Coffee extraction: A review of parameters and their influence on the physicochemical characteristics and flavour of coffee brews

Journal Pre-proof Coffee extraction: A review of parameters and their influence on the physicochemical characteristics and flavour of coffee brews Nancy Córdoba, Mario Fernandez-Alduenda, Fabian L. Moreno, Yolanda. Ruiz PII:

S0924-2244(19)30569-2

DOI:

https://doi.org/10.1016/j.tifs.2019.12.004

Reference:

TIFS 2675

To appear in:

Trends in Food Science & Technology

Received Date: 12 July 2019 Revised Date:

22 November 2019

Accepted Date: 8 December 2019

Please cite this article as: Córdoba, N., Fernandez-Alduenda, M., Moreno, F.L., Ruiz, Y., Coffee extraction: A review of parameters and their influence on the physicochemical characteristics and flavour of coffee brews, Trends in Food Science & Technology (2020), doi: https://doi.org/10.1016/ j.tifs.2019.12.004. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

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Coffee extraction: A review of parameters and their influence on the physicochemical characteristics and flavour of coffee brews. Córdoba Nancya, Fernandez-Alduenda Marioc, Moreno Fabian Lb, and Ruiz Yolandab * a

Biosciences Doctoral Program, Universidad de La Sabana, AA 53753, Chía, Colombia Agro-industrial Process Engineering, Universidad de La Sabana, AA 53753, Chía, Colombia c Coffee Quality Institute, Aliso Viejo, California, Estados Unidos *Corresponding author. Tel.: +57 1 8615555. E-mail address: [email protected]

b

Abstract Background: The physicochemical characteristics and flavour of coffee are related to the volatile and non-volatile compounds produced during roasting, which reach the coffee cup upon brewing. Scope and approach: This review focuses on interpreting the contribution that various parameters have during the coffee extraction process (coffee brewing). Coffee brewing methods and their extraction parameters were analysed in terms of phenomenological explanations and their effect on the physicochemical and flavour characteristics of brewed coffee. Key findings and conclusions. Many brewing methods have been developed to achieve a myriad of coffee flavour characteristics. Although several well-known brewing techniques have been adopted in the coffee industry, little associated relevant scientific data is available. Overall, these methods vary by extraction pressure, coffee/water ratio, water quality, contact time, particle size distribution, and temperature. An overview shows that all these factors modify the extraction of bioactive and volatile compounds that affect the flavour profile of the beverage. However, more in-depth explanation of the mass and energy transport phenomena would be useful to improve the understanding of the relationship between extraction variables and coffee flavour. Thus, phenomenological explanations are included to impart a better understanding of physicochemical and flavour changes in coffee beverages. Additionally, several gaps in knowledge relating to the extraction process are identified; and new trends in coffee extraction, including the cold brew method, are also discussed. Keywords: Coffee brewing, solid-liquid extraction, flavour, transport phenomena, physicochemical characteristics, volatile compound. Introduction The extraction of coffee is the final step in its production process before it is consumed. Coffee brewing is a solid-liquid extraction wherein the process parameters have a significant impact on the extraction kinetics of the different chemical compounds present in roasted coffee. Although coffee extraction generally only takes a few minutes, the process directly affects the final quality of the brew. Coffee extraction is carried out at different scales, such as industrial extraction, to produce instant coffee or extraction in domestic devices to produces a single cup of coffee (Moroney, Lee, O‫׳‬Brien, Suijver, & Marra, 2015). Coffee brewing methods change depending on the geographic, cultural, and social environment and individual preferences (Caporaso, Genovese, Canela, Civitella, & Sacchi, 2014; Gloess et al., 2013; Mestdagh, Glabasnia, & Giuliano, 2017). However, in all cases, the intimate contact between water and roasted coffee solids is required for the transfer of coffee flavours from the ground bean into the water matrix (Moroney et al., 2015; Petracco,

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2001) where the final objective, regardless of the methods used, is to produce coffee beverages with the highest possible quality. Both subjective and objective components contribute to the overall quality of food and beverages. Specifically, the objective components include the physicochemical, sensory and nutritional characteristics whereas, the subjective components refer to the individual perception of the final consumers(Giacalone, Fosgaard, Steen, & Münchow, 2016; Toledo, Pezza, Pezza, & Toci, 2016). In coffee beverages, the flavour is considered to be one of the most important characteristics that determine its quality and overall consume acceptance. According to the New Oxford American Dictionary, “flavor (Brit. flavour) is the distinctive quality of a particular food and drink as perceived by the taste buds and the sense of smell” (Jelen, 2012). In coffee beverages, certain features (smell, taste, colour, and body) are relevant and highly valued qualitative attributes (Nunes, Coimbra, Duarte, & Delgadillo, 1997). Processing parameters and variables in the coffee extraction process contribute greatly to the flavour, quality, and consumer acceptance of the coffee extraction process. Moreover, specific extraction variables, such as extraction time, water composition and temperature, pressure, particle size, and the coffee/water ratio affect the extraction process and the flavour. Aspects such as the chemistry of coffee brewing have received considerable attention (Moroney et al., 2015). However, other aspects related to physical phenomena and their effect on the extraction kinetics of the chemical compounds are still poorly studied in the coffee extraction. Overall, studies of coffee brewing (coffee extraction) show a lack of an engineering approach. Some critical parameters, such as the diffusivity, permeability of the packed beds, and transport phenomena often being overlooked. As a result certain myths and misconceptions surrounding coffee extraction have been promoted (Corrochano, Melrose, Bentley, Fryer, & Bakalis, 2015). The effect and phenomenological explanations of extraction variables on chemical composition and flavour are presented in this review. A better understanding of the coffee extraction process can lead to its improvement, making it replicable based on beverage quality. Likewise, this will suggest new alternatives for process design, equipment, and products aimed at coffee consumer satisfaction. Although there is a need in the market to provide new sensory experiences to consumers, it has been suggested that there are still substantial knowledge gaps concerning the extraction process exist. Indeed, the complexity of the coffee extraction process coupled with a lack of applied knowledge makes the brewing process for high-quality coffee more an art than a science. It is difficult to produce high-quality coffee beverages in a reproducible manner. Thus, this lack of reproducibility acts as a significant bottleneck for development of a sustainable business model. Consequently, this review describes the current state and advancement in the science of coffee extraction, emphasising current practices, the main parameters involved in the coffee extraction process, phenomenological explanations, and their effect on the physicochemical and flavour characteristics of the coffee beverage. Trends in the coffee extraction process and coffee brewing methods are also discussed. 1. The coffee extraction process Coffee extraction is carried out by different methods to produce the brew. This process involves (1) water absorption by the coffee grinds, (2) mass transfer of soluble compounds from the ground coffee into the hot water, and (3) separation of the resulting extract from coffee solids (Petracco, 2001; Wang, William, Fu, & Lim, 2016). During the coffee brewing, volatile and non-volatile flavour compounds are removed from ground coffee with hot water and distributed between water, oil, and solid phases (Steen, Waehrens, Petersen,

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Münchow, & Bredie, 2017). The coffee extraction process is considered a solid-liquid operation, the mechanism for which is favoured by increased surface area per unit volume of extractable solids and decreased particle radial distances within the solids. Both aspects are favorued by decreased particle size. In this way, the coffee extraction process has been modelled using Fick’s law of diffusion, which determines the solute quantity, diffusing from a solid particle surrounded by a liquid (Petracco, 2001, 2005) A model of caffeine diffusion in coffee infusions depending on particle size was proposed by Spiro & Selwood (1984). The authors assumed that water uptake by the particles is complete after significant extraction of soluble compounds. Each particle is considered a uniform sphere, surrounded by a Nernst diffusion layer of effective thickness δ, which decreases with increased stirring. At a given time, t, caffeine concentration in the centre and the periphery of a coffee particle is c' and c'1, respectively. c1 is the caffeine concentration on the solution side of the particle, while c represents caffeine in the bulk stirred solution outside the Nernst layer. In the steady state, caffeine flux (the net caffeine amount leaving the coffee per unit time) is given by the following kinetic equations (Eq. 13):
=  ( −  )/

(1)


=  (  −   )

(2)


=  ( − )/

(3)

Equations (1) and (3) are applications of Fick's first law of diffusion. In these equations, D represents the caffeine diffusion coefficient in the subscripted medium, and A represents the coffee particle surface area. Equation 2 represents caffeine transfer across the interface defined by k1 (coffee to solution) and k-1 in the opposite direction. Since the 1980s, the caffeine extraction kinetic model was used to study the coffee extraction process. Previous studies suggested that coffee extraction is primarily based on Fick's laws of diffusion. According to the steady-state theory, the variation of caffeine concentration c in the bulk solution outside the coffee varies with time t and follows a firstorder kinetic equation (Equation 4), where  is the equilibrium of caffeine concentration in the coffee solution (Spiro & Page, 1984; Spiro & Selwood, 1985).  



