Ozone is generated commercially within the food industry by one of two generally accepted procedures: by passing an oxygen-containing gas through either a source of ultraviolet (UV) radiation or a high-energy electrical field. The first method is known as photochemical (UV), the second is corona discharge (CD), sometimes referred to as ‘plasma techniques’.
There are several less commercially mainstream methods of making ozone (electrolysis, radiochemical, reaction of elemental phosphorus with water).
Virtually all food processing applications use CD ozone generation. A few applications (air fumigation and some recent modified air packaging applications) use UV radiation. Some applications involve both CD and UV, although not applied at the same time.
Ozone is a metastable molecule produced from elemental oxygen.
Clearly, ozone cannot be generated by thermal activation of oxygen, since the standard free energy of formation of ΔG° (1 atm) = +161.3 kJ/mol.
Generation of ozone.
The technologies involved in CD ozone generation are varied, but all such ozone generators operate fundamentally by passing a dried, oil-free, dustfree, oxygen-containing gas through a high-energy electrical field between two special electrodes, one of which serves as the ground electrode, and the other as the dielectric (current-bearing) medium.
As oxygen molecules pass through the electrical field, they are caused to split apart, forming very active atomic oxygen radicals that can combine with intact oxygen molecules to produce molecular ozone, O3.
It is generally accepted that CD ozone generators are classified into three types: low frequency (50–100 Hz), medium frequency (100–1000 Hz) and high frequency (>1000 Hz). Some 85–95% of the electrical energy supplied to a CD ozone generator produces heat, inherently reducing generated O3 output and capacity/energy efficiency.
Consequently, transference and heat-removal systems are standard items for CD ozone generators. Accordingly, CD systems utilise one or more of the following cooling methods: air, water with oil or Freon, or water.
Any CD system requires a power supply to produce a high-voltage spark. Depending on the manufacturer, the power supply used to create the electrical field can be anything from a simple, off-the-shelf oil furnace transformer to advanced electronic technology and control subsystems capable of manipulating electrical power characteristics to enhance power generation efficiency and O3 output concentrations.
Here is the flow diagram of the various steps involving the generation, application and control of ozone in a food processing plant
Ultraviolet (UV) (photochemical) ozone generation.
Light is measured on a scale called an electromagnetic spectrum and its increments are referred to in terms of nanometres (nm).
Low-pressure mercury UV lamps are used as a means of ozone production as well as for air treatment. A major advantage lies in the emission spectrum of the mercury discharge, because mercury emits with high efficiency two resonance lines with wavelengths of 254 and 185 nm (Voronov 2008).
The photons with 185 nm wavelength are responsible for ozone production, whereas photons having the 254 nm wavelength are effective in changing the DNA of microorganisms, thereby preventing their ability to reproduce.
An important point to make is that the target wavelength (254 nm) of UV lamps commonly used today for direct inactivation of microbial contaminants cannot be used to generate ozone, because ozone is actually destroyed at this wavelength. UV254 systems, referred to as UV sterilizers or germicidal sterilizers, inactivate microorganisms by affecting their DNA, disrupting their ability to reproduce.
Water to be treated with ozone is passed by a 254 nm UV lamp separated from a process stream by a quartz glass sleeve. It is the wavelength and intensity of the light itself that impacts the organism, not ozone.
Most commercial UV lamps are made from a form of quartz that contains impurities which absorb the 185 nm emission entirely, so that they produce no ozone . Consequently, when purchasing UV bulbs for the purpose of maximising the output of ozone, it is important that the UV equipment supplier understands the objective and provides the proper ozone-generating bulb(s).
Ozone Generation:
Ozone is generated using an ozone generator, which can be either a corona discharge or ultraviolet light generator. It converts oxygen gas (O2) into ozone (O3) through chemical reactions.
Ozone Delivery:
The generated ozone is delivered to the desired application points within the food processing plant.
This is typically achieved by injecting the ozone gas into a carrier gas, such as dry air or pure oxygen, to facilitate distribution and application.
Food Contact Applications:
Ozone can be applied directly to food products, surfaces, or equipment to achieve disinfection, sanitation, and decontamination.
Ozone Rinse or Spray: Ozone-enriched water or ozonated water is sprayed or used to rinse food products, surfaces, or equipment.
Ozone Immersion: Food products are immersed in an ozone solution or tank for a specific period to achieve desired disinfection or preservation effects.
source : http://higiene.unex.es/Bibliogr/Ozono/Ozone%20in%20Food%20Processing.pdf
Air Treatment Applications:
Ozone can be used to treat the air within the food processing plant to eliminate airborne pathogens and reduce odors.
Ozone Injection: Ozone is injected into air handling units or ventilation systems to disinfect and deodorize the air circulating in the plant.
Ozone Chambers: Enclosed spaces or rooms can be designed specifically for ozone treatment to target airborne contaminants or pathogens.
Packaging Applications:
Ozone can be employed in packaging processes to extend the shelf life of perishable food products.
Ozone Packaging: Ozone gas is introduced into the packaging material or sealed containers to inhibit the growth of bacteria, fungi, and other spoilage microorganisms.
Ozone Control and Monitoring:
Ozone levels are monitored using ozone sensors placed strategically throughout the food processing plant.
An ozone controller system manages and regulates the ozone generation, delivery, and application processes based on predefined parameters and feedback from the sensors.
Safety measures, such as alarms, emergency shutdown systems, and proper ventilation, are implemented to ensure safe operation and minimize ozone exposure risks.
Ozone Decomposition:
After ozone has served its purpose, it needs to be decomposed or converted back into oxygen to avoid residual ozone in the plant.
This can be achieved through ozone destruct units or by allowing sufficient contact time for natural ozone decay.
Effect of ozone on food
Microbial studies to date typically show that the mandatory 5 log reductions of spoilage and potentially pathogenic species most commonly associated with fruit and vegetable juices may be achieved. A number of studies report the effects of ozone on quality parameters of treated fruits and vegetables.
Ozone is not universally beneficial and in some cases may promote oxidative spoilage in foods. Surface oxidation, discolouration or development of undesirable odours may occur in substrates from excessive use of ozone.
Recently researchers in Spain evaluated the effects of continuous and intermittent applications of ozone gas treatments, applied during cold storage to maintain postharvest quality during subsequent shelf life, on the bioactive phenolic composition of Autumn Seedless table grapes after long-term storage and simulated retail display conditions.
The most notable effect of ozone on the sensory quality of fruits reported in the literature is the loss of aroma. Ozone-enriched cold storage of strawberries resulted in reversible losses of fruit aroma.
Effects of ozone exposure on fatty acids and lipids that are susceptible to oxidation were studied but contradictory results were reported.
Ozone has been shown to reduce microorganism levels on red meat and poultry carcasses and cuts. Although it is likely there are a number of mechanisms involved in the mode of action of ozone against microflora,one is the oxidation potential of ozone.
The use of ozone-containing slurry ice was investigated as a new refrigeration system for the storage of farmed turbot (Psetta maxima). With this purpose in mind, an ozone generator device was coupled to a slurry ice system working at subzero temperature (−1.5 °C) which does not effect the fish.
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