Unraveling the Wide Variation in the Thermal Behavior of Crystalline

Article Cite This: Cryst. Growth Des. 2018, 18, 1070−1081

pubs.acs.org/crystal

Unraveling the Wide Variation in the Thermal Behavior of Crystalline Sucrose Using an Enhanced Laboratory Recrystallization Method Yingshuang Lu,† Danielle L. Gray,‡ Leilei Yin,§ Leonard C. Thomas,∥ and Shelly J. Schmidt*,⊥ †

Department of Food Science and Human Nutrition, University of Illinois at Urbana−Champaign, 399A Bevier Hall 905 South Goodwin Avenue, Urbana, Illinois 61801, United States ‡ George L. Clark X-Ray Facility & 3M Materials Laboratory, University of Illinois at Urbana−Champaign, 70 Noyes Laboratory, 505 South Mathews Avenue, Urbana, Illinois 61801, United States § Beckman Institute, Imaging Technology Group, University of Illinois at Urbana−Champaign, B604A Beckman Institute 405 North Mathews, Urbana, Illinois 61801, United States ∥ DSC Solutions, 27 East Braeburn Drive, Smyrna, Delaware 19977, United States ⊥ Department of Food Science and Human Nutrition, University of Illinois at Urbana−Champaign, 367 Bevier Hall, 905 South Goodwin Avenue, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: Recently, we found that sucrose from beet sources exhibited only one large endothermic DSC peak, whereas sucrose from most cane sources exhibited two peaks. Thus, our objective was to unravel the cause of this wide variation in thermal behavior by investigating both commercial and recrystallized sucrose samples, using a variety of analytical techniques, including DSC, HPLC, SXRD, and Micro-CT. With the aid of recrystallization method enhancements and compositional changes, sucrose crystals were intentionally altered to produce a variety of thermal behaviors, including DSC curves exhibiting one or two endothermic peaks or a single peak with either a low (144 °C) or a high (190 °C) Tmonset value. SXRD results for all sucrose crystals studied were consistent with the known structure of sucrose. Thus, polymorphism is not the cause of thermal behavior variation, but rather, the variation is attributed to the influence of occlusion composition and chemistry on thermal decomposition. Micro-CT supported this assertion by revealing the development of large cavities within the sucrose crystal during heat treatment when occlusion composition and chemistry was conducive to thermal decomposition (e.g., low ash content and pH), but showed impeded cavity formation when occlusions contained inhibitory attributes (e.g., high ash content, sulfite, or water removal via grinding).

1. INTRODUCTION The crystallization and melting behaviors of sucrose have been studied over a long period of time.1−3 During our investigation of a large number of sucrose samples (18 beet and 31 cane samples), a distinct difference was observed between the differential scanning calorimetry (DSC) curves of beet and cane sucrose sources at a 10 °C/min heating rate.4 In general, sucrose from beet sources exhibited only one large endothermic DSC peak with an average onset temperature (Tmonset) of 188.5(0.43) °C, whereas sucrose from most cane sources exhibited two endothermic DSC peaks, one small and one large peak, yielding average Tmonset values of 153.6(6) and 187.3(1.7) °C, respectively.4 Example DSC curves with labeled Tmonset values of each sugar source are given in Figure 1. Previous studies have also revealed that the appearance of the small endothermic DSC peak is associated with the formation of thermal decomposition components in cane sucrose sources, which lead to the more general assertion that the varied thermal © 2017 American Chemical Society

behavior of sucrose is connected to the composition and chemistry of the mother liquor occlusions trapped within the sucrose crystals, which are formed during the crystallization process.5,6 In the literature, the terms “occlusions” and “inclusions” are often used interchangeably; however, Harvey provides the following definitions of these terms: inclusions form by the potential interfering ions whose size and charge are similar to a lattice ion and may substitute into the lattice structure by chemical adsorption, provided that the interferent precipitates with the same crystal structure, whereas occlusions form when rapid precipitation traps a pocket of solution within the growing precipitate.7 Based on these definitions, we will use the term “occlusion” herein for our research findings, but when Received: November 1, 2017 Revised: December 9, 2017 Published: December 14, 2017 1070

