A predictive spatial model for roasted coffee using oxygen isotopes of α-cellulose

Fraudulent region-of-origin labeling is a concern for high-value, globally traded commodities such as coffee. The oxygen isotope ratio of cellulose is a useful geographic tracer, as it integrates climate and source water signals. A predictive spatial model ("isoscape") of the δ¹⁸ O values of coffee bean cellulose is generated to evaluate coffee region-of-origin claims.

The oxygen isotope ratio of α-cellulose extracted from roasted coffee beans was measured via high-temperature conversion elemental analyzer/isotope ratio mass spectrometry (TC-EA/IRMS) and used to calculate the δ¹⁸ O value of coffee bean water. The ¹⁸ O enrichment of coffee bean water relative to the δ¹⁸ O value of local precipitation was modeled as a function of local temperature and humidity. This function was incorporated into a mechanistic model of cellulose δ¹⁸ O values to predict the δ¹⁸ O values of coffee bean cellulose across coffee-producing regions globally.

The δ¹⁸ O values of analyzed coffee bean cellulose ranged from approximately +22‰ to +42‰ (V-SMOW). As expected, coffees grown in the same region tended to have similar isotope ratios, and the δ¹⁸ O value of coffee bean cellulose was generally higher than the δ¹⁸ O value of modeled stem cellulose for the region. Modeled δ¹⁸ O values of coffee cellulose were within ±2.3‰ of the measured δ¹⁸ O value of coffee cellulose.

The oxygen isotope ratio of coffee bean cellulose is a useful indicator of region-of-origin and varies predictably in response to climatic factors and precipitation isotope ratios. The isoscape of coffee bean cellulose δ¹⁸ O values from this study provides a quantitative tool that can be applied to region-of-origin verification of roasted coffee at the point-of-sale.