Refrigeration is an important means of maintaining fresh quality while extending the postharvest life of highly perishable tropical and subtropical fruits. Rapid postharvest cooling and cool storage treatments are outlined. However the application of refrigeration to fruits of tropical and subtropical origin is limited by their sensitivity to cold stress (termed 'chilling injury') at temperatures below about 13°C. The mechanism of chilling injury is discussed briefly as well as areas of research being studied to elucidate the physiological and biochemical processes involved. Methods used for the alleviation of chilling injury are given. The effects of preharvest and postharvest chilling temperatures are discussed in relation to various fruits such as tomato, pineapple, atemoya, avocado, banana, mango, papaya and guava.
The quality of fresh fruit and vegetables at harvest is determined by factors such as climatic environment, crop cultural practices, variety, maturity, pests and diseases. Postharvest quality maintenance is also influenced by factors which may be physiological, pathological, physical or combinations of these three (1).
In postharvest storage, the factors which define the storage environment are temperature, relative humidity and atmospheric composition. Of these, temperature management is the most important factor in the maintenance of fresh quality by reduction in the rate of respiration, transpiration, enzymic activity, and growth and spread of micro-organisms (2). The overall benefits of cooling of fresh produce are increased profits and greater market flexibility (3).
Rapid cooling of fresh produce involves the removal of field heat by the use of refrigeration. The initial cooling, termed precooling, also requires energy for removal of heat of respiration, heat transmission through cool rooms and heat leakage from outside air. Methods for cooling fresh produce are room cooling, forced air cooling, hydrocooling, vacuum cooling and ice packing after precooling. Of these, forced air cooling has proved to be the most effective method of fast cooling.
Whilst the postharvest storage of produce at low temperature is beneficial, all aspects of metabolism of the produce are not suppressed to the same extent. Continued activity of some metabolic systems at low temperature can lead to cellular dysfunction and collapse. Metabolic disturbances occurring at reduced temperature are usually categorised as chilling injury (which affect tropical and subtropical fruits in particular) and physiological disorders (which affect mainly deciduous tree fruits such as apple, pear, stonefruits and citrus) (4).
There are two limiting storage temperatures for fresh produce - freezing temperature and chilling temperature. With fruits which have optimal storage temperatures close to 0°C (such as grape and kiwi fruit), care must be taken not to freeze the commodity. However most horticultural produce of tropical origin is susceptible to physiological injury at temperatures below about 12° - 13°C, well above freezing. Chill-sensitive plants and their fruits are subject to injury at all stages of their development (5). Susceptibility to chilling injury and its manifestations vary widely among different fruits, different varieties of the same fruit, and the same crop grown in different areas (4). Chilling injury causes the release of metabolites, such as amino acids, sugars and mineral salts from cells, which, together with the degradation of cell structure, provide an excellent substrate for the growth of pathogenic organisms - a serious postharvest problem even with sound fruit of tropical or subtropical origin. Internal browning may also be caused by membrane breakdown, allowing enzymes and substrates to mix within cells.
Information concerning the physiological and biochemical responses of plants to chilling stress has been accumulating rapidly in recent years (6,7). Responses of horticultural produce which have received attention include changes in membrane structure and function, cessation of protoplasmic streaming, alteration in respiration rates and patterns, changes in ethylene synthesis and many biochemical and compositional changes associated with chilling injury (8). However there is a general lack of consensus in the literature concerning stress. Some ambiguous areas remain and numerous hypotheses have been put forward to explain the mechanism of chilling injury.
The primary effect of temperature on plant cell membranes is believed to be on the fluidity of the membrane lipids (6). The membrane lipids, which are in a mobile condition at higher temperatures, enter a gel-like state and become immobile below a critical temperature. For example there is a marked change in the rate of respiration of isolated mitochrondria of tomato, a chill-sensitive fruit at about 10°C, but no change for cauliflower buds, a chilling-resistant tissue (4). This effect occurs at temperatures around 10° - 15°C for most tropical commodities and is therefore correlated with the onset of chilling injury.
As in other stress conditions, some plants react temporarily to an abrupt lowering of temperature but can recover and in fact adapt by regular exposure to chilling temperatures. Other plants and plant parts lack this capacity and are rapidly and apparently irreversibly damaged by even short exposure to chilling temperatures. Tropical and subtropical plants and fruits fall into this category.
