Formation of aliphatic acids by carbohydrate degradation during roasting of coffee

 A major fraction of the acidity generated on coffee roasting can be attributed to formation of the four aliphatic acids formic, acetic, glycolic and lactic. Addition of sucrose, glucose or fructose to green coffee beans gave, compared to untreated beans, significant increases in the yields of the four acids on subsequent roasting. In addition, model studies using these three carbohydrates confirmed sucrose as the principal green bean precursor of the acids. Arabinose, erythrose and 1,6-anhydroglucose were identified as reaction products of sucrose thermal degradation and also subsequently served as precursors for acid formation. Isotopic labelling experiments indicated that known degradation pathways could be used to explain the formation of the four aliphatic acids from sucrose. Received: 7 January 2000     

An impression of coffee carbohydrates

Coffee is one of the most popular beverages in the world. It is consumed for its refreshing and stimulating properties. Carbohydrates are the major constituents of coffee beans and serve various functions like binding of aroma, stabilization of foam, formation of sedimentation, and increased viscosity of the extract. The principal low molecular weight carbohydrate is sucrose and no evidence of other simple oligosaccharides has been found. Polysaccharide fraction from green coffee is dominated by arabinogalactan, galactomannan, and cellulose. The polysaccharide content is reduced during roasting due to degradation to low molecular weight carbohydrates (viz., mono and oligosaccharide) and become more extractable. Various methods that can be employed to extract the carbohydrate from roasted coffee are sequential extraction, acid hydroloysis, hot water extraction, enzymatic extraction etc. Carbohydrates from coffee can be quantitatively determined by liquid chromatography, high performance anion exchange chromatography, size exclusion chromatography, and high performance liquid chromatography. Besides improving the organoleptic quality of the coffee beverage, carbohydrates also possess various biological activities such as lowering colon cancer risk. Besides their sheer mass, a variety of evidences testify to the important contribution that the polysaccharide content makes to the character of the final brew. Although a number of chemical and enzymatic methods have been devised to isolate and quantify the carbohydrates of R&G coffee, till date hot water extraction is the only method which can be accepted as a most feasible process and hence, there is wide scope of further research for the efficient and economically viable technology for extraction of carbohydrates from coffee.     

Formation of 5-hydroxymethyl-2-furfural (HMF) and 5-hydroxymethyl-2-furoic acid during roasting of coffee

The formation of 5-hydroxymethyl-2-furfural (HMF) and 5-hydroxymethyl-2-furoic acid (HMFA) during roasting of coffee was studied. At 240 degrees C the maximum concentration of HMF occurs after 3 min with a quick degradation up to 10 min when most of the HMF has disappeared again. Similar to 5-hydroxymethyl-furfural, HMFA is formed in coffee but not in a model system consisting of sucrose, alanine with or without chlorogenic acid. It was shown that HMFA is produced from different precursors than HMF namely glyceraldehyde and pyruvate. The comparison of the laboratory scale roasting with industrial roasting showed that 5-hydroxymethyl-furfural decreases with a higher degree of roasting whereas HMFA did not change. In the laboratory scale experiments, the highest concentration of 5-hydroxymethyl-furfural in coffee (909 microg/g) was obtained after 3 min and the maximum concentration of HMFA after 4 min (150 microg/g). Industrially roasted coffee contained up to 350 microg/g 5-hydroxymethyl-furfural and 140 microg/g HMFA.     

From the green bean to the cup of coffee: investigating coffee roasting by on-line monitoring of volatiles

