Gadung (Dioscorea hispida) tuber starch modified by freeze moisture treatment and heat moisture treatment: a study of functional, pasting, and physicochemical properties

ABSTRACT

Gadung starch has limitations in several functional properties and thermal stability. Modifying starch through temperature treatments such as Freeze Moisture Treatment (FMT) and Heat Moisture Treatment (HMT) potentially improves these weaknesses. This research aimed to determine the effects of FMT and HMT on the functional, pasting, and physicochemical properties of gadung starch. This study consisted of a variety of treatments, namely native starch, FMT, HMT 100°C, HMT 110°C, FMT + HMT 100°C, and FMT + HMT 110°C. The results showed that FMT and HMT treatments significantly affected the functional, pasting, and physicochemical properties of gadung starch. The combination modification treatment of FMT + HMT 110°C increased swelling volume by 1.44 times, and water absorption capacity increased by 1.37 times. This was confirmed by the formation of porous and amorphous starch granules. Modified starch also had the best heat stability, as indicated by a low breakdown viscosity of 31.00 ± 9.90 cP. FMT+HMT110°C modified treatment was the best treatment because it had more pores, cracks, and higher swelling volume. The starch crystallinity index of modified starch decreased from 35.75 to 31.58 and changed the crystallinity type from B-type to A-type. The starch modification did not cause changes in functional groups but reduced crystallinity as indicated by the smaller ratio of the bands at 1045/1022 and the higher ratio at 1022/995 from FTIR spectra. Thus, FMT and HMT could effectively improve the characteristics of gadung starch by forming heat-stable amorphous starch.

KEYWORDS:

• Starch granule
• swelling volume
• gelatinization
• crystallinity
• porous starch

Introduction

Gadung (Dioscorea hispida) starch has a high gelatinization temperature but has limitations in several functional and pasting properties such as low-swelling volume, low absorption capacity, and low thermal stability, so its use is limited.^[1,2] Apart from that, gadung starch also has low solubility and is difficult to form a strong gel.^[3] Therefore, starch modification is needed to improve these properties so they can be extensively applied.

Some researchers have modified gadung starch, including by chemical modification, namely by H₂O₂/UV photo-oxidation^[4] and acetylation,^[5] but chemical modification is less safe and can cause chemical residues in the product. Physical modification of starch by temperature treatment at a certain humidity can significantly improve the properties of starch and is known as a method that is easy, cheap, and environmentally friendly.^[6–9] Physical modifications that can improve starch stability can be conducted through heat moisture treatment (HMT). Modification of gadung starch has also been carried out physically with heat moisture treatment.^[1,2] This modification produces gadung starch that is heat resistant, but the absorption capacity and swelling volume are still low.

HMT was proven effective in improving stability at high temperatures and acidic conditions. HMT is carried out by heating above the gelatinization temperature (84–120 ℃), at limited water content (<35% W/W), and within a certain period (15 min − 16 hours).^[10,11] The heating temperature of HMT greatly determines the characteristics, especially the stability of the starch. However, HMT can reduce swelling volume, solubility, and absorption capacity.^[12–15] Therefore, a treatment that can improve these functional properties is needed, namely through freeze moisture treatment (FMT).

Freeze treatment of starch has been proven to improve functional properties through the formation of a more amorphous and porous granule structure.^[16–20] FMT is a physical modification by hydrothermal treatment at freezing temperatures which continues to be developed because it can be used to produce porous starch effectively. FMT can be carried out at freezing temperatures with a certain moisture level, then sublimated or evaporated at low pressure until the water content is reduced and a dry and porous starch is formed.^[21,22] Porous starch granules have higher starch solubility and water absorption capacity, so they can be applied in the food industry as an absorbent, supporting the encapsulation of food compounds, and in products that require high water absorption.^[23,24] Thus, the combination of physical modification by FMT and HMT was chosen because it has the potential to increase starch resistance to heat and has good absorption and swelling capabilities, and this modification can be carried out easily, safely and environmentally friendly.

