1. Introduction
Seaweeds are very important natural resources from the oceans that are employed as human foods and animal feeds in their whole form, and as sources of polysaccharides (mainly alginates, carrageenans and agar), carotenoids, lipids, vitamins, minerals, dietary fiber, proline and amino acids for use in food and pharmaceutical industry [
1]. Seaweeds have been included for a long time in the traditional diet of East Asian countries such as Japan, Korea and China; more recently, their presence in all forms in the diet of Western countries has been progressively increasing [
2].
Seaweeds are considered healthy foods because, despite their low caloric content, they are rich in important nutrients such as protein, essential amino acids, vitamins, minerals and some bioactive compounds [
1]. Seaweeds are also an excellent source of both soluble and insoluble dietary fiber. Among red algae, the genus
Gracilaria contains a broad diversity of valuable contents for human nutrition and are one of the world’s most cultivated and valuable marine seaweed [
3]. Its lipid content is low (1–5% dry weight, DW) [
1], but it contains docosahexaenoic acid (DHA) which is recognized as the most important
n-3 polyunsaturated fatty acid (PUFA) to reduce the risk of cardiovascular diseases [
4,
5]. In particular,
n-3 PUFAs act as excellent antioxidants, strengthening the cell membrane, repairing damaged cells and tissues, improving heart function and fighting against cancer [
6].
n-3 PUFAs were also found to prevent the growth of atherosclerotic plaque that affects blood clotting and blood pressure and improve the immune function, while
n-6 PUFAs decrease low-density lipoprotein cholesterol and may also decrease high-density lipoprotein, cholesterol which reduces heart disease risk [
7].
With respect to their protein content, the most abundant amino acids in
Gracilaria species are aspartic acid, alanine, glutamic acid and glutamine. These amino acids provide the typical flavor of algae and accumulate in response to stress conditions [
8].
Gracilaria is also a good source of both soluble and insoluble dietary fiber, so it can be employed as a potential alternative to cereal-based fiber in Western countries [
1]. Soluble dietary fiber helps to increase viscosity and reduce glycemic response and plasma cholesterol in humans [
1]. Insoluble dietary fiber improves the bulking effect caused by water absorption in feces and thus contributes to weight management, improvement of cardiovascular and gastrointestinal functions and cancer prevention [
1]. Polysaccharides isolated from red seaweeds show potent antibacterial, antiviral, antioxidant, anticoagulant and anti-inflammatory activity [
9].
Seaweeds such as
Gracilaria can concentrate minerals from seawater and reach a mineral content 10–20 times higher than that of terrestrial plants [
10]. Consequently, they are a valuable source of minerals, with important human nutrition functions [
11,
12]. Chlorophyll, an important pigment constituent present in algae, has positive effects on inflammation, oxidation and wound healing [
13]. Chlorophyll acts directly as a reducer of free radicals and has the potential to protect lymphocytes against oxidative DNA damage by free radicals [
14]. Moreover, a large number of potentially bioactive compounds such as phenols, polyphenols, terpenes, steroids, halogenated ketones and alkanes, fucoxanthin, polyphloroglucinol and bromophenols have been isolated [
15,
16,
17].
However, the nutrient profile of seaweeds such as
Gracilaria is influenced by different factors such as seaweed species, habitat, maturity stage, season, water temperature and the sampling conditions and method employed in the determinations [
1,
2].
Gracilaria edulis and
G. corticata is abundantly available in almost all seasons in Palk Bay, on the southeast coast of India, rather than other
Gracilaria sp. Both
G. edulis and
G. corticata are commercially important and commonly edible seaweeds in India. These two algae exhibited in a previous work high biological activities (proximate composition, antioxidant, antibacterial, and biopreservative effects in seafoods during preservation and extended shelf life than other
Gracilaria species in a previous work [
18]. Thus, the present study sought to evaluate and compare the chemical composition (proximate composition, lipid profile, amino acids, vitamins and pigments such as chlorophyll and carotenoids) and physicochemical properties of both,
Gracilaria corticata and
Gracilaria edulis from the Thondi coast of Palk Bay, southeast India.
2. Results
Proximate, polysaccharide content and fatty acids profile of both
G. corticata and
G. edulis in a DW basis are shown in
Table 1.
The crude polysaccharide content found for G. corticata and G. edulis was 49.64 g/100 g and 38.02 g/100 g, respectively. The moisture content (in dried seaweeds) of G. corticata and G. edulis was 8.40 g/100 g and 10.40 g/100 g, respectively. With respect to proximate composition, important differences were obtained for the two seaweeds investigated. Carbohydrates and fat content were significantly higher in G. corticata, whereas protein content was significantly higher in the case of G. edulis.
