It has traditionnally been accepted that drying of Artemisia annua should take place in the shade because drying in the sun would destroy many useful molecules. But recent papers have questionned this belief and have studied the effect of ultraviolet radiation on the accumulation or decrease of medicinal compounds in plants (WJ Zhang et al., Fitoterapia, 2009, 80, 207-18). A study in Turkey showed that UV-C radiation had remarkable promoting effects on the accumulation of secondary metabolites in the calli of a grape cultivar (E Cetin, Biological Research, 2014, 47 :37).
Essentials oils : Supposedly it should be preferable to dry at temperatures below 60°C because volatile components might evaporate. But this is not confirmed in the scientific literature. The decrease was even lower for sun drying than for shade or oven drying in the case of Ocimum basilicum. At temperatures of 60°C the composition of the oils however changes. (M Hassanpouraghdam et al., J Essential Oil Bearing Plants, 2010, 13, 759-766). This needs further studies.
Artemisinin: Jorge Ferreira (J Agric Food Chem, 2010 Feb 10, 58(3) 1691-8) evaluated the effect of freeze, oven, shade, and sun drying on the leaf concentration of artemisinin, dihydroartemisinic acid, artemisinic acid and on the leaf antioxidant capacity. Freeze-dried samples had the lowest artemisinin concentrations as compared to the other drying methods.. A significant decrease (82% average) in dihydroartemisinic acid was observed for all drying procedures, with a simultaneous, significant increase in artemisinin. The average bioconversion was 43% for oven- and shade-dried plants and 94% for sun-dried plants, reiterating the hypothesis that dihydroartemisinic acid , not artemisinic acid, is the main biosynthetic precursor of artemisinin and suggesting that sun drying improves this bioconversion. Similar results had been found in Tasmania (JC Laughlin, Proc Int Conf on MAP, Acta Hortic 576, ISHS 2002 315-318). Artemisinin increased in the plants 21 days after harvest, either in the sun or the shade, more than in oven dried samples.
Phytosterols, saponins (H Pham et al., 2015, Technologies, 2015, 3, 285-301 and fatty acids are not much affected by high drying temperatures. Only above 80 °C noticeable decreases are found (M Garrysiak et al., Eur J Lipid Sci Technol 2015, 117, 493-490).
Flavonoids and polyphenols : Only minor and variable changes are noticed. A large study on Mediterranean herbs show that air dried herbs contain more flavonoids and polyphenols (T Rababah et al., 2015, In J Agric & Biol Eng, 8, 145-149). Other authors find that the flavonoid and phenolic content stay constant in Camellia sinensis at 60°C. But diversity in the methods of drying leads to differennt phenolic content and composition (T Hajmepour et al., Int J Pharmac Sci and Res 2012). (S Roshanak et al., J Food Sc Technol. 2015, 53, 721-29). Flavonoids are the main sunscreen chemicals of plants as confirmed by the studies of Pr Gareth Jenkin at the University of Glasgow. Flavonoids are even used as natural antioxidants and UV light stabilizers for plastics like polypropylene (MD Samper et al., J Appl Polym Sc. 2013, 129, 1707-1716). Exposure of plants to UV‐B causes ﬂavonoid productIon. The higher temperatures in sun drying may also inhibit the enzymatic degradradation of flavonoids and other constituents due to the longer period it takes to dry in the shade.
Vitamin C sharply decreases (S Roshanak op.cit.). The same sharp decrease in Vitamin C was noticed for sun drying of apricots (M Madrau et al., Eur Food Res Technol, 2009, 228:441. This may be consisered as a positive effect as Vitamin C is antagonistic with most antimalarial treatments.(see my blog« Vitamin C and malaria beware ! » on www.malariaworld.org).
