Phenolic compounds, sweetness and amino acids content of two onion (Alliumcepa L.) cultivars, white (Giza-6) and red (Giza-20) onions and their relationship with antioxidant activities were assessed. Data analysis showed that the white onion variety has higher values for protein, ash, fiber, total carbohydrates and total energy while red once has the lowest dry matter content (12.61%). Regarding mineral and vitamins levels, the white onion variety represents higher levels of K, Ca, Mg, P, Zn, Cu and Mn while Fe, S, Se and vitamin C were higher in red variety. A white variety shows higher value of total amino acids (1460 mg/100g FW) than red once [1345 mg/100g FW) and the opposite was observed for sulphur-containing amino acid (cysteic, S-carboxymethyl cystein (S-CM cystein), cystine and methionine].
The total single sugars detected in red onion (4.78 g/100g FW) are superior to in white (3.91 g/100g FW). Glucose and fructose levels are higher in the red onion than the white once and the opposite with sucrose. Concerning pungency, white variety can be classified as very sweet (6.24 µmol pyruvic acid/g FW) andred as sweet (8.37 µmol pyruvic acid/g FW). A negative correlation in sucrose and glucose and a positive correlation in fructose, sulphur, vitamin C, sulphur-containing amino acids and phenolics content with pungency werealso observed. The phenolic acids, flavonols, anthocyanins and total phenolics content in red variety(81.59, 70.38, 7.56 and 187.17 mg/100g FW, respectively) were higher than for white once (72.47, 32.49, 4.90 and 131.65 mg/100g FW, respectively). Consequently, antioxidant activity was higher for the red variety. Statistical analysis indicates that total phenolic compounds beside other factors including Se and sulphur-containing amino acid contents play the major role in the antioxidant activity of onion bulbs.
Conclusively, the present data indicates that white onion variety shows higher nutritional value while the potential healthbenefits related to the presence of antioxidant compounds and other factors were higher in onion red variety.