Processing of coffee by wash/wash process includes various processing steps like harvesting, cleaning, pulping, fermentation, drying and storage. Individual processing steps should be carefully performed to avoid any chances of contamination and to prevent damage during processing.
Harvesting and cleaning:
1. For preparation of coffee by wet process, only ripe coffee beans should be harvested.
2. If over ripe or green cherries are harvested, should be stored out on a clean surface.
3. Clean bags should be used for collection of harvested cherries
4. Where fertilizer or cattle feed bags are used, they should be washed thoroughly in running water many times before being used.
Pulping:
1. Proper cleanliness should be maintained in pulper house
2. The pulper machine, washing machine, tanks, vats, trays etc. should be kept clean
3. Cherries should be graded into uniform size before pulping
4. Pulper machine should be adjusted to suit the size of berries
5. After pulping, fruit skin and unpulped cherries should be removed from pulped cherries
6. Only pulped cherries are taken for fermentation
Fermentation: Fermentation is low cost and widely used technology. This process is used for removing mucilage layer from pulp cherries. Removing of mucilage can be carried out by different method such as natural fermentation, treatment with alkali, enzymatic method and frictional removal method. However, over fermentation or under fermentation should be prevented in each case.
1. Natural fermentation method: The sticky mucilage surrounding the beans is not easily removed by traditional pulpers and hence fermentation is carried out to hydrolyze these layers so that it can be easily removed during washing process. The mucilage breaks down in the process of fermentation. In the case of Arabica variety, it is completed in about 24-36 hours whereas it takes 72 hours in Robusta variety. Coffea robusta has thicker and stickier mucilage and hence takes longer time for fermentation.
Fermentation takes longer time in cool weather than in warm condition. When correctly fermented, mucilage is removed easily and parchment does not stick to hand after washing. Parchment is a naturally occurring papery substances that surrounds the coffee bean. Parchment coffee means green coffee beans contained in parchment skin.
2. Treatment with alkali: In this method, the beans after pulping are dipped in the 10 % NaOH solution. Removal of mucilage by alkali takes 1 hours for Coffea arabcica (have thin mucilage) and 1.5 to 2 hours for Coffea robusta (have thick mucilage). After completion of treatment, parchment is no longer slimy and makes a rattling noise. In subsequent step, 3 – 4 times washing of parchment coffee with clean water is needed for removal of alkali.
3. Enzymatic method: Pulped coffee is treated with pectinolytic enzymes and is allowed to hydrolyze (digest) overnight. The mucilage. The fermentation time is shorter in enzymatic process. However, the concentration of enzymes and ambient temperature affect the fermentation time.
Washing and Drying:
1. Parchment coffee obtained after fermentation should be washed with clean water as many times as required to have pebble clean parchment.
2. Parchment coffee can be soaked in 1 % sodium metabisulfate (KMS) for overnight for enhancing the quality of coffee.
3. Basket lined with polyethene sheets can be used for carrying wet parchment coffee to the drying yard. Excess water should be drained off before drying for quick removal of moisture.
4. Clean tiles or concrete yards can be used for drying.
5. Damaged and defective beans can be sorted out during drying of coffee beans.
6. Coffee beans kept for drying in yard should be heaped and kept covered during night to prevent moisture gain.
7. Coffee should be dried to prescribed test weight standards and should be stored in clean gunny bags or sacks in well ventilated and clean warehouse.
8. Coffee beans should not be stored with pesticides, fertilizers any other commodities like spices that give intense odor and volatiles. These volatiles may contaminate the flavor of coffee beans.
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.