UV-LIGHT AND SCOPOLETIN
Scopoletin and coumarins appear to be the only molecules where the content signifantly increases after sundrying. (H Al-Oubaidi et al., World Journal of Pharmaceut Sc., 2014, ISSN 2321-3086). Sun drying is used in Africa for cassava chips. In Benin, cassava is one of the most important plants grown. The conservation of fresh cassava roots is very difficult because they are highly perishable products subject to contamination by fungi, bacteria and other germs (Rafiatou Ba et al., Int J Appl Biol and Pharmac Technol. 20167, ISSN 0976-45550). The study showed that sun drying is a major factor promoting the accumulation of scopoletin. Scopoletin inhibits the growth of Aspergillus flavus and the production of aflatoxin in dried cassava roots (GJB Gnonlonfin et al., J Food Safety, 2011, 31 553-558). More important even, in sun dried chips the scopoletin content stays high after 3 months of storage. UV-C irradiation also is used to induce scopoletin accumulation in oranges to avoid postharvest decay (G D’hallewin et al., J Amer Soc Hort Sci 1999, 124, 702-707). Tobacco plants confronted by contamination in the soil increase the content of scopoletin in stems for protection (Y Cohen et al., Phytopathology, 1981, 71-2:209), (JL Song et al.,J Asian Nat Prod Res., 2016. Jun14, 1-7). Alternaria alternata a tobacco pathotype is inhibited by the fluorescent scopoletin (Huanhuan Sun et al., J Exper Botany, 2014, doi :10.1093/jxb/en). Scopoletin also acts as insect feeding deterrent (A Tripathi et al.,Insect Science,2011, 18, 189-194). US patent 6337095 finds concentration of 0.3 % of scopoletin in the stems and only 0.2 % in the leaves and uses the stems for commercial scopoletin extraction.
All Artemisia plants are rich in scopoletin, with Artemisia annua, afra and apiacea top ranking around 0.1 % dry weight (Eur J Clin Pharmacol. 1996;50:225-30).This is mucher higher than in other plants known to contain scopoletin like Avena sativa, Nicotia tabacum (tobacco), Melia Azedirach, Manihot esculenta (cassava) and even higher than in noni juice (Morinda citrifolia) famous for its scopoletin content.
Scopoletin has anti-nociceptive and antiinflammatory activities (Z Chen et al., J Ethnopharmacol 2013, 25, 501-6). Recent studies have discovered and described its antitumor activities (EJ Seo et al., Molecules 2016, 21, 496).
Scopoletin is known as ion channel opener and has a significant effect on erythrocyte membrane ion motive ATPases. The stimulatorx effect is stronger on Na-K-ATPase, followed by Ca-ATPase and then Mg-ATPase. Ion motive ATPases play a crucial role in malaria (CA Ezeokonkwo et al, Nig J Nat Prod and Med 2001, 05, 37-40).
Scopoletin and other coumarins inhibit CYP3A4. The inhibition of CYP3A4 by grapefruits is mainly related to furanocoumarins (bergamottin, psoralens). Scopoletin can markedly affect the pharmacokinetics of artemisinin and increase the plasma concentration, more than other molecules present in Artemisia like arteannuin-B, artemisinic acid, casticin, chrysoplenol (Chao Zhang et al., Asian Pacific J of Trop Med. June 2016.05.04)
The effect of Artemisia plants on CYP3A4 was confirmed by assays run at the Vrije Universiteit Brussel (Masterproef. K Lazaridi 2014) in the Laboratory of Pr Kris Demeyer. They worked with Artemisia plants from different origins, including the species Artemisia abrotanum, Artemisia apiacea, Artemisia pontica, Artemisia herba alba, Artemisia absinthium, Artemisia afra. The CYP3A4 inhibition was surprisingly high for all Artemisia samples, up to 6 times higher than for ketoconazol (0.11 µg/mL) or for diluted grapefruit juice. This is surprising because grapefruit juice has the reputation to be the strongest CYP3A4 inhibitor from plant origin. Surprising is also the fact that all Artemisia samples show this strong CYP3A4 inhibition without a particular strength for one of these 7 species.
SUN DRYING AND BACTERIA
But the most important benefit of sun drying could be the killing effect of UV light for bacteria and molds. In other drying techniques, especially at room temperature, they might have ample time to develop.
The thresholds of microbial count of medicinal plant material according to the European Pharmacopeia 2005 are
- For herbal medicinal products to which boiling water is added before use are 10⁷ for aerobic bacteria, 10⁵ for molds and 10² for E.coli.
- For preparations for oral and rectal administration 10⁴ for anaerobic bacteria, 10² for molds and nil for E.coli
Post-harvest processes such as collection of plant material in the field, transport to the farm are often suspected to increase mibrobial contamination of medicinal plants. When the bulk of harvested material is not ventilated, auto-heating provides favourable conditions for micro-organism growth. It is important to reach as soon as possible a moisture content < 10%. in the plants (Rocha RP, J Med Plants Res, 5(33) 7076-7084, 2011),
Immediate drying of the harvested plants in the sun could prevent these problems.