Ricotta cheese was subjected to PL treatments at increasing fluence (1.3, 3.1, 7.5, 15.0 J/cm2) and stored at 4 °C. Just treated samples presented microbial counts (Pseudomonas spp., Enterobacteriaceae, yeasts) lower than those of the untreated one. PL at 1.3 and 3.1 J/cm2 allowed delaying microbial spoilage during storage but higher fluences favoured microbial growth and off-odour formation, probably due to the surface nature of PL technology. The penetration depth of the UV-C fraction of PL resulted of only 1.5 mm, hindering microbial inactivation at the core and promoting local modification of ricotta cheese constituents. FTIR, SDS-PAGE and DLS analysis demonstrated that PL induced the formation of small protein particles, able to interact with lipids and carbohydrates, and rearrange into larger aggregates. Aggregation reduced protein solubility and occurred following exposure of hydrophobic protein groups, enhancement in S–S bonding and changes in α-helix and β-sheets structures. Proteins photoreaction was confirmed by the formation of melanoidins and carbonyls.
