Study Investigates Impact Of Roast Degree On Caffeine Content In Coffee – CoffeeTalk

A recent study has investigated the relationship between roast degree and caffeine content in brewed coffee, revealing a complex interplay of factors that challenge previous conclusions. Researchers examined 30 distinct combinations of green coffee variety, roast degree, and brew time to evaluate their effects on extraction yield and caffeine levels. The study used an AeroPress brewer, a consistent 15:1 mass ratio of brew water to ground coffee, and refractometry and high-performance liquid chromatography (HPLC) to quantify extraction yield and caffeine content.

The findings indicated that extraction yields tended to decrease for roast batches that experienced mass losses exceeding 12–14%. Caffeine concentrations also decreased in 10-minute brews for roast batches with drop temperatures beyond 400–420 °F. Under uniform brewing conditions, dark roasts generally showed lower caffeine concentrations compared to their lighter and medium-roasted counterparts. However, interestingly, when factoring in identical extraction yields, dark roasts typically revealed higher caffeine concentrations than lighter roasts.

These results suggest that the volatilization or decomposition of soluble compounds, along with the increased porosity resulting from roasting, may act as competing mechanisms influencing the concentrations of compounds in brewed coffee. This contributes to the ongoing debate surrounding coffee brewing and its intricacies, paving the way for further research and understanding in the domain of caffeine extraction.

The concentration of caffeine in brewed coffee has become a significant point of interest due to the beverage’s global popularity and its associated health implications. Over the past few decades, extensive research has focused on the relationship between the degree of roast and caffeine content in coffee, showing a surprising level of disagreement. One explanation for the conflicting findings may lie in the challenges of isolating the effects of roast degree and minimizing influences from other experimental factors.

This comprehensive study meticulously documented and quantified every step of the process, including sourcing green coffee, roasting, degassing, grinding, sieving, brewing, and characterization. Currently, there is no universally accepted method for precisely quantifying the degree of roast. The study addressed variations in particle size distributions from grinding, which significantly affect extraction kinetics during brewing.

In addition to grind size distributions, the microstructure of roasted coffee grains significantly influences extraction dynamics. During the roasting process, internal pressure builds within the coffee seeds, leading to increased volume and porosity. This study uniquely explores the correlation between porosity, caffeine concentration, and extraction yield as influenced by the roasting degree, representing the first quantitative comparison of these factors in the literature.

A study conducted by researchers at Royal Coffee in Emeryville, California, aimed to observe variations in roasting behavior and extraction characteristics of Ethiopian coffee. The study focused on two types of Ethiopian green coffee: the natural process coffee, specifically the Organic Chelbesa Raised Bed variety (Crown Jewel CJO1503), and a fully washed process coffee from the Yirgacheffe region (Ethiopia Yirgacheffe Grade Zero Washed Adame Kebele—28183).

Natural process coffees are produced by allowing the coffee cherries to dry and ferment in the sun for several days before the removal of the husks and surrounding fruit. In contrast, washed process coffees, sometimes referred to as “wet-processed,” have their outer fruit removed prior to fermentation, with water used to eliminate the remaining mucilage layer before grading and sorting. The natural green coffee was grown at elevations ranging from 1,900 to 2,200 meters above sea level, with moisture content recorded at 10.5% and a freely settled density of 0.707 g/mL. The washed green coffee, grown at altitudes between 1,950 and 2,100 meters, had a moisture content of 9.6% and a freely settled density of 0.692 g/mL.

The study used an Aillio Bullet v2 electric drum roaster, handling batches of 500 grams of green coffee. Bean masses were measured with an Adam Equipment ADP-800L balance, while temperatures were recorded using an NTC thermistor probe placed inside the roaster drum. The desired roast degree for each batch was achieved by following a standardized recipe and adjusting the drop times based on the onset of the first crack (FC).

Plots showcasing bean probe temperature against roast time, as well as the rate of rise (ROR) versus roast time, were generated for each batch, reflecting differences in the natural and washed Ethiopian coffee roasts. These plots, which include markers for first and second crack events, provide insight into the distinct roasting profiles of the two coffee types examined.

The study revealed a consistently decreasing trend in the rate of rise (vs. roast time) throughout the darkest roast (R7) profile. Upon finalizing roast recipes for each variety of green coffee, the team adjusted the remaining roast degrees by terminating the process early. The study successfully achieved five distinct degrees of roast using the outlined method: R0, R1, R3, R5, and R7 for each green coffee examined.

