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Coffee processing Technology |
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Coffee Plant |
Coffee is an evergreen shrub or small tree. The coffee plant is a woody perennial evergreen dicotyledon. Coffee flowers are white and sweet smelling, producing green berries, which turn red when ripe. The ripe coffee fruits are called cherries or grapes. The coffee–cherry contains a mucilaginous pulp with two greenish grey seeds or beans, each covered by a thin membrane, the silver skin, and both are enclosed in a common husk-like membrane or parchment. Sometimes a single bean fills the berry instead of two and the seed is called a pea berry since it is like a pea. The beans grow in pairs of seeds in cherries. The ripe berries are picked for processing.
Geographically coffee grows between the Tropic of cancer and the Tropic of Capricorn (torrid zone). Outside this area coffee cannot survive when there is opportunity for frost. Higher the altitude, the quality of coffee will be better. The limiting factor is the frost danger zone. Most of the coffee is grown in volcanic soils. Temperature of 20±3°C and annual rainfall between 1500 and 2000 mm are ideal for coffee. Average coffee yield is around 500 kg / ha. The annual production of coffee in India is 294000 metric tones in an area of 3, 54,840 ha. The plantations in the south India (Karnataka, Kerala and Tamil Nadu) are the cradles of Indian coffee. |
The different species of coffee are given below: |
Coffee Cherry |
Coffee processing methods Processing of coffee consists of removal of the skin, pulp, parchment and silver skin. There are two main methods used in coffee processing viz., washed (wet) process and natural (dry) process. In the wet process, the ripe fruit is squeezed in a pulping machine which removes most of the soft outer pulp. The product is called washed coffee. The washed coffee is then dried to 12 per cent moisture content and then yellow parchment layer is removed in a hulling machine and the finished green coffee results. In the natural method, the fruit is allowed to remain on the tree after the fully ripe stage and is partially dried before harvesting. After harvesting it is dried and dehulled. |
Steps in coffee processing
For natural coffee, the processing goes directly from transportation to drying. |
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(i) Storage of harvested coffee In the case of washed coffee, ripe cherries are dumped into a tank partially filled with water. The floats, which are considered inferior, are removed. (ii) Classification of coffee cherries Classifying systems of coffee employ the principles of floatation and screen separation by size. Systems of this type require large amounts of water. The cherries are drawn from the bottom of storage tank to a stone and dirt remover. The berries that sink with stones and dirt pass down the U-shaped chamber. Stones and dirt collect at the bottom being heavier than coffee. The floats by-pass the U-shaped trap. Separate canals lead the floats and sinkers into rotating screens. |
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(iii) Pulping The pulping operation has two steps: first, the fruit is squeezed between the roughened surface of either a rotating cylinder or a disk and a stationary member with a smooth slotted surface. The squeezing action detaches skin and flesh i.e. the pulp from the screen. In the second step, the seeds are separated from the pulp by means of a plate fixed at right angles to the cylinder or disk with a precisely regulated clearance of 1.5 mm. The seed is then taken to a stream of water for next operation. (iv) Removal of mucilage The freshly pulped coffee seeds are covered with a slippery mucilaginous layer. The presence of this layer delays drying and handling will be hard. The mucilage can be removed by: a) Natural fermentation, (a) Natural fermentation It is used for 90-95 % of present washed coffee production. The pulped coffee is placed in tanks, water is drained and the coffee is held until the mucilage is dispersible. When the digestion is complete coffee beans are washed. The time required for digestion varies from 6 to 72 hours depending on temperature and thickness of mucilage. For a typical, average coffee plantation at altitudes between 600 to 1200 m, the natural fermentation time is 24 hours. (b) Fermentation with enzymes To accelerate digestion of mucilage, pectic enzymes containing pectase, pectinase etc. as active ingredients are added to reduce the digestion time to 8 hours, which improves quality. (c) Chemical method Sodium hydroxide solution of 3-5 % concentration or 6-8 % sodium bicarbonate solution is used and the digestion time is varied between 30 minutes to 1 hour for removal of mucilage. (d) Warm water Equal quantities of pulped coffee and water are mixed and heated to 50±3°C as quickly as possible. The warm water breaks down the mucilage gel and the coffee is washed after 3 min. (e) Pulping (v) Washing Washing is done with the help of washer, which is a low pressure, low power machine which removes mucilage by attrition. It is a trough vibrating longitudinally and contains fixed and moving baffles. The coffee beans are rubbed against each other and on the baffles and by the time the coffee has traveled from one end of the trough to the other end and the mucilage is rubbed off. In vertical type washer, a vertical steel cylinder is covered at top. A vertical shaft with arms is rotated at 300 rev / min. Baffles are attached to the inside of stationery cylinder. The pulped coffee and water are fed at the bottom. The mucilage is removed by the time the coffee overflows from top outlet. The maximum loss of solids during fermentation is 4.8 % in 24 hours which is the average time required for natural fermentation. The quick removal of mucilage using washers will avoid this weight loss. |
