Light pulse of short duration and high intensity penetrates the material. The light pulse that penetrates the material does not transmit but dissipates in the material in the form of heat. A temperature gradient is created between the surface and the inner layer of the material, and heat is transferred from the surface to the inner layer in the form of conduction. Heat conduction continues until the material temperature reaches a constant steady state. The amount of heat dissipated and the thermal properties of the material determine the time required. The duration of the light pulse is shorter than that of heat conduction, and the energy of the light pulse is stored on the surface of the material for a very short period of time, during which little or no heat conduction occurs, which in effect instantaneously heats the surface of the thin layer to a temperature higher than the steady state temperature achieved by heating with a continuous beam of light having an equal amount of average power.
Applications of light pulses
Light pulses can kill microorganisms on sterile packaging materials, liquid foods, solid foods such as meat and fish, and baked foods. Light-treated meat products can extend shelf life in the refrigerated state. Potatoes, tomatoes, bananas, apples and prepared foods such as macaroni and rice can be treated with light pulses to extend shelf life. Tomatoes, although susceptible to mold infection under refrigerated conditions, could be stored under refrigeration after light pulse treatment for 30 d. Similarly, pulse treatment of packaged slices of white bread kept them fresh for up to 2 weeks, whereas untreated slices quickly became moldy. Not only did light pulsing reduce the number of Listeria monocytogenes and Salmonella monocytogenes inoculated in the meat, but the nutritional composition of the meat changed very little.
Light pulsing increases the shelf life and safety of meat. In addition to the sterilization of food, light pulses are used in the following products and fields:
① Manufacturing ingredients and finished products for cosmetics.
② Equipment, products, fixtures and areas requiring a high degree of cleanliness.
③ Medical and dental equipment before use.
④ Food processing equipment to minimize the extent and potential for cross-contamination.
⑤ Treated or semi-treated wastewater to minimize microbial loads.
⑥ Air or other gases or gaseous compounds to reduce microbial load.