Effect of Hydrothermal Treatment on Formation and Structural Characteristics of Slowly Digestible Non-pasted Granular Sweet Potato Starch

Transcription

1 Starch/Stärke 57 (2005) DOI /star Sang Ick Shin Ho Jong Kim Hyo Jeong Ha Seung Hee Lee Tae Wha Moon Effect of Hydrothermal Treatment on Formation and Structural Characteristics of Slowly Digestible Non-pasted Granular Sweet Potato Starch Department of Food Science and Technology, School of Agricultural Biotechnology, and Center for Agricultural Biomaterials, Seoul National University, Seoul, Korea The formation and structural characteristics of slowly digestible non-pasted granular starch in sweet potato starch were investigated under various hydrothermal treatment conditions. The moisture content of the sweet potato starch was adjusted to 20, 50 or 90%, and the starch was heated at 40, 55 or 1007C for 12 h in a dry oven. The relative crystallinity of the hydrothermally treated samples was decreased with increasing temperature, and the X-ray diffraction patterns of the samples were altered from C b - type to A-type. Microscopic observations did not reveal any changes in the starch granules of any samples except those with moisture contents of 50 and 90% that were heated at 1007C. When gelatinization parameters were examined, samples with moisture contents of 50 and 90% that were heated at 557C and samples of all moisture contents that were heated at 1007C had peak temperatures higher than that of raw starch but gelatinization enthalpies lower than that of raw starch. The swelling factor of the samples heated at 407C did not change significantly, whereas that of samples heated at 55 and 1007C was decreased at increased moisture levels. The sweet potato starch with 50% moisture content that was heated at 557C had the highest content of granular slowly digestible starch, about 200% that of raw starch, although our study did not involve further hydrothermal treatment conditions. Further study is required to complete a process for more efficient production of heat stable and slowly digestible starch. Keywords: Slowly digestible starch; Sweet potato starch; X-ray diffraction; Differential scanning calorimetry; Swelling factor 1 Introduction Starch is the main carbohydrate reserve in plants and is an important factor in human nutrition. For nutritional purposes, starch is generally classified based on its rate of digestion into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) [1]. The plasma glucose and insulin responses in humans vary with the physical form of the starchy food digested; this may be related to differences in the enzyme digestion rates of starches [2]. Foods containing SDS may be helpful in controlling and preventing diabetes. Such foods may also be advantageous to satiety, physical performance, improved glucose tolerance, and reduced blood lipid levels in both healthy individuals and those with hyperlipidemia [3]. Correspondence: Tae Wha Moon, Department of Food Science and Technology, School of Agricultural Biotechnology, and Center for Agricultural Biomaterials, Seoul National University, Seoul , Korea. Phone: Fax: , twmoon@snu.ac.kr. Sweet potato starch is used as an ingredient in bread, biscuits, cakes, ice cream, and noodles. It can also be converted to glucose or isomerized glucose syrup, both of which are used in a variety of foods, including candies, ice creams, and jams [4]. The action of a-amylase on sweet potato starch granules has been studied by several researchers [5 7]. Sweet potato starch is more susceptible than potato starch but less susceptible than cassava starch to a-amylase digestion. Hydrothermal treatment induces changes in the physicochemical properties of starch without destroying the granule structure [8, 9]. Jacobs et al. [9, 10] reported that annealed starch showed resistance to enzyme digestion and that heat-moisture treated starch exhibited increased enzymatic susceptibility owing to reduced relative crystallinity. Heat-moisture treatment of starch changes the crystallographic pattern of the starch granules, which leads to the conversion of a fraction of amorphous amylose to the crystalline form [9]. Piacquadio et al. [11] also reported that the extent of enzyme digestion increased as the heating time Research Paper

