ABSTRACT
Effects of inulin with short and long-chain on pasting, texture and rheological properties of sweet potato starch (SPS) were investigated. Subsequently, the interaction mechanism between inulin and SPS was discussed. Both short and long-chain inulin could decrease all the viscosity parameters and raise the pasting temperature (GT) of SPS-inulin mixes. With the increase of inulin addition, the hardness and gumminess of SPS gel decreased, while tan δ and temperature of G’ Max rose. Inulin could inhibit SPS granule swelling, and made SPS granules concentrate in the smaller ones. Inulin also remarkably lowered the content of leached amylose (p < 0.05). The hydrogen bond was the mainly interacting force between SPS and inulin. Additionally, G’ and G” of SPS-SI gel was lower than those of SPS-LI samples at the same addition level. Generally, short-chain inulin could significantly affect GT and paste viscosity of SPS than long-chain inulin at lower addition level.
En el presente estudio se investigaron los efectos de la inulina de cadena corta y larga en el pegado, la textura y las propiedades reológicas del almidón de batata (SPS). Posteriormente, se examinó el mecanismo de interacción entre la inulina y el SPS. Tanto la inulina de cadena corta como la de cadena larga pudieron disminuir todos los parámetros de viscosidad y aumentar la temperatura de pegado (GT) de las mezclas de inulina y SPS. Con el aumento de la adición de inulina, la dureza y la gomosidad del gel SPS disminuyó, mientras que el color marrón δ y la temperatura de G’ Max aumentó. La inulina pudo inhibir la hinchazón de los gránulos de SPS, e hizo que estos se concentraran en los más pequeños. La inulina también disminuyó notablemente el contenido de amilosa lixiviada (p < 0.05). El enlace de hidrógeno fue la fuerza de interacción principal entre el SPS y la inulina. Además, la G’ y la G” del gel de SPS-SI era más baja que la de las muestras de SPS-LI al mismo nivel de adición. En general, la inulina de cadena corta puede afectar significativamente la GT y la viscosidad de la pasta de SPS que la inulina de cadena larga a un nivel de adición más bajo.
1. Introduction
Sweet potato (Ipomoea batatas L.), as one kind of dicotyledonous plants, is a nutritious food source for human and animals, and is the sixth most-planted crops in the world (Abegunde et al., Citation2013; Lai et al., Citation2016; H. Wang et al., Citation2020). The annual yield of sweet potato in China reaches 117 million tons, which accounts for nearly 90% of the world’s total output. Starch, taking up about from 50% to 80% of the dry weight, is the main component of sweet potato tuber (Ji et al., Citation2019; Lai et al., Citation2016). Sweet potato starch (SPS) could be widely used in baked food, noodles, candy products and so on (Azeem et al., Citation2020; Monthe et al., Citation2019; Yadav et al., Citation2014). Because of the higher peak viscosity, worse stability of freeze-thaw and shear, and easier retrogradation of native starch, the utilization of SPS in starch-based foods was limited (Hoover, Citation2001; Hung & Morita, Citation2005). In order to meet the raw material demand of SPS-based products, chemical, physical and enzymatic methods could be employed to improve the physicochemical properties of SPS (Gou et al., Citation2019; Guo et al., Citation2019; Zhang et al., Citation2020; Zhou et al., Citation2017). Green, safe and efficient modification of SPS would be favoured by producers and consumers. Recently, non-starch hydrocolloids had attracted increasing attention due to that they could effectively modify the properties of starch (Charoenrein et al., Citation2011; Fang et al., Citation2020).
Inulin, predominately obtained from chicory root, is a linear polysaccharide, which is composed of D-fructose linked by β (2→1) glycosidic bonds (Luo et al., Citation2017; Mensink et al., Citation2015). The chain length of inulin varies in the range of 2 to 60 monomers. Depending on the degree of polymerization (DP), inulin could be classified into short-chain inulin (SI, DP value ≤10), long-chain inulin (LI, DP value ≥ 23), and natural inulin (NI, DP value in range of 2 and 60) (Luo et al., Citation2020). Inulin could be applied in baked foods, meat and dairy products to replace the fat and sugar, to improve the viscosity and texture, and to supplement the dietary fiber (Gao et al., Citation2016; Paola & Sancho, Citation2018; Y. Zhang et al., Citation2020).
Recent reports showed a growing interest in attention to the effects of inulin with different chain-length on physical and chemical properties of starch (Kou et al., Citation2018; Krystyjan et al., Citation2015; Luo et al., Citation2017; R. Wang et al., Citation2019; Witczak et al., Citation2014; Ye et al., Citation2018; Zhang et al., Citation2019). Kou et al. (Citation2018) reported that inulin with different chain-length could affect the texture of wheat starch gel, and natural inulin significantly decreased the stability of the gels, while both short and long-chain inulin maintained the stability of starch gel. R. Wang et al. (Citation2019) found that addition of inulin with different chain-length reduced the viscosity and raised the pasting temperature of rice starch. Additionally, short-chain inulin exerted stronger effect on viscosity parameters than long-chain inulin. According to Luo et al. (Citation2017), inulin with short-chain influenced the retrogradation of wheat starch more effectively than long-chain inulin. However, to the best of our knowledge, such researches remained less in focusing on the effect of inulin with different chain-length on the properties of SPS. Therefore, the present research was carried out to explore the influences of inulin with short and long-chain on the pasting, rheological, and textural properties of SPS. Subsequently, the impact mechanism between inulin and SPS were discussed.
2. Materials and methods
2.1. Materials
Short-chain inulin (DP≤ 10, inulin content was 90.80%) and long-chain inulin (DP>23, inulin content was 100%) was obtained from Beneo-Orafti Company, Belgium. Sweet potato starch (the total starch content was 86.53%) was purchased from Beijing Ershang Company, China. All the other chemical reagents utilized in this paper were analytically pure.
