Effect of soluble amylose separation on the faster cooking time of sago analog rice

Abstract

This study aims to observe the effect of soluble amylose separation on the cooking time of analog sago rice. This rice is made from 50% sago starch, 5% sago fiber, 15% sago starch that has been partially pregelatinized, and each is mixed with 30% white rice flour, 30% red rice flour, and 30% black rice flour. Each mixture is extruded using a twin-screw extruder at a moisture content of 30%, barrel temperatures of 60, 70, 90, and 90 °C (zones 1–4), and screw speeds of 20–50 rpm. A post-installation washing step is carried out to separate surface amylose (in the form of mucus), followed by drying. Observations of the morphology show that the total porosity of the rice after separation is greater than before amylose separation, resulting in a shorter cooking time. Smaller crystallinity results in a shorter cooking time. A greater onset of gelatinization results in a shorter cooking time. Antioxidant analysis shows that washing does not remove antioxidants. Analog sago rice without amylose separation showed a cooking time of 14–16 min, while analog sago rice with separation treatment showed a cooking time of 5–9 min. This amylose-washed analog sago rice is intended for applications requiring a faster cooking process, such as emergency relief operations or fast-paced urban lifestyles.

Introduction

The increasing demand for convenient and functional foods has driven the development of starch-based products with shorter rehydration and cooking times without compromising nutritional quality and texture. The shift in consumption patterns toward instant products demands innovative approaches to starch structural engineering, particularly for granular staple foods. In this context, starch systems with rapid rehydration capabilities are a key focus because these characteristics are strongly influenced by amylose distribution, gelatinization level, surface morphology, and changes in crystalline structure.

Sago (Metroxylon sagu) is a tropical starch source with great potential for alternative food development. Sago starch has a high carbohydrate content (up to 88.67%)¹, a relatively low glycemic index of 28², and is gluten-free, making it relevant for functional food formulations. In addition, sago is known as a plant that is tolerant to marginal land conditions and climate change³, making it a strategic resource in sustainable food diversification. However, the natural characteristics of sago starch, which tend to have a stable crystalline structure and a certain swelling capacity, can limit the rehydration rate in solid grain-based products.

The development of analog rice as a substitute for conventional rice has been widely reported⁴,⁵. Various studies have shown that extrusion techniques and formulation engineering are capable of producing grains with textural properties similar to rice⁶. However, most of the reported analog rice still requires a relatively long cooking time, similar to native rice⁷. Literature specifically exploring the mechanisms of accelerated cooking time through starch structural engineering—including modification of the degree of gelatinization, amylose redistribution, and integration of fiber and bioactive components in the sago starch matrix—is still limited. In addition, the relationship between changes in surface morphology, starch crystallinity, and functional properties such as antioxidant capacity and resistant starch formation in instant analog rice systems has not been comprehensively studied.

The study of developing instant sago analog rice has both scientific and practical urgency. Conceptually, this system offers an interesting model for understanding how starch structural engineering can modify the hydration and gelatinization kinetics of granular products⁸. The integration of pigmented rice flour (red and black rice) as a source of phenolic compounds also opens up opportunities for increasing functional value through the contribution of antioxidant activity and other bioactive components⁹.

This study contributes to the literature by (i) expanding the understanding of the structure-function relationship in instant granular sago starch systems, (ii) evaluating the implications of changes in crystallinity and morphology on cooking time and texture, and (iii) integrating nutritional and bioactivity aspects in the development of alternative staple foods based on local resources. This approach is expected to strengthen Sago’s position as a strategic raw material in sustainable instant food innovation.

