Coffee Drying Research Articles

Modeling of Forced and Natural Convection Drying Process of a Coffee Seed

HighlightsThe forced and natural convection drying processes of a single coffee grain can be predicted.The moisture distribution profiles depict the drying phenomena and water concentration zones.With accurate predictive drying curves, coffee growers can ensure high-quality coffee beans.Abstract. Different coffee drying technologies face complex tasks in ensuring an acceptable final seed moisture content. This research performed a Finite Element Analysis (FEA) study, simulating a single coffee bean's drying process as a transient mass diffusion model under mechanical and natural convection conditions, so the drying behavior and data of both case scenarios can be foreseen and controlled by a predictive Finite Element Model (FEM). A wet bean was 3D-scanned and digitized as the FEA simulation geometry; the water diffusion between the grain and the atmosphere was defined by a diffusion coefficient subject to the drying air temperature and the grain's moisture content. Three cases were studied: mechanical grain drying at three different temperatures (50, 45, and 40°C) in a forced convection environment; variable natural convection drying under environmental conditions (wet and dry season); and constant natural convection (wet and dry season), including the variation in day/night temperature and relative humidity. The results agree well with the data found in the literature, obtaining the graphical moisture distribution of the phenomena, predictive drying curves, diffusion coefficients, and isotherms. Both simulated drying scenarios provide essential information for coffee growers to improve and control their drying processes, thus obtaining high-quality grains, reducing contamination by microorganisms, and ensuring the integrity of their products. Keywords: Coffea arabica, Coffee Drying, Coffee Seed, Finite Element Model, Moisture Diffusion.

Development and validation of a heated drying air diffusion system to optimize rotary dryers and final coffee quality

The final quality of pre-processed coffees is influenced by the applied drying technology. Thus, the aim of the study was to develop and validate a heated air flow diffusion system to optimize and reduce the drying time of rotary dryers and improve the final quality of coffee. Computational fluid dynamics was used for the simulation of the air fluid dynamics in the combustion chamber of the heat generator. It was observed that the energy losses in the upper and lower walls of the heat generator chamber were higher with an increase in the convective heat transfer coefficient. It was found that the rate of fluid flow presented a fully developed profile, in which the higher speed value was found in the central region of the outlet. The reduction in moisture content during coffee drying was directly proportional to the increase in temperature. The Midilli model shows the best fit to describe the drying curves of the coffee. The effective diffusion coefficient increases with increasing temperature of the drying air. It was observed that the adjustments of the fluid dynamics in the burning of gas and the adaptation of the diffuser system significantly influenced the drying time and final quality of naturally processed and pulped coffees. In conclusion, the adapted technological set, a rotary dryer with gas heating and diffusion of heated air, had a high performance in the final quality of the coffee, and for this reason it is recommended to producers and the industry.

The effect of drying temperature on cup quality of coffee subjected to mechanical drying

The objective of the work was to study the effect of drying temperature on cup quality of the robusta coffee subjected to mechanical drying in comparison with conventional sun drying. The robusta coffee processed by wet (parchment coffee) and dry (cherry coffee) methods were subjected to drying at different temperature regimes (40 oC, 50 oC and 60 oC) in a rotary mechanical dryer. The results of the study indicated that as the drying temperature increased, the time of drying reduced. Sun drying of parchment coffee took 48 hours (approximately seven days) to attain the desired moisture content of 11-12 per cent, while mechanical drying reduced the drying time to 16 to 24 hours. Similarly, cherry coffee subjected to sun drying took 88 hours (approximately 15 days), while mechanical drying reduced the drying time to 32 to 48 hours. The cup quality rating of coffee dried by different drying methods revealed that sun-dried robusta parchment coffee scored the highest cup rating. As the drying temperature increased, the cup quality ratings decreased. A similar cup quality rating was also observed with cherry coffee. These results indicate a considerable reduction of drying time when coffee beans are dried in a mechanical dryer. However, there is a need to regulate the drying temperature, which otherwise would negatively impact the quality of coffee. The drying temperature should not exceed 40oC for preserving the innate quality of robusta coffee because the high drying rates provoked by high temperatures can cause damage to the coffee quality due to the damage caused to the cell membranes. Overall, mechanical drying is more advantageous to sun drying in-terms of drying hours (indirectly reduces dependency on manpower) and preservation of innate quality of the coffee.

