This defect is most often referred to as ‘shells’ or sometimes ‘ears’, we’ve even heard the term ‘elephant ears’ while researching this topic. They are thus nicknamed for obvious reasons – they look just like that part of the anatomy. These should not be confused with broken beans which will be covered in the next lesson. This defect is problematic for farmers because, like peaberrys, yields can be reduced when plants produce a high number of empty beans. Scientific literature is not robust on this subject with the most comprehensive research dating back to the 1940s. It is reported that this defect is only encountered with interspecific hybrids such as the Timor hybrid which was a natural crossing of robusta and arabica. (Ferwerda, F. P., 1948)
The issue arises in hybrids of plants with differing numbers of chromosomes e.g. Arabica and robusta are the parents of the Timor hybrid but have different numbers of chromosomes: Coffea arabica has 44 chromosomes whereas Coffea robusta has only 22. (T.M. Wormer, 1964) Research in the fifties from A.J.T. Mendes et al., (1954) located a recessive gene responsible for the trait in Mondo novo plants in Brazil.
General appearance and cross-sections of endosperms, showing stages of reduction (a, b, c) in beans of liberica-arabica hybrids as compared with the endosperm in a normal bean (d). Endosperm reduction may proceed much further than is indicated in (a). Image from Ferwerda, F. P., (1948)
How Empty Beans Impact Roasting
In a roast batch, empty beans behave differently to other beans. Heat is likely to penetrate these beans more quickly. Their characteristic sharp edges and narrow diameter means that moisture can escape from them more quickly which allows heat to penetrate more easily. Too many shells in a roast batch causes unevenness in the roasting and may introduce charred flavours into a brew where empty beans are too numerous.