The effect of roasting on boron isotope ratio in coffee beans: Implications for provenance studies of roasted coffee

We determined delta B-11 values of green and roasted coffee beans from 20 locations worldwide and conducted laboratory experiments with the aim to investigate boron isotope fractionation during roasting. Authentic single origin roasted coffees were found to be isotopically lighter than their green bean counterparts, with an average difference of 1.5 parts per thousand. This isotope fractionation can be explained as arising from partial dissociation of boric acid in capillary water of green beans, where B-11 isotopes are preferentially partitioned into molecules of undissociated boric acid and are then volatised during roasting. However, boron isotope fractionation induced by roasting was significantly smaller than between-origin variations in delta B-11 values of green coffee beans that had the range of similar to 54 parts per thousand. This implies that delta B-11 isotopic composition of roasted coffee retains the geographical origin information within delta B-11 values of green beans when regional differences in boron isotopic composition of coffee are considered.