2 422 S. I. Shin et al. Starch/Stärke 57 (2005) increased at subgelatinization temperatures (50 557C) as a result of changes in the internal structure of starch granules. Although SDS is not commercially available, its production process and structural properties have been reported [12, 13]. A process for making SDS from debranched rice starch was studied by Guraya et al., who reported that a debranching enzyme content of 10% and cooling at 17C were optimal for the formation of starch with reduced digestibility [12]. SDS was also made from debranched waxy sorghum starch by controlling the storage conditions and the degree of debranching. The structural properties of SDS were investigated, and it was reported that SDS contained a small proportion of imperfect crystallites [13]. The objective of the present study was to determine the effect of hydrothermal treatment on SDS formation and to investigate the structural characteristics of non-pasted granular sweet potato SDS. The study addresses the hypothesis that hydrothermal treatments affect both the crystalline and amorphous regions in starch, which has its effect on the amount of SDS. Sweet potato starch was obtained from Samyang Genex Co. (Daejeon, Korea). Pancreatin (P-7545, activity 86USP/g) was purchased from Sigma Chemical Co. (St. Louis, MO, USA) and amyloglucosidase (AMG 300L, activity 300 AGU/mL) from Novozymes Inc. (Bagsvaerd, Denmark), where USP and AGU stand for United States Pharmacopia and amyloglucosidase activity, respectively. All other chemicals were of analytical reagent grade. 2.1 Preparation of hydrothermally treated starches The initial moisture content of the sweet-potato starch used was determined to be 11% according to the AACC 44 15A method [14]. Moisture content of sweet potato starch was adjusted to 20, 50, or 90% by adding distilled water. The samples were sealed in containers, kept at room temperature for 24 h for equilibration, and then heated in a dry oven for 12 h at 40, 55, or 1007C. The containers were subsequently opened, and the samples were air-dried at room temperature to a moisture content of about 10%. The samples were then ground so that they would pass through a 100 mesh sieve. Sweet potato starches with A% moisture content heated at BºC are referred to here as SP B7C/A%. Raw sweet potato starch is referred to as SP R (Fig. 1, Tab. 1). 2 Materials and Methods 2.2 X-ray diffraction X-ray diffraction analysis was performed with an X-ray diffraction image processor (Dip 2030, Indigo 2, Oxford Model, HX-200w/c, Mac Science, Tokyo, Japan) (analysis parameters: 50 kv and 90 ma, CuK radiation l = Fig. 1. Schematic of the hydrothermal treatment of starch.

3 Starch/Stärke 57 (2005) Formation of SDS by Hydrothermal Treatment 423 Tab. 1. Abbreviations. Abbreviation Definition tape mounted on an aluminum specimen holder, coated with a thin film of gold (30 nm), and examined at 20 kv. RDS SDS RS SP R SP 407C/20% SP 407C/50% SP 407C/90% SP 557C/20% SP 557C/50% SP 557C/90% SP 1007C/20% SP 1007C/50% SP 1007C/90% T o T p T c A c A a SD Rapidly digestible starch Slowly digestible starch Resistant starch Raw sweet potato starch Sweet potato starch with 20% moisture content heated at 407C Sweet potato starch with 50% moisture content heated at 40ºC Sweet potato starch with 90% moisture content heated at 407C Sweet potato starch with 20% moisture content heated at 557C Sweet potato starch with 50% moisture content heated at 557C Sweet potato starch with 90% moisture content heated at 557C Sweet potato starch with 20% moisture content heated at 1007C Sweet potato starch with 50% moisture content heated at 1007C Sweet potato starch with 90% moisture content heated at 1007C Onset temperature of gelatinization Peak temperature of gelatinization Conclusion temperature of gelatinization Crystalline area Amorphous area Standard deviation 2.5 Swelling factor The swelling factor of hydrothermally treated starch was measured by the method of Tester and Morrison [16]. A starch sample (100 mg) was suspended in 5 ml of distilled water and kept in a water bath at 707C for 30 min. This sample was cooled in ice, 0.5 ml of blue dextran solution (5 mg/ml) was added, and the sample was inverted to mix with the solution. After centrifugation at 3,0006g for 10 min, the absorbance of the supernatant was read at 620 nm. The values were the average of three determinations. 2.6 Determination of starch fractions An enzyme solution was prepared according to the method of Brumovsky and Thompson [17] with slight modification. Pancreatin (9 g) was added to 90 ml of water in a beaker and stirred using a magnetic stirrer for 10 min. This suspension was then centrifuged at 1,5006g for 10 min. The cloudy supernatant (45 ml) was transferred to a volumetric flask containing 2.65 ml amyloglucosidase solution and brought to 100 ml with distilled water nm, nickel filter). The sample was scanned through the 2y range from 37 to 307. Relative crystallinity was calculated according to the method of Nara and Komiya [15] using Origin 5.0 (Microcal Inc., Northampton, MA, USA). 