| Sea cucumber belongs to Holothuroidea in Echinodermata.Various species of sea cucumber can be found in the world,its nutritional ingredients are not only rich,but also are balanced and reasonable.The industry of sea cucumber has developed rapidly in recent years,and specifically in 2012,the production capacity has reached 171,000 tons with the value of more than 40 billion.The main edible part of sea cucumber is the body wall,in which the protein content is very high.The special physical characteristics of sea cucumber during heating treatment,such as elasticity and hardness,may mainly depend on the changes of collagen.In the present study,the crude collagen fibers(CCF)and the pepsin-solubilized collagen(PSC)were extracted from the body wall of sea cucumber.And the amino acid compositions,denaturation temperatures,and structure changes during the heating process of two types of collagen has been comparative studied thoroughly.This study was to further reveal the changes of the mechanism of collagen in the body wall of sea cucumber during heating process,providing a theoretical basis for various optimization and development of deep processing of sea cucumber products.The amino acids of CCF and PSC were analyzed.The glycine contents of both two types of collagen of higher levels were 34.05 ± 0.95%and 36.44 ± 1.72%,accounting for higher than one-third of the total,in which followed by glutamic acid,proline,alanine,proline and hydroxyproline.The denaturation temperatures of CCF and PSC were 46.1 ? and 35.3 ?,respectively by differential scanning calorimetry(DSC),which suggested CCF was generally more thermal stable than PSC.Through preliminary analysis of the structural changes were conducted by circular dichroism spectroscopy(CD)during the heating process.The results suggested the unheated soluble collagen was consisted of the complete triple helix structure,which showed a strong positive absorption peak at 220 nm,and a strong negative absorption peak at 202 nm;when the sample was heated into 28 ? a significant change occurred in spectra,and the conformation of PSC changed dramatically;,whereas,the triple helix structure of PSC has been completed unwrapping when the temperature reached 30 ? which showed the positive absorption peak disappeared completely.Fourier transform infrared spectroscopy(FTIR)combined curve-fitting technique were used to analyze the secondary structure of CCF and PSC,the results revealed that the secondary structural differed,and both of them had a high amount of a-helical structure;CCF showed much high proportion of a-helical structure than PSC.Differential Spectral analysis of CCF and PSC during the heating process showed that both the peak of amide I band near 1657 cm-1 reduced dramaticly and blue shifted,a-helical structure was significantly damaged,the secondary structure of CCF and PSC occurred significant changes first time at 35 ? and at 45 ?,respectively.During the cooling process,the changes of CCF showed substantially adverse trends compared to the changes during heating.Because of the poor thermal stability of PSC,the structure of the amide I band was difficult to maintain under the prolonged heating conditions,and the peak was reduced again after the first rebound trend.Two-dimensional spectrum analysis was used to determine the secondary structure changes sequence of CCF and PSC during heating.In the heating and cooling process,the a-helix of CCF characterized by wave number 1657 cm-1 showed a strong auto-peak;and the ?-sheet structure also has significant cross-peaks.The secondary structure changes sequence was judged by spectral rules,during the heating and cooling process,the ?-sheet changes occurred before a-helix structure damaged;While during the heating and cooling process,the ?-helix,?-sheet,and 3-helix of PSC was dramatically changed,in which the changes sequence can be obtained by spectrum analysis;similarly,?-sheet structure changed before ?-helix in the heating process,and the sequence during the cooling process showed the adverse trends. |
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