- The food industry is facing an increased demand for more plant-based proteins and oils, but traditional extraction techniques use solvents to remove the oil before protein recovery can begin.
- An established approach using water as the solvent aided with enzymes (EAEP) has gained traction for extracting proteins and lipids consecutively. The process requires less water when put through a two-step countercurrent extraction that recycles extract from a previous run.
- The EAEP process has been successfully applied to almonds, rapeseed, peanut, and soybean, and a new study found it effective for green coffee beans.
- The EAEP multi-stage counter-current method recovers 72% of protein and 48% lipids from green coffee beans, while reducing the water usage by 60% compared with the singleextraction process.
- The EAEP process could be applied to spent coffee grounds from daily coffee consumption to provide an upcycled product from a common waste stream.
In 2018, global coffee production reached about 10 million tons, trading at around $24.4 billion per ton. Roasted and ground, the beans are steeped in water to produce one of the most widely consumed and enjoyed beverages around the world, but coffee beans offer more than a pep in your step.
Coffee beans are 60% carbohydrates, but the beans contain a small fraction of lipids (17%) and protein (11%). Coffee lipids are primarily triglycerides (75%) with a smaller fraction of free fatty acids, such as palmitic, oleic, linoleic, and stearic acids, as well as sterols, phosphatides, tocopherols, and ceramides. In addition, coffee beans are rich in bioactive compounds with antioxidant properties.
"Green coffee oil has attracted considerable interest from the scientific and industry communities, which has been attributed to the presence of bioactive compounds with desirable functional and biological properties," said Juliana Maria Leite Nobrega de Moura Bell, associate professor in Food Science and Technology at the University of California, Davis, USA. "We have applied environmentally friendly bioprocessing strategies to maximize lipid and protein extraction from this untapped resource."
Unlike roasted beans used for coffee, green coffee beans have not been exposed to heat, leaving the nutrients and oil in the purest form. Traditional extraction practices use solvents, like hexane, to remove the lipids from the crushed bean flour. This approach has been used by industry for decades on products like soybean, because it is efficient and effective, extracting up to 95% of lipids from the seed matrix. Today, consumers expect their foods to be produced without the use of flammable and hazardous solvents, which highlights the need for developing greener extraction approaches.
To preserve the properties of green coffee oil, industry adopted the norm of cold, mechanical pressing where the beans are crushed and pressed at 26°C. This process results in a high-quality, pure oil, but the yield is low, often only 3% of the coffee bean weight. The remaining oil is trapped in the crushed cake. The cake must then be treated with solvents to remove the remaining lipids before protein extraction can begin. Higher oil yields can be obtained from roasted coffee beans, but the heat might alter the properties of the bioactive compounds. Because the extraction method is pivotal in the ability to harvest all of the beneficial compounds from the green coffee beans, de Moura Bell sought alternate techniques to harvest this potentially valuable dual commodity.
MAXIMIZING YIELD, SAVING WATER
The team has devised sustainable, environmentally-friendly techniques to extract lipids and proteins from a variety of seeds. The process begins upstream. The seeds are crushed by grinding or flaking to increase the surface area and disrupt the structure of the intracellular matrix. The aqueous extraction process (AEP) uses water to solubilize proteins from the ground seeds. With the protein removed, the matrix becomes more porous, allowing easier removal of lipids.
Enzymatic-aqueous extraction processes (EAEP) use enzymes to enhance the extraction. By developing the right cocktail of enzymes, it is possible to increase the protein and lipid extraction yield. For example, carbohydrase disrupts the integrity of the cell wall, and protease frees the oil and assembles larger proteins into smaller peptides that become more soluble in the extraction medium.
The end product of the AEP-EAEP process is a slurry that is centrifuged into spent solids and a liquid phase, which is separated into the cream and skim. The cream is a lipid-rich portion of the extraction that exists as an oil-in-water emulsion. The skim is the protein-rich fraction that contains most of the extracted protein. Because the skim is not purified, it contains proteins in many different structures along with other soluble compounds, like carbohydrates and phenolics. The remaining spent solids contain the unextracted compounds. Each fraction can potentially be converted into food, animal feed, and even fuel with additional processing.
