Article pubs.acs.org/JAFC
Effects of Granule Size of Cross-Linked and Hydroxypropylated Sweet Potato Starches on Their Physicochemical Properties Jianwei Zhao,†,‡ Zhenghong Chen,§ Zhengyu Jin,‡ Piet Buwalda,§ Harry Gruppen,† and Henk A. Schols*,† †
Laboratory of Food Chemistry, Wageningen University, Post Office Box 17, 6700 AA Wageningen, Netherlands School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu 214122, People’s Republic of China § AVEBE Food Innovation Centre, 9640 AA Veendam, Netherlands ‡
ABSTRACT: Sweet potato starch was modified by cross-linking, hydroxypropylation, and combined cross-linking and hydroxypropylation, and the starches were subsequently sieved to obtain differently sized granule fractions. The effects of granule size of native and modified sweet potato starch fractions and all fractions were investigated with respect to their physicochemical properties. The large-size granule fraction (27−30 μm) showed a 16−20% higher chemical phosphorylation and a 4−7% higher hydroxypropylation than the small-size granule fraction (14−16 μm). The large-size granule fractions of native and modified sweet potato starches showed lower transition temperatures (0.7−3.1 °C for peak temperature of gelatinization) and lower enthalpy changes (0.6−1.9 J/g) during gelatinization than the small-size granule fractions, making the sweet potato starch different from cereal starches. The large-size granule fraction of native starch showed a higher paste viscosity (78−244 cP) than the corresponding small-size granule fraction. In addition, cross-linking and hydroxypropylation affected the paste viscosity of the large-size granule fraction significantly more than that of the small-size granule fraction when compared to the corresponding parental starch fractions. The large-size granule fraction of native and dual-modified starches showed a lower syneresis after freeze−thaw treatments than the small-size granule fractions. The difference in swelling power between large- and small-size granule fractions was not significant. In general, the large-size granule fraction of sweet potato starch was more susceptible for cross-linking and hydroxypropylation and the physicochemical properties were changed to a higher extent compared to the corresponding small-size granule fraction. KEYWORDS: cross-linking, hydroxypropylation, physicochemical property, paste viscosity, syneresis
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sweet potato starch.7 It has also been reported that the processability and quality of dried and cooked sweet potato starch noodles made from the small-size granule fraction were significantly better than those made from the parental starch preparation and much better than those made from the largesize granule fraction.8,9 In addition, it was reported that the small-size granule fractions of acetylated yellow pea, cow pea, and chick pea starch exhibited larger swelling volumes and higher peak viscosities compared to the corresponding largesize fractions.10 Until now, the improved functional properties of small-sized sweet potato granules have not been explained. In addition, the effect of granule size on the functional properties of hydroxypropylated sweet potato starch and the effect of granule size of dual-modified sweet potato starch have not been investigated thus far. In this study, sweet potato starch was modified with crosslinking and/or hydroxypropylation, and the granules were sieved to obtain the large- and small-size granule fractions. All fractions were investigated for their physicochemical properties. The effects of starch granule size on the starch properties of the various modified starches are discussed.
INTRODUCTION Starch is one of the main ingredients used in the food industry. Starch is often modified to overcome shortcomings of native starches, such as cooking loss and syneresis of frozen food. Cross-linking and hydroxypropylation are two important types of modification. Cross-linking reinforces the molecule structure of granular starch by covalent bonds between starch chains, thereby improving the stability of starch under acid, heat, and shear force conditions.1 Hydroxypropylation allows starch to have good clarity, higher viscosity, reduced syneresis, and improved freeze−thaw stability.2 Cross-linking of starch is often combined with other modifications, such as hydroxypropylation, to obtain even better functional properties.3 Sweet potato (Ipomoea batatas Lam.) is one of the most important economic species of tropical root and tuber crops, which can grow in great abundance on marginal soils.4 Isolated sweet potato starch is widely used as a food ingredient for noodles, bakery foods, snack foods, and confectionery products.5 To overcome limitations of native sweet potato starch, chemical modifications have been performed to extend the range of food applications. Cross-linking reduced the swelling power and apparent viscosity and increased the gelatinization temperature, enthalpy, storage modulus, loss modulus, and complex viscosity of sweet potato starch.6 Hydroxypropylation increased the swelling power, freeze− thaw stability, and flow behavior index and decreased the transition temperature and enthalpy of gelatinization, consistency index, apparent viscosity, and complex viscosity of © 2015 American Chemical Society
