Quality Attributes in Centella asiatica(L.) Urban: Comparison
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Centella asiatica is well known for its miraculous therapeutic properties in various systems of traditional medicine across the world. However, significant variation in its pharmacological activities has been reported due to the unavailability of quality raw material and non-standardized formulations. A number of research papers have been published on the collection of C. asiatica plants from different regions for the identification of a suitable agroclimate with elite germplasms. Efforts have been made to standardize production and post-harvest practices for the availability of quality raw material with a high centelloside content. 

  • Centella asiatica
  • biomass
  • drying
  • madecassoside
  • asiaticoside

1. Ethnomedicinal Use of Centella asiatica

The literature survey showed that the plant has a long history of ethnomedicinal use in Asian countries [7,8][1][2]. A large number of ethnomedicinal reports are from South East Asia, especially the Indian subcontinent including India, Pakistan, Bangladesh, Sri Lanka, Nepal, Bhutan and Myanmar. Apart from the Indian subcontinent, its traditional uses have been reported in other Asian countries like China, Japan, Cambodia, Indonesia, Malaysia, Thailand and Vietnam. The ethnomedicinal uses of C. asiatica have been reported in ancient medicinal texts such as Indian ayurvedic pharmacopeia [9[3][4],10], Chinese herbology medicine [11][5] and Japanese Kampo medicine [12,13][6][7]. There are very few reports of the use of this plant as a fresh vegetable from other parts of the world as well [14][8]. Ethnomedicinally, the healing properties of C. asiatica are vast and vary with ancient cultures and tribal groups [15][9]. The plant is well known for the treatment of various mild and chronic diseases such as neurological disorders, diabetes, hepatitis, anemia, skin diseases [16][10], diarrhea, ulcers, fever and amenorrhea [17][11]. The plant has been widely used as an antioxidant, antibacterial, antiviral and anti-cancer agent [18,19][12][13]. The highest number of ethnomedicinal uses of C. asiatica has been reported in various ethnic communities in India and also in the ancient texts of the Indian System of Medicine (ISM). The plant has been mentioned as “Medhya Rasayana” or Brahma Rasayana for the management of mental exhaustion, nervous weakness, insomnia and memory loss and the improvement of overall mental health in the ISM [20][14]. It has also been used in the Ayurveda and Unani systems for the treatment of ailments like body aches, ulcers, stomach disorders, asthma, leprosy, leukorrhea, urethritis, loose bowels, dysentery, and mental illness [21,22,23][15][16][17]. There are several reports of the use of C. asiatica as “food medicine” for the management of various disorders by ethnic communities in India. Fresh leaves of this plant are consumed as a salad for the management of gastrointestinal disorders like constipation by the ethnic communities of the district of Bandipore, Kashmir, India [24][18]. Topical application of the fresh leaf paste of this plant is used by ethnic communities in Kolli Hills and Kani Tribals, Tamil Nadu, India, to treat fever and cold [25,26][19][20]. In India, Mizo communities in the western part of Mizoram used its leaf decoction for the treatment of asthma and eye problems [27][21]. The tribes of the Similipal forest, Mayurbhanj, Odisha, India, used one teaspoonful of the leaf paste daily for the treatment of diarrhea and dysentery [28][22]. The fresh whole plant is used as a cure for stomach problems and for boosting immunity and mental health by the Aka, Miji and Monpa communities of the West Kameng district, Arunachal Pradesh India [29,30][23][24]. Various ethnic communities in Kerala, India, used this plant for the treatment of hair