Electric dryers can be used where temperature control is more precise and drying time can be reduced.
Some researchers claim that soaking, i.e. keeping parchment coffee under clean water for a number of hours after it has been demucilaged lessens bitterness and favors quality. This could be an interesting option for quality conscious millers because the parchment could be soaked after the mechanical removal of mucilage. Although this procedure requires tanks and batch processing and causes weight losses, the water will not be contaminated and can easily be recycled. Weight losses will be lower than in fermentation provided that coffee is soaked for a shorter period than for the fermentation process. The weight gains due to mechanical mucilage removal may be lost if drying does not start shortly after demucilaging. The interval between mucilage removal and drying should not exceed 6 hours. Furthermore, the mechanical removal of mucilage allows the design of more compact mills and the construction of compact modules for wet processing. [ Pencegah Penyakit Jantung , 2011]
Drying. During drying operation significant moisture reduction from 53% to about 12% was reported. Electric dryers can be used where temperature control is more precise and drying time can be reduced, but chances of over-drying or un-uniform temperature distribution inside dryer, may affect the final produced coffee quality. Dried coffee seeds are passed through the hulling machine to remove the hull and by mechanical aspiration the hulls are separated from the seeds. Hulled coffee seeds are manually inspected and defected/discolored seeds are removed from the whole lot. Electronic colour sorters may also be used to eliminate defective seeds, as alternate to manual sorting. Sorted good quality beans are graded based on seed size, colour, and other standard tests to estimate the brewing quality of the finished coffee. Out of wet and dry processing method , wet processing of coffee seeds results in better quality green coffee [9, 10].
Coffee is dried by increasing the temperature of the bean to evaporate water. In sun drying beans are heated by direct exposure to the sun and by radiation from a heated surface (in the case of drying grounds). Convection and wind move the saturated air away. In mechanical drying beans are heated by the passage of hot air which also carries the moisture away. Temperatures must be monitored during natural and artificial drying. Coffee temperatures should not exceed 40°C for parchment and 45°C for cherries. It is often thought that overheating can only occur in mechanical dryers. Maximum tolerated temperatures may well be exceeded in sun drying if the beans are not revolved frequently or, in the case of fine Arabica beans, protected by plastic sheets, a tarpaulin or a roof during the hottest hours of the day. Temperature control becomes more critical in the later stages of the drying process when moisture levels are low. [ Journal , 2011]
In the early stages, there is a lot of water to be removed and relatively high air temperatures are not likely to induce the beans to overheat. At this stage, air flow to remove the surface moisture is more important than the temperature itself. As coffee moisture decreases water must move from the center to the periphery of the beans at a speed that depends on the outer air temperature and on the intrinsic physical characteristics of the bean itself. Since there is no more free moisture near or at the surface, the beans heat quickly. At this point the difference between air and coffee temperatures are considerably reduced and the drying temperature must be carefully monitored to avoid damaging the beans. In sun drying, temperature is controlled by revolving the coffee frequently. Parchment should be covered before it becomes too hot. In machine drying, the temperature is controlled by fuel feeding, air flows, etc. Coffee moisture is high, 50-55 % or even more, at the beginning of the drying period so high air temperatures may be used without risk of overheating the coffee.
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