Results

Proximate analysis

Proximate analysis was conducted to determine moisture, ash, fat, starch, protein, and crude fiber. The results of the proximate analysis of sago analog rice that has been separated with soluble amylose mixed with white rice are called instant sago analog rice mixed with white rice (ISARWR), instant sago analog rice mixed with red rice (ISARRR), and instant sago analog rice mixed with black rice (ISARBR) are presented in Table 1. The starch content of ISARBR (64.06 ± 0.01^c % w/w) was significantly higher than that of ISARRR (62.78 ± 0.01^b % w/w) and ISARWR (61.62 ± 0.01^a % w/w). The lipid content of ISARWR (0.10 ± 0.006^a % w/w), ISARRR (0.11 ± 0.002^a % w/w) and ISARBR (0.10 ± 0.001^a % w/w) insignificantly different. The low lipid content is advantageous in preventing rancidity and sourness, thereby contributing to a longer shelf life of the product¹⁰. Ash content reflects the total mineral composition of a food material. Ash is the inorganic residue that remains after the combustion of a sample at high temperatures. The ash content of ISARBR (0.62 ± 0.01^a % w/w) lowest, significantly different, compared to that of samples mixed with ISARRR (0.63 ± 0.01^b % w/w) and ISARWR (0.51 ± 0.01^b % w/w). The protein content of the ISARRR (2.80 ± 0.10^c % w/w) was significantly higher compared to the ISARWR (2.09 ± 0.10^a % w/w) and ISARBR (2.39 ± 0.10^b % w/w. The addition of red, white, and black rice flours contributed to increased protein levels. Previous studies have reported that the protein content of native sago starch is approximately 0.2%¹¹. Extrusion processing has been shown to alter the interactions responsible for maintaining protein conformation, although the primary structure, bound by peptide bonds, remains intact¹². Analog rice products with higher protein content have been reported to exhibit lower glycemic activity, as protein can inhibit glucose absorption in the small intestine¹¹. The crude fiber content of ISARRR (1.22 ± 0.01^c % w/w) was significantly higher than that of ISARWR (0.22 ± 0.01^b % w/w and ISARBR (0.76 ± 0.01^a % w/w. This difference is attributed to the addition of 5% sago fiber. Dietary fiber is closely associated with the glycemic index (GI) of foods and contributes to lowering the GI value¹³. In its intact form, fiber can act as a physical barrier, hindering the transport of food and enzyme activity, thereby modulating the digestion process.

Table 1.

Proximate of ISARWR, ISARRR, and ISARBR

Instant Rice	Moisture (% w/w)	Ash (%w/w)	Starch (%w/w)	Lipid (%w/w)	Protein (% w/w)	Crude fiber (% w/w)
ISARWR	7.60 ± 0.10a	0.51 ± 0.01b	61.62 ± 0.01a	0.10 ± 0.006a	2.09 ± 0.10a	0.22 ± 0.01b
ISARRR	6.38 ± 0.10b	0.63 ± 0.01b	62.78 ± 0.01b	0.11 ± 0.002a	2.80 ± 0.10c	1.22 ± 0.01c
ISARBR	6.37 ± 0.01b	0.62 ± 0.01a	64.06 ± 0.01c	0.10 ± 0.001a	2.39 ± 0.10b	0.76 ± 0.01a

Results are means ± standard deviations of triplicate analysis. Values with the different letters in the same column were significantly different (p ˂ 0.05).

Cooking time

One of the most important parameters of analog rice is cooking time, which indicates the duration required to cook the rice until fully gelatinized and edible¹⁴. The cooking time results are presented in Table 2. The cooking time of ISARWR was the shortest at 5 ± 1.09^b min, followed by ISARRR at 7 ± 1.45^bc min and ISARBR at 9 ± 1.99^c min, all under 10 min. In contrast, sago analog rice mixed white rice (SARWR) required longer cooking times 14 ± 0.69^a min, sago analog rice mixed red rice (SARRR) at 15 ± 1.51^a min, and sago analog rice mixed black rice (SARBR) at 16 ± 1.49^a min—all exceeding 10 min.

Table 2.

Amylose, total porosity, and cooking time of Sago Analog Rice before and after separation

	Before separation			After separation
	SARWR	SARRR	SARBR	ISARWR	ISARRR	ISARBR
Amylose (%)	39.17 ± 3.65a	35.01 ± 1.06a	37.66 ± 1.64a	30.28 ± 0.01b	29.05 ± 0.01b	28.38 ± 1.35b
Total porosity (%)	0.675 ± 0.03a	0.475 ± 0.04b	4.43 ± 0.03c	5.55 ± 0.64 d	1.55 ± 0.64e	18.95 ± 1.34 f
Cooking Time (min)	14 ± 0.69a	15 ± 1.51a	16 ± 1.49a	5 ± 1.09b	7 ± 1.45bc	9 ± 1.99c

Results are M ± SD of triplicate analysis. Values with different superscript letters in the same row are significantly different (p < 0.05).

Post-cooking washing of the sago-based analog rice resulted in the greatest amylose loss in ISARWR and the least in ISARBR. Subsequent drying after washing facilitated the reassociation of amylose-to-amylose and amylose-to-amylopectin chains, with the highest degree of reassociation observed in ISARBR. Direct quantification of amylose loss after separation in SARWR was 8.89%, SARWR was 5.96%, and SARBR was 9.28%, as shown in Table 2.

Surface morphology observation

The surface morphology of the six sago analog rice before and after amylose washing was investigated using FE-SEM micrographs shown in Fig. 1. The morphological structure observed at 5000× magnification shows that the three types of sago analog rice before separation (a–c) and sago analog rice after amylose separation (d–f). The six rices showed crushed and damaged grains. This structural degradation is caused by gelatinization and retrogradation, which result in grain aggregation and adhesion. During the production process, water is absorbed, causing the grains to swell, and the gelatinization process disrupts intramolecular hydrogen bonds, which causes structural damage¹. The FESEM images show that amylose separation has resulted in an increase in total porosity from SARWR (0.675 ± 0.03^a%) to ISARWR (5.55 ± 0.64^d%), from SARRR (0.475 ± 0.04^b%) to ISARRR (1.55 ± 0.64^e%), and from SARBR (4.43 ± 0.03^c%) to ISARBR (18.95 ± 1.34^f %). The total porosity of the six sago analog rices is shown in Table 2.

