Stewart Postharvest Review

Stewart Postharvest Review An international journal for reviews in postharvest biology and technology

Impact of environmental conditions on fruit and vegetable quality M Cecilia do Nascimento Nunes Department of Food Science and Human Nutrition, Center for Food Distribution and Retailing, University of Florida – IFAS, Gainesville, Florida, USA

Abstract Purpose of review: This review demonstrates through data published in some of the most recent scientific publications how environmental conditions, namely temperature, humidity and atmosphere can significantly impact the quality of fresh fruits and vegetables. Findings: Although several studies have shown that ambient temperature and atmosphere modifications significantly affect the quality of horticultural commodities, not many have considered the impact of humidity alone on the quality of fresh fruits and vegetables. Because ambient humidity is probably one of the most difficult environmental conditions to control, the majority of studies have used protective packaging to maintain acceptable humidity levels around the produce and at the same time create a modification of the atmosphere, which together help to maintain quality and extend shelf-life. A vast number of studies carried on controlled atmosphere (CA) storage and modified atmosphere packaging (MAP) have shown that these technologies in combination with proper temperature management extend shelf-life and maintain the quality of fruits and vegetables. However, optimum atmosphere conditions cannot be generalised as they are greatly dependent on numerous factors related to the produce, packaging materials and environment. Directions for future research: Temperature variations often encountered during distribution and retailing constitute one of the major concerns when establishing optimum CA or MAP conditions. In order to maintain quality and reduce waste, ready applicable research should focus on the impact of real life conditions on the quality of fresh fruits and vegetables and on ways to solve major breaks in the cold chain. Furthermore, while ambient humidity might not constitute a concern when produce is packed, when unpacked, the low humidity levels often encountered during distribution and retailing may lead to important quality and economic losses. Therefore, future research should also focus on the effects of different levels of ambient relative humidity alone and in combination with temperature on the quality of fresh produce. Finally, consideration should be given to the development or improvement of “intelligent” and biodegradable films that can create an optimum atmosphere and maintain humidity levels within requirements for each specific produce and at the same time be attractive to the consumer. Keywords: temperature; humidity; atmosphere; appearance; texture; composition

Abbreviations Ascorbic Acid AA Controlled Atmosphere CA Chilling Injury CI Modified Atmosphere Packaging MAP Relative Humidity RH

© 2008 Stewart Postharvest Solutions (UK) Ltd. Online ISSN:1945-9656 www.stewartpostharvest.com

Correspondence to: M Cecilia do Nascimento Nunes, Department of Food Science and Human Nutrition, University of Florida-IFAS, SW 23rd Drive, Bldg. 685, Gainesville, FL 32611-0720. Tel: +1 352 392 1978 ext. 401; Fax: +1 352 392 1988; email: [email protected] Stewart Postharvest Review 2008, 4:4 Published online 01 August 2008 doi: 10.2212/spr.2008.4.4

Nunes / Stewart Postharvest Review 2008, 4:4

Introduction In the last few years there has been considerably increased interest in the study of storage requirements for fresh fruits and vegetables, mainly due to changes in production trends and marketing. In order to increase food availability, a partial alternative can be provided by improved storage and conservation as a way of reducing postharvest losses and providing better quality and more nutritious fruits and vegetables to the consumer. Fruits and vegetables supply several vital components to the human organism and are important constituents of a healthy and well balanced diet. Fruits and vegetables are the major sources of vitamins A and C in the human diet [1, 2] and constitute a rich source of phytochemicals and other bioactive components with potential anti-carcinogenic and cardiovascular risk reduction properties [3–6]. In addition, fruits and vegetables are good sources of fibre, water and minerals and they bring to our daily food consumption diversity in colour, texture and flavour. However, appearance, texture, flavour and levels of these important bioactive compounds in fruits and vegetables are greatly influenced by postharvest conditions such as precooling method, storage temperature, humidity and atmosphere composition, type of packaging and distribution method [7, 8*–11]; and if fruits and vegetables are handled under improper conditions, a great part of these health benefits may be significantly lost.

Effects of temperature on fruit and vegetable quality Environmental conditions, in particular temperature, have a major impact on the appearance, texture, composition and eating quality of fruits and vegetables. Temperature is, in fact, the element of the postharvest environment that has the greatest impact on the quality of fresh fruits and vegetables. Good temperature management is the most important and simplest method of delaying produce deterioration, and optimum preservation of fruit and vegetable quality can only be achieved when the produce is cooled to its optimum temperature as soon as possible after harvest. However, delays before cooling and poor temperature management inevitably occur in commercial handling during field, transport and storage operations, thereby reducing the quality and maximum potential shelf-life of fresh fruits and vegetables. Several recent studies have confirmed that prompt precooling of fruits and vegetables after harvest significantly reduces loss of quality during storage and extends shelf-life. For example, compared with asparagus room cooled to 1 or 10ºC, forced-air cooling of asparagus to 1ºC after exposure to 20ºC for 5 h extended the shelf-life of the spears for at least 7 days by reducing loss of moisture and maintaining a better texture [12]. Similar results were found when precooling of broccoli was delayed, that is, a delay of cooling for 24 h at 20°C before storage at 5°C reduced the shelf-life of broccoli florets to about 9 days, compared with a maximum shelf-life of about 18 days when broccoli was immediately cooled after harvest