With respect to fatty acids profile, total fatty acid content, expressed as g fatty acids methyl esters (FAME)/100 g total fat, of
G. corticata and
G. edulis was 5.49 ± 0.30 g/100 g and 3.92 ± 0.13 g/100 g, respectively (
Table 2). The main saturated fatty acids (SFAs) found in both
G. corticata and
G. edulis were palmitic acid (C16:0), margaric acid (C17:0) and stearic acid (C18:0). With respect to PUFAs, linoleic acid (C18:2
n-6), α-linolenic acid (C18:3
n-3), stearidonic acid (C18:4
n-3) and DHA (C22:6
n-3) were found in both seaweeds. In the case of monounsaturated fatty acids (MUFAs), only oleic acid (C18:1) was detected in relevant amounts in both
G. corticata and
G. edulis. Margaric, linoleic and stearidonic acids were found in similar amounts in
G. corticata and
G. edulis. Palmitic, stearic, α-linolenic acid were found in higher amounts in
G. corticata than in
G. edulis. Overall, in G.
corticata, SFAs accounted 49.4% of total fatty acids, MUFAs accounted a 3.3% and PUFAs accounted a 47.3%, whereas in the case of
G. edulis, SFAs accounted 43.9% of total fatty acids, MUFAs accounted a 27% of total fatty acids, and PUFAs accounted a 29%.
The protein of
G. corticata and
G. edulis is shown in
Table 3. The total amino acid content was higher in
G. corticata (76.60 ± 5.14 mg/g), than in
G. edulis (65.42 ± 3.58 mg/g). These values are comparable to their corresponding crude protein content of 22.84 ± 0.87 and 25.29 ± 0.67 g/100 g, respectively, indicating that the amount of non-protein nitrogenous materials in these red seaweeds is low.
Nine essential amino acids (EAAs), and 11 non-essential amino acids (NEAAs), were found in both G. corticata and G. edulis. Total EAAs where significantly higher in G. edulis (35.55 ± 1.75 mg/g) than in G. corticata (22.76 ± 1.81 mg/g), whereas total NEAAs where higher in G. corticata (36.14 ± 3.33 mg/g) than in G. edulis (29.86 ± 1.83 mg/g). The EAAs/total amino acid ratio suggests that more than 50% of the amino acids found in G. edulis are EAAs. The results also indicate a good ratio of essential amino acids to non-essential amino acids in G. corticata (0.62 ± 0.54 mg/g) and G. edulis (1.19 ± 0.95 mg/g). It was noted that a much higher concentration of the essential amino acid threonine (20.57 ± 0.62 mg/g) was found in G. edulis than in G. corticata. Contrariwise, alanine content was much higher in G. corticata (21.11 ± 0.54 mg/g) than in G. edulis (1.46 ± 0.18 mg/g). Aspartic acid content was similar in both seaweeds.
The mineral content of
G. corticata and
G. edulis is shown in
Table 4.
G. corticata showed a higher content of Mg (463.23 ± 8.87 mg/kg) and Fe (1072.48 ± 20.97 mg/kg) than
G. edulis. Moreover,
G. edulis was found to possess more of trace elements like Zn (42.73 ± 2.12 mg/kg) and Cu (14.61 ± 0.46 mg/kg) than
G. corticata. In view of the present results, both
G. corticata and
G. edulis contain an adequate amount of minerals, which suggests that these seaweeds could act as important sources of mineral supplements which are essential for human nutrition.
For both
G. corticata and
G. edulis, the presence of water-soluble vitamins (vitamin B
1, vitamin B
2, vitamin B
3, vitamin B
6, vitamin B
9 and vitamin C) and fat-soluble vitamins (vitamin A and vitamin E) was found, as shown in
Table 5.
G. corticata had a higher vitamin A (2.67 ± 0.31 mg/g vs. 2.14 ± 0.17 mg/g) and vitamin B
9 contents (1.00 ± 0.07 mg/g vs. 0.45 ± 0.06 mg/g) than
G. edulis, whereas
G. edulis showed a significantly higher content of vitamin B
2 (1.54 ± 0.39 mg/g vs. 0.05 ± 0.01 mg/g) and vitamin B
6 (4.77 ± 0.23 mg/g vs. 3.79 ± 0.30 mg/g) than
G. corticata.
The methanolic extracts of G. corticata and G. edulis at 1 mg/mL concentration indicate the presence of three major compounds, chlorophyll a, chlorophyll b and carotenoids, in G. corticata (Retention factor (Rf) value = 0.97, 0.92 and 0.95, respectively) and G. edulis (Rf = 0.96, 0.96 0.84, respectively). G. corticata and G. edulis contained 8.96 ± 0.39 µg/g and 17.14 ± 0.55 µg/g of chlorophyll a and 7.74 ± 0.33 µg/g and 8.44 ± 0.63 µg/g of chlorophyll b, respectively. With respect to the carotenoid content, it was higher for G. corticata (12.82 ± 0.50 µg/g) than for G. edulis (2.99 ± 0.56 µg/g).
