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Polyphenol composition and antioxidant potential of mint leaves

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Nicole BrownJenny A. John & Fereidoon Shahidi

Abstract

Soluble and insoluble/bound phenolic compounds and an aqueous infusion of two mint species (Medina and Hasawi), were tested for their total phenolic content, 2,2′-azino-bis-3-ethylbenzthiazoline-6-sulphonate (ABTS) and 1,1-diphenyl-2picrylhydrazyl (DPPH) radical scavenging activity, iron (III) reduction, iron (II) chelating and oxygen radical absorbance capacity (ORAC). Furthermore, the potency of the mint extracts in the inhibition of radical-induced DNA scission, human low-density lipoprotein (LDL) cholesterol oxidation, formation of thiobarbituric acid reactive substances (TBARS) in a cooked ground meat system and LPS (lipopolysaccharide)-stimulated cyclooxygenase-2 (COX-2) expression in J774A.1 mouse macrophage cells were monitored. Results showed that the soluble phenolics had a higher phenolic content and antioxidant activity than the insoluble-bound extracts and aqueous infusions in most of the assays. Both varieties exhibited notable antioxidant activities and inhibition of LDL cholesterol oxidation, DNA scission and COX-2 gene expression at transcriptional level. However, Medina mint was a more potent antioxidant than the Hasawi mint. High performance liquid chromatography with online tandem electrospray ionization mass spectrometry (HPLC/ESI-MS/MS) analysis of the extracts revealed that rosmarinic acid was the major phenolic compound present in both mint samples.

Introduction

Spices and herbs are known to serve as powerful antioxidants. Members of the Lamiaceae family such as mint, basil, rosemary, sage, savory, oregano, and thyme, are particularly well known for their use in food and traditional medicine (Park 2011). Extracts of rosemary and sage are routinely used to control oxidation of food lipids (Zheng and Wang 2001; Shahidi and Zhong 2010). These products are now commercially available in the deflavored form. Aqueous infusions obtained from mint leaves have long been used to treat anorexia, hypertension, and many spasmolytic and gastrointestinal problems (Mimica-Dukic and Bozin 2008). In addition, mint has been shown in clinical trials to treat headaches through analgesic properties, reduce painful muscle spasms in patients undergoing endoscopy of the upper and lower gastrointestinal (GI) tract, and reduce abdominal pain and dyspepsia (McKay and Blumberg 2006). In vitrobased studies have also demonstrated the potential anti-depressant effects of mint extracts (Lopez et al. 2010). Many of these medicinal effects of mint are closely associated with the high content of phenolic compounds; the phenolics from mint tea and mint extracts have been shown to exhibit antimicrobial and antiviral activities (Mimica-Dukic and Bozin 2008). Additionally, mint extract has been shown to increase cellular superoxide dismutase activity, a natural protective mechanism against oxidative damage (Mimica-Dukic et al. 1996).

Phenolics belong to an important class of compounds responsible for the antioxidant activity of mint. Previous studies have found the total polyphenolic content of peppermint leaves to be approximately 19–23% with total flavonoids of 12% (McKay and Blumberg 2006). Mata et al. (2007) reported that the main active components in mint were eriocitrin, rosmarinic acid, luteolin 7-O-rutinoside, hesperidin, and small quantities of pebrellin, gardenin B and apigenin. Another study determined the major polyphenols in mint as rosmarinic acid, caffeic acid, ferulic acid and eugenol (Tahira et al. 2011).

Phenolic compounds are, however, not distributed evenly throughout different parts and sections of the plant. The outer layers of plants often contain higher levels of phenolics than their interior parts. For example, many different studies have shown that there is a higher phenolic content in the peels of fruits and vegetables than in the flesh (Kujala et al. 2000; Tomas-Barberan et al. 2001). At a cellular level, insoluble-bound phenolics are often linked to components of cell walls and soluble phenolics are located within plant cell vacuoles as either free phenolic acids, esterified to pectins and arabinoxylans, or cross-linked to cell wall polysaccharides in the form of dimers (Naczk and Shahidi 2004). In the present study, a modified version of the method described by Krygier et al. (1982) and Naczk and Shahidi (1989) was used to extract both the soluble and insoluble-bound phenolics.

In this work, the antioxidant activity and phenolic content of two mint varieties grown in Saudi Arabia were determined. Solvent extracts of both soluble and insoluble phenolics were prepared and dechlorophyllized to determine their potential use as a natural antioxidant. Furthermore, an aqueous mint infusion was prepared and analyzed to quantify the amount of polyphenols extracted during normal cooking procedures or in tea preparation as used traditionally, and to determine its efficiency to act as an antioxidant. A cooked meat model system was also used, in addition to other in vitro tests of antioxidant activity, to determine the effectiveness of mint samples as natural food preservatives. The phenolic compounds present in the mint extracts were identified using high performance liquid chromatography with online tandem electrospray ionization mass spectrometry (HPLC/ESI-MS/MS). Since no information on determination of polyphenols in mint cultivars from local sources in Saudi Arabia is available, this investigation provides basic information for further work on antioxidant potential in herbs and medicinal plants.

Reference:

https://fppn.biomedcentral.com/articles/10.1186/s43014-019-0001-8

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