Kale: Review on nutritional composition, bio-active compounds, anti-nutritional factors, health beneficial properties and value-added products

Abstract There has been an increasing trend in recent times for taking more of green leafy vegetables (GLV) portion in the human diet. Among various GLVs available for human consumption, some are confined to a specific region and few are available in many parts of the world. Kale (Brassica oleracea L. var. acephala) is among the latter group which belongs to Brassicaceae family. This review summarizes the nutritional composition and anti-nutritional factors of kale available in different parts of the world. Consideration was also given for summarization of the studies reported on health benefits, pharmacological activities and different food products. It is noted from the literature that kale is a good source of fiber and minerals like potassium with higher calcium bioavailability than that of milk. Kale also contains prebiotic carbohydrates, unsaturated fatty acids and different vitamins while the anti-nutritional factors such as oxalates, tannins and phytate are present in higher concentrations. Research studies are reported different health beneficial activities of the kale like protective role in coronary artery disease, Anti-inflammatory activity, Antigenotoxic ability, gastro protective activity, inhibition of the carcinogenic compounds formation, positive to gut microbes, anti-microbial against specific microorganisms. However, in case of value-added products kale was reported limited usage like, in baked products and beverages. Finally, concluded that, kale has good potential to use in different food and nutritional applications.


PUBLIC INTEREST STATEMENT
Kale is widely consumed Green leafy vegetable in worldwide; it is providing high bioavailability of calcium, better than milk and good concentration of the Iron. In addition, kale reported better concentrations of the probiotic carbohydrates, organic acids, unsaturated fatty acids, carotenoids, phenolic acids and different vitamins. The in vitro and in vivo studies reported various health benefits for the consumers like coronary artery disease, anti-inflammatory activity, antigenotoxic ability, gastro protective activity etc. However, value-added products from the kale was reported in very limited areas and the most common foods reported are bread incorporate with kale, juice, puree. With all the reported nutritional and health benefits, kale reported good concentrations of the Anti nutritional components.

Introduction
Kale (Brassica oleracea L. var. acephala) is a green leafy vegetable in Brassicaceae family (Fahey, 2003). Initial evidence of kale is from the eastern Mediterranean and Asia Minor regions. Kale was considered as the food crop since 2000 B.C, this is evidenced by the Theophrastus report in 350 B.C. on curved and wrinkled kale. Leonard (2019) reported that, kale has spread over the centuries across the world through immigrants, travelers and merchants. Kale plant is an annual crop and its size and nutritional variation depends on the variety and growing conditions (Lefsrud et al., 2007). The growth of this plant depends on the agricultural practices employed and geo-climatic conditions and generally, it will be ready after two months of sowing. Different varieties of kale are available they include, green kale, dwarf kale, marrow-stem kale, tronchuda kale, curly leaf kale, scotch kale, tree kale and bore kale. Kale leaves are generally consumed as fresh and unprocessed as salad or cooked and used as garnish and they are usually sold in fresh, canned and frozen forms (Fahey, 2003).
The vegetables of Brassicaceae family have specially gained attention due to their sulfur containing phyto-nutrients that promote health. In Africa, kale is regarded as nutritious and its consumption provides good health (Emebu & Anyika, 2011). Popular articles have described about the health benefits, its nutritional composition (Megan, 2020) and consumer acceptance (Bryan, 2020). The Brassicaceae exhibit positive cardiovascular protective roles preventing gastrointestinal tract cancer (Raiola et al., 2018). Glucosinolates, flavonoids (glycosylated flavanols) and phenolic (kaempferol, quercetin and isorhamnetin) compounds are responsible for antioxidant and free radical scavenging properties (Cartea et al., 2011;Lin & Harnly, 2009). The United States Center for Disease Control has assessed the vegetables for their nutritional quality with ≥10% Recommended Daily Allowance (RDA) of 17 essential nutrients especially those are strongly associated with reducing risk of heart disease and other noncommunicable diseases. Among those, kale has been ranked as the 15 th (Di Noia, 2014).
Although kale has been widely studied for its nutritional highlights, reviews on the consolidation of the research findings are hardly find. Hence, the objective of the present paper is to review the nutritional composition, bio-active compounds, anti-nutritional factors present in kale and health beneficial properties and value-added products of kale reported from different researchers around the world.
The energy levels of kale vary from 58.46-66 kcal per 100 g on fresh weight basis (Emebu & Anyika, 2011;Thavarajah et al., 2019) which is higher compared to other salad crops (Gupta & Rana, 2003) and vegetables of brassica family (Fahey, 2003) as well as other temperate vegetables cultivated in different parts of the world (mean 23.18 kcal per 100 g on fresh weigh basis) (Roy & Chakrabarti, 2003). For low energy requirements, the nutrition experts always suggest to consume more amount of GLVs as they are rich source of moisture with low amount of the carbohydrates and fats (Pandey et al.,  of the energy compared to other carbohydrates due to the inability of small intestine to absorb sugar alcohols properly (Grembecka, 2015;Mäkinen, 2016). These are widely used in food products for people with diabetes (Manisha et al., 2012). The sorbitals are non-carcinogenic (Hayes, 2001) and prevents the formation of the teeth cavities (Anonymous, 2011). Very few common fruits and vegetables are reported with sorbitols (Lee, 2015). Kale has other common sugars like, glucose and fructose and sucrose, arabinose (73.5 mg/100 g), mannose (241 mg/100 g) and xylose (59.9 mg/ 100 g) (Thavarajah et al., 2016). Xylose is generally found in smaller concentrations in many fruits and vegetables like berries, oats and mushrooms (Hassan et al., 2011).