Therefore, the combination of FMT and HMT is expected to improve the thermal stability, absorption and swelling ability of starch, so that it can be applied more widely in various products that require good stability and better functional properties. The combination of modifications by FMT and HMT of gadung starch is still limited and not widely used. Therefore, this study aimed to determine the effects of modified FMT, HMT, and the combination of FMT + HMT on starch characteristics such as functional properties, pasting, and physicochemical properties of gadung starch.

Materials and methods

Material

Yellow gadung (Dioscorea hispida Dennst) tubers with a harvest age of 6–9 months were obtained from agricultural land at Saguling, Bandung, West Java, Indonesia. In addition, other materials were used, such as silica gels, distilled water, and lime were obtained from local market Bandung, Indonesia, and other reagents such as KI, NaOH, and AgNO₃ were obtained from Merck KGaA (Darmstadt, Germany).

Preparation of gadung starch

Gadung starch was prepared by wet extraction. First, the gadung tuber was washed to remove contaminants, then peeled and sliced to obtain a thickness of 0.3 cm. Gadung slices were soaked in lime solution with a concentration of 15% overnight to neutralize HCN and then washed until no lime solids were attached to the tubers as well as to remove remaining HCN and dioscorin. Gadung slices were ground and added with water at a ratio of water and starch at 3:1 (w/v), then filtered up to two times. All filtrates were precipitated for 24 h, washed with clean water, and dried at 50°C for 16 h using a cabinet drier (Shel Lab SMO28–2, Sheldon Manufacturing, Inc, USA). The dried gadung starch was then ground again using a grinding machine of FCT-Z500 Fomac, PT. Putra Chandra Sentosa, Indonesia, and sieved with a size of 100 mesh. The gadung starch obtained was then tested for HCN content to ensure that the gadung starch obtained was safe and free of HCN.

Modification of gadung starch by FMT and HMT

The treatments for modification of gadung starch were conducted, including native gadung starch as control, FMT with 70% water content, HMT 100°C, HMT 110°C, the combination of FMT + HMT 100°C, and FMT + HMT 110°C. The order of treatment was FMT first then HMT. FMT was conducted first by adding distilled water so that the water content reached 70% in thin-walled containers, then frozen at −30°C, and evaporated at the vacuum pressure of 50 ± 2°C for 6 h, and then measured the water content. In the second stage, the HMT of gadung starch was adjusted for water content by spraying it with distilled water, and then stirred until it reached a moisture level of ± 30%. The water content was measured again to ensure the water content was correct. The starch was incubated at 4–5°C for 24 h in the refrigerator. The gadung starch was packed in aluminum foil, and then heated in an oven (Memmert Universal Laboratory Oven UNB400, Memmert GmbH + Co.KG, Germany) at 100°C and 110°C for 16 h. Gadung starch was then dried at 50°C to a moisture of about 9–10%. Gadung starch was then crushed by a grinder and sieved with a size of 100 mesh.

Functional properties

Solubility and swelling volume were determined based on Collado and Corke,^[25] by weighing 0.35 g of gadung starch which was suspended in 12.5 mL of distilled water, then the total volume was read and homogenized using a vortex for 30 sec. The starch suspension was heated at a temperature of 80°C for 30 min in a water bath, then cooled for 1 min in cold water, and centrifuged for 30 min. The supernatant volume was read to calculate the swelling volume and then dried at 110°C for 24 h in an oven to calculate the solubility. The swelling volume and solubility were calculated using the following formula.