[13]. In blueberry fruit, reducing the cooling delays after harvest from 16 to 2 h significantly reduced mass and firmness losses during subsequent storage at 4°C and 95% relative humidity (RH) [14]. In order to reduce decay and loss of quality during storage, strawberries should also be precooled immediately or not more than 2–3 h after harvest [15]. Prompt vacuum cooling also helped to retain firmness, as well as ascorbic acid (AA) and chlorophyll contents in lettuce during subsequent storage for 2 weeks at 1ºC [16]. Prompt hydro-cooled rambutan fruit had better quality and longer shelf-life than non-cooled fruit. Hydro-cooling reduced rambutan pericarp browning and weight loss, minimised losses in AA and titratable acid, and delayed soluble solids increase [17]. In general, the lower the storage temperature within the limits acceptable for each type of commodity, the longer the storage life. For each horticultural commodity there is assumed to be an optimal storage temperature at which the rate of produce deterioration is minimised. Storage of fruits and vegetables at their optimum temperature retards aging, softening, textural, colour and flavour changes, as well as slows undesirable metabolic changes, moisture loss and losses due to pathogen invasion. Many recent studies have confirmed that maintenance of an optimum constant temperature from the field to the store is crucial for maintaining fruit and vegetable quality. However, fruits and vegetables are often handled, transported and displayed under inadequate conditions, leading to loss of large amounts of produce at the retail or consumer levels. Fruit or vegetable rejection often results from deterioration in appearance as a result of exposure to too low or to high temperature. In fact, appearance (ie, colour changes, shrivelling, wilting, chilling injury (CI) symptoms, decay) is one of the most important factors that determines the market value of fresh fruits and vegetables and is greatly influenced by temperature [18**]. For example, the colour of ‘Golden Delicious’ apples stored at 20°C became lighter, less green and more yellow after 30 days, compared with freshlyharvested fruit [19]. During storage, the colour of raspberries became less bright red and less vivid than at the time of harvest (lower L*, hue and chroma values), but the lower temperatures tended to better maintain the red colour of the fruit [20, 21]. Compared with ‘Medallion’ yellow summer squashes stored at 0, 5 or 10°C, which maintained a bright yellow colour during storage, fruit stored at 15 or 20°C showed a significant decrease in L* value after 8 days, meaning that the colour of the squashes turned from a bright yellow to a dark yellowish-orange colour [22]. Conversely, colour changes in broccoli during storage are associated with an increase in L* and chroma values, and decrease in hue angle and chlorophyll content, as a result of yellowing of the florets [23]. Although depending on the cultivar there might be some variation related to the onset of yellowing, this process is greatly affected by temperature and may occur within a few days if broccoli is held at ambient temperatures [18**]. For example, broccoli stored at 4°C retained its green colour and fresh appearance during storage for 7 days, whereas 2


Figure 1. Appearance of broccoli heads after 3 days of storage at 0, 5, 10, 15 and 20°C (from left to right) and 90–95% RH.

broccoli stored at 20°C showed traces of yellowing after 3 days, and after 7 days the heads were completely yellow, showed some mould development and released an unpleasant odour [24]. Figure 1 illustrates how increasing the storage temperature significantly enhances the development of yellowing in broccoli stored for 3 days at 0, 5, 10, 15 and 20°C. After 3 days at 20°C, broccoli florets were completely yellow, whereas even after 20 days of storage at 0°C complete floret yellowing was not attained [18**]. In bell pepper harvested at different colour stages from green to green-orange, development of orange colouration occurred after 10–15 days at 22ºC. On the other hand, the colour of peppers stored at 7ºC for 20 days did not change until fruit were transferred to 22ºC [25]. The appearance of mangoes exposed to ambient warm temperatures was impaired not only by discolouration as a result of accelerated ripening, but also by the development of shrivelling and fruit softening due to moisture loss, whereas CI symptoms developed when mangoes were exposed to chilling temperatures [18**]. Losses of sub-tropical and tropical crops are often due to exposure to temperatures below 10°C, which causes the development of CI. Symptoms of CI in low temperaturesensitive crops may vary with the type of produce and cultivar, maturity at harvest, the temperature at which symptoms become evident and length of exposure to that temperature, and are usually aggravated when produce are removed from the chilling temperature. For example, in mangoes stored at temperatures below 10°C, skin injuries (ie, discolouration or uneven colouration, pitting, greyish scald) are usually the first symptoms of CI to develop, and the intensity of the injury often increases after holding the fruit at ambient temperatures [26, 27]. Internal injury, such as pulp discolouration, usually occurs later than skin discolouration. For example, mangoes exposed to temperatures between 4 and 8°C developed greyish scald-like spots on the skin, followed by the development of large dark brown areas on the skin. Severe skin discolouration was accompanied by uneven pulp softening and poor eating quality due to reduced volatile production [27, 28]. In peaches, lack of juiciness, often associated with mealiness, and hard or woolly texture is a common

