Effect of adding vegetable oils to starches from different botanical origins on physicochemical and digestive properties and amylose–lipid complex formation

Abstract

Coconut oil, rice bran oil and sunflower oil were added to rice, corn, banana and mung bean starches and the effect on physicochemical properties, amylose–lipid formation and digestive properties were investigated. Starch samples were heated while oil was added and starch treated without oil addition served as the control. Starches with different botanical origins complexed diversely with vegetable oils. The RDS content in corn and rice starches with oil addition decreased, while SDS and RS fractions increased. By contrast, the RS content of treated banana and mung bean starches decreased compared with native starch but RS and SDS contents increased when oil was added compared with the control sample. The A-type crystalline polymorph of corn and rice starches changed to a mixed A + V form, whereas native mung bean (C_(A)-type) changed to B-pattern and banana starch remained unchanged (B-type). FTIR spectra indicated new peaks corresponding to starch-lipid complexes. Starches added with oils and the control showed lower peak viscosity, trough viscosity final viscosity and setback but higher pasting temperature and delayed pasting time compared to native starch. Heat-moisture treatment with added vegetable oil showed promise as a process to prepare functional starch high in SDS and RS.

Introduction

Starchy foods are an important source of energy for humans and animals. Starch is classified into three fractions based on digestibility rate by α-amylase. Rapidly digestible starch (RDS) is determined by the amount of glucose released within the first 20 min, while slowly digestible starch (SDS) is completely digested in the small intestine at a slower rate than RDS at between 20 and 120 min. Resistant starch (RS) resists digestion in the small intestine and is fermented by microflora in the large intestine to produce short-chain fatty acids which benefit health (Liu et al., 2022). RS is currently being intensively investigated due to its physiological benefits as dietary fiber. SDS and RS are also classified as dietary fibers and have functional properties that help reduce the development of chronic ailments such as obesity, diabetes, and cardiovascular disease (Hasjim et al. 2010).

Recently, amylose–lipid complexes have been widely studied. Amylose undergoes a transformation from double helices to a single helix and forms complexes with other ligands (Putseys et al. 2010). Amylose complexes with lipids are known as amylose–lipid complexes that have been proposed as resistant starch type 5 (RS5). These complexes occur naturally in starch and also during gelatinization of starch in the presence of lipids (Putseys et al. 2010). RS5 is thermally stable and, thus, suitable for food processing that requires heating such as bread-making ingredients, giving a lower glycemic index value (Hasjim et al. 2010). Amylose–lipid complexes are found in native starch granules as a result of gelatinization. Many factors affect amylose–lipid complex formation including moisture content, reaction temperature, heating time, food processing methods, edible oil types and amylose content (Putseys et al. 2010). Previous studies mainly focused on complexing free fatty acids with starch (Cervantes-Ramírez et al. 2020; Meng et al. 2014; Raphaelides and Georgiadis 2008; Reddy et al. 2018; Wang et al. 2016). Ai et al. (2014) reported that adding 10% of free fatty acids to cassava starch (29% amylose content) reduced enzymatic hydrolysis by 66.15% with palmitic acid 66.4%, oleic acid 66.8% and linoleic acid 72.0%. Only a few studies (Farooq et al. 2018; Luangsakul and Ritudomphol 2018; Krishnan et al. 2020) have evaluated the effect of adding cooking oils to starch on RS formation and physicochemical changes.

