# Evolution of Aromatic Profile of _Toruslapora delbrueckii_ Mixed Fermentation at Microbrewery Plant
## 4 Discussion  
The use of non-conventional yeasts in the brewing process was recently proposed with the aim to produce beers with distinctive aromatics note or to develop a new technology to increase the typicity  
Figure 2: Sensory analysis of beer produced in the microbrewry plant by the _T. delbrueckii_ mixed fermentation. From pure cultures of _S. cerevisiae_ ( \(-\)\(-\)\(-\)\(-\)) and mixed cultures of _S. cerevisiae/T. delbrueckii_ ( \(-\) ). (**A**) Olfactory analysis; (**B**) gustatory analysis. DMS, dimethyl sulfide. * = Significantly different (Fisher ANOVA). of specialty beers such as low-calorie beer, low alcohol beer, novel flavored beer, and gluten-free beer [8, 12, 13, 16, 21, 22, 23, 24]. In previous studies, the use of _T. delbrueckii_ (strain DiSVA 254) in mixed fermentation with _S. cerevisiae_ starter strains was investigated at a laboratory scale [6, 12]. The results indicated a promising behavior of this yeast for use at microbrewery plants. Indeed, the interactions between _S. cerevisiae_ and _T. delbrueckii_ produced beers characterized by a distinctive aromatic profile (fruity/citric notes, fruity/esters notes, and full-bodied attributes) [12]. For these reasons, the application of _S. cerevisiae_/_T. delbrueckii_ was assessed at the microbrewery plant evaluating the evolution of the volatile compounds during the fermentation process. The first relevant aspect for its application at the industrial level was the duration of fermentation. Similarly, to the laboratory-scale trials, the brewing process carried out with _T. delbrueckii_ mixed fermentation showed a comparable fermentation time to that exhibited by the _S. cerevisiae_ starter strain showing good competitiveness with _S. cerevisiae_ in the co-culture. This aspect is crucial for its application in a microbrewery where for economic reasons the fermentation process should not exceed 10-15 days. Regarding the evolution of aroma compounds, generally higher alcohols did not show a significant difference between mixed and pure fermentations. Regarding \(\beta\)-phenyl ethanol content, known for the rose and floral aroma with an odor threshold of 10 mg/L [18], the results showed an increase in mixed fermentation exhibiting a different trend by a previous study [6, 12]. An increase of \(\beta\)-phenyl ethanol was observed by [PERSON] et al. [25] and [PERSON] et al. [26] highlighted that the production of this compound was determined by _T. delbrueckii_ and _S. cerevisiae_ strains used in fermentation but also by the fermentation condition. Esters compounds produced by esterification between alcohol and short- or long-chain fatty acids are important compounds that can affect the aroma of the beer [27]. Phenyl ethyl acetate is known for the floral, sweet, honey, and fruity aroma with a threshold of 3.8 mg/L [16] was significantly affected by the presence of _T. delbrueckii_ as previously reported [6, 12, 26]. This study, confirming a previous study [12], showed the significant increase of ethyl hexanoate, fruity esters associated with apple flavor [27], when _T. delbrueckii_ was used in mixed fermentation while a different trend was observed with different _T. delbrueckii_ and _S. cerevisiae_ strains [25, 26]. \(\alpha\)-terpineol, the terpene responsible for balsamic/fruit notes, was detected only in mixed fermentation and highlighted that these aroma compounds were related to _T. delbrueckii_. Regarding the evolution of the main aroma compounds during fermentation, the _S. cerevisiae_ pure culture and mixed fermentation exhibited a different trend. In particular, the evolution of acetaldehyde content is related to a different metabolic pathway of _S. cerevisiae_ and _T. delbrueckii_. Indeed, in _S. cerevisiae_ fermentation the acetaldehyde content decreased during the fermentation process, while in mixed fermentation the content of this carbonyl compound remains similar from beginning to end. The same trend was also exhibited for the main alcohol compounds. Few works are present in the literature regarding the application of _T.