# Phytochemical and Functional Properties of Fruit and Vegetable Processing By-Products
## 3 Results
### Bioactive Compounds and Antioxidant Capacity  
Vegetable and fruit by-products were compared for their bioactive compound content in terms of soluble phenolic content (TPC), flavonoids (FLC), carotenoids (TCC), chlorophyll a (Chl A), and chlorophyll b (Chl B), and their antioxidant activity (AA). The statistically significant differences of the phytochemical profile (\(p\) < 0.05) are shown in Table 2.  
Phenolic compounds are biologically active plant metabolites involving phenols with one or more hydroxyl groups, and they are effective in neutralizing free radicals [41]. Moreover, they can exhibit anticancer, antimicrobial, and antimutagenic properties [42]. However, the bioaccessibility and the bioavailability of phenolic compounds strongly depend on the microstructure of fruits and vegetables. Higher benefits for human health can be obtained in free-form macromolecules, rather than in conjugated form [43]. In our study, TPC content ranged from the 1934 \(\pm\) 124 mg 100 g\({}^{-1}\) GAE dw of black carrot peel and the 1915 \(\pm\) 43 mg 100 g\({}^{-1}\) dw of mango peel to the 8 \(\pm\) 1 mg 100 g\({}^{-1}\) GAE dw of orange carrot peel. A considerable amount of TPC was also found in samples of black plum peel (1576 \(\pm\) 33 mg 100 g\({}^{-1}\) GAE dw) and apple peel (1228 \(\pm\) 25 mg 100 g\({}^{-1}\) GAE dw). Generally, fruit and vegetable peels show a high level of phenolic compounds, especially for root vegetables [44]. However, phenolic accumulation can depend on species, varieties, climatic conditions, vegetative stage, and agronomic management [45]. Flavonoids are a class of polyphenic secondary metabolites found in plants, characterized by the ability to scavene free radicals. They can protect against cardiovascular diseases and certain cancers [41]. Apple peel and black plum peel showed the highest values of flavonoid content (2434 \(\pm\) 124 mg 100 g\({}^{-1}\) CAE dw and 2396 \(\pm\) 75 mg 100 gCAE dw, respectively), whereas the orange carrot pomace and parsnip showed the lowest (6 \(\pm\) 0 mg 100 g\({}^{-1}\) CAE dw and 5 \(\pm\) 0 mg 100 g\({}^{-1}\) CAE dw, respectively). In general, vegetable and fruit peels are rich source of flavonoids; however, industrial processing could have a negative impact on FLC, strongly reducing it, especially if subjected to high temperatures [46]. Fruit and vegetable by-products provide a wide range of natural bioactive pigments (secondary metabolites), including carotenoids and chlorophylls. These pigments can be used as natural colorants in food and beverages preparations, as dietary supplements and pharmaceuticals, or as antioxidants with positive effects on human health [47]. Carotenoids are a fat-soluble group of pigments which can vary from yellow to orange and red. They can be associated with several beneficial effects, such as the reduction in the risk of cardiovascular diseases, cancer, neurovegetative diseases, macular degeneration, diabetes, and liver diseases [48; 49; 50]. Chlorophylls are oil-soluble pigments with a green color, and they can have two different structures: chlorophyll a, with a methyl (-CH\({}_{3}\)) group at the 7-carbon position and a blue-green color, and chlorophyll b, with an aldehyde (-CHO) group at the 7-carbon position and a blue-yellow color [51]. Chlorophylls a and b can exhibit strong antioxidant, anti-inflammatory, and antimutagenic properties [52]. In the present study, the total carotenoid content ranged from the 5.8 \(\pm\) 0.9 g\({}^{-1}\) 8 CE dw of parsnip peel to the 753.2 \(\pm\) 3.1 ug g\({}^{-1}\) 8 CE dw of orange carrot peel. The highest value of chlorophyll a was found for cucumber peel (21.50 \(\pm\) 0.30 ug g\({}^{-1}\) ChIAE dw), followed by black carrot peel (3.80 \(\pm\) 0.20 ug g\({}^{-1}\) ChIAE dw), whilst the lowest was for black plum pomace (0.13 \(\pm\) 0.03 ug g\({}^{-1}\) ChIAE dw) and parsnip peel (0.11 \(\pm\) 0.04 ug g\({}^{-1}\) ChIAE dw).