Hybrid drying of pulped arabica coffee cherry beans (Coffea arabica L. cv. Catuai) using a hexagonal microwave dryer designed by numerical simulations

AbstractThe consumption of specialty coffees has been increasing significantly in recent years. Their quality is influenced by several factors and the drying method is the most important. In order to reduce drying time and maintain the quality of the coffee, several techniques have been used, among which the microwave‐assisted convective drying stands out. In this work, a hexagonal microwave dryer was developed using numerical simulation techniques; then some parameters were evaluated in drying of special coffee using air at room temperature and microwaves heating, named hybrid drying. The results showed that the simulations reaches good results, being possible its use in the prediction of the electromagnetic fields and dimensioning of hexagonal cavities. In addition, the prototype performed well in the coffee drying, influenced by the coffee mass, temperature and time, reducing the moisture content from 50.4 to 10 g/100 g wet basis in 5 hr. The hybrid drying was advantageous compared with convective or microwave drying processes.Practical applicationsSpecial coffees require long drying times so that there is no loss of product quality. The application of hybrid drying by including microwave heating in the drying equipment reduces the energy cost and the processing time, providing an improvement in the processing of specialty coffees. Through this work, it was possible to verify the scale‐up potential of the microwave cavities by simulation and the hybrid drying of special coffees. The use of numerical simulation techniques for the design and scale‐up of microwave cavities will help researchers and the industry to dimension cavities for use in several scientific areas, further spreading hybrid drying via microwave.

Modeling and simulation of heat and mass transfer during drying of coffee in a box dryer using C++ programming language

Abstract Drying is one of the stages of coffee processing and has been considered one of the factors that impact the quality of the final drink. In this sense, the present study developed a phenomenological model for a box-type dryer implementing in C / C ++ programming language. The profile of the exhaust air temperature was obtained over the drying time. The model was verified by comparing data obtained from the literature, thus guaranteeing its functionality to make predictions about the air temperature profile in equipment used by small coffee producers, which may contribute to the increase in product quality.

Design of Measurement of Coffee Seed Water Content Using Load Cell Sensor On Coffee Dryer

The conventional method of drying coffee beans that depend on sunlight (drying) has a number of drawbacks, one of which is that the drying productivity takes a long time. In general, coffee drying is carried out until it reaches a moisture content of 12.5% to achieve a quality bean standard. Mathematically, the moisture content can be assessed by paying attention to the initial weight of the coffee beans and the final weight of the coffee beans. So that the use of the Load Cell sensor to read the depreciation value of the coffee bean weight on a coffee dryer powered by Hybrid Collector and LPG coffee beans. Research that has been carried out during 200 minutes of drying in a coffee dryer, the Load Cell sensor is able to read the depreciation value of the coffee bean weight by 114 grams from the initial weight of 978 grams. Therefore, it can be concluded that the final moisture content of coffee beans has reached 12.01% and the final weight of coffee beans is 864 grams.

Drying kinetics of organic parchment coffee beans (Coffea arabica L.) using microwave fluidised bed: Semi-theoretical modeling

The aim of this study was to model the drying kinetics of microwave and fluidised bed of parchment coffee bean (Coffea arabica L.) using a semi-theoretical approach. A completely randomized design with six treatments: three with microwave drying (MD) and the rest combined with fluidized bed drying (MFBD) was carried out. The drying curves were established until reaching a final moisture content between 10 to 12% w.b. The results indicated that the drying time decreased as the microwave power level increased. When MFBD was used, drying times increased approximately 4-fold compared to MD treatments. The values of effective diffusivity for coffee beans treated with MD and MFBD ranged from 0.913 to1.72×10-9 m2.s-1 and 0.23 to 0.42×10-9 m2.s-1, respectively. Although all the models presented high goodness of fit, the two-term model presented the highest R2 (0.9855 – 0.9978) and lowest RMSE (0.0128-0.0285). In general, all models adequately fitted the experimental data so they can be used to predict the drying kinetics, therefore they could be very useful in equipment design and optimization of the coffee drying process.

