Most defects in coffee cause discolouration to beans. However, the roasting process tends to mask the appearance of defects such as ‘black bean’ (dead or decaying beans that have suffered microbial attack after coming in contact with the ground for extended periods), making them impossible to visually identify. This excellent macro lens video from Thompson Owen of Sweet Marias provides a close up view an overview of the defects that traders of green coffee beans watch out for.
From a café QC standpoint, we are concerned with coffee after it is roasted. In this chapter we review the defects that can be visually identified after roasting, and we explain the flavour taint that is associated with each defect.
There are various modern standards for grading coffee and defining defects around the world, such as the SCA and the Institute of Brazillian Coffee systems. They are all very similar and are all derived from the New York Sugar and Coffee Exchange (NYSCE), which was founded in 1882. The NYSCE developed a grading system to rate the green coffees imported from Brazil and Central/Latin America (R J Clarke, 1987).
The NYSCE established the black bean defect as a measure of comparison for evaluating the relative seriousness of other types of defects. The system is called ‘the black bean equivalent’. For example, five immature beans in a sample size of 300g are equivalent to one black bean defect, whereas the presence of only three shells is equivalent to one black bean defect. With the possible exception of stones and twigs, taints such as these usually impart defective flavours to brewed coffee. (Please note, the well known SCA green grading protocol uses 350g samples.)
The scale of the issue is enormous. For example, the total Brazilian coffee output is estimated to produce 20% defects (A S Franca et al, 2004). At every stage of coffee production, farmers, millers, traders, and roasters are on the lookout for defects in green coffee.