Sensory and physicochemical quality of rooibos (Aspalathus linearis) infusions depend on brewing water characteristics

Abstract

• Mineral-rich brewing water is detrimental to rooibos infusion quality. • Infusion colour darkens when brewing with mineral-rich water. • Atypical ‘date pudding’ aroma, salty taste and thick mouthfeel appear. • Infusions brewed with mineral-rich water have higher soluble solids content. • Mineral-rich water causes decreased levels of phenolic compounds. Rooibos herbal tea is popular worldwide owing to its distinctive sensory characteristics and phenolic composition, related to its bioactivity. Yet the effect of brewing water on the sensory profile and phenolic composition of rooibos infusions remains unexplored. Six different water types available to consumers and varying in pH and mineral content, namely municipal tap water (W6) and four bottled waters (W2 - W5), as well as deionised water (W1), were selected. Rooibos infusions brewed using rooibos of varying quality ( n = 18) were evaluated. Mineral-rich brewing water (W3 and W5; electrical conductivity > 30 mS/m vs < 10 mS/m) substantially altered the sensory profiles of the rooibos infusions. These include the emergence of atypical ‘date pudding’ aroma and flavour, salty taste and thick mouthfeel. ‘Sweet spice’ aroma, ‘caramel’ aroma and sweet taste were ( p < 0.05) intensified ( p < 0.05), while characteristic rooibos sensory attributes were attenuated ( p < 0.05). These infusions appeared visibly darker and more intensely red, with slightly higher ( p < 0.05) soluble solids content (ca 1.6 vs 1.4 g/L). A significant ( p < 0.05) reduction in phenolic concentration occurred (e.g. aspalathin < 1.5 vs > 4 mg/L), likely due to reduced extraction efficiency and/or compound degradation. Low-mineral waters effectively preserved the characteristic rooibos sensory attributes. Interactions between the water type and rooibos quality were limited. The results indicate that low-mineral water is optimal for maintaining infusion quality. These findings have direct implications for rooibos producers, tea merchants and consumers, highlighting the need to consider water quality as a critical factor in delivering consistent flavour, optimal phenolic content, and overall infusion quality.