 

=

Although these studies focus on caffeine kinetics, overall conclusions regarding the mass phenomena and microscopic changes occurring in the coffee extraction process can be made. For instance, using Equation 4 to study caffeine extraction has allowed for the conclusion that caffeine diffusion through the intragranular pore space is the rate-limiting step in the extraction process (Spiro & Selwood, 1985). Likewise, the effect of water counter-flow, particle swelling, caffeine association with other soluble compounds, and physical matrix restraints have been correlated to the low caffeine diffusion coefficient inside the coffee particles (Spiro, Toumi, & Kandiah, 1989). However, this equation is only appropriate for systems defined as a dilute suspension where the bulk fluid is well mixed. The rates of coffee extraction and the physicochemical and flavour characteristics of coffee beverages depend on a large number of process variables. Among the most

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studied are the green coffee quality (Farah, Monteiro, Calado, Franca, & Trugo, 2006; Hečimović, Belščak-Cvitanović, Horžić, & Komes, 2011; Smrke, Kroslakova, Gloess, & Yeretzian, 2015), roasting degree (Dias et al., 2018; Gloess et al., 2014; Masi, Dinnella, Barnabà, Navarini, & Monteleone, 2013; Petisca, Pérez-Palacios, Farah, Pinho, & Ferreira, 2013), grinding (Andueza, Paz De Peña, & Cid, 2003; Baggenstoss, Poisson, Kaegi, Perren, & Escher, 2008; De Oliveira, Corrêa, De Oliveira, Baptestini, & VargasElías, 2016; Masayuki et al., 2003; Uman et al., 2016), and some coffee brewing methods (Caporaso et al., 2014; Gloess et al., 2013; Ludwig et al., 2012; Pérez-Martínez, Caemmerer, De Peña, Concepción, & Kroh, 2010; Sanchez & Chambers, 2015). As shown in Fig., multiple variables affect the coffee extraction process. Firstly, coffee quality is affected by the flavour, which is associated with chemical substances present in green coffee beans (Borém et al., 2013), also called precursors. These compounds become transformed into volatile and non-volatile compounds during the roasting process. Many of the compounds are extracted into the coffee brew, the rates of which are intimately related to the extraction parameters including, the particle size, extraction time, pressure, temperature, water quality, coffee/water ratio, and brewing method. Consequently, a change in any of these variables will subsequently affect the extracted materials, which, in turn, will impact the flavour and quality of the coffee brew. Although green coffee beans composition and the roasting process play a key role in the final brew quality, this review is focused on the primary variables involved in the extraction operation itself. The physics and phenomenological explanations of these extraction parameters and their effects on the chemical composition and flavour of the coffee brew are discussed. 1.1 Methods of coffee extraction Extraction methods can be categorised by the extraction tools as well as by the key process parameters influencing the final flavour profile (Mestdagh et al., 2017). Fig. 2 shows the classification of the leading coffee extraction methods that are used to evaluate the chemical and sensory composition of coffee beverages under different extraction conditions. Despite numerous extraction methods, coffee extraction can be classified into three main categories: decoction methods (boiled coffee, Turkish coffee, percolator coffee, and vacuum coffee), infusion methods (filter coffee and Napoletana), and pressure methods (Plugger, Moka, and espresso)(D’Agostina, Boschin, Bacchini, & Arnoldi, 2004; Moroney et al., 2015; Petracco, 2001). Broadly, decoction is a batch operation requiring prolonged contact between solids and water; while the infusion method requires hot water to flow through a coffee bed, allowing a short contact time with each elementary volume of water. Finally, in the pressure method, a driving force (pressure) is required for water flow through a compact bed (coffee cake) made from coffee grounds (Petracco, 2001). 1.2. Trends in coffee extraction methods The number of studies on coffee extraction related to specific coffee brewing methods is shown in Fig. 3. Despite numerous coffee extraction methods in the market, the most frequently studies coffee brewing methods are those related to the espresso (pressure) method, followed by the filter method (infusion). Filtered and espresso coffee beverages have been broadly studied, primarily because these types of coffee brews are commonly consumed. The consumption of Mocha, French, and Turkish coffee is more geographically restricted, and hence, these coffee types have been studied to a lesser extent (Toci & Boldrin, 2018). Studies on the espresso method are often focused on the different process variables such as temperature, extraction time, roasting degree, pressure, coffee/water ratio, water

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quality, and grinding. The impact on physicochemical characteristics and the aromatic compounds present in the brew are also studied. However, new market trends have motivated other methods to make espresso coffee (EC). For example, single-serve brewer machine extraction uses coffee grounds packaged in capsules or pods. Recent studies of these systems show continuing growth (Albanese, Di Matteo, Poiana, & Spagnamusso, 2009; Gloess et al., 2013; Parenti et al., 2014; Rahn & Yeretzian, 2019; Wang et al., 2016). Even more recently, novel analytical approaches using real-time extraction monitoring of volatile organic compounds (VOCs) during EC brewing with single-serve coffee capsules have been proposed (Sánchez-López, Zimmermann, & Yeretzian, 2014). Emerging studies focus on new coffee brewing methods that are already well-known by baristas and consumers but lack scientific data. For instance, Angeloni, Guerrini, Masella, Bellumori, et al., (2019), studied three espresso systems, one cold brew system, and four filter methods (V60, Aeropress, French Press, and Moka). The maximum caffeine and chlorogenic acid concentrations were found in ECs, while Moka and filtered coffees were 3-6 times less concentrated. In general, the bioactive compound concentration was higher for the espresso group than the filter group. Most coffee brewing methods use hot water during the extraction process. However, new coffee brewing trends have focused on cold extraction. Retail sales of cold brew coffee in the US have grown 460% from 2015-2017 (Mintel, 2017), demonstrating increased popularity. Cold brew coffee is prepared at room temperature (20 to 25°C) or at lower temperatures over a longer period than that for traditional hot brewing methods, with steeping times fluctuating from 8-24 hrs (Cordoba, Pataquiva, Osorio, Moreno, & Ruiz, 2019; Fuller & Rao, 2017). Several cold brew methods include dripping, immersion, or even French press. Because the extraction conditions (time/temperature) are unique, cold brew preparation provides a beverage typically characterised by sweetness and chocolate notes, which can be different from hot-brewed coffee (Angeloni, Guerrini, Masella, Innocenti, et al., 2019; Mestdagh, Davidek, Chaumonteuil, Folmer, & Blank, 2014). Despite the growing popularity of cold brew coffee, there are few scientific papers regarding their chemical attributes and flavour profiles. Several coffee retailers advertise cold brew coffee, claiming smoother taste, less bitterness, sweet taste, and low acidity. However, few published scientific studies performed cold brew chemical and flavour analysis, which emphasises a lack of information to support the claims of sellers and enthusiasts. Additionally, the effect of the extraction variables on the flavour and chemical characteristics remains unclear. Recent studies found significant differences in the concentrations of caffeine, chlorogenic acid, and trigonelline, and in the flavour profile between different cold extraction methods (dripping vs. stepping) and times (3,6,9, and 18 hr) (Kim & Kim, 2014). Fuller & Rao analysed the extraction kinetics and equilibrium concentrations of caffeine and 3-chlorogenic acid (3-CGA) in cold brew coffee and hot French press methods under different roast degrees and grinding conditions. Higher 3CGA and caffeine concentrations were observed in cold brew coffee made with mediumroasted beans, reaching equilibrium at between 6 and 7 hr of extraction. 3-CGA concentrations and pH were comparable between the cold and hot brews (Fuller & Rao, 2017). However, Lane and co-workers found that the caffeine concentration in cold brew coffee did not differ from that in hot coffee (Lane, Palmer, Christie, Ehlting, & Le, (2017). Recently, Angeloni, Guerrini, Masella, Innocenti, et al., (2019) measured the caffeine and cinnamic acid concentrations, as well as certain physicochemical and sensory characteristics in cold brew coffees that were prepared using two cold brew methods. They reported that extraction at higher temperature (22 °C) increased the total solids content, and caffeine, caffeoylquinic acid, and 5-chlorogenic acid concentrations compared to

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extraction at 5°C. Similarly, they observed differe nces in flavour profiles between cold brew methods and temperatures, as measured in terms of bitterness, sweetness, sourness, and global intensity (Angeloni, Guerrini, Masella, Innocenti, et al., 2019). Due to recent investigations indicating the potential health benefits of coffee intake, some studies on cold brew coffee have evaluated bioactive compounds. The polysaccharides isolated from cold brew have been shown to improve macrophage function and the intestinal immune system (Shin, 2017). Furthermore, analysis of the acidity and antioxidant activity of cold brew coffee among light-roast coffees from different coffee-producing countries indicates that cold brew extracts had lower acidic compound concentrations than hot brew coffee extracts prepared from the same beans. However, hot brew coffees had higher antioxidant levels than their cold brew counterparts (Rao & Fuller, 2018). Due to worldwide consumption, filtered coffee and EC are the most studied. In the coffee industry, the new trend is to develop and use novel coffee brewing methods. However, many new methods lack detailed analysis, creating gaps that drive misinformation regarding coffee extraction and the lack of process reproducibility. 2. Physicochemical characteristics of brewed coffee Coffee extraction is an essential process that determines the coffee brew characteristics. During the process, water-soluble components including chlorogenic acids, caffeine, nicotinic acid, soluble melanoidins, and volatile hydrophilic compounds are extracted (Farah, 2012). Although the lipid fraction is not water-soluble, part of it reaches the coffee beverage when high temperature and pressure are used. The polyphasic nature of the beverages and the non-volatile and volatile compound concentration determine the physicochemical and sensory properties of different coffee beverages (Illy & Viani, 2005). 2.1 Total solids and extraction yield. Total solids are the material present in coffee beverages and their overall concentration is often perceived as ‘strength’ by consumers. The total dissolved solid (TDS) ratio is the proportion of dissolved material mass in the beverage to the total beverage mass. Extraction yield (EY) is the ratio of the mass of extracted coffee solubles to the mass of the coffee grains used. EY and TDS are often reported as percentages. The chemical composition of total coffee solids can vary depending on green coffee quality, roasting process, and brewing method (Petracco, 2001). Because of varying aqueous solubility, the chemical compounds in roasted coffee are extracted at different rates. Therefore, EY will determine the flavour properties of the brew (Wang et al., 2016). According to Brewing Control Charts of the Specialty Coffee Association (SCA) used to balance the coffee brew flavour profile, the EY should be 18-22%, with 0.79-1.38% TDS. This classic chart was developed in the 1950s by Ernest Lockhart at the Coffee Brewing Institute and shows the relationship between EY and TDS percentages at a given brewing ratio. The chart displays various acceptability and rejection zones for each parameter (Lingle, 2011). Even though the coffee brewing chart remains widely used, its applicability is rapidly decreasing because of new coffee brewing methods. Thus, it is necessary to consider additional extraction parameters. New tools to measure the relationship between extraction parameters and the physicochemical and attributes of coffee flavour are necessary. These tools will help satisfy the coffee consumer, who is looking for new sensory experiences via a unique coffee drink. 2.2 pH and total acidity. Measurements of pH quantify the aqueous hydrogen ion concentration and provide a measure of deprotonated acid molecule levels in a sample. Total titratable acidity is a

