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Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer
BIOMAC-13886; No of Pages 8 International Journal of Biological Macromolecules xxx (xxxx) xxx
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Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer Thaís Paes Rodrigues dos Santos a,c,⁎, Célia Maria Landi Franco b, Magali Leonel c a b c
Center for Tropical Roots and Starches (CERAT), College of Agronomic Science (FCA), São Paulo State University (UNESP), Botucatu, São Paulo, Brazil Department of Food Engineering and Technology, Institute of Biosciences, Language, and Physical Sciences, São Paulo State University (UNESP), São José do Rio Preto, São Paulo State, Brazil Center for Tropical Roots and Starches (CERAT), São Paulo State University (UNESP), Botucatu, São Paulo State, Brazil
a r t i c l e
i n f o
Article history: Received 7 August 2019 Received in revised form 8 November 2019 Accepted 11 November 2019 Available online xxxx Keywords: Spray dryer Pregelatinization Thermal properties Instant food
a b s t r a c t Native sweet potato starch was gelatinized at temperatures between 57 and 69 °C and then spray dried, to verify the changes caused by this process. The spray dried starch granules initially swelled with the increase in preheating temperature, displaying a larger diameter, but shrank upon colling. There was an increase in the gelatinization temperature and a decrease in the enthalpy change of the spray dried starches relative to those of native starch. Spray dried starch preheating at 69 °C showed highly reduced enthalpy change (80%) and relative crystallinity. Based on the parameters studied, the best preheating temperature was 67 °C. Under this condition, the starch can be applied in more viscous products that require a greater thickness, and with great process yield (65%). This study showed the feasibility of the application of this process to produce sweet potato starches with specific properties for different purposes in the food and non-food industries. © 2018 Elsevier B.V. All rights reserved.
1. Introdution Sweet potato (Ipomoea batatas L.) is the seventh most important food crop in the world. In 2017, the world production of sweet potato roots was 112.8 million tons [1]. Starch, the main component of the sweet potato root, it is a biopolymer composed of amylose and amylopectin, which are made up of chains of glucose molecules that differ from each other in size and shape. Amylose displays α(1,4) glycosidic bonds and has a linear helical structure, with few branches. Amylopectin has a highly branched structure with α(1,4) glycosidic bonds, and 4–5% of α(1,6) glycosidic bonds, which are responsible for the formation of branches. Native or processed starches of sweet potato are mainly used as an ingredient for noodles, snacks, cakes, breads, etc. However, the food application of native starch is limited owing to some of its undesirable Abbreviations: D[3.2], surface-weighted; D[4.3], volume-weighted; d(0.5), is the granule size at which 50% of all the granules by volume are smaller; AML, amylose; RC, relative crystallinity; To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; ΔT, range of temperature; ΔH, enthalpy change; GD, gelatinization degree; SP, swelling power; SS, solubility. ⁎ Corresponding author at: Center for Tropical Roots and Starches (CERAT), São Paulo State University (UNESP), José Barbosa de Barros Street, 1780, Jardim Paraíso, Botucatu, São Paulo, 18.610-307, Brazil. E-mail address: [email protected] (T.P.R. dos Santos).
characteristics, such as its paste instability, poor paste clarity, and poor solubility. The number of studies on the modification of sweet potato starch and/or its application in the food and non-food industries has been growing. The aim of starch modification is to improve the natural characteristics of the polysaccharide that are considered undesirable for industrial use, and it can be performed by physical, chemical, and/or enzymatic methods. The physical modification of starch is considered a green method because the process does not produce wastes. Pregelatinization can convert native starch into a partially or fully gelatinized form to obtain a gel at room temperature. The gelatinization process can cause chemical and physical changes to the starch granules by rearranging the intra- and intermolecular hydrogen bonds between the water and starch molecules and can partially or completely destroy the crystalline zones in the starch depending on the intensity of the process [2]. Spray drying requires a short period of preheating at a temperature approximate to that for starch gelatinization. Thus, the intensity of changes on the starch properties can be controlled by the preheating temperature. Partially gelatinized starch converts starch from an amorphous state to a more ordered or crystalline state that has different digestion properties, providing benefits for the dietary management of metabolic disorders [3]. In general, pregelatinized starches, called instant starches, are used as thickeners in many food products that receive minimal heat
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Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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processing, such as instant soups and instant desserts. These products require only addition of hot water with stirring before serving, and also disperse readily with highshear stirring or when mixed with sugar or other dry ingredients [4]. This processed starch can be primarily used as a thickener in many instant products, such as baby food, broths, soups and desserts, powders in beverages, and meat and convenience products such as creams, mayonnaise, frozen pasta, baking products, and premixes; as main ingredients, as bulking agents, thickeners, or stabilizers in many other food products; and in pharmaceutical formulations, as binders and disintegrants. Given the increased use of pregelatinized starch and the industry demand for specific technological properties of the product as well as knowledge about new non-conventional starch sources, this study was conducted to investigate the effects of preheating and spray drying on the physicochemical, structural and thermal characteristics of sweet potato starch. The best conditions for obtaining partially pregelatinized starches were also studied, the information of which could provide new uses of sweet potato starch in industrial processes. 2. Materials and methods 2.1. Materials Sweet potato roots (cv. Canadense) were collected on São Manuel Experimental Farm (CERAT/UNESP, São Manuel city, São Paulo state, Brazil). The roots were planted on March 2015 and were harvested 150 days after planting. Starch was extracted from the roots 24 h after harvesting, following the methodology described by Santos et al. [5]. The sweet potato starch was characterized by physicochemical analysis: moisture, ash, lipids, protein and starch were 11.78, 0.45, 0.89, 0.22, and 86.66% (wet basis), respectively, and pH 6.12 [6]. 