2.2. Preparation of SPS-inulin mixes
SPS-inulin mixes, which were denoted by SPS-SI, and SPS-LI, were obtained by partial replacement of SPS with inulin (0, 2.5%, 5%, 10%, respectively). And the addition of inulin with short and long-chain was shown in .
Table 1. SPS-inulin mixes formulation composed with inulin with short and long-chain.
Tabla 1. Formulación de mezclas de SPS-inulina elaboradas de inulina con cadena corta y larga
2.3. Pasting properties
Pasting properties of SPS and SPS-inulin mixes were detected by using rapid viscosity analyzer (RVA4500, Perten, Sweden). After 3.0 g samples were blended with 25 mL distilled water in RVA container, the slurries were prepared. The pasting parameters (peak viscosity, trough viscosity, breakdown, final viscosity, setback and pasting temperature) of samples were measured according to the reported method of Wan et al. (Citation2017). The results were the average values of three replications.
2.4. Texture analysis
The texture of SPS-inulin mixes gels was measured according to the method of Zhang et al. (Citation2019). The pastes obtained from RVA analysis of 2.3 were transferred to the plastic containers. After being kept for 24 h at 4°C, the gels were detected under TPA mode by using a texture analyzer (TA-XT2i, Stable Micro System, England). The samples were compressed using P/50 probe at the speed of 4 mm/s to reach 30% strain. The hardness, springiness, cohesiveness, gumminess, chewiness, and resilience was recorded. Results were the average values of three replicates.
2.5. Dynamic rheological properties
2.5.1. Preparation of SPS-inulin mixes paste
The suspension was obtained by simply blending 9 g SPS-inulin mixes with 150 mL distilled water, followed by agitating at 20 oC for 20 min, and then heated at 95 °C for 30 min in water bath with the stirring rate of 200 rpm. Finally, the paste of SPS-inulin mixes was obtained. During heating, three layers plastic wraps were covered on the mouth of beaker.
2.5.2. Frequency sweep tests
The samples obtained with the method of 2.5.1 were employed to explore the dynamic rheological properties using Haake Mars 60 rheometer (Thermo Fisher Scientific, Germany). Frequency sweep tests were carried out over the frequency of 0.1 to 10 Hz with 1% strain at 25 °C. The storage modulus (G’), the loss modulus (G”), and tan δ (G”/G’) was obtained.
2.5.3. Temperature sweep tests
The suspension prepared with 6 g samples and 100 mL distilled water was agitated at 20 oC for 20 min, and then loaded onto the plate. Dynamic oscillatory properties of temperature sweep were determined according to the published method (Zhang et al., Citation2019). P60Til Parallel-plate geometry at gap of 1.00 mm was used. Temperature sweep tests were conducted at a frequency of 1 Hz and the strain of 0.4%. The temperature of samples increased from 25 to 110 °C at the rate of 5 °C/min, and then decreased to 25 °C at the same rate. G’ was obtained, and the maximum value of G’ (G’Max) was analyzed.
2.6. Measurement of the leaked amylose content
The leaked amylose amount of samples was evaluated according to the published method with some modifications (Zhu et al., Citation2011). After the slurry was prepared by mixing 2 g SPS-inulin samples with 200 mL distilled water, it was heated in water bath for 20 min at 95 oC, and then cooled to 25 °C. The supernatant of pastes was collected after being centrifuged at 12 000 × g for 20 min at 25 oC. The obtained supernatant was mixed with 0.5 mol/L NaOH (6 mL), and then heated in water bath for 15 min at 95 oC. After being cooled to 25 oC, the solution was diluted to 100 mL with distilled water in volumetric flask. 1 mL prepared sample solution, 1 mL acetic acid solution (1 mol/L), and 2 mL iodine reagent (0.01 mol/L) was transferred to volumetric flask in turn. Then, the mixes solution was diluted to 100 mL by using the distilled water. Finally, they were stored in darkroom for 20 min at ambient temperature. The absorbance was measured at the wavelength of 620 nm.
2.7. Detection of granule size
The paste was obtained with the method of 2.5.1. The granule size of samples was analyzed with BT-9300 H particle-size analyzer (Bettersize, Dandong, China) according to the published method (W. Kim & Qin, Citation2014). The solvent was distilled water, and its flow velocity was set at 60 mL/s.
2.8. Interaction force between inulin and SPS
The interaction force was evaluated according to the reported method (Tang, Citation2013). 6 g SPS-inulin mixes was prepared by blending 5.85 g SPS with 0.15 g SI or LI, followed by dissolving in 100 mL 0, 0.2 mol/L thiourea, or 0.2 mol/L NaCl, respectively. The resulted slurry was heated in water bath for 20 min at 95 oC. After being cooled to ambient temperature (25 oC), G’ of SPS-inulin paste was detected according to the method of 2.5.2.
2.9. Analysis of FT-IR spectroscopy
After being prepared with the method of 2.5.1, the samples were freeze-dried, and then characterized by TENSOR 27 FT-IR (Bruker Optics, Billerica) according to the method of Zhang et al. (Citation2019). The scanning range was from 4000 cm−1 to 400 cm−1, and the spectral resolution was set at 4 cm−1.
2.10. Statistical analyses
The results were expressed as mean ± standard deviation. All data were subjected to analysis using one-way analysis of variance. Significant differences among the mean was evaluated by LSD test at 5% level.
3. Results and discussion
3.1. Effect of inulin with short and long-chain on pasting properties of SPS
Compared with SPS without inulin, addition of inulin reduced the peak viscosity, final viscosity, trough viscosity, setback and breakdown of SPS mixes. Similar results were found when rice, potato, and wheat starch was replaced or added with inulin (Witczak et al., Citation2014; Ye et al., Citation2018). The highest drop of SPS paste viscosity was observed when long-chain inulin was added, while the smaller was short-chain inulin ().
