chilling injury of horticultural crops

 Introduction

Chilling injury is primarily a disorder of crops of tropical and subtropical origin, although certain physiological disorders will appear in temperate crops only when they are stored at low temperatures. Chilling injury is not the same as freezing injury, which is a result of damage from ice crystals formed in tissues stored below their freezing point. The minimum safe temperature for chilling sensitive commodities will be well above their freezing point. The critical temperature for chilling injury varies with the commodity, but it generally occurs when produce is stored at temperatures below 10°–13°C. Therefore, crops which are susceptible to chilling injury often have a short storage life as low temperatures cannot be used to slow deterioration and pathogen growth. Chilling injury may occur in the field, in transit or distribution, in retail or home refrigerators. The effects of short periods of chilling may be cumulative in some commodities.

The primary cause of chilling injury is thought to be damage to plant cell membranes. The membrane damage sets off a cascade of secondary reactions, which may include ethylene production, increased respiration, reduced photosynthesis, interference with energy production, accumulation of toxic compounds such as ethanol and acetaldehyde and altered cellular structure. As plant structures differ in both susceptibility to damage and ability to repair these membranes, symptoms vary greatly between commodities. Chilling injury is a time by temperature problem. If the produce is stored below the critical temperature for short periods, the plant can repair the damage. If exposure is prolonged, irreversible damage occurs and visible symptoms often result. Injury occurs sooner and is more severe, the lower the temperature is below the threshold temperature. Detection and diagnosis of chilling injury is often difficult, as products often look sound when removed from the chilling temperature, but symptoms may occur when the produce is placed at higher temperatures. Symptoms which appear at higher temperatures may appear almost immediately, or may take several days to develop. Symptoms also may not be visible externally.

Climatic conditions during the growing season affect the sensitivity of crops to chilling injury. Chilling injury may increase in colder growing seasons and crops grown in Ontario may be more susceptible than the same cultivars grown in warmer climates.

 Figure 1. Pitting, Shrivelling And Yellowing Of Cucumber Held At 0°C For 4 Days.

Potential Symptoms of Chilling Injury

• surface lesions
  • pitting, sunken areas and discolouration
  • occurs most frequently in products with a firm, thick peel such as citrus or cucumbers
    (Figure 1)
• water-soaking of tissues
  • occurs most frequently in fruit and vegetables with thin or soft peels such as peppers, asparagus and grapes (Figure 2)
• water loss /dessication /shrivelling (Figure 3)
• internal discolouration (Figures 4 and 5)
• tissue breakdown
• failure of fruit to ripen, or uneven or slow ripening (Figure 6)
• accelerated senescence/ethylene production
• shortened storage or shelf life
• compositional changes e.g., flavour and texture
• loss of growth or sprouting capability
• increased decay due to leakage of plant metabolites, which encourage growth of micro-organisms, especially fungi (Figure 7) 

Figure 2. Water-Soaking Of Asparagus Tips Due To Storage At 0°C. Shrivelling And Secondary Rot Is Also Occurring.

Figure 3. Shrivelling And Pitting Of Pepper With Chilling Injury. Note Lower Susceptibility Of The Red Variety.

Figure 4. Low Temperature Induced Internal Browning Of "Newton" Apple Stored At 0°C.

Figure 5. Flesh Reddening And "Woolly" Texture Of Nectarines Stored At 2°C For 3 Weeks.

Figure 6. Abnormal/Uneven Ripening Of Green Tomatoes Stored At 3°C And Subsequently Ripened At Room Temperature.

Figure 7. Breakdown And Subsequent Decay Of Cantaloupes Stored At 0°C For 4 Days.

Minimizing Chilling Injury

Although chilling injury is most easily prevented by storing susceptible crops above their critical temperatures, this is not always possible when only one storage facility is available for several crops. In this situation, there are methods to reduce the severity of chilling injury; however, not all of these methods will be appropriate for all crops.

