| In the traditional corn starch industrial production, the water solution of SO2 is used to steep the corn. SO2 can break the disulfide bond of protein and destroy the network structure of protein. It is conducive to the release of corn starch. In the steeping process, the addition of 0.5% lactic acid can promote the expansion of corn water and solution of protein. However, the SO2 process has long steeping time, pollution of the environment, large amount of waste water discharge, which has restricted the development of starch production industry. To find a suitable method to replace the SO2 steeping process can reduce the environmental pollution caused by the production of corn starch. To provide theoretical basis for the new technology of wet milling, the bonding mechanism of starch and protein in corn endosperm, the effect of different forces on corn starch extraction were studied. In this paper the research is as a part of the national natural fund project named "The starch flocculation mechanism of microbial fermentation acid liquid". The acid liquid with the flocculating activity (used to separate sweet potato starch) is applied to the process of corn starch production. A new reagent combined with Chinese traditional acid liquid was used to study environmentally friendly corn wet-milling. The outline and results of the study are as following:1. In corn endosperm, corn starch granules are wrapped in protein matrix, which is difficult to be separated by physical method. The study of bonding mechanism of corn starch and protein could provide the theoretical basis for the separation of corn starch. The analysis of characteristics groups changes of corn starch, corn flour, conjugates of corn starch with protein, the mixture of corn starch and protein were determined by IR and Raman spectra. The changes of characteristics groups were used to speculate bonding forces of starch and protein. The results showed that association effect was existed in starch and protein molecular in maize endosperm. This may be caused by intermolecular hydrogen bonds. The structure of disulfide bonds changed significantly in the separation process of starch and protein. The disulfide bonds of proteins were presented as twisted-twisted-trans type in the corn endosperm. In the process of separation of starch and protein, disulfide bond are broken and re-combined to three common types of disulfide bond. The protein re-combined with starch when it is oxidized again and presented as twisted-twisted-trans type. Therefore, the main force was disulfide bonds and hydrogen bonds were next in importance.2. Corn starch and protein combined closely. In the production process of corn starch the bonding forces between starch and protein need to be broken in order to separate corn starch. The effects of bonding forces on corn starch extraction by different chemical treatments were studied in order to improvise the corn starch extraction process. A number of low-toxicity or nontoxic reagents (such as L-cysteine, urea, NaCl, SDS, and so on.) added to a corn slurry to investigate their effects on the disruption or weakening of common molecular forces, namely, hydrogen bonding, disulfide bond, hydrophobic and electrostatic interactions, or a combination of these. The results showed that starch and corn protein could be easily separated by disruption of the disulfide bond with L-cysteine. The treatment sequence-hydrogen bondâ†'disulfide bondâ†'electrostatic interactionâ†'was found to have the greatest effect on the yields of corn starch extraction. Scanning electron microscopy showed that the structure of the starch granules was not destroyed by the reagent.3. L-cysteine is an amino acid with active in (-SH), which is capable of reducing the disulfide bond of the protein and the protein molecules and destroy spatial structure of protein to release the starch. Therefore, L-cysteine is suitable to replace SO2 in wet milling. In order to investigate the effect of L-cysteine solution on the separation of corn starch and protein, corn was milled after steeping with acid liquid for 24 h and L-cysteine was added. Hydrosulphonyl content and starch mass was determined to study the effects of L-cysteine on the combination of corn starch and protein. Due to the sedimentation of corn starch and protein in natural conditions is not easy to be separated, so acid liquid with flocculating activity was added to steeping process. Acid liquid can separate corn starch and protein effectively after treatment. And there is conducive to the determination of starch content. The results showed that the effects on combination of corn starch and protein were great when L-cysteine content was 1%, acting time 4.2 h, solution pH6.5.4. In the process of wet milling, the lactic acid bacteria in the corn produce lactic acid, which accelerate the water absorption and expansion, and accelerate the solution of protein. It is favorable for the