$\mathrm{Swelling}\mathrm{Volume}=\frac{\mathrm{Total}\mathrm{volume}-\mathrm{Supernatant}\mathrm{volume}(\mathrm{mL})}{\mathrm{Dry}\mathrm{sample}\mathrm{weight}(\mathrm{g})}$

$\mathrm{Solubility}=\frac{\mathrm{Weight}\mathrm{of}\mathrm{dry}\mathrm{supernatant}\left(\mathrm{g}\right)}{\mathrm{Starch}\mathrm{sample}\mathrm{weight}\left(\mathrm{g}\right)}\mathrm{x}100\mathrm{\%}$

The water absorption capacity (WAC) was determined based on Kadan et al.^[26] 1 g of gadung starch was added to 10 mL of distilled water and then homogenized using a vortex mixer for 30 sec. The starch suspension was incubated at 25°C for 30 min, then centrifuged for 30 min. The WAC was calculated based on the amount of water absorbed in the starch. The following formula calculated the WAC:

$\mathrm{WAC}=\frac{\mathrm{Total}\mathrm{weight}\mathrm{of}\mathrm{distilled}\mathrm{water}\left(\mathrm{g}\right)-\mathrm{Weight}\mathrm{of}\mathrm{supernatant}\left(\mathrm{g}\right)}{\mathrm{Weight}\mathrm{of}\mathrm{sample}\left(\mathrm{g}\right)}$

Color chromaticity

Color chromaticity of gadung starch was performed using the chromameter instrument (Spectrophotometer CM-5, Konica Minolta, Inc, Tokyo-Japan). Gadung starch was placed in a dish, and the chromaticity was read. Testing was done with Hunter color system L*, a*, and b* . The total color difference value (∆E) was calculated using the following formula:

$\mathrm{\Delta }\mathbf{E}\mathit{\hspace{0.17em}}=\mathit{\hspace{0.17em}}{\left[{\left(\mathrm{\Delta }{\mathbf{L}}^{\ast }\right)}^{\mathbf{2}}+\mathit{\hspace{0.17em}}{\left(\mathrm{\Delta }{\mathbf{a}}^{\ast }\right)}^{\mathbf{2}}+\mathit{\hspace{0.17em}}{\left(\mathrm{\Delta }{\mathbf{b}}^{\ast }\right)}^{\mathbf{2}}\right]}^{\mathbf{1}/\mathbf{2}}$

Pasting properties

The pasting properties of gadung starch were determined using a Rapid Visco Analyzer (Perten RVA-SM2, Warriewood Australia).^[27] 3.5 g of gadung starch was added to 25 mL of aquadest in the RVA tube, and then mounted on the RVA instrument. The test was conducted for 12 min, where the initial temperature was maintained at 50°C for 1 min, then heated to 95°C and retained for 5 min, then the temperature was reduced to 50°C again and retained for 5 min. The parameters that can be obtained through RVA analysis were gelatinization temperature, peak viscosity, hold viscosity, final viscosity, breakdown viscosity, and setback viscosity. These parameters were calculated using Microsoft Excel software by Microsoft 365.

Granule morphology

The morphology of gadung starch granules was carried out using a scanning electron microscope (JEOL SEM-JSM6510, JEOL Ltd, Tokyo-Japan). Gadung starch was put on a holder, then coated with gold and placed in the SEM instrument. Starch granule morphology was read at magnification scales of 7000.

Functional group analysis

The functional groups of gadung starch were determined by “FTIR spectrometer of Nicolet iS10, Thermo Fisher Scientific Inc, Waltham, MA USA” integrated with ATR. 2 mg of gadung starch was put down in the sample holder area and then pressurized. The spectrum of functional groups was displayed with a spectral of 400 to 4000 cm⁻¹. FTIR was also used to determine the ratio of wave numbers 1045/1022, 1022/995 and 1045/995 to determine the crystallinity, degree of order (DO) and degree of double helix (DD).

Crystallinity analysis

Gadung starch crystallinity was determined using a fine-focus copper X-ray tube (SmartLab, 30 mA, 40 kV, Rigaku Holdings Corporation, Tokyo-Japan) with an SC70 detector. 1 g of gadung starch was put down in a glass sample holder as a flat plate with holes, and then X-rays were emitted. The diffractogram was recorded at 2θ from 3° to 40° with a step size of 0.02° at 1 deg/min, and the crystallinity index was determined according to Frost et al.^[28]

Statistical analysis

The data obtained were analyzed using ANOVA, then continued with the Duncan test using SPSS Statistics ver 23.0 to detect differences between treatments at p < .05.

