Changes in the Aromatic Profile of Espresso Coffee as a Function of

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Changes in aromatic profile of espresso coffee as a function of grinding grade and extraction time: a study by electronic nose system. Carla Severini, Ilde Ricci, Mauro Marone, Antonio Derossi, and Teresa De Pilli J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/jf505691u • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Journal of Agricultural and Food Chemistry

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Changes in aromatic profile of espresso coffee as a function of grinding grade and

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extraction time: a study by electronic nose system.

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C. Severini*, I. Ricci, M. Marone, A. Derossi, T. De Pilli

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Department of Science of Agricultural, Food and Environment (SAFE)

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University of Foggia - Via Napoli 25, 71122 – Foggia, Italy

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*Corresponding author: Phone: +39 0881 589222 E-mail: [email protected]

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ACS Paragon Plus Environment

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Abstract

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The changes in chemical attributes and aromatic profile of espresso coffee were studied

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taking into account the extraction time and the grinding level as independent variables.

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Particularly, by using an Electronic Nose System the changes of the global aromatic

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profile of EC were highlighted. The results showed as the major amount of organic

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acids, solids and caffeine were extracted in the first 8 seconds of percolation. The

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grinding grade significantly affected the quality of EC probably as effect of the particle

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size distribution and the percolation pathways of water through the coffee cake. The use

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of Electronic Nose System allowed to discriminate the fractions of the brew as a

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function of percolation time and also the regular coffees obtained from different

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grinding grades. Particularly, the aromatic profile of a regular coffee (25 mL) was

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significantly affected by grinding level of coffee ground and percolation time which are

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two variables under the control of bar operator.

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Keywords

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Espresso coffee, Electronic Nose System, aroma profile, extraction time, grinding grade

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Introduction

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The Italian espresso coffee (EC) is the most consumed coffee beverage in the world.

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Particularly, since its appreciated sensorial properties, over than 50 millions of cups are

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consumed every day 1. Espresso coffee may be defined as ‘a brew obtained by

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percolation of hot water under pressure through compacted cake of roasted ground

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coffee, where the energy of the water pressure is spent within the cake’ 2.

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As well known, the quality of EC is affected by several variables, some of which are

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managed by the industry such as the coffee varieties, roasting conditions, roasting

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degree, the mixture of roasted coffee varieties as well as the storage conditions 3-7. Other

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variables are under the control of the barista (i.e. the technician of the bar), such as

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temperature and pressure of water, the grinding grade, the weight of coffee ground

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(dose) and the pressure on the upper surface of coffee cake (tamping); also, these

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variables significantly affect the physical, chemical and sensorial attributes of espresso

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coffee 1, 7-12.

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Moreover, the extraction time is of crucial importance since the overall quality of EC is

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the result of the equilibrium of hundreds of chemical compounds 13. In general, a regular

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coffee of 25 mL is obtained with an extraction rate of 1 mL/s but several differences are

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commonly observed. For instance, the so-called the ristretto coffee (about 15 mL) and

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the lungo coffee (about 30 mL) are often consumed in Italy and in other countries

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Under this point of view, for a correct management and an accurate standardization of

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EC quality, a precise description of the kinetic extraction of all chemical compounds is

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of great importance, but this aspect has not subjected to scientific experiments in

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details; indeed, the majority of the papers available on literature focused their attention

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on the extraction and analysis of aromatic compounds, sugars, solids, lipids, proteins,

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caffeine, etc. 13,15,16.


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.

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17-19

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Also, even though the EC is considered the most aromatic coffee brew

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papers focused their efforts on the changes of volatiles over extraction time separating

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the brew in different fractions

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quantitative characterization of volatiles of EC cup 21,22.

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For

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GC/olfactometry (GC/O) over than 1000 volatiles and 70 odorants have been identified

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from espresso coffee

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consuming 25. On the other hand, sensory analysis has been extensively used to evaluate

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and discriminate the sensorial attributes of coffee, particularly the aroma and flavour,

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but some disadvantages of this technique include subjectivity and poor reproducibility

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26-28

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In the last years, one of the most promising applications in routine quality control of

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foods and beverages is the Electronic Nose (EN) that is a new technology which enables

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to obtain the sensory analysis for the detection of the overall aromatic profile of

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samples. In particular, the Electronic Nose attempts to emulate the mammalian nose by

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using an array of sensors simulating mammalian olfactory responses to the aroma

