Review
The Potential of Roselle (Hibiscus sabdariffa) Plant in Industrial Applications: A Promising Source of Functional Compounds
2024 年 73 巻 3 号 p. 275-292
詳細
2024 年 73 巻 3 号 p. 275-292
Roselle is an annual botanical plant that widely planted in different countries worldwide. Its different parts, including seeds, leaves, and calyces, can offer multi-purpose applications with economic importance. The present review discusses the detailed profile of bioactive compounds present in roselle seeds, leaves, and calyces, as well as their extraction and processing, to explore their potential application in pharmaceutical, cosmetic, nutraceutical, food and other industries. Roselle seeds with high phenolics, fiber, and protein contents, which are suitable to use in functional food product development. Besides, roselle seeds can yield 17-20% of roselle seed oil with high content of linoleic acid (35.0-45.3%) and oleic acid (27.1- 36.9%). This unique fatty acid composition of roselle seed oil makes it suitable to use as edible oil to offer the health benefits of essential fatty acid. Moreover, high contents of tocopherols, phenolics, and phytosterols were detected in roselle seed oil to provide nutritional, pharmaceutical, and therapeutic properties. On the other hand, roselle leaves with valuable contents of phenols, flavonoids, organic acid, and tocopherols can be applied in silver nanoparticles, food product development, and the pharmaceutical industry. Roselle calyces with high content of anthocyanins, protocatechuic acids, and organic acids are widely applied in food and colorant industries.
Hibiscus is a common flower plant grown worldwide with more than 300 species. Roselle (Hibiscus sabdariffa L.) is one of the hibiscus flower plants, belonging to the Malvaceae family. Roselle is an annual botanical plant with multi-purpose functionality. Roselle is very easy to grow and can be found in all warm countries, including China, Egypt, India, Indonesia, Malaysia, Mexico, Philippines, Saudi Arabia, Sudan, Thailand, and Vietnam. It is a new commercial crop in Malaysia, which was brought from India to Malaysia1). China and Thailand are the largest producers of roselle in Asia for industry purposes.
Roselle is a bushy shrub that can grow 0.5-3 m tall with green or red stalks and red or pale-yellow calyces. The plant grows well in warm and humid climates, where it takes about 3-4 months to grow to maturity for the calyces to be harvested. Across the globe, roselle has been given common or vernacular names specific to the language used by the local people in different countries or regions. For instances, it is known as rozelle in English-speaking countries, rosella in Australia, rosela in Indonesia, mesta in Indian subcontinent, asam paya in Malaysia, sorrel in Africa, krajeab in Thailand, and many other, as summarised in a review2). Generally, every part of roselle plants like calyces, leaves, seeds, and stems can be utilized for different applications.
Roselle plant is widely used for medicinal and nutritional purposes. Roselle calyces are normally used to prepare beverages, jam, jelly, sauces, and pickles with various important nutritional benefits3). A previous study showed that roselle calyces present a high amount of anthocyanin, antioxidants, amino acids, minerals, and vitamin C. Besides that, the leaves are normally cooked as a vegetable in Africa as the leaves contain high minerals such as phosphorus, calcium, magnesium, and potassium. Meanwhile, roselle seeds are normally discarded as waste during processing. Nevertheless, the chemical composition of roselle seeds revealed that they contain appreciable amount of protein, lipid, and fiber2). Roselle seeds contain 17-20% of oil content, which can act as a cheaper source of edible oil to provide with high amount of unsaturated fatty acids. The stem of the roselle plant can be used for fiber crops also4). Thus, the whole roselle plant has high functionality to act as a good economic source for diverse applications.
A limited number of reviews have been conducted on roselle. To address this knowledge gap, the extraction and processing, characterizations, and applications of different parts of roselle plants (seeds, leaves, and calyces) are discussed in this review. The physicochemical properties, bioactive compounds composition, and functionality of seed oil, seed extract, leaf extract, and calyx of roselle are reviewed to stimulate directions for future research and application.
The global roselle market size is expected to reach USD244.9 million by 2027, rising at a market growth of 10.4% compound annual growth rate (CAGR) from 2021 to 20275). One of the factors that contributes to the market growth of roselle is its versatility, roselle being a multifaceted plant with endless potential. Roselle seeds, leaves, and calyces have various uses and applications in food and beverages, pharmaceutical and nutraceutical, as well as cosmetic. Such a plant that can be used to its full creates less agricultural losses, thus reduces the environmental impact of the waste and waste disposal costs. Besides, it helps to satisfy the demands for sources of pharmaceutical and nutraceutical, more food to feed the growing world population, and food for feeding animals for human consumption. Proper use of the roselle plant could generate economic gains for the countries, provide beneficial health effects and reduce prevalence of diseases, mitigate environmental problems caused by mismanagement of agricultural losses, and many more. These altogether help in achieving at least three out of 17 Sustainable Development Goals (SDGs), which include “SDG2: End hunger, achieve food security and improved nutrition and promote sustainable agriculture”, “SDG3: Ensure healthy lives and promote well-being for all at all ages”, and “SDG12: Ensure sustainable consumption and production patterns”.
Raw roselle seed is known to exhibit a bitter taste due to the presence of anti-nutrients6). Interestingly unlike other seed foods, anti-nutrients such as alkaloid, oxalate, phytate and saponin were reported to be low in roselle seed and were not influenced by age, parts and cultivar of the plant7). It is projected that food processing techniques may favorably improve the safety and overall quality of this plant seed. To date, processing methods such as milling, drying, toasting, boiling, fermentation, soaking, sprouting, and enzymatic treatment have been proposed to modulate the palatability and nutritional values of the roselle seed8). Several processing methods might require to be performed to achieve the targeted outcome.
3.1.1 Milling
Roselle seed is generally milled into flour before being utilized in various food. Milling reduces cooking time and improves the palatability of the products created using seeds. Previous study determined the influence of oxygen and different light conditions on the oxidative stability of milled sun-dried roselle seeds recommended the milled roselle seed to be flushed with nitrogen and stored in the dark to retain its flavor9). A total of 85 volatile compounds including aldehydes, alcohols, ketones, furans, and acids were identified in the packed milled roselle seeds stored for 7 months. This study recommended the milled roselle seed be flushed with nitrogen and stored in the dark to retain its flavor.
