Module 9. Coffee
Lesson
32
COFFEE
PROCESSING
32.1 Introduction
The previous lesson dealt with origin and
classification of coffee. In this lesson the processing of coffee bean will be discussed.
The green bean has no comestible value for humans and must be roasted before
use for developing the desired colour and flavor, enjoyed as a stimulant
beverage.
32.2 Green Bean Processing
32.2.1 Roasting process
It is a time-temperature dependent process,
whereby chemical changes are induced by pyrolysis
within the coffee beans, together with marked physical changes in their
internal structure. The required change takes place with a bean temperature
from 190°C upwards; bean temperature up to 240°C may be reached in less than 12
minutes.
Batch operated horizontal rotating drum roaster
with either solid or perforated walls, in which hot air from a furnace/burner
passes through the tumbling green coffee beans. Green coffee beans under
movement are subjected to heat by conduction from hot metal surfaces, or
convection from hot air, or more generally a mixture of both methods of heat
transfer, together with contribution by radiation.
A typically sized roaster holds 240 kg of green
coffee, with an outturn (charging to discharging) of 15 min. The furnace or
burner will be either oil or gas fired.
Other roasters include:
·
Vertical
static drum with blades
·
Vertical
rotating bowl
·
Fluidized
bed
·
Pressure
roasting
The latest roasters have shorter roast times
i.e. of the order of 3-5 min. Fast-roasted coffee is advantageous because of
lower bulk density and ‘high yield’ on brewing.
The
degree of roast may vary from ‘Very light to very dark’.
Consumer
preference is usually ‘medium roast’.
32.2.1.1 Physico-chemical changes in coffee
32.2.1.1.1 Chemical changes
The chemical changes include Maillard type reactions and caramelization of sucrose. The composition of
roasted coffee is furnished in Table 32.1.
Volatile complex comprising of furan
derivatives, pyrazines, pyridines, benzenoid aromatics, aliphatics, alicyclics and various sulphur compounds. These are
important for the flavour/aroma in medium-roast Arabica coffee.
Some compounds are generated by straight pyrolysis of single compounds e.g. chlorogenic
acids in generating phenols; there is overall 40% residual content for a medium
roast. The change in chlorogenic acid content is used
as analytical measure of ‘degree of roast’.
Similarly, coffee oil leads to formation of
small amounts of aldehydes and hyrdrocarbons.
The coffee oil is practically unaffected, as is the caffeine content.
Newly formed residuum of ~ 25% by weight of
roasted coffee is melanoidins/humic
acids. The loss of mass is 2-3% on dry basis for ‘Light roast’, whereas
it is up to 12% on dry basis for ‘Very dark roast’. The beans lose
15-20% of their weight, but increases up to 25% in size.
32.2.1.1.2 Physical
changes
The physical changes that occur include:
·
Change
in colour.
·
Formation
of cavities/cracking of surface.
·
Void
volume is 47% of ‘medium roast bean’ vs. 0% in green bean.
32.2.2 Cooling
In batch operation, the roasted beans have to
be quickly discharged at the end of required roasting period into a cooling
car, or vessel, allowing upward passage of cold air.
In addition, water may be sprayed from within
the rotating drum, just before the end of the roast – so called ‘Water
quenching’.
32.2.2.1
Advantages of water quenching
·
Assists
in necessary cooling.
·
Adds a
small percentage of water by weight to roasted beans, thereby assists uniformity
of particle size in subsequent grinding.
Table
32.1 Composition of roasted coffee
|
Component |
Typical
average content for (%) |
|
|
Arabica |
Robusta |
|
|
Alkaloids
(caffeine) |
1.3 |
2.4 |
|
Trigonelline
(including roasted byproducts) |
1.0 |
0.7 |
|
Proteinaceous
‘Protein’
Free amino acids |
7.5 0 |
7.5 0 |
|
Lipids
(Coffee oil with unsaponifiable) |
17.0 |
11.0 |
|
Sugars:
Sucrose
Reducing sugars |
0 0.3 |
0 0.3 |
|
Polysaccharides (unchanged
from green) |
33.0 |
37.0 |
|
Lignin |
3.0 |
3.0 |
|
Pectins |
2.0 |
2.0 |
|
Acids
Residual chlorogenic
Quinic
Aliphatic |
2.5 0.8 1.6 |
3.8 1.0 1.6 |
|
Minerals
(oxide ash) |
4.5 |
4.7 |
|
Caramelized/condensation
products (Melanoidins, etc.) |
25.5 |
25.5 |
|
Total |
100.0 |
100.0 |
32.2.3 Grinding
Multistage twin horizontal rollers up to 4
stages may be used to ensure more uniform particle size distribution.
1st and 2nd stages –
Essentially performs cracking or crushing the beans into smaller units.
3rd
and 4th stages – Leads to progressively finer grinding.
The grind size required is related to
subsequent method of brewing to be adopted and whether for home use or
subsequent large scale extraction i.e. coarse, medium, fine, very
fine. The ground size of roasted and ground coffee beans for different
applications is presented in Table 32.2.
Table
32.2 Grind size of roasted and ground coffee beans
|
Grind
size |
Actual
size (µm) |
|
Fine
grind |
430
(Europe), 800 (USA) |
|
Coarse
grind* |
850
(Europe), 1130 (USA) |
* for household percolators
The
number of different screen sizes numbered by aperture size within the range of
1400 µm to 250 µm. Newer method performs sizing by
laser beams.
32.2.4 Packaging
Roasted and ground (R & G) coffee releases substantial
quantities of entrapped CO2 gas which develops high internal
pressure, leading to bursting of package.
