Enzymatic conversion of sweet potato granular starch into fermentable sugars: Feasibility of sweet potato peel as alternative substrate for α-amylase production

- The α-amylase production using sweet potato peel as substrate is reported for the first time.
- An efficient approach for enzymatic hydrolysis of sweet potato granular starch was established.
- A. niger crude extract was able to efficiently hydrolyze the sweet potato granular starch.
- Substantial yields of sugars were achieved.

Currently, the conventional process for bioethanol production from starch-based feedstocks incurs high costs. Alternatively, the granular starch hydrolysis process makes starch gelatinization unnecessary and reduces energy requirement, thereby conferring economic advantages. The aim of this study was to establish an effective approach for enzymatic conversion of sweet potato granular starch into fermentable sugars. The effects of some factors on α-amylase production by A. niger were evaluated using a Central Composite Rotational Design (CCRD) combined with Response Surface Methodology (RSM). An improved yield of 214.28 U mL−1 was achieved after 6.5 days of cultivation, at pH 6.0, using 2.0% sweet potato peel as substrate. The α-amylases produced were more active at 60 °C at pH 4.5. Hydrolysis of sweet potato granular starch was also optimized by RSM and CCRD, using A. niger crude extract. It proceeded optimally at pH 4.0 at 50 °C, with an α-amylase loading value of 31.25 U g−1 dry feedstock and a reaction time of 37 h. Under these conditions, a saccharification rate of 78.33% was obtained. This work reports, for the first time, the production of remarkable yields of α-amylase through the use of sweet potato peel. In addition, it describes the achievement of expressive levels of glucose from sweet potato granular starch hydrolysis. These results open up new possibilities for rendering bioethanol production from sweet potato economically competitive.

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