In the early s in addition to above mentioned

In the early 2000s, in addition to above-mentioned plant derived compounds, several peptide fragments from protein sources (e.g. gelatin, egg yolk, meat, fish, chicken, legumes) have been also confirmed as potential natural antioxidants and extensive studies about bioactive peptides have been conducted (Gibbs et al., 2004, Rajapakse et al., 2005, Wu et al., 2003). The antioxidant activity of proteins was thought to encompass both free radical scavenging by amino ampa receptors residues and chelation of prooxidative transition metals. Sulfur containing amino acids such as cysteine and methionine were defined as antioxidant compounds with their radical scavenging ability via their sulfhydryl groups (Elias, McClements, & Decker, 2005). Moreover, it was shown that antioxidant compounds could be occurred during food processing. Franke and Iwainsky (1954) firstly demonstrated that Maillard reaction products (MRPs), occurred as a consequence of reaction between carbonyl groups and amino groups, could exert antioxidant activity in food systems. Then, the mechanisms underlying the antioxidant activity of melanoidins were reported as: 1) trapping of positively charged electrophilic metabolites, 2) scavenging of oxygen radicals, 3) metal chelation, and 4) synergism (Lingnert and Eriksson, 1980, Morales and Jiménez-Pérez, 2004, Morales et al., 2008). Rufián-Henares and Morales (2007) isolated melanoidin fractions from several amino acid-glucose model systems and indicated that low molecular weight compounds bounded to melanoidins exert antioxidant activity higher than those of the pure melanoidins. Melanoidins content of heat-treated foods (e.g. coffee, breakfast cereals, bread, biscuits, chocolate, balsamic vinegar) and their antioxidant potential were investigated in several studies (Borrelli et al., 2003, Delgado-Andrade and Morales, 2005, Delgado-Andrade et al., 2005, Fogliano and Morales, 2011). Taking into account their daily intake, it was reported that melanoidins contributed 20.2% to the daily antioxidant capacity intake (Pastoriza & Rufián-Henares, 2014). It was also highlighted that although breakfast cereals contained more melonoidins than different types of coffee, coffee melanoidins were more antioxidant than others, this leading to higher contribution of coffee melanoidins to daily antioxidant intake. Table 2 gives the total antioxidant capacity of selected foods according to the USDA Food Composition Database, and their major antioxidant compounds (Haytowitz & Bhagwat, 2010). It is obvious that diets rich in cereal grains and protein sources in addition to fruits and vegetables, could be considered as excellent antioxidant sources. As another perspective, it is important to consider the serving size as well as concentrations to make an overall evaluation about antioxidant potential of foods. Wu et al. (2004) categorized the foods into four groups ranked by their hydrophilic or lipophilic oxygen radical absorbance capacity (ORAC) per serving. It was reported that berries, bean, apple, cherry, black plum, walnut, broccoli raab, red cabbage, pistachio, red grape were among the high antioxidant sources category; pumpkin, onion, tomato, corn, bread, lettuce, cucumber, celery, watermelon were in the low antioxidant sources category. Some researchers considered that spices were also important natural antioxidants with their physiological health effects although they contribute daily diet in small quantities (Srinivasan, 2005). Antioxidants in food occur as a stable mixture of two or more antioxidant individuals and create a complex network. To achieve a complete understanding of the cooperative action of antioxidant and its effects on total antioxidant mechanism, interaction among these antioxidants were also started to investigate (Pedrielli & Skibsted, 2002). It was reported that combination of α-tocopherol and flavonoids or ascorbic acid showed higher antioxidant potential than the sum of their antioxidant effects seperately due to regeneration of antioxdiant by another and this type of interaction called as synergism (Barclay et al., 1985, Jørgensen et al., 1999). However, it was noticed that some combinations of antioxidants such as α-tocopherol combination with rosemary extract or BHA combination with peanut extract, caused less antioxidant capacity than simple addition of their individual effects, called as antagonistic interaction (Banias et al., 1992, Duh and Yen, 1998). Over the last decade, numerous studies focusing on antioxidant synergies between ascorbic acid, tocopherols, carotenoids, and phenolic compounds were performed to understand the synergy mechanism both in foods and in in vitro physiological conditions (Wang & Zhu, 2017). It was also considered that the concentration of antioxidants determined their interaction type (antagonistic or synergistic) with other antioxidants. Phenolic compounds exert their antioxidant activity at relatively low concentrations while, at higher concentrations, since they themselves were susceptible to oxidation, they could behave as pro-oxidants due to their involvement in initiation reactions (Robards et al., 1999). Therefore, several studies have investigated the relationship between concentration and antioxidant capacity of phenolic compounds. It is better to bear in mind that the favorable dietary antioxidant doses to maintain the balance between oxidation and anti-oxidation processes must be determined.