Flaxseed milk has been one of the most emerging plant-based dairy alternatives, due to its nutritional benefits and unique flavor profile. However, the key aroma compounds can be highly influenced by processing approaches. It is essential to clarify the different preparation steps on its sensory characteristics. This study aimed to systematically investigate the effects of various preparation stages (including microwave treatment, soaking, and homogenization) on the aroma compounds and endogenous enzyme activities in flaxseed milk. Processing techniques were then optimized to enhance the overall flavor quality of flaxseed milk for consumer acceptability. Solid-phase microextraction was employed to combine with the gas chromatography-mass spectrometry (SPME-GC-MS). Nine compounds of the key aromas were identified and quantified in the flaxseed milk. Among them, the (E,E)-2,6-nonadienal and 1-octen-3-ol greatly contributed to the characteristic aroma. Additionally, the activities of five endogenous were measured, such as the enzymes-lipoxygenase (LOX), peroxidase (POD), lipase (LPS), hydroperoxide lyase (HPL), and phospholipase A2 (PLA2). A systematic analysis was also made on their correlation with the formation of aroma compounds. Enzymatic reactions were dominated in the lipid oxidation and volatile compound generation. The key factors were then taken from the sensory attributes of flaxseed milk. The results demonstrated that the activities of all five endogenous enzymes decreased significantly during processing, which directly impacted the generation of volatile aroma compounds. A strong negative correlation was observed between the enzyme activity and the production of aldehydes, pyrazines, alcohols, and phenols. The enzymatic degradation was dominated to modulate the flavor profile of flaxseed milk. Microwave treatment was found to particularly enhance the aroma release. The total content of the nine key aroma compounds significantly increased from 215.90 μg/kg to 587.32 μg/kg during processing. The reason was primarily attributed to the heat-induced acceleration of Maillard reactions and lipid oxidation. These pathways of reaction were facilitated to form the volatile compounds, especially the pyrazine-derived aroma compounds. Pyrazines were also characterized by roasted, nutty, and caramel-like notes. There was a positive contribution to the overall flavor perception of flaxseed milk. Furthermore, microwave treatment reduced the release of undesirable aldehydes during lipid oxidation. Thereby, the off-flavors were minimized to improve the sensory acceptability of the final products. The soaking was also used to modify the flavor profile of flaxseed milk. The overall sensory experience was improved to effectively reduce the content of undesirable volatile compounds, including the aldehydes and ketones that are associated with the rancid and grassy notes. The (E, E)-2, 4-Heptadienal was also reduced, due likely to the enzymatic deactivation and the leaching of certain precursors into the soaking medium. Their transformation into volatile compounds was prevented from negatively impacting the aroma quality. Homogenization further contributed to the flavor enhancement. The release and dispersion of aroma compounds were facilitated over the flaxseed milk matrix. The stability and consistency of the products were improved for the more uniform distribution of volatile compounds, indicating the enhanced attributes of sensory. The mechanical forces during homogenization also disrupted the cellular structures, allowing for the better extraction and retention of desirable aroma compounds. Therefore, there were complex interactions between different preparations and the aroma profile of flaxseed milk. An effective strategy of aroma enhancement was coupled with microwave treatment with controlled soaking and homogenization. The sensory quality of flaxseed milk was also optimized after treatment. The desirable compounds of pyrazine-based aroma were formed to reduce the off-flavors after the lipid oxidation. Microwave treatment also served as a valuable tool to improve the flavor characteristics. Meanwhile, the soaking effectively mitigated the off-flavors. The homogenization also further enhanced the sensory perception under the uniform distribution of volatile compounds. A theoretical foundation was provided for the refining processing in the plant-based dairy industry. Additional parameters were further considered to enhance the aroma and overall quality of flaxseed milk, such as the temperature, enzyme inhibitors, and alternative processing. Additionally, the sensory evaluation and consumer preference analysis were also essential to validate the practical applicability in the commercial production of flaxseed milk. Ultimately, the flavor profile of flaxseed milk was optimized to serve as a sustainable and appealing alternative to conventional dairy products.

Flaxseed is one of the eight major oil crops in China, and flaxseed oil is a healthy vegetable oil rich in α-linolenic acid, which is highly favored by consumers. Volatile flavor substances are a key attribute affecting the quality of flaxseed oil and its popularity among consumers. This article provides a systematic overview of recent advances in the research on volatile flavor substances in flaxseed oil, with a focus on the techniques and methods for the extraction, analysis and detection of volatile flavor substances in flaxseed oil from the perspective of molecular sensory science, the material basis and aroma characteristics of volatile flavor in flaxseed oil, and the factors affecting the flavor formation of flaxseed oil. Moreover, it also summarizes the possible formation pathways of some key flavor substances during the processing of flaxseed oil. It is hoped that this review will provide a theoretical basis for improving the flavor of linseed oil.

In this study, the flavor components of double-low fragrant rapeseed oils (cv. “Zhongyouza 19”) from nine different planting areas were qualitatively and quantitatively analyzed by headspace solid phase microextraction (HS-SPME) and gas chromatography-mass spectrometry coupled with olfactometry (GC-MS-O). The results showed that a total of 63 aroma-active compounds were identified using two columns (polar and non-polar) at a total concentration of 222.0 to 468.9 mg/kg. The total concentration of aroma compounds was highest in the Hunan sample and lowest in the Jiangxi sample. Out of the identified flavor substances, 38 had odor activity value (OAV) greater than 10. The results of partial least squares-discriminant analysis (PLS-DA) and sensory evaluation showed that 2-methylpyrazine, 2,3,5-trimethylpyrazine, and 2-methyl-3,5-diethylpyrazine were the main contributors to roasted aroma notes, 3-butenylisothiocyanate provided spicy notes, and 5-hexenenitrile and 4-methylthionitrile led to green notes. Based on variable important in the projection (VIP) values, nine key differential substances were selected, namely, methanthiol, 2-methylpyrazine, 2,3-diethyl-5-methylpyrazine, dimethyl disulfide, 5-methylthiopentonitrile, 5-hexenenitrile, (E,Z)-2,4-pentadienonitrile, hexanal and dimethyl trisulfide. They could be considered as potential flavor markers for the discrimination of double-low fragrant rapeseed oils from different planting areas. This research can provide a theoretical guide for the screening of raw materials for fragrant rapeseed oils.

The plant milk market has a promising future and flavor is a critical quality attribute of plant milk. The good flavors of plant milk such as milky, fruity, nutty, and cereal-like aromas are well-liked by consumers, while the undesirable flavors such as bitterness, bean-like odor, green odor, and mushroom-like odor severely limit the development of the plant milk industry. Based on the current state of development and future trends in raw material screening and processing technology for plant milk production and research on the flavor composition of plant milk, this paper summarizes the methods used for the extraction and identification of volatile substances from plant milk and the material basis of its flavor composition, highlighting the formation pathways of aroma and off-odor, the interaction between macromolecules and small molecules in plant milk, and the influential factors of and the regulatory strategies for the formation of aroma and off-odor in plant milk. This review is expected to provide scientific research ideas and strategies for maximizing the preservation of the good flavor of plant milk and avoiding the generation of undesirable flavors.