کاربرد یک میدان فراصورت یکنواخت سطح پایین برای تسریع زیست فراوری انزیمی پنبه Application Of A Low-Level, Uniform Ultrasound Field For The Acceleration Of Enzymatic Bio-Processing Of Cotton

INTRODUCTION Since the middle 1990s, the use of various enzymes in the textile industry has considerably increased, especially in the processing of natural, high value fibers such as cotton. A major advantage of enzymatic bio-processing is that the application of enzymes is much more environmentally benign and the reactions catalyzed are very specific, thus assuring a higher performance. In contrast, the traditional use of harsh organic/inorganic chemicals for cotton processing generates large quantities of toxic wastewater effluents, much less specific, often inducing undesirable side effects, such as reduction in the polymerization degree of cellulose. The enzymes used in cotton bioprocessing, acting as catalysts, speed up complex bio-chemical reactions such as the hydrolysis of cellulose (by cellulases), pectins (by pectinases), starches (by amylases), and triglyceride-based compounds in fats and oils (by lipases). Once they act as catalysts, relatively small concentrations of enzymes are required; if the applied conditions are favorable to the specific enzyme, the action will be repeated several times during the process. Other potential benefits of enzymatic bio-processing include cost reduction through energy and water savings, and improved product quality. Even a larger acceptance of enzymatic bio-processing by the textile industry in the near future will probably result from increasing legislative pressures, from the part of the governments worldwide, to sharply decrease the quantity and toxicity of textile wastewater effluents. In recent years, the high worldwide demand for energy and unstable and progressively more expensive petroleum sources imposed the development of new alternative transportation fuels,1,2 such as bio-ethanol from various biomass feedstocks, including the underutilized sources of plant cellulose, such as cotton gin and lint waste. Currently, the cost-competitive production of cellulosic bio-ethanol is prohibited mostly by the high cost and low efficiency of enzymatic hydrolysis of plant celluloses. Despite the recent,3 substantial reduction in the production cost of cellulolytic enzymes, the actual conversion of plant cellulose into sugars still remains an expensive and slow step. One of the most critical stages of this conversion of plant celluloses into biofuels employs hydrolysis reactions between a highly specific enzyme and the matching substrate (e.g. cotton gin/lint waste cellulose with cellulase), soluble sugars, to be easily converted into ethanol in a subsequent step, thus resulting. The typical applications of enzymes for bio-processing of cotton and cotton waste celluloses are summarized4-7 in Table 1. In addition to the numerous advantages of enzymatic bio-processing of cotton and cotton waste celluloses, several critical shortcomings – such as added processing costs and most important, slow reaction rates – should be mentioned. Enzymatic bioprocessing of cotton, like any other wet processing system, involves transfer of mass (enzyme macromolecules) from the processing liquid medium (enzyme solution) across the surface of the substrate. The detailed mechanism of enzymatic reactions, quite complicated, is still being investigated. In very general terms, the enzymatic reaction could be described according to the stages from Figure 1. At least two stages of the enzymatic reaction (1 and 4) involve transport of the enzyme macromolecules and of the enzymatic reaction products to and from the substrate surface. Since both stages are controlled by diffusion, the overall reaction rate of enzymatic hydrolysis is governed by the diffusion rate of the enzyme macromolecules. Generally, the large threedimensional enzyme macromolecules have very low diffusion rates and also tend to react with the outlaying cellulose fibers from the cotton yarn, which could result in excessive fiber damage. It was suggested8 that sonication of the enzyme processing solution under certain specific conditions could provide a far more efficient transport mechanism for the “bulky” enzyme macromolecules throughout the immediate border layer of liquid at the substrate surface. DISCUSSION The general trend observed during the experimental studies on enzymatic bioscouring of cotton fabrics and hydrolytic conversion of cotton waste celluloses into sugars indicates that the introduction of a low-level uniform ultrasound field into the reaction chamber considerably enhanced the performance of enzymes by significantly increasing their overall reaction rates. The beneficial effects of the introduction of ultrasonic energy could be summarized as follows: a) acceleration of the transport of the enzyme macromolecules toward the substrate/fiber surface through the border layer of the liquid at the liquid–solid interface. The concentration of enzyme macromolecules in this layer is a controlling factor, which defines the overall reaction rate; b) vigorous agitation of the normally immobile border layer of the liquid at the liquid–solid interface, caused by sonication, helps the enzyme macromolecules to position themselves “fittingly” onto the substrate; c) prevention of any possible agglomeration of enzyme macromolecules, which could decrease enzyme activity; d) improved removal of the enzymatic hydrolysis products from the reaction zone, which accelerates the overall enzymatic reaction rate; e) “opening up” of the surface of the substrate/fibers as a result of the mechanical impacts produced by the collapsing cavitational bubbles. CONCLUSIONS • It appears that sonication of the enzyme processing solution does not reduce the specific activity of the enzyme macromolecules in any significant way. • At a laboratory scale, the introduction of ultrasonic energy in the reaction chamber during the enzymatic biopreparation of cotton fabrics or enzymatic bio-conversion of cotton waste celluloses into sugars resulted in a significant improvement in enzyme efficiency. • The combination of enzymatic biopreparation and enzymatic bio-conversion of cotton waste celluloses with a low-level, uniform ultrasound irradiation could significantly advance these new “green chemistry” processes and make them more suitable for widespread industrial implementation. This could considerably reduce the amount of wastewater effluents, energy consumption and overall processing costs. • This study also provides a good potential for intensifying other technological processes that involve various types of enzymes and matching substrates. One can assume that, practically, any solid/liquid system that involves a reaction between the enzyme macromolecules and the solid substrate would greatly benefit from the introduction of ultrasonic energy into the system.