Kinetics of change in colour and rosmarinic acid equivalents during convective drying of lemon balm (Melissa officinalis L.)

• Colour and active ingredients of lemon balm are extremely heat sensitive.
• Course of a* and b* is described by a first-order reaction kinetics model.
• The intensity of colour changes depends on moisture content level.
• Hue represents the visual impression of colour deterioration in leaves adequately.
• Browning of leaves can be associated with depletion of rosmarinic acid equivalents.

The course of colour changes of lemon balm (Melissa officinalis L.) was examined during hot-air drying at temperatures ranging from 30 to 90 °C, constant specific humidity of 10 g kg-1 dry air and uniform air airflow of 0.2 m s-1. CIELAB colour data was fitted to a first-order reaction kinetics model, in which the temperature dependence of the rate constant is modelled by the Arrhenius-type relationship. Total hydroxycinnamic acid derivatives were quantified by the photometric method and expressed as rosmarinic acid equivalent content. The a* and b* values were the most sensitive parameters to describe the colour changes of M. officinalis during drying. Redness was increased while yellowness gradually decreased as drying progressed towards the equilibrium moisture content. Colour degradation immediately started at the beginning of the drying process when moisture content was still high in the material. The rosmarinic acid equivalents were decreased with a progressive increase of temperature of drying air and it was assumed to be responsible for the browning of leaves due to enzymatic oxidation, especially at higher temperatures. Hue proved as best to express the optical impression of colour change. Low drying temperatures are recommended to avoid colour deterioration and to preserve high rosmarinic acid equivalent content.

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