Native starches from different botanical sources have diverse physical properties, compositions and molecular structures. Cereal starches have A-type crystal patterns and are more densely packed. Tuber starch presents B-type crystal patterns, while legume starch generally gives a C-type pattern consisting of a combination of A- and B-type crystallites which are more resistant against enzymatic hydrolysis than cereal starches (Jane 2009). The types of granules, size, shape and percentage vary depending on the starch granule distribution from different sources (Jane 2009). The diverse characteristics of different starch types interact with edible oils in contrasting ways. Krishnan et al. (2020) found that cooking fat changed the RS formation in rice, while the addition of palm oil significantly affected the physicochemical properties and in vitro digestibility in rice starch. The starch-oil complex also showed V-type peak forms in X-ray diffractograms, indicating the formation of tightly packed crystalline amylose–lipid complexes (Farooq et al. 2018). Investigation of these factors on RS5 formation using native starch and available cooking oil may be more practical, with lower cost and more commercial availability for food factory applications than using fatty acids. Vegetable oils are classified by their saturated fatty acids (non-double bond; SFAs), monounsaturated fatty acids with one double bond (MUFAs,) and polyunsaturated fatty acids with two or up to six double bonds (PUFAs,). However, vegetable oil has a specific fatty acid distribution depending on the plant source. Therefore, this study was carried out to investigate the effects of adding different vegetable oil types including coconut oil (high-SFA), rice bran oil and sunflower oil (high-MUFA) to starches from different botanical origins. The physicochemical properties, amylose–lipid complex formation and digestion properties of the starches were examined.

Materials and methods

Starch preparation

Commercial corn, mung bean, rice starches, coconut oil, rice bran oil and sunflower oil were purchased from a supermarket in Maha Sarakham, Thailand. Banana starch was obtained from unripe banana fruits. Green banana fruits (Musa sapientum Linn, ABB group, Kluai Namwa), at the second stage of ripening, 115–120 days after bloom were purchased from a local market in Maha Sarakham, Thailand. Banana starch was isolated following the procedure of Vatanasuchart et al. (2012), with slight modifications. The starch was dried in an oven at 45 °C to obtain approximately 10% moisture content (wet basis, wb) then finely ground, passed through a 70-mesh sieve, packaged in plastic bags and kept in a refrigerator until required for analysis.

Preparation of amylose-oil complexes

Starch-oil complexes were prepared following the methods of Reddy et al. (2018) with some modifications. The vegetable oil solution was prepared by dissolving 0.8 g (2% of starch, dry basis, db) of each oil into ethanol (35%) with the ratio of oil to ethanol 2:15 w/v. The oil solution was gently added to raw starches (40 g) of banana, mung bean, corn and rice. The starch-oil mixtures were mixed well using a hand dough mixer before being heated uncovered at 80 °C in a water bath for 10 min to evaporate the ethanol. The mixture was added with distilled water (360 mL) and vigorously stirred by hand to obtain a starch slurry. The slurry was then heated in a water bath at 95 ± 2 °C for 1 h and then cooled to 70 °C with continuous mechanical agitation to prevent coagulation. The starch paste samples were cooled to 50 °C and left at this temperature for 2 h to obtain starch-oil complexes. The starch-oil complexes were then cooled to room temperature, washed three times with 50% ethanol and centrifuged at 3000 rpm for 15 min. The final precipitated complex was collected and dried in an air oven at 45 °C to obtain moisture content lower than 10% before grinding into a fine powder using a coffee grinder. The control samples were prepared following the same procedure without oil addition.

Determination of apparent amylose content

Apparent amylose contents of native starch and starch-oil complexes were determined using the method of Juliano (1971) with slight modifications. Starch (100 mg) was dispersed in 1.0 mL ethanol (95%) and then added with 9 mL of 1 M NaOH. The mixture was boiled with stirring at 300 rpm for 20 min before cooling to room temperature and diluted to 100 mL with deionized water. The diluted solution (0.5 mL) was mixed with 5 mL of deionized water, followed by addition of 0.5 mL 1 M acetic acid and 1.0 mL I₂/KI solution (0.0025 M I₂ and 0.0065 M KI). The absorbance was measured at 620 nm (Shimadzu UV-1800 UV/visible scanning spectrophotometer).

Determination of complexing index (CI)

The complex formation between starch and lipid was determined following the procedure of Meng et al. (2014). Starch-oil complex powder (5.0 g) was mixed with 25 mL of distilled water in a 50 mL centrifuge tube for 3 min using a vortex mixer before centrifuging for 15 min at 3000 rpm. Then, 500 µL of the supernatant was added with 15 mL of deionized water and 2 mL of an iodine solution (2.0% w/w KI and 1.3% w/w I₂ in deionized water) in a 25 mL test tube and mixed well. The UV absorbance was measured at 620 nm (Shimadzu UV-1800 UV/ visible scanning spectrophotometer). Starch pastes that contained only starch were used as a reference. The CI was calculated using the equation below:

$$CI\left(\%\right)=\left(\frac{ABSofreference-ABSofstarch-oilcomplex}{\left(A,B,S,,,o,f,,,r,e,f,e,r,e,n,c,e\right)}\right)\times 100$$

where ABS _reference is the absorbance value of starch-only pastes and ABS _{starch-oil complex} is the absorbance of the starch-oil complex.

