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In this study, two thermostable endoglucanases (Rem_GH5EG and Rem_GH7EG) from Rasamsonia emersonii were heterologously expressed in Pichia pastoris and characterized to evaluate their potential for industrial biomass saccharification. Rem_GH5EG demonstrated markedly superior catalytic efficiency toward barley ß-glucan (kcat/Km = 6.3 × 10^-3/mg mL/min), while Rem_GH7EG exhibited a preference for carboxymethyl cellulose (kcat/Km = 1.17 × 10^-3/mg mL/min). Notably, Rem_GH5EG showed optimal activity at 90 °C with a half-life (t_1/2) of 2 h, whereas Rem_GH7EG was active at 70 °C with a half-life (t_1/2) of 1 h, highlighting its suitability for high-temperature hydrolysis processes. Moreover, pre-conditioning of steam and acid pretreated unwashed rice straw slurry with Rem_GH5EG at 90 °C effectively reduced viscosity-related mass transfer limitations, thereby enhancing the hydrolytic efficiency of benchmark cellulase. These findings underscore the industrial relevance of Rem_GH5EG as the more promising candidate for developing efficient enzyme cocktails for biomass saccharification.