2.3 Gelatinization parameters The gelatinization parameters of hydrothermally treated starches were investigated using a differential scanning calorimeter (Seiko DSC 120, Seiko Instrument, Inc., Chiba, Japan). Indium was used as a calibration standard. A 5 mg sample was placed in a Seiko high-pressure stainless steel pan, and 10 ml of distilled water was added. The sample pan was sealed and kept at room temperature for 1 h. Distilled water was used as a reference. The sample pan was heated from 30 to 1207C at 57C/min. 2.4 Microscopic observations Microscopic observations of heat-moisture treated starches were performed using a scanning electron microscope (JSM 5410LV, JEOL Ltd., Tokyo, Japan). A dried, finely ground sample was placed on double-sided Scotch For the determination of the starch fraction, samples (500 mg, wet basis) were placed into 50 ml triangle flasks, and 15 ml of sodium acetate buffer (ph 5.2) and five glass balls were added to each flask. The flasks were equilibrated in a shaking water bath with a stroke speed of 150 strokes/min at 377C for 10 min. Prepared enzyme solution (10 ml) was then added to each flask. After 120 min, 50 ml of hydrolyzate was transferred to a tube containing 1 ml of absolute ethanol and mixed. These samples were analyzed for glucose by the glucose oxidase and peroxidase assay. The remaining material in the flask was analyzed for total starch by the method of Brumovsky and Thompson [17]. The remaining material was boiled in a cooker for 30 min and then the flask was cooled in ice for 10 min. Ten milliliters of 7 M KOH were added, and the mixture was stirred for 30 min. Two hundred microliter of the contents was added to a 2 ml tube containing 1 ml of 1 M acetic acid. A40mL aliquot of AMG solution (diluted 1: 5.25 with water from the original AMG solution) was added to the tube. The tube was incubated at 707C for 30 min. Afterwards, a 100 ml aliquot of the contents was added to a 2 ml tube containing 1 ml of absolute ethanol and analyzed for glucose by the glucose oxidase and peroxidase assay.

4 424 S. I. Shin et al. Starch/Stärke 57 (2005) RDS and SDS were measured after incubation with both pancreatin and amyloglucosidase at 377C for 20 and 120 min, respectively. RS was the starch not hydrolyzed after the 120 min incubation period [1]. 2.7 Glucose oxidase and peroxidase assay A 100 ml volume of the sample was added to a 2 ml tube containing 1.5 ml of glucose oxidase and peroxidase reagent (YD Diagnostics Co., Seoul, Korea). The tube was incubated in a water bath at 377C for 20 min. The absorbance of the sample at 505 nm was then read. Tab. 2. Relative crystallinity of hydrothermally treated starch. Samples Relative crystallinity [A c /(A c 1A a )] SP R 0.30 SP 407C/20% 0.30 SP 407C/50% 0.29 SP 407C/90% 0.29 SP 557C/20% 0.27 SP 557C/50% 0.27 SP 557C/90% 0.27 SP 1007C/20% 0.25 SP 1007C/50% 0.20 SP 1007C/90% Statistical analysis All samples were analyzed in triplicate, and standard deviations were calculated using Microsoft Excel Results and Discussion 3.1 X-ray diffraction and relative crystallinity The X-ray diffraction patterns and relative crystallinities of the samples are given in Fig. 2 and Tab. 2, respectively. The SP R sample had peaks at 5.87, 11.77, 15.47, 17.47, 23.67, and 277. Hizukuri [18] reported that X-ray diffraction peaks at 5.57, 157, 177, and 227 or 237 are characteristic of a C-type starch that is common to most sweet potato starches. The C-type starches have been subclassified as C a -, C b -, and C c -type on the basis of their resemblance to either A-type, B-type, or a type between A and B, respectively [18]. By this classification, the sweet potato starch used in this work belongs to the C b -type. The peak at 57 was broad in the SP 557C/90% sample and was not present in the SP 1007C group. In addition, the 177 peak of these samples was split slightly into two peaks (177 and 187). These results indicated that the X-ray diffraction pattern of sweet potato starch was changed from C b -type to A-type by hydrothermal treatment. Hoover and Vasanthan [19] reported the alteration of the X-ray diffraction pattern from B- to A- (or C- type) for potato starch following heat-moisture treatment. A shift from C- to A- type following heat-moisture