There is general agreement that apples destined for medium or long-term storage should be in an unripe condition at the point of harvest but close to the onset of ripening (sustained increase in ethylene production and in respiration rate).
Picking too early is to be avoided since fruits are likely to have a tough texture, high acidity, low sugar and poor flavour development. Early picked fruit may also have insufficient red coloration (important for dessert cultivars) and may have increased susceptibility to storage disorders such as superficial scald, bitter pit and core flush.
Late picked fruit are likely to be too soft for the market and to develop disorders such as senescent breakdown during storage or marketing. The general appearance of the fruit may be adversely affected and the background colour may be too yellow for market requirements. In the case of some cultivars such as Gala, skin can become greasy when picked over-mature.
During the period leading up to harvest, changes are taking place in many of the characteristics of the fruit. By picking fruit at frequent intervals and storing these under recommended conditions for varying periods relationships have been established between harvest parameters and ex-store quality.
Harvest maturity parameters commonly used include total soluble solids (usually measured with a refractometer), titratable acid, total soluble solids to acid ratio, starch content, background colour and firmness.
In some cases several parameters are combined to provide a single harvest maturity index. Perhaps the most publicised index is that produced by Streif (1996) which is calculated as follows:
Firmness (kg) measured with an 11 mm probe x 9.81
Soluble solids concentration (%) x starch score (1-10)
In the UK it is usual to measure starch by cutting fruit at the equator and applying a solution of iodine to the cut surface. The area of blue-black stain is assessed as a percentage of the surface area often with the aid of transparent gauges. These gauges should be available from advisers or marketing groups.
The information presented in Table 6 will enable growers to convert % starch to the 1-10 scale used in the calculation of the Streif index. In practice the percentage starch coverage can only be estimated with an accuracy of about plus or minus 2.5% even with the help of the transparent gauges referred to earlier. Therefore it is essential to include at least 10 apples in the sample for maturity testing.