Delayed light emission as a means of quality evaluation of fruits and vegetables.

DLE is probably produced by all fruits, vegetables, and plant materials undergoing photosynthesis. However, the intensity and duration of the emitted light vary widely depending upon many factors. Because of the strong dependence of DLE on the chlorophyll content, variation in the DLE can be expected among different varieties of the same product. Therefore, quality evaluation based on DLE measurements require careful selection of measuring criteria such as duration and intensity of illumination, dark period, and temperature. It is necessary to obtain precise values of DLE for a particular product under a set condition; and the measurement conditions should be carefully validated to establish a standard measuring criteria. In general, the quality evaluation of fruits and vegetables is based on three major aspects: maturity and/or ripeness evaluation, composition analysis, and internal and external defect detection. DLE measurements so far have focused mainly on maturity evaluation because of the definite relationship between the emitted light and chlorophyll concentration, which changes with maturity. It should be noted that chlorophyll alone is not sufficient; it is the complex of in situ chlorophyll and related compounds in plant materials that jointly contribute to the DLE. Composition of fruits such as sugar content has been related to DLE. Recently, DLE measurements have been applied to detect certain physiological stress responses and defects such as chilling injuries. However, these areas of research have not yet been fully explored. Further investigation relating to compositional quality and internal and external defects of fruits and vegetables with DLE measurement may offer solutions to certain complex quality evaluation problems. The duration of DLE after excitation involves a matter of at least several seconds. From a mechanical standpoint, this extended period of light emission offers a convenient time in which to separate excitation and measurement. However, the exponential decay necessitates very precise control of time between illumination and measurement. The broad activation spectrum permits wide choice and simple design of excitation sources. However, size, shape, and varietal variations of plant materials make uniform DLE measurements difficult. It is suggested that DLE measuring instruments be designed to make several measurements over the entire surface of the specimen. An average of such measurements would reduce the effect of surface nature and shape of the product. Reporting the DLE intensity per unit surface area of the product will eliminate the dependence of area of excitation on DLE.(ABSTRACT TRUNCATED AT 400 WORDS)