DOI: 10.1021/acs.cgd.7b01526 Cryst. Growth Des. 2018, 18, 1070−1081

Crystal Growth & Design

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underwent preferential caramelization when the sucrose crystals were heated. Eastmond reviewing the work of Thomas and Williams, stated that results such as those of Thomas and Williams demonstrate that lattice imperfections are important as reaction sites.18 Vaccari investigated factors affecting the growth and resultant crystal structure of sucrose, including the presence of mother liquor occlusions.21 Trapping of mother liquor solution is related to the instability of the surface structure, which is due to the high growth rate of the various crystal faces. Rapid growth rates can be achieved through specific conditions of supersaturation, temperature, and stirring. In addition, boiling of the sugar solution during the crystallization process can also cause disturbances of the surface of the crystal. Boiling is usually used in traditional sugar refining to retain the conditions of supersaturation for high yield, but has adverse consequences on the quality of the resultant crystals. During boiling, vapor bubbles tend to form mainly on the crystal surface, thus promoting inclusion of mother liquor. This is explained by the rapid evaporation of the solution on the surface of the crystals resulting in a local increase in supersaturation, as well as a local increase of the growth rate and an increase of the surface instability. The formation of cavities promotes the trapping of solution. To avoid this phenomenon, it would be necessary to crystallize at low supersaturation (with very long times) and without boiling (cooling crystallization) conditions, which are in contrast with normal practices utilized in the sugar refining industry. The morphology of sucrose grown in aqueous solution has been studied by a number of researchers.21−32 When sucrose crystals are grown in a pure aqueous solution, there are 15 possible faces, 8 of which are considered the most important faces.33 The absence of some faces is because the faster growing faces become smaller and smaller until they disappear, whereas the slower growing faces gradually become larger and larger and the final external morphology is only composed of the slower growing faces.21 Thus, typically 8 faces of sucrose are seen because the other 7 faces are fast growing faces, which eventually disappear. The morphology of pure crystalline sucrose should always be the same regardless of plant source, since the molecular structure of the sucrose crystal is determined by physical constraints.24 However, crystalline sucrose obtained via the standard refining process is never totally pure, since other compounds, in addition to sucrose, are unintentionally extracted from the plant source during the refining process. In addition, compounds are also intentionally added to aid in the refining process, which are not able to be completely removed. These nonsucrose compounds (impurities) can be incorporated into the crystal lattice, affecting its morphology; they can also be incorporated into the mother liquor occlusions within the crystal lattice, affecting its composition and chemistry, and they can remain on the surface of the crystals after removal of excess mother liquor by centrifugation. For example, raffinose, a trisaccharide present in sugar beet, is an example of a plant source compound known to affect crystal morphology. The presence of raffinose, even in rather low concentrations, results in dramatic elongation of the b axis, yielding a crystal that appears long and thin (crystal image shown in Vaccari,21 Figure 4 therein). For the interested reader, Vaccari provides a detailed discussion regarding the effects of a variety of impurities on sucrose morphology.21

Figure 1. DSC curves of “as is” analytical grade Sigma cane, “as is” US beet, and “as is” US cane samples at a 10 °C/min heating rate.

describing the findings of other researchers we will use the terminology presented in their original article(s). For the interested reader, an animation of the entrapment of mother liquor in growing sugar crystals is available on the Nordic Sugar and YouTube Web sites.8 The presence and significance of mother liquor occlusions within crystals was recognized as far back as 1903 by Richards: “substances crystallizing from a solution enclose within their crystals small quantities of the mother-liquor” and that this entrapment was exceedingly common, “It is no careless exaggeration to state that in all my chemical experience I have never yet obtained crystals from any kind of solution entirely free from accidentally included mother-liquor; and, moreover, I have never found reason to believe that anyone else ever has”.9 In the specific case of sucrose, the presence of water inside the sucrose crystal, observed by light microscopy, was reported by Powers.3,10 Powers stated that the included water was “due to the “growing in” of the mother syrup by the layers during the growth of the crystal”.10 Powers also linked the amount of water in the crystal to the size of the crystal, with large crystals (approaching an inch in length) containing more water (0.1 to 0.4%) compared to smaller crystals (0.01 to 0.04%).10 It is interesting to note that Powers explained the wide variation in specific gravity and melting point values for sucrose given in the literature, 1.58 to 1.60 g/cm3 and 160 to 186 °C, respectively, to the presence of these water inclusions.10 Over the years, Powers continued to study various aspects of “included water” in sucrose crystals, including the mechanism of inclusion formation, identity of nonsucrose constituents held in the syrup inclusions, gaseous inclusions, as well as crystal growth and structure.11−15 In addition to Powers, a number of others researchers have studied mother liquor occlusions in sucrose, including Thomas and Williams,16 Mackintosh and White,17 Eastmond,18 Guo and White,19 Grimsey and Herrington,20 and Vaccari.21 Thomas and Williams studied the defects, or lattice imperfections, of crystalline sucrose using optical and electron microscopy.16 They found that water was present in dislocation cores within the sucrose crystal structure, which could be liberated upon heating and by mechanical means. Thomas and Williams demonstrated that prolonged heating at 120 °C under vacuum gave rise to “decomposition volcanoes” on the surface of the sucrose crystal, again likely situated at dislocation sites.16 They also noted that regions of higher imperfection density 1071



