Ordinary Water Leaves Rooibos Unchanged, Stellenbosch Finds
Stellenbosch University and ARC researchers tested rooibos infusions against water of different pH and mineral content. Ordinary water left the cup unchanged; only water at the pH and mineral extremes measurably dulled it.
Researchers at Stellenbosch University and the Agricultural Research Council have run the first direct sensory test of how brewing water changes a cup of rooibos.
Dalene de Beer, Erika Moelich, Brigitte von Pressentin du Preez, Chantelle Human and Elizabeth Joubert published the study in the South African Journal of Botany on February 17, testing rooibos infusions against water of varying pH and mineral content, then scoring the results with a trained sensory panel alongside chemical analysis.
Most water left the cup alone. Infusions brewed with water in a pH range of 5.3 to 7.8, and with electrical conductivity, a measure of dissolved mineral content, below 10 millisiemens per metre, came out sensorially and chemically close to infusions made with deionised water, the researchers' clean baseline.
Only the extremes did measurable harm. Water with both a high pH and high electrical conductivity produced a noticeably worse cup: muted floral and sweet notes, an unfamiliar aroma, a salty taste, a thicker mouthfeel, and a darker colour than the baseline infusion.
The same research group ran a similar water-type panel on honeybush, rooibos's fynbos cousin, in 2025, and the finding holds up on rooibos too: ordinary domestic water, tap included, is very unlikely to reach the pH or mineral extremes the panel flagged as the ones that matter.
The result leaves the Ministry's standing brewing advice exactly where it was. A full boil, a generous steep, and whatever tap is nearest remain the whole method. What is new is that rooibos has, at last, had its own cup put to the test, not borrowed conclusions from a different plant.
Sources: de Beer D, Moelich EI, von Pressentin du Preez B, Human C, Joubert E, "Sensory and physicochemical quality of rooibos (Aspalathus linearis) infusions depend on brewing water characteristics," South African Journal of Botany, vol. 191 (2026), pp. 193 to 203, published online 17 February 2026, DOI: 10.1016/j.sajb.2026.02.023.