The life and quality of fruit depends on the conditions under which it is held after harvest. Every person in the marketing chain must be aware of these conditions and ensure the best possible post-harvest handling and storage. As a result, high quality produce will be delivered to the consumer. Ideal post-harvest handling and storage conditions vary according to fruit kind and varieties. Temperature and humidity levels and approximate storage life for the main fruit kinds and varieties are given in the table.

Storage temperature

The rate at which any chemical reaction goes depends on the temperature. The respiration reaction in fruit is no exception. For every 10°C rise in temperature the rate roughly doubles, at least in the range from 10° to 30°C. Below 10°C the effect of temperature is even more pronounced. A low respiration rate means slower metabolism and therefore a longer life. To obtain the maximum potential life it is important that fruit be cooled down as soon as possible after harvest. Delay in cooling allows the ripening process to continue, and for some short-life fruit, for example, apricots, the fruit may quickly become too mature. Also, the fruit might use a fair proportion of its available life before being cooled, and thus only a limited extension of life is possible. There are three temperature limits to the life of fruit. They are high temperatures, freezing, and chilling injury. Within the limits of freezing or chilling injury, the longest life is obtained by using the lowest storage temperature.

High-temperature effects
At temperatures over about 30°C the respiration process changes. Enzymes that promote the complex chemical reactions in the living fruit become inactive. At about 35°C abnormal ripening occurs. Tomatoes and bananas, for example, turn very soft and have a watery appearance, known commercially as "boiling". Red pigments in tomatoes and stone fruit turn yellow, and in most fruit both flavour and aroma are not normal.

Freezing
The lower temperature limit for all fresh produce is its freezing point. Water freezes at 0°C; as most fruits are about 80-90% water, it might be expected that they would freeze at a similar temperature. However, the sugars and other soluble solids dissolved in the cellular liquid lower its freezing point. Freezing points for fruit vary from about -2.5°C for figs, pomegranates and mature grapes to around -0.8°C for stone fruit, citrus and berry fruit. Once the freezing point is reached, water from the cells is frozen in the spaces between the cells, and the cells themselves are damaged. When thawed out the cells are unable to resume their normal functions.

Chilling injury
In some fruit a peculiar change takes place at low temperatures but above freezing point. The cells lose their normal activity below a critical temperature and visual symptoms slowly appear, usually as brown pitting or staining of the skin, or browning and softening of the flesh. Susceptible fruit held below the critical temperature will show more severe symptoms the lower the temperature and the longer the exposure to that temperature. Some fruit, though, may show chilling injury more rapidly if stored just below the critical temperature.
Fruit affected by chilling may fail to ripen normally. This is common with bananas and pineapples. Often the injury may not be seen in store, but develops within a day or two at warm temperatures. Common examples are breakdown of Jonathan apples and superficial scald of green apples. The susceptible fruit includes all the tropical fruits and citrus, as well as tomatoes, stone fruit and some apple varieties.

The critical temperatures vary from 5° or 6°C up to about 12°C. Recommended storage temperatures for fruit in commercial storage are based on knowledge of each variety and the effects of maturity and growing district, all of which can alter the critical temperature.

Storage atmosphere

During respiration oxygen is used and carbon dioxide is given off by the living fruit. The rate of a chemical reaction depends on the concentrations of both the materials used and those produced. If we want to slow down the respiration rate, we can reduce the amount of oxygen available to the fruit, or we can allow the carbon dioxide to accumulate. Normal air contains about 21% oxygen and 0.03% carbon dioxide. Holding fruit under refrigeration at low oxygen and/or high carbon dioxide is the basis of controlled atmosphere (CA) storage.

CA storage involves both temperature and atmosphere control. It reduces the rate of the living process to as slow a rate as possible, but still keeps the fruit alive. It is not capable of improving the fruit, but can only preserve the quality obtained during the growing period and fixed at the time of harvest.

