Catalyzes the metal-independent reduction of curcumin to dihydrocurcumin (DHC) as an intermediate product, followed by further reduction to tetrahydrocurcumin (THC) as an end product. It also acts on 3-octene-2-one, 3-hepten-2-one, resveratrol, and trans-2-octenal. 2 NADP(+) + tetrahydrocurcumin = curcumin + 2 H(+) + 2 NADPH Inhibited by thiol-specific reagents (p-chloromercuribenzoate and 5,5'-dithio-bis-2-nitrobenzoate). Optimum pH is 5.9. The enzyme is most stable in the pH range of 4.5 to 12.0, with 90% of the initial activity being retained even at pH 12.0. Optimum temperature is 35 degrees Celsius. The enzyme is most stable from 10 to 50 degrees Celsius. Homodimer. Belongs to the NADP-dependent oxidoreductase L4BD family.