diseases, diabetic ulcers, piles, jaundice, anemia, skin diseases and mental illness [31,32,33,34,35][25][26][27][28][29]. The plant has been used very profusely in other countries of the Indian subcontinent. In an ethnomedicinal study reported in the district of Rajshahi, Bangladesh, 125 g of the leaves was boiled with water, and 1 cup of the decoction was taken with honey every morning and evening to treat blood disorders. For the management of fever, 31.25 g of the leaf juice of C. asiatica was mixed with 31.25 g of the leaf juice of Nyctanthes arbor-tristis and taken every morning on an empty stomach [36][30]. The ethnic communities in Jessore district in Khulna Division, Bangladesh, indicated that the application of C. asiatica fresh leaf paste twice daily for 7 days around the nipple resulted in an increase in breast milk after childbirth [37][31]. The “Garos” community in Madhupur, Tangail, Bangladesh, used whole-plant juice to stop excess menstruation and also whole-plant paste for the treatment of skin diseases [38][32]. Tripura traditional healers of the Chittagong Hill Tracts region of Bangladesh used C. asiatica for gastric disorders, stomach pain and the treatment of diarrhea [39][33]. Paste prepared from C. asiatica young leaves was used in the treatment of eczema and headache, and whole plants were used as a vegetable for the management of dysentery by locals in the village of Dohanagar, Patnitala Upazilla of Naogaon district, Bangladesh [40][34]. Kavirajes, traditional medicinal practitioners from the Chalna area of Bangladesh, use the whole plant for the treatment of dog bites, asthma, itching, leucorrhea, malaria, tumors and wounds and as a carminative [41][35]. In Gosiling gewog in Tsirang district in the southern region of Bhutan, the ethnic communities reported that the oral administration of the paste of fresh C. asiatica leaves helped as a treatment for pneumonia and internal wounds [42][36]. The plant is used both as a leafy vegetable and a medicinal herb in Myanmar. It has been used for the treatment of diabetes, skin diseases (eczema, leprosy, itching, rashes and sores), mental illness, blood problems, dysentery, urine retention, painful urination, blood in the urine and syphilitic infections. It has shown snake-poison-neutralizing capacity. The leaf extract, together with sugar and honey, has been given daily to treat colds and fever as a restorative product [43][37]. In Nepal, tea prepared from C. asiatica leaves is used as a detoxicant and diuretic and also for the treatment of diarrhea, hypertension, urinary tract infections and poor memory [44][38]. The leaf and whole-plant juice has been used as an antiseptic for skin infections [23][17]. Around four teaspoons of the leaf juice obtained by squeezing 50 leaves are taken orally in the morning for 2–3 weeks to cool the body and stomach [45][39]. Among other Southeast Asian countries, C. asiatica is used as a vegetable and traditional medicine in Malaysia, Indonesia and Thailand [14][8]. In Malaysia and Indonesia, the whole plant is eaten fresh as a vegetable in a salad, in soup or as an appetizer and is also believed to treat memory loss and act as an anti-aging herbal medicine [46,47][40][41]. The Kadazandusun community of Malaysia cook this plant with coconut milk or shredded coconut to prepare soup. In these countries, the plant has been used as tea for treating hypertension, diarrhea and urinary tract infections, as a detoxicant and diuretic and to lower blood pressure and decrease heart rate [23][17]. In Thailand, the fresh leaves of C. asiatica have been eaten with sour chopped meat salad or fried noodles [48][42]. It has been reported that the freshly prepared juice from the young and tender leaves of C. asiatica is a rich source of vitamin A and is commonly taken for thirst-quenching purposes or as a cooling drink to reduce “inner heat” [24][18].

