Milk is one of the most consumed liquid foods worldwide. The latter is a consequence of its well-accepted flavor, high nutritional value and low price. The co-existence of proteins, lipids and riboflavin (RF, vitamin B2; an endogenous photosensitizer found in milk), along with the harsh conditions encountered during milk processing (e.g. high temperatures, high pressures, light exposure) results in the formation of oxidants. Caseins represent 80% of the total protein content in milk, and due to their amphiphilic nature, they stabilize the oil-water interface; this results in exposure to both hydrophilic and lipophilic oxidants. We hypothesized that exposure of caseins (α-, β-, and κ-casein) to light in the presence of RF, or peroxyl radicals (ROO•) would trigger modifications to the amino acid side-chains of caseins, with downstream consequences for structure and function. We also predicted that the high casein concentrations in milk would facilitate chain reactions and damage propagation. Our results demonstrate that both ROO• and RF-mediated oxidation generates multiple different oxidation products (from Trp, Tyr, Met, His, Cys) and crosslinks including diTyr (from multiple caseins) and disulfide bonds (with κ-casein). At low casein concentrations (1 mg/mL) higher yields of crosslink products were detected by SDS-PAGE. However, at high protein concentrations (10–27 mg/mL) a great extent of total amino acid consumption and modification were detected, consistent with complex oxidation mechanisms. Thus, for example, an increase in the length of chain reactions from 2.21 to 10.52 was evidenced for oxidation of κ-casein at 1 mg/mL and 20 mg/mL, respectively. These results highlight the importance of understanding, at a molecular level, the processes occurring during oxidation of proteins in complex biological matrices such as foodstuffs, to enable the development of new strategies to improve industrial processing methods