symptom that develops when peaches are exposed to chilling temperatures. Flesh textural changes are usually observed before the development of flesh browning in chilled peaches, whereas the flavour of peaches was lost approximately 5 days prior to the appearance of visual symptoms of internal breakdown due to chilling damage [29*]. In ‘Horn of Plenty’ yellow summer squashes, pitting of the skin, peel discolouration, scalding and minor signs of decay developed after approximately 5 days at 0 or 5°C, while in ‘Medallion’ squashes signs of CI became objectionable after 3 days at 0 or 5°C [22]. Finally, in ‘Sunny’ green tomatoes, flaccidity and delayed, uneven and non-uniform ripening occurred when the fruit were stored at or below 7.5°C for more than 5 days [30]. Development of decay is another concern if fruits or vegetables are exposed to inadequate temperatures and usually increases as storage progresses or when fruits are transferred to ambient temperatures after being exposed to chilling temperatures [18**]. For example, decay of ‘Marsh’ grapefruits increased after exposure at a chilling temperature (4°C), affecting 24% of the fruit stored for 2 months and 67% of the fruit after 4 months [31]. Storage of green tomatoes at 2.5°C for only 3 days resulted in uneven ripening and development of decay due to CI [30]. In fruits sensitive to high temperatures, decay generally increases as storage progresses or temperature rises. In blackberries, decay and percentage of leaky fruit increased during storage at 2°C and after 14 days 20– 33% of the fruit were decayed, while 39–50% were leaky [32]. While ‘Patriot’ blueberries stored at 0 or 5°C developed the smallest amount of decay after 12 days (5% decay in fruit stored at 0°C and 7.5% decay in fruit stored at 5°C, respectively), fruit stored at 10, 15 and 20°C developed 10, 13 and 18% decay, respectively [33]. Softening of the fleshy tissues of some fruits and vegetables is one of the most important changes occurring during storage and also has a major impact on consumer acceptability. Changes in the textural quality of vegetables include decreased crispness and juiciness or increased toughness. Crispness is expected in fresh apples, peaches and green onions, but tenderness is desired in asparagus and green beans. In the 3


case of leafy vegetables, as they lose water they wilt, shrivel, and become flaccid, losing their expected attractive appearance. For example, as storage temperature increased ‘Navelina’ oranges became softer, peel thickness decreased and the level of dehydration increased due to loss of moisture. During 84 days at temperatures between 20 and 23°C, dehydration increased from 0.06 to 2.52 kg/m2, while peel thickness decreased from 4.3 to 2.9 mm. The albedo became thinner and more compact, the density of the peel decreased, turgidity forces increased and firmness decreased due to aging of the fruit [34]. In ‘Pink Lady’ apples stored for 25 weeks at 1°C and then transferred to 20°C for 7 days, firmness decreased significantly from an initial value of 91.7 N to 74.8 N after storage [35]. Firmness of ‘Killarney’ raspberry stored at 0°C was considered unacceptable after approximately 6 days, while at 5°C fruit softened faster and reached an objectionable firmness after 3 days [21]. On the other hand, asparagus tend to become less turgid and tougher as storage time and temperature increases. For example, storage for 3 days at 21ºC significantly increased asparagus strength mainly in the last portion of the stem [36], while in asparagus stored at 2ºC the shear force increased by 70.2% after 14 days of storage [37]. Loss of water is also a major cause of postharvest deterioration and is significantly affected by time and temperature. Loss of water often impairs appearance, as well as the compositional and eating quality of stored fruits and vegetables. The rate of water loss from horticultural crops is however dependent on the type of fruit or vegetable and is greatly related to the physiological and morphological characteristics of each individual fruit or vegetable [38]. For example, the rate of weight loss from tomatoes stored at 5 or 12°C was 0.15 and 0.49% per day, respectively, while in tomatoes stored at temperatures between 25 and 27°C the weight loss rate increased to 0.68% per day [39]. In broccoli stored under domestic refrigeration conditions (4 to 8°C), weight loss was about 1.4% after only 7 days, while when held at ambient temperatures (12–22°C) weight loss increased to 9.0% [40]. ‘Lanes Lane’ oranges stored at 3°C showed a weight loss of 0.09 and 1.6% after 20 and 30 days, respectively, and upon transfer to 22°C for 30 days weight loss increased to 16% [41]. Finally, weight loss in raspberries exposed to simulated harvest-to-consumer conditions was about 10% and was mostly affected by the last 24 h at 20°C, while the further 3 days at 2°C caused smaller losses [20]. Depending on the maximum postharvest life, weight loss at 20ºC and 85–95% RH was greatest (45%) in mushroom and least (2%) in tomato. Tomato, witloof chicory, blueberry, raspberry, mango, papaya and strawberry lost less than 5% of their initial weight; asparagus lost 8% of its initial weight; yellow summer squash, snap beans, peach and green bell pepper lost between 11 and 25% of their initial weight; and mushroom lost more than 40% of the initial weight after storage. At these levels of weight loss, appearance was already considered objectionable and when weight loss increased slightly above such levels, quality deteriorated at a faster rate and the

