K Venkatesh, K. Basavaraj, R. Naidu
The success of coffee trade more than any other commodity depends on quality. Nowhere in the chain from coffee tree to coffee cup is quality more important than at the green coffee stage, where grading and sorting to select the uniform beans for roasting is a critical step in developing taste and aroma for specific market preference. In the cured bulk coffee the beans are not uniform in size, shape, density and colour, when suck a bulk coffee is roasted it does not produce uniform roast or a clean cup.
Therefore, it is essential to segregate the coffee beans based on the above cited parameters which will enable to carry out a more unifonn roast and to prepare a better cup. It may not be possible at the curing works to enhance the quality of the badly processed coffee on the estate. But the curing works could bring about quality deterioration, if the right type of machinery is not installed, if effective and careful drying is not carried out or if the coffee is not stored under ideal and controlled conditions.
This deterioration in quality would be irreversible. Thus curing works has the onerous responsibility of receiving of coffee from the growers, drying it to the required standard (if necessary), milling (peeling/dehusking), grading sorting as per the prescribed standards, bulking, packing, storing and finally making available to the market (fig. I)
Hence, a curing works is an important processing house in preserving and enhancing the quality of coffee. Every curing works should possess the following machinery to carry out the operations efficiently.
- Sample huller
- Moisture meter
- Mechanical dryer
- Precleaner & destoner
- Huller
- Peeler/polisher
- Winnower
- Unhulled coffee separators i.Graders
- Catadors / pneumatic separators
- Gravity Separators
- PB band separators
- Electronic colour sorters
- Bulking machine
Quality control measurers to be adopted at the curing works to preserve and enhance the quality of green coffee.
An ideal curing works should possess the basic requirements like adequate and good drying yards or mechanical dryers, ideal and efficient machinery; ideal and adequate storage structures sound management, etc.
a) Checking the moisture content of coffee beans:
Immediately on receipt of coffee from the growers, the moisture content of the clean coffee should be checked
DRY CHERRY COFFEE
|
DRY PARCHMENT COFFEE
|
PRE CLEANING AND DESTONING
|
HULLING
|
PEELING/POLISHING
|
WINNOWING
|
GRADING BY SHAPE & SIZE
|
GRADING BY DENSITY
|
SORTING BY COLOUR
|
BULKING
|
PACKING
|
STORING
|
DESPACHING
|
| Fig.1 - Flow Chart illustrating the processing stage at the curing works. |
by using a standard moisture meter (Kappa/Teketor moisture meter) to ascertain whether the coffee is dried to the prescribed moisture level. If the moisture of the coffee beans is more than the prescribed standard, the raw coffee can not be processed efficiently and the quality of coffee can not be preserved during storage.
b) Redrying of raw coffee:
If the coffee received from the growers are found to be under dried, the same should be dried at the curing works to bring down the moisture content to the prescribed moisture level (Ara.pmt: 10-10.50: Ara.cherry: 10.50-11.00: Rob.pmt:l0-l0.50 :
Rob.chy:1l-11.50%). Drying of coffee to the prescribed moisture standard not only enables efficient dehusking/peeling but also provides storage stability of coffee. For redrying the raw coffee, the curing works should possess adequate and good drying yards or mechanical dryers. About 1200 sq. ft. area is required for spreading the uncured coffee equivalent to one tonne of clean coffee. The flooring of the drying yard should be made of tiles, bricks or cement/concrete. Mechanical dryers can be used for redrying coffee and these dryers will enable drying of coffee even during a wet inclement weather.
c) Storage:
Coffee can be stored in the raw fonn or cured form. However, when coffee is stored in the raw fonn, the husk! parchment cover provides protection against moisture. The moisture content of coffee for satisfactory storage should not be more than 11.50% since at this level, mould growth and enzymatic activity is minimal.
It is generally considered that when the moisture content of the coffee exceeds 13.00% certain moulds develop and when the moisture content exceeds 18.00% yeasts and bacteria also develop. Mustly flavours may easily develop when under dried coffee is stored. Depending upon the relative humidity, temperature of the air and the length of time in storage, certain biochemical changes take place in the coffee, resulting in discolouration of beans (bleached) change in the size of the beans and also loss in cup quality.
