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.

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. Used coffee grounds are rich in sugars, which comprise about half of their weight. A further 20% is made up of proteins, and a further 20% is

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidit ...

s. 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). The quantity of

caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class and is the most commonly consumed Psychoactive drug, psychoactive substance globally. It is mainly used for its eugeroic (wakefulness pr ...

remaining in used coffee grounds is around 48% of that in fresh coffee grounds. There are significantly less

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and Precipitation (chemistry), precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' is widel ...

s in used coffee grounds than fresh coffee grounds.

Production

On average, 1 tonne of green coffee produces approximately 650 kg of spent coffee grounds, and over 15 million tonnes of spent coffee grounds are generated annually. In keeping with a life cycle approach to

sustainability Sustainability is a social goal for people to co-exist on Earth over a long period of time. Definitions of this term are disputed and have varied with literature, context, and time. Sustainability usually has three dimensions (or pillars): env ...

, 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.

Knockbox

A knockbox (informally known as a "bash bin", "coffee column", "slam piece" or "bang bang") is a device used to store spent

espresso Espresso (, ) is a concentrated form of coffee produced by forcing hot water under high pressure through finely ground coffee beans. Originating in Italy, espresso has become one of the most popular coffee-brewing methods worldwide. It is cha ...

grounds, called a puck, after a shot of espresso has been pulled. 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 In atmospheric chemistry, is shorthand for nitric oxide () and nitrogen dioxide (), the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tr ...

when burnt.

In gardens

In gardens, coffee grounds may be used for

compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical, and biological properties. It is commonly prepared by Decomposition, decomposing plant and food waste, recycling organic materials, and man ...

ing or as a

mulch A mulch is a layer of material applied to the surface of soil. Reasons for applying mulch include conservation of soil moisture, improving soil fertility, fertility and health of the soil, reducing Weed control, weed growth, and enhancing the v ...

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 Blueberries are a widely distributed and widespread group of perennial flowering plants with blue or purple berries. They are classified in the section ''Cyanococcus'' with the genus ''Vaccinium''. Commercial blueberries—both wild (lowbush) ...

, although due to acids being leached from the grounds while in use, they typically have a neutral pH, 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. Used coffee grounds are particularly noted as a

soil amendment A soil conditioner is a product which is added to soil to improve the soil’s physical qualities, usually its fertility (ability to provide nutrition for plants) and sometimes its mechanics. In general usage, the term "soil conditioner" is often ...

. Spent coffee grounds have phytotoxic properties which can be reduced through composting. Gardeners have reported the use of used coffee grounds as a borer, slug and snail repellent, but this has not yet been scientifically tested. Some commercial coffee shops run initiatives to prevent the grounds from going to waste, including

Starbucks Starbucks Corporation is an American multinational List of coffeehouse chains, chain of coffeehouses and Starbucks Reserve, roastery reserves headquartered in Seattle, Washington. It was founded in 1971 by Jerry Baldwin, Zev Siegl, and Gor ...

' "Grounds for your Garden" project, and community sponsored initiatives exist, such as "Ground to Ground" or the 'Green Coffee Shop Scheme' in Cambridgeshire, UK.

In fortune telling

divination Divination () is the attempt to gain insight into a question or situation by way of an occultic ritual or practice. Using various methods throughout history, diviners ascertain their interpretations of how a should proceed by reading signs, ...

and

fortune-telling Fortune telling is the spiritual practice of predicting information about a person's life. Melton, J. Gordon. (2008). ''The Encyclopedia of Religious Phenomena''. Visible Ink Press. pp. 115–116. The scope of fortune telling is in principle ...

, the patterns of coffee grounds are used for predictions.

In the home

Dried used coffee grounds were recommended to fill

pincushion A pincushion (or pin cushion) is a small, stuffed cushion, typically across, which is used in sewing to store pins or Sewing needle, needles with their heads protruding to take hold of them easily, collect them, and keep them organized. Pincus ...

s. Used coffee grounds have other homemade uses in wood staining, air fresheners, and body soap scrubs.

Agricultural uses

Initiatives have succeeded using coffee grounds as a

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

for the cultivation of mushrooms (including oyster mushrooms). 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. Application of 10 kg used coffee grounds per square metre has been suggested as part of a

crop rotation Crop rotation is the practice of growing a series of different types of crops in the same area across a sequence of growing seasons. This practice reduces the reliance of crops on one set of nutrients, pest and weed pressure, along with the pro ...

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 Legumes are plants in the pea family Fabaceae (or Leguminosae), or the fruit or seeds of such plants. When used as a dry grain for human consumption, the seeds are also called pulses. Legumes are grown agriculturally, primarily for human consu ...

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. 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.

Industrial uses

Coffee grounds may be used industrially in

biogas Biogas is a gaseous renewable energy source produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, Wastewater treatment, wastewater, and food waste. Biogas is produced by anaerobic ...

production or to treat

wastewater Wastewater (or waste water) is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of do ...

. 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.

Biodiesel Biodiesel is a renewable biofuel, a form of diesel fuel, derived from biological sources like vegetable oils, animal fats, or recycled greases, and consisting of long-chain fatty acid esters. It is typically made from fats. The roots of bi ...

may be produced from coffee grounds, either directly by extracting the oils using solvents, by mixing the grounds with methane and a catalyst, or by using the grounds to feed bio-producing algae. In concrete making, 10-15% of the sand used may be replaced with used coffee grounds. It has been suggested to recover caffeine from used coffee grounds for commercial applications in agrifood, cosmetic, nutraceutic or pharmaceutic industries. 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. 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.

References

{{Coffee Coffee derivatives Horticulture Composting Food waste