fruit or vegetable appeared unacceptable, due to severe softening, colour deterioration, shrivelling, wilting or dry appearance [38]. The composition and nutritional value of fruits and vegetables can also be greatly affected by temperature. In general, an increase in temperature leads to a decrease in acidity, soluble solids, total sugars and AA contents. On the other hand, some pigments such as anthocyanin and lycopene, with the exception of chlorophyll, which showed a decrease with increased temperature, may continue to be synthesised during storage, and their syntheses appear to be dependent on temperature. Cantaloupe stored for 7 days at 2°C or for 1 or 2 weeks at 10°C showed a decrease in acidity, soluble solids, fructose and glucose contents [42], whereas ‘Galia’ muskmelons showed a decrease in soluble solids from 12.9 to 10.2% after 14 days at 5–6°C plus 3 days at 17°C [43]. In honeydew melons, storage for up to 24 days at 5°C had no significant effect on sugar content, whereas storage at 10°C resulted in a significant decline in the sugar content of melons [44]. In four apple cultivars, soluble solids, acidity and volatile contents also decreased during storage for 12 months at 0°C [45]. In snap beans stored at 8ºC, total sugar content initially increased, but a decrease was observed subsequently, when storage was extended to 18 days. The hydrolysis of starch to release soluble sugars and the possible synthesis of some sugars that occurred during the first days of storage may explain the initial increase in sugar content of snap beans stored at 8ºC. Subsequently, the drop in the sugar content observed after 11 days at 8ºC might be related either to an increase in the respiration metabolism, which involves the consumption of simple sugars, or to the condensation of sugars [46]. AA content is also affected by storage time and temperature, even when fresh fruits and vegetables are handled under optimum conditions. For example, the AA content of broccoli stored at 1°C decreased progressively during storage, from an initial value of 2.15 mg/g dry weight to a minimum value of 1.45 mg/g dry weight after 20 days of storage [23]. While in honeydew melons there was a slight decline in AA content during storage at 5°C, a significant decrease was observed when melons were stored at 10°C [44]. Conversely, the AA content of cucumber exposed to 5°C declined faster and was lower than that of fruit stored at 10°C, whereas fruit stored at 20°C had the higher AA content after 16 days of storage. The lower AA content of cucumber stored at 5°C compared with that of fruit stored at 10°C was attributed to the antioxidant effects of the reactive oxygen species that induced chilling stress in the fruit stored at lower temperature, rather than to the increased activity of AA oxidation enzymes [47]. When several apple cultivars were stored at 20°C, the AA content decreased from initial average values of 5.9 to 14.5 mg/100 g fresh weight to about 3.3 to 13.1 mg/100 g fresh weight after 10 days [48]. In papaya, AA tended to decrease during storage, regardless of the storage temperature. However, after 20 days of storage the AA content in fruit stored at 5°C was 4


Figure 2. Quality curves for asparagus (A), Boston-type lettuce (B) and peach (C) stored at 0, 5, 10, 15 and 20°C and 85–95% RH. The dotted line corresponds to the limit of acceptability before the quality becomes unacceptable.

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higher than that of fruit stored at 13°C [49], but the greatest decrease in AA (from 750 mg to 393 mg/ 100g dry weight) was observed in papaya stored for 14 days at 0°C compared with those stored at higher temperatures [50]. Thus, after 14 days at 0, 5, 10, 15 and 20°C, the AA content was reduced by about 48, 36, 42, 41 and 43%, respectively [50]. Reductions in the AA content of papaya fruit exposed to chilling temperatures were probably due to a same degenerative process of the cell wall structure that usually takes place during ripening. That is, the damage in the cell membranes that generally occurs during exposure of chilling sensitive fruit to cold temperatures may be compared to that occurring during senescence and thus might have also contributed to accelerated AA oxidation in fruit exposed to 0°C [51]. Synthesis of lycopene in tomato and watermelon seems to be dependent on temperature. In fact, lycopene synthesis occurs at higher rates when tomatoes are exposed to temperatures between 12 and 30°C, and at 32°C lycopene synthesis is completely inhibited [52*]. For example, lycopene content increased in tomato stored for 3 weeks at 20°C, but no changes were observed in fruit stored at 4°C for the same period of time [53]. Similarly, the lycopene content of tomato stored for 7 days at temperatures between 25 and 27°C was significantly higher than that of tomato stored at either 5 or 12°C [39]. However, while at 12°C the synthesis of lycopene was much higher than at 5°C [39], at 30°C the formation of lycopene was inhibited [54]. In the same way, the lycopene content of watermelons increased during storage and synthesis was accelerated when storage temperature increased. Lycopene content of ‘Black Diamond’, ‘Summer Flavour 800’ and ‘Sugar Shack’ watermelons stored at 13°C was similar to that of fresh watermelon after 14 days of storage, while in melons held at 5°C lycopene content decreased by 12–24%. On the other hand, compared with fresh watermelons, the lycopene content increased by 12–40% in fruit stored at 21°C [55]. Anthocyanin production also seems to be governed by temperature. For example, when blueberries were harvested before attaining full blue colour, a considerable increase in total phenolic and anthocyanin contents occurred during storage at 5°C [56]. Similarly, storage of ‘Mekker’ raspberries for 9 days at 0° C resulted in increased anthocyanin concentration [20]. However, when stored for more than 7 days at 10°C, total phenolic and anthocyanin contents of raspberry decreased compared with initial values at harvest [57]. Although total anthocyanin content in strawberry harvested three-quarter coloured increased by about 13% in ‘Chandler’, 18% in ‘Oso Grande’ and 25% in ‘Sweet Charlie’ strawberry cultivars during storage for 8 days at 1°C, ‘Sweet Charlie’ was the only cultivar that showed an increase in total anthocyanins comparable to that observed in field-ripened strawberry [58]. The flavour of fruits and vegetables can also be influenced by temperature due to either production or inhibition of specific volatile compounds. For example, strawberries stored at 5 or 10°C produced higher levels of aroma volatiles than fruit 5