The study focuses on the roasting process of coffee, specifically the development phase and brewing method. Commercial roasters typically track the relative percentage of roast time spent in three distinct phases: drying, middle, and post-FC development. The drying phase begins with the introduction of green coffee into the heated roaster and concludes when the coffee’s moisture content is sufficiently reduced to enable critical chemical reactions. This phase is marked by sensory indicators, such as the color transformation from green to yellow and the emergence of a hay-like aroma.

The middle phase, also known as the “browning,” “caramelization,” or “Maillard” phase, begins and continues until the onset of first crack (FC), marking the transition into the final roasting phase. This crucial segment, commonly referred to as the development phase, plays a significant role in coffee roasting. The term “development” denotes the process of reducing the temperature gradient between the internal core and the external surface of the roasting beans. For the purposes of this study, the period following the onset of FC until the conclusion of the roast is designated as post-FC development time.

The volume distribution density in relation to coffee particle size was assessed across three different roast batches of natural Ethiopian coffee. A brewing apparatus was utilized for the experiment, featuring an AeroPress positioned atop a glass beaker, supported by a force stand designed with a manual force application lever system and a digital force gauge. The brewing method employed in this study prioritized relevance for the general coffee consumer while ensuring consistent control over brewing parameters.

For optimal brewing, a 15:1 ratio of prepared brew water (150 grams) to ground coffee (10 grams) was maintained throughout the study. After adding the ground coffee to the inverted AeroPress, the coffee bed was gently agitated to ensure even leveling. A digital kettle, equipped with a T-type thermocouple probe, heated the brew water to a precise 212 ± 1°F. A timer was initiated upon the heated water’s first contact with the coffee grounds. Once the target mass of brew water was reached within the AeroPress, the device was promptly rotated and placed atop a glass beaker, which was then positioned on a force stand to apply consistent plunge forces during extraction.

The study also explores the impact of applied force and extraction time on the flavor profile of coffee. Researchers determined a target plunge force of 1000 N after preliminary trials showed that the brewer could fracture under an applied load of 1100 N. The study varied brew times—1, 2, and 10 minutes—to control the extraction yield of the coffee.

For brew characterization, a handheld refractometer (VST Lab Coffee III) was utilized to measure total dissolved solids (TDS). Extraction yield (EY) was calculated through a specific formula that factors in brew concentration, mass of the brew, and mass of coffee grinds used. Each sample underwent testing three times over the course of one minute to ensure accuracy.

This thorough examination into the brewing process provides valuable insights that could enhance coffee flavor profiles and extraction methods, offering an opportunity for coffee enthusiasts and professionals alike to refine their brewing techniques.

A study using high-performance liquid chromatography (HPLC) techniques was conducted to analyze caffeine and select chlorogenic acids (CGAs) in various coffee samples. The HPLC system included a 4.6 mm x 5 μm column, utilizing a mobile phase comprising methanol, acetonitrile, 1M acetic acid, and ultrapure water. Spectrophotometric-grade methanol and acetonitrile, along with glacial acetic acid diluted with ultrapure water, were carefully selected for the analysis to ensure high-quality results.

The experimental setup maintained a consistent temperature of 40°C for both the column and detector flow cell. Samples were processed at a flow rate of 1.000 mL/min, with an overall run time of 9.5 minutes. The acetonitrile and 1M acetic acid proportions in the mobile phase were held steady at 3% and 10%, respectively, while methanol and ultrapure water proportions varied throughout the run. The methanol concentration was strategically adjusted during the process to optimize the separation of compounds.

To maximize the detection of caffeine, three-dimensional absorbance data was collected across a spectral range of 210 to 700 nm, with chromatograms specifically extracted at 272 nm for the caffeine peak. The researchers utilized a set of five external caffeine standards ranging from 100 to 300 ppm to accurately determine caffeine concentrations, attaining a high linearity (R² = 0.9995) for calibration curves. The variation in caffeine concentration for replicate injections was found to be less than 0.5% relative standard deviation (RSD), indicating robust methodology.

In addition to caffeine analysis, the study focused on measuring the relative concentrations of CGAs, which serve as indicators of the degree of coffee roasting. Chromatograms were extracted at 333 nm to enhance the signal strength for CGA absorbance peaks. The quantification of 4-caffeoylquinic (4-CQA) and 5-caffeoylquinic (5-CQA) acids was carried out through peak integration, supported by tandem high-resolution mass spectrometry that confirmed the correct isomer assignments.