Glucose, fructose and sucrose are the major soluble sugars found in kale. The glucose ranges from 993 to 5800 mg/100 g, fructose 545-7200 mg/100 g and sucrose 39.3-3400 mg/100 g (Ayaz et al., 2006;Hagen et al., 2009;Thavarajah et al., 2016;Wang, 1998). This broad range in carbohydrates may be attributed to the difference in species, agricultural practices and other agro-climatic conditions. It is reported that kale grown at temperatures above 25° C is bitter in taste compared to the one grown at temperatures between 7-21 °C (Thavarajah et al., 2016;Wang, 1998). The kale grown under cooler temperatures is reported to contain higher concentration of the water-soluble prebiotic and have sweeter taste and superior nutritional quality.
The non-digestible carbohydrates and lignins are known as the dietary fiber (Cleary, 2003;El Khoury et al., 2012) which stimulates immunity and enhances mineral absorption (Lee & Mazmanian, 2010;Whisner & Castillo, 2018) and reduces the risk of colon cancer (Pool-Zobel, 2005) and risks due to the obesity (Cerdó et al., 2019;Ejtahed et al., 2019). It is reported that prebiotic carbohydrates can reduce excess circulation of glucose in blood (Davani-Davari et al., 2019), reduce cholesterol levels (Nakamura &Omaye, 2012) and improve insulin sensitivity. Table 1 indicates that the percentage of dietary fiber in dry kale is 36.8% (Kahlon , Chiu and Chapman, 2008) whereas in fresh kale, it is found to be 1.94-8.39% (Emebu & Anyika, 2011;Manchali et al., 2012;Sikora & Bodziarczyk, 2012). This variation in fresh base may be attributed to maturity and the proportion of the moisture removed in the dry samples (Barrett et al., 2010). Recommended Dietary Allowance (RDA) for dietary fiber is 25 g/day for adults 18 years and above for normal bowel function and human gut health (Anonymous, 2010;Phillips & Cui, 2011). Thavarajah et al. (2016) have reported a total identified prebiotic carbohydrates of 1900 mg/ 100 g and other pre-biotic carbohydrates of 5500 mg/100 g (Table 2) in kale. Dietary prebiotics are considered as non-digestible fiber and can pass through the upper part of the intestine and promotes the growth of beneficial microbes settled in large intestine by acting as substrate (Scantlebury & Rgibson, 2004). The prebiotic carbohydrates are categorized from dietary fiber lactulose (disaccharide), inulin (polysaccharide), fructo-oligosaccharides, gluco-oligosaccharides (Lannitti & Palmieri, 2010).
Organic acids like citric, malic and oxalic acids are usually found in GLVs (Flores et al., 2012). Citric acid in kale was reported to be 386-2231 mg/100 g and malic acid 124-151 mg/100 g (Ayaz et al., 2006;Wang, 1998). In GLVs, the organic acid concentration depends on degree of maturity of the plant with variations in different parts of the plant (Batista-Silva et al., 2018). Further, the content of organic acid in GLVs also depends on the gene expression in the seeds due to the environment and agronomic practices (Kader, 2008); Mu et al. (2018) have reported that the concentration of organic acids govern the organoleptic properties especially the sourness in different fruits and vegetables. Oxalic, malic and citric acids act as antioxidants due to their ability to chelate metals (Kayashima & Katayama, 2002).
In case of Calcium, kale is appreciated for its high concentrations and excellent absorbability compared to other salad crops (Gupta & Rana, 2003) and brassica vegetables (Fahey, 2003;Heaney et al., 1993). Kale was reported to have 58.8% of absorption in calcium which is higher than milk (32%). This fractional absorption percentage of kale is comparable to cauliflower (58.6%), but lesser than Brussels sprouts (63.8%) (Connie & Aren, 1994). However, availability of vegetables like Cauliflower and Brussels sprouts is difficult for poor people in under developed countries, while kale is the best source for the calcium at low cost. Kale is also reported to have high amount of magnesium compared to other vegetables of brassica family (Fahey, 2003). The amount of potassium and magnesium in fruits and vegetables play a potential role in the management of bone mineral density (Tucker et al., 1999).
The GLVs were reported to be a good choice for iron in vegan food habits. However, it depends on the composition of ascorbic acid (promoter), dietary fiber, oxalates and tannins (inhibitors) (Chiplonkar et al., 1999). Kale is found to have 5-10 mg/100 g of iron (Gopalan et al., 1989), which is higher compared to spinach (2.71 mg/100 g) (Bhattacharjee et al., 1998) and other brassica vegetables (Fahey, 2003). Hence, kale is the best source for fortification to enhance the iron content. The iron content in kale ranges from 1.1 to 12.19 mg/100 g, Zn 0.045-394 mg/100 g and 0.8-14.73 mg/100 g (Emebu & Anyika, 2011;Manchali et al., 2012;Sikora & Bodziarczyk, 2012). Zinc content in kale is reported to be higher than in all other common brassica vegetables and spinach (530 µg/100 g) (Bhattacharjee et al., 1998). The deficiency of zinc is considered as a worldwide public health problem resulting in 1.4% deaths around the globe (Fischer Walker et al., 2009). People in sub-Saharan Africa are identified with iron and zinc deficiencies due to the consumption of cereal-based diets for energy and micronutrients (Joy et al., 2014) and GLVs provide contribution of zinc in human diet. Cereals are composed of considerable amount of anti-nutritional factors (phytate and tannins) which reduces the bioavailability of the iron and zinc in the diet (Hunt, 2003;Kruger et al., 2015). Kale has a higher quantity of manganese compared to the vegetables of brassica family (Fahey, 2003) and spinach (Bhattacharjee et al., 1998).
