Table 3.

Types of pre-treatment methods.

Sr. No	Pre-treatment method		Key Highlights	References
1	Chemical pre-treatment	Acid Pre-treatment	Hemicellulose can be broken down by acid pre-treatment into pentoses, which can then open the chain of lignocellulose's fibre bundles. Diluted sulphuric acid pre-treatment is the most well-liked. The suitable temperature is typically 100–180 °C, whereas acid concentration is between 0.5 and 10%. Since acid is corrosive, it exerts a load on the reaction equipment at high temperatures and can further degrade sugars into by-products making the fermentation process difficult.	[[57], [58], [59]]
		Alkali pre-treatment	This method eliminates acetyl groups, lignin or damages the lignin structure to promote polysaccharide reactivity. Sodium hydroxide, sodium carbonate, calcium hydroxide, hydrogen peroxide, ammonia, and other common alkaline reagents are a few examples. However, the alkali pre-treatment reaction takes a long time, and neutralisation is required after the reaction.	[[60], [61], [62], [63], [64]]
		Organic Solvent Pre-treatment	To achieve delignification ad cellulose accessibility, some organic solvents like acetone, glycerine, ionic liquids (ILs), ethanol, aqueous tetrahydrofuran, green solvent of imidazole and IL etc are frequently used. This method is environmentally safe with a low level of toxicity. However, it's expensive and problems with organic solvent recovery exist.	[[65], [66], [67]]
2	Physical pre-treatment	Traditional coarse crushing	In the process of bio-converting biomass, substrates are often mechanically crushed beforehand to lower the particle size. To obtain a particle size at mm and μm scale, conventional mechanical crushing often requires cutting, milling, and grinding. It is an environmentally friendly method. No chemical liquid waste. Studies show that a smaller biomass particle size can result in a greater enzymatic hydrolysis efficiency and glucose output.	[68,69]
		Ultrafine grinding	In this method, biomass is milled to obtain a particle size in the micron range. This method increases the cellulose surface area and enlarges the pore size, thus more enzyme is adsorbed onto the substrate making enzyme hydrolysis effective. However, this method involves high energy consumption. Ball milling is another method used that facilitates enzyme hydrolysis by removing lignin, reducing cellulose crystallinity, and increasing the specific surface area.	[[70], [71], [72], [73], [74], [75]]
		Alternative physical pre-treatment	Microwave, ultrasound, and light irradiation are examples of physical treatments that have been used to improve hydrolysis. These pre-treatments primarily employ the physical forces to destroy biomass structure and release cellulose.	[[76], [77], [78], [79], [80]]
3	Physicochemical pre-treatment	Hydrothermal pre-treatment	It is a method to decompose biomass with water under high temperature and high-pressure conditions. It is a green, pollution -free method, but high temperature causes by-products.	[[81], [82], [83]]
		Steam explosion pre-treatment	In this method, the fibrous nature of the lignocellulose is attacked through saturated steam at low pressure and high temperature. Steam explosion treatment is generally advantageous in that there are no adverse environmental effects and low waste stream recycling costs.	[84,85]
		AFEX treatment	In order to depolymerize the fibre structure, AFEX (Ammonia fibre expansion) uses the expansion effect generated from the rapid decompression of steam nitrogen when the liquid nitrogen reaches steam conditions at high heat and pressure. However some disadvantages including high operating cost, liquid nitrogen cost, release of toxic fumes into the environment, and the problems with ammonia recycling and recovery limit the usage of this treatment method.	[[86], [87], [88]]
4	Biological pre-treatment	Using bacteria or fungi	It involves the degradation of lignin components by aerobic bacteria or fungi. It requires less energy, no release of hazardous compounds, and less inhibitors formation. However, it involves issues like low sugar yields, lengthy reaction times (30–60 days), and microbial instability.	[[89], [90], [91], [92]]