Results and discussion

Swelling volume and solubility

Swelling volume and solubility were used to measure the interaction between water and starch in crystalline and amorphous regions.^[2,10] The swelling volume and solubility of modified gadung starch by FMT and HMT can be seen in Figures 1 and 2.

Figure 1. Swelling volume of modified gadung starch by FMT and HMT. Graphs marked with different letters indicate significant differences at p < .05.

Figure 2. Solubility of modified gadung starch by FMT and HMT. Graphs marked with different letters indicate significant differences at p < .05.

FMT and HMT significantly affected the swelling volume of gadung starch (p < .05). The swelling volume increased with the modification of FMT, HMT, or a combination of FMT+HMT. These results were in accordance to the research by Subroto et al.^[22] who found that FMT modification of adlay starch could increase the swelling volume. Adebowale et al.^[29] also reported that modification of HMT in red sorghum and corn starch could increase the swelling volume. Modified gadung starch by FMT + HMT 110°C had the highest swelling volume of 18.09 ± 0.83 mL/g. These results showed that the FMT caused physical damage to the starch granules, making them porous due to the evaporation or sublimation of ice crystals and water in the granules and making it easier for water penetration and hydration when HMT was carried out. The increase in the swelling volume during the HMT was due to the damage to the hydrogen bonds of the starch granules during the heating treatment so that the starch became more hydrophilic and could bind more water so that the swelling ability of the starch increased.

Figure 2 shows that the FMT and HMT significantly affected the solubility of gadung starch (p < .05). The solubility of gadung starch modified by FMT, HMT, and their combination of FMT+HMT was lower than native starch. The most significant decrease in solubility was found in HMT 110°C, which reduced the solubility from 32.33 ± 0.58% to 2.49 ± 0.06%. These results were in accordance with other studies, which showed that HMT modification caused a significant decrease in starch solubility.^[2,12] This was due to heated starch in limited water caused the starch granules to become stiffer and produced double helix amylopectin side-chain groups, which were more stable and able to inhibit amylose leaching so that its solubility decreased.^[8,30] The solubility of modified gadung starch by combining FMT + HMT was higher than that modified by HMT alone. This can be caused by FMT encouraging the emergence of pores in the granules due to the freezing and sublimation or evaporation during modification, then the pores made it easier for water to diffuse into the granules, and there was an increase in amylose leaching from the amorphous area.^[23,24,31]

Water absorption capacity

Water absorption capacity (WAC) was used to determine the ability of starch to retain water absorbed by starch. WAC was related to the properties of starch and granule composition after adding water. The WAC of modified gadung starch can be observed in Figure 3. Figure 3 shows that FMT and HMT modifications increased the WAC of gadung starch (p < .05). HMT 100°C increased the WAC from 1.09 ± 0.01 g/g to 1.87 ± 0.09 g/g. The results of this study were in accordance with Adebowale et al.^[29] and Li et al.^[32] who reported that HMT modification was able to increase the WAC value of red sorghum and potato starch. This indicated that HMT modification caused an increase in hydrophilic groups in starch granules due to the amorphous part of the starch granules experiencing swelling, which caused some of the hydrogen bonds between the amorphous and crystalline parts to break and bind with water molecules, so that the hydrophilic groups increased.^[33] The more hydrophilic groups make the water enter the granules and interact through hydrogen bonds, the higher the capacity of starch to absorb water. On the other hand, FMT could increase the cracks and pores on the surface of starch granules, making it easier for starch granules to absorb more water.

Figure 3. WAC of modified gadung starch by FMT and HMT. Graphs marked with different letters indicate significant differences at p < .05.

