Retronasal perception and flavour thresholds of iron and copper in drinking water

Drinking water flavour has a strong role in water quality perception, service satisfaction, willingness to pay and selection of water sources. Metallic flavours are often caused by the dissolved iron and copper, commonly found in groundwater or introduced to tap water by corroding infrastructure. Taste thresholds of iron and copper have been investigated by several studies; however, reported results and test methods vary considerably. This study determined the taste thresholds of ferrous and cuprous ions in room temperature reagent water by using the one-of-five test with multi-nation panellists in the United States. For ferrous and cuprous ions, individual thresholds ranged from 0.003 to >5 mg l(-1) and 0.035 to >5 mg l(-1), respectively. Population thresholds were determined by logistic regression and geometric mean methods as 0.031 and 0.05 mg l(-1) for ferrous ion, and 0.61 mg l(-1) for cuprous ion by both methods. The components of metallic sensation were investigated by use of nose-clips while panellists ingested iron and copper solutions. Results showed that metallic sensation has a significant odour component and should be treated as a flavour instead of a taste. Ferrous, cuprous and cupric ions also produced weak bitter and salty tastes as well as astringent mouthfeel. In comparison, ferric ion produced no sensation.