AbstractBackground and objectivesProtein powders contain metastable radicals that are released upon hydration. The influence of protein source, free radical content, and hydration condition on luminescence was investigated.FindingsHydration of rice, soy, pea, and egg albumin generated a burst of luminescence in the absence of added enhancers/stimulates. Rice proteins were generated from 3 to 10 times more luminescence than other protein samples. Hydrating proteins with 25 mM H2O2 increased the luminescence intensities from all food proteins. Luminescence maximums occurred between 550–600 nm for rice protein, 500–600 nm for soy protein, and two peaks from egg albumin (500–550 nm and <450 nm). Carbon‐centered metastable radicals were most abundant in rice protein which contained between 3.87 × 1016–53.44 × 1016 spins per gram. Plotting intrinsic luminescence versus spins per gram (including all types of proteins examined) revealed an r‐square of 0.771.ConclusionsBased on peak shape, g‐value (2.0050 ± 0.0003) and power saturation (4–8 mW), metastable radicals in proteins were identified as carbon‐centered. Both intrinsic and added sources of oxidative stimuli strongly correlate with the generation of chemically stimulated luminescence from hydrated proteins.Significance and noveltyThese findings are the first to correlate the quantity of metastable radicals in rice proteins with intrinsic luminescence generated upon hydration and provide spectral characterization of luminescence from these proteins.