Product quality and energy consumption when drying peppermint by rotary drum dryer

Peppermint is one of the most important aromatic plants because it has wide range of industrial uses and is widely accepted by the public for its flavoring and pharmaceutical properties. Hot air or natural drying of medicinal and aromatic plants, including peppermint, is a common postharvest operation. Drying the product reduces biochemical and microbiological degradation. However, many studies in the literature report the results of drying only a small amount of plant material (less than 1 kg) and do not consider the performances of industrial scale dryers in terms of their effect on product quality, energy consumption and throughput. A rotary drum dryer was used to dry 15 kg of fresh chopped peppermint plant. The rotary drum dryer was programmed to operate under two different drying schemes: constant air temperature drying and rectangular wave-shaped drying air temperature profiles. The drying durations were 15–18 h and 12–15 h for constant temperature profile and rectangular wave-shaped temperature profiles, respectively. Hot air drying caused considerable darkening of the dried peppermint leaves. Essential oil contents of peppermint samples dried by the two different drying procedures were relatively unaffected by drying scheme (2.08–2.7 mL 100 g dry matter−1). The menthol content of the leaves increased from 32.52% (fresh) up to 44.52% (dried), their menthone content decreased from 24.85% (fresh) down to 9.07% (dried). However, the variations within replications were relatively high in terms of the menthol (25.39–44.52%) and menthone (9.07–18.49%) contents of the dried leaves. These variations might have been caused by the uncontrolled morphological and physiological changes of the fresh peppermint materials associated with their maturity. These variations are the main obstacle to obtaining reproducible results and therefore dried peppermint products with consistent quality. The specific energy consumption values ranged from 7.88 to 15.08 MJ (kg water removed)−1. Daily fluctuations in ambient air conditions directly affected the specific energy consumption of rotary drum dryer.