Table 6 shows the swelling capacity (SWC), water-holding capacity (WHC) and oil-holding capacity (OHC) of
G. corticata and
G. edulis. In general, as temperature varied, the SWC and WHC of
G. corticata and
G. edulis powder varied, due to an increase in the solubility of the dietary fiber and the presence of protein in
G. corticata and
G. edulis. However, it also reaches significant differences for the case of SWC, whereas no statistical differences were obtained for WHC or OHC. The SWC of
G. edulis were higher than
G. corticata at both 25 °C and 37 °C (8.66 ± 0.53 mL/g vs. 7.90 ± 0.32 mL/g, and 7.70 ± 0.60 mL/g vs. 5.70 ± 0.65 mL/g, respectively).
With respect to the WHC of G. corticata and G. edulis, values of 4.03 ± 0.39 and 4.09 ± 0.28 g/g, respectively, were obtained at 25 °C, reduced to 3.96 ± 0.58 g/g in G. corticata and 3.64 ± 0.18 g/g in G. edulis at 37 °C. In this study, both G. corticata and G. edulis exhibited similar OHC values (about 2 g/g) at both 25 °C and 37 °C.
3. Discussion
With respect to proximate content, the moisture of
G. corticata and
G. edulis was lower than most results obtained for
Gracilaria sp. in general, such as the 12.15 g/100 g obtained for
G. acerosa [
6] the 19.2 g/100 g for
G. edulis [
8], and the 12.86 g/100 g for
G. edulis [
1], but higher than the 5.32 g/100 g obtained for
G. changii [
19]. In this work,
G. edulis showed a higher ash content in a DW basis than
G. corticata. Similarly, it was reported an ash content of 8.70 g/100 g in
G. edulis [
8], whereas other authors reported a higher ash content (40.30 g/100 g) in
G. changii [
19] than those found in the present work. A high ash content shows the presence of appreciable amounts of diverse minerals found in both seaweeds. A similar observation was for
G. changii [
19] in which were found an ash content of 6.40 g/100 g. Interestingly, total dietary fiber is known to have physiological properties for the prevention and treatment of cancer, obesity and diabetes [
20,
21]. Therefore,
G. corticata and
G. edulis may have the potential to be used as a source of dietary fiber in the nutraceutical industries.
Other authors found much lower crude protein contents in
Gracilaria spp. than those found in the present work. Thus, it was reported a crude protein content of 6.68 g/100 g for
G. edulis [
8], 0.61 g/100 g for
G. acerosa [
6], 12.57 g/100 g in
G. changii [
19] or 19.70 g/100 g for
G. cervicornis [
22]. Moreover, the high protein content of
G. corticata and
G. edulis indicates that these seaweeds may be considered as potential marine plant sources of protein [
22]. Proteins from seaweeds can have antibacterial, antioxidant, immunostimulating, antithrombotic and anti-inflammatory activities. Consequently, they can be used for prevention and treatment of hypertension, diabetes and hepatitis among other positive effects in the organism [
20].
The total carbohydrate content of both
G. corticata and
G. edulis was markedly lower than that reported [
8] for
G. edulis (10.2 g/100 g) or the 29.44 g/100 g reported for
G. changii [
19]. However, other authors found lower carbohydrate content in
Gracilaria species, such as
G. acerosa, for which was reported a carbohydrate content of 1.05 g/100 g [
6]. The wide variation in the carbohydrate content observed in red and brown seaweed species might be due to the influence of different factors like salinity, temperature and sunlight intensity [
2]. Moreover, carbohydrate content is also influenced by biomass, which reveals the link between growth and carbohydrate content [
23].
In general, seaweeds have a low fat content [
23]; that makes seaweeds low-calorie foods and in the present work both seaweeds contained fat amounts of 7.07 g/100 g DW seaweed (
G. corticata) and 4.71 g/100 g DW seaweed (
G. edulis). These results are lower than those obtained by other authors [
8], whose reported a crude lipid content for
G. edulis of 8.30 g/100 g but significantly that the 0.3% reported for
G. changi [
19], or the 1.7–3.6% reported for
G. fisheri and
G. tenuistipitata [
5]. Thus,
Gracilaria content in fat can widely vary depending on the species and source.
