Study On The Shrinkage Behavior Of Pigskin Collagen

Leather manufacturing is one of the most ancient human skills,which greatly improving the quality of people’s lives by providing a variety of useful articles for daily use and handicrafts.Tannning is the most important step in leather manufacturing process,which can improve the physical and chemical properties of animal skin.The aim of tannning is to improve the thermal stability of animal skin.In the leather industry,the moisture and heat stability of leather is an important index to measure the quality of leather.Usually,the hydrothermal shrinkage temperature of leather sample is used to evaluate the tanning effect of tanning agent.Therefore,it is of great importance to investigate the mechanism of hydrothermal shrinkage of animal skin,which has important guiding significance for the development of new and efficient tanning agents.So far,researches have made great contribution to clarify the mechanism of hydrothermal shrinkage of animal skin.However,the hydrothermal shrinkage behavior of collagen fiber in solution could not reflect the real hydrothermal shrinkage behavior of skin.In addition,a lot of theoretical calculations have been made on the mechanism of hydrothermal shrinkage of animal skins.At present,there are few in situ studies on the hydrothermal shrinkage of animal skins.In addition,the changes of collagen molecules,fibrils and fibers in the process of leather shrinkage,especially the synergism among them,are still not clear.Herein,this work takes pigskin as the research object,and uses different detection methods to systematically study the structure and property changes of pigskin during the process of hydrothermal shrinkage,and summarizes the mechanism of hydrothermal shrinkage of pigskin.This paper mainly carries out following three specific research works:1.Pigskin was pretreated to remove the fat,followed by extracting collagen using the acetic acid-pepsase combine method firstly.Then the different detection method and comparative research were applied to study the hydrothermal shrinkage behavior of collagen in solution and the pigskin collagen at solid state.This work was carried out to explore the different methods in the study of collagen denaturation differences and features,and further to establish an effective method for detecting the hydrothermal shrinkage process of pig skin at molecular level.The results indicate that the Fourier transform infrared spectrum(FT-IR),X-ray diffraction(XRD)and atomic force microscope(AFM)are effective method to detect solid state collagen denaturation.FT-IR and AFM can qualitatively detect collagen denaturation.XRD can quantitatively detect the thermal denaturation of collagen.The hydrothermal denaturation of collagen in solution can be qualitatively determined by circular dichroism(CD)spectroscopy,optical rotation and ultra-sensitive differential scanning calorimetry.It should be stressed that the ultra-sensitive differential scanning calorimetry is the most accurate testing method.FT-IR,XRD and AFM are more suitable for detecting and studying the hydrothermal shrinkage behavior of pigskin.2.The hydrothermal shrinkage behavior of pretreated fresh pigskin collagen was studied in situ,with the focus on the changes of microstructure and macroscopic morphology during the hydrothermal shrinkage process of pigskin,in chapter three.The changes of physical and chemical properties,such as water content and solid content,as well as the changes of macroscopic properties,such as thermal stability and mechanical properties of pigskin were detected.To explore the variation rule of collagen molecular triple-helix structure,fibrils and fibers and their synergism with macroscopic morphological changes in the process of hydrothermal shrinkage of pigskin collagen,and to speculate the mechanism of hydrothermal shrinkage of pig skin collagen.The results show that the hydrothermal shrinkage process of pigskin collagen could be divided into four stages.In the first stage,when the hydrothermal treatment temperature lower than 50 oC,the appearance and microstructure of pigskin did not change significantly,and the water content of pigskin increased slightly: In the second stage,between 50 oC and 66 oC,under the influence of humidity and heat treatment,a large number of water molecules entered between the three polypeptide chains of collagen molecules,which increased the diameter of collagen molecules and thus slightly increased the diameter of collagen fibers.In the third stage,between 66 oC and 70 oC,the thermal unstable region of the C-terminal of collagen molecule first loses the t triple-helix structure to form small nodules,which further expand and combine to cause pigskin contraction.In the process of pigskin shrinkage,the diameter of collagen fibers increases sharply,and the space between the collagen fibers is compressed,forming a more compact spatial arrangement than the original expanded form.At the fourth stage,above 70 oC,the residual collagen fiber structure gradually disappears and collagen degrades.3.In chapter four,the hydrothermal shrinkage behavior of tanning pigskin collagen by tannic acid was studied in situ.The back skin of adult pigs was selected as raw material,then the epidermis,hair and fat were removed by pretreatment.And the 10%(w/v)tannic acid solution was used for tanning the pretreated pigskin.Then the hydrothermal shrinkage behavior of tannic tanning pigskin by tannic acid was studied to explore the tanning mechanism of tannic acid on pigskin collagen.The appearance,micro-morphology,FT-IR spectra,physical properties,dissolution amount of tannic acid and thermal degradation of tanning pigskin by tannic acid were systematically investigated.The results show that tannic acid permeated into the pigskin tissue,deposited on the surface of collagen fibrils,occupied the space between collagen fibrils,and blurred the band structure of collagen fibrils with alternating light and dark.With the increase of temperature,tannin gradually peeled off from the surface of collagen fibrils,and then the hydrothermal shrinkage process of collagen was the same as that of untanned pigskin collagen.However,due to the filling effect of tannic acid on the gap between collagen fibers and the interaction between tannic acid and collagen,the moist and hot shrinkage temperature of tannic acid tanned pigskin is significantly increased.