The effects of solvent and time of extraction on the antioxidant properties and phenolic profiles of 13 herbs and spices used for food seasoning and preservation were investigated. The profiles of phenolics in plant material were analysed by UPLC, and the chemometric approach was used for the deeper evaluation of relationships between phenolic compounds and the antioxidant properties of the samples. The best extraction conditions enabling to obtain the highest total phenolics and the antioxidant activity were: long time extraction (24 h) with 50% aqueous ethanol. The most potent antioxidant was clove with the ABTS—2495.85 µmol/g, DPPH—1443.35 µmol/g and FRAP—1310.91 µmol/g, which resulted from the high total phenolic content (TPC—167.22 mg GAE/g) and total flavonoid content (TFC—26.22 mg QE/g). Wide variations in the antioxidant activity (ABTS: 7.31—2495.85 µmol/g) and TPC (0.87–167.22 mg GAE/g) were observed with black seed extract showing one of the lowest values of the parameters studied. High linear correlation (above 0.95) calculated for the parameters showed significant contribution of phenolics to the antioxidant activity of spices. UPLC analysis together with PCA (principal component analysis) confirmed this relation and ABTS, FRAP, DPPH, TFC, TPC, total phenolic acids, protocatechuic acid,t-cinnamic acid,p-hydroxybenzoic acid determined sample distribution along PC1 (principal component 1), whereas total flavonoids determined by UPLC, kaempferol, catechin and luteolin along PC2. All samples were discriminated with 100% classification’s propriety according to GDA (general discrimination analysis) which proved huge diversity among phenolic profiles and the antioxidant properties of tested samples.

The influence of stereochemistry on the radical scavenging activity of catechins was investigated by studying the effect of pH on the antioxidant properties of catechin epimers. The difference in the pH-dependent Trolox equivalent antioxidant capacity (TEAC) profiles was observed only in case of gallocatechin gallate (GCG) and epigallocatechin gallate (EGCG), indicating the influence of steric structure on the TEAC antioxidant activity of these galloyl moiety-containing catechins. Based on comparison of the pH-dependent TEAC values to theoretically calculated parameters, including homolytic OH bond dissociation energy and ionization potential (IP) as well as theoretically predicted structures of the most stable monoanions of GCG and EGCG, it was concluded that due to steric hindrance in GCG molecule, the IP value of GCG monoanion increases reflecting lower radical scavenging capacity of GCG in comparison with EGCG. It results in the difference in the pH-dependent TEAC profiles of these two catechin epimers at pH above 3.5. This effect does not occur for other pairs of catechin epimers of this study.

This study evaluated the effect of increasing manganese (Mn) nutrition on the content of antioxidative compounds such as vitamin C, lycopene and polyphenols, and the antioxidant activity of tomato (Lycopersicon esculentum Mill., cvs ‘Alboney F1’ and ‘Emotion F₁’) fruit. Plants were grown in rockwool using a nutrient solution with the following content of Mn (mg dm^–3): 0.0, 0.3, 0.6, 1.2, 2.4, 4.8, 9.6 and 19.2. The level of vitamin C and lycopene decreased with the increasing Mn nutrition. Since the colour of fruits was correlated with the change in carotenoid content, the decrease in lycopene content promoted the reduction of redness and increase of yellowness of fruits. However, total polyphenol content and antioxidant activity significantly increased when plant were exposed to toxic levels of Mn. Observed changes could be the result of the oxidative stress induced by high concentrations of Mn. Polyphenolic compounds play a crucial role in the plant’s response to Mn stress and affect predominantly the total antioxidant properties of fruits, which could be used as a source of phenolics. Moreover, total phenolic content measurement, as an easy and inexpensive method, could be used as an indicator of Mn-induced stress in fruits of tomato.