## 3 Results and Discussion
### Yoghurt Active (pH) and Titractable Acidity  
The acidity of the yoghurt, both the active acidity (pH) and the tittatable acidity (% of lactic acid), is shown in Table 9. Titractable acidity was measured on storage day 0, shortly after the yoghurt was removed from the incubation chamber, whereas the changes in active acidity (pH) were monitored during the storage period at weekly intervals, commencing on storage day 7. As shown, there were no differences (\(p>0.05\)) in both the tittatable and the active acidity (pH) between the different groups and within the same storage period. The tittatable acidity of yoghurt from all treatment groups exceeded the minimum threshold of 0.6% lactic acid recommended by the Codex Alimentarius for commercial products [85]. It is important to note that tittatable acidity was measured only on day 0 to assess the potential of growth of pathogens, such as _Salmonella_ spp. and _Listeria monocytogenes_[57].  
As expected, the pH values were reduced during the storage period. The pH values were within the range of 3.69-4.64 reported for commercially available sheep milk yoghurt [69]. [PERSON] et al. [25] found lower levels of lactic acid content and higher pH values in laboratory produced samples. Parameters such as lipolysis can hydrolyse milk fat into free fatty acids which can lower the pH of the yoghurt. The rate and the extent of lipolysis are also related to the endogenous antioxidants present in the milk [86]. The  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Storage Day/Variable**} & \multicolumn{4}{c}{**Treatment**} \\ \cline{2-6}  & **Control** & **UOP** & **POP** & **SEM** & **Significance** \\  & **(n = 4)** & **(n = 4)** & **(n = 4)** & & \\ \hline \hline \multicolumn{6}{c}{_Active acidity (pH)_} \\ \hline
7 & 4.06 b  & 4.08 & 4.08 & 0.034 & NS \\
14 & 3.92 a  & 3.96 & 3.99 & 0.026 & NS \\
21 & 3.88 a  & 3.90 & 3.97 & 0.027 & NS \\ \hline \hline \multicolumn{6}{c}{_Tittatable acidity (g lactic acid/100 g yoghurt)_} \\ \hline
0 & 1.38 & 1.38 & 1.32 & 0.044 & NS \\ \hline \hline \end{tabular}  
* UOP = unprocessed orange peel; POP = processed orange peel; NS = non-significant; superscripts a, b within the same column difter at \(p<0.05\)  
\end{table}
Table 9: Changes in active pH during storage and tittatable acidity.  
decrease in pH as well as the increase in lactic acid content during cold storage is linked to post-acidification, which occurs due to the residual activity of microorganisms. These microorganisms continue to produce lactic acid by catabolizing lactose, even during cold storage, gradually enriching the yoghurt with lactic acid [56; 87]. [PERSON] et al. [88] recommend pH values between 4.0 and 4.4 to prevent an excessively acidic taste in commercial yoghurt. In this study, however, the pH values of yoghurt samples from all treatment groups fell below the minimum recommended level on storage days 14 and 21.  
In relation to changes in the active acidity (pH) during storage and within the same treatment, no statistically significant differences (\(p>0.05\)) were observed in yoghurt on the UOP and POP treatments, whereas the pH decline resulted in differences (\(p<0.05\)) in samples from the Control treatment. [PERSON] et al. [25] reported no differences in the pH values in laboratory produced samples during storage for 15 days.