Analysis of raw material
Microstructural analysis of starch granules
The results of observing starch granules using SEM in Fig 1a show that the structure of the starch granules in rice flour is polyhedral in shape. This shape is a general characteristic of starch granules in rice. According to
Zhang et al., (2023), different types of rice still have the same starch granule shape, namely polyhedral.
Meanwhile, Fig 1b shows that the structure of the starch granules in pre-gelatinized rice flour is damaged, loses its polyhedral shape, and forms an irregular matrix. Damage to the starch granule structure occurs due to the gelatinization process in the pre-gelatinized flour. When the gelatinization process of starch granules is heated in water, the starch granules swell and then burst at higher temperatures, resulting in changes in the structure of the starch granules, which provide different water absorption properties
(Gopadile et al., 2024). The irregular structure (crystalline) of starch granules that have been damaged makes the shape of the starch granules irregular, making it easier for water to penetrate the starch granules, thereby increasing the absorption capacity of the starch granules
(Jia et al., 2023).
The changes that occur in the starch granules in pre-gelatinized flour, which can be seen in Fig 1b, occur due to the damage to the starch granule structure; the amylose molecules interact with each other to form a gel and can no longer return to their original granule form
(Richardson et al., 2004). Pre-gelatinized rice flour has irregular starch granules, making it easier to hydrate. This allows rice flour to absorb water at cold temperatures. Starch granules’ high hydration level increases absorbency and viscosity (
Cornejo-Ramírez et al., 2018).
FTIR analysis
FTIR was used to analyze rice flour compounds before and after pre-gelatinization treatment, as seen in Fig 2. The same peak was detected in both samples at 3000-3500 cm
-¹ and 1500-2000 cm
-¹, indicating no significant difference in chemical composition between regular rice flour and pre-gelatinized rice flour. At 3000-3500 cm
-¹, the broad peak reflects the stretching vibration of the O-H groups, related to the presence of hydroxyl groups and hydrogen bonds in starch. This shows that even though the gelatinization process occurs, the basic chemical structure of the hydroxyl groups in the starch molecules does not change.
At waves of 1500-2000 cm
-¹, the peaks that appear are associated with the vibration of minor functional groups, such as proteins and lipids (
Kong and Yu, 2007). The peak positions and intensities in these bands were similar for both samples, indicating that pre-gelatinization treatment did not significantly change the primary or secondary structure of the protein. The similarity in these waves confirms that the pre-gelatinization process does not cause the formation of new chemical groups or significant molecular changes in rice flour. This suggests that pre-gelatinization influences physical properties, such as granule structure and hydration, more than the basic chemical structure of the flour.
Analysis of pasting properties
Table 2 show the pasting properties of rice flour. Rice flour has a peak viscosity value of 4712 cP with a peak time of 9.47 minutes. The rice flour pasting properties graph in Fig 3 shows several stages: peak viscosity, breakdown viscosity, and final viscosity. This is similar to the results obtained by
Copeland et al., (2022) on the pasting properties graph for rice flour, which has peak viscosity, breakdown viscosity and final viscosity. The pattern formed from this graph shows the process of swelling of starch granules, increasing viscosity.
On the other hand, pre-gelatinized rice flour produces a different graphic pattern, as seen in Table 2. In this graph, pre-gelatinized rice flour shows no peak or breakdown viscosity. The viscosity is already formed at cold temperatures and increases gradually without decreasing. This phenomenon can be explained by the gelatinization process, which changes the structure of starch granules. When the temperature increases, the starch granule structure breaks down, resulting in a decrease in viscosity (breakdown viscosity). However, in pre-gelatinized rice flour, the previous gelatinization process has caused permanent damage to the starch granules. This damage allows pre-gelatinized rice flour to absorb water and form viscosity at cold temperatures without experiencing viscosity breakdown because no longer intact granules can break. The damage to starch granules during the gelatinization process carried out on pre-gelatinized rice flour gives pre-gelatinized rice flour the characteristics of absorbing water and producing viscosity at cold temperatures
(Nakorn et al., 2009).
The characteristic of pre-gelatinized rice flour, which can form viscosity at cold temperatures, provides benefits in making white bread, especially in helping trap gas produced during fermentation So that it can produce better characteristics of white bread. However, a too-high viscosity results in the dough being too thick, making the dough stiff so that gas during fermentation is challenging to trap. Dough that is too stiff can reduce the volume of the bread because it loses the ability to trap gas effectively
(Waziiroh et al., 2021).