Received: Revised: Accepted: Published: 4646
January 5, 2015 April 20, 2015 April 23, 2015 April 23, 2015 DOI: 10.1021/jf506349w J. Agric. Food Chem. 2015, 63, 4646−4654
Article
Journal of Agricultural and Food Chemistry
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Paste Properties of Starch Samples. The pasting properties of starches were measured using a rapid visco analyzer (RVA-4, Newport Scientific, Warriewood, New South Wales, Australia). The starch slurry (7%, w/v, in distilled water, dry weight basis, 27 g of total slurry) was prepared in a RVA canister. A paddle was placed into the canister, and the blade was vigorously stirred through the sample up and down to avoid lump formation. Next, the canister was inserted into the instrument for measurement. The sample was equilibrated at 50 °C for 1 min, heated to 95 °C in 7.5 min, held at 95 °C for 5 min, cooled to 50 °C in 7.5 min, and held at 50 °C for 2 min. Swelling Power of Starch Samples. The swelling power of starches was measured as described before,5,13 with some modifications. Starch was weighed (0.1−0.2 g, dry weight basis) into a 15 mL polypropylene tube, and 12 mL of distilled water was added, followed by mixing. The tubes were equilibrated at 25 °C for 5 min and subsequently heated for 30 min in a water bath at a given temperature (55−95 °C range) with shaking to avoid lump formation. The sample was cooled to 25 °C and centrifuged (1000g at 25 °C for 15 min). The supernatant was removed by pipetting, and the swollen starch sediment was weighed. The swelling power was the ratio in weight of the swelled starch sediment to the initial weight of dry starch.14 Results used for calculation were means of triplicate measurements. Freeze−Thaw Stability of Starch Samples. Freeze−thaw stability was determined as syneresis as described before.15 In short, starch was suspended (6%, w/w, dry weight basis, 5 g of slurry) in distilled water in a 10 mL screw-capped tube. The tubes were tightly capped and heated in a 95 °C water bath for 30 min with agitation by inverting the tubes 5 times at 3 min intervals during the first 15 min of heating. These tubes were cooled and stored at 4 °C for 24 h before being subjected to 7 freeze−thaw cycles. The tubes were centrifuged (1500g at 25 °C for 15 min) to remove the excluded water. During each cycle, the tubes were put into a freezer at −20 °C for 22 h and thawed at 30 °C in a water bath for 1.5 h. After centrifugation (1500g at 25 °C for 15 min), the water eliminated from the gel was weighed. The syneresis was expressed as the proportion of the weight of the separated water to the total weight of the initial paste. Duplicated tests were used for each cycle. Refrigerated Storage Stability of Starch Samples. Syneresis of starch gel without freeze−thaw treatment was measured by storing the gel at 4 °C. Starch pastes were prepared as described above. The tubes were stored in a refrigerator at 4 °C for several days. With a 1 day interval, the excluded water was measured after centrifugation (1000g at 25 °C for 20 min). The weight difference of the gel before and after 4 °C storage was measured as water loss. The syneresis was calculated as the weight percentage of water loss based on the parental starch paste weight. Statistical Analysis. The statistical analysis was performed using SAS 9.2 for Windows (SAS Institute, Cary, NC). The analysis of variance was conducted using Tukey’s range test using honestly significant difference (HSD) values at a 0.05 significance.
MATERIALS AND METHODS
Starch Samples. Native sweet potato starch (NT-SPS, SuShu2) having a amylose content of 19.3% (w/w, dry basis), was isolated as described previously.5 The modified starches were prepared at the AVEBE Research and Development facilities (Veendam, Netherlands). All of the modifications were conducted in aqueous suspensions of the granular starches. The cross-linked sweet potato starch (CL-SPS) was obtained by adding 0.002 mol of trimetaphosphate/mol of glucose to the starch suspension. A 39% (w/w) suspension of starch (1 kg) was prepared with tap water, and 0.70 mol of Na2SO4 was dissolved in this suspension at room temperature. The pH of the suspension was adjusted to 10.5 with dropwise 4.4% (w/w) NaOH. Subsequently, 0.002 mol of sodium trimetaphosphate (STMP) was added for reaction of 6 h at 35 °C, and then the suspension was neutralized to pH 5.5 with H2SO4 (10 N) and dewatered over a Büchner funnel. The product was washed with tap water (1:5) and dried at 40 °C in an oven. The hydroxypropylated sweet potato starch (HP-SPS) was obtained by adding 0.2 mol of propylene oxide/mol of glucose residues. A 39% (w/w) suspension of starch (1 kg) was prepared with tap water, and 0.70 mol of Na2SO4 was dissolved in this suspension at room temperature. The pH of the suspension was adjusted to 10.5 with dropwise 4.4% (w/w) NaOH. A total of 0.2 mol of propylene oxide was added with a dropping funnel to the closed vessel. After 24 h of reaction at 35 °C, the suspension was neutralized to pH 5.5 with 10 N H2SO4 and dewatered over a Büchner funnel. The filter cake