Fig. 1. Surface morphology of sago analog rice before and after soluble amylose separation.

FESEM (field emission scanning electron microscope) before separation of amylose from a SARBR (Sago Analog Rice Black Rice) at 5000× magnification (mag.); b SARRR at 5000× mag.; c SARWR at 5000× mag. and after separation of amylose from d ISARBR (Instant Analog Sago Rice Black Rice) at 5000× mag.; e ISARRR at 5000× mag.; f ISARWR at 5000× mag. It appears that a–c show smaller porosity compared to d–f.

Starch crystallinity

The X-ray diffractograms of the three types of ISAR were presented in Fig.2. The ISARWR exhibited major diffraction peaks at 2θ angles of 17.61° and 32.84°, with a crystallinity degree of 17.45%. ISARRR displayed main peaks at 2θ angles of 12.95°, 19.70°, and 34.47°, with a crystallinity degree of 19.40%. Meanwhile, ISARBR showed predominant peaks at 2θ angles of 12.79°, 18.20°, and 37.99°, and had the highest crystallinity degree of 24.33%. All three instant sago analog rice samples exhibited V-type crystalline patterns, which are characteristic of pregelatinized starch, typically resulting from the formation of amylose–lipid complexes or related compounds¹⁵. The crystallinity of starch is influenced by factors such as amylopectin content and chain length, crystallite size, and the orientation of double helices relative to the X-ray beam¹⁶. Reducing crystallinity from SARRR 31.28%⁵ to ISARRR 19.40% affects cooking time. The lower the crystallinity, the faster the cooking time. As shown in Fig. 2 and Table 2, the crystallinity and cooking time of ISARWR are smaller than those of ISARRR and ISARBR.

Fig. 2. The X-ray diffractograms of the three instant sago analog rice.

The peaks on the graphs indicate the crystallinity of a ISARWR, b ISARRR, and c ISARBR.

Antioxidant, total phenol, and resistant starch analysis

Antioxidant analysis refers to the measurement of a compound’s or extract’s ability—such as that of ISAR to scavenge or neutralize free radicals. Free radicals are unstable molecules that can damage cells, proteins, and DNA, and are associated with various degenerative diseases such as cancer, cardiovascular diseases, and premature aging. The DPPH (2,2-diphenyl-1-picrylhydrazyl) assay was employed to evaluate antioxidant capacity by measuring the ability of the extract to reduce DPPH radicals. The antioxidant concentration was calculated based on the percentage of DPPH radical inhibition by the sample extract. The antioxidant contents of ISARWR (446 ± 0.02^a AAEµg/g), ISARRR (448 ± 4.62^a AAEµg/g), and ISARBR (448 ± 0.02^a AAEµg/g). AAE µg/g (Ascorbic Acid Equivalent micrograms per gram). Insignificant difference among them. This indicates that all three types of ISAR possessed comparable antioxidant capacities, effectively scavenging free radicals. The addition of 30% white rice, red rice, or black rice to the formulation contributed to the enhancement of antioxidant levels in the final products.

Inhibitory activity refers to the percentage of free radicals that are successfully neutralized by a given concentration of antioxidant compounds. The inhibitory activities of ISARWR (71 ± 0.01^a %), ISARRR (82 ± 0.11^c %), and ISARBR (75 ± 2.63^b %). These values indicate that free radicals were effectively eradicated by the antioxidants present in each formulation.

Total phenolic content (TPC) measures the concentration of phenolic compounds in the extract of ISAR. Phenolic compounds are a group of bioactive substances known for their strong antioxidant properties, with gallic acid being a common example. The Folin–Ciocalteu assay is the most widely used method for determining TPC, and results are expressed as µg GAE/100 g extract (gallic acid equivalents). Generally, higher total phenolic content correlates with stronger antioxidant potential. The TPC of instant sago analog rice mixed with white rice, red rice, and black rice was 20 ± 9.36^a µg GAE/100 g extract, 86 ± 14.66^b µg GAE/100 g extract, and 145 ± 38.21^c µg GAE/100 g extract, respectively, with significant differences among the samples. This variation is believed to be influenced by the polyphenol content present in sago starch and the added rice types. According to Duque et al.¹⁷, the total polyphenol content of sago fine flour fractions (28.3–61.7 µg GAE/100 g sample). These findings confirm that all three formulations of ISAR possess antioxidant capabilities and that the addition of 30% white rice, red rice, or black rice contributes to the enhancement of antioxidant levels.

