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New techniques to manage post-harvest diseases of fruits and vegetables
By: Madhu Prakash Srivastava*
Introduction
“Substantial amounts of vegetables and fruits are lost to spoilage after harvest. This thrashing can range from 10-50% depending on the product and country”.
Currently, synthetic chemicals are the primary resources of checking post harvest diseases of vegetables and fruits. Public worry about food safety, though, increased concern to find out the efficient alternatives to unsafe chemical pesticides to control post harvest diseases of perishables. The eventual aim of recent research programmes in this area has been the advancement and assessment of various alternative control strategies to trim down reliance on synthetic fungicides.
In India, post-harvest losses in vegetables at different regions of the country have been estimated, which were for Delhi 7.2-34.7%; Maharashtra 15-20 % and Uttar Pradesh 4-10 %. In Delhi, the percentage loss reported by the retailers who have permanent shop was around 6.75% and around 8.8% for pushcart vendors. Post harvest pathogen not only affects the produce quantitatively but also qualitatively. A perusal of literature on the changes occurring during pathogenesis in various fruits clearly revealed that the quantity of various free and bound amino acids and organic acids was altered and a gradual decrease in sugar and ascorbic acid content was observed with the advancement of diseases .
A number of enabling technologies are available for optimizing product quality through manipulation of nutrition, water and light to minimize post harvest disorders and quality deterioration as well as to optimize carbon assimilation, distribution and accumulation in harvested organs. Common practices used for the control of post-harvest diseases of fruit are controlled atmosphere storage, refrigeration and fungicides.
Physical agents
Physical agents have been used to control the post-harvest diseases of fruits and vegetables. A different type of physical method applies in the control of plant pathogens. There are various types
Ultraviolet light treatment (UV Treatment)
Low dose of ultraviolet light especially UV-C hormesis have emerged as alternative technology to avoid chemical fungicides. Low dose UV C hormesis was shown to induce resistance in postharvest commodities to harvest decay and to extend shelf life of fruits by delaying the ripening and senescence process. Application of a low dose of UV-C light reduced the development of post-harvest decay in horticulture crops such as onion, sweet potato, apple peach, citrus fruits, bell pepper, tomato, carrot and strawberry. UV-C therapy effectively reduced storage rot 60-90 % compared with 100 % decay for the non-irradiated control.
Heat treatment
Pre-storage heating holds potential as a nonchemical method for control of post-harvest diseases by directly inhibiting pathogen growth, activating the natural resistance of the host and slowing down the ripening process. Heat treatments are promising and have been used with success in eradication or suppressing the development of fungi on the surface as well as those situated just below the surface as a result of pre harvest infection. Post-harvest curing at 34–36°C for 48–72 h effectively controls citrus decay and reduces chilling injury symptoms.
Irradiated Fruits
Emission and propagation of energy through material medium in the form of waves is called irradiation. Both ionizing and non-ionizing irradiation have been used for post-harvest storage studies. Irradiation basically controls the post-harvest diseases by sterilizing the fruits. Microwave treatments were found effective against fungi viz- Botrytis cinerea and Penicillium expansum.
Low Pressure storage
Storage life is influenced by atmospheric pressure and at low pressure it is extended. Low pressure (180-190 mm) has been reported to reduce fruit ripening. At low atmospheric pressure the availability of O2 for respiration is reduced besides controlled storage, use of fungicides under reduced atmospheric pressure helps from rotting of storage fruits by protecting them.
Low storage temperature
Low temperature also reduces the ripening and the respiration rates. At 13oC fruits have been kept in the best condition. The temperature requirement for slow ripening depends upon the stage of maturity; green fruit at 15oC, orange green fruit at 10oC and red fruit at 8oC have been kept for a longer period. However, under very low temperature conditions chilling injury is caused and such situation arises below 10oC. Alternations of low and high temperatures, 2oC and 20oC respectively have prolonged storage tissues. At ambient temperature fruits can be stored for longer duration.
