FACTS AND INFORMATION OF USING OZONE AIR PURIFICATION IN STORAGE
Gaseous ozone has been shown to effectively control mould and bacteria on various fruits and vegetables. The main benefit of using gaseous ozone is that mould and bacteria are controlled both in the air and on the surface of the product. The end result to the produce grower/packer is a dramatic reduction of product spoilage. Additionally, gaseous ozone oxidises and destroys ethylene gas that is released when fruits and vegetables begin to ripen. Thus, the use of gaseous ozone can reduce ripening while in storage.
Ozone can destroy bacteria, fungi and yeasts and can control odours in fruit cold storage rooms. However, several factors must be recognized and taken into account.
Bactericidal Activity
Ozone is a less effective bactericide in air than in water; therefore, it must be present in relatively high concentrations to be effective. At the low concentrations of 0.2 mg 03/m3 of air, ozone encourages, rather than discourages bacterial growths. In addition, some bacteria apparently can acclimate to low levels of ozone after prolonged treatment.
Fungicidal Activity
Only small concentrations of ozone are required to prevent the growth of fungi on mould and fungus-free surfaces. However, higher ozone concentrations are necessary for the destruction of existing fungal colonies. The fungicidal activity of ozone increases with increasing relative humidity. Since ozone does not penetrate deeply into the fruit, its major action is on the fruit surface. These facts indicate that airborne microorganisms must be swollen (by moisture) in order for ozone to be most effective.
Yeasts and Mildew
Yeasts are more readily attacked by ozone than are bacteria and the spores of mildew fungus are even more resistant to ozone than are bacteria. However, ozone controls blue mildew, which normally grows well under cold storage conditions at 0oC, and imparts a mildewy taste and odour to fruits.
Effect of Relative Humidity
The rate of decomposition of ozone in air increases with increasing relative humidity. Thus for optimum treatment efficiency in humid cold storage rooms, the ozone must be thoroughly and evenly distributed quickly, otherwise decomposition will occur before the ozone is able to contact its intended microorganism targets. However, rapid decomposition of ozone at high relative humidity is also beneficial, in that once feeding of ozone into the cold storage room is ceased, workmen can enter the room a short time later without concern for breathing harmful quantities of ozone.
The optimum relative humidity for ozone application is in the range of 90 to 95%. Under these conditions, ozone effectively controls surface microorganism growth without causing the fruit to lose weight. Shrinkage of apples also is prevented under these conditions.
Ozone Treatment Time
Ozone can be applied for a few hours several times per day. Concentrations of 2 to 3 cm 03/m air are applied over 2 to 3 hours, two to three times per day. Much higher concentrations (up to 400 mg/m3) do not improve the fungicidal effect, in most cases.
Effects of Temperature
Ozone treatment of food materials loses its germicidal effect above 10oC, when the normal concentrations are applied. Thus, ozone inhibition of microorganism growth above this temperature is insufficient.
On the other hand, below this temperature, metabolism of microorganisms is so slow that ozone treatment can overwhelm it. Therefore, at lower temperatures, the required ozone treatment time becomes shorter.
Fruit Storage Conditions
Because ozone acts only on the surface of most fruits, the fruit being stored must be packaged so as to allow free circulation of ozone-containing air, at least under forced circulation conditions. The concentration of ozone must be sufficiently high to allow for its decomposition on the walls of the storage room, on the wooden crates and other objects present, and still remain in sufficient concentration to provide its desired bactericidal and fungicidal actions.
Ozone Control of Odours
Ozone treatment controls odours in storage rooms when applied between changes of fruit to be stored. However, the rotting smell is not removed. Lower temperatures slow the rate of odour control, but relative humidity does not affect it. Levels of 0.01 to 0.04 cm3 03/m3 of air eliminate mildewy odours and impart a fresh smell to the air.
Fruit storage boxes develop an odour in the 80 to 90% relative humidities of the storage room. These odours can be controlled effectively with ozone treatment.
Engineering Considerations
Continuous ozonation of cold storage room air can be effected in combination with a central air cooling system, or by application in conjunction with separate cooling units used for each storage area.
