## 1 Introduction  
The grains such as rice, wheat, oats, malt, and barley are widely used as ingredients for improving the functional properties in diverse foods [1; 2]. In particular, grain-based probiotictic functional foods are becoming increasingly popular and have considerable potential as dairy replacements [2; 3]. Grains can be used as substrates to produce fermented foods by lactic acid bacteria (LAB) [4; 5]. Several studies have demonstrated the fermentation of LAB in grain substrates [4; 5; 6; 7; 8]. [PERSON] and [PERSON] found that grain-based nondiary products were suitable for the growth of probiotic LAB strains for fermentation [6]. [PERSON] et al. utilized whole grains as a substrate for fermenting LAB to produce a grain-based beverage [4]. The role of LAB in rice fermentation has also been reported [8; 9; 10; 11]. Steamed breads such as _idli_ and _puto_, which are made with rice, are fermented by _Leuconostoc_ strains, with _Leuconostoc mesenteroides_ in particular initiating relatively rapid growth in rice [8; 9]. _Chicha_ and _Haria_, which are rice-based ethnic fermented beverages of east and central India and Brazil, are fermented simultaneously by LAB and yeast [10; 11]. Fermentation by LAB can change the taste and aroma of substrates and has been used to improve the organoleptic quality of fermented foods [2]. LAB can produce large amounts of lactic acid via diverse metabolic pathways and form volatile compounds derived from amino acids, peptides, and fatty acids upon further bioconversions [12; 13]. Some studies have reported the formation of volatile compounds in grain substrates and grain-based products by LAB [1; 14; 15]. [PERSON] et al. demonstrated the formation of volatile compounds by _Lactobacillus plantarum_ NCIMB 8826 (NCIMB 8826; National Collection of Industrial and Marine Bacteria, UK) grown in grain substrates (wheat, oats, malt, and barley) [14]. Those authors identified 60 compounds, with the most abundant in all grain substrates (wheat, oats, malt, and barley) being linoleic acid, oleic acid, 5-hydroxymethylfurfural, and acetic acid, respectively [14]. In addition, [PERSON] et al. employed headspace gas chromatography analysis to determine volatile compounds formed from barley and malt fermented by _Lactobacillus reuteri, Lac. plantarum,_ and _Lactobacillus acidophilus_[1]. They found that these _Lactobacillus_ strains can produce large amounts of certain major flavor compounds (acetaldehyde, ethanol, acetone, diacetyl, and ethyl acetate), which affects the organoleptic qualities of the fermented products [1]. In particular, fermentation by LAB generates characteristic aroma compounds such as acetaldehyde, acetone, butan-2-one, butane-2,3-dione (diacetyl), and 3-hydroxybutan-2-one (acetoin) in fermented products [15]. Butane-2,3-dione and 3-hydroxybutan-2-one, which have creamy and buttery aroma notes, can serve as major flavor compounds in various dairy products fermented by LAB [13; 16; 17]. LAB can also contribute to the formation of nonvolatile compounds that can act as precursors of characteristic aroma compounds as well as tastants themselves in fermented products such as ripened cheese, sourdough, butter, buttermilk, fermented vegetables, and yogurt [18]. [PERSON] et al. investigated the effects of citrate on the formation of butane-2,3-dione and 3-hydroxybutan-2-one by different LAB strains [19]. The heterofermatative LAB did not produce butane-2,3-dione or 3-hydroxybutan-2-one in the absence of citrate, whereas all of the homofermatative LAB produced significant amounts of butane-2,3-dione and 3-hydroxybutan-2-one in the presence of citrate. In addition, lactones can be formed from unsaturated fatty acids such as oleic acid and linoleic acid during fermentation [13; 20]. It has been reported that these lactones can contribute significantly to the flavor characteristics of fermented rice foods such as rice beer and Chinese rice wine [13; 21; 22].