# Non-Thermal Technologies in Food Processing: Implications for Food Quality and Rheology
## 5 Ultraviolet and Pulsed Light  
UV light inactivates microorganisms primarily through the induction of damage to their DNA/RNA through the creation of dimers between the pyrimidine nucleotide molecules in the nucleic acid backbone which disrupt replication and transcription and lead to cell death [139]. The synthesis of those dimers (pyrimidine 6-4 pyrimidine photoproducts, or 6-4 PPs, and cyclobutane pyrimidine dimers, or CPDs) interferes with the microbes' regular DNA construction and other biological processes [139]. Similarly, PL treatment destabilises microbial cells with the incorporation of intense energy pulses, which bring about rapid photothermal and photochemical annihilation. Structural havoc on bacterial cell membranes results from the high-energy pulses, yielding intracellular material leakage and oxidative stress [12]. Ultraviolet light (UV) has a low penetrating capacity and is used to disinfect food surfaces or packaging. Originally, UV light was only used in transparent liquid, but with improvements in reactors and optimum hydraulics, the delivery and exposure of target microorganisms in different food matrices and foods with varying rheologiesto UV light is now possible. Studies have investigated the effects of UV irradiation on the rheology of fish gelatin. The results from the study by [PERSON] and [PERSON] [128] demonstrated that the gel strength of fish gelatin increased after the treatment with UV irradiation for 30 and 60 min. The viscosity of the irradiated samples was seen to reduce while the melting enthalpy changed significantly. Like UV, the pulsed light (PL) processing method has gained prominence in the food industry. In PL processing, a highly energised, short array of pulses from the white light broad band are passed through food, and this is more effective for food processing and preservation as normal UV light is 1000 times weaker than pulsed light [140; 141]. In recent years, several scientific publications have elaborated on the principles of pulsed light for food processing, including its mechanism for microbial inactivation [142; 143; 144; 145]. PL processing is useful for different solid and liquid foods and has been applied in fruits, fish, meat, and vegetables to remove microbes [147; 148; 149; 150]. UV and PL technologies affect the food rheology by modifying treated products' textural and structural properties. UV treatment has been shown to enhance the gel strength of fish gelatin, resulting in stronger gel networks and reduced viscosity. This is also because of the cross-linking of the UV-induced polypeptides of collagen, leading to higher protein-protein interactions and a firmer gel matrix [151]. UV also leads to partial protein denaturation, exposing reactive amino acid residues to cross-linking, making the gel stronger. The UV light may further lead to the oxidation of sulfhydryl (-SH) groups and, upon forming disulfide (-S-S) bonds, and so a stable harder gel network is created [152]. Similarly, PL processing effectively maintains fresh produce's texture, appearance, and microbial safety. In polysaccharide-rich foods, PL treatment breaks hydrogen bonds and induces depolymerisation, reducing viscosity with the concurrent gelation of a structure of reinforced strength in protein-based systems [139]. Transient thermal and photochemical changes induced by the high-energy pulses reorder water-macromolecule interactions influencing the overall rheological behaviour [12]. These methods are highly effective in preserving the food quality, particularly in maintaining the microbial safety, sensory attributes (such as the colour, texture, and flavour), and nutritional composition. However, their application is limited by their penetration capacity and the nature of the food matrix [128; 141]. The optimal UV and PL treatment conditions for various food matrices have been determined by different studies.