Effect of coffee cherry maturity on the performance of the drying process of the bean: Sorption isotherms and dielectric spectroscopy

The study of the dynamic sorption isotherms of parchment coffee revealed that the maturity stage has an effect on the water availability of the bean, and that such dependence might affect the bean drying. The cultivar had an impact on the dispersion at high water activity values (aw > 0.8); hence, depicting dissimilar conditions before the drying is actually performed. The Guggenheim-Anderson-de Boer (GAB) sorption model revealed that stages 2, 3 and 7 present higher monolayer moisture content (Xm), K values close to one for all stages indicated a multilayer behaviour close to free water mobility and low C values for stages 2 and 7 represented a stronger water binding to the monolayer for these stages. Under static conditions and the drying conditions defined according to the moisture content of the beans, the unripe stages depicted an accelerated water activity decay that might promote stalling after roasting due to low values of the attribute (aw < 0.4). In a second phase of the study, the dielectric spectroscopy technique was used to evaluate the effect of the maturity and the drying category, and the response depended on the drying category rather than on the maturity stage except for the drying category with the higher moisture content range. The dielectric constant was utilized to predict both the moisture content and the water activity of the beans. The kernel-based ridge method with radial basis function for the moisture content and sigmoid function for the water activity model had good performance (R2: 0.82–0.88). A feature selection model based on a pipelined framework to include the whole spectra selected the 0.3 GHz to 0.9 GHz range as the optimal estimator, which was consistent with the performance of the single frequency models at 0.3 GHz. Although the errors were lower for the feature selection model, the single frequency is preferable as it has minimal complexity. As a non-linear dependence was evidenced, the machine learning strategies were necessary to respond to this condition. Consequently, the technology could be considered for the assessment of the water features of the bean during coffee drying or the storage of green coffee.

Supply Chain Uncertainties of Small-Scale Coffee Husk-Biochar Production for Activated Carbon in Vietnam

Between 2014 and 2019, an innovative small-pyrolysis system (the PPV300) using agrowaste as feedstock was transferred and adapted from Switzerland to Vietnam by a United Nations Industrial Development Organization (UNIDO) project. The coffee husk resulting from the processing of coffee beans is usually disposed of in Vietnam or burned inefficiently to dry coffee beans. Small-scale pyrolysis of coffee husk using the PPV300 avoids smoke emissions and local air pollution while providing an energy source for coffee drying and biochar as a by-product. This paper investigates the uncertainties in the “coffee husk to activated carbon” supply chain in Vietnam and discusses the pros and cons of different supply chain setups using a framework derived by Chopra and Meindl (2013). According to the analysis, a number of actors (an intermediary without previous direct involvement in the coffee supply chain, a coffee processor, or a hybrid between farm and processor), each with advantages and disadvantages, would be suitable from a supply chain perspective to attain an efficient strategy that would keep the price of the biochar low. However, in order to be attractive for one activated carbon producer, several PPV300 systems are needed to reach a significant economy of scale. Sufficient husk sourcing and storage capacity is also needed. If the purpose is simply to produce biochar for activated carbon, processors and intermediaries could consider a simpler and cheaper design than the PPV300. In conclusion, supply chain uncertainties and economic viability can be optimized when the PPV300 is used by coffee processors or intermediaries, who are able to make use of the co-products generated (biochar, heat, and wood vinegar). In addition to its financial viability, all of the other co-benefits of this technology should be taken into consideration (reduced smoke and environmental pollution, avoided health costs, greenhouse gas savings, etc.) for a proper assessment of its economic attractiveness.