Color chromaticity

Color chromaticity measurements were carried out using a chromameter based on the reflection of light by the sample surface, which was indicated by the L*, a*, and b* values, and the quantity of color change was manifested as ΔE. The color chromaticity of gadung starch modified by FMT, HMT, and their combinations is presented in Table 1. Table 1 shows that the modification treatment significantly affected the color chromaticity of gadung starch. L* in all treatments was in the range of 91.55 to 94.79, which designated that all samples were light white. However, HMT and its combination with FMT decreased the L* . On the other hand, modification of FMT, HMT, or a combination of FMT+HMT increased the a* and b* of gadung starch. The highest a* was found in the FMT + HMT 110°C with about 0.73 ± 0.01, while the highest b* was found in the HMT 110°C, namely 9.48 ± 0.03. The results showed that the a* (redness) was inversely proportional to the L* but directly proportional to the b*. The increase in the a* and b* values of gadung starch could be caused by a non-enzymatic browning reaction due to heat treatment, especially during HMT modification. The 70% FMT treatment had the lowest ∆E value, namely 0.47, which showed a very small color difference with native starch. The highest ∆E value was found in the HMT 110°C with a value of 5.10. The HMT treatment and the combination of FMT+HMT increased the ∆E value, ranging from 3.29 to 5.09, which indicated a color change compared to native starch. This was due to the effect of using high temperatures, which caused a non-enzymatic browning, that increased the chromaticity of the brownish-yellow color in starch.^[34,35] However, the color change was very small, and the L* value was still high (≥90), which indicated that gadung starch was still bright white.

Table 1. Color chromaticity of modified gadung starch by FMT and HMT.

Treatment	Color Chromaticity
	L*	a*	b*	$\mathrm{\Delta }$E
Native starch	94.79 ± 0.01^f	−0.29 ± 0.01^b	5.62 ± 0.01^a	0.00 ± 0.00^a
FMT	94.67 ± 0.02^e	−0.34 ± 0.01^a	6.07 ± 0.02^b	0.49 ± 0.02^b
HMT 100${ }^{\circ }\mathit{C}$	93.26 ± 0.01^d	0.44 ± 0.01^d	9.24 ± 0.02^e	3.98 ± 0.26^e
HMT 110${ }^{\circ }\mathit{C}$	91.55 ± 0.00^a	0.48 ± 0.00^e	9.48 ± 0.03^f	5.09 ± 0.03^f
FMT + HMT 100${ }^{\circ }\mathit{C}$	93.20 ± 0.01^c	0.37 ± 0.01^c	9.08 ± 0.01^d	3.29 ± 0.02^d
FMT + HMT 110${ }^{\circ }\mathit{C}$	92.30 ± 0.02^b	0.73 ± 0.01^f	7.72 ± 0.02^c	3.41 ± 0.01^c

The treatment means marked by different letters for the same column indicate differences between treatments (p < .05).

Starch granule morphology

The surface of starch granules can change when subjected to heating or cooling treatment. The morphology of gadung starch granules modified by FMT, HMT, and their combination of FMT + HMT can be seen in Figure 4. The granules of native gadung starch were polygonal in shape with a smooth and non-porous surface. Modified gadung starch by FMT had a rougher and slightly porous granule surface than native gadung starch. This could be caused by the lyophilization or evaporation process during FMT. This process caused forced distortion of the granules due to the release of water or ice crystals from the starch, which was caused by the pressure that forms in the starch granules so that water molecules diffused out of the granules.^[18] This caused damage to the granules and the formation of cracks and pores on the surface of starch, which was followed by an increase in WAC.^[22,24]

Figure 4. Morphological characteristics of modified gadung starch by FMT and HMT at a magnification of 7000X. Pores and cracks were marked by red arrows.

Figure 4 also shows that the modification of HMT 100°C and HMT 110°C caused some starch granules to experience structural deformation and cracks on the surface of the starch granules. This could be caused by loss of physical integrity due to restructuring and partial gelatinization on the surface of starch granules when heated at high temperatures.^[36] These results are in accordance with research by Zavareze and Dias,^[30] which stated that modification of HMT at limited water content resulted in cracks in starch granules. Similar results were also shown by Na et al.^[37] who reported the effect of HMT on the formation of cracks in the morphology of low-digestible sweet potato starch granules. HMT caused water molecules to penetrate into the amorphous regions of starch and swell when heated and then shrink when cooled so that the starch granules became cracked and porous.^[38] The treatments of the combination of FMT + HMT 100°C and the combination of FMT + HMT 100°C produced more porous granules with a rough and cracked surface. However, most cracks and granule pores occurred at FMT + HMT 110°C. This was due to the porous starch granules resulting from FMT facilitating optimal water penetration during the HMT process. The amylose leaching during FMT could also encourage starch granule deformation. In addition, heating starch with a certain water content caused the surface of the starch granules to become rougher so that when more water entered, the network structure of the starch granules would weaken, and more wrinkles or cracks were formed.^[39]