Figure 3. Appearance of intact (A and B) and intentionally bruised plus inoculated with Botrytis cinerea (C and D) strawberries exposed to semi-constant (A and C) and fluctuating (B and D) temperature regimes.

stored at 0°C [59]. However, refrigeration, as well as short term high temperature storage may cause an irreversible decrease in the volatile content and alter the flavour of tomatoes. In fact, volatile compounds in tomatoes stored for 3 days at 6° C significantly decreased and, after transfer to ambient temperature some of the compounds were further reduced [60], whereas in tomatoes stored for 4 days at 20°C an increase in the attribute “tomato-like” odour, flavour and after taste were detected [61]. Similarly, after storage of cantaloupe for 7 days at 2°C there was a general decrease in the aroma profile of the fruit, particularly for esters and alcohols [42]. Figure 2 shows quality curves (based on changes in colour, taste, aroma and development of decay or CI symptoms) for asparagus, lettuce and peach stored at five different temperatures (0, 5, 10, 15 and 20ºC and 85–95% RH). Shelf-life rapidly declines with increasing temperature and after 10, 6, 5, 4 and 3 days at 0, 5, 10, 15 and 20ºC, respectively, the quality of asparagus spears was considered unacceptable (Figure 2A); lettuce maintained an acceptable quality up to 11, 9, 7, 5 and 3.5 days at 0, 5, 10, 15 and 20ºC, respectively (Figure 2B), whereas the quality of peaches was considered objectionable after 20 days at 0ºC, 16 days at 5ºC, 6 days at 10 and 15ºC, and after 4 days at 20ºC (Figure 2C). In the same way, the shelf-life of mangoes stored at 2 and 5°C was reduced to about 6–8 days due to increased fruit softness and CI, whereas increased softness reduced the shelf-life of mangoes stored at 12 and 15°C to 6 and 3 days, respectively. Fruit softening followed by changes in colour were the major quality limiting factors for mangoes stored at 20°C, and limited their shelf-life to 2 to 3 days [26]. Fluctuating temperatures often encountered during shipping and distribution of fruits

and vegetables may also result in increased loss of quality. For example, strawberries exposed to fluctuating temperatures during handling were softer, had a higher weight loss and lower AA content than fruit held at constant temperatures. In addition, strawberries exposed to fluctuating temperatures developed darker colour, had evident dryness of the skin and were considered unacceptable for sale [62]. Figure 3A shows intact strawberries, while Figure 3B shows fruit that were intentionally bruised and inoculated with Botrytis cinerea and then exposed to two different temperature regimes, constant and fluctuating. It is clear that intact or even bruised plus inoculated strawberries from the constant temperature regime (Figures 3A and C) had a better overall appearance compared with fruit that were exposed to a fluctuating temperature regime (Figures 3B and C). Similarly, papaya handled in a fluctuating cold or warm temperature regime lost more weight, developed objectionable colour, was softer and more shrivelled, had more decay and had lower soluble solids, acidity and AA contents than papaya handled in the semi-constant temperature regime. In addition, papaya handled in the fluctuating temperature regime that included exposure to 1ºC for 2 h developed CI symptoms after being transferred to 20ºC for 7 days [51]. Pre-storage heat treatments (ie, hot water, air or vapour) normally used to control pests and diseases have been successfully applied to some tropical and sub-tropical fruits before exposure to chilling temperatures to prevent development of CI symptoms and decay, as well as to enhance colour, taste and texture. However, exposure to temperatures that are too high and for extended periods of time may cause detrimental effects on fruit visual and eating quality. For example, in ‘Valencia’ oranges a pre-storage hot water treatment at 48°C for 12 h or a curing treatment at 53°C for 6 h was effective in reducing CI and decay in fruit stored at 4°C for 6 months [63]. Similarly, exposing oranges to 33°C for 65 h also reduced the incidence of decay in fruit stored at 4°C for 2 months followed by 7 days at 20°C [64]. On the other hand, exposure of oranges to a hot air treatment at 37°C for 48 h resulted in increased weight loss, decreased juice yield, firmness and AA content, and had a negative effect on fruit taste and flavour [65]. In papaya, exposure to 42°C for 6 h before storage at 5°C resulted in reduced CI symptoms [66]. Yet, exposure of papaya fruit to hot moist air between 48.5 and 50°C and 100% humidity for 4 h resulted in severe injury compared with exposure to dry air (50% humidity) at the same temperature. Thus, during subsequent storage at 5°C, hot-dry air heated papayas showed less sensitivity to CI and accumulated more sugars than non-heated fruit, whereas internal colour, soluble solids, weight loss, carotene and lycopene concentrations were similar in heated and non-heated fruit [67]. Conditioning mangoes at 40°C for 8 h, before a heat treatment at either 45°C for 30 min or 47°C for 15 min, accelerated starch degradation and consequently soluble solids was higher in conditioned fruit than in non-conditioned ones [68]. However, visible symptoms of skin injury such as translucence, shrivelling, dimples, brown discolouration and 6


Figure 4. Appearance of yellow summer squash and zucchini packed in an expanded polystyrene tray covered with a polyvinylchloride film (left) or non-packed (right) after 4 days of simulated retail display at 14°C and 85% RH (non-packed) or 98–100% RH (packed).

decay, as well as visible symptoms of pulp injury such as starchy bands of variable thickness, small starch islands near the stem, and air-filled voids at the stem and near the seed, developed when mangoes were immersed in a 47°C water bath for 25 min [69*]. Finally, exposure of tomatoes to 38°C and 95% RH for 24 h resulted in severe heat injury characterised by scalding, irregular ripening and colour development, brown spots, cracking, fluid leakage and development of decay, whereas fruit heated to 34°C for 24 h were only slightly injured and developed better colour [70]. Besides, after 30 days at either 4 or 10°C, tomatoes that were heated for 24 h at 34 or 38°C had greater weight loss, and less AA and lycopene contents than non-heated fruit [71].

