## 2 Results and Discussion  
### Usability of the Sono-Heat-Exchanger (SHE) and Oil Yields  
Prototype development requires identification and control of risks that arise from a specific context of use and the characteristics of the user (for example level of education, and intellectual and technical skills). In order to demonstrate that these requirements are satisfied, it is necessary to validate usability. The IEC (International Electrotechnical Commission) 62,366 and the Human Factors Engineering Guidance Document of the FDA [25] provide guidance on how this validation must be carried out. It is required that the representative users (olive millers) are involved and the protocol is applied in its representative context of use (usage environment; the olive mill). Indeed, it is necessary to run through the core tasks within a participatory observation of genuine users in an actual or simulated user environment and let the security functions run. Only when users are able to achieve the targets, thus proving that user requirements are met, is the usability of product validated. To this aim, three different olive millers and mills were selected to test the performance of the sono-heat-exchanger (SHE). The oils were produced by processing olive fruits of cultivars collected from several geographical areas in the Apulia region, and with a different maturity index, according to data shown in Tables 1 and 2 and in Figure 1.  
The SHE is essentially a piping through which the olive paste passes. If the heat pump and the driver are turned off, it is a simple through pipe, while, if they are turned on, the olive paste is simultaneously subjected to the exchange of thermal and ultrasonic energy. Therefore, in order to assess the effect of the innovative SHE system on the quantity and quality of the extracted EVOO, batches of olives were selected considering cultivar, harvesting area, and ripeness index. Each batch was divided into identical portions and processed both with and without ultrasounds, at a temperature of 24 \({}^{\circ}\)C, according to a randomized scheme. The malaxation was not applied when the SHE turned on and this part of the plant was used only to transit the product into the decanter. The oil yields were calculated as the difference from the total oil content in the fruit and the total oil content in the olive pomace; it ranged from 12.1% to 24.1% depending on the cultivar and the maturity index of the olives, as shown in Figure 1. When applying ultrasound during milling, the oil extraction yields were always higher than those derived by the traditional process. Furthermore, combining the results shown in Figure 2 and the maturity index in Table 1, it is clear that the positive effect of the ultrasound treatment on oil yields was more evident when the olives were unripe than after fruit ripening. Indeed, it is more difficult to extract oil from green firm olives since the oil is contained in the cell vacuoles, which are not so easy to rupture. Ripening of fleshy olive fruit is one of the lasts developmental phases of fruit ontogeny, and it involves many changes including the accumulation of pigments and sugars and the process of flesh softening allowing an easier release of oil from very ripe fruits. Another interesting aspect is that the oils extracted from green fruits contain higher amounts of phenolic compounds, which increase the shelf life and the health quality of the EVOO, also increasing its commercial value. Having available a technology that allows the miller to anticipate the harvest without penalizing yields, reducing production times and obtaining an oil with a higher market value, contributes to increased profits and improved income. It has been previously demonstrated the use of the sono-heat-exchanger during milling allows energy savings [1; 9].