The coffee beans may also get damaged by insects, if stored under unsanitary conditions. Therefore, to retain the inherent quality of coffee, the raw/ cured coffee should be stored in ideal and controlled conditions. Coffee should be stored in the godowns built as per the standards of Bureu of Indian Standards.
Flooring of the godowns shall be either cement concrete, granite stone or any other good quality stone slabs. The roof may be either of reinfored concrete or asbestos cement sheets. Galvanized steel sheets and perplex or fibre glass sheets should not be used for roofing. Godowns should have adequate ventilation and air circulation by the provision of air vents, ventilators and exhaust fans. The coffee should be stored in a cool and dry environment with storage temperature not exceeding 20°C to 25°C and humidity in the range of 50 % to 70% to retain the quality. Ideal dunnage should be provided to all the stacks of coffee to prevent the dampness of floor affecting the coffee· and to provide better aeration. Sufficient air space (not less than 1/3rd of the stack height) should be provided between the top most layer of the coffee stack and roof of the godown. Sufficient space should be provided between the walls of godown and coffee stacks.
The coffee stored in the godown should be protected from rain, rodents, insects, birds, etc. Fertilizers, pesticides, chemicals, spices, etc., should not be stored where coffee is stored. Storing of raw coffee : The well dryed coffee or the redryed coffee should not be stored on the drying yards. But it should be stored in the godowns. The floor area required for storing raw coffee equivalent of one tonne of clean coffee is 13.50 sq. ft. for stack height of 15 feet.
Interms of volume, the space required is 202.50 cubic feet. The optimum stack height recommended for raw coffee is 15 ft. The stack height could be increased subject to a maximum of 24 bags equivalent to a height of 18 ft.
d) Precleaning & destining:
The raw coffee may contain any impurities e.g. pebbles, metal pieces, twigs, twines and other foreign bodies. These need to be removed during precleaning to protect the curing machinery. The precleanders are designed to clean and screen coffee, prior to feeding to the next processing stage. The precleanders should effectively remove stones, metal pieces, sand rubble, leaves, twigs, twines and other unwanted matter.
e) Peeliug and polishing:
The peeler peels the dried parchment layer of wet processed coffee. An efficient peeler should peel the coffee completely cool viz. without high pressure and heat, other wise the arabica parchment coffee tends to loose the bean shape and colour. The polisher removes the silver colour. The polisher removes the silver skin. Polishing usually done to give the cured coffee an attractive appearance. Excessive polishing may impart 'metallic'offtaste in the coffee beans. By the use of 'phosphor bronze' liners instead of cast iron liners in the polishers, the metallic off-taste can be minimized and it also imparts bluish colour to the beans.
f)Hulling / Dehusking:
The huller (dehusker) removes the entire dried outer coverings of the original cherries of dry processed coffee. The huller should not be used for peeling the parchment. An efficient huller should not damage coffee beans.
g) Winnowing:
The winnowers will help in separating the husk / dry parchment covers from the cured coffee.
h) Grading by size:
Primary grading is carried out based on the size of the beans. Grading is done with the help of flat screen graders drum graders.
Standard Coffee screen dimensions
| Screen number | ISO dimensions
(in mm) |
| 10 |
4.00 |
| 11 |
4.50 |
| 12 |
4.75 |
| 13 |
5.00 |
| 14 |
5.60 |
| 15 |
6.00 |
| 16 |
6.30 |
| 17 |
6.70 |
| 18 |
7.10 |
| 19 |
7.50 |
| 20 |
8.00 |
i) Grading by shape:
The pea berry beans are separated from the flat beans by using PB band separators.
j) Grading by density:
Secondary grading is done based on densimetric classification of individual grades. Catadors (pneumatic air separators) and gravity separators are used for the densimetric classification of coffee. The catadors use air to separate beans of different denstities. Gravity separators are also used to seperate heavy beans and light beans. The choice between installing catadors or gravity separators (or both) depends on the product to be processed and the export quality demanded. Idealy best results will be achieved if the coffee is first separated by size, then the main accept grades should be passed through catadors and only then over a gravity separator.
