Effects of oat lipid-starch complexation on starch structure, freeze-thaw stability and pasting characteristics under Ultra-high pressure treatment

Abstract

Ultra-high pressure (UHP) is an emerging green non-thermal technology that can modify macromolecules like starch, altering its structure and properties. Using oat starch as the raw material and adding 0–15 % oat lipids, starch-lipid complexes were prepared by treatment at 0–500 MPa. This study investigated how UHP-induced complexation between oat lipids and starch affects the microstructure and physicochemical properties of oat starch. The results showed that at 400–500 MPa, lipids promote the formation of starch granules into network-like aggregates or large clusters, reducing relative crystallinity and short-range molecular order. The crystal structure type transitions from type A to type V, while increasing the double helix structure of starch molecules. These structural changes significantly influenced the gel texture properties of starch, enhancing its thermal stability and freeze-thaw stability. Furthermore, the formation of oat starch-lipid complexes via hydrogen bonding under UHP treatment delayed the gelatinization of oat starch and inhibited its retrogradation.

Graphical abstract

Highlights

• •The effect of oat lipids on the modification of oat starch under UHP was studied.
• •Using UHP and oat lipids improve thermal and freeze-thaw stability of starch.
• •Provides a new green method for preparing oat starch-lipid complex.

1. Introduction

Oats are a grass species belonging to the Avena genus within the Poaceae family. Oats are rich in nutrients, containing significant amounts of starch, lipids, protein, and β-glucan, as well as various minerals, vitamins, and phenolic compounds. The starch content in oats can reach up to 60 % (Zhu, 2017). Unlike starch in other grains, oat starch has smaller particle size, well-developed particle surfaces, and is less prone to degradation, making it highly valuable in food processing. The lipid content of oats is 5–10 %, with most being unsaturated fatty acids (Zhang et al., 2023). They also contain sterol esters, glycerides, phospholipids, and glycolipids, making them highly nutritious (Li et al., 2021). Oats are widely cultivated in multiple regions of China, with the largest cultivation area in Inner Mongolia. However, the deep processing of oat-based foods in Inner Mongolia is insufficient, necessitating urgent research in this area.

Natural oat starch exhibits properties such as low shear stress resistance, low gel transparency, thermal decomposition, and high viscosity, which impose certain limitations on its applications in food production and processing (Falsafi et al., 2019). Therefore, modification of native oat starch is necessary to improve its physical and chemical characteristics. Various methods exist for starch modification, with the primary approaches typically being chemical, physical, and enzymatic modification. Among these, chemical modification is widely adopted in both laboratory and industrial settings due to its low cost, short processing time, and simple procedures. However, chemical modification methods and potential residual chemicals may pose risks to human health. In contrast, physical modification offers higher safety as it does not involve chemical treatment, while still effectively improving the starch's physicochemical properties, structure, and functionality (Rostamabadi et al., 2022). Consequently, physical modification methods have gained significant development and application. Common physical modification techniques include heat treatment, extrusion, ultrasonication, microwave treatment, and high-pressure processing (Rashwan et al., 2024).

Ultra-high pressure (UHP), also known as high hydrostatic pressure (HHP), is a new non-thermal food processing technology. Compared with traditional heat treatment, UHP is green and can change the structure of macromolecules by breaking non-covalent bonds at room temperature, which can be used for the modification of macromolecules such as starch (Błaszczak et al., 2005), and related studies have pointed out (Almeida et al., 2023) that in recent years, UHP has been utilized to improve a wide range of natural starch's undesirable properties. Regarding the effect of ultra-high pressure (UHP) treatment on oat starch properties, one study has demonstrated that UHP significantly influences the structure, gelatinization characteristics, and in vitro digestibility of oat starch (Zhang, Zhang, Wang, et al., 2022). Oat starch and lipids are key nutrients in oats that play a crucial role in determining various properties of food. Therefore, the interaction between starch and lipids influences the properties of food (De Pilli & Alessandrino, 2020). Some researchers have investigated the improvement of lipid-induced effects on the rheological and thermal properties of starch by mixing lipids into starch (Li et al., 2021). However, mixing lipids with starch does not form stable composites, and there is a lack of research on their microstructural aspects. Other researchers have improved the thermal stability and in vitro digestibility of starch by preparing starch-lipid composites through thermal treatment (Shen et al., 2024). However, this thermal treatment involves high-temperature heating of starch to gelatinize it and form composites with lipids, which may lead to material decomposition and loss during heating. In contrast, ultra-high pressure processing can prepare stable starch-lipid composites while avoiding the potential loss of starch and the formation of other substances caused by high-temperature heating, which may damage starch functional groups and chemical bonds. Currently, no scholars have utilized ultra-high pressure processing, a non-thermal treatment method, to prepare oat starch-lipid complexes and conduct related research. Therefore, for the deep processing and development of oat-based foods, it is necessary to investigate the effects of oat lipids on the structure and properties of starch under ultra-high pressure conditions.

This study aims to prepare oat starch-lipid complexes using ultra-high pressure (UHP) technology and investigate the effects of oat lipids on the structural properties and related characteristics of oat starch under UHP treatment. Using hull-less oats as raw material, oat starch and oat lipids were extracted. With reference to the natural lipid content in oats (5–10 %), varying proportions of oat lipids (0–15 %) were incorporated into oat starch. The mixtures were then subjected to UHP treatment at different pressure levels (0–500 MPa) to form oat starch-lipid complexes. We will observe granule morphology and determine the following properties: relative crystallinity, short-range molecular order, thermal properties, gel texture properties, freeze-thaw stability, and pasting characteristics. Comparative analysis will be conducted on the structure and properties of different samples versus native oat starch (0 % lipid, 0 MPa). This approach will elucidate how oat lipids influence the microstructure and related characteristics of oat starch under UHP treatment.

2. Materials and methods

2.1. Materials

Naked oat (purchased from Inner Mongolia Academy of Agricultural and Animal Husbandry Sciences, origin: Inner Mongolia Autonomous Region, China), petroleum ether (AR), sodium hydroxide (AR), and neutral protease (Sareptase,50 U/mg, Shanghai Macklin Biochemical Co., Ltd.).

