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Digital determination of the ripeness of apples: Visual assessment of starch degradation replaced by image analysis

The visual assessment of the starch degradation index (SI) serves as a reliable method for determining the degree of ripeness of apples. This assessment is carried out in order to make an early prediction on the opening of the harvest window for...

However, this conventional and simple method has one major drawback: subjectivity. A digital method for determining starch degradation based on image analysis – independent of the individual person carrying out the assessment– would yield objective data and would hence represent a good alternative for use in actual practice.

Digital determination of the ripeness of apples: Visual assessment of starch degradation replaced by image analysis

Why use a starch measuring device?

The starch measuring device calculates the intensity and distribution of the starch contained in the apple on the basis of a digital image analysis. The starch is first dyed dark by immersing an equatorial slice, cut using a two-edged knife, into an iodine solution (Lugol‘s Solution).

In the course of many years of research, the „Storage and Postharvest Biology“ working group has visually analyzed samples of the most important apple varieties having a wide range of starch degradation; this was done using both the 1-5 scale developed at the Laimburg Research Centre and with the international 1-10 scale (Ctifl).

The value calculated by the starch measuring device was compared with the visual assessments of the same samples conducted by the experts; this was to calibrate the device as precisely as possible. The variability – i.e., the subjectivity of the visual assessment of an expert group – was evaluated over the course of several years of development and compared with the values yielded by the device.

Measurement not only of the starch …

Besides having the advantage of precision and reproducibility of the analyses, the starch measuring device also allows the data to be stored in the form of digital image files and in the form of numerical values.

For a holistic interpretation of the ripeness parameters, however, one needs to have a great deal of experience; only then can the proper harvest time be determined. In doing so, one must note that the speed of starch degradation depends not only on ripeness, but also upon many other factors – specifically, it depends also on the amount of fruit hanging on the branch, seasonal climatic influences, or the temperature prevailing during the ripening process. For this reason, it is important that not only the starch value be correctly determined; rather, such other parameters of ripeness and quality as firmness of the fruit, sugar concentration, and acidity as well as base and top color must be taken into account and properly interpreted in combination with the starch value.

Assessment of experts still indispensable

The „Amilon“ (Isolcell) starch measuring device is already available on the market and represents an important advancement in the objective determination of the starch value. However, in order to obtain a correct interpretation of the measurement values, the expertise and professional knowledge of experts is still needed: The measurement values must be interpreted in the context of other parameters such as the physiological ripeness or the commercial quality (color, size, etc.), in order to predict the ideal harvest time and guarantee a high quality and long-term storability of the fruit.

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