Ozone in Cold Storage & its benefits in the Cold Storage of foodstuffs & flowers
Using Ozone in Cold Storage dramatically reduces the potential for spreading germs from handling foods like meats, fish, fruits and vegetables. Microorganisms such as bacteria, moulds and yeast that can grow in refrigerators, cause spoilage and decrease shelf-life. Organic or bacterial “slime” grows on produce and refrigerator coils, pans, and in drain lines that can block or restrict flow, causing cross-contamination to the other foods in the refrigerator.
Ethylene is produced from all parts of higher plants including the leaves, stems, roots, flowers, fruits, tubers, and seeds. During the life of the plant ethylene production is induced during certain stages of growth such as germination, ripening of fruits, abscission of leaves, and senescence of flowers. Plant reactions to ethylene include stimulation of the aging process in leaves & flowers, the shedding of leaves & flowers, ripening of fruit and a climacteric rise in respiration in some fruit which causes a release of additional ethylene.
- Ethylene shortens the shelf life of many fruits by hastening fruit ripening and floral senescence.
- Ethylene will shorten the shelf life of cut flowers and potted plants by accelerating floral senescence and floral abscission.
- Flowers and plants which are subjected to stress during shipping, handling, or storage produce ethylene causing a significant reduction in floral display. Flowers affected by ethylene include carnation, geranium, petunia, rose, and many others.
Ethylene can cause significant economic losses for florists, markets, suppliers, and growers. Researchers have developed several ways to inhibit ethylene but inhibiting ethylene synthesis is less effective for reducing post-harvest losses since ethylene from other sources can still have an effect.
Using Ozone in Cold Storage will increase shelf life by controlling ripening and reducing decay. Common items that benefit from Ozone in Cold Storage are Apples, Avocados, Bananas, Berries, Citrus, Cucumbers, Grapes, Onions, Pears, Peppers, Potatoes, Stone Fruit and Tomatoes. Ever increasing concerns regarding extending shelf-life, control of decay, control of ripening, quality and appearance of food without depending on chemicals have driven increased demand for safe, proven food storage alternatives such as ozone.
Ozone in Cold Storage kills airborne and surface microorganisms, shuts down the sporulation process, and consumes ethylene produced by ripening. After oxidizing microorganisms, ozone immediately reverts to pure oxygen, leaving no residue and maintaining taste, texture and smell characteristics in the product’s natural state. Growers, packers, and processors are able to extend product life and marketability, as well as decrease decay losses of fresh produce naturally. Ozone can be used alone or as a complementary measure with various post-harvest preservation techniques.
Ozone in Wholesale Cut Flower Storage
The shelf life of cut flowers in storage is affected by ethylene gas production and the spread of disease. Ozone will effectively control the accumulation of ethylene gas produced by some varieties of flowers and will increase the safe storage period for many varieties of cut flowers. Ozone will also destroy airborne organisms and reduce losses caused by the spread of disease.
Keep Flowers Fresh Longer: Ozone water neutralizes surface micro-organisms that cause cut flowers to decay. Ozone destroys the ethylene gases produced by aging and decay of plant. It also helps reduce algae, fungus, mildew, spores and root rot. During the oxidation of ozone, extra oxygen is created, which helps flowers retain their freshness longer. Florists and nurseries experience the benefits of longer shelf-life and better plants using ozone.
Ozone in Fruit and Vegetable Cleaning and Storage
During post- harvest storage, surface infections on produce can spread and cause significant losses of the crop. The use of ozonized water for washing and disinfecting can reduce these losses and maximise returns.
Ozonized water is a convenient way to wash fruit and vegetables and offers the following advantages over traditional chlorine based sanitisers.
- The level of dissolved ozone in the water can be accurately and simply controlled using ORP (Oxidation Reduction Potential) measurement.
- The disinfectant effect of ozone is not dependant on the pH level of the water.
- Ozone will leave no residue on the produce that may affect taste or reduce shelf life.
- Ozone is generated as required and therefore needs no special storage.
- Ozone has no ongoing chemical purchase or disposal costs.
- Ozone leaves no residue in the water and therefore makes reuse simply a matter of filtration.
The shelf life of produce in cold storage can also be extended by the use of gaseous ozone. Ozone in the air within a cold storage room can retard the growth of microorganisms in the air and on the surface of the produce. Ozone is also effective in breaking down ethylene gas which is given off by some fruits and accelerates the ripening process. The reduction of ethylene gas will effectively reduce the ripening effect of mixed produce held in the same area.