2. Therapeutic Uses in Modern Medicine

C. asiatica has been used as a remedy for several health problems in modern medicine [5][43]. The plant has been useful in improving cognitive function and memory [56][44]. It has shown its effectiveness in the management of mild cognitive impairment (MCI) among the elderly, aged 65 years and above [57][45]. Administration of C. asiatica has protective effects against colchicine-induced cognitive impairment and associated oxidative damage as well as anticonvulsant, antioxidant and CNS-protecting activity in animal models [58][46]. C. asiatica extract can have an influence on neuronal morphology and promote higher brain function during the postnatal developmental stage in mice [56][44]. The fresh leaf extract could potentially be used to improve neuronal dendrites in conditions involving stress, neurodegenerative disorders and memory disorders [59][47]. Triterpenes from this plant have shown antidepressant effects in various studies. Triterpenes significantly decreased the serum level of corticosterone and increased the amount of monoamine neurotransmitters in the rat brain [2][48]. It has been indicated that the constituents/active principles of the plant stimulate neuronal growth, improve memory and prevent neurodegeneration [60][49]. Asiatic acid isolated from this plant enhanced memory and learning in male Sprague–Dawley rats [61][50]. This chemical has the ability to reverse the effects of valproic acid treatment on cell proliferation and spatial working memory [62,63][51][52]. Triterpenoid saponins and pectin found in C. asiatica have been shown to exhibit immunomodulatory and immunostimulatory activities, respectively [64,65][53][54]. C. asiatica extract has been reported to have protective effects against ethanol-induced gastric mucosal lesions [66][55]. The ulcer-protective effect of the fresh juice is believed to be attributed to its ability to strengthen mucosal defensive factors [64][53]. Two phytoconstituents, asiatic acid and madecassic acid, possess significant anti-inflammatory activities [67][56]. Ethanolic and methanolic extracts of C. asiatica have demonstrated significant protective effects and the ability to lower blood glucose levels to normal in glucose tolerance tests conducted in alloxan-induced diabetic rats [68][57]. Asiaticoside has been associated with enhanced wound-healing activity, primarily attributed to increased collagen formation and angiogenesis. It exhibited significant wound-healing activity in animal models of normal and delayed healing [69][58]. It is also used for the treatment of scars or wounds by increasing the activity of myofibroblasts and immature collagen. It is also used in porcine skin as it stimulates the epidermis by activating the cells of malphigian. Direct application of asiaticoside is also used to treat wounds in rats and increase the ability or strength of newly formed skin. It is used in the treatment of hypertrophic scars and keloids and to prevent new scar formation [1][59]. Asiatic acid shows cytotoxic effects on human ovarian cancer cells [70][60]. The antioxidant activity of the plant extract prevented ethanol-induced gastric mucosal lesions by strengthening the mucosal barrier [69][58]. The plant essential oil, produced by steam distillation, was found to be an excellent antioxidant. Its antioxidant activity was comparable to that of the synthetic antioxidant butyl hydroxy anisole [71][61]. Two new flavonoids, castilliferol and castillicetin, from C. asiatica showed effective potential in preventing aging [72][62]. The total triterpenic fractions of C. asiatica (TTFCA) have been found to be effective in the management of venous insufficiency and related symptoms, such as ankle edema, foot swelling and capillary filtration rate [73][63]. The plant possesses good antimicrobial properties. Ethanol and petroleum ether extracts of the C. asiatica plant have exhibited significant activity against several fungal strains such as Aspergillus niger, Aspergillus flavus and Candida albicans [64][53]. Methanol, chloroform and acetone extracts of C. asiatica have been found to exhibit a notable inhibitory effect on the growth and sporulation of Colletotrichum gloeosporioides [74][64]. The ethanol extract also exhibited prominent activity against Aspergillus niger and Bacillus subtilis, followed by methanol and water [56][44]. Essential oil extracted from C. asiatica exhibited antibacterial activity against Gram-positive (Bacillus subtilis and S. aureus) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa and Shigella sonnei) bacteria [75][65].

3. Nutritional Uses

C. asiatica serves as a good source of various macro- and micronutrients. This plant is relatively low in proteins (2.4%), carbohydrates (6.7%) and fat (0.2%). In terms of dietary fibers, it contains insoluble dietary fiber (5.4%) and soluble dietary fiber (0.49%). The mineral contents (mg/g of dry weight) include phosphorus (17.0), iron (14.9) and sodium (107.8) [28,29,30][22][23][24]. C. asiatica is reported to be rich in various vitamins such as vitamin C (48.5 mg), vitamin B1 (0.09 mg), vitamin B2 (0.19 mg), niacin (0.1), carotene (2649 μg) and vitamin A (442 μg) per 100 g approximately [76][66]. Biochemical experiments show higher total sugar and protein contents in in vitro raised plants than field-grown plants while total starch content was lower in micro-propagated micro-shoots [77][67].

4. Culinary and Nutraceutical Uses

The plant is used as a vegetable in many countries in South East Asia, especially India. In the North East part of India, it holds good economic value as it is sold in open markets and can be grown easily without requiring significant investments. Consuming two to three leaves of C. asiatica on an empty stomach in the morning promotes a healthy digestive system and enhances immunity against seasonal and chronic diseases [78,79][68][69]. Various ethnic communities in North East India also use it for recreational and medicinal purposes. It is eaten as a salad or cooked as a soup or an appetizer. It is also cooked as a vegetable together with the main meal. Herbal tea is also prepared by steeping either a dried or fresh plant in boiled water and letting it brew for a few minutes before drinking [80][70]. It has been used in various polyherbal formulations for commercial applications and health-related purposes. C. asiatica is believed to nourish the hair follicles and scalp, reduce hair loss and breakage, strengthen hair strands and promote healthy hair growth. It is used in hair color, anti-dandruff formulations, organic shampoos, hair oils, hair gels, hair conditioners and other hair care products [68][57]. It is known for its potential to brighten and nourish the skin, reduce pigmentation, fine lines and dark circles and provide overall skin rejuvenation. It is used in skin creams, skin toners, mask packs, cleansing balms, skin moisturizers, facial serums and other skincare formulations [68][57].

