Title : Direct solar leaf-dehydration: physiochemistry, and practice on the island of Crete
Abstract:
There is increased interest in the use of direct solar energy treatments for the dehydration of leaves prior to their use both in culinary applications, as well as their long-term storage. The treatment also has application as a first-step, prior to traditional organic-solvent free leaf hydrodistillation (immersion in boiling water) for the production of essential oils and hydrosols. The latter used in; leaf tea infusions, decongestant, topical scalp treatment, and water-base aromatherapy. In the leaf dehydration process itself, the aim is to reduce the water content of the leaves, in order to prevent enzymatic degradation and slow microbial growth within the leaf matrix, while retaining its volatile and non-volatile bioactive compounds. Both temperature and desiccating airflow are important parameters to ensure that this narrow band of molecular-weight biomolecules maintains their efficacy and are not degraded. Over the past two years, experimental off-grid direct solar leaf dehydration studies within a box-like reactor have been performed on the island of Crete in the eastern Mediterranean Sea (35.31o N, 24.31o E at an altitude of 119 m), demonstrating that this traditional top-down approach in dehydration leaves under adverse atmospheric Sahara dust episodes and intense solar irradiance levels. This paper presents details on the reactor design and its maintenance and explores the fixed and dynamic strategies for controlling the relative humidity and air temperature, airflow, and the process endpoint for successful solar batch dehydration. The treatment was carried out on the following Lamiaceae family culinary and medicinal leaves: Laurus nobils, Ocimum basilicum, Ocimum basilicum var, minimum, Salvia officinalis, and Salvia Rosmarinus. Treatment efficacy depends on the time of harvest, season, and the soil type in which the plants are grown.
It is suggested that the abiotic (heat and sunlight) induced stress that cause visible leaf morphological change, as characterized by the standard leaf rolling score metric (1, 2, 3, 4, and 5, where 1 is minimal stress, and 4 is maximum stress), needs to be modified with a prefix of ‘plus’ for adaxial rolling, and a prefix of ‘minus’ for abaxial rolling. With this modification, state-space representation of solar-induced leaf shrinkage and rolling (where LRS is plotted on the vertical axis and solar dehydration time plotted on the horizontal axis) quantifies a leaf’s hydroresponse to solar dehydration. The advantage of such vector graphics is the possibility to map the morphological, and physiological, if not biochemical, leaf phenotype response in a two-dimensional representation.