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The sensors respond to a broad range of volatiles which have a high affinity with

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aldehydes, alcohols, ketons, etc. These compounds are drawn across the sensor array

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and induce a reversible physical and/or chemical change in the sensing material, which

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causes a change in electrical properties, such as conductivity. These changes are

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transducer into electrical signals, which are pre-processed and conditioned before

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identification by a pattern recognition system

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32

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aromatic profiles while other researchers, used EN to differentiate commercial coffee

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brands, different coffee varieties or different roasting grades, the presence of defects on

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roasted coffee as well as to estimate the shelf-life of packaged coffee 33-39. Furthermore,

instance,

by

using

16-20

very few

while a wide literature reports the qualitative and

chromatography/mass

spectrometry

(GC/MS)

and

23,24

; nevertheless these techniques are expensive and time

.

30

. Aishima

31

29

.

and Pornpanomchai et al.

, by using the EN, classified different types of instant coffee on the basis of their


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the Electronic Nose System was applied on espresso coffee to determine the best time

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for packaging

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countries and roasting degrees

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commercial coffee blends 41. Particularly, in this work the authors reported that a correct

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distinction of the brews was not statistically significant.

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On the basis of above considerations, the purpose of this paper was to study the changes

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of the espresso coffee during extraction to obtain useful information for the better

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managing of the brew quality. More specifically, the main objective was to use the

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Electronic Nose System to study the changes of the overall aromatic profile of espresso

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coffee taking into account the extraction time and the grinding level of coffee powder as

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independent variables.

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of coffee ground, to discriminate the coffee from different production 24

as well as to classify beans, ground and brew of

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Material and methods

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Raw materials and espresso coffee preparation

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Roasted coffee beans (medium-dark roasting: L*=21.50, a*=5.92) were supplied from

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ESSSE caffè S.p.A. (Anzola dell’Emilia, Bologna, Italy). The beans were ground by an

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automatic grinder with flat grinding blades (Mod. Super Jolly Coffee Grinder for

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Grocery, Mazzer, Italy) having 8 levels of grinding: 1 for the finest point and 8 for the

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coarsest. Water “Leggera” (Gaudianello, spa) used for brew preparation was locally

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purchased. Table 1 reports the main physical and chemical characteristics of the water

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as specified on the label, except the pH value which was experimentally measured.

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Espresso coffee samples were prepared by using an EC machine mod. V220

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(Vibiemme, Italy) with filter holder monodose applying the following experimental

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conditions: 7.0 g of ground coffee, a 60 mm of holder filter diameter, pressure on the

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upper surface of ground coffee cake of 1500 g, water temperature of 92°C and 9 atm of

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relative pressure. According to scientific literature, these conditions may be considered ACS Paragon Plus Environment

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7,8,11-13,42

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as the most common for EC preparation

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chosen on the basis of preliminary experiments in which different weights were used to

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press ground coffee.

. Instead, the value of pressure was

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Experimental design

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Espresso coffee samples were always prepared within 3 minutes from the grinding of

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beans which was performed at three levels: 6, 6.5 and 7, respectively correspondents to

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a fine, fine-coarse and coarse coffee ground. The choice of these grinding levels was

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performed on the basis of preliminary experiments in which the particle size distribution

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of the ESSSE caffè s.p.a. coffee ground commonly used in the bars, was determined.

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Once the variability of coffee ground was defined, we did choose the grinding levels

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which enables to well express the differences in terms of particle size distribution

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commonly detectable in the bars. Then, the particle size distribution for each grinding

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level was described shaking 100 g of coffee ground with four sieves (600, 400, 250 and

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180 µm) until constant weight. Table 2 shows the distribution’s percentage of the

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particle sizes for each grinding level.

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Each espresso coffee sample was divided in three fractions (F) collecting the brews

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respectively obtained in the first 8 s (Ft1), from 9 to 16 s (Ft2) and from 17 to 24 s (Ft3)

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of extraction. These extraction times were chosen dividing in three fractions the time

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necessary to prepare a regular coffee that is 25 s. The Extraction times were measured

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using a digital timer. The regular espresso coffee of 25mL, from each grinding level,

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was prepared measuring the volume of brew through a graduate cylinder. At least 20

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replicates were performed for each espresso obtaining a total of 240 samples.