Kale is found to have good amount of the selenium ranging from 0.009 to 0.0023 mg/100 g (Manchali et al., 2012;Thavarajah et al., 2016) compared to other brassica and green leafy vegetables (Fahey, 2003). According to the research of Navarro-Alarcon and Cabrera-Vique, (2000), this good amount of the Selenium is important in several selenoproteins with essential biological functions. Kale is reported as having good concentrations of phosphorus in rage of 0.52-513 mg/ 100 g compared to other salad vegetable crops (Gupta & Rana, 2003) except for Cress among the Brassica family vegetables (Fahey, 2003).
Sodium intake is necessary for humans and RDA may be vary from adequate intake and UL (Anonymous, 1998). It is indicated in Table 3 that, 4.69-170 mg/100 g of sodium (Emebu & Anyika, 2011;Manchali et al., 2012;Sikora & Bodziarczyk, 2012) is found in kale which is higher compared to other vegetables of brassica family (Fahey, 2003). Although, less than 500 mg/day Na is sufficient for physiological requirements, usually, the average consumption of sodium is more than recommendations (William et al., 2015).
Cobalt deficiency has not been reported generally and hence it is considered as a non-essential mineral (Yamada, 2013). The RDA of Cobalt is very low (2.4 µg/day) compared to other minerals (Bhattacharya et al., 2016). Cobalt is toxic to muscles with much exposure and higher concentrations will increase in red blood cells number (polycythemia) (Squires et al., 1994). It is reported that kale has 0.02 mg/100 g of Cobalt which is enough to achieve RDA (Ayaz et al., 2006). Uriu-Adams and Keen (2005) have reported the RDA of copper as 9 mg/day for an adult with a tolerable upper intake level (UL) of 10 mg/day. Kale was reported to have optimum quantity of copper in the range of 0.18-0.51 mg/100 g which is higher compared to other vegetables of brassica family (Fahey, 2003).
The RDA of Lithium is 100 μg/day for adult human which helps in the stabilization of nerve system activities (Schrauzer, 2002). An amount of 0.01 mg/100 g of lithium (Ayaz et al., 2006) is found in kale which is higher than that is found in other brassica vegetables (Fahey, 2003). It is reported that kale is found to have optimum concentration of Molybdenum as 0.29 mg/100 g compared to the other brassica and leafy vegetables (Fahey, 2003). Ayaz et al. (2006) have reported that kale contains the heavy metals like Arsenic (0.07 mg/ 100 g), Barium (1.59 mg/100 g) Cadmium (0.01 mg/100 g), Chromium (0.26 mg/100 g), Lead (0.02 mg/100 g), Titanium (0.04 mg/100 g), Strontium (25.2 mg/100 g) and Nickel (0.2 mg/ 100 g). All these metals are toxic when they reach higher concentrations (Jaishankar et al., 2014) but kale is reported to have very low concentrations within safety levels (Fahey, 2003).
Kale is usually consumed in fresh as salad or minimally cooked. Slow cooking has reported no change in the kale's mineral concentrations (Gupta & Rana, 2003). The broad range of minerals and variation of the results among the authors reports are attributable to genetic (Phuke et al., 2017), environmental and analytical differences (Howard et al., 1998). However, some authors are reported the mineral information on dry weight basis, which created much difficulty for comparison between different reports (Ayaz et al., 2006;Fadigas et al., 2010). Table 4 gives the composition of different amino acids in kale. Less amount of Cystine has been found in kale compared to other GLVs grown in Africa (Ntuli, 2019). Cysteine content in kale is in the range of 34.0-58 mg/100 g (Ayaz et al., 2006;Lisiewska et al., 2008). Large amount of cystine is found in animal foods, lentils and seeds (Piste, 2013). Apart from kale, cruciferous vegetables like, cabbages, broccoli and allium vegetables such as onions, leeks and garlic are the best source of this amino acid (Doleman et al., 2017). Table 4 that the concentration of Glutamic acid in kale ranges from 33.20-450 mg/100 g (Ayaz et al., 2006;Eppendorfer & Bille, 1996;Lisiewska et al., 2008), which is lower than that is found in Hibiscus cannabinus and Haematostaphis barter (Kubmarawa et al., 2009), Korean spinach (Yoon et al., 2016). But, the concentration of Glutamic acid in kale is higher than that of the Nigerian spinach (A. hybridus), Bitter leaf (V. amygdalina), Pumpkin leaf (T. occidentalis) and Water leaf (T. triangulare) (Arowora et al., 2017).

Vitamins and selected carotenoids in kale
Kale is reported to have high concentration of vitamin C than all other salad vegetables and vegetables of Brassicaceae family (Fahey, 2003;Gupta & Rana, 2003). Edelman and Colt (2016) have reported that the amount of vitamin C in kale is much higher than that of in Duck-weed and also other GLVs of Africa (Uusiku et al., 2010). It is in the range of 62.27-969 mg/100 g and is considered as the best source for vitamin C, satisfying the RDA for both males and females (Acikgoz, 2011;Hagen et al., 2009;Murtaza et al., 2006;Sikora & Bodziarczyk, 2012). Vitamin C RDA is 90 mg-120 mg/day (Aly et al., 2010). The deficiency of vitamin C leads to scurvy with disturbances in collagen metabolism and a tendency to bleed (Mayland et al., 2005).