Effects of relative humidity on fruit and vegetable quality Although few studies have evaluated the effects of the ambient RH alone on the quality of fresh fruits and vegetables, the RH of the surrounding air is also an important factor that

needs to be controlled during storage of fruits and vegetables. Humidity and temperature together are particularly critical in minimising the difference in water vapour pressure between the produce and the environment. The humidity of the surrounding environment should be maintained at a level that minimises the water vapour pressure deficit. Therefore, when the RH is too low, transpiration is enhanced, resulting in loss of moisture. For example, while weight loss of grapefruits stored at 30% RH was about 0.4–0.5% per day, in fruit stored at 90% RH the rate of weight loss was reduced to 0.3% per day; with weight loss of fruit stored at low RH being two times greater than that of fruit stored at 90% RH [72]. The rate of fruit or vegetable transpiration, and thus loss of moisture, can be reduced by: raising the RH; by lowering the air temperature; by minimising the difference between the air and fruit temperatures; reducing air movement; and by protective packaging. The use of protective packaging (plastic containers or film wrapping) creates a higher RH in the environment and consequently helps to reduce loss of moisture during distribution and retailing. However, fungal decay may constitute a problem when fruits and vegetables are exposed to high temperatures combined with high humidity levels. For example, the percentage of strawberries affected by decay significantly increased as storage temperature and RH increased. That is, after 4 days, 3.9, 9.3 and 11.7%, of the fruit stored at 10°C and 75, 85 or 95% RH, respectively, were affected by decay, whereas the percentage of decay increased to 59.2% when strawberries were held at 20°C and 75% RH, and 90% of the fruit were affected by decay when held at 20°C and 85 or 90% RH [73]. Softening, discolouration, increased flabbiness, loss of turgidity, wilting and shrivelling, and dryness are visual symptoms generally associated with loss of moisture [38]. Because a decrease in firmness is strongly related to increased weight loss, raising humidity levels during postharvest handling helps to reduce fruit and vegetable desiccation and excessive softening. High RH during storage of ‘Late Lanes’ oranges not only reduced fruit moisture loss and maintained fruit firmness, but also reduced CI symptoms [41, 74]. Besides, reduced humidity during storage not only results in loss of moisture and orange dehydration, but may also lead to peel damage. In fact, rind breakdown was observed in ‘Navelina’ and ‘Navelate’ oranges with a weight loss higher than 2% that were transferred from low (30–45%) to high humidity (90–95%) storage [75]. Water loss due to low ambient humidity levels was also suggested as a potential trigger for the development of postharvest pitting in cucumber and zucchini stored at non-chilling temperatures. In fact, postharvest pitting was eliminated when produce was exposed to either high humidity levels (99–100% RH) or bagged [76]. Figure 4 illustrates the importance of maintaining a high RH around the produce during distribution and retailing. Yellow summer squash and zucchini were packed in an expanded polystyrene tray covered with a polyvinylchloride film or non-packed for bulk display and exposed to simulated retail display condi7


tions at 14°C and 75–85% RH (non-packed) or 98–100% RH (packed). After 4 days, packed produce clearly had a better appearance than un-packed produce, which appeared dry, pitted and discoloured (Figure 4). Other studies have also reported that wrapping green bell peppers in plastic films contributed to better green colour retention compared with non-wrapped fruit. In addition, the hue angle of green bell peppers packed in different materials gradually decreased (became less green and more yellow) during storage, but it showed the lowest value in unpackaged fruit. While bell peppers packed in a chitosan film lost 10–11% of their initial weight during 16 days at room temperature (27°C and 65% RH), fruit packed in low density polyethylene film lost 2 to 2.5% of their weight during the same period of time [77]. Weight loss was significantly higher in non-packed than packed broccoli, and increased with increased temperature and storage time. After 30 days, weight loss in broccoli stored at 0°C attained 1 and 5% when packed and nonpacked, respectively, while after 25 days of storage at 5°C weight loss in packed and non-packed broccoli was about 1.5 and 17%, respectively. Weight loss significantly increased in non-packed broccoli stored at 10°C and after 10 days broccoli had already lost 22% of its initial weight, compared with a 2% weight loss in packed broccoli [78]. Weight loss of non-packed broccoli stored at 1°C and 90% RH reached values higher than 45% after 20 days of storage, while in broccoli packed in plastic films weight loss averaged 7% after 28 days at 1°C [23]. In grapefruit stored at 20°C and 30% RH weight loss was two times higher than that of fruit stored at 90% RH. Therefore, after 20 days weight loss was about 12% for fruit stored at 30% RH and 6% for those stored at 90% RH. Besides, when grapefruits were transferred from 30 to 90% RH the rate of weight loss was significantly reduced [72]. In litchi stored at 20ºC and 50, 70, 80 or 90% RH weight loss was faster and significantly higher in fruit stored at 50% RH (approximately 15% after 3 days) than in fruit stored at 90% RH (less than 5% after 3 days) [79]. While there are only a few publications on the effects of RH alone on the composition or nutritional value of fruits and vegetables, most likely due to the difficulty in controlling the ambient humidity, protective packaging which allows a high RH around the produce, may help to reduce the loss of some health related compounds, which is usually caused by excessive loss of water. When fruits and vegetables are filmpacked, the humidity around the produce is maintained near the saturation point (ie, 100%) however, atmosphere modification may occur if the permeability of the film to CO2 and O2 is very low. In this case, it is hard to separate the effects of high RH alone from the effects of the atmosphere composition on the quality of the produce inside the package. On the other hand, when the permeability of the film is high to CO2 and O2 but low to water vapour then the effect of humidity alone can be evaluated. For example, after 20 days of storage at 1°C, a sharp decrease in chlorophyll, total phenolic and AA contents was observed during storage of non-packed