2.2. Spray drying process The spray drying process was carried out according to the methodology described by Fu et al. [2] and Santos et al. [7]. Dispersions of starch (8%, m/m, dry basis) were prepared by first mixing the starch in distilled water at 40 ± 1 °C. The mixtures were then transferred to thermostatic water baths set at 57, 60, 63, 66, or 69 °C (treatments: T57, T60, T63, T66, and T69, respectively), with continuous stirring. After reaching the desired temperature, the starch dispersions were continuously stirred in the water bath for 10 min. Evaporation of water was minimized by using a plastic film to cover the beaker. After this time, the hot starch dispersions were dried in a spray dryer (Model MSD 0.5, Labmaq, São Paulo, Brazil) fitted with a double-atomizer nozzle with a nozzle hole of 0.7 mm diameter. The inlet and outlet temperatures were 170 and 110 °C, respectively, and the feed rate was 1.0 L h−1. The flow rates of the compressed air and hot air were 0.40 L min−1 and 3.8 m3 min−1, respectively, and the air pressure was set at 6 bars. 2.3. Morphology of granules The morphologies of the native and spray dried starch granules were evaluated by scanning electron microscopy (Evo-LS15, Carl Zeiss, Oberkochen, Baden-Württemberg, Germany). Starches samples were applied to an aluminum stub with double-sided tape and coated with layer of gold (20 nm) in metallizer for 220 s (BAL-TEC SCD 050, 220 seg). 2.4. Particule size analysis The average size and size distribution of the native and spray dried starch granules were determined using the laser light diffraction technique with a He\\Ne laser (Mastersizer 2000, Laser Scattering Spectrometer Mastersizer S, Model MAM 5005 - Malvern Instruments Ltd., Worcestershire, UK) [2,8].
2.5. X-ray diffraction pattern and relative crystallinity X-ray patterns were examined using a goniometer unit (MiniFlex 600, Rotaflex, Rigaku, Tokyo, Japan), with copper Kα radiation (λ = 0.1542 nm) [7] and the relative crystalline was calculated as described by Nara & Komiya [9]. 2.6. Amylose content For determination of amylose content, starches samples were defatted with 85% (v/v) methanol in reflux with a Soxhlet extractor for 24 h. The amylose content was determined according to the ISO6647 method [10]. 2.7. Thermal properties The gelatinization properties of the native and spray dried starches were evaluated with a differential scanning calorimeter (Pyris 1, Perkin Elmer, Norwalk, Connecticut, USA) as described by Santos et al. [5]. The degree of gelatinization (DG) of the spray dried starches samples was calculated by the ratio of the enthalpy change of the spray dried starch and the enthalpy change of the native starch [11]. 2.8. Swelling power (SP) and solubility (S) Swelling power and solubility was evaluated according Schoch [12], as described by Santos et al. [7]. 2.9. Statistical analysis Data were tested by analysis of variance, and when appropriate, the difference among means was determined using Tukey's multiple comparisons test at the 0.05 probability level. The effect of each pretreatment temperature on the starch characteristics was evaluated by regression analysis. Regression coefficients were considered significant at the 5% probability level. All measurements were carried out in triplicate and the data are presented as the mean ± standard deviation. 3. Results and discussion 3.1. Effects of preheating temperatures on the characteristics of spray dried starches The statistical parameters corresponding to each of the responses showed high F-values for the individual or quadratic effects of the dependent variables. The accuracy of each model was assessed from R2, the values of which were all higher than 0.70 for the models for crystallinity, mastersizer parameters, gelatinization temperatures, and enthalpy change (ΔH) of gelatinization. Additionally, the R2 value for each of the responses was found to be close to that of the other responses, which was indicative of the accuracy of the models. 3.2. Morphology and distribution of granule average size Native sweet potato starch displayed granules with a smooth surface, and oval and polygonal shape, as well as a few depressions (Fig. 1, native), similar to the observations reported in the literature [13–15]. The granule sizes were highly variable, with 65.50% being between 11 and 20 μm in diameter (average 16.49 μm), which was also similar to results reported in the literature [15–17]. The changes on the structure surface and shape of the starch granules were more drastic with the increase in preheating temperature. The starch granules were partially gelatinized and swollen during the spray drying process, but shrank upon cooling and displayed wrinkles, which is typical of macromolecular particles produced by spray drying
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Fig. 1. Morphology of native and spray-dried sweet potato starch. a, 1000×; b, 2000×.
[18]. These properties should promote the flowability of the starch, which is important for use as a direct compression excipient [19]. The changes in the morphology of the spray dried starch granules are shown in Fig. 1. As shown in the figure, the shape and surface of the spray dried granules changed with the increases in preheating
temperature. The starches preheated at 57 and 60 °C exhibited similar characteristics to those of native starch, indicating that these temperatures did not affect the starch granules. However, the median diameter of spray dried granules of these treatments showed decreased (13.90 and 15.32 μm, respectively), which may be due to the moisture content
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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of native starch that was transferred out during the spray drying, causing the granules to shrink and their size to decrease [20]. However, when the starch was preheated at 63 and 66 °C, larger granules and more truncated shapes were observed, with the formation of agglomerates. This change can be explained due to the increase of gelatinization temperature, which lead the starch granules to absorb more water, allowing for the expansion of the granules and their partial gelatinization in the interior. The pretreatments at 63 and 66 °C led to an increase in the average size of the granules to 18.84 and 20.44 μm, respectively, which were higher than that of the native starch. These pregelatinized starches exhibited granule size distribution more homogeneus, which is appropriate and contributes to a good compressibility, tablet disintegrability and flowability, thus, it became a binder for the production of rapid orally disintegranting tablets [21,22]. With the increase in temperature to 69 °C, part of the granules disintegrated and thus their average size decreased to 18.80 μm, as observed in the microscopy image (Fig. 1, T69). Under these conditions, the granule structure become frangible and the large granules disintegrated. This explains why the median diameter of starch granules first increased and then decreased when the temperature was increased from 63 to 69 °C. Fu et al. [2] and Santos et al. [7] observed a similar result with corn and Peruvian carrot starch, respectively, processed in a spray dryer. In the present study, the spray drying process resulted in an increase in the proportion of granules of 6–15 μm in size in the samples pretreated at 57 and 60 °C, whereas a higher proportion of granules of 16–60 μm in size was observed in the samples pretreated at 63 to 69 °C (Fig. 2A and B).
