# Zero Defect Manufacturing in the Food Industry: Virgin Olive Oil Production
## 2 Related Work
### State-of-the-Art Technology at the VOO Elaboration Process  
The VOO elaboration process can be regarded as a multistage system because it consists of multiple stages required to finish the final product. The VOO process chain consists of seven major processing stages, starting with the fruits reception in the oil mill yard and finishing with the virgin olive oil storage. Basically, after the reception, the olives are washed to remove the leaves, stalks, earth and impurities that arrive with them. Once the fruits are cleaned, they are stored in hoppers and fed into the mill, where they are crushed to form the olive paste. The paste is kneaded in a thermomixer (malaxer) where it is slowly stirred and heated in order to improve the conditions for the oil separation. This separation is carried out in a decanter centrifuge and yields olive oil and pomace as a by-product. The humidity and impurity content of this oil is still undesirably high, so further separation is performed in a vertical centrifuge. After this operation, the oil might be filtered or directly pumped to a final tank to be stored. In multistage systems, the quality of the final product is determined by complex interactions among the stages. That is, the quality characteristics at one stage are not only influenced by local variations at that stage, but also by variations propagated from upstream stages [22]. In the case of VOO production, the quality of the final product is deeply conditioned by the ripeness and sanitary state of the olive fruits. Process variables at the different production stages have only limited authority to influence the final features of the produced VOO, and when properly tuned, they can only preserve the potential quality offered by the fruit. As the incoming olives set an upper bound on the VOO quality, plenty of research has been devoted to the characterisation of the olives at the reception stage. In [23], the authors presented a computer vision system, composed of two devices working in the visible spectra and installed before and after the washing stage. The system automatically classified olivesthat were picked from the ground or from the tree. The analysis of multispectral images to discriminate olive fruits by their firmness was performed in [24] using a laboratory set-up. Infrared images were also analysed in [25; 26]. In the first reference, olives were classified according the presence of defects under controlled image acquisition conditions. The image acquisition for the second reference was performed under the regular operating conditions of an olive oil mill. Moreover, the computer vision set-up was installed after the washing stage to perform the detection of defects on individual fruits. Cameras working on the visible spectra have also been included at the kneading stage. In [27], the main indicators of the olive paste state (viscosity, granularity and presence of olive oil over the paste) were obtained from images acquired inside the thermomixer. Then, this information was used to suggest the values of the process variables according to the olive paste elaboration state. With regard to the product quality assessment, European legislation provides classes of olive oil based on its physico-chemical and organoleptic characteristics [28]. The sensory analysis of olive oils has to be carried out by both smell and taste assessments by means of a panel test, constituted by a group of trained and selected assessors (EEC 2568/91; EC 640/08). To date, there is no methodology able to suitably replace human sensory panels but some attempts have been made to automate this process. As recently reviewed in [29], electronic noses have been widely used for the classification and quality control of edible oils.