11.6.3 Modified and controlled atmospheres

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Figure 11.19 Storage life of kiwifruit can be greatly extended by controlled atmospheres. Under standard conditions (humidified air, 0°C) firmness declines exponentially over time, reaching limited acceptability by 8 weeks. Storage life ends once fruit firmness drops below about 0.9 kgf. Fruit are then soft enough to eat. Softening in cold store was slowed and storage life greatly extended by holding fruit in atmospheres containing either 5% CO2 + 2% O2 (top curve) or 8% CO2 + 16% O2 (middle curve).

(Based on McDonald and Harman 1982)

Rates of metabolism can be reduced and storage life further extended by modifying atmospheric composition during storage. Elevated CO2 and reduced O2, used either separately or together, can delay ripening and slow onset of senescence (Figure 11.19). These methods were originally developed on a commercial scale for apples, but have been progressively applied to many other fruits and vegetables. Container shipping helped their introduction because a sealed container makes it easier to maintain the required atmospheres along with reduced temperatures. Active control is achieved through sensors and scrubbers that help control flow and composition of recirculating atmospheres. Ethylene is also removed this way. Such storage using active control systems became known as controlled atmosphere or CA storage (Kader et al. 1985).

Another form of gaseous modification is achieved by passive methods. Fruit respiration is used to reduce the concentration of O2 and increase that of CO2 inside an enclosed space. The fruit is prevented from becoming anaerobic by making such enclosures out of plastic films that are partially permeable to O2 and CO2. In this way a balance is set up between removal of O2 by the fruit, and its replacement through inward diffusion from outside air. Such storage, known as modified atmosphere or MA storage, is very dependent on interaction between the rate of respiration (itself highly dependent on fruit temperature), film permeance, presence of any punctures in the film, and the surface to volume ratio of the container. Despite such problems, MA storage is potentially more flexible than CA storage, because fruit are in smaller independent packages that can usually be moved intact throughout handling and retailing.