In case of Thiamine, it is reported to exist between 0.110-0.9 mg/100 g of kale as indicated in Table 5 which is comparable with those of other salad vegetables as well as vegetables of Brassicaceae family (Fahey, 2003;Gupta & Rana, 2003). Agte et al. (2000) reported that GLVs are the best source of Vitamin B1. Gupta, Gowri, et al., (2013) has reported that kale has high concentration of thiamine than Amaranthus gangeticus, Chenopodium album, Centella asiatica, Amaranthus tricolor, Trigonella foenum-graecum. The concentration of Riboflavin reported in kale is considered to be reasonably good which varies between 0.13-0.9 mg/100 g (Fahey, 2003;Gupta & Rana, 2003) though it is lower than that of spinach and Duck-weed (Edelman & Colt, 2016). A similar concentration of vitamin B2 is found in vegetables of Brassicaceae family (Fahey, 2003). Agte et al. (2000) has analyzed 24 varieties of GLVs for riboflavin concentrations and reported that kale is better among all. Uusiku et al. (2010) has found that kale has the best concentration of vitamin B2 compared to several other GLVs from Africa. Catak and Yaman (2019) has analyzed the profiles of vitamin B3 in several fruits and vegetables and kale showed a better concentration than in Broccoli, Brussels sprouts, Spinach and other brassica vegetables (Fahey, 2003). Similarly, it is reported that kale has better concentration of niacin than other common salad crops (Gupta & Rana, 2003). The concentration of niacin in kale was reported as 1.00 mg/100 g.
Kale is considered as a good source of vitamin B5 and it ranges from 0.091 to 0.9 mg/100 g. Hasan et al. (2013) have found no traces of Vitamin B5 in some of the indigenous GLVs of Bangladesh and Indian spinach. It is found that pantothenic acid in kale is lower than that of other brassica vegetables (Fahey, 2003) Kale is considered as a good source of vitamin B6 among other GLVs and the amount of vitamin B6 in kale is reported to be 0.27-2.5 mg/100 g (Fahey, 2003), which is better than other commonly consuming Brassicaceae family vegetables. It is also reported that kale has better concentrations of the pyridoxine compared to Indian spinach, red and green amaranth leaves and duck weed (Edelman & Colt, 2016;Hasan et al., 2013).
Kale is reported to have higher concentrations of folic acid to the extent of 29 mg/100 g than amaranth, mint, spinach and other common brassica vegetables (Agte et al., 2000). However, it has smaller concentrations of vitamin B9 (Fahey, 2003). Takeiti et al. (2009)  It is reported that kale is a moderate source of vitamin A having 8900 IU with the retinal equivalence (RE) as 890 µg/100 g (Fahey, 2003). The RE reported in GLVs from Africa is 99-1970 µg/ 100 g (Uusiku et al., 2010) and also vitamin A in P. aculeata (OPN leaves) is 2333 IU/100 g (Takeiti et al., 2009) which is lower than that in kale. Raju et al. (2007) have reported vitamin A (retinal equivalent) as 641-19,101 µg/100 g in medicinal plants which is higher than in kale.
Vitamin E (α-tocopherol equivalent) is reported as 0.800 mg/100 g of kale (Fahey, 2003). Achikanu et al., (2013) have reported that Ficus capensis, Solanum melongena, Mucuna prurient, Solanum macrocarpon, Solanum nigrum, Moringa oleifera lam, Solanum aethiopicum, Cridoscolus acontifolius were have higher concentration of Vitamin E. Usually the best source of Vitamin E are the oils and fats and the GLVs are considered as its poor source (Choo et al., 1996).
From the samples of kale grown in Boston and Montreal, vitamin K is reported as 573 µg/100 g (Catani et al., 2005) and 6.21-16.57 µg/100 g (Booth Sarah et al., 1993), respectively. Compared to other fruits and vegetables, kale is reported to have good concentration of the vitamin K. Novotny et al. (2010) have reported that the bioavailability of phylloquinone from kale is 4 · 7%. However, kale is reported as one of the best sources of vitamin K compared to other commonly consuming vegetables (Booth Sarah et al., 1993).
Lutein, violaxanthin and neoxanthin are the major carotenoids and their concentrations in kale are presented in Table 5. Lutein is in the range of 5.06-38.16 mg/100 g (De Azevedo & Rodriguez-Amaya, 2005; M. G. Lefsrud et al., 2005;M. Lefsrud et al., 2007). Lutein cannot synthesize in humans and should be obtained through food composed of vegetarian diet (fruits and vegetables) (Calvo, 2005). Lutein, zeaxanthins are the stereo isomers which usually coexists in nature and GLVs like kale and spinach are their best sources (Shegokar & Mitri, 2012). Holden et al. (1999) have reported that 40 mg/100 g of lutein + zeaxanthin is available in kale. In contrast, only less than 1 mg is reported in 100 g of yellow-orange color food crops like carrots, peaches, corn, papaya and oranges. Researchers have reported that lutein acts as an antioxidant and it is very important for skin health (Shegokar & Mitri, 2012;Stahl & Sies, 2004). Finally, kale is reported as the best source of lutein than other orange to yellow fruits and vegetables. Perera and Yen (2007) have reported that violaxanthin and neoxanthin are abundant in green parts of the plants like GLVs. They cannot be used by the humans but, their concentration can influence the total carotenoid intake of an individual. Biehler et al. (2012) have reported that yellow bell peppers were especially rich in violaxanthin (4.4 mg/100 g) followed by spinach (2.8 mg/ 100 g) and creamed spinach (2.5 mg/100 g) and kale is also reported to have similar amounts of the Violaxanthin (3.36 mg/100 g). De Azevedo and Rodriguez-Amaya (2005); Žnidarčič et al. (2011) have reported 0.35-1.07 mg/100 g of neoxanthin in five leafy vegetable and found that neoxanthin is lower than violaxanthin in all. Similar conclusions are reported by De Sá and Rodriguez-Amaya (2003) and also stated that Violaxanthin in GLVs usually surpasses neoxanthin. The same trend of the results was observed in the kale that 1.694 mg/100 g of neoxanthin is found (De Azevedo & Rodriguez-Amaya, 2005) which is less than the violaxanthins.

