Han Zhang

Camellia oil with a high oleic acid content is widely used for frying. To comprehensively describe the quality change in camellia oil during frying, the changes in composition, deterioration indicators, and volatile profiles were investigated. The results showed that tocopherols mainly degraded in the early stage of frying, followed by unsaturated fatty acids (UFA). This caused the carbonyl value and total polar compounds level to significantly increase. Moreover, frying promoted the accumulation of volatile compounds in terms of type and abundance, especially aldehydes, which are related to the degradation of UFA. Principal component analysis showed that the frying of camellia oil was divided into three stages. First, the camellia oil with a heating time of 2.5–7.5 h showed excellent quality, where tocopherol played a major role in preventing the loss of UFA and was in the degradation acceleration stage. Subsequently, as tocopherol entered the degradation deceleration stage, the quality of camellia oil heated for 10.0–15.0 h presented a transition from good to deteriorated. Finally, tocopherol entered the degradation stagnation stage, and the quality of camellia oil heated for 17.5–25.0 h gradually deteriorated, accompanied by a high level of volatile compounds and deterioration indicators. Overall, this work comprehensively determined the deterioration of camellia oil during intermittent frying and offered valuable insights for its quality evaluation.

To establish a practical model for evaluating the oxidation of frying oil using aldehydes, the aldehydes of 10 commercial oils during frying at 180 °C were identified using headspace-gas chromatography/mass spectrometry, and the changes of common aldehydes and their correlation with carbonyl values (CV) were analyzed. The results showed that the total peak area of aldehydes increased significantly with heating time, which was related to the fatty acid and tocopherol contents of the oils. There were four common aldehydes with different trends during frying, namely, pentanal, hexanal, (E)-hept-2-enal, and nonanal. Moreover, pentanal with a high correlation with CV was selected as the quality evaluating index of frying oil due to its stable accumulation over time. Based on the linear fitting relationships between CV and pentanal, as well as the initial content ratio of linoleic acid to palmitic acid and total tocopherols in oils, a predictive model was established for evaluating the quality of frying oils with high precision and non-reagent by using mass spectrometry. In summary, this work provides theoretical support for using aldehyde as the quality evaluation index of frying oil and provides a new idea for evaluating oil deterioration from the perspective of volatile compounds.

The volatile compounds present in ten edible oils (six refined, two roasted, and two natural pressed) with different fatty acid compositions were measured to study volatile profile and flavor characteristics. The volatile profile of the refined oils was found to be linked to the fatty acid composition. The percentage of alcohol in the volatile profiles, varied from 38.59 to 74.37%, was smaller in refined oils with more linoleic and linolenic acid than those in oils with more oleic acid, while the opposite trend was observed for the percentage of aldehyde, ranging from 20.27 to 37.19%. The total peak area and number of volatile compounds detected in roasted oils were much greater relative to those in natural pressed oils. Ten characteristic compounds were detected exclusively in roasted sesame oil, and seven compounds were found to be unique to roasted perilla oil. 2-Methylpropan-1-ol and hexanal were the only volatile compounds common to all ten oils.

The decomposition of tocopherol homologs and their influences upon the decomposition of unsaturated fatty acids were explored by studying the reduction in tocopherol homologs and fatty acid contents in 10 commercial edible oils during deep frying at 180 °C. The results showed that the decomposition of total tocopherols was faster than for the total unsaturated fatty acid (TUFA). With the decomposition of TUFA, the decomposition rates of tocopherol homologs were in the order γ-tocopherol > α-tocopherol > δ-tocopherol in 10 oils, regardless of the initial content and composition of tocopherol and fatty acids. It was also found that higher oil unsaturation led to slower tocopherol decomposition during frying, with more tocopherol remaining after the procedure. The result of multiple regression analysis (ΔTUFA_t = 0.049 Δα_t + 0.044 Δγ_t + 0.052 Δδ_t + 0.733) revealed that the effects of tocopherol homologs on the decomposition of unsaturated fatty acid in 10 oils during frying were in the same order, that is, γ-tocopherol > α-tocopherol > δ-tocopherol.

