In　this　study,　the　molecular　transformation　of　sludge　biopolymers　during　hydrothermal　treatment　with　the　temperature　ranging　from　25　degrees　C　to　200　degrees　C　was　examined　and　was　seen　to　significantly　affect　the　macrophysical　properties　(dewaterability　and　rheological　property)　of　sludge.　The　results　showed　that　the　sludge　dewaterability　and　flow　ability　under　high　shear　stress　deteriorated　by　a　hydrothermal　process　at　25　degrees　C　to　120　degrees　C,　but　the　deterioration　alleviated　above　the　temperature　threshold　of　120　degrees　C.　The　consistence　of　changes　in　sludge　dewaterability　and　rheological　property　in　HT　process　was　mainly　attributed　to　the　variation　in　gel　properties　of　soluble　biopolymer.　Two-stage　changes　in　biopolymer　transformation　were　identified,　beginning　with　a　solubilization　stage　from　25　degrees　C　to　120　degrees　C　in　which　a　biopolymer　with　a　gel-like　network　structure　was　released　into　liquid　phase,　creating　flow　resistance　under　high　shear　stress　such　that　sludge　dewaterability　deteriorated.　The　second　stage　was　identified　as　a　conversion　stage　(120　degrees　C-200　degrees　C)　in　which　proteins　and　polysaccharides　hydrolyzed　and　experienced　a　Maillard　reaction,　leading　to　the　degradation　of　the　biopolymer　network　structure.　The　newly　formed　recalcitrant　Maillard　products　showed　weak　flow　response　to　high　shear　stress,　allowing　for　an　improvement　in　sludge　dewaterability.　The　pathways　of　a　Maillard　reaction　were　identified　via　gas　chromatography-mass　spectrometer　(GC-MS),　nuclear　magnetic　resonance　spectroscopy　(＇H　NMR)　and　two-dimensional　correlation　spectral　analysis　(2D-COS)　of　Fourier-transform　infrared　spectrometer　(FTIR),　etc.　Three-dimensional　excitation-emission　matrix　(3D-EEM)　proved　to　be　an　applicable　method　for　tracking　Maillard　reaction　in　sludge　hydrothermal　process　due　to　the　distinctive　fluorescence　characteristics　of　Maillard　products.　This　study　further　clarifies　the　obscure　process　of　sludge　hydrothermal　treatment　and　will　help　improve　the　accuracy　of　subsequent　research.　(C)　2021　Elsevier　B.V.　All　rights　reserved.