# Optimization of Valorization of Chicken MDCM to Produce Soluble Protein and Collagen Peptides
## 3 Results and Discussion
### Residual Bone Treatment  
#### 3.3.1 Treatments and Optimization Parameters  
According to preliminary experiments, pressurized hot water extraction proved the most promising way to separate collagen peptides. The variables in optimizing the conditions were temperature (140-190 \({}^{\circ}\)C), treatment time (15-100 min), and dry matter content (5-30%). The responses were the nitrogen yield of the solution (describes protein yield) and the mineral content of the solid residue. A statistical test design (CCF) was used in the series of experiments, and the results were modeled by response surface modeling. Good, reproducible, and predictive models were obtained for both responses. The nitrogen yield indices were \(R^{2}\) = 0.96, \(Q^{2}\) = 0.86, validity = 0.77, and reproducibility = 0.94. Correspondingly, \(R^{2}\) = 0.98, \(Q^{2}\) = 0.96, validity = 0.68, and reproducibility = 0.98 were obtained for the mineral content. The results show that a high temperature and long exposure time significantly improved the yield (Figure 3).  
The equation to describe the model in the case of collagen yield is  
\[\begin{array}{c}\text{Nitrogen yield}=8.78\text{X}_{1}+6.07\text{X}_{2}+9.13 \text{X}_{3}+5.73\text{X}_{2}{}^{2}-8.68\text{X}_{3}{}^{2}-4.75\text{X}_{1} \text{X}_{2}-\\ 1.85\text{X}_{1}\text{X}_{2}-3.26\text{X}_{2}\text{X}_{3}+69.87,\end{array}\]  
where X\({}_{1}\) = temperature, X\({}_{2}\) = dry matter (%), and X\({}_{3}\) = treatment time (min). The equation to describe the model in the case of mineral content is  
\[\begin{array}{c}\text{Mineral content}=3.97\text{X}_{1}-0.52\text{X}_{2}+3. 32\text{X}_{3}+0.62\text{X}_{2}{}^{2}-2.56\text{X}_{3}{}^{2}-0.97\text{ X}_{1}\text{X}_{3}+\\ 80.26.\end{array}\]  
The values of the optimal point variables predicted by the model were 190 \({}^{\circ}\)C, 83 min, and 5% dry matter content. The model predicted a nitrogen yield of 86% for the extract and a mineral content of 86% for the remaining bone fraction. The model was validated by performing an extraction experiment in optimal conditions. The measured nitrogen yield was 86.9% \(\pm\) 2.3%, and the mineral content was 84.5% \(\pm\) 0.2%. The measured values were very close to the values predicted by the model; thus, the modeling was successful. In addition, the yields were high; as such, PHWE appears to be a promising method for isolating collagen peptides from chicken bones. In the second test series, PHWE treatment was performed with constant dry matter content (20%) and extraction time (53 min), and the temperature was varied (110-190 \({}^{\circ}\)C).  
Figure 3: Factors affecting nitrogen and mineral yield in the processing of chicken MDCM clean bones.  
The samples were analyzed for nitrogen content and mineral content. The results confirmed the observation of a previous series of experiments that raising the temperature increased the yield of collagen peptides (Figure 4). The purpose of this series of experiments was to further investigate the peptide composition of the extracted fractions and the effect of temperature on the composition.