Acid extraction is to use a certain concentration of acid solution to extract collagen under certain conditions, mainly using low ion concentration acidic conditions to destroy intermolecular salt bonds and Schiff bases, and cause fiber swelling and dissolution. Collagen extracted by acid method is usually Become acid-soluble collagen. The acid dissolution method can dissolve the collagen molecules without cross-linking and can also dissolve the collagen fibers containing aldimine cross-linking bonds, and then release them into the solvent. The acid method is a relatively common and effective method for extracting collagen. The collagen extracted by the low temperature acid method maintains its triple helix structure to the greatest extent and is suitable for the preparation of medical biological materials and raw materials. The usual practice is to add an appropriate concentration of acid solution to the pretreated bone meal according to a certain material-to-liquid ratio and stir and extract at 0-25 °C for a certain period. When using the acid method to extract collagen, it should be noted that the extraction temperature should not be too high, to avoid the destruction of the biological activity of collagen. After sampling and pretreatment, the homogenate was extracted with acid at low temperature and centrifuged to obtain acid-soluble collagen (ASC). The acids used as solvents mainly include hydrochloric acid, phosphoric acid, formic acid, acetic acid, malic acid, citric acid, etc., but most studies focus on acetic acid extraction. For example, Maria Sadowska et al. used 0.5mol/L citric acid to extract collagen at room temperature protein, its extraction rate is slightly lower than that of acetic acid extraction. Citric acid is widely used in the extraction of collagen in the food industry because it does not produce color and odor.
During the acid treatment, the reaction is strong, the hydrolysis is complete, and the amino acid mixture is mostly generated. When using acid extraction, depending on the acid concentration, hydrolysis temperature, hydrolysis time and other conditions, the collagen hydrolyzate with uneven molecular weight can be obtained. However, tryptophan will be destroyed, serine and tyrosine will also be partially destroyed, and the equipment will be seriously corroded during the complete hydrolysis of moderate concentration of acid. Therefore, acidity, temperature, time, and other influencing factors should be accurately controlled for biomedical collagen dissolution by acid method. Due to various deficiencies, the acid method is rarely used alone, and is generally combined with the enzymatic method. For example, using pig skin as raw material, collagen is extracted under the synergy of citric acid (pH8.6) and pepsin. Add 0.05moL/L citric acid solution (pH 2.5-3) containing pepsin to the treated pig skin for a period, and then salt out with NaCl, the final extraction rate is 12.35%, and the extract keeps intact three strands Helical structure of type I collagen. Others used chick sternal cartilage as raw material, digested with pepsin for multiple times under the condition of 0.5moL/L acetic acid, centrifuged at 4°C and 20000r for 20min, and finally applied DEAE-Sephadex A-50 for ion exchange chromatography, and then dialyzed, and then salted out with NaCl, and finally purified collagen type II was obtained.
