Antioxidant components change
Lipid oxidation results in undesirable taste and flavor, and oils with high levels of lipid oxidation may lost nutritional value and generate toxic compounds (Peng et al., 2017; Yang et al., 2017). Antioxidant components in oils are thus important with respect to their dietary effects when consumed by humans. The past literature has reported that the natural antioxidant activity of refined oil may be lower than that of crude oil (Cicero et al., 2018). The result show that the α–tocopherol levels of the peanut oils generated by roasting at 120℃, 140℃, and 160℃ in this study were 72.33, 60.39, and 55.72 μg/g, respectively; their γ–tocopherol contents were 67.31, 72.91, and 70.51μg/g, respectively; their total phenolic contents were 18.31, 29.63, and 36.61 GAE μg/g, respectively; and their total flavonoid contents were 4.27, 3.96, and 4.44 QE μg/g, respectively (Table 3). In analyzing the phytosterol derivatives of the peanut oils, we identified squalene, campesterol, stigmasterol, stigmast–5–en–3–ol, and stigmasta–5,24(28)–dien–3–ol. The results further showed that as the roasting temperatures of the peanut oils increased, the levels of squalene, campesterol, stigmasterol, and stigmast–5–en–3–ol contained in the oils also increased.
The antioxidants in oils improve their oxidative stability and prevent their oxidative degradations, either by delaying the oxidation reaction by reacting with free radicals or by inhibiting the propagation step by reacting with alkoxy and alkyl peroxy radicals (Redondo–Cuevas et al., 2018). The results of this study further showed the DPPH clearing capacity of 2.5% peanut oil was 42.02~52.34%, while the FRAP was 151.22~328.64 Trolox μg/g (Table 3.). The oil generated by roasting at 160℃ had the best antioxidant capacity among the three varieties. Roasting could increase the release of phenols by bound phenolic compounds of peanuts brown skin, and the formation of Maillard reaction products like melanoidins (Taş and Gökmen, 2017; Różańska et al., 2019). These substances protect tocopherols from heat degradation during roasting. However, while phytosterols are important, the interactions between the antioxidants have synergistic effects (Chenet al., 2016).
The olfactory sensations of edible oils are very important, and these sensations combine the effects of an oil’s constituents on the taste and olfactory organs. The processing techniques used in producing oils would affect significantly the major volatile components concentrations, and hence determine their flavor quality. In this study, we detected 20 volatile compounds in the peanut oils generated by different roasting temperatures, including 7 N–heterocyclic compounds, 5 alkane compounds, 4 O–heterocyclic compounds, and 2 aldehyde compounds, as well as alcohol and sulfide (Table 4). Roasting affects the production of volatile compounds, particularly those derived from N–heterocyclic compounds (such as pyrazine and pyrrole), with the formation of alkylated pyrazines occurring via automatic condensation or condensation with other aminoketones of α–aminoketones in the Strecker degradation (Siegmund and Murkovic, 2004; Dun et al., 2019; Yang and Chian, 2019). Peanuts contain abundant amounts of essential precursors for the Maillard reaction, and the carbon skeleton of the pyrazines is derived in the Maillard reaction from carbohydrate degradation, while the pyrazinic nitrogen originates directly from amino acids (Siegmund and Murkovic, 2004; Dun et al., 2019). We found that some volatile compounds were formed during lipid oxidation, with dioxygen leading to the formation of hexanal, 2–hepten–1–ol, and nonanal. Hexanal emerged from linoleic acid, whereas nonanal is an oleic acid derivative that mainly imparts a fresh and fatty flavor (Haiyanet al., 2007). The results showed that high roasting temperatures induce the formation of volatile compounds through the Maillard reaction and lipid oxidation. In particular, high roasting temperatures result in the removal 2,3,4–trithiapentane, helping to prevent foul odors.