Optimising high-temperature oxidation of Cyclopia species for maximum development of characteristic aroma notes of honeybush herbal tea infusions

The effect of oxidation temperature and time on the sensory characteristics of Cyclopia genistoides, C. subternata, C. maculata and C. longifolia was investigated to establish the optimum temperature × time combination for production of high quality honeybush herbal tea. The plant material was subjected to high-temperature oxidation (also known as “fermentation”) at 80 °C and 90 °C for 8, 16, 24 and 32 h. Descriptive sensory analysis was used to establish changes in the intensities of aroma, taste and mouthfeel attributes. The major positive aroma attributes, “fynbos-floral” and “fynbos-sweet”, were consistently high for all species, irrespective of fermentation temperature and can thus be considered defining aroma attributes of honeybush tea. Other positive aroma attributes such as “apricot/apricot jam” and “woody” contributed to species-specific profiles. No “new” sensory attributes developed during fermentation, the positive and negative sensory attributes were merely intensified and reduced, respectively. In general, these herbal teas became more “sweet” and “floral” with increasing fermentation time, while the “green” aroma notes decreased considerably, especially when the plant material was fermented for 16 h and longer. Fermentation for 32 h, as opposed to 24 h, mostly did not improve the sensory profiles of the infusions. Hence, considering cost of heating and production through-put, fermentation time should be limited to 24 h. Minor manipulation of the positive aroma profile can be achieved by changing the fermentation conditions from 80 °C/24 h to 90 °C/16 h. The “rose geranium” aroma was more prominent in C. subternata and C. genistoides infusions, reaching low, but detectable, intensity levels when the plant material was fermented at 80 °C/24 h. Infusions of C. genistoides and C. longifolia had a prominent bitter taste when fermentation time was limited to 8 h. Longer fermentation times were effective in reducing bitterness to acceptable levels in C. longifolia, but not in C. genistoides.