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To study the key aroma components and flavor profile differences of Cheddar cheese with different maturity and from different countries, the flavor components of 25 imported commercial Cheddar cheese samples in the China market were determined by gas chromatographymass spectrometry. The quality and quantity of 40 flavor compounds were analyzed by gas chromatographyolfactometry among 71 aroma compounds determined by gas chromatography-mass spectrometry. Combined with odor activity value calculation, principal component analysis (PCA) was conducted to analyze the relationship among 26 flavor compounds with odor activity values >1 and the maturity of Cheddar cheese. The PCA results showed significant differences between the group of mild Cheddar cheese and the groups of medium Cheddar cheese and mature Cheddar cheese, and no significant differences were observed between medium Cheddar cheese and mature Cheddar cheese. According to the results of PCA and consumers' preference test, representative Cheddar cheese samples with different ripening times were selected for the flavor profile analysis. Partial least squares regression analysis was conducted to obtain the relationship between sensory properties and flavor compounds of different Cheddar cheeses. Based on partial least squares regression analysis, 1-octen-3-one, hexanal, acetic acid, 3-methylindole, and acetoin were positively correlated with milky, sour, and yogurt of mild Cheddar cheese. Dimethyl trisulfide, phenylacetaldehyde, ethyl caproate, octanoic acid, and furaneol and other compounds were positively correlated with fruity, caramel, rancid, and nutty notes of the medium and mature Cheddar cheeses.

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Casein-derived antioxidant peptides by using microbial proteases have gained increasing attention. Combination of two microbial proteases, Protin SD-NY10 and Protease A “Amano” 2SD, was employed to hydrolyze casein to obtain potential antioxidant peptides that were identified by LCMS/ MS, chemically synthesized and characterized in a oxidatively damaged HepG2 cell model. Four peptides, YQLD, FSDIPNPIGSEN, FSDIPNPIGSE, YFYP were found to possess high 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging ability. Evaluation with HepG2 cells showed that the 4 peptides at low concentrations (< 1.0 mg/ml) protected the cells against oxidative damage. The 4 peptides exhibited different levels of antioxidant activity by stimulating mRNA and protein expression of the antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px), as well as nuclear factor erythroid-2-related factor 2 (Nrf2), but decreasing the mRNA expression of Kelch-like ECH-associated protein 1 (Keap1). Furthermore, these peptides decreased production of reactive oxygen species (ROS) and malondialdehyde (MDA), but increased glutathione (GSH) production in HepG2 cells. Therefore, the 4 casein-derived peptides obtained by using microbial proteases exhibited different antioxidant activity by activating the Keap1-Nrf2 signaling pathway, and they could serve as potential antioxidant agents in functional foods or pharmaceutic preparation.