Resistant starch (RS) is a type of starch (a complex carbohydrate) that resists digestion in the small intestine and passes into the large intestine, where it is fermented by gut microbiota. Because it is resistant to digestive enzymes such as amylase, RS is not readily converted into glucose, thereby avoiding rapid spikes in blood sugar levels. It functions similarly to dietary fiber. Instant sago analog rice primarily contains RS type 3 (RS3), which refers to starch that has undergone cooking and subsequent cooling, leading to the formation of a crystalline structure that is difficult to digest due to retrogradation. Among the formulations tested, ISARBR (17 ± 0.22^b g/100 g) and ISARRR (17 ± 0.09^b g/100 g) exhibited the highest RS content, the lowest RS was significantly ISARWR (12 ± 0.15^a g/100 g). RS3 formation occurs through the extrusion process, where gelatinized starch forms RS3, not through washing. The difference in RS results from the type of rice flour, not simply amylose separation. The values of antioxidant activity, inhibition percentage, total phenolic content, and resistant starch content for the instant sago analog rice formulations mixed with white rice, red rice, and black rice are summarized in Table 3.

Table 3.

Antioxidant, inhibition activity, TP, and RS of ISARWR, ISARRR, and ISARBR

Instant rice	Antioxidant (AAE µg/g)	Inhibition activity (%)	Total phenol (µg GAE/100g extract)	Resistant starch (g/100g)
ISARWR	446 ± 0.02a	71 ± 0.01a	20 ± 9.36a	12 ± 0.15a
ISARRR	448 ± 4.62a	82 ± 0.11c	86 ± 14.66b	17 ± 0.09b
ISARBR	448 ± 0.02a	75 ± 2.63b	145 ± 38.21c	17 ± 0.22b

Results are M ± SD of triplicate analysis. Values with different superscript letters in the same column are significantly different (p < 0.05).

Gelatinization profile

Pasting properties (gelatinization profiles) are presented in Table 4 and Fig. 3. In this study, ISARWR exhibited a higher peak viscosity compared to formulations ISARRR and ISARBR. The presence of sago starch in the instant analog rice appears to act as a heat conductor, facilitating more efficient heat transfer directly to the starch granules¹⁸. A small breakdown value was observed in ISARR and ISARBR, whereas ISARWR showed a breakdown value of zero. This indicates that ISARWR has greater stability during heating and shear processes¹⁹. The setback value of ISARWR was the highest among ISARRR and ISARBR. Setback is associated with the retrogradation capacity and gel-forming ability of starch pastes²⁰. A higher setback is generally linked to a higher amylose content²¹, besides that increased setback values also indicate the formation of a cohesive paste and a firm gel, with greater retrogradation potential and lower storage stability at low temperatures²². Therefore, setback is an important indicator in determining starch quality and the stability of cooked starch pastes²³.

Table 4.

Gelatinization profile of ISARWR, ISARRR, and ISARBR

Instant Rice	Pasting parameters
	BoG		MV	PT	SoHP		SoCP		EoCP		EoFHP		BV(MV-SoCP)	SV(EoCP-SoCP)
	(BU)	(°C)	(BU)	(°C)	(BU)	(°C)	(BU)	(°C)	(BU)	(°C)	(BU)	(°C)	(BU)	(BU)
ISARWR	10	89.7	20	95.2	15	94.5	20	95.1	40	50.7	39	50	0	20
ISARRR	0	30.1	8	94.9	6	94.5	7	95	17	50.7	16	50	1	10
ISARBR	0	30	8	95.1	4	94.8	6	95	15	50.9	15	50	2	9

BoG beginning of gelatinization, MV maximum viscosity, PT pasting temperature, SoHP start of holding period, SoCP start of cooling period, EoCP end of cooling period, EoFHP end of final holding period, BV breakdown viscosity, SV setback viscosity.

Fig. 3. Gelatinization profiles of instant sago analog rice formulations.

a ISARWR, b ISARRR, and c ISARBR.

The decrease in setback viscosity is associated with the content of anthocyanins and proanthocyanidins in red rice and black rice, where the thermal process causes some anthocyanins and proanthocyanidins to degrade, impacting the gel structure of starch and the tendency for retrogradation. The higher the content of anthocyanins and proanthocyanidins, the longer the heating, the lower the setback viscosity value, as well as reducing the hardness and cohesiveness of the gel, which is important in starch-based food processing²⁴. Figure 3 and Table 4 show that the beginning of gelatinization of ISARWR is higher than that of ISARRR and ISARBR, resulting in a shorter cooking time.