3.1.2 Drying and toasting
Roselle seed is usually dried to a recommended safe moisture level (approximately 7.0-8.5% wet basis) as the post-harvest treatment to preserve the seed quality. Previous study reported the effects of raw-freeze-dried and sun-dried (2-3 days) methods on the nutritional composition of roselle seeds grown in Malaysia10). Overall, the protein and dietary fiber contents of sun-dried roselle seeds were found to decrease by 5.4% and 28.3% to 33.45% and 18.3% compared to raw-freeze-dried roselle seeds, respectively; whereas the ash and lipid contents were not significantly different from each other at approximately 7.4% and 24.7%, respectively. Further, the obtained results indicated that there was no significant difference in all the amino acids studied except lysine and leucine between the two drying methods. The results obtained are not uncommon as similar findings were also observed by a study11), in which insignificant differences were found in the nutritional qualities of samples dried using sun-drying versus freeze-drying. Previous study examined the effects of crushing, toasting and fermentation on the nutrient and phytochemical compositions of roselle seeds grown in Nigeria. The roselle seeds that were toasted at 100°C for 30 min displayed insignificant difference in proximate composition compared to crushed and fermented roselle seeds, but phytochemicals such as flavonoids, tannin, phlobatannin in roselle seeds were destroyed by heating significantly as reported in the study12).
3.1.3 Boiling
Boiling is another process commonly employed to treat roselle seeds, it involves cooking the seeds in water at a high temperature. The boiling process was thought to cause leaching of nutrients into the surrounding water that would reduce the nutritional value of the seeds. Interestingly, Tounkara et al.13) reported no significant difference between the raw and boiled roselle seeds grown in Mali in their proximate compositions. The boiling process was also expected to cause protein denaturation in the seeds that would increase its protein digestibility. Nevertheless, no significant difference was determined between rats fed with dried roselle seed powder and those fed with boiled roselle seed powder in protein efficiency ratio, net protein ratio, apparent digestibility, true digestibility, and biological value14). Together, these results implied the boiling process had negligible effects on the nutritional quality of roselle seeds.
3.1.4 Fermentation
In Sudan, roselle seed is naturally fermented to improve its nutritional and organoleptic properties. Fermentation for 6 days improved the in vitro protein digestibility of the cooked roselle seed through protein denaturation. The protein digestibility was found to decrease on the ninth day of ferment, which might be associated with the increased glutelin fractions that retarded the protein-enzyme interactions15). Fermentation is known to improve protein’s digestibility through the destruction of trypsin inhibitors that hinder the protein’s digestion, as well as through partial denaturation of the complex storage protein into smaller forms16),17). Therefore, similar results were also observed by a previous study, which reported the increased protein and mineral contents together with the reduced anti-nutrient contents of roselle seeds after natural fermentation18).
The total phenolic content (TPC) of roselle seed documented varies according to the types of solvent used for extraction and extraction method. Table 1 shows the TPC of roselle seed as determined in previous studies. It was noted that the majority of the studies were conducted in Malaysia, except for three that were done in the Republic of Mali, Vietnam, and Iran. Details of the extraction method and solvent, as well as the state of the seed used (not defatted or defatted), are also summarised in Table 1. The TPC of roselle seed was always quantified based on a calibration curve constructed using gallic acid and the result was expressed as gallic acid equivalent (GAE). However, results could be expressed on a different weight basis. For instance, some studies reported TPC as mg GAE per every g of dried seed or mg GAE per every g of the extract while some studies did not specify the expression. Comparison between studies is only fair when TPC was expressed on a similar weight basis.
Total phenolic content of roselle seed determined by Folin–Ciocalteu method.
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Mohd-Esa et al.19) extracted phenolic compounds in different parts of the roselle plant using water and 80% methanol. The TPC of roselle seed (not defatted, freeze-dried) was observed to be higher than the other parts of the roselle plant, which included the calyx, leaf, and stem. In the same study, the 80% methanol extracts showed higher TPC than the water extracts, implying the relatively less polar 80% methanol was able to extract more phenolic compounds in the roselle plant. The TPC of roselle seed was 4.87 mg of GAE/g seed and 2.97 mg of GAE/g seed, respectively, when 80% methanol and water were used as extraction solvents. Extracting phenolic compounds from not defatted dried roselle seed too but using 100% methanol, Nyam et al.20) reported a lower TPC (0.188 mg GAE/g seed) as compared to Mohd-Esa et al.19). This observation could be due to a shorter extraction time of 1 hour that was employed by Nyam et al.20) while Mohd-Esa et al.19) performed extraction for 2 hours.
The major phenolic compounds in defatted roselle seed were p-coumaric acid and ferulic acid21). The researchers utilized methanol and subcritical water (low-polarity water) for phenolic compound extraction. It was found that defatted seed had 1.32 to 1.73 mg phenolic compounds/g dried seed when methanol was used as extraction solvent. Meanwhile, under various subcritical water extraction conditions, the extract had 5.47 to 24.26 mg phenolic compounds/g extract.
As expected, Cissouma et al.22) and Tran-Thi et al.21) reported TPC for defatted dried roselle seed that was lower than that of Mohd-Esa et al.19). Interestingly, the two former studies used roselle seed from foreign lands whereas the latter study used roselle seed originating from Malaysia. This implies probably little compositional variation between roselle seeds from different geographical regions. Having said that, the majority of the studies did not indicate the developmental stage of the roselle plant used, which is another factor that may affect the biosynthetic pathways of phenolic compounds and hence the TPC determined. Phenolic compounds in defatted dried roselle seed were extracted using deionized water, 30% methanol, 30% ethanol, and 30% acetone (polarity in decreasing order) and the TPC determined were 1.66 mg GAE/g seed, 1.72 mg GAE/g seed, 1.77 mg GAE/g seed, and 1.99 mg GAE/g seed, respectively (TPC in increasing order)22). A similar type of inverse relationship was also noted in the study of Mohd-Esa et al.19), whereby extraction of phenolic compounds in roselle plants was more efficient using a relatively less polar solvent. As the polarity of the extraction solvent decreased further particularly when methanol was used such as that in the study of Tran-Thi et al.21), the TPC of defatted dried roselle seed was found in the range of 2.06 to 3.57 mg GAE/g seed, depending on the solvents used for defatting the dried seed (hexane, methanol followed by hexane, or petroleum ether).
The expression of TPC of roselle seed is sometimes based on g of extract, as mentioned. Phenolic compounds were extracted in defatted seed using 70% ethanol at room temperature for 4 hours (2 rounds of 2 hours) conventionally on a magnetic stirrer and the TPC of the extract was 18.3 mg GAE/g extract23). Moreover, extraction time could be significantly reduced to just 10 minutes in the presence of microwave radiation, and comparable TPC (17.91 mg GAE/g extract) of defatted seed in 70% ethanol24). In the same study, the TPC of roselle seed extract decreased to 9.66 mg GAE/g extract when extraction time was reduced to 4 minutes, implying that longer extraction time favors extraction efficiency and more phenolic compounds could be extracted. With more advanced technology, such as subcritical water extraction adopted by Tran-Thi et al.21), the extract had TPC in the range of 9.78 to 71.79 mg GAE/g extract under different extraction parameters set. Subcritical water is water that has its polarity reduced. More phenolic compounds could be extracted as the polarity of the extraction solvent decreased, this observation is coherent with all previous studies.
