## 3 Results
### Analysis of Milk: Storage Method Comparison  
On the packaging of the studied milk, the words \"once opened, keep refrigerated and use within 3/4 days\" are reported. We studied how the electrical characteristics of the milk change after being opened, if stored in the refrigerator or if left outside at room temperature. For this purpose, four different samples from the same milk package were prepared. One was left at room temperature, the others were stored in the fridge for 4, 5 and 7 days, respectively. Figure 12a shows the Nyquist diagram of the freshly opened milk, after 3 days and after 4 days at room temperature. A change in the characteristics can already be observed after 3 days. We did not go beyond 4 days, because on the fifth day the alteration of the milk became visible to the eye, that is, lumps and curds had become evident. From Figure 12b-d, the effect of cooling on the electrical characteristics of the milk is evident. From Figure 12b, it can be seen how the characteristics of the milk, stored in the refrigerator for 4 days, once the sample has been brought back to room temperature, overlap with those of the freshly opened milk. This can be considered evidence of the fact that storage in the fridge keeps the quality of the milk unaltered. In Figure 12c, that is, after 5 days of storage in the fridge, the differences with respect to the freshly opened milk begin to be noticed, while after a week the differences are considerable (Figure 12d).  
From the graphs in Figure 12 it is evident that the most noticeable variations occur at high frequencies (the area of the graph furthest from the origin), and this seems to suggest that the properties of the milk change, while the properties of the interface between the  
Figure 12: (**a**) variation of the Nyquist diagram of a freshly opened milk sample, after three and four days of storage at room temperature; (**b**) variation of the Nyquist diagram of a freshly opened milk sample, after 4 days of storage in the fridge as soon as it comes out of the fridge and brought back to room temperature; (**c**) the same as (**b**) but after 5 days of storage; (**d**) the same as (**b,c**) but after a week of storage.  
electrode and the milk do not change. The variations in the milk's electrical characteristics can be explained by means of the breakdown of casein micelles, that release free ions (calcium, above all) that contribute to increase in the conductance of the samples [17]. In addition, the deformation of the fat globules and the acidification process of the milk contribute to the changes in the milk conductance and susceptance [17; 43].  
If, on the other hand, a milk sample is compared with a sample of the same type of milk but expired, probably due to the prolonged deterioration process compared to that observed in a few days, a change in the behavior of the electrode/milk interface is also noted. That is, as can be seen from Figure 13, there is a modification of the electrical characteristics even for frequencies lower than the relaxation frequency of the electrode polarization, and, as a consequence, also the \(n\) parameter, which is usually quite constant for the different milk samples from the same brand, varies appreciably.  
The fitting operation on the curves in Figure 13 showed that the capacitance of the milk increased by an amount of 40% after two months past the expiration date and by 100% after seven months past the expiration date. That is, the circuital interpretation of the data could also allow the expired milk to be distinguished.