1.1. Antihypertensive Properties Of Apple Peel         

Apples have sufficient supply of flavonoids containing practicable benefits to health. In North America about 22% of dietary phenolics like flavonoids and phenolic acid are obtained from eating apples [40]. Hypertension is a major problem regarding to health and is increasing day by day in different parts of world. In regard of controlling high blood pressure Angiotensin converting enzyme (ACE) is considered as a major therapeutic target. Latest work shows that apple peel extract rich in flavonoids and its constituents are competitive inhibitors of ACE [41] (Table 3).

Compounds found in apple peel contain anthocyanins and quercetin are useful and advantageous for reducing blood pressure level and have antihypertensive effects [42]. Quercetin component and different kind of flavonoids found in high concentration of an apple peel, lower blood pressure due to its effects like antioxidants, angiotensin-converting enzyme inhibition and improved endothelium function [43].  Pro-anthocyanidins is another component in apple peel that has been studied for its antihypertensive effect. It is beneficial in prevention of oxidation of LDL cholesterol, blood pressure reduction and fat metabolism improvement [44]. Apple now has been identified in several mechanism like lowering of blood cholesterol particularly LDL, weight loss, antioxidant action and prevention of atherosclerosis [41].

It is reported that apple have ability to lower the cholesterol predominately because of presence of phytochemicals like epicatechin, β-carotene, proanthocyanidin-B1 and catechin [45]. Rupasinghe et al. [46] reported that medicinal (pharmaceutical and nutraceutical) methods using apple peel extracts have been derived for treating cardiovascular disease, that lessens cholesterol levels and inhibit low density lipoprotein (LDL) oxidation. Atanassova & Bagdassarian [47] reported that rutin lowers the degree at which mitochondrial damage will occur and improves functions and structure of cardiac mitochondria.

1.2. Antioxidative Properties of Apple Peel Extract

Apple flesh extracts has lower antioxidant actions than apple peel extract [40]. Lipid oxidation, notably oxidation of polyunsaturated fatty acids is prominent issue in the food industry influencing quality of food and health of consumers. Apple peel is considered a natural supply of antioxidant that all components of apple such as quercetin, chlorogenic acid, catechin, and phloridzin are powerful antioxidants. Flavonoids decreases the possibility of cardiovascular disease by increasing the release of endothelial nitric oxide inducing vasodilatation [48].

 Anthocyanidins have high antioxidant activity that protects LDL cholesterol oxidation by their action. Peeled and unpeeled apples have high antioxidant activity and suppresses the growth of cancer and helpful in chronic disease prevention. A component pro-anthocyanidins has antioxidant property. It combines with the phospholipids and limits the entrance of oxidants on surface of membrane and restricts the oxidants movement within the membrane [49]. Apple peel has high antioxidant activity compared to the flesh. The peels from an average sized apple have an antioxidant action equal to 820g of vitamin C. Concentration of the phenolics extract is responsible for total antioxidant activity [50].

1.3. Antiproliferative Property

Apple have antiproliferative activity. It is reported that a combination of phytochemical in apple are powerful in blockage of the expansion of tumor cell. Apple has high antiproliferative activity when compared with other fruit [51]. Apple without peel is less effective against Hep G2 cell proliferation than apple with peel. So, it is noted that peel of apple has more antiproliferative activity. Wolfe et.al suggested that peel of apple has higher rate in reduction of Hep G2 cell proliferation as compared to whole apple [52].  It is also seen that the antioxidants of apple is not directly associated in the inhibition of cell proliferation of tumor but they indirectly reduces the cell proliferation [53].

1.4. Anti-Inflammatory Properties

Martín, M. A. & Ramos [54] reported that flavonols such as epicatechin have anti-inflammatory and antioxidant properties and they shows biological effect on target and are chief constituent of human diet. Quercetin have antioxidant property which leads to anti-inflammatory activity and its inhibitory action on enzymes to stop inflammation process with successive inhibition of inflammation mediators like leukotrienes and prostaglandins describe that flavonoid (quercetin) found in apples, red grapes and blueberries have anti-inflammatory properties shown in chronic inflammatory diseases and joint health [54].

1.5. Antidiabetic Property

Apple not only reduces the probability of asthma, diseases related to heart and cancer but it is also noted that eating apple is also related with the low risk of diabetes. It is noted that around 10,000 people who eats apple have lower risk of type II diabetes. As mentioned, quercetin is present in apple in rich amount and its intake help in the reduction of type II diabetes. Apple consumption is also related with increased function of lung and weight loss. Weight loss is noted in middle age Brazilian women by consuming apple [52].

1.6. Ant Carcinogenic Properties Of Apple Peel

Abundance of phytochemicals like catechin, quercetin, chlorogenic acid, and phloridzin are present in apple peel and these considerably act as antioxidants [55]. Triterpenoids isolated from apple peels have strong anti-proliferative activity and might be partly responsible for the apple’s anticancer activity [56]. Rutin is effective against prevention of some cancers and have antioxidant activity like other polyphenols [57]. Biological activity of flavonoids has protective effect in hepatic tumor cells [58,59]. 

Apples possesses antiproliferative activity against tumor cell growth as well as antioxidant activity that prevents oxidative damage in cellular components [60]. Apple peel instead of apple is used for antioxidant and antiproliferative activity because of high level of quercetin glycosides [38].

Abraham ., et al., Cher et al, and Hall et al., [61-63] describes the  antiproliferative effects of apple peel extract increases the maspin expression which inhibits the growth of cancer cells in breast [61–63]. Phytochemicals in fruits and vegetables have synergistic and additive effect that manages their anticancer and antioxidant actions [64]. Interaction of anthocyanins and Proanthocyanidins increases the potency of flavonoid-rich fruits against cancer, metabolic syndrome and cardiovascular diseases [65].

 Apple peel constitutes greater quantity of flavonoids, anthocyanins and phenolics and these compounds possess antiproliferative activities [66]. Apple peel have shown strong antioxidant and anti-proliferative activity than apple flesh, specifying that apple peel contains more bioactive phytochemicals. This also gives the idea that presence of high phenolic content, high antioxidant activity and high proliferative activity in apple peel shows that these give health benefits upon consuming and proved to be a valuable source of antioxidants and bioactive compounds [38].

Apple contain larger quantity of biologically active compounds but phenolic distribution in apple peel and apple flesh is different. Both contains chlorogenic acid, Phloretin glycosides, phloridzin but peel have additional flavonoids that are not present in flesh. By evaluating nutritional quality of apple peel contents of phenol, flavonoids and antioxidant activity along with tumor cell growth is higher in peel flesh, regardless of cultivar [67]. Apple extracts from skin and flesh have a dosage-dependent activity which inhibits the proliferation of colon carcinoma and the inhibitory effect was significant in extracts of apple peel [49].

Table 3: Presence and absence of polyphenolic compounds in different Commercial varieties of apple peel and flesh.