broccoli, whereas a smaller decrease was observed in these same compounds in broccoli packed in a macro-perforated film with no atmosphere modification [23]. In strawberry fruit stored at 0.5, 10 or 20ºC and 75, 85 or 95% RH, flavonoid and total phenolic contents also decreased with decreasing RH (from 95 to 75%), regardless of the storage temperature [73]. Similarly, anthocyanin and phenolic content of litchi stored at 20ºC and 50, 70, 80 or 90% RH significantly decreased as the RH decreased. In addition, fruit pericarp browning occurred faster when fruit where stored at 50% RH than at 90% RH; severe browning developed after 3 days at 50% RH and after 10 days at 90% RH [79].

Effects of atmosphere on fruit and vegetable quality The use of low temperatures combined with controlled atmosphere (CA) storage or modified atmosphere packaging (MAP) have been used successfully with beneficial effects in extending shelf-life and maintaining quality of many fruits and vegetables. CA refers to a controlled addition or removal of gases in the storage environment, resulting in an atmospheric composition different from that of air. Unlike CA, the purpose of MAP is not to create a fixed gas composition throughout the storage period or shelf-life of the produce inside the package. MAP creates a predetermined gas composition that may change over time depending on the respiration rate of the produce inside the package, the ambient temperature and RH, as well as the barrier properties of the packaging material that determines the specific gas composition at the equilibrium. The predetermined gas composition inside the package can be achieved passively or actively. A passive MAP is achieved by sealing the produce inside the package using a barrier film with specific permeability to CO2 and O2. In this case, the O2 consumed and CO2 released by the respiration of the produce inside the package, and the O2 and CO2 passing through the film attain a steady state at which the amount of O2 consumed and CO2 produced inside the package equals the amount of O2 and CO2 passing through the film. Conversely, an active MAP is achieved by replacing the air inside the package with a known mixture of gases that creates an initial atmosphere inside the package. Both CA and MAP usually involve the reduction of O2 or the elevation of CO2 levels of the surrounding atmosphere. However, the use of CA or MAP should always be considered as a supplement to proper temperature and RH management. If combined with proper refrigeration, one of the major benefits of these methods is delaying fruit and vegetable ripening and all the biochemical and physiological changes associated with senescence, such as increased respiration rate, texture, colour and compositional deterioration, development of decay, and other breakdown processes that occur during ripening. The concentrations of gases used in CA or MAP depend on the species, origin, its maturity or ripeness stage, the storage duration, temperature, RH and the tolerance of the fruit or vegetable to high levels of CO2 or low levels of O2. Furthermore, limits of tolerance can be different at temperatures 8


above or below those temperatures recommended for each commodity. Overall, an ideal controlled or modified atmosphere treatment is one that will prevent or delay ripening and senescence of a specific fruit or vegetable without causing any detrimental effect to their quality after removal from modified atmosphere conditions to normal air. The residual effects of a modified atmosphere, that is the effects after transfer of the fruit or vegetable to normal atmosphere, may include reduction of respiration and ethylene production rates, maintenance of colour and firmness and delayed development of decay. However, atmosphere modifications, which are largely influenced by variation in temperature that often occurs during distribution, storage and retailing, may create undesirable responses such as induction of fermentation. When the ambient temperature increases, the respiration of the produce inside the package tends to increase more than the permeation of the package creating fermentative conditions. In addition, development of objectionable flavours, reduction of aroma biosynthesis, and induction of tissue injury and alteration of microbial fauna may also take place [80, 81]. Another problem associated with temperature fluctuations during handling and marketing of MAP produce is the development of high humidity levels inside the package, which leads to condensation on the film and on the produce surface. The presence of free water inside the package may promote the development of decay and may also block O2 diffusion into the tissues and through the film, causing fermentation [80]. In the 1920s, Kidd and West studied how O2 and CO2 affected the shelf-life of apples, pears, plums and berries, and their research showed that low O2 and moderately high CO2 levels could extend the shelf-life of fruit [82]. Since then, researchers from all over the world have conducted comprehensive work on the effects of atmosphere modifications on the physiology, biochemistry and overall quality of fruits and vegetables. In the last 5 years, a large number of scientific publications have reported the effects of atmosphere modification (CA or MAP) on the quality of an assortment of fruits and vegetables. The two published volumes of the Proceeding of the Eight International Controlled Atmosphere Research Conference [83**] regroup many research works performed worldwide on the effects of CA and MAP on the quality of various horticultural commodities, including summaries of CA requirements and recommendations for specific horticultural crops. Overall, low O2 or high CO2 reduce ethylene synthesis and sensitivity of the produce to ethylene, consequently delaying ripening, as well as retarding colour deterioration by reducing chlorophyll, anthocyanins and carotenoids losses, and by preventing oxidative browning [81, 83**, 84]. Textural changes such as flesh softening or toughening are also reduced when CO2 and O2 are maintained within the tolerance limits of the produce, yet softening may be accelerated if CO2 or O2 are outside the tolerance limits. CA and MAP reduce the use of carbohydrates and acids as a result of the decrease in the produce respiration rate, resulting in slower sugar and acid sugar losses. In addition, AA and ß-