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lower final heating temperature (85 °C), no agitation, a closed system, and the use of centrifugation to remove surface mother liquor. The yield was much lower than in the literature recrystallization method; however, single crystals with greatly improved appearance were obtained. 2.2.2a. Preparation of Saturated Sucrose-HPLC Grade Water Solution. The reported saturation concentration values at 70 and 75 °C of sucrose are approximately 76 and 77.5 g per 100 g of solution, respectively.42 Saturated sucrose solutions were prepared by adding 19 g of analytical grade Sigma cane sucrose and 6 g of HPLC grade water into a 50 mL centrifuge tube. Sample tubes were then warmed to 85 °C using a water bath for about 1 h and occasionally shaken by hand to help dissolve the sucrose until no crystals remained. The temperature of the saturated solution was then allowed to drop spontaneously and continually in a 25 °C incubator to allow nucleation to occur in a closed system. Care was taken to avoid any shaking or unnecessary movement during crystallization to encourage homogeneous nucleation. After approximately 24 to 48 h, the newly grown, single crystals reached a desirable size (1 to 3.5 mm) and were ready to be harvested. 2.2.2b. Addition of Impurities. Sucrose recrystallization experiments were also carried out with the addition of impurities. In this study, either potassium sulfate (K2SO4) at 1 wt % (dry mass of impurities to sucrose) or potassium sulfite (K2SO3) at 0.5% was added to the Sigma cane sucrose prior to being dissolved in the HPLC grade water. It is important to note that these impurity levels were used so as to ensure adequate incorporation of sulfate or sulfite into the sucrose crystals during the crystallization process, not as a reflection of the levels used in processing of commercial sucrose. The saturated sucrose solution preparation and recrystallization procedures outlined in section 2.2.2a were then carried out. 2.2.2c. Harvest Crystals Using Centrifugal Filtration. The newly grown, single crystals, with the desired morphology, were carefully transferred from the 50 mL centrifuge tube using tweezers into the Vivaspin 20 mL centrifugal concentrator tube (Vivaproducts, Inc. Littleton, MA) and filtered, using centrifugal filtration (Eppendorf Centrifuge 5810R, Hamburg, Germany) at 3600 rpm for 25 min, to remove the mother liquor from the surface of the crystals. No wash water was used during centrifugal filtration. The harvested crystals with minimal mother liquor solution at the surfaces were then placed into a Petri dish, covered, and conditioned under ambient environmental conditions (20 to 30%RH, 20 to 25 °C) for 48 h. The laboratory recrystallized samples were then transferred to 15 mL glass vials, capped, and further sealed with parafilm for storage until analysis. The morphology information for commercial and laboratory recrystallized sucrose samples was recorded using a Leica M205C Microsystem (Leica, Heidelberg Germany), equipped with polarized light. The mass of a single crystal ranged from approximately 3 to 9 mg. 2.2.3. DSC Analysis and HPLC Sample Preparation. Thermal analysis, as well as sample preparation for HPLC analysis, was carried out using a TA Instruments Q2000 DSC (New Castle, DE), equipped with a refrigerated cooling system (RCS 90). The DSC was calibrated for enthalpy and temperature using a standard indium sample (Tmonset of 156.6 °C, ΔH of 28.71 J/g, TA Instruments, New Castle, DE) prior to sample scanning. Hermetic aluminum Tzero pans and lids (TA Instruments, New Castle, DE) were used for all calibration and sample measurements, including an empty pan as the reference. Dry nitrogen, at a flow rate of 50 mL/min, was used as the purge gas. For thermal analysis experiments, sucrose samples were equilibrated at 25 °C and then heated at a rate of 10 °C/min to 220 °C. A 3 mg sample mass was used for the commercial samples, whereas for the laboratory recrystallized Sigma samples, a single crystal was used, due to the relatively large mass of the crystals (3 to 9 mg). An end temperature of 220 °C was selected so as to ensure coverage of the entire endothermic peak for all samples tested. Thermal analysis experiments were conducted in duplicate. For sample preparation for HPLC analysis, selected sucrose samples (approximately 5.0 mg) were heated to predetermined target temperatures unique to each sucrose sample, based on previous research by Lu et al.5 HPLC analysis was also carried out on samples

