T = Measurement temperature; CAME = canola oil-FAME; PME = palm oil-FAME; SME = soybean oil-FAME; Q10 = oxidation reactivity ratio; Avg = average value; RF = response factor.
a Average values from n = 2 replicate measurements. Standard deviations (SD) in parentheses (blank if SD = 0).
b Calculated from Eq. 12.
c Calculated from Eq. 13.
The ‘Q10’ ratio is known as the relative oxidation reactivity for fatty derivatives (Frankel, 2005c; Ghosh et al., 2019; Pereira et al., 2015). For oxidation, this ratio is typically between 2 and 3, indicating that a 10 °C increase in T decreases IPR by 50-66 %. Four FAME in Table 1 exhibited average Q10 ratios of 2.0-2.7, while MeC18:2 had an average Q10 = 1.82, a value that was only 9.2 % below the lower threshold. An examination of the individual Q10 data for MeC18:2 shows that the ratio tended to decrease as T increased, with the Q10 < 2.00 occurring at higher temperatures. This may indicate that MeC18:2 was impacted by thermal decomposition as well as oxidative degradation at higher temperatures.
The IPR of all five of the FAME studied could only be ranked at T = 100 °C. This ranking, in descending order was:
PME > CaME > MeC18:1 > SME > MeC18:2 (14)
where the IPR of PME was only slightly greater than that of CaME, with a probability (p ) = 0.298 that the two values were equivalent. In general, when IPR values were similarly ranked for two or more FAME (two FAME at 80 °C; four FAME at 90, 110 and 120 °C; three FAME at 130 °C), they followed this ranking. The only exception was observed for CaME and PME, where CaME had higher IPR values at T = 110-130 °C. In this T-range, deviations between IPR values for CaME and PME were significant (p ≤ 0.012). It was noted earlier that analysis of the fuel properties of PME (Table S1 in the supporting information) suggested the sample experienced a small degree of degradation before its use in the study. This is the most likely explanation for the otherwise unexpected observation that the IPR of CaME was greater than that of PME, especially at T ≥ 110 °C.