Shell Separation in Coated Foods

In a previous article, we mentioned that temperature can cause shell separation in coated products. Here, we would like to add another important factor—the gelatinization and viscosity of starch.

Corn starch is widely used as a thickening agent and functional additive in food processing, with a gelatinization temperature range of 67-72°C. At this temperature, its viscosity is at its highest and relatively stable. However, 

when we prepare batter, we often mix it with other additives, such as salt, which can slightly lower the gelatinization temperature. Therefore, attention should be paid to the following points:

1. Excessive cooking time and high temperatures that exceed the starch gelatinization temperature will inevitably reduce its adhesive strength.

2. Overmixing can cause starch granules to break due to shear force, leading to a decrease in viscosity.

3. Allowing the mixture to rest for too long after mixing can lead to the parallel arrangement of starch chains, resulting in sedimentation and crystallization, which also reduces adhesive strength.

The following article provides a very professional perspective on this issue for reference.

Gelatinization of Starch and Starch Paste

Starch is a naturally occurring product of photosynthesis, existing as tiny, insoluble particles that are resistant to enzymatic hydrolysis. Direct applications of these particles are rare; instead, their gelatinization properties are utilized. When heated in the presence of water, starch granules absorb water and swell to form a viscous paste, referred to as starch paste. Understanding the gelatinization properties of starch is crucial for its application.

1. Gelatinization of Starch

Starch granules are insoluble in water but can absorb a small amount of water, causing them to slightly swell. Ordinary corn starch and potato starch have equilibrium moisture content in water of about 28% and 33%, respectively. This phenomenon of water absorption and swelling is reversible, as the original granule structure returns upon drying.

When starch is mixed with water and stirred continuously, the granules become suspended, forming a white suspension known as starch milk. Heating the starch milk causes the granules to absorb more water and swell further. At a certain temperature, the original starch structure is destroyed, and the granules swell into a viscous gel. This phenomenon is called gelatinization, and the temperature at which it occurs is termed the gelatinization temperature, resulting in the formation of starch paste.

The gelatinization temperature varies among different starch varieties and even within granules of the same starch type. Larger granules gelatinize more readily at lower temperatures, while smaller granules are more resistant and require higher temperatures. The size of starch granules can vary significantly (2–150 μm). When heated, larger granules gelatinize first, followed by smaller granules. Gelatinization temperatures are not fixed values but ranges, typically differing by about 10°C. For example, corn starch has a gelatinization temperature of 62–72°C, while potato starch is between 56–68°C.

The gelatinization process is an endothermic reaction where heat breaks the hydrogen bonds between starch molecules, allowing the granules to swell and absorb water, destroying the crystalline structure and causing the disappearance of the polarizing cross. A common method to determine gelatinization temperature utilizes this property: the disappearance temperature of the polarizing cross indicates the onset of gelatinization. This method involves using a polarizing microscope and an electric heating stage, making it simple to operate with reliable results. A small sample of starch (approximately 0.1% to 0.2% concentration) is mixed with water and placed on a slide, surrounded by glycerol or mineral oil, and heated at a rate of about 2°C/min. As observed under the polarizing microscope, the disappearance of the polarizing cross indicates the onset of gelatinization, while the temperature at which about 98% of the granules gelatinize indicates the completion of the process. Smaller granules may not fully gelatinize and can be disregarded. Based on the number of gelatinized granules, it can be estimated that approximately 50% of the granules gelatinize at temperatures of 62–67–72°C for corn starch and 56–63–68°C for potato starch and 52–57–64°C for cassava starch.

Several compounds affect the ease of starch gelatinization. Some compounds promote gelatinization and lower the temperature, such as sodium hydroxide, urea, dimethyl sulfoxide, salicylates, thiocyanates, and iodides. Sodium hydroxide has a strong effect and can cause starch to gelatinize at room temperature. Other compounds, like sodium sulfate, sodium chloride, and sodium carbonate, can hinder gelatinization, including sucrose. 