AEP-EAEP has an advantage over other extraction techniques because it combines lipids and proteins extraction in one process. Despite these benefits, the AEP-EAEP extraction process is not perfect.
de Moura Bell is experimenting with different enzyme cocktails, pHs, and temperatures to optimize the lipid yield from different seeds. She has managed to capture up to 97% of lipids from soybean (https://doi.org/10.1016/j.fbp.2021.08.004). The one wrinkle with the AEP-EAEP technique is the extracted oil is trapped as an emulsion. A scientist must apply additional processing to liberate the lipids for future applications. The AEP-EAEP technique also consumes a lot of water, requiring time and energy to centrifuge the slurry to obtain the protein and lipid fractions.
To address the second challenge, de Moura Bell developed a multistage, counter-current extraction method to minimize water use without reducing extraction yields (Figs. 1 & 2). This approach retains the skim from the second extraction and sets it aside. A fresh water and enzyme cocktail is added to the depleted insoluble fraction to draw the last of the oil and protein from the cake. The retained skim is then applied to a fresh sample of ground seeds. This approach enhances the concentration gradient, drawing more protein and lipids from the depleted matrix.
The multistage, counter-current extraction uses less water, which reduces the amount of time and energy needed to centrifuge the slurry. Even better, this technology can be scaled for industrial applications. According to Bell, the process uses stirring tanks and decanters found in food processing facilities, allowing fast, scalable processing that requires less energy and overall cost. Furthermore, AEP-EAEP and the multistage, counter-current extraction is a sustainable way to acquire lipids and proteins, producing clean, high-value extracts from one process.
DEVELOPING THE BEST BREW
For a time, AEP-EAEP extraction had been successfully applied to soybean, rapeseed, almond, and peanuts but not green coffee beans. de Moura Bell was intrigued by coffee, because of the aforementioned abundance of proteins, lipids, and bioactive compounds in the beans.
de Moura Bell sought to improve the lipids yield from the green coffee beans beyond the meager amount obtained from cold pressing. This required experimenting with the enzyme cocktail, solid-to-liquid ratio, and reaction time to generate the highest protein and lipid yields while preserving the physiochemical properties of the compounds in each fraction.
With all great brews, the study begins with the beans. In this case, de Moura Bell used Brazilian Arabic green coffee beans. The beans were ground to a flour and sieved to 850 mm, an optimal grain size fraction to immerse in the water and enzyme wash. Before the extraction, an analysis of the coffee flour found it contained 9.4% lipids, 12.7% proteins, and 5.3% moisture.
Coffee goes cosmetic
The cosmetics industry has long sought coffee for the lipid, phenolic and bioactive compounds that provide photoprotective, anti-aging, and hydration properties (https://doi.org/10.3390/cosmetics10010012). The traditional extraction process using solvents, diminishes the quality of the oil that can be used by the industry. Cold pressing produces a high-quality and pure oil but with paltry yields. What’s an industry to do?
Upcycling is a movement that takes items from a waste stream and imbues them with a new function and added value, while reducing contributions to landfills. Coffee is an attractive candidate for an upcycled product. The home-brewed coffee waste stream offers a potential revenue source for industry consideration. The grounds thrown out after every brewed cup could provide a highvalue side stream to increase overall sustainability in a circular economic model.
A 2014 study analyzed the composition of spent coffee flour (https://doi.org/10.1007/s11947-014-1349-z ). The study results show that brewed coffee grounds contain up to 2% lipids and 17% protein. By pairing coffee grounds with de Moura Bell’s extraction process, it may be possible to obtain the compounds and oils from this waste stream, addressing some of the cosmetics industry’s needs.