Flavonoids, phenolic compounds, glucosinolates in kale
Flavonoids are group of polyphenolic compounds found widely in plants (Cao et al., 1997) and possess strong antioxidant properties due to the phenolic hydroxyl groups (Subhasree et al., 2009). Different types of flavonoids presented in the kale are presented in Table 6. The flavonoids content of kale is reported as 661-892 of mg/100 g as shown in Table 6 (Olsen et al., 2009;Sidsel et al., 2009;Susanne et al., 2010). It is reported that GLVs are the good source of flavonoids and other anti-oxidant vitamins and compounds (Subhasree et al., 2009). Adefegha and Oboh (2011)  Kaempferol is reported in the range of 58-537 mg/100 g of kale samples (Olsen et al., 2009;Sidsel et al., 2009;Susanne et al., 2010). Bahorun et al. (2004) have reported that Kaempferol in Chinese cabbage (9.6 mg/100 g), onion (4.5 mg/100 g), Mugwort (12.5 mg/100 g), Broccoli (4.6 mg/ 100 g), Cauliflower (1.2 mg/100 g), Tomato (0.7 mg/100 g), Carrot (0.6 mg/100 g) and these concentrations are reported as lower than that of kale. Lako et al. (2007) have analyzed the Fijian fruits and vegetables and reported less than 1 to 34 mg/100 g of Kaempferol. In general, kale has good concentration of the Kaempferol as compared to other vegetables.
Phenolic compounds are not nutrients but, the dietary intake provides health-protective effects (Cheynier, 2012). They can be divided into phenolic acids, flavonoids, tannins, coumarins, lignans, quinones, stilbens, and curcuminoids (Agati et al., 2012). Phenolic compounds are reported to have health benefits including, antibacterial, anti-inflammatory and anti-mutagenic activities (Chandrasekara & Josheph Kumar, 2016). Kale is reported for 201.67-1167 mg/100 g of total phenolic content (Murtaza et al., 2006;Olsen et al., 2009;Sidsel et al., 2009;Sikora & Bodziarczyk, 2012;Susanne et al., 2010). Johari and Khong (2019) have reported that the total phenolic content of Pereskia bleo as 252.0-408.2 mg/100 g. Aryal et al. (2019) have reported the total phenolic content of wild vegetables from Western Nepal such as Alternanthera sessilis, Basella alba, Cassia tora, Digera muricata, Ipomoea aquatica, Leucas cephalotes, Portulaca oleracea and Solanum nigrum to be 770.6-2926.5 mg/100 g. Hossain et al. (2017) have reported that green amaranth, water spinach leaf and Indian spinach leaf had a total phenolic content of 93.33, 92.14 and 91.95 mg GAE/100 g, respectively. Obeng et al. (2019) have reported that, Solanum macrocaron (Gboma), Talinum fruticosum (Ademe), Corchorus olitorius (Yevogboma) and Amaranthus spp. (Atormaa) have a total phenolic content of 0.014-0.982 mg/100 g by the fresh weight. Research reports have clearly identified the quantitative and qualitative differences of polyphenols in fruits, vegetables and GLVs. These differences can be attributed to the method of extraction, processing and growing conditions and varietal differences. Hydroxycinnamic acids are the natural phenyl propenoic acid compounds which are the metabolic products of cinnamic acid with 3-6 carbon backbone (Johari & Khong, 2019). Hydroxycinnamic acids are very important sources for antioxidants and possess the role in the stability of flavor, color and nutritional bioavailability of foods (Wilson et al., 2017). Very little research is done on Hydroxycinnamic acids concentration in the kale. Olsen et al. (2009) have reported that kale has 204 mg of Hydroxycinnamic acids per 100 g kale. Dietary sources of Hydroxycinnamic acids include apples, blueberries, cereals, cherries, cinnamon, coffee, ginger, grapes, lettuce, olives, oranges, pears, pineapples, plums, potatoes, prunes, spinach, strawberries, sunflower seeds, turmeric and herbs like basal, marjoram, oregano, rosemary, sage and thyme (El-Seedi et al., 2012). Compared to fruits and vegetables, kale has less concentration of this phenolic acid. Some of the plant sources like tea showed huge amount of the Hydroxycinnamic acids but kale has these acids similar to other GLVs like spinach.
Glucosinolates are the plant secondary metabolite characteristics of the Cruciferae family. Glucosinolate containing plants include mustard, wasabi, cabbage, swede, rapeseed, kale, turnip are the source for food and feed for humans and animals (Cartea et al., 2011). Kale is reported to have 41 µmol/g by dry weight (Velasco et al., 2007) which is comparable with other brassica vegetables. Recent studies have reported a positive nature of glucosinolates, which include regulation of inflammation, stress, antioxidant activities and antimicrobial properties (Melrose, 2019). A comprehensive analysis of glucosinolates among broccoli, brussels sprouts, cabbage, cauliflower and kale has reported that, Brassicaceae vegetables contained wider glucosinolates among the other vegetables (Carlson et al., 1987). Broccoli contains glucoraphanin as the primary glucosinolate whereas brussels sprouts, cabbage, cauliflower and kale have higher levels of sinigrin and progoitrin with very little amounts of glucoraphanin (Jeffery & Stewart, 2004). Ayaz et al. (2006) have reported different types of fatty acids in kale by dry weight basis and presented in Table 4. Usually, the fat content in kale is reported as 11.8% on dry weight basis (Kahlon, Chiu and Chapman, 2008) and 0.26-0.74% by fresh weight basis (Emebu & Anyika, 2011;Manchali et al., 2012;Sikora & Bodziarczyk, 2012). Among the reported results, the unsaturated fatty acids are higher than the saturated fatty acids. As reported by Ayaz et al. (2006), the total saturated fats are 30.0 µg/g whereas, the total unsaturated fats are 129 µg/g of kale. Among the saturated fatty acids kale is reported to have C14:0 (Myristic acid), C15:0 (Pentadecylic acid), C16:0 (Palmitic acid), C18:0 (Stearic acid), C20:0 (Arachidic acid), C22:0 (Behenic acid), C24:0 (Lignoceric acid). Among all these saturated fatty acids, C16:0 is reported as 18.7 µg/g whereas other fatty acids such as C14:0, C15:0, C18:0, C20:0, C22:0, C24:0 are reported to have 0. 70, 0.33, 5.92, 0.72, 0.71 µg/g, respectively (Ayaz et al., 2006). Saturated fatty acids are reported as the reason for elevated lipid levels in human blood and considered as non-essential because they can synthesize in human body . All saturated fatty acids from 8 to 16 carbon atoms are responsible to raise the serum LDL cholesterol levels when they are consumed through human diet (Forouhi et al., 2018). However, stearic acid does not raise the serum LDL cholesterol levels due to rapid conversion into oleic acid in the body (Denke & Grundy, 1991). Kale is reported to have low composition of the saturated fatty acids compared to some of the GLVs (Adeyeye et al., 2018).