2.2. Isolation and purification of oat starch

Naked oats are crushed with a pulverizer. It was passed through an 80 mesh sieve and then pulverized twice with an ultra-micro pulverizer to obtain oat flour for spare use. The extraction method was referred to the previous study (Shah et al., 2016) and improved. Oat powder was weighed, excess petroleum ether (AR) was added, stirred and placed on a shaker to shake for 30 min, and then centrifuged in a centrifuge (3500 r/min, 15 min) after shaking. After centrifugation, the upper layer of yellow liquid was discarded and the procedure was repeated three times. The dried defatted oat flour was added with sodium hydroxide solution (pH = 10) at a ratio of 1:10 and stirred in a water bath at a temperature of 35 °C for 2 h. At the end of the reaction, it was filtered and centrifuged. After centrifugation, the upper layer of liquid was discarded, the yellow powder layer of the precipitate was scraped off, the white precipitate was retained, washed with distilled water and centrifuged. The crude oat starch was then freeze-dried with a lyophilizer to obtain crude oat starch. Protease enzymes are efficient in extracting oat starch (Kaur et al., 2022) and therefore were utilized to purify the starch. Crude oat starch was weighed and added to distilled water at a material-liquid ratio of 1:6 with stirring in a water bath, and neutral protease was added at a dosage of 7 U/g at an enzyme digestion temperature of 45 °C and an enzyme digestion time of 2 h. After the reaction, oat starch was washed with distilled water and centrifuged to obtain purified oat starch, which was freeze dried and then prepared for use. The purity of the oat starch was measured to be 96.14 ± 0.22 %, indicating high purity.

2.3. Isolation of oat lipids

Weigh the appropriate amount of oat flour into a beaker, according to the material-liquid ratio of 1:5 to add petroleum ether (AR), with a low-temperature shaker shaking, shaking time of 1 h, shaking is completed, and then put into the centrifuge for centrifugation (3500 r/min, 15 min). After centrifugation, the upper layer of yellow liquid was poured out and collected. Repeat the above operation three times to collect the upper layer of yellow liquid until the last centrifuged upper layer of liquid was colorless, then stop collecting. The collected liquid was poured into a rotary evaporator for distillation under reduced pressure to remove petroleum ether at 40 °C for 3 h. After distillation, the oat lipids were removed from the evaporation flask and sealed for refrigeration. The purity of the isolated oat lipid was 98.58 ± 0.48 %, indicating high purity.

2.4. Preparation of oat starch-lipid complexes by UHP treatment

Oat lipids were added into oat starch in the proportion of 0 %, 5 %, 10 % and 15 % respectively for compounding, and then distilled water was added to formulate a suspension of 15 % (m/v). After the preparation was completed, it was stirred well and packed into polyethylene sealed bags and processed with vacuum packing confidential seal. The sealed samples were subjected to ultra-high pressure (UHP) treatment using an ultra-high pressure device (HHP-600, Kefa High Pressure Food Processing Inc. China) at 0 MPa, 100 MPa, 200 MPa, 300 MPa, 400 MPa and 500 MPa, respectively. After the UHP treatment, the oat lipids and starch in the samples would be complexed to form starch-lipid complexes, and then the samples were freeze-dried by a freeze-dryer. The moisture content of all samples was measured to be below 2 % (Table S1). The samples were sealed and refrigerated for future use.

2.5. Microstructures

2.5.1. Scanning electron microscopy (SEM)

The oat starch-lipid complex was examined using a scanning electron microscope (Regulus 8100, Hitachi High-Tech Corporation, Japan) to observe the morphology of starch granules. The dried sample powder was fixed on a metal stub with black double-sided adhesive tape and observed at an acceleration voltage of 3.0 kV with an 800× magnification to analyze the granular structure of the starch.

2.5.2. X-ray diffraction (XRD)

The oat starch-lipid complex was analyzed using an X-ray diffractometer (MiniFlex600, Rigaku Corporation, Japan) to obtain the X-ray diffraction (XRD) patterns. The testing method was slightly modified from a previous study (Gu et al., 2022). Spectra were collected under Cu K-α radiation with a tube voltage of 40 kV and current of 40 mA. Continuous scanning was performed over a 2θ range of 5–45° with a step width of 0.05° and a scanning speed of 5°/min. Based on the obtained XRD patterns, the relative crystallinity of the samples was calculated as the ratio of crystalline area to total area.

2.5.3. Fourier-transform infrared spectroscopy (FTIR)

The oat starch-lipid complex was analyzed using a Fourier transform infrared spectrometer (FTIR, Thermo Fisher Corporation, USA) to obtain FTIR spectra. The testing method was adapted from a previous study (P. Guo et al., 2018). The oat starch-lipid complex was thoroughly mixed and ground with high-purity potassium bromide (KBr) in an agate mortar, then pressed into pellets using a pellet press. The samples were scanned in the wavenumber range of 400–4000 cm⁻¹ with 16 scans per spectrum. The short-range molecular order was determined by calculating the absorbance ratio values at 995/1022 cm⁻¹ and 1045/1022 cm⁻¹.

2.6. Textural properties

The gel textural properties of the oat starch-lipid complexes were determined using a texture meter (CTX texture meter, AMETEK Brookfield, USA).The gel test method was modified from a previous study (Liu et al., 2021). The oat starch-lipid complex was reconstituted into a 12 % (m/v) suspension by adding distilled water and heated in a boiling water bath with stirring for 20 min. After cooling to room temperature, the oat starch-lipid gel was stored in a refrigerator at 4 °C for 24 h. The sample was then stored in the refrigerator at 4 °C for 2 h to make an oat starch-lipid gel. Before testing, the samples were equilibrated at room temperature for 1 h. The test mode was selected as TPA mode with TA4 probe, 50 % compression ratio, 1.0 mm/s test speed, 1.0 mm/s rise speed and 5.0 g trigger force.

2.7. DSC thermal properties

The thermal properties of the oat starch-lipid complex were determined using differential scanning calorimetry (DSC) with a differential scanning calorimeter (DSC200, Hitachi High-Tech Corporation, Japan). The testing method was modified from a previous study (W. Zhang et al., 2025). Briefly, 3 mg of the oat starch-lipid complex was weighed into an aluminum crucible, mixed with 9 μL of deionized water, and hermetically sealed. After equilibrating at room temperature for 12 h, the samples were scanned from 30 °C to 200 °C at a heating rate of 10 °C/min. An empty aluminum crucible was used as the reference. The measured parameters included the onset temperature (To), peak temperature (Tp), conclusion temperature (Tc), and enthalpy change (ΔH).

