Comprehensive study of coffee infusions composition targeting espresso properties
Year: 2019
Type: dissertation
Abstract: The different coffee brews as espresso, filtered, or instant coffee exhibit distinct physico-chemical characteristics due to the intrinsic properties of roasted coffee as well as to the conditions used to prepare the brews (e.g. amount of coffee to extract, volume and temperature of extraction, and grinding degree). Nevertheless, all coffee brews are prepared with only two ingredients: roasted ground coffee and water. Therefore, all coffee brews contain carbohydrates, proteins, caffeine, chlorogenic acids, lipids and volatile compounds that vary in a certain range, constituting a complex mixture of compounds. They are related with properties that can be valued in formulations with defined organoleptic (e.g. body, flavour, colour or texture) and/or nutritional and health benefits (e.g. dietary fibre). Thus, it may be hypothesized that the modulation of a coffee extraction process may allow to obtain extracts that can be more related to instant, filtered or espresso coffee according to the desired brew characteristics. To fulfil this hypothesis, coffee infusion experiments were conducted to infer how operational variables affect the efficiency and/or selectivity of coffee compounds. The use of the same roasted coffee in all experiments eliminated the variability related to coffee species/origin and/or roasting degree of coffee. Response surface methodology (RSM) was used to comprehensively study and compare two distinct extraction methods: a conventional coffee infusion process at atmospheric pressure (solid-liquid extraction) and a coffee microwave-assisted extraction (MAE). The use of closed-vessel systems allows to achieve, in MAE, temperatures higher than the water boiling point. The extraction yield and the extracts chemical composition, namely, carbohydrates content and composition, caffeine, and chlorogenic acid (5-CQA), as well as colour properties were used as responses to describe the systems. The conventional infusion process was delineated according to a face-centered central composite design with 4 factors (time and temperature of extraction, mass-to-volume ratio (w/V) and grinding degree) in a wide range of conditions (10-360 min, 20-80 ºC, 1-6 g/30 mL, and three grinding levels). The conditions applied allowed to extract up to 30% of coffee compounds. Temperature was the major factor for coffee extracts differentiation, regarding both overall and carbohydrates yield of extraction and composition. It was estimated the contribution in the extract of the two main coffee carbohydrates: galactomannans (GM) and arabinogalactans (AG). The distinct extract composition across the design space was more related to GM extraction than AG, as the variation of AG content was lower (7.5-12.0%) when compared to GM (5.4-18.3%), with GM/AG ratio varying from 0.7 to 1.9. The viscosity of the coffee solutions was positively related to the GM content of the extract. In cold brew conditions, mainly the extraction of AG was promoted. However, prolonged times promoted the increase of GM extraction. Moreover, the extraction of GM from coffee powder increased with the increasing of temperature. The higher temperature and lower w/V ratio lead to browner coffee extracts. Caffeine and 5-CQA were extensively extracted in the conditions tested. For this reason, their relative content in each extract varied due to the content of the other extracted compounds. The effect of coffee MAE was studied by a Box-Behnken design with 3 factors and 3 levels: time (1-10 min), temperature (120-180 ºC) and w/V ratio (2-6 g/60 mL) on the overall extraction yield (24-47%w/w), and in parameters as carbohydrates content (18-43% w/w), sugars composition, caffeine (4-7% w/w) and 5-CQA (1-2% w/w). In this case, temperature was also the major factor for coffee extracts differentiation, with MAE allowing considerably higher overall extraction yields than those obtained with the conventional system, mainly when performed at 180 ºC. This was associated to a substantial increase in AG extraction over GM, resulting in extracts approaching instant coffee composition. Almost all arabinose and galactose presented in roasted coffee powder was extracted during the experiments at 180 °C, while the residue was still rich in GM and cellulose. Moreover, there was a yellowing of the extracts. Indeed, MAE prove to be a reliable method to produce extracts that can be used as defined food/brew ingredients, as instant coffee-like extracts, providing also a GM and cellulose rich residue that can also be valued as a source of dietary fibre under a circular economy concept. As polysaccharides account for a considerable amount of espresso coffee brews and contribute to their organoleptic properties (viscosity and foam stability), they were used as target to resemble espresso properties under the milder conventional infusion process. The optimized conditions regarding espresso similarity were scaled up and concentrated via freeze- and spray-drying and compared to commercial espresso and instant coffee samples regarding content and composition in sugars, protein, melanoidins, lipids, caffeine, 5-CQA, and volatile compounds, and physical properties such as colour, dissolution in water, viscosity, pH, and foamability. The scaled-up extract exhibited a sugars composition similar to espresso coffee, with predominance of GM over AG, contrary to instant coffee, even the ones labelled as “espresso coffee”. FTIR analysis confirmed the overall resemblance of infusion and espresso coffee extracts composition, except on the lower amount of lipids present. All extracts were able to produce foam when submitted to CO2 injection or when an effervescent agent was used. The volatile profile, including key odorants, for the infusion freeze-dried and the espresso coffee extracts was similar. Even though there was a loss of volatiles intensity in comparison with the ready-made espresso coffee sample, it resembled the fresh brewed espresso coffee. Although the lower amount of lipids and volatile compounds with impact on aroma is still a challenge to pursuit the target of approaching espresso composition by infusion methods, this can be overcome with the addition of roasted coffee oil, rich in volatile compounds, in a future formulation, aiming to improve the organoleptic quality of the instant-espresso coffee product.
Author: Guido R. Lopes
Language: English
Topic: Coffee research and impacts
Subfield: Pharmacology (medical)
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W3158443354