Kale is reported to have ω-3, 6, 7 and 9 fatty acids in good concentrations (Ayaz et al., 2006). Unsaturated and poly unsaturated fatty acids (PUFA) are reported to have numerous benefits like preventing Coronary Heart Diseases (CHD) and deaths related to CHD (Mozaffarian et al., 2010). Among different fatty acids, linoleic acid is reported for overall health benefits (Jandacek, 2017). Researchers have reported that, ω-3, 6, fatty acids are very important to patients surviving from myocardial, cardiovascular disease (Dunbar et al., 2014) and anti-inflammatory effects, positive effect on obesity, improved endothelial function, reduced blood pressure, lowered triglycerides in blood (Patterson et al., 2012), for alteration of chemotherapeutic drugs toxicity, protection from skin and oral cancers (M. Johnson et al., 2019). Calder (2015) has reported that, n-3 PUFA are very important in immunomodulatory and antiinflammatory properties. Consumption of fatty acids help in prevention of many inflammatory related diseases like diabetes (Lee et al., 2014) and cardiovascular disease (Phang et al., 2013). Simopoulos (2004) has reported that Purslane, Spinach, Butter crunch Lettuce, Red Leaf Lettuce, Mustard are good source for omega fatty acids which contain more PUFA fatty acids compared to that of kale. M. Johnson et al. (2013Johnson et al. ( ), (2018, 2019) had reported that the GLVs serve as a major dietary reservoir of the essential PUFAs and the consumption of GLVs determines the liver fatty acid composition. Uddin et al. (2014) have reported that, kale has low sources of omega-3 fatty acids which are similar to broccoli.

Anti-nutritional factors in kale
Oxalic acid and its salts are present in number of plant based foods that have an adverse effect on mineral bioavailability of Calcium and other minerals (Bhandari & Kawabata, 2004). Tea, rhubarb, spinach and beet are reported for high oxalate-containing foods (Noonan, 1999). Kale was reported as the rich source of oxalates. Erdogan and Onar (2011) have reported that the oxalate content as 297 mg/100 g by fresh weight and 2302 mg/100 g by dry weight of kale. These contents are less compared to the oxalates in Chard and Spinach. The oxalate content (0.08 mg/100 g) reported by Emebu and Anyika (2011) is very much lower than reported by Erdogan and Onar (2011). This difference can be attributed to variations in the method of analysis, the species and agro-geological conditions. P Agbaire (2011) have reported that Vernomia anydalira (Bitter leaf), Moni esculenta (cassava leaf), Teiferia occidentalis (Ugu leaf), Talinum triangulare (water leaf), Amaranthus spinosus (Green vegetable) has 0.076-0.106 mg/100 g of oxalates by dry weight base. Kale is a rich source of Calcium and oxalate content is very important consideration for bio-availability of Calcium. Noonan (1999) has reported that processing methods like soaking and heat processing of high oxalate food samples have reduced the oxalate content.
Nitrates in kale is reported to be 201.6 mg/100 g by fresh weight basis and 1563 mg/100 g by dry weight basis of kale (Erdogan & Onar, 2011). It is noted that the nitrate content of kale is higher than in spinach and lower than in chard. Nitrate in kale is 11.1 and 85.7 mg/100 g by fresh and dry weight basis, respectively (Erdogan & Onar, 2011). Compared to spinach and chard kale is reported to have more concentration of Nitrate. Dennis and Wilson (2003) have reported that, nitrate content in the samples of kale from USA was reported as 178.0 mg/kg. Nitrate is a usual component of plants which is found due to microbiological attack. Nitrate in the soil is utilized by plant as nitrogen in protein synthesis. Photosynthesis is a key for protein synthesis in plant, however, photosynthesis is decreased as the light levels fall and this situation leads to nitrate accumulation in cell fluids (Keeton, 2011). The nitrate concentrations in vegetables grown under subdued light are reported to be higher than that is grown under bright light. Overall, nitrate content in plant is determined by genotype and growing conditions (Anjana & Iqbal, 2007).
Erdogan and Onar (2011) have reported that kale has phytate content as 0.12 mg/100 g and tannin as 0.15 mg/100 g of the kale grown from the Nigeria. P Agbaire (2011) has reported that Vernomia anydalira (Bitter leaf), Moni esculenta (Cassava leaf), Teiferia occidentalis (Ugu leaf), Talinum triangulare (Water leaf), Amaranthus spinosus (Green vegetable) have 0.58-0.811 mg/ 100 g of phytate, while kale is reported to have least concentration of the phytate than the above leafy vegetables. P.O. Agbaire (2012) has analyzed the anti-nutritional factors in GLVs and reported that the phytate is in the range of 0.412-1.3 mg/100 g which is higher than that of kale. The tannin content reported by P.O. Agbaire (2012) is 0.004-0.026 mg/100 g in different GLVs which is lower than the phytate content in kale (Erdogan & Onar, 2011).