Determination of digestion properties of starch

The digestion properties of starch were evaluated through starch fractions (RDS, SDS, and RS) which were determined using a Megazyme digestible and resistant starch assay kit (K-DSTRS) (Megazyme International Ireland Ltd., Bray, Ireland) according to Englyst et al. (1992). Glucose was measured with GOPOD reagent using a Megazyme glucose assay kit to reflect the rate of starch digestion rate in vivo.

Measurement of X-ray diffraction (XRD)

X-ray diffraction (XRD) patterns of native starch and starch-oil complexes were obtained using TTRAX III Multipurpose System, Rigaku, Japan. This generated XRD patterns using Cu Kα radiation (λ = 0.1542 nm) produced in a sealed tube. The XRD machine was operated at 30 mA and 40 kV as an X-ray source in the range of 4° to 35° (2θ angle) with a scanning step width of 0.04 and count time 1 s/step (5–35 degrees).

Determination of swelling power

The swelling power was determined following the modified method of Li and Yeh (2001). Starch-oil complex samples and native starch (0.1 g, db) were added with distilled water (10 mL) in a 50 mL centrifuge tube, mixed well and placed in a water bath at 55, 65, 75, 85 and 95 °C for 30 min with a low level of stirring. The solution was cooled to room temperature and centrifuged (3000 rpm for 15 min). The supernatant was removed and the sediment was collected and dried at 105 °C until achieving constant weight (W1). The water solubility index (WSI) was calculated as follows:

$$WSI=\frac{W1}{(Weightofsample)}\times 100\%$$

Swelling power (SP) was calculated as follows:

$$SP(\frac{g}{g})=\frac{W1}{(Weightofsample)\times (100-WST)}$$

Measurement of pasting properties

Pasting properties of the starches were measured using a Rapid Visco Analyzer (RVA) (Newport Scientific Instruments, Sidney, Australia). The modified starch sample was dispersed in water (10%, w/w), held at 50 °C for 1 min, then heated to 95 °C at a rate of 6 °C/min, then held at 95 °C for 5.5 min and cooled from 95 to 50 °C at a rate of 6 °C/min. The suspension paste was held at 50 °C for 2 min. The paste was stirred at 160 rpm throughout the experiment.

Measurement of Fourier-transform infrared spectroscopy (FTIR)

FTIR of the defatted native and starch-oil complex samples was conducted using a Nicolet iS50 FT-IR Spectrometer (Thermo Fisher Scientific, USA). Each spectrum was scanned at room temperature and recorded at a transmittance mode from 4000 to 400 cm⁻¹.

Statistical analysis

Experimental data were processed using analysis of variance (ANOVA) and expressed as mean value and standard deviation. Duncan’s multiple range test was conducted to assess significant differences among experimental mean values (p < 0.05). All statistical computations and analyses were conducted using SPSS version 23.0 for Windows.

Results and discussion

Effect of oil addition on complexing index (CI)

The CI values were used to determine the degree of complexity between amylose and oils. Figure 1A shows that starches from different botanical sources and different oil types had diverse CI values. Rice bran oil indicated the highest CI values with all starch types. These results concurred with Krishnan et al. (2020) who found that brown rice cooked with rice bran oil had the highest CI, with a significant increase in RS content compared to other oils. Krishnan et al. (2020) have also reported that CI positively correlated with RS at 0.94 (p < 0.001) and SDS negatively correlated with RDS at − 0.97.

Fig. 1.