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Physical and chemical analyses


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The pH of the samples was measured by a pH-meter mod. Basic 20 (Crison, Allen)

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previously calibrated with three buffer at pH = 4.00, 7.00 and 9.00. Also, acidity was

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measured by titratation of 25 mL of coffee brew at room temperature with NaOH 0.1 N

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until pH of 7.00

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about 3 mL of espresso coffee at 105°C until constant weight 44.

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Caffeine content was measured by using the method described by Skoog et al. 45, with

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some modifications. Each EC brew (fractions and regular coffee) was previously

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filtered with 0.22 µm nylon filter (Olim Peak, © Teknokroma Anlítica, Spain) and

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directly injected (injection volume = 20 µL) into HPLC binary pump (Waters mod.

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1525, Massachusetts, USA) equipped with a detector set at 254 nm (Waters mod. 2487,

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Massachusetts, USA). Peak separation was achieved on a C18 Column (Hibar® 125-4 –

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LiChrospher® 100, 5 µm) at 25°C, by using a mobile phase (V/V) of water (74%),

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methanol (25%) and acetic acid (1%) (J.T. Baker) previously seep through a filter in

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cellulose acetate of 0.45 µm (VWR International, USA) and submitted to an ultrasound

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pulse (Elmasonic S 60 H, Elma Schmidbauer GmbH, Germany) for 20 minutes in order

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to remove the air. Flow rate of pump was of 0.9 mL/min. The concentration was

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calculated by using the equation obtained by the linear regression of external caffeine

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standard at different concentration (Sigma Aldrich, USA) and the results were expressed

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as mg/mL.

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. The total solids content was measured gravimetrically by drying

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Determination of aromatic profile by Electronic Nose System.

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Aromatic profile was characterized by an electronic nose system αFOX Sensor Array

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System 2000 (Alpha M.O.S., Toulouse - France) equipped with six metal oxide

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semiconductor sensors (T30/1, T70/2, P10/1, P10/2, P40/1, PA/2) and an autosampler

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mod. HS 100 (Alpha M.O.S., Toulouse - France). The sensors measure the changes of

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the electrical resistance generated by the adsorption–desorption of the volatile organic ACS Paragon Plus Environment

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compounds. Dry air with impurities specified as H2O < 5 ppm, CnHm < 5 ppm, O2 +

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N2 > 99.95%, and O2 = 20 ± 1% was used as carrier gas by using a flow rate of 150

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mL/min. 1 mL of each coffee samples was placed in vials of 10 mL, sealed with rubber

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stopper and maintained in oven at 87°C in agitation at 300 rpm for 3 minutes before the

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injection of 500µL. A flow rate of 150 mL/min was used for the analysis. The sensor

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responses were detected every 1 s for a total time, t, of 300 s (a total of 300 points for

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each sample). The response of the sensors was expressed as Rt/R0 where R is the

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resistance of the sensor and the subscripts 0 and t refer to the initial value and at each

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time t. After each analysis the system was purged for 240 s with filtered air before a

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new injection to allow re-establishment of the instrument base line.

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Statistical analysis

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For each grinding grade, significant differences among physicochemical parameters of the

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fractions and the regular coffees were determined by one-way ANOVA. Tukey's test

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(p 0.05), exhibiting values ranged


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between 4.983±0.419 and 5.177±0.021; these results were in accordance with the data

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reported from Parenti et al.

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et al. 7 and Caporaso et al. 47 who respectively showed average values of ~5.6 and ~5.8.

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Similar results were observed for titrable acidity for which only the samples obtained by

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using a grinding level of 7 showed values significantly lower than the others. Moreover,

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taking into account the total solid content, no differences were observed (p > 0.05)

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stating as the three grinding grades did not affect this parameter for the regular EC

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samples. Our results, showed values ranged between 44.342±0.745 mg/mL and

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55.115±4.747 mg/mL which were in accordance with Parenti et al.

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total solid content of 59.48 mg/mL. However a high variance in total solids content of

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EC has been reported in literature 7,8,47.

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Caffeine contents of the regular EC between 3.212±0.267 mg/mL and 4.179±0.209

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mg/mL were observed. These values were higher than those reported from Caporaso et

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al. 47 and Parenti et al. 46 which showed values always lower than 2.44 mg/mL.

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Although our results showed some discrepancies when compared with other authors,

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our findings may be considered within the natural variance of the chemical composition

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of EC brews. As well known, the espresso coffee may be affected by several variables

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such as roasting grade, mixture of roasted coffee varieties, water temperature, etc. 3-7. In

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addition, the volume of espresso coffee, which considerably affect the chemical

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composition of brew, often is highly variable among the scientific publications 7,8,46,47.

