2. Produce quality factors
2.1. Respiration of fruit and vegetables
2.1.1 Fresh fruit and vegetables are alive. They respire (breathe) by taking up oxygen (O2) and giving off carbon dioxide (CO2) and heat. The carbon dioxide released during respiration may induce storage disorders in some produce if the concentration is allowed to build up in the surrounding air. The maximum concentration of CO2 for some fruits is given in Appendix 1, Table 1 under ventilation. Fresh fruit and vegetables also transpire i.e. they lose water. After harvest, losses caused by respiration and transpiration cannot be replenished. Thus fruit and vegetables are perishable and the higher the rate of respiration or transpiration the higher the rate of senescence.
The rate of deterioration depends on the environmental conditions following harvest. Respiration and transpiration rates fall as the temperature is lowered. For most produce, the respiration rate also falls if the O2 level in the surrounding air is lowered and the CO2 level is raised. Using changed atmosphere conditions is called Controlled Atmosphere (CA) or Modified Atmosphere (MA) storage. The respiration rate of fruit and vegetables can be reduced (to extend the shelf life) by altering the composition of the air in the surrounding atmosphere. This can be achieved by reducing the oxygen content, increasing the nitrogen content or a combination of both.
Modified atmosphere packaging (semi-permeable packaging), allows the fruit to breathe, but as the carbon dioxide level increases, respiration is slowed; the production of ethylene falls and the ripening process is slowed down. The difficulty with modifying or controlling the atmosphere is that prolonged exposure to high levels of carbon dioxide may affect the colour and/or flavour of the fruit. It is important that the set limits for oxygen and carbon dioxide are not exceeded. A very low level of oxygen or a high level of carbon dioxide would cause suffocation of the fruit (refer to Table 1&2 in
Appendix 1).
Alteration of temperature and atmosphere provide the main ways of reducing deterioration rates and prolonging storage life of produce. CA systems are designed to maintain an atmosphere different from normal, usually with low oxygen and increased carbon dioxide contents. This enhances the storage life of some produce, when used in conjunction with refrigeration. CAUTION: CA could be fatal to humans due to the low oxygen content. CA compartments must be appropriately marked and must be adequately ventilated prior to entry.
2.1.2. Fruit and vegetables vary in their rate of respiration and transpiration and in their response to low temperatures and changed gas compositions; this variation depends on a number of factors including growing conditions, variety and maturity. There is no single, ideal condition that is suitable for storage of all varieties of produce.
2.1.3. The heat released by respiring produce depends upon the type of produce, its maturity, and its temperature. Respiration heat varies over a large range and must be removed throughout storage and transport.
2.2. Ethylene
2.2.1. All fruits release small amounts of ethylene during growth and some release much larger amounts during ripening. Excess ethylene emissions can accelerate the ripening process of produce. CA storage reduces the rate of ethylene production and sensitivity to ethylene of some produce. Ripening can be induced in some fruit (eg. bananas) by introducing ethylene into the surrounding air. Ethylene can increase the rate of deterioration of some produce and may cause off-flavours. Thus produce that is sensitive to ethylene, (eg. bananas, avocados and kiwi fruit) must not be stored or transported with produce that release ethylene (refer to Section: 9 Stowage). Leafy vegetables are particularly sensitive to ethylene, with the leaves turning yellow in colour, russet spotting and abscission occurring with prolonged exposure. Cucumbers also turn yellow in colour and become soft. Ethylene in the atmosphere can also be removed by absorption with activated carbon, potassium permanganate, or catalytic oxidisation. Ethylene must be removed from storage and transport enclosures by venting or scrubbing. Exhaust gases from internal combustion engines may contain levels of ethylene that can cause damage to produce. Therefore the vents (air freshening devices) fitted to integral containers should be left closed during inland transport periods.
