Terrace Drying in Brazil: Facilities and Control of Quality and Safety during Drying

Transcription

1 Terrace Drying in Brazil: Facilities and Control of Quality and Safety during Drying (Taken from Hygienic Coffee Drying prepared for this resource by Juarez de Sousa and Consuelo Domenici Roberto, University of Viçosa, Brazil 1 ) 1. Introduction: In Brazil, freshly harvested natural coffee generally has a wide range of moisture content (70% - 25% w.b.) because the coffee is strip harvested late to minimise green cherry content which has the effect of presenting a high tree-dried cherry proportion. In other coffee producing countries, coffee is commonly selectively picked or selectively stripped (bearing shoots are stripped but only those bearing mostly ripe coffee), to provide a relatively uniform coffee to the drying yard. On arrival at a Brazilian drying facility coffee is usually subjected to water flotation to separate soft ripe and green cherries (which sink in water) from partially dried hard cherries (which float in water). These two groups of coffee are spread out on separate parts of the drying terrace using hand carts (Fig. 1) for water drainage, and equipped with a door located in the perforated bottom. The door is opened as the car is pulled along leaving a layer of coffee of regular thickness on the surface of the terrace. The thickness of the coffee layer is approximately 4 cm at the beginning of the drying process and may be increased up to 10 cm when the coffee approaches the final moisture content. Figure 1: Diagrammatic representation of a hand cart At the start of terrace drying, when the coffee moisture content is high, or when the natural cherries or pulped cherries are removed from the hydraulic separator or from the coffee washer machine, and spread out on the drying terrace, the surface becomes completely wet. In case some part of the terrace drying surface is not exposed to the immediate drying of excess water, the product will be highly susceptible to mould development, due to the high degree of water availability beneath the coffee layer. To remove this liquid water, the terrace drying operator should open up the coffee layer and expose bare yard surface to the sun, until at least the fifth drying day, by forming parallel coffee lanes, as illustrated in Fig. 2a. The coffee lanes should be redistributed and redone at regular time intervals (not longer than 60 minutes) using a scraper pushed by hand (Fig. 2b). Specific design details are presented below. The terrace manager should be careful to leave open lanes to be exposed for sun drying and heating so as to indirectly dry coffee in the next lane (Fig. 3). In opening the coffee lanes, the terrace manager 1 The views expressed in this publication are those of the author(s) and do not necessarily reflect the views of the Food and Agriculture Organization of the United Nations. Page 1 of 5

2 must take care regarding solar orientation. The coffee lane must follow the same direction of the manager s shadow (Fig. 2c). a b c Fig. 2: a. Terrace after spreading with wet cherry; b. Stirring of cherry on cement terrace; c. Terrace manager taking care regarding solar orientation. Fig. 3: Formation and stirring of coffee ridges during cement terrace drying After about five drying days, once the coffee is partially dry, at about 15:00, the coffee is gathered into heaps or piles oriented along the highest slope of the terrace, and covered with plastic sheet or tarpaulins. In this way, the sun's heat absorbed during the day is partially conserved during the night, guaranteeing uniform moisture redistribution in the mass of coffee seeds. The following morning, at about 09:00, the heaps or piles should be uncovered and removed from the place they rested overnight, in order to dry the area beneath. Soon afterwards, the product should be spread out on the drying terrace, repeating the operations done on the previous days until reaching a moisture content for storage (12% w.b.) or until 35% moisture content is reached if mechanical drying is to be employed. The terrace drying of washed coffee follows similar procedures used in the drying of natural coffee. The main difference is that washed pulped coffee has very uniform initial moisture content and requires lesser drying time than natural or whole fruit coffee. Green cherry coffee cannot be pulped, and should be dried separately from the Page 2 of 5