There are also times when TPC was expressed on an unknown basis. Once again, relatively less polar solvents demonstrated better extraction power than the more polar ones. For instance, the roselle seed extracts prepared using deionized water, methanol, and acetone (decreasing order for solvent polarity) had TPC of 1.67 mg GAE/g dry weight, 1.86 mg GAE/g dry weight, and 2.01 mg GAE/g dry weight, respectively25). Likewise, it was also documented that the TPC of roselle seed was 4.86 to 7.67 mg GAE/g when 60% ethanol was used as extraction solvent; as the concentration of ethanol increased from 80% to 100%, TPC increased from 10.95 to 16.30 mg GAE/g to 9.48 to 20.18 mg GAE/g26).
Roselle seed is generally receiving lesser attention compared to its calyces and hence has not had any commercial food applications to date. Nevertheless, roselle seed contains considerable amounts of protein, oil, carbohydrate and dietary fiber, which can be industrially exploited for the production of value-added compounds based on a sustainable circular economy concept.
In North Western Nigeria, roselle seed is pounded into roselle seed cake and used as the main ingredient to prepare a soup condiment called Daddawan Batso27). The dried roselle seed can also be used as a coffee substitute or fermented with spices to prepare a food known as Mungza Ntusa28). Furundu is a traditional protein-rich fermented roselle seed food that often serves as a meat substitute during famine times in Sudan. The shelf life of Furundu was approximately one year, but interestingly it has become less popular lately due to a lack of accessibility to roselle seed, together with the competitive advantages of other food items29).
There is a large potential to utilize roselle seed in value-added food products. For instance, roselle seed is added to bakery products like cookies20),30),31), biscuits32), and bread33),34). The bakery products incorporated with roselle seed flour generally showed higher content of protein, lipid, dietary fiber and minerals, together with improved antioxidant activities compared to the control. Most importantly, these bakery products were accepted by the sensory panel in the studies, which implied roselle seed flour’s potential to improve the nutritional provision of wheat flour through substitution. Roselle seed extract (RSE) has been reported to be added to beef patties to retard lipid oxidation19). The results obtained suggested that the patties incorporated with RSE demonstrated better antioxidant properties compared to butylated hydroxytoluene (BHT) and α-tocopherol.
Despite human research being necessary to deepen the understanding of roselle seed’s nutraceutical and pharmaceutical importance, most studies to date are limited to in vitro and in vivo bioassays. A previous study investigated the effects of defatted sun-dried roselle seed powder at different doses on lipid profiles of hypercholesterolemia rats, which the defatted sun-dried roselle seed powder was found to demonstrate hypocholesterolemic effects in rats at higher dosages such as 50 and 150 g/kg35). The exhibited hypocholesterolemic effects by roselle seeds might be contributed by the binding effects of dietary fiber with bile acid, which led to increased fecal excretion2). Additionally, the binding capacity of bile salts by defatted roselle seed hydrolysates was studied36). In vitro digestion of defatted roselle seed proteins by Alcalase, Flavourzyme, and sequential Alcalase-Flavourzyme exhibited bile-acid binding capacity that could potentially have cholesterol-reducing properties.
Solvent extraction, cold pressing, and supercritical fluid extraction are the three common methods used to extract roselle seed oil (RSO). Roselle seed is considered a valuable source of oil content, which yielded around 8.75 to 18.98% of RSO37). Solvent extraction can be performed by a Soxhlet extractor with an organic solvent such as petroleum ether or hexane with a mass ratio of 1:3 (seed to solvent) at 40-60°C3),38). Solvent extraction could deteriorate bioactive compounds and unsaturated fatty acids due to the high temperature applied during the process. Solvent extraction of RSO normally yielded 18.98-20.0%. High extraction yield could achieve by solvent extraction, but higher free fatty acids (FFA) value (2.41%) and peroxide value (PV) (4.57 mEq/kg) were detected in RSO compared to other extraction methods. Cold pressing and supercritical fluid extraction presented FFA values of 0.30-0.71% and PV of 1.01-2.14 mEq/kg in RSO, which showed higher oxidation status caused by the temperature applied in solvent extraction37). Besides that, solvent extraction normally takes 6-9 h to process. The chemical solvent used in solvent extraction may contaminate the sample, which is required to proceed to the refining process39). The whole process is considered time-consuming.
Cold pressing such as hydraulic- and screw-pressing are another two methods of extraction applied in the extraction of RSO. There are no heat and chemicals involved in cold pressing. Therefore, the consumer would prefer this kind of green extraction method. However, low extraction yield is the biggest challenge for cold pressing. The oil content of 10-12% obtained by cold pressing would eventually limit its application in the oil industry. A previous study showed that screw-pressed and hydraulic-pressed gave the oil extraction yield of RSO of 12.17% and 9.35%, respectively37). Another study reported the optimum yield of screw-pressed was 11.95% at 45°C with 30 rpm of speed40).
Supercritical fluid extraction is another green extraction method that can preserve the bioactive compounds in the oil and stabilize the oxidation status. Bioactive compounds like tocopherols, phenolics, phytosterols, and unsaturated fatty acids are sensitive to heat and can be preserved from supercritical fluid extraction. Supercritical carbon dioxide extraction (SC-CO2) can conduct at low temperature with a shorter extraction time. This method is advantageous as it is free of chemicals and it uses carbon dioxide as a non-active solvent to protect the extracted sample from high-temperature degradation. Thus, this method is environmental-friendly as CO2 is non-toxic, inexpensive, and can be recycled. Seeds quality, pressure, temperature, and extraction time are the factor that can affect the extraction process of SC-CO2 extraction of RSO. A previous study showed the SC-CO2 extraction of RSO yielded 8.75%, which showed a lower yield than solvent extraction37). Another study conducted the SC-CO2 extraction of RSO with optimized parameters of 30 MPa of pressure, a temperature of 40°C, an extraction time of 180 min, and a flow rate of 5 mL/min, which yielded around 6.22-16.17% of oil. This study showed that when the pressure increased at low temperature, the overall extraction oil yield increased also. The highest oil yield (16.17%) with 4.74 mg/100 g of γ-tocopherol was obtained for SC-CO2, compared to 23.8% of oil yield with 1.32 mg/100 g of γ-tocopherol for solvent extraction with petroleum ether for 8 h at 60°C. The results showed that SC-CO2 successfully preserved the tocopherol content in RSO39).
