Use of Enzymatic Hydrolysate from Agroindustrial Asparagus Waste as Substrate for the Production of Polyhydroxyalkanoate by Bacillus thuringiensis

Abstract

Polyhydroxyalkanoate (PHA) has unique physicochemical and mechanical properties like conventional plastics; however, its high production cost makes it unsuitable for commercial use. Therefore, the purpose of the present study is to use low-cost and bioavailable raw materials such as agro-industrial waste of asparagus husk, as substrate for obtaining PHA by Bacillus thuringiensis. The proximal characteristics and structural carbohydrates of the waste were previously determined using HPLC. The pretreatment conditions were optimized using a Plackett-Burman design and response surface of the central compounds, evaluating temperature, %NaOH, time, % solid/liquid and solvent. Likewise, the enzymatic hydrolysates of the optimal conditions of the pretreatment were used, using an enzymatic solution with cellulase activity at 45°C at 100 rpm for 72 h. To produce PHA, a mineral-based medium, supplemented with enzymatic hydrolysate from the optimal pretreatment, was utilized. This study examined the effects of varying initial inoculum concentrations (0.25, 0.5, and 0.75 g/L) and percentages of enzymatic hydrolysate supplement (%, v/v). The process was conducted at 30°C and agitated at 125 rpm for 72 h. Maximum production of PHA was obtained with 0.138 g/L from an initial inoculum of 0.75 g/L of B. thuringiensis and a 47% supplement of the enzymatic hydrolysate. The PHA biopolymer was identified by its chemical characteristics by FTIR and correlated by HPLC with a standard. This study contributes to the use of agro-industrial waste to obtain biologically-based bioplastic through a low-cost process aligned with the circular economy strategy.