2.8. Freeze-thaw stability

The freeze-thaw stability of oat starch-lipid complexes was evaluated by measuring syneresis after subjecting them to freeze-thaw cycles. Based on a previously reported method (Li et al., 2024) with modifications, the oat starch-lipid complexes were dispersed in distilled water to prepare a 5 % (m/v) suspension. The suspension was heated in a boiling water bath with continuous stirring for 30 min, cooled to room temperature, and then stored in a − 18 °C freezer for 24 h. After freezing, the samples were thawed at 30 °C for 2 h and centrifuged (3500 r/min, 15 min). The weights of the samples before centrifugation and the supernatant after centrifugation were recorded. This freeze-thaw process was repeated four times, and the syneresis rate was calculated. The formula is as follows:

$$\text{Syneresis}=\frac{\text{Weight of supernatant after centrifugation}\left(\mathrm{g}\right)}{\text{Weight of sample before centrifugation}\left(\mathrm{g}\right)}\times 100\%$$

2.9. Pasting properties

The pasting characteristics of oat starch-lipid complexes were determined using a rapid viscosity analyzer (RVA viscometer), the test method was referred to Zhang, Zhang, Wang, et al. (2022). Distilled water was added to the oat starch-lipid complexes and configured into a suspension with a concentration of 5.5 % (m/v), and the temperature control program was set and then put into a Rapid Viscometer Analyzer (RVA-4500, Perten Instruments, Sweden) for testing.

2.10. Statistical analysis

The experimental data were measured three times independently, plots were generated using Origin 2024 software, data were processed using SPSS 27 software and one-way analysis of variance (ANOVA) was done and Duncan's Multiple Range Test was performed to assess the differences between the experimental means (p < 0.05).

3. Results and discussion

3.1. Microstructure analysis

3.1.1. SEM analysis

Fig. 1 shows the SEM images of oat starch subjected to different ultra-high pressure (UHP) treatments and oat starch-lipid complexes with varying lipid contents. It can be observed that the untreated oat starch granules exhibit an irregular spherical shape, appearing small and intact, which is typical of oat starch granules (Kaur et al., 2022). After treatment at 100 MPa, partial fragmentation of the granules occurred, but they remained relatively intact. At 200–300 MPa, the degree of fragmentation increased, and aggregation began to appear. Under 400 MPa, extensive granule fragmentation and aggregation were observed. At 500 MPa, the starch granules underwent deformation and aggregated into a network-like chain structure. Upon the addition of lipids, the surface of the starch granules became smoother, and as the lipid content increased, the granules exhibited greater adhesion. Compared to pure oat starch, the oat starch-lipid complexes maintained higher structural integrity and smoother surfaces after UHP treatment, consistent with previous research (Y. Wang et al., 2025). This suggests that lipids protect starch granules, delaying their disruption under high pressure. With increasing lipid content, the formation of a network-like chain structure became more pronounced under the same UHP conditions, likely due to the adhesive nature of lipids, which promotes granule aggregation under high pressure. Notably, at 500 MPa, the granules formed large non-network clusters, indicating that excessive pressure combined with high lipid content leads to extensive agglomeration rather than an ordered network structure.

Fig. 1.

SEM images of oat starch-lipid composites with different ultra-high pressure treatments and different lipid contents.

3.1.2. XRD analysis

Fig. 2 displays the XRD patterns, where oat starch treated at 0–400 MPa exhibits strong diffraction peaks at 15°, 17°, 18°, 20°, and 23°, characteristic of an A-type crystalline structure (Zhang, Zhang, Zhang, et al., 2022). As shown in Table 1, the relative crystallinity of oat starch gradually increased under 0–300 MPa treatment, attributed to the enhanced intermolecular interactions induced by ultra-high pressure (UHP), which refined the crystalline structure. However, at 400 MPa, the relative crystallinity began to decrease, indicating partial disruption of the crystalline structure, though the A-type configuration was retained. After 500 MPa treatment, the relative crystallinity of oat starch significantly decreased, and the crystalline structure transformed into a V-type polymorph, demonstrating that UHP treatment can alter the crystalline type of starch molecules, with the critical transition pressure lying between 400 and 500 MPa. This observation aligns with previous findings (Yan et al., 2024). Upon lipid incorporation, a minor peak emerged at 13°, corresponding to the V-type characteristic peak formed by starch-lipid complexes, confirming the formation of oat starch-lipid composites (Ren et al., 2020). With increasing pressure, the crystalline structure of the oat starch-lipid complex also transitioned from A-type to V-type at 500 MPa, with its main diffraction peaks exhibiting higher intensity than those of pure oat starch treated at the same pressure. This suggests that lipid complexation facilitates the transformation of oat starch crystalline structure into the V-type polymorph.

Fig. 2.

XRD spectra of oat starch-lipid complexes with different lipid contents: (A) 0 %, (B) 5 %, (C) 10 %, and (D) 15 % lipids.

Table 1.

XRD parameters of oat starch-lipid complex.

Pressure (MPa)	Relative crystallinity (%)
	Lipid 0 %	Lipid 5 %	Lipid 10 %	Lipid 15 %
0	38.65 ± 0.06b	40.54 ± 0.29a	41.36 ± 0.18a	37.99 ± 0.39a
100	39.29 ± 0.37b	37.67 ± 0.23b	36.52 ± 0.28b	33.43 ± 0.64bc
200	40.49 ± 0.32a	37.14 ± 0.22b	34.52 ± 0.55c	32.18 ± 0.35cd
300	41.43 ± 0.75a	36.05 ± 0.12c	31.36 ± 0.38d	33.76 ± 0.54b
400	37.18 ± 0.34c	34.92 ± 0.42d	30.15 ± 0.51e	30.98 ± 0.43d
500	31.40 ± 0.36d	30.97 ± 0.31e	33.65 ± 0.21c	29.13 ± 0.57e

Different letters in the same column indicate significant differences (p < 0.05).