4.2.1 Fatty acid composition
Table 2 summarizes the fatty acid composition (% area) of RSO reported in previous literature. Different extraction methods, seed varieties, and environmental conditions may contribute to the variations of fatty acid profiles of RSO. From the data, linoleic acid (35.02-45.29%) is the dominant fatty acid, followed by oleic acid (27.07-36.90%), palmitic acid (17.19-21.90%), and stearic acid (3.63-7.96%). The percentages of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA) are 21.18-28.50%, 27.07-37.26%, and 36.87-46.93%, respectively. The unique proportions of SFA: MUFA:PUFA makes it suitable for cooking and drying. Vegetable oils rich in both oleic and linoleic acids are highly potential to use as supplementation to substitute saturated fats in our diet. This can help to reduce the risk of getting cardiovascular diseases. Linoleic acid and linolenic acid are essential fatty acids that are required by our body to maintain skin integrity, cell membrane structure, immune system, and eicosanoid formation. The synthesis of eicosanoids offers anti-inflammatory and anti-thrombotic functions to our body.
Fatty acid composition of RSO samples in previous literature.
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Minor identified fatty acids were lauric, myristic, palmitoleic, linolenic, arachidic, and gondoic acids. There was no low molecular weight fatty acid (less than C13) detected in the previous studies, which can enhance the oil stability3). Different extraction methods (solvent, screw-pressed, hydraulic-pressed, and SC-CO2) caused a slight variation in the fatty acid compositions of RSO37). However, the variations did not vary much. The same trend went to the effect of boiling the roselle seeds13). The induction period indicates the oxidative stability index of the edible oils. A previous study reported the induction period of RSO was 24.88 h at 100°C4), which was quite stable compared to soybean oil (10.9 h) and sunflower oil (11.0 h)41). RSO has the potential to use as cooking oil or functional oil as it provides a rich source of MUFA and PUFA, which offer health benefits to humans.
A noteworthy addition to the abovementioned composition of RSO is three unusual fatty acids, namely epoxyoleic (4.5%), sterculic (2.9%), and malvalic acids (1.3%) reported in a study42). Epoxyoleic acid, an epoxy fatty acid, demonstrated the ability to survive digestion and was deposited in the rats as other fatty acids43). In the same study, epoxyoleic acid showed no adverse effect in the animals at a level of 7% of the animals’ diet for a period of 28 days. Sterculic and malvalic acids, on the other hand, are cyclopropenoid fatty acids that also found in cottonseed oil. Little is known about the metabolism and physiological role of sterculic and malvalic acids in the human body, despite review had highlighted the emerging therapeutic effects of sterculic acid in certain diseases including cancer, Alzheimer’s disease, and atherosclerosis44).
4.2.2 Tocopherols
Tocopherols and tocotrienols are lipid-soluble antioxidants that contribute to vitamin E content in vegetable oils. The total tocopherols and tocotrienols contents were found in RSO as 99.68 mg/100 g, which was composed of 3.48 mg/100 g of tocotrienols and 96.2 mg/100 g of tocopherols4). γ-Tocopherol (67.58 mg/100 g) represented the most abundant tocopherol, followed by α-tocopherol (27.54 mg/100 g), δ-tocopherol (0.89 mg/100 g), and β-tocopherol (0.20 mg/100 g). α-Tocotrienol (2.11 mg/100 g), β-tocotrienol (0.52 mg/100 g), and γ-tocotrienol (0.85 mg/100 g) were detected in minor composition in RSO. Tocopherols and tocotrienols could offer antioxidant and anti-inflammatory activities. γ-Tocopherol could retard the oxidation of unsaturated fatty acids in vegetable oils, which contributes to the high oxidative stability of RSO during storage4),45).
SC-CO2 extraction successfully retained the highest tocopherol content (289.78 mg/100 g) in RSO, compared to other extraction methods, such as solvent extraction (267.35 mg/100 g), hydraulic-press (234.01 mg/100 g), and screw-press (215.23 mg/100 g)37). The SC-CO2 of RSO was optimized at a pressure of 20 MPa, a temperature of 80°C, and a flow rate of 20 mL/min, which successfully yielded 89.75 mg/100 g oil tocopherol in RSO46). Total tocopherols of 200.1 mg/100 g was detected in RSO45). γ-Tocopherol (74.5%) represented the most abundant tocopherol in RSO, and the relative abundance of α-, γ-, and δ-tocopherol was 25:74.5:0.5. Total tocopherols of 246 mg/100 g were reported in RSO, which composed by 153 mg/100 g of γ-tocopherol, 69 mg/100 g of α-tocopherol, and 24 mg/100 g of δ-tocopherol38). The tocopherol content in RSO is higher than those reported in vegetable oils, such as corn oil (70.7-100.6 mg/100 g), rapeseed oil (61.7-82.3 mg/100 g), soybean oil (72.5-132.8 mg/100 g), sunflower oil (73.7 mg/100 g), and walnut oil (63.4 mg/100 g)47).
4.2.3 Phenolics
The presence of the hydroxyl group able to scavenge the free radicals make the phenolic compounds to be a strong antioxidant that can improve the shelf life of the oil. Flavonoids, phenolic acids, and tannins are the main categories of dietary phenolic compounds. Naeem et al.37) reported the total phenolic content of RSO samples was in the range of 13.74 to 22.18 mg GAE/g due to different extraction methods. The RSO extracted by SC-CO2 retained the highest total phenolic content (22.18 mg GAE/g), while the screw-pressed RSO showed the lowest phenolic content (13.74 mg GAE/g). On the other hand, hydraulic-pressed RSO showed 17.25 mg GAE/g of phenolics, and solvent-extracted RSO showed 18.1 mg GAE/g of phenolics. El-Deab and Ghamry38) reported 18.37 mg GAE/g of phenolic content in RSO, and Al-Okbi et al.4) reported a total phenolic content of 56.31 mg GAE/g and flavonoids content of 4.99 mg catechin/g in RSO samples. These phenolic contents could act as natural antioxidants to preserve the shelf life of oil against oxidation, especially for oil with a high proportion of unsaturated fatty acids.
