Tunisian tomato by-products, as a potential source of natural bioactive compounds

Abstract Consumption of tomato and tomato products is positively related to the reduction in cardiovascular disease and several types of cancer, thanks to the presence of natural compounds, such as antioxidants. Peels and seeds fractions of tomato, collected after industrial processing in Tunisian industries, were analysed for nutritional and antioxidants composition in perspective of its utilisation. Proximate composition, fatty acids profile, carotenoids, such as lycopene and beta-carotene, polyphenols contents, demonstrated the good potential of these residual products as a source of natural compounds, useful for food and nutraceuticals applications.


Introduction
Food wastes account as a source of valuable nutraceuticals and deal with the aims of feeding fast growing population in twenty-first century (Galanakis 2012). Perspectives originate from the enormous amounts of food-related materials that are discharged worldwide and the existing technologies, which promise the recovery, recycling and sustainability of high-added value ingredients inside food chain (Galanakis 2012).
Tomato (Solanum lycopersicum) is one of the most widely cultivated vegetables in the Mediterranean countries, and it is well known that is one of the principle sources of the beneficial carotenoids, such as β-carotene and lycopene that may protect against cardiovascular diseases and several types of cancer (Pannellini et al. 2010). These epidemiological leads have stimulated a number of researches aimed to the optimisation of extraction of these compounds from residual material of tomato processing (Kalogeropoulos et al. 2012). Tunisia is one of the most important producers of canned tomato in the Mediterranean countries. In 2011, 865,000 tons of tomatoes were processed and 86,500 tons of wastes are disposed (Tunisian Ministry of Industry). The processing of tomatoes generated 10-30% of by-products represented mainly by peel and seeds (King & Zeidler 2004). For these components was reported high nutritional value represented by essential amino acids and minerals, a high content of unsaturated fatty acid content with over 50% linoleic acid (Lazos & Kalathenos 1988). In most of the previous studies, antioxidants have been measured, mainly in whole tomatoes or processed tomato products (Lenucci et al. 2006;Pinela et al. 2012). In general, there is a lack of studies on the levels of antioxidants in the peel and seed fractions of tomato. The aim of this paper was to evaluate whether peels and seeds fraction of tomato pomace TP, collected after industrial processing of tomato, can be exploited as source of bioactive compounds, for nutraceutics applications.

Proximate composition
The results of proximate composition are shown in Table S1. Significantly higher levels of lipids and proteins (26.53 and 30.23 g/100 g dW respectively) were detected in the seeds fraction of TP. These values fall in the range reported for the composition of seeds from tomato processing (eller et al. 2010) and are consistent with results reported by elbadrawy and Sello (2011) and by Navarro-González et al. (2011). The ash content of peels and seeds is in accordance with values of Rossini et al. (2013).

Fatty acids composition
The fatty acids content was determined in seeds that are the main sources of oil (Table S2). The profile is dominated by total unsaturated fatty acids that account for 74.8%. In this class, high percentage of oleic acid, C 18:1ω-9 and linoleic acid, 18:2ω-6 was founded. Results are in accord to data of tomato by-products from Botineṣtean et al. (2012). Saturated fatty acids that account for 25.18% are dominated by palmitic acid 16:0, which represents the 18.2% (Table S2). The differences in individual contents of fatty acids may be due to the cultivars origin and environmental factors. The high content of essential linoleic fatty acid confirms the high nutritional value of these components that can be considered a good source of edible oil, especially for children.

