Temperature

The earliest attempts at temperature management were dependent on fruit being held in cold caves, or using cold night air to prolong the storage life, but experience showed that a ‘best’ temperature can be sharply defined, and may differ between species or even cultivars (Sevillano et al. 2009). To obtain the maximum benefit from cold temperatures, the temperature must be as low as possible without causing damage to the fruit; this is termed the lowest safe temperature. Below the lowest safe temperature, but at non-freezing temperatures, the fruit may develop symptoms of chilling injury (See Section 14.4). At even lower temperatures, generally in the range –0.5°C to –1.5°C, freezing occurs which irreversibly damages a living product. Because of this, –0.5°C is usually the lowest temperature used for storage of fruit, including some apple cultivars, berries or 'Hayward' kiwifruit. Temperatures at which chilling symptoms occur are around 8^oC for subtropical species and may be anything up to 14°C for some tropical fruit: for example unripe banana and mango need to be shipped at 13–14°C. However, it is not only tropical and sub-tropical fruit that are susceptible to chilling injury; even 'Hayward' kiwifruit, which is stored at 0°C or just below, may develop chilling injury.

At 0°C, respiration is reduced to a level that is just enough to maintain cell function. Sugar is slowly consumed during this process so that fruit with a low sugar content at harvest are less durable. Commodities such as kiwifruit, which are picked with large supplies of carbohydrate in the form of starch, have an additional source of sugar to utilise, giving longer storage lives than those entirely reliant on soluble reserves, such as grapes.

Low-temperature storage has played an important part in the development of successful fruit export industries in Australasia, because of the great shipping distances between orchard and consumer. The success of kiwifruit has been largely due to its ability to be stored at 0°C for 6 months or more with no detrimental effect on flavour or texture.

Associated with low-temperature storage is a wide range of techniques to manage temperature changes en route to storage (Kader 2002). There are strong differences between species in their temperature management requirements. Elements of temperature management that need to be considered include the timing of cooling after harvest, the rate of cooling and the final storage temperature. Temperature management may also be viewed as a two-stage process, the removal of the field heat and then temperature maintenance during storage. While it is generally considered that the field heat should be removed from fruit as soon as possible after harvest, there are circumstances where delays may be advantageous for the postharvest performance of the fruit. So whilst highly perishable berryfruit tend to be cooled as soon as possible after harvest, kiwifruit and some stonefruit benefit from a delay at ambient temperature before cooling. Exactly what happens during this delay period is not clear; it may simply be a continued progress of fruit development or the loss of a small amount of water. However, the delay tends to make the fruit more tolerant of storage at low temperatures. In the case of 'Hayward' kiwifruit, the delay period is termed ‘curing’ and is specifically applied to reduce the incidence of stem-end rots caused by Botrytis. As a beneficial side effect, the low temperature tolerance of the fruit is also increased. In this sense, curing in kiwifruit is not the same as the curing for wound healing of the skin that is commonly referred to for sweet potatoes.

The rate of cooling is dependent both on what is required commercially and what can be tolerated by the fruit. Simply placing fruit, either in bulk bins or packed, in a coolstore will result in the fruit being cooled, the rate of which will depend on the initial fruit temperature, the cooling capacity of the refrigeration equipment, the airflow in the store and any insulating effects from the packaging, especially if the fruit are packed in boxes with polyliners and held on pallets. The rate of cooling can be increased by forced air cooling, also termed precooling, in which cold air is actively drawn past the fruit. This is a rapid method for removal of field heat, after which temperature management in a coolstore removes the smaller heat load that results from continued respiratory activity during storage. In some cases fast precooling may induce high incidences of chilling damage. This is one reason why 'Hayward' kiwifruit is not always precooled, but may be cooled from about 14–18°C at harvest to about 2°C after about 5 days, with a further 5–7 days to reach the final storage temperature.

Managing the rate of cooling of fruit to avoid chilling injury may be as simple as allowing the fruit to cool slowly, as in the case of 'Hayward' kiwifruit described above, or there may be clearly defined stages of cooling whereby fruit are cooled to an intermediate temperature, held for a period of days before the temperature is reduced to the final storage temperature. In all these instances of slow cooling, there is a trade off between the conditioning effect that increases tolerance to low temperatures and the progression of fruit development that occurs more rapidly at higher temperatures, and reduces storage life of the fruit.