Volatile organic compounds (VOCs), emitted from green coffee beans, during coffee roasting and from a cup of coffee, were all analysed by proton-transfer-reaction mass spectrometry. Firstly, the headspace (HS) of green beans was investigated. Alcohols dominate the HS, but aldehydes, hydrocarbons and organic acids were also abundant. Secondly, we roasted coffee under two different conditions and monitored on-line the VOCs emitted during the process. In a first roasting series, a batch of beans was roasted. After an initial drying phase, dominated by evaporation of water and methanol, the HS concentrations of VOCs such as acetic acid, acetaldehyde, pyridine and methylbutanal rapidly increased and went through a maximum at medium roast level. In a second series, just six beans were roasted. We observed sporadic bursts of some volatiles (furans, butanal, 2,3-pentanedione), coinciding with popping sounds. Other VOCs showed smooth time-intensity profiles (pyridine, pyrazine). These experiments gave a real-time insight into the complex processes taking place during roasting. Finally, the HS of coffee extracts, prepared from beans roasted to different roast levels, were analysed. Most VOCs showed a maximum concentration at medium roast level (e.g. pentanedione, furfural, 5-methyl furfural), while others showed a gradual increase (e.g. pyrrol) or decrease (e.g. methanol).     

Mitigation of acrylamide and hydroxymethyl furfural in instant coffee by yeast fermentation

Acrylamide being known as carcinogenic and hydroxymethyl furfural (HMF) being known as cytotoxic compounds are heat induced process contaminants found in instant coffee. Today's instant coffee production method involves roasting of coffee beans, grinding, flavor and aroma separation, extraction, concentration, and drying steps. During roasting, acrylamide and HMF are formed in varying amounts depending upon the degree of heat treatment as a result of the Maillard reaction. This study was conducted in order to reduce the concentrations of acrylamide and HMF in instant coffee. Instant coffee (20%, w/v) was mixed with sucrose (0–10, w/v) and baker's yeast (Saccharomyces cerevisiae, 1–2%, w/v) in a tightly closed glass vessel. The mixture was fermented at 30 °C for 48 h. The kinetics of acrylamide and HMF degradation was investigated. HMF and acrylamide contents were reduced exponentially at varying rates, depending upon fermentation medium and time. After 24 h, HMF concentration was decreased by 61.2%, 75.7%, 93.6% and 99.2% in the fermentation media containing none, 1%, 5%, and 10% of sucrose, respectively. After 48 h, acrylamide concentration was decreased by about 70%. These results revealed that yeast fermentation is promising for the mitigation of HMF and acrylamide in instant coffee.     

焙炒条件对咖啡风味影响的研究

以云南产的两种咖啡豆为分析对象．研究了焙烤过程中咖啡抽提液一些成分的变化以及对咖啡香气的影响。利用高效液相色谱对在焙烤过程中葫芦巴碱、绿原酸、烟酸、咖啡因含量的变化进行分析，并对绿原酸／咖啡因比率与色度值的相关关系建立了相应的模型。     

Effect of coffee physical structure on volatile release

The mechanisms of volatile release from soluble coffee powders with different roasting degrees were studied. The presence of volatiles in the headspace during coffee humidification was analyzed by gas chromatography. Small amounts of volatiles were observed at low water activities (a_w), independently from the roasting degree; as the a_w increased headspace volatiles rose and then decreased as the moisture further increased. The changes in a_w and volatiles went along with the changes of coffee structure from a free-flowing powder to a sticky viscous fluid. The mechanism of volatiles release was controlled by both kinetic and thermodynamic factors. The former prevailed at low a_w in glassy systems; while the latter became important at high a_w when, due to increased mobility, equilibrium conditions were approached. Modified state diagrams were used to predict the critical temperature and a_w at which structural collapse and volatile release occurred. As far as equilibrium condition was achieved, coffee volatiles were partially re-adsorbed in the liquid phase.     

Arabinogalactan as a potential furfural precursor in roasted coffee

A fraction C₁ which was very rich in polysaccharides and did not contain sucrose was isolated from high molecular weight water-soluble material obtained from green coffee. After roasting, furfural was identified in fraction C₁ by means of headspace-GC/MS analysis. Roasting of the fraction C₁ clearly affected the arabinogalactan promoting a high loss of arabinose residues. We suggest that furfural found in fraction C₁ originates mainly from arabinogalactan, which, together with sucrose appears to be the major furfural precursor in roasted coffee.     