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measure of all acidic protons in a sample, including non-dissociated protons that can be neutralised by the addition of a strong base (Rao & Fuller, 2018). Total acidity and pH have been widely used to characterise coffee beverages by perceived acidity. Usually, the perceived acidity is primarily attributed to several carboxylic acids, including acetic, malic, formic, lactic, chlorogenic, and quinic acids (Farah, 2012; Gloess et al., 2013; Santos, Lopo, Rangel, & Lopes, 2016). Furthermore, some melanoidins contain incorporated chlorogenic acids that may contribute to acidity perception. Some studies successfully correlated the titratable acidity with sourness intensity (Fuse, Kusu, & Takamura, 1997). Conversely, other studies report no correlation between pH, titratable acidity, and perceived acidity (Andueza, Vila, Paz de Peña, & Cid, 2007; Gloess et al., 2013). There have been many efforts made to understand the relationship between pH, titratable acidity, and perceived acidity. However, the lack of agreement on coffee brew acidity relationships may be explained by the fact that changes in coffee brew acidity are slight, and the most common methods to measure acidity (alkali titration) cannot detect small changes (Gloess et al., 2013). Regarding acidity perception, the interaction between the acids and the taste buds is not entirely understood (Petracco, 2001). The perceived acidity may overlap with other sensory attributes. Further, some environmental factors can influence its perception. For instance, the temperature may affect the perception of acidity. In general, acidity is more pronounced at lower coffee brew temperature (Gloess et al., 2013) 2.3 Non-volatile compounds in coffee beverages. Overall, coffee brews mainly comprise carbohydrates, chlorogenic acids, lipids, organic acids, melanoidins, minerals, and nitrogen compounds, such as caffeine and trigonelline (Ludwig, Clifford, Lean, Ashihara, & Crozier, 2014). Lipid concentration in non-filtered coffee brews (i.e., espresso) is higher than in brews prepared using filter devices. Indeed, the filter paper retains many lipophilic molecules (Moeenfard, Silva, Borges, Santos, & Alves, 2015; Rendón, dos Santos Scholz, & Bragagnolo, 2017, 2018). Lipids play an essential role in coffee brew flavour since they form emulsions that retain aromatic compounds. They further improve the texture perception in coffee. Galactomannans and arabinogalactans are the major polysaccharides in coffee brew (Moreira et al., 2011). Regarding coffee beverage flavour, these polysaccharides bind the aroma (Clifford, 1985), stabilise foam (Nunes et al., 1997), and increase extract viscosity (Arya & Rao, 2007). Acetic, formic, malic, lactic, phosphoric, quinic, and chlorogenic acids are frequently quantified in coffee brew. Despite their thermolability, chlorogenic acids are present in relatively high amounts in coffee and are the principal components responsible for the functional properties of coffee, such as antioxidant capacity, anti-inflammatory effects, anticarcinogenic, and antimutagenic properties (Farah, 2012; Ludwig et al., 2014). These acids have been associated with the astringency, bitterness, and acidity of the beverage. Specifically, those present in high amounts, such as caffeoylquinic and feruloylquinic acids, have been found to be related to undesirable flavour. Likewise, chlorogenic acids are phenol and catechol precursors that arise during the roasting process and can form unpleasant sensory notes in the brew (Angeloni, Guerrini, Masella, Innocenti, et al., 2019; Farah, 2012; Sunarharum, Williams, & Smyth, 2014) Regarding nitrogen compounds, the most abundant in the coffee brew is caffeine and trigonelline. Their concentration in the coffee brews varies depending on the extraction process conditions and the volume consumed. Analytical and sensory studies have observed that the alkaloids caffeine and trigonelline represent a maximum of 10-30% and 1% of the total bitter taste intensity of a coffee beverage, respectively (Ginz & Engelhardt, 2000). Furthermore, additional compounds including, phenylindanes (Frank, Blumberg, Kunert, Zehentbauer, & Hofmann, 2007), as well as certain furan-2-yl methylated benzene

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diols, and triols (Kreppenhofer, Frank, & Hofmann, 2011) have also been reported to contribute to the bitter and astringent taste of coffee. Coffee brews are considered one of the main sources of melanoidins in the human diet (Ludwig et al. 2014). Therefore, they are widely studied for biological activities such as antioxidants, metal chelation, enzymatic modulation, antimicrobial, and as dietary fibre sources (Bekedam, Loots, Schols, Van Boekel, & Smit, 2008; Ludwig et al., 2014; RufiánHenares & Pastoriza, 2015). In addition to their colour contribution, melanoidins modulate flavour release, due to the presence of varied functional groups. Melanoidins may also covalently bind with odorants compounds (Hofmann, Czerny, Calligaris, & Schieberle, 2001). Due to the extreme complexity in their formation and chemical composition, these molecules are not fully understood (Bekedam et al., 2008). 2.4 Volatile compounds in coffee beverages Aroma is a crucial attribute that defines the quality and consumer acceptance of coffee products (Lee, Cheong, Curran, Yu, & Liu, 2015). More than 1000 volatile compounds are found in roasted coffee. However, several studies suggest that only approximately 30-50 of these compounds are responsible for coffee aroma (Cantergiani et al., 2001; Czerny & Grosch, 2000; Sanz, Maeztu, Zapelena, Bello, & Cid, 2002). The identification of the numerous volatile compounds in coffee highlights that coffee aroma has been widely studied. However, the complex, key odour compounds are not wholly elucidated. Although it is well-known that coffee brewing affects coffee flavour, few studies have evaluated the key aroma compounds in coffee brews. Recently, Toci & Boldrin, detailed the key odour compounds studied in filtered and EC beverages. They also determined that there is scarce information for other types of coffee brewing (Toci & Boldrin, 2018). In this review, extensive search of the Scopus scientific database was performed, using the target words "espresso," "French press," "plunger," "cold brew," "moka pot," Turkish," "coffee," and "volatile compounds." The search identified few studies pertaining to the Moka, French press, Turkish, and cold brew coffee methods. However, the analysis identified 117 volatile compounds across all evaluated coffee brew methods. Fig. 4 shows the number of volatile compounds reported in the coffee brews. Overall, most compounds reported are pyrazines (28), furans (23), aldehydes (13), ketones (14), and pyrroles (10), which represent more than 70% of the volatile compounds in coffee beverages (Fig. 4a). Although these numerous compounds are not directly associated with perceived coffee flavour, they have been found to influence the odour of a particular food when the concentration is higher than the odour threshold, often by additive or synergistic effects (Jelen, 2012). Likewise, the volatile compounds present in the final coffee brew depend on the extraction technique. Fig. 4b shows that a higher number of pyrazines have been reported in the coffee brews made by Turkish, espresso, and filter coffee methods. Furan compounds have been reported less in coffees prepared using the Moka method, and certain compounds, such as thiols, which more strongly influence coffee flavour, have not been reported in the Turkish and French press methods. Finally, other sulphur compounds have only been reported in Turkish and filtered coffee. However, some volatile compounds including thiols are challenging to measure as they are unstable due to chemical interactions with other coffee components in an aqueous solution (Quintanilla-Casas, Dulsat-Serra, Cortés-Francisco, Caixach, & Vichi, 2015), hence, their identification can be limited by the specific analytic techniques employed for quantification. The main techniques used for VOCs identification are gas chromatography-mass spectrometry and/or olfactometry (GC–MS, GC-O) and gas chromatography (GC) coupled to flame ionisation detection (FID). Additionally, various forms of headspace sampling

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methods, mainly solid-phase microextraction (SPME), have been used (Bressanello et al., 2017; Kim, Ko, Kang, & Park, 2018). Recently, studies of volatile coffee compounds have focused on direct monitoring or on-line VOCs formation. Proton transfer reaction time-offlight mass spectrometry (PTR-ToF-MS) has been used to determine the VOCs extraction kinetics in espresso brewing. Some studies show that this type of methodology is a fast and straightforward tool to study VOCs extraction dynamics (Sánchez-López et al., 2014). The application of this novel technique in espresso brewing also shows that the aroma compound extraction profiles change with different brewing parameters. In espresso brewing methods, increased pressure and temperature result in higher VOCs extraction, mainly affecting the aroma balance during the last extraction stage (Sánchez-López, Wellinger, Gloess, Zimmermann, & Yeretzian, 2016). With the use of new instrumental techniques, it is now possible to perform relevant chemical compound kinetic studies under different extraction conditions. This knowledge can lead to understanding the release kinetics and compound degradation related to coffee flavour, allowing the development and optimisation of new processes and products. 3 Coffee extraction parameters and their effect on coffee flavour quality. During coffee extraction, the process parameters are interdependent. It is, therefore, difficult to adjust any single factor without subsequently impacting others. For instance, in espresso, changing the grind size distribution will change brewing pressure, thus affecting flow rate and contact time. An overview of the specific influence of coffee extraction parameters on the chemical and flavour characteristics of brewed coffee is presented in Table 1. The data shows that the process parameters have been more studied in traditional coffee brewing methods such as espresso coffee, while other methods have received less attention. The following sections provide more detail regarding the role and relevance of each parameter on the physicochemical characteristics and coffee flavour. 3.1 Extraction time The interaction time between water and roasted coffee grounds (extraction time) is a key factor for compound extraction related to the coffee flavour and quality. The extraction efficiency describes the quantity of a specified component released from the coffee matrix in a given time. Sugars, organic acids, and caffeine are extracted efficiently within the first seconds of preparation, rapidly reaching >90% extraction yield (Severini, Ricci, Marone, Derossi, & De Pilli, 2015). In contrast, less soluble compounds are only become extracted after a long time or in the presence of larger water volume. Among these compounds, several bitter or astringent tastants may be present (Mestdagh et al., 2017). In espresso brewing, more than 70% of the antioxidants (save for dicaffeoylquinic acids, (diCQA)) are extracted during the first 8 sec. Though extraction begins later in filter coffee brewing (after 75 sec), extraction efficiency is higher, mainly for less polar antioxidant compounds such as diCQA (Ludwig et al., 2012). In filter methods, coffee grounds rapidly release soluble material during the first 2 min of brewing. The soluble yield during this period is approximately 18-22%, which represents 65-75% of the available flavour material. After 5 min of brewing, the water removes more than 80% of the available soluble material (Lingle, 2011). Thus, lower EY (under-extracted coffee) leads to a sweet-acidic cup profile dominated by highly water-soluble compounds such as sugars and acids (Mestdagh et al., 2017), while longer brewing time may cause higher extraction of certain compounds that are highly susceptible to oxidation and degradation (Petracco, 2001), thereby affecting the sensory profile. Alternatively, in the cold extraction where long contact times are required due to the low temperature used in the process (room temperature), 3-CGA and caffeine concentration have been described