An extreme example of temperature treatment prior to storage is where fruit may be treated at high temperatures (40–50°C) for disinfestation, and in particular to kill fruit fly, after which the fruit ripening may be slower than would occur naturally.

The expression of chilling injury symptoms may be reduced in long-term storage by intermittently warming the fruit. However, whilst there are numerous reports of such treatments in the scientific literature, the practicalities of the procedure and detrimental side effects to fruit quality make it commercially uncommon.

Relative humidity

Once harvested, fruit will continuously lose water to a point where quality will be affected. In some species, a small amount of water loss may accelerate ripening (e.g. avocado), but in all fruit there eventually comes a point at which loss of water, usually first seen as shrivelling, results in the fruit becoming unacceptable. Water loss from the fruit is driven by the vapour pressure gradient between the fruit and the surrounding environment. While the capacity for air to hold water is reduced at low temperatures, there is always a gradient driving water from the fruit into the coolstore atmosphere. The less fruit there is in a coolstore, the greater the water loss from each fruit before an equilibrium relative humidity is reached. Water may be lost from the coolstore atmosphere by condensation on the refrigeration coils that are colder than the room atmosphere, and the greater the temperature differential between the coils and atmosphere the greater the loss of water. When storage is at about 0°C, this can be seen by ice developing on the coils that must be removed by defrosting.

In preventing quality loss of harvested fruit, the relative humidity of the storage environment is one of the first aspects considered, since fruit will lose water more rapidly at lower relative humidity. This is mostly an issue where fruit are held unpacked or in bulk in a coolstore, and water loss is exacerbated where there is only a small volume of fruit in the store, air flow is high and there is a large temperature differential on the refrigeration coil. In other circumstances, such as for kiwifruit that may be stored for months, the fruit is packed into fibreboard packs with a polyethylene liner or bag. In these circumstances, it is the bag that creates a high humidity environment for the fruit and limits the fruit’s water loss. A very high relative humidity in the store environment where packed fruit are held may be detrimental to the integrity of the fibreboard packaging, which would soften and lose its strength.

Fig 11.19.png

Figure 11.20. Storage life of kiwifruit can be greatly extended by controlled atmospheres. Under standard conditions (humidified air, 0°C) firmness declines exponentially over time, reaching limited acceptability by 8 weeks. Storage life ends once fruit firmness drops below about 0.9 kgf. Fruit are then soft enough to eat. Softening in cold store was slowed and storage life greatly extended by holding fruit in atmospheres containing either 5% CO₂ + 2% O₂ (top curve) or 8% CO₂ + 16% O₂ (middle curve). (Based on McDonald and Harman (1982) Sci. Hort. 17, 113-123)

The storage life achievable by refrigerated storage can be extended by modifying the store atmosphere by reducing the oxygen and increasing the carbon dioxide concentrations. Elevated CO₂ and reduced O₂, used either separately or together, can delay ripening and slow the onset of senescence (Figure 11.20). When both high CO₂ and low O₂ concentrations are combined then the beneficial effects may be additive. These methods were originally developed on a commercial scale for apple, but have been progressively applied to many other fruit. Container shipping helped their introduction because a sealed container made it easier to maintain the required temperature and atmosphere regimes.

For the bulk storage of fruit in bins, packs of fruit on pallets, coolstores, ships’ holds and individual shipping containers, an active process called controlled atmosphere (CA) may be operated. In these, the concentrations of O₂ and CO₂ are monitored and maintained at predetermined levels. Initial low O₂ concentrations may be achieved through the use of nitrogen generators or O₂ scrubbers, or the fruit may be allowed to reduce the O₂ concentration through respiratory activity. To prevent the O₂ concentration from becoming too low, air can be exchanged with the atmosphere. CO₂ accumulates from respiration, but can be prevented from increasing excessively by absorbing it with lime, by removal with an activated carbon scrubber or by purging from the store with nitrogen. In a closed CA system it is also possible to scrub ethylene out of the atmosphere. The removal of ethylene is particularly important for ethylene sensitive fruit such as kiwifruit, where even low levels (e.g. 30 ppb) in the store atmosphere can reduce the storage life of the fruit.