There were significant differences among the various treatments for the parameters D[3.2], D[4.3], and d(0.5) (Fig. 3A, B, and C, respectively), which showed negative quadratic effects in the regression analysis. The results indicated that the preheating at 67 °C had resulted in higher D [3.2], D[4.3], and d(0.5) values for the spray dried starch (Fig. 4A, B, and C, respectively). 3.3. Amylose content, X-ray pattern and relative crystallinity The amylose content of native sweet potato starch was similar to that reported in previous studies (19.1–27.2%) [13,14,17,23,24]. The pretreatment temperatures and spray drying did not decrease the amylose content of the starch samples in any significant way (Figs. 3D and 4D). However, the slight decrease in amylose content in the spray dried starches compared with native starch may be attributed to additional interactions between the amylose-amylose and/or amylose-amylopectin chains during the preheating process [25]. The native starch displayed a Ca-type diffraction pattern, with the most intense peaks being at 15, 17, 18, and 23° 2θ (Fig. 2C) and a shoulder peak at approximately 18° 2θ. Previous studies have shown that native starch granules of sweet potato exhibited the diffraction pattern, which has higher proportion of short-branched chains of amylopectin [15,17,26,27]. There was decrease gradually in peak height with increase in the preheating temperature (Fig. 2C). Similar changes were observed by Fu et al. [2] with corn starch. When preheating temperatures was above 69 °C the peaks occurring at 2θ = 15°, 17°, 18°, and 23° disappeared. This
Fig. 2. Granule average size diameter (A) and granule size distribution (B) of native and spray-dried sweet potato starch; DXR diffractogram (C) and DSC curves (D) (to DXR and DSC: native sweet potato starch. T57. T60. T63. T66. and T69 from up to bottom).
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Fig. 3. D[3.2] (A), D[4.3], (B), d(0.5) (C), amylose (D), relative crystallinity (E), swelling power (F), and solubility (G) of Native starch, T57, T60, T63, T66, and T99. The error bars represent the standard deviations of means. Different letters marked above the bars indicate statistical differences by Turkey's test (p b .05). Results of three replicates.
observation suggested that the crystalline form was converted to amorphous form [2]. A similar result has been obtained from starches from Dioscorea modified by pregelatinization by freeze drying [21].
In this present study, the relative crystallinity decreased progressively with the preheating temperature, with a 29% decrease observed with the starch from the 63 °C pretreatment. However, the decrease in relative crystallinity in the starch from 66 °C pretreatment was twice
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Fig. 4. Effect of temperature of preheating on spray-dried sweet potato starch. A, D[3.2]; B, D[4.3]; C, d(0.5); D, Amylose content; E, Relative crystalline; F, G, and H, onset, peak and conclusion temperature, respectively; I, Range of temperature; J, Enthalpy change; K, Gelatinization degree; L, Swelling power; M, Solubility.
that of the 63 °C pretreatment. With pretreatment above 69 °C, the peaks disappeared and the decrease in relative crystallinity was 70%, which suggested that the native crystalline form had converted largely into the amorphous form. A negative quadratic effect of temperature on the crystallinity was observed, showing that for the temperatures evaluated in this study, higher preheating temperatures, resulted in a lower relative crystallinity of the spray dried starch (Fig. 4E).
3.4. Thermal properties The onset, peak, and conclusion (Tonset, Tpeak, and Tconclusion) gelatinization temperatures of native sweet potato starch were 63.83, 68.94, and 73.50 °C, respectively. These results were similar to those reported in the literature, as were the values of ΔH, which were cited to be between 12.3 and 13.01 J g−1 [23,24,26,28]. Higher temperatures and a
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Table 1 Changes in thermal properties of spray dried sweet potato starch in function of temperature of pretreatment. Samples
To (°C)
Tp (°C)
Tc (°C)
ΔT (°C)
ΔH (J g−1)
GD (%)
Native starch T57 T60 T63 T66 T69
63.83 ± 0.1f 64.61 ± 0.3e 66.09 ± 0.0c 68.47 ± 0.0b 65.65 ± 0.1d 74.52 ± 0.1a
68.88 ± 0.2c 68.71 ± 0.5c 69.32 ± 0.2c 71.09 ± 0.4b 68.74 ± 0.2c 77.20 ± 0.3a
73.73 ± 0.2bc 72.96 ± 0.5c 72.85 ± 0.0c 74.28 ± 0.7b 73.26 ± 0.3bc 79.87 ± 0.6a
9.88a 8.35b 6.77 cd 5.82de 7.61bc 5.35e
12.73 ± 0.2a 8.74 ± 0.1b 8.16 ± 0.1bc 8.24 ± 0.2c 3.53 ± 0.1d 1.72 ± 0.3e
– 30.10 ± 1.0d 34.72 ± 1.0c 34.90 ± 1.0c 71.75 ± 1.0b 87.33 ± 1.1a
ΔT, range of temperature (ΔT = Tconclusion − Tonset); ΔH, enthalpy change; GD, gelatinization degree (GD = [1 − (ΔH of processed starch / ΔH native starch)] × 100). Values followed by the same lower-case letter in colun do not differ statistically from each other. Tukey's HSD test (p b .05). Results of three replicates.