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as rapidly increasing over the initial 180 min and reaches equilibrium by approximately 400 min (Fuller & Rao, 2017). Regarding VOCs, studies have determined that certain compounds, such as furans, can be extracted in different amounts depending on the coffee brewing method (Rahn & Yeretzian, 2019). These compounds can be reduced during the brewing time by 35% after 1 min of extraction, and more than 60% after 5 min using water near its boiling point (La Pera et al., 2009). While solubility and volatility of relevant chemical compounds during extraction are important, compound polarity is also crucial. Some studies show that compound polarity is one of the main drivers of extraction kinetics. Highly polar compounds are extracted at the beginning of extraction and less polar compounds at the end of extraction (Sánchez-López et al., 2016; Mestdagh et al., 2014). Certain polar compounds, such as 2,3-butanedione, can be extracted more rapidly than compounds with moderate polarities, such as damascenone (Mestdagh et al., 2014). In the same way, brewing time influences the hexanal and methanethiol concentration in coffee brews (Caporaso et al., 2014). Methanethiol gives intense cabbage-like, cheese, garlic, and sulphur odour, while hexanal is responsible for a green note (Flament, 2002). During espresso brewing, maximum VOCs intensity occurs between 2 to 24 s, although 95% are extracted in less than 10 sec (Sánchez-López et al., 2014). Given the differences in the extraction kinetics of coffee chemical compounds, the contact time between coffee and water is critical to obtain coffee brews with a particular reproducible flavour profile. 3.2 Water temperature. In the coffee extraction process, temperature is the driving force favouring the extraction of chemical compounds present in coffee grounds. At higher temperatures, the kinetic energy of water molecules is higher (Mestdagh et al., 2017). Increased mobility amplifies the possibility of leaching out compounds from the coffee bed due to higher physical forces (Mestdagh et al., 2017). The temperature increases also favours the solubility of many of these compounds. However, high temperatures can also cause VOCs release, which can affect the sensory perception of the coffee brew. The difference in the chemical compound concentration in coffee beverages often occurs when these compounds exhibit different solubility, which generally increases with temperature (Mestdagh et al., 2017). Low temperatures produce beverages with the lowest concentration, low extraction percentage, and total solids (Angeloni, Guerrini, Masella, Innocenti, et al., 2019; Parenti et al., 2014). Hence, the total solids and caffeine content are significantly higher when water at 110°C is used (Albanese et al., 2009). A temperature increase from 88 to 98 °C is favourable for lipid extraction (Moeenfard et al., 2015). Studies have shown that using increasing gradient temperature profiles (88 to 93 °C) resulted in increased caffeine, a few acidic co mpounds, and chlorogenic acid extraction. Coffee brews produced under these conditions are characterised by balanced astringency and bitterness, good crema colour, well-balanced aroma intensity, body, and flavour. Decreasing gradient temperature profiles (93 to 88°C) decrease 5-caffeoylquinic acid (5-CQA) (Salamanca, Fiol, González, Saez, & Villaescusa, 2017). Temperature also affects the saturated vapour pressure of aromatic compounds. Higher temperatures lead to greater VOCs evaporation, and consequently, a higher release of volatile compounds (Sánchez-López et al., 2016). A higher concentration of some volatile compounds, such as guaiacol and pyrazines are present in coffee beverages made using water at a temperature ≥96 °C (Andueza, Maeztu, et al., 2003; Flament, 2002 ). These compounds are associated with smoky, nutty, hazelnut-like, and roasty sensory notes

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(Caporaso et al., 2014). Conversely, a significant reduction in furan compounds is observed when high temperatures (near at 100ºC) are used (La Pera et al., 2009). Currently, temperature variation has driven the development of new methods of coffee brewing. Some methods are carried out at temperatures lower than room temperature (less than 25 °C). The low temperature facilitates the extraction of high-polarity compounds. However, other compounds need more contact time to reach the coffee brew. Thus, under these temperature conditions, more extraction time is required, usually between 6 to 24 hrs. Since temperature often significantly influences aqueous compound solubility, differences in the chemical compositions between hot- and cold brew coffees have been described (Fuller & Rao, 2017). Hot brews contain higher concentrations of total titratable acids and higher antioxidant activity than cold brew coffees, suggesting that hot brewing methods tend to extract more non-deprotonated acids than cold brew methods (Rao & Fuller, 2018). Furthermore, cold brew coffees have distinct chemical differences when prepared at 25 °C and 4 °C. A higher concentration of total solids, caffeine, total caffeoylquinic acids (CQAs), and 5-CQA concentrations are present in cold brews prepared at 25 °C compared to those brewed at 4 °C (Angeloni, Guerrini, Masella, Innocenti, et al ., 2019). Furthermore, coffees brewed at 22 °C are described by intense sweetness, fruity an d floral flavours, medium bitterness and acidity, and a creamy body. Odour-active compounds such as furans, pyrazines, ketones, aldehydes, pyrroles, esters, lactones, furanones, and phenols have also been identified in these coffee brews (Cordoba et al., 2019). 3.3 Pressure Pressure is a process parameter required in the most well-liked coffee brewing methods as the espresso coffee, where a pressure gradient is needed to lead hot water (90 ± 5 °C) through a coffee packed bed, to extract its soluble material (Corrochano et al., 2015). A pressure field within a fluid generates potential energy that can be easily transformed into kinetic energy, thus giving speed to elementary masses of fluids. The energy expended during this operation produces interesting effects, such as driving micron-size solid particles or oil droplets into the cup, which may dramatically change the beverage properties and enhance the sensory character (Petracco, 2001). An advantage of pressure is that aromatic compounds cannot easily evaporate from the coffee bed when pressure is applied. Instead, these compounds end up in-cup to a higher extent, compared to nonpressurised or lower pressure extraction methods (Sánchez-López et al., 2016). Pressure is also required to foam or crema formation. When pressure is applied, CO2 present in ground coffee is forced into the water phase. As dissolved CO2 is slowly released, some solids follow and form a dense and stable crema on top of the beverage. Brewing methods lacking pressure cannot form this cream or foam (Mestdagh et al., 2017), which makes a difference upon consumption. In the espresso brewing method, a pressure range of 7-9 bar and a temperature of 92 ºC is ideal for the most efficient extraction of some bioactive compounds such as caffeine, trigonelline, and nicotinic acid (Caprioli et al., 2014). Coffee beverages brewed using 9 bar pressure, have been described as having high foam consistency and aroma intensity, with few negative flavour notes (Andueza, Maeztu, Dean, & Cid, 2002; Caprioli et al., 2012). Increased pressure (up to 11 bar) can increase the viscosity, body intensity, and odour intensity. However, these coffee brews have lower consumer acceptance (Andueza et al., 2002). Other studies show that 11 bar of pressure increases VOCs extraction over the entire extraction time (25 s), with significantly higher intensities during the last 10 sec of

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extraction (Sánchez-López et al., 2016). Pressure effects on certain coffee biomolecules were also studied. Higher pressures (e.g., 12 bar) extract more total lipids (Parenti et al., 2014). Moreover, increasing the pressure from 7 to 11 bar serves to increase proportionally the diterpene levels, however, pressure up to 14 bar diminishes these molecules (Moeenfard et al., 2015). 3.4 Coffee/water ratio in the brewing The coffee/water ratio is the relationship between the mass of coffee grains to the mass of water used in the coffee brewing. However, it is often reported as coffee mass per water volume. Its importance goes beyond the ingredient amounts used in the process. This parameter is associated with phenomena that govern the processes between coffee and water. For example, the packing bed and its porosity determines water flow and mass transfer during extraction. So far, few studies have examined the direct effect of the coffee/water ratio on chemical and sensory characteristics of the coffee brew. Some studies performed in espresso brewing show that using a higher coffee/water ratio improves the extraction of caffeine, chlorogenic acid, and compounds related to bitterness and astringency (Andueza et al., 2007). Using 6.5-9.5 g of coffee in espresso brewing is related to increased diterpene concentration (31.92-42.53 mg/L, respectively) (Moeenfard et al., 2015). In other studies, using single-serve coffee machines demonstrated that increasing the coffee quantity also increases the ratio between the titratable acidity/total polyphenol concentration (TA/TPP), without affecting the EY. Indeed, using more water during brewing decreases the TA/TPP concentration ratio and changes the coffee flavour (Wang et al., 2016). Overall, an incorrect coffee/water ratio will result in beverages with underdeveloped flavour (too much coffee or too little water) or over-extracted beverages with bitter or weak flavour (too little coffee or too much water) (Lingle, 2011). The amount of coffee added to a brew will determine the physical characteristics of the packing bed. Several studies show that an excessive amount of coffee does not allow sufficient expansion of coffee grounds during wetting, which causes over-compaction, disrupted percolation, and solid deposition in the cup (Petracco, 2005). A high amount of coffee also decreases bed permeability. An extremely low bed permeability can result in excessive extraction pressures, low flow rates, and extended extraction times, possibly leading to over-extraction (Corrochano et al., 2015). In a fixed pressure gradient, permeability determines the flow rate across the bed, brewing time, and water residence time. Thus, the influence of permeability on mass transfer must be considered when analysing brewing processes and formulation parameters (coffee/water ratio) on coffee brew quality (Corrochano et al., 2015). 3.5 Particle size The particle size of the roast and coffee grounds determines the extraction rates of several chemical compounds, which define the final flavour of the coffee beverage. Grinding increases the extraction surface area, thereby increasing the interface between water and coffee. This condition promotes the transfer of soluble and emulsifiable materials into the brew. At the same time, the breaking of coffee bean tissue cells stimulates the release of CO2 and VOCs (Andueza, Paz De Peña, et al., 2003). The grinding operation is used to break down the roasted coffee into particles or smaller fragments. This process is influenced by factors such as the variability of coffee beans, moisture (Baggenstoss, Perren, & Escher, 2008), and the roasting degree (Andueza, Paz De Peña, et al., 2003; Lingle, 2011). During the grinding process, different particle sizes and shapes can be produced in the same sample. The range of particle sizes is