This investigation provides valuable insights into the chemical composition of roasted coffee and highlights the effective methodologies applicable to the analysis of complex mixtures. Further exploration into porosity determination is anticipated in the continuing research, which aims to deepen the understanding of coffee quality and its brewing characteristics.

A recent study on Ethiopian coffee roasting processes has revealed significant differences between natural and washed batches. Natural coffee batches exhibited higher mass losses during roasting compared to their washed counterparts, with washed coffee consistently showing mass decreases that were 2–3% lower than those observed in natural roasting batches. The study also found that drop temperatures varied between the natural and washed coffee types, even within common roast designations. Washed Ethiopian batches demonstrated consistently higher density and color measurements, coupled with lower drop temperatures, suggesting that these batches achieved lighter roast levels compared to natural Ethiopian coffees. Experts in the specialty coffee community may classify the R0 and R1 washed Ethiopian batches as “underdeveloped,” as reflected by their respective roasting mass losses of 8.9% and 11.6%.

Extraction yield measurements indicated a trend where darker roasts (R5, R7) produced lower extraction yields compared to lighter and medium roasts (R0, R1, R3). Natural Ethiopian roast batches exhibited a wider range of extraction yields, spanning 15-21%, in contrast to the 16-20% range observed in washed Ethiopian coffee brews. This variance in extraction yields correspondingly influenced caffeine concentrations in the brews. While 1-min and 2-min brew sets shared similar extraction trends with the 10-min brew sets, the latter yielded the highest extraction results.

Recent studies have highlighted the significant influence of roast degree on the concentrations of chlorogenic acid (CGA) and caffeine in brewed coffee samples, particularly in natural and washed Ethiopian coffee. The caffeine concentrations in natural Ethiopian brews ranged from 134 to 165 mg per 8 oz cup, while washed Ethiopian coffee showed a slightly higher range at 145 to 165 mg per cup. Notably, the washed Ethiopia samples exhibited more restricted extraction yields and caffeine concentrations, which brings attention to the darker roast levels typically found in natural Ethiopian coffee.

Further findings indicate a strong correlation between roast degree and CGA content. As observed in earlier studies, increasing the roast degree leads to substantial decreases in CGA concentrations in brewed samples. Consistently, darker roasts produced from natural Ethiopian beans had lower CGA concentrations compared to their washed counterparts with identical roast batch classifications.

The study reveals that the porosity of coffee beans affects caffeine sublimation rates and extraction kinetics during brewing. As the degree of roast increases, the porosity of the coffee beans also rises, creating additional pathways and surface areas that enhance diffusion processes, ultimately leading to higher initial caffeine extraction rates.

The study analyzed 90 brew samples and identified notable trends among various green coffee varieties regarding extraction yields, caffeine content, and roasting characteristics. The findings indicate that extraction yields peaked in batches that experienced a roasting mass loss of 12–14%, typically associated with lighter roasts. In contrast, darker roast batches demonstrated a decrease in extraction yields.

The highest caffeine concentrations in 10-minute brews were found in roast batches with a 14–16% mass loss, categorizing them as light to medium roasts. However, caffeine concentrations began to decline in darker roast batches. This behavior suggests significant volatilization of caffeine through sublimation during the latter stages of roasting, implying that the sublimation temperature for caffeine could surpass the previously documented 352°F (178°C) at atmospheric pressure.

Interestingly, the study noted that when consistent extraction yields are achieved across different roast levels, caffeine levels tend to increase with the degree of roast. Although this seems contradictory to the decreasing caffeine concentrations observed in darker roasts, it is offset by lower extraction yields in these brews. As a result, even though darker roasts may contain less caffeine overall, the remaining extractable material constitutes a larger portion of the total brew. Conversely, while lightly roasted coffee beans may hold higher caffeine concentrations, the extraction process tends to be less efficient due to their lower porosity.

These findings contribute valuable insights into the intricate relationship between coffee roasting processes and caffeine extraction, paving the way for enhanced brewing techniques and a better understanding of consumer preferences. Researchers have identified two primary mechanisms that influence the efficiency of extraction: porosity, which is enhanced through the roasting process, and the volatilization or breakdown of compounds that occurs during roasting.

The findings indicate a clear correlation between extraction yield, porosity, caffeine content, and the degree of roast. However, the researchers emphasize the need for further studies to explore whether these insights are applicable to various roasting, grinding, and brewing methods. As coffee enthusiasts continue to seek the perfect brew, understanding these dynamics can pave the way for improved brewing techniques and a richer coffee experience.

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