Table 2. Pasting characteristics of SPS replaced by inulin with short and long-chain.
Tabla 2. Características de pegado del SPS sustituido por inulina de cadena corta y larga
The breakdown could reflect the stability of starch granules during being heated. The smaller the breakdown, the stronger the thermal stability of SPS. Addition of inulin resulted in the decrease of breakdown of SPS, indicating that inulin improved the heating stability of starch granules. As for inulin with different chain-length, at the lower addition levels (≤5.0%), the influence of short-chain inulin on breakdown was greater than that of long-chain inulin. While, at the higher addition levels (10%), long-chain inulin had much stronger impact on breakdown than that of short-chain inulin. Witczak et al. (Citation2014) reported the similar result.
Addition of inulin decreased the setback values of SPS, which indicated that inulin could improve SPS storage stability. With the increase of short-chain inulin addition amount, the setback of SPS decreased first and then increased slightly, and at the addition level of 5%, the value of setback was the lowest (1383 cP). While this value of SPS decreased all along with the increase of long-chain inulin content. Compared to inulin with long-chain, inulin with short-chain has much smaller molecules that could easily combine with water at room temperature, so that the retrogradation of SPS could be markedly inhibited by short-chain inulin even at smaller addition-level. When the addition amount of inulin was much higher, short-chain inulin could not continuously inhibit the starch from aging, but slightly promote the retrogradation of SPS, which might be related to the crystal-forming between inulin and inulin when the addition of inulin was much higher (Glibowski et al., Citation2014).
Additionally, the GT of SPS rose with the addition of inulin with different chain-length (). R. Wang et al. (Citation2019) reported the similar result, pasting temperature of rice starch rose after being blended with inulin. As far as the two kinds of inulin were concerned, when the addition of inulin was less than 10%, the pasting temperature of SPS replaced by short-chain inulin was slightly higher than that done by long-chain inulin. The similar results were observed by Li (Citation2017), who reported that the effect of short-chain inulin on the gelatinization of rice starch was more obvious than that of long-chain.
3.2. Effect of inulin with short and long-chain on texture of SPS gel
The influence of short and long-chain inulin on the texture of SPS gel was shown in . With the increase of inulin content, the hardness of SPS gels showed a downward trend. When addition level of inulin was 2.5%, the hardness of SPS gels replaced by short-chain inulin and long-chain inulin decreased to 3.21 N and 3.55 N, respectively. While their value was not significantly different from that of SPS without inulin (p > .05). When the addition amount of inulin increased to 10.0%, the hardness of SPS-SI and SPS-LI mixes gel decreased to 1.85 N and 2.71 N, respectively, and their value was much lower than that of pure SPS gel (p < .05). Diao (Citation2015) described the similar result that rice starch gel texture became soft after being mixed with inulin. Inulin addition caused a decrease in the hardness of starch gel, which may be related to that the network structure of inulin gel was much weaker than that of starch gel (Li, Citation2017).
Table 3. Texture parameters of gel prepared by SPS and SPS-inulin mixes.
Tabla 3. Parámetros de textura del gel preparado con mezclas de inulina y SPS y de solo SPS
With the increase of inulin content, both the resilience and springiness of SPS increased first and then decreased. When inulin addition level was 10%, springiness and resilience of SPS-SI mixes gel decreased greatly, while these of SPS-LI declined slowly. Additionally, the gumminess of SPS gel gradually declined with the addition of inulin with both short and long-chain.
3.3. Effect of inulin with short and long-chain on dynamic rheological properties of SPS gel
3.3.1. Frequency scanning
The results of G’ and G” of SPS-inulin mixes gels varying with the frequency was presented in . G’ and G” of all the SPS-inulin mixes exhibited a slight frequency dependency. Additionally, the values of G’ were much higher than those of G” within the measured frequency (). This result suggested that the gel of SPS-inulin mixes had solid-like property under the measured frequency range.
Figure 1. The change in G’ of SPS-inulin mixes gel with frequency scanning. G’, the storage modulus; SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-SI-5, sweet potato starch substituted by short-chain inulin with the level of 5%; SPS-SI-10, sweet potato starch substituted by short-chain inulin with the level of 10%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%; SPS-LI-5, sweet potato starch substituted by long-chain inulin with the level of 5%; SPS-LI-10, sweet potato starch substituted by long-chain inulin with the level of 10%.
Figura 1. Cambio en la G’ del gel elaborado con mezclas de inulina y SPS detectado mediante barrido de frecuencia. G’, el módulo de almacenamiento; SPS, almidón de batata; SI, inulina de cadena corta; LI, inulina de cadena larga; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-SI-5, almidón de batata sustituido por inulina de cadena corta a nivel de 5%; SPS-SI-10, almidón de batata sustituido por inulina de cadena corta a nivel de 10%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%; SPS-LI-5, almidón de batata sustituido por inulina de cadena larga a nivel de 5%; SPS-LI-10, almidón de batata sustituido por inulina de cadena larga a nivel de 10%
Figure 2. The change in G” of SPS-inulin mixes gel with frequency scanning. G”, the loss modulus; SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-SI-5, sweet potato starch substituted by short-chain inulin with the level of 5%; SPS-SI-10, sweet potato starch substituted by short-chain inulin with the level of 10%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%; SPS-LI-5, sweet potato starch substituted by long-chain inulin with the level of 5%; SPS-LI-10, sweet potato starch substituted by long-chain inulin with the level of 10%.