Effect of oil addition on complexing index (CI) A and apparent amylose content B of native banana, mung bean, corn and rice starches with 2% addition of different types of vegetable oil. Note: Means ± SD with different superscripts in the same graph are significantly different, while means ± SD with the same superscript in each group are not significantly different (p > 0.05)

Apparent amylose content

Amylose contents of native corn, rice, banana and mung bean starches were 32.66%, 22.98%, 29.64% and 31.35%, respectively (Fig. 1B). When oil was added to corn and rice starches, their apparent amylose contents decreased compared with native starch. The formation of amylose–lipid complexes reduced the ability to complex with iodine (Jane 2009). Similar trends were observed in control samples of rice and corn starches (Fig. 1B) because lipids present in native starch can also form amylose–lipid complexes reducing the amylose available to form complexes with iodine (Derycke et al. 2005; Zhang et al. 2019a, b). Moran (2021) also reported that amylose–lipid complexes are commonly found in native cereal starch granules, mostly containing free fatty acids, phospholipids and lipids. Increases in the apparent amylose content of banana and mung bean starches with oil added were observed compared with native starch. These increases were attributed to degradation of the banana and mung bean starch molecules that formed helical structures with iodine-binding complexes instead of complexes with oil (Cordeiro et al. 2018).

Digestion properties of native starch and starch-oil complexes

In corn Fig. 2A and rice starches Fig. 2B, addition of oil resulted in a decrease in RDS content compared with the control whereas SDS and RS increased. The RS content of each starch increased with addition of different types of oils. RS contents of corn starch with coconut, rice bran and sunflower oil were 13.37%, 15.21% and 10.75%, respectively and higher than the control sample (3.65%). Rice starch without oil showed the lowest RS content (2.98%), while RS contents of rice starch added with coconut, rice bran and sunflower oil significantly increased by 4.98%, 4.32% and 3.39%, respectively. The resistance of starch to amylase enzyme increased because the crystalline structure changed from the A pattern to a single V-amylose (Putseys et al. 2010). Enzyme hydrolysis of granular starch to reduce digestion depends on the botanical origin of the starch and also on the amount of oil added. Farooq et al. (2018) reported that addition of oil reduced starch digestion in vitro in brown rice and glutinous rice but did not affect sticky rice and depended on the amount of amylose in the rice. The A-type crystal structural pattern (corn and rice) was more sensitive to digestive enzymes than type B crystals because the A granules had relatively large openings with pores, whereas the B-type granules had fewer defined holes and channels without pores (Kim and Huber 2008). The RDS, SDS and RS values of native banana starch were 0.99%, 21.67% and 76.47%, respectively. The RS content of unripe banana flour in this study was higher than reported by Moongngarm et al. (2014) at 48.88% in unripe banana harvested at 105 days after bloom due to differences in banana cultivars and growing conditions. Figure 2C shows that single treatment of banana starch with sunflower oil gave 51.92%, while banana starch containing coconut and rice bran oil had RS contents of 42.85% and 43.41%, respectively. In B-type starch crystals, adjacent double helices are mainly linked by hydrated bridges as a result of the limited extent of hydrogen bonds. Rupture of hydrated water bridges caused the two adjacent double helices to move apart and reorientate in a non-perfectly parallel crystalline array (Hoover and Vasanthan 1994). The RDS, SDS and RS contents of native mung bean starch were 6.65%, 40.09% and 54.66%, respectively. The RDS content of mung bean starch with rice bran and sunflower oil significantly reduced from 38.01% in the control to 0.38% and 0.26%, respectively while a significant increase was recorded in SDS content (63.32% with rice bran oil and 73.33% with sunflower oil (Fig. 2D). Rice and corn starches showing A-type structure were more susceptible to degradation than the B-type indicated in banana and C_(A)-type in mung bean starches. The A-type had a more porous starch granule structure than the B-type and C_(A)-type since enzymatic damage was only on the granule surface, whereas enzymatic hydrolysis was deeper with A-type crystallinity (Kim and Huber 2008). The starch granules with B or C pattern are generally more resistant to amylolytic digestion than those with A pattern because this difference could be attributed to chain lengths of the branch-chain double helices and the packing of double helices in starch granules (Jiang et al. 2010).

Fig. 2.