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However, from the table 3 the analysis of the changes in chemical attributes of the three

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fractions of EC showed structured variations. The pH values significantly increased as a

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function of the fraction of brew for each grinding grade. Of course, this result may be

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caused from the progressive reduction of organic acids still contained into the coffee

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ground during the solid-liquid extraction. Moreover, comparing the pH values obtained

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from different grinding grades while keeping constant the same fraction, no significant

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but slightly lower than the values reported from Andueza


46

who showed a

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differences (p > 0.05) were observed for the Ft1 samples, while a significant increases

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(p < 0.05) were observed for the second (Ft2) and the third fraction (Ft3) of espresso

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coffee. These results probably were caused by a dilution effect since that the increasing

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of the grinding grade significantly affected the percolation rates with values of

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1.66±0.19 mL/s, 2.89±0.59 mL/s and 6.51±0.26 mL/s respectively when grinding level

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of 6, 6.5, and 7 were used. More specifically, the increase of percolation rate could be

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caused by the effect of the different particles size distribution (as reported in table 2) on

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the microstructure of the coffee cake. A higher porosity and/or better percolations

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pathway, which are expected when increasing grinding grade, could improve the

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extraction rate. However, since the pH values of the first fraction of EC samples were

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practically constant, it is possible to suppose that the major amount of organic acids

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were extracted in the first 8 second of percolation. Similar observations may be

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performed for titrable acidity, total solids and caffeine contents for which it was

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observed a progressive decrease as a function of the coffee fractions and the grinding

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grade. Particularly, for caffeine content the HPLC diagram of Ft1, Ft2 and Ft3 samples

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obtained by using a grinding level of 6 are reported in figure 1 showing as caffeine

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content decreased as a function of extraction time in a range of 5.231 and 0.749 mg/mL.

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The study of aromatic profile of each fraction of the EC and of the regular coffees

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obtained by using different grinding grade is reported in figures 2 - 5.

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Figure 2 shows the loading plot of the sensors which contributes in the discrimination of

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samples as well as the factor coordinates of the fractions of espresso coffee prepared

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with the three grinding grades. PC1 accounted for the 98.77% of the variation of

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samples while the PC2 accounted for the 1.01% stating as the first two PCs allowed to

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accurately explaining the variation of aroma profile of EC samples (99.78%). All the

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sensors have been significant for detecting differences in terms of odour compounds of

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EC samples. More specifically, the sensors T30/1, P10/2, PA/2 and T70/2 showed the ACS Paragon Plus Environment

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highest contribution in the discrimination of the majority of samples with the exception

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of the samples 6-Ft1 which were better recognized by the sensors P10/1 and P40/1.

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Particularly, the samples 6-Ft1 were clearly separated from the other coffee fractions

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stating as during the first 8 seconds of percolation with the finest coffee ground, a

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significant different aromatic profile was developed. However, the samples 6-Ft2 and 6-

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Ft3 were also well discriminated, but their differences were better highlighted from the

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responses of the other four sensors.

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When considering the three fractions of ECs prepared with the grinding grade 6.5 (6.5-

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Ft1, 6.5-Ft2, 6.5-Ft3) it was observed a good discrimination in the direction of the

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sensors T30/1, P10/2 and PA/2 which gave the most important contribution for

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detecting significant differences in the aroma profile of these samples. Similarly, the

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three fractions of the samples obtained from the coarse coffee ground (7-Ft1, 7-Ft2 and

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7-Ft3) were well discriminated from the above three sensors. These results confirmed as

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there is a significant change in the type and/or amount of volatiles of EC during

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extraction. Moreover, significant differences were also observed among the three

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grinding grades used for preparing coffee brews. By increasing the grinding grade for

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each fraction of EC (Ft1, Ft2 and Ft3), the samples were well discriminated proving as

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the use of different grinding grade may significantly modify the aromatic profile of

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coffee brew. Again, it is reasonably to suppose that this result was caused by a different

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level of extraction as a consequence of the variation in particle size distribution of

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coffee ground. Particularly, as the particle sizes are smaller as the solid-liquid surface is

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greater and, as previously reported, the percolation rate is lower. Of course, the greater

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solid-liquid surface could have increased the amount of volatiles extracted from coffee