Figura 2. Cambio en la G” del gel elaborado con mezclas de inulina y SPS detectado mediante barrido de frecuencia. G”, el módulo de pérdida; SPS, almidón de batata; SI, inulina de cadena corta; LI, inulina de cadena larga; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-SI-5, almidón de batata sustituido por inulina de cadena corta a nivel de 5%; SPS-SI-10, almidón de batata sustituido por inulina de cadena corta a nivel de 10%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%; SPS-LI-5, almidón de batata sustituido por inulina de cadena larga a nivel de 5%; SPS-LI-10, almidón de batata sustituido por inulina de cadena larga a nivel de 10%
G’ of inulin-SPS mixes showed a downward trend with the increase of inulin addition, indicating the elastic gel formed by the interaction of inulin and SPS molecules was weak and prone to be out of shape (Kou et al., Citation2018). Additionally, inulin could also make G” of SPS gel decrease. When inulin addition amount was the same, both G’ and G” of samples containing short-chain inulin was lower than those of samples containing long-chain inulin.
The tan δ of SPS-inulin mixes gel was larger than those of SPS without inulin (), which showed the viscosity of samples with inulin changed faster than elasticity, demonstrating that SPS-inulin mixes gel had stronger liquid-like behavior. Similar result was reported by W. W. Kim and Yoo (Citation2011), who found the tan δ of acorn starch gel became larger with the addition of galactomannan. The tan δ of SPS-inulin mixes gel enhanced with the increase of inulin addition. When addition level of inulin was 2.5%, tan δ of SPS replaced by inulin with short-chain was lower than that did by long-chain inulin. While the tan δ of SPS-LI mixes was higher than that of SPS-SI mixes when inulin addition level was above 2.5%.
Figure 3. The change in tan δ of SPS-inulin mixes gel with frequency scanning. SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-SI-5, sweet potato starch substituted by short-chain inulin with the level of 5%; SPS-SI-10, sweet potato starch substituted by short-chain inulin with the level of 10%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%; SPS-LI-5, sweet potato starch substituted by long-chain inulin with the level of 5%; SPS-LI-10, sweet potato starch substituted by long-chain inulin with the level of 10%.
Figura 3. Cambio en el tan δ del gel elaborado con mezclas de inulina y SPS detectado mediante barrido de frecuencia. SPS, almidón de batata; SI, inulina de cadena corta; LI, inulina de cadena larga; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-SI-5, almidón de batata sustituido por inulina de cadena corta a nivel de 5%; SPS-SI-10, almidón de batata sustituido por inulina de cadena corta a nivel de 10%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%; SPS-LI-5, almidón de batata sustituido por inulina de cadena larga a nivel de 5%; SPS-LI-10, almidón de batata sustituido por inulina de cadena larga a nivel de 10%
3.3.2. Temperature scanning
As shown in , in the initial heating period (40–70 oC), the changes in G’ were quite small, and the G’ value was not significantly different between SPS and SPS-inulin mixes. When heating temperature increased from 70 to 97 oC, G’ values were sharply raised until to the maximum. The G’ values dropped with further heating (). Similar results were reported by Gonzalez-Reyes et al. (Citation2003), and Singh et al. (Citation2007).
Figure 4. The change in G’ of SPS-inulin mixes with temperature scanning. G’, the storage modulus; SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-SI-5, sweet potato starch substituted by short-chain inulin with the level of 5%; SPS-SI-10, sweet potato starch substituted by short-chain inulin with the level of 10%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%; SPS-LI-5, sweet potato starch substituted by long-chain inulin with the level of 5%; SPS-LI-10, sweet potato starch substituted by long-chain inulin with the level of 10%.
Figura 4. Cambio en la G’ de las mezclas de inulina y SPS detectado mediante barrido de temperatura. G’, módulo de almacenamiento; SPS, almidón de batata; SI, inulina de cadena corta; LI, inulina de cadena larga; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-SI-5, almidón de batata sustituido por inulina de cadena corta a nivel de 5%; SPS-SI-10, almidón de batata sustituido por inulina de cadena corta a nivel de 10%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%; SPS-LI-5, almidón de batata sustituido por inulina de cadena larga a nivel de 5%; SPS-LI-10, almidón de batata sustituido por inulina de cadena larga a nivel de 10%
In addition, the temperature of G’Max rose along with the increase of inulin addition. SPS without inulin showed the lowest G’Max temperature with the value of 93.20 °C. SPS-LI with 10% inulin addition had the highest G’Max temperature, which was 96.04 °C. This behaviour may be due to good water retention ability of inulin (Luo et al., Citation2012).
As illustrated in , at the same addition level, the G’Max temperature of SPS-LI mixes was higher than that of SPS-SI blends, which might be related to the fact that the long-chain inulin could absorb more water than short-chain inulin at higher temperature, much easily inhibit the water-swelling of SPS and result in the higher increase of G’Max temperature (Luo et al., Citation2017, Citation2020).
3.4. Effect of inulin with short and long-chain on amylose content leaked from SPS
During the gelatinization of SPS, amylose more easily leaked out from starch granule than amylopectin. After being blended with inulin, the leaked amylose content of SPS-inulin mixes fell significantly than that of SPS without inulin (p < .05) (). Additionally, the leakage content of amylose dropped remarkably with the increase of inulin addition. Similar results were reported by Li (Citation2017), Zhu et al. (Citation2011), and Li (Citation2017) observed that adding inulin resulted in decrease of the leakage of amylose content of wheat starch. Zhu et al. (Citation2011) described that with the addition of xanthan gum, the leakage content of tapioca starch amylose reduced.
Figure 5. Effect of inulin with short and long-chain on the leaked amylose content of SPS. SPS, sweet potato starch; Different letters above the bars of standard deviations indicate significant differences among samples (p < 0.05).
