## 1 Introduction  
Due to the limited use and strong environmental impact of petroleum, bio-based chemicals are gaining strength as concrete sustainable alternatives to the fossil oil-based economy [1]. In fact, the products obtained by fermentation are expanding beyond the traditional high-value low-volume compounds, such as pharmaceuticals, and are beginning to compete with the traditional high-volume synthetic production of commodity chemicals [2]. However, to fully win the battle, products based on renewable resources should be more competitive in terms of quality, technical performance, environmental impact, and at least be comparable for price [3]. Thus, it becomes strictly necessary to maximize productivity and minimize production costs. Whereas productivity has been greatly improved due to biotechnological research [4; 5], cost-efficiency depends on technological variables and the cost of raw materials [6]. Pure nutrients (i.e., glucose and amino acids) are expensive and make most of the developed biotechnological processes economically unfeasible. Therefore, great attention has been focussed on the biotechnological potential of agro-industrial waste, such as cassava bagasse [7], sugarcane bagasse [8], sugar beet pulp [9], coffee husk and pulp [10], apple pomace [11], oil-cakes [12], wheat/rice bran [13] etc., for their use as raw materials in the production of several value-added products. Special attention must be paid to product recovery and purification costs [14]. Taking into account the actual market volumes and relevance of current or future biorefinery operations, lactic acid, together with glycerol, malonic and propionic acids, is largely considered at the top of the list of potential C3 building-block chemicals obtainable from renewable resources [15]. Due to its versatile applications as an acidulant, flavour enhancer and preservative, lactic acid has occupied a prime position in food, pharmaceutical, cosmetic and other chemical industries. Recently, use of lactic acid has expanded; it has been used as monomer in polymerization to form polylactic acid (PLA), a polymer of great interest because it is biodegradable [16]. Unfortunately, PLA production is now restricted worldwide due to the high production costs, although the increasing market demand is expected to reach 2170 billion USD by 2020 [17]. It has been reported that the cost of raw materials for the fermentative production of lactic acid accounts for about 35% of the total manufacturing costs [18]. Thus, the efficiency and economics of the up-stream fermentation is still a key factor, and the nutrients supply plays a vital role in the improvement of overall process sustainability. There have been several attempts to produce lactic acid from low-cost resources, such as rice bran [19], whey permeate [20], solid waste [21] and green microalgae [22]. Nowadays, lignocellulosic and starchy raw materials represent the most attractive substrates as feedstock for the production of lactic acid due to their abundance and cheapness [23]. Up to now, the principal factor, which limits their utilization on a large scale, is that these materials are not directly available for fermentation. Expensive pretreatments are necessary to remove lignin, separate cellulose and hemicellulose in order to increase the accessible surface area to the hydrolytic enzymes before microbial fermentation [24]. Up to now, lactic acid bacteria (LAB) have been the most common microorganisms used for the production of lactic acid at an industrial scale [25] since they are able to convert numerous mono- (both hexose and pentose) and di-saccharides [26], are tolerant to broad conditions of temperature (ranging from 20 to 55 \({}^{\circ}\)C) and pH (they can survive at pH 5 and lower), and are highly versatile. Depending on the metabolic pathways, they can produce l- or d-lactic acid (by homo-, hetero- or mixed-acid fermentation) [27]. In spite of the wide use and advantages of LAB strains for lactic acid production, economic feasibility often remains problematic.