Determination of the color

The effect of anthocyanin extracts in black rice²⁵ and proanthocyanidins in red rice²⁶, contributes significantly to visible color intensity. The color properties of ISARWR, ISARRR, and ISARBR show a general decrease in luminosity (L)*, indicating reduced brightness. The a* value reflects changes in redness, while the b* value indicates shifts in yellow to blue tones, both before and after cooking, as presented in Fig. 4, Fig. 5, and Table 5. Images (a) SARWR, (b) SARRR, and (c) SARBR show sticky surfaces with residual mucilage, indicating the presence of unwashed dissolved amylose. In contrast, (d) ISARWR, (e) ISARRR, and (f) ISARBR display clean, non-sticky granules following the washing and drying process, which facilitated faster recooking times. a* value increased with the addition of red and white rice, but decreased in the formulation with black rice. Meanwhile, the b* value declined across all formulations after cooking. Among the samples, the red rice blend exhibited the highest L* value, suggesting the smallest change in brightness upon cooking. The highest a* value was observed in the black rice formulation, indicating the deepest purple. ΔE (total color difference) value for ISARBR was 127.21 ± 1.26^c, the highest among the samples, indicating a significant color change caused by the addition of black rice. In contrast, the ΔE values for ISARWR (89.62 ± 9.50^b) and ISARRR (74.50 ± 1.49^a).

Fig. 4. Appearance of sago analog rice before soluble amylose separation.

a SARWR, b SARRR, and c SARBR. Before soluble amylose separation, dissolved amylose is visible covering the rice.

Fig. 5. Appearance of instant sago analog rice after soluble amylose separation.

a ISARWR, b ISARRR, and c ISARBR. After soluble amylose separation, rice granules are more visible.

Table 5.

Color change of ISARWR, ISARRR, and ISARBR

Rice color before cooking	L0*	a0*	b0*	ΔE*
SARWR	24.26 ± 0.04b	1.18 ± 0.01b	1.36 ± 0.02a	89.62 ± 9.50b
SARRR	45.89 ± 1.57 d	6.61 ± 0.25e	13.67 ± 0.62 d	74.50 ± 1.49a
SARBR	46.18 ± 0.16 d	9.07 ± 0.05c	14.56 ± 0.03c	127.21 ± 1.26c
Instant Rice Color after cooking	L*	a*	b*
ISARWR	17.80 ± 3.02a	1.26 ± 0.20a	1.11 ± 0.04a
ISARRR	29.15 ± 0.46b	6.87 ± 0.02e	10.17 ± 0.01b
ISARBR	23.03 ± 0.63c	9.04 ± 0.02 d	10.45 ± 0.03b

Results are M ± SD of triplicate analysis. Values with different superscript letters in the same column are significantly different (p < 0.05).

Instant Sago Analog Rice texture

The texture parameters of ISAR before cooking, as shown in Table 6, include hardness and fracturability, whereas post-cooking texture parameters (Table 7) include hardness, adhesiveness, cohesiveness, chewiness, and elasticity. Before cooking, ISARBR (19.98 ± 2.64^c kg) exhibited significantly higher hardness than ISARWR (14.95 ± 2.02^a kg) and ISARRR (16.31 ± 2.64^b kg), indicating a denser and more compact granular structure. This increased firmness suggests stronger molecular associations within the starch matrix, likely due to higher amylose interactions and enhanced crystalline structure. Consistently, ISARBR also showed the highest fracturability (1.32 ± 0.13^c mm), reflecting greater brittleness and supporting the presence of a more ordered crystalline network prior to hydration.

Table 6.

Texture of sago analog rice before cooking

Analog Rice	Hardness (kg)	Fracturability (mm)
ISARWR	14.95 ± 2.02a	1.19 ± 0.10a
ISARRR	16.31 ± 2.64b	1.25 ± 0.10b
ISARBR	19.98 ± 2.64c	1.32 ± 0.13c

Results are M ± SD of triplicate analysis. Values with different superscript letters in the same column are significantly different (p < 0.05).

Table 7.

Texture of instant sago analog after cooking

Instant rice	Hardness (g)	Adhesiveness (g s)	Springiness	Cohesiveness	Gumminess	Chewiness
ISARWR	4454 ± 609.02a	−419.3 ± 110.12a	0.85 ± 0.02a	0.65 ± 0.01a	3279 ± 469.3a	2772 ± 355.14a
ISARRR	5063 ± 440.72b	−222.90 ± 62.52b	0.86 ± 0.07a	0.71 ± 0.03b	3314 ± 377.8a	2978 ± 349.5a
ISARBR	5554.8 ± 602.80c	−200.40 ± 38.52b	0.90 ± 0.16a	0.74 ± 0.02b	3948.0 ± 454.5b	3278 ± 862.26a

Results are M ± SD of triplicate analysis. Values with different superscript letters in the same column are significantly different (p < 0.05).

