ± 1.4% 78.9 ± 1.9% 25.1 ± 0.5% 29.8 ± 2.0% 42.3 ± 2.1% 45.0 ± 2.8%
Ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel. Extraction efficiency of acrylamide (defined in Eq. (1)).
60
60
Extraction efficiency, %
Extraction efficiency, %
50
40
30
20 batch 2, 100 ˚C, 300 bar, Ethanol 9.5%
80 ˚C, 200 bar 80 ˚C, 300 bar 100 ˚C, 300 bar
50
40
30
20
10 10
0 0
100
200
300
400
500
600
Extraction time (min)
Fig. 2. Typical trend of the acrylamide extraction process versus time evaluated by UV measurement at 272 nm.
3.2.1. The influence of temperature and pressure The efficiency of the extraction process depends on temperature and pressure. Fig. 3 reports a selection of the experimental tests in order to point out the combined effect of the temperature and pressure on the extraction efficiency of acrylamide. It can be observed that while a temperature increase of 20 °C strongly affects the extraction efficiency, a pressure increase of 100 bar is almost negligible. As a general rule, at constant temperature, an increase in pressure would increase the solvent density and its solvation power. However, the pressure of the supercritical solvent does not seem to significantly affect the solvation power towards acrylamide in the experimental range considered in this work. This is probably connected to the fact that pressure values are so high that the compressibility of the supercritical fluid is minimum in the thermal range here considered. Since the extraction process is mainly affected by temperature a deeper investigation of this variable on the extraction efficiency was carried out. Fig. 4 reports a plot of the different results obtained on the coffee beans from batch 1 without the addition of cosolvent. The figure clearly shows that at low temperatures (25–50 °C) the extraction efficiency is scarcely affected by temperature while it increases when the temperature is raised above
10
15
20
25
30
35
mSF/mcoffee (g/g)
Fig. 3. Combined effect of temperature and pressure on the efficiency of the acrylamide extraction process plotted versus the ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel (batch 2, n = 2, 9.5% w/w ethanol).
60 °C. This can be explained by the effect of temperature on the solubility of acrylamide in the supercritical fluid, which is more complex than that of pressure. The effect of temperature on the solubility of a solute in a supercritical fluid results from the combination of two factors: an increase in temperature decreases the solvent density and its solvation power, but it also raises the vapor pressure of the solute enhancing its solubility in the supercritical fluid (Pereira and Meireles, 2010). This can explain the trend observed in Fig. 4: the two opposite factors are roughly balanced at low temperatures while the vapor-pressure-increase overcomes the density-decrease phenomenon when the temperature is raised above 60 °C. To better explain the above discussion the vapor pressure of acrylamide was estimated at different temperatures through DIADEMÒ 2004 (companion software available for the DIPPR 801 database); the calculation error is reported to be lower than 25%. The vapor pressure trend versus temperature is reported in Fig. 5 and shows a steep increase above 60 °C. The melting point of acrylamide is 84.5 °C (Carpenter and Davis, 1957). It is well known that the melting point of a pure solid can be depressed significantly under
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M. Banchero et al. / Journal of Food Engineering 115 (2013) 292–297 25
Extraction efficiency, %
Extraction efficiency, %
40
30
20
10
20
15
10
5
0
0 0
20
40
60
80
100
120
0%
140
Temperature (˚C)
700
Vapor pressure (Pa)
600 500 liquid acrylamide 400 300 melting point
200 solid acrylamide 100 0 30
40
50
60
70
80
90
100
110
120
Temperature (˚C)
Fig. 5. Vapor pressure of acrylamide versus temperature – calculated through DIADEMÒ 2004.
the influence of high pressure CO2 (Lucien and Foster, 2000). This implies that, during the supercritical extraction process, acrylamide may reasonably approach the liquid state at 60–70 °C, which would correspond to a huge increase in the vapor pressure trend. The visual observation of the coffee beans after the supercritical treatment showed a not-negligible color-change when they were processed at 120 °C probably due to the occurrence of some chemical reactions related to coffee roasting. This could be a drawback since it could involve the removal of the newly formed aroma and taste compounds by means of the supercritical solvent. For this reason, a maximum temperature of 100 °C was chosen to conduct the following experimental tests. 3.2.2. The influence of the cosolvent addition and duration of the extraction process Ethanol (5–9.5% w/w) was added to increase the polarity of the supercritical fluid without affecting the single-phase characteristics of the solvent mixture (Secuianu et al., 2008; Blanch-Ojea et al., 2012). In Fig. 6 the extraction efficiency obtained without the addition of ethanol is compared with those obtained with different ethanol content: all the experiments were conducted on the same batch of coffee beans (batch 1) and at the same working conditions (80 °C, 200 bar, 345 min of extraction time). The figure clearly shows that adding 9.5% w/w of cosolvent involves a 60% increase in the extrac-
Fig. 6. The effect of the content of ethanol on the efficiency of the acrylamide extraction process plotted versus time (batch 1, n = 2, 80 °C, 200 bar, 345 min of extraction time).