Chemical agents
Calcium chloride (CaCl2) and Sodium bicarbonate
A post-harvest calcium and sodium treatment was reported safe and effective methods of improving the quality and shelf life of fresh fruits. Selected organic and inorganic salts are active antimicrobial agents and have been widely used in the food industry. Among these, Calcium delays ripening and particularly softening by altering intracellular and extra cellular processes. It also reduces disorder and decay losses. Sodium bicarbonate (SBC) and potassium sorbate are used for controlling pH, taste and texture, and they also exhibit broad-spectrum antifungal activity .The potential of bicarbonate salts for the control of post-harvest pathogens has been demonstrated in citrus, carrot, bell pepper and melon. Sodium bicarbonate at a concentration of 2% (w/v) has potential for controlling Rhizopus, Alternaria and Fusarium decay on ‘Galia’ and ‘Ein Dor’ fruit.
Chitosan
Chitosan and its derivatives, including glycolchitosan, were reported to inhibit fungal growth and to induce host-defence response in plants and harvested commodities. Chitosan, a high molecular weight cationic polysaccharide, is soluble in dilute organic acids, and have been used as a preservative coating material for fruits. It has ability to form a semi-permeable film and chitosan coating have definite potential to modify the internal atmosphere as well as decrease transpiration losses in fruits. Chitosan coatings have been found to extend the storage life of fresh fruit and that too without causing anaerobiosis. Moreover, they have also been reported to reduce decay by inhibiting the growth of several fungi.
Organic Fungicides
A number of fungitoxic chemicals for controlling postharvest diseases have been developed. These chemicals are mostly used as dilute solutions into which the fruit or vegetables are dipped before storage or as solutions used for washing or hydrocooling of fruits or vegetables immediately after harvest. Benomyl, triforine, dichloran etc. are used as dips, sprays or wax formulations. Taken together, all these factors have resulted in reframing of government policies which not only allows restricted use of fungicides but also provides the impetus to develop alternative and effective natural methods of controlling post-harvest diseases.
Biological agents
Biocontrol
In the recent past, biological control has emerged as an effective strategy to combat major postharvest decays of fruits. However, compared to the long-standing interest in biological control of soil borne pathogens research into biological control of post-harvest decays is still in its infancy. Thus, biological control of post-harvest diseases of fruit and vegetables offers a viable alternative to the use of present day synthetic fungicides. Today biological control of postharvest diseases of fruit has become an important field for research. Microbial antagonists have been reported to protect a variety of harvested perishable commodities against a number of post-harvest pathogens Post-harvest treatment of fruits with microorganisms recovered from fruit surfaces is being developed as an alternative method for control of post harvest diseases of Citrus, Apples, and other fruits and vegetables. A number of yeasts and bacteria have been reported to inhibit post-harvest decay of fruit effectively. Utilization of antagonistic yeasts as an alternative appears to be a promising technology.
Botanicals
Drawbacks of synthetic chemical methods have increased interest in developing further alternative control methods, particularly those that are environmentally sound and biodegradable. Thus, replacement of synthetic fungicides by natural products (particularly of plant origin), which are non-toxic and specific in their action, is gaining considerable attention. Because of greater consumer awareness and concern regarding synthetic chemical additives, foods preserved with natural additives have become popular. This has led researchers and food processors to look for natural food additives with a broad spectrum of antimicrobial activity .The plant kingdom represents an enormous reservoir of potential fungicidal compounds that could be useful alternatives to synthetic fungicides.
Recently, there have been several attempts to use naturally occurring compounds for the control of postharvest decay. Plants also produce a variety of essential oils and volatile substances that could have potential as antifungal preservatives for harvested commodities. Both plant essential oils as well as similar compounds in wood smoke have shown promise as natural antimicrobials. Essential (volatile) plant oils occur in edible, medicinal and herbal plants, which minimize questions regarding their safe use in food products. Essential oils and their constituents have been widely used as flavouring agents in foods since the earliest recorded history and it is well established that many have wide spectra of antimicrobial action.
