3.4. Thermal stability of Senjed peel aqueous extracts
Effect of heating (at 185 ℃) on thermal stability of Senjed peel aqueous
extracts for different intervals is shown in Fig. 1. Up to 30 min
heating time, extracts were almost stable, but after 40 min a gradual
decrease in antioxidant activity was observed with the increase in
heating period. The decrease in antioxidant activity was not significant
(p<0.05 ) up to 40 min but became pronounced after 50
min heating time. At the 80 min heating interval, extracts exhibited
34.37% antioxidant activity (59.08% remaining antioxidant activity).
Iqbal & Bhanger (2007) reported that the decrease in antioxidant
activity methanolic extracts of garlic was not significant
(p< 0.05 ) up to 40 min but became pronounced after 50
min heating time at 185 ℃. The stability of some phenolic antioxidants
against thermal oxidation was reported by Hamama & Nawar (1991) as
follow: BHT > PG > BHA > TBHQ
after 60 min heating at 185 ℃. Comparing the results, it is observed
that the thermal stability of Senjed peel aqueous extracts is higher
than synthetic antioxidants. Liu et al. (2016) reported that as heating
temperature and heating time increased, losses of TBHQ in palm oil
increased due to increasing volatility of TBHQ. Losses of antioxidant
activity, after longer heating times at high temperatures, may be due to
vaporization and volatility, and various chemical reactions occurring
during oxidation, leading to the formation of hydroperoxides,
hydrolysis, polymerization, chemical decomposition, which lead to
deterioration in oils and fats giving rancidity (Liu et al., 2016;
Warner & Knowlton, 1997). These results reveal that Senjed peel aqueous
extracts is a potential source of natural antioxidants, which is
applicable in food systems even at high processing temperatures.