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INCORPORATING COLD CHAIN of UV-C can prolong the postharvest life and maintain the quality of fruits . These effects include delay of senescence process and fruit ripening ( Gonzalez-Aguilar et al ., 2007a ), induction of natural defence and elicitors against fungi and bacteria ( Alothman et al ., 2009a ).
Exposure to hermetic doses of UV radiation triggers a series of biochemical events within the plant tissue , and a number of quite distinct responses have been identified . Some responses involve the synthesis of enzymes that have activity against molds , while others result in the generation of a host of so-called phytoalexins , which are inhibitory to microorganisms . These effects are produced by the use of very low UV doses , and the time scale for the induction of such events is measured over hours or even days .
Resistance to infection by pathogens is correlated with the induction of plant defence mechanism ( Gonzalez- Aguilar et al . 2007b ) and DNA damage ( Charles et al ., 2009 ). This is manifested through the stimulation of anti- fungal chemical species such as phytoalexins ( scoparone and resveratrol ), flavonoids , and degrading fungal cell wall enzymes ( chitinases , glucanases ) ( El- Ghaouth et al ., 1998 ).
The induction of plant defence systems can also trigger the accumulation of these compounds and other phytochemicals such as carotenoids and vitamin C which exhibit antioxidant potential , improving the nutritional status of the fruit ( Alothman et al ., 2009a , 2009b ; Gonzalez- Aguilar et al ., 2007a , 2007b ). All published work on the delivery of low doses of UV to fresh produce has concerned itself with only relatively small numbers of fruit treated under laboratory conditions , and little consideration has been given to how produce may be treated on a large scale under industrial conditions .
Shama ( 2007 ) considered the prospects of treating fruits with UV on a commercial scale . According to this author , any process for irradiating produce must fulfil certain essential requirements ( Shama , 2007 ) such as : produce should not be subjected to any form of mechanical handling during irradiation that might cause it to become damaged ; there should be provision for both varying the UV dose delivered and controlling the dose ; UV-C treatment should not add unduly to processing costs ; the design of equipment should enable high throughputs to be treated ; ideally , a wide variety of different types of fruit and vegetables should be treatable .
POSTHARVEST UV RADIATION : EFFECTS AND POSSIBLE TECHNOLOGICAL APPLICATIONS Ready-to-use fruit and fresh vegetables The market sales of ready-to-use fruit and fresh vegetables have grown rapidly in recent years due to the health benefits associated with these foods . Its growth has heightened awareness about the microbiological and physiological parameters associated with quality in fresh ready-to-eat vegetables due to the relevance for industry and its economic impact .
Chlorine solutions have been widely used to sanitise fruit and vegetables in the fresh-cut industry and continue being the most commonly used sanitisers due to their efficacy , cost-effectiveness ratio and ease of use . However , chlorine has been associated with the possible formation of carcinogenic chlorinated compounds ( Rico et al ., 2007 ). This preoccupation urges fresh-cut industry to find new alternatives . These alternatives must satisfy the consumers and maintain a balance between sensory and quality . For this reason , exploration and enhancement of new alternatives is essential . There is a real need to find alternatives for preservation of fresh-cut fruit and vegetables in order to improve the efficacy of washing treatments .
Alternatives or modified methods have been proposed , as antioxidants , irradiation , ozone , organics acids , modified atmosphere packaging , whey permeate , etc .; however , none have yet gained widespread acceptance by the industry ( Rico et al ., 2007 ). New techniques for maintaining quality and inhibiting undesired microbial growth are demanded in all the steps of the production and distribution chain . Allende et al . ( 2006 ) summarised and discussed some of the new techniques available in the fresh-cut industry , such as the combination of sanitisers with other methods , combinations of physical and chemical treatments , the use of ultraviolet-C radiation , modified-atmospheres , heat shocks and ozone treatments , alone or in different combinations , in order to control microbial growth and maintain quality during storage of fresh-cut produce .
