## 3 Results and Discussion
### Yoghurt Colour  
Changes in the colour attributes during storage are shown in Table 10. As seen, there were no significant differences (\(p>0.05\)) in lightness (L*), hue angle, and the whiteness index, whereas significant differences (\(p<0.01\)-\(p<0.05\)) were found in redness (a*), yellowness (b*), and chroma (colour saturation index) between the treatments. With regard to the redness, yellowness, and chroma values, the differences are mainly observed between the yoghurt on the Control treatment and yoghurt produced from milk from ewes receiving the unprocessed orange peels. Specifically, samples from the UOP treatment had lower redness (a*) and colour saturation index values compared to the Control samples, while their yellowness values were higher than those of the Control samples.  
The colour measurements were conducted on the surface of intact yoghurt samples. Since the yoghurt was produced from non-homogenised milk, regarding to the distribution of fat globules, a crust was formed. This crust had a non-even surface that could influence the measurements. The colour of the crust is usually more yellow than the colour of the yoghurt beneath it.  
Yoghurt colour from all groups was different than the values reported by [PERSON] et al. [86], who reported lower a* values but higher and positive b* values in sheep milk yoghurt stored for 14 days. Colour parameters are influenced by yoghurt composition. Protein content affect a* values, where a higher protein content results in a greener colour (negative a* values), as shown for yoghurt from the UOP group [46]. However, despite the higher fat content of the UOP group yoghurt, there were no differences in the L* values as expected, since increased fat content raises L* values, as fat contributes to the white appearance of yoghurt [46]. Citrus food residue remaining after juice extraction are a good source of carotenoids [10]. The significant differences observed in yellowness (b*) values might be related to a higher carotenoid content in the UOP feed. The white colour of yoghurt is a typical quality attribute, whereas the whiteness index correlates with the lightness values (L*) [46].  
Total colour difference (\(\Delta\)E\({}_{\text{Lab}}\)) between the Control and treated samples (UOP and POP) exceeded the value of 1 in all examined periods indicating that minor colour differences could be detected by the human eye (Figure 5). According to [PERSON] et al. [89] the perception of colour difference (\(\Delta\)E\({}_{\text{Lab}}\)) depends on both the colour observed and the sensitivity of the human eye. A \(\Delta\)E\({}_{\text{Lab}}\) value \(<\) 1 indicates that colour differences are undetectable to the human eye, \(\Delta\)E\({}_{\text{Lab}}\) values between 1 and 3 represent minor colour differences that can be detected, and \(\Delta\)E\({}_{\text{Lab}}\) values \(>\) 3 signify colour differences that are easily noticeable to the human eye. The total colour difference values of \(\Delta\)E\({}_{\text{Lab}}\) exceeded marginally the value of 1 between samples on the Control and POP treatments on storage days 7 and 14, whereas it was 2.22 on storage day 14. Regarding differences between samples on the Control and UOP treatments the smallest difference, also slightly higher than 1, was observed on day 21.