Starch fractions classified based on digestion of corn A, rice B, banana C and mung bean D with 2% addition of different types of vegetable oil. Note.¹: Means ± SD with different superscripts in the same graph are significantly different, while means ± SD with the same superscript in each group are not significantly different (p > 0.05)

X-ray diffraction

Native corn and rice starches displayed typical A-type patterns, generalized by peaks (2θ) at 15.2°, 17.0°, 17.5°, 18.0° and 23.2° (Farooq et al. 2018). Starch-coconut, rice bran and sunflower oil complexes displayed peaks (2θ) at 15.0°, 17.2°, 17.9°, 19.8° and 23.2°, with a new maximum position in the V-type format at 12.8° and 19.8°, as shown in Fig. 3A and B that related to naturally present amylose–lipid complexes. The V-type pattern was outstanding, although fragments of the A-type pattern were still visible in corn and rice starches that were added with oil. X-ray diffraction results were in agreement with Luangsakul and Ritudomphol (2018) who found that cooked rice with oil showed A + V patterns with reflections at 15.2°, 17.3°, 18.2°, 20° and 23° (2θ). Native banana starch displayed a strong diffraction signal (2θ) at 17.0° and four weaker peaks at 15.0°, 16.3°, 17.1°, and 23.2°, respectively (Fig. 3C) showing a B-type pattern (Waliszewski et al. 2003). All banana starches treated with vegetable oil exhibited no change in crystalline pattern, except for a slight reduction in diffraction peak intensities displaying type B-pattern. Mung bean starch (native) showed 2θ peaks at 15.0°, 17.0°, 17.4°, 18.0° and 23.2° displaying C_(A)-type, mixed between A and B type crystals that differ in compactness, while both showed a double helical structure (Zhang et al. 2019a, b). Mung bean starch heated without added oil showed reduced 2θ peak intensity at 15.0°, 17.0°, 18.0° and 23.2°, with a broader peak at 17.1° to 18.1° and another at 19.8° (Fig. 3D). The C_(A)-type pattern disappeared, indicating that the crystal structure was destroyed during heat treatment. However, when oil was added to the starch system and heated, an additional peak at 17° shown in banana and mung bean starches had high intensity, indicating that this peak corresponded to recrystallization of the B-polymorph structure (Li et al. 2015).

Fig. 3.

X-ray diffraction patterns of corn A, rice B, banana C and mung bean starches D with 2% addition of different types of vegetable oil

Swelling power

The swelling power of native starch increased at higher temperatures from 55 to 95 °C, as shown in Fig. 4. Changes in swelling power and viscosity varied depending on the starch source. Native rice starch with the smallest granules showed the highest swelling power (Fig. 4B). This positively correlated with short chain amylopectin that easily bound to water molecules through hydrogen bonds (Singh et al. 2010). However, when oil was added to corn, rice, banana and mung bean starches, the starch-oil complex showed significantly decreased swelling power (p < 0.05). The amylose–lipid complexing process retarded starch granule swelling because the lipid formed an insoluble layer around the granules, thereby preventing the entry of water. Amylose–lipid complexes interact via helical bonds that did not allow interaction with water molecules (Raphaelides and Georgiadis 2008). Moreover, the degree of starch crystallization and amount of amylose–lipid complexes also affected the swelling power of starch (Wang et al. 2016).

Fig. 4.

Swelling power of native starches and starch-oil complexes of corn starch A, rice starch B, banana starch C and mung bean starch D

Pasting properties (RVA)

Paste viscosities of starchy foods depended on the starch sources and oil types added. Starches with oil added showed low peak viscosities Table 1. The decrease of pasting viscosity after adding oil under thermal processing occurred because of starch granule damage and partial gelatinization leading to reduction of water absorption, swelling and reduced polymerization of the starch chains to form amylose–lipid complexes (Liu et al., 2022). Pasting temperatures of starches with oil varied from 74.72 to 85.97 °C in corn, 82.51 °C to 89.37 °C in rice, 82.15 °C to 82.08 °C in banana and 74 °C to 91.58 °C in mung bean starch. Increase in the gelatinization temperature indicated strong and compact structure of the starch-oil complex making it difficult for water molecules to penetrate the matrix of the starch system and dissolve the starch. Amylose–lipid complex formations limited starch granule swelling and were resistant to heating (Birt et al. 2013). The pasting time of both starch-oil complex samples and control was delayed from 4 min in native starch to 7 min, approximately. This may be due to an increase in the stability of the starch granule to swell and collapse, indicating that a higher temperature is required for starch gelatinization (Dorantes-Campuzano et al. 2022).