tion efficiency of acrylamide, which is raised from 12.5 ± 0.9% to 20.3 ± 1.4%. Experiments performed at 80 °C and 200 bar with a 5% w/w of ethanol showed intermediate values of extraction efficiency after 345 min of extraction times (14.9 ± 0.7%). The high extraction efficiency connected to the ethanol addition was confirmed when the experiments were conducted with the coffee beans from batch 2, which had a higher initial acrylamide content. Experiments performed at 80 °C, 200 bar and 9.5% w/w ethanol showed an extraction efficiency of 22.7 ± 0.4% after 345 min, which raised up to 38.4 ± 1.4% when temperature was increased up to 100 °C. Eventually, a set of experiments were conducted to investigate how the duration of the process affected the extraction efficiency. Longer extraction times, in fact, correspond to higher mSF/mcoffee ratios. The experiments were conducted with the coffee beans from batch 2 at the maximum ethanol content (9.5% w/w) and the results are reported in Fig. 7 that shows the effect of the mSF/ mcoffee ratio on the extraction efficiency at two different temperatures (80 and 100 °C). It can be observed that an increase in the mSF/mcoffee ratio involves a considerable increase in the extraction efficiency, as it was expected. In the experimental range here investigated, a maximum extraction efficiency of 63.8 ± 1.1% was observed when the system was operated at 80 °C and a maximum value of 78.9 ± 1.9% when it was run at 100 °C. This last value is the highest extraction efficiency that was found in this work. 90 80 Extraction efficiency, %
Fig. 4. Efficiency of the acrylamide extraction process versus temperature without any ethanol addition (batch 1, n = 2, 200 bar, 345 min of extraction time).
5% 9.5 % Ethanol content (% w/w)
70 60 50 40 30 80 ˚C, 200 bar 100 ˚C, 200 bar
20 10 0 0
10
20
30
40
50
mSF/mcoffee (g/g)
Fig. 7. The effect of the ratio between the total mass of CO2 flowed during the extraction test and the coffee amount in the extraction vessel on the efficiency of the acrylamide extraction process at different temperatures (batch 2, n = 2, 9.5% w/ w ethanol).
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3.3. The impact on the organoleptic properties of the coffee brew – preliminary considerations
properties of coffee in order to meet the aroma and taste standards suitable to Lavazza coffee consumers.
The impact of the supercritical treatment on the organoleptic properties of coffee was not the aim of this preliminary work. Anyway, this aspect is very important in perspective of a future industrial application of this technology. As it was explained in Section 1 conventional roasting is generally carried out at 220–250 °C (Guenther et al., 2007) while the pre-roasting treatment was conducted at much lower temperatures (107–151 °C). This resulted in the production of a significant amount of acylamide to be extracted but also in minimum formation of the aroma and taste constituents of coffee. This last point is quite advantageous from an organoleptic point of view. In this way, in fact, the majority of aroma and taste compounds has still to be formed and cannot be removed by the supercritical treatment. This was confirmed by exploratory degustation tests, which were conducted by coffee testing experts on the coffee brews prepared with some samples of the coffee beans that had been previously subjected to the supercritical treatment. These degustation tests, which were conducted according to the standards generally adopted by Lavazza S.p.A., also pointed out that the supercritical removal of acrylamide must be followed by a cleaning step with pure supercritical CO2 in order to remove the ethanol residues. It can be preliminary concluded, then, that the supercriticalacrylamide-mitigation strategy is expected to only slightly modify the sensory properties of coffee. Future research will be conducted to optimize the supercritical treatment in order to match an efficient removal of acrylamide with the aroma and taste standards suitable to Lavazza coffee consumers.
Acknowledgment
4. Conclusion The use of supercritical extraction to reduce the acrylamide content in coffee has been investigated in this work for the first time. The work consisted into two steps. In the first step a preroasting treatment was set-up with the aim of obtaining high amounts of acrylamide in the coffee beans to be treated with the supercritical fluid. The experiments resulted in the preparation of two different batches: one with a medium (batch 1, 0.37 mg/kg) and one with a high acrylamide content (batch 2, 0.90 mg/kg). The supercritical extraction was the second step. The efficiency of acrylamide removal ranged from 8% to 45% when the duration of the extraction was below 525 min, which corresponds to a consumption of supercritical solvent equal to approximately 19 g/g of coffee beans. When the extraction time was raised up to 1305 min, a maximum extraction efficiency of 79% was found, which corresponds to a consumption of supercritical solvent of 47 g/g of coffee beans. While a pressure variation in the 200– 300 bar range did not significantly affect the results, temperature was the variable that affected the extraction process at the highest extent since the extraction efficiency was highly increased above 60–80 °C. The addition of ethanol (up to 9.5% w/w) resulted in a significant increase in the extraction performance due to the change in polarity of the supercritical solvent mixture. The best working conditions were found to be equal to 100 °C, 200 bar and 9.5% w/w ethanol. These results show that supercritical extraction can be a valid tool to remove acrylamide from coffee. Furthermore, the caffeine content in the coffee beans was not significantly affected by the supercritical treatment. Anyway, this is a first important result and the goal to propose supercritical extraction as a powerful strategy to mitigate the acrylamide content in coffee requires further steps. Future research will match the optimization of the acrylamide-extraction process with the investigation of the organoleptic
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