Screening Of The Strains Producting Acid Protease And Its Application In Production Of Corn Starch

During the production of corn starch, the wet-milling processing technique has been widely applied in domestic and overseas. The steeping condition of traditional wet-milling procedure in corn starch production is that the corn was steeped into the steeping water containing SO₂ concentration of 0.2 %～0.25 % at 50℃±2℃for 48 hours. Because the follow-up procedures are affected significantly by the steeping technique, which effects directly the output and quality of corn starch, so the steeping technique is an essential key process in production of corn starch. However, the problems of soaked time too long and sulfurous acid pollution could give rise to the energy waste and be unable to realize green production, which has been the bottleneck of restricting the development of whole starch production industry.The purpose of using sulfurous acid is to destroy the disulfide bond of the protein which warps the starch grain, thereby protein molecules are loosened and the starch particles dissociates out. Based on the functions of the sulfur dioxide in steeping water, the objective of this thesis is to explore to screen the high-yielding strains of acid-protease instead of the functions of the sulfur dioxide in the corn steeping conditions. Firstly, the strains of acid-protease were screened out in the natural breeding conditions, and then bred to acquire the high-yielding strains of acid-protease by ultraviolet(UV) mutagenic treatment. Secondly, the best conditions of the ferment were achieved by optimizing fermentation medium and condition of culture, then the acid protease was extracted, and the enzyme properties were measured. Finally, the acquired acid protease was exploratively applied in the corn steeping technique of corn starch production. The research contents and results are listed as follows:1. According to the most microbes of breeding acid protease being made of the fungus, the breeding conditions are designed and screened at 50±2 degrees and pH3.5～5.5. Then the breeding acid protease vitality and amylase activity were identified, finally the strain T26 was determined as the purpose strain. Gotten through the microscopic observation of the morphology and colony of strain T26 and combined the identification of molecular biology, the strain T26 was confirmed to be part of the Aspergillus fumigates strain.2. The screened strain T26 was carried out by ultraviolet (UV) mutagenic treatment, and then the experiments of the vitality and stability for the breeding acid protease were achieved. Subsequently, mutant strain UV11 was obtained, and the activity of proteinases was increased into 6680 u/ml. Finally, the best conditions of the ferment were achieved by optimizing fermentation medium and condition of culture, and the most suitable fermentation conditions were that: soybean meal 4 %, wheat bran 8 %, NH₄Cl 3%, K₂HPO₄ 0.2 %, CaCl₂ 0.5 %, initial pH=5, inoculation volume 1.5 %, temperature 50℃, and fermentation cultivation 5 days.3. The enzymologic properties of obtained proteinase were analyzed, including the characters of the suitable pH, the suitable substrate concentration, the suitable temperature, and thermal stability, etc. the best bleeding conclusions of acid protease were obtained by mutant strain UV11: the pH 4.5, the noculation volume 2 %, the temperature 50℃.4. In the conditions of the laboratory, the steeping technique of traditional corn starch production process was improved by enzymatic processing technique, then the best steeping conditions were obtained: Firstly, the corn was steeped into the steeping water of temperature 50℃, lactic acid 0.5 %, and SO₂ 0.08 % for 12 hours; Then, the peel and embryo were removed; Finally, after added the fermentation liquor of 12 % UV11 into the steeping water, the treated corn needed be continued steeping for 10 hours again. In contrast to traditional wet-milling producre, the proposed enzymatic processing technique can shorten the steeping time of 26 hours in corn starch production. The obtained starch can accord with the quality regulation of the starch by the enzymatic technique, which can also improve the recover rate of the various components of the corn starch.