**2. Materials and methods**

**Figure 1.** Graphical illustration of complex nitrogen nutrients and price margin.

mentation of most targeted bioactive compounds [14].

86 Renewable Resources and Biorefineries

united nation, goal 3 and 12 stated the need for the good health and well-being of the consumer and to ensure the sustainable production and consumption patterns of food product. Acetoin (3-hydroxy-2-butanone or acetylmethylcarbinol) is widely used in the food industry as a flavor enhancer, giving a buttery taste [1]. It can also be used as a building block for various chemicals such as alkyl pyrazines, diacetyl, and acetyl butanediol [1, 2]. Currently, most of the commercial acetoin is produced by chemical synthesis from 2,3-butanedione and 2,3-butanediol. However, the use of such chemically derived acetoin is restricted to food and cosmetic industries because of safety concerns [3]. The production of acetoin by microbial fermentation has been reported to gain increasing interest due to its safety and environmental friendliness [3, 4]. Intensive efforts have been taken to enhance microbial production of acetoin, including screening of high-yield acetoin-producing strains [5, 6], metabolic engineering breeding [7–9], and fermentation optimization [9–11]. The price of chemically generated acetoin (racemic) is over \$ 16.2/kg, and biologically generated dietary acetoin can be two times more expensive [12]. One of the chemicals that can use acetoin as a building block is 1,3-butadiene (BD), which has a worldwide market of 11 Mt./year [13]. However, acetoin is only a minor by-product of 2,3-butanediol production, and its accumulation usually needs complex and expensive growth factors. Apart from carbon, which acts as feedstock for most fermentations, nitrogen is another significant nutrition compositional element needed for the active microbial growth for the fer-

Studies have shown the potential of low-cost media and complex nitrogen sources like urea, peptone, yeast extract, beef extract, soya bean [14–16] for acetoin production but yet to ascertain effectively the optimized condition. Nitrogen source is one of the main contributors in the total material cost of commercial acetoin production. Hence, replacing a costly nitrogen source with less expensive ones for acetoin production may improve the economics of the process. Corn steep liquor (CSL) has been identified as a potential nitrogen source in biochemical industries and a good substitute for other expensive complex media. It is a major by-product found during cornstarch processing. it is a low-cost source of proteins (**Figure 1**), amino acids, minerals, vitamins, and trace elements and can be used as a rich and effective
