Coffee Processing
- 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 a 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.
Physico-chemical changes in coffee
- Chemical Changes
The chemical changes include Maillard type reactions and caramelization of sucrose. The composition of roasted coffee is furnished below table
The volatile complex comprising of furan derivatives, pyrazines, pyridines, benzenoid aromatics, aliphatics, alicyclic 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 an overall 40% residual content for a medium roast. The change in chlorogenic acid content is used as an analytical measure of the degree of roast.
Similarly, coffee oil leads to the formation of small amounts of aldehydes and hydrocarbons. The coffee oil is practically unaffected, as is the caffeine content.
The newly formed residuum of ~ 25% by weight of roasted coffee is melanoidins/humic acids. The loss of mass is 2-3% on a dry basis for Light roast, whereas it is up to 12% on a dry basis for a Very dark roast. The beans lose 15-20% of their weight but increase up to 25% in size.
- Physical changes
The physical changes that occur include:
- Change in colour.
- Formation of cavities/cracking of the surface.
- Void volume is 47% of medium roast bean vs. 0% in the green bean.
- Cooling
In batch operation, the roasted beans have to be quickly discharged at the end of the required roasting period into a cooling car, or vessel, allowing upward passage of cold air.
Also, water may be sprayed from within the rotating drum, just before the end of the roast so-called Water quenching.
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.
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 |
Grinding
Multistage twin horizontal rollers up to 4 stages may be used to ensure more uniform particle size distribution.
1st and 2nd stages Essentially perform cracking or crushing the beans into smaller units.
3rd and 4th stages Lead to progressively finer grinding.
The grind size required is related to the 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 below
Grind size of roasted and ground coffee beans
Grind size | Actual size (m) |
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 m to 250 m. The newer method performs sizing by laser beams.
Packaging
Roasted and ground (R & G) coffee releases substantial quantities of entrapped CO2 gas which develops high internal pressure, leading to the bursting of package.
The usual packaging material is laminated.
Packaging under vacuum
It allows a low percentage of oxygen content in the headspace to be established within the package and accommodate the release of CO2. Alternatively, CO2 scavenger may be used.
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 the headspace is below 1.0%.
In Europe, the use of plastic packages to which a non-return valve is securely attached allows the release of excess CO2, when the internal pressure exceeds a certain predetermined level.
A Glance at the Processing Steps
Green bean treatment
Cleaning
Blending
Storage
Roasting
Roast bean treatment
Storage
Grinding
Conditioning
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.
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.
Concentration
Falling film and plate evaporators
Freeze concentration
Membrane filtration systems
Concentration: It serves to increase the solids content in extract before freeze or spray drying.
a) Thermal concentration Multistage non-recirculating evaporators operating under vacuum in a plug flow model.
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.
Drying
Nozzle Tower spray dryer
Fluidized bed spray dryer
Continuous freeze dryers (CONRAD)
Batch freeze dryers (RAY)
Agglomeration
Rewet agglomerators (RWA)
Packing
Domestic and Catering Methods of Brewing
Brewing is the 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:
Steeping/ Slurrying of R & G coffee with water, with or without agitation, followed by sedimentation or filtration or both.
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.
Extraction
Extraction of coffee solids can be carried out by
Fast instant coffee extraction.
Conventional batch percolators.
Continuous countercurrent extractors.
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.
The 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).
Flavour Quality of Coffee Brew
The factors determining the flavour quality of brew include:
The choice of blend used.
The degree of roast.
Brewing conditions.
Choice of grind.
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.
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 the 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.
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.
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