Antioxidant composition
epidemiologic studies demonstrated that especially processed tomato products are associated with a 30-40% reduction in prostate cancer risk (Pannellini et al. 2010). our results showed that the skin contains the highest levels of lycopene, respect to the seeds (Table 1) in accord to Sharma and Le Maguer (1996), who showed that lycopene is mostly associated with the water insoluble fraction and the skin. Lycopene showed higher amount comparing to the literature (Toor & Savage 2005;Kaur et al. 2008). The variation in the lycopene content of tomatoes is probably due to the differences in their growing conditions. It has also been demonstrated that some extraction parameters can have an impact on lycopene recovery. Nunes and Mercadante (2004) extracted lycopene from tomato skin, using methanol and ethanol and high temperature and their experimental values ranged from 2.55 to 9.85 mg/100 g. Ilahy et al. (2016) recovered from 423.7 to 893 mg/kg of lycopene from peels of different Tunisian cultivars. β-carotene is the precursor of vitamin A, and it is a second abundant carotenoid in tomatoes and tomatoes' by-products, after lycopene. Results show higher value of β-carotene in the peel fraction (Table 1). The content of β-carotene in peel fraction evaluated in our study was higher than other studies carried on tomato fruit (Pinela et al. 2012).
Regarding polyphenolic compounds, it tend to accumulate in the dermal tissues of plant to protect against ultraviolet radiations, to act as attractants in fruit dispersal, and as chemicals defence against pathogens and herbivorous. high levels were detected both in the peel and seed fractions (Table 1). Results are in agreement with those of Chérif et al. (2010) since they cited a value of 151.60 mg/100 g from tomato waste. George et al. (2004) reported that the phenolic content in the skin of different field grown tomato genotypes ranged from 10 to 40 mg catechins equivalents/100 g. The values of phenolics in the skin and pulp of New Zealand tomatoes fall in the same range (Toor & Savage 2006). Tomatoes were identified as the most important suppliers of phenolics in human diet. elbadrawy and Sello (2011) found that the essential phenolic acids in peel fraction of tomato are caffeic acid, vanillic acid, catechin and gallic acid. other studies report that tomato peel contains several flavonoids with beneficial effects for human health such as rutin and naringenin (Navarro-González et al. 2011). Ferreres et al. (2010) indicated higher values of phenolics in the seed fraction, mainly flavonoids such as kaempferol, quercetin, rutin and rhamnoside. Rutin has been associated with markedly decreased hepatic and cardiac levels of triglycerides (Fernandez et al. 2010) and with anti-inflammatory properties. Naringenin has been suggested as an antioxidant, an anti-inflammatory and a regulator of fat metabolism and sex hormone metabolism. Finally, quercetin has been reported to exhibit antioxidative, anticarcinogenic, anti-inflammatory, anti-aggregation and vasodilating effects.
TP contains not only high amounts of carotenoids but it also contains tocopherols (Table 1). Tocopherol is among the most biologically active compounds. It is essential for normal growth and development, and its deficiency often leads to clinical abnormalities (Lenucci et al. 2006). Tocopherols due to their long side chain and aromatic rings are apolar molecules, and their extraction was carried out peel and seeds fraction and they have been extracted with different organic solvents. Recovery of tocopherols from peel and seeds fraction was, respectively, between 1.62 and 1.31%.

Antioxidant activity
To our knowledge, this is the first time that the antioxidant activity in the peel (TP) and seeds (TS) fraction of tomato by-products are reported, as dPPh and reducing power (Figures 1  and 2). Scavenging the stable dPPh radical model and reducing power determination are the widely used methods to evaluate the whole antioxidant activity of extracts (Chung et al. 2005). Among polyphenols and carotenoids, the highest antioxidant activity was reported for peel extracts, respect to the seeds (p < 0.05). Among the tocopherols, the higher value of  the radical inhibition was recorded in the seeds extract (p < 0.05) (Figure 2). The total reducing power resulted higher in the carotenoid extract from seeds, respect to the peel extract (p < 0.05) and in tocopherols extracts (Toc TP) (p < 0.05). due to the higher levels of all major antioxidant compounds, the skin fraction of tomato by-products had a significantly higher antioxidant activity compared to seeds fraction. Antioxidant activity in both the hydrophilic and lipophilic extracts of tomatoes was measured, and it was found that the lipophilic extracts of skins and seeds were the major contributor to the total antioxidant activity.
This work was carried out to study the nutritional and the chemical composition of wastes deriving from the tomato processing line then separated into peels and seeds. Results show that tomato by-products should be regarded as a potential source of bioactive compounds with added value useful for nutraceuticals and pharmaceuticals applications. Lycopene is the major component in tomato peels. Seeds are rich in proteins and fatty acids. These results indicate that lycopene, essential fatty acid and proteins can be extracted with substantial success from by-products of the Tunisian tomato processing industry. According to the Food and Agriculture organization, almost one-third of the edible parts of food produced for human consumption (accounts about 1.3 billion tn/year) gets lost or wasted globally, reflecting not only the food processing wastes, but also the 'food losses' (Galanakis 2012). Tomato by-products from Tunisian value chain should be regarded as a potential source of natural compounds and treated not as a waste, but as a resource of oils and antioxidants, for nutraceuticals applications