The decomposition of unsaturated fatty acids and tocopherols in 10 commercial edible oils during deep frying was investigated. The dominant tocopherol in oils rich in polyunsaturated fatty acids (PUFAs) was γ-tocopherol, except for natural perilla oil (δ-tocopherol dominant), and the main tocopherol in oleic acid-rich oils was α-tocopherol. The PUFA-rich oils had higher tocopherol contents than the oleic acid-rich oils. Both the reduction rate of total unsaturated fatty acid (TUFA) and total tocopherol (TToc) were linear with frying time (t). The decomposition rate of TToc is faster than that of TUFA since the slope values obtained from fitting equations (Y = k t) k_TToc (1.520–14.483) were obviously larger than for k_TUFA (0.155–0.270). By establishing a dynamic decomposition index, unsaturated fatty acids and tocopherol in oils showed dynamic decomposition over multiple frying cycles. The obtained results showed that decomposition characteristics of oils are related to their fatty acid compositions.

The changes in the fatty acid composition of eleven edible oils (soybean, rapeseed, natural and refined sesame, corn, safflower, olive, rice bran, natural and refined perilla, coconut) are investigated during deep frying. French fries are intermittently fried for 5 h each day for 6 consecutive days at 180 °C in these oils. The results suggested that all of the oils contained large amounts of unsaturated fatty acids (83.49–95.28%) before deep frying, except for coconut oil (7.13%). With the extension of frying time, the content of unsaturated fatty acids in the oils decreased linearly (Y = −AT + 100, R² = 0.955–0.990) except in the coconut oil. The equation (A = 0.113X + 0.203, R = 0.836) can be used to determine the decomposition rate of unsaturated fatty acids (A) in a variety of unsaturated fatty acid‐based oils. The ratio of palmitic acid to oleic acid in the oils before deep frying (X) has an important impact on the degradation of a variety of unsaturated fatty acid‐based oils, and can be used as an evaluation index for predicting the degradation rate of such oils before deep frying.Practical Applications: Changes to the unsaturated fatty acid content of a variety of unsaturated fatty acid‐based oils during deep frying can be predicted using this novel evaluation index. A clear understanding of the deterioration process of frying oils can be obtained when the index is combined with other indicators of deterioration. The index can facilitate timely judgments to prevent the over‐use of frying oils according to the waste oil standards of individual countries. This would maximize the utilization of oil while reducing the production of substances that are harmful to the human body.With the extension of frying time, the content of unsaturated fatty acids in the oils decreased linearly (Y = −AT + 100). The equation (A = 0.113X + 0.203) can be use to determine the decomposition rate of unsaturated fatty acids in a variety of unsaturated fatty acid‐based oils. X is the ratio of palmitic acid to oleic acid in the oils before deep frying, and can be use as an evaluation index for predicting the degradation rate of a variety of unsaturated fatty acid‐based oils before deep frying.

The determination of γ-nonalactone as one of the important odor-active compounds in freshly cooked non-scented rice is reported. It was evaluated by gas chromatography-olfactometry (GC-O) analysis and identified by gas chromatography-mass spectrometry (GC-MS) analysis in the headspace above the freshly cooked non-scented rice samples extracted by using a modified headspace solid-phase microextraction (SPME) method. This component had a mass spectrum with a characteristic ion peak at m/z 85 (100%) and a linear retention index (RI) of 2,023 on a DB Wax column, consistent with those of an authentic sample of γ-nonalactone. The odor characterization of a strong, sweet, coconut-like aroma of this compound was also validated by GC-O comparison with the authentic compound.

The thermal properties of Bombyx mori silk fibers subjected to heat treatment were examined by thermogravimetry, differential thermal analysis, Fourier transform infrared absorption spectrometry (TG‐DTA‐FTIR) and scanning electron microscopy (SEM). The color, size, and shape of B. mori cocoon shells were observed as they were heated from 25 to 550°C. Only 1% of the original cocoon shell weight remains as cocoon ash after treatment at 550°C. Inorganic components, such as calcium, potassium, sulfur, magnesium, etc., were detected in the cocoon ash by energy dispersion fluorescent X‐ray spectrometry. A sharp decrease in weight was observed in the TG data beginning around 280°C, and an endothermic peak appeared at 308°C, as evidenced by the DTA curves. The IR bands observed at 2380 cm⁻¹ (OH stretching), 1760 cm⁻¹ (CO stretching), 1503 (NH stretching), 1085 cm⁻¹ (CN stretching) and 965 cm⁻¹ (NH₂ stretching) become stronger as an exothermic reaction beginning at 280°C takes place. This is probably due to cleavage of the main chain and the accompanying decomposition of the silk fibers. Similarly, a SEM micrograph of silk fibers treated at 300°C shows a microtubule in the middle of the silk fibers of about 25‐μm diameter. This suggests that the thermal reactions starts in the middle of the silk fiber and forms a microtubule. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1817–1820, 2002