Cocoa pod husk valorization: alkaline-enzymatic pre-treatment for propionic acid production

Abstract

Lately, cocoa pod husk (CPH) has received some attention from researchers due to its significant content of cellulose, hemicelluloses and pectin, antioxidant capacity, potential for energy generation, physicochemical characteristics and possibility to be used as adsorbent material or activated carbon. In this work, alkaline (NaOH 2.3% w/v) and enzymatic treatment [Cellic® CTec 2 (2.4% v/v)] were developed before propionic acid (PA) production using fermentable sugars derived from cocoa pod husk hydrolysate (CPHH). The physical, structural and morphological characteristics of raw and treated samples of CPH lignocellulosic matrix were assessed using tools such as scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetry, and X-ray diffraction. The results of these analyses revealed the effectiveness of CPH sequential alkaline and enzymatic treatment for glucose production, reaching maximum concentration of 60.5 g·L⁻¹ and a yield of 275 mg glucose·g⁻¹ CPH. Subsequently, a medium composed by glucose from CPHH (7.5 g·L⁻¹), as low-cost feedstock, glycerol (7.5 g·L⁻¹) and yeast extract (10 g·L⁻¹), was prepared to obtain PA using Propionibacterium jensenii DSM 20274. PA concentration reached a maximum of 10.28 ± 1.05 g·L⁻¹ and a productivity of 0.08 ± 0.01 g·L⁻¹·h⁻¹ after 72 h of fermentation. To the best of our knowledge, there is no literature available on the bioconversion of CPH for the specific production of PA or other organic acids. Thus, the potential of CPH to be used as substrate for PA production was demonstrated with good perspectives.

Graphic abstract