Figura 5. Efecto de la inulina de cadena corta y larga en el contenido de amilosa escapada del SPS. SPS, almidón de batata; Las distintas letras sobre las barras de las desviaciones estándar indican diferencias significativas entre las muestras (p < 0.05)
When the addition amount of inulin was less than 5%, the leached amylose content of SPS mixed with short-chain inulin was significantly lower than that of sample blended with long-chain inulin at the same addition level (p < .05). When the amount of inulin was 10%, the leached amylose content of SPS-LI mixes was only 17.23%, which was lower than that of SPS-SI (17.45%), while the difference between them was not significant (p > 0.05).
3.5. Effect of inulin with short and long-chain on the granular size distribution of SPS
As compared with SPS alone, the median diameter size of SPS-inulin mixes treated for 20 min at 95 oC became smaller, and decreased with inulin addition level rose (). With the increase of inulin amount, the volume fraction in the larger range (>45 μm) of SPS granule dropped and was lower than that of SPS without inulin, while those of the smaller (<45 μm) grew. When inulin addition level was 10%, compared to SPS alone, the volume fraction of the smaller sized particle of SPS blended by inulin with short and long-chain increased by 3.16% and 5.26%, respectively (, ). These results suggested that the swelling of SPS granules could be inhibited by inulin, and its granules was mostly concentrated in the smaller ones. A similar result was reported by Zhang et al. (Citation2019).
Table 4. Effect of inulin with different chain-length on granular size distribution of SPS.
Tabla 4. Efecto de la inulina con diferente longitud de cadena en la distribución granular del tamaño del SPS
Figure 6. Differential distribution of granule size of SPS replaced by inulin with different chain-length. SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-SI-5, sweet potato starch substituted by short-chain inulin with the level of 5%; SPS-SI-10, sweet potato starch substituted by short-chain inulin with the level of 10%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%; SPS-LI-5, sweet potato starch substituted by long-chain inulin with the level of 5%; SPS-LI-10, sweet potato starch substituted by long-chain inulin with the level of 10%.
Figura 6. Distribución diferencial del tamaño del gránulo de SPS reemplazado por inulina con diferente longitud de cadena. SPS, almidón de batata; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-SI-5, almidón de batata sustituido por inulina de cadena corta a nivel de 5%; SPS-SI-10, almidón de batata sustituido por inulina de cadena corta a nivel de 10%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%; SPS-LI-5, almidón de batata sustituido por inulina de cadena larga a nivel de 5%; SPS-LI-10, almidón de batata sustituido por inulina de cadena larga a nivel de 10%
In addition, the median diameter size of SPS-SI was slight lower than that of SPS-LI when the addition amount of inulin was smaller (<5%), while it was higher than that of SPS-LI when inulin addition level was ≥5%.
3.6. Interaction force between SPS and inulin molecules
Generally speaking, the interaction such as electrostatic or hydrophobic force, hydrogen bonding may exist in the non-starch hydrocolloids-starch blended system (Zhang et al., Citation2019). Sodium chloride (NaCl) mainly influences the electrostatic force in the blended system, and thiourea mainly destroys the action of hydrogen bond (Li, Citation2017). The G’ of SPS-inulin mixes decreased slightly in NaCl solution, which indicated that it was not electrostatic interaction between inulin and SPS. While, the G’ values of SPS-inulin mixes gel decreased remarkably in thiourea solution. These results suggested that the hydrogen bond was the main interaction force between inulin and SPS. Similar results were found by Li (Citation2017), and Zhang et al. (Citation2019). Additionally, it could be seen from that the gel of SPS-LI was more susceptible to thiourea than that of SPS-SI.
Figure 7. Effects of NaCl and thiourea solution on G’ of SPS-inulin mixes. G’, the storage modulus; SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-SI-5, sweet potato starch substituted by short-chain inulin with the level of 5%; SPS-SI-10, sweet potato starch substituted by short-chain inulin with the level of 10%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%; SPS-LI-5, sweet potato starch substituted by long-chain inulin with the level of 5%; SPS-LI-10, sweet potato starch substituted by long-chain inulin with the level of 10%.
Figura 7. Efectos de la solución de NaCl y tiourea en la G’ de las mezclas de inulina y SPS. SPS, almidón de batata; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-SI-5, almidón de batata sustituido por inulina de cadena corta a nivel de 5%; SPS-SI-10, almidón de batata sustituido por inulina de cadena corta a nivel de 10%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%; SPS-LI-5, almidón de batata sustituido por inulina de cadena larga a nivel de 5%; SPS-LI-10, almidón de batata sustituido por inulina de cadena larga a nivel de 10%
3.7. Infrared spectroscopy analysis of SPS replaced by inulin with short and long-chain
The infrared spectra of SPS replaced by inulin with different chain-length were detected in the range of 4000 cm−1 and 400 cm−1, and the results are presented in and . The peak band at about 3700 cm−1 was related to – OH stretching, which suggested that free OH group existed in the system of SPS partially replaced by inulin. Additionally, at about 3700 cm−1, the peak intensity of SPS-SI was slightly higher than that of SPS-LI, which might be due to the more hydrogen bonds and the stronger hygroscopicity of short-chain inulin (Li, Citation2017).
Table 5. Effect of inulin with short and long-chain on absorption peak of SPS.
Tabla 5. Efecto de la inulina de cadena corta y larga en el pico de absorción de SPS
Figure 8. Infrared spectra of SPS-inulin mixes. SPS, sweet potato starch; SI, short-chain inulin; LI, long-chain inulin; SPS-SI-2.5, sweet potato starch substituted by short-chain inulin with the level of 2.5%; SPS-LI-2.5, sweet potato starch substituted by long-chain inulin with the level of 2.5%.
