Maintaining the quality of fresh produce refrigerated storage

7.10.1 Introduction

As discussed in section 7.8.2, the storage/shelf-life of fresh produce is considerably extended if respiration can be slowed down using refrigeration. Lists of recommended storage conditions for a wide range of fruits and vegetables are given in a number of publications (Kader, 1992; Snowdon and Ahmed, 1981; Thompson, 1996). Following precooling, it is important that the cold chain is maintained throughout the life of the product. This means that refrigeration should take place throughout transportation (Eksteen, 1998) and storage and preferably be maintained during retailing and in the home of the consumer. Typically, road and sea containers are refrigerated, as are the storage units at exporters, importers and retail distribution centres. Air freight is rarely cooled and relies on adequate pre-cooling, good pack insulation and the speed of transport to maintain adequate quality (Frith, 1991). The cool chain tends to be broken in the retail store where fruits and vegetables are rarely displayed in chilled cabinets.

7.10.2 Control of humidity

Most cool stores or refrigerated containers are refrigerated by a direct expansion system (Thompson, 1992). Fans are usually necessary to circulate the storage air over the evaporator coils and then through the produce in the cooling space. Heat is removed from the cooling space, when the refrigerant gas is allowed to expand in the evaporator coils. The temperature gradient between the coil and the produce is accompanied by a vapour pressure deficit, which increases water loss from the produce. To reduce water losses during longer term storage it is important to have as small a difference between coil temperature and produce storage temperature as possible. For produce particularly susceptible to water loss, for example leafy vegetables, an indirect cooling system may be used. Storage air is cooled to about 1-2°C and humidified to a RH of over 98% by passing it through a shower of cold water that has been cooled by mechanical refrigeration.

7.10.3 Control of ethylene

The presence of ethylene can stimulate senescence and give rise to a number of disorders as described in section 7.6.1. Good store management is needed to ensure that ripening fruit is not stored together with unripe fruit or other produce which is sensitive to ethylene (Dover, 1989). Exhaust gases from vehicles contain ethylene and must be kept well apart from produce stores. For fruits and vegetables which only produce low levels of ethylene, adequate ventilation from a clean air source is usually sufficient to keep ethylene at safe levels. Where ventilation is not sufficient to manage ethylene levels, ethylene can be destroyed by oxidation. Store air can be passed over the oxidising compound, potassium permanganate held on an inert substrate. Alternatively, ultraviolet (UV) light is in use commercially to destroy ethylene. The UV generates ozone production. It is believed that the ethylene is destroyed by active intermediates produced during the formation of the ozone (Reid, 1992). Ethylene can also be destroyed using catalytic converters by heating the air to over 200°C in the presence of a suitable catalyst such as platinum (Knee et al., 1985).

7.10.4 Control of chilling injury and low temperature sweetening

Chilling injury in tropical and sub-tropical crops may limit the use of refrigeration to temperatures well above freezing. Chilling injury is dependent not only on the temperature but the length of exposure at that temperature. The early stages of chilling injury are believed to be reversible and some produce can tolerate chilling temperatures for short periods of time without development of symptoms. A range of methods is available to limit chilling injury (Wang, 1991). These include stepwise reduction in storage temperature, or intermittent warming during storage (e.g. nectarines and peaches). Some fruits may become less susceptible to chilling when held under appropriate modified atmospheres, for example mango, avocado.

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