Arabica coffee flow properties assessed using different roasts and particle sizes during storage

Abstract

Flowability of agricultural products is an important factor to be considered at post-harvest, thus impacting directly on number of operations and the design of machinery. This study aimed to evaluate and determine the K coefficient and flow properties as a function of different levels of roasting, grain size, temperature and storage. Coffee beans were roasted at medium light and moderately dark, then ground at fine, medium and coarse sizes. An additional coffee lot was not ground. Samples were stored at 10 ºC and 30 ºC and analyzed during storage (0, 30, 60, 120 and 180 days), regarding internal and external friction coefficients, angle of internal friction, effective angle of internal friction, wall friction angle (concrete, rough steel and wood) and lateral pressure coefficient (K coefficient). Angle of internal friction varied significantly due to particle size and roast degree. Moderately dark roast with fine particle size led to higher values (about 29.5 º) of wall friction angle. The wood sample was the material with the highest values of wall friction angle, followed by the concrete and steel samples, when compared at the same storage temperature, roast degree and particle size. Whole coffee was classified as free-flowing. Fine particle size leads to cohesive flow characteristic, according to the flow function. Coffee roasted at medium light, kept whole requires storage facilities with higher pressure support, accordingly to the K coefficient values.