Pasting properties

Pasting properties are used to determine the gelatinization pattern of starch, which is related to measuring the viscosity of starch when given heating and cooling treatments. Pasting properties of gadung starch modified by FMT, HMT, and their combination of FMT+HMT are shown in Figure 5 and Table 2. FMT and HMT significantly affected all parameters of pasting properties (p < .05). HMT treatment, or in combination with FMT, increased the pasting point of gadung starch. However, a single FMT modification did not increase the pasting point significantly if not combined with HMT. The increment in the pasting point of HMT-modified starch was due to the transformation of amorphous amylose into a helical form without breakage of the amylopectin and amylose chains so that the starch granules become more resistant to rupture when heated.^[30] Modification of FMT, HMT, and their combination caused a decrease in the peak viscosity of gadung starch, where the most significant decrease occurred in the FMT + HMT 110°C, namely decreasing from 3671.00 ± 143.54 cP to 2288.00 ± 284.26 cP. FMT reduced peak viscosity due to physical damage to starch granules by forming ice crystals during the freezing process so that water penetration for hydration increased.^[40] On the other hand, HMT treatment led to interactions between amorphous and crystalline areas to cause rearrangement to form increasingly compact amylopectin-amylopectin, amylose-amylopectin, and amylose-amylose bonds.^[7,41,42] This reduced solubility in water and limited swelling of starch granules, so that the peak viscosity of HMT-modified gadung starch was lower than native gadung starch.

Figure 5. Pasting curves of modified gadung starch by FMT and HMT.

Table 2. Pasting properties of modified gadung starch by FMT and HMT.

Treatment	Pasting Point (°C)	Peak Viscosity (cP)	Hold Viscosity (cP)	Final Viscosity (cP)	Breakdown Viscosity (cP)	Setback Viscosity (cP)
Native starch	75.67 ± 0.29^a	3671.00 ± 143.54^c	682.00 ± 124.45^a	873.50 ± 170.41^a	2989.50 ± 267.99^b	191.00 ± 45.96^b
FMT	75.50 ± 1.15^a	3658.00 ± 52.33^c	919.00 ± 2.83^a	1475.00 ± 4.95^ab	2739.00 ± 49.50^b	556.00 ± 7.78^c
HMT 100 °C	82.24 ± 1.68^b	3075.50 ± 496.39^a	2015.00 ± 436.99^b	2015.00 ± 436.99^bc	113.00 ± 53.40^a	0.00 ± 0.00^a
HMT 110 °C	79.44 ± 0.04^ab	2477.00 ± 79.20^ab	2442.00 ± 80.61^bc	2599.00 ± 33.94^cd	38.00 ± 1.41^a	157,00 ± 46.67^b
FMT + HMT 100°C	81.13 ± 0.59^b	3484.00 ± 134.35^a	3446.00 ± 111.72^c	3384.50 ± 113.84^d	35.00 ± 22.63^a	1504.500 ± 1.41^It is
FMT + HMT 110°C	81.50 ± 0.08^b	2288.00 ± 284.26^bc	2257.00 ± 274.36^b	4950.50 ± 232.64^It is	31.00 ± 9.90^a	1150.00 ± 9.90^d

The treatment means marked with the same lowercase letter are not significantly different at the 5% level according to the Duncan test.