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ground. From the figure 2, it is possible to observe as the samples 6-Ft2 cannot be

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discriminated from the samples 6.5-Ft1 and 7-Ft1 stating as the coffee brew extracted

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from 9 to 16 s by using the finest coffee ground showed similar aromatic profile than ACS Paragon Plus Environment

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the brews obtained during the first 8 seconds of extraction with the medium-coarse and

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the coarse coffee ground. According to our results, Clarke

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reported as the coffee brews prepared with a coarse coffee ground showed the lowest

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aromatic profile. Bhurmiratana et al. 49 demonstrated that the grinding of roasted coffee

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beans increased the surface area and, consequently, influenced the release of aromatic

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compounds. Again, these results support the idea that different aromatic profile of EC

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may be obtained when different grinding grades are used and/or when the extraction

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time is not constant.

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With the aim to better compare the difference in aromatic profile among the fractions of

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EC and of the regular coffee, the figures 3 and 4, show the results of PCA for samples

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respectively prepared with grinding grades of 6 and 6.5. In all cases the PC1 accounted

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for more than the 97% of the variance of the samples while the PC2 explained a

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maximum of the 1.93%. Considering the samples prepared with a grinding grade of 6 a

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clear discrimination of each of the three fractions was observed (Figure 3). Again, these

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differences could be a result of a dilution effect due to the progressive reduction of the

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chemical substances and volatiles still contained into the coffee ground, going forward

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the extraction time of coffee brew. However, the regular coffee of 25 mL seems to have

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an aromatic profile comparable with the second fraction of the brew collected from 9 to

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16 s of extraction. This is in accordance with the above results; particularly, by

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considering the flow rate of 1.66 mL/s, ~ 15 s are necessary to obtain a regular coffee of

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25 mL which is in the range of time for obtaining an EC cup with all the Ft1, and part of

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the Ft2 samples. Probably, the dilution effect produced by the addition of Ft2 to Ft1 led

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the regular EC to have an aromatic profile similar to the only Ft2 sample. Similarly,

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Nicoli et al.

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aromatic concentrations in the consecutive fractions of espresso and moka coffees.

15

and Severini et al.

16

48

and Andueza et al.7

observed a progressive reduction of chemical and


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When grinding grades of 6.5 and 7 were used for EC preparation, a good discrimination

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was again observed among the three different fractions of the brew. Nevertheless, by

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considering the PC1 axes, where the maximum variance explained resides, the regular

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coffees appeared to be practically overlapped with the Ft1 sample when using a

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grinding grade of 6.5 (Figure 4), while the regular coffee was slightly separated when a

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grinding grade of 7 was used (data not shown). Again, considering the volume of brew

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of 28.858±2.318 mL and 41.847±1.701 mL (Table 3), respectively measured for

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grinding grade 6.5-Ft1 and 7-Ft1, it is reasonable to suppose as these fractions

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respectively showed similar and lower concentration of odour compounds when

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compared to the respective regular coffees (25 mL). Some studies supported our results

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showing as the fine grade produced a low volume of coffee brew respect to a coarse

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grinding level 6,7. In addition, Clarke and Macrae 4 reported that the brews obtained with

309

fine ground coffee exhibited highest extraction of soluble solids and volatile

310

compounds, while Andueza et al. 7 did found that the particle size are inversely related

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to the extraction of organic acids and the sensorial aromatic perception.

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In figure 5 the results of PCA obtained by analyzing the only regular coffees achieved

313

from different grinding levels are shown. In this case both the PCs had a significant

314

contribution in the discrimination of samples with the PC1 which accounted for the

315

73.24% and the PC2 for the 22.96%. The three regular coffees were well distinguished

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proving as different aromatic profiles were obtained by preparing a cup of coffee of 25

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mL by using coffee powder with different grinding levels; nevertheless in comparison

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with the difference observed among each fraction, the three types of regular coffees

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appeared to be nearest. According to the previous results, the regular coffees prepared

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with fine-coarse and coarse coffee ground respectively contain the entire Ft1 sample and

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only a small part of it, while by using the fine coffee ground the regular coffees contain


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both the fractions Ft1 and Ft2. Under this consideration, it can be stated as that the

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regular coffees collected the majority of odour compounds.