After cooking, ISARBR maintained significantly higher hardness (5554.8 ± 602.80^c g) compared to ISARRR (5063 ± 440.72^b g) and ISARWR (4454 ± 609.02^a g). This persistence of firmness after thermal treatment suggests that the starch matrix in ISARBR resists structural breakdown to a greater extent, likely due to long-chain amylose associations forming stable gel networks during gelatinization.

ISARBR also demonstrated the highest adhesiveness (−200.40 ± 38.52^b g s), indicating greater surface stickiness after cooking, which may be associated with partial amylose leaching and increased surface interaction. Although springiness values were not significantly different among treatments, ISARBR showed a numerically higher elasticity (0.90 ± 0.16), suggesting improved structural recovery after deformation. The highest cohesiveness, gumminess, and chewiness were likewise observed in ISARBR, indicating a more internally bonded and mechanically resistant structure. Since gumminess and chewiness are positively correlated with hardness, these results confirm that the black-rice formulation forms a stronger three-dimensional starch network, contributing to enhanced textural integrity after cooking²⁷.

Discussion

The removal of surface amylose during processing results in granule brittleness, which is visible in FESEM images as pores and cracks²⁸. This more open structure reduces the physical resistance of the granules to hot water, thus accelerating the gelatinization rate, which directly explains the reduction in cooking time. At the same time, the released amylose forms an amylose–lipid complex with a V-type crystal pattern in XRD²⁹. This complex is relatively stable, but not sufficient to maintain the hardness of the product, resulting in a lower hardness in the final texture. Thus, the microstructural changes resulting from amylose removal significantly influence the acceleration of cooking and the modification of the texture profile³⁰.

Low-amylose starch cannot maintain grain integrity³¹. For analog rice, the grain structure must not disintegrate during soaking/cooking, not stick together, and not become mushy. Low-amylose starch, or waxy starch, forms a highly cohesive and sticky gel. It cannot form stable grains, causing agglomeration and a soft, rice cake-like texture. Therefore, using low-amylose rice flour or waxy starch does not produce analog grains that meet rice quality standards, even though the cooking time may be short³².

In extruded sago analog rice, post-processing washing is an important texture-correction step because it removes part of the leached/free amylose on the grain surface formed during extrusion and drying. If not removed, this surface amylose promotes rapid retrogradation, dense recrystallization, and adhesive film formation during cooking and cooling, which leads to hard, compact, and clumped grains. Washing reduces this surface amylose, thereby decreasing intergranular adhesion and post-cooking hardening. It may also shorten cooking time by improving water penetration and making hydration faster and more uniform. Although this step may be considered less efficient for industrial processing because it requires additional water and handling, it is still justified because it improves texture and reduces cooking time, which can also help lower cooking energy demand³³. The use of low-amylose rice is optional in achieving short cooking times.

Extrusion modifies starch, but it does not remove the soluble amylose fraction. High-shear and high-temperature extrusion breaks down starch granules, but amylose remains soluble and migrates to the surface of the product. Precisely because extrusion has high shear, the soluble amylose fraction increases, not decreases. In other words, extrusion does not replace the function of amylose breakdown; in fact, it makes breakdown more relevant because free amylose is more abundant. Functional modification through extrusion does not automatically improve cooking behavior. Extrusion modifies gelatinization, molecular fragmentation, viscosity changes, and crystal denaturation. However, the effects on cooking quality are not always positive. Analogous sago rice products that still contain free amylose exhibit longer cooking times, a tougher texture, clumping, and loss of grain integrity. This suggests that engineering modification through extrusion is insufficient to directly lead to the structural problems caused by soluble amylose³⁴.

Although direct substitution with low-amylose rice or glutinous rice flour may seem simpler, this approach can be used to produce grains with a soft and sticky structure. Low-amylose starch tends to produce overly cohesive gels, preventing the formation of stable analogous grains. Therefore, an extrusion-washing step is still necessary to achieve a balance between rapid rehydration and intact grain texture³⁵. The observed increase in hardness in ISARBR can be attributed to the presence of long-chain amylose molecules, which can form stronger molecular associations and result in a higher degree of crystallinity, thereby increasing the overall chewiness of cooked rice²⁷.

Collectively, these studies demonstrate that deliberate and controlled structural changes or adjustments to achieve specific functional effects on amylose dissociation provide a structural framework for engineering starch-based instant staple foods that align accelerated cooking performance with mechanical integrity, thereby advancing sustainable sago innovation.