Reducing Citrus Postharvest Losses from Mould
Decay of postharvest citrus fruit is most often caused by fungal pathogens. The most common postharvest fungal diseases of citrus are Diplodia stem-end rot (Lasiodiplodia theobromae), Phomopsis stem-end rot (Phomopsis citri [teleomorph Diaporthe citri]), and green mold (Penicillium digitatum). Sour rot (Galactomyces citri-aurantii), anthracnose (Colletotrichum gloeosporioides), and, less frequently, Alternaria stem-end rot (black rot) (Alternaria alternata) and brown rot (primarily Phytophthora palmivora and P. nicotianae) can also cause commercially important losses of citrus fruit. To reduce postharvest citrus losses the certain factors need to be actively managed. As we are currently doing work for a citrus facility in Delmas, this article touches on issues that we are addressing.
- Certain citrus fruit lose water at low relative humidities after harvest and become prone to stem-end rind breakdown and physiological injuries that can cause fruit to decay. To reduce fruit water loss, the fruit is handled as quickly as possible in a high humidity environment and a protective wax coating is applied to retard desiccation.
- Fruit turgidity, freshness and enhanced healing of minor injuries can be maintained by keeping relative humidity high during handling, storage, and transit thereby reducing susceptibility to green mold.
- When fruit is stored in plastic containers the relative humidity should be 90%–98%. If fruit is packed in cardboard boxes, the humidity should be lower (85%–90%) to prevent carton deterioration.
Effective sanitation practices such as the regular removal of fruit, leaves and other trash from the floor and machinery during postharvest handling can greatly reduce the incidence of decay.
- Decayed fruit should be separated from sound fruit immediately to prevent contamination of the line by fungal inoculum.
- Decayed fruit should not be left near the packinghouse because spores may be transferred back into the packinghouse.
- Hot water (at least 70°C) or an approved sanitizing agent (e.g. chlorine, peroxyacetic acid, etc.) should be used to clean fruit-contact surfaces at the end of each day.
- Approved quaternary ammonia (QA) compounds are often used.
- Empty pallet bins should be clean and free of debris before each trip to the field.
When cleaning citrus, soak tanks introduce the possibility of fruit infection from fungal contamination in the water. Traditionally chlorine has been used to disinfect water. Chlorine kills decay fungi on contact, but it has little or no residual activity. Other methods of disinfecting cleaning water include the use of ultra violet units and the introduction of ozone gas into the water.
Water disinfection using Ultra Violet
Using ultra violet to disinfect water for citrus washing is a non-chemical method of disinfection which has been used in the beverage industry for years. UV kills all known pathogens and spoilage microorganisms, including bacteria, viruses, yeasts and moulds (and their spores). It is a low maintenance, environmentally friendly technology which eliminates the need for chemical treatment while ensuring high levels of disinfection. UV is the part of the electromagnetic spectrum between visible light and X-rays. The specific portion of the UV spectrum between 185-400nm (also known as UV-C) has a strong germicidal effect, with peak effectiveness at 265nm. At these wavelengths UV kills microorganisms by penetrating their cell membranes and damaging the DNA, making them unable to reproduce and effectively killing them. A typical UV disinfection system consists of a UV lamp housed in a protective quartz sleeve which is mounted within a cylindrical stainless steel chamber. The liquid to be treated enters at one end and passes along the entire length of the chamber before exiting at the other end.
Water disinfection using Ozone
Ozonizing water for citrus washing can achieve a disinfection efficiency of 99.9 percent (of the water) with a concentration of 1 mg/l ozone and will require a retention time of only 57 seconds. On the contrary, using chlorine to achieve a 99.9 percent disinfection efficiency at the same water temperature and ph. value (15°C and 7 respectively) with 1 ppm chlorine will require a retention time of 75 minutes. Chlorine always leaves an oxidation or disinfection by-product whereas ozone simply reverts back to oxygen. During the oxidation or disinfection process, only one oxygen atom is used for the chemical reaction. Another benefit of ozone is that it is generated on-site and as needed. There is no chemical to store and no residual disinfection by products to monitor.
Disinfection of air to reduce the threat of Mould
Mould and mildew are generic terms for various types of fungi. Fungi produce enormous quantities of microscopic spores. These spores are always present in the environment and are spread by air currents. When these spores find a hospitable environment they will germinate. If small patches of germinating spores are ignored you will get a mould bloom or outbreak. A mould bloom is basically millions of fungi producing enormous quantities of spores.
To reduce the threat of mould it is essential to maintain an environment that is not hospitable for the germination of mould spores. An unhospitable environment would have a temperature of between 20 – 22 degrees C, and a relative humidity of 65% or less. This environment, unfortunately, is also unhospitable for the storage of citrus fruit.
To maintain an environment conducive to good fruit storage and to reduce the threat from moulds, it has become common to install ultra violet and ozone air sanitation units in citrus fruit storage.
The Principles of Ozone and Ultraviolet use to reduce postharvest citrus losses
Postharvest green mould, caused by Penicillium digitatum, and postharvest blue mould caused by Penicillium italcum are among the most economically important postharvest diseases of citrus worldwide (1). In California, USA, Valencia oranges for preparation of fresh juice are stored at temperatures from 3 – 5°C. Currently both diseases are controlled mainly by application of the fungicides imazalil, sodium ortho-phenyl phenate or thiabendazole (1). Alternative methods are needed for reducing postharvest decay, as the widespread use of fungicides has resulted in resistant strains of the pathogens (2). In addition, concerns about the effects of fungicide residues to human health and the environment have also increased the need for alternatives to fungicide usage.