2. Starch Paste

The properties of starch paste differ among various starch types, and no two starch pastes are identical.

Viscosity is the most important property of starch paste. The viscosity is commonly determined using viscosity curves, with the Brabender viscometer being a standard instrument. A starch sample is placed in a cup, and a needle-type stirrer is immersed. The mixture is heated at a rate of 5°C/min to 95°C and held for 1 hour, then cooled to 50°C at 1.5°C/min, maintaining that temperature for another hour. The instrument automatically plots the viscosity curve, measured in Brabender units (Bu). 

Potato starch gelatinizes most easily at lower temperatures, rapidly increasing in viscosity but exhibiting poor stability as it quickly decreases. Cassava starch is more difficult to gelatinize, requiring higher temperatures for an increase in viscosity, which is lower than that of potato starch but has better stability. Corn and wheat starches gelatinize with difficulty and only at higher temperatures, showing minimal viscosity increase but rising viscosity upon cooling. Different starches exhibit varying gelatinization properties.

As temperature increases, starch granules absorb water, swell, and increase in viscosity. The largest granules swell the most, but their structural integrity is weaker, making them susceptible to shear forces from stirring, leading to a significant decrease in viscosity. Upon cooling, the starch chains tend to associate, resulting in an increase in viscosity.

Starch paste exhibits adhesive and elastic properties, indicated by the length of the paste strand. For example, inserting a wooden stick into potato starch paste and pulling it out results in a long strand that is difficult to break, indicating strong adhesive and elastic properties. In contrast, using corn starch results in a shorter, easily breakable strand, indicating weaker properties. Potato starch is referred to as long-strand paste, while corn starch paste is referred to as short-strand paste.

As the hot starch paste cools, viscosity increases, forming a semi-solid gel. Corn and wheat starch gels have higher gel strength, while potato and cassava starch gels are weaker. Glutinous corn starch gels are difficult to form. Gels have varying degrees of transparency, with potato starch gels being highly transparent and corn starch gels being opaque.

During the cooling and gelation of starch paste, the molecular chains tend to align parallel to each other, forming a crystalline structure that is insoluble in water. As the gel reaches a certain point, the colloidal structure may break down, resulting in a white precipitate and water separation, a phenomenon known as retrogradation. Corn starches exhibit strong retrogradation and weak gel stability, affecting their applications, while potato and other tuber starches exhibit weaker retrogradation. Glutinous corn starch lacks amylose, resulting in very weak retrogradation. Various studies have measured retrogradation in different starch pastes.

Research separating amylose from wheat, corn, and potato starches has shown that wheat and corn starch exhibit the strongest retrogradation, while potato starch is weaker. Retrogradation primarily results from the association of amylose molecules, while amylopectin does not exhibit this behavior due to its branched structure, which inhibits association among amylose molecules. However, at higher concentrations or lower temperatures, amylopectin molecules may also slightly associate, but to a much lesser extent. The degree of polymerization of amylose molecules involved in retrogradation is typically between 100 and 200. Corn starch contains approximately 27% amylose, with a degree of polymerization between 200 and 1200, showing strong retrogradation, while it also contains 0.6% lipid compounds that promote retrogradation. Potato starch contains 20% amylose, with a degree of polymerization around 1000–6000, resulting in weaker retrogradation. Increased concentration accelerates retrogradation, while lower temperatures approaching 0°C also speed up retrogradation. Retrogradation occurs most rapidly at pH 7, while it is slower above pH 10 and below pH 2.

Starch exists as granules; however, in both food and non-food applications, it is primarily used in its gelatinized form. The properties of starch paste are critical for its application. Previously, starch varieties with desired properties were selected, but with the advancement of industrial science and technology, some starch varieties no longer meet the requirements of new processes. Consequently, modified starches have been developed for both food and non-food applications, yielding excellent results. Currently, modified starch technology has reached a high level, capable of producing new products with virtually any required properties, advancing rapidly.