Saccharification Enzymes and Process Optimization in Starch Hydrolysis

Amyloglucosidase, a key enzyme in the saccharification process, was introduced in the early 1960s and has since become a staple in the industry. This saccharification enzyme plays a crucial role in breaking down α-1,4-linkages rapidly. However, the hydrolysis of the highly branched amylopectin’s α-1,6-linkages is a slower process. To achieve the desired final degree of saccharification (DX) within 48 to 96 hours, the dosage of amyloglucosidase (AMG) is carefully adjusted.

At the onset of saccharification, there is a rapid formation of dextrose, but this rate gradually decreases towards the end of the process. This decline is attributed to the accumulation of branched dextrins and the increasing concentration of dextrose, which accelerates reversion, i.e., the re-polymerization of dextrose into isomaltose and other saccharides. If saccharification is not halted at the point of maximum DX, the dextrose level will gradually decrease towards chemical equilibrium, typically around 85% dextrose at 30% DS (dry substance) and 60°C.

The addition of a pullulanase like Promozyme, in conjunction with AMG at the beginning of saccharification, facilitates the rapid hydrolysis of α-1,6-linkages in branched dextrins. This results in fewer branched oligosaccharides accumulating towards the end of the process, shifting the point at which reversion surpasses dextrose formation to a higher DX level.

The maximum DX achievable with AMG and Promozyme depends significantly on the enzyme dosage ratio (PUN:AGU) and the dry substance level. The impact of Promozyme is most pronounced at PUN:AGU ratios below 0.5.

During saccharification, the dry substance concentration increases by approximately 10%, mainly because each molecule of dextrose released by hydrolysis absorbs one molecule of water. Higher DS levels favor re-polymerization (reversion), leading to a reduction in the maximum attainable DX. An initial DS level of around 30% w/w is typically chosen as an economic compromise.

Saccharification time, necessary to achieve maximum DX, is influenced by both the PUN:AGU ratio and enzyme dosage level. A saccharification temperature of 60°C is standard, as higher temperatures reduce enzyme stability, while lower temperatures slow the reaction rate and increase the risk of microbial infection.

Optimal enzyme performance is observed at an initial pH of 4.3, measured at 60°C. The pH tends to drop slightly during saccharification, and this drop must be considered when selecting the initial pH set point, ensuring it does not fall below 3.5.

Continuous saccharification is theoretically capable of achieving the same final DX as a batch process under identical conditions. However, achieving this requires either a plug-flow reactor or an infinite series of infinitely small continuous-flow stirred tank reactors (CSTR). Due to retention time variance, a series of at least eight tanks is recommended to avoid a final DX lower than theoretically possible.

Inactivation of AMG is necessary shortly after achieving the maximum DX to prevent excessive reversion of glucose to isomaltose. Promozyme, when used, lowers the risk of reversion, and inactivation can be achieved through ion exchange, carbon treatment, or heating the liquid.

The saccharified liquid undergoes filtration to remove insoluble impurities, followed by carbon treatment for the removal of soluble organic impurities and ion exchange. The use of Dextrozyme™, a balanced mixture of amyloglucosidase and pullulanase, offers advantages in glucose yield and saccharification time compared to AMG alone.

A membrane reactor has been developed for continuous hydrolysis of liquefied corn starch, demonstrating increased productivity compared to batch reactors. Enzymes like cellulases, pentosanases, and proteases aid in purifying saccharified wheat starch by removing impurities adhering to starch granules.

Wheat starch, known for forming hard-to-filter precipitates, benefits from enzymes like cellulases and pentosanases to aid in refining processes, particularly in cases where impurities adhere to starch granules. This comprehensive overview encompasses various aspects of saccharification enzymes and their role in optimizing the starch hydrolysis process, providing valuable insights into the intricacies of this important industrial process.

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