higher ΔH reflect a stronger or more ordered crystalline molecular structure [28,29]. After spray drying, the starches showed an increase in gelatinization temperature and a decrease in the range of temperatures and in the ΔH, compared with those of native starch (Table 1). The treatments generally resulted in a progressive increase in the gelatinization temperatures (Tonset, Tpeak, and Tconclusion) with the increase in the preheating temperature (except at 66 °C, where the temperatures showed lower values). At the higher preheating temperatures (i.e., from 60 °C upward), the gelatinization temperatures of the spray dried starches (Fig. 4F, G, and H). These changes may be due to the processing conditions which allowed gelatinization of the crystalline structure of the starch granules, as observed in slight curves at higher temperatures obtained by differential scanning calorimetry (Fig. 2D). This process also led to a reduction in the gelatinization temperature range, showing a more homogeneous distribution of the crystals. The ΔH of the spray dried starches decreased significantly from 12.73 to 1.72 J g−1 when the preheating temperature was increased to 69 °C, implying that this treatment lead to almost completely gelatinized starch (Fig. 2D). The reduction in the ΔH occurs because water primarily attacks the amorphous regions in the granules and these regions play an important role in the thermodynamics of gelatinization. In addition, the pregelatinized starch can swell and produce a gelatinous barrier like a hydrophilic matrix. This property is desirable for control drug release, so this starch could be used in direct compression tablets in the pharmaceutical industry [19]. In this present study, gelatinization of the starch granules was achieved through spray drying at relatively high temperatures (inlet and outlet temperatures of 200 and 110 °C, respectively) which could also have affected the gelatinization temperature of the spray dried starches [30]. The regression analysis showed a negative quadratic effect on ΔH, meaning a lower ΔH value with a higher preheating temperature (Fig. 4J). A higher degree of gelatinization was observed for the spray dried starches pretreated at 66 and 69 °C. The higher the degree of gelatinization, the more molecular material expands in water during gelatinization before drying. Consequently, when the pregelatinized starch is redispersed in water, the expanded starch material dissolves in water and establishes the polymer network [31]. This polymer network becames a suitable ingredient in low-fat dressing applications [32]. Starch gelatinization is affected by several parameters such as the baking process, baking temperature, and time; besides, the formulation and the accessibility to water remains an important factor controlling the degree of starch gelatinization. In study with partial substitution of flour on cake quality or total substitution on bread with pregelatinized starches was observed retard of staling and an increase in crumb water content, the authors related these results with the higher water holding capacity of pregelatinized starches [33,34]. 3.5. Swelling power (SP) and solubility (S) Native sweet potato starch showed swelling power and solubility values of 37.04 g g−1 and 29.44%, respectively, which were higher than the indices reported in the literature, which ranged from 25.2 to
31.1 g g−1 and from 9.3% to 16.0%, respectively [16,17,35]. The swelling power indicates the ability of water to penetrate the starch granules. The swelling power and solubility of the spray dried starches ranged from 31.24 to 46.21 g g−1 and from 28.36% to 35.04%, respectively (Fig. 3F and G). The increase in swelling power and solubility in some treatments may have occurred due to the weak interactions formed, which accompanied by the formation of a less stable structure and increased leaching of the amylose molecules resulted in an increase in the surface and water inflow in granules [36]. This process can result in increase in the size, swelling, and solubility of the starch granules [13,2]. Although the regression analysis showed no significant effect on the swelling power and solubility (Fig. 4L and M), it was possible to observe that the starch sample preheated at 57 °C exhibited similar swelling power to that of native starch. However, when the starch was preheated at 66 and 69 °C, the swelling power and solubility increased relative to those of native starch and of the starches preheated at lower temperatures. Higher swelling power and solubility values were observed for the starch preheated at 66 °C, which was in concordance with the particle size results. When starch was preheated at high temperatures, the solid content from the previously gelatinized starch granules leached out easily. The amorphous areas absorbed water rapidly, allowing the granules to swell, and with this may break into small pieces with relative ease, which was observed when the starch dispersion was preheated at 69 °C. Thus, smaller granules could not hold more water, which also contributed to the decrease in its swelling power. 4. Conclusions The different conditions of spray drying process led to a significant change in the morphological and physicochemical characteristics of sweet potato starch. Spray drying resulted in the formation of agglomerates and increased the average size of the granules. Higher pretreatment temperature led to increased gelatinization temperatures and decreased ΔH of the pregelatinized starch, as well as increased swelling power and solubility. This process resulted in partially gelatinized starches with desirable characteristics for use in products requiring gelatinization at milder temperatures. Preheating at 67 °C was the best condition for the production of gelatinized sweet potato starch. In conclusion, this preheating and spray drying process could improve the functional properties of sweet potato starch, thereby increasing its applicability in the food and non-food industries. Acknowledgment The authors thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/Brazil) (Process: BEX 9551/14-0) and CNPq (Process 302827/2017-0) for financial support. References [1] FAO - Food and Agriculture Organization of the United Nations, FAOSTAT: Production-Crops, Available in http://www.fao.org/faostat/en/#data/QC 2017.
