Engineering:Used coffee grounds

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Short description: Waste product from brewing coffee
Used coffee grounds
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Used coffee grounds in boxes.
Usage

Used coffee grounds are the result of brewing coffee, and are the final product after preparation of coffee. Despite having several highly-desirable chemical components, used coffee grounds are generally regarded as waste, and they are usually thrown away or composted. As of 2019, it was estimated that over 15 million tonnes of spent coffee grounds are generated annually. Due to this quantity of waste and the chemical properties of used coffee grounds, they have several potential uses.

In the late 19th century, used coffee grounds were used to adulterate pure coffee.[1]

Chemical composition

Most used coffee grounds are similar in chemical composition, although coffee grounds used to make instant coffee have fewer chemicals in them due to a more extensive extraction process.[2] Used coffee grounds are rich in sugars,[3] which comprise about half of their weight. A further 20% is made up of proteins, and a further 20% is lignins.[4] The dry coffee grounds contain significant amounts of potassium (11.7 g/kg), nitrogen (27.9 g/kg), magnesium (1.9 g/kg), and phosphorus (1.8 g/kg).[5] The quantity of caffeine remaining in used coffee grounds is around 48% of that in fresh coffee grounds.[6] There are significantly less tannins in used coffee grounds than fresh coffee grounds.[7]

Production

On average, 1 tonne of green coffee produces approximately 650 kg of spent coffee grounds,[8] and over 15 million tonnes of spent coffee grounds are generated annually.[9] In keeping with a life cycle approach to sustainability,[10] this large quantity of waste requires waste management plans. Due to the amount of spent coffee grounds generated and the chemical properties of spent coffee grounds, the usage of spent coffee grounds is avidly investigated.[6]

Knockbox

An espresso tamper, grinder, and knockbox

A knockbox (informally known as a "bash bin", "coffee column", "slam piece" or "bang bang") is a device used to store spent espresso grounds, called a puck, after a shot of espresso has been pulled.[11][12] It is generally made out of stainless steel or plastic, and has a sturdy bar known as a "bash bar", against which a portafilter is tapped to release the puck. Because the espresso puck is typically very hot after extraction, knockboxes need to be sturdy. Further, they are also subject to repetitive knocking and tapping to dislodge espresso after use, and thus rubber and stainless steel are the most commonly used materials. In various coffee houses they are seen as drawers which are integral within the bar structure, or as stainless steel boxes usually located near the grinder for convenience.

Usage

Precautions

It is not recommended to burn dried used coffee grounds, as they give off hazardous nitrogen oxides when burnt.[4]

In gardens

Composting worms moving about in used coffee grounds.

In gardens, coffee grounds may be used for composting or as a mulch[13] as they are known to slowly release nitrogen into the soil. They are said to be especially appreciated by worms and acid-loving plants, such as blueberries,[14] although due to acids being leached from the grounds while in use, they typically have a neutral pH,[15] and red wiggler growth and survival has been experimentally tested and found to be reduced in treatments using used coffee grounds as the primary feedstock for the worms.[16] Used coffee grounds are particularly noted as a soil amendment.[15] Spent coffee grounds have phytotoxic properties which can be reduced through composting.[17] Gardeners have reported the use of used coffee grounds as a borer,[18] slug and snail repellent,[13][19] but this has not yet[when?] been scientifically tested.[20] Some commercial coffee shops run initiatives to prevent the grounds from going to waste, including Starbucks' "Grounds for your Garden" project,[21] and community sponsored initiatives exist, such as "Ground to Ground" [22] or the 'Green Coffee Shop Scheme' in Cambridgeshire, UK.[23]

In fortune telling

Main article: Unsolved:Tasseography

In divination and fortune-telling, the patterns of coffee grounds are used for predictions.

In the home

Dried used coffee grounds were recommended to fill pincushions.[24] Used coffee grounds have other homemade uses in wood staining, air fresheners, and body soap scrubs.[13]

Agricultural uses

Oyster mushroom mycelium on coffee grounds.