3 natural ripe cherry, tree-dried coffee and washed coffee (if this is prepared). The drying of the green cherry must be done in a slower way than for ripe or washed coffee. The manager must choose one area of the drying terrace with smaller solar radiation, and spread the coffee in deeper layers or piles. Fast drying or excessive temperatures in mechanical drying of green cherry causes darkening of the surface of the bean that represents a defect in coffee grading. 2. Differences Among Drying Surfaces: The amount of input energy required to dry coffee varies somewhat according to the surface on which it is dried. In a drying experiment conducted by Lacerda Filho ( see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality ) the efficiencies of drying terraces constructed of different materials (brick, concrete, asphalt, and hard soil) were determined and the coffee quality was also evaluated. Table 1 shows that on average about kj/kg is required to reach dryness with about ±6% variation attributable to the surface materials tested. The product subjected to sun drying in compacted-soil terraces resulted in coffee of poor quality compared to that dried in brick, concrete, and asphalt terraces. Aside from this measure of efficiency, other factors that affect the viability of the surface exist. In our research, practical problems were observed especially with asphalt paving where adherence, durability and uniformity of the surface, high porosity, and vegetation emergence were noted. The asphalt readily cracks up through weathering, or plant growth, thus potentially contributing foreign matter to the end product. Table 1: Terrace drying of cherry coffee under identical solar conditions. Initial m.c. = 62% (w.b.). Type of drying terrace m.c. after 16d of sun drying (% w.b.) Specific energy requirement (kj kg -1 ) Soil ,870 Brick ,600 Concrete ,970 Asphalt ,900 Source: Lacerda Filho ( see: Selected bibliography under Section 3, specifically the section on Coffee Processing and Quality ) Page 3 of 5

4 a b Fig. 4: a. Emergence of vegetation through asphalt drying surface; b. Cracks in asphalt surface can retain water or contaminated material and loose fragments may be easily mixed with product. Whenever possible, the drying terrace should be divided in blocks, in order to facilitate the drying of different lots second origin, moisture content, and quality. To facilitate rainwater drainage, the drying terrace should be built with steepness in the range of 0.5 to 1.5% with drainage plates located in the lower part of the terrace. The drains, measuring 0.4 x 0.25m, should be built in steel plate with 50% of perforation, with squared holes of 4mm of side, to impede the passage of the coffee beans in case of heavy rain. In the case of adopting circular perforations, the same perforation percentage should be used, with holes of smaller dimensions (maximum diameter of 2.0mm). It is advised the construction of protection walls measuring 0.25m of height 0.15m of thickness around the drying terrace to avoid losses or mixtures of different types of coffees. Costs of building a yard are dependent on the local economic conditions, but a calculation for the construction of a cement yard in Brazil is given below for reference. Table 2 shows the material needed for the construction of a 150 m 2 terrace (10m of width for 15m of length) paved with cement (1:4:8 with 8 cm of thickness) and finished with 2cm mortar 1:3. Table 2: Material needed for the construction of a 150 m 2 cement drying terrace as in Brazil, (NB - R$1 = approx. US$1.8 in 2000). Discrimination Unity Quantity Unity Price (R$) Total price (R$) Contribution of overall cost (%) Tractor service h 2 35,00 70,00 3,71 Bricklayer d 16 25,00 400,00 21,20 Bricklayer assistant d 29 10,00 290,00 15,37 Cement bag 58 11,00 638,00 33,82 Sand m ,00 121,00 6,41 Broken stone m ,00 275,00 14,58 Plywood m 75 0,80 60,00 3,18 Bricks (20 x 20) one 250 0,13 32,50 1,73 TOTAL Cost/m 2 = R$18,86 / US$ 10, ,50 100,00 Page 4 of 5

5 3. Summary of the Terrace Drying Process: Do not mix coffees from different lots; Spread cherry coffee on the same day it is harvested, or parchment coffee on the same day it is washed in layers of 3 to 5 cm depth. Proceed to form the layers into furrows or lanes. In case there is great percentage of green fruits, deeper layers must be used (about 10 cm of height); Mix or stir the coffee at least eight times a day, adjusting the orientation of the lanes so that the worker's shadow is in front or behind him: so that the sun falls along the lanes, not across them; At the end of the afternoon (about 15:00) of the second drying day, rake the coffee into layers of 15 to 20 cm height. At about 09:00 on the following day, re-spread for drying. This procedure will accelerate the drying and will avoid rewetting.; In case of rain during this period, shift the coffee into stacks at the highest part of the drying yard and cover with water-proof material. When the rain stops, the terrace manager must uncover and stir the deep layers until the terrace surface has dried at which time the coffee can be re-spread and the regime described above resumed; Never dry cherry coffee during in deep layers before 35% moisture content is reached. This is the point at which mechanical drying can be initiated if available. At this point, coffee will not stick together when squeezed in the hand. Slowing the rate of drying by increasing the layer thickness at this point is necessary in order to allow heat exchange and equilibration within and between coffee fruits to ensure homogeneous drying; Continue the regime of shifting the coffee into stacks at about 15:00 each afternoon, covering them with plastic sheets overnight and spreading the coffee again at about 09:00, when the humidity of the air is lower; and Continue the process until 12% (d.b.) or less has been reached and confirm with moisture meter if available. To store, collect the coffee during the morning before the coffee is heated by the sun. Page 5 of 5