A major difference in the refining of beet and cane sucrose sources is that beet sugar processing routinely includes a sulfitation step, whereas cane sugar refining usually does not.34,35 Among sugar cane processors worldwide, there is mixed interest in the use of sulfitation. In the United States, sulfitation has rarely been used in cane raw sugar factories since the 1950s.36 However, in China, cane sugar refineries routinely include sulfitation steps for juice clarification.37 The use of sulfitation in the refining of sugar beets results in the formation of sulfite and sulfate ions in the sugar juice,38 which, in turn, can be incorporated in the mother liquor occlusions within the beet sugar crystals. Thus, the composition and chemistry of the mother liquor occlusions are directly influenced by the plant source and sugar refining process employed. In the research herein, we hypothesized that the presence of the small endothermic peak in most “as is” crystalline cane sucrose DSC curves is associated with the onset of thermal decomposition of sucrose within mother liquor occlusions, initiated by hydrolysis and mediated by the composition and chemistry of the sucrose crystal.6 Thus, the objective of this study was to unravel the wide variation in the thermal behavior of crystalline sucrose by characterizing and comparing the physicochemical properties of the sucrose crystals from commercially available white refined beet and cane sources, as well as from our own enhanced laboratory recrystallization method. A variety of analytical techniques were applied to approach this research objective, including Differential Scanning Calorimetry (DSC), High Performance Liquid Chromatography (HPLC), Single Crystal X-ray Diffraction (SXRD), and X-ray Micro Computed Tomography (MicroCT) analyses.

2. MATERIALS AND METHODS 2.1. Materials. Analytical grade crystalline sucrose (#S0389; ≥99.5%) was purchased from Sigma-Aldrich Co. (St. Louis, MO). White refined beet (US beet) and white refined cane (US cane) samples were obtained directly from United Sugar (US) Corporation (Clewiston, FL). These commercially available sugars were tested “as is” without further purification. Potassium sulfite (K2SO3; ≥97%), potassium sulfate (K2SO4; ≥99%), potassium iodide (KI anhydrate; ≥99%), and the sugars (sucrose, glucose, and fructose) used as standards for HPLC analysis were purchased from Sigma-Aldrich Co. (St. Louis, MO). HPLC grade water (Fisher Scientific Inc., Pittsburgh, PA) was used for the preparation of standard and sample solutions. 2.2. Methods. 2.2.1. General Literature Recrystallization Method. Preliminary sucrose recrystallization experiments were carried out based on the general method reported in the literature, where a saturated solution is heated, crystallization is initiated by agitation, and the sample is cooled to room temperature.39,40 Saturated solution components, Sigma sucrose (100 g), 1 wt % of K2SO4 (dry mass of impurity to sucrose), and HPLC grade water (25 mL), were mixed and heated on a hot plate in a beaker to 128 °C. After this temperature was reached, the solution was removed from the heating source. At this point, the solution was vigorously hand-stirred for 30 s using a spatula to initiate crystallization. The temperature of the solution was allowed to drop to room temperature. The resultant crystal mass was placed over P2O5 and dried overnight before use. The appearance of the recrystallized sample was recorded using a Canon PowerShot Digital Camera. Samples were subjected to DSC analysis in both the unground and ground state. Grinding was carried out using a mortar and pestle. The ground crystals used passed through a No. 100 U.S.A. standard testing sieve with 100 mesh and 150 μm opening size. 2.2.2. Enhanced Laboratory Recrystallization Method. In order to produce large, single crystals of enhanced quality (fewer surface defects) a slow cooling crystallization method was employed.41 The enhancements to the method included a lower sucrose concentration, 1072



