# Optimization of Valorization of Chicken MDCM to Produce Soluble Protein and Collagen Peptides
## 3 Results and Discussion
### Residual Bone Treatment
#### 3.3.2 Molecular Weight Distribution and Peptide Profiles  
The molecular size distribution was determined from the residual bone PHWE extracts to assess the effect of temperature on collagen cleavage. According to the results of size exclusion chromatography, the majority of the proteinaceous compounds in the PHW extracts of chicken bones were in the range of 0.5-13.7 kDa in MW with all extraction temperatures applied (Figure 5). The proportion of MW range of 0.5-13.7 kDa compounds varied from 61% (110 \({}^{\circ}\)C) to 75% (190 \({}^{\circ}\)C) between the applied temperatures. The results showed that an increase in the extraction temperature resulted in increased protein fragmentation. In particular, the proportion of proteins with MW higher than 44.2 kDa was reduced by increasing the extraction temperature. At an extraction temperature of 190 \({}^{\circ}\)C, the degradation products were the smallest; the proportion of compounds with MW smaller than 0.5 kDa was higher in comparison to the extracts produced at the lower temperatures. This indicates that the free amino acids and di- and tripeptides were more abundant in the PHW extract produced at 190 \({}^{\circ}\)C than in extracts produced at lower temperatures. Figure 4: The response surface of the enzymatic hydrolysis model of MDCM clean bones showing the relationships between nitrogen and mineral yield, temperature, and hydrolysis time. The skin is the largest organ of the human body and acts as a barrier that protects us against various external damages. It is also an important organ, regulating temperature and fluid-electrolyte balance and metabolism, for example [27]. Collagen and its hydrolysates have attracted particular attention because of their skin-enhancing properties. Cosmetic and edible cosmetic formulations use hydrolyzed collagen for its superior solubility at a neutral pH, its easy dermis penetration, and its water-binding properties [28]. Recently, many studies have reported skin antiaging and healthcare properties in small collagen peptides [29]. Chicken bones, as sustainable residual materials, have received special attention as a source of collagen (reviewed by [PERSON] et al. [30]). The present study shows that PHWE treatment is a promising method for producing collagen peptides with a low molecular weight from chicken bones, a sustainable residual material from food processing. The nitrogen yield from the purified bones was high (87%), indicating that most of the chicken bone protein was extracted. The results are in line with the findings of [PERSON] et al. [31], who studied PHWE treatment with chicken bones at 120-135 \({}^{\circ}\)C. They also found that increasing the treatment temperature improved the recovery of collagen. In addition to PHWE treatment, other extraction methods have been studied to recover collagen from chicken bones; attempts have been made with an NaCl solution containing small amounts of NaNO\({}_{2}\), ascorbate, and Na\({}_{4}\)P\({}_{2}\)O\({}_{7}\), combined with homogenization [21] and alkali extraction [32]. Chicken foot collagen extraction has been reported with papain and pepsin enzymes in an acetic acid solution [33] and with dilute acid treatment [34]. Moreover, extraction studies of turkey bones have been performed with dilute alkaline NaCl solutions [35] and acid and alkaline treatment [36]. These methods have yielded 46% collagen peptides at best. It can, therefore, be stated that, among the methods studied so far, PHWE seems the most efficient way of extracting chicken collagen peptides. In this study, the smallest collagen peptides were produced at temperatures above 150 \({}^{\circ}\)C. However, a large proportion of the total protein in the bone extracts was small peptides below 10 kDa already at a rather mild temperature between 110 and 140 \({}^{\circ}\)C. The results indicate the suitability of the chicken bone PHW extracts for cosmetic use, but more research is needed to investigate the skin enhancing effects on the cellular level and in vivo.