3.1.3. FTIR analysis

Ultra-high pressure treatment alters the double helix structure and short-range molecular order of starch molecules by breaking non-covalent bonds such as hydrogen bonds (N. Wang et al., 2023). FTIR can detect changes in the molecular chain conformation, crystalline structure, and helical arrangement of starch. The FTIR spectra are shown in Fig. 3. As illustrated in Fig. 3A, the main absorption peaks of oat starch appear at 3393 cm⁻¹, 2932 cm⁻¹, 1154 cm⁻¹, 1080 cm⁻¹, and 1020 cm⁻¹. No new characteristic peaks appeared or disappeared after different pressure treatments, indicating that ultra-high pressure treatment did not generate new functional groups or chemical bonds in oat starch. However, as shown in Fig. 3B, C, and D, after adding lipids to starch, new absorption peaks emerged at 2850 cm⁻¹ or 1750 cm⁻¹. These peaks result from the C O vibration of carbonyl groups or the stretching vibrations of –CH₂ and –CH₃ groups in lipids (Shen et al., 2024). This phenomenon indicates that oat lipids combined with amylose and formed helical starch-lipid complexes (Chumsri et al., 2022).

Fig. 3.

FTIR spectra of oat starch-lipid complexes with different lipid contents: (A) 0 %, (B) 5 %, (C) 10 %, and (D) 15 % lipids.

According to previous research (Sevenou et al., 2002), the band at 1022 cm⁻¹ represents the amorphous region of starch, while 1045 cm⁻¹ corresponds to the crystalline region. The ratio between 995 cm⁻¹ and 1022 cm⁻¹ reflects the double-helix structure of starch, and the ratio between 1045 cm⁻¹ and 1022 cm⁻¹ indicates the short-range molecular order of starch. As shown in the Table 2, after 100–300 MPa treatment, both R_995/1022 and R_1045/1022 values of oat starch increased, suggesting enhanced double-helix structure and short-range molecular order. This is attributed to the improved intermolecular interactions and perfected crystalline structure induced by ultra-high pressure. However, after 400–500 MPa treatment, both ratios decreased, indicating significant damage to starch granules and the beginning of crystalline structure disintegration (Zhang, Zhang, Wang, et al., 2022), which reduced the double-helix structure and short-range molecular order. When lipids were incorporated and treated at 0–500 MPa, the R_995/1022 values increased compared to pure oat starch, demonstrating that lipids promoted the formation of double-helix structures in starch molecules. In contrast, the R_1045/1022 values decreased, suggesting that lipids reduced the short-range molecular order of starch. At the same lipid content, ultra-high pressure treatment decreased both R_995/1022 and R_1045/1022 values compared to 0 MPa treatment, consistent with the reduced relative crystallinity detected by XRD. These observations differ significantly from studies on starch-lipid complexes prepared by conventional heat treatment (Lu et al., 2019). This discrepancy may arise because heat treatment tends to disrupt the double-helix structure of starch, while UHP treatment facilitates the formation of complexes between lipids and the single-helix structure of amylose, thereby delaying double-helix dissociation. The combination of lipid complexation and ultra-high pressure disrupted the crystalline structure of starch, leading to reduced short-range molecular order. Combined with XRD results, we conclude that under ultra-high pressure treatment, oat starch and oat lipids form complexes primarily through hydrogen bonding.

Table 2.

FTIR parameters of oat starch-lipid complex.

Lipid (%)	Pressure (MPa)	Value
		R995/1022	R1045/1022
0	0	0.776 ± 0.014de	0.858 ± 0.004bc
	100	0.748 ± 0.013e	0.849 ± 0.018c
	200	0.810 ± 0.011bc	0.865 ± 0.00bc
	300	0.855 ± 0.009a	0.906 ± 0.002a
	400	0.835 ± 0.010ab	0.874 ± 0.002b
	500	0.795 ± 0.005cd	0.821 ± 0.003d
5	0	1.048 ± 0.002a	0.705 ± 0.010a
	100	1.030 ± 0.004b	0.667 ± 0.007b
	200	1.007 ± 0.003d	0.669 ± 0.010b
	300	1.035 ± 0.004b	0.668 ± 0.007b
	400	1.018 ± 0.005c	0.668 ± 0.009b
	500	1.019 ± 0.001c	0.664 ± 0.006b
10	0	1.071 ± 0.001c	0.716 ± 0.008a
	100	1.082 ± 0.003b	0.702 ± 0.006a
	200	1.051 ± 0.001d	0.673 ± 0.007b
	300	1.171 ± 0.008a	0.654 ± 0.006bc
	400	1.053 ± 0.005d	0.666 ± 0.007bc
	500	1.022 ± 0.001e	0.647 ± 0.009c
15	0	1.144 ± 0.001a	0.695 ± 0.008a
	100	1.073 ± 0.003d	0.673 ± 0.006ab
	200	1.064 ± 0.003e	0.690 ± 0.010a
	300	1.091 ± 0.003c	0.664 ± 0.003b
	400	1.108 ± 0.001b	0.692 ± 0.005a
	500	1.016 ± 0.002f	0.664 ± 0.007b

Different letters in the same column indicate significant differences (p < 0.05).

3.2. Textural properties analysis

Table 3 shows the gel texture parameters of oat starch-lipid complexes. It can be observed that ultra-high pressure (UHP) treatment reduced the hardness, gumminess, and chewiness of oat starch gels, while increasing their springiness, cohesiveness, and responsiveness. This is because UHP treatment disrupts the structure of starch granules, weakening the texture of the starch gel, while the pressure causes the starch granules to aggregate, affecting cohesiveness and related indicators (Vittadini et al., 2008). After the addition of lipids and UHP treatment, hardness, gumminess, and chewiness significantly decreased, and springiness slightly increased. These phenomena occur because lipids can reduce the water-holding capacity of oat starch (Putseys et al., 2010). Moreover, after UHP treatment and the formation of starch-lipid complexes, changes in the starch structure are induced, reducing the water-binding and gel-forming capacity of oat starch and weakening the gel strength (Ai et al., 2013), leading to a decrease in the hardness, gumminess, and chewiness of the starch gel. This indicates that under ultra-high pressure conditions, oat lipids can weaken the strength of starch gels and effectively influence the gel texture properties of oat starch.

Table 3.

The gel textural properties parameters of oat starch-lipid complex.