A matter of considerable importance, which affects the ripening process of fruit, is the time delay between harvesting and transfer to an environment where this process can be controlled. The atmospheres in which fruits are kept or carried have a significant effect on the ripening process. The atmosphere must be regulated to control this process, as per the client’s requirements allowing the fruit to arrive un-ripened at its destination. The presence of O2 allows the production of ethylene, which in turn activates the ripening process. Ventilation to remove the ethylene from the atmosphere or controlling the atmosphere by replacing O2 with CO2 is the methods used to control this process.
2.3. Temperature
The amount of deterioration of quality of produce is a function of both time and temperature. A small drop in temperature at high temperatures gives only a small reduction in the rate of deterioration, but a small drop in temperature (even a drop of 1°C) at low temperatures can give a large reduction in the rate of deterioration. For example, 5 days storage of cabbage at 5°C gives the same amount of deterioration due to respiration as 9 days storage at 0°C.
2.4. Chilling injury
Chilling Injury: A physiological disorder caused by low temperature stress, causing sensitive produce to discolour or reduce their physical appearance.
2.4.1. Produce exposed to temperatures below a critical level (dependent on the commodity, but above the freezing temperature) could suffer from ‘Chilling Injury’. The degree of damage is dependent on the sensitivity of the produce, the duration of exposure and temperature. Some produce suffers 'chilling injury’ if stored at temperatures below about 12°C. The tolerance of such sensitive produce to low temperatures varies widely between different species and varieties, eg. bananas will develop chilling injury below 12°C. This can limit the extent to which reduction in temperature can be used to extend storage life.
For some of this produce, the reduced deterioration as a result of low temperature storage must be balanced against the risk of damage due to chilling injury. For example, with oranges, an increased deterioration rate at higher temperatures may be due to fungal or bacterial decay as well as that caused by a higher respiration rate, and it may be preferable to risk chilling injury and transport at a lower temperature.
The transport of some chilling sensitive produce below their (minimum long term) chilling temperature is possible provided the combination of transport temperature and supply chain duration means that all the produce will be sold and consumed before the chilling injury is noticeable. This strategy involves some risks and requires a very good understanding of the produce being shipped, the duration of the supply chain and the rate of development of chilling injury symptoms that will impact on marketability of the product.
2.1. Respiration of fruit and vegetables
2.1.1 Fresh fruit and vegetables are alive. They respire (breathe) by taking up oxygen (O2) and giving off carbon dioxide (CO2) and heat. The carbon dioxide released during respiration may induce storage disorders in some produce if the concentration is allowed to build up in the surrounding air. The maximum concentration of CO2 for some fruits is given in Appendix 1, Table 1 under ventilation. Fresh fruit and vegetables also transpire i.e. they lose water. After harvest, losses caused by respiration and transpiration cannot be replenished. Thus fruit and vegetables are perishable and the higher the rate of respiration or transpiration the higher the rate of senescence.
The rate of deterioration depends on the environmental conditions following harvest. Respiration and transpiration rates fall as the temperature is lowered. For most produce, the respiration rate also falls if the O2 level in the surrounding air is lowered and the CO2 level is raised. Using changed atmosphere conditions is called Controlled Atmosphere (CA) or Modified Atmosphere (MA) storage. The respiration rate of fruit and vegetables can be reduced (to extend the shelf life) by altering the composition of the air in the surrounding atmosphere. This can be achieved by reducing the oxygen content, increasing the nitrogen content or a combination of both.
Modified atmosphere packaging (semi-permeable packaging), allows the fruit to breathe, but as the carbon dioxide level increases, respiration is slowed; the production of ethylene falls and the ripening process is slowed down. The difficulty with modifying or controlling the atmosphere is that prolonged exposure to high levels of carbon dioxide may affect the colour and/or flavour of the fruit. It is important that the set limits for oxygen and carbon dioxide are not exceeded. A very low level of oxygen or a high level of carbon dioxide would cause suffocation of the fruit (refer to Table 1&2 in
Appendix 1).
Alteration of temperature and atmosphere provide the main ways of reducing deterioration rates and prolonging storage life of produce. CA systems are designed to maintain an atmosphere different from normal, usually with low oxygen and increased carbon dioxide contents. This enhances the storage life of some produce, when used in conjunction with refrigeration. CAUTION: CA could be fatal to humans due to the low oxygen content. CA compartments must be appropriately marked and must be adequately ventilated prior to entry.
