## 1 Introduction  
Nowadays, a huge amount of solid wastes and by-products is generated by the beer industry. Brewer's spent grain (BSG) is one of the most produced by-products, representing ca. 85% of the total beer industry byproducts [1]. BSG is a lignocellulosic material resulting from work preparation, being mainly composed by 70% fibers (cellulose, hemicellulose and lignin), 30% proteins and some minerals and lipids [2]. Currently, BSG is mainly utilized as animal feed. However, the high amount generated, together with its interesting composition with high biotechnological value, has driven the search for green environmental and sustainable alternatives to this readily available low-cost biomass. Several studies reported the utilization of BSG in different fields of application: biogas and energy production, for the extraction of sugars, proteins and antioxidants, as well as for microorganisms cultivation [1, 3]. Fermentation processes using BSG as a carbon and/or nutrient source, can generate several added-value products such as organic acids, amino acids, vitamins, ethanol and butanediol [2]. The production of volatile fatty acids (VFA) through the conversion of the organic material involves the two first stages of the anaerobic process (hydrolytic and acidogenicstages) [4]. VFA production under anaerobic conditions can be performed by pure or mixed cultures starting from several wastes. The use of mixed microbial cultures is advantageous since: (1) possessing a diversity of microbial pathways turns easier the conversion of substrates of complex composition; (2) no necessity of sterilization, thus saving energy [5; 6]. As such, BSG can be used as carbon source for VFA production, which can be used for several industrially interesting applications, namely, bioenergy, biological nutrient removal, and bioplastics production, such as polyhydroxyalkanoates (PHA) [4; 7]. In the overall process of VFA synthesis, the hydrolysis is the rate-limiting step during the anaerobic conversion of the complex structure and composition of solid waste. A previous study on the acidogenic fermentation of raw BSG demonstrated that the bioconversion of solid BSG into VFA is possible. However, the use of solid waste to feed the fermentation reactor caused some operational constrains, namely, equipment clogging, and the need of using high hydraulic retention times (HRT) values, which limits the process efficiency [8]. To overcome these constrains, several pre-treatments, including physical (crushing and grinding), chemical (alkaline and acid) [9; 10], physico-chemical (auto-hydrolysis, hot water, steam and supercritical fluids) and biological methods (enzymes, fungi and bacteria) [10; 11] can be applied to the solid feedstocks before their fermentation. These pre-treatments improve the hydrolysis and biodegradability of the feedstock, through the production of a liquor rich in fermentable sugars. Acid catalyzed processes are the commonest used and are divided in two general approaches based on concentrated-acid/low temperature and dilute-acid/high temperature hydrolysis [12]. The efficiency of the hydrolysis step determines the amount of readily available sugars which may impact the fermentation process yield and productivity. In this study we hypothesized that a two-stage process to convert BSG into organic acids, consisting of an acid hydrolysis followed by acidogenic fermentation, would avoid the operational problems related to the use of solid BSG and would lead to enhance the overall productivity. The first step, consisting of an acid hydrolysis, was performed to extract sugars from solid BSG and thus facilitating the subsequent step of acidogenic fermentation, where organic acids are produced. Given that one of the potential applications for the organic acids is their use as precursors for PHA production, the impact of organic load (OL) and pH in the acidogenic batch fermentation, namely on organic acids profile and concentration, and on foreseen polymer composition was evaluated.