咖啡主要烘焙风味物质的形成及变化规律

咖啡以其独特的风味和功能特性受到人们的喜爱。烘焙温度及烘焙时间决定了咖啡的烘焙程度,并对咖啡风味物质的产生及变化产生重要影响。咖啡主要烘焙风味物质是与咖啡风味直接相关且代表性强的香气成分,本文通过综合分析烘焙程度与咖啡主要风味物质的形成和变化规律的关系,为系统了解和掌握精品咖啡烘焙工艺提供科学参考。      

High‐resolution 1H NMR investigation of coffee

Recent successful applications of NMR spectroscopy and imaging in food science prompted the authors to use these new techniques for the analysis of espresso coffee. The target of this work was to identify an experimental procedure to obtain maximum information from high‐resolution ¹H spectra. Only the espresso coffee was considered as the true food, since it includes all aromatic substances responsible for the coffee aroma and taste. A big effort was made in order to select an easy, quick and non‐degrading procedure to analyse the espresso cup content, preventing artificial or natural degradation effects with a particular care in the measurement of labile and volatile components. The procedure was tested on two arabica and one robusta samples; each sample was prepared with three roasting degrees to follow the chemical changes due to the roasting process. Moreover, one water extract was prepared also from ground green coffees. As a comparison, each coffee extract was prepared both with a standard ‘espresso coffee machine’ and with a ‘mocha’ as used in the domestic setting. Significant differences among the three samples were observed. Moreover, for all samples, as a function of the roasting degree, a clear trend in the chemical composition was observed. © 1999 Society of Chemical Industry     

Peroxyl radical-scavenging activity of coffee brews

The ORAC_FL assay was used in non-automated mode to evaluate the specific peroxyl radical scavenging properties of the aqueous soluble components of green and roasted Arabica and Robusta coffee samples. A relationship between ORAC_FL and the concentration of CQAs (caffeoyl quinic acids) was found for the extracts from green coffee beans. Aqueous extracts from roasted coffee beans possessed equal or stronger scavenging power than that obtained for the green coffee beans extracts and the scavenging activity depended on the variety of coffee and the roasting conditions. Brews from Robusta coffee beans showed the highest ORAC_FL. The best scavenging properties for the brews from Arabica coffee beans were detected in samples prepared from coffee beans roasted under light conditions. The data indicate that, during roasting, a complex network of reactions takes place leading to the formation of a wide number of compounds possessing specific scavenging properties. Under mild roasting conditions, caffeoyl quinic acids appear to be the main components responsible for the free radical scavenging power of coffee brews. In contrast, Maillard reaction products may be the principal components with free radical scavenging activity in more severely (medium and dark) roasted coffees.     

A comparative study of chemical attributes and levels of amines in defective green and roasted coffee beans

Differences in chemical attributes (proximate composition, water activity, sucrose, acidity and pH levels) and amine levels between defective and healthy coffee beans were studied. Before roasting, significant differences (p < 0.05) were observed for the ash contents of the coffee samples, with the highest values found for black beans. Moisture content was higher for non-defective beans in comparison to defective beans. Non-defective coffee beans had higher lipids contents than both sour and black beans. There were no significant differences (p > 0.05) for protein levels between defective and non-defective beans. After roasting, protein levels remained constant, there was a small decrease in ash contents and a slight increase in oil contents of black and sour beans. Both black and sour beans had higher acidity levels than immature and non-defective beans. Acidity levels decreased after roasting. Water activity levels also decreased with roasting, with slightly higher levels for defective beans in comparison to non-defective ones. Sucrose levels were much higher in non-defective beans, and the lowest values were in black beans, prior to roasting. After roasting, only traces of sucrose were found. Total amine levels were much lower for black beans, in comparison to the other coffee samples. Putrescine was the prevailing amine in all samples. Histamine was only detected in the defective coffee samples. Small amounts of serotonin, cadaverine and tryptamine were found in some of the samples. After roasting to a light degree, only traces of serotonin were detected and no amines were detected after roasting to medium and dark degrees.     