CA storage uses a refrigerated chamber which has been made gastight. The fruit in the chamber uses up some of the oxygen; a CA generator is used to hasten the oxygen reduction.

The minimum concentration of oxygen for most fruit is around 1.5%, although various fruit may require high levels or tolerate brief exposure to lower levels. Generally, below about 1.2% oxygen anaerobic ("without air") respiration takes place, resulting in the production of alcohols, lactic acid or acetaldehyde. These give the fruit objectionable flavour and aroma, and the fruit has a very much shortened life.

Low-oxygen atmospheres are used in commercial CA storage of apples and pears. About a 30-50% increase in their storage life can be obtained, and both apples and pears can outturn in good eating condition several months later than they would otherwise have been available. Carbon dioxide build-up is also useful. For most apples and pears about 2-5% is the effective limit above which carbon dioxide poisoning can occur. This shows as flesh browning, tissue collapse and a much shortened storage life. Berry fruit and cherries can tolerate carbon dioxide concentrations of up to around 20%, and a useful extension of their life is obtained.

Detailed information on particular fruits or varieties should be obtained before attempting commercial CA storage.

Storage humidity

It has been said that "fruit is mainly water in a fancy package". This is generally true because most fruits contain more than 80% water. To attract the buyer's attention fruit must look fresh and not be shrivelled. The consumer wants fruit that has its characteristic crispness or juiciness. The loss of quite small amounts of water will quickly affect these qualities.

Water is lost from fruit at a rate that depends on the variety, size and maturity of the fruit, its packaging, and the temperature, humidity and velocity of the air around the fruit. Small fruit, with a large surface-to-volume ratio, can lose moisture more rapidly than large fruit. Grapes, cherries, small apricots and berries are all prone to rapid shrivelling under adverse conditions. The free water in the fruit is in equilibrium with the water vapour in the air spaces within the fruit. The humidity of internal air in the fruit is usually about 95-98% relative humidity (RH). This air and the air surrounding the fruit tend to come to equilibrium, but the skin of the fruit forms a barrier to the movement of water vapour.

Most fruits have a natural waxy cuticle or skin coating which restricts the movement of water vapour from within the fruit to the outside air. This coating may thicken at maturity, as can be seen in apples. The effect is noticeable in the reduced likelihood of mature apples shrivelling as compared to immature apples. Golden Delicious apples have a much thinner cuticle than Granny Smith, and so will shrivel more easily. If the natural wax is removed by detergent washing, moisture loss will be increased.

Movement of water vapour from within the fruit (about 95% RH) to the outside air occurs when the outside air is relatively dry. The air can hold a greater amount of water vapour if its temperature is increased. The tendency, then, is for water vapour to move from the fruit more quickly when the air temperature rises. As the temperature of the important layer of air around the fruit is close to the fruit temperature, it is essential to get the fruit temperature down as soon as possible after harvest.

Prompt cooling is probably more important than the conditions under which the cooling is done. However, the higher the humidity of the air surrounding the fruit, the lower will be the movement of water vapour from the fruit. The relative humidity of the air, whether in precooling or during storage, should be as high as possible to reduce moisture loss and consequent shrivelling. Commercial forced-air cooling systems using very high humidity air are now available, as well as the hydro-coolers (refrigerated water) used for precooling operations.

Physical damage to fruit will increase moisture loss. This can be through loss of the cuticle by abrasion, or by cuts which allow the water vapour to pass directly out of the fruit. Damage to the cells, whether by cutting or crushing, allows them to quickly dry out. The velocity of the air across the fruit surface is important only when the air is at a lower relative humidity. Once constant storage conditions have been reached, air at equilibrium humidity with the fruit will not remove any moisture from the fruit. Packaging treatments assist in reducing moisture loss. They prevent high-humidity air being swept away from around the fruit while in the coolstore and during marketing. They can, however, have an undesirable effect if they slow down the cooling rate by preventing the cooling air reaching the fruit.