Starch is an important component in many foods and industrial products as a thickener, stabilizer, and water retention agent. However, native starch has some limitations that make it unsuitable for certain applications. Starch modification can improve its functional properties for specific uses, including enhancing freeze-thaw stability in frozen foods. The most common methods for achieving this involve genetic, physical, and chemical modifications.
Genetically modified starch comes from plants that have been modified to produce new fatty acids or carbohydrates that are not found in the original plant. To improve the freeze-thaw stability of starch, the amylose content can be reduced or the chain length of the amylopectin can be decreased. Genetic engineering can create a waxy starch with short-chain amylopectin by downregulating certain starch synthase genes. Large-scale field trials of waxy mutants have been conducted on crops like maize, barley, and potato, which showed greater freeze-thaw stability and less changes in dough stickiness and extensibility during frozen storage.
Starch can be changed physically using moisture, heat, shear, or radiation, without the need for chemicals. This is becoming more popular in food products and can be done in different ways like alcoholic-alkaline, micronization, and drum-drying. Pregelatinized starch is a common type of physical modification and is used in various food products, like frozen food, to increase their shelf life.
Pregelatinized starches are precooked and dried to form a stable suspension in cold water, and are widely used in the textile, food, and foundry industries. They have unique properties, such as easy dispersal and water absorption, and can form a gel at room temperature that protects molecules and microorganisms like yeast. Pregelatinized starch can help decrease the content of free water, which may enhance cell viability and reduce firmness in baked goods. It can also be a source of fermentable sugars for yeast, increasing proofing speed, and can enhance the quality of frozen noodles.
Chemical modification changes the properties of starch by adding a functional group to the molecule. This alters its ability to form a gel, its composition, and how it behaves when cooked and cooled. Different factors affect how the starch is modified, such as the source of the starch and the type of chemical used. Common chemical modifications include acetylation, cationization, acid hydrolysis, oxidation, and cross-linking. Adding groups like acetyl, phosphate, and hydroxypropyl can improve the stability of the starch during freezing and thawing.
Acetylation is a common chemical method of modifying starch. It involves esterifying native starch with acetic anhydride or vinyl acetate in the presence of an alkaline catalyst. Acetylation increases solubility, swelling power, and viscosity, but decreases the gelatinization temperature of starch. Studies have shown that acetylation and cross-linking can alleviate retrogradation of normal rice starch and improve its freeze-thaw stability. Cross-linking starch involves chemically linking a small number of starch polymer chains using a bi- or polyfunctional reagent. Dual modification using both cross-linking and phosphorylation has been proposed as a method to improve freeze-thaw stability.
Adding hydroxypropyl groups to starch through a chemical reaction with propylene oxide is an effective method for enhancing the quality of starch paste. This modification improves the shelf life, freeze-thaw stability, cold-storage stability, and clarity of the paste. The hydroxypropyl groups weaken the internal bond structure of starch granules, which prevents water from separating during freeze-thaw cycles and reduces the likelihood of syneresis.
To measure freeze stability, researchers measure the amount of liquid in starch gel after it’s centrifuged. Many studies have used this method to test the freeze stability of different starches, such as hydroxypropylated potato starch, waxy wheat/barley starch, pea starch, and maize/tapioca starch. They found that these starches showed reduced syneresis during freeze-thaw cycles, indicating improved freeze stability. In one study, researchers estimated the energy needed to break down recrystallized starch molecules using differential scanning calorimetry (DSC) after 10 freeze-thaw cycles, and found that hydroxypropyl distarch phosphate completely inhibited the recrystallization of starches.
Researchers conducted a study on the freeze stability of hydroxypropyl potato starch pastes using a rheological method. They found that changes in complex modulus and phase angle were related to syneresis, as confirmed by centrifugation and DSC measurements. The study showed that the destabilization process can be predicted using this method. Further research found that increasing molar substitution and cross-linking delayed destabilization and syneresis. Cooking conditions and concentration also affect freeze stability, with increased pasting extent and starch concentration enhancing stability. The amount of intermingled amylose and amylopectin was found to be the main controlling factor in rheological response and syneresis.