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Figure 2. a. Appearance of analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 using the general literature recrystallization method. b. Appearance of analytical grade Sigma cane sucrose recrystallized in HPLC water using the enhanced laboratory recrystallization method. c. Appearance of analytical grade Sigma cane sucrose recrystallized with 0.5% K2SO3 in HPLC water using the enhanced laboratory recrystallization method. d. Appearance of “as is” analytical grade Sigma cane sucrose. e. Appearance of “as is” commercial US beet sucrose. f. Appearance of “as is” commercial US cane sucrose. that received no heating (25 °C). The selected sucrose samples, with target temperatures list in parentheses after each sucrose sample, were “as is” analytical grade Sigma cane (150, 160, 200 °C), “as is” US beet (190, 200 °C), Sigma cane recrystallized in pure HPLC water using the enhanced laboratory recrystallization method (140, 150, 160 °C), and Sigma cane recrystallized with 0.5% K2SO3 in HPLC water using the enhanced laboratory recrystallization method (160, 170, 180, 190, 200 °C). Samples were equilibrated at 25 °C and then heated at a rate of 10 °C/min in the DSC. After reaching each target temperature (actual temperatures were approximately 1.5 °C lower than target temperatures due to thermal lag), the system was quickly equilibrated back to room temperature at a cooling rate of approximately 35 °C/ min. 2.2.4. HPLC Analysis. Approximately 5 mg of each sucrose sample (with [prepared in the DSC cell as described in section 2.2.3] and without heating) was dissolved into 100 mL of HPLC water and then transferred to a 2 mL screw thread robovial with silicone septa caps before injection (Fisher Scientific Inc., Pittsburgh, PA). Detection of sucrose and two initial thermal decomposition indicator components (glucose and fructose) was carried out based on AOAC Official Method.43 Carbohydrates were separated by anion exchange chromatography and detected by pulsed amperometric detection at a gold working electrode. A Dionex IC3000 HPLC equipped with a gradient pump, Dionex Carbopac PA1 guard, and analytical columns, as well as an electrochemical detector with disposable carbohydratecertified gold electrodes were used. A 150 mM solution of sodium hydroxide was used as the eluent at a flow rate of 1.0 mL/min. The temperature of the column was set at 30 °C. The flow rate was 1 mL/ min with 10% acetonitrile/0.1% acidified water solution. The water was acidified with 85% phosphoric acid. The limit of detection (LOD) for sucrose, glucose, and fructose was 0.5 ppm. The temperature at which the initial thermal decomposition component (glucose) was detected (Donset) was labeled on the corresponding DSC curve. HPLC analysis was carried out in duplicate for all samples. 2.2.5. Single Crystal X-ray Diffraction Measurements. For selected sucrose samples, unit cell parameters were determined for an individual, representative crystal both before and after heat treatment. This was done by collecting a short series of omega scans at room temperature using a Bruker D8 Venture Duo system (Mo Kα radiation), equipped with a four-circle κ-axis diffractometer and motorized Photon 100 CMOS detector. Data were harvested and the unit cells were indexed and refined using APEX II software (Bruker AXS, Inc., Madison, WI). The unit cell parameters were then compared to sucrose parameters contained in the Cambridge Crystallographic Data Centre (CCDC), specifically Brown and Levy

(refcode SUCROS).44 Each crystal used for SXRD unit cell collection was selected using a Leica M205C Microsystem (Leica, Heidelberg Germany) under polarized light and morphology information for each sample was recorded. To additionally confirm that the small differences in unit cell parameters from sucrose source to sucrose source had little effect on the known bulk structure, a full crystal structure determination was performed on a representative crystal of each sucrose source prior to heat treatment. Intensity data for the full crystal structures were collected on either a Bruker Siemens Apex II platform diffractometer (used for analytical grade Sigma cane, US Beet, and US Cane samples) or a Burker D8 Venture Duo system (used for Sigma recrystallized in HPLC water (Lab Method) and Sigma recrystallized with 0.5% K2SO3 (Lab Method) samples). The collection, cell refinement, and integration of intensity data were carried out with the APEX2 software (Bruker AXS, Inc., Madison, Wisconsin, USA). Multiscan absorption corrections were performed numerically with SADABS.45 All structures were refined with the full-matrix least-squares SHELXL program.46 2.2.6. Micro-CT Measurements. X-ray Micro Computed Tomography (Micro-CT) was used to produce 3D images to nondestructively and noninvasively reveal the internal structure of the crystal samples.47 The X-ray microscope takes multiple projection images at different viewing angles to provide the original 2D images. A computer then utilizes these 2D projection images to reconstruct 3D volumetric data to reveal the internal structure. The Xradia Bio Micro-CT (MicroXCT-400), utilized in this study, is a high-resolution 3D Xray imaging system, which is optimized for nondestructive imaging of complex internal structures. Voxel size and optical magnification of CT scans were selected based on the crystal size. X-ray voltage was set at 40 kV and a total of 901 projection images were collected over 360° angle for each sample scan. The number of actual sample images, however, may vary depending on the original size and geometry of the crystal. Preliminary experiments, using the MicroXCT-400, were carried out to collect images of recrystallized analytical grade Sigma cane sucrose grown in saturated sucrose solution, in order to differentiate the solid crystalline phase from the surrounding saturated sucrose solution. In addition, in order to visualize the mother liquor occlusions entrapped in the sucrose crystal, 10% KI (weight of dry matter), which has much higher X-ray attenuation compared to crystalline sucrose and serves as a contrast agent for Micro-CT scan, was recrystallized with the analytical grade Sigma cane sucrose using our enhanced laboratory recrystallization protocol outlined previously. For Micro-CT analysis, “as is” Sigma cane, “as is” US beet, “as is” US cane, Sigma cane recrystallized in HPLC grade water (Lab Method), 1073



