INFLUENCE OF STORE ENVIRONMENT ON CALCIUM REQUIREMENTS OF FRUIT

Ageing of apple fruits is characterized by progressive cellular disorganization including breakdown of membranes which leads, in particular, to increased leakage of ions from cortical cells.

In areas of calcium-deficient cells, progression from incipient lesions to typical visible symptoms of bitter pit and breakdown depends not only on the actual calcium concentration but also on cell metabolic rate. Since, in commercial practice, fruit senescence is retarded by storage under refrigeration and controlled atmosphere (CA), the calcium threshold concentration required for high resistance to these disorders must be defined in relation to the appropriate storage conditions.

Although the incidence of bitter pit in Cox apples may be markedly reduced by lowering the storage temperature from 3°C to 0°C, this lower temperature cannot be adopted commercially because of the sensitivity of the variety to injury when stored below 3°C.

Increasing the relative humidity of the store promoted senescent breakdown in Cox. A rise from 85 per cent to 95 per cent R.H. increased the incidence of breakdown from 1 per cent to 17 per cent. Breakdown induced in this way may be controlled by postharvest treatment with 2 per cent calcium chloride, suggesting that a higher calcium requirement exists for apples stored in high humidities.

CA storage has considerable commercial significance in preventing bitter pit development. Thus, when samples of apples from 54 commercial Cox orchards were stored in air (3°C), 28 developed more than 10 per cent bitter pit compared with only two samples in 2 per cent oxygen (<1 per cent carbon dioxide). CA has a curative effect on incipient pit lesions and, in practice, this allows for a calcium threshold of 4.5 mg/100 g to be adopted for Cox stored in CA compared with 5 mg/100 g which is necessary for air storage. This apparently small difference has considerable practical importance since it enables the UK fruit industry to allocate a higher proportion of the crop for long-term storage.