Thermostable microbial α-amylases: production, characterization, molecular basis, and industrial applications — a review

Abstract

Microbial α-amylases (EC 3.2.1.1) represent the cornerstone of global industrial starch bioprocessing, valued for their ability to maintain high catalytic velocity and conformational integrity under elevated operating temperatures valued for their ability to maintain catalytic activity under the elevated temperatures characteristic of industrial bioprocessing, with specific poly-extremotolerant variants exhibiting additional resilience across challenging pH regimes, surfactant matrices, and oxidative conditions. This review presents a critical and systematic synthesis of current knowledge on thermostable α-amylases, bridging molecular classification, biosynthesis mechanisms, bioprocess engineering, and downstream applications. We elucidate the contrasting canonical Ca² + -dependent coordination architectures against alternative structural networks that confer a Ca² + -independent biochemical phenotype. The review evaluates diverse microbial reservoirs- spanning thermophilic and hyperthermophilic bacteria, archaea (e.g., Pyrococcales ), thermophilic fungi (e.g., Thermomyces lanuginosus ), and haloalkaliphilic isolates from geothermal, marine, compost, and agro-waste environments. We critically compare submerged (SmF) and solid-state (SSF) fermentation paradigms across normalized volumetric productivity and downstream recovery metrics, highlighting the statistical transition from empirical one-factor-at-a-time (OFAT) screening to interaction-aware response surface designs (CCD and Box–Behnken). Analytical limitations of standard activity assays (DNS, Nelson–Somogyi, iodine dextrinizing, and synthetic chromophores) and kinetic modeling practices are evaluated, emphasizing non-linear regression over distortive linear transformations. Furthermore, we address core technological controversies and industry trade-offs, including inconsistent characterization practices, particularly the frequent conflation of temperature optima with long-term thermostability and the use of non-standardized conditions for reporting residual activity, thermal half-life, kinetic parameters, and enzyme stability, the high cost of downstream processing, and the narrow taxonomic base of industrially exploited strains, and the translation of bench-scale kinetics to high-dry-solids industrial reactor matrices. Finally, we outline key research bottlenecks and future engineering trajectories—including metagenomic bioprospecting of unculturable extremophiles, machine-learning-assisted structure prediction (characterizationFold), and circular-economy biorefinery integration—poised to define the next generation of industrial biocatalysis.