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Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac
Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer Thaís Paes Rodrigues dos Santos a,c,⁎, Célia Maria Landi Franco b, Magali Leonel c a b c
Center for Tropical Roots and Starches (CERAT), College of Agronomic Science (FCA), São Paulo State University (UNESP), Botucatu, São Paulo, Brazil Department of Food Engineering and Technology, Institute of Biosciences, Language, and Physical Sciences, São Paulo State University (UNESP), São José do Rio Preto, São Paulo State, Brazil Center for Tropical Roots and Starches (CERAT), São Paulo State University (UNESP), Botucatu, São Paulo State, Brazil
a r t i c l e
i n f o
Article history: Received 7 August 2019 Received in revised form 8 November 2019 Accepted 11 November 2019 Available online xxxx Keywords: Spray dryer Pregelatinization Thermal properties Instant food
a b s t r a c t Native sweet potato starch was gelatinized at temperatures between 57 and 69 °C and then spray dried, to verify the changes caused by this process. The spray dried starch granules initially swelled with the increase in preheating temperature, displaying a larger diameter, but shrank upon colling. There was an increase in the gelatinization temperature and a decrease in the enthalpy change of the spray dried starches relative to those of native starch. Spray dried starch preheating at 69 °C showed highly reduced enthalpy change (80%) and relative crystallinity. Based on the parameters studied, the best preheating temperature was 67 °C. Under this condition, the starch can be applied in more viscous products that require a greater thickness, and with great process yield (65%). This study showed the feasibility of the application of this process to produce sweet potato starches with specific properties for different purposes in the food and non-food industries. © 2018 Elsevier B.V. All rights reserved.
1. Introdution Sweet potato (Ipomoea batatas L.) is the seventh most important food crop in the world. In 2017, the world production of sweet potato roots was 112.8 million tons [1]. Starch, the main component of the sweet potato root, it is a biopolymer composed of amylose and amylopectin, which are made up of chains of glucose molecules that differ from each other in size and shape. Amylose displays α(1,4) glycosidic bonds and has a linear helical structure, with few branches. Amylopectin has a highly branched structure with α(1,4) glycosidic bonds, and 4–5% of α(1,6) glycosidic bonds, which are responsible for the formation of branches. Native or processed starches of sweet potato are mainly used as an ingredient for noodles, snacks, cakes, breads, etc. However, the food application of native starch is limited owing to some of its undesirable Abbreviations: D[3.2], surface-weighted; D[4.3], volume-weighted; d(0.5), is the granule size at which 50% of all the granules by volume are smaller; AML, amylose; RC, relative crystallinity; To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; ΔT, range of temperature; ΔH, enthalpy change; GD, gelatinization degree; SP, swelling power; SS, solubility. ⁎ Corresponding author at: Center for Tropical Roots and Starches (CERAT), São Paulo State University (UNESP), José Barbosa de Barros Street, 1780, Jardim Paraíso, Botucatu, São Paulo, 18.610-307, Brazil. E-mail address: [email protected] (T.P.R. dos Santos).
characteristics, such as its paste instability, poor paste clarity, and poor solubility. The number of studies on the modification of sweet potato starch and/or its application in the food and non-food industries has been growing. The aim of starch modification is to improve the natural characteristics of the polysaccharide that are considered undesirable for industrial use, and it can be performed by physical, chemical, and/or enzymatic methods. The physical modification of starch is considered a green method because the process does not produce wastes. Pregelatinization can convert native starch into a partially or fully gelatinized form to obtain a gel at room temperature. The gelatinization process can cause chemical and physical changes to the starch granules by rearranging the intra- and intermolecular hydrogen bonds between the water and starch molecules and can partially or completely destroy the crystalline zones in the starch depending on the intensity of the process [2]. Spray drying requires a short period of preheating at a temperature approximate to that for starch gelatinization. Thus, the intensity of changes on the starch properties can be controlled by the preheating temperature. Partially gelatinized starch converts starch from an amorphous state to a more ordered or crystalline state that has different digestion properties, providing benefits for the dietary management of metabolic disorders [3]. In general, pregelatinized starches, called instant starches, are used as thickeners in many food products that receive minimal heat
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Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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processing, such as instant soups and instant desserts. These products require only addition of hot water with stirring before serving, and also disperse readily with highshear stirring or when mixed with sugar or other dry ingredients [4]. This processed starch can be primarily used as a thickener in many instant products, such as baby food, broths, soups and desserts, powders in beverages, and meat and convenience products such as creams, mayonnaise, frozen pasta, baking products, and premixes; as main ingredients, as bulking agents, thickeners, or stabilizers in many other food products; and in pharmaceutical formulations, as binders and disintegrants. Given the increased use of pregelatinized starch and the industry demand for specific technological properties of the product as well as knowledge about new non-conventional starch sources, this study was conducted to investigate the effects of preheating and spray drying on the physicochemical, structural and thermal characteristics of sweet potato starch. The best conditions for obtaining partially pregelatinized starches were also studied, the information of which could provide new uses of sweet potato starch in industrial processes. 2. Materials and methods 2.1. Materials Sweet potato roots (cv. Canadense) were collected on São Manuel Experimental Farm (CERAT/UNESP, São Manuel city, São Paulo state, Brazil). The roots were planted on March 2015 and were harvested 150 days after planting. Starch was extracted from the roots 24 h after harvesting, following the methodology described by Santos et al. [5]. The sweet potato starch was characterized by physicochemical analysis: moisture, ash, lipids, protein and starch were 11.78, 0.45, 0.89, 0.22, and 86.66% (wet basis), respectively, and pH 6.12 [6]. 2.2. Spray drying process The spray drying process was carried out according to the methodology described by Fu et al. [2] and Santos et al. [7]. Dispersions of starch (8%, m/m, dry basis) were prepared by first mixing the starch in distilled water at 40 ± 1 °C. The mixtures were then transferred to thermostatic water baths set at 57, 60, 63, 66, or 69 °C (treatments: T57, T60, T63, T66, and T69, respectively), with continuous stirring. After reaching the desired temperature, the starch dispersions were continuously stirred in the water bath for 10 min. Evaporation of water was minimized by using a plastic film to cover the beaker. After this time, the hot starch dispersions were dried in a spray dryer (Model MSD 0.5, Labmaq, São Paulo, Brazil) fitted with a double-atomizer nozzle with a nozzle hole of 0.7 mm diameter. The inlet and outlet temperatures were 170 and 110 °C, respectively, and the feed rate was 1.0 L h−1. The flow rates of the compressed air and hot air were 0.40 L min−1 and 3.8 m3 min−1, respectively, and the air pressure was set at 6 bars. 2.3. Morphology of granules The morphologies of the native and spray dried starch granules were evaluated by scanning electron microscopy (Evo-LS15, Carl Zeiss, Oberkochen, Baden-Württemberg, Germany). Starches samples were applied to an aluminum stub with double-sided tape and coated with layer of gold (20 nm) in metallizer for 220 s (BAL-TEC SCD 050, 220 seg). 2.4. Particule size analysis The average size and size distribution of the native and spray dried starch granules were determined using the laser light diffraction technique with a He\\Ne laser (Mastersizer 2000, Laser Scattering Spectrometer Mastersizer S, Model MAM 5005 - Malvern Instruments Ltd., Worcestershire, UK) [2,8].