Polysaccharides are polymers composed of at least 10 monosaccharides linked by glycosidic bonds [
9]. Recently, seaweed polysaccharides have been given large attention by the scientific community due to their outstanding bioactivities and correspondingly low toxicity [
9]. They have been shown to have other beneficial health effects, including their prebiotic effect and antioxidant or anti-inflammatory activity [
20]. The polysaccharide content obtained in the present work was higher than the polysaccharide extracted from
Gracilaria species in previous works, such as 29.08 g/100 g [
24], 27.20 g/100 g [
25], 21.40 g/100 g [
26], and 32.80 g/100 g [
27]. Contrariwise, it was also reported a higher polysaccharide content in
G. debilis [
28], in the range 52–67 g/100 g. A previous work [
29] reported that the polysaccharide yield from
Gracilaria species varies due to seasonal variations, physiochemical factors, environmental conditions and extraction methods. Additionally, the variations in the polysaccharide content of
Gracilaria can vary depending on atmospheric temperature at the time of extraction [
26]. Hence the present study significantly indicates that the crude polysaccharides present in
G. corticata and
G. edulis may exert varied biological activity [
25].
With respect to the fatty acids composition, those of seaweeds often differ from those of terrestrial plants whereby seaweeds have a higher proportion of PUFAs than terrestrial vegetables. Red seaweeds are particularly rich in SFAs and PUFAs which have nutritional applications that lead to their extensive use in food, feed, cosmetic, biotechnological and pharmaceutical applications [
30,
31]. Variation in fatty acid content may also be due to the season of collection as well as other abiotic factors such as nutrition, salinity, light and temperature [
8,
20]. In the present work, total fatty acids were significantly lower than those obtained by other authors [
8], who found 11.41 g/100 g in
G. edulis. According to this work [
8], the most abundant fatty acids in both seaweeds were palmitic, stearic and α-linoleic acid acids. The same fatty acids were also found abundant in
G. changii [
20]. However, our results were significantly lower than those obtained in
G. changii for DHA content, in which DHA were found as the most abundant fatty acids, with a 48.36% of total fatty acids. The results of the present study revealed that both seaweeds are rich in SFAs and especially in PUFAs, which provide important health benefits. With respect to most commonly found n-3 PUFA, eicosapentaenoic acid (EPA) and DHA, it is common that their contents vary dramatically from
Gracilaria spp. and even into the same species [
32]. No EPA presence were found for the seaweeds tested in the present work. The presence of this n-3 fatty acid in
Gracilaria spp. is inconstant, because it was found in
G. gracilis [
20], but it was not detected in
G. changii [
19] or
G. edulis [
8]. Fatty acids overall profile obtained in this work were significantly different than 57.5% SFAs, 18.3% MUFAs and 18.4% PUFAs reported for
Gracilaria sp. [
3] or the 7.5% SFAs, 38.3% MUFAs and 51.2% PUFAs 18.4% reported for
Gracilaria changii [
19].
The protein composition found in this work for
G. corticata and
G. edulis was lower than those found in a previous work [
20], which reported an amino acid content of 91.90 mg/g in
G. changii. The EAAs/total amino acid ratio was higher than those previously reported [
6,
19,
26]. Aspartic acid content, that is important for the organoleptic point of view because it was reported that it is responsible for the special flavor and taste of seaweeds [
33], was in similar contents in both seaweeds.
Seaweeds are one of the richest sources of minerals and trace elements, because the cell-wall polysaccharides and proteins of seaweed contain sulfate, anionic carboxyl and phosphate groups which act as binding sites for metal retention [
34]. With respect to the mineral content,
G. corticata showed a higher content of Mg (463.23 mg/kg) and Fe (1072.48 mg/kg) than
G. edulis. Moreover,
G. edulis was found to possess more of trace elements like Zn (42.73 mg/kg) and Cu (14.61 mg/kg) than
G. corticata. Both seaweeds had a higher or similar content of minerals like Zn, Cu, Mg and Fe when compared with the content of
G. acerosa [
6],
G. edulis [
8],
G. fisheri and
G. tenuistipidatata [
5] or
G. changii [
19], with the exception of Mg in
G. edulis which were lower than those found for other previous works as
G. changii [
19]. The ability of seaweeds to accumulate metals will depend on a variety of factors such as location, exposure, salinity, temperature, pH, light, nitrogen content, season, plant age, metabolic processes or the affinity of the plant for each element among others [
35]. In view of the present results, both
G. corticata and
G. edulis contain an adequate amount of minerals, which suggests that these seaweeds could act as important sources of mineral supplements which are essential for human nutrition.