Lipid (%)	Pressure (MPa)	Hardness (g)	Springiness (mm)	Cohesiveness	Gumminess (g)	Chewiness (mJ)	Responsive ness(mJ)
0	0	1150.42 ± 73.19a	0.90 ± 0.01b	0.77 ± 0.01a	889.16 ± 67.75a	801.37 ± 57.13a	0.45 ± 0.04b
	100	609.68 ± 67.00b	0.95 ± 0.03ab	0.78 ± 0.11a	472.61 ± 43.62b	471.96 ± 10.36b	0.57 ± 0.08ab
	200	571.99 ± 48.21bc	0.97 ± 0.03ab	0.83 ± 0.11a	470.00 ± 27.68bc	456.40 ± 38.73bc	0.62 ± 0.09a
	300	538.17 ± 51.10bc	0.98 ± 0.02ab	0.79 ± 0.08a	421.17 ± 26.77bcd	412.95 ± 29.43bc	0.61 ± 0.09a
	400	516.53 ± 9.63c	1.01 ± 0.02a	0.80 ± 0.03a	412.79 ± 2.53cd	410.94 ± 7.58c	0.61 ± 0.03a
	500	497.15 ± 45.07c	1.05 ± 0.12a	0.82 ± 0.10a	405.90 ± 16.93d	425.69 ± 56.20bc	0.61 ± 0.09a
5	0	438.40 ± 23.37a	4.30 ± 0.20a	0.59 ± 0.07a	165.90 ± 36.18a	12.04 ± 0.22a	2.42 ± 0.18a
	100	286.63 ± 5.88b	1.69 ± 0.15d	0.47 ± 0.01b	135.70 ± 2.93ab	3.48 ± 0.29b	0.40 ± 0.05cd
	200	177.20 ± 3.60d	2.90 ± 0.18b	0.57 ± 0.06ab	84.73 ± 2.12b	2.39 ± 0.13c	0.33 ± 0.03cd
	300	253.30 ± 7.71c	2.34 ± 0.04c	0.47 ± 0.02bc	111.56 ± 4.27b	2.57 ± 0.18c	0.74 ± 0.09b
	400	244.23 ± 7.68c	2.38 ± 0.10c	0.52 ± 0.01ab	133.50 ± 2.55ab	2.33 ± 0.17c	0.28 ± 0.03d
	500	265.93 ± 7.91bc	1.45 ± 0.07d	0.35 ± 0.03c	98.57 ± 5.02b	1.28 ± 0.03d	0.54 ± 0.01bc
10	0	156.13 ± 9.84ab	2.07 ± 0.09b	0.58 ± 0.07a	88.77 ± 4.16a	1.63 ± 0.07 a	0.49 ± 0.05abc
	100	159.67 ± 4.05ab	1.84 ± 0.07c	0.36 ± 0.01c	58.20 ± 2.47c	1.05 ± 0.03b	0.57 ± 0.07ab
	200	165.13 ± 13.69a	2.06 ± 0.08b	0.45 ± 0.03bc	95.87 ± 5.86a	1.08 ± 0.12a	0.66 ± 0.10a
	300	164.90 ± 2.12a	2.39 ± 0.07a	0.46 ± 0.01b	76.30 ± 1.22b	1.79 ± 0.05a	0.67 ± 0.05a
	400	147.23 ± 12.78ab	1.69 ± 0.11c	0.39 ± 0.02bc	56.83 ± 5.02cd	0.94 ± 0.08b	0.41 ± 0.03bc
	500	130.97 ± 2.34b	1.72 ± 0.08c	0.37 ± 0.01bc	46.13 ± 1.44d	0.85 ± 0.06b	0.34 ± 0.01c
15	0	196.97 ± 4.02c	2.07 ± 0.08c	0.43 ± 0.02b	73.66 ± 2.35c	1.51 ± 0.08b	0.67 ± 0.02c
	100	188.20 ± 1.83b	3.65 ± 0.14a	0.55 ± 0.03a	93.53 ± 0.51a	3.53 ± 0.08a	0.80 ± 0.01b
	200	131.00 ± 3.54e	2.65 ± 0.16b	0.45 ± 0.02b	56.50 ± 1.61d	1.42 ± 0.04bc	0.66 ± 0.03c
	300	106.33 ± 1.64f	2.74 ± 0.22b	0.45 ± 0.01b	54.53 ± 2.74d	1.29 ± 0.01c	0.60 ± 0.02c
	400	222.33 ± 3.96a	1.79 ± 0.06c	0.34 ± 0.01c	81.60 ± 1.45b	1.02 ± 0.03d	0.75 ± 0.05b
	500	157.33 ± 3.14d	1.80 ± 0.03c	0.33 ± 0.02c	47.70 ± 1.43e	1.04 ± 0.04d	0.58 ± 0.01c

Different letters within columns present significant differences (P < 0.05).

3.3. DSC thermal properties analysis

Table 4 shows the thermal properties of oat starch-lipid complexes. The gelatinization temperature reflects the thermal stability of oat starch. After UHP treatment, the To, Tp, and Tc of oat starch gradually increased, indicating enhanced thermal stability, which is consistent with previous research (Chou et al., 2020). The ΔH value is primarily related to the energy required for the disruption of double-helical structures and the melting of crystalline regions (Dominguez-Ayala et al., 2023). After treatment at 100–300 MPa, the ΔH value of oat starch gradually increased, whereas at 400–500 MPa, it began to decrease. This is because pressure initially promotes the formation of double-helical structures, but at 400–500 MPa, these structures undergo dissociation, which aligns with the FTIR analysis. After adding different proportions of lipids, the To, Tp, and Tc of oat starch increased. Moreover, under 100–300 MPa treatment, the To (onset temperature), Tp (peak temperature), and Tc (conclusion temperature) of the complexes gradually increased. However, after 400–500 MPa treatment, To, Tp, and Tc exhibited substantial elevation. This phenomenon is likely attributed to the formation of type II starch-lipid complexes at these pressures, which require higher melting temperatures. The ΔH values primarily showed a decreasing trend. With increasing lipid content and pressure, the ΔH reduction became more pronounced. Although lipid incorporation enhanced the double-helical structure of starch molecules, it significantly reduced relative crystallinity. This resulted in decreased ΔH during gelatinization, consistent with XRD analysis results. Collectively, these findings demonstrate that oat starch-lipid complex formation markedly improves the thermal stability of starch.

Table 4.

DSC thermal properties parameters of oat starch-lipid complex.