The usual packaging material is laminates.
32.2.4.1
Packaging under vacuum
It
allows a low percentage of oxygen content in headspace to be established within
the package and accommodate release of CO2. Alternatively CO2
scavenger may be used.
32.2.4.2 Degas over a
sufficient time period
The R & G coffee is allowed in bulk to
degas over a sufficient time period to a low level, followed by gas purging
whilst individual packages are being filled.
Gas purging is used to ensure that the residual
oxygen in headspace is below 1.0%.
In Europe, use of plastic packages to which a
non-return valve is securely attached allows release of excess CO2,
when internal pressure exceeds a certain predetermined level.
32.3 A Glance at
the Processing Steps
32.3.1 Green
bean treatment
·
Cleaning
·
Blending
·
Storage
32.3.2
Roasting
32.3.2.1 Roast bean treatment
·
Storage
·
Grinding
·
Conditioning
32.3.2.2 Extraction
·
Fast
instant coffee extractors (FIC)
·
Conventional
batch percolators
·
Continuous
counter current extractors (CONTEX)
·
FIC
extraction unit
It
reduces extraction time by 50% compared to batch percolators. Water is directed
through the ground coffee in two stages. The process results in two completely
separated extract fractions viz., aroma and hydrolysis. After extraction, the
extract is filtered and centrifuged.
32.3.2.3 Extraction treatment
·
Aroma
recovery
·
Clarification
·
Clarification
It is a
system consisting of filters and centrifuges to separate insoluble parts from
the extract to achieve international standards.
32.3.2.4 Concentration
·
Falling
film and plate evaporators
·
Freeze
concentration
·
Membrane
filtration systems
·
Concentration: It serves to increase the solids
content in extract prior to freeze or spray drying.
a) Thermal
concentration – Multistage non-recirculating
evaporators operating under vacuum in a plug flow mode.
b) Membrane filtration
– The
aroma fraction of the extract can be pre-concentrated using ‘reverse osmosis’
in a membrane filtration system.
c) Freeze
concentration – By cooling the extract to subzero temperatures,
excess water is removed as ice crystals.
Freeze and thermal concentration, membrane filtration,
Falling film and plate evaporators are used for the
purpose of concentration.
32.3.2.5 Drying
·
Nozzle
Tower spray dryer
·
Fluidized
bed spray dryer
·
Continuous
freeze dryers (CONRAD)
·
Batch
freeze dryers (RAY)
32.3.2.6 Agglomeration
·
Rewet agglomerators (RWA)
32.3.2.7 Packing
32.4 Domestic
and Catering Methods of Brewing
Brewing
is extraction of soluble substances contributing to the basic taste plus of
volatile substances for overall flavour. Roast coffee must be ground before
brewing.
The two main mechanical principles are:
32.4.1
Steeping/ Slurrying of R & G coffee with water, with or
without agitation, followed by sedimentation or filtration or both.
32.4.2
Percolation in fixed beds of R & G coffee held in an
open or closed container. Water may be passed through either in a single pass
under gravity or under pressure (including steam, as in Espresso making), or in
a multipass.
32.5 Extraction
Extraction of coffee solids can be carried out
by
·
Fast
instant coffee extraction.
·
Conventional
batch percolators.
·
Continuous
countercurrent extractors.
32.6 Factors in Brewing
·
Coffee-to-water
weight ratio
·
The
appliance used for brewing.
·
The
temperature employed.
Of the components of roasted coffee, only some
will be extracted completely with variable amounts of the others to reach ~ 28%
w/w total maximum and 21% optimum under household brewing conditions, by hot or
boiling water – so called ‘yield’.
Mechanical operation involved is a means of
separating the undesired so-called ‘Spent coffee grounds’ from the required
brew formed by sufficient contact with water. The brew should contain as little
of spent ground particles as possible and must be presented hot (i.e. 50-55°C).
32.7 Flavour Quality
of Coffee Brew
The
factors determining flavour quality of brew include:
·
The
choice of blend used.
·
The
degree of roast.
·
Brewing
conditions.
·
Choice
of grind.
32.8
Filter coffee
South Indian Coffee, also known as Filter Coffee is a
sweet milky coffee made from dark roasted coffee beans (70-80%) and chicory (20-30%), especially popular in the
southern states of Tamil Nadu and Karnataka.
The most commonly used coffee beans are Arabica and Robusta.
Outside India, a coffee drink prepared using a
filter may be known as Filter Coffee or as Drip
Coffee as the water passes
through the grounds solely by gravity and not under pressure or in longer-term
contact.
Fig. 32.1 South Indian metal based coffee
filter
32.9 Aromatization of Coffee
It is a term applied to a process, whereby
essentially the headspace coffee aroma volatiles are made available by plating
coffee aroma oil, prepared by expression methods from roast coffee, or other
sources onto the soluble coffee, usually at the packing stage. This is a
treatment imparted to improve the flavour and aroma. The powder lacks full
flavour and aroma of freshly brewed coffee. The flavour and aroma constituents
are trapped and recovered during roasting, grinding and extraction and from
oils pressed from coffee bean. The cold CO2 does not damage the
flavour and aroma compounds in coffee oil and it is easily separated from
extracted oil for recompression and reuse.
After CO2
removal of the oil, the ‘Roasted and Ground coffee’ is still highly suitable
for extraction of water soluble solids in the regular extraction battery
operation.
32.9.1 Aroma recovery
The
extract fractions are stripped of their volatiles in an aroma recovery unit.
After being stripped from the concentrate in a flash evaporator, the aroma is
recovered in a 2-stage condenser system.