An overview of coffee flavour: Influence of process, composition and sensory properties

Abstract

Coffee is one of the most widely consumed beverages in the world, and it began to take its place as a culture among consumers since the mid-16th century. While the first coffee houses became common in Turkey, Iran, Syria and Arabia in the 15^th century, Europe was introduced to coffee in the 16^th century (1). According 2023/24 data the coffee consumption of the regions is as follows: Europe 53.7, Asia & Pacific 45.7, North America 30.9, South Amerika 28.0, Africa 12.5 and Caribbean, Central America & Mexico 6.1 million 60-kg bags (2). The countries that consume the most coffee per capita are the Scandinavian countries, and according to 2023 data, Finland (12 kg) ranks first in the world in per capita consumption, followed by Norway (9.9 kg), Iceland (9  kg), Denmark (8.7 kg), the Netherlands (8.4 kg), Sweden (8.1 kg), Belgium (6.8  kg), Luxembourg (6.5 kg), Canada (6.5 kg) (3).

The fact that coffee taste is the most important parameter for the consumer, requires detailed investigation of coffee taste. Coffee flavour and distinctive sensory quality are affected by bean type, geographical location, climate, different agricultural practices, and process parameters. Studies on coffee flavour are mostly limited to determining the basic flavour compounds depending on a single coffee type, style, or geographical origin, and depending on the type of coffee studied and the basic flavour components obtained from one study differ from the data obtained from another study (4). In addition, the effect of the flavour compounds on sensory properties has not been fully determined. Starting from the cultivation of coffee beans, the processing, roasting, grinding, brewing methods and presentation of coffee to the consumer are important factors that affect the taste of coffee. In order to understand the taste of coffee, it is important to examine it from a broad perspective.

Although the sensory properties of coffee have been studied for many years, the importance of coffee flavour in terms of industry and science has increased with the increase in coffee consumption around the world. A sensory description describing coffee flavour has been used in many studies. Some of these sensory attributes were determined as; astringent, body, bitter taste, burnt smell, burnt taste (5), sweet caramel, earthy, roasted, sooty (6), roasted/burnt, spicy, bitter, sour, sweet, salty, astringent, woody, fermented, earthy and tobacco-like (7). Attributes such as coffee, brown, bean-like, hazelnut, cocoa, floral, fruity, green, ashy, sweet aromatic, sour aromatic and pungent have also been used as different aroma terms (8). Studies have shown that brewing enhances the sweet-caramel aroma in Coffea arabica , while it highlights the spicy, sharp and earthy aromas in Coffea robusta . It has been determined that different roasting degrees applied to C. arabica and C. robusta cause differences in astringency, body, bitter taste, and burnt odour (5).

In addition to aroma and taste, texture, mouthfeel and chemesthesia (sensitivity of the mucosa) are other components that affect the perception of flavour and are changed by the interaction of the food structure with its mouth-coating feature (9). While the texture and mouthfeel characteristics of coffee are defined as full-bodied, astringent, round, smooth, thick, coarse, granular, hard, oily, and sticky, the overall impression is pure, non-persistent, clear, sharp, moderate, round, soft, balanced, strong. Characters such as, heavy, hard, light, plain, simple, ripe, winey, rich, sharp, astringent, alkaline, easy to swallow were also used (10, 11). In another study conducted in Italy, espresso coffee was described with thick, lingering, fluid, resistant to tongue movement, syrupy, viscous, velvety, pasty, creamy, mouth-covering, smooth, round, granular, full-bodied and rich in character (12).