A more recent approach to CA storage is termed dynamic CA storage, in which the O₂ concentration in the store is determined by the response of the fruit. Dynamic CA optimises the CA process, since using a predetermined atmosphere tends to err on the side of safety by setting the O₂ concentration well above the lowest safe level to allow for the variability in low O₂ tolerance amongst fruit from different orchards or seasons. Although this eliminates the risk of fruit becoming anaerobic, it also reduces the potential benefit. While early attempts at dynamic CA utilised ethanol sensors to detect if fruit metabolism was becoming anaerobic, it was the development of a fluorescence sensor that could give a rapid measurement of the fruit response to low O₂ stress that allowed the commercialisation of dynamic CA. The sensor is placed over a sample of the fruit in the store, the O₂ concentration is decreased until a response is detected from the fruit and then the O₂ concentration is increased slightly above the low O₂ stress point. The procedure can be repeated throughout the storage period so that the O₂ concentration can be continually matched to the capacity of the fruit to withstand low O₂.

An alternative way of utilising the beneficial effects of low O₂ and high CO₂ is termed modified atmosphere (MA) storage. In this system, fruit respiration is used to reduce the concentration of O₂ and increase that of CO₂ inside an enclosed space, usually the export box or retail packs. The fruit is prevented from becoming anaerobic by making such enclosures out of plastic films that are partially permeable to O₂ and CO₂. Both gases come to an equilibrium based on respiration rate, the specific permeability of the film, the surface to volume ratio of the package and the amount of fruit in the package. Hence, this form of storage is highly dependent on being able to control the fruit temperature, since this determines the rate of respiration. The independence of having fruit in smaller packages that can be moved intact throughout handling and retailing suggest that MA may be more versatile than CA, although in practice any inability to maintain adequate cold-chain conditions can result in fruit spoilage as packages turn anaerobic at higher than desired temperatures.

Coating fruit in waxes or other compounds may act in a similar way to MA, by modifying the gas permeability of the fruit skin, thereby reducing the flow of O₂ in and CO₂ out of the fruit. As with MA, if the restriction of oxygen flow into the fruit is too great, the fruit may turn anaerobic and ferment.

How do altered atmospheres delay ripening and retard senescence? There are several possibilities, mostly involving fruit respiration and ethylene metabolism. One common observation is that fruit respiration is suppressed in response to the changed atmosphere. This could occur via acidification of the cytosol, resulting from an elevated CO₂ concentration redirecting metabolism towards alcohol or lactate/succinate or malate production rather than CO₂ production. Another alternative is a direct effect of ultra-low O₂ concentrations (<2%) on cytochrome c oxidase in the mitochondrial electron transfer pathway, preventing that enzyme from functioning properly.

Fruit differ with respect to critical values for tolerance to low O₂ or high CO₂ concentrations, and ideally we might make a model for predicting the tolerance limits for a new cultivar or fruit from specific background information on its physiological behaviour. However, there is a key problem in manipulating atmospheres by static modelling approaches. The critical gas composition exists within the flesh of a fruit, not in the environment around it, while differences in genetic background cause each cultivar to behave differently with respect to metabolism and thus internal gas composition. Species vary in their response to the altered atmospheres of CA, and can even differ according to cultivar and harvest. This variation is seen in both the final concentration of CO₂ and O₂ within stored fruit, and in the time taken to equilibrate. Normally, an internal 0.5% (0.5 kPa) partial pressure is the minimum O₂ level tolerable, and 10% (10 kPa) is the maximum for CO₂.

Conditions during storage are especially critical because optimum levels of CO₂ and O₂ are on the threshold between aerobic respiration (desirable) and anaerobic respiration (undesirable). Fruit differ in their sensitivities to anaerobic respiration, but are normally intolerant of prolonged periods (>3 days), after which disorders and off-flavours appear. Yet the anaerobic metabolites ethanol and acetaldehyde are common volatiles of many ripe fruit, and treatment with these metabolites, or short anaerobic periods before storage, can have beneficial effects on storage life in some fruit, although they are not used commercially. Fruit in which ethanol and acetaldehyde have been induced are able to metabolise these compounds without tissue damage. The effect of anaerobic metabolism is therefore likely to be a question of degree: how much anaerobic metabolism and how sensitive is the tissue?