Initiatives have succeeded using coffee grounds as a substrate for the cultivation of mushrooms (including oyster mushrooms).[25][26] The use of spent coffee grounds in this application has the advantage of the used coffee grounds needing no pre-treatment to be usable as a mushroom substrate.[6]

Application of 10 kg used coffee grounds per square metre has been suggested as part of a crop rotation system, where for the first six months, the field is allowed to lie fallow with a layer of coffee grounds on it suppressing weed growth, then the coffee grounds are plowed in and legumes are grown, which fix their own nitrogen. Application of an equal amount of horse manure at the same time as the coffee grounds has been shown to nearly eliminate negative effects of fresh used coffee grounds.[27]

It has been proposed to use spent coffee grounds to feed ruminants, pigs, chickens and rabbits, but the high lignin content makes this an undesirable use.[28]

Industrial uses

Coffee grounds may be used industrially in biogas production or to treat wastewater.[20] Bioethanol may also be produced from the sugar content of spent coffee grounds, after it is defatted as a pre-treatment, it is typically hydrolysed by dilute acid.[4]

Biodiesel may be produced from coffee grounds, either directly by extracting the oils using solvents,[29] by mixing the grounds with methane and a catalyst,[30] or by using the grounds to feed bio-producing algae.[31] Used coffee grounds have also been investigated as an energy source through hydrothermal liquefaction, producing a char with very high energy content.[32]

In concrete making, 10-15% of the sand used may be replaced with used coffee grounds.[33]

It has been suggested to recover caffeine from used coffee grounds for commercial applications in agrifood, cosmetic, nutraceutic or pharmaceutic industries.[34]

In 2021, Gloucestershire-based football club Forest Green Rovers trialed a kit made from 35% used coffee grounds combined with recycled plastic.

Cafe uses

A method of safe disposal of coffee ground waste has been tested to assist with noise management in cafés. Using coffee waste and urea resin to create sound absorbing panels, the noise generated in cafés can be reduced while coffee waste is recycled. The coffee grounds are dried, mixed with the resin, then pressed and heated.[35]

Different particle sizes of the coffee grounds were tested. The Coffee Waste Sound Absorption (CWSA) that had the highest sound absorption coefficient was observed at 0.4g/cm3 density. While the panels were applied to the ceiling of the café, they could reduce reverberation time from 1.2 s without sound absorbing material to 0.7 s with CWSA. The sound pressure level was reduced by 7 decibels.[35]