separation of corn starch and protein. The lactic acid bacteria are often added to starch production to increase the steeping efficiency. The lactic acid bacteria were replaced with acid liquid which with flocculating activity. It not only can accelerate corn steeping speed but also can accelerate the solution of protein. Moreover, the acid liquid has the flocculation activity, which can make the starch granules aggregate, accelerate the starch deposition, and accelerate the separation of the corn starch and protein. Corn acid liquid was fermented by Lactobacillus paracasei fermentation liquid with flocculating activity which was isolated from the sweet potato starch acid liquid and was used to steep corn. The effect of acid liquid on steeping process was studied. The results showed that, when the inoculation amount was 10%, the content of lactic acid was 2.8%, and the content of the protein was 334.48 μmol/g, the water absorption rate and protein solubility were also higher. The best steeping time was 48h. At 30℃, the water absorption rate and the increment of soluble protein were higher, and the content of thiol group was also higher. Therefore corn acid liquid can promote corn swelling, the combination of starch and protein in endosperm become loose, promote the soluble protein digestion, weakening the link between starch and protein, and it is conducive to the corn starch extraction.5. Lactobacillus paracasei subsp.paracasei with flocculating activity in the acid liquid can not only produce lactic acid and promote the steeping effect, and accelerate the separation of the starch. Therefore, corn acid liquid is suitable for application to corn starch wet milling. Acid liquid and L-cysteine were combined to steep corn and the steeping effects were compared with other steeping liquid. It provides a research foundation for further use of acid liquid and L-cysteine in corn starch production.Corn granules were steeped in different steeping liquid at 30℃. The water absorption and soluble protein content in solution were determined every once in a while to compare the effect of different steeping liquid on corn granules. The results show that, acid liquid steeping effect was better than that of lactic acid. The steeping effect of acid liquid+L-cysteine was little lower than acid liquid+SO2, resulting from the larger molecular of L-cysteine. Therefore, in the follow-up study broken corn can be used to increase the contact area of corn and L-cysteine. The water state of corn granules steeped with acid liquid was studied by the low field nuclear magnetic resonance. The bound water increased rapidly with the increased steeping time at 0-8 h. The fluctuation of weak flow water in corn granules was large before 12h; the weak flow water was stable at 36 h-50 h. The free water changed slowly at 0-36 h and increased rapidly after 36 h.6. L-cysteine was used to replace SO2 in traditional steeping process to reduce the pollution of SO2. L-cysteine was added to acid liquid of corn which fermented by Lactobacillus paracasei subsp.paracasei LI. Chopped corn steeped in the steeping liquid and milled with water after a certain time of steeping and sieved to remove impurity such as fiber. Starch settled quickly under the action of Lactobacillus paracasei subsp.paracasei LI which has the flocculating activity. Effect of various factors on the increment of soluble protein in steeping liquid and starch extraction rate were studied by the change of acid liquid amount, L-cysteine amount, steeping time, steeping temperature, pH, and inoculum size of Lactobacillus paracasei. The optimum extraction condition was determined by response surface method. The optimum parameters were that:steeping time 48h, L-cysteine amount 1.5%, pH 7, acid liquid amount 1:4, steeping temperature 30 ℃, inoculum size of Lactobacillus paracasei 20%. The maximum extraction rate of starch was 93.21%. The results showed that extraction rate of starch extracted by synergy of acid liquid and L-cysteine wet milling and the ideal, and it reduce environment pollution caused by SO2 wet milling7. In order to apply the corn starch separated by synergistic effect of L-cysteine and acid liquid to starch products, the physicochemical properties of corn starch isolated with L-cysteine and flocculating activity acid liquid (ALS) were studied. Starch isolated with acid liquid and SO2 (ASS) and commercial corn starch (CS) was examined to compare the effect of different isolation methods on the starch properties. The results indicated that ALS had the lowest amylose content, smallest average granule size, highest swelling power, and solubility. Scanning electron microscopy showed that many pores were on the surface of ALS granules. The X-ray diffraction pattern of three starches exhibited typical A-type starch patterns. The peak viscosity and the breakdown of ALS were higher than ASS and CS. |
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