treatment has also been observed for arrowroot and cassava starches [20]. Thus, it appears that hydrothermal treatments induce a structural alteration depending on heating conditions and moisture content. The intensity and relative crystallinity of the peaks of SP 557C/20%,SP 557C/50%,SP 557C/90%,SP 1007C/20%, and SP 1007C/ 50% at 57, 117, 157, 177, and 237 were lower than those of SP R (Fig. 2, Tab. 2). The X-ray diffractogram of SP 1007C/90% showed a disappearance of the split peak, broadening of most peaks (157, 177, ), and a large decrease in relative crystallinity (Fig. 2, Tab. 2). The decrements of intensities and crystallinities implied that the crystalline region of starch was disrupted by hydrothermal treatment. Some researchers have reported decreased intensities of X-ray diffraction for hydrothermally treated cassava, barley, and maize starches [20 22]. Gunaratne and Hoover [23] reported that double helical movement during heat-moisture treatment could disrupt starch crystallites and change crystallite orientation. This would explain the observed changes in crystallinity and intensity following hydrothermal treatment. 3.2 Gelatinization parameters Fig. 2. X-ray diffractogram of hydrothermally treated sweet potato starch. The thermal properties of various hydrothermally treated starches are shown in Tab. 3. The onset temperature of gelatinization (T o ) of the SP 407C and the peak temperature

5 Starch/Stärke 57 (2005) Formation of SDS by Hydrothermal Treatment 425 Tab. 3. Thermal properties 1 of hydrothermally treated starches. Sample Transition temperature [7C] T o 2 T p 3 T c 4 T c -T o 5 SP R SP 407C/20% SP 407C/50% SP 407C/90% SP 557C/20% SP 557C/50% SP 557C/90% SP 1007C/20% , SP 1007C/50% SP 1007C/90% : values are averages of three determinations. 2: SD, 2.17C, n =3. 3: SD, 0.37C, n =3. 4: SD,27C, n =3. 5: SD, 2.97C, n =3. 6: SD, 0.4 mj/mg, n =3. DH [mj/mg] 6 of gelatinization (T p ) of the SP 557C group (except samples with 20% moisture content) were higher than those of SP R, and the conclusion temperature (T c ) and T c -T o values of the SP 407C and SP 557C groups were lower than those of SP R. However, the T p value of the SP 407C group was similar to that of SP R. It is generally recognized that the melting temperature of starch crystallites is dependent on the water content as water decreases the temperature of glass transition and eventually the temperature of the melting of the crystallites [24, 25]. Thus, increase in T o can explain the melting of starch crystallites which have a lower melting temperature at the particular water content of the treatment than the treatment temperature. This partial melting will result in a decrease of enthalpy and T c -T o and an increase in T o as the weakest crystals are eliminated first (Tab. 3). Therefore, the increases in T o and the decreases in T c -T o of the SP 407C and SP 557C groups indicate that hydrothermal treatment results in a decrease of the destabilization effect of the amorphous region [8, 23]. Jacobs and Delcour [9] suggested that a change in the amorphous fraction by hydrothermal treatment was related to an increase in order without an increase in crystallinity. This implies that hydrothermal treatment converted the internal structure in the amorphous region of the samples to a more rigid structure than that of SP R. However, decrease in T c depending on the treatment temperature and moisture contents is difficult to explain with previous studies. Usually, T c increases at increasing treatment temperature and water content [9, 26, 27]. The gelatinization enthalpies of the SP 407C group and SP 557C/20% showed no difference from that of SP R suggesting no disruption of the double helices under these treatment conditions [8]. However, the enthalpies of SP 557C/50% and SP 557C/90% were lower than that of SP R indicating that some of the double helices present in the crystalline and amorphous regions of the granule might be disrupted under these particular conditions [23]. Vasanthan and Bhatty [28] proposed that a situation in which starch has higher enthalpy but lower gelatinization temperature could be explained by changes in the compactness of the amorphous region within starch. Therefore, the high T p and low enthalpies of SP 557C/50% and SP 557C/90%, as compared with SP R, suggest that these samples contained a higher amount of effective crosslinks in the amorphous region and a slightly disrupted crystalline region. The gelatinization parameters of the SP 1007C group were different from those of the