Figura 8. Espectro de infrarrojos de las mezclas de inulina y SPS.SPS, almidón de batata; SI, inulina de cadena corta; LI, inulina de cadena larga; SPS-SI-2.5, almidón de batata sustituido por inulina de cadena corta a nivel de 2.5%; SPS-LI-2.5, almidón de batata sustituido por inulina de cadena larga a nivel de 2.5%
The peak band at about 3200 cm−1 was attributed to the associative-OH stretching (Li, Citation2017). This band of SPS shifted to lower wave number (3200 cm−1) after being blended with inulin, which reflected that the hydrogen bond between starch and inulin was strengthened. The hydroxyl stretching vibration band of SPS alone was 3338.67 cm−1, while the band of SPS replaced by inulin with short and long-chain was 3330.95 and 3334.81 cm−1, respectively. This indicated that the shorter chain-length of inulin was, the stronger the hydrogen bond between inulin and SPS molecules was. Similar result was obtained by Li (Citation2017). Additionally, as shown in , compared to SPS alone, there were no new absorption peak bands observed in SPS-inulin mixes.
3.8. Possible influence mechanism of inulin on pasting of SPS
Based on the results obtained in this paper and related published articles, the possible influence mechanism of inulin on pasting of SPS was depicted in the form of schematic diagram ().
Figure 9. The schematic diagram demonstrating the potential effect mechanism of inulin on pasting of SPS. SPS, sweet potato starch.
Figura 9. Diagrama esquemático del posible mecanismo de efecto de la inulina en el pegado de SPS.SPS, almidón de batata
Inulin, a fructan-type polysaccharide with linear molecular structure, has higher water retention ability, better texture properties of gel, and lower viscosity (Gao et al., Citation2016; Karimi et al., Citation2015; Paola & Sancho, Citation2018). After being blended with SPS and water, inulin could competitively absorb water with SPS granules during pasting, limited the availability and mobility of water, and reduced the amount of water available for SPS. These resulted in the increase of the gelatinization temperature, and the inhibition of both swelling and leakage of amylose of starch granules (, -B2). Although SPS granules could absorb the water from the continuous phase during pasting, the viscosity of SPS-inulin mixes decreased (), this could be attributed to that, (1) The hygroscopicity of inulin molecules caused that the amylopectin of SPS could not absorb enough water to form a completely viscous substance (Li, Citation2017); (2) The viscosities of inulin were much lower than those of SPS ().
Simultaneously, inulin could interact with the leached amylose (Li, Citation2017), and hinder them to diffuse into the solution. Additionally, some of the leached amylose wrapped on the surface of starch granules (Alamri et al., Citation2012), and the non-leaking amylose could make the molecules inside starch granule more entangled, these resulted in the inhibition of the unwinding of the amylopectin molecules (-C2 and 9-D2). Since most of the hydroxyl groups are outside of its molecule structure, inulin possessed larger hydrogen bonding area, and was easy to get close to the amylose through hydrogen bonding. Therefore, the formed hydrogen bonding distance between inulin and starch was short and their binding force was larger.
Compared with long-chain inulin, short-chain inulin had more hydroxyl groups outside the molecule structure, and exhibited stronger ability of binding water (Luo et al., Citation2012). Additionally, short-chain inulin was more easily inserted into starch molecule than long-chain inulin during heating, which in turn affected the crystallization structure of SPS. Therefore, short-chain inulin could significantly affect the gelatinization temperature and paste viscosity of SPS than long-chain inulin at a lower addition level (<5.0%) (). At higher temperature (80–90°C), the compact and highly regular crystal structure of long-chain inulin might be destroyed, resulting in the more hydroxyl groups being exposed, which enhanced its hygroscopicity and water retention ability (Li, Citation2017; Luo et al., Citation2017, Citation2012). Therefore, long-chain inulin could significantly affect the viscosity of SPS than short-chain inulin at higher addition level (≥5%) ().
4. Conclusion
This paper explored the influences of inulin with different chain-length (short, and long-chain) on pasting, texture and rheological properties of SPS. Compared with SPS without inulin, addition of inulin reduced the peak viscosity, final viscosity, trough viscosity, setback and breakdown of SPS samples, and enhanced the GT. With the increase of inulin content, the hardness of SPS gels showed a downward trend, while both the springiness and resilience of SPS first increased and then declined. The G’ and G” of SPS-SI gel was lower than those of SPS-LI samples at the same addition level. The temperature of G’Max went up with the increase of inulin addition. SPS without inulin showed the lowest G’Max temperature, while SPS-LI with 10% inulin addition had the highest G’Max temperature. At the same addition level, the G’Max temperature of SPS-LI mixes was higher than that of SPS-SI blends. Inulin could inhibit the swelling ability of SPS granules, and made SPS granules concentrate in the smaller. Inulin could make the content of leached amylose decline significantly (p < .05). The hydrogen bond was the interacting force between SPS and inulin. Generally, inulin with different chain-length affected the pasting, texture, and rheological properties of SPS in varying degrees. These results could provide some scientific basis for the use of inulin in SPS-based foods.
Ethical approval
This article does not contain any studies with human participants or animals performed by any of the authors.
Disclosure statement
All authors declare that there are no conflict of interest.