Effects of roasting degree on radical scavenging activity,phenolics and volatile compounds of Arabica coffee beans (Coffea arabica L. cv. Catimor)

We investigated the effect of roasting degree on volatile and phenolic compounds of coffee beans (Coffea Arabica L. cv. Catimor). The colour, 1,1‐diphenyl‐2‐picrylhydrazyl radical‐scavenging activity and phenolic acids of the aqueous extracts were studied. For the colour, L and b values were increased with an increase in roasting degree. Increasing roasting degrees led to a decrease in radical‐scavenging activity. Maximum radical‐scavenging activity was observed for the light‐roasted coffee. Chlorogenic acid was the most predominant amongst the ten phenolic acids identified, in green and all roasted beans. Syringic acid, p‐coumaric acid, gallic acid and sinapic acid increased with an increase in roasting degree. The volatile compounds were analysed using gas chromatography and mass spectroscopy. Aldehyde was the major volatile compound in green coffee. Our study has demonstrated that light‐roasted coffee gave the most desirable quality of roasted coffee with respect to phenolic content and radical‐scavenging activities.     

Isolation and characterisation of a coffee melanoidin fraction

BACKGROUND: Melanoidins contribute to the colour and flavour of coffee brews and are also reported to be physiologically active components. However, structural information about coffee melanoidins is rare and current models are supported by few data. RESULTS: Using hydrophobic interaction chromatography (HIC), melanoidin fractions were isolated from different high‐molecular‐weight fractions of coffee brews obtained by ultrafiltration. Generally, melanoidin fractions isolated by HIC showed three‐ to fourfold higher antioxidant activities than the remaining high‐molecular‐weight material in the ultrafiltration fractions. Melanoidin fractions showed an intense brown colour: aqueous solutions with a concentration as low as 40 µg mL^?1 were distinguishable from pure water. The melanoidin fractions contained less than 6% each of releasable carbohydrates and amino acids. The molecular masses of the melanoidins were estimated to be between 3 and 22 kDa, irrespective of the ultrafiltration fraction from which they were obtained. Two‐dimensional nuclear magnetic resonance studies revealed an enrichment of phenolic/aromatic/olefinic structural units and did not support the idea of intact ferulate or caffeate moieties integrated into the melanoidin polymer. CONCLUSION: These studies support melanoidin models describing them as high‐molecular‐weight compounds themselves as opposed to models describing them as low‐molecular‐weight chromophores bound to polysaccharides or proteins. Phenolic constituents are more likely integrated into melanoidins as condensed phenolics than as intact hydroxycinnamates from chlorogenic acids. Copyright © 2008 Society of Chemical Industry     

Antiglycative and antioxidative properties of coffee fractions

In this work the inhibitory activity of coffee low molecular weight compound (LMWC) and high molecular weight compound (HMWC) fractions against in vitro advanced glycation end-products (AGEs) formation was investigated. The HMWC fraction was characterised for its content in total phenolic groups, proteins and carbohydrates. The chlorogenic acids of LMWC fraction were identified by liquid chromatography coupled with tandem mass spectrometry. HMWC inhibited bovine serum albumin glycation by acting as radical scavenger and Fe-chelator in the post-Amadori phase of the reaction and by inhibiting dicarbonyl reactive compounds production during glucose autoxidation. LMWC fraction was able to inhibit protein glycation and dicarbonyl reactive compounds formation more than HMWC fraction. Chlorogenic acids are the main compounds responsible for the antiglycative activity of LMWC fraction.     

The kinetics and mechanism of caffeine infusion from coffee: The effect of roasting

Rates of extraction of caffeine into water at 80°C were measured for green and roasted Kenyan arabica coffee beans. The green beans were roasted to controlled extents in a Moda coffee roaster, ground, and sieved to a size range 0.85–1.18 mm. Certain physical properties (weight loss, colour, bean volume and infusion pH) were determined for each product. The half-time of caffeine infusion did not change significantly on light roasting but dropped by 40% on more severe treatment and by a further 30% when roasted to scorching. The activation energy also decreased on strong roasting. The kinetic parameters have been correlated with the physical properties of the coffees and with the chemical and physical changes of roasting.     

