## 3 Results and Discussion
### Stabilisation Process of Chlorophylls
#### 3.2.2 Effect of pH  
Figure 2 presents the effect of various pH values (4-9) on the amaranth purees stabilised with Cu and Zn concentrations of 210 mg/L and 1500 mg/L, respectively, at 80 \({}^{\circ}\)C  
Figure 1: Effect of metal (**a**) CuSO\({}_{4}\) and (**b**) ZnCl\({}_{2}\) concentrations on greenness (\(-\)a*/b*) and total chlorophyll derivative content (mg/g) of amaranth purees. The study was performed at pH 6 after heating at 80 \({}^{\circ}\)C for 15 min. Each value is expressed as mean \(\pm\) standard deviation (\(n=3\)) of triplicate analysis. The bars with different lowercase (a–g) and uppercase letters (A–F) indicate significant differences (\(p<0.05\)) in [PERSON]’s HSD test.  
for 15 min. Thus, the results indicated that the pH value significantly affected the greenness and chlorophyll derivative content of both Cu- and Zn-amaranth pures (\(p<0.05\)). Moreover, these results are consistent with the findings of [PERSON] et al., that the formation of metallo-chlorophyll derivatives in heated vegetables was a pH-dependent process [52].  
In Figure 2a, Cu-amaranth pures produced the highest greenness value (\(-0.41\)) with no significant difference (\(p>0.05\)) between pH from 6 to 9. Comparatively, Zn-amaranth pures are significantly (\(p<0.05\)) the highest greenness value (\(-0.29\)) at pH 8. Subsequently, the greenness value reached a plateau above pH 8. [PERSON] et al. reported similar results that the avocado pures treated with copper chloride showed larger negative values of \(\mathrm{a^{*}/b^{*}}\) (greener colour) than the avocado pures treated with ZnCl\({}_{2}\)[19]. In contrast, [PERSON] et al. reported that the green colour of pandan leaf extracts was highest at pH 5 and lower at pH 7 and 8 [25]. Hence, the contradicting results of pH may be associated with variations in the plants' raw materials and chlorophyll derivative contents.  
Figure 2: Effect of pH on greenness (\(-\mathrm{a^{*}/b^{*}}\)) and total chlorophyll derivative content (mg/g) in (a) 210 mg/L of CuSO\({}_{4}\) and (b) 1500 mg/L of ZnCl\({}_{2}\) stabilised amaranth pures after heating at 80 \({}^{\circ}\)C for 15 min. Each value is expressed as mean \(\pm\) standard deviation (\(n=3\)) of triplicate analysis. The bars with different lowercase (a–e) and uppercase (A–D) letters indicate significant differences (\(p<0.05\)) in [PERSON]’s HSD test.  
The highest chlorophyll derivative content (0.65 mg/g FW) was observed in Cu-amaranth purees (prepared with 210 mg/L of CuSO\({}_{4}\)) at pH 6. Nevertheless, the value decreased at pH > 6 (Figure 2a). Based on these results, metallo-chlorophyll complexes rapidly formed when the pH was increased from 4 to 8.5. On the contrary, these formations decreased at pH 10 [49]. Therefore, the Cu-amaranth purees at pH 6 were used for subsequent experiments. Meanwhile, the chlorophyll derivative content of Zn-amaranth pures at pH 7 and 8 were 0.28 and 0.22 mg/g FW, respectively. However, the colour of the puree at pH 8 was brighter than the puree at pH 7 (Figure 2b). Based on these observations, the Zn-amaranth puree treated at pH 8 was selected for subsequent experiments.