5. Major Phytochemicals

The major secondary metabolites of C. asitica are saponin-containing triterpene acids and their sugar esters. Saponins are ubiquitous secondary plant metabolites that are produced through the isoprenoid pathway, which results in a hydrophobic triterpenoid structure (aglycone) with a hydrophilic sugar chain (glycone) [17][11]. The major phytochemical constituents such as asiatic acid, madecassic acid, asiaticosides, madecassoside, asiaticoside A and asiaticoside B have been characterized [81][71]. The quantitative estimation of major secondary metabolites in methanol extract by HPTLC indicated the presence of madecassoside, asiaticoside and its sapogenin asiatic acid [82,83][72][73]. Madecassoside, asiaticoside, madecassic acid and asiatic acid from C. asitica have also been quantitated by HPLC [84][74]. The content of asiaticoside was 1.20 µg/mL in C. asiatica by HPLC [85][75]. The methanolic extract of the aerial part of the plant resulted in the discovery of three novel compounds, namely centellin, asiaticin and centellicin [86][76]. The whole-plant essential oil is a colorless mild-scented oil with a yield of 0.06%. The GC-MS spectra revealed forty components, accounting for 99.12% of the oil content. The GC-MS analysis of essential oil also reported p-cymene (44%) as a major constituent [87][77]. The GC-MS of essential oil chiefly contains sesquiterpenes (68.80%), comprising α-humulene (21.06%), β-caryophyllene (19.08%), bicyclogermacrene (11.22%), germacrene B (6.29%) and germacrene D (4.01%) [88][78].

6. Centelloside Content in Accessions

Accessions have been collected for the identification and domestication of superior genotypes for gainful cultivations. The selection and propagation of elite genotypes with a higher content of the major centellosides, viz. asiatic acid, madecassic acid, asiaticoside and madecassoside, have been a major objective in C. asiatica production in the past few years. It has been reported that the concentration of asiatic acid, madecassic acid, asiaticoside and madecassoside varies between 0.02 and 3.2%, 0.02 and 3.06%, 0.018 and 4.3% and 0.01 and 4.8%, respectively, in various accessions collected from India. The ecological niche modeling approach revealed that the areas with high climatic suitability for the production of C. asiatica with a high content of these centellosides are largely confined to the Western Ghats, North East, Eastern Himalaya and Western Himalaya in India [89,90][79][80]. The large leaf of C. asiatica collected from Varanasi, Uttar Pradesh (central India), showed a low concentration of asiatic acid (0.05%), asiaticoside (0.31%) and madecassoside (0.31%) compared to the above-mentioned agroclimates [80][70]. Among the five accessions collected from Assam, Meghalaya, Uttar Pradesh, Kerala and Uttaranchal, the samples collected from Assam were found to have the highest content of asiaticoside (21.660 mg/g dry weight), madecassoside (11.176 mg/g dry weight), madecassic acid (0.740 mg/g dry weight) and asiatic acid (1.640 mg/g dry weight) [91][81]. In accessions collected from south India, the highest content of madecassoside was 5.67 ± 0.08% (dry weight of the whole plant), while the highest content of asiaticoside was 1.70 ± 0.02%. The combination of asiaticoside and madecassoside in C.asiatica collected from Kerala was reported to be 6.18 ± 0.26% (dry weight of the whole plant) [92][82]. The range of asiaticoside content in C. asiatica collected from Central Nepal was reported to be between 0.24% and 8.13%, while the range for asiatic acid content was between 0.29% and 0.66%. The study suggests that plants collected from Central Nepal exhibit higher levels of these metabolites compared to plants from the Eastern and Western regions of Nepal [94][83]. C. asiatica leaves from the local market in Pathumthani Province, Thailand, indicated asiatic acid, madecassic acid, asiaticoside and madecassoside content of 3.39, 4.4, 10.69 and 19.84 mg/g dry weight basis, respectively [95][84].