• Minimize the length of time the crop is exposed to the chilling temperature: If exposure is minimal, the damage can be reversed and no visual symptoms will occur.
• Preconditioning: Stepwise cooling of the commodity can allow the fruit to adapt to the cooler temperatures and minimize chilling injury development.
• Intermittent Warming: Warming the commodity to room temperature at intervals during storage before permanent injury has occurred will allow the product to recover and prevent chilling injury symptoms.
  • This treatment may, however, cause undesirable softening and increase decay and may cause condensation to form on the product.
• Cultivar selection: Certain cultivars are more resistant to chilling.
• Pre-harvest Nutrition: Proper pre-harvest nutrition can minimize chilling susceptibility. Calcium treatment may stabilize cellular membranes and reduce chilling injury in certain commodities.
• Maturity/Ripeness selection: Generally riper fruit is less susceptible to chilling injury. Ripe tomatoes, bananas and avocados tolerate lower temperatures than unripe fruit. Peaches and nectarines which are ripened for 1–2 days after harvest prior to storage are less susceptible to low temperatures.
• Specialty Storages: High humidity can minimize dessication due to chilling injury. Controlled or modified atmospheres (generally O₂ <5%, CO₂ >2%) can slow plant metabolism and slow chilling injury development in certain crops (e.g., peaches, nectarines, okra, avocado). Controlled atmospheres can also allow longer storage of chilling sensitive crops when stored above their critical temperature.
  • Controlled atmospheres may in some cases further stress crops and increase chilling injury susceptibility (e.g., some apple cultivars, cucumbers, tomatoes, asparagus and citrus).
• Other Treatments: Other methods which are still in experimental stages include treatment with hormones or other chemicals to stabilize plant membranes and induction of chilling resistance by exposure to other stresses such as high temperature or low oxygen concentration. 

Table 1. Examples Of Fruits, Vegetables And Floriculture Products Susceptible To Chilling Injury.¹

Commodity	Recommended Minimum Storage Temperature (°C)2	Potential Chilling Injury Symptoms3
Anthurium	> 13	darkening and water-soaked appearance
Apple	0–7	core or flesh browning, fermented flavour, spongy texture, susceptibility and symptoms vary with cultivar
Asparagus	2–4	occurs primarily at the tips - darkened and water-soaked followed by bacterial soft rot
Avocado	7–13	darkening of vascular tissues, discolouration of flesh and skin, off-flavours and odours, abnormal ripening
Banana	> 13	green fruit: brown under peel discolouration. Ripe fruit: brown to black peel discolouration, off-flavours, abnormal ripening
Basil	7–10	wilting, water-soaked appearance, darkening
Bean (snap)	7–10	russeting, pitting
Cantaloupe	2–5	pitting, surface decay
Cucumber	7–10	pitting of surface, lenticel area affected first, followed by Fusarium and other rots
Egg-plant	7–13	scald-like browning, pitting, flesh browning, decay and loosening of capstems, Alternaria rot
Grapefruit	10–15	brown pitting of rind, watery breakdown of internal and external tissues, fermented odour
Honeydew Melon	7–13	water-soaking of the rind, softening, greying or browning, surface becomes soft and sticky resulting in increased decay
Lemon	10–14	as for grapefruit, plus red blotch
Lime	9–12	as for grapefruit
Mango	> 13	greyish skin discolouration, pitting, uneven ripening, poor flavour, increased susceptibility to Alternaria rot
Okra	7–10	pitting
Orange	2–5	as for grapefruit
Orchid, cattleya	7–10	discolouration of column first, then sepals and petals
Papaya	7–13	pitting, olive or brown discolouration, abnormal ripening
Peach/Nectarine	-0.5–1	(critical temperature 2–8) – internal breakdown, mealiness, abnormal ripening, flesh browning or reddening
Pepper	7–13	water-soaked appearance, sheet pittting, darkening, predisposition to Alternaria and Botrytis
Pineapple	7–13	flesh watery, followed by browning or blackening
Poinsettia	> 13	leaf drop, wilting
Potato	3–10	mahogany browning, sweetening
Pumpkins/winter squash	10–15	rot, primarily Alternaria
Sweet potato	> 13	flesh discolouration, internal breakdown, increased decay, off- flavours, hard core when cooked
Tomato – ripe – green	7–13 > 13	rubbery texture, watery flesh, irregular ripening, seed browning
Watermelon	10–15	pitting, loss of flavour, fading of red colour
Zucchini/summer squash	5–10	surface pitting, rapid decay

Sources:

1. Produce Handler’s Guide, Agriculture and Agri-food Canada; Postharvest Technology of Horticultural Crops, University of California; The Commercial Storage of Fruits, Vegetables and Nursery Stocks, U.S.D.A.
   
2. Optimum temperature varies with cultivar, storage duration, maturity and pre-storage conditions.
   
3. Symptoms usually appear upon return to warm temperatures and will vary with cultivar, storage duration, maturity and pre-storage conditions.
4. http://www.omafra.gov.on.ca/english/crops/facts/98-021.htm