Lipid (%)	Pressure (MPa)	To (°C)	Tp (°C)	Tc (°C)	∆ H (J/g)
0	0	57.40 ± 0.56e	63.67 ± 0.50d	78.40 ± 0.49d	8.41 ± 0.18b
	100	54.60 ± 0.44f	63.60 ± 0.70d	77.07 ± 0.58e	8.50 ± 0.20b
	200	60.20 ± 0.43d	77.10 ± 0.61c	82.37 ± 0.41c	8.81 ± 0.20ab
	300	84.10 ± 0.49c	91.37 ± 0.35b	105.60 ± 0.38b	9.41 ± 0.09a
	400	88.47 ± 0.34a	99.13 ± 0.23a	109.53 ± 0.26a	9.32 ± 0.27a
	500	86.50 ± 0.38b	100.43 ± 0.67a	109.37 ± 0.38a	8.97 ± 0.19ab
5	0	69.13 ± 0.66f	77.53 ± 0.33f	91.33 ± 0.38d	7.83 ± 0.11a
	100	72.23 ± 0.37e	81.07 ± 0.64e	104.67 ± 0.63c	6.33 ± 0.16b
	200	77.27 ± 0.37d	86.93 ± 0.97d	111.07 ± 0.66b	5.28 ± 0.12c
	300	86.60 ± 0.76c	100.2 ± 0.41c	109.40 ± 0.92b	4.90 ± 0.23cd
	400	125.60 ± 0.59b	154.53 ± 0.96b	169.63 ± 3.27a	4.49 ± 0.18d
	500	163.87 ± 0.79a	175.40 ± 0.26a	168.17 ± 0.26a	4.06 ± 0.04e
10	0	72.17 ± 0.44e	80.26 ± 1.59e	85.32 ± 0.52e	7.32 ± 0.12a
	100	83.00 ± 0.06d	96.30 ± 2.05c	109.93 ± 0.58c	6.64 ± 0.09b
	200	83.57 ± 0.22cd	96.93 ± 0.12c	107.63 ± 0.23d	6.43 ± 0.10b
	300	85.30 ± 0.65c	92.27 ± 0.37d	106.57 ± 0.39d	5.75 ± 0.07c
	400	163.77 ± 0.61b	171.47 ± 0.56b	175.07 ± 0.49b	5.24 ± 0.04d
	500	174.60 ± 1.31a	183.60 ± 1.86a	187.50 ± 0.25a	4.93 ± 0.07e
15	0	83.47 ± 0.38d	96.30 ± 0.67d	107.57 ± 0.34e	6.76 ± 0.04a
	100	83.20 ± 0.42d	96.43 ± 0.38d	109.30 ± 0.40d	6.37 ± 0.06b
	200	85.53 ± 0.54c	98.03 ± 0.66c	107.00 ± 0.53e	5.95 ± 0.05c
	300	158.20 ± 0.50b	165.20 ± 0.49b	182.83 ± 0.73b	5.37 ± 0.06d
	400	162.83 ± 0.80a	172.33 ± 0.38a	187.23 ± 0.38a	5.48 ± 0.11d
	500	159.17 ± 1.13b	165.43 ± 0.35b	177.93 ± 0.74c	5.89 ± 0.02c

Different letters within columns present significant differences (P < 0.05).

To: onset temperature, Tp: peak temperature, Tc: conclusion temperature, ∆ H: enthalpy of gelatinization.

3.4. Freeze-thaw stability analysis

Syneresis rate is a key indicator reflecting freeze-thaw stability and is commonly used to evaluate the freeze-thaw stability of starch. A lower syneresis rate indicates better freeze-thaw stability, while a higher rate suggests poorer stability. Fig. 4 shows the changes in syneresis rate of UHP-treated oat starch and UHP-prepared oat starch-lipid complexes with varying lipid content after four freeze-thaw cycles. As observed, the syneresis rate of the samples gradually increased with the number of freeze-thaw cycles. This is because each freeze-thaw cycle disrupts the network structure of the samples, enhancing the mobility of water molecules (Ye et al., 2018). As shown in Fig. 4A, after treatment at 100–500 MPa, the syneresis rate of oat starch decreased, indicating improved freeze-thaw stability. This is attributed to the fact that UHP treatment strengthens the intermolecular interactions of starch, delaying the structural breakdown of the samples during freeze-thaw cycles. When lipids were added, the syneresis rate slightly increased compared to pure oat starch, likely because the lipids had not yet formed complexes with the starch. Free lipids repelled water molecules, thereby reducing freeze-thaw stability. However, after 100–500 MPa treatment, the syneresis rate significantly decreased. This is due to the formation of stable starch-lipid complexes, which enhance the structural integrity of starch and improve freeze-thaw stability.

Fig. 4.

Freeze-thaw cycle bar chart of oat starch-lipid complexes with different lipid contents: (A) 0 %, (B) 5 %, (C) 10 %, and (D) 15 % lipids.

Additionally, the double-helical structure can influence the freeze-thaw stability of starch. An increase in double-helical structures helps mitigate structural damage caused by water molecules and ice crystals during freezing (Meng et al., 2025). Meanwhile, a reduction in relative crystallinity can also enhance the freeze-thaw stability of starch (J. Li et al., 2024). According to FTIR and XRD results, oat starch-lipid complexes with varying lipid contents exhibited higher double-helical structures but lower relative crystallinity compared to native oat starch. Combined with the observed changes in syneresis rate, these findings demonstrate that the formation of oat starch-lipid complexes under ultra-high pressure (UHP) treatment can effectively improve the freeze-thaw stability of starch.

3.5. Pasting properties analysis

As shown in Fig. 5 and Table 5, ultra-high pressure (UHP) treatment significantly reduced the peak viscosity (PV), trough viscosity (TV), final viscosity (FV), breakdown (BD), and setback (SB) of oat starch, while increasing the peak time (PT) and pasting temperature (Pt). The peak viscosity (PV) is associated with short-range molecular order (Li et al., 2022). Combined with FTIR analysis results, UHP treatment decreased the short-range molecular order of oat starch, thereby reducing its peak viscosity (PV). Previous studies have also shown that UHP treatment can lower starch viscosity during gelatinization (Kim et al., 2012). Breakdown (BD) is negatively correlated with the thermal stability of starch (Zhang, Zhang, Wang, et al., 2022). The decrease in breakdown (BD) indicates that UHP treatment improved the thermal stability of oat starch. The increase in peak time (PT) and pasting temperature (Pt) suggests that UHP treatment delays starch gelatinization, which corresponds to enhanced thermal stability. The next stage after gelatinization is retrogradation. Setback (SB) reflects starch retrogradation and is positively correlated with it (Guo et al., 2015). The reduction in setback (SB) demonstrates that UHP treatment can effectively inhibit the retrogradation of oat starch.