7. Production Practices

7.1. Open and Protected Cultivations and Hydroponic Production

The cultivation of C. asiatica has been carried out in open air under full sunlight and protected (in glasshouses/shade net houses) with reduced light intensity to enhance productivity and quality. An increase in fresh herb yield (18%) and dry matter yield (41%) was reported in cultivation under 50% sunlight in a shade net house compared to cultivation in full sunlight. The asiaticoside concentration was slightly high in 50% shade (0.91%) compared to open cultivation (0.90%) in full sunlight [96][85]. Accessions collected from Bhowali, Uttarakhand, India (accessions A), and Bengaluru, Karnataka, India (accession M), were cultivated under open air in Lucknow, Uttar Pradesh, India, and Bengaluru, Karnataka, India. The open cultivation of accession M in Lucknow produced higher fresh herb yield, dry weight yield and asiaticoside, madecassoside, asiatic acid and madecassic acid content compared to cultivation in Bengaluru [85][75]. This showed that agroclimate–genotype interactions play a significant role in centelloside content in C. asiatica. The asiatic acid content was high in organic cultivation (8.50%) compared to a non-organic cultivation system (7.67%) in an experiment conducted in Nagpur, Maharashtra, India [97][86]. A significant increase in dry biomass has been observed on the supplementation of inorganic nitrogen fertilizer at 50, 75, 100, 125 kg/ha and 100:60:60 kg/ha of NPK along with a basal dose of FYM (5 t/ha) in Arka Divya and Arka Prabhavi cultivars of C. asiatica in open-field and 50%-shade cultivation in Bengaluru, India. The total centelloside contents were higher in open cultivation (4.87%) compared to 50% shade (4.57%) at 50 Kg/ha of nitrogen fertilizer supplementation along with a basal dose of FYM (5 t/ha). The study showed that the open-field cultivation of C. asiatica would be more profitable compared to 50% shade due to a better benefit–cost ratio in the selected agroclimatic conditions [98][87]. A field test involving different lines of C. asiatica, specifically the induced tetraploid line and diploid lines, was conducted.

7.2. Production in Tissue Culture System

Considering the economic importance of C. asiatica, attempts have been made for its production in a tissue culture system. The effects of a number of elicitors, viz. yeast extract, CdCl2, CuCl2 and methyl jasmonate (MJ), on asiaticoside production in the whole-plant cultures of C. asiatica were studied. MJ elicited the production of asiaticoside in leaves (9.56 ± 0.91 mg/g dry weight), petioles (1.85 ± 0.07 mg/g dry weight), roots (0.17 ± 0.01 mg/g dry weight) and whole plants (4.32 ± 0.35 mg/g dry weight) in eight-week-old culture [110][88]. For quality planting material generation, in vitro multiplication resulted in the harvesting of over 25,000 plantlets within 160 days from a single shoot tip explant [111][89]. The use of 2 mg/L of 2, 4-D and 1 mg/L of kinetin in the establishment of the cell suspension culture of C. asiatica proved effective in supporting cell growth and enhancing flavonoid production [112][90]. When comparing centelloside production in semisolid and liquid media, the production of asiatic acid (1.02 ± 0.03 mg/g fresh weight) in semisolid media was nearly 3-fold compared to liquid media. The leaf callus produced a maximum amount of 1.46 ± 0.06 mg/g fresh weight asiatic acid in semisolid media [113][91]. Tissue culture supplemented with 100 µM of MJ produced asiaticoside (1.11 mg/g dry weight) and madecassoside (0.62 mg/g dry weight) in 15-day-old whole plants [114][92]. Similarly, tissue culture of C. asiatica supplemented with sucrose produced fresh plant weight (9.1 g/50 mL culture) and dry weight (1.37 g/50 mL culture) on the 24th day of harvest. It indicated that C. asiatica cells were able to accumulate a significant amount of biomass under the influence of sucrose. The highest dry cell weight of 27.4 g/L and the elicited concentration of asiaticoside (4.26 mg/g dry weight), madecassoside (2.34 mg/g dry weight), madecassic acid (0.71 mg/g dry weight) and asiatic acid (1.4 mg/g dry weight) were observed under sucrose supplementation [123][93]. Asiaticoside (45.35 mg/g dry weight) was very promising on the 24th day of culture at 30 g/L sucrose supplementation [115][94]. Tissue culture of C. asiatica colonized with root endophytic fungus, Piriformospora indica, induced asiaticoside production in leaves (5.3 mg/g dry weight) and the whole plant (2.3 mg/g dry weight) in 45 days of culture [116][95]. C. asiatica supplemented with a 3% culture filtrate of Trichoderma harzianum produced 9.63 mg/g of asiaticoside on a dry weight basis on the 10th day of culture [117][96]. Tissue culture supplemented with the auxin and cytokinin growth hormones produced the highest asiaticoside in leaves (16.72 mg/g) compared to roots (6.42 mg/g) and adventitious roots (11.42 mg/g) on the 60th day of culture [121][97]. Supplementation of 2 mg/L 2,4-dichlorophenoxyacetic acid, 1 mg/L kinetin and 30 g/L sucrose resulted in a 4.5-fold increase in the concentration of asiaticoside (45.35 mg/g dry weight) compared to planta leaves [115][94]. Tissue culture, supplemented with 1 mg/L BAP and 1.5 mg/L naphthalene acetic acid (NAA) elicited with MJ, produced 0.1434 ± 0.004 mg/mL, 0.2004 ± 0.0023 mg/mL and 0.016 ± 0.0001 mg/mL asiaticoside in shoot, callus and cell suspension culture, respectively, and 0.2681 ± 0.010 mg/mL and 0.0656 ± 0.0026 mg/mL asiatic acid in shoot and callus culture, respectively [118][98]. Supplementation of diploid and tetraploid cultures of C. asiatica with MJ (50 and 100 µM concentration) induced centelloside production in hairy root culture. The 20–28-day diploid and tetraploid hairy root culture produced asiaticoside (25.87 and 14.39 µg/mg dry weight), madecassic acid (0.79 and 1.85 µg/mg dry weight) and asiatic acid (2.24 and 5.69 µg/mg dry weight), respectively, at a 50 µM concentration [119][99]. Similarly, 50 μM MJ demonstrated the highest production increase in centellosides, phenols and flavonoids in the shoot cultures of C. asiatica [126][100]. For shoot induction, Murashige and Skoog (MS) media supplemented with 0.5 mg/L 6-Benzylaminopurine (BAP) and 0.1 mg/L NAA resulted in an 82 ± 2.2% success rate [128][101], whereas MS media supplemented with 4 mg/L BAP and NAA in combination with 2 mg/L 2,4-D resulted in a 92% success rate [125][102]. High frequencies of multiple shoot regeneration were reported for shoot tip explants cultured in MS media supplemented with 4.0 mg/L BAP and 0.1 mg/L NAA. Compared to an average of 10.2 ± 0.38 shoots per explant [127][103], 2.0 mg/L BAP supplementation resulted in 15.24 nodes per shoot after 30 days of culture [129][104].