4.2.4 Phytosterols
Phytosterols are exhibited as a non-glyceride portion in the plant oils, which offer anti-inflammatory, anti-bacterial, anti-fungic, anti-ulcerative, and anti-tumor activities. Phytosterols help to reduce cholesterol levels in the human body. The total phytosterol content of RSO was 4573±268 mg/kg. β-Sitosterol (71.90%) was reported as the most abundant phytosterol detected in RSO, followed by campesterol (13.61%) and ∆-5-avenasterol (5.94%). Other minor phytosterol contents were detected in RSO, such as stigmasterol (3.53%), cholesterol (1.35%), and clerosterol (0.60%)45). Another study reported 749.8 mg/100 g oil of total phytosterol content in crude RSO, which was composed mainly of β-sitosterol (559.3 mg/100 g), campesterol (107.0 mg/100 g), stigmasterol (50.6 mg/100 g), and β-sitostanol (32.9 mg/100 g)3). The phytosterol content in RSO is comparable with those of vegetable oils in the market, such as corn oil (669.9 mg/100 g)3), olive oil (176.3-193.1 mg/100 g), rapeseed oil (767.1-823.8 mg/100 g), soybean oil (267.1-326.6 mg/100 g) and sunflower oil (294.6-375.5 mg/100 g)48).
β-Sitosterol has an inhibitory effect on colon cancer in both humans and laboratory animals3). RSO represents a good source of phytosterol to provide health benefits. Previous study found the SC-CO2 of RSO modified with ethanol to act as an entrainer could successfully recover 108.74% of phytosterol content in RSO, compared to the values obtained using SC-CO2. An entrainer such as ethanol could enhance the solubility and achieve a phytosterol content of 726.28 mg/100 g oil in RSO. The optimum parameters used were temperature of 40°C, 40 MPa pressure, and flow rate of 20 mL/min in the presence of 2 mL/min ethanol as entrainer49).
Previous literature reported RSO extracted from roselle seeds is edible with no toxicity. A toxicity study of RSO was conducted on mice and found the highest safe dose level of RSO was 10 g/kg of mouse body weight, which indicated the safe consumption of RSO as edible oil4). RSO was given at a 10% dosage to normal healthy and dyslipidemic rats for 4 weeks. The results showed that no symptom of toxicity or disturbance was observed in the functions of the kidneys and liver. The dosage of 10% RSO was around 12 g RSO/kg rat body weight if food intake and rat body weight have been taken into account, which corresponds to about 78 g/70 kg man body weight for humans. Thus, RSO is safe for human consumption and can be applied as edible oil or functional ingredients in food product development.
RSO can be incorporated into functional food product development to replace oil ingredients to offer unsaturated fatty acids and bioactive compounds. The substitution of RSO at a concentration of 75% in mayonnaise successfully retarded the increase of PV, and it reached 4.26 meq/kg oil at the end of storage, compared to PV of 29.36 meq/kg oil in the control sample. This is due to the antioxidant activity of RSO, which improved the oxidative stability of food products38).
RSO and roselle seed flour can also be incorporated with wheat flour to make functional cookies. The incorporation of RSO to replace margarine in 15-35% of the dosage reduced the spread factor of cookies, resulting in the production of softer cookies. This is because the viscoelastic property of gluten structure in a dough mixture was reduced by oil emulsion to improve the elastic texture of gluten in the aqueous phase. The texture analysis showed that the crushing, cutting, and penetration force used to break the cookies decreased with the incorporation of RSO at 5, 10, 15, 20, 25, and 30% to replace the use of margarine in the cookies formulation. The incorporation of RSO at a level of 15-35% offered the most superior quality attributes (appearance, flavor, crispiness, taste) and acceptability in the overall final baked products, showed by sensory evaluation of the previous study31).
Some vegetable oils have been suggested to use as a fuel due to their characteristics are closer to diesel, and they are renewable sources. Bothon et al.50)suggested RSO has a high potential to use for the transesterification process to produce biodiesel. RSO presented its kinematic viscosity (~4 mm2/s), cetane number (~55), and density (0.87 g/cm3), following the U.S. and European standards. Hasni et al.51) optimized the transesterification process of biodiesel production from RSO using the response surface methodology. The parameters such as methanol to oil ratio, temperature, usage of calcium oxide catalyst derived from waste eggshells, and agitation speed were optimized at 6:1, 67.5°C, 1%, and 750 rpm, respectively. The methyl esters produced from RSO presented kinematic viscosity of 4.55 mm2/s, acid value of 0.027 mg KOH/g, cloud point of 3°C, pour point of 1°C, flash point of 161°C, cetane index of 49 min, and density 856 kg/m3, with 95.01% of yield, comply within the range of ASTM D6751 and EN 14214 standard specifications. Thus, RSO could be a potential feedstock for biodiesel.
5.1.1 Drying technologies
Drying of roselle leaves can be achieved by different methods, either by direct sun-drying, artificial drying, or solar-assisted drying. The direct sun drying method involves the exposure of the leaves to the sun or spreading the leaves on a surface under ambient conditions and left to dry52). Although direct sun drying is environmentally friendly, its low drying efficiency contributes to its inherent drawbacks, which can have a negative effect on the roselle leaf. Compared with the artificial drying method, the ascorbic acid content in sun-dried roselle leaves was significantly lower53). Due to exposure to the open environment, microbial growth and contamination by dust and insect are also some of the problems of direct sun drying52).
Conventional air drying, infrared drying, and freeze drying are examples of artificial drying. Freeze drying was compared to oven drying, microwave drying, and crossflow drying, and the results concluded that freeze drying retained higher phenolic and flavonoid contents53). It can be explained by the prolonged drying and severe heating of conventional air drying that caused irreversible physical and chemical changes in the products52). While freeze drying can yield high-quality products with minimal oxidative and thermal degradation. The freeze drying method can lead to the high efficiency of polyphenol compound extraction, as the formation of ice crystals in the plant cells will lead to greater rupturing of the plant cell structure54).
Aside from sun drying and artificial drying method, a more simplified yet reliable drying method needs to be developed, to ensure the drying process is affordable and accessible to small farmers in developing countries. In addition, the united nations pronouncement of vision 2030, which focuses on sustainable development goals, highlighted the need for renewable energy55). Thus, solar dryers that harvest radiative energy from sunlight have the potential to be a practical alternative to incompetent sun drying and artificial drying method. According to the previous study, a solar dryer system for roselle leaf was developed to harvest solar energy flux over the single-pass solar air heating collector (SPSAHC) system and double-pass solar air heating collector (DPSAHC). The developed system could reduce the moisture content of roselle leaves to 9.2% (wet basis) after 14 hours of drying and meet the international standard requirement55).