Fig. 5.

Pasting curve chart of oat starch-lipid complexes with different lipid contents: (A) 0 %, (B) 5 %, (C) 10 %, and (D) 15 % lipids.

Table 5.

The pasting properties parameters of oat starch-lipid complex.

Lipid (%)	Pressure (MPa)	PV (cp)	TV (cp)	FV (cp)	BD (cp)	SB (cp)	PT (°C)	Pt (min)
0	0	676.00 ± 1.15a	368.00 ± 0.58a	1567.80 ± 1.12a	308.00 ± 1.73a	1199.80 ± 0.56a	94.80 ± 0.06e	8.53 ± 0.01e
	100	315.33 ± 0.67d	222.00 ± 0.58c	880.67 ± 0.67d	93.33 ± 1.20c	658.67 ± 0.33d	96.90 ± 0.06bc	8.83 ± 0.01c
	200	536.00 ± 1.15b	223.33 ± 0.67c	942.67 ± 0.67c	312.67 ± 1.76a	719.33 ± 0.67c	95.70 ± 0.06d	9.09 ± 0.01a
	300	396.00 ± 1.15c	259.00 ± 0.57b	1165.33 ± 0.67b	137.00 ± 1.00b	906.33 ± 0.33b	96.77 ± 0.03c	8.85 ± 0.01c
	400	226.00 ± 4.00f	182.00 ± 1.15e	784.67 ± 0.67f	44.00 ± 3.06e	602.67 ± 0.67f	97.00 ± 0.06ab	8.78 ± 0.01d
	500	286.00 ± 0.01e	200.00 ± 1.10d	845.33 ± 0.67e	86.00 ± 1.15d	645.33 ± 1.33e	97.17 ± 0.09a	9.00 ± 0.01b
5	0	543.33 ± 4.37c	352.00 ± 1.15d	1812.00 ± 0.58b	191.33 ± 3.33b	1460.00 ± 1.73b	96.10 ± 0.83b	8.76 ± 0.01c
	100	614.00 ± 5.13b	444.33 ± 6.17a	1722.67 ± 4.37c	169.67 ± 6.23cd	1278.33 ± 7.36d	96.47 ± 0.38ab	9.29 ± 0.01a
	200	407.33 ± 3.48e	231.00 ± 6.08e	1692.67 ± 6.36d	176.33 ± 5.90c	1461.67 ± 8.76b	96.10 ± 0.32b	8.39 ± 0.01d
	300	542.67 ± 5.21c	383.00 ± 2.65c	1730.67 ± 5.70c	159.67 ± 2.60d	1347.67 ± 3.48c	96.37 ± 0.44ab	8.37 ± 0.01d
	400	480.00 ± 4.62d	351.00 ± 0.58d	1357.33 ± 4.37e	129.00 ± 4.93e	1006.33 ± 4.33e	97.40 ± 0.47ab	8.81 ± 0.05c
	500	681.00 ± 4.36a	426.33 ± 6.15b	1988.67 ± 6.36a	254.67 ± 5.81a	1562.33 ± 8.41a	97.90 ± 0.21a	8.97 ± 0.12b
10	0	510.00 ± 2.89c	256.00 ± 2.31e	1436.00 ± 2.31f	254.00 ± 4.51a	1183.00 ± 4.93d	96.17 ± 0.29a	9.11 ± 0.01ab
	100	582.00 ± 3.51a	410.00 ± 1.73a	1940.28 ± 1.58a	172.00 ± 3.79b	1530.28 ± 1.67a	96.77 ± 0.48a	9.33 ± 0.17a
	200	426.00 ± 5.20e	359.33 ± 4.37c	1489.67 ± 4.41e	66.67 ± 9.06d	1130.33 ± 7.13e	96.27 ± 0.61a	8.58 ± 0.01d
	300	562.00 ± 1.15b	394.00 ± 2.31b	1764.00 ± 1.15c	168.00 ± 3.06b	1370.00 ± 3.06c	96.43 ± 0.09a	8.68 ± 0.01cd
	400	465.67 ± 1.20d	347.33 ± 11.67c	1533.33 ± 0.67d	118.33 ± 10.71c	1186.00 ± 11.59d	96.50 ± 0.49a	8.86 ± 0.09c
	500	580.00 ± 2.89a	314.67 ± 4.48d	1796.00 ± 3.06b	265.33 ± 5.93a	1482.33 ± 5.84b	96.73 ± 0.03a	8.89 ± 0.01bc
15	0	575.33 ± 3.18a	353.67 ± 4.41b	1384.67 ± 6.96b	221.67 ± 6.33a	1031.00 ± 9.07d	96.07 ± 0.34a	8.56 ± 0.01e
	100	481.33 ± 3.53b	388.00 ± 1.15a	1550.00 ± 1.15a	93.33 ± 2.40d	1162.00 ± 2.00a	96.30 ± 0.12a	8.72 ± 0.01c
	200	409.33 ± 3.48c	312.67 ± 6.44c	1360.00 ± 1.15c	96.67 ± 3.18d	1047.33 ± 5.36c	96.83 ± 0.30a	8.81 ± 0.01b
	300	353.33 ± 2.96e	278.33 ± 2.96d	1228.00 ± 4.16e	75.00 ± 1.73e	949.67 ± 3.76f	96.17 ± 0.33a	8.62 ± 0.01d
	400	416.67 ± 3.53c	253.67 ± 2.40e	1343.00 ± 2.08d	163.00 ± 2.00b	1089.33 ± 0.33b	96.17 ± 0.35a	8.52 ± 0.01f
	500	394.00 ± 1.15d	256.67 ± 0.67e	1221.67 ± 6.89e	137.33 ± 0.67c	965.00 ± 6.66e	96.70 ± 0.49a	8.97 ± 0.01a

Different letters within columns present significant differences (P < 0.05).

PV: peak viscosity, TV: trough viscosity, FV: final viscosity, BD: breakdown, SB: setback, PT: peak time, Pt: pasting temperature.

