Impact of steam pasteurization on the sensory profile and phenolic composition of rooibos (Aspalathus linearis) herbal tea infusions

Summary

The effect of steam pasteurization of fermented rooibos leaves and stems on the sensory characteristics and phenolic composition of infusions was determined. The extent to which this processing step changes the sensory profile and whether compositional changes influence taste and astringency of the beverage was determined. These were achieved by examining the changes in the concentrations of soluble solids (SS), total polyphenols (TP) and 14 individual non-volatile monomeric phenolic compounds, as well as the changes in 17 aroma, flavor, taste and mouthfeel attributes of rooibos infusions. Steam pasteurization significantly reduced the SS, TP and aspalathin contents, as well as the “total color” (area under the curve: 380 to 520 nm). Neither the intensities of the taste attributes, sweetness and bitterness, nor the levels of individual phenolic compounds changed significantly, except that of aspalathin which were significantly reduced. A small but significant decrease in the astringency of rooibos infusions was observed. The intensities of most of the aroma and flavor attributes decreased significantly as a result of steam pasteurization. “Green” and “caramel” notes exhibited the largest reductions in attribute intensity. The prominent “green” flavor of unpasteurized rooibos was frequently changed to a “hay-like” flavor after steam pasteurization.