3.1. General findings and background of the studies included in the review
A total of 503 articles were identified through database search in Scopus and ScienceDirect. 109 duplicates and 318 irrelevant articles were removed based on titles and abstract. 64 articles were retrieved for full text screening and 22 articles were excluded based on the established inclusion and exclusion criteria. Finally, 42 articles were included in this review. The flow diagram of study selection is illustrated in Fig. 1.
The articles selected were spread across the globe and almost half of the publications (n = 21) were published in Asia such as Thailand (n = 8), India (n = 6), China (n = 5), Saudi Arabia (n = 1) and Malaysia (n = 1). Meanwhile, articles from other countries were published in Brazil (n = 5), South Africa (n = 4), Mexico (n = 3), Egypt (n = 2), Argentina (n = 2), Serbia (n = 1), France (n = 1), Greece (n = 1), Spain (n = 1), Colombia (n=) and Italy (n = 1). The global distribution of articles is illustrated in Fig. 2. This might be because SSF is used in food processing and the manufacture of traditional fermented foods in the Orient and Asia for centuries. However, there has been increasing awareness in the west on the valorization of waste to valuable products via SSF. Besides that, most of the articles included were published in the year of 2019 (n = 16) and 2020 (n = 10), followed by the year of 2018 (n = 7), 2021 (n = 7) and 2022 (n = 3).
3.1 Solid State Fermentation (SSF)
In today’s world rapid modernization, there are rising technological advancements which are discovered daily. SSF is an exceptional method used since ancient times and it is a fermentation process in the absence of free-flowing water on a solid substrate. SSF is in the spotlight for scientists and industries globally in recent years, due to several advantages related to solid waste management, biomass energy conservation and production of valuable products. Moreover, SSF has more advantages compared to submerged fermentation (SmF) for enzyme production such as utilization of agro-industrial wastes as substrate, higher product concentration, more stable extracellular bioproducts, lower energy requirements and easier downstream processing. Conesa et al. (2020) reported that higher bioethanol produced via SSF due to complex carbohydrates hydrolysis that allows gradual generation of fermentable sugars and reduces substrate inhibition, hence causing an increase in bioethanol productivity.
There are several factors influencing SSF process, which is shown in Fig. 3.
3.1.1 Effect of moisture
Moisture content is an essential aspect in SSF processes as it directly impacts the growth of microorganism. High moisture content promotes particle agglomeration, thus reducing gas transfer and heat dissipation. On the other hand, low moisture content causes decreased solubility of substrate nutrients, thus affecting microbial growth and productivity. A recent study reported 1:2 (w/v, substrate: water) as the most suitable initial moisture ratio for cellulase production by Aspergillus fumigatus, whereby a decrease and increase of moisture content may bring a negative impact towards enzyme production (Fadela et al., 2021). Besides that, Tai el al. reported that 70 and 75% moisture resulted in significantly high (p < 0.05) FPase titers (2.13 U/g and 2.33 U/g, respectively) and CMCase titers (1.26 U/g and 1.11 U/g, respectively) but no obvious effect on xylanase production (Tai et al., 2019).
3.1.2 Effect of inoculum type and size
Inoculum size in the fermentation process is a way to measure the growth of microorganism. Optimum inoculum size is vital for maximum fermentation productivity as lower inoculum size may not be sufficient to initiate microbial growth or it will consume an extended time for microbes to multiply. On the other hand, higher inoculum size will lead to restricted mass transfer while inhibiting microbial growth and reducing fermentation productivity. There are reports which shows that higher inoculum concentration brings negative impact on the yield of bioethanol (El-Hefnawy et al., 2022) which may be due to limitations on the availability of nutrients from substrate.
The use of single or mixed culture also affects the efficiency of SSF. A study confirmed that the use of mixed culture of Saccharomyces cerevisiae, Scheffersomyces stipitis, and Schwanniomyces occidentalis resulted in an increase of ethanol production (Estrada-Martínez et al., 2019). Besides that, type of microorganism that is chosen for fermentation plays an important role. Boonchuay et al. (2018) reported that sugar consumption and ethanol fermentation by S. cerevisiae was negatively affected by high temperature; however, a fair performance was observed in ethanol production by a thermotolerant yeast strain KY618709 at elevated temperature (Boonchuay et al., 2018). Furthermore, SSF processes that employed thermotolerant yeasts such as Kluyveromyces marxianus ATCC-3690 enabled the reduction of processing steps, which decreased the risk of contamination, escalation of saccharification efficiency and eventually resulted in cost reduction (da Silva et al., 2018). Kumar et al. reported that co-culture of S. cerevisiae with a microbe that is isolated from apple pomace soil exhibited elevated bioethanol production from apple pomace via SSF (Kumar et al., 2020). Furthermore, in a study conducted by Salafia et al., enzymatic hydrolysis of pineapple waste releases xylose and arabinose that are unable to be fermented by wild Saccharomyces spp. Thus, the use of mixed cultures and/or recombinant yeasts or the development of microbes that able to ferment hexoses and pentoses simultaneously will lead to an increase in the efficiency of ethanol production (Salafia et al., 2022).