8. Methods of Post-Harvest Processing

Drying in a hot-air oven (40–60 °C) has been the most common method of post-harvest processing. The shade-drying method has also been practiced to reduce the cost [90,110][80][88]. For the drying of C. asiatica plants produced in tissue culture, freeze drying has been largely practiced due to the low volume of the material [110,114,120][88][92][105]. There are reports that oven drying (OD) and solar drying (SD) have resulted in a decrease in antioxidant activity as well as total centelloside content in comparison to shade drying (SHD), microwave drying (MD) and freeze drying (FD). Freeze drying has been identified as the most efficient method as the bioactive content, antioxidant activity and color were higher in comparison to SD, SHD, OD and MD [133][106]. The presence of a high concentration of flavonoids in C. asiatica leaf, root and petiole has been reported for FD [134][107]. Microwave blanching in a heat-pump-assisted dehumidified dryer at 40 °C shows the highest total phenolic compound content of about 4.7 ± 0.08 mg/g. Additionally, the microwave blanching and heat pump drying (HPD) methods offer the advantage of reduced drying time compared to conventional drying methods [135][108]. Mixed-mode natural-convection SD was used for drying a bulk quantity of C. asiatica. The dryer was capable of handling an initial moisture content of 80% and drying it completely within 4 h. Furthermore, the solar tunnel dryer provided protection against external elements such as rain, insects and dust, ensuring the integrity and cleanliness of the material [136][109]. The use of SD was effective in reducing the moisture content of C. asiatica from 88.3% to 15.9% within a 12 h period. The average temperature inside the dryer was reported to be 45.4 °C, while the relative humidity was measured at 25.8%. The results suggested that the solar dryer was suitable for drying the plant due to the combination of low drying air temperature and a high moisture evaporation rate [137][110]. The solar-assisted dehumidification system achieved maximum values for pick-up efficiency (ηP) at 70%, solar fraction (SF) at 97% and the coefficient of performance (COP) at 0.3. These values were obtained when drying C. asiatica, with an initial wet basis moisture content of 88% and a final moisture content of 15%, using an air velocity of 3.25 m/s. The results suggest that the solar-assisted dehumidification system is suitable for drying heat-sensitive products like C. asiatica due to the fact that the drying process is conducted at low air temperature and low relative humidity [138][111].

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