was washed with tap water (1:5). The starch was dried in an oven at 40 °C. The cross-linked and hydroxypropylated sweet potato starches (CLHP-SPS) were first cross-linked and subsequently hydroxypropylated. Starches were fractionated by sieving on an analytical sieve shaker (20 μm sieve, model AS200 digit, Retsch, Haan, Germany) at a vibrational amplitude of 1 mm and rinsing with tap water. When the rinsing water was clear and no starch granules could be detected visually, the sieving was stopped. The large-size granule fraction (>20 μm) and the small-size granule fraction (20 μm) and small-size (20 μm) was around 20% higher than that found for the small-size granule fraction (20 μm, large-size granule fraction; and 20 μm 20 μm 20 μm 20 μm 20 μm, oversize granules of 20 μm sieve; 20 μm 20 μm 20 μm 20 μm 20 μm, oversize granules of 20 μm sieve; and 20 μm, large-size granule fraction; and native > dualmodified > cross-linked starches (Figure 3). The small-size granule fractions of hydroxypropylated sweet potato starch showed a slightly higher swelling power than the large-size granule fraction and parental starch fraction, indicating that the higher amylopectin content in the smallsize granule fraction increases the swelling power more than the hydroxypropylation. The swelling power of the parental hydroxypropylated starch was lower than that of sieved starch fractions. Swelling power indicates the extent of water absorption and volume expansion of the starch granules. The volume includes the starch granule itself and the interspace between swollen granules. It is probable that the volume of
hydroxypropyl-modified granules. This indicated that the change of the transition temperature and enthalpy of gelatinization after dual modification predominantly resulted mainly from the hydroxypropylation. Pasting Properties of Starches. The pasting curves of native sweet potato starch showed a high peak viscosity and rapid shear thinning at high temperatures (Figure 2A). The peak viscosity of the large-size granule fraction is slightly lower than that of the small-size granule fraction, probably because of the lower phosphorus and lower amylopectin content of the large-size granule fraction (Table 1). The viscosity of the parental starch was lower than that of the large- and small-size granule fractions, which was also found for potato starch.29 This may be due to an optimal packing of small and large granules in the parental starch paste, resulting in a lower viscosity. The pasting curve of cross-linked starches expressed a low but stable viscosity (20−80 cP) and no obvious pasting peak (Figure 2B). The large-size granule fraction had the higher degree of cross-linking than the small-size granule fraction, resulting in even a lower pasting viscosity. This indicates that the viscosity of starch paste is sensitive to the degree of crosslinking. As expected, it was found that the hydroxypropylation increased the peak viscosity and decreased the trough and final viscosity of starch pasting (Figure 2C). The introduction of hydroxypropyl groups increased the hydrophilicity of starch molecules, resulting in swelling to a greater extent, creating a high peak viscosity.3,12 The large-size granule fraction showed a higher viscosity than the small-size granule fraction, probably 4650
DOI: 10.1021/jf506349w J. Agric. Food Chem. 2015, 63, 4646−4654
Article
Journal of Agricultural and Food Chemistry
interspace between granules affects the swelling power. Parental starch has a heterogeneous size of starch granules, where small granules may fill in the interspace between large granules, leading to a relatively small total volume and small total weight increase compared to the same dry weight of more uniform starch granules. The cross-linked as well as the dual-modified starch showed a lower swelling power than the native starch. There was not a significant difference in swelling power between the large- and small-size granule fractions of cross-linked starch. The swelling tendency of sweet potato starch in different granule size fractions is different from that reported for wheat starch, in which the small-size fraction had the lowest swelling power. Most likely, this can be explained by the high level of amylose−lipid complexes present in the small-size fraction of wheat starch.30 Effects of Starch Modification on the Refrigeration and Freeze−Thaw Properties. Low-temperature storage stability involves the sensitivity of starch to retrogradate when cooling and stored, in which the process may result in syneresis of the starch gel, resulting in excluded water from the gel. In our experiment, we used 6% starch pastes heated at 95 °C, before subjecting the gel to the storage conditions. As a consequence of the swelling power of the parental and modified
Figure 3. Swelling power of native (NT-SPS), cross-linked (CL-SPS), hydroxypropylated (HP-SPS), and dual-modified (cross-linked and hydroxypropylated, CLHP-SPS) sweet potato starches at various temperatures. Parental, unfractionated starch; >20 μm, large-size granule fraction; and 20 μm, large-size granule fraction; and 20 μm, large-size granule fraction; and 20 μm 78 7 >20 μm 5.8 27 >20 μm 0.2 25
>20 μm 244 39 20 μm 0.4 50
