###### Abstract  
In this research, oats were fermented with _Lactobacillus plantarum_, _Lactobacillus acidophilus_, _Lactobacillus casei_, _Lactobacillus bulgaricus_ and _Streptococcus thermophilus_ for 48 h at 37 \({}^{\circ}\)C. The purpose of this work was to compare the growth capacities of the five lactic acid bacteria (LAB) in the oat matrix and the effects of fermentation on the contents of the bioactive components of oat, such as \(\beta\)-glucan, polyphenols, flavonoids and volatile compounds at different time (0, 4, 8, 12, 24, 36 and 48 h). After 48 h of fermentation, the number of living _L. acidophilus_ in oat reached 7.05 \(\times\) 10\({}^{9}\) cfu/mL, much higher than that of other strains. _S. thermophilus_ retained the greatest \(\beta\)-glucan content, and _L. casei_ had increased total polyphenol and total flavonoid contents. The proportion of free and bound polyphenols and flavonoids in all samples was changed by microbial action, indicating that forms of polyphenols and flavonoids can be transformed during the fermentation process, and the changes varied with different strains. The samples with _L. plantarum_, _L. acidophilus_, and _L. casei_ fermentation contained more alcohols, whereas those with _S. thermophilus_ and _L. bulgaricus_ fermentation had more aldehydes, which revealed that the composition of volatile components was related to strains. The results indicate that oat substrate is a good medium for LAB growth. This study provides a reference for the use of different strains to achieve different fermentation purposes and a theoretical basis for the further processing of oat and fermented oat beverages.  
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bacteria (LAB) are probiotics commonly used in food production, especially in dairy fermentation. However, exposure to lactose intolerance, milk allergy, and cholesterol content, and the increase in vegetarianism, have led people to pay more attention to the development of fermented products from plant sources [4; 10]. In fact, the enzymes produced by LAB, as well as their metabolic capacity, are vital for the synthesis of some beneficial substances and could facilitate their application in the oat beverage market [11]. In the process of fermentation, the glycosidic bonds of some substrates such as polyphenols and flavonoids are hydrolyzed by the \(\beta\)-glucosidase of LAB, which may release and increase their concentration, thus improving the potential value of oat the fermented beverage. At the same time, the activation of the peptidase system increases the digestibility of proteins and the level of limiting amino acids. In addition, phytase activity has been increased in low pH environment for better hydrolysis of phytates and enhanced mineral bioaccessibility [12]. There have been some reports of oat-based fermented products, such as a beverage containing 25% of oats fermented with _Lactobacillus plantarum_ LP09, which improved polyphenol utilization, antioxidant activity and flavor [13]. [PERSON] et al. [14] found that co-fermentation of oat milk by _Lactobacillus_ and _Bifidobacterium_ decreased phytic acid content and increased the bioavailability of amino acids, polyphenols and vitamins. [PERSON] et al. [15] combined the prebiotic \(\beta\)-glucan of oats with the potential benefits of probiotics to produce a functional product that could lower cholesterol levels.  
There is much literature on the development of beverages produced from grains, but studies on the selection of appropriate fermentation conditions and strains are few [13]. In this paper, oats were fermented by different LAB strains, and the growth ability of strains and changes in oat bioactive substances and volatile compounds were monitored to provide guidance for the development of fermented oat beverages.