A study by Harriet Jarlett, published in Postharvest Biology and Technology in May 2013, explored the impacts of ozone on the amounts of certain proteins in a tomato fruit, to try and unravel why low levels of ozone gas can protect fruit and vegetables from disease, and increase their shelf life. The team found that changes in the proteins observed when a tomato fruit was treated with ozone after it had been harvested were associated with defence and signalling responses. The mechanistic relationships are still poorly understood but responses are believed to be associated with physical and chemical defence responses, as well as enhanced repair capacity,’ says Professor Jeremy Barnes, who co-authored the paper. The scientists also showed that storage of fruit in enhanced levels of ozone inoculates it against pathogens for up to two weeks afterwards, even once it is removed from the ozone-enriched atmosphere (3). ‘If you re-introduce a pathogen to a tomato that’s been given ozone it doesn’t grow anywhere near as voraciously as it does in a non-treated tomato,’ says Barnes. ‘One of the most surprising outcomes of our study is the fact that shifts in protein profiles were sustained for a short period following removal of the fruit from the ozone enriched environment and this was consistent with ‘memory’ or ‘vaccination effects’ reported by the authors – effects consistent with enhancement of product shelf-life.'(3)
Whilst I am certain that there is still going to be a lot of debate about the ‘memory’ or vaccination effects’ of ozone, there is no doubt that Ozone (the triatomic form of oxygen O3) is one of the most powerful oxidants in nature. It has been proven that ozone storage can lower natural levels of decay in a variety of citrus fruits in commercial storage conditions, by
- lowering the amount of spores produced in the storage environment
- destroying ethylene
- Maintaining firmness and assisting in resistance to disease.
- Reducing the level of pesticide residues on fruit.
Another postharvest storage technology being employed today is Ultraviolet Germicidal Irradiation (UVGI) units. Although the use of ultraviolet light is well established for water treatment, air disinfection, and surface decontamination, its use is still limited in food treatment and in postharvest technology in particular. Ultraviolet light utilizes its energy to break down the organic molecular bonds of the micro-organisms, there by rendering it unable to reproduce and ‘in effect’ killing it. In an ultraviolet air sanitizer, the particles and organisms present in the air enter the unit and whilst passing the UV rays, their molecular bonds are broken and they are destroyed. These units destroy all the harmful particles and micro-organisms in the air. They include particles like dust and dust mites, mildew, allergens, pollens, smoke, vehicle fumes, moulds, dead skin flakes and dander as well as organisms like bacteria, viruses, fungi and other germs.
The best possible cold room storage conditions would be a combination of traditional methods, and the use of UVC units, and Ozone produced by UVC. We recently completed an installation where we installed UVC units behind the cooling coils to ensure that the water droplets in the chiller, and the chiller itself remained sanitised. This was also to ensure that the air in the room was also sanitised. UVC units were also installed by the inlets to remove any pathogens brought in by flushing (1 air change every 24 hours). In addition we installed 185nm UV Ozone producing units to assist with surface disinfection and ethylene control. Pre-installation tests revealed colonies of pathogens too numerous count. Tests taken 1 month after installation indicated only 1 or 2 colonies.
Another issue that needs to be addressed is the period between harvesting and arrival at the storage facility. Even if the grower has taken every possible precaution to keep his produce disease free, there will always be possible contaminants between the farms at the storage facility. These could include the storage bins, delivery vehicles etc.
In an article on the News 24 website dated 29 May 2014, “South Africa’s citrus industry’s future is in jeopardy according to the Citrus Growers’ Association of Southern Africa. This statement came after the European Commission’s committee on plant health authorised stricter import requirements from South Africa’s fruit industries. According to a report by the SABC, the new restrictions are being put in order to protect the Europe’s crops from citrus black spot. It is estimated that 70% of the EU’s citrus fruit comes from SA. The CGA also said that the citrus industry brings in R8bn a year and is responsible 12 000 jobs”. (4,5) According to a Business day Live report Spain has been eying South Africa’s citrus market share in Europe and has been lobbying quite fiercely for the EU to cut its imports of citrus fruit from South Africa.(4,5) Black spot, caused by Guignardia citricarpa, is another one of the main diseases affecting citrus fruits and usually occurs in the growing and packing process. UV-C irradiation was not able to control citrus black spot on fruit at a satisfactory level but the incidence of black spot lesions was lower on postharvest. (6)
The benefits of ultraviolet and ozone in cold storage are fact, but without the conventional ‘housekeeping’ of storage facilities, and sanitary measures right down the chain, from the growers to the delivery mechanisms we may find ourselves bumping heads with Spain and the EU again.