3.1.3 Effect of pH
pH is an essential factor in SSF as it influences the growth of microorganisms. The complex chemical composition of substrate used in SSF has a buffering effect, but it is difficult to monitor the fluctuation of pH due to the absence of free-flowing water and heterogenous condition of substrate. Furthermore, optimum pH range is dependent on the type of organism used for SSF. Filamentous fungi can grow well in an extensive pH range of 2–9, with an optimum range of 3.8 to 6.0. Yeasts can grow in wide pH range from 2.5 to 8.5, with an optimum pH range of 4 to 5. In a recent study that was conducted on SSF of seaweed at varying pH conditions for the production of bioethanol, pH 4 was identified as an optimal pH for the maximum yield of bioethanol by yeast (El-Hefnawy et al., 2022). Fadela et al. discovered that cellulase was produced by Aspergillus fumigatus in pH 7 to 10 and confirmed the alkalophilic nature of produced cellulase (Fadela et al., 2021).
3.1.4 Effect of temperature
The efficiency of SSF process is highly dependent on temperature since microbial growth and secondary metabolites production is correlated with temperature and heat transfer processes. Several studies demonstrated that SSF is highly influenced by incubation temperature which varies according to the microorganism employed in the fermentation process. According to studies conducted by El-Hefnawy et al, (2022), the optimum temperature for highest yield of bioethanol by yeast was recorded at the temperature of 35°C (El-Hefnawy et al., 2022). Lower or higher temperature will lead to the retardation of yeast growth and lower bioethanol production. Besides that, the highest cellulase production by A. fumigatus was recorded at the temperature of 30°C, a drop or rise of temperature of 30°C will cause a decline in cellulase production (Fadela et al., 2021). Passadis et al. evaluated different temperature for optimum enzymatic saccharification of starch and cellulose and the temperature of 35°C is favorable, given that higher energy is required to maintain higher temperature (Passadis et al., 2022).
Silva et al. utilized a thermotolerant yeast, Kluyveromyces marxianus ATCC-36907 for SSF of Carnauba straw and the ethanol production was higher at temperatures of 40°C and 45°C (da Silva et al., 2018). This discovery is vital as employing thermotolerant yeast in SSF can reduce the number of processing steps, reducing the risk of contamination, increasing saccharification efficiency, reducing the inhibitory feedback of cellulase and eventually minimizing production cost.
3.1.5 Effect of particle size
Small substrate particle size enables a large surface area for the attachment of microbes. However, too small particles size may cause agglomeration of substrate, leading to disrupted oxygen transfer which is vital for microbial growth. On the other hand, large substrate particle size allows better aeration but the surface area for microbial attachment will be limited. As per recent study conducted by Conesa et al. (2020), a significant increase in microbial growth is observed for grinded persimmon waste due to easier access to nutrients. Thus, it is important to conduct a thorough study and assessment on optimum particle size in order to ensure maximum efficiency of ethanol production.
3.1.6 Effect of substrate and nutrients supplementation
In SSF system, nutrients are mainly derived from degradation of organic compounds that are present in the solid substrate. However, the lignocellulosic biomass may contain limited nutrients to induce initial microbial growth due to their complex structure and recalcitrant nature. Thus, supplementation of nutrients that are not available or scarce in the substrate such as nitrogen, carbon, minerals, and vitamins may influence the productivity of SSF. In a recent study, nutrient supplementation has led to rise in ethanol production using oil palm trunk residue by 1.43 folds and it reached maximum value at 12-hour fermentation, due to availability of vital nutrients for cell growth and metabolite production (Nutongkaew et al., 2020). Zapata et al. observed that Trichoderma asperellum and Trichoderma reesei colonies grow larger on sawdust that is supplemented with yeast extract and peptone. These media modifications have also altered T. reesei morphology with denser and highly branched mycelia, causing larger surface area for enzyme-substrate interaction and resulting an elevated enzyme production (Zapata et al., 2018). The increase of solid loading during SSF leads to an increase of ethanol production as reported by Passadis et al. Higher solids loadings imply higher carbohydrates loading, higher glucose production and higher ethanol production (Passadis et al., 2022). A study conducted by Wang et al. demonstrated that N2 periodic pulsation could significantly improve the efficiency of ethanol fermentation due to availability of more nutrition and better gas exchange (L. Wang et al., 2021).