The FMT alone did not reduce the breakdown viscosity of gadung starch significantly. However, HMT and the combination of FMT+HMT reduced the breakdown viscosity significantly. This was due to the HMT carried out after FMT making it easier to arrange the crystal matrix and restructure the amylose-amylopectin to increase stability. These results were in accordance with research by Marta et al.^[43] and Subroto et al.^[12] who reported that HMT was able to increase the stability of hot paste on breadfruit starch and potato starch, respectively. A decrease in breakdown viscosity accompanied an increase in hot paste viscosity. The combination treatment of FMT + HMT 110°C increased the stability of heating and stirring. However, it was not significantly different from other modified starches except for FMT alone. HMT, which was supported by the formation of a porous starch granule surface caused an increase in the regularity of the crystal matrix due to amylose leaching. Amylose then formed a complex with amylopectin or other components so that the stability of the paste during heating increased.^[8,17]

Table 2 also shows that FMT modification increased the setback viscosity of gadung starch. This could be due to the reassociation of starch granules during the FMT modification process.^[44] Meanwhile, the HMT treatment had a lower setback viscosity than the other treatments. This indicated that the HMT of gadung starch had a lower retrogradation ability when cooled. The decrease in setback viscosity in HMT modification was caused by the formation of amylose-lipid complexes, amylose-amylose, and amylose-amylopectin, which reduced the ability of starch, especially amylose, to re-bond with each other.^[45,46] The combination of FMT+HMT treatment increased the setback viscosity, which indicates that more starch granule reassociation occurs. These results were in line with Li et al.^[44] who reported that starch modification using the Microwave freeze-drying method produced very high setback viscosity for chinese yam flours.

Based on the amylograph in Figure 5, native gadung starch and FMT had a B-type viscosity pattern which was characterized by high peak viscosity and breakdown viscosity during heating.^[47] These results were in conform with Kumoro et al.^[5] who showed that gadung starch had a B-type viscosity pattern like starch from other tubers. Meanwhile, HMT-modified gadung starch and the combination of FMT + HMT had a C-type viscosity pattern, indicated by the absence of peak viscosity. Peak viscosity was relatively constant and even increased during heating.^[48] The characteristics of starch, which were stable in the heating process, were suitable for application in products that were sterilized by heating at high temperatures, such as sauces in canning products. Based on pasting properties, the modified starch FMT + HMT 110°C combination had the best thermal stability, indicated by the low breakdown viscosity (Table 2), and high WAC (Figure 3). This combination modification had the highest swelling volume (Figure 1) and a more porous granule morphology (Figure 4). Therefore, the FMT + HMT 110°C combination modification was further evaluated regarding crystallinity and functional groups compared to native starch.

Crystallinity

The crystallinity diffraction patterns of native gadung starch and modified by FMT + HMT 110°C are presented in Figure 6. Based on Figure 6, there was an alteration in the crystallinity of gadung starch after the modification by FMT + HMT 110°C. Native gadung starch had a diffraction pattern marked by the appearance of small diffraction peaks at 2θ of 5.83° and 34.42°, strong peaks at 2θ of 17.28° and 29.4°, and a broad peak at 2θ of 22.14° and 24.26°. This showed that gadung starch was classified as a B-type diffraction pattern. The diffraction pattern of gadung starch by FMT + HMT 110°C changed significantly. Modification by FMT + HMT 110°C caused the minor peaks at 2θ of 5.83° and 34.42° to disappear and a significant decrease at the 2θ of 34.42°. This indicated that the crystallinity type of modified gadung starch changed from B-type to A-type. These results were the same as those reported by Vermeylen et al.^[49] that hydrothermal treatment with HMT on potato starch changed the crystal type from B-type to A-type. Meanwhile, the strong peak at 2θ of 17.28° and broad peaks at 2θ of 22.14° and 24.26° still existed. Apart from that, native gadung starch and modified FMT + HMT 110°C had a diffraction pattern marked by the appearance of diffraction peaks at 2θ of 29.41°. This peak indicated that the starch leads to a semi-crystalline structure.^[50] However, this peak was lower in FMT + HMT 110°C than native starch which indicated that the modified starch has a weaker semi-crystalline structure.