Table 1.

Effect of adding vegetable oils to starches on pasting properties of native starches and starch-oil complexes

Starch type	Added oil type	Viscosity in rapid visco units (RVU)					Peak time (min)	Pasting temperature (°C)
		Peak viscosity	Trough	Breakdown	Final	Setback
Corn (A-type)	Native starch	345.85 ± 9.51a	214.19 ± 2.80a	114.99 ± 8.10a	331.43 ± 5.17a	117.24 ± 2.37a	4.96 ± 0.03c	74.72 ± 0.43c
	Control	121.40 ± 2.37b	107.69 ± 2.03b	13.71 ± 0.33b	165.96 ± 3.39b	58.26 ± 1.35b	7.00 ± 0.00a	83.17 ± 0.02b
	Coconut oil	93.34 ± 1.44d	86.24 ± 0.85e	7.10 ± 0.59bc	122.11 ± 3.56d	34.87 ± 1.95c	6.96 ± 0.03a	85.97 ± 0.43a
	Rice bran oil	109.15 ± 1.05c	102.88 ± 1.27c	6.26 ± 0.21c	139.73 ± 3.26c	36.84 ± 1.99c	6.90 ± 0.03b	85.33 ± 0.30a
	Sunflower oil	102.23 ± 3.26c	95.69 ± 3.30d	6.53 ± 0.04c	132.64 ± 5.93c	36.95 ± 2.63c	7.00 ± 0.00a	83.08 ± 0.78b
Rice (A-type)	Native starch	291.89 ± 9.07a	233.66 ± 5.78a	58.22 ± 3.80a	435.69 ± 6.58a	194.67 ± 8.28a	6.38 ± 0.10c	82.51 ± 0.14b
	Control	105.61 ± 3.84d	95.05 ± 2.79c	10.55 ± 1.06bc	125.58 ± 1.01d	30.52 ± 1.77d	7.00 ± 0.00a	83.00 ± 0.56b
	Coconut oil	98.94 ± 8.70d	92.11 ± 9.28c	6.83 ± 0.57c	129.99 ± 4.18d	24.75 ± 6.79d	6.95 ± 0.04a	89.37 ± 1.85a
	Rice bran oil	121.91 ± 9.38c	119.61 ± 9.2b	2.30 ± 0.09d	164.95 ± 1.25c	45.33 ± 6.78c	6.77 ± 0.04b	80.71 ± 0.03c
	Sunflower oil	137.38 ± 0.34b	123.80 ± 3.22b	13.58 ± 2.88b	187.61 ± 2.11b	63.80 ± 1.10b	5.60 ± 0.11d	78.81 ± 0.49d
Banana (B-type)	Native starch	434.89 ± 7.84a	275.94 ± 8.75a	158.97 ± 0.91a	502.67 ± 9.95a	226.72 ± 1.20a	4.48 ± 0.07b	82.15 ± 0.17b
	Control	144.75 ± 2.33d	127.89 ± 1.87c	16.86 ± 0.46e	205.16 ± 6.24d	77.27 ± 4.77e	6.97 ± 0.04a	88.81 ± 0.82a
	Coconut oil	201.02 ± 2.07c	179.21 ± 1.35b	20.19 ± 0.62d	303.94 ± 3.79c	123.11 ± 2.35d	7.00 ± 0.00a	78.81 ± 1.78c
	Rice bran oil	210.38 ± 6.82b	180.53 ± 6.15b	29.84 ± 0.67b	327.92 ± 6.30b	154.05 ± 9.80b	7.00 ± 0.00a	81.65 ± 0.87b
	Sunflower oil	195.23 ± 1.82c	173.18 ± 4.36b	25.38 ± 0.50c	307.05 ± 2.80c	137.20 ± 1.48c	7.00 ± 0.00a	82.08 ± 1.27b
Mung bean (C (A)-type)	Native starch	453.19 ± 7.08a	286.64 ± 3.87a	169.89 ± 6.13a	484.22 ± 8.00a	197.58 ± 6.16a	4.22 ± 0.04b	74.00 ± 0.47d
	Control	148.33 ± 7.10b	128.27 ± 4.72c	17.39 ± 4.70b	214.59 ± 4.93b	88.53 ± 7.74b	7.00 ± 0.00a	83.67 ± 1.23c
	Coconut oil	107.08 ± 3.53e	98.08 ± 4.42d	9.03 ± 5.34c	148.80 ± 6.24e	53.73 ± 7.47d	7.00 ± 0.00a	89.49 ± 0.83b
	Rice bran oil	120.41 ± 4.37d	102.30 ± 4.49d	18.11 ± 0.20b	171.05 ± 4.99d	68.75 ± 1.61c	7.00 ± 0.00a	91.58 ± 0.93a
	Sunflower oil	137.51 ± 2.56c	118.52 ± 2.49c	18.98 ± 0.76b	200.21 ± 4.63c	81.68 ± 2.29b	7.00 ± 0.00a	83.27 ± 1.61c

