2025 年 72 巻 4 号 論文ID: 7204202
Short linear maltodextrin (SLMD) is a novel maltodextrin synthesized from starch using the combined enzymatic actions. SLMD exhibits unique aggregating and solidifying properties. In this study, we prepared SLMD aggregates, solidified materials under various conditions, and investigated their crystallinity. Aggregates formed in the 50 % SLMD solution at 4 °C (AGG-4), 25 °C (AGG-25), and 50 °C (AGG-50) showed clear X-ray diffraction peaks. A B-type crystal diffraction pattern was observed for AGG-4, whereas an A-type pattern was observed for AGG-25 and AGG-50. Kneading SLMD with a limited quantity of water produced solidified slurries at 4 °C (SS-4) and 25 °C (SS-25). SS-4 exhibited a C-type structure with low crystallinity, whereas SS-25 showed an A-type structure with high crystallinity. In addition, B-type crystals were detected in the aggregates in the emulsions solidified with vegetable oil. Therefore, SLMD crystals occurred in different forms in the aggregates or solidified bodies under various conditions.
SLMD, short linear maltodextrin; DP(n), (number-average) degree of polymerization; CL(n), (number-average) chain length; XRD, X-ray diffraction
Short linear maltodextrin (SLMD) is a novel maltodextrin synthesized from starch by the combined action of branching and debranching enzymes [1]. SLMD comprises DP 6-12 linear chains and the number-average values of both degree of polymerization (DPn) and chain length (CLn) are approximately 8.5 [1]. Its unique physicochemical properties enable several potential applications for SLMD in the preparation and modification of starch-based or starch-containing foods. Most notably, SLMD tends to aggregate in aqueous solutions. This aggregation is reversed by reheating and repeatable. Furthermore, SLMD powder absorbs water under high relative humidity conditions and forms a solid body without any evident stickiness [1]. This phenomenon may possibly involve the crystallization of the SLMD short linear chains.
Regarding crystallization of α-glucan-related saccharides, the semi-crystalline nature of native starch granules is well known, and two primary allomorphic types, A- and B-type, can be distinguished using X-ray diffraction (XRD) [2]. The A-type crystalline form primarily occurs in cereal starches, whereas the B-type occurs in tubers and high-amylose starches. In addition, the C-type crystalline form is a mixture of A- and B-types and is characteristic of most legume starches. Crystallization temperature [3] and CLn of amylopectin [4] are correlated with the crystalline type of starch granules. The crystalline form is prepared by the aggregation of α-glucan- or sucrose-derived linear malto-oligosaccharides, which are generated by processing α-glucans or sucrose with enzymes such as debranching enzymes [5, 6, 7, 8], phosphorylase [9], or amylosucrase [10]. The polymorphic form of the crystalline aggregate appears to be determined by CL, saccharide concentration, and aggregation temperature. Thus, factors such as shorter CL, higher concentration, and higher temperature favor the formation of the A-type crystalline form [6, 7], whereas the CL primarily determines the crystalline allomorphic form [9].
Previously, we observed aggregate formation by SLMD despite exhibiting a shorter CLn than those of the examples mentioned earlier, which has raised significant questions regarding the mechanism and conditions underlying the aggregation of short linear malto-oligosaccharides. In addition, whether the SLMD aggregates are crystalline remains unclear. Furthermore, we aimed to determine whether the crystallization of SLMD powder is accompanied by the formation of a solid body. In this study, we prepared aggregates and solidified SLMD under various conditions and investigated the relationship between the aggregation/solidification conditions and crystallinity of the products.
We previously reported that 10 % SLMD solution produced aggregates on cooling at 4 °C [1]. In this study, a larger quantity of aggregates was prepared by increasing the SLMD concentration to 50 %. Aggregate formation occurred at all three tested temperatures (4, 25, and 50 °C, designated as AGG-4, AGG-25, and AGG-50, respectively). In total, 7.3, 7.2, and 6.0 g of AGG-4, AGG-25, and AGG-50 were obtained from 19.8, 19.6, and 40.0 g of SLMD, respectively (recovery rates: 37, 37, and 15 %). AGG-4 and AGG-25 were spherical and approximately 10 μm in size; in addition, AGG-25 was slightly larger in size than AGG-4. AGG-50 exhibited an irregular shape with a few rounded protrusions, some of which were > 20 μm (Fig. S1; see J. Appl. Glycosci. Web site).
XRD analysis showed no diffraction peaks for SLMD (Fig. 1), confirming its amorphous nature. In contrast, the SLMD aggregates showed clear diffraction peaks, which indicated that the aggregates were crystalline (Fig. 1). The diffraction pattern of AGG-4 indicated a B-type crystalline form, whereas those of AGG-25 and AGG-50 indicated an A-type crystalline form.
