A Brief Introduction to Starch: "Knowledge" and "Action" on the Tip of the Tongue

Introduction

When you hear the word "starch," what's the first thing that comes to mind? Is it a wholesome bowl of whole grains? Or perhaps it's those calorie-packed candied chestnuts? Maybe it's freshly baked cakes and pastries still warm from the oven? Or do you think of the soft and chewy texture of tapioca pearls? I'll come clean—right at this moment, all I crave is a steaming bowl of spicy beef noodles to satisfy my hunger after several hours of work.

Starch is a polysaccharide composed of glucose molecules and belongs to the category of high-molecular-weight carbohydrates. Consuming excessive amounts of starch can lead to obesity and digestive problems, including bloating. Long-term consumption of a high-carbohydrate diet may even increase the risk of developing conditions such as diabetes and cardiovascular diseases [1]. As a food lover who wants to satisfy both taste buds and maintain a healthy figure, let's embark on a journey to find a win-win solution.

First, let's understand the structure of starch before tailoring our approach to address the issue.

Structure of Starch

Starch consists of four hierarchical structures: granules, shells (growth rings), blocklets, and molecules. What we see with our naked eye is starch in the form of granules (a, 1-100μm). The shape and size of starch granules can vary depending on the plant source, and they can be oval, spherical, lens-shaped, polygonal, hollow, or irregular.

Imagine the entire starch granule as an onion. The central point of the onion represents the amorphous core region, which is also called the hilum and is proportional in size to the amylose content in straight-chain starch. Moving outward from the core, we encounter layers of alternating amorphous shells and semi-crystalline shells (b), also known as growth rings (120-500nm).

(Structure of Starch at Different Hierarchical Levels [2]: (a) Starch Granules; (b) Amorphous and Semicrystalline Growth Rings; (c) Thin Layers of Amorphous and Crystalline Regions; (d) Blocklets; (e) Amylopectin Double Helical Structure; (f) Nanocrystals; (g) Amylopectin Molecular Structure; (h) Amylose Molecular Structure)

Now, let's take a closer look at the two different "onion layers" that compose starch. The secondary structures that make up these layers are called blocklets (20-50nm), and there are two types of blocklets. One type of blocklet is small (about 20nm), has defects in its structure, and consists solely of amylose. These blocklets form the amorphous shell. The other type of blocklet (d) is a spherical semi-crystalline structure, containing both amorphous and crystalline layers (c). It is composed of both amylose and amylopectin, forming the semi-crystalline shell.

The molecules that make up blocklets represent the smallest structural units of starch. Among them, amylose (g) forms the amorphous layers by intertwining in a left-handed helical manner. Amylopectin forms a double helical structure (e) by intertwining in a double helical manner, with gaps created at the ends of branched chains being randomly filled with amylose to form the crystalline layers. Additionally, lipids and polyphenolic substances also contribute to the composition of blocklets.

(The author took a polarized light microscopy image of cassava starch from home, and in the image, you can observe a cross-shaped dark region with the navel point of the starch granule at its center, known as the birefringent cross phenomenon. This is caused by the differences in the density and arrangement of starch molecules in the two layers, resulting in anisotropy.)

Different Types of Starch

Starch is composed of both amylose and amylopectin, but when there are differences in the length and arrangement of the double-helical structures of amylopectin, it leads to the formation of A/B-type crystalline structures, resulting in three different types of starch: A-type, B-type, and C-type starch. Typically, A-type starch contains only A-type crystals, B-type starch contains only B-type crystals, while C-type starch contains both A and B-type crystals simultaneously. From a microscopic perspective, the differences in amylopectin branching may directly result in structural differences in blocklets, leading to distinct crystalline structures in each type of starch. These structural differences give rise to distinct X-ray diffraction patterns, which can be used for differentiation, as shown in the following image. There is also a special V-type starch, which is formed by a mixture of amylose and lipids, among other substances.