Phytate has anti-nutritional activities in human body by strong chelation of calcium, iron and zinc to form insoluble complexes and contributes to the deficiency of iron and zinc (Lopez et al., 2002). On the other hand, phytate has a positive nutritional role as an antioxidant and anti-cancer agent (Kumar et al., 2010). Phytate is reported to be contributing 60 to 80% of total phosphorus in cereals, legumes, nuts and oilseeds. The lower concentrations of phytate are found in roots, tubers, fruits and berries (Reddy, 2001).

Studies reported on the health benefits of the kale
Consumers considering kale consumption provide better health, to confirm this, researchers reported limited in vitro and in vivo studies, and they are summarized in Table 7. Only few researchers established kales positive role in management of macular disease, bilirubin metabolism, protective role in coronary artery disease, Anti-inflammatory activity, Antigenotoxic ability, Gastro intestinal protective activity, inhibition of the carcinogenic compounds formation, positive to gut microbes, anti-microbial nature against specific microorganisms. However, there are clear gaps and researchers can work on different aspects related to the health and pharmacological activities of the kale.

Studies reported on the value-added products from the kale
Kale is widely consuming as part of the diet, but very limited studies were reported on the value-added products and they are summarized in Table 8. The products reported from the kale are incorporation of kale in bread, fresh kale juice sterilized with radiation, fermented kale juice by spontaneous and induced fermentations (L. plantarum BFE 5092 and L. fermentum BFE 6620), beverages with addition of apple juice, kale purée, dried kale leafs and kale leaves chlorophyll microcapsules. Even though kale is a famous GLV, conversion of the kale to different value-added products are not studied well. Still there is a huge scope for development of value-added products from kale.

Conclusions
Kale is one of the oldest GLVs in the world, known for its best source of fiber in dry conditions and also for providing good concentration of prebiotic carbohydrates while it has been the poor source of fat, energy and carbohydrates. Kale is a better source of potassium and calcium. The bioavailability of the calcium in kale is very high which is better than milk. The amino acid composition of kale is balanced and contains more unsaturated fatty acid than the saturated. Kale is also a good source of vitamin A and β-carotenes and also for flavonoids like, Quercetin, kaempferol. In addition, kale has good concentrations of the phenolic compounds hydroxycinnamic acids. With better mineral compositions, kale contains high concentration of oxalates which is a major antinutritional component. Kale also has glucosinolates along with tannins, phytates and nitrogen compounds (Nitrates and Nitrites). In case of the health benefits, limited studies only reported in vitro and in vivo studies and established that kales potential role in management of macular disease, bilirubin metabolism, protective role in coronary artery disease, Anti-inflammatory activity, Antigenotoxic ability, gastro protective activity, inhibition of the carcinogenic compounds formation, positive to gut microbes, anti-microbial against specific microorganisms. Kale is usually consumed as a salad crop similar to other green leafy vegetable with minimal processing. However, the value-added products and research on product developments from the kale leaf is not reported well, except for its drying and preparation of juice. However, the role of kale in health promotion also investigated in narrow. It can be concluded that kale is a potential leafy vegetable for dietary recommendations for all age groups and it have very good potential for food and health based products.
In future line of work researchers can intensively work on kale utilization in different foods and kale based value-added food products for wider age groups consumers. Scholars can also carry research on isolation of bio-active components from kale and their effective utilization in nutrition. In addition, researchers can also work to determine kale role in nutrition, health and pharcological Authors concluded that, daily kale consumption increased CYP1A2 activity as determined by caffeine metabolite ratios by 16.4% and 15.2% after one and two weeks of feeding, respectively. Also, daily kale consumption modified Bilirubin metabolism such that serum conjugated Bilirubin decreased from 19.4% of total Bilirubin on day 1 to 14.3% and 9.5% on days 8 and 15, respectively. Charron et al. (2020) 3 In vitro binding of bile acids by kale and other vegetables The in vitro binding of bile acids by kale in comparison with other brassica vegetables was determined using a mixture of bile acids secreted in human bile at a duodenal physiological pH of 6.3.
The results of study revealed that, Bile acid binding for spinach, kale and brussels sprouts was significantly higher than for broccoli and mustard greens. These results point to the health promoting potential of spinach = kale = brussels sprouts > broccoli = mustard greens > cabbage = green bell peppers = collards, as indicated by their bile acid binding on dry matter basis. Kahlon et al. (2007) 4 Phenolic acid contents of kale extracts and their antioxidant and antibacterial activities Nine phenolic acids were identified and quantified by HPLC-MS in leaves and their antimicrobial properties are determined against different micro organisms. Kale and papaya supplementation in colitis induced by Trinitrobenzenesulfonic (TNBS) acid in the rat Researchers evaluated the effect of dried vegetables as a prebiotic and intestinal anti-inflammatory in the rat colitis model. Rats received, orally, 500 mg/kg of rat weight of three treatments of dried vegetables: papaya, kale and the mixture of both vegetables (60% of kale plus 40% of papaya). After two weeks of feeding the evaluation was done to determine anti-inflammatory activity.
