Dilute acid and alkaline pretreatment of spent tea leaves to determine the potential of carbon sources

Tea is one of the most commonly consumed beverages in the worldwide. Therefore, spent tea leaves (STL) are abundant low-cost lignocellulosic materials and can be evaluated as a carbon source for the production of value-added products by fermentation. The aim of this study was to optimize the dilute acid and alkaline pretreatment conditions of STL. The optimal conditions were determined by response surface method (RSM) in terms of fermentable sugars concentration (FSC) and total phenolics concentration (PHC). Independent variables were selected as temperature (115–135 °C), pretreatment time (20–60 min), solid-to-liquid ratio (1:8–1:16 w/v), and dilute acid (1–5% v/v) or alkaline (1–3 N) ratio. It was found that the optimum conditions for dilute acid pretreatment were 131.67 °C, 20 min, 1:10.26 w/v, and 1.58% v/v, which yielded as 21.56 g/L, 0.228 g/g, and 1.53 g/L for FSC, FSY, and PHC, respectively. Nevertheless, the best dilute alkaline pretreatment conditions were 116.42 °C, 20 min, 1:11.23 w/v, and 1.68 N, which achieved as 28.18 g/L, 0.316 g/g, and 5.26 g/L for FSC, FSY, and PHC, respectively. Nevertheless, based on the statistical evaluation, it was found that RSM models victoriously fitted the experimental data related to the pretreatment of STL. Consequently, acid and alkaline pretreated-STL can be considered as suitable carbon source for value-added products’ production by fermentation.