(For example, wheat starch exhibits four prominent diffraction peaks at approximately 15°, 17°, 18°, and 23° (2θ angle); potato starch displays four strong diffraction peaks at around 6°, 17.5°, 22.5°, and 24°, along with a broader peak at around 15°; pea starch shows diffraction peaks similar to potato starch up to 13°, and beyond 13°, it exhibits diffraction peaks resembling wheat starch.)

• A-Type Starch

A-type starch is commonly found in cereal crops such as corn, wheat, and rice. It predominantly consists of short amylopectin chains (DP < 20) and tends to form a single A-type crystalline structure. A-type starch granules are characterized by the presence of pores or channels that extend from the surface to the starch hilum (navel point). These starches belong to the monoclinic crystal system, have low water content within the crystal structure, and involve water molecules in the formation of starch double-helical structures. As a result, A-type starches have a compact and stable structure.

Due to the aggregation of amylopectin branch points in both the crystalline and amorphous regions, as well as the presence of channels extending from the crystalline region to the starch hilum (navel point) on the granule surface, A-type starches are more susceptible to enzymatic attack, leading to an inside-out digestion pattern for starch. Additionally, the shorter branches in A-type starches accelerate the rate of enzymatic digestion of the starch. However, the presence of branch points within the crystalline region also provides protection against acid hydrolysis.

• B-Type Starch

B-type starch is typically found in tubers and high-amylose cereal seeds, such as potatoes. It primarily consists of long amylose chains (degree of polymerization > 23) and tends to form a single B-type crystalline structure. The granules of B-type starch are smooth on the surface and lack pores or channels. B-type starch belongs to the hexagonal crystal system and can accommodate a higher content of water molecules within its crystal structure. Some of these water molecules participate in building the starch double-helical structure, while others are distributed within the hexagonal unit cells of the starch chains. As a result, B-type starch is more susceptible to hydrolysis and external influences such as heat treatment.

Ever wondered why potatoes are easier to cook than rice?

Potatoes are easier to cook because the amylopectin, which forms the branched structure in their starch, has all its branching points concentrated in the amorphous region and has a smooth surface. This makes it less prone to enzymatic digestion but more susceptible to acid hydrolysis. Additionally, potatoes contain more long-chain branched amylopectin, making them more resistant to digestion compared to A-type starch.

• C-Type Starch

C-type starch is typically found in foods like sweet bananas, legumes, and certain root vegetables. It predominantly consists of medium to long branched amylopectin chains and can exhibit both A-type and B-type crystalline structures. Its hydrolysis and digestive properties typically fall somewhere between those of A-type and B-type starches. The crystalline structure of C-type starch is more complex because it can include both A-type and B-type arrangements, depending on the plant source.

Currently, it is believed that C-type starch exists in at least four crystalline models:

• A/B-type crystals coexist inside and outside of the same starch granule. For example, in yam starch hydrolysis under acidic conditions or pea starch, internal dissolution is observed, and X-ray powder diffraction (XRPD) shows A-type crystal diffraction peaks.

• In some starch granules, B-type crystals are mainly distributed at the distal hilum, while the proximal hilum region mainly contains A-type crystals. This can be observed in the gelatinization process of lotus root starch, where gelatinization starts at the distal hilum and gradually moves toward the center.
  

• A/B-type crystals are found in different starch granules.
  

• A/B-type crystals can coexist within the same starch granule or in different starch granules. For example, sweet potato starch can contain all three types (A/B/C), but it is categorized as C-type starch.
  

• V-Type Starch

V-type starch structures contain hydrophobic guest molecules such as lipids or polyphenols, forming complexes with the helical structures of amylose. These structures are rarely found in natural starch, so they are not the focus of my investigation and will not be elaborated on further in this article.

How to Choose Starchy Foods

Now that we've discussed the classification of starch, let's talk about how to enjoy starchy foods without overburdening your body. First, when starch is consumed, it is broken down into glucose by amylase and absorbed for utilization. The rate at which this process occurs can affect blood glucose levels. Rapid spikes in blood sugar levels over time can increase the body's burden, leading to issues like fat accumulation, increased blood viscosity, and decreased sensitivity to insulin. Therefore, choosing the right starchy foods is crucial. After looking at the chart below, you'll have a better understanding:

Investigating further, I came across an unexpected breakthrough: leftover rice, steamed buns, and other starchy foods that have cooled down can actually help "reduce calorie intake" to some extent. Let's dive into the analysis in the following sections.

