The interest in substituting traditional fossil fuel-based materials with biobased chemicals has been increasing in the last 15 years (Fadlallah et al., 2021; Ferreira-Filipe et al., 2021; Weiss et al., 2012; Wilson and Schwarzman, 2009). This switching has become more necessary because of the concerns about greenhouse gas emissions and supply chain uncertainty. As goal 13 of the 2030 United Nations sustainable development agenda states, impactful actions, such as developing new technology, must be taken to reduce greenhouse gas emissions and stop climate change (United Nations, 2016). However, the benefits of substituting fossil-fuel synthetic materials with biobased chemicals still need to be evaluated against the associated environmental impacts (Mann et al., 2020; Van Schoubroeck et al., 2018; Weiss et al., 2012). Different aspects, such as technical, economic, and social issues, must be systematically studied before switching to a biobased economy (OECD, 2022). The current focus of the biobased economy is a graduate change from energy production to the production of high-value biobased materials and chemicals (Diep et al., 2012).

An example of shifting toward the biobased economy and production of high-value biobased materials is noted in the biorefining industry (Blair and Mabee, 2021). Biorefining is a group of processes where different biomass feedstocks are converted into a spectrum of biobased products, such as chemicals, materials, and energy in an analogous approach to the existing petroleum refineries (Annevelink et al., 2022). Biorefineries can use forest and agricultural wastes as new resources in integrated facilities capable of producing multiple products sequentially (Arpit Singh et al., 2022; Matharu et al., 2016; Yoo and Kim, 2021). Traditionally, well-established industries, such as pulp and paper, starch, and sugar, use conventional conversion processes for biorefining (Diep et al., 2012). However, developing a more efficient approach, where biomass conversion to biobased chemicals is prioritized before energy conversion, is part of the strategies for maximizing the sustainability potential of biorefining (Clark et al., 2012; Gajula and Reddy, 2021). Therefore, by applying the principles of green chemistry (i.e., utilizing a more energy-efficient process, using renewable feedstocks, reducing the production of derivatives, and producing chemicals more susceptible to degradation) to biorefining, decisive steps can be taken to mitigate climate change while meeting the industry demands (Clark et al., 2012; Heo et al., 2022).

Biopolymers are found in different living organisms and can be extracted as leading products, by-products, or wastes by various processes, including biorefining (Baranwal et al., 2022; Demuner et al., 2019). Lignin is an aromatic biopolymer constituting approximately 30% of the lignocellulose biomass. Biologically, lignin offers rigidity and protective properties to the cell walls in vascular plants (Liu et al., 2018). As the second most abundant biopolymer, it is highly available and mainly obtained as a by-product from the delignification of wood in the pulping production process (Haile et al., 2021). Lignin is chemically composed of phenylpropane units arranged in complicated tridimensional structures. Lignin is rich in functional groups such as phenoxy, methoxy, carboxyl, and carbonyl (Katahira et al., 2018). Owing to its chemical properties and extensive availability, lignin is strongly considered a green alternative to oil-based and synthetic chemicals and materials (Mariana et al., 2021; Moretti et al., 2021). Starch is another biopolymer highly utilized in diverse applications in green chemistry (Al-Douri, 2021; Stanisz et al., 2020). Starch is a polysaccharide made of anhydroglucose units (AGU) and the second most abundant carbohydrate in many plants, which functions as an energy reserve macromolecule (Le Corre et al., 2010). Starch's biodegradability, extensive accessibility, and low cost make it often a preferred feedstock of green polymeric formulations (Falua et al., 2022; Salehizadeh et al., 2018).

Despite the abundant availability and low production cost of lignin and starch, the industrial use of these natural polymers requires chemical modification to strengthen their properties (Balakshin et al., 2021; Zha et al., 2019). The structural complexity and variability of lignin restrict many possible applications, which is the reason for its modification (Ragauskas et al., 2014). The chemical modification of lignin is often considered to be included in biorefining processes to create new lignin-based green polymers with enhanced physicochemical properties proper for high-value utilization (Laurichesse and Avérous, 2014; Suota et al., 2021). For instance, carbon fibers (Souto et al., 2018), polymer alloys (Gonçalves et al., 2022), adhesives (Ang et al., 2019), resins (Van Nieuwenhove et al., 2020), fillers (Jeong et al., 2012), flocculants (Wang et al., 2020a, Wang et al., 2020b), and dispersants (Zhu et al., 2021) are among the most produced and studied lignin-based materials. Although >50% of commercial starch is used for non-food applications, its natural properties do not always meet the requirements for a wide variety of specialized applications (The European Starch Industry Association, 2017). Also, the industrial use of starch needs to be carefully targeted because starch is an essential component of human food (Wang et al., 2020a, Wang et al., 2020b; Yang et al., 2022). Generally, starch is chemically modified to increase its solubility in water and enhance its mechanical, thermal, and reactive properties (Lermen et al., 2022). Starch-based green polymers are mainly used as glues (Chen et al., 2019), flocculants (Liu et al., 2017b), dispersants (Zhu et al., 2013), films (Tang et al., 2022), hydrogels (Ma et al., 2022), and packaging materials (Tapia-Blácido et al., 2022).

As presented in this review paper, the blend of lignin with starch has received significant attention in the last decade. Most studies have focused on reinforcing the starch matrices with lignin to enhance moisture/water sensitivity, thermolability, UV protection, and mechanical weakness. Therefore, this review article explores the information associated with producing starch-lignin materials as a new approach to creating biobased materials. Moreover, the challenges, current trends, and future opportunities related to the production of starch-lignin materials are also discussed. First, this review paper discusses the general description, sources, and chemical modification of lignin and starch. Furthermore, the discussion will focus on starch-lignin materials and their properties and production approaches, which will be followed by a review of the characterization methods of starch-lignin macromolecules.