Understanding the Fundamentals of Chilling Injury
Chilling injury is a form of cold damage that affects certain plants and produce when they are exposed to low, non-freezing temperatures. Unlike freezing injury, which is caused by the formation of ice crystals within plant tissues, chilling injury occurs when temperatures are typically between 32°F and 59°F (0°C to 15°C). The severity of the damage depends on several factors, including the specific temperature, the duration of exposure, and the individual plant's sensitivity. Symptoms may not appear immediately but often become visible after the product is moved to a warmer environment. This makes identifying the cause of the damage a challenging but important task for growers, distributors, and consumers.
The Cellular Mechanism Behind the Damage
At a cellular level, chilling injury is a complex physiological response to cold stress. The primary event is a phase change in the plant's cell membranes. These membranes, which are normally in a fluid (liquid-crystalline) state, become more rigid and less fluid (gel state) at low temperatures. This transition disrupts normal metabolic function and leads to a cascade of problems:
- Increased Membrane Permeability: The rigid membranes become leaky, causing an uncontrolled leakage of cellular contents, including water and solutes.
- Metabolic Dysfunction: Enzyme activity is disrupted, affecting key processes like photosynthesis and respiration. This can lead to a build-up of metabolic intermediates and a reduction in energy production (ATP).
- Oxidative Stress: The imbalance in metabolism leads to the overproduction of reactive oxygen species (ROS), which cause oxidative damage to cellular components like lipids and proteins.
- Hormonal Imbalance: Chilling can trigger increased production of ethylene, a plant hormone that accelerates ripening and senescence (aging), further degrading product quality.
Recognising the Symptoms
The visual signs of chilling injury are varied and depend heavily on the type of product affected. These symptoms significantly impact the quality and marketability of produce.
- External Symptoms:
- Surface Pitting: Small, sunken spots appearing on the peel or skin (common in cucumbers, bell peppers, and citrus).
- Discoloration: The development of brown, gray, or purplish patches on the surface (seen in bananas, eggplants, and basil).
- Water-Soaked Appearance: A translucent or waterlogged look to tissues, which can become soft and mushy.
- Internal Symptoms:
- Internal Browning: The flesh or seeds turn brown (can occur in tomatoes and melons).
- Failure to Ripen: Chilled fruits like tomatoes may fail to ripen properly or develop blotchy, uneven colour.
- Other Effects:
- Increased Decay: The damage makes the produce more susceptible to decay-causing pathogens, like fungi.
- Off-Flavours: The metabolic disruption can lead to the development of unpleasant tastes and aromas.
- Wilting: Leaves and shoots of plants can wilt and curl.
Susceptible Crops and the Risk Factors
Plants and produce of tropical and subtropical origins are generally most sensitive to chilling injury. Some key examples include:
- Fruits: Bananas, mangos, papayas, melons, avocados, and citrus fruits like lemons and grapefruit.
- Vegetables: Cucumbers, eggplant, tomatoes, peppers, okra, green beans, and sweet potatoes.
- Plants: Basil and certain ornamental plants.
Risk factors that influence the severity of the injury include:
- Duration of exposure: Longer exposure to cold increases damage.
- Temperature: Temperatures just above freezing are often the most damaging.
- Product Maturity: The maturity stage at harvest can affect sensitivity. For instance, green tomatoes are more sensitive than ripe ones.
Prevention and Mitigation Strategies
Preventing chilling injury is the most effective approach, and several postharvest and cultivation techniques can be employed:
- Temperature Management: Storing and transporting produce at or above its specific chilling threshold temperature is the most critical step. For example, basil should be stored above 54°F (12°C).
- Temperature Conditioning (Acclimation): Gradually lowering the temperature of produce after harvest can help it adapt to colder conditions before longer-term storage.
- Intermittent Warming: Periodically warming the produce during storage can reverse or slow down the development of chilling injury symptoms.
- Controlled Atmosphere Storage: Storing products in an atmosphere with modified oxygen and carbon dioxide levels can help reduce chilling sensitivity.
- Heat Treatments: A brief hot water dip or hot air treatment before cold storage can induce resistance to chilling injury by activating protective enzymes.
- Careful Handling: Minimizing physical damage during harvesting and transport can reduce susceptibility to injury and subsequent decay.
- For Garden Plants: For outdoor plants, wait until the danger of frost has passed before pruning and ensure adequate watering, as dry soil can exacerbate cold damage.
Chilling Injury vs. Freezing Injury: A Comparison
To better understand chilling injury, it's helpful to distinguish it from freezing injury.
| Feature | Chilling Injury | Freezing Injury |
|---|---|---|
| Temperature Range | Above freezing (approx. 32°F-59°F or 0°C-15°C) | Below freezing (32°F or 0°C and below) |
| Mechanism | Membrane phase transition and metabolic dysfunction | Ice crystal formation within plant tissue, mechanically rupturing cells |
| Appearance | Discoloration, pitting, internal browning, water-soaked tissue | Water-soaked, mushy texture; complete tissue collapse after thawing |
| Reversibility | Often cumulative and irreversible, though symptoms can be mitigated with specific treatments. | Irreversible tissue damage upon thawing, as cell membranes are destroyed. |
| Susceptibility | Mainly tropical and subtropical plants; some temperate crops | All plants, especially non-acclimated or younger tissue |
Conclusion
Chilling injury is a significant concern in the agricultural and horticultural industries, representing a non-freezing form of cold damage that can severely compromise the quality of sensitive plants and produce. From cellular membrane dysfunction to visible symptoms like discoloration and pitting, the effects are widespread and can lead to substantial financial loss. However, by understanding the underlying mechanisms and implementing smart prevention strategies, such as proper temperature management, acclimation, and postharvest treatments, it is possible to minimize chilling injury and maintain the quality and shelf-life of these sensitive commodities. Ongoing research continues to explore new methods, including genetic engineering, to enhance crops' tolerance to low temperatures. For further resources on postharvest technology, you can visit the UC Davis Postharvest website..
