## 1 Introduction  
Lactic acid is a compound with a unique characteristic of having both hydroxyl and carboxylic acid groups, making it a commonly used precursor for wide ranging applications in the cosmetic, pharmaceutical, polymer, textile, food, and chemical industries. Some of the known applications of lactic acid include: (1) dehydration to acrylic acid; (2) polymerization to poly-lactic acid (PLA); and (3) condensation to 2,3-pentanedione [1; 2; 3]. Lactic acid is also often utilized as an alternative raw material for production of commodity chemicals such as propionic acid, pyruvate acid, and propylene glycol [1]. Lactic acid and lactates are considered to have direct effect on dietary mineral supply and are used for athletes training, metabolic rate modulation, weight management, lowering the glycemic index of bread, and improved mineral supplementation in nutritional formulas for infants and people using proton inhibitors [4]. These applications drive the demand for lactic acid, with an annual global capacity estimated to exceed 1960 kt by 2020 [5]. Majority (-90%) of lactic acid currently available in the market is produced through fermentation with glucose, sucrose (from cane and beet), whey (containing lactose and maltose), and dextrose (from hydrolyzed starch) as commonly used carbohydrates for commercial applications. The remaining lactic acid in the market is produced through chemical synthesis from acetaldehyde and hydrogen cyanide [1; 2; 6]. With respect to feedstock flexibility, the fermentation route is preferred and can use substrates based on local or regional availability. In addition, studies on lactic acid production from lignocellulosic biomass are currently being pursued, which could widen the geographical regions for lactic acid fermentation [5; 7; 8; 9]. Lactic acid fermentation is typically carried out in a batch mode of operation. During the fermentation, the pH of the system decreases as more and more lactic acid is produced. The decrease in pH causes microbial inhibition that eventually shuts down the process. The usual practice to overcome this limitation is to add a base (i.e., calcium hydroxide) during the fermentation to neutralize and partially precipitate the acid as calcium lactate. At the conclusion of the batch run, more calcium hydroxide is added to precipitate majority of the lactic acid. The precipitates are then filtered and treated with sulfuric acid to recover the lactic acid. Despite its effectiveness, this downstream process for lactic acid purification utilizes large quantity of unrecoverable calcium hydroxide and sulfuric acid that end up as solid wastes (i.e., calcium sulfate) which require treatment before disposal [1; 6; 10]. To improve lactic acid productivity, genetic and metabolic engineering [11; 12; 13; 14], fed-batch fermentation mode [15; 16], and continuous fermentation [17] can be exploited. Among these options, the continuous fermentation offers the possibility of applying an extractive fermentation strategy to remove or recover the lactic acid as it is produced during the process thereby eliminating the pH-induced microbial inhibition [1]. Extractive fermentation includes liquid-liquid extraction (sometimes referred to as biphasic fermentation or phase-partitioning bioreactor) [10; 18], adsorption [19; 20], and membrane separation (e.g., electrodialysis) [21; 22; 23]. The liquid-liquid extraction is limited by solvent availability, while both adsorption and membrane separation are limited by biofouling during the fermentation process. Solvents that have been evaluated for extractive fermentation through liquid-liquid extraction include long chain aliphatic tertiary amines (e.g., trioctylamine, alanine), long chain alcohols (e.g., octanol, decanol), and their mixtures [1; 24]. Despite their high extraction efficiency, as indicated by their distribution coefficients, these solvents are not suitable for extractive fermentation due to their microbial toxicity, making them less prone to biodegradation, which could pose tremendous environmental burden [24].