other groups, SP 407C and SP 557C (Tab. 3). The gelatinization temperature of SP 1007C increased and the enthalpy and T c -T o value of SP 1007C decreased with increasing moisture content. The gelatinization parameters (T o, T p, T c, and T c -T o )ofsp 1007C/20% were consistent with previous studies [8, 9, 19, 27], which reported that gelatinization parameters increased upon heat-moisture treatment. In case of the SP 1007C group, a substantial part of the crystallites melts, which results in a strong decrease of enthalpy. At the same time, the crystallites with a melting temperature over 1007C are annealed, which is presumably a cause of the higher final melting temperature T c. Also, increase in gelatinization parameters of the SP 1007C group should be due to the change of crystalline type (C?A-type) because A-type crystals melt at a higher temperature than B-type crystals [29]. Cooke and Gidley [30] showed by X-ray and NMR spectroscopy that gelatinization enthalpy values mainly reflect the loss of double helices rather than of crystalline order. The large decrease in enthalpy for SP 1007C/50% and SP 1007C/90%, therefore, implies the disruption of double helices in the crystalline region by these treatments. Furthermore, the higher T p and lower enthalpy of SP 1007C/50% and SP 1007C/90% compared with SP R suggests that most of the semi-crystalline structure is destroyed and little concentrated crystalline region remained within the starch structure after these treatments. Among the SP 100ºC group, the T c -T o values of SP 1007C/20% and SP 1007C/50% were greater than that of SP R, but the T c -T o value of SP 1007C/90% was lower. In general, the increase in T c -T o indicates that internal changes between the components of starch during hydrothermal treatment may lead to the formation of crystallites of different stabilities [8]. Thus, SP 1007C/20% and SP 1007C/50% may contain various

6 426 S. I. Shin et al. Starch/Stärke 57 (2005) double helices which have different thermal stabilities. Although most of the crystalline region in SP 1007C/90% appeared to be lost following hydrothermal treatment, a portion of the crystallites having resistance to heat treatment remained and caused the changes in enthalpy and relative crystallinity (Tabs. 2 and 3). Conclusively, in the course of a hydrothermal treatment employed in this study two events might have occurred. First, the partial melting of starch crystallites with a lower melting temperature than the treatment temperature results in a decrease of DH and an increase in T o. Secondly, the annealing of starch crystallites implying that polymer crystallites exhibit improved perfection when heated at a temperature not too much below their melting temperatures and upward shift in T c in case of the annealing also affects the highest melting crystallites. This annealing effect is stronger for crystals with a melting temperature more closely to the treatment temperature. 3.3 Microscopic observations Various hydrothermally treated starches were observed under a scanning electron microscope (SEM). The surface of the SP R sample at magnification appeared to be smooth with no evidence of cracks (Fig. 3). The external appearances of the SP 407C and SP 557C groups and the SP 1007C/20% sample were not different from that of SP R (electron micrographs not shown). Franco et al. [21] and Kawabata et al. [31] suggested that intermolecular bonds between amylose and amylopectin molecules were formed within granules during heat-moisture treatment. The SEM results for the SP 407C and SP 557C groups and the SP 1007C/20% sample, therefore, imply that internal alterations may be generated by the hydrothermal treatment without external alterations. The SP 1007C/50% sample, however, displayed aggregation of granules, distorted granule shape, and cracks and hollows on the granule surfaces (Fig. 3). Kawabata et al. [31] previously reported the formation of cracks on the surfaces of heat-moisture treated corn and potato starches. Most of the SP 1007C/90% sample appeared to lose original starch granule shape (Fig. 3). 3.4 Swelling factor The swelling factors of various hydrothermally treated starches are presented in Tab. 4. The swelling factors of SP 407C/20%,SP 407C/50%,SP 407C/90%, and SP 557C/20% samples were not significantly different from that of SP R. On the other hand, those of the SP 557C/50%, SP 557C/90%, and SP 1007C/20% samples were lower as compared to the SP 407C group and SP R. Swelling is known to begin in the relatively mobile amorphous fractions and in more restrained amorphous regions immediately adjacent to the crystal- Fig. 3. SEM of hydrothermally treated sweet potato starch. (A) SP R (B) SP 1007C/50% (C) SP 1007C/90%