Environmental trade-offs associated with bioenergy from agri-residues in sub-tropical regions: A case study of the Colombian coffee sector

The coffee sector generates vast amounts of residues along its value chain. Crop residues, like coffee stems, are burned in the field, used for domestic cooking or coffee drying in processing plants having significant environmental and health implication to rural communities. This research investigated the environmental impacts of replacing current practices with modern bioenergy applications in the Colombian coffee sector. A biomass gasification system to produce decentralised energy from coffee stems was considered, and the environmental impacts of such bioenergy implementation were evaluated. A lifecycle assessment was conducted to quantify the environmental performance of this bioenergy system and compare it to current residues uses and energy needs that feature small-to large-scale coffee farms. The results show that deploying modern bioenergy could result in reductions in 48–86% greenhouse gas (GHG) emissions and up to 98% less particulate matter formation when current practices and fossil-based energy are replaced. However, negative impacts should be considered as substituting grid electricity, largely generated from hydro-electricity, could increase GHGs by 68% and fossil fuel consumption by 73%. The results also show the relevance of understanding the environmental performance of bioenergy systems compared to reference scenarios; this allowed to evaluate and identify environmental trade-offs from modern bioenergy implementations. To maximise benefits and minimise the limitations of these systems, it is important to conduct whole-system assessments that inform on the wider environmental impacts of using agri-residues for bioenergy generation in a region- and system-specific context.

Preliminary study of drying of natural coffee by cyclical pressure changes

A “special coffee” is obtained with an adequate drying process, which allows the preservation of volatile substances responsible for excellent cup quality. The aim was to carry out preliminary studies of drying of natural coffee by implementing a Cyclic Pressure Changes (CPCD) dryer, comparing it with solar drying and with a mechanical drying system by forced convection of low-temperature air. The drying times and rates of the systems used were compared, as well as the respective valuation of the cup quality. Drying times were 767 h for solar drying system, without reaching the desired moisture content; 153.5 h for mechanical drying system and 449 h for the CPCD system. However, the global cup tests showed a better quality of the grain obtained with the CPCD system (87 points), than the one obtained with the mechanical drying at 35°C (84 points).

Propuesta de implantación de invernadero de secado de café con cubierta parabólica y estructura modular adaptada

El presente trabajo propone un invernadero destinado al secado de café formado por una cubierta solar parabólica y estructura modular adaptada. Se inició a través de un levantamiento planimétrico elaborado por diferentes softwares de ingeniería y arquitectura como: SolidWorks y Revit Architecture. Para el experimento se utilizó principios de ingeniería inversa, tomando como base, una estructura ya existente para desarrollar un modelo de acople modular, con el fin de establecer interacciones entre mecanismos y estructura. El estudio demostró que el diseño de los acoples de armado, facilitan su movilización, reducen costos y permiten la aplicación de materiales accesibles para el secador parabólico solar (marquesina), adicionalmente se presentan curvas de secado de café en un secador solar tipo parabólico y su tiempo representados en Statgraphics. El resultado del modelo con acoples de armado modular se asoció correctamente con resultados experimentales existentes, permitiendo realizar comparaciones entre tiempo y eficiencia del secado de café.

Secador de camada fixa: uma adaptação para melhorar a secagem de café natural no Brasil

A secagem de café por meio de secadores de camada fixa tem sido amplamente utilizada por cafeicultores brasileiros. Entretanto, frequentemente são relatados elevados gradientes de teor de água formados nestes secadores, bem como o aparecimento de microrganismos na massa de café. Desenvolveu-se o presente trabalho com a finalidade de monitorar a performance destes secadores e seu efeito na qualidade final do produto, comparando à uma adaptação feita em secadores de camada fixa convencional. Durante a secagem foram monitorados em diversos pontos na câmara de secagem as seguintes características: temperatura do ar de secagem, do ar intergranular, do ar de exaustão e do ar ambiente; umidade relativa do ar de secagem, do ar ambiente e do ar de exaustão e teor de água do produto em diferentes pontos e alturas na câmara de secagem (0,12; 0,36 e 0,60 m). Dentre as avaliações do produto seco, foram realizadas: análise de cor e análise microbiológica. De acordo com os resultados obtidos, a secagem com inversão do fluxo de ar resultou em um gradiente de teor de água de 0,4% (b.u.), enquanto a secagem convencional ocasionou um gradiente de 8% (b.u.). Em relação à coloração do produto nos diferentes manejos de secagem, a secagem com inversão resultou em produtos mais claros. Para a análise microbiológica, foram encontrados fungos dos gêneros Cladosporium, Fusarium e Aspergillus, dentre outros. Sendo que a média da incidência destes fungos foi maior nas amostras da secagem convencional que na secagem de camada fixa com inversão do fluxo do ar.