'Hayward' kiwifruit is a good example of where CA storage can be successful in prolonging storage life. Both low O₂ (2%) and high CO₂ (5%) can independently improve firmness retention during storage, with a synergistic effect when used in combination. However, whilst effective in retarding ripening, there are risks to the fruit. The greatest firmness retention is achieved by a rapid establishment of the CA, although too rapid establishment of 5% CO₂ can result in increased physiological disorders and rots. Also, concentrations of CO₂ at about 10-15% can result in a differential softening of the fruit flesh and core, resulting in a core that is firm relative to the pericarp tissues.

With ethylene having a pivotal role in the ripening of many (but not all) fruit, the use of the ethylene action inhibitor 1-MCP has been investigated for prolonging the storage life of a wide range of species through retarding fruit ripening and softening (Watkins 2008). 1-MCP is usually applied after harvest as a gas treatment in a sealed store, container or tent, with the active ingredient released from a powder by dissolving in water. The commercial delivery of 1-MCP is by the SmartFresh^(SM) system (www.agrofresh.com).

Successful use of 1-MCP to delay ripening depends on the physiology of the fruit, most likely on the natural rate of replacement of the ethylene receptors that are blocked by 1-MCP. Since binding of 1-MCP to existing ethylene receptors is irreversible, a single period of exposure can delay ripening for several to many days, depending on the rate of synthesis of new receptors. There has been a rapid uptake of 1-MCP use for commercial storage of some apple cultivars, although for other cultivars the treatment has little effect on fruit softening. The rapid uptake for apple is associated with the way in which apple fruit ripen, which involves only limited softening and with firmness retention being a key quality component, i.e., people like crisp apples. This contrasts with the physiology of other species in which ripening involves a softening of the fruit coordinated with changes in flavour and colour. For example, while 1-MCP prolongs storage life in species such as avocado, pear and banana, obtaining uniform ripening afterwards may be difficult (Watkins 2008). This may be because the softening, flavour, and colour aspects of ripening have varying sensitivities to ethylene (Johnston et al., 2009) that are affected differently by partial suppression of ethylene perception and the climacteric, resulting in poorer flavour and colour. In stonefruit such as peach, the ripening inhibition is rapidly overcome, and repeated exposure to 1-MCP may be necessary, which can be commercially unfeasible. For all cultivars, careful optimisation of maturity stage, 1-MCP concentration, exposure frequency and duration and storage temperature is required.

Fig 11.20.png

Figure 11.21. Postharvest incidence of the storage disorder watercore in Fuji apple is related to picking date (and thus fruit maturity). Watercore index represents the percentage fruit volume occupied by water-soaked tissue. Fuji is prone to this disorder, especially when fruit are picked mature. Early harvesting thus becomes an important control method. (Original data courtesy F.R. Harker)

When fruit are put into storage, they are on a slow path to senescence and death, and a number of disorders can arise during that time. Several storage disorders have physiological origins, which may be chilling related, and are often highly specific to species, cultivar, season and even growing region. Fruit maturity at picking is one important factor (Figure 11.21), with less mature fruit generally being more susceptible to chilling injury.

Sensitivity to storage disorders depends on many factors, including maturity at harvest, a lack or imbalance of nutrients and adverse growing conditions. Even if fruit are susceptible at harvest, the expression of disorder symptoms is dependent on storage conditions and duration, and symptoms may not always develop. The development of chilling injury is often described as a time by temperature relationship, i.e. it develops sooner at lower temperatures. This is true for damage that is a direct result of exposure to low temperature and which is seen almost immediately after exposure. However, many chilling disorders develop only after long periods in storage and are associated with an inability of fruit to ripen correctly at low temperatures (e.g. kiwifruit, peach, avocado). It seems that at low temperatures the natural highly co-ordinated process of ripening is disrupted by an element that is temperature sensitive. If removed from storage early enough, no symptoms of chilling develop when the fruit ripens at higher temperature.