Discrimination between defective and non-defective Brazilian coffee beans by their volatile profile

The coffee roasted in Brazil is considered to be of low quality, due to the presence of defective coffee beans that depreciate the beverage quality. In view of the fact that coffee flavour is directly related to the volatile compounds produced during roasting, the objective of the present study was to perform a comparative evaluation of the volatile fraction of defective (black, immature, sour) and healthy coffee beans, in order to find possible chemical markers for detection of defective coffee beans in roasted coffee. Volatiles extraction and concentration was performed by solid phase micro-extraction (SPME) of the roasted coffee headspace, using a triple phase (divinylbenzene/carboxen/polydimethylsiloxane) fiber. Analysis of the volatile profiles was performed by GC–MS. The results obtained showed that the proposed methodology was adequate for extraction, concentration and analysis of the coffees volatile profile. Several substances were identified as possible markers for differentiating black, sour and immature beans from healthy coffee beans. Statistical analysis of the data by principal components (PCA) demonstrated that the volatile profile enables the differentiation of healthy and defective coffees. The data were separated into two major groups, one represented by immature and black beans and the other by healthy and sour coffee beans. Such results indicated that black and sour beans can be associated to fermentation of immature and of healthy beans, respectively.     

An investigation of coffee roasting using high performance gel filtration chromatography

Changes in the molecular weight distribution of components in green coffee during roasting were investigated using high performance gel filtration chromatography (HPGFC) with TSK-PW 4000 columns. The chromatographic separation of sample extracts obtained from green and roasted coffees, after different degrees of roasting, was monitored by refractive index (RI) detection and UV detection at 280, 325 and 420 nm. The chromatograms obtained with RI detection showed significant differences in molecular weight profiles. UV detection showed similar patterns at 280 and 325 nm, suggesting that phenolic compounds were probably bound to proteins. The formation of pigments was clearly demonstrated when chromatography was carried out with detection at 420 nm and the coloured material was found to be distributed throughout the molecular weight range studied. The use of HPGFC provided a more rapid analysis compared with traditional gel filtration and proved to be useful in monitoring changes in molecular weight which occur during coffee roasting.     

Studies on acrylamide levels in roasting, storage and brewing of coffee

The content of acrylamide in coffee reaches a peak early in the roasting process, reflecting occurrence of both formation and destruction of acrylamide during roasting. Levels of acrylamide in the fully roasted product are a small fraction of the peak reached earlier. Glucose and moisture in green coffee do not show a significant correlation with acrylamide in roasted coffee. Pre-roasting levels of asparagine show a correlation only in Arabica coffee. The main factors affecting the level of acrylamide in roasted coffee appear to be the Arabica/Robusta ratio, with Robusta giving higher levels; time and degree of roast, with both shorter and lighter roasting at the edges of the normal roasting range giving higher levels; storage condition and time, with clear reduction at ambient storage. This storage reduction of acrylamide followed second order reaction kinetics with an activation energy of 73 KJ/mole. The acrylamide in roasted coffee is largely extracted into the brew and stable within usual time of consumption. As these four main factors also substantially affect the sensorial characteristics of the brew, and as modifications of the process have to comply with the consumer-accepted boundaries of taste profiles, only small effects on the acrylamide level are expected to be achievable.     

Evaluation of sensory attributes of coffee brews from robusta coffee roasted under different conditions

Robusta coffee beans with a different initial moisture of 5, 7.5 and 10% were convectively roasted at 230 °C, microwaved at 700 W, and roasted by the coupled convective-microwave method. Sensory attributes of brews prepared from these coffee samples were evaluated. Final temperature of microwaved coffee beans was lower than that of the beans processed by the two other methods, which resulted in a higher content of volatile aroma compounds and a lesser degree of charring of their surface. Lower initial humidity of coffee beans shortened the time of roasting. However, the aroma developed upon roasting of the moistest beans was the most intense and pleasant. Modification of roasting conditions increased shifting of the overall acceptability of coffee infusions by 2 points in a 10-point hedonic scale, which implies that, if roasting conditions are adequate to the type of coffee, its sensory characteristics can be improved. Thus, optimization of roasting parameters can increase the share of robusta in well-accepted commercial coffee blends, a convenient fact because of the significant difference in price between the latter and arabica coffee.