11.6.5 Storage disorders

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Figure 11.20 Postharvest incidence of the storage disorder watercore in Fuji apple is related to picking date (and thus fruit maturity). Watercore index represents the percentage fruit volume occupied by waterlogged tissue. Fuji is prone to this disorder, especially when fruit are picked mature. Early harvesting thus becomes an important control method.

(Original data courtesy F.R. Harker)

When fruit, vegetable or flower products are put into post-harvest storage, they are on a slow path to senescence and death, and a number of disorders can arise during that time. Several storage disorders have physiological origins, and are often highly specific to species, cultivar, season and even growing region. Fruit maturity at picking is one important factor (Figure 11.20) and four examples of postharvest physiological disorders in apple are listed below to illustrate our partial understanding of the problems that do occur, and to provide a glimpse of a large and complex area of postharvest physiology.

Bitter pit is a brown, bitter pitting of the skin in some cultivars, particularly Cox’s Orange Pippin. It is primarily a response to inadequate calcium content in the skin, and can be greatly reduced by spraying fruit on the tree with calcium-containing solutions during the later stages of fruit development.

Scald is a brown, sunburn-like discolouration of the skin surface, particularly in cultivars like Granny Smith. It appears to be connected with the nature of the cuticular wax and lipids and with the production of free radicals, and can be greatly reduced by application of a ‘free radical mop’ like diphenylamine.

Core flush is a browning of internal fleshy tissues sur-rounding the core of a fruit, and may have more than one cause. One factor seems to be the O2 supply to the core, since conditions potentially causing anaerobiosis (large size, a closed and airtight calyx and a low-O2 atmosphere) increase incidence.

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Figure 11.21 NMR images from the equatorial plane of an apple show watercore (waterlogging) as an intense white region. The first scan (left) was taken from a Fuji apple with severe watercore at the time of harvest. The second scan (right) was taken of the same fruit after cool storage for 15 weeks at 0°C, when symptoms had disappeared due to resorption of intercullar water. Scale bar = 1 cm.

(Original images courtesy C.A. Clark)

Watercore is a condition where there are glassy, water-logged sections of tissue towards the centre of the fruit, typically centred around the vascular bundles. Severe watercore leads to anaerobiosis, development of fermentation flavours, and core browning similar to core flush. Fuji is an especially susceptible cultivar. Watercore is more severe in sweet fully mature fruit (Figure 11.20) and involves a breakdown in transport of sorbitol across cell membranes. As outlined earlier (Section 11.3.3) sorbitol is the main soluble carbohydrate supply for early growth in apple fruits. Unlike other storage disorders watercore becomes less severe or even disappears during storage (Figure 11.21) presumably because pericarp cells eventually resorb intercellular water and sugar and allow airspaces to reform.

 

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