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characterised by the particle size distribution (PSD) (Von Blittersdorff & Klatt, 2017). Moisture content and the way the coffee is roasted influences the PSD. The porosity and brittleness of coffee beans increase with roasting degree. Particularly when coffee is roasted for a short time (Von Blittersdorff & Klatt, 2017), the coffee beans are more finely broken down during grinding due to the loss of cell-wall elasticity and the increase in brittleness (Illy & Viani, 2005). However, when coffee beans are roasted slowly, they form a matrix with homogeneous pore sizes and higher density. Though these coffees require more grinding energy, the PSD can be more homogeneous (Von Blittersdorff & Klatt, 2017). During grinding, high water content in the roasted beans leads to larger particles, whereas low water content results in PSDs with a smaller mean particle size (Baggenstoss, Perren, et al., 2008). Computer simulations of espresso brewing show that coarse particles result in large channels and low tamping, which allows high flow velocities of extraction water and low extraction yields (Cappuccio & Liverani, 1999). Conversely, if the particle size is too small, the filters can clog, and over-extraction can occur from increased exposure time (Baggenstoss, Perren, et al., 2008). Likewise, small, irregular particles release their soluble substances faster and lead to a more intense, more concentrated coffee in the cup (Von Blittersdorff & Klatt, 2017). In coffee brewing, the shape and particle size also affect the wettability degree, which affects extraction rates and the diffusion of chemical compounds into the cup. Little information is available on coffee grinding dynamics; however, some authors have indicated the caffeine diffusivity is much higher in pre-swollen coffee beans (wet coffee grounds) (Spiro et al., 1989). During coffee brewing, water reaches and fills coffee cell cavities, increasing coffee particle volume up to 20-23% in 10-15 min after wetting. Given these phenomena, studies suggest that the particle diameter in the coffee extraction process cannot be considered constant but is variable as a function of time. However, only a few studies have investigated the effects of water on the physical properties of coffee and water absorption kinetics into roasted coffee particles (Mateus, Rouvet, Gumy, & Liardon, 2007). Some conclusions can be extracted from existing studies on the PSD in the coffee extraction process. Nevertheless, the relationship between particle size and shape and the physical and hydrodynamic processes during the extraction process in different methods of coffee preparation is understudied. Furthermore, not much information is available on coffee particle size changes during the process (shape and wettability) and its effect on the flavour related compounds extraction. Some studies conclude that the presence of the pores between the grains (intergranular) and pores within the grains (intragranular) control extraction efficiency in two main stages–a rapid dissolution of coffee from the particle surface, followed by a much slower diffusion of coffee through the intragranular pore network to the particle surface (Corrochano et al., 2015; Moroney, Lee, O’Brien, Suijver, & Marra, 2017; Moroney et al., 2015; Moroney, Lee, Brien, Suijver, & Marra, 2016) 3.6 Water quality. After the roasted coffee, water is the second essential ingredient for coffee brewing, and its ionic content is crucial in coffee brewing. The flavour compounds in coffee beans exist as aprotic, charge-neutral species, and as a collection of acids and conjugate salts. Therefore, the dissolved mineral content in the water affects the dissolution and extraction of these organic molecules. Controlling Na+, Mg2+, and Ca2+ levels in the extraction by water enables extraction of the compounds present in the roasted and grind coffee at

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different rates (Hendon, Colonna-Dashwood, & Colonna-Dashwood, 2014). Despite the importance of water composition in the extraction process, few scientific papers have focused on this aspect. Specific studies were carried out before the 1980s, wherein the effect of water in the sensory quality of coffee beverage was evaluated (Lockhart, Tucker, & Merritt, 1955; Pangborn & Trabue, 1971). These studies determined that carbonates produce a bitter and flat coffee, whereas distilled water can produce coffee brews with excessive sourness. Likewise, carbonates and bicarbonates with excessive Na+ affect brewing time, both having a retarding effect in direct proportion to their concentration. In addition, water hardness can indirectly affect beverage quality by reducing the effectiveness of heat transfer, thereby influencing the extraction temperature (Navarini & Rivetti, 2010). More recent studies on espresso brewing demonstrate that the bicarbonate ions naturally present in the water represent an extra CO2 source available for foaming (Navarini & Rivetti, 2010). Mechanisms and mineral composition of water in the coffee extraction have also been studied using computational chemistry. This theoretical approach was applied to study the binding energies of five coffee acids (lactic acid, malic acid, citric acid, quinic acid, and chlorogenic acid), caffeine, and a representative flavour component eugenol. The authors concluded that Mg2+-rich water is the most suitable to extract the most coffee constituents (i.e., instant coffee). However, to achieve the best flavour balance for a given light-roast coffee, both Ca2+ and Mg2+ are necessary for the water composition. Notably, there is no one particular water component that produces consistently flavourful extracts from all roasted coffees. Instead, there is water with the best extracting ability (i.e., cationrich), and the resultant flavour depends on the balance between the cations in solution and the quantity of bicarbonate present (acting as a buffer)(Hendon et al., 2014). For many years, water was considered an essential ingredient in the extraction of coffee components and brew characteristics. However, literature shows that water composition is more reported in studies related to espresso brewing than other methods. In many studies on the coffee brewing, water properties are still neglected. Nevertheless, a recent water quality handbook published by the SCA (A systematic guide to water fundamentals) (Specialty Coffee Association (SCA), 2018) may prove useful in future studies to better understand the role that water plays in various coffee extraction processes. 3.7 Physical and transport phenomena in the coffee extraction process Currently, there are no models showing a direct correlation between extraction parameters and the flavour or sensory profile in brewed coffee. Studies have proposed two model categories–those that model the flow through a porous packing bed, and those that model the mass transfer using a specific compound (usually caffeine). Overall, these models describe espresso methods. Recently, some studies have proposed extraction kinetics models that can be applied to filtered methods. Regarding specific coffee brewing methods, in the early 1960s, the first efforts to develop a mathematical model of the espresso brewing process were reported, when the key role of the pressure drop from hydraulic resistance of the ground coffee cake was first recognised. In this model, the extraction process (percolation) was assumed to be hydraulically defined by an equation with five variables (Equation 5), which were considered adequate to characterise the process in its macroscopic physical aspect (Petracco, 2005):

14

(

! = ")

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∆$() * %(),'()

where: V= Volume of beverage in the cup T= Temperature inside the coffee cake τ = Percolation time ∆p = Pressure difference between the top and bottom of the cake R = Hydraulic resistance of the cake. This approximation uses the integral of Darcy’s law, where “The loss of pressure in a pipe is proportional to the flow rate of the liquid through it” (∆p=RQ, where Q is the flow and R is the hydraulic resistance of the cake) (Petracco, 2005). During espresso brewing, the pressure and resistance R (Eq. 5) depend on time, either through various bed structure changes or temperature changes. However, in this model, other additional variables that could affect the hydraulic resistance R, such as the amount and particle size of ground coffee, and bed compaction, are not included. Thus, these general and theoretical models do not fully explain the extraction process phenomena. They only include flow through the porous bed, while neglecting other fundamental phenomena, such as mass transfer. Recently, several studies have addressed new approaches to formulating theoretical models explaining the different phenomena in coffee extraction. Fig. 5 shows how the coffee matrix was considered in the new models and evaluates mass transfer and mechanical and chemical interactions between the flow and the porous matrix at the macro and micro scale. Multiscale semitheoretical models of coffee extraction physics consider the doubly porous nature of the coffee bed, where the system is represented by intergranular pores (h-phase), intragranular pores (v-phase), and coffee solids (s-phase) (Moroney et al., 2017; Moroney et al., 2015). The general coffee extraction model at the macro scale consists of conserved equations for coffee and liquid in the three phases. The models describe the advection (transport of a substance by bulk motion) of coffee in the fluid flow. Darcy’s Law and the Kozeny-Carman equation are used to explain the flow rate at which solutes are carried through the different phases of the coffee matrix. The models explain ground coffee extraction in two situations, in a well-stirred, dilute suspension of coffee grains and in a packed coffee bed. Although the models use mass transfer diffusion coefficients, the diffusion coefficient for caffeine in water is used to resolve the models. However, this does not reflect the effective diffusivity of all soluble compounds from coffee to the system (Moroney et al., 2016). Overall, models and studies of the coffee extraction phenomena have established that the coffee bed properties are relevant to mass transfer. During coffee extraction, bed permeability is dynamic. The initial water invasion induces wetting of the coffee bed, followed by the solubilisation of soluble, low molecular weight compounds and VOCs. Simultaneously, coffee bed particles swell because of water-insoluble polysaccharides present in roasted coffee, which geometrically rearrange owing to the water flow and/or pressure (Mestdagh et al., 2017). Thus, a low bed permeability can lead to high extraction pressures, low overall flow rates, and extended extraction times and over-extraction (Corrochano et al., 2015). 4

Conclusions and future trends

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Even though the coffee industry is continuously developing new methods, there are evident gaps in this field. Many of the extraction variables have been widely evaluated in traditional coffee preparation methods, such as espresso coffee, whereas the study of the other extraction methods has been neglected. Regarding flavour, there is much information on the aroma and correlation with VOCs. On the other hand, some non-volatile compounds, such as caffeine, trigonelline, chlorogenic acids, and melanoidins, have been related to coffee brew bitterness and astringency. Lipids and some polysaccharides have been associated with the textural properties (body and aftertaste). No clear relationship has been described between the titratable and perceived acidity of coffee, nor between chemical compounds and coffee brew sweetness. The coffee extraction process is an extremely complex operation because of several variables that have a direct effect on the chemical composition and sensory profile of the brew. Modifying variables during the extraction process has led to the development of innovative procedures in the coffee industry. Most of these new techniques are a variation of the basic extraction categories (i.e., espresso and filtered). Changes to some process variables produce beverages with novel sensory profiles. Although the extraction process is a solid-liquid operation, different coffee extraction studies rarely include interpretation of the physical, hydrodynamic, and phenomenological changes produced by the different coffee brewing methods. New studies have recently emerged that apply mathematical modelling to better understand phenomena associated with the coffee extraction process. Although it is widely believed that the coffee extraction process has been thoroughly studied, most studies have only evaluated some physicochemical characteristics of beverages brewed using the filtered and espresso extraction techniques. There is a limited body of knowledge in current coffee brewing research showing the extraction kinetics of the diverse compounds present in roasted and ground coffee. Understanding these phenomena is of paramount importance for the standardisation of brewing methods that are highly reproducible at the chemical and sensory level. Recently, analysis of extraction kinetics for chemical compounds has gained relevance. New analytical tools have been applied to generate better understanding of brewing mechanisms associated with the kinetics of diverse VOCs during the coffee extraction process. These emerging analytical tools attempt to elucidate the associated phenomena in real-time, generate more accurate results, and reduce cost and analysis time. Despite the complexity of the coffee extraction process, studies of physical, hydrodynamic, and mass/heat transfer phenomena could provide valuable insights on the connection between extraction/brewing and the chemical composition that impacts the sensory profile of the beverage. Further research will create tools and new applied knowledge for the development of reproducible, cost-effective, and high-quality beverages that maximise consumer value and preferences. Acknowledgements The authors thank the Universidad de La Sabana and Coffeelands Program of the Catholic Relief Services (CRS) for their support and funding in this investigation through the ING180-2016 project. Furthermore, Nancy Cordoba acknowledges COLCIENCIAS for the doctoral scholarship (Grant number 727-2015). Declaration of interests. The authors have no interests to declare. References Albanese, D., Di Matteo, M., Poiana, M., & Spagnamusso, S. (2009). Espresso coffee (EC) by POD: Study of thermal profile during extraction process and influence of water temperature on chemical-physical and sensorial properties. Food Research