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and Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) crystals were scanned prior to heat treatment, using Micro-CT. Then each crystal was heated to 165 °C at 10 °C/min using the DSC, except for the Sigma cane recrystallized in HPLC grade water sample, which was heated to a 140 °C end temperature, due to its lower DSC Tmonset value. After heating (approximately 1.5 °C lower than target temperature), the system was quick-cooled, equilibrated back to room temperature at a cooling rate of approximated 35 °C/min. Then the same crystal was rescanned using Micro-CT under the same experimental conditions, in order to observe any changes occurring inside the crystal due to the heating process. Image analysis and reconstruction were carried out using FEI Avizo visualization and analysis software (version 9.0.1, Visualization Sciences Group, Mérignac Cedex, France). The percent porosity (porosity%) values were obtained by analyzing the 2D images of each sucrose sample, both before and after heat treatment. The porosity% value, which is the ratio of the pore volume to the total volume of sample, was then calculated using eq 1

Porosity% = (Vp/Vt ) × 100%

commercial sucrose crystals as compared to our laboratory recrystallized sucrose crystals. Another interesting observation is that the white refined beet sugar sample was consistently shinier, when visually examined, than the white refined cane sucrose samples. Generally, a dull appearance relates to defects within the crystalline structure of a material.48 Though our laboratory recrystallized sucrose was single, large crystals with fewer apparent defects, the growth rate was slow (24 to 48 h) and the yield was low. Generally, a faster growth rate results in rougher crystal surfaces and deeper cavities.21 A low yield was predictable based on the enhancements employed. The degree of supersaturation was low, which means the driving force for crystallization is low. A closed container was used, forcing crystallization to occur without water evaporation; thus, the maximum amount of crystals that could grow was dependent on the difference in solubility between the starting and ending temperatures of crystallization.30 However, by applying our enhanced laboratory recrystallization method, we did not obtain the large mass of agglomerated crystals as was obtained when using the general literature recrystallization method,39,40 nor did we harvest the large conglomerates, which represent a collection of crystals joined together randomly as the recrystallized sucrose, obtained by Lee and Chang.49 Instead, we were able to grow and harvest large, single crystals with fewer surface defects. In addition to the growing conditions, the enhanced morphology and quality of our laboratory recrystallized sucrose samples was also dependent on the centrifuge conditions applied in this study. It is known that in both beet and cane sugar refining, the crystals in massecuite are separated from the surrounding syrup or molasses by centrifugal machines.34,38,50 In sugar refineries, water or dilute mother liquor is sometimes used during centrifugation to rinse the crystals, improving sugar quality.38 In order to mimic the procedure used in sugar refineries, the newly grown crystals, in this study, were transferred into centrifugal concentrator tubes and filtered, using centrifugal filtration, at 3600 rpm for 25 min. However, to avoid possible dissolution effects of using a rinse, no rinsing step was included. The centrifuge conditions used herein, selected to obtain better separation effects, were at a higher speed and longer time than those typically used in the sugar industry (1000 rpm [standard speed] or 1600 to 2200 rpm [high speed] for 10 min). In addition, our recrystallized sucrose crystals were harvested once they grew to relatively large sizes, resulting in less surface area for mother liquor contact as compared to smaller crystals. These conditions resulted in purging the mother liquor from the surface of the crystals with greater ease in the centrifugal apparatus and better purging efficiency.50 3.2. DSC Analysis. The DSC curves of “as is” analytical grade Sigma cane sucrose, recrystallized Sigma cane sucrose with 1% K2SO4 using the general literature recrystallization method,39,40 unground and ground, as well as recrystallized Sigma cane sucrose with 1% K2SO4 using our enhanced laboratory recrystallization method are plotted in Figure 3. As can be observed from Figure 3, the small endothermic DSC peak was not inhibited by addition of 1% K2SO4, as reported by others,39,40 using either the literature method or our own laboratory recrystallization method, unless a grinding step was added prior to scanning in the DSC. Beckett et al. reported using a grinding step prior to DSC measurement,39 which likely explains their conclusion that addition of 1% K2SO4 is responsible for the absence of the small endothermic DSC

