The liquefaction process refers to a procedure where, under high-temperature conditions, the structure of starch is destroyed and gradually decomposed and dissolved under the action of enzymes. During this process, starch transitions from an insoluble, rigid, and stable crystalline structure to a soluble, dispersed, and amorphous liquid state.

For enzymes to hydrolyze starch, they first need to break down the rigid granular structure of starch to access the interior of the starch structure. However, sometimes the liquefaction process may proceed in reverse—i.e., starch reverts from a soluble state to an insoluble aggregated or crystalline state. This process is called starch retrogradation, meaning starch undergoes aging.

Starch retrogradation is an indication of low liquefaction efficiency. Retrograded (aged) starch is insoluble and difficult to be hydrolyzed by glucoamylase, which directly affects the yield of starch sugar and leads to a decline in production efficiency.

For liquefied starch with a DE value (Dextrose Equivalent) greater than 10%–12%, the iodine test reaction of the resulting dextrin is negative, indicating the absence of aged starch. Conversely, a positive reaction of liquefied starch in the iodine test signifies the presence of aged starch.

In industrial production, the iodine test is a fast, simple, and intuitive method for checking liquefaction results, and it is currently widely adopted by factories.

Starch retrogradation is a complex process influenced by multiple factors, such as starch concentration, reaction temperature, and reaction time.

The retrogradation of starch in the liquefied solution significantly impacts starch conversion rate and filtration speed. The prevention of aged starch formation can be controlled and prevented from several aspects:

1. Starch Concentration

Starch concentration is directly related to liquefaction efficiency. A low starch concentration results in low viscosity of the liquefied starch, good heat transfer performance, high water activity, and easy contact between enzymes and starch—making liquefaction easy to control and naturally reducing the amount of aged starch. In contrast, a high starch concentration leads to increased viscosity, decreased water activity, reduced heat transfer efficiency, and difficulty in the contact and catalytic hydrolysis of starch by enzymes, often resulting in the formation of insoluble starch.

From an enterprise's economic perspective, factories tend to increase starch concentration for energy conservation and production volume enhancement. However, this must be controlled within a certain range and based on a specific standard—i.e., checking for the obvious presence of aged starch.

2. DE Value

The DE value of the liquefied solution reflects the degree of starch hydrolysis by enzymes, i.e., the length of the hydrolyzed dextrin molecular chains.

A low DE value of liquefaction indicates a high degree of starch polymerization, and the iodine test reaction usually shows a blue or purple color, signifying the presence of insoluble starch. 

Typically, the iodine test reaction of liquefaction only turns negative when the DE value is greater than 10%–12%. Therefore, the DE value of liquefaction should not be controlled at an excessively low level.

3. Reaction Temperature​

High temperatures can break down the crystalline structure of starch. Thus, thoroughly disrupting the crystalline structure in starch granules through heating is crucial for eliminating starch retrogradation.

As the temperature gradually increases, the crystalline structure of starch is progressively destroyed. However, even when the temperature reaches 95°C, it cannot guarantee the complete dissolution of all starch granules.

This is because some starch molecules are bound to liposomes, forming a highly stable structure that requires maintenance at a high temperature of approximately 110°C for 5–8 minutes to be broken down. Even under the action of enzymes, a temperature of around 105°C is still necessary.

Therefore, maintaining a high temperature is essential to prevent the formation of aged starch.

4. Liquefaction Reaction Time

A prolonged liquefaction time or slow heating/cooling rates during liquefaction will accelerate the retrogradation of the liquefied solution. Hence, the liquefaction time should not be too short or excessively long; it is generally appropriate to control it within 90–120 minutes. 

The liquefaction equipment should be equipped with a good thermal insulation layer to ensure excellent heat insulation performance and prevent temperature drop due to rapid heat dissipation during the liquefaction process. 

In addition, the heating rate at the start of liquefaction and the cooling rate after the end of liquefaction should be fast, and injectors and heat exchangers with good performance should be selected.