k) Sorting by colour:
The sorting of defective beans and disco loured beans is essential to achieve uniform colour and quality of the cured coffee. This is usually carried out manually. Beans sorting area should have sufficient natural light. Sorting on the tables is ideal and efficient than on the floor. However, removal of undesirable beans on the basis of colour has always been a labourious, time consuming and not very efficient procedure and for this reason most of the curing works are now equipped with a series of electronic colour sorters which can effectively separate defective / disco loured beans like blacks, browns, greens, etc. The 'off-flavoured' beans such as stinkers can be effectively removed by passing through a band of Ultra Violet light.
l) Bulking:
To obtain lots of marketable size and the best possible uniformity in quality (both in physical appearance and in the cup), small lots of coffee are bulked. This is done either manually or by using the bulking machine.
m) Aspiration and disposal of waste products:
Aspiration is of enormous importance. A curing works without proper aspiration facilities will have an unacceptable working environment and dust saturation will threaten the health of workers. In addition coffee dust is combustible and under certain circumstances can cause explosions. Coffee dust also proves a favourable medium for insect infestation and also helps spread of mould spores, etc., throughout the cunng establishment. Most modern equipments are fitted with aspiration systems, but their effectiveness should always be verified as it varies. Care must also be taken to ensure that dust collecting chambers are situated outside the curing works. The disposal of enormous quantity of husk/hull obtained during hulling/peeling causes environmental problem. Therefore, curing works should provide outside silos into which the husk/hulls are blown. Then they can be emptied into closed - body trucks for transportation off the site.
n) Packing:
Bags usually made of jute or other natural fibres are used for packing the raw / cured coffee. The quality of bags used for packing the coffee may affect its quality in the cup. For instance, the 'baggy / oily' off taste arises from the mineral oil used in the manufacture of jute bags in which coffee is stored. Similarly, the 'spicy' flavour often encountered in the cured coffee is due to the usage of bags which were previously used for the storage of spices. Coffee is very sensitive to foregn odur. Therefore, care has to be taken to use only jute bags which are free from odour. Odour free jute bags (clean, used DWF/B Twill bags, size 44 x 26 1/2 ) should be used for packing raw coffee. These gunny bags should not have been used previously for any products like fertilizers, cement, coal etc. Bags should not have holes and should not be tomed. Cured coffee should be packed in new hydro carbon free bags (treated with vegetable oil, size: 36 x 28). The cured coffee is packed in quantity of 60 to 75 kg /bag.
0) Storing of cured coffee:
The area required for storing one tonne of cured coffee is 7.50 sq ft. for stack height of 14 feet. The maximum
stack level prescribed in 14 bags height. In terms of volume, the space required for storing one tonne of clean coffee is 105 cubic ft.
p) Control measures against storage pests: Prophylactic treatment to the coffee godowns should be carried out to eradicate the storage pests before storing the coffee and also control measures should be adopted against rodents and insect pests whenever required. Presence of any living or dead insects in coffee is a serious quality defect.
q) Maintaining of conditions: hygienic Absolute hygienic conditions should be maintained in the curing works since coffee can easily absorb any odour. Presence of excreta of animal in coffee is very serious quality defect.
r) Transport of coffee: Usually to cure the coffee a grower, trader transports the raw coffee through trucks from the estate to the curing works. The coffee baggage for transport should be covered with tarpaulin to prevent the absorption of moisture on the way. During transport if the coffee is exposed to rain there will be serious loss in the quality of coffee. Therefore, proper care has to be taken during transport of coffee to avoid the exposure of coffee to rain / high humidity. The influence of relative humidity and ambient temperature on coffee bean moisture are important when the beans are undergoing ocean shipment (as in containers). Damage to the beans would occur when moisture that was liberated in a warm location, condensed as water droplets on coffee in cool location & this may cause of mould growth. However with the use of ventilated conditioners greater control over changes in temperature & moisture is possible, leading to less damage from moisture loss, sweating and condensation.
References:
1. Indian coffeeC>its flavour profile, 1987, coffee Board Publication
2. Commodity hand book - coffee an exporters' guide, 1992 ITC publication
(Reproduced from March 1999 Indian Coffee)