Note¹: Mean value ± SD with different superscripts in the same column are significantly different, while means ± SD with the same superscript in each starch group are not significantly different (p > 0.05)

Fourier-transform infrared (FTIR) spectroscopy

The FTIR spectra corresponded to natural bending and stretching energies of molecular bonds in the samples. When edible oil was added to the starch, FTIR spectra showed three new peaks at 1746, 2854 and 2926 cm⁻¹ as presented in Fig. 5. The peak at 1746 cm⁻¹ was attributed to C = O stretching vibration of an ester group, confirming the formation of ester carbonyl groups of oil from triglycerides (Grotowska and Wawrzeńczyk 2002), while peaks at 2926 cm⁻¹ and 2854 cm⁻¹ were attributed to stretching vibrations of C–H in the molecular chain of CH₂ and CH₃ stretching (Cervantes-Ramírez et al. 2020).

Fig. 5.

FTIR spectra of corn A, rice B, banana C and mung bean starches D with 2% addition of different types of vegetable oil

Conclusions

Adding vegetable oil to starch altered the polymorphic crystal structure depending on the botanical origin of the starch. The crystal structure of native corn and rice starch (A-type) changed to a mixed A + V form, native mung bean starch (C_A-type) changed to a B-pattern, while banana starch remained unchanged as a B-pattern. Adding oil to starch decreased viscosity when pasting temperature increased. Adding oil to starch reduced RDS content, increased the formation of starch-oil complexes, and also increased SDS and RS contents in corn and rice starches. The RS content of treated banana and mung bean starch decreased compared with native starch but RS and SDS increased when oil was added compared with the control sample. These results suggested that different botanical origins of starches diversely interact with vegetable oils. Mung bean starch with rice bran oil and sunflower oil (2%) gave maximum improved SDS and RS formation. It was a promising process to prepare functional starch high in SDS and RS with possible use as a functional ingredient in food products. Further studies related to modified starch are required to investigate the stability of amylose–lipid complexes in food processing, starch digestibility and the glycemic response.

Abbreviations

CI:

Complexing index

RDS:

Rapidly digestible starch

RS:

Resistant starch

SDS:

Slowly digestible starch

SP:

Swelling power

WSI:

Water solubility index

FTIR:

Fourier-transform infrared spectroscopy

RVA:

Rapid visco analyzer

XRD:

X-ray diffraction

Author contributions

RP conceived, carried out the experiments, methodology, formal analysis, and wrote the MS; AM funding acquisition, supervised the work, methodology, wrote the MS, and edited the manuscript.

Funding

This research was financially supported by the Thailand Research Fund, Thailand (TRF; Royal Golden Jubilee Ph.D. (RGJ-PhD) Program (PhD/0062/2560), the National Research Council of Thailand and Faculty of Technology, Mahasarakham University, Thailand.

Declarations

Conflict of interest

The authors declare that they have no competing interests.