Wet processing method

This technique of processing ensures a much higher quality product. It is always used for preparing mild Arabica coffee, particularly in Central and Latin America. This method is currently used in Papua New Guinea farms, Keronge coffee is no exception. It requires processing equipment (for the initial cleaning/classification-pulping-fermentation-washing and drying). The method is good when there is an abundant supply of clean water, and the harvesting of ripe fruits only. An important difference from the dry processing method is that most of the outer coverings of the coffee bean are removed before drying.

1.         Receiving

It is important that only ripe fruits are harvested, as existing pulpers cannot deal with green berries or berries that have dried on the tree. The crop must be transported as quickly as possible to the processing centre to avoid heating of the mass, which can result in irreparable damage (production of discoloured beans, sticker beans, etc.). The crop is generally unloaded into a receiving tank equipped with an overflow weir to remove the floaters. This simple system fulfills several functions: separating floating from non-floating berries, eliminating sand and stones. And removing leaves/twigs, etc. the floaters, made up of berries dried on the tree and insect-attacked are then generally processed by the dry method.

2.         Pulping

            Pulping consists of removing the exocarp (outer) and the major part of the mesocarp (fleshy portion), which is referred to as the ‘pulp’. If is a delicate operation, for if the bean itself is damaged it will be susceptible to microbial attack and penetration by undesirable substances. The principle on which the pulpers work is essentially the ‘tearing off’ of the exocarp and mesocarp of the berries, which operation is carried out under running water. Conventional pulping does not remove all of the mesocarp, which the mucilage is adhering to the parchment surrounding the actual coffee beans together with the testa. The yield expressed as dry green coffee varies with the species: Arabica at 15% - 19%, robusta at 18% - 23% and Arabica at 13% - 16%. The various types of pulper are described in the sections following.

3.         Separation/Classification

            The berries are not perfectly sorted in the receiving tank; there is a further opportunity after they have left the pulper, again by flotation by different means. This is particularly appropriate for mild Arabica coffee, since, for example, floating parchment coffee (i.e. with the pulp and mucilage removed) contains very few healthy beans. The following methods can be used at this stage.

            i)          Draining

                        Using small vertical boards acting as a spillway for circulation in running water, the lightest beans go the furthest, and the heaviest are stopped by the first boards.

            ii)         Use of Fermentation Tanks

                        Fermentation to remove mucilage is described on following notes on fermentation together with the operation of the tanks.

            iii)        Aagaard Densimetric Grader

                        This grader is based on the jigging principle used to separate minerals from coal. Jigging consists of placing the heterogeneous matter on a sieve submerged in water. The sieve moves to create ascending and descending currents, and with each pulsation the beans are suspended and resettled. The heaviest beans move towards the bottom of the layer and lightest move towards the top. When this machine is positioned between the pulper and repasser, it separates the mass into three categories: small berries that have not been pulped, heavy beans that are removed by a scraper conveyor, and light beans.

4.         Fermentation

            The objective of fermentation is to hydrolyse the mucilage, already mentioned, in order to facilitate its final removal during subsequent washing. If mucilage remains present during drying, there is the risk of undesirable fermentation, which is detrimental to the quality of the coffee. Biochemically speaking, the hydrolysis of pectin is caused by a pectinase already in the fruit, but the reaction is accelerated by different micro-organisms, such as Saccharomyces, which also a have pectinolytic properties. The rate of hydrolysis depends upon the temperature; consequently, it is necessary to adapt the length of the fermentation period to existing ambient conditions. Some micro organisms can cause off flavours to develop, particularly in prolonged fermentation. It is important to prevent the development of harmful species (moulds, aerogenic coli) by encouraging the development of acidogenic species.

Fermentation can be carried out either ‘dry’ or under water.

Fermentation is most often carried out in concrete tanks, which vary considerably in size. For small scale production like the ones practice in Papua New Guinea copper dishes, bucket and canoes are used. The length of the fermentation time differs mainly as a function of climatic conditions and the condition of the crop.