British Black Tea

Ask anyone in the UK about their tea and the conversation eventually reaches water. Hard water in London and the South East produces a scummy, muted cup. Soft water in Scotland and the North yields a bright, astringent one. The ideal sits somewhere between: soft enough to avoid scum, hard enough to retain body. The scum problem is well documented in food chemistry. It forms when calcium and bicarbonate ions react with oxidised tea polyphenols (theaflavins and thearubigins) at the liquid surface, creating an insoluble film. The reaction requires both calcium and bicarbonate to be present: remove either and the scum largely disappears. This is why softened or filtered water produces a cleaner-looking cup.

But very soft water has its own problems. Without any mineral content, black tea can taste thin and overly astringent. A small amount of calcium (around 17 ppm) rounds out the mouthfeel without triggering heavy scum formation. Bicarbonate at 50 ppm provides enough buffering to moderate the perceived sharpness of tannins without flattening the tea entirely.

The Tea Association of the United States recommends water with 50–150 ppm TDS for optimal tea brewing. Research on black tea infusion quality suggests a hardness sweet spot between roughly 17–68 ppm (1–4 grains), where colour is bright, flavour is full, and surface scum is minimal. This profile targets the lower middle of that range. For English breakfast, Assam, and Ceylon styles (the backbone of British tea drinking), this water brings out malt and honey notes without the chalky flatness that hard water imposes or the tannic bite that distilled water can produce. Yorkshire Tea's research into why their blend tastes different in different parts of the country comes down to exactly this: the mineral content of the water changes what the same tea leaves deliver in the cup.