3.2 Utilization of Lignocellulosic Wastes as Substrate
Nowadays, the production of lignocellulosic waste has expanded dramatically due to rapid economic development, massive urbanization, and growing population due to diverse lifestyle adaptations. Management of widely available lignocellulosic waste has drawn the interest of researchers globally due to its conversion into valuable products that include bioethanol. The capacity of lignocellulosic biomass production in various environments has caught attention as the biomass is directly linked to agricultural practices. Lignocellulosic biomass is comprised of cellulose, hemicellulose, and lignin as major building blocks. The composition of these components depends on the type of lignocellulosic biomass. The usage of lignocellulosic wastes as substrate in recent studies and its composition are summarized in Table 1.
Pretreatment is crucial to the fermentation of lignocellulosic biomass in order to improve the accessibility of lignocellulosic sugars or nutrients for the growth of microorganism. The polysaccharides in the cell wall are closely packed, thus forming extremely recalcitrant structures that are resistant to direct enzyme attack and they are usually encapsulated by lignin (Himmel et al., 2007). There were significant differences which were discovered between untreated and heat-treated biomass, such as sterilization stage that eliminated the contamination of other microorganisms present in the biomass, thus increasing the growth of microorganism in interest. Zapata et al. reported that enzymes activity was low when sawdust was utilized as a substrate for SSF and it was suggested that the delignification of sawdust prior to SSF with Trichoderma should be studied thoroughly as lignin is the main challenge for enzyme production (Zapata et al., 2018). Silva et al. evaluated three different pretreatments to optimize the enzymatic hydrolysis of carnauba biomass, namely hydrothermal (HT), alkaline (AL), and alkaline acid (AA) pretreatment. The AL pretreatment exhibited the best method for pretreatment of carnauba biomass as it could significantly remove both lignin and hemicellulose (75.60% and 64.02% respectively) (da Silva et al., 2018). In addition, steam explosion is one of the biomass pretreatment methods which is classified as a green technology due to its utilization of only lignocellulosic biomass with water and has successfully hydrolysed and delignified water hyacinth prior to SSF process (Figueroa-Torres et al., 2020).
High enzyme cost is one of the vital challenges to make SSF process viable commercially. Thus, it would be ideal to establish a low-cost process for enzyme production by utilizing abundantly available lignocellulosic biomass. Furthermore, SSF is a promising fermentation technology because of its ability to utilize lignocellulosic biomass to support the growth of microorganisms. A number of studies were conducted last few years on cellulases, xylanases and other enzymes production by solid state fermentation using lignocellulosic wastes including wheat straw and cotton oil cake (Singh et al., 2022), rice straw (Bala et al., 2020; Jha et al., 2019; Kaur et al., 2020; Singhajutha et al., 2020), water hyacinth (Figueroa-Torres et al., 2020), sugar beet pulp (Fadela et al., 2021), oil palm frond (Tai et al., 2019) and wheat bran (de Barros Ranke et al., 2020; Gama et al., 2020; Zhao et al., 2019).
Table 1
Lignocellulosic wastes that were utilized as substrate and its compositions
| Substrates used | Approximate Compositions of Substrate (%) | Reference |
| Cellulose | Hemicellulose | Lignin | |
| Persimmon waste (Peel and Calyx) | 6.37 | 4.84 | 1.86 | (Conesa et al., 2020) |
| Carnauba Straw | 25 | 10 | 35 | (da Silva et al., 2018) |
| Pineapple leaves | 62.37 | 22.38 | 5.45 | (Imman et al., 2021) |
| Apple pomace | 20.8. | 23.02 | 17.7 | (Kumar et al., 2020) |
| Sugarcane baggase Wheat bran Soybean meal Rice husk | 47.3 34.0 18.0 44.3 | 22.0 30.0 6.0 26.0 | 28.0 5.0 3.3 16.0 | (Pereira Scarpa et al., 2019) |
| Corn stover | 33.2 | 24.9 | 20.8 | (Dong et al., 2019) |
| Oil palm trunk residue | 13–55 | 12–20 | 1.30–13.00 | (Nutongkaew et al., 2020) |
| Cassava peels | 14.26 | 33.64 | 12.87 | (Aruwajoye et al., 2020) |
| Vetiver grass | 45.22 | 32.07 | 14.45 | (Subsamran et al., 2019) |
| Wheat bran Milled rice straw | 36 30 | 28 25 | 8 19 | (Zhao et al., 2019) |
| Alstroemeria sp. waste | 36 | 14.7 | 15.2 | (Zamora Zamora et al., 2021) |
| Sweet lime peel | 25.4 | 9.4 | 23.6 | (John et al., 2022) |
3.3 Production of Bioethanol by Solid State Fermentation
Currently, bioethanol is one of the most widely utilized biofuels throughout the globe and has successfully reduced the consumption of crude oil from fossil fuels while diminishing environmental pollution. Ethanol productions are traditionally carried out by submerged fermentation, however pasts research indicated SSF as a more practical method due to utilization of agro-industrial biomass and less water requirement. Saccharomyces cerevisiae was extensively used for the bioconversion of biomass such as apple pomace (Kumar et al., 2020), seaweed (El-Hefnawy et al., 2022), persimmon waste (Conesa et al., 2020), sweet sorghum stalks (Li et al., 2021), pineapple waste (Salafia et al., 2022), corn cob (Sewsynker-Sukai & Gueguim Kana, 2018), cassava peel (Aruwajoye et al., 2020), potato peel (Chohan et al., 2020), municipal food wastes (Estrada-Martínez et al., 2019), damaged rice grains (Mihajlovski et al., 2018) and sugarcane bagasse (Jugwanth et al., 2020) to bioethanol by solid state fermentation.