Following lipid addition, compared with native oat starch, the peak viscosity (PV), breakdown (BD), and setback (SB) decreased, while peak time (PT) and pasting temperature (Pt) increased. However, without UHP treatment, the lipid's effect on starch was not pronounced. After UHP treatment, when lipids formed complexes with starch, both PV and BD showed significant reduction, whereas PT and Pt exhibited notable elevation. Combined with DSC and FTIR analysis results, this demonstrates that starch-lipid complexation can substantially enhance starch's thermal stability and delay gelatinization. For starch-lipid complexes prepared with lower lipid ratios under UHP treatment, their SB values showed no significant difference compared to native oat starch. However, when incorporating 15 % lipids followed by 100–500 MPa treatment, the resulting oat starch-lipid complexes consistently displayed lower SB values than native oat starch. This confirms that UHP-induced complexation between oat lipids and starch can effectively inhibit starch retrogradation.

3.6. Mechanism analysis

As shown in Fig. 6, in the absence of lipids (0 % lipid), relative crystallinity was positively correlated with hardness, gumminess, and responsiveness. Double helix structure showed positive correlations with responsiveness, springiness, and cohesiveness, but negative correlations with chewiness, gumminess, and hardness. Short-range molecular order was positively correlated with responsiveness and negatively correlated with chewiness. After the addition of lipids, all structural indicators showed positive correlations with textural properties. In the group with 5 % lipid addition, double helix structure was significantly positively correlated with responsiveness, relative crystallinity was significantly positively correlated with chewiness, and short-range molecular order was significantly positively correlated with cohesiveness and springiness. Based on the relevant data, the structural changes led to a significant decrease in hardness, gumminess, and chewiness of the starch gel, while springiness, cohesiveness, and responsiveness increased. Regarding thermal properties, relative crystallinity and short-range molecular order were negatively correlated with melting point but positively correlated with gelatinization enthalpy. A decrease in relative crystallinity and short-range molecular order resulted in an increase in the starch melting point and a decrease in gelatinization enthalpy. Meanwhile, relative crystallinity, double helix structure, and short-range molecular order were all positively correlated with syneresis. A reduction in relative crystallinity and short-range molecular order, along with an increase in double helix structure, reduced the syneresis of the starch. Furthermore, at a lipid addition level of 15 %, relative crystallinity was positively correlated with pasting viscosity; double helix structure was positively correlated with pasting viscosity and breakdown (BD); and short-range molecular order was positively correlated with breakdown (BD) and setback (SB), showing a significant positive correlation with pasting viscosity. Under these conditions, changes in starch structure resulted in reduced pasting viscosity, breakdown (BD), and setback (SB).

Fig. 6.

(A) (B) (C) (D) Heat maps showing the correlation between structural and performance parameters of the samples at lipid addition levels of 0 %, 5 %, 10 %, and 15 % (red indicates positive correlation, blue indicates negative correlation, * denotes “structure-performance parameters” significantly associated with performance indicators (p < 0.05), * indicates p < 0.05, and ** indicates p < 0.01).(E)Mechanism diagram of the effect of oat lipids on starch structure and related properties under ultra-high pressure treatment. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Based on Fig. 6E and relevant data, it is indicated that under ultra-high pressure conditions, oat lipids and starch form complexes via hydrogen bonding, leading to structural modifications in the starch. A significant decrease in hardness, gumminess, and chewiness, along with an increase in springiness, cohesiveness, and responsiveness, demonstrates substantial changes in the textural properties of the starch. The crystalline structure transitioned from type-A to type-V, with an increase in double-helix structures and a reduction in both relative crystallinity and short-range molecular order. These structural alterations resulted in an elevated melting point, decreased gelatinization enthalpy, and enhanced thermal stability of the starch. The improvement in thermal stability and delayed gelatinization were mutually reinforcing. The decrease in relative crystallinity and short-range molecular order, coupled with an increase in double-helix content, reduced syneresis and improved the stability of the starch during freeze-thaw cycles. Furthermore, the addition of lipids reduced the pasting viscosity, breakdown (BD), and setback (SB) values, indicating that lipids contribute to delayed gelatinization and inhibited retrogradation of the starch.

4. Conclusions

Using oat starch and oat lipids as raw materials, oat starch-lipid complexes were prepared by ultra-high pressure (UHP) technology, and their microstructure and related physicochemical properties were characterized. The results showed that under UHP treatment, oat lipids promoted the aggregation of starch granules and induced the formation of network-like or large clustered aggregates at high pressure levels (400–500 MPa). As pressure increased, the relative crystallinity of the complexes decreased more significantly compared to oat starch alone, and the crystal structure type transitioned more readily from A-type to V-type, particularly under high pressure (400–500 MPa). Additionally, complexation with lipids increased the double-helix content while reducing the short-range molecular order of starch. These structural changes led to a significant decrease in the hardness, gumminess, and chewiness of the starch gel, along with an increase in springiness, cohesiveness, and responsiveness. The melting point of starch was elevated, while the gelatinization enthalpy, syneresis, pasting viscosity, breakdown (BD), and setback (SB) values were reduced.

In summary, under ultra-high pressure conditions, oat lipids and oat starch can form starch-lipid complexes that alter the microstructure of starch, enhance its thermal and freeze-thaw stability, and delay gelatinization while inhibiting retrogradation. These effects were particularly pronounced at high pressure levels (400–500 MPa). Therefore, this study reveals how UHP-induced complexation between oat lipids and starch influences the structure, freeze-thaw stability, and pasting properties of starch, providing a novel feasible approach for preparing oat starch-lipid complexes and offering theoretical support for future deep processing and research on oat-based foods. However, this study has certain limitations. For instance, starch retrogradation can affect food quality, yet the raw material used here was native starch without further retrogradation treatment. Thus, future research should focus on the structural and functional changes in retrograded starch treated with both ultra-high pressure and lipids.

CRediT authorship contribution statement

Jingyu Xie: Writing – original draft, Visualization, Conceptualization. Minjun Sun: Methodology, Investigation. Rui Huo: Validation, Formal analysis. Ying Miao: Conceptualization. Yangyang Chen: Investigation. Meili Zhang: Writing – review & editing, Supervision, Project administration, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary material.

Data availability

Data will be made available on request.