## 4 Discussion  
Many compounds contribute to the flavor and aroma of beer, and sometimes just a little difference in their intensity emission may produce an entirely different flavor. In this paper, the volatile chemical composition of beer before and after spirulina addition was investigated through SPME-GC/MS and PTR-ToF-MS. Our results showed that the volatile profiles of the two beer samples differed for a series of compounds. With the aim to better compare the volatile compounds emitted in the presence of spirulina, we focused our attention on the volatile organic compounds (VOCs) with the most aromatic impact on beer. Therefore, all signals linked to terpene emission were discarded from the discussion because they are linked to the addition of hops [32]. The obtained results with both survey techniques used showed that some signals and their intensity emission were different between beer with and without spirulina. For example, the reduced ethanol emission in beer with spirulina highlighted a lower fermentative performance of the yeast when adding spirulina during the fermentative process. This finding is likely linked with the antimicrobial spirulina effect that may have interfered with the fermentation process reported by some authors [33; 34]. Similarly, since ester compounds synthesized through the complex metabolic pathways of yeasts play an important role in the final flavor of beers, we tried to gain a clear understanding of the interaction between ester production and spirulina addition [35]. In particular, compounds such as ethyl acetate (\(m/z\) 89; solvent-like aroma), isoamyl acetate (\(m/z\) 131; banana aroma), ethyl caporate (\(m/z\) 145; aniseed, apple-like aroma), phenyl ethyl acetate (\(m/z\) 165; roses, honey), ethyl caprylate (\(m/z\) 173; sour apple aroma) and ethyl caprate (\(m/z\) 201; floral odor) showed an higher emission in the sample without spirulina. Furthermore, the emission peaks linked to ester compounds clearly indicated a different behavior for the two analyzed beers, highlighting how the spirulina addition reduced the fermentative activity of yeasts. Our chemical analyses conducted on spirulina biomass showed the presence of a high amount of lactic acid. Generally, the addition of lactic acid after the primary fermentation makes the beer more acidic due to the effect of the _Lactobacillus_ bacteria. In fact, _Lactobacillus_ is part of a family of bacteria called \"lactic acid bacteria\". The addition of spirulina, in a limited percentage, during the fermentation process allows the creation of a tasty beer but with undoubted alterations in smell and taste compared to conventional beer [34]. From a functional point of view, in the present study, we evaluated the beneficial properties derived from the addition of spirulina to beer. In general, its consumption has been associated with antioxidant, anti-inflammatory, hypoglycemic, lipid-lowering and immunomodulatory effects [36; 37; 38; 39]. Recently, it has also been reported to inhibit the oxidative response induced by highly active antiretroviral therapy in HepG2 cells, likely through the modulation of Nrf2 signaling [4]. In line with this evidence, we evaluated the effects of adding spirulina to craft beer in terms of antioxidant power and improvements in cellular defenses in a model of human cholangiocarcinomaytes. The obtained results showed that the spirulina extract contained polyphenic compounds, including a high amount of tannins. These data are in agreement with the results obtained for both _Arthrospira platensis_ and _Spirulina maxima_[40; 41; 42]. However, spirulina had little effect on total polyphenol and tannin levels in both beer samples. Beer is generally characterized by the presence of polyphenic compounds, such as phenolic acids and tannins, flavones and flavonoids [43], which contribute to its typical organoleptic characteristics and stability as well as to its antioxidant properties [44; 45; 46].