(1)

where, in the study herein, Vp is the volume of void space (gas filled cavities) within the crystal and Vt is the total volume of crystal, including the void space. In addition, for enhanced visualization, the 3D structure of each sucrose crystal, before and after heating treatment, was reconstructed from the 2D images using the Avizo software volume rendering functions. In the 3D images, blue coloring was used for the matrix, which represents the bulk portion of the sucrose crystal lattice, and red coloring was used for the void space (gas filled cavities) observed in sucrose samples, both before and after heat treatment.

3. RESULTS AND DISCUSSION 3.1. Sucrose Recrystallization. The appearance of analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 according to the literature recrystallization method,39,40 is recorded in Figure 2a. Instead of harvesting a single crystal exhibiting classic morphology as discussed by Vavrinecz,33 the literature recrystallization method produced large masses of agglomerated crystals (Figure 2a). As a result, the literature recrystallized sample needed to be cut into small pieces, in order to seal into the DSC pans for thermal analysis and HLPC sample preparation. In contrast, Beckett et al. ground their recrystallized samples,39 instead of cutting them, before placing them into the DSC pan for analysis, the implications of which are discussed in detail in section 3.2. The appearance of the crystals produced using our enhanced laboratory crystallization method, analytical grade Sigma cane recrystallized in HPLC water and analytical grade Sigma cane recrystallized with 0.5% of K2SO3, is shown in Figure 2b and c, respectively. By applying our enhanced laboratory recrystallization methodlower sucrose concentration, lower final heating temperature (85 °C compared to 128 °C), no agitation, a closed system, and the use of centrifugation to remove surface mother liquorlarge size (approximately 1 to 3.5 mm), single crystals with fewer surface defects were obtained. The appearance of three commercially available sucrose samples (“as is” analytical grade Sigma cane, “as is” US beet, and “as is” US cane) are shown in Figure 2d to f, respectively. As can be observed, the analytical grade Sigma cane (Figure 2d) is somewhat larger in size (0.9 to 1.2 mm) compared to the US beet (Figure 2e) and US cane (Figure 2f) sucrose crystals, which are screened during commercial processing to yield an average particle size of approximately 0.40 to 0.60 mm for regular granulated sugar (also called table sugar). In addition, more surface defects (cracks, twinning) were observed in the 1074



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Figure 3. DSC curves of “as is” analytical grade Sigma cane sucrose, analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 using the general literature recrystallization method (unground and ground), and analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 using the enhanced laboratory recrystallization method at a 10 °C/min heating rate.

Figure 4. DSC curves of “as is” Sigma cane, “as is” US beet, Sigma cane recrystallized in HPLC water using the enhanced laboratory recrystallization method, and Sigma cane recrystallized with 0.5% K2SO3 using the enhanced laboratory recrystallization method at a 10 °C/min heating rate, labeled with the DSC Tmonset temperature and the temperature at which the earliest thermal decomposition component (Donset: glucose) was detected using HPLC.