Separation of soluble amylose can shorten cooking time. ISARWR is the fastest, while ISARBR has the longest cooking time. All three samples of instant sago analog rice showed no significant differences in antioxidant content. The highest inhibition activity was observed in ISARRR, and the lowest in ISARWR. The highest total phenolic content was found in ISARBR, and the lowest in ISARWR. The highest resistant starch content was found in ISARRR and ISARBR. The three types of instant sago analog rice displayed a morphological structure consisting of many coarse and damaged grains that stuck together. The crystallinity of ISARBR is the highest, while ISARWR has the lowest crystallinity. The highest maximum viscosity, breakdown, and recovery values are found in ISARWR. The highest hardness and adhesiveness are found in ISARBR. The values for extensibility, cohesiveness, elasticity, and chewiness showed no significant differences among the three samples of instant sago analog rice.

The ability to control cooking time, texture, and functional properties through amylose separation provides a significant opportunity to develop instant staple foods tailored to different population groups, including health-conscious consumers, emergency food programs, and modern markets prioritizing convenience. Therefore, soluble amylose separation represents a promising technological strategy to improve both the functionality and market competitiveness of instant sago analog rice.

Methods

Raw material

Sago, partially pregelatinized sago starch (PPSS)¹, and sago fiber are produced by PT Bangka Asindo Agri (PT BAA) in Sungai Liat-Bangka, Indonesia. White rice, red rice, and black rice are produced by Hasil Bumiku in Bantul, Indonesia.

Producing Sago Analog Rice

The addition of Partially Pregelatinized Sago Starch (PPSS) in the production of sago analog rice was previously conducted⁵. (1) The process involved the following steps. Formulation: Three formulations were developed. Formulation 1 consisted of 50% sago starch, 15% PPSS, 5% sago fiber, and 30% white rice flour. Formulations 2 and 3 used the same base composition, but the white rice flour was replaced by red rice flour and black rice flour, respectively. (2) Ingredient preparation and mixing: all ingredients were accurately weighed and mixed. (3) Moisture content adjustment: the initial moisture content was measured, and distilled water was added accordingly to achieve 30% moisture; (4) Blending: the mixture was stirred until a homogeneous blend was obtained. (5) Extrusion: the homogeneous mixture was extruded using a twin-screw extruder at a feed moisture of 30%, with barrel temperatures of 60 °C for zones 1 and 2, and 80 °C and 90 °C for zones 3 and 4, respectively. Screw speed was maintained at 30–50 rpm. (6) Drying: the extruded product was sun-dried. (7) Cooking: the dried analog rice was cooked using a rice cooker. (8) Washing: the cooked analog rice was washed to remove dissolved amylose. (8) Final drying: the washed rice was sun-dried again to reduce moisture content. (9) Final product: the process yielded three types of instant sago analog rice based on the type of rice flour used (white, red, and black).

Proximate analysis

The first test conducted was moisture, lipid, protein, ash, total starch, and crude fiber, which were calculated based on the AOAC methods 925.10, 920.85, 992.15, 923.03, 996.11, and 991.42, respectively³⁶. Total amylose content was measured by following Hoover and Ratnayake³⁷, a modification from the work of Mcgrance³⁸.

Cooking time

The cooking time determination, following the AACC (2000) procedure³⁹. The test was performed using a water-to-sample ratio of 10:1. Specifically, 5 g of the sample were placed in 50 mL of water. The mixture was then cooked at a temperature of 100 °C. Cooking was considered complete when the rice granules, upon being pressed firmly between two glass slides, no longer exhibited a visible white core, indicating that the sample was fully gelatinized.

Surface morphology and porosity analysis by FESEM–ImageJ

The surface morphology and porosity were determined by scanning electron microscopy analysis using Thermoscientific, Quattro S type, The Netherlands, with 5000× magnification. A thin gold layer first covered the surface of the samples. After that, the sample was placed into the FE-SEM, and the surface image was obtained using a secondary electron (SE) detector with a working distance (WD) of 8–9 mm and HV 20 kV Total.

The surface porosity of analog sago rice was quantified from FESEM micrographs using ImageJ software. For each sample, micrographs were captured from at least three different fields of view to ensure representative surface characterization. The FESEM micrographs were imported into ImageJ (National Institutes of Health, Bethesda, MD, USA; version 1.54 g for porosity analysis.

Starch crystallinity

The spectrum pattern and the crystallinity of materials were analyzed using XRD (Panalytical, x’pert3 powder, Malvern, The Netherlands). The measurement was performed using a tube with 2Ɵ: 5°–80° at 40 kV, 20 mA, and a speed of 2° per minute.