Green coffee oil formulations are already found in face cream, albeit probably not from the grounds of your morning coffee. The green coffee bean formulations have been found to increase the hydration of the outermost layer of skin by lowering trans-epidermal water loss and increasing sebum levels. Studies attribute this outcome to the essential fatty acids, sterols, alkaloids, tocopherols, and carotenes in the green coffee bean oil. These cosmetic formulations also protect against solar radiation by boosting the SPF protection (https://doi.org/10.3389/fphys.2019.00519).
Imagine a day when you can submit your used coffee grounds to aid an industry that aims to make the world look and feel good. That could put more pep in your step.
The research team focused on four commercial enzymes for the experiments: alkaline protease, neutral protease, cellulase, and hemicellulose. Alkaline protease, an endoprotease produced by Bacillus licheniformis, breaks down peptide bonds along the nonterminal amino acid. The neutral protease is another endoprotease produced by B. subtilis and catalyzes the hydrolysis of protein peptide bonds into small molecule peptides under neutral, weak acid, or weak alkaline conditions. Cellulase is produced by Trichoderrma reesei and degrades cellulose into soluble sugars. Finally, hemicellulose, produced by Aspergillus niger, degrades the hemicellulose in plant cell walls.
They ran the process with all four enzymes and compared the protein and lipid results from each experimental batch to the results from the AEP without the addition of enzymes. All reactions were performed at 50°C and tested at different pH conditions depending on the enzyme (Fig. 3). Each slurry was centrifuged and the liquid fraction containing the cream and skim were transferred to a separatory funnel and refrigerated at 4°C overnight.
UNDER OPTIMUM EXTRACTION CONDITIONS, PROTEIN EXTRACTABILITY IN THE SINGLE-STAGE EXTRACTION PROCESS INCREASED FROM 65 TO 70%, WHILE OIL EXTRACTABILITY REMAINED CONSTANT (46-48%).
From the experiments, the team showed that the majority of the extracted oil (65%) was concentrated in the cream and free-oil fractions. The skim contained more than 90% of the extracted protein. Additional research is needed to optimize the separation of the lipid and protein into the cream and skim fractions.
The 0.5% alkaline protease produced the highest protein (62.23%) and lipid (47.7%) yields. The neutral protease produced smaller yields (protein 59.17% and 44% oil). Both extractions required 30 minutes for the reaction time and an additional 30 minutes for the counter-current extraction. The addition of cellulase and hemicellulose before the proteases did not significantly improve protein and lipid extractability.
While enzymes improved the solubility of the extracted protein in acidic media, de Moura Bell was determined to optimize the process. Her lab was able to reduce the alkaline protease by 80% by lowering the concentration from 0.5% to 0.1%. They also found the enzymes were more effective in a dilute solid-to-liquid ratio (1 part ground flour to 17 parts water). Under optimum extraction conditions, protein extractability in the single-stage extraction process increased from 65 to 70%, while oil extractability remained constant (46-48%). The two-stage countercurrent extraction process yielded higher oil (58%) and protein (72%).
According to de Moura Bell, more work is needed to understand how to leverage the biological properties of the protein and lipids obtained from green coffee beans. She found the bioavailability of protein extracts from green coffee beans to be quite favorable, with a digestibility around 97%. This product could be attractive to different markets. At this time, the nature of the green coffee beans and extraction conditions impart a green hue to the final protein product, which may be a turnoff for consumers. The current study focused on Brazilian Arabic green coffee beans. Other coffee varieties could produce differing results in terms of functional or biological properties, but de Moura Bell cautions it will depend on the parameters of the extraction.
Despite the potential of its protein and oil extracts, de Moura Bell does not believe coffee the beverage is facing any stiff competition in the near term.
"As a niche market, green coffee oil could grow as we see movement in consumers looking at plant-based supplements and nutraceuticals for potential health benefits, but more research is needed to evaluate biological properties, which takes time," said Bell. "And coffee as a drink, that is sacred."
About the Author
Stacy Kish is a science writer for INFORM and other media outlets. She can be contacted at firstname.lastname@example.org