(vi) Drying of green coffee beans The berries are sun dried by spreading them out on drying floors and the coverings are removed by hulling. The thickness of layer in natural sun drying is 50-100 mm and the coffee is stirred at frequent intervals. The beans are later cured in curing sheds. The product obtained is known in trade as cherry or native coffee. (b) Mechanical drying Mechanical drier consists of horizontal tunnel, the top of which is enclosed by trays with bottoms of wire screen or perforated metal. Wet coffee is placed in the trays and heated air under sufficient pressure is passed through the coffee. The source of heat may be wood, coal, oil, hot air or steam of dried coffee pulp. |
Sun-drying of Coffee |
(vii) Dehulling Hulling machine removes the dried skin, pulp and parchment from the dried cherry. The hulling machine consists of iron screw with helical pitch increasing towards the discharge end. Hulling is achieved by creating friction among the beans. The broken parts of the parchment along with dust fall through a perforated steel plate at the bottom of the screw by means of suction air to a cyclone separator. An average hulling loss for arabica parchmen coffee is 20 %. |
Coffee beans with shell |
Green Coffee Beans
Roasted coffee beans |
(viii) Roasting Coffee is then taken to the roasting plant. Large size particles are removed in a rotary screen and light materials are removed by air. In small plants, green coffee is blended within the roasting cylinder. In large plants, blending is done before the beans enter the cylinder. Raw or green coffee has no flavour or aroma and has an unpleasant taste. Roasting of green coffee affects the most important physical and chemical changes in the coffee because flavour is developed here. For use as a beverage, it is roasted, powdered and brewed and the aqueous extract used as a beverage with or without the addition of milk, sugar and other substances.During roasting many physical and chemical changes occur. The beans swell in size to almost double of their original size, the dull-green colour changes to brown and the characteristic coffee aroma develops. The beans lose their hard horny structure and become brittle, with the outer surface still smooth and firm. During roasting, pressure develops in the beans and this appears to be necessary for the proper development of coffee flavour. The flavour is due to a mixture of numerous components rather than a definite chemical entity and is apparently produced during roasting. Some moisture is lost during roasting and carbon dioxide is produced in large quantity, some of it escapes and rest get absorbed within the texture of the roasted bean. Carbohydrates decompose, caramalize and perhaps in combination with other substances, contribute to the aroma of the beverage produced from the roasted beans. Oxidative changes can be prevented by the presence of carbon dioxide in roasted coffee. On storage, carbon dioxide is lost and so are the flavour and aroma. Moisture also has a profound effect on the flavour of coffee. Coffee exposed to moisture loses all its flavour in a relatively short time. (ix) Grinding The roasted coffee is made into powder with the help of a Burr mill or Grinder and finally the coffee powder is packed in suitable packaging material for marketing and further consumption. Quality of CoffeeThe wet method gives better quality coffee with a bluish-green colour (green coffee). Each variety of coffee has its own flavour and other characteristics. Generally, marketed coffee is a blend of different varieties of coffee beans. The blends are controlled for flavour, aroma, colour and strength from the roasted bean. |
Quality of Coffee The wet method gives better quality coffee with a bluish-green colour (green coffee). The green seeds are then graded and packed. Green coffee may be stored for prolonged periods with no adverse effects. Each variety of coffee has its own flavour and other characteristics. Generally, marketed coffee is a blend of different varieties of coffee beans. The blends are controlled for flavour, aroma, colour and strength from the roasted bean. For further information entrepreneurs can contact following training institute:
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T. Arumuganathan* Assistant Professor (Horticulture) NADP-PF Cell, e mail:indunathan@gmail.com* Department of Agricultural Processing, TNAU, Coimbatore-3 e-mail: indunathan@gmail.com |
