Abstract
Roasting affects the physicochemical and nutritional qualities of flaxseed oil (FSO). The FSO samples were extracted from the roasting flaxseeds at 10-, 20-, and 30-min points and at different temperatures (140°C, 160°C, and 180°C). A total of 61 volatile compounds were identified, and the quantity of the volatile compounds increased significantly (p < 0.05) after roasting. The maximum aldehyde (25.83%) and heterocyclic content (29.26%) was obtained from the samples roasted at 200°C for 20 and 30 min, respectively. The predominant fatty acid in FSO samples was linolenic acid (46.01%-49.35%), which changed dynamically during roasting. The loss of α-, γ-, and δ-tocopherol after roasting was 28.73, 109.78, and 6.67 mg/100 g, respectively. The principal component analysis and hierarchical cluster analysis results showed good discrimination of the different FSO samples into three groups, which were mainly related to the roasting time. Therefore, it can be concluded that roasting time has a stronger effect on the volatile composition of FSO than the temperature during the roasting process. This work provides a basis for improving the aroma of FSO. PRACTICAL APPLICATION: The roasting process is used to extract flaxseed oil (FSO) from flaxseeds. Studying the physicochemical properties and quality characteristics of FSO under diverse roasting conditions is an important step in producing FSO in the food industry, which can give precise instructions to produce flaxseed oil in factories. The results of this study document the volatile constituents generated in FSO samples extracted from flaxseeds during roasting, which may help manufacturers, who are trying to develop natural and artificial FSO flavors.
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Key Points
Roasting is an important pretreatment method, and it is also a key step in producing the characteristic aroma of FSO before pressing. A series of chemical reactions occur under high temperature and specific oxygen conditions. The volatile and nonvolatile products produced by the reactions have an important influence on the flavor and quality of FSO. Currently, volatile aroma substances known in vegetable oils are mainly alcohols, aldehydes, acids, furans, pyrazines, thiazoles, and pyrroles. FSO has a distinctive volatile composition with a characteristic odor that originates in flaxseeds during the roasting process. The objectives of the present study were (1) to identify the major volatile constituents of FSOs pressed
under diverse roasting conditions, and to visually assess their significant differences using principal component analysis (PCA) and hierarchical cluster analysis (HCA), and (2) to assess the changes in FAs and total tocopherols in FSO produced during roasting. This study provides a theoretical basis for the processing control for the characteristic aroma of FSO, and focused on the characteristics of FSOs extracted from flaxseeds subjected to roasting at 160◦C (n = 3), 180◦C (n = 3), or 200◦C (n = 3) for 10, 20, or 30 min in a rotary electric oven. Both PV and p-AV increased significantly (p < 0.05) during the roasting process in the range of 0.56−1.30 mmol/kg and 4.99−34.41, respectively. The contents of total UFA and tocopherols decreased by 0.01−1.00% and 10.31−25.22%, respectively. The results showed that roasting did not affect the content and stability of FAs during oxidation. Furthermore, 61 volatile compounds were detected in the FSO samples by HS-SPME-GC-MS. The amount of volatile compounds increased significantly (p < 0.05) compared with the initial amount after roasting, especially for aldehydes and heterocyclics. The PCA and HCA results from the different FSO samples showed significant differences in the three groups, which were mainly related to the roasting time. It can be concluded that the roasting time has a stronger effect on the volatile composition of FSO than the roasting temperature during the roasting process.