The formation of coffee flavour begins with the development of the fruit in the coffee plant. Green coffee beans contain over 1000 substances with different chemical and physical properties. Insoluble (cellulose, hemicellulose) and soluble (arabinose, fructose, galactose, glucose, sucrose, raffinose, stachyose, etc.) carbohydrates, lipids, chlorogenic acids and nitrogen-containing compounds are considered as basic aroma precursors (13). Sucrose, glucose, and fructose are responsible for the formation of acids and other volatile compounds during roasting, as Maillard reaction takes place between carbohydrates and amino acids (14). Polysaccharides are responsible for the retention of volatiles and contribute to flavour formation. Nitrogen-containing compounds such as alkaloids (caffeine and trigonelline) and proteins, non-volatile aliphatic acids (citric, malic and quinic acids), volatile acids (acetic, butanoic, decanoic, formic, hexanoic, isovaleric, propanoic acids) are broken down by roasting to form important flavour active metabolites such as pyridines and pyrroles (15). Differences in chemical composition of coffee types affect the taste and aroma obtained from these types. For example, C. arabica and C. robusta are quite different in terms of taste. The caffeine content of C. robusta beans is higher than C. arabica, and their volatile component contents are also different. 2-methylisoborneol, determined in Robusta coffee, causes the typical earthy flavour (16). Differences have been founded even among C. arabica varieties due to environmental conditions. It has been stated that environmental factors such as geographical origin, climate, altitude and temperature rise, and shading are effective on coffee quality (17).

The development of complex coffee flavour continues with various coffee processing and preparation techniques. In the processing of green coffee beans, dry processing, wet processing, or semi-wet processing methods are used (17). The results obtained by the fermentation and washing process are main differences of mentioned methods and affect the taste of the coffee. While "hard" coffee with a medicinal taste is obtained with dry processing, better quality, less bodied, high acidity, and aromatic coffee is obtained with wet processing. It has been stated that coffees obtained by the half-wet method have a medium body (18).

Roasting, grinding, and brewing applied to green coffee beans are important processes that affect the taste of the coffee drink. Chemical reactions related to the colour, taste and aroma of coffee occur during roasting (17). During the roasting process, coffee flavour is formed by Maillard reactions, Strecker degradation, pyrolysis reactions, and the breakdown of trigonelline, quinic acid, pigments, and lipids (19).

The coffee flavour is revealed through the grinding process applied for extraction or infusion in the preparation of the coffee drink (8). The degree of grinding and particle size affect the extraction and therefore the quality of the coffee drink. A very finely ground coffee causes bitter coffee due to excessive extraction, while a coarsely ground coffee reduces extraction due to the decrease in surface area, resulting in a weak coffee (20).

Coffee brewing is a crucial step in transferring coffee aromas from the ground beans to the beverage. Extraction time, water temperature, applied pressure, particle size, coffee/water ratio and water quality affects the volatile substances extracted from coffee (21).

Non-volatile compounds found in roasted coffee beans are alkaloids (caffeine, trigonelline), chlorogenic acids, carboxylic acids, carbohydrates and polymeric polysaccharides, lipids, protein, melanoidins and minerals, and these compounds are important for coffee aroma. Variations in coffee growing and processing conditions affect the presence of these components in roasted coffee beans. Compared to C. arabica, C. robusta has been stated to contain higher amounts of caffeine as green or roasted beans (22, 23).

Volatile compounds formed during roasting of coffee beans are decisive in coffee quality. The mechanism of coffee aroma formation is quite complex and is formed by the interaction of many reactions during coffee beans and roasting. Mechanisms that are effective in the formation of important aroma volatiles during roasting include the Maillard reaction, Strecker degradation, degradation of sulfur amino acids, hydroxy amino acids, proline and hydroxyproline, trigonelline, chlorogenic acid and quinic acid, degradation of pigments and lipids (19).

The key compounds found in coffee are not enough to explain coffee flavour. Similarly, sensory determination of aroma attributes of coffee is insufficient to explain what causes a specific sensory property without chemical data. Evaluating sensory data and physicochemical measurements together is possible with chemometrics, known as multiple data analysis tools. Principal component analysis (PCA) and partial least squares (PLS) regression analysis are used to determine the components that cause specific aroma differences in complex matrices such as coffee (15).

Coffee flavour is affected by differences in the processing and preparation stages, starting from the cultivation of coffee. Differences in these factors cause changes in the flavour and aroma components of green and roasted coffee beans and brewing stages. The effect of volatile and non-volatile components on flavour perception of coffee, consumer preference and coffee pleasure are important. Although information about the chemical composition of coffee flavour is important, the reliable sensory data regarding the aroma composition of coffee is insufficient to explain the importance and contribution of these components to flavour. Matching sensory data with the components that make up coffee flavour will provide a better understanding of coffee flavour. Although studies on coffee flavour have been ongoing for nearly 100 years, detailed studies are needed to understand formation of coffee flavour.