Based on the capillary electrophoresis-based CL analysis of the aggregates, the molar percentages of saccharides with DP ≤ 8 decreased compared with that of SLMD. The major component chains showed DP of 8-16, which indicated the aggregation of the SLMD long-chain saccharides (Fig. S2; see J. Appl. Glycosci. Web site). The abundance of saccharides with DP ≤ 5 became negligible in the aggregates, which is consistent with the finding of a previous study where crystallization of saccharides with DP ≥ 6 was observed in a mixture of saccharides of various CLs [7]. In the current study, saccharides with DP ≥ 40 were not observed in the aggregates. A previous study showed that the average CLn of waxy maize and waxy potato starch was 24.1 and 32.1 glucose units, respectively, and both starches presented bimodal chain-length distribution with low and high molecular weight peaks in gel permeation chromatography analysis [6]. The SLMD-derived aggregates showed short CL and monomodal molecular weight distribution, which differs from that of previously reported starch-derived aggregates.
Solidified slurries were produced after kneading SLMD with a limited quantity of water and storage at 4 °C or 25 °C (designated as SS-4 and SS-25, respectively) (Fig. 2(a)). SS-25 was non-sticky, whereas SS-4 was slightly moist. XRD analysis showed that SS-4 and SS-25 contained C-type crystals with low crystallinity and A-type crystals with high crystallinity, respectively (Fig. 3(a)). We have previously reported that when the powder of SLMD was stored at 25 °C and 94 % relative humidity, the weight increase by water absorption of SLMD ceased at 18 h, and a solid body was obtained [1]. We tested this scenario in this study at 25 °C and 94 % relative humidity and at 4 °C and 96 % relative humidity (designated as SB-25 and SB-4, respectively). Before drying, 4.0 and 4.2 g of SLMD yielded 4.6 and 4.8 g of SB-4 and SB-25, respectively (weight increase 15 and 14 %). XRD analysis indicated that SB-25 contained an A-type crystalline structure (Fig. 3(b)). In contrast, SB-4 was slightly sticky with no diffraction peaks detected. In a previous experiment at 25 °C and 94 % relative humidity, the weight increase reached equilibrium within 18 h and that SB-4 was allowed to absorb moisture for 99 h [1]; therefore, this result indicated that crystallization did not occur at 4 °C even if moisture is absorbed until the weight gain reaches equilibrium.
Solidification at 25 °C showed that crystalline SLMD is produced using a small quantity of water (SS-25) or even moisture absorbed from the air (SB-25). Usually, linear α-glucans are obtained in crystalline form as precipitates in water. Among these precipitates collected from the solution, the saccharides that did not aggregate are separated and washed out with water [6, 7, 8, 9, 10]. However, notably, a solid formed by a SLMD slurry exhibited a significant degree of crystallinity, despite the fact that the SLMD solid still contains short saccharide chains of DP ≤ 5 (Fig. 3). Such short chains do not participate in crystallization and are typically expelled from the crystallites [6, 7, 8, 9, 10]; however, they can coexist in the SLMD solid. This indicates that crystallization is extremely efficient during the solidification process at low water concentration.
In contrast, solidification at 4 °C showed that SLMD crystallization was not effective at low temperatures and with a small quantity of water. Analysis of the aggregates in the 50 % SLMD solution (AGG-4) showed that B-type crystals were formed in the presence of sufficient quantity of water. However, low water concentration was not conducive to crystallization, which resulted in a C-type crystalline structure with low crystallinity (SS-4) or completely amorphous structure (SB-4).
To determine whether solidification occurs even in the presence of other ingredients, SLMD was emulsified with water and liquid vegetable oil and stored at 4 °C. During storage, the emulsion changed to a semisolid state (Fig. 2(b)). We obtained 2.1 g of the aggregate from 46.8 g of semi-solidified emulsion. Although the crystallinity was low, B-type crystals were detected in the aggregates (Fig. 3(c)). This result confirms that crystallization through solidification occurs in emulsions in the presence of other components.
This study shows that SLMD possesses unique aggregating and solidifying properties, which have potential food-based applications. Furthermore, aggregation and solidification are accompanied by crystallization of the short linear saccharides of SLMD. The appropriate crystallization temperature determines and controls the crystal-type of the aggregates (Table S1; see J. Appl. Glycosci. Web site). Hizukuri proposed the following functional relation between the carbohydrate concentration (C, %) and the crystallization temperature (T, °C) for each crystalline type of A-, C-, and B-type: 2.5T + C > 84, 72 < 2.5T + C < 84, 2.5T + C < 72, respectively, using an amylodextrin of DPn 12.6 in the ranges 8-55 °C and 31.2 to 46.5 % [3]. In the present study, we expanded the range of validity of this formula and showed that it can be applied to solidification at a high concentration of 70 %. However, when the powder absorbed moisture, A-type crystals were formed as predicted by the formula at 25 °C, but crystallization did not occur at 4 °C. As the SLMD powder was manufactured in a completely amorphous state, we could dissolve it in water at ambient temperature and crystallize it into a different type of crystalline polymorph. Moreover, the solidification property is particularly distinctive because it can be solidified by simply kneading with a small quantity of water but also in the presence of other ingredients such as liquid oil in an emulsion. Furthermore, the solidifying property has potential applications in the food industry such as the preparation of low-calorie fat spreads. In addition, the high crystallinity of the solidified material prevented the ready absorption of moisture. This property is advantageous for food-industry applications such as non-sticky sugar coating. Future studies should explore the properties of crystalline and solidified materials and their applications.
