3.3. Effect of liquefaction conditions on the fermentation kinetics and process yield
The ethanol production rate (rp) was the highest in during the first day of the SSF in all studied variants of experiment (Table 2) and its productivity was the best in variant Tf. After subsequent days of fermentation the production of ethanol decreased and between third and fourth day of the SSF no significant increase in productivity was observed. The ethanol yield parameters (Ydm, Ysugar and Yp) were also the highest for mashes liquefied at final temperature of starch gelatinization until the end of the process.
The final ethanol yield in all studied samples was very high ranging ca. 411–425 g of ethanol from 1 kg of raw material dry matter what corresponded to ca. 92–96% of practical yield based on the sugars content in the raw material. Moreover increase in ethanol yield after the third day of studied SSF experiments were insignificant so these processes could be shortened to 72 h without major loss in product efficiency. Obtained results were higher in comparison to previously reported in earlier studies dealing with waste bread conversion to ethanol. Ebrahimi et al. [15] obtained 350 g of ethanol from 1 kg of waste wheat bread dry matter in a SHF process at substrate loading of 350 g kg1. Kawa-Rygielska et al. [16] studied the possibility of improving the SHF process of waste wheat–rye bread to ethanol conversion by applying complex enzymatic preparations in the mashing stage. The ethanol yield was improved to ca. 366 g kg1 of raw material dry matter in comparison to control mashed without supportive enzymes (ca. 352 g kg1). Pietrzak and Kawa-Rygielska [38] studied the possibility of applying the direct starch to ethanol conversion using granular starch hydrolyzing enzyme (GSHE) on the ethanol fermentation process of waste wheat–rye bread, which is the process similar to traditional SSF but with the omitting of high temperature liquefaction. They stated that direct conversion process without bread waste pretreatment was slightly less efficient than the SHF, and low-temperature enzymatic pretreatment of raw material with a multi-directional enzymatic preparation improved the ethanol yield in comparison to the SHF. The results obtained in present study proved that the simultaneous saccharification and
fermentation is more efficient in comparison to separate hydrolysis and fermentation which was also reported earlier [33]. The SSF is also less energy demanding, time consuming and cost efficient than SHF because no need of separate starch saccharification step is done (optimal temperature for glucoamylase activity is ca. 50 C which also acts very slow) and two processes (saccharification and fermentation) acts is one reactor so the investment cost is lower [20]. Srichuwong et al. [29] reported that liquefaction of sweet potato cultivar, which starch exhibited low temperature of gelatinization, at temperatures close to final temperature of gelatinization (66.2 C) resulted in achievement of ethanol efficiency similar to the processes where the used liquefaction temperature of starch was above 80 C what was also achieved in present study. The authors compared their research with sweet potato cultivar having high final temperature of gelatinization (ca. 85 C) and liquefaction at temperatures below this resulted in substantial decrease in process efficiency. This suggests that the SSF process efficiency strongly depends on the thermal properties of starch in used raw material and optimization of liquefaction conditions, based on gelatinization parameters, could result in savings of energy by reduction in liquefaction temperature without loss in product yield. However this require further study with different raw materials. The pre-treatments (drying, dry-grinding) used in this study at a laboratory scale would probably not be economically feasible in the case of industrial scale production of ethanol from bread residues, however they were necessary to keep the homogeneity of the raw material. Because fresh bread has a dry matter content of ca. 500 g kg1, drying could be completely omitted. The dry-grinding could be in this case replaced by hydromechanical shredding in the water slurry what could be combined with the enzymatic hydrolysis. To prevent the raw material from the excessive mold growth, what reduce the ethanol yield [17], the storage time for the raw material should be minimized.
3.3. ผลของเงื่อนไข liquefaction ผลผลิตจลนพลศาสตร์และกระบวนการหมักอัตราการผลิตเอทานอล (rp) ได้สูงสุดในระหว่างวันแรกของ SSF ในย่อย studied ทั้งหมดของการทดลอง (ตารางที่ 2) และผลผลิตของถูกสุดในตัวแปรรหัส หลังจากหมักวันต่อมา ผลิตเอทานอลที่ลดลง และ ระหว่างวันที่สาม และสี่ของ SSF เพิ่มผลผลิตอย่างมีนัยสำคัญที่ดำเนินการ พารามิเตอร์ (Ydm, Ysugar และ Yp) ผลผลิตเอทานอลก็ยังสูงสุดสำหรับ mashes หมุนอุณหภูมิสุดท้ายของ gelatinization แป้งจนจบกระบวนการThe final ethanol yield in all studied samples was very high ranging ca. 411–425 g of ethanol from 1 kg of raw material dry matter what corresponded to ca. 92–96% of practical yield based on the sugars content in the raw material. Moreover increase in ethanol yield after the third day of studied SSF experiments were insignificant so these processes could be shortened to 72 h without major loss in product efficiency. Obtained results were higher in comparison to previously reported in earlier studies dealing with waste bread conversion to ethanol. Ebrahimi et al. [15] obtained 350 g of ethanol from 1 kg of waste wheat bread dry matter in a SHF process at substrate loading of 350 g kg1. Kawa-Rygielska et al. [16] studied the possibility of improving the SHF process of waste wheat–rye bread to ethanol conversion by applying complex enzymatic preparations in the mashing stage. The ethanol yield was improved to ca. 366 g kg1 of raw material dry matter in comparison to control mashed without supportive enzymes (ca. 352 g kg1). Pietrzak and Kawa-Rygielska [38] studied the possibility of applying the direct starch to ethanol conversion using granular starch hydrolyzing enzyme (GSHE) on the ethanol fermentation process of waste wheat–rye bread, which is the process similar to traditional SSF but with the omitting of high temperature liquefaction. They stated that direct conversion process without bread waste pretreatment was slightly less efficient than the SHF, and low-temperature enzymatic pretreatment of raw material with a multi-directional enzymatic preparation improved the ethanol yield in comparison to the SHF. The results obtained in present study proved that the simultaneous saccharification andfermentation is more efficient in comparison to separate hydrolysis and fermentation which was also reported earlier [33]. The SSF is also less energy demanding, time consuming and cost efficient than SHF because no need of separate starch saccharification step is done (optimal temperature for glucoamylase activity is ca. 50 C which also acts very slow) and two processes (saccharification and fermentation) acts is one reactor so the investment cost is lower [20]. Srichuwong et al. [29] reported that liquefaction of sweet potato cultivar, which starch exhibited low temperature of gelatinization, at temperatures close to final temperature of gelatinization (66.2 C) resulted in achievement of ethanol efficiency similar to the processes where the used liquefaction temperature of starch was above 80 C what was also achieved in present study. The authors compared their research with sweet potato cultivar having high final temperature of gelatinization (ca. 85 C) and liquefaction at temperatures below this resulted in substantial decrease in process efficiency. This suggests that the SSF process efficiency strongly depends on the thermal properties of starch in used raw material and optimization of liquefaction conditions, based on gelatinization parameters, could result in savings of energy by reduction in liquefaction temperature without loss in product yield. However this require further study with different raw materials. The pre-treatments (drying, dry-grinding) used in this study at a laboratory scale would probably not be economically feasible in the case of industrial scale production of ethanol from bread residues, however they were necessary to keep the homogeneity of the raw material. Because fresh bread has a dry matter content of ca. 500 g kg1, drying could be completely omitted. The dry-grinding could be in this case replaced by hydromechanical shredding in the water slurry what could be combined with the enzymatic hydrolysis. To prevent the raw material from the excessive mold growth, what reduce the ethanol yield [17], the storage time for the raw material should be minimized.
การแปล กรุณารอสักครู่..