2.5. X-ray diffraction pattern and relative crystallinity X-ray patterns were examined using a goniometer unit (MiniFlex 600, Rotaflex, Rigaku, Tokyo, Japan), with copper Kα radiation (λ = 0.1542 nm) [7] and the relative crystalline was calculated as described by Nara & Komiya [9]. 2.6. Amylose content For determination of amylose content, starches samples were defatted with 85% (v/v) methanol in reflux with a Soxhlet extractor for 24 h. The amylose content was determined according to the ISO6647 method [10]. 2.7. Thermal properties The gelatinization properties of the native and spray dried starches were evaluated with a differential scanning calorimeter (Pyris 1, Perkin Elmer, Norwalk, Connecticut, USA) as described by Santos et al. [5]. The degree of gelatinization (DG) of the spray dried starches samples was calculated by the ratio of the enthalpy change of the spray dried starch and the enthalpy change of the native starch [11]. 2.8. Swelling power (SP) and solubility (S) Swelling power and solubility was evaluated according Schoch [12], as described by Santos et al. [7]. 2.9. Statistical analysis Data were tested by analysis of variance, and when appropriate, the difference among means was determined using Tukey's multiple comparisons test at the 0.05 probability level. The effect of each pretreatment temperature on the starch characteristics was evaluated by regression analysis. Regression coefficients were considered significant at the 5% probability level. All measurements were carried out in triplicate and the data are presented as the mean ± standard deviation. 3. Results and discussion 3.1. Effects of preheating temperatures on the characteristics of spray dried starches The statistical parameters corresponding to each of the responses showed high F-values for the individual or quadratic effects of the dependent variables. The accuracy of each model was assessed from R2, the values of which were all higher than 0.70 for the models for crystallinity, mastersizer parameters, gelatinization temperatures, and enthalpy change (ΔH) of gelatinization. Additionally, the R2 value for each of the responses was found to be close to that of the other responses, which was indicative of the accuracy of the models. 3.2. Morphology and distribution of granule average size Native sweet potato starch displayed granules with a smooth surface, and oval and polygonal shape, as well as a few depressions (Fig. 1, native), similar to the observations reported in the literature [13–15]. The granule sizes were highly variable, with 65.50% being between 11 and 20 μm in diameter (average 16.49 μm), which was also similar to results reported in the literature [15–17]. The changes on the structure surface and shape of the starch granules were more drastic with the increase in preheating temperature. The starch granules were partially gelatinized and swollen during the spray drying process, but shrank upon cooling and displayed wrinkles, which is typical of macromolecular particles produced by spray drying
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Fig. 1. Morphology of native and spray-dried sweet potato starch. a, 1000×; b, 2000×.
[18]. These properties should promote the flowability of the starch, which is important for use as a direct compression excipient [19]. The changes in the morphology of the spray dried starch granules are shown in Fig. 1. As shown in the figure, the shape and surface of the spray dried granules changed with the increases in preheating
temperature. The starches preheated at 57 and 60 °C exhibited similar characteristics to those of native starch, indicating that these temperatures did not affect the starch granules. However, the median diameter of spray dried granules of these treatments showed decreased (13.90 and 15.32 μm, respectively), which may be due to the moisture content
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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of native starch that was transferred out during the spray drying, causing the granules to shrink and their size to decrease [20]. However, when the starch was preheated at 63 and 66 °C, larger granules and more truncated shapes were observed, with the formation of agglomerates. This change can be explained due to the increase of gelatinization temperature, which lead the starch granules to absorb more water, allowing for the expansion of the granules and their partial gelatinization in the interior. The pretreatments at 63 and 66 °C led to an increase in the average size of the granules to 18.84 and 20.44 μm, respectively, which were higher than that of the native starch. These pregelatinized starches exhibited granule size distribution more homogeneus, which is appropriate and contributes to a good compressibility, tablet disintegrability and flowability, thus, it became a binder for the production of rapid orally disintegranting tablets [21,22]. With the increase in temperature to 69 °C, part of the granules disintegrated and thus their average size decreased to 18.80 μm, as observed in the microscopy image (Fig. 1, T69). Under these conditions, the granule structure become frangible and the large granules disintegrated. This explains why the median diameter of starch granules first increased and then decreased when the temperature was increased from 63 to 69 °C. Fu et al. [2] and Santos et al. [7] observed a similar result with corn and Peruvian carrot starch, respectively, processed in a spray dryer. In the present study, the spray drying process resulted in an increase in the proportion of granules of 6–15 μm in size in the samples pretreated at 57 and 60 °C, whereas a higher proportion of granules of 16–60 μm in size was observed in the samples pretreated at 63 to 69 °C (Fig. 2A and B).