5.1.2 Conventional extraction technologies
Conventional extraction methods, including maceration and infusion, have been widely applied because of their simplicity. Maceration is one of the commonly used solid-liquid extraction methods, which depends on the convective and conductive processes to heat the product56). Previous study carried out maceration on roselle leaves (32 g) with 500 mL of solvent for 4 days of extraction. While for the infusion method, it utilized thermal treatment to leach out the compounds. The leaching processes may involve dissolution or a simple physical solution57). Roselle leaves were infused with 15% w/v of water at 100°C for 15 min58). However, the conventional method required a huge amount of organic solvents and involved the use of high temperature or agitation to enhance the mass transfer of solute across the roselle leaf59). In addition, the conventional methods are time-consuming and required high temperatures, causing a higher cost of production. The quality of the extract will be affected due to the use of high temperatures and prolonged extraction time will damage the thermolabile compounds.
5.1.3 Non-conventional extraction technologies
The non-conventional extraction method has been developed because of technological developments to improve the roselle leaves extraction process. Sonication-assisted extraction (SAE) method has been proposed as a reliable and fast method, an alternative to traditional methods in extracting valuable compounds from roselle leaves60). This method utilizes ultrasound mechanical vibrations to damage vegetative cell walls tissue, making it easier for the solute diffuse across the tissue and extract the valuable compounds61). The ultrasonic power, pulse duty cycle, solid-to-liquid ratio, the solvent used, and temperature are important factors that influence the process optimization56). The SAE method was evaluated with amplitude ranging from 15-45% and time ranging from 5 min to 75 min60).
5.2.1 Phenols and flavonoids
Table 3 shows the bioactive compounds detected in roselle leaf. Aside from supplying color and imparting taste, phenolic compounds play an essential role in providing health-protective effects, including antioxidant, anti-ulcer, and hepatoprotective. Moreover, phenolic compounds can also increase collagen expression and decrease reactive oxygen species production, highlighting their role as skin care agents62). There were three phenolic compounds detected in the roselle leaf including neochlorogenic acid (6.84 mg/g), chlorogenic acid (0.85 mg/g), and cryptochlorogenic acid (2.13 mg/g). While for the flavonoid compounds in roselle leaf, quercetin (5.18 mg/g) and kaempferol (1.55 mg/g) were detected63). Many scientific investigations have revealed that quercetin and kaempferol have beneficial effects on non-alcoholic fatty liver disease, hyperlipidemia, inflammation, and diabetes. A previous study also indicated that quercetin and kaempferol are thought to act synergistically in delaying the proliferation of cancer cells64). High total phenolic content was detected in 80% v/v methanol-extracted roselle leaves (2.20±0.02 mg GAE/g)19). The roselle leaves extract also demonstrated strong DPPH free radical scavenging activity (89.80±0.33%) than the synthetic antioxidant, BHT (24.60±6.75%). Thus, roselle leaves extract can offer high antioxidant activity to slow down the progress of developing chronic disease and protect the human body from free radicals.
Bioactive compounds detected in roselle leaf.
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Besides that, polyphenol compounds have a chemical structure similar to the substrate for the enzyme tyrosinase. Thus, it can act as a competitive inhibitor for tyrosinase. Tyrosinase functions as a key regulatory enzyme that is responsible for the synthesis of melanin pigment. However, excessive accumulation of melanin pigment will cause dermatological disorders such as age spots and melasma. In addition, tyrosinase is also responsible for enzymatic browning which will damage the vegetable and fruit. It has been reported that the roselle leaf extract (RLE) showed inhibitory activity (11±3%) against the enzyme tyrosinase65).
5.2.2 Organic acid
The roselle leaves extract contained a high percentage of organic acid, including citric acid (0.21%), ascorbic acid (1.37%), tartaric acid (2.30%), oxalic acid (74.30%), and malic acid (21.79%), with the last two predominating66). Malic acid was widely used in the cosmetic industry to control acne and act as exfoliating agent67). Despite its constant use in cosmetology, malic acid showed a significant antiproliferative effect on human epidermal keratinocyte68). While citric acid and ascorbic acid, both of them demonstrated strong antioxidant properties against free radicals66).
5.3.1 Nanotechnologies
The leaf extracts had been widely used in the synthesis of silver nanoparticles, as a green alternative to the chemical method. Silver in the form of nanoparticles have been widely used in the medical and dentistry fields, particularly for their application in antibacterial and anticancer therapy. Additionally, silver nanoparticles can be employed to effectively limit bacterial growth without resulting in bacterial resistance69). RLE has been reported to use as the reducing agent and capping agent for the synthesis of silver nanoparticles69). Another study revealed that roselle leaf synthesized silver nanoparticles demonstrated antimicrobial activity against the Aggregatibacter actinomycetemcomitans70).
5.3.2 Food products
Protein obtained from the roselle leaf demonstrated the potential to be used as a viable alternative source of low-cost protein substitute in dietary ingredients or supplements for the food industry. The crude protein content in roselle leaf (27-28%) was higher than in the calyces (16-17.5%) and comparable to the protein content in Moringa oleifera leaf71). Aside from being eaten raw in the form of salad, roselle leaf with a rhubarb-like flavor can be processed into seasoning powder after roasting. In addition, roselle leaf powder can also be used as a cheaper substitute for individual amino acid intake.
5.3.3 Folk medicine and pharmaceutical
Traditionally, the RLE was utilized to stimulate the intestinal peristalsis and treat different types of ailments such as hypotensive, febrifuge, choleretic, and diuretic in Mexico, India, and Africa72),73). On the other hand, the natural gums and mucilage with non-toxic, capable of chemical modification, readily available, and biodegradable properties gained increasing attention in the pharmaceutical industry. Previous study showed the mucilage can be isolated from the roselle leaf and applied as a suspending agent in the pharmaceutical industry. The mucilage extracted from roselle leaf at a concentration of 2.0% demonstrated good suspending action and comparable sedimentation profiles with the commercial Calcimax suspension like pH, re-dispersibility, and rheological characteristics74).
Table 4 summarizes the pharmaceutical benefits of RLE, which suggested its high potential to use in the pharmaceutical industry. The usage of conventional antibiotics had resulted in an increasing number of multidrug-resistant infections. Thus, there is increasing consumer demand for naturally derived antibiotic drugs. The ethanolic RLE was found to be effective against Escherichia coli, Salmonella typhimurium, and Listeria monocytogenes at a concentration of 5 mg/mL75). Previous study also revealed that ethanolic RLE can significantly inhibit the growth of Pseudomonas aeruginosa (25 mg/mL), Proteus vulgaris (25 mg/mL), Klebsiella aerogenes (25 mg/mL), Staphylococcus aureus (12.5 mg/mL), Bacillus cereus (12.5 mg/mL), Escherichia coli (50 mg/mL), and Moraxella catarrhalis (25 mg/mL). Thus, roselle leaves extract can act as a good anti-microbial agent76).