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International, 42(5–6), 727–732. https://doi.org/10.1016/j.foodres.2009.02.027 Andueza, S., Maeztu, L., Dean, B., & Cid, C. (2002). Influence of Water Pressure on the Final Quality of Arabica Espresso Coffee. Application of Multivariate Analysis. Journal of Agricultural and Food Chemistry, 50, 7426–7431. Andueza, S., Maeztu, L., Lucía, P., Ibañez, C., Paz de Peña, M., & Concepción, C. (2003). Influence of extraction temperature on the final quality of espresso coffee. Journal of the Science of Food and Agriculture, 83, 240–248. https://doi.org/10.1002/jsfa.2720 Andueza, S., Paz De Peña, M., & Cid, C. (2003). Chemical and Sensorial Characteristics of Espresso Coffee As Affected by Grinding and Torrefacto Roast. Journal of Agricultural and Food Chemistry, 51(24), 7034–7039. https://doi.org/10.1021/jf034628f Andueza, S., Vila, M. A., Paz de Peña, M., & Cid, C. (2007). Influence of coffee/water ratio on the final quality of espresso coffee. Journal of the Science of Food and Agriculture, 87, 586–592. Angeloni, G., Guerrini, L., Masella, P., Bellumori, M., Daluiso, S., Parenti, A., & Innocenti, M. (2019). What kind of coffee do you drink? An investigation on effects of eight different extraction methods. Food Research International, 116, 1327–1335. https://doi.org/10.1016/j.foodres.2018.10.022 Angeloni, G., Guerrini, L., Masella, P., Innocenti, M., Bellumori, M., & Parenti, A. (2019). Characterization and comparison of cold brew and cold drip coffee extraction methods. Journal of the Science of Food and Agriculture, 99(1), 391–399. https://doi.org/10.1002/jsfa.9200 Arya, M., & Rao, L. J. M. (2007). An impression of coffee carbohydrates. Critical Reviews in Food Science and Nutrition, 47(1), 51–67. https://doi.org/10.1080/10408390600550315 Baggenstoss, J., Perren, R., & Escher, F. (2008). Water content of roasted coffee: Impact on grinding behaviour, extraction, and aroma retention. European Food Research and Technology, 227(5), 1357–1365. https://doi.org/10.1007/s00217-008-0852-8 Baggenstoss, J., Poisson, L., Kaegi, R., Perren, R., & Escher, F. (2008). Coffee roasting and aroma formation: Application of different time-temperature conditions. Journal of Agricultural and Food Chemistry, 56(14), 5836–5846. https://doi.org/10.1021/jf800327j Bekedam, E. K., Loots, M. J., Schols, H. A., Van Boekel, M. A. J. S., & Smit, G. (2008). Roasting effects on formation mechanisms of coffee brew melanoidins. Journal of Agricultural and Food Chemistry, 56(16), 7138–7145. https://doi.org/10.1021/jf800999a Borém, F. M., de Oliveira, P. D., Isquierdo, E. P., Giomo, G. D. S., Saath, R., & Cardoso, R. A. (2013). Scanning electron microscopy of coffee beans subjected to different forms of processing and drying. Coffee Science, 8(2), 218–225. Bressanello, D., Liberto, E., Cordero, C., Rubiolo, P., Pellegrino, G., Ruosi, M. R., & Bicchi, C. (2017). Coffee aroma : Chemometric comparison of the chemical information provided by three different samplings combined with GC – MS to describe the sensory properties in cup. Food Chemistry, 214, 218–226. https://doi.org/10.1016/j.foodchem.2016.07.088 Cantergiani, E., Brevard, H., Krebs, Y., Feria-Morales, A., Amadò, R., & Yeretzian, C. (2001). Characterisation of the aroma of green Mexican coffee and identification of mouldy/earthy defect. European Food Research and Technology, 212(6), 648–657. https://doi.org/10.1007/s002170100305 Caporaso, N., Genovese, A., Canela, M. D., Civitella, A., & Sacchi, R. (2014). Neapolitan coffee brew chemical analysis in comparison to espresso, moka and American brews. Food Research International, 61, 152–160. Retrieved from

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Figure captions Fig. 1. Diagram of the main parameters in the coffee extraction process and their relation to the physicochemical and flavour characteristics of the coffee brewed: Raw material (green coffee beans), roasting degree along with the main process variables. Fig. 2. General technical description of the leading coffee extraction methods. Decoction: Boiled/Turkish (A) and Vacuum (B). Infusion: Filtered (C). Pressure: Plunger/ French Press (D); Stove-top coffee maker/Moka (E) and Espresso (F). Fig. 3. Number of publications (1993-2018) on the coffee extraction process using different coffee brewing methods: ( ) Cold brew; ( ) Filtered (FC); ( ) Moka pot; ( ) Espresso (EC); ( ) French Press, ( ) Turkish. Fig 4. Aromatic compounds in coffee beverages. (a) Total percentage of the classes of the volatiles compounds reported in coffee brews. (b) Number of volatile compounds by classes in each coffee brewing methods: ( ) Turkish, ( ) French Press, ( ) Espresso, ( ) Moka pot, ( ) Filtered. Fig. 5. Macro and microscale mass transfer occurring in the coffee bed during the coffee extraction process (Moroney et al., 2015).

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Table 1. Typical values of the extraction parameters and their effect on chemical compounds and flavour of the coffee brews. Main evaluated parameters

Additional parameters

Tº (ºC) ♦Tº1=88 ♦Tº2=92 ♦Tº3=96 ♦Tº4=98

♦t (s): set up 21. ♦P(bar): fixed 9. ♦C/W (g/L): 187. ♦PS: n.g (fine grinding**). ♦WQ: n.g.

P (bar)

♦t (s): set up 21. ♦Tº(ºC): fixed 92. ♦C/W (g/L):187g/L. ♦PS: fine grinding** (50% >500 µm, 21% >400 µm and <500 µm, 25% >300 µm and <400 µm; 2% <200 µm). ♦WQ: n.g. ♦t (s): Set up 2 ♦Tº (ºC): Fixed 92 ♦P(bar): Fixed 9 ♦C/W (g/L): 187 ♦WQ: n.g.

♦P1 = 7 ♦P2 = 9 ♦P3 =11

PS (µ µm) § ♦Very fine (VF)=450 ♦Fine (F)= 550 ♦Coarse (C)= 550600

C/W (g/L) ♦C/W1=162.5 ♦C/W2=187.5 ♦C/W3=212.5

♦t (s): Set up 21 ♦Tº(ºC): Fixed 92 ♦P(bar): Fixed 9 ♦PS: n.g. ♦WQ: n.g.

Chemical compounds

Flavour

Espresso brewing method ♦88ºC: ↑ 3-methylbutanal, 2methylpropanal, 2-methylbutanal. ♦92ºC: ↑trigonelline, CQA. ↓ Pyrazines. ↑Sulphur compounds, aldehydes & ketones. ♦96ºC: ↓ trigonelline & CQA. ↑ pyrazines. ♦98ºC: ↓ trigonelline & CQA. ↑ hexanal ♦7bar: ↓lipids & CQAs. ↓methanethiol acetaldehyde, propanal, 3-methylbutanal, 2,3-butanedione. ♦9bar: ↑ lipids, CQAs, ↑odour compounds. ↑Methanethiol & propanal. ♦11bar: ↓ lipids & CQAs, ↑2methylbutanal, 3-methylbutanal, 2-ethyl3,5 dimethylpyrazine.

♦88ºC: ↑ odour, flavour, body & overall acceptability. ♦92ºC: ↑ freshness, fruity, malty & buttery (positive notes), flavour & overall acceptability. ♦96ºC: ↑bitterness, burnt/roast flavour perceived. ♦98ºC: presence of woody/papery, burnt/roast & acrid (negative notes). ♦7bar: ↑acrid, straw, malty, cereal notes. ♦9bar: ↑ key odorants related to freshness, fruity, malty & buttery. ♦11bar: ↑ bitterness, astringency, odour & aftertaste intensity, notes cereal/ malty notes, burnt/roasty. ↓ overall acceptability.

(Andueza, Maeztu, et al., 2003)

♦VF: slightly over-extracted, presence of woody/papery, fermented, burnt/roasty notes. ♦F: ↑body, woody/papery, fermented, burnt/roasty notes. ♦C: ↓ development of aroma & flavour. Presence of the acrid, burnt & rubbery notes.

(Andueza, Paz De Peña, et al., 2003)

♦↑ C/W: ↑compounds related bitterness & astringency. ♦C/W1: ↑malty/cereal, ↓acidity. ♦C/W2: acidity, bitterness, astringency, aftertaste & odour intensity, malty/cereal & woody /papery notes. ♦C/W3: ↑acidity, burnt /roasty, acrid, fermented flavours & aftertaste intensity.

