Decay Detection in Citrus Fruits Using Hyperspectral Computer Vision

Introduction

The citrus industry, with an annual production over 89 million tons, is the most important of the World regarding fruits and vegetables production, giving an idea of its relevance on the World economy. It is relevant too on the European Union, where its production arises sums around 10 million tons, of which the 35% is produced on the Valencian Region.

In fact, Spain is the first World fresh citrus exporter, with the 34% of the global market, and the fourth in production level. According to sources of the Generalitat Valenciana, during the 2006-2007 season, the valencian citrus production increased to 4.2 tons (Figure 1), which represents the 80% approximately of the total citrus production in Spain. If only the data of the most important citrus is observed, the Comunitat Valenciana exported the 89% of the oranges and the 97% of the mandarins.

Figure 1.

Estimation and balance of the production of citrus

Of these exportations, the 82% approximately is addressed to European Union countries, while the rest is exported to so different countries as Japan or Canada, where the valencian citrus have strongly penetrated and have a market more and more important. As an example, the exportations to a so demanding market as the United States of America have risen from 4,200 to 80,000 tons in the last 10 years. This fact is due to the effort done by the producers in order to offer products of greater quality and by the researches, both from the point of view of the fruit, and from the technology for its cultivation.

The most of this effort has been focused on getting varieties resistant to plagues or illnesses or varieties marketable out of its usual period, on the biological fight against plagues and on new technologies on post-cultivation treatments, especially in automatic quality estimation and detection of damaged and ill fruits systems.

One of the fundamental requirements for a fruit to be exportable is that there is no appreciable damage on it. It must have the suitable size and colour and, furthermore, have a completely damage free skin. These machines, which technology for the candidate system has been in research for more than 15 years, allow to estimate in an individual way the quality of each fruit, classifying and separating it depending on, among other factors, the presence, quantity and severity of its damages. These machines ensure that the fruits in the market are in the best conditions and comply all the standards. The distinction among different kinds of damages on the fruit skin is of great importance, because all the markets do not have the same requirements or standards.

Figure 2 shows examples of some of the most common damages that appear in citrus. Attending to the economic losses that they could generate, the damages on the citrus skin can be classified into two categories: defects that continue evolving once the fruit has been recollected and defects that do not evolve. The defects that do not evolve are those that cause an aesthetic damage and thus, they reduce the value of the product but do not hamper its commercialization in second markets. Among these there are: damages produced by meteorology as hail, wind or persistent raining, damages derived from biochemical alterations due to the incorrect fertilization, watering and phytosanitary treatments, damages caused by plagues that colonize the fruit surface but do not affect the intern qualities, etc.

Figure 2.

Common defects produced on citrus by: branching (a), peduncle loss (b), peduncle puncture (c), phytotoxicity (d), trips (e), Oystershell Scale (f), sooty mould (g), cold (h), oleocelosis (i), Penicillium digitatum(j), antracnosis (k), and fly bite (l).