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Table 4 shows the maximum value of the responses of each sensor to the volatiles of EC

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samples. A reduction of the response of all sensors as a function of the fractions of EC

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brew was observed for each grinding grade. As reported from several authors, the

327

reduction of the intensity of the response of the sensors may be related to the

328

concentration of aromatic compounds

329

for EC samples may be related to a decrease of the volatiles concentration in the brew

330

due to the dilution effect above discussed. The maximum response observed for the Ft1

331

samples confirmed the hypothesis that the maximum extraction of odour molecules

332

occurs during the first seconds of percolation process. For all grinding levels the

333

reduction of sensors intensity resulted more pronounced from Ft1 to Ft2, while the gap

334

between Ft2 and Ft3 appeared practically negligible. For the regular espresso coffee (25

335

mL), only slight variations in the response of each sensor were observed, confirming the

336

low differences in terms of aromatic profile as highlighted in figure 5.

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In conclusion, the chemical attributes and the aromatic profiles of EC were proved to be

338

significantly affected by the extraction time and the grinding grade of coffee powder.

339

Particularly, the majority of organic acids, solids and caffeine contained into the coffee

340

ground were extracted during the first 8 second of percolation. Also, by using the

341

Electronic Nose System, it was possible to prove as both extraction time and the

342

grinding level significantly affect the overall aromatic profiles of EC samples; indeed

343

through PCA a high discrimination was obtained among the brew fractions and the

344

grinding levels. This result highlights the importance to make more efforts on the

345

control of these two variables in the bar with the aim to assure a constant aromatic

346

quality of espresso coffee served every day.

37,50

. Similarly, the reduction observed in table 4

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Acknowledgment

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The authors gratefully acknowledge ESSSE Caffè S.p.A. for providing coffee samples.

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Arcangela Del Mastro and Ofelia Alessandrino (University of Foggia) are

351

acknowledged for their support during the experiments.

352 353

Conflict of interest

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The authors declare no competing financial interest.

355 356

References

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(1) Illy, E.; Navarini, L.. Neglected Food Bubbles: The Espresso Coffee Foam. Review

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Article, Food Biophysics 2011, 6, 335–348. (2) Illy, A.; Viani, R.; Suggi Liverani, F. Espresso Coffee: The Science of Quality. Elsevier Academic Press 2005, pp. 16–19. (3) Sivetz, M., and Desrosier, N.W., 1979. Coffee Technology. AVI Publ. Co., Inc., Westport, Conn. (4) Clarke, R. J.; Macrae, R. Coffee. Volume 2. Technology. Elsevier Applied Science. Publ. London. 1987.

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(5) Lerici, C.R.; Dalla Rosa, M.; Nicoli, M.C.; Severini, C. Influences of Processing

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conditions on the properties of coffee beverage. In: Trends in Food Processing II.

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Singapore: Ang How Ghee, Inst. of Food Sci. And Technol, 1987, pp. 94-98.

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Figure captions

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Figure 1. HPLC diagram of caffeine contents for each fraction (Ft1, Ft2, Ft3) from

497

grinding level 6 (fine)

498 499

Figure 2. Principal component projection of loading plot of the six sensors and score

500

plot of espresso coffee samples in different experimental conditions.

501 502

Figure 3. Principal component projection of loading plot of six sensors and score plot of

503

the fractions and of regular coffees (25mL) obtained from grinding grade 6.

504 505

Figure 4. Principal component projection of score plot of fractions and regular coffee

506

(25mL) obtained from grinding grade 6.5 (fine-coarse).

507 508

Figure 5. Principal components projection of score plot of regular coffee (25mL)

509

obtained from three grinding grades (6, 6.5, 7).

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Tables Table 1. The physicochemical characteristics of mineral water ‘Leggera’ (Gaudianello, Italy) as listed on the bottle’s label Analytical parameters

Values

pH Conductivity at 20°C µS/cm Total Residue at 180°C (TDS) mg/L Silica (SiO2) mg/L Potassium (K+) mg/L Calcium (Ca++) mg/L Magnesium (Mg++) mg/L Chloride (Cl-) mg/L Sulphates (SO4--) mg/L Bicarbonates (HCO3-) mg/L Nitrates (NO3-) mg/L

7.30* 510 416 105 32 46 14 26 14 306 4

*

Mean value of 6 samples measured in triplicate.

Table 2. Distribution (%) of particle size in each grinding grade of coffee powder (mean values ± standard deviation)

Particle size (µm) >600 400

Changes in the Aromatic Profile of Espresso Coffee as a Function of

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