Low temperature effects
The temperature at which products freeze is a function of the concentration of dissolved solutes—such as sugar—within the cells. Pure water freezes at 0°C. A product such as lettuce, which is mostly water, will freeze at approximately –0.2°C. In contrast, the high sugar content (up to 10% at harvest) of products such as carrots and sweetpotato, means they may not freeze until temperature falls to –1.8°C.
When products freeze, water forms into ice crystals within and between cells, resulting in dehydration of the cells themselves. Expanding ice crystals can also pierce the cell walls. When the product thaws the cells collapse, resulting in the water-soaked appearance and loss of structural integrity typical of freezing injury. Freezing can effectively ‘kill’ the product, which is unable to resume its normal metabolic activity. Damage is most likely when the vegetable freezes slowly, resulting in the formation of larger ice crystals. Food processing facilities producing frozen vegetables rely on rapid blast freezers to limit the size of ice crystals and maintain the integrity of the frozen product.
A few vegetables, such as cabbage, have some tolerance to freezing and can recover if defrosted slowly.
Chilling injury is most commonly a problem for fruit and root vegetables that originate in tropical or warm temperate regions. It occurs when these products are held above their freezing point but below a temperature at which physiological damage occurs. The reasons why chilling injury occurs are still not well understood, however, it appears to relate to malfunction of cellular membranes and disruption of normal processes within the cells.
Injury is not simply an issue of temperature, but also of exposure time. Most chilling sensitive products can withstand limited exposure to temperatures below their normal temperature threshold, but as storage time increases damage becomes more likely.
Symptoms of chilling injury may not be obvious during storage, but develop after products are returned to warmer temperatures. Common symptoms include the development of sunken, pitted areas, water-soaked lesions, internal browning, and accelerated development of surface rots and decay. Fruiting vegetables can fail to ripen normally, as well as develop off flavours and odours. Weight loss is often rapid, with products deteriorating quickly once on retail display.
Preventing or reducing chilling injury
Vegetables vary in their susceptibility to chilling injury according to variety, growing conditions and maturity. Chilling sensitivity can be unpredictable; an environment or treatment that reduces chilling injury in one product may increase it in another. There are a number of ways susceptibility to chilling injury can be reduced:
- Growing a healthy plant. For example, greenhouse cucumbers grown under optimal temperature and humidity were less susceptible to chilling injury than those grown with fewer environmental controls.
- Harvesting when mature. Mature, fully red capsicums and chillies are less sensitive to chilling temperatures than green fruit.
- Plastic packaging. Increasing the RH around the product appears to have some effect. If the packaging also modifies the atmosphere—by increasing CO2 and reducing O2—then this can further reduce chilling sensitivity.
- Heat treatments such as hot water dips or showers. Short exposures to high temperatures can induce heat shock proteins in the product. The effect is somewhat similar to a vaccination, providing protection against later exposure to cold temperatures. For example, a postharvest treatment of 30 seconds under a shower of 55°C water delayed the onset of chilling injury in green capsicums stored at 2°C by 4 – 6 weeks.
- Low temperature conditioning. Reducing the temperature gradually after harvest can effectively acclimatise the vegetable to low temperatures. For example, one day at 18°C or two days at 10°C both delayed the appearance of chilling injury in zucchini stored at 5°C.
- Intermittent warming. Raising the temperature briefly appears to allow plant tissues to recover from cold stress, metabolising damaging compounds produced at chilling temperatures and replenishing their reserves.
- Exposure to ultraviolet (UV) light. Like heat treatments, irradiation with UV light can also produce defence compounds in vegetables. It may also reduce symptoms by killing microbes on the vegetable skin.
- Plant growth regulators or antioxidants. Products that may reduce chilling injury include abscisic acid, polyamines, methyl jasmonate, gibberellins and salicylic acid.
For chilling sensitive products, selecting the best storage temperature is a balance between the development of low temperature damage and the rots, yellowing and water loss that can occur at warmer temperatures.
For some vegetables, storage life is longer at temperatures that can cause chilling damage than at ‘safe’ temperatures, where rots, yellowing and water loss become major problems. Capsicums are an example of this, with storage life maximised at close to 0–2°C in spite of their chilling sensitivity.