Physical and mechanical properties of Arabica coffee (Coffea arabica L.) cherries and beans in Rwanda

Abstract

Arabica coffee is worldwide predominated and accounts for about 97% of Rwandan coffee production. Up to 70% of exported washed coffee is pulped using high-capacity coffee pulping machines while the remaining is pulped by smallholder farmers. However, the lack of information on coffee beans can affect depulping activity, especially when using locally-made pulping machines. Therefore, this study aimed to investigate Arabica coffee's physical and mechanical properties from eight districts of Rwanda to improve the pulping process. The coffee parameters were investigated using precision measuring instruments and statistically analysed using One-Way ANOVA, one-sample t-test, and the Turkey multi-comparison method in GraphPad Prism 9.3.1 software. The investigation found the mean length of 15.40 mm, 12.12 mm and 11.85 mm, the width of 13.03 mm, 8.26 mm and 8.15 mm, the thickness of 13.03 mm, 4.88 mm and 4.87 mm for Arabica cherries, wet parchment beans and dried parchment beans, respectively. Their corresponding average mass of 1.53 g, 0.43 g, and 0.173 g, the density of 1.006 g/mL, 0.894 g/mL and 0.390 g/mL for Arabica cherries, wet parchment beans, and dried parchment beans, respectively. The Arabica showed a true volume of 1.531 mL and 0.271 mL for cherries and dried coffee beans, respectively. The wet parchment coffee was dried from 29.9 to 10.8% wb. The dried parchment beans showed a coefficient of static-friction and fracture force of 0.585 and 29.85±6.81 kgf, respectively. The other parameters evaluated for cherries and dried parchment coffee beans include moisture content, true volume, density, porosity, sphericity, and angle of repose. The results showed that coffee's size, mass, densities, and diameters from different districts are significantly different at Pvalue < 0.05 and also showed a perfect correlation. Each of the above coffee parameters provides the general information needed to develop a pulping machine for cherries. Therefore, the small-scale coffee pulping machine should be developed, and properly adjusting the pulping unit to enhance the performance of the machine based mainly on the size, fracture force, and density of the coffee.