(Andueza et al., 2007)

♦VF: ↑trigonelline, lipids, caffeine & CQAs. ↑2-methylpropanal, 2methylbutanal, 3-methylbutanal, 2,3butanedione & 2,3-pentanedione. ♦F: trigonelline, lipids, caffeine & CQAs. ↑ 2-methylpropanal, 2 methylbutanal, 3-methylbutanal. ♦C: ↓trigonelline, lipids, caffeine & CQAs. ♦Increasing C/W: ↑ caffeine & CQAs ♦C/W1: ↓ caffeine, CQAs, & volatile sulphur compounds. ↑ ethylpyzine percentage. ♦C/W2: caffeine, CQAs & VOCs. ♦C/W3: ↑ caffeine, CQAs & guaiacol

24

Ref.

(Andueza et al., 2002)

C/W (g/L). ♦C/W1: 630; ♦C/W2: 315; ♦C/W3:157.5; ♦C/W4:78.8; ♦C/W5: 57.3; ♦C/W6:42.0. T (ºC) ♦Tº1=Updrawn gradient (88–93). ♦Tº2=Downdrawn gradient (93–88) ♦Tº3=Fixed 90 Equipment ♦ECA: settable P (bar)= 7, 9, 11 Tº (ºC)= 88, 92, 98. ♦ECB: (unsettable) P (bar)= ~2-9 Tº (ºC)= ~87-98 Equipment ♦Conventional Machine (BM) t (s)= 25-30 Tº (ºC)=92. P (bar)= 12. C/W (g/L): 290 ♦Hyper capsules (HIP): Tº (ºC):92. P (bar):12. C/W (g/L): 260 ♦I-EC SystemCapsules (IT): Tº (ºC):92. P (bar): 12. C/W (g/L): 276

♦t, Tº & PS: n.g. ♦P(bar): 20 ♦WQ: Commercial

♦t(s): 25. ♦P(bar):9. ♦C/W (g/L): 300 ♦PS (µm): 200-630§ − ♦WQ (mg/L): HCO3 : 2+ − 113, Ca : 27.7, Cl : 2+ + 5.7, Mg : 4.5, K :4.9, + 2Na :11.9, SO4 :11.2. ♦t (s): fixed 25. ♦C/W (g/L): 300 ♦PS: n.g (fine grinding**) ♦WQ: n.g ♦PS: 29% >500 µm; 250 µm < 47.4% < 500 µm; 125 µm < 22.2% < 250 µm; and 1.4% < § 125 µm . ♦WQ (mg/L): TDS: 148, Hardness:14ºF, + Free CO2: 3.3, K : 0.5, 2+ 2+ Ca : 30.1, Mg : 15, − Cl : 1.5 2− 3− SO4 : 10.7, HCO : 3− + 157, NO : 5; Na : 1.4; pH: 8.1; conductivity (µS/cm): 249

♦Increase water volumes: ↓ higher polar odorants & ↑Low polar compounds.

♦A balanced extraction of non-volatile taste components, contributing to body, bitterness, astringency or acidity.

♦Overall: Higher polar components (2,3butanedione), were released much faster than the lower (β-damascenone) ♦Tº1=↑TPC extraction, TS, 5-CQA ♦Tº2=↑total lipids, extraction yield to Arabica washed coffees. ♦Tº3=↑caffeine & pH

♦Tº1: balance, astringency & bitterness. Good colour of crema, well balanced aroma intensity, body & flavour.

♦ECA: ↑ aroma intensity (92ºC/9bar). ↑ positive key odorants the final fractions (21-25s).

♦Tº2 & Tº3: ↓ foam index, viscosity, body & level of pleasant odours. ↑ bitterness & astringency. ♦ECA: ↑ positive contribution of the key odorants at 92 °C / 9 bar. ↓ negative flavour notes. ↑ aroma intensity.

♦ECB: ↑ proteins, lipids & positive key odorants in the first fractions (0–10 s).

♦ECB: ↓ positive odorants in the intermediate and last fractions.

♦pH: ↑ BM ↓HIP & IT.

♦HIP: ↑ foam index, foam persistency, foam thickness.

♦Lipid concentration: ↑IT & HIP systems ↓ conventional system (BM) ♦Conventional system (BM): ↑2methylbutanal, 3- methylbutanal, 2- ethyl3,5-dimethylpyrazine, 2-ethyl pyrazine, 2ethyl-6- methylpyrazine, diacetyl & 2,3pentanedione, guaiacol. ↑VOCs detected directly above in the cup.

25

♦IT system: ↓ viscosity (less body) than BM & HIP. ♦BM: ↓foam index, ↓ foam thickness, ↓foam persistency (about 2min), which is close to the threshold of acceptability.

(Mestdagh et al., 2014)

(Salamanca et al., 2017)

(Caprioli et al., 2012)

(Parenti et al., 2014)

WQ (mg/L) 2+ ♦WQ1 =Ca : 32.9, 2+ -3 Mg 6.5, HCO :106. 2+ ♦WQ2 =Ca : 55.0, Mg2+:12, HCO-3:198. 2+ ♦WQ3 =Ca : 80, 2+ -3 Mg :26, HCO : 360. 2+ ♦WQ4=Ca : 171, 2+ -3 Mg : 27,8, HCO : 574 Coffee capsules. ♦Ristretto Forte. t(s): 14.2; C/W(g/L): 150.3 † PS:345-55µm , ♦Espresso Alba. t(s): 28.9; C/W (g/L): 82.6 PS: 330-47µm†. ♦Espresso Classic t(s): 22 C/W(g/L): 86.7 † PS:346-57µm . ♦Espresso Intense. t(s): 23.5; C/W(g/L): 83.3 † PS: 331-48µm ♦Lungo Crema (LC) t (s): 41.1; C/W (g/L): 48.1 † PS: 343-49µm . ♦Lungo Fortissimo t (s): 42.0; C/W: 46.11 g/L † PS: 341-56µm

♦C/W (g/L): 350 Other parameters not given.

♦CO2 present in the roasted coffee can additionally contributed in the foam formation. ♦Bicarbonate ions help to foaming but foam collapsing very quickly.

♦Water quality affects the foam volume, persistency & texture. ♦WQ1 & WQ2 (↓ ↓bicarbonate ions): ECs with foam layer with a desired fine texture. ♦WQ4 (↑bicarbonate ions): ECs foam nondesired with large bubbles unacceptable for consumers.

(Navarini & Rivetti, 2010)

For all capsules:

♦VOCs: Maximum intensity 2 - 24 s.

Not given

♦Tº&P: n.g. ♦WQ: Alkalinity 4 dH°, Hardness 6dH.

♦In the first 10s almost 95% of the VOCs are extracted

(SánchezLópez et al., 2014)

♦At the beginning: Most of the coloured compounds are extracted. ♦More polar compounds are extracted faster. ♦Aroma profile of the extracts was different for the six capsule types. ♦There were differences in the VOCs extraction dynamics for each coffee (capsule).

26

P (bar) / Tº (ºC) ♦P, Tº1: 9/92. ♦P, Tº2: 7/92. ♦P, Tº3: 11/92. ♦P, Tº4: 9/82. ♦P, Tº5: 9/96

♦t (s): 25. ♦C/W (g/L): 18 ♦PS: n.g. ♦WQ (mg/L): Total mineralization: 130; 2− 3− HCO : 71, SO4 : 8.1, + 2+ Na : 11.6, Ca : 11.5, Mg2+: 8

♦Increasing Tº: ↑VOCs intensity, especially t > 14 s. ↑solubility, ↑extraction. ♦11 bar: ↑ VOCs over the entire extraction time than at 7 bar. ♦7 & 9 bar: No differences in VOCs families. ♦↑P& ↑Tº: ↑VOCs extraction.

♦Least polar compounds are the most affected. Impacting the aroma balance in the last stage of the extraction and the cup.

(SánchezLópez et al., 2016)

Comparison between coffee extraction methods

Brewing methods. ♦EC1: t(s): 28.7. Tº(ºC):92. P(bar): 9. C/W (g/L): 267. † PS: 400-220 µm . ♦EC2: PS: n.g t(s): 24. Tº(ºC): 90. P(bar): 8.25. C/W (g/L): 267. ♦EC3: PS & Tº: n.g t(s): 24. P (bar):19. C/W (g/L):183.3. ♦MC: Tº & P: n.g t (s): 224. C/W (g/L): 68.18. † PS: 400-220 µm . ♦FPC: P: ~1bar. PS: 1,000–1,025 µm† T (s): 240. Tº(ºC): ~90. C/W (g/L): 55. ♦FC: P (bar): ~1. † PS: 1000-1025 µm t(s): 348. Tº(ºC): ~90. C/W (g/L): 55.6.

Extraction efficiency per gram of coffee ♦EC1: Semi-automatic machine. ♦EC2: Fully automatic machine ♦EC3: Nespresso ♦MC: Moka ♦FPC: French Press ♦FC: Filtered ♦WQ: n.g for any method

♦3-CQA & 5-CQA: ↑EC3. ↓EC1 & EC2 ♦Extracted acids: ↓EC1 & EC2. ↑EC3. ♦Intensity aroma compounds: ↑EC3 ↓EC2 & EC1. ♦Percentage fatty acids: ↑EC2 ↓ EC3 ♦Extraction efficiency VOCs: ↑ intensity ECs. However, MC & FC also showed a high headspace intensity.

♦ECs: ↑texture/body, strong roast & bitter flavour, prolonged aftersensation. ♦EC1 & EC2: ↑overall & roasty aroma intensity. ♦EC1: fine, darker crema than EC2. ♦EC3: the highest quantity of the darkest crema. ♦FC & FPC: weak but very well-balanced flavour profile.

♦CQAs: ↑ECs. ↓MC & FC

♦FC & FPC: modest aromaticity, weak roasty & bitter notes, pronounced sweetness.

♦Fatty acids: ↑ FPC. ↓ FC. Relatively high amount of fatty acids for MC.

♦MC: no crema, middle-range sensory values, same flavour profile as EC3.

♦Main fatty acids: hexadecanoic & octadecadienoic acids (more than 80 % in total).