See also

  • Ecological effects of coffee
  • Sustainable coffee

References

  1. ↑ Pendergrast, Mark "Uncommon grounds : the history of coffee and how it transformed our world" 2010 Basic Books. ISBN 978-0-465-02404-9
  2. ↑ Pérez-Burillo, Sergio; Cervera-Mata, Ana; Fernández-Arteaga, Alejandro; Pastoriza, Silvia; Rufián-Henares, José Ángel; Delgado, Gabriel (November 2022). "Why Should We Be Concerned with the Use of Spent Coffee Grounds as an Organic Amendment of Soils? A Narrative Review" (in en). Agronomy 12 (11): 2771. doi:10.3390/agronomy12112771. ISSN 2073-4395. 
  3. ↑ Mussatto, Solange I.; Carneiro, Livia M.; Silva, João P.A.; Roberto, Inês C.; Teixeira, José A. (January 2011). "A study on chemical constituents and sugars extraction from spent coffee grounds". Carbohydrate Polymers 83 (2): 368–374. doi:10.1016/j.carbpol.2010.07.063. 
  4. ↑ 4.0 4.1 4.2 McNutt, Josiah; He, Quan (Sophia) (March 2019). "Spent coffee grounds: A review on current utilization". Journal of Industrial and Engineering Chemistry 71: 78–88. doi:10.1016/j.jiec.2018.11.054. 
  5. ↑ Ballesteros, Lina F.; Teixeira, José A.; Mussatto, Solange I. (December 2014). "Chemical, Functional, and Structural Properties of Spent Coffee Grounds and Coffee Silverskin". Food and Bioprocess Technology 7 (12): 3493–3503. doi:10.1007/s11947-014-1349-z. 
  6. ↑ 6.0 6.1 6.2 Campos-Vega, Rocio; Loarca-Piña, Guadalupe; Vergara-Castañeda, Haydé A.; Oomah, B. Dave (September 2015). "Spent coffee grounds: A review on current research and future prospects". Trends in Food Science & Technology 45 (1): 24–36. doi:10.1016/j.tifs.2015.04.012. 
  7. ↑ Choi, Bogyoung; Koh, Eunmi (August 2017). "Spent coffee as a rich source of antioxidative compounds". Food Science and Biotechnology 26 (4): 921–927. doi:10.1007/s10068-017-0144-9. PMID 30263620. 
  8. ↑ Murthy, Pushpa S.; Madhava Naidu, M. (September 2012). "Sustainable management of coffee industry by-products and value addition—A review". Resources, Conservation and Recycling 66: 45–58. doi:10.1016/j.resconrec.2012.06.005. Bibcode: 2012RCR....66...45M. 
  9. ↑ Kamil, Mohammed; Ramadan, Khalid Mustafa; Awad, Omar I.; Ibrahim, Thamir K.; Inayat, Abrar; Ma, Xiao (July 2019). "Environmental impacts of biodiesel production from waste spent coffee grounds and its implementation in a compression ignition engine". Science of the Total Environment 675: 13–30. doi:10.1016/j.scitotenv.2019.04.156. PMID 31026637. Bibcode: 2019ScTEn.675...13K. 
  10. ↑ Gebreeyessus, Getachew Dagnew (August 2022). "Towards the sustainable and circular bioeconomy: Insights on spent coffee grounds valorization". Science of the Total Environment 833. doi:10.1016/j.scitotenv.2022.155113. PMID 35427619. Bibcode: 2022ScTEn.83355113G. 
  11. ↑ "Best Coffee Knock Boxes". https://dipacci.com.au/search?type=product&q=COFFEE+KNOCK+BOX. 
  12. ↑ "Coffee machine accessories: Bang Bang" (in en). https://www.sunbeam.com.au/kitchen-and-home/coffee/coffee-machine-accessories/bang-bang. 
  13. ↑ 13.0 13.1 13.2 "Don't Throw Out Your Leftover Coffee Grounds!". Huffington Post. 4 August 2014. http://www.huffingtonpost.com/hometalk/7-astonishing-ways-hometa_b_5632711.html. 
  14. ↑ Martin, Deborah L; Gershuny, Grace, eds (1992). "Coffee wastes". The Rodale book of composting. Emmaus, PA: Rodale Press. p. 86. ISBN 978-0-87857-991-4. https://books.google.com/books?id=N6sx5-OM_psC&pg=PA86. Retrieved January 5, 2010. 
  15. ↑ 15.0 15.1 "Coffee Grounds Perk up Compost Pile With Nitrogen" (in en). Life at OSU. 10 June 2009. http://today.oregonstate.edu/archives/2008/jul/coffee-grounds-perk-compost-pile-nitrogen. 
  16. ↑ Liu, K.; Price, G.W. (September 2011). "Evaluation of three composting systems for the management of spent coffee grounds". Bioresource Technology 102 (17): 7966–7974. doi:10.1016/j.biortech.2011.05.073. PMID 21704514. Bibcode: 2011BiTec.102.7966L. 
  17. ↑ Stylianou, Marinos; Agapiou, Agapios; Omirou, Michalis; Vyrides, Ioannis; Ioannides, Ioannis M.; Maratheftis, Grivas; Fasoula, Dionysia (December 2018). "Converting environmental risks to benefits by using spent coffee grounds (SCG) as a valuable resource". Environmental Science and Pollution Research 25 (36): 35776–35790. doi:10.1007/s11356-018-2359-6. PMID 29860699. Bibcode: 2018ESPR...2535776S. 