Analysis product
Specific volume analysis
Based on Fig 3, it can be seen that as the substitution of pre-gelatinized rice flour for rice flour increases, the specific volume of gluten-free white bread increases. The increase in specific volume shows that the density of gluten-free white bread decreases, which indicates that gluten-free white bread is more hollow and lighter. This decrease in density occurs because gluten-free white bread traps more air, which increases the bread’s volume. The substitution of pre-gelatinized rice flour, which can absorb cold water, contributes to the viscosity of the dough, which plays an important role in increasing the ability of the dough to trap gases produced during fermentation.
When the pre-gelatinized flour substitution exceeded 20%, a specific volume decreased, indicating an increase in the density of gluten-free white bread. This decrease is caused by the dough’s viscosity being too high so that the dough becomes too thick and less elastic, inhibiting the dough from expanding during fermentation. As a result, the dough’s ability to trap gas decreases, which makes the bread volume smaller. On the other hand, a viscosity that is too low can also cause a dense bread structure because the dough loses the ability to hold gas, which ultimately reduces the volume of the bread
(Waziiroh et al., 2021). Although the specific volume of gluten-free white bread obtained increased, this result differs significantly from the specific volume of white bread made from wheat flour. Based on research by
Pham et al., (2005) and
Iacovino et al., (2024), white bread made with wheat flour has a specific volume ranging from 2 to 4 ml/g.
Bread pore analysis
The analysis results show that the bread porosity obtained ranges from 17.65-28.05%, as seen in Fig 4. Bread porosity increases with increasing substitution of pre-gelatinized rice flour from 0% to 20%.
This decrease is caused by too much pre-gelatinized flour, causing excessive water absorption and making the dough too thick and less flexible. The number of bread pores also showed significant changes along with increasing substitution of pre-gelatinized rice flour, as seen in Fig 5, with pores ranging from 63.33-484.33. The number of pores increased at rice flour substitution levels of up to 30%. This increase can occur due to adequate viscosity for good gas-trapping power, which supports effective gas distribution and trapping during fermentation.
Proximate analysis
As seen in Table 3, pre-gelatinized rice flour can absorb water better in cold conditions because hydrogen bonds in starch molecules are broken during the gelatinization process, damaging the structure of starch granules. The gelatinization process causes the starch to swell. It causes damage to the starch granules, making their structure irregular and providing access for water to bind with the starch, thereby increasing water absorption
(Jia et al., 2023). As a result, more water molecules bind to the starch granules, making it more difficult for the water in the bread to evaporate during the baking process.
Rice, as the flour used in gluten-free white bread, contains minerals such as magnesium, zinc, iron, potassium, calcium, manganese, and copper, which can be found in rice
(Summpunn et al., 2023). Lack of mineral intake such as magnesium, zinc, copper, and iron can reduce the ability of the immune system or even interfere with controlling inflammation throughout the body
(Weyh et al., 2022).
The proteins in rice flour are divided based on their solubility: around 4-6% albumin, 6-13% globulin, 79-83% glutelin and 2-7% prolamin
(Jayaprakash et al., 2022). Even though rice flour has a relatively low protein content, rice flour is the primary source of protein content in this gluten-free white bread. Protein levels measured in various treatments of gluten-free white bread showed that there were no significant differences between treatments, as seen in Table 3, with protein content values ranging from 2.49-3.22%.
In the fat content analysis, the main source of fat content in gluten-free white bread comes from additional ingredients such as margarine and rice flour. As the main source of fat content in white bread, Margarine plays a role in the stability of the gas bubbles produced in white bread dough during fermentation (
Renzyaeva, 2013). It can be seen in Table 3 that the results of the analysis of fat content in gluten-free white bread show that there is no significant difference between various levels of pre-gelatinized rice flour substitution with fat content values ranging from 0.62-0.92%.
In the carbohydrate analysis, it can be seen in Table 3 that the results of the carbohydrate content analysis show a decrease in carbohydrate content along with increasing substitution of pre-gelatinized rice flour with carbohydrate content values ranging from 52.41-57.46%. The gelatinization that pre-gelatinized flour has undergone causes the granule structure to be damaged, which changes the shape of the starch granules to become irregular and increases their surface area, causing the absorption capacity to increase
(Liu et al., 2017) in line with the statement of
Jia et al., (2023), which states that gelatinization destroys the crystalline (ordered) structure of starch granules, turning them into irregular structures and providing more free hydroxyls that can bind with water. This means that the starch can retain more water molecules so that the water content increases as pre-gelatinization rice flour increases, as seen in Table 3.