Physicochemical Properties of Starch

One of my research interests is the behavior of corn starch gels. Corn starch has interesting properties as shown by these couple of videos:

Previously, I discussed the general chemistry of starch and corn starch is similarly made up of amylose and amylopectin arranged in starch granules. Corn starch undergoes gelation, gelatinization and retrogradation on heating, cooling and storage. As corn starch is also widely used by the food industry especially as a gelling agent, thickener or bulking agent its physicochemical behavior is important. One of my undergraduate research projects has been to study the texture and rheology of corn starch gels. So I was excited to come across this article [1].

According to The Dictionary of Food Ingredients [2], cornstarch is:

The starch made from the endosperm of corn [[3]], containing amylose and amylopectin starch molecules. When starch is heated in water it forms a viscous, opaque paste. The paste forms semisolid gels upon cooling and has the ability to form strong adhesive films when spread and dried. Cornstarch is not freeze-thaw stable and is used widely except when clarity or the lack of gel formation is required. It exists as fine or course powders. The coarse starch is sometimes termed pearl starch. It is used in sauces, puddings. pie fillings. and salad dressings. the typical usage level is 1 to 5 percent. It is also termed common, regular or unmodified cornstarch.

For any of cooks reading – you often use cornstarch [3] to make a gravy.

The objective of this research paper was:

to characterize the corn varieties grown in India on the basis of the physicochemical, thermal, pasting and gel textural properties of their starch. This will be useful in selecting the appropriate variety for end use suitability.

Thus, this article was a characterization of different corn starches. The researchers measured many properties including amylose content, swelling power, solubility, water binding capacity, thermal properties – to obtain gelatinization temperature, pasting properties, and texture. All these properties of starch can be used to determine what is the best use of the particular starch.

I was interested in the techniques they used as that is directly applicable to the research in my lab. I was especially interested in their texture measurements. Once they had a gelled starch:

…the gel was compressed at a speed of 0.5 mm/s to a distance of 10 mm with a cylindrical plunger (diameter = 5 mm). The compression was repeated twice to generate a force–time curve from which hardness (height of first peak) and springiness (ratio between recovered height after the first compression and the original gel height) was determined. The negative area of the curve during retraction of the probe was termed adhesiveness. Cohesiveness was calculated as the ratio between the area under the second peak and the area under the first peak. Gumminess was determined by multiplying hardness and cohesiveness. Chewiness was derived from gumminess and springiness and was obtained by multiplying these two.

This type of analysis is a texture profile analysis (TPA).

Some definitions:
Hardness:
• Force required to deform the product to given distance, i.e., force to compress between molars, bite through with incisors, compress between tongue and palate.
• Determined as maximum force required to compress gel 10 mm in first compression
Adhesiveness:
• Force required to remove the material that adheres to a specific surface (e.g., lips, palate, teeth).
• Determined from the negative area after the probe was removed after the first compression
Cohesiveness:
• Degree to which the sample deforms before rupturing when biting with molars.
• Determined as the ratio of the areas under the second to first compressions
Springiness:
• The resilience rate at which the sample returns to the original shape after partial compression.
• Determined as the ratio of the max force for the second compression to hardness.
Gumminess:
• Energy required to disintegrate a semi-solid food to a state ready for swallowing.
• Determined as hardness x cohesiveness
Chewiness:
• Number of chews (at 1 chew/sec) needed to masticate the sample to a consistency suitable for swallowing.
• Determined as gumminess multiplied by springiness.

Until I read this article I had only been measuring hardness. Now I could tweak my texture analyzer method and I would be able to put numbers on other texture properties of the starch.

A typical texture graph looks like this:

I carried out a series of experiments with different starch gels. Comparing the controls (5% starch solutions heated in a microwave to >85 ^oC showed that the methods are consistent.

The researchers found that the different corn varieties produced starches with different physicochemical properties. In particular, they showed that the African Tall had a lower amylose content and also had the weakest (hardness was lowest) gel.

I am more interested in the effects of adding food additives on gel strength (hardness) and other properties. This is shown clearly in the next picture where the control gel is compared to one with added ingredients (I’m not say what is in my experimental gel yet (other than starch) as I still need to formally write this up for publication):

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[1] Link will only work if you have access to Food Chemistry.

Reference:
Sandhu & Singh (2007) Some properties of corn starches II: Physicochemical, gelatinization, retrogradation, pasting and gel textural properties. Food Chemistry; 101, (4), 2007, Pages 1499-1507

[2] This is a great book for anyone interested in what and why are those ingredients in the food label. I have the third edition, but the fourth is available from Amazon

Reference
Igoe & Hui (1999) Dictionary of Food Ingredients

[3] Another of those England and America being separated by having the same language moments. Corn is England is maize. However, corn starch is corn flour. You know, just to confuse.