It has been reported that UV – C affects several physiological processes in plant tissues and , what it is more important , damages microbial DNA ( Kuo et al ., 1997 ; Lucht et al ., 1998 ). Lado and Yousef ( 2002 ) reported that UV – C radiation from 0.5 to 20 kJ / m 2 inhibited microbial growth by inducing the formation of pyrimidine dimers which alter the DNA helix and block microbial cell replication . Therefore , cells which are unable to repair radiationdamaged DNA die and sub-lethally injured cells are often subject to mutations .
It has been reported that hormetic doses of UV-C can prolong the postharvest life and maintain the quality of fruits .
A number of in vitro studies have demonstrated the efficiency of UV – C radiation on microbial inhibition ( Gardner and Shama , 2000 ). Abshire and Dunton ( 1981 ) found that some species ( Pseudomonas aeruginosa ) were more sensitive than others ( Micrococcus radiodurans and Candida albicans ). Consistent with this , Sumner et al . ( 1995 ) found that UV – C was effective in destroying Salmonella typhimurium on agar plates . Selma et al . ( 2008 ) investigated the disinfecting efficacy of ozone ( O3 ) and UV-C illumination , and their combination for reduction microbial flora of fresh-cut onion , escarole , carrot , and spinach wash waters collected from the industry .
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In the tests conducted on tomatoes , changes in the physical and biochemical modifications that occur in the epidermal cell in response to UV-treatment may be conducive to an improved ability of the plant tissue to resist pathogen attack .
They achieved a maximum microbial reduction of 6,6 log CFU mL-1 after 60 min treatment with O3-UV and concluded that O3 and O3 – UV are alternatives to other sanitisers used in the fresh-cut washing processes . The use of these technologies would allow less frequent changing of spent water and the use of much lower sanitiser doses . Nevertheless , in specific applications where levels of undesirable microbial and chemical constituents are lower , UV treatment alone could be an appropriate treatment considering cost-effectiveness criteria .
Postharvest treatments of either ClO2 or fumaric acid combined with UV-C can be useful for maintaining the quality of strawberries , including the sensory evaluations scores ( Kim et al ., 2010 ). These authors examined the combined effect of aqueous chlorine dioxide ( ClO2 ) or fumaric acid with ultraviolet-C ( UV-C ) on postharvest quality of ‘ Maehyang ’ strawberries . The strawberries were treated with distilled water , 50 mg L−1 ClO2 , 0.5 % fumaric acid , 5 kJ / m 2 UVC irradiation , and a combination of 50 mg L−1 ClO2 / 5 kJ / m 2 UV-C and 0.5 % fumaric acid / 5 kJ / m 2 UV-C .
The combined treatment of fumaric acid / UV-C reduced the initial populations of total aerobic bacteria and yeast and molds in the strawberries by 2.25 and 2.01 log CFU g−1 , respectively . Sensory evaluation results indicated that the combined treatment provided better sensory scores than did the control .
Tomatoes The use of UV-light could be employed to improve tomato nutritional qualities and lycopene content without inducing significant changes to the physical properties of tomatoes during post-harvest storage . Liu et al . ( 2009 ) treated harvested mature green ( breaker-stage ) tomatoes with short bursts of UV-C , red light or sun light for up 21 days . The concentration of ycopene in tomato exocarp was significantly increased after 4 days and dramatically enhanced by UV-C or red-light treatments .
However , the concentration of β-carotene was not affected by UV-C or red-light treatments and decreased by sun light treatment during 21 days of storage , compared to the control samples . The colour ( a * and b * values ) and force required to penetrate the tomatoes were , to a small but significant extent , influenced by the light treatments . The total soluble refractive solids of all tomato samples remained the same throughout storage .
Liu et al . ( 2011a ) studied gene expression of tomato fruit in response to postharvest UV-C irradiation ( 4 kJ / m 2 ), during 24 h after the treatment . They concluded that UV-C
In the study , UV-C treatments seemingly inhibited lesion development on the mushroom surface .
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