## 1 Introduction  
Carrots are one of the most significant root vegetables globally. In 2022, Spain's production alone reached 387,907 t [1]. However, an estimated 25-30% of this harvest is discarded due to physical imperfections that render it unsuitable for the commercial market [2]. Carrot discards (CD) contain high levels of free sugars and structural carbohydrates [3, 4], making them a potential feedstock for the production of biofuels and/or bioproducts (e.g., 2,3-butanediol (2,3-BDO)) using biological processes. 2,3-BDO is considered an important industrial platform, characterized to be green, natural, and sustainable. In addition, it is known as an interesting, valuable commercial chemical [5, 6]. Its favorable physicochemical properties--including its colorless, odorless, and hygroscopic nature, high solubility in various solvents, and excellent biodegradability--underpin its use in numerous applications [7]. These include rolesas a liquid fuel, cryoprotectant, fumigant, food additive, as carrier for pharmaceuticals, as intermediate for solvents and high-value-added products production, as cosmetic ingredient (i.e., as humectant, embilient, and antiseptic ingredient), and in the painting sector, among others [8; 9]. Although 2,3-BDO is currently produced industrially by cracking petroleum-derived hydrocarbons, there is growing interest in its biological production from renewable, carbohydrate-rich residues. This bio-based route offers a path towards more sustainable, cost-effective, and large-scale synthesis [7]. In this way, according to [PERSON] et al. [9], the 2,3-BDO global market could increase up to $220 million by 2027. A wide array of microorganisms have been identified as potent 2,3-BDO producers, including species of _Enterobacter_, _Klebsiella_, _Ralstonia_, _Serratia marcescens_, _Paenibacillus_, and _Bacillus_, among others [7]. Among them, the 2,3-BDO production by _Bacillus_ strains (such as _B. licheniformis_ and _B. amyloliquefaciens_) is greatly recommended, since they are considered nonpathogenic (class 1) strains. Therefore, the 2,3-BDO industrial production using this type of strain will require of less strict safety regulations, and then, it could be cheaper and less complex [10; 11]. Other than 2,3-BDO, _Bacillus_ strains are able to produce considerable other by-products, such as acetoin, ethanol, and lactic, acetic, and citric acids, among others [12]. Acetoin is a naturally occurring flavor compound found in numerous foods, including fruits and fermented products like wine. As an important platform chemical, it is used extensively as a flavor and fragrance agent in the food and cosmetic industries, with global consumption reaching thousands of tons annually. Beyond these roles, acetoin serves as a crucial building block for synthesizing high-value chemicals, including optically active \(\alpha\)-hydroxyketone derivatives, liquid crystal composites, and pharmaceuticals. It can also be catalytically converted into other valuable compounds such as diacetyl, acetoin acetate, and 2,3,5,6-tetramethylpyrazine [9; 13]. Currently, industrial-scale acetoin production relies predominantly on chemical synthesis from fossil-based feedstocks. These processes often involve free-radical substitution reactions, which pose significant environmental concerns. While the direct cost of bio-based acetoin production may currently be higher than conventional chemical routes, the resulting product is considered \"natural\". This classification makes it far more acceptable for use in the food, cosmetic, and pharmaceutical industries, where consumer preference and regulatory standards favor bio-derived ingredients. Consequently, the development of highly efficient and sustainable biological methods for acetoin production represents a significant research priority with substantial application value [13; 14]. The conversion of complex carbohydrates in vegetable residues like CD into fermentable sugars requires enzymatic hydrolysis.