Starch Gelatinization and Retrogradation

Have you ever noticed how rice grains go from hard to soft and from small to large when you cook rice? This phenomenon is caused by starch gelatinization. Starch granules absorb water and swell in hot water, leading to structural breakdown (IIa), exposing the straight-chain starch in the amorphous region and the hilum, resulting in a starchy texture, and gradually eliminating the crystalline structure (IIb). The crystalline structure gives rice grains a certain hardness, which disappears once the crystal structure is completely disrupted.

Now, when we take out leftover rice from the previous night, have you noticed that the rice grains become harder again? This happens because the gelatinized starch undergoes retrogradation as it cools down. During the cooling process to room temperature, the straight-chain starch exposed during gelatinization re-crystallizes through intermolecular hydrogen bonding, forming a network of double helices (IIIa), giving rice grains a certain hardness once more.

Furthermore, long-term storage of raw starch can also lead to retrogradation. This occurs because the short chains on the outer layers of branched starch form double helices and stack up in an ordered crystalline structure, causing starch to retrograde (IIIb). However, the retrogradation of cooked starch can only be attributed to straight-chain starch.

The image shows the structural changes that occur in starch granules from gelatinization to retrogradation [10].

06 The Connection Between Starch Retrogradation and Absorption

During the process of starch retrogradation, resistant starch is formed, which is known as RS3 starch (as shown in the diagram below).

In a sense, resistant starch can be considered a "hero" in controlling calorie intake. It behaves in the human gastrointestinal tract somewhat like enoki mushrooms, passing through the digestive system in a way that typically prevents it from being hydrolyzed by starch enzymes or absorbed in the small intestine. Instead, it undergoes fermentation by gut microbiota in the large intestine, producing short-chain fatty acids. Resistant starch has physiological functions such as regulating gut microbiota, promoting gastrointestinal motility, and preventing inflammation. This can help reduce the potential risk of conditions like obesity and diabetes, aligning with the dietary preferences of those concerned with their health and appearance.

Will all types of starchy foods produce resistant starch when cooled?

(So, relatively reducing calories is achieved with cold rice, not with reheated egg fried rice or with rice cakes stored in the refrigerator...)

Can all foods containing straight-chain starch undergo retrogradation?

No, not all of them. This is because the moisture content of starchy foods is also a crucial factor influencing the formation of resistant starch. Take white rice porridge as an example; it typically has a moisture content of over 85%. Imagine each straight-chain starch molecule surrounded by a significant amount of water molecules. How can they come close to each other to form a double helix structure and subsequently undergo retrogradation into resistant starch? Therefore, when you put white rice porridge into the fridge, not only does it not reduce in calories, but it also provides a breeding ground for bacteria in the refrigerator.

Furthermore, the recommendation from nutritionists to consume whole grains is well-founded. Refined grains, after processing that removes bran, germ, and most of the essential nutrients, also disrupt the protective layers outside starch, making them more easily digestible. If they are further ground into flour, such as the commercially available "healthy" multi-grain flours, they may become less beneficial (although they can still be used for milk tea, of course).

Tips for healthy diets

In summary, the general rule is to eat larger, coarser, and drier foods, but this should be accompanied by a healthy digestive system and moderation. Don't pick sesame and lose watermelon. For example, incorporate coarse grains (with larger granules) like potatoes and whole grains into your diet for better nutrition and health. Reduce consumption of porridge (as it can cause rapid blood sugar spikes and may not be conducive to long-term health). Wait a bit for freshly cooked rice to cool before consuming, and so on.

Finally, if anyone wants to discuss health and weight loss experiences with me, feel free to leave a comment! In the future, we intend to compile the latest research developments in weight loss supplements to share with everyone, so stay tuned!