# Brewer's Spent Grains: Possibilities of Vaporization, a Review
## 3 Possible Uses of BSG
### Source of Phenolic Compounds  
Phenolic compounds in plants are made up of several components: phenolic acids, phenolic alcohols, flavonoids, tannins, stilbene, and lignans [57; 58]. BSG is a generous source of hydroxycinnamic acids (HCA) [9], which accumulate in cell walls are considered the most important source of antioxidantsin cereals, both in free form and the bound form [17]. Phenolic compounds are considered natural antioxidants associated with some chronic diseases, such as cardiovascular disease, neurodegenerative diabetes, and cancer [9]. Phenolic acids, including caffe acid, vanillic acid, quercetin, and epigallocatechin-3-gallate, inhibit cyclooxygenase isoforms, reducing the risk of cancer according to the method evaluated by DNA fragmentation and the Hoechst staining test [11]. Phenolic acids are grouped into hydroxycinnamic acids (HCA) and hydroxybenzoic acids (HBA), the latter in low amounts in BSG. Ferulic acid (FA) and p-cumaric acid (\(p\)-CA) are the most abundant HCAs in BSG, with values between 35-490 mg/100 g dry matter for FA and 6.7-180 mg/100 g dry matter for \(p\)-CA [17]. Ferulic acid is the target product of BSG; it is an antioxidant found in cell walls with a key role in the development and the protection of the plant. Ferulic acid has similar properties in the human body, protecting the skin and keeping it young, with effects similar to vitamin C [1]. It is a stable and gentle compound with the skin that reduces the negative effects of free radicals, considerably slowing the aging of the skin. It has also been approved as a preservative in many countries to prevent food oxidation [9; 11]. For these reasons, extraction and recovery of these compounds is of niche interest for research for both the food industry and the pharmaceutical/cosmetic industry [17]. The important steps in phenolic compounds extraction include pretreatment, extraction, isolation, and purification. There is an upward trend of \"green chemistry\" or \"green extraction\" that implies the use of solvents and substances obtained from renewable materials but which could keep the quality of the gained extraction [57]. The extraction methods and the used treatments are microwave-assisted extraction (MAE) and saponification with NaOH 1-4 M (alkaline hydrolysis is the most used). Contents of 27.31 \(\pm\) 0.69 \(\upmu\)g/mL ferulic acid and 0.732 \(\pm\) 0.020 mg galic acid equivalent (GAE)/mL phenolic acid were obtained at a concentration of 1 M. Other methods imply acid hydrolysis, enzymatic hydrolysis, ultrasound extraction, liquid-liquid extraction or solid-liquid extraction, water bath extraction with solvents, supercritical CO\({}_{2}\) extraction, and cosolvent ethanol (extraction temperature 40 \({}^{\circ}\)C, 240 min, 35 MPa CO\({}_{2}\) pressure, and 60% ethanol [9; 17]. Enzymatic hydrolysis proved to be less efficient and with a higher cost of the analyses due to the high cost of pure enzymes (cellulose, \(\alpha\)-amylase, pectinase) [17]. Microwave-assisted extraction (MAE) is an efficient, promising, and fast technique. Microwave hydrodiffusion and gravity (MHG) is a new bioactive compounds plant extraction technique that mixes the microwave heating with the gravity, the whole process deploying at atmospheric pressure without the water implication or another solvent [57]. Solid-state fermentation (SSF) is when the microbial growth and formation of products take place almost in the absence of water; the substrate contains moisture to allow growth and metabolism of microorganisms. Submerged fermentation (SmF) assumes that microorganisms are grown in a liquid medium containing the necessary nutrients [59]. Studies have shown that SSF has a higher yield than SmF in the production of bioactive compounds in addition to lower costs and secondary compounds that occur in a shorter time than SmF without the need for aseptic conditions.