Post harvest Management of Horticultural Crops (2+1)

Chilling Injury

Metabolic disturbances occurring at sub-ambient temperature are generally divided into:

1. Chilling injury- cellular process expressed in short (fast)time frames
2. Low temperature associated disorder- cellular process expressed in long (slow) time frames


1. Chilling Injury

Chilling injury typically results from “exposure of susceptible produce, especially that of tropical or sub-tropical origin, to temperatures below 10-150C”.
However, the critical temperature at which chilling injury occurs varies among commodities. Chilling injury is completely different to freezing injury(which results when ice crystals form in plant tissues at temperatures below their freezing point). Both susceptibility and symptoms of chilling injury are product and even cultivar-specific. Moreover, the same commodity grown in different areas may behave differently in response to similar temperature conditions.


Symptoms of Chilling Injury

• Skin pitting - is a common chilling injury symptom that is due to collapse of cells beneath the surface. The pits are often discoloured. High rates of water loss from damaged areas may occur, which accentuates the extent of pitting.
• Browning or blackening of flesh tissues - is another common feature of chilling injury (e.g. avocado; Chilling-induced browning in fruit typically appears first around the vascular (transport) strands. Browning can result from the action of the polyphenoloxidase (PPO) enzyme on phenolic compounds released from the vacuole during chilling, but this mechanism has not been proven in all cases.
• Water-soaking - in leafy vegetables and some fruits (e.g. papaya)
• De-greening of citrus fruit is slowed by even mild chilling.
• Fruit that has been picked immature may fail to ripen or ripen unevenly or slowly after chilling (e.g. tomato).
• Development of off-flavour or odour (low O2 ¬levels)
• Rotting - chilling injury causes the release of metabolities (e.g. amino acids, sugars) and mineral salts from cells. Leakage of metabolites and ions, together with degradation of cell membranes, provides substrates for growth of pathogenic organisms, especially fungi. Such pathogens are often present as latent infections or may contaminate produce during harvesting and postharvest operation. Thus, rots is another common symptom of chilling injury, particularly upon removal from low-temperature storage.
Symptoms of chilling injury normally occur while the produce is at low temperature. However, they sometimes chilling injury appear when the produce is removed to a higher temperature and deterioration may then be quite rapid, often within a matter of hours.

Produce	Lowest safe storage temperature (0C)	Symptoms
Avocado	5-12	Pitting, browning of pulp and vascular strands
Banana	12	Brown streaking on skin
Cucumber	7	Dark-coloured, water-soaked areas
Eggplant	7	Surface scald
Lemon	10	Pitting of flavedo, membrane staining, red blotches
Lime	7	Pitting
Mango	12-13	Dull skin, brown areas
Melon	7-10	Pitting, surface rots
Papaya	7-15	Pitting, water-soaked areas
Pineapple	6-15	Brown or black flesh
Tomato	10-12	Pitting, Alternaria rots

Management of Chilling Injury

1. Maintaining critical temperature - The safest way to manage chilling injury is to determine the critical temperature for its development in a particular produce and then not expose the commodity to temperatures below that critical temperature (Eg. Safe storage temperature for apple is 0-20C and care should to taken to not to store apple below this critical temperature to avoid chilling injury ). However, it has been found that exposure for a short period to chilling temperatures with subsequent storage at higher temperatures may prevent the development of injury. This conditioning process has been effective in managing

• black heart in pineapple
• Woolliness in peach
• Flesh browning in plum.
2. MAS - Modified atmosphere storage may also reduce chilling injury in some commodities.
3. Maintaining high RH - both in storage at low temperature and after storage can minimize expression of chilling injury symptoms, particularly pitting (e.g. film-wrapped cucumbers).


Mechanism of chilling injury

The critical temperature, below which chilling injury occurs is an integrated genotypic but expressed in phenotypic characteristic of the particular organ. Highly chilling-sensitive fruits, such as banana and pineapple, have relatively high critical temperatures such as 120C or higher. It has even been suggested that the critical temperature may be greater than 200C for some pineapple cultivars. Chilling-insensitive fruits, such as apple and pear, have much lower critical temperatures, around 00C. Of course, low-temperature storage at/ below 10C is not possible for fresh fruit, vegetables or flowers because of freezing damage.
The cellular events of chilling injury can be separated into primary and secondary events. Primary events are transiently reversible, but become irreversible, particularly with the onset of cell death and tissue necrosis.

The main primary events in chilling injury are:

• low temperature-induce changes in the properties of cell membranes due to changes in the physical state of membrane lipids (membrane phase change)
• production of reactive oxygen species (eg. hydrozen peroxide) that oxidize leading to altered enzymatic activities and structural proteins (e.g. tubulin) are disrupted.
Secondary events are:
The physical changes in membrane lipids alter the properties of their parent membranes. Consequently,

• Ion and metabolites moves across affected membranes
• Activities of membrane-bound enzymes are disrupted
• The overall consequence of membrane disturbance is breakdown of sub-cellular compartmentation, which is readily measured as increased ion leakage from chill-injured tissues.
• Changes in the relative activities of enzymes lead to imbalanced metabolism and can ultimately result in cell death.
• Accumulation of toxic compounds (e.g. acetaldehyde)
• Structural proteins of the cell cytoskeleton (eg. tubulin) dissociate in chilling-sensitive tissues at low temperatures.




Fig. 2. Time Sequence of events leading to chilling injury