  18. ↑ (in en) Southern Cultivator. J.W. & W.S. Jones. 1853. https://books.google.com/books?id=xGxNAAAAYAAJ&dq=%22coffee%20grounds%22%20%2Bgarden&pg=PA156. 
  19. ↑ "NORTH COAST GARDENING: Winter vegetable growing". Eureka Times-Standard. 24 December 2014. http://www.times-standard.com/lifestyle/20141224/north-coast-gardening-winter-vegetable-growing. 
  20. ↑ 20.0 20.1 Chalker-Scott, Linda (2009). "Coffee grounds— will they perk up plants?". Master Gardener. Puyallup Research and Extension Center, Washington State University. https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/coffee-grounds.pdf. 
  21. ↑ "Coffee for Your Plants? Starbucks Offers Free Coffee Grounds for Gardeners". Starbucks.com. https://news.starbucks.com/news/starbucks-coffee-grounds-for-the-garden. 
  22. ↑ "About Us | Coffee Grounds to Ground". Groundtoground.org. 24 May 2010. http://groundtoground.org/about/. 
  23. ↑ "Green Coffee Shop Scheme" (in en). 2019-05-08. https://cambridgefoodhub.org/services/green-coffee-shop-scheme/. 
  24. ↑ (in en) The Universal Household Assistant: A Cyclopedia of what Everyone Should Know.... A.L. Burt. 1884. p. 347. https://books.google.com/books?id=1F1f2s4I7LIC&dq=coffee+grounds+pincushion&pg=PA347. 
  25. ↑ Dillen, Nina (2015-06-18). "Zelf oesterzwammen kweken op basis van ... koffiegruis? Zo doe je dat". https://www.hln.be/nina-kookt/zelf-oesterzwammen-kweken-op-basis-van-koffiegruis-zo-doe-je-dat~aa08de83/. 
  26. ↑ "PermaFungi |". 28 March 2014. https://www.permafungi.be/?lang=nl. 
  27. ↑ Yamane, Koji; Kono, Mitsuaki; Fukunaga, Taiji; Iwai, Kazuya; Sekine, Rie; Watanabe, Yoshinori; Iijima, Morio (January 2014). "Field Evaluation of Coffee Grounds Application for Crop Growth Enhancement, Weed Control, and Soil Improvement". Plant Production Science 17 (1): 93–102. doi:10.1626/pps.17.93. Bibcode: 2014PlPrS..17...93Y. 
  28. ↑ Mussatto, Solange I.; Machado, Ercília M. S.; Martins, Silvia; Teixeira, José A. (July 2011). "Production, Composition, and Application of Coffee and Its Industrial Residues". Food and Bioprocess Technology 4 (5): 661–672. doi:10.1007/s11947-011-0565-z. 
  29. ↑ Kondamudi, Narasimharao; Mohapatra, Susanta K.; Misra, Mano (2008-12-24). "Spent Coffee Grounds as a Versatile Source of Green Energy" (in en). Journal of Agricultural and Food Chemistry 56 (24): 11757–11760. doi:10.1021/jf802487s. ISSN 0021-8561. PMID 19053356. https://pubs.acs.org/doi/10.1021/jf802487s. 
  30. ↑ Patel, Prachi (2017-05-18). "A simpler route to biodiesel from used coffee grounds" (in en-US). https://www.anthropocenemagazine.org/2017/05/a-simpler-route-to-biodiesel-from-used-coffee-grounds/. 
  31. ↑ Coxworth, Ben (2022-11-02). "Coffee grounds used to both feed and support biodiesel-producing algae" (in en-US). https://newatlas.com/science/coffee-grounds-feed-support-biodiesel-algae/. 
  32. ↑ Papamatthaiakis, Nikolaos; Barbero-López, Aitor; Eronen, Eemeli; Jänis, Janne; Mola-Yudego, Blas; Haapala, Antti (2025-07-09). "Hydrothermal Liquefaction of Coffee Silverskin and Spent Coffee Grounds: Bioenergy and Biochemical Potential" (in en). BioEnergy Research 18 (1): 65. doi:10.1007/s12155-025-10867-3. ISSN 1939-1242. 
  33. ↑ Christie, Damian (31 October 2019). "From coffee to concrete: Researchers offer a new solution". Quarry. https://www.quarrymagazine.com/2019/10/31/from-coffee-to-concrete-researchers-offer-a-new-solution/. 
  34. ↑ Vandeponseele, Alexandre; Draye, Micheline; Piot, Christine; Chatel, Gregory (2 April 2021). "Study of Influential Parameters of the Caffeine Extraction from Spent Coffee Grounds: From Brewing Coffee Method to the Waste Treatment Conditions". Clean Technologies 3 (2): 335–350. doi:10.3390/cleantechnol3020019. 
  35. ↑ 35.0 35.1 Yun, Beom Yeol; Cho, Hyun Mi; Kim, Young Uk; Lee, Sung Chan; Berardi, Umberto; Kim, Sumin (1 May 2020). "Circular reutilization of coffee waste for sound absorbing panels: A perspective on material recycling". Environmental Research 184. doi:10.1016/j.envres.2020.109281. 



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