Thus far, chilling damage has been described as a single disorder, yet there are numerous symptoms that may develop in the fruit flesh or skin that differ among species and cultivars. In addition, there are disorders that develop as fruit start to senesce, irrespective of storage duration or temperature, and that may have similar symptoms to chilling injury in the fruit flesh.

Five examples of postharvest physiological disorders in apple are described below (Figures 11.22, 11.23) to illustrate our partial understanding of the problems that occur, and to provide a glimpse of a large and complex area of postharvest physiology.

Picture11.22.png

Figure 11.22. Physiological disorders of apple fruit. The top left panel shows bitter pit, a disorder associated with calcium deficiency. It can be partially controlled by preharvest sprays of calcium salts directly onto the fruit. The top right panel shows superficial scald, a low temperature disorder of the skin that can be controlled by 1-MCP treatment prior to cool storage. The bottom left panel shows soft scald, a low temperature disorder with symptoms of brown lesions that extend into the flesh. Incidence can be increased by over-maturity of the fruit at harvest and preharvest climatic conditions. The bottom right panel shows core flush, a browning within the core line, that is a form of senescent breakdown.

Bitter pit is a brown, bitter pitting of the skin in some cultivars, particularly ‘Cox’s Orange Pippin’. It occurs as sunken discoloured pits in the skin with spongy, dry brown flesh beneath. It is primarily a response to inadequate calcium content, and can be greatly reduced by spraying fruit on the tree with calcium-containing solutions during the later stages of fruit development.

Superficial scald is a brown discolouration of the skin surface, particularly in cultivars like ‘Granny Smith’. It appears to be connected with the accumulation of the hydrocarbon α–farnesene in susceptible cultivars, the oxidation products of which are brown and may cause cell collapse. Superficial scald can be reduced by a postharvest dip in an antioxidant free-radical scavenger like diphenylamine (DPA). As a postharvest chemical treatment, DPA is being phased out, and in some circumstances the use of 1-MCP before cool storage may mitigate scald expression, since the production of α–farnesene is promoted by ethylene.

Less is known about the factors that affect the occurrence of soft scald, which can occur most frequently on cultivars ‘McIntosh’ and ‘Jonathan’. Soft scald or deep scald develops as sharply-defined brown lesions on the skin that usually extend into the flesh. Soft scald is a low temperature disorder, partially avoided by slow (delayed) cooling or by storing at slightly warmer temperatures. Its causes are unclear, but incidence is increased by factors including over-maturity of the fruit at harvest, and by dull, cool, wet summers.

Core flush, most serious in ‘McIntosh’, is a browning of internal fleshy tissues surrounding the core of a fruit, and may have more than one cause. One factor seems to be the O₂ supply to the core, since conditions potentially causing anaerobiosis (large size, a closed and airtight calyx and a low-O₂ atmosphere) increase incidence. It is most serious in fruit stored for long periods at around 0°C, and may be greatly reduced by storage at 4°C under CA.

Fig 11.21.jpg

Figure 11.23. NMR images from the equatorial plane of an apple show watercore (waterlogging) as an intense white region. The first scan (left) was taken from a Fuji apple with severe watercore at the time of harvest. The second scan (right) was taken of the same fruit after cool storage for 15 weeks at 0°C, when symptoms had disappeared due to reabsorption of apoplastic water. Scale bar = 1 cm. (Original images courtesy C.A. Clark)

Watercore (Figure 11.23) is a condition where there are glassy, waterlogged sections of tissue towards the centre of the fruit, typically centred around the vascular bundles. Severe watercore leads to anaerobiosis, development of fermentation aromas, and core browning similar to core flush. Fuji is an especially susceptible cultivar. Watercore is more severe in sweet fully mature fruit (Figure 11.21) and involves a breakdown in transport of sorbitol across cell membranes. As outlined earlier (Section 11.3.2), sorbitol is the main soluble carbohydrate supply for early growth in apple fruit. Unlike other storage disorders, watercore becomes less severe or even disappears during storage (Figure 11.23) presumably because pericarp cells eventually take up intercellular water and sugar and allow airspaces to reform.