## 3 Results and Discussion
### Effect of Nitrogen Sources  
In order to find more economical and efficient alternative nitrogen sources, ethanol fermentations were performed in hydrolyzed waste potato mash media by using poultry meal, hull and fines mix, feather meal, and meat and bone instead of yeast extract as the nitrogen source. Figure 3 shows ethanol, glucose and cell populations using four different nitrogen sources in fermentation medium.  
Figure 3: Ethanol, glucose and cell populations in the fermentation broth with different nitrogen sources: Yeast extract (**a**), Feather meal (**b**), Poultry meal (**c**), Meat bone meal (**d**), Hull and fines mix (**e**).  
Figure 2: Cont.  
yield (32 g/L ethanol concentration in final broth) was observed with feather meal, which also had very good results for growth rate (0.28 log CFU/mL/h), production rate (3.59 g/L/h), and consumption rate (5.06 g/L/h). Final ethanol concentrations for hull and fines mix and meat bone meal were less than yeast extract with 24.59 and 25.54 g/L ethanol, respectively, whereas 30.80 g/L ethanol obtained while yeast extract was nitrogen source. Moreover, these two nitrogen sources fell behind in growth and production rates (0.194 log CFU/mL/h and 1.97 g/L/h and 0.13 log CFU/mL/h and 2.36 g/L/h, respectively). Overall, the maximum growth rate was observed with yeast extract and feather meal (both 0.28 log CFU/mL/h). However, maximum production rate was obtained with yeast extract (3.68 g/L/h) with a 30.8 g/L ethanol production.  
Although poultry meal has the highest ethanol yield, it has lower growth and production rates compared to yeast extract and feather meal. Statistical analysis showed that there is a significant difference among production rates of different nitrogen sources (\(p<0.05\)). Compared to yeast extract by Dunnett Test and it was concluded that no nitrogen source produces ethanol significantly different than yeast extract (\(p>0.05\)).  
According to the results, it can be concluded that all of the animal by-products investigated in this study provide nitrogen for growth of yeast in ethanol fermentation. However, a limitation of the use of alternative nitrogen sources was that these animal-by-products were not 100% soluble, maybe due to partial hydrolysis during manufacturing of these animal by-products, which caused plugging in the tubes of the reactors.  
A comparison of poultry meal, hull and fines mix, feather meal and meat bone meal for ethanol production on carob extract by using _S. cerevisiae_ was reported by [PERSON] _et al._[20]. In their study, maximum production rate and ethanol yield among four alternative nitrogen sources were determined by addition of meat bone meal, whereas none of the other evaluated nitrogen sources reached the yield of ethanol and growth rate when yeast extract was added. [PERSON] and [PERSON] [21] also studied the effect of nitrogen sources for ethanol fermentation with S. cerevisiae in a chemically defined media to evaluate seven different animal by-products as a nitrogen source instead of yeast extract. For these studies, yeast extract was compared with poultry meal, hulls and fines mix, beef stock, blood meal, feather meal, pork stock and meat and bone meal [21]. Pork stock and meat and bone meal were reported as giving the two highest ethanol yields as alternative nitrogen sources, however, the second highest production rate after yeast extract was obtained with poultry meal [21]. These animal by-products could be alternative nitrogen sources for ethanol production instead of yeast extract although all of them had lower production rates than the yeast extract.