Antioxidant analysis

An antioxidant activity test was conducted using a diphenylpicrylhydrazyl (DPPH) reagent. Initially, 0.5 mM (DPPH) stock solution was made in ethanol. Each extract was added with 1 mL of DPPH stock solution and left for half an hour. After that, the absorption density was assessed in a spectrophotometer (Thermo Scientific UV-Visible Spectrophotometers, Thermo Fisher, Waltham, MA, USA) at a wavelength of 517 nm and was measured using a linear regression equation from the DPPH standard curve. The percentage of inhibition was calculated using Eq. (1)⁴⁰:

$$\%Inhibition=\frac{(Absorbblank-absorbsample)}{Absorbblank}\mathrm{x}100\%$$ 1

Total phenol

Total phenolic content in a sample can be determined using the Folin–Ciocalteu method, as standardized by AOAC 2017.13⁴¹. The Folin–Ciocalteu reagent reacts with phenolic compounds to form a blue-colored complex. Phenol extraction: The sample is dissolved in 80% ethanol. Then, 0.5 mL of Folin–Ciocalteu reagent is added to 0.5 mL of the sample solution. The mixture is incubated for 5 minutes. Subsequently, 2 mL of 7% Na₂CO₃ solution is added and mixed thoroughly. The reaction is allowed to proceed for 30–60 min at room temperature in the dark. Absorbance is then measured using a spectrophotometer at a wavelength of 765 nm. The total phenolic content is quantified by comparing the absorbance to a standard calibration curve prepared with gallic acid and expressed as µg GAE/100 g extract (micrograms Gallic Acid Equivalent per 100 g extract).

Pasting properties

Brabender Viscograph-E (GmbH & Co. KG, Germany) was utilized to examine the pasting properties. The experiment was performed by heating and cooling cycles with constant stirring (75 rpm) on the 500 g sample. The heating was performed from 50 °C to 93 °C, and the achieved temperature was maintained for 15 min. After that, the temperature declined to 50 °C, and was kept for 10 min. This process was performed once, presenting the result as a viscosity profile.

Determination of color

Color analysis using Konica Minolta’s chromameter. Quantitation of color values could be done on solids, powders, and pastes. Color analysis standards were based on an international system developed by CIE, whose analysis results are outlined in variables L* = brightness, a* = red, and b* = yellow-blue.

The samples of the third ISAR were determined by the SC-10 colorimeter, and their L*, a*, and b* values were determined. The total color difference (ΔE) was calculated using

$$\mathrm{\Delta }E=\left({\left(\mathit{Lo}-L\right)}^2+{\left(\mathit{ao}-a\right)}^2+{\left(\mathit{bo}-b\right)}^2\right)$$ 2

where L, a, and b were the color parameters of the rice after cooking, and L0, a0, and b0 were the color parameters of the rice before cooking.

Resistant starch

Enzymatic Method⁴² This is the official AOAC method and the most widely used. The sample is heated and digested by artificial digestive enzymes (amylase & glucoamylase) to simulate digestion in the small intestine. The digested starch is then converted into glucose. The remaining (undigested) is considered as resistant starch (RS), precipitated, dried, then weighed or analyzed for glucose.

Sago Analog Rice texture

Texture analysis method of rice before cooking. Mechanical Texture Test (Texture Profile Analysis/TPA) using Texture Analyzer (e.g., TA. XT Plus from Stable Micro Systems)/Parameters measured: Hardness is the maximum force required to crush rice grains. Fracturability is the point at which the rice grains break. Sample: 10–20 whole, clean, and dry rice grains. Texture Analyzer tool set: Probe: flat plate P/5; mode: single compression; speed: 1 mm/s; rice is placed on the platform one by one; run the compression test until the rice breaks; data recorded in the force-time graph is the peak force value = hardness. After cooking, the rice was separated from the water and ready to be analyzed using a Texture Analyzer (TA. XT Plus Stable Micro System Type plus100 series, producer Schroff, United Kingdom). Measurement Conditions of Texture analyzer–TA. TX-plus was working in the mode: TPA; Pre-Test Speed was 1.0 mm/s; the Test Speed was 1.0 mm/s; Post-Test Speed was 1.0 mm/s; and Distance/Strain was 95%.

Statistical analysis

Some measurements were conducted in triplicate. The results obtained in this investigation were presented as the average ± standard deviation. The data was analyzed using the Statistical Package for the Social Sciences 26.0 (SPSS). The statistical techniques of analysis of variance (ANOVA) and Duncan’s multi-range test were employed to ascertain the presence of statistically significant differences (p < 0.05).

Author contributions

Each author made substantial contributions to the material presented in this manuscript. Sabirin conducted the experiments, analyzed the data, and wrote the manuscript. Mardonius Budi Kusarpoko, Dyah Primarini Meidiawati, and Bambang Triwiyono contributed to the conception and design of the study and the review of the manuscript. Abdullah Darussalam, Yanuar Sigit Pramana, and Ika Mulawati Purwanti Noviana participated in experiments involving the analysis of the starch, proximate, antioxidant, inhibition activity, TPC, resistant starch, color, pasting properties, crystallinity, and texture. Fidrianto and Arry Darmawan participated in experiments that contributed to the acquisition and analysis of data or their interpretation.

Data availability

All data supporting the findings of this study are available within this article.

Competing interests

The authors declare no competing interests.