Materials of Construction
Under the normal ozone concentrations used in fruit storage (5 mg 03/m3) or less), corrosion effects are minimal and without much consequence. Ducts and piping should be constructed of aluminium or stainless steel. Sealing materials on do ors and windows should be made of ozone- resistant materials.
Food Type And Application | Continuous Ozone Exposure Conditions | Comments and/or References | ||
| Minimum Residual Level in | Minimum Treatment Time, minutes | |||
| Air mg/m3 (ppm) | Water mg/L (ppm) | |||
| Molds on Newtown apples stored 3 mos. | 1-2 | 3 months | Smock & van Doren, 1939; no mould growth; no scald | |
| Spores (P. expansum; S. fructicola) on appples | 0.6; 85-90% RH | 3-4 hrs | Smock & Watson, 1942 | |
| Fruit storage – general conditions | 90% RH”}”>1-3; >90% RH | 2-3 hrs/day | Kuprianoff, 1953 | |
| Thornless blackberries storage | 0.1-0.3 @ 2ΕC | 12 days | Barth et al., 1995; O3 suppressed fungal growth 12 days | |
| Grapes storage | 0.1 | 20-40 | Sarig et al., 1996; O3 reduced fungal berry decay | |
| Strawberries | 2.7 | 0.5 | Lyons-Magnus, 1999; Reduced E. coli, SPC | |
| Chinese cabbage washing | 2-3 | < 60 | 90% redn in total bacterial counts”}”>Kondo et al., 1989; > 90% redn in total bacterial counts | |
| Broccoli washing Carrots washing Broccoflower washing | 1.1 0.64 1.08 | 10 10 10 | Hampson et al, 1994; 3-log redn of APC 2-log redn of APC 1-2 log redn of APC | |
| Carrots storage | 15 ΦL/L air | 8 h/day @ 2ΕC | Liew & Prange, 1994; gave some disease protection with minimal change (lighter color) | |
| Broccoli florets washing | 1 | 10-50, then 4 days storage | Zhuang et al., 1996; microbial growth inhibited | |
| Broccoli washing Broccoflower washing Carrots washing | 1 1 2001 | 6.0/1-log 7.5/1-log 9.6/1-log | Hampson/ Fiori, 1997; for higher log-enacts, increase [O3] or contact time with wash water | |
From this study in 1941 one can see the advantages of using ozone in Apple Storage in spores reduction.
Spore Counts in Ozone and Non-ozone Cold Storage Rooms for Apples (Smock and Watson, 1941)
Storage Number | Temperature ΕF | Date | Ozone Conc’n, ppm | Total spores/yd3 atmosphere | |
Ozone | No Ozone | ||||
1 | 40 | 11/22/40 | 1.5* | 16,000 | 1,380,000 |
1 | 40 | 12/26/40 | 1.5* | 2,760 | 4,280,000 |
2 | 32 | 2/26/41 | ….** | 910,000 | 1,111,000 |
3 | 32 | 11/18/40 | 0.4*** | 46,000 | …….. |
3 | 32 | 12/21/40 | 0.4 | 81,000 | …….. |
* Ozone was used continuously in this room. *** Ozone was used only for an hour or two each day. | |||||
Conclusions: In the experiments described above, the use of 1-2 parts per million of ozone used an hour or two each day in the apple storage controlled the growth of surface moulds.
The use of ozone was found to reduce materially the mould spore count in apple storage rooms.
Careful handling of the fruit is the most important consideration in rot control. The use of ozone while not a control is a step in the right direction from the standpoint of apple rots. It checked the spread of rots on scabby apples significantly. Ozone prevents any aerial growth of the rotting fungus and may prevent spreading of spores in this way. Ozone definitely checks the rate of spore germination and actually plasmolyzes mould spores.
Ozone Solutions have also applied ozone gas in storage areas to remove ethylene and prevent bacterial cross contamination in fruit such as grapes, pomegranates, peaches and pears thereby extending shelf life dramatically.
In cheese, eggs or meat storage ozone can play an important role