Various treatments have been tried to alleviate chilling injury and its symptoms (8). Some are applied directly to the commodity and others involve manipulation of the environment. Examples of such treatments are:
(i) conditioning at near-chilling temperatures before chilling;
(ii) intermittent warming treatments during chilling;
(iii) pretreatment with calcium or ethylene;
(iv) controlled atmosphere and hypobaric storage.
However at the present time, chilling injury of tropical and subtropical produce is best avoided by avoiding or minimising exposure of the produce to chilling temperatures both in the field and postharvest.
Examples of cold stress on fruits
Postharvest responses of fruit and vegetables to chilling stress are often greatly influenced by preharvest field factors. Tomatoes are a good example where maturity at harvest, field chilling, transit chilling, market chilling and the home refrigerator all contribute to the potential losses of product and quality (5).
Other fruits also show the cumulative effects of low temperature, for example blackheart or endogenous brown spot, an internal symptom of chilling injury in pineapple, may be induced by chilling temperatures in the field or postharvest (9). Further, chilling already initiated in the field may not be evident at harvest but can manifest itself fully during postharvest handling and may also contribute to increased severity of blackheart in cool storage. Chilling symptoms in pineapple can occur at temperatures up to 21°C with a maximum susceptibility observed at 15° - 18°C. However, exposure for only a short period to chilling temperatures with subsequent storage at higher temperatures may prevent the development of blackheart (10).
All banana cultivars are extremely sensitive to chilling injury. Exposure of the fruit to 5°C for only 24 hours will induce obvious symptoms (11). Mild symptoms are a slight darkening of the skin, vascular tissues and latex vessels. More severe symptoms are dull yellow to brown skin, failure to ripen, loss of flavour, pulp hardening and susceptibility to physical injury. Chilling in the field also is known to affect the fruit.
The Australian grown atemoya, African Pride, is susceptible to a field chilling condition known as russeting. This is a skin discolouration which is induced by low night temperatures during late autumn and manifests itself as small discrete dark spots which eventually coalesce to form a uniform dark skin discolouration. This superficial injury is of considerable importance in the market returns on affected fruit from southern Queensland and northern New South Wales.
The same chilling symptom can be induced by postharvest storage at temperatures below 15°C (12,13) and can then progress during storage to severe skin and pulp. injury.
Chilling injury in avocados is characterised by a grey-brown discolouration of the vascular tissue and the flesh, uneven ripening, off-odours and off-flavours with pitting and scald-like browning of the skin. Fruit may appear satisfactory while in storage but develop symptoms during post-storage ripening(14). Avocado cultivars differ in sensitivity to chilling injury, and for a particular variety, chilling sensitivity depends on the stage of the ripening process (15,16). Chilling injury of avocados in storage can be reduced by calcium infusion (17) and by controlled atmospheres(18).
Mango fruits are highly sensitive to chilling injury at temperatures below about 13°C. Symptoms are a greyish, scald-like discolouration of the skin, failure to ripen, non-uniform ripening, poor flavour and susceptibility to postharvest decay(19). As with many chill-sensitive fruits, partial ripening reduces the fruit's sensitivity to chilling injury(20).
In the papaya, symptoms of chilling injury at storage temperatures below 12°C are delayed or uneven ripening, blotchy skin colour, marbled flesh, off-flavours and off-odours and susceptibility to decay. Guava fruit stored at temperatures below 5°C develop skin discolouration and gelatinous pulp (21).
Refrigeration is a valuable tool in maintaining fresh quality while extending the postharvest life of highly perishable tropical and subtropical fruits. However the limiting factor for fresh produce of tropical or subtropical origin is chilling sensitivity at temperatures below about 13°C for even relatively short exposures, either in the field or postharvest. Temperature management to avoid such chilling conditions remains the best means of avoiding the problem.
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(16) Young, R.E. and Kosiyachinda, S. (1976). Yearbook Calif. Avoc. Soc. 59: 73-76.
(17) Chaplin, G.R. and Scott, K.J. (1980). HortScience 15: 514-515.
(18) Scott, K.J. and Chaplin, G.R. (1978). Trop. Agric. (Trinidad) 55: 87-90.
(19) Mathur, P.B., Singh, K. K. and Kapur, N.S. (1953). Indian J. Agr. Sci. 23: 65-77.
(20) Mukerjee, P. K. and Srivastava, R.B. (1979). Prog. Hort. 10: 63-69.
(21) Brown, B.I. (1981). M.Sc.Thesis. The Univ. of New South Wales, Kensington, N.S.W.
DATE: November 1985
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