Administration of the mixture was able to modulate the bacterial flora in healthy rats, as well as in rats with colitis induced by TNBS. In addition mixture of kale and papaya showed intestinal anti-inflammatory effect in the colitic rats. Lima et al. (2010) 7 Kale on Genotoxic and Anti-genotoxic potential in Different Cells of Mice The researchers were performed this study using the comet assay, on leukocytes, liver, brain, bone marrow and testicular cells,and using the micronucleus test (MN) in bone marrow cells. In this study, eight groups of albino Swiss mice were used, control (C), positive control (doxorubicin 80 mg/kg (DXR)) and six experimental groups, which received 500, 1000 and 2000 mg/kg of kale extract alone while a further three groups received the same doses plus DXR (80 mg/kg).   The impact of kale leaves on bread making was assessed. This study revealed that baking of non-processed kale in bread induced relatively low losses of flavonoids, but high losses of glucosinolates break down products, carotenoids and chlorophylls. Additionally, in kale an increase in hydroxycinnamic acid derivatives was found after bread preparation. Hence, breads with added fresh kale could enrich health-promoting secondary plant metabolites in baked goods.
Klopsch et al. starter strains were investigated for their application in fermentation of African kale. The strains utilized simple sugars in the kale to quickly reduce the pH from pH 6.0 to pH 3.6 within 24 h. The strains continued to produce both D and L-lactic acid up to 144 h, reaching a maximum concentration of 4.0 g/L. Although vitamins C, B1 and B2 decreased during the fermentation, the final level of vitamin C in the product was an appreciable concentration of 35 mg/100 g and shelf life also extended. Researchers concluded that, controlled fermentation of kale offers a promising avenue to prevent spoilage and improve the shelf life and safety. 5 Drying of kale in convective hot air dryer Authors studied the effect of air temperature and sample thickness (10, 20, 40 and 50 mm) on the drying kinetics of kale using a convective airdryer at a fixed airflow rate of 1 m/s and drying air temperatures of 30, 40, 50 and 60 °C. The drying rate increased with drying air temperature but decreased with layer thickness. The effective diffusivity for 10 mm thick layers was found to increase with the drying air temperature and ranged between 14.9 and 55.9 × 10 −10 m 2 /s. The effect of temperature on diffusivity could be expressed by an Arrhenius type relationship with a high R 2 of 0.9989. The activation energy of kale was found to be 36.115 kJ/mol.

Mwithiga and
Olwal (2005 Fresh kale juice treated with gamma irradiation Researchers evaluated gamma radiation treatment on shelf life of natural kale juice. The total aerobic bacteria in fresh kale juice, prepared by a general kitchen process and the bacteria survived in the juice in spite of gamma irradiation treatment was determined. Two typical radiation-resistant bacteria, Bacillus megaterium and Exiguobacterium acetylicum were isolated and identified from the 5 kGy-irradiated kale juices. The growth of the surviving B. megaterium and E. acetylicum in the 3-5 kGy-irradiated kale juice retarded and/or decreased significantly during a 3 day post-irradiation storage period. This study suggested that 3-5 kGy of gamma irradiation may be effective for prolonging the shelf-life of natural kale juice from a microbiological point of view. D. Kim et al. (2007) 7 Fresh Ashitaba and kale juice treatment with gamma irradiation In this study, examined the effects of irradiation on the microbiological, chemical and sensory properties of ashitaba and kale juices for industrial application and possible shelf-life extension. Irradiation of 5 kGy induced higher than 2 decimal reductions in the microbial level, which was consistently maintained during storage for 7 days under refrigerated conditions. Total content of ascorbic acid in vegetable juice decreased upon irradiation in a dose-dependent manner, in contrast, flavonoids did not change, whereas that of polyphenols increased upon irradiation. This study recommended irradiation sterilizing fresh vegetable juice. Jo et al. (2012) 8 Beverages based on apple Juice with addition of frozen and freeze-dried kale leaves Authors were determined the polyphenols, glucosinolates and ascorbic acid content including antioxidant activity of beverages on the base of apple juice with addition of frozen and freeze-dried kale leaves. Upon enrichment with frozen (13%) and freeze-dried curly kale (3%), the naturally cloudy apple juice showed an increase in phenolic compounds by 2.7 and 3.3-times, accordingly. The antioxidant activity of beverages with the addition of curly kale ranged from 6.6 to 9.4 μmol Trolox/mL. Prepared beverages were characterized glucosinolates content at 117.6-167.6 mg/L and ascorbic acid content at 4.1-31.9 mg/L. Sensory acceptability of prepared juice reported high acceptability.
Ró Ża et al. (2017) 9 Whey protein isolatekale leaves chlorophyll (WPI-CH) microcapsules The authors were reported whey protein isolate-kale leaves chlorophyll (WPI-CH) microcapsules were prepared by spray drying. Effect of inlet air drying temperatures on the physicochemical properties and antioxidant activity of WPI-CH microcapsules were investigated The moisture content of WPI-CH (20% addition) microcapsule was decreased by 21.1% with the inlet air drying temperature increased from 120 to 180 °C. The encapsulation efficiency and solubility of chlorophyll were enhanced by 3.78% and 7.79%, respectively. Furthermore, DPPH scavenging capacity of WPI-CH microcapsules under different addition of chlorophyll were increased from 42.9% to 74.3%, 52.7%-82.7% and 71.8%-85.3%, respectively. This method concluded as promising to preserve chlorophyll with WPI. Zhang et al. (2019) 10 Kale purée: Thermal processing impact on process intensity and storage on quality The authors were focused on investigating quality changes of thermally processed kale purée. Low, medium and high processing intensities (carried out at 70, 90 and 128 °C) were used. The physicochemical properties, consumer acceptability of the puree is largely dependent on the treatment intensity. The high intensity treatments resulted in the least favorable quality characteristics (distinct brown color, chlorophyll and vitamin C destruction as well as a phase separation after storage). Enzymes were inactivated with increasing thermal load. Form this study concluded that, intermediate thermal process intensity seems the best choice to create a high quality kale product that is reasonably quality stable under refrigerated conditions. Wibowo et al. (2019) properties. Research should conduct on the loss of nutrient in kale by different preservation, processing or cooking methods.