In ethanol production, the biomass would undergo pretreatment, saccharification of pretreated biomass, sugar fermentation and ethanol recovery (Cardona et al., 2010). The ethanol production cost is highly influenced by 4 factors namely energy, water, enzymatic formulations, and yeast. In addition, Bala et al. reported that the utilization of enzymatic cocktail of Myceliophthora thermophila that was obtained by SSF of rice straw is one of the strategies which is considered cost-effective and eco-friendly in bioethanol production. Despite the presence of sugars in the solid substrate, SSF enables gradual generation of fermentable sugars for further fermentation to bioethanol. This process reduced the exposure of microorganism to high sugar content and substrate inhibition was less likely to occur (Conesa et al., 2020).
3.4 Production of Postbiotic by Solid State Fermentation
In a typical ethanol production, saccharification of the pretreated biomass is often accomplished with commercial cellulolytic enzymes, which is one of the steps that contributes the most to the overall cost of manufacturing process (Mesa et al., 2016). Although the use of commercial enzymes is practical, the process may not be economically feasible due to high cost of these enzymes. Thus, one of strategies that should be considered to reduce the production cost is production of postbiotic which includes enzymes and other metabolites by SSF. SSF is one of promising fermentation technologies for bulk enzyme production due to its ability to utilize lignocellulosic residues for the growth microorganism.
In a recent study conducted by Kaur et al. (2020), an in-house preparation of enzymes by SSF from rice straw using Aspergillus niger resulted in production of a broad range of enzymes including the complete cellulase system along with xylanases and mannanases (Kaur et al., 2020). Furthermore, they reported cost reduction of the crude cellulase-hemicellulase consortium as compared to the costs of cellulases production in other studies. Guillaume et al. worked on biocatalyst production by SSF of Aspergillus tubingensis on a mix of wheat bran and rapeseed and characterization of enzymatic composition of the biocatalyst was conducted. A total of 131 proteins were identified and they are mainly hydrolases with more than half (58.0%) is protease (aspergillopepsin). Besides that, enzymes such as pectinases, endoglucanase, β-glucosidase, hemicellulases and xylanases were also identified. The crude extract of biocatalyst was then applied in the fermentation of ethanol, showing greater performance and an increase in ethanol production compared to single and combined purified enzymes which might be due to synergistic effect of various enzymes in the crude extract (Guillaume et al., 2019). The composition of enzymes complex produced varies greatly based on the type of microorganism and substrate used. A secretome analysis of thermophilic mould Myceliophthora thermophila by SSF on rice straw was conducted by Bala et al. and it showed good titres of xylanase and cellulase enzymes with the presence of various proteins along with inducible CAZymes that were required for the degradation of lignocellulosic biomasses (Bala et al., 2020).
3.5 Comparative data for the production of bioethanol and postbiotics by using conventional methods and SSF
Bioethanol and postbiotic can be produced either by submerged fermentation (SmF) or solid-state fermentation (SSF). SSF involves growing microorganisms in the absence of free-flowing water with maximum substrate concentration. Different studies compared the production of bioethanol and enzymes by SSF and SmF. Higher yields of SSF compared with SmF were observed in the study of Passadis et al. where the ethanol yields were higher (up to 30%) during SSF (Passadis et al., 2022). Besides that, ethanol is produced rapidly with higher concentration and higher yield during SSF, indicating superiority over SmF. In a study conducted by Conesa et al., results revealed that SSF had more advantages compared to direct fermentation, as ethanol yields were significantly higher for SSF (p-value < 0.05) due to higher solid load, implying that more fermentable sugars were available but substrate inhibition phenomena were reduced (Conesa et al., 2020).