## 3 Results  
### Physicochemical Parameters  
Firstly, both sourdough and baker's yeast bigas used for breadmaking were chemically characterized in terms of dry matter percentage, pH, total titratable acidity (TTA), and main fermentative metabolites (Table 2). As expected, the differences identified were consistent with the microflora metabolic products of each different leavening agent adopted, with a significantly higher acidification and a drop in the pH detected in sourdough biga attributable to the LAB production of organic acids [58].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \(p\)**-Value 1** & **Sourdough Biga** & **Baker’s Yeast Biga** \\ \hline
**Dry matter (\% dm)** & * & 52.90 & 59.50 \\ \hline
**pH** & ** & 3.85 & 5.06 \\ \hline
**Total titratable acidity** & *** & 0.12 & 0.02 \\ (**meq lactic acid/g dm)** & * & 0.06 & 0.02 \\ \hline
**Acetic acid** & * & 0.03 & n.d. 2 \\ (**mmol/g dm)** & * & 0.06 & n.d. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical and chemical characterization of sourdough and baker’s yeast biga: dry matter (dm %), pH, total titratable acidity (TTA), and main fermentative metabolites. Data presented are the mean of three replicates.  
Regarding cooked bread samples (Table 3), no significant effect was observed with regard to water activity and dry matter percentage. As expected [39], the free acidity increased together with the growing percentage of the flaxseed cake used for fortification. Furthermore, free acidity was significantly affected also by the leavening agent employed, with considerably higher values for sourdough breads (Table 3), showing--according to the two-way ANOVA analysis--a significant interaction between the two main factors (\(p<0.001\)).  
These results are related to the free fatty acids content, in particular unsaturated fatty acids, of flaxseed cake. Moreover, the detected difference between breads produced with the same percentage of flaxseed cake but with different leavening agents could be due to both the interference of organic acids produced by the LAB of sourdough together with the activity of lipase. Lipase, which is responsible for the hydrolysis of fatty acids, was differently influenced by the pH associated to the leavening agent systems considered. As reported in literature, indeed, lipase activity in flaxseed is promoted at a pH close to 4.5 [59]; this value is comparable to the pH of sourdough biga and far below that of brewer's yeast (Table 2).  
Regarding FA composition, we observed a significant increase of \(\alpha\)-linolenic acid with the rising percentage of the flaxseed cake (Table 3). On the contrary, the high level of flaxseed reduced significantly the percentage of C16:0 and C18:2n-6 (Table 3). We also observed the highest level of C18:19 for SB10 sample, while all the others did not show significant differences.  
As shown in Table 4, while the main fermentative metabolites concentration was related to the different leavening agent used, it did not appear to be significantly influenced by the level of fortification, thus indicating that the fermentation activity seemed not to be significantly affected by the addition of flaxseed cake.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & \(p\)**-Value 1 [FOOTNOTE:1]Footnote 1: Significance level—\(\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\text{\texttexttexttexttexttexttexttext  
The only exception was ethanol concentration in baker's yeast bread, which significantly increased together with the percentage of flaxseed cake added and therefore seemed to promote yeast metabolism in the range of fortification considered, with a strong interaction (\(p<0.001\)) between the two factors considered.