11.6.1 Temperature

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Fruit were originally held in cold caves at prevailing temperatures, but experience showed that a ‘best’ temperature can be sharply defined, and may differ between species or even cultivars (Wang 1991). At one extreme, freezing irreversibly damages a living product so that –0.5°C is generally the lowest temperature used. Apple cultivars are often best stored at or close to this minimum. In contrast, some fruits of tropical origin (avocado, banana) suffer chilling injury (Section 14.4) and store best between 7–10°C or 12–13°C respectively. Even persimmon, a temperate plant in many respects, has an optimum storage temperature of around 10°C, tolerating 0°C for only short periods.


Figure 11.18 Paradoxically, heat treatment extends subsequent storage life of persimmons at low temperature (6.5 weeks at 0°C). Untreated fruit (far right) suffer from cold injury and disintegrate following removal from cold store. Fruit pretreated at 50°C for more than 2 h suffer heat damage and the polyphenols become oxidised (top left). However, there is a window of treatment intensity that allows fruit storage at 0°C without damage for a full term. This window is shown as a band of light-coloured fruit rising from the bottom left to the right (0.5-1.5 h at 50°C, 1.5-6 h at 47°C, and 3-6 h at 44°C).

(Original data courtesy A.B. Woolf)

At 0°C, respiration is reduced to a level that is just enough to maintain cell function. Sugar is slowly consumed during this process so that fruit with a low sugar content at harvest are less durable. Commodities such as kiwifruit, which are picked with large supplies of carbohydrate in the form of starch, have an additional source of sugar to utilise, giving longer storage lives than those entirely reliant on soluble reserves, such as grapes.

Low-temperature storage has played an important part in the development of successful fruit export industries in Australasia, because of the great shipping distances between orchard and consumer. The success of kiwifruit as a new fruit has been largely due to its ability to be stored at 0°C for six months or more with no detriment to flavour or texture (see Beever and Hopkirk 1990).

Associated with low-temperature storage is a wide range of techniques to manage temperature changes en route to storage (Kader et al. 1985; McDonald 1990). Again there are strong species differences. Speed of cooling and methods employed for heat extraction are both important. There can be passive or forced air cooling, evaporative cooling of leafy vegetables, use of dry air or moisture-laden air and harsh or gentle techniques. For example, kiwifruit packed into trays and held on standard pallets can take a week to cool from ambient temperatures around 15–20°C to their storage temperature of –0.5–0°C. Forced air cooling can do this in only 8–10 h, yielding fruit that are much firmer, easier to handle and longer lasting.

Temperature change can be usefully imposed as a sequence of steps. In ‘ramping down’, temperature is reduced to an intermediate point, held there for a brief time, then again reduced to the final storage temperature. In ‘preconditioning’, fruit is held for a longer time at a pretreatment temperature, then taken rapidly to its final storage temperature. This is becoming an important commercial method for increasing tolerance to low temperatures. Preconditioning temperatures may be elevated, as in heat preconditioning, or close to the temperature which normally causes chilling damage. In each case, the pretreatment is intended to increase uniformity of fruit behaviour and allow fruit to adjust metabolically to reduced temperatures.

One further variant that sometimes gives better storage behaviour is to pretreat fruit to a temperature as high as 47°C (Figure 11.18), or else impose ‘intermittent warming’ where fruit is occasionally brought back to room temperature (Paull 1990). High-temperature effects were uncovered during a search for alternatives to pest fumigation and are now used quite empirically to good effect.