7 Starch/Stärke 57 (2005) Formation of SDS by Hydrothermal Treatment 427 Tab. 4. Swelling factors of hydrothermally treated sweet potato starches. Samples Swelling factor 1 SP R 13.8 SP 407C/20% 13.3 SP 407C/50% 13.3 SP 407C/90% 12.4 SP 557C/20% 13.1 SP 557C/50% 8.6 SP 557C/90% 8.4 SP 1007C/20% 7.5 SP 1007C/50% 3.3 SP 1007C/90% 2.3 1: SD, 0.5, n =3. Values are averages of three determinations. line regions [8]. Therefore, the decrease in swelling factor is caused by the interaction between or among starch components in amorphous regions of the granule [19]. Tester et al. [26] also suggested that the free amylose actively restricted swelling by cross-linking of amylopectin crystallites although the short exterior chains of amylopectin forming double helices together probably precluded this happening to a great extent. Tester and Morrison [16] suggested that the swelling factor is primarily a property of amylopectin and that amylose is a diluent. The decrease, therefore, in the swelling factors of the SP 557C/50%,SP 557C/90%, and SP 1007C/20% samples could be attributable to an increase in effective cross-links resulting from an improved order in the amorphous regions. The samples SP 1007C/50% and SP 1007C/90% showed a large decrease in swelling factor compared with SP R and the other samples (Tab. 4). This result could be attributed to interplay of three factors. The first one is connected to the change in the packing arrangement (C?A type) of hydrothermally treated starch. Hoover and Vasanthan [19] reported that A-type starches have lower swelling factors than C- or B-types ones. Secondly, there is a partial crystal melting during hydrothermal treatment (Tabs. 2 and 3). Presumably, a portion of the organization is recovered during drying of the treated granules by a retrogradation process within the granules. Especially, in the samples treated at 90% moisture, there is sufficient water present that partial retrogradation can occur. This would induce the organized domains of limited size which are not detected by DSC or X-rays. The reduced swelling can then be explained by assuming that in these domains starch chains originating from different molecules are connected with each other rather than chains from the same molecule. The latter is predominant in the crystallites in unmodified starch granules where neighboring chains within the same amylopectin molecule are connected. Therefore, after the hydrothermal treatment, the amount of effective cross-links may be higher than that of the initial starch crystallites. Finally, the unraveling of double helices during hydrothermal treatment could induce the decrease in swelling factor although it also can occur during normal gelatinization. Gunaratne and Hoover [23] reported that the disruption of crystallites upon hydrothermal treatment could contribute to a large decrease in the swelling factor. These factors may explain the large decrease in the swelling factors of the SP 1007C/ 50% and SP 1007C/90% samples. 3.5 Relation between formation and structural change of slowly digestible starch The results of the in vitro digestion of hydrothermally treated sweet potato starches are presented in Tab. 5. The rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) contents of SP R were 17.1, 15.6, and 67.3%, respectively. These data on RDS, SDS and RS only refer to the sweet potato starch ingredient without further processing. The amounts of the RDS, SDS, and RS fractions of the SP 407C/20%,SP 407C/50%, SP 407C/90%, and SP 557C/20% samples were similar to those of SP R (Tab. 5). This trend was consistent with the results for the swelling factors, gelatinization enthalpies, and relative crystallinities of the SP 407C group and SP 557C/20%. These values in the SP 407C group and the SP 557C/20% sample showed no significant differences from those of SP R (Tabs. 2, 3, and 4). However, for the SP 557C/50%,SP 557C/90%, and SP 1007C/20% samples, the amounts of RDS and SDS were higher and the amount of RS was lower than the amount in SP R.It Tab. 5. RDS, SDS and RS fractions of hydrothermally treated sweet potato starch. Samples RDS [%] 1 SDS [%] 2 RS [%] 3 SP R SP 407C/20% SP 407C/50% SP 407C/90% SP 557C/20% SP 557C/50% SP 557C/90% SP 1007C/20% SP 1007C/50% SP 1007C/90% : SD,2.1%, n=3. 2: SD,3.8%, n=3. 3: SD,4.5%, n=3. Values are averages of three determinations.