27

(Gloess et al., 2013)

Brewing methods ♦EC: t (s): 25. Tº (ºC): 95. C/W (g/L): 280. ♦FC: t (s): 120. Tº (ºC): ~90 C/W (g/L): 83.3. ♦NC: t (s): 300. Tº (ºC): Initial ~90/ final 60. C/W (g/L): 106.2 ♦MC: t (s): ~180. Tº (ºC): ~100. C/W (g/L): 141.3 Brewing time (s). ♦Espresso Coffee (EC). t1: 0-8, t2: 8-16, t3: 16-24, tf: 24. ♦Filtered coffee (FC) t1: 0-75, t2: 75-150, t3: 150-225, t4: 225-300, t5: 300-375, tf: 375

♦EC: Espresso ♦FC: Filtered ♦NC: Napolitan ♦MC: Moka: ♦WQ: distilled water for all methods. ♦PS: n.g for all methods.

♦EC: t (s): 24. C/W (g/L): 155.6 Tº, P, WQ: n.g PS: n.g. ♦FC: t (s): 375. Tº (ºC): ~90. C/W (g/L): 58.3. PS & WQ: n.g

♦TPC: ↑EC, ↓ FC, MC & NC. ♦Caffeine: EC > MC > FC > NC. ♦Furans content: ↑EC & FC, ↓NC & MC. ♦VOCs: Main differences between brewing methods in 2-furanmethanol acetate, 2,5-dimethylfuran & furfuryl methyl ether. ♦Guaiacol: ↑MC. ♦2-ethyl-6-methylpyrazine, 2-ethyl-3,5dimethylpyrazine & 2-methyl-3-transpropenylpyrazine: ↑MC, ↓EC. ♦2,3-pentanedione: ↑ FC with respect to all methods. ♦β- damascenone: ↑NC. ♦Methanethiol: ↓NC, ↑ EC. ♦Aldehydes: ↑ EC, ↓NC & MC

Not given.

♦EC: Increase time: ↓antioxidant capacity. t1 (0-8s): ↑70% antioxidant capacity, ~70% of 3-4-5CQA & ~50% of diCQAs extracted. t2 (8-16s): ~ 17% of 3-4-5CQA & ~30% diCQAs extracted. t3 (16-24s): ↓12% antioxidant capacity. ↓14% of 3-4-5CQA & ~20% diCQAs extracted.

Not given

♦FC: ↑CQAs in t5. ↑ extraction efficiency mainly less polar antioxidant (diCQA) increasing the turbulence & contact time.

28

(Caporaso et al., 2014)

(Ludwig et al., 2012)

Brewing methods. ♦FC: filtered coffee ♦FPC: French Press coffee. PS & Roasting degree (RD) in Cold brew coffee (CBC) ♦RD: Medium (MR) & Dark (DR). § ♦PS: Medium (MG)& § Coarse grinding (CG) ♦MR-MG(µm): 841 (26.2%); 400 (53.3%). ♦MR-CG(µm): 841 (70.6%); 400 (23.1%). ♦DR-MG(µm): 841 (38.1%); 400 (45.4%). ♦DR-CG(µm): 841 (77.8%); 400 (17.5%) CBC-Dripping & CBC-Stepping ♦t (h): t1: 6.5 t2: 3.3 ♦Tº(ºC): Tº1: 22 Tº2: 5 ♦FPC. t (min): 5. Tº(ºC): 95.

FC & FPC ♦t (s): 300. ♦Tº (ºC): 91. ♦C/W (g/L): 100 § ♦PS: 200-500µm . ♦WQ: n.g. ♦CBC: t (min): 1440 Tº (ºC): 21-25 C/W (g/L): 100. WQ: n.g ♦FPC t (min): 6 Tº (ºC): 98 C/W (g/L): 100. WQ: n.g

♦FC: ↓TS, ↓cafestol (1.07-2.3 mg/L), ↓ diterpene ♦FPC: ↑ cafestol (30.12 - 62.31 mg/L, ↑diterpene. ♦Cafestol: Paper filter retained 12.41%, spent coffee 87.45% & the brew 0.15%. ♦CBC: ↑3-CGA & caffeine over the first 180 min. After shown slow until reach the equilibrium ~ 400 min in all coffee evaluated.

Not given. (Rendón et al., 2017)

Not given

(Fuller & Rao, 2017)

♦CBC-stepping: ↑ sugar caramelization flavours & sweet taste. ♦CBC-dripping: ↑ bitter attributes. ♦FPC: ↑ bitter intensity. ♦CBC-stepping: ↓ bitter intensity. ♦Sour intensity: ↑CBC at 22ºC, ↓ CBC at 5ºC. ♦Sweetness: ↑CBC-stepping & CBCdripping and ↓FPC.

(Angeloni, Guerrini, Masella, Innocenti, et al., 2019)

♦Particle size did not impact 3-CGA & caffeine concentrations. ♦3-CGA & pH: CBC comparable with FPC. ♦Caffeine: ↑ CBC coarse grind than FPC.

For all methods: ♦C/W (g/L): 100. ♦PS: n.g (coarse)**. ♦WQ (mg/L): TDS: + 148, Free CO2: 3.3, K : 2+ 2+ 0.5, Ca : 30.1, Mg : − 2− 15, Cl : 1.5. SO4 : 3− 10.7, HCO : 157 3− + NO : 5; Na : 1.4; pH: 8.1. Conductivity (µS/cm): 249, Hardness: 14ºF.

♦CBC dripping at 5ºC: ↑ pH than FPC. ♦CBC-dripping: ↑caffeine than CBCstepping. ♦FPC & CBC dripping at 22ºC: Similar caffeine content. ♦CBC-stepping: no differences in caffeine between t1 & t2. ♦CBC dripping at 22ºC: ↑ CQAs ♦Increasing temperature: ↑TS, ↑5CQA & ↑caffeine.

29

CBC & FPC methods

PS & brewing time in CBC. ♦CBC. t (h): t1:14; t2: 22 PS: Medium (501-700 µm) & Coarse grinding (701–900 § µm) . Coffee origin. Huila & Nariño

1168 1169 1170 1171 1172 1173 1174 1175

♦CBC t (h): 7. Tº (ºC): 21-25. C/W (g/L): 100. PS: n.g. WQ: Filtered ♦FPC. t(min): 6. Tº (ºC): Boiling point. C/W (g/L): 100. PS: n.g WQ: Filtered ♦CBC. Tº (ºC): ~20 C/W (g/L): 60. WQ: Filtered ♦FPC. t (min): 6. Tº (ºC): ~92. C/W (g/L): 85. PS: Coarse (701– § 900 µm) . WQ: Filtered

♦CBC vs FPC: Comparable pH values (4.85 to 5.13) ♦TA, antioxidant activity & CQAs: ↑FPC, ↓CBC. ♦5-CQA the most abundant CQA isomer in CBC & FPC. ♦Correlation total CQA & total antioxidant activity: ↑ CBC ↓ FPC.

Not given.

(Rao & Fuller, 2018)

♦CBC-CG-22: ↑TDS, extraction yield, pH, TA & TPC. The type of coffee used mainly affected the TA & pH.

CBC-CG-14h: ↑higher scores sensorial evaluation. Strong sweetness, fruity & floral flavours, medium bitterness, acidity, creamy body.

(Cordoba et al., 2019)

♦CBC-CG-14h- Odour active compounds: presence of the furans, pyrazines, ketones, aldehydes, pyrroles, esters, lactones, furanones, & phenols.

BM: Brewing method; EC: Espresso Coffee; FC: Filtered coffee; FPC: French Press coffee; NC: Napolitan Coffee; MC: Moka coffee; CBC: Cold brew coffee; t: time; Tº: Temperature; P: pressure, C/W: coffee water ratio; PS: Particle size; WQ: water quality, CQAs: caffeoylquinic acids. TS: Total solids. TDS: Total dissolved solids. TA: titratable acidity. TPC: Total phenolic content. di-CQA: dicaffeoylquinic acids. 3CGA: 3-chlorogenic acid, CQAs: caffeoylquinic acids, 5-CQA: 5-caffeoylquinic acid. VOCs: Volatile Organic compounds. n.g: Not given. One upward arrow (↑) indicates an increase or high values within conditions evaluated. One downward arrow (↓) indicates a decrease or low values within conditions evaluated. One double horizontal arrow ( ) indicates intermediate values under the conditions evaluated. ** It designates that the level of grinding or particle size was reported, but the measuring method is not described. § Particle size was measured by

30

1176 1177

the sieve analysis method, and the data are reported whit relation to the mass or percent of coffee used in the measure (mostly, 100g of coffee grounds). † Particle size distribution measured by laser light diffraction methods.

31

Extraction Methods

Process variables

Green Coffee bean

*Extraction Time *Water Temperature *Pressure *Coffee: water ratio *Particle size *Water quality.

Coffee Coffee Roasting degree

Dissolved solids + water

Physicochemical and flavour characteristics

Brewed coffee (extract)

Decoction Methods Boiled/Turkish

Infusion Method Filtered

Vacuum

Hot Water Coffee grounds

Mixture: Coffee grounds/water

Steam/ Pressure

Mixture: Coffee grounds/water

Filter

Coffee grounds

Funnel

< Water

A

Heat

Coffee Brew

B

Heat

C

Pressure Methods Plunger/French Press

Stove-top coffee maker/ Moka

Espresso

Pressure (to down plunger/filter) Pressurised Hot Water

Coffee Brew

Filter basket

Filter

Coffee (podwer)

Steam/ Pressure

Coffee grounds/ Hot water

D

Water

Heat

Brewing chamber

Coffee grounds (cake)

Filter

Coffee Brew

E

F

2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 1997 1996 1993 0

1

2

3

4

5 6 7 numbers of scientific papers

8

9

10

11

12

Thiols 4% Pyridines 5%

Furanones Other sulf comp

Terpens 2%

Furanones 6%

Thiols

Phenols 6%

Terpens

Other sulf comp 2%

a)

Pyrazines 24%

Pyrroles 8%

Furans 20%

Ketones 12% Aldehydes 11%

Pyrroles

Pyridines

Pyrazines

Phenols

Ketones

Furans

b)

Aldehydes 0

2

4

6

8

10

Number of compounds

12

14

16

18

20

Highlights • Coffee brewing is a complex process, with many parameters affecting final quality. • Aromatic compounds are differentially extracted in the brewing process. • Temperature, pressure, particle size, and water quality contribute to coffee flavor profile. • Further studies should address these parameters in new brewing methods. • Phenomenological explanations are essential to understanding the coffee extraction process