For both
G. corticata and
G. edulis, the presence of water-soluble vitamins (vitamin B
1, vitamin B
2, vitamin B
3, vitamin B
6, vitamin B
9 and vitamin C) and fat-soluble vitamins (vitamin A and vitamin E) was found.
G. corticata had a higher vitamin A (2.67 mg/g vs. 2.14 mg/g) and vitamin B
9 contents (1.00 mg/g vs. 0.45 mg/g) than
G. edulis, whereas
G. edulis showed a significantly higher content of vitamin B
2 (1.54 mg/g vs. 0.05 mg/g) and vitamin B
6 (4.77 mg/g vs. 3.79 mg/g) than
G. corticata. With respect to previously published works, the vitamin content reported for
Gracilaria species is widely different between the different authors [
1,
6,
8]. Perhaps the more remarkable difference in vitamin content is that in the present work both
G. corticata and
G. edulis showed a significantly higher vitamin A content (2.67 and 2.07, respectively) than those previously reported for
G. acerosa [
6] or for
G. edulis [
1].
Another important difference was found for the case of vitamin C that showed a higher content than those previously reported [
6,
8] for
G. edulis or
G. acerosa, respectively. The variation in vitamin content may be due to some environmental factors such as salinity, atmospheric temperature, seasonality and methods of preservation and processing [
6].
G. corticata and
G. edulis contained 8.96 µg/g and 17.14 µg/g of chlorophyll
a and 7.74 µg/g and 8.44 µg/g of chlorophyll
b, respectively. In a previous work describing the composition of several seaweeds [
36] it was reported that chlorophyll
a and
b in red seaweeds ranged from 68 to 162 µg/g and from 25 to 46 µg/g, respectively. Specifically, for
Gracilaria spp., it was reported high chlorophyll
a of 577.89 µg/g and low chlorophyll
b of 1.11 µg/g in
G. changii [
19].
With respect to carotenoid content, a higher total carotenoid content was reported for
G. changii than in the present study (74.22 µg/g) [
19]. Carotenoids such as
β-carotene, lutein, zeaxanthin and antheraxanthin have been identified in red seaweed, including
Gracilaria species. Further, seaweed carotenoids, especially
β-carotene, are preferred by the market of natural products, because they are a mixture of cis and trans isomers, which may possess anticancer activity [
37].
The SWC, WHC and OHC properties of seaweeds are generally related to their content and type of polysaccharides as well as protein which links to the cell wall of polysaccharide [
5]. Previous works described that variations in temperature can widely vary physicochemical properties of seaweeds, due to increase in the solubility of the dietary fiber and the presence of protein [
5,
8]. However, in our work, only were found significant variations in the case of SWC. Previous works [
8] reported a SWC of 20 mL/g in
G. edulis, higher than those found in the present study. Similarly, a SWC at 37 °C of 7.68 mL/g in
G. changii was reported [
19], whereas for
G. acerosa [
6] an SWC at 37 °C of 5 mL/g was reported.
With respect to the WHC of
G. corticata and
G. edulis, a similar observation was also made in a previous work [
8], that reported a WHC for
G. edulis of 3.08 g/g. Other authors found better WHC than in the present work for other
Gracilaria spp., such as
G. fisheri, for which a WHC of 5.53 g/g was reported [
5], and
G. changii [
26], for which WHC values of 6.15 g/g at 24 °C and 9.93 g/g at 37 °C were reported. Both SWC and WHC of seaweeds might be attributed due to different protein content and increases in the number and nature of the water binding sites on the protein molecules [
38].
OHC is another functional property of food ingredients used in formulated foods for consumption. Ingredients with high OHC values allow the stabilization of food emulsions and high-fat food products [
19]. For other
Gracilaria spp., it was reported [
8] that
G. edulis showed an OHC of 1.64 g/g, which is very similar to the OHC values of the present study. Moreover, for
G. changii [
19] OHCs of 3.11 g/g at 24 °C and 1.17 g/g at 37 °C were reported. The low oil absorption capacity of red seaweeds is generally related to the hydrophilic nature of the changed polysaccharides (agar, carrageenan, fucans and alginates) of soluble dietary fiber [
39]. The results of the present study for physicochemical properties confirmed that
G. corticata and
G. edulis could be considered as a source of food ingredients including proteins, dietary and soluble fiber [
39].