8 428 S. I. Shin et al. Starch/Stärke 57 (2005) has been reported that starch digestibility of cereals is increased by heat treatment in comparison to that of raw cereals [32]. In particular, SP 557C/50% showed the highest SDS content (31.0%), with a value about double that of SP R. The RS fraction of SP 1007C/20% was increased compared to those of SP 557C/50% and SP 557C/90%. This trend was also in agreement with the changes in structural parameters such as swelling, gelatinization enthalpy, X-ray pattern, and relative crystallinity for SP 557C/50%,SP 557C/90%, and the SP 1007C group (Tabs. 2, 3, and 4). It is well known that differences in the in vitro digestibility of starches by a-amylase can be attributed to the interplay of factors such as surface area, swelling, crystalline type, interaction between amylose chains, and crystallinity [23, 26, 33]. The decrease in the swelling factors (Tab. 4) of the samples indicates a higher amount of effective cross-links in the amorphous regions. This change may directly and indirectly control enzymatic hydrolysis [26]. The increase in the RS fraction of SP 1007C/20% may be a result of partial retrogradation (or association) of amylose chains within the amorphous region, which would decrease accessibility to a-amylase [23]. The decreases in the gelatinization enthalpies (Tab. 3) and relative crystallinities (Tab. 2) of the samples indicate the loss of double helices within the granule [30] and crystallite disruption [23], respectively. These structural changes in starch could affect the a-amylase hydrolysis of starch. Gunaratne and Hoover [23] suggested that the initial step of a-amylolysis corresponds to the adsorption of a-amylase on the granule surface. Thus, crystallite disruption near the granule surface upon heat-moisture treatment of true yam and potato starches could facilitate the rapid entry of a-amylase into the granule interior. Planchot et al. [33] suggested that one of the limiting factors in the rate of enzymatic hydrolysis could be the nature of the granule surface with respect to crystallinity. They also suggested that the alteration of starch from C b -type to A-type in the samples could cause the crystalline region in the samples to be more susceptible to a-amylase. The A-type polymorph is more susceptible to a-amylase hydrolysis than are the B- and C-type polymorphs and that this difference is a result of different blocklet sizes [1, 34]. The alteration of hydrothermally treated starch could induce an increase in the extent of amylolysis compared with that of raw starch. Overall, the changes of the amorphous and crystalline regions can explain the increase in the RDS and SDS contents of the SP 557C/50%,SP 557C/90%, and SP 1007C/20% samples. SDS composed a greater proportion of total starch in the SP 1007C/50% (22.3%) and SP 1007C/90% (17.7%) samples than in SP R. However, distinct changes in the RDS and RS fractions were observed for SP 1007C/50% and SP 1007C/90% (Tab. 5), which could be explained by the results of electron microscopy and by differences in the swelling factor, gelatinization enthalpy, and relative crystallinity (Figs. 2 and 3, Tabs. 2, 3 and 4). The increases in the RDS fractions of SP 1007C/50% and SP 1007C/90% imply that they contained structures which can be more easily attacked by a- amylase, compared with other hydrothermally treated samples. Scanning electron microscopy revealed porosities and cracks on the surfaces of the samples (Fig. 3B and C). Large decreases in the swelling factor, gelatinization enthalpy, and relative crystallinity also indicate that the granular structure was disrupted to a great extent and that disordered structures were formed. These factors would explain the increase in the RDS fraction and the decrease in RS seen in SP 1007C/50% and SP 1007C/90%. 4 Conclusions The optimum conditions for the preparation of slowly digestible granular sweet potato starch were intermediate temperature (557C) and moisture content (50%). Changes seen in the extent and rate of enzymatic hydrolysis were due to the alteration of the internal granular structure. Hydrothermal treatment was found to alter swelling, gelatinization enthalpy, crystalline type, and crystallinity. Overall, the structure of slowly digestible granular sweet potato starch contained very rigid amorphous regions and partially disrupted crystalline regions. This study suggests that hydrothermal treatment affects the formation of slowly digestible granular sweet potato starch and provides basic information for the efficient preparation of SDS. Further study is required to complete a process for more efficient production of heat stable SDS. Acknowledgements This work was supported by the Korea Research Foundation (Grant KRF F00054), funded by the Korean Government (MOEHRD). References [1] H. N. Englyst, S. M. Kingman, J. H. Cummings: Classification and measurement of nutritionally important starch fractions. Eur. J. Clin. Nutr. 1992, 46 (suppl. 2), S33 S50. [2] G. Collier, K. O Dea: Effect of physical form of carbohydrate on the postprandial glucose, insulin, and gastric inhibitory polypeptide responses in type 2 diabetes. Am. J. Clin. Nutr. 1982, 36, [3] I. Björck, N.-G. Asp: Controlling the nutritional properties of starch in foods a challenge to the food industry. Trends Food Sci. Technol. 1994, 5,

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