Beneficial properties of herbal tea leaves can be affected by oxidase enzymes activity on harvesting, as these enzymes utilize phenolics as the main substrate. The purpose of this study was to investigate whether there is any correlation between the oxidase enzymes activity and the degree of phenolic degradation. Eight herbal tea leaves underwent ambient air‐drying process and their total phenolic content change as well as polyphenol oxidase and peroxidase enzyme activity change for each fresh leaf were determined after ambient air‐drying. Results showed that except for two plants, polyphenol oxidase activity had a positive linear correlation with total phenolic degradation (y = 1.929x + 21.082; R2 = 0.8909), but peroxidase activity showed no such relationship. Plants with rosmarinic acid as their main phenolics, showed a much better correlation. Practical applications The process of preserving herbal tea leaves for their bioactivities by drying is crucial for the purpose of commercialization. Herbal industries have been using, among others, air‐drying method as a means of preservation because of its cost effectiveness. However, the presence of oxidase enzymes may lead to degradation of beneficial compounds. In our study, by identifying the correlation between oxidase enzyme activity for both polyphenol oxidase and peroxidase with that of total phenolic degradation, we are able to understand the actual effect of oxidase enzymes on phenolics degradation. Hence, through the correlation and solely based on its oxidase enzyme activity, mainly polyphenol oxidase, we could determine whether air‐drying will be suitable for a particular herbal tea leaf. This will result in a much faster and efficient decision in choosing a preserving method for the herbal tea leaf.

Different drying methods and blanching were investigated as to their effects on antioxidant and oxidase activities of Thunbergia laurifolia leaves. Results showed that oven‐drying had the highest degradation of total phenolic content (TPC) and antioxidant activity at >85%, while freeze‐drying had the lowest at <20%. However, inactivation of oxidase enzymes by blanching at 100°C resulted in a lesser decrease in TPC for oven‐drying at 50 and 100°C (51% and 65%, respectively), indicating the importance of inactivating the oxidase enzymes for lower degradation of phenolics on drying. The high‐performance liquid chromatography analysis showed that its major antioxidant, rosmarinic acid, degraded tremendously in the presence of oxidase enzymes, but only degraded slightly upon inactivation of oxidase enzymes. Hence, this work showed that by controlling the enzymatic activity, the preservation of phenolics with specific bioactivity in herbal tea leaves can be achieved. Practical applications Thunbergia laurifolia leaves have been frequently consumed in the form of a tea or pill due to its medicinal properties. Processing of fresh herbal plant leaves by drying is required to preserve antioxidant phenolic compounds and quality of the plant leaves. Although the drying effects on the antioxidant properties have been studied, the factors that cause the change in properties have not been investigated in‐depth. Controlling the factors that affect the phenolic content can help to preserve the beneficial antioxidants when processing the leaves by drying. The result of this study will be of relevance and beneficial to the herbal tea industry.

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