Additional information
Funding
References
- Abegunde, O. K., Mu, T. H., Chen, J. W., & Deng, F. M. (2013). Physicochemical characterization of sweet potato starches popularly used in Chinese starch industry. Food Hydrocolloids, 33(2), 169–177. https://doi.org/https://doi.org/10.1016/j.foodhyd.2013.03.005
- Alamri, M. S., Mohamed, A. A., & Hussain, S. (2012). Effect of okra gum on the pasting, thermal, and viscous properties of rice and sorghum starches. Carbohydrate Polymers, 89(1), 199–207. https://doi.org/https://doi.org/10.1016/j.carbpol.2012.02.071
- Azeem, M., Mu, T. H., & Zhang, M. (2020). Influence of particle size distribution of orange-fleshed sweet potato flour on dough rheology and simulated gastrointestinal digestion of sweet potato-wheat bread. LWT-Food Science and Technology, 131, 109690. https://doi.org/https://doi.org/10.1016/j.lwt.2020.109690
- Charoenrein, S., Tatirat, O., Rengsutthi, K., & Thongngam, M. (2011). Effect of konjac glucomannan on syneresis, textural properties and the microstructure of frozen rice starch gels. Carbohydrate Polymers, 83(1), 291–296. https://doi.org/https://doi.org/10.1016/j.carbpol.2010.07.056
- Diao, S. C. (2015). Effect of inulin on physicochemical properties of rice starch and the quality of rice-cakes [Master’s thesis]. Anhui Agricultural University. http://www.cnki.net
- Fang, F., Luo, X., BeMiller, J. N., Schaffter, S., Campanella, O. H., Hayes, A. M. R., Woodbury, T. J., & Hamaker, B. R. (2020). Neutral hydrocolloids promote shear-induced elasticity and gel strength of gelatinized waxy potato starch. Food Hydrocolloids, 107, 105923. https://doi.org/https://doi.org/10.1016/j.foodhyd.2020.105923
- Gao, J., Brennan, M. A., Mason, S. L., & Brennan, C. S. (2016). Effect of sugar replacement with stevianna and inulin on the texture and predictive glycaemic response of muffins. International Journal of Food Science and Technology, 51(9), 1979–1987. https://doi.org/https://doi.org/10.1111/ijfs.13143
- Glibowski, P., Pikus, S., Jurek, J., & Kotowoda, M. (2014). Factors affecting inulin crystallization after its complete dissolution. Carbohydrate Polymers, 110, 107–112. https://doi.org/https://doi.org/10.1016/j.carbpol.2014.03.080
- Gonzalez-Reyes, E., Mendez-Montealvo, G., Solorza-Feria, J., Toro-Vazquez, J. F., & Bello-Perez, L. A. (2003). Rheological and thermal characterization of Okenia hypogaea (Schlech. & Cham.) starch. Carbohydrate Polymers, 52(3), 297–310. https://doi.org/https://doi.org/10.1016/S0144-8617(02)00291-6
- Gou, M., Wu, H., Saleh, A. S. M., Jing, L., Liu, Y., Zhao, K., & Li, W. (2019). Effects of repeated and continuous dry heat treatments on properties of sweet potato starch. International Journal of Biological Macromolecules, 129, 869–877. https://doi.org/https://doi.org/10.1016/j.ijbiomac.2019.01.225
- Guo, L., Tao, H., Cui, B., & Janaswamy, S. (2019). The effects of sequential enzyme modifications on structural and physicochemical properties of sweet potato starch granules. Food Chemistry, 277, 504–515. https://doi.org/https://doi.org/10.1016/j.foodchem.2018.11.014
- Hoover, R. (2001). Composition, molecular structure, and physicochemical properties of tuber and root starches: A review. Carbohydrate Polymers, 45(3), 253–267. https://doi.org/https://doi.org/10.1016/S0144-8617(00)00260-5
- Hung, P. V., & Morita, N. (2005). Physicochemical properties and enzymatic digestibility of starch from edible canna (Canna edulis) grown in Vietnam. Carbohydrate Polymers, 61(3), 314–321. https://doi.org/https://doi.org/10.1016/j.carbpol.2005.04.021
- Ji, L., Sun, H., & Mu, T. (2019). Analysis of physicochemical and hygienic indexes of 12 different kinds of sweet potato starch products. Food Science and Technology, 44(3), 245–251, 255. http://www.cnki.net
- Karimi, R., Azizi, M. H., Ghasemlou, M., & Vaziri, M. (2015). Application of inulin in cheese as prebiotic, fat replacer and texturizer: A review. Carbohydrate Polymers, 119, 85–100. https://doi.org/https://doi.org/10.1016/j.carbpol.2014.11.029
- Kim, W., & Qin, Y. (2014). Effect of heating temperature on the particle size distribution in waxy wheat flour. Journal of Cereal Science, 59(2), 228–233. https://doi.org/https://doi.org/10.1016/j.jcs.2013.12.011
- Kim, W. W., & Yoo, B. (2011). Rheological and thermal effects of galactomannan addition to acorn starch paste. LWT-Food Science and Technology, 44(3), 759–764. https://doi.org/https://doi.org/10.1016/j.lwt.2010.11.009
- Kou, X., Luo, D., Li, Y., Xu, B., Zhang, K., Li, P., Liu, J., Li, X., & Han, S. (2018). Effect of inulin with different degree of polymerisation on textural and rheological properties of wheat starch-effect of inulin on gel properties of starch. International Journal of Food Science and Technology, 53(11), 2576–2585. https://doi.org/https://doi.org/10.1111/ijfs.13852
- Krystyjan, M., Ciesielski, W., Khachatryan, G., Sikora, M., & Tomasik, P. (2015). Structure, rheological, textural and thermal properties of potato starch – Inulin gels. LWT-Food Science and Technology, 60(1), 131–136. https://doi.org/https://doi.org/10.1016/j.lwt.2014.07.056
- Lai, Y. C., Wang, S. Y., & Gao, H. Y., Nguyen, K. M., Nguyen, C. H., Shih, M. C., Lin, K. H. (2016). Physicochemical properties of starches and expression and activity of starch biosynthesis-related genes in sweet potatoes. Food Chemistry, 199, 556–564. https://doi.org/https://doi.org/10.1016/j.foodchem.2015.12.053
