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Coffee processing at home

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Although there are several coffee species, most coffee is made from the seed or bean of either Coffea arabica (Arabica coffee) or Coffea canephora (Robusta coffee). Arabica trees normally produce berries 8-15 mm in diameter, and Robusta produces berries approximately 10 mm in diameter. Commercially, Robusta is regarded as having inferior quality to Arabica coffee and is used mainly as a filler in instant coffee blends.

Coffee berries are picked when they ripen to a bright deep red colour, though there are a few cultivars that ripen to a deep yellow colour. The coffee or 'green bean' lies within the fruit, and is surrounded by the parchment membrane, pulp or mucilage and outer skin.

Coffee processing in the home is very time consuming. Small-scale processing equipment is now available in Australia. Equipment is also available from the United Kingdom at considerable expense. Processing involves six main steps outlined below.

1. Pulping

This step involves removing the skin and pulp, and should be carried out as soon as possible after harvesting, certainly within 24 hours. It is necessary to remove all green unripe and black overripe dry berries before pulping, as these will reduce the quality of the coffee.

Two home methods can be used for pulping. One is to squeeze each individual berry by hand, and the other is to use a piece of wood to tamp the berries in a bucket until all seeds have been forced out from the skin. After this operation, fill the bucket with water and stir the skins and seeds. Pour away the skins before they settle. Repeat this process to remove all the skins and pulp, and any coffee beans that float. Good coffee beans will not float in water. Remove by hand any remaining skins or unpulped coffee berries.

2. Fermentation

Fermentation by natural enzymes breaks down the insoluble mucilage around the parchment layer, i.e. the slippery layer you can feel with your fingers.

Place the coffee beans in a plastic bucket to avoid the effects iron has on quality and add water to cover the beans. Fermentation may be complete in 18-24 hours, depending on the surrounding temperature.

To check whether the fermentation phase is complete, gently wash a handful of the beans. If they come clean and feel gritty (not slippery), then adequate fermentation has been achieved and the beans can be washed. Wash in agitated water and drain. Repeat this process until the water becomes clear. This normally takes approximately three washes.

To strain off the water, use a colander or some fine, net-like material such as an onion bag, as this will prevent the loss of beans. During the washing process, discard any floating beans.

3. Drying

Coffee beans must be dried before the parchment can be removed and beans roasted.

The simplest method of drying is sun drying.  Physical aids such as wire drying racks or other fine mesh supports allow for the flow of air and enhance drying. The beans should be spread out in a thin layer no more than 3 cm thick, stirred three times a day, and be protected from rainy weather. Drying in this way can take 5-30 days, depending on weather.

Alternatively, you can use a home food dehydrator. This equipment uses electric heating elements to control drying. Only dehydrators with variable temperature controls are suitable as drying temperature must be kept at 40oC for the entire drying period.

As with sun drying, beans must be stirred three times a day. Drying in this manner can be completed in several days and there is no risk of the beans being harmed by the weather.

Whichever method of drying you use, the parchment on the coffee bean will dry to a pale straw colour and be brittle to touch. At this stage, test the dryness of the beans by removing the parchment by hand off several coffee beans. If dry, the bean inside should be greyish blue in colour, hard, and likely to break when bitten between the teeth, if soft and chewy continue the drying process.

Inadequate drying - greater than 12 per cent moisture - will cause mouldiness and stale aroma during storage. After correct drying, store for at least a fortnight in cans, jars or heavy gauze bags to allow moisture to distribute evenly throughout the coffee beans. For longer term storage before parchment removal, air-tight containers are recommended.

4. Parchment removal (hulling)

Before roasting, remove the thin tough parchment layer from the beans. Place the beans, a small quantity at a time, in a food processor or similar type of blender. Use plastic blades to avoid breaking the coffee beans. Blend at low speed for approximately 30 seconds to remove the parchment from the beans. Then use a hair dryer or similar piece of equipment to blow away the unwanted lighter parchment from the beans. Another technique is to rub the dried beans on concrete under a hessian bag or similar.

The very thin membrane that may remain on the green bean is the silver skin. It is not considered necessary to remove this before or after roasting. However, it may detract from the visual appearance of the roasted beans and can be removed by gently rubbing the beans following roasting. At this stage, the coffee can be stored in sealed jars for roasting as required.

5. Roasting

The green coffee beans must be roasted to develop the typical coffee aroma and flavour. During roasting, several changes occur to the beans. These include loss of moisture, caramelisation of sugars, change in colour and increase in size.

Roast the beans in large baking dishes in the oven. Spread the beans thinly and stir frequently to prevent burning and to give an even roast. As a rough guide, a single layer of beans will roast in 12 minutes at 230-250oC, while beans at a depth of 25 mm may take 30 minutes at this temperature. Roasting can also be carried out in a fry pan or using a popcorn machine.

After the coffee has been roasting for a short time, the colour of the beans changes to a yellowish brown which gradually deepens in colour as they cook. As the beans are heated, they shrivel until half cooked, then swell, and begin to open out as they increase in size. The colour and flavour of the beans will be influenced by the length of roasting, for example, light brown beans (a light roast) will have a weaker flavour than brown/black beans (a dark roast).

The extent to which you will roast the beans depends upon individual flavour preferences. Over-roasting gives a burnt flavour. Adequately roasted beans should crack easily between the fingers. Once roasted, remove the beans from the oven, spread thinly and cool as quickly as possible with a fan, or the beans will continue to cook from their own heat. 

6. Grinding, storing and brewing

The cooled coffee beans may then be ground  to the desired extent. This is determined by the type of brewing extraction to be used. The finer the grind the greater the extraction of the flavour when the coffee is brewed. For example:

  • expresso - very fine
  • percolator - fine
  • filter coffee (dripolator) - fine to medium
  • pot infusion (plunger) - medium to coarse.

Ground coffee goes stale rapidly and loses flavour due to oxidation of certain oils. Packaging ground coffee in air tight containers and storing in the refrigerator will help to reduce flavour loss. However, the best quality brew is obtained when using freshly ground beans. It is recommended to store roasted beans in an air tight container in the refrigerator and grind the required amount just before use.

The most important aspect of brewing is not to let coffee stand once brewing is complete. Over brewing destroys the flavour. The water need not be boiling, but just off the boil.

When using plunger coffee makers (pot infusion), brew coffee for 3-5 minutes before drinking. Coffee made by filtration using dripolators should be served once the cycle is complete. Residual coffee should not be left sitting on the hot plate as overbrewing will result and destroy the best flavour. Percolators, by design, tend to overbrew coffee.

Individual taste preferences may influence your selection of brewing method, as well as roasting criteria.