Economic analysis of low-temperature grain drying

The main objective of this study was to simulate the economic feasibility of low-temperature grain drying systems considering Brazilian conditions and using three drying capacities, seven grain types and two furnace fuels. For this, 42 scenarios were simulated and compared among themselves by using an economic analysis based on the cash flow model with project lifetime of 20 years. The indices net present value, payback period, benefit-cost ratio and internal rate of return were applied during the analyses. Simulations showed that drying of coffee and beans in large systems presented higher economic feasibility, regardless of the furnace fuel used. All simulated scenarios were cost-effective provided that at least two drying cycles are performed per year. Labor costs, social taxes, grain type and drying capacity most affected the profitability of this kind of investment, while the furnace fuel less influenced the evaluated economic indices.

Efect of lnf (cna - cnb) enzyme complex in the drying process and the coffee quality

The world demand for quality coffee is growing and the market is increasingly demanding for sustainable practices, leading coffee growers to search for new coffee processing processes always aiming for maximum quality. The objective of this work was to verify the benefits of using the LNF CNA - CNB enzyme complex in the postharvest process, in order to compare the results of drying time and the final beverage quality of the product. The research was carried out at Fazenda Iracema, municipality of Machado - Minas Gerais, with mechanically harvested coffees from the cultivar Mundo Novo, with 80% maturation, and the treatments were: natural and washed coffee, with and without application of enzymatic complex and drying in suspended, concrete and mixed courtyards (initially in the yard and finished in rotary dryers), the enzyme complex dosage was 1 ml CNB + 2.5 ml CNA per liter of water, applied to 60 liters of coffee. The experimental design was in randomized blocks, in a 2 x 2 x 3 factorial scheme (2- types of “washed or natural” processing; 2- with and without enzymatic application; 3 - “suspended, concrete and drying” drying forms), with 4 repetitions per treatment, totaling 48 experimental plots. The coffees were dried to 11.5% humidity and evaluated by the SCAA. It was concluded that the use of LNF (CNA - CNB) enzymatic complex in the coffee drying process did not influence the final drying time results, nor did it influence the coffee beverage aroma attributes. The drying forms directly influence the drying time. Enzymes; post-harvest; specialty coffees.

The potential of coffee stems\xa0 gasification to provide bioenergy for coffee farms: a case study in the Colombian coffee sector

The coffee industry constitutes an important part of the global economy. Developing countries produce over 90% of world coffee production, generating incomes for around 25 million smallholder farmers. The scale of this industry poses a challenge with the generation of residues along with the coffee cultivation and processing chain. Coffee stems, obtained after pruning of coffee trees, are one of those abundant and untapped resources in the coffee supply chain. Their high lignocellulosic content, the low calorific value ranging between 17.5 and 18 MJ kg−1 and the low ash content make them a suitable solid fuel for thermochemical conversion, such as gasification. This research evaluates the feasibility of using these residues in small-scale downdraft gasifiers coupled to internal combustion engines for power and low-grade heat generation, using process modelling and the Colombian coffee sector as a case study. The producer gas properties (5.6 MJ Nm−3) and the gasifier’s performance characteristics suggest that this gas could be utilized for power generation. A cogeneration system efficiency of 45.6% could be attainable when the system’s low-grade heat is recovered for external applications, like in the coffee drying stage. An analysis of the energy demand and coffee stems availability within the Colombian coffee sector shows that the biomass production level in medium- to large-scale coffee farms is well matched to their energy demands, offering particularly attractive opportunities to deploy this bioenergy system. This work assesses the feasibility of providing coffee stem–sourced low-carbon energy for global coffee production at relevant operating scales in rural areas.

Thin-layer drying of parchment Arabica coffee by controlling temperature and relative humidity.