The pharmaceutical benefits of roselle leaves.
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The effect of RLE was investigated on human prostate cancer. After 24 hours of treatment, the results showed that the RLE suppressed the growth of human prostate cancer cells, with a 60% reduction in cell number (p<0.01) when treated with 0.5 mg/mL RLE. In addition, roselle leaves extracts also inhibited the binding activity of nuclear factor kappa B (NF-κB) and afterward suppressed the gene and protein expression level of matrix metalloproteinase-9 (MMP-9), highlighting roselle leaves’ potential as an anticancer drug. The suppression of MMPs and their regulatory pathways have been considered promising targets for chemotherapeutic agents and anticancer drugs. The in vivo results also demonstrated that RLE feeding at a non-toxic concentration of 1.0% could significantly suppress the prostate tumor cell by 50%77). Chiu et al.78) studied the in vitro anticancer activity of roselle leaf against melanoma cells and revealed the potential of roselle as an anti-melanoma agent. With IC50 values of 250 µg/mL, roselle leaves extract inhibited the expression of B-cell lymphoma 2 (Bcl-2) and enhanced the expression of fast ligand (Fas-L), caspase-3, -8, and -9. These results indicated that the RLE could induce apoptosis in melanoma cells through an intrinsic pathway.
The anti-hyperlipidemia activity of roselle leaf extract capsules was investigated in human subjects. After 90 days of the intervention period, the experimental group’s serum triacylglycerol (143.3±73.9 mg/dL) and low-density lipoprotein (LDL) cholesterol (127.6±24.1 mg/dL) level were significantly reduced compared to placebo group (serum triacylglycerol- 130.6±51.2 mg/dL; LDL cholesterol- 132.0±17.1 mg/dL)80). Farombi and Ige79) also reported the anti-hyperlipidemia activity of RLE act in the cholesterol induced hyperlipidemic rats. The RLE riches in polyphenols can significantly reduce the experimental group LDL-cholesterol: high-density lipoprotein (HDL)-cholesterol risk ratio, atherogenic index, serum glutamic pyruvic transaminase (SGPT) level, serum glutamic oxaloacetic transaminase (SGOT) level, and alkaline phosphatase (ALP) activity in compared with the control group. In addition, the HDL-cholesterol level in the experimental group (26.27±0.86 mg/dL) was significantly increased and higher than the lovastatin standard group (20.47±0.97 mg/dL)80).
The anti-inflammatory activity of 70% (v/v) methanolic extract of roselle leaves against lipopolysaccharide treated murine macrophage RAW 264.7 cells. The cells treated with RLE demonstrated dose-dependent inhibition of nitric oxide synthetase (NOS), with inhibition ranging from 9.1-57.9%. The roselle leaves extract promising anti-inflammatory activity was contributed by the major compounds detected, including chlorogenic acid, kaempferol, and quercetin. This study highlighted the potential application of roselle leaf as a safe functional supplement to relieve human chronic inflammatory disease63).
The hepatoprotective effect of roselle leaves was studied on alcohol-induced hepatotoxic activity in rats. The roselle leaves extract reduced the serum levels of ALP (64.32%), alanine transaminase (ALT) (79.71%), aspartate transaminase (AST) (72.47%), and bilirubin (72.13%) significantly in the orally administered treatment group (400 mg/kg). The histopathological evaluation further supported the RLE hepatoprotective effect, in which the result revealed that the normal liver cellular shape was retained as compared to the standard drug - silymarin81).
Extraction of bioactive compounds can be carried out on fresh or dried calyces, with the latter being commonly used and studied. Fresh calyces are either kept frozen (–20°C) or dried (solar, air, or freeze-dried) and kept under low temperatures (–28°C – 4°C) before extraction.
Extraction with solvents such as ethanol, methanol, acetone, and water under different conditions is commonly used to obtain the extract of roselle calyces. Ethanol is regularly used in the extraction of phenolic compounds as it is economic, non-toxic and safe for human consumption. Different ratios of solvent:water affects the extraction efficiency of total phenolic and flavonoid contents82),83). Generally, the extraction yield is improved with an increase in solvent proportion84). However, a recent study reported a decrease in the extract yield but an increase in total phenolic and total flavonoid when the concentrations of solvents were increased83). These can be due to the different degrees the of polarity of solvents that facilitate the extraction of compounds with different solubility in the solvents. Besides that, the temperature is another factor that governs extraction efficiency. Extraction yield is increased with increased temperature, but reduced when the temperature becomes too high (90°C and above)83). Therefore, the temperature should be kept low (below 50°C) to prevent thermal degradation of polyphenols and reduce antioxidant activity. The extraction is also frequently carried out in a slightly acidic environment to maintain the flavylium cation form of anthocyanins, which has high stability in a highly acidic medium84). Therefore, ethanol or methanol acidified with weak organic acids or low concentrations of strong acids are used as the extraction solvent.
Conventional extraction methods such as maceration, heating, boiling and solid extraction are commonly used. Recently a few non-conventional extraction methods such as ultrasonic-assisted extraction (UAE) and microwave-assisted extraction (MAE) are applied to improve the extraction process. These methods can overcome the shortcomings of conventional methods by producing a higher extraction yield in a shorter time with fewer solvents used. For instance, Yusoff and Leo24) reported a three-fold increment in extraction yield for MAE when compared to solid extraction. A recent study reported improved anthocyanin extraction with UAE, with the use of water as solvent. These methods facilitate the extraction of compounds through disruption and damage of cell walls via ultrasound cavitation or thermal energy produced by electromagnetic waves86),87). Meanwhile, non-conventional methods are termed as more green and sustainable methods as well as feasible and economic for industrial application.
Roselle calyces contain valuable components that determine the quality of the plant, which include color, flavor and aroma. They are known to contain polyphenols and organic acids that are related to antioxidant and antibacterial properties88).
6.2.1 Anthocyanins
Anthocyanins, classified as flavonoids, are water-soluble pigments responsible for the red color of fruit and flower of plants. These compounds are relatively unstable, and their structures and stability can be affected by factors such as pH, temperature, light, oxygen, metals, organic acids, sugars, enzymes, and co-pigmentation82). Color variation, stability and extractability of anthocyanins are significantly influenced by pH. Anthocyanin exists primarily in the form of flavylium cation at low pH (in red), it forms a quinoidal base when pH increases above pH 5 (blue or violet) and eventually forms colorless carbinol/chalcone with increasing pH88). Therefore, the addition of acids improves the efficiency of anthocyanins extraction by creating a favorable environment for the formation of flavylium chloride salts85).
