## 4 Health Benefits and Composition of Kiwi By-Products
### Kiwi Peel/Skin  
As a fruit, kiwi fruit is commonly eaten without its peel, which ends up in household waste, and the waste of juice-producing companies and other kiwi fruit producers. However, discarding the skin can and should be avoided as it is a nutritious part of the fruit, but unfortunately it is unusable. Depending on the variety, the peel can be green, brown, yellow, or orange, while its texture is usually fluffy-hairy. Throughout the literature, the content of phenolic compounds in the peel is consistently reported, which indicates high antioxidant capacity. For the variety _A. deliciousa_, a phenolic content of 13 mg GAE/g dw and 0.91 mg GAE/g fw was reported and, specifically, the compounds protocatechuric acid, chlorogenic acid, caffeic acid, rutin, p-hydroxybenzoic acid, and quercetin, were observed [34; 36]. These compounds are also present in the pulp, but not at such high values. Epicatechin, with a concentration of 163 ug/g, quantitatively takes the highest position among the polyphenic compounds. This phenolic superiority, especially flavonoids that appear in kiwi fruit skin, supports all of its aforementioned antioxidant, antibacterial, and anticancer properties. Their activities can be quantified and compared to those of other parts of the fruit, by subjecting such extracts to a series of assays (DPPH, ABTS). Furthermore, the type of solvent, its solubility, and its content are considered to be important factors in the reported occurrence of antioxidants. After measuring the antioxidant potential of kiwi fruit peel with DPPH (107 g/mL) and ABTS (258 g/mL) assays, the ethanolic extracts were determined to be excellent antioxidants. Many studies have been carried out specifically on the phenolics of the different kiwi fruit peels, mostly reaching the aforementioned conclusion. For example, in another study, the values varied depending on the ethanol percentage, thus 96% ethanol extracts had a phenolic concentration of up to 1.54 CtE mg/g, while for 50% ethanol extracts, the concentration decreased to 0.55 CtE mg/g [11; 37]. Moreover, metals appear in small quantities, but they do not cease to exist and add nutritional value. The most important percentages are given by manganese, potassium, calcium, and sodium (820 mg/100 g, 230 mg/100 g, 230 mg/100 g, and 90 mg/100 g, respectively) [34]. Comparing the aforementioned values with those in Table 3, it is made apparent that the different peels are more enriched in metals than the juice/flesh of the fruit. Carotenoids are also present, with a total amount of approximately 5.70 mg/g DW, with the main representative, as expected, being beta-carotene. In the same survey, the presence of lycopene is mentioned, a red pigment, mainly found in tomatoes, which is rarely found in kiwi fruit, especially in the species _A. deliciousa_. Based on the USDA, kiwi fruit has lycopene content that is non-negative [36; 40]. Chlorophylls were extensively tested in a project, which used extracts of peel and pulp from two species of the _A. deliciousa_ family, to make flour. The amount that was found in the peel extracts was 2-12 mg/100 g of the flour. The wide range of the amount is due to the growing conditions of the fruit and the stage of ripening, as the composition of the peel is time sensitive and changes photochemically with the passage of days [41]. The category of pigments also includes, as mentioned, anthocyanins, with cyanidin-3-O-sambubioside standing out in the skin of kiwi [12]. The last, but most important of the two components that remain to be analyzed, are vitamin E and plant/dietary fibers. The fiber content of kiwi is mainly due to its skin, rather than its flesh. That is, when kiwi juice or the raw part of the fruit without the peel is consumed, the fibers that were expected to pass through the body do not even reach the oral cavity, as they are discarded together with the peel. The main source of fiber is the carbohydrates contained and, especially the polysaccharides, in the skin, which help in the development of insoluble and soluble fibers.