Assessment of the Quality of Heated Oils by Differential Scanning Calorimetry

Abstract

Differential scanning calorimetry (DSC) was used to follow changes in the thermal characteristics of commercial frying oils (sunflower seed, colza, and groundnut) during heating at 180 °C for up to 10 h. DSC tracings of oils scanned from 10 to −80 °C were characterized by a single crystallization peak (CP) at −43 to −48 °C. Heating of oils resulted in a progressive shift of the CP to lower temperatures and reduced enthalpies of crystallization (EC). The temperature of the CP decreased as much as 27 °C, and the EC was reduced by almost 90% after heating for 10 h. These changes in thermal characteristics correlated well with the appearance of polar compounds, the increase in viscosity, and color changes of the oil. DSC is a fast method of analysis, requires small samples (15−60 mg) with minimal preparation, and may be implemented directly in fried products. Keywords: Differential scanning calorimetry; oil; frying; quality; enthalpy; viscosity; color

Impact of Frying on Fatty Acid Profile and Rheological Behaviour of Some Vegetable Oils

“…Fresh oils showed relative Newtonian behaviour with correlation coefficients greater than 0.99 at 25°C and slight non-Newtonian behaviour after 4, 8, 12 and 16 h of frying. These results are in agreement with those of Gloria and Aguilera [27]; Santos et al [28] and Quinchia et al [29] who found that the flow behaviour of some edible vegetable behaved as non-Newtonian fluids. Also, the results are in agreement with Huang and Sathivel [30] who reported that the crude salmon oil behaved as non-Newtonian fluids.…”

“…The tendency of viscosity to increase during frying of the oil has been found to correlate with formation of polymers [27]. The increasing in viscosity indicates that polymers, which are the higher molecular weight fraction of the degradation products, are increased by increasing frying time.…”

Heating effects on some quality characteristics of date seed oil

“…The presence of two distinct peaks may be due to the appearance of a second polymorphic form. This hypothesis is supported by the fact that the oxidized product extracted from heated sunflower oil did not show any thermal transitions in the selected temperature range (Gloria & Aguilera, 1998). In microwave heated oil, the oxidation products such as polar compounds, hydroperoxydes, aldehydes, dimers and polymers may disturb the rearrangement of different polymorphic form in oils (Tan, Che Man, Jinap, & Yussof, 2002).…”

“…It has been also reported that the formation of oxidation products that do not crystallise in the scanning temperature range was responsible for the reduction in enthalpy of crystallisation with frying time (Gloria & Aguilera, 1998). The reduction in the enthalpy of crystallisation with frying time was attributed to the formation of degradation products that do not crystallize in the scanning temperature interval.…”

Effect of frying on the rheological and chemical properties of palm oil and its blends

“…Fats and oils are largely responsible for the pleasant creamy taste of foods; therefore, its physiological characteristic is important (Belitz et al 2009). Frying causes oil to undergo a wide range of reactions, namely hydrolysis, oxidation, isomerization, and polymerization due to high temperature and absorption of water and oxygen (Go'mez-Alonso et al 2003;Gloria and Aguilera 1998). The deteriorated oil has nutritional and toxicological implication on human health since the toxic substances may accumulate in oil through the food stuff fried in it; final products fried in deteriorated oil possess unpleasant taste, odor and texture (Tena et al 2009).…”