This paper presents thin‐layer drying of parchment coffee (Coffea arabica). Thin‐layer drying of parchment coffee was conducted under controlled temperatures (50°C, 60°C, and 70°C) and relative humidities (10%–30%). The temperature of the drying air was important for drying at a high temperature, which results in the rapid removal of moisture and reduced time for drying. Nine thin‐layer drying models (Newton, Page, Henderson and Pabis, logarithmic, two‐term, modified Henderson and Pabis, two‐term exponential, approximation diffusion, and modified‐Midilli) were fitted to the experimental data for parchment coffee. The drying parameters of parchment coffee were related to temperature and relative humidity. The best model was the modified‐Midilli model, which can be used to design the optimal dryer. The effective moisture diffusivity of parchment coffee drying was determined by minimizing the sum of squares of the deviations between the experimental data for the moisture content and the predicted values of thin‐layer drying. The effective moisture diffusivity as a function of the temperature at each relative humidity was expressed by the Arrhenius‐type equation.

Evaluation of the Energy Density for Burning Disaggregated and Pelletized Coffee Husks.

Coffee husks represent about 12 wt % of coffee grains, generating a significant impact on the environment because of its inadequate disposal. In Colombia, this waste presents an energy resource opportunity equivalent to over 49,106 TJ per year. However, several challenges related to this type of biomass, such as the moisture content, the irregular shapes, and the low bulk density, make its use difficult in current burners. Thus, in this paper, the combustion of coffee husk pellets was studied in detail to design a high-efficiency burner to produce energy for coffee drying. The pellets were prepared in a pelletizer with 15% moisture and 20% yield and burned in a bench-scale lateral reactor to determine the energy density. It was found that the combustion properties of coffee husk depend on the specifics of the pelleting process. The energy density values were Iv = 0.789 MW/m3 and Ig = 0.007 MW/m2, which could be used to design the combustion chamber for coffee husk burning.

Effect of drying and mechanical pre-drying previous to storage in the quality of coffee grain (Coffea arabica L.)

The aim of this study was to evaluate the effect of drying and pre-drying conditions of samples of parchment coffee stored during 6 months under controlled conditions of relative humidity of 70% and temperature of 25°C quantifying the Electrical Conductivity (EC), Potassium Leaching (KL) and Fatty Acidity (FA) of the grain. The coffee drying consisted on combined processes using air temperatures of 50°C and 60°C and two periods of 4 and 8 hours in the mechanical pre-drying and subsequently taken to commercial moisture content under patio drying, and two control treatments consisting of complete mechanical drying and complete solar drying. The results of the tests showed the following values: 6.35 - 16.34 Sm cm-1 / g DM (EC), 2.0 - 4.45ppm / g DM (KL) and 0.76 - 1.57mL KOH / 100g MS (FA), where the statistical analysis shows that there is no effect of the drying type on the studied variables. However, the highest accumulated averages in the storage time were observed for the complete mechanical drying. The values did not show deterioration of the cellular structure attributed to the frame of reference of drying with high temperature or prolonged storage. The tests of Electrical Conductivity (EC) and Fatty Acidity (FA) shows sensitivity to the storage time while the Potassium Leaching (KL) test did not constitute an indicator of grain deterioration attributable to thermomechanical stress or storage time.

Physico-chemical characteristics and sensory attributes of coffee beans submitted to two post-harvest processes

Post-harvest processes (PHP) interfere with the quality of the coffee beverage. Recently, biochemical phenomena and critical points concerning coffee quality in PHP were identified. The objective was to evaluate the chemical composition and sensory attributes of green beans and roasted coffee beans from natural (CN) and semi-dry (CD) PHP. Samples were processed by coffee growers from a Brazilian coffee-producing region. The physico-chemical variables of green and roasted beans and sensory attributes were grouped in order to apply a multiple factorial analysis. This analysis showed that the description of CN and CD coffees depended on the group of variables employed. Certain aroma and flavor precursors, such as sucrose, proteins and 5-CQA, were associated with the year of production, whereas lipids, phenolic compounds, caffeine and chlorogenic acids, were associated with the PHP. Effects attributed to the occurrence of germination during drying were observed in both processes and had reflexes on the sensorial attributes. The results of the samples prepared by the producers are similar to the results obtained in laboratory experiments by other authors. The same attributes were found in the description of both processes, suggesting that they depend on the content of precursors in the green coffee beans. The formation of the precursors was influenced by factors related to peeling and drying of the green coffee. The control of these favor factors enabled the success of each PHP.