2.1.2. Fruit and vegetables vary in their rate of respiration and transpiration and in their response to low temperatures and changed gas compositions; this variation depends on a number of factors including growing conditions, variety and maturity. There is no single, ideal condition that is suitable for storage of all varieties of produce.
2.1.3. The heat released by respiring produce depends upon the type of produce, its maturity, and its temperature. Respiration heat varies over a large range and must be removed throughout storage and transport.
2.2. Ethylene
2.2.1. All fruits release small amounts of ethylene during growth and some release much larger amounts during ripening. Excess ethylene emissions can accelerate the ripening process of produce. CA storage reduces the rate of ethylene production and sensitivity to ethylene of some produce. Ripening can be induced in some fruit (eg. bananas) by introducing ethylene into the surrounding air. Ethylene can increase the rate of deterioration of some produce and may cause off-flavours. Thus produce that is sensitive to ethylene, (eg. bananas, avocados and kiwi fruit) must not be stored or transported with produce that release ethylene (refer to Section: 9 Stowage). Leafy vegetables are particularly sensitive to ethylene, with the leaves turning yellow in colour, russet spotting and abscission occurring with prolonged exposure. Cucumbers also turn yellow in colour and become soft. Ethylene in the atmosphere can also be removed by absorption with activated carbon, potassium permanganate, or catalytic oxidisation. Ethylene must be removed from storage and transport enclosures by venting or scrubbing. Exhaust gases from internal combustion engines may contain levels of ethylene that can cause damage to produce. Therefore the vents (air freshening devices) fitted to integral containers should be left closed during inland transport periods.
A matter of considerable importance, which affects the ripening process of fruit, is the time delay between harvesting and transfer to an environment where this process can be controlled. The atmospheres in which fruits are kept or carried have a significant effect on the ripening process. The atmosphere must be regulated to control this process, as per the client’s requirements allowing the fruit to arrive un-ripened at its destination. The presence of O2 allows the production of ethylene, which in turn activates the ripening process. Ventilation to remove the ethylene from the atmosphere or controlling the atmosphere by replacing O2 with CO2 is the methods used to control this process.
2.3. Temperature
The amount of deterioration of quality of produce is a function of both time and temperature. A small drop in temperature at high temperatures gives only a small reduction in the rate of deterioration, but a small drop in temperature (even a drop of 1°C) at low temperatures can give a large reduction in the rate of deterioration. For example, 5 days storage of cabbage at 5°C gives the same amount of deterioration due to respiration as 9 days storage at 0°C.
2.4. Chilling injury
Chilling Injury: A physiological disorder caused by low temperature stress, causing sensitive produce to discolour or reduce their physical appearance.
2.4.1. Produce exposed to temperatures below a critical level (dependent on the commodity, but above the freezing temperature) could suffer from ‘Chilling Injury’. The degree of damage is dependent on the sensitivity of the produce, the duration of exposure and temperature. Some produce suffers 'chilling injury’ if stored at temperatures below about 12°C. The tolerance of such sensitive produce to low temperatures varies widely between different species and varieties, eg. bananas will develop chilling injury below 12°C. This can limit the extent to which reduction in temperature can be used to extend storage life.
For some of this produce, the reduced deterioration as a result of low temperature storage must be balanced against the risk of damage due to chilling injury. For example, with oranges, an increased deterioration rate at higher temperatures may be due to fungal or bacterial decay as well as that caused by a higher respiration rate, and it may be preferable to risk chilling injury and transport at a lower temperature.
The transport of some chilling sensitive produce below their (minimum long term) chilling temperature is possible provided the combination of transport temperature and supply chain duration means that all the produce will be sold and consumed before the chilling injury is noticeable. This strategy involves some risks and requires a very good understanding of the produce being shipped, the duration of the supply chain and the rate of development of chilling injury symptoms that will impact on marketability of the product.