The advantage of essential oils is their bioactivity in the vapour phase, a characteristic that makes them attractive as possible fumigants for stored product protection. There have been some studies on the effects of essential oils on post-harvest pathogens. Some of the essential oils have been reported to inhibit post-harvest fungi in in vitro conditions. The potential of essential oils to control post harvest decay has also been examined by spraying and dipping the fruit and vegetables. A promising recent development involves incorporating these antimicrobials into packaging materials, rather than the food itself. This concentrates the antimicrobial at the surface of the product, which is where noxious organisms grow and reduces interference from food constituents. Although the fungitoxic properties of the volatile constituents of higher plants have been reported, little attention has been paid to the fungitoxicity of these substances when combined. This information is desirable since the fungitoxic potency of most of the fungicides has been reported to be enhanced when combined.
Discussion
Although many alternatives to chemical control have been investigated, none, when used alone, is as effective as fungicides. Hot air treatment either reduced or completely eradicated decay of apple fruit caused by Penicillium expansum but the pathogen was not completely eradicated in the case of decay by Colletotrichum acutatum. Heat treatment, while a good eradicant, has no residual activity. The reduction of decay by biological control is generally more variable than for fungicides since biocontrol is affected more by environmental factors. There is also a narrower spectrum of activity than is found with chemical control.
Similarly, SBC is not effective in providing protection if fruit are infected after treatment. Integrating different physical control options such as radiation and ultraviolet illumination was found to be effective against fungi sensitive to low gamma doses such as Colletotrichum spp. Combining physical and chemical alternatives has also been extended to combine radiation and fungicide applications. In this case, both dosages could effectively be reduced to provide cumulative protection. Integrating hot water treatments with SBC and fungicides have been known to be effective in reducing decay.
Combining fungicides in natural or edible waxes has also resulted in increased effectiveness of the products compared to using the products on their own. Korsten et al. (1991) also described successful control of mango postharvest diseases when the antagonist was incorporated into the natural waxes applied on the packing line. Shrink or plastic wrapping the fruit after heat treatments has also proven to be an effective integrated approach. Fungicides used at low concentrations when combined with biocontrol agents have been shown to be effective against several postharvest diseases. Biocontrol products could effectively be integrated with chemicals used at lower concentrations or when used with softer chemicals or disinfectants.
Combining chemical elicitors such as chitosan with Bacillus subtilis was found to increase the effectiveness of postharvest biocontrol treatments of Penicillium spp. on citrus. Other combinations such as calcium salts and sodium bicarbonate with biocontrol agents proved similarly. Adding SBC to the heated or antagonist treated fruit had little effect on decay caused by either pathogen, but on non-heated fruit, it slightly reduced decay caused by P. expansum. An increase in control of decay on oranges caused by Penicillium digitatum and Penicillium italicum occurred when Bacillus subtilis antagonists were combined with SBC. Combining SBC with another antagonist also improved decay control of P. digitatum on oranges and grapefruit.
At present quite a few promising biological approaches that include the natural plant based antimicrobial substances (volatile aromatic compounds, acetic acid, essential oils, jasmonates, glucosinolates, plant extracts and propolis), the application of microbial antagonists (bacteria, fungi, yeasts), the antimicrobial substances from soil (deoxyfusapyrone and fusapyrone) and the natural animal-based antimicrobial substances like chitosan have been advanced to curb the menaces of post harvest diseases in perishables. Compounds that activate host plant defense responses potentially recommend socio environmentally potent alternative methods for disease control. Amalgamation of the above complementary techniques could well lead to efficient control of post harvest diseases.
*Department of Botany, University of Lucknow, Lucknow 226007, E-mail:
[email protected]
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