There were significant differences among the various treatments for the parameters D[3.2], D[4.3], and d(0.5) (Fig. 3A, B, and C, respectively), which showed negative quadratic effects in the regression analysis. The results indicated that the preheating at 67 °C had resulted in higher D [3.2], D[4.3], and d(0.5) values for the spray dried starch (Fig. 4A, B, and C, respectively). 3.3. Amylose content, X-ray pattern and relative crystallinity The amylose content of native sweet potato starch was similar to that reported in previous studies (19.1–27.2%) [13,14,17,23,24]. The pretreatment temperatures and spray drying did not decrease the amylose content of the starch samples in any significant way (Figs. 3D and 4D). However, the slight decrease in amylose content in the spray dried starches compared with native starch may be attributed to additional interactions between the amylose-amylose and/or amylose-amylopectin chains during the preheating process [25]. The native starch displayed a Ca-type diffraction pattern, with the most intense peaks being at 15, 17, 18, and 23° 2θ (Fig. 2C) and a shoulder peak at approximately 18° 2θ. Previous studies have shown that native starch granules of sweet potato exhibited the diffraction pattern, which has higher proportion of short-branched chains of amylopectin [15,17,26,27]. There was decrease gradually in peak height with increase in the preheating temperature (Fig. 2C). Similar changes were observed by Fu et al. [2] with corn starch. When preheating temperatures was above 69 °C the peaks occurring at 2θ = 15°, 17°, 18°, and 23° disappeared. This
Fig. 2. Granule average size diameter (A) and granule size distribution (B) of native and spray-dried sweet potato starch; DXR diffractogram (C) and DSC curves (D) (to DXR and DSC: native sweet potato starch. T57. T60. T63. T66. and T69 from up to bottom).
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Fig. 3. D[3.2] (A), D[4.3], (B), d(0.5) (C), amylose (D), relative crystallinity (E), swelling power (F), and solubility (G) of Native starch, T57, T60, T63, T66, and T99. The error bars represent the standard deviations of means. Different letters marked above the bars indicate statistical differences by Turkey's test (p b .05). Results of three replicates.
observation suggested that the crystalline form was converted to amorphous form [2]. A similar result has been obtained from starches from Dioscorea modified by pregelatinization by freeze drying [21].
In this present study, the relative crystallinity decreased progressively with the preheating temperature, with a 29% decrease observed with the starch from the 63 °C pretreatment. However, the decrease in relative crystallinity in the starch from 66 °C pretreatment was twice
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Fig. 4. Effect of temperature of preheating on spray-dried sweet potato starch. A, D[3.2]; B, D[4.3]; C, d(0.5); D, Amylose content; E, Relative crystalline; F, G, and H, onset, peak and conclusion temperature, respectively; I, Range of temperature; J, Enthalpy change; K, Gelatinization degree; L, Swelling power; M, Solubility.
that of the 63 °C pretreatment. With pretreatment above 69 °C, the peaks disappeared and the decrease in relative crystallinity was 70%, which suggested that the native crystalline form had converted largely into the amorphous form. A negative quadratic effect of temperature on the crystallinity was observed, showing that for the temperatures evaluated in this study, higher preheating temperatures, resulted in a lower relative crystallinity of the spray dried starch (Fig. 4E).
3.4. Thermal properties The onset, peak, and conclusion (Tonset, Tpeak, and Tconclusion) gelatinization temperatures of native sweet potato starch were 63.83, 68.94, and 73.50 °C, respectively. These results were similar to those reported in the literature, as were the values of ΔH, which were cited to be between 12.3 and 13.01 J g−1 [23,24,26,28]. Higher temperatures and a
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Table 1 Changes in thermal properties of spray dried sweet potato starch in function of temperature of pretreatment. Samples
To (°C)
Tp (°C)
Tc (°C)
ΔT (°C)
ΔH (J g−1)
GD (%)
Native starch T57 T60 T63 T66 T69
63.83 ± 0.1f 64.61 ± 0.3e 66.09 ± 0.0c 68.47 ± 0.0b 65.65 ± 0.1d 74.52 ± 0.1a
68.88 ± 0.2c 68.71 ± 0.5c 69.32 ± 0.2c 71.09 ± 0.4b 68.74 ± 0.2c 77.20 ± 0.3a
73.73 ± 0.2bc 72.96 ± 0.5c 72.85 ± 0.0c 74.28 ± 0.7b 73.26 ± 0.3bc 79.87 ± 0.6a
9.88a 8.35b 6.77 cd 5.82de 7.61bc 5.35e
12.73 ± 0.2a 8.74 ± 0.1b 8.16 ± 0.1bc 8.24 ± 0.2c 3.53 ± 0.1d 1.72 ± 0.3e
– 30.10 ± 1.0d 34.72 ± 1.0c 34.90 ± 1.0c 71.75 ± 1.0b 87.33 ± 1.1a
ΔT, range of temperature (ΔT = Tconclusion − Tonset); ΔH, enthalpy change; GD, gelatinization degree (GD = [1 − (ΔH of processed starch / ΔH native starch)] × 100). Values followed by the same lower-case letter in colun do not differ statistically from each other. Tukey's HSD test (p b .05). Results of three replicates.
