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Antioxidant synergy of Mediterranean herbs and algal polysaccharides: potential in the development of functional beverages

Herbal extracts and algae contain a wide range of bioactive molecules with positive effects on human health. However, their application in the production of functional beverages is still insufficient despite the fact that various plant species and their potential use as ingredients in different categories of functional beverages have been explored. Salvia officinalis and Thymus serpyllum are aromatic herbs from the Lamiaceae family native to the Mediterranean area. Due to their high antioxidant capacity, which is directly correlated with a high content of various polyphenolic and volatile compounds (Mrkonjić et al., 2021), they present a valuable source for the production of functional beverages. On the other hand, green alga Ulva lactuca is an important source of sulfated polysaccharide ulvan, while brown algae Fucus virsoides and Cystoseira barbata are important sources of sulfated polysaccharide fucoidan. All of these polysaccharides show a wide range of biological activities such as antioxidant, anti-inflammatory and antitumor, however they are strongly dependent on chemical composition of polysaccharides. In order to test the possibility in the development of functional beverage on the basis of selected Mediterranean herbs extracts enriched with algal polysaccharides, the aim of this research was to evaluate antioxidant capacity of S. officinalis and T. serpyllum extracts, polysaccharides of U. lactuca, F. virsoides and C. barbata and their mixtures. S. officinalis and T. serpyllum were purchased from Suban Ltd. (Strmec, Croatia), while U. lactuca, F. virsoides and C. barbata were harvested from coastal region of Zadar (Croatia) in February 2020. S. officinalis and T. serpyllum (2 g) were each extracted with ddH2O (100 mL) using ultrasonic bath at 50 C/30 min. Obtained extracts were analysed for total polyphenols using Folin-Ciocalteau method (Repajić et al., 2018) and mixed in 1:1 ratio for further experiment. Conventional polysaccharide extraction was performed at 80 C/3 h using 0.1M H2SO4 (Dobrinčić et al., 2021). Extracted polysaccharides were analyzed as follows: total sugars by colorimetric phenol-sulfuric acid method (Dubois et al., 1956), L-fucose concentration by colorimetric assay with L-cysteine (Dische and Shettles, 1948), sulfate content was quantified after acid hydrolysis (1M HCl at 105 °C for 5h) by turbidimetric BaCl2-gelatin method (Dodgson and Price, 1962) and uronic acid content was measured with modified sulfamate/m-hydroxydiphenyl colorimetric method (Filisetti-Cozzi and Carpita, 1991). In order to evaluate antioxidant synergy of Mediterranean herbs and algal polysaccharides, three different amounts of polysaccharides (0.25, 0.5 and 1 g) from each alga were mixed with 10 mL of herbal extract. The ability of prepared extracts (herbal extract, algal polysaccharides and their mixtures) to scavenge the DPPH radical was assessed spectrophotometrically. Content of total sugars in all three algal polysaccharides ranged from 57.28 to 61.21%, while sulfate group content ranged from 33.55 to 41.86%. U. lactuca polysaccharides had significantly lower content of fucose and uronic acid, while C. barbata polysaccharides contained the highest fucose content. The highest content of uronic acid was found in F. virsoides polysaccharides. Herbal extracts contained 27.00 (S. officinalis) and 15.03 mg GAE g-1 (T. serpyllum) of total polyphenols and their DPPH radical scavenging activity was 242.03 and 215.9 mg L-1, respectively. Furthermore, mixture (1:1 ratio) of these extracts showed DPPH radical scavenging activity of 230.53 mg L-1, while DPPH radical scavenging activity of U. lactuca, F. virsoides and C. barbata polysaccharides was 43.51, 67.64 and 71.14 mg L-1, respectively. Addition of polysaccharides into the herbal extract increased DPPH radical scavenging activity (280.03 to 299.9 mg L-1) regardless of the algal species and the amount added. However, increase of polysaccharides addition from 0.25 to 1 g increased scavenging activity in F. virsoides and C. barbata herbal mixtures, but not in herbal mixture of U. lactuca. Various biological activities of algal polysaccharides are often associated with their chemical composition, e.g. high levels of L-fucose, high degree of sulfation and low levels of contaminants such as uronic acid and protein (January et al., 2019). However, significant discrepancies in structural properties makes it impossible to precisely relate the bioactivity of polysaccharides with just one structural property but rather a combination of several properties. Green algae U. lactuca is a source of ulvan whose major monosaccharide unit is L-rhamnose, while L-fucose is predominant monosaccharide in brown algae polysaccharides. Lower fucose and sulfate group content, despite low level of uronic acid, could explain lower DPPH radical scavenging activity of U. lactuca polysaccharides. On the other hand, S. officinalis and T. serpyllum extracts contain various polyphenols which are known as strong antioxidants. The chemical structure of polyphenols varies upon number of aromatic rings and hydroxyl substituents which enable polyphenols to scavenge free radicals and reacting oxygen species through direct reaction with free radicals and from the chelation of free metals (Leopoldini et al., 2011). Obtained results revealed a synergistic action of polyphenols from Mediterranean herbs and algal polysaccharides by which antioxidant activity of herbal extracts increased with the addition of algal polysaccharides, even in low amounts. These observations present an excellent base for the further development of functional beverages enriched with natural antioxidants.

Salvia officinalis ; Thymus serpyllum ; algae ; polyphenols ; polysaccharides ; antioxidant activity

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