Maltodextrin sample. Powdered SLMD sample (AmyloSoln) was manufactured by Showa Sangyo Co., Ltd. (Tokyo, Japan) using a slightly modified method that has been described previously [1]. The molecular weight distribution of SLMD (AmyloSoln) (Fig. S3; see J. Appl. Glycosci. Web site) was confirmed to be identical to that of SLMD used previously [1] by gel-permeation chromatography.
Preparation of aggregates. Aggregates were prepared from 50 % (wt/wt) SLMD solution at different temperatures for 48 h. For aggregation at 4 °C (AGG-4) and 25 °C (AGG-25), the 50 % solution was prepared at ambient temperature. In the case of aggregation at 50 °C (AGG-50), the 50 % solution was created at 50 °C. The aggregates were formed during storage. Next, the suspension was centrifuged at 2,000 × G, and the supernatant was discarded. The collected precipitate was washed three times by resuspending in water at a volume equal to that of the initial solution, followed by dispersion using a vortex mixer and centrifuged at 2,000 × G. The precipitates were then dehydrated using suction filtration through a No. 2 filter paper and air-dried on a tray. The dehydrated products were passed through a 60-mesh sieve before being subjected to further analysis. The aggregates were observed using a microscope (RH-2000; Hirox Co., Ltd., Tokyo, Japan) at 1,000 × magnification.
Preparation of solidified slurry. Water was added to SLMD (weight ratio, SLMD:water = 7:3), and the mixture was kneaded by hand in a plastic bag, poured into a silicon mold, and kept at 4 °C (SS-4) or 25 °C (SS-25) for 48 h in a sealed bag. The slurry was solidified and removed from the mold, dried at ambient temperature, crushed in a mortar, and passed through a 60-mesh sieve.
Preparation of solid SLMD through moisture absorption. SLMD powder was stored at 4 °C under 96 % relative humidity (SB-4) or at 25 °C under 94 % relative humidity (SB-25) in a saturated potassium nitrate aqueous solution atmosphere for 99 h. During storage, the SLMD particles formed solid bodies. The solids were dried at ambient temperature, crushed in a mortar, and passed through a 60-mesh sieve.
Preparation of aggregates in a solidified emulsion containing vegetable oil. All ingredients, that is, 10 g SLMD, 10 g granulated sugar (Mitsui DM Sugar Co., Ltd., Tokyo, Japan), 3 g skim milk (Megmilk Snow Brand Co., Ltd., Tokyo, Japan), 14 g canola oil (Showa Sangyo Co., Ltd.), and 10 g water, were kneaded manually in a plastic bag to obtain a stable emulsion. The emulsion (46.8 g) was transferred to a vial and stored at 4 °C for 72 h to obtain a semi-solid. Two volumes (100 mL) of water to the solid were added, followed by dispersion using a vortex mixer, centrifugation at 2,000 × G. During this processing, the emulsion collapsed into oil- and aqueous-phase, and thus the insoluble precipitate of aggregates was collected by discarding the liquid phases. This procedure was repeated three times. Next, the precipitate was air-dried on a tray, crushed in a mortar, and passed through a 60-mesh sieve.
XRD analysis. XRD was performed using an X-ray diffractometer (MiniFlex600-C; Rigaku Co., Tokyo, Japan). The X-ray source was CuKα, and an incident slit of 1.25° and longitudinal slit of 10 mm were used. The scanning speed was set at 10°/min and the scan range was 3°-35°.
Chain-length distribution analysis using capillary electrophoresis. CL distribution analysis of SLMD and the prepared SLMD aggregates was performed using fluorophore-assisted capillary electrophoresis (FACE) with a P/ACE MDQ Carbohydrate System (Sciex LLC, Framingham, MA, USA), according to previously described methods [11, 12].
Atsushi Kawano, Tomohiro Yamamoto, Yuya Shinagawa, and Hironori Yoshida are employees of Showa Sangyo Co., Ltd.
The authors thank Starch Technologies Co., Ltd. for the CL distribution analysis.
