How Long to Steep for the Most Out of a Cup, According to a Beirut Lab

You have been told, correctly, that rooibos forgives a long steep and a hard boil. A food-science team at Beirut Arab University and two other Lebanese universities has now put a number on what that steep is actually pulling out of the leaf.

Mayyas Othman, Nada El Darra and colleagues ran a commercial red rooibos through a twelve-run experimental design, varying water temperature from 53.8 to 96.2 degrees Celsius (about 129 to 205°F) and steeping time from roughly 4 to 46 minutes, then mapped how the two variables interact. The model's own predicted peak, confirmed afterward by a dedicated trial run specifically to test it rather than by any of the twelve original runs, was 96.24 degrees Celsius for 11.42 minutes. But the researchers also validated a gentler point nearby, 90 degrees Celsius (about 194°F) for 10 minutes, close enough to that peak in both phenolic content and antioxidant activity, and asking for less heat and less time, that they settled on it as the practical answer: 49.94 milligrams of gallic-acid-equivalent phenolic compounds per gram of dry leaf, with antioxidant activity to match. That temperature and time sit right where the Ministry's own kettle already lands, a touch under a full boil, kept on the heat about as long as the standing advice already calls for.

Chemical analysis of that extract identified 43 separate compounds, led by the flavonoids isoorientin and rutin. The same extract, tested separately in a lab dish against two common bacteria, slowed the growth of both Staphylococcus aureus and Pseudomonas aeruginosa. Against three cancer cell lines, two showed a modest, statistically real reduction in growth that never reached the threshold researchers use to call an effect meaningful, and the third showed no effect at all. None of it was tested in a person, and the Ministry's standing line holds regardless of what a lab dish shows for one extract at one concentration: rooibos is caffeine-free and rich in antioxidants, said plainly, and no more than that.

The study used a single commercial product, one grower's own processing choices, so the researchers flagged raw-material variability as the open question a wider trial would need to settle. Until then, the kettle needs no thermometer. A full boil already sits close enough to the number this lab found, and a cup steeped the way the Ministry has always recommended was, it turns out, already close to as good as the leaf gets.

Sources: Othman MM, Azakir B, Khazaal S, Rabie M, Sokhn ES, Maroun RG, Debs E, Louka N, El Darra N, "Optimized extraction of polyphenols from rooibos tea (Aspalathus linearis) and their biological activities," Frontiers in Nutrition, vol. 13 (2026), published 6 March 2026, DOI: 10.3389/fnut.2026.1778749.