higher ΔH reflect a stronger or more ordered crystalline molecular structure [28,29]. After spray drying, the starches showed an increase in gelatinization temperature and a decrease in the range of temperatures and in the ΔH, compared with those of native starch (Table 1). The treatments generally resulted in a progressive increase in the gelatinization temperatures (Tonset, Tpeak, and Tconclusion) with the increase in the preheating temperature (except at 66 °C, where the temperatures showed lower values). At the higher preheating temperatures (i.e., from 60 °C upward), the gelatinization temperatures of the spray dried starches (Fig. 4F, G, and H). These changes may be due to the processing conditions which allowed gelatinization of the crystalline structure of the starch granules, as observed in slight curves at higher temperatures obtained by differential scanning calorimetry (Fig. 2D). This process also led to a reduction in the gelatinization temperature range, showing a more homogeneous distribution of the crystals. The ΔH of the spray dried starches decreased significantly from 12.73 to 1.72 J g−1 when the preheating temperature was increased to 69 °C, implying that this treatment lead to almost completely gelatinized starch (Fig. 2D). The reduction in the ΔH occurs because water primarily attacks the amorphous regions in the granules and these regions play an important role in the thermodynamics of gelatinization. In addition, the pregelatinized starch can swell and produce a gelatinous barrier like a hydrophilic matrix. This property is desirable for control drug release, so this starch could be used in direct compression tablets in the pharmaceutical industry [19]. In this present study, gelatinization of the starch granules was achieved through spray drying at relatively high temperatures (inlet and outlet temperatures of 200 and 110 °C, respectively) which could also have affected the gelatinization temperature of the spray dried starches [30]. The regression analysis showed a negative quadratic effect on ΔH, meaning a lower ΔH value with a higher preheating temperature (Fig. 4J). A higher degree of gelatinization was observed for the spray dried starches pretreated at 66 and 69 °C. The higher the degree of gelatinization, the more molecular material expands in water during gelatinization before drying. Consequently, when the pregelatinized starch is redispersed in water, the expanded starch material dissolves in water and establishes the polymer network [31]. This polymer network becames a suitable ingredient in low-fat dressing applications [32]. Starch gelatinization is affected by several parameters such as the baking process, baking temperature, and time; besides, the formulation and the accessibility to water remains an important factor controlling the degree of starch gelatinization. In study with partial substitution of flour on cake quality or total substitution on bread with pregelatinized starches was observed retard of staling and an increase in crumb water content, the authors related these results with the higher water holding capacity of pregelatinized starches [33,34]. 3.5. Swelling power (SP) and solubility (S) Native sweet potato starch showed swelling power and solubility values of 37.04 g g−1 and 29.44%, respectively, which were higher than the indices reported in the literature, which ranged from 25.2 to
31.1 g g−1 and from 9.3% to 16.0%, respectively [16,17,35]. The swelling power indicates the ability of water to penetrate the starch granules. The swelling power and solubility of the spray dried starches ranged from 31.24 to 46.21 g g−1 and from 28.36% to 35.04%, respectively (Fig. 3F and G). The increase in swelling power and solubility in some treatments may have occurred due to the weak interactions formed, which accompanied by the formation of a less stable structure and increased leaching of the amylose molecules resulted in an increase in the surface and water inflow in granules [36]. This process can result in increase in the size, swelling, and solubility of the starch granules [13,2]. Although the regression analysis showed no significant effect on the swelling power and solubility (Fig. 4L and M), it was possible to observe that the starch sample preheated at 57 °C exhibited similar swelling power to that of native starch. However, when the starch was preheated at 66 and 69 °C, the swelling power and solubility increased relative to those of native starch and of the starches preheated at lower temperatures. Higher swelling power and solubility values were observed for the starch preheated at 66 °C, which was in concordance with the particle size results. When starch was preheated at high temperatures, the solid content from the previously gelatinized starch granules leached out easily. The amorphous areas absorbed water rapidly, allowing the granules to swell, and with this may break into small pieces with relative ease, which was observed when the starch dispersion was preheated at 69 °C. Thus, smaller granules could not hold more water, which also contributed to the decrease in its swelling power. 4. Conclusions The different conditions of spray drying process led to a significant change in the morphological and physicochemical characteristics of sweet potato starch. Spray drying resulted in the formation of agglomerates and increased the average size of the granules. Higher pretreatment temperature led to increased gelatinization temperatures and decreased ΔH of the pregelatinized starch, as well as increased swelling power and solubility. This process resulted in partially gelatinized starches with desirable characteristics for use in products requiring gelatinization at milder temperatures. Preheating at 67 °C was the best condition for the production of gelatinized sweet potato starch. In conclusion, this preheating and spray drying process could improve the functional properties of sweet potato starch, thereby increasing its applicability in the food and non-food industries. Acknowledgment The authors thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/Brazil) (Process: BEX 9551/14-0) and CNPq (Process 302827/2017-0) for financial support. References [1] FAO - Food and Agriculture Organization of the United Nations, FAOSTAT: Production-Crops, Available in http://www.fao.org/faostat/en/#data/QC 2017.
Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105
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Please cite this article as: T.P.R. dos Santos, C.M.L. Franco and M. Leonel, Gelatinized sweet potato starches obtained at different preheating temperatures in a spray dryer, , https://doi.org/10.1016/j.ijbiomac.2019.11.105