الفهرس 
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Abstract
Fish is the cheapest source of good quality animal protein readily available to the masses throughout the world and its demand is always on the increase. Fish wastes such as skin, scales, bones and fins are the major by-products in the fishery and aquaculture industries have received significant attention in recent years as an important source of collagen. So, using fish skin as an alternative source of collagen converts this low-value raw material to a value-added product. Collagen is an important biomaterial in medical applications due to its special characteristics, and enormous applications in the food and pharmaceutical industries and cosmetics. Because of this wide area of applications, the study on characteristics and functional properties of collagen extracted from fish skin is very important and illustrates the differences and similarities in the characteristics of these extracted collagens.
The present research was undertaken in order:
1. To utilize the fish skin waste source of collagen for this reason two extraction methods were used to obtain acid-soluble collagen (ASC) of catfish and Nile tilapia skin pepsin-soluble collagen (PSC) from Nile tilapia skin to determine the chemical composition (moisture, protein, fat and ash content), ultraviolet absorption spectrum, amino acid analysis, FT-IR Spectrum, SDS-PAGE, Thermal denaturation temperature (Td) of extracted collagens and color values L, a. and b.
2. To investigate the functional properties of extracted collagen such as solubility, oil and water absorption, emulsifying activity and stability, foaming ability, foam stability and gelation properties to produce new functional beverage containing collagen obtained from catfish skin.
The results obtained could be summarized as follows:
1. The yield of catfish (Clarias Lazera) skin was 5.40% whereas, the yields of Nile tilapia (Oreochromis niloticus) skin and scale were 4.85 and 2.41% respectively of whole fish.
2. The yield of ASC was higher from the skin of the catfish and Nile tilapia skin contained (5.20 and 4.30%) respectively followed by PSC of Nile tilapia skin (1.84 %). Whereas the Nile tilapia scale contained a very low content of ASC (0.58%) (based on the wet weight) which indicated that the skin of catfish and Nile tilapia was best suitable for collagen extraction than the Nile tilapia scale.
3. The proximate composition analysis revealed a presence of higher protein content (32.71 and 31.04%) in the catfish and Nile tilapia skin whereas, the skins of catfish and Nile tilapia contained low moisture 56.65% and 59.49% respectively. Ash contents were low in catfish and Nile tilapia skin (2.92 and 1.59%) and usually high in the Nile tilapia scale (16.27%). Skins of catfish and Nile tilapia skin were therefore considered as a good source of raw materials for the extraction of collagen-based on their protein content. On the other hand, ASCs of catfish and Nile tilapia skins had a low moisture content of 14.03% and 15.69% respectively. Moreover, the protein content for ASCs of catfish and Nile tilapia skins was 83.20% and 82.48% respectively, whereas the fat and ash contents in all extracted collagen very low.
4. The maximum absorption of ASCs isolated from catfish and Nile tilapia skins were near at 220 and 233nm respectively. Whereas, PSC of Nile tilapia skin was near at 236 nm.
5. Amino acid composition of all extracted collagen indicated the Glycine was abundant. Moreover, the content of imino acids (proline and hydroxyproline) was (173.3, residues per 1000 residues) for ASC of catfish skin was higher than ASC and PSC of Nile tilapia skin (172.1 and 164.18 residues per 1000 residues).
6. The FTIR Spectrum confirmed that the presence of a triple-helical structure characteristic of all extracted collagen.
7. SDS-PAGE results indicated that all extracted collagen were type I collagen which consisted of at least two α chains (α1 and α2).
8. The denaturation temperature (Td value) of ASC from catfish collagen was (27 °C) whereas Td value of ASC and PSC from Nile tilapia skin was 26⁰C and 25⁰C respectively.
9. The color L*)lightness( values were 76.23, 67.62 and 58.01 for ASC (catfish), ASC (Nile tilapia) and PSC (Nile tilapia) respectively, It’s obvious the L* values of ASCs samples were significantly (p<0.05) higher than PSC of Nile tilapia skin.
10. The results of functional properties showed the highest solubility of all extracted collagen was in the acidic region pH (2–3)and the ASCs obtained from catfish and Nile tilapia skin and PSC indicated higher solubility at low NaCl concentration (0 and 2 %).
11. ASCs of catfish and Nile tilapia skin and PSC of Nile tilapia skin had good water and oil absorption capacity. Emulsify activity (EA) of ASCs of catfish and Nile tilapia skin were 53.31 and 51.50% respectively, whereas PSC of Nile tilapia skin was 54.20%. The foaming capacity (FC) of ASCs extracted from catfish and Nile tilapia skin was 125.25% and 165.50% respectively and 114.5% for PSC. Whereas, foaming stability (FS) of ASCs of catfish and Nile tilapia skin and PSC of Nile tilapia skin was 120.62%, 130.50% and 92.5% respectively. Gelation studies revealed that all extracted collagen have good gelation properties (LGC) as the index of gelation capacity, lower LGC of Both ASCs of catfish and Nile tilapia skin was 3% whereas PSC of Nile tilapia skin was 4 %.
12. In concerning the development of healthy and functional drinks containing collagen the results indicated the orange beverage containing fish skin collagen was the most acceptable beverage.
13. The addition of collagen in an amount of 0.9% increased the viscosity of beverages and does not have an impact on pH and total soluble solids (TSS). It’s noticeable that all samples with added collagen had significantly (P<0.05) highest (L*) values compared to others.
Consequently, collagen has shown to be an important functional ingredient in the beverage industries. Sensory evaluation of this beverage indicated that the odour characteristic of fish raw materials was absent and does not have a negative impact on the sensory characteristics of beverages.