Evaluating Wastes as Low-Cost Substrates for Use in Microbial Citric Acid Production

Introduction

Using fermentable substrates as a carbon source is one of the most interesting topics and challenges in biotechnology. For this purpose, it is important to provide low-cost and easily available raw materials (Monrroy et al., 2019). Microbial-based organic acid production, rather than chemical production, can reduce environmental concerns. This progress in production accelerates efficient and environmentally friendly fermentation (Sharma et al., 2021). It is of great interest to make the biotechnological CA production more economical and sustainable through its use of cheap renewable carbon sources such as agricultural industrial wastes and their by-products. These substrates, which are suitable for the production of CA, are rich in carbohydrate and other vital nutrients (Dhillon et al., 2013). The use of food wastes as a substrate in CA production helps to create an effective recycling process to increase the added value (Nayak & Bhushan, 2019). Because, one of the recent problems in this industry has been the inability to make enough profit in CA production due to high raw material and energy costs. Waste generated during food processing accounts for 39% of the total food waste in European Union (EU) countries. However, these wastes can adversely affect human health and the ecosystem by creating significant environmental problems such as greenhouse gas emissions, and water and soil pollution. CA is mainly produced by Aspergillus niger using submerged fermentation (SmF) but the solid-state fermentation (SSF) technique attracts considerable attention due to its advantages. Using a similar environment to the microorganism’s natural environment, low bacterial contamination risk under low substrate moisture and reduced liquid phase, reduction to the volumes of waste material and liquid effluents, operation in non-sterile conditions, simple fermentation facilities and lower energy consumption of SSF are considered to be advantageous over SmF (Roukas & Kotzekidou, 2020). SSF has gained importance in recent years as an alternative method for CA production to use agro-industrial wastes (Dhillon et al., 2011a). On the other hand, moisture content, substrate particle sizes, nutrient sources, incubation temperature, pH, and inoculum level are very effective in the growth of microorganisms and the formation of the final product (Ali et al., 2016). It is also emphasized that optimizing the medium composition is effective in improving CA production by A. niger and Yarrowia lipolytica (Hu et al., 2019). Finally, nutrient-rich biomass turns into a suitable substrate for the growth of microorganisms for industrial processes when the suitable conditions are provided. The objective of this study is to compile the wastes used as substrate in the production of CA from the past to present, together with the factors affecting the production, to shed light on future studies.