- Li, Y. (2017). Interactions between inulin and wheat starch [Master’s thesis]. Henan University of Science and Technology. http://www.cnki.net
- Luo, D., Li, Y., Xu, B., Ren, G., Li, P., Li, X., Liu, J., & Han, S. (2017). Effects of inulin with different degree of polymerization on gelatinization and retrogradation of wheat starch. Food Chemistry, 229, 35–43. https://doi.org/https://doi.org/10.1016/j.foodchem.2017.02.058
- Luo, D., Xi, D., Zhang, K., An, Y., Huang, J., & Kang, H. (2020). Gelling properties of long-chain inulin. Journal of the Chinese Cereals and Oils Association, 35(4), 151–157. http://www.cnki.net
- Luo, D., Xu, W., Chen, R., Liu, J., Xu, B., & Chen, H. (2012). Solubility and gelatin textural properties of inulin. Transactions of the Chinese Society for Agricultural Machinery, 43, 118–122, 129. http://www.cnki.net
- Mensink, M. A., Frijlink, H. W., van der Voort, M. K., & Hinrichs, W. L. J. (2015). Inulin, a flexible oligosaccharide I: Review of its physicochemical characteristics. Carbohydrate Polymers, 130, 405–419. https://doi.org/https://doi.org/10.1016/j.carbpol.2015.05.026
- Monthe, O. C., Grosmaire, L., Nguimbou, R. M., Dahdouh, L., Ricci, J., Tran, T., & Ndjouenkeu, R. (2019). Rheological and textural properties of gluten-free doughs and breads based on fermented cassava, sweet potato and sorghum mixed flours. LWT–Food Science and Technology, 101, 575–582. https://doi.org/https://doi.org/10.1016/j.lwt.2018.11.051
- Paola, D., & Sancho, B. (2018). Effects of replacing starch by inulin on the physicochemical, texture and sensory characteristics of gummy jellies. CyTA-Journal of Food, 16(1), 1–10. https://doi.org/https://doi.org/10.1080/19476337.2017.1327462
- Singh, J., McCarthy, O. J., Singh, H., Moughan, P. J., & Kaur, L. (2007). Morphological, thermal and rheological characterization of starch isolated from New Zealand Kamo Kamo (Cucurbita pepo) fruit – A novel source. Carbohydrate Polymers, 67(2), 233–244. https://doi.org/https://doi.org/10.1016/j.carbpol.2006.05.021
- Tang, M. (2013). Effect of xanthan on the retrogradation of rice starch and preliminary study on its mechanism [Master’s thesis]. Jiangnan University. http://www.cnki.net
- Wan, J., Zhou, G. H., Luo, S. J., Wang, R. S., Liu, C. M., Zhang, X., & Liu, F. (2017). A study of the effect of amino acids on pasting and short-term retrogradation properties of rice starch based on molecular dynamics simulation. Starch - Stärke, 69(9–10), 9–10. https://doi.org/https://doi.org/10.1002/star.201600238
- Wang, H., Yang, Q., Ferdinand, U., Gong, X., Qu, Y., Liu, M., Gao, W., Ivanistau, A., & Feng, B. (2020). Isolation and characterization of starch from light yellow, orange, and purple sweet potatoes. International Journal of Biological Macromolecules, 160, 660–668. https://doi.org/https://doi.org/10.1016/j.ijbiomac.2020.05.259
- Wang, R., Wan, J., Liu, C., Xia, X., & Ding, Y. (2019). Pasting, thermal, and rheological properties of rice starch partially replaced by inulin with different degrees of polymerization. Food Hydrocolloids, 92, 228–232. https://doi.org/https://doi.org/10.1016/j.foodhyd.2019.02.008
- Witczak, T., Witczak, M., & Ziobro, R. (2014). Effect of inulin and pectin on rheological and thermal properties of potato starch paste and gel. Journal of Food Engineering, 124, 72–79. https://doi.org/https://doi.org/10.1016/j.jfoodeng.2013.10.005
- Yadav, B. S., Yadav, R. B., Kumari, M., & Khatkar, B. S. (2014). Studies on suitability of wheat flour blends with sweet potato, colocasia and water chestnut flours for noodle making. LWT-Food Science and Technology, 57(1), 352–358. https://doi.org/https://doi.org/10.1016/j.lwt.2013.12.042
- Ye, J., Yang, R., Liu, C., Luo, S., Chen, J., Hu, X., & Wu, J. (2018). Improvement in freeze-thaw stability of rice starch gel by inulin and its mechanism. Food Chemistry, 268, 324–333. https://doi.org/https://doi.org/10.1016/j.foodchem.2018.06.086
- Zhang, L., Wang, X., Ji, H., Sun, J., Ma, H., & Liu, X. (2020). Effect of dry heating with short-chain inulin on physicochemical properties of sweet potato starch. CyTA-Journal of Food, 18(1), 216–228. https://doi.org/https://doi.org/10.1080/19476337.2020.1732468
- Zhang, L., Wang, X., Li, S., Sun, J., & Liu, X. (2019). Effect of inulin on the pasting, textural, and rheological properties of sweet potato starch. CyTA -journal of Food, 17(1), 733–743. https://doi.org/https://doi.org/10.1080/19476337.2019.1645738
- Zhang, Y., Dong, M., Zhang, X., Hu, Y., & Zhou, G. (2020). Effects of inulin on the gel properties and molecular structure of porcine myosin: A underlying mechanisms study. Food Hydrocolloids, 108, 105974. https://doi.org/https://doi.org/10.1016/j.foodhyd.2020.105974
- Zhou, D. N., Zhang, B., Chen, B., & Chen, H. Q. (2017). Effects of oligosaccharides on pasting, thermal and rheological properties of sweet potato starch. Food Chemistry, 230, 516–523. https://doi.org/https://doi.org/10.1016/j.foodchem.2017.03.088
- Zhu, L., Zhang, Y., Hong, Y., & Gu, Z. (2011). Gelatinization mechanisms of tapioca starch-xanthan gum system. Food Science, 32(3), 81–85. http://www.cnki.net