**4. Presence of secondary metabolites**

Peanut is a reservoir of secondary metabolites like flavonoids, polyphenols, phytosterols, stilbenes ( **Table 2**). The evaluation of peanuts role in a heart-healthy diet has increased in the last decade [54]. Extraction procedures would play a big role in getting these bioactive components since extracting solvent, isolation procedures, purity of active compounds, as well as the test system and substrate to be protected by the antioxidant affects its function [55].

The flavonoid content in peanuts was determined, which is second only to walnuts [56]. Studies [57, 58] reported that peanut seeds had an isoflavonoid content of daidzein and genistein in the greatest amounts with a content of 49.7 mg/100 g and 82.6 mg/100 g, respectively. A-type proanthocyanidins was determined in peanuts [59]. Luteolin was the principal antioxidative component from the methanolic extracts of peanut hulls [60]. Mature, red peanut skins contain about 17% by weight of procyanidins, nearly 50% of which are low molecular weight oligomers [61]. Catechins, A-type and B-type procyanidins dimers, trimers, and tetramers were also detected in chemically purified peanut skin aqueous and ethanol extracts [45]. Furthermore, higher concentrations of compounds mentioned were observed in raw peanut skins than roasted peanut skins.

The polyphenolic content of raw and dry roasted peanut samples containing varying levels of oleic acid (normal, mid, and high) were determined [62, 63]. Normal oleic acid peanuts had higher concentrations of individual polyphenolics than mid- and high-oleic peanuts. Free p-coumaric acid, three esterified derivatives of p-coumaric, and two esterified derivatives of hydrobenzoic acid were identified as the predominant polyphenolics. Whole raw peanuts had a mean of 25 mg/ kg of p-coumaric acid (from a range of 8 to 66 mg/kg among cultivars) and the value increased to an average of 69 mg/kg when peanuts were roasted at 175 °C for 10 min.

Peanuts as a source of phytosterol has been getting a lot of attention with new research findings identifying phytosterols like beta-sitosterol, sitosterol in peanuts and peanut products as cancer growth inhibitors, as well as protectors against heart diseases [64]. The phytosterol contents of peanuts and peanut products were analyzed. Results show that among the four cultivars studied, the Valencia peanuts in raw, dry roasted, and oil roasted, contained the highest phytosterol concentration [65]. Studies with sitosterol or mixtures of plant sterols have shown that they reduce serum cholesterol levels in humans by approximately 10%. This discovery has resulted in subsequent research to evaluate the effects of sitosterol derivatives on cholesterol absorption and serum cholesterol levels [48].

Stilbenes contain two phenyl compounds connected by a 2- carbon methylene bridge. They occur in nature in a rather restricted distribution. Stilbenes like isoflavonoids, are also classified as phytoestrogens. Most stilbenes in plants act as antifungal phytoalexins, compounds that are usually synthesized only in response to infection or injury. The most studied one is resveratrol. Resveratrol is one of the major stilbene phytoalexin compounds produced by grape berries and peanuts in response to stress like fungal infection, the presence of heavy metal ions, or ultraviolet (UV) irradiation [66]. Resveratrol was found to be present in substantial amounts in the leaves, roots, and shells of peanuts, but very little was found in developing seeds and seed coats of field-grown peanuts [67]. The phytoalexin content of peanuts, however, increases during germination and is enhanced by microbial infection, postharvest induction procedures such as soaking and drying; wounding (slicing and incubation); UV light exposure, among others. Raw peanuts soaked in water for about 20 hours and dried for 66 hours increased the resveratrol content between 45 and 65 times after the soaking treatment [66]. Boiled peanuts contain more resveratrol than peanut butter and roasted peanuts. Resveratrol has been associated with reduced CVD and reduced cancer risk. Resveratrol has been shown from in vitro, ex vivo, and animal studies to have many attributes that may provide protection from atherosclerosis, antiproliferative, and proapoptotic properties against breast, colon, prostatic, and leukemia cells [68].