Anthocyanins are reported to have potent antioxidant properties and therapeutic potentials such as anti-inflammatory, anti-lipidemic, and anti-diabetic properties83). Anthocyanins contribute about 50% of roselle’s antioxidant capacity, making them the major source of antioxidant activity against free radicals89). Generally, there are four anthocyanins in roselle calyx: delphinidin-3-sambubioside, cyanidine-3-sambubioside, delphinidin-3-glucoside and cyanidine-3-glucoside that exhibit antioxidant properties85),90). Among these compounds, the first two that can be found in greater proportions are linked to anti-hypertensive, anti-carcinogenic, and hypocholesterolemic effects67).
6.2.2 Protocatechuic acid
Protocatechuic acid (PCA) is a phenolic acid that occurs naturally in roselle calyx. It is heat-resistant and exhibits antimicrobial activity. A few studies revealed the in vitro antimicrobial effect of roselle calyx extract against several pathogenic bacteria such as E. coli, S. typhimurium, L. monocytogenes, S. aureus, B. cereus, P. aeuroginosa, K. pneumoniae and Bacillus subtilis91). Similar observations were reported by a previous study92), suggesting the potential use of roselle calyx extract as an additive to prevent food spoilage and as a consumable antibacterial agent.
PCA displays strong antioxidative properties and further study revealed its anti-tumor promotion effect. PCA of roselle calyx extract was found to induce apoptosis in human gastric cancer cells, with inhibition of up to 50% of cell viability at 0.95 mg/mL93). Meanwhile, the normal mouse embryo fibroblast cells were less responsive to the cytotoxic effect of the extract, demonstrating the ability of PCA to selectively kill tumor cells. These findings highlight the potential use of roselle calyx in food preservation and overcoming the issue of antibiotic resistance. The chemotherapeutic activities of roselle extract require further investigation to explore its relevance in human gastric cancer treatment.
6.2.3 Organic acids
Several organic acids are found in the extract of roselle calyx. They contribute to the acidic taste and pose certain therapeutic properties (as shown in Table 5). The presence of organic acids, together with other bioactive compounds in the roselle calyx promotes its health benefits. However, most studies focused on the main compounds such as anthocyanins and protocatechuic acid, without exploring the individual pharmacological capability of these organic acids67). Future study on the individual health benefits of these organic acids is essential to explore more beneficial properties of roselle calyx.
Organic acids in roselle calyces.
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6.3.1 Food products and beverages
Roselle calyces appear as the most valuable part of the plant as they are used as food as well as natural flavoring and colorant. Generally, roselle is considered a medicinal plant and is traditionally consumed by different cultures. Roselle calyx is rich in dietary fiber, vitamins, minerals, and a variety of bioactive compounds such as anthocyanins and organic acids. Fresh calyces can be eaten raw in salads and used in cooking (to make soups and sauces, as a seasoning in curries) and desserts (to make jams, jellies, puddings, pickles etc.). They can be sun or air-dried and ground into powder for storage.
One of the common ways to extract the soluble compounds in roselle calyx is by hot water infusion of the fresh calyx. The infusion has a variety of health benefits, such as treating cancers, digestive disorders, high cholesterol and reducing high blood pressure82). Hot and cold beverages that are made from aqueous extraction of fresh or dried calyces are popular and widespread in Asia and Africa. It is also one of the common ingredients found in many herbal tea blends due to its proven hypocholesterolemic and anti-hypertensive effects in humans94). In Sudan, sour tea is produced from ground dried calyces and is known as herbal tea with medicinal properties88). In China, roselles calyces are traditionally used as a natural remedy to treat hypertension, fever and cancer, and to improve the digestive system and leukemia28). In other countries, beverages derived from roselle calyx are named differently: hibiscus tea, sorrel tea, flor de jamaica, bissap, drink of the Pharaohs, zoborodo, da Bilenni, Sudan tea, etc.88),94).
A few studies reported the association of consuming food containing anthocyanins with the prevention and management of type 2 diabetes95). Several studies also documented the antioxidant properties of roselle calyces extract, which potentially help to reduce chronic diseases such as hypertension, dyslipidemia, and cardiovascular diseases95). Therefore, roselle calyx can be included in the daily diet and its extract can be value added to food products and health supplements.
6.3.2 Natural food colorant
Color provides the first impression of food quality as it plays an important role in affecting consumers’ preference and acceptance of a particular food. Without the inclusion of colorants, many confectionary products, gelatine desserts, snacks, cakes, ice cream, and puddings appear to be less attractive. The interest in natural colorants has increased due to the growing demand for natural products. Roselle calyces produce a vivid red color with a pleasant acidic taste when soaked in the water, making it a suitable source of natural food flavoring and colorant. The incorporation of roselle calyx extract (15-20 g/100 mL) into the preparation of yogurt dressing increased consumer preference for color and significantly increased the total polyphenol and flavonoid contents96). This suggests that the use of roselle calyx extract as a food colorant not only satisfies the color preference but also enhances the nutritional value of the food products.
Roselle-derived food colorant powder has similar solubility to synthetic food coloring, but lower color stability due to factors such as enzymatic reactions, pH, light, oxygen, sugar, and temperature88). Further studies on suitable encapsulation techniques are therefore desired to overcome this inadequacy and extend its use in the food product industry.
Roselle is well-known for its high phytochemical composition along with various applications and pharmaceutical benefits. In this paper, most of the previous studies on extraction and processing, phytochemical characterization, as well as applications of roselle have been reviewed to find out its present status and stimulate its future applications. Roselle seeds yield RSE and RSO rich in phytochemicals, such as phenolics, tocopherols, and phytosterols. Pharmaceutical benefits such as anti-cancer, anti-hyperlipidemia, and anti-inflammatory were reported in RLE, which suggested RLE be used to develop as a pharmaceutical drug. The high antioxidant activity and unique color of roselle calyx make it suitable to use in the preparation of food, herbal drink, and hot and cold beverages. Roselle plant can be widely utilized for its nutritionally important phytochemicals in pharmaceutical, cosmetic, nutraceutical, food and other industries.
No funding was received in this review.
All the tables are original in this review.
All data are available for publication.
All authors were involved in writing, editing, reviewing and approving the final manuscript.
No potential conflict of interest was reported by the authors.
