Introduction: of spent coffee grounds and their potential


Coffee is one of the most popular
beverages in the world and is the second most traded commodity in the world
after petroleum 1. It is produced in over 80 countries worldwide and an
average of 3.5 billion cups of coffee are drank every day 2.However, due to
an increase in coffee production as a result of increased yields and an
increase in consumption there is a need for the management of coffee residues
or by-products. The main coffee by-products are spent coffee grounds (SCG),
silverskin and the coffee husks 3. One tonne of coffee can result in over
600kg of residues. These residues can by used as a by-product to increase its
value. This idea is known as valorization. Here we will focus on the
valorization of spent coffee grounds and their potential uses.



Coffee is
produced by brewing roasted coffee beans. Coffee is grown in about 80
countries, all tropical countries, however coffee consumption is highest in
Europe and the United States (Heeger et., 2017). Three countries are
responsible for over half the world’s coffee production, namely Brazil, Vietnam
and Columbia. In 2016-2017 approximately 151.62 million 60 kilogram bags were
produced worldwide (Yilmaz et al., 2017).  Green coffee production increased by roughly 17% between 2000 and 2012,
more than likely due to an increase in yield (Campos-Vega et al., 2015) .  


There are approximately 70
different types of coffee beans (Yilmaz et al., 2017). The most popular and
used are Coffea Arabica and Coffea Robusta (Y.K. Tastan, 2009). Coffee is the
common name for the Coffea trea belonging to the Rubiaceae family. Coffee trees
start producing fruits 3 or 4 years after they are planted and it takes 8 to 10
months for the fruits to ripen. Arabica is one type of coffee bean, from which
75% of the world’s coffee is made. It was the first discovered coffee bean and
is very difficult to grow. To produce one kilogram of roasted coffee, over
eight thousand Arabica coffee beans are required. Robusta is a newer type than
Arabica and is more resistant to growing in harsher conditions. It is mostly
used for instant coffee due to its cheapness and it contains almost twice as much
caffeine content than Arabica beans (Yilmaz et al., 2017)

Harvesting and Processing

cherry consists of skin, pulp, mucilage, parchment, silverskin and coffee bean.
The coffee bean is found in the centre of the cherry. It is the bean that is
used for the brewing of the beverage. Extracting the coffee bean from the
coffee cherry is quite a complex procedure and leads to a lot of waste
products. When the fruits are ripe they are harvested and processed into coffee
beans. This can be done in two ways, either by dry processing or wet
processing. In dry processing the coffee cherries are dried out in the sun and
then de-hulled. The husk is the by-product from this process(Esquivel & Jiménez, 2012) . In the wet processing of coffee
cherries water is used to separate the ripened fruit from the unripe fruits.
The ripe fruit sink to the ground whereas the unripe coffee cherries remain on
the surface. The addition of a pulper helps remove the skin and the pulp. The
next step is a fermentation step, here the mucilage and the remnants of the
pulp are removed. Finally the coffee beans are de-hulled, which means the
parchment is taken off and dried (Esquivel and Jiménez, 2012, Pandey et al.,
2000). The coffee bean is now ready to be roasted which gives a typical coffee
flavour, aroma and colour (Murthy and Naidu, 2012).








Image –
(R. Campos-Vega et al., 2015)


Spent Coffee Grounds (SCG)

Spent coffee grounds is the residue
obtained during the brewing process (Cruz et al., 2012). Spent coffee grounds
contain many organic compounds such as lipids, carbohydrates, carbonaceous, and
nitrogen containing compounds which can be exploited as a source of value added
products 4. According to Karmee (2018) roughly nine million tonnes of spent
coffee grounds were dumped in landfills in 2014. With an increased focus on
global warming and effective waste management, alternative options for
disposing of and valorizing this by-product are being utilised. SCG can be used
as a source of renewable energy with R. Campos-Vega et al. giving the example
of Nestlé, the world’s biggest food company pledging to use SCG in 20 of their
factories to reduce waste and help protect the environment. Currently specialised
agencies collect the SCG from the soluble coffee industry and sell it on for
different purposes. This is only  recent
phenomenon as the potential uses of SCG were only established in the last
decade with very little research done in the area prior to this period. This is
highlighted by the statement from Campos-Vega et al., that half of the total
number of papers published in this area since 1973 have been in the last four