Why Does Rice Flour Release Heat While Hydrating?
Now, let's think about some physics that matter in our food and kitchen. Have you ever hydrated roasted rice flour to make puttu? I did, and I felt heat while mixing it with my hands. Do you know the reason?
To understand the heat-retaining property of roasted rice flour, I selected two types of flour. One was raw rice (IR8)flour prepared by soaking, blending, and roasting. The other one I bought from a store (by soaking boiled rice, then blending, and roasting). The raw rice flour was waxy, and the heat released was higher than the other during hydration. There was also a difference in taste.
We can get rice flour from either white rice, brown rice, or glutinous rice. To make the flour, remove the husk of rice or paddy and obtain raw rice. Soak the raw rice and grind the rice into flour. [8]
For making puttu, we usually use roasted rice flour. Some changes occur while we heat the rice flour. All of us know that the water content of rice flour evaporates while heating. Let's look into it and understand what other factors change while roasting.
Rice is a staple food in Kerala. There are different varieties of rice. Earlier studies have shown that the presence of fat plays a significant role in retaining heat. In the case of waxy rice flour, research reports indicate a very high heat release, which reduces after defatting (1).
The following changes occurred during roasting:
- Moisture content decreases
- The heat-retention capacity increases. Waxy rice holds more heat compared to other rice flour.
- The increase in the amount of heat absorbed or released per gram of the sample during thermal decomposition.
- The highest peak temperature occurs during the thermal decomposition of particles[3].
- An increase in the initial temperature of thermal decomposition of the sample.
- The thermal stability at high temperatures.
The positive impact on the thermal stability of rice particles. The gelatinized starch molecules form a more stable structure, enabling them to withstand high temperatures more effectively and thus enhance the thermal stability of rice.[4-5]
The improved resistance of roasted rice to physical and chemical changes during high-temperature processing and storage indicates its enhanced thermal stability[6-7].
Why do we feel heat on our hands?
I think we feel the heat during the hydration of roasted flour because:
- It has a heat-retaining capacity only in a dry position.
- Gelatinized starch molecules are stable and help to retain heat
- During hydration, the stability of gelatinized starch molecules decreases, and they will not be able to withstand high temperatures.
- This decrease in stability leads to the release of excess heat, and we feel the heat while mixing it with our hands.
Starch gelatinization
It is a process of breakdown of the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water. The result is the irreversible dissolution of starch granules in water. Water acts as a plasticizer.
Process
Three main processes happen to the starch granule: granule swelling, crystallite and double-helical melting, and amylose leaching.
- Granule swelling: During heating, starch absorbs water in the amorphous space of starch, which leads to a swelling phenomenon. [9]
- Melting of double helical structures: Water then enters via amorphous regions into the tightly bound areas of double helical structures of amylopectin. At ambient temperatures, these crystalline regions do not allow water to enter. Heat causes such regions to diffuse, the amylose chains begin to dissolve to separate into an amorphous form, and the number and size of crystalline space decreases. Under the microscope in polarized light, starch loses its birefringence and extinction cross. [10]
- Amylose leaching: Penetration of water thus increases the randomness in the starch granule structure and causes swelling. Eventually, amylose molecules leach into the surrounding water, and the granule structure disintegrates.
The gelatinization temperature of starch depends upon plant type and the amount of water present, pH, types and concentration of salt, sugar, fat, and protein in the recipe, as well as starch derivatization technology used. Some types of unmodified native starches start swelling at 55 °C, and other types at 85 °C.[11] The gelatinization temperature of modified starch depends on, for example, the degree of cross-linking, acid treatment, or acetylation.
- It is the temperature at which most of the starch granules in a sample lose their birefringence.
- The gelatinization temperature of milled rice ranges from about 55–79°C
The decomposition and reduction of heat-sensitive acidic substances occur during the roasting of rice.
Chemical reactions and the production of new compounds can result from the roasting process of rice. These compounds contribute to the aroma and flavor of the roasted rice.
The change in the roasting of rice, during which the roasting process results in the formation of new compounds, such as pyrazine
Pyrazine production occurs through oxidation, reduction, dehydration, decomposition, and other reaction pathways. On the other hand, medium roasting has been identified as the optimal degree for achieving the best quality characteristics in rice roasting [7-8].
Exothermicity reduces in the following cases:
- In the presence of protein
- In the absence of lipids (i.e., defatted flour).
Heat retaining capacity increases:
- In the presence of fat and protein.
- Waxy rice holds more heat than other rice flour.
Research reports suggest that roasting treatment may be an effective way to significantly increase the RS (resistant starch) content by controlling the water content of rice before roasting. These changes may lead to the denaturation and recrystallization of starch, resulting in the formation of RS [3].
Reference:
1 Vasudeva Singh et al., J. Agricultural and Food Chemistry, American Chemical Society, 2000
Online ISSN: 1520-5118.
2 Liu B., Xu C.R., Gao M., Zhang C.J., Jia J.H., Zang N., and Dou B.X Effects of different roasting degrees on quality characteristics of compound brown rice flour. J. Food Saf. Qual. 2022;13:2144–2150. (In Chinese) [Google Scholar]
3 Ruifang Yang, et al, Starch Properties of Roasting Rice from Naturally High-Resistant Starch Rice Varieties," Molecules. 2023 Sep; 28(17): 6408.
Published 2023 Sep. doi: 10.3390/molecules28176408
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8 Wikipedia
9 Jenkins, P J, and A M. Donald. "Gelatinisation of Starch: a Combined Saxs/waxs/dsc and Sans Study." Carbohydrate Research. 308 (1998): 133-147. Print.
11 ^ Hans-Dieter Belitz, Werner Grosch, Peter Schieberle, Food chemistry, Edition 3, Springer, page: 318-323, year: 2004, ISBN 3-540-40818-5, ISBN 978-3-540-40818-5
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