## 1 Introduction  
Modern consumers demand food products that are healthy and safe. Fortification with natural bioactive compounds is a means to enhance the nutritional and health benefits of foods and beverages. For instance, milk is commonly fermented with lactic acid bacteria to produce wholesome, shelf-stable, probiotic-rich foods such as yogurt, cheese, and sour milk [1, 2]. The consumption of probiotic products provides health benefits such as treatment of diarrhea, alleviation of lactose intolerance, and enhancement of immunity [3]. Traditionally, probiotics were commonly recognized to be derived from dairy-based products. In recent times, non-dairy foods have been used to isolate potential probiotics [4]. _Bacillus subtilis_, a spore-foaming non-lactic acid producer, has also been used as a probiotictic in fermented foods and supplements [5]. _Bacillus_ spp. are valuable microorganisms for the production of legume-based fermented foods. They can also generate valuable bioactive compounds such as poly-\(\gamma\)-glutamic acid (\(\gamma\)-PGA), hydrolytic enzymes, peptides, and probiotics [6, 7]. _Bacillus_ spp. are used to produce traditional Korean fermented foods like chungkookjang, characterized by its strong smell and sticky consistency. Nevertheless, fermentation of milk with _Bacillus_ spp. is uncommon because of the unpleasant odor of the resulting products. \(\gamma\)-PGA is an anionic, naturally occurring homo-polyamide composed of d- and l-glutamic acid units connected by amide linkages between \(\alpha\)-amino and \(\gamma\)-carboxylic acid groups. \(\gamma\)-PGA is water soluble, biodegradable, edible, and non-toxic to humans and the environment. This biopolymer is one of the main components of the mucilage produced by _Bacillus_ strains, such as the sticky substance found in chungkookjang. The Korea Food and Drug Administration certifies \(\gamma\)-PGA as a functional ingredient for boosting immunity and calcium absorption in humans [8]. Therefore, potential applications of \(\gamma\)-PGA and its derivatives in food, cosmetics, and medicines are of interest [9]. Lactic acid bacteria are considered to be probiotics and are widely used in food fermentation because of their contribution to human health; they also have a generally recognized as safe (GRAS) status. Lactic acid fermentation enhances the safety, organoleptic attributes, nutritional value, and health benefits of foods [10]. Some foods produced by lactic fermentation are yoghurt, kefir, sauerkraut, and kimchi. Lactic acid bacteria such as _Lactobacillus brevis_, _L. plantarum_, _L. delbrueckii_, _L. paracasei_, and _Lactococcus lactis_ show glutamate decarboxylase activity, which is needed for the production of gamma-aminobutyric acid (GABA) [11]. _Lactobacillus brevis_, as a functional dairy starter, has been used for manufacturing novel fermented dairy foods with high GABA content [12]. GABA has numerous roles in physiological processes, including neurotransmission as well as hypotensive, diuretic, and tranquilizer effects. Natural and conventional treatments such as mechanical and cold stimulation encourage GABA biosynthesis. The addition of exogenous glutamic acid or gibberells increases GABA production [13; 14]. Biosynthesis of GABA may be much more promising than chemical synthesis since it involves a simple bioreaction process with high catalytic efficiency. The biosynthesis of GABA depends on the bioconversion of glutamate to GABA, which is catalyzed by glutamate decarboxylase (GAD) [15]. In this study, a novel co-fermentation using _Bacillus subtilis_ HA and _Lactiplantiobacillus plantarum_ EJ2014 was optimized to produce milk fortified with bioactive compounds and improved flavor. Fermentation with _B. subtilis_ in the presence of milk containing MSG aimed to produce \(\gamma\)-PGA and peptides. The second lactic acid fermentation with _Lactiplantiobacillus plantarum_ was performed to produce GABA and stabilize the co-fermented milk.