carotene losses as well as flavour deterioration are reduced when fruits and vegetables are stored under adequate CA or MAP conditions [84]. Finally, elevated CO2 controls sporulation or growth of numerous fungal decay organisms [85]. CA or MAP storage was reported to retard discolouration, yellowing and chlorophyll losses, reduce nutrient losses and maintain texture and general appearance of green vegetables. For example, quality loss of broccoli packaged under MAP at 1°C was significantly reduced when compared with broccoli stored in air. Broccoli stored in air showed higher weight loss, yellowing, chlorophyll degradation and stem hardening, as well as a rapid decrease in total antioxidant activity, AA and total phenolic compounds, whereas in broccoli packaged under modified atmosphere visual quality, total antioxidant activity, AA and total phenolic compounds remained almost unchanged during a 28-day storage period [23]. Glucoraphanin, an anti-carcinogenic glucosinolate, also seems to be better preserved when broccoli florets are stored at low temperatures under a modified atmosphere than in air [13, 24]. For example, storage of broccoli florets under 10% CO2 and normal O2 concentrations resulted in a higher glucoraphanin content and better visual quality for up to 20 days compared to air-stored broccoli. In this case, the elevated CO2 concentration favoured the induction of glucorophanin biosynthesis or the reduction of glucorophanin degradation, whereas a 20% CO2 concentration and normal O2 accelerated the hydrolysis of glucorophanin. In addition, low concentrations (1%) or absence of O2 may also contribute to a reduction in glucorophanin biosynthesis or promote its breakdown [13]. Storage of cauliflower under CA (3% O2 and 5% CO2) at 0°C resulted in less weight loss, slower decline in curd lightness (higher L* values) and higher vividness (lower chroma values) compared with cauliflower stored in air [86]. When green celery stalks were stored at 4°C for 35 days under CA, the respiration rate was reduced by 30% compared with airstored samples; the higher the CO2 levels (from 5 to 25%) the higher the inhibition of stalk elongation; CA also reduced decay, and prevented butt and cut browning and pithiness. An atmosphere with 5% O2 and 15% CO2 resulted in no development of decay and green celery stalks with best overall quality, whereas a CA with 25% CO2 produced browning of the internal petioles [87]. In Chinese chives, storage at 20°C and low oxygen (1 and 3% O2) decreased the respiration rate, the onset of leaf yellowing and maintained the chlorophyll content, compared with air-stored chives which developed yellowing of the leaves after 5 days of storage [88]. An atmospheric composition of 10% O2 and 10% CO2 in combination with a storage temperature of 5°C prevented red discolouration of chicory heads and leaf head discolouration [89], whereas the shelf-life of green beans was extended and nutritional value preserved when stored in 3% O2 and 3% CO2 at 8°C [46]. External appearance, texture and composition were best maintained and shelf-life extended when green asparagus spears were stored under MAP at 2°C. Shelf-life of green asparagus packaged under modified atmosphere and 9


Figure 5. Appearance of strawberry fruit harvested three-quarter or full red coloured after storage for 2 weeks in air or CA at 10°C.

2 Weeks’ storage

3/4 Coloured 15% CO2 + 5% O2 at 10oC

Full ripe 15% CO2 + 5% O2 at 10oC

3/4 Coloured Air at 10oC

Full ripe Air at 10oC

stored at 2°C was 12 days longer than those stored under normal air [37]. In tropical and sub-tropical fruits, CA or MAP reduces the respiration rate and ethylene biosynthesis, delays ripening, alleviates CI symptoms and reduces decay incidence, while maintaining good fruit quality. For example, in guava, respiration rate and ethylene production were reduced when fruit were stored at 8°C in atmospheres containing six combinations of O2 and CO2 (2.5 O2 + 2.5% CO2, 2.5% O2 + 5% CO2, 5% O2 + 2.5% CO2, 5% O2 + 5% CO2, 8% O2 + 5% CO2 and 10% O2 + 10% CO2). In addition, CA storage was effective in maintaining fruit firmness and retarding changes in soluble solids, acidity, AA and total phenols contents compared with air storage at the same temperature. CI and decay incidence were also reduced during ripening of fruit stored in optimal atmosphere compared with air-stored fruit. However, guavas stored under low O2 levels (5%) [90]. Compared with air storage, ‘Hass’ avocado fruit held at 5°C in static (5% O2 and 5% CO2) or dynamic (