What are A-type and B-type starches?
Starch is a polymeric carbohydrate found in most green plants, composed of amylose (linear) and amylopectin (branched) molecules. In cereals like wheat and barley, these molecules are packed into discrete granules that exhibit a bimodal size distribution, categorized as A-type and B-type. A-type granules are the larger, lenticular or disk-shaped particles, while B-type granules are the smaller, spherical or polygonal ones. Although B-granules are more numerous, A-granules account for the majority of the starch's weight.
Granule morphology and composition
The most apparent difference lies in granule size and shape. A-granules typically have diameters between 10 and 45 µm, appearing as flattened disks or lenticular shapes. B-granules, conversely, are much smaller, usually under 10 µm in diameter, and have a more spherical or polygonal shape.
Chemical composition, particularly the ratio of amylose to amylopectin, also distinguishes these starch types. While total amylose content can be similar in bulk starch, A-granules generally contain higher concentrations of amylose and lower protein content compared to B-granules. The higher amylose concentration in A-granules contributes to their higher gelatinization enthalpy.
Gelatinization and pasting properties
When starch is heated in water, it undergoes gelatinization, where granules swell and amylose leaches out, leading to increased viscosity. A and B starches respond differently to this process, which significantly impacts the texture of final food products.
- Gelatinization Temperature: B-type granules typically have higher peak and completion gelatinization temperatures than A-type granules, despite starting to gelatinize at a similar temperature.
- Viscosity: During pasting (heating and stirring), A-granules tend to exhibit higher peak, final, and setback viscosities compared to B-granules. This means A-starch forms a thicker, more stable paste, while B-starch results in a thinner paste.
- Enthalpy: The gelatinization enthalpy (ΔH), representing the energy required for the process, is higher for A-granules than for B-granules.
Digestibility and applications
The structural differences also affect how easily these starches are digested. The larger, more compact structure of A-granules makes them more resistant to enzymatic digestion than the smaller, more porous B-granules in their raw state. However, gelatinized A-type starches have a higher proportion of slowly digestible starch (SDS) than gelatinized B-type starches.
These variations are exploited in different industrial applications. For instance, the higher viscosity and gelling properties of A-starch are useful for thickening agents, while the texture differences can be leveraged for specific applications like pasta. Understanding these granular differences is critical for optimizing the quality of flour-based products.
Comparison of A Starch and B Starch
| Feature | A-Type Starch (AS) | B-Type Starch (BS) |
|---|---|---|
| Granule Size | Larger (>10 µm) | Smaller (<10 µm) |
| Granule Shape | Lenticular, flattened disks | Spherical, polygonal |
| Amylose Content | Higher amylose content | Lower amylose content |
| Protein Content | Lower protein content | Higher protein content |
| Granule Porosity | More porous internal structures (pinholes, channels) | Less porous structure |
| Gelatinization Temp | Lower peak and completion temperatures | Higher peak and completion temperatures |
| Pasting Viscosity | Higher peak and final viscosity | Lower peak and final viscosity |
| Digestibility (Raw) | Less susceptible to enzyme digestion | More susceptible to enzyme digestion |
| Gelatinization Enthalpy | Higher enthalpy (ΔH) | Lower enthalpy (ΔH) |
The structural impact on food quality
The functional properties of starch are directly influenced by the ratio and characteristics of A and B granules present. This ratio is important for determining overall wheat starch composition and behavior. For example, the higher amylose content and more compact structure of A-granules contribute to stronger gel formation and better freeze-thaw stability in certain food products. Conversely, B-granules can have different pasting characteristics that are beneficial for other applications, such as certain noodle products where a lower viscosity is desired.
For food manufacturers, controlling the ratio of A and B starch granules can be a way to manipulate the texture, viscosity, and shelf-life of their products. This is especially relevant in the production of baked goods, pastas, and sauces where starch plays a critical role as a texturizing or thickening agent. The ability to separate and utilize these different granule types opens up possibilities for creating novel food products with specific, desired functional properties. Further research continues to explore the full potential and applications of these distinct starch components.
Conclusion
The difference between A starch and B starch is not merely a matter of size but encompasses distinct variations in shape, chemical composition, and physicochemical properties. A-type granules are larger, lenticular, and contain more amylose, leading to higher viscosity and gelatinization enthalpy. B-type granules are smaller, spherical, and exhibit higher gelatinization temperatures but lower pasting viscosities. These intrinsic differences determine their behavior during processing and their impact on the final texture, stability, and digestibility of foods. Recognizing these distinctions is essential for food scientists and manufacturers to innovate and control the quality of starch-based products.
