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.