UV radiations’ ability to kill bacteria, molds, yeasts and viruses was recognized in the early 1800s. Its implementation was only possible after the invention of the Finsen UV lamps for which Niels R. Finsen was awarded a Nobel prize in 1903. One of the earliest practical applications of UV sanitizing was disinfecting the municipal water in Marseille, France.
Commercialization of UV lamps was pioneered by the Westinghouse company which produced units for sanitizing hospital equipment. Shortly after, UV sanitizing lamps were installed in drug manufacturing as well as food, dairy and beverage production plants.4
How does UV sanitizing work?
Ultraviolet rays encompass the invisible spectrum of non-ionizing electromagnetic radiation with a wavelength of 100–400 nanometers (nm). UV sanitizing referred to as UV-C rays cover the wavelength range from 100–280 nm. They are most effective at 240–260 nm.2
UV-C light is used effectively in sanitizing and disinfecting surfaces by eliminating most of bacteria, viruses, molds and odors.
Exposure of infected surfaces to UV-C light allows the rays to penetrate the microorganisms cell membranes. Subsequent photochemical reactions with the thymine constituent of the cell’s DNA/RNA produce thymine dimers. The latter arrest the metabolism of pathogenic cells and prevent them from further multiplication.
UV-C destruction of microorganisms is an exponential process, i.e. the higher the exposure, the more effective is the microbial destruction.5 For example, UV-C exposure necessary to kill 99% of microorganisms is twice that required for killing 90% of them.
Ultraviolet rays dosing is the energy per m2 of surface for a specified exposure time (min) to obtain >1 logarithmic reduction in microbial population.5
The dose required to kill a given pathogen depends on its cell structure. For example, bacteria such as Salmonella, Listeria and E. Coli have thin cell walls and can be easily killed by UV-C rays. Thick cell-walled mold spores, however, can partially block UV-C rays and thus require higher radiation doses.
Selection of UV-C light equipment depends mainly on the type of the pathogen and its original and target counts as well as prevailing moisture and temperature conditions in the treated area. UV-C exposure levels used in sanitizing food manufacturing operations range from 10 to 100 mJ/cm2.
UV-C rays are produced essentially from mercury vapor produced by specialized lamps. Two types of UV light sources can be used in UV disinfection processes:6
- Low pressure mercury discharge tubes: includes conventional and amalgam types. The conventional type is the most energy efficient source. It converts electrical input into UV-C light output at wavelength 254 nm, i.e. within the germicidal effect. The amalgam type (mercury plus indium or gallium), on the other hand, provides three-times the light output of conventional tubes and is less sensitive to ambient temperatures.
- Medium (high) pressure mercury discharge lamps: electrically not very efficient but can produce higher UV-C light intensities. Therefore, they can achieve a target light exposure in a much shorter irradiation time.
Several lamps can be installed together with suitable reflectors e.g. walls coated with reflecting materials such as stainless steel to allow most of the radiation to bounce back into the target surface.
UV-C rays are currently used in food manufacturing plants for continuous decontamination of meat processing, bakeries and cheese conveyor belts. It has also been used in sanitizing packaging containers, tubes, films and foils.
In bakeries, microbial contamination takes place mostly after baking i.e. during cooling, slicing and packaging. Freshly baked bread leaves the oven warm and at high moisture levels, i.e. an ideal medium for mold and fungus contamination. Cooling air, depending on the outside conditions, contains high levels of Bacillus subtilis spores, a source of mold.
Traditionally, production of mold-free bread was achieved using a combination of reduced water activity (aw) and preservatives such as sorbates and propionates. Current consumer demands for preservatives-free bread, however, has forced bakeries to invest in new preservation technologies such as vacuum cooling, infrared, cold-plasma and ultraviolet light.
Only few research studies have focused on sanitizing bakeries and baked goods with UV-C rays. A study on treating baguettes with UV-C radiation (32kGy) confirmed its potential in increasing the product shelf life by 100% compared to 26% for calcium propionate and 106% for potassium sorbate.7
Despite the proven benefits of UV sanitizing in various food manufacturing processes, the baking industry still lags behind. New promising improvements in UV-C systems have been introduced recently such as by installing spiral coolers operating in a continuous mode to provide seamless transportation of the baked goods from the oven to the slicing and packaging stages. Another approach involves installing UV radiation lamps above and around the sides of conveyor belt to properly cover all surfaces of product and food-contact surfaces, thus reducing the risk of producing non-conforming out-of-spec products.
Advantages of UV sanitizing
- It is a non-thermal method, therefore no physicochemical changes to the food/baked product should be expected
- Effective against all types of pathogens (bacteria, viruses, yeasts, fungi, molds, etc.)
- The possibility of reducing contamination by up to 99.99% in food products represents a clear competitive advantage for food manufacturers
- Eliminates the need for harsh chemicals such as quaternary ammonium compounds and sodium hypochlorite or heat treatment
- HACCP compliance
Limitations of UV sanitizing
- Extremely low penetration power and complete absorption by outer surfaces
- Intensity of rays decreases as the distance from the source becomes larger; hence decreasing the overall killing capacity or effectiveness of the sanitizing treatment4
- UV-C rays are ineffective in dead points (covered by surfaces above them or in crevices)4
- Some bacteria strains can develop resistance to UV radiation, thus the occasional need for combining UV treatment with chemical disinfection methods to ensure proper cleaning and sanitation4
- Capital cost required for system installation
- Potential health and safety concerns. UVC-rays can produce secondary emissions of UV-A and UV-B rays which can severely irritate eyes and cause skin inflammation. Suitable shields can prevent personnel from being directly exposed and affected by radiation.2
The following table reports the approximate lowest dose (J/m2) of UV radiation at 253.7 nm to kill 90% (i.e. log reduction of 1 or produce a 10-fold decrease) of various microorganism populations:2
|UV DOSE (J/m2)
|Bacillus subtilis (vegetative form)
|Bacillus subtilis (spore form)
The FDA has approved the use of UV-C for treating and sanitizing foods and food processing equipment (21 CFR 179.39).8
- Stanga, M. “Disinfectants and Sanitation Technology” Sanitation: Cleaning and Disinfection in the Food Industry, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2010, pp. 524–554.
- Stanga, M. “Environmental Sanitation” Sanitation: Cleaning and Disinfection in the Food Industry, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2010, pp. 459–471.
- Kopper, G. “Hygiene in Primary Production” Food Safety Management: A Practical Guide for the Food Industry, Academic Press, Elsevier Inc., 2014, pp. 559–620.
- VC TECHNOLOGY: HISTORY AND EXPLANATIONS. https://www.sportsturfnw.com/uvc-equipment/uvc-technology-history-and-explanations/.
- Marriott, N.G., Schilling, M.W., and Gravani, R.B. “Sanitizers” Principles of Food Sanitation, 6th edition, Springer International Publishing AG, 2018, pp. 175–183.
- Lagunas-Solar, M.C. “Pulsed Ultraviolet Radiation Processing” Encyclopedia of Food Safety, Volume 3, Academic Press, Elsevier, Inc., 2014, pp. 225–238.
- Doulia, D., Katsinis, G., & Mougin, B. Prolongation of the microbial shelf life of wrapped part baked baguettes. Int. J. Food Prop. 2000, 3, 3, pp. 447-457.
- FDA. Irradiation in the production, processing and handling of food. 21 CFR.179.31. https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfCFR/CFRSearch.cfm?fr=179.39.