Figure 6. The diffractogram of native and modified gadung starch by FMT + HMT 110°C.

Based on Figure 6, the modification of gadung starch through FMT+HMT 110°C caused a decrease in the starch crystallinity index from 35.75 to 31.58. Changes in peak intensity in modified gadung starch could be caused by the loss of double helix chains in starch crystals and the forming of a matrix that was more amorphous than native starch.^[7] The decrease in peak intensity in starch modified by FMT + HMT 110°C was also associated with partial gelatinization of starch granules that occurred in the modification process.^[36] The decreased crystallinity in the modified starch by FMT + HMT 110°C could be caused by forming a porous granule surface that facilitated retrogradation as indicated by the high setback viscosity value. The decrease in the peak intensity of starch treated by freeze-drying could occur due to the release of the double helix in the starch crystals. These results were in accordance with Jayanthi et al.^[17] which reported that freeze-drying of starch from various tropical tubers caused a decrease in starch crystallinity.

Functional groups

The FTIR spectra for the functional groups of native gadung starch and modified by FMT + HMT 110°C are presented in Figure 7. Starch modified by FMT + HMT 110°C did not show significant spectral changes. The FTIR spectrum of starch peaked at wave numbers 3397–3398 cm-1, indicating the presence of hydroxyl groups (O – H). The peak at the wave number 2924 cm⁻¹ exhibited the presence of the C – H group. The next peak with a wave number of 1640 cm⁻¹ indicated the presence of a C=O group, and the peak at 1022 cm⁻¹ marked the presence of a C – O group.

Figure 7. FTIR spectra of native and modified gadung starch by FMT + HMT 110°C.

The results showed that the starch modification treatment with a combination of FMT + HMT 110°C did not cause a change in the absorption peak, indicating no formation of new functional groups in the starch. The modification of FMT + HMT 110°C caused a decrease in intensity, especially at peaks with wave numbers 2852 and 1744 cm^−1, due to the strengthening of stretching vibrations in the C – H and C=O bonds. This result may be caused by the partial gelatinization that occurred in the HMT process, which caused the breaking of the double helix bonds at high temperatures.^[51] The FMT also caused starch to be partially retrograded, resulting in the re-formation of a double helix structure composed of short chains.^[52] In addition, the ratio of the bands at 1045/1022 and bands at 1022/995 were important for knowing the crystalline and amorphous regions, and the ratio of the bands at 1045/995 was important for knowing the degree of the double helix.^[53] The results indicated that the smaller ratio of the bands at 1045/1022 in the gadung starch was modified by FMT + HMT 110°C, namely 0.8866, compared to native starch, namely 0.8881. The ratio of bands at 1022/995, gadung starch modified by FMT + HMT 110°C, was higher, namely 1.2968, compared to native starch, namely 1.2687. This indicated that modified gadung starch FMT + HMT had a larger amorphous region than native starch. In addition, the ratio of bands at 1045/995, gadung starch modified by FMT + HMT 110°C, was lower, namely 0.9366, compared to native starch, namely 1.1268. This indicated that modified gadung starch FMT + HMT had a lower degree of double helix region than native starch.

Conclusion

The starch modification by FMT and HMT affected the characteristics of gadung starch significantly. This modification increased the swelling volume, WAC, thermal stability, and porosity but reduced the solubility and reduced the crystallinity of gadung starch. The combination of modification by FMT + HMT 110°C was the best treatment, which was able to increase the swelling volume by 1.44 times and water absorption capacity by 1.37 times, the best heat stability with a low breakdown viscosity of 31.00 ± 9.90 cP, and starch granules which were porous and amorphous, change the crystallinity type from B-type to A-type, without changes to new functional groups. Thus, FMT and HMT-modified gadung starch are potentially suitable for application in various products that require good absorption capacity and stability toward heating, such as heat-resistant sauce, instant noodles, and other instant products.

Acknowledgments

The authors would like to thank the Rector of Universitas Padjadjaran and The Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Universitas Padjadjaran [1776/UN6.3.1/PT.00/2024].