peak around 150 °C. As reported by Lu et al., physical grinding alone, without the addition of impurities (e.g., K2SO4), was sufficient to cause the small endothermic DSC peak in analytical grade Sigma and white refined US cane sucrose samples to disappear.6 Beckett et al. mainly attributed the appearance of the peak at 150 °C to impurities in the sucrose, especially the mineral salt content;39 however, they did not consider the impact of sample grinding and associated loss of water on the presence of the small peak. However, grinding of the crystals before analysis extends beyond an inert sample preparation step, as stated by Richards, “It is usually considered as a sufficient precaution to powder the material finely and expose it to the air for a short time, in order to allow the undesirable included water to evaporate”.9 By repeating the K2SO4 impurity recrystallization work done by Beckett,39 with and without grinding the sample before DSC measurement, we were able to differentiate which aspect (impurities versus grinding) had a significant impact on the presence of the small endothermic DSC peak around 150 °C. As shown in Figure 3, analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 (Lit. Method) without grinding resulted in an even larger small DSC peak compared to “as is” analytical grade Sigma cane sucrose. However, after grinding, the small DSC peak in analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 (Lit. Method) was completely eliminated. This result is mainly attributable to the mechanical disruption of the mother liquor occlusions distributed throughout the sucrose crystal, allowing the water to evaporate and, thus, inhibiting the thermal induced hydrolysis process that would have occurred during heating if the water were present.6 Based on these results, it was noted that K2SO4 is less reactive than the related salt K2SO3. Thus, the laboratory recrystallization method was also carried out using K2SO3. The DSC curves of “as is” analytical grade Sigma cane sucrose, “as is” US beet sucrose, Sigma cane sucrose recrystallized in HPLC water (Lab Method), and Sigma cane sucrose recrystallized with 0.5% K2SO3 (Lab method) are plotted in Figure 4. A somewhat surprising result, shown in

Figure 4, was that by recrystallizing analytical grade Sigma cane sucrose in HPLC grade water using the enhanced laboratory recrystallization method, the large endothermic peak typically observed in white refined cane sucrose (e.g., Sigma cane “as is” curve in Figure 4, with a small peak Tmonset of approximately 151 °C and a large peak Tmonset of approximately 188 °C) completely disappeared, leaving only one, comparably large, endothermic DSC peak with a Tmonset of approximately 144 °C. At the other extreme, recrystallization of Sigma cane sucrose with the addition of 0.5% potassium sulfite (K2SO3), using our enhanced laboratory recrystallized method, resulted in elimination of the small endothermic DSC peak; thus, only the large endothermic DSC peak was present, with a Tmonset of approximately 190 °C. As expected, the “as is” US beet sucrose exhibited one large peak with a Tmonset of approximately 188 °C, in agreement with Lu et al.4,6 In general, for most crystalline materials, the presence of even a small quantity of impurities will lower the melting point by a few degrees and broaden the melting transition temperature range. Because impurities cause defects in the crystalline lattice, it is easier to overcome the intermolecular interactions between the molecules, and consequently, a lower temperature is required for melting in the presence of impurities.48 Interestingly, in this study, analytical grade Sigma cane sucrose recrystallized in very pure HPLC grade water, due to the partitioning effect, the newly grown crystals have fewer impurities as compared to the “as is” analytical grade Sigma crystals, but exhibited the lowest Tmonset value (144 °C), whereas Sigma recrystallized with impurities (0.5% K2SO3) exhibited the highest Tmonset value (190 °C). 3.3. HPLC Analysis. HPLC analysis indicates that the initial thermal decomposition component, glucose, was first detected at 160 °C for “as is” analytical grade Sigma cane, 200 °C for “as is” US beet, 150 °C for recrystallized Sigma cane in HPLC water (Lab Method), and 200 °C for recrystallized Sigma cane with 0.5% K2SO3 (Lab Method), respectively, as prepared in the DSC at a heating rate of 10 °C/min (Figure 4). Overall, the 1075



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Table 1. Unit Cell Parameters of Selected Beet and Cane Sucrose Samples Obtained Using Single Crystal X-ray Diffractiona sample ID

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

volume (Å3)

space group

temp (°C)

Sucrose Referenceb Sigma cane “as is” Sigma cane 165 °Cc US beet “as is” US beet 165 °Cc US cane “as is” US cane 165 °Cc Sigma rec. in HPLC water Sigma rec. in HPLC water 140 °Cc Sigma rec. w/0.5% K2SO3 Sigma rec. w/0.5% K2SO3 165 °Cc

7.7585(4) 7.763(3) 7.770(4) 7.766(6) 7.752(3) 7.741(3) 7.749(4) 7.740(6) 7.741(5) 7.7667(14) 7.7595 (10)

8.7050(4) 8.703(4) 8.689(5) 8.690(7) 8.692(3) 8.686(4) 8.700(4) 8.668(7) 8.685(6) 8.7026(11) 8.7092(8)

10.8633(5) 10.858 (6) 10.878(6) 10.848(8) 10.844(5) 10.834(5) 10.861(6) 10.829(8) 10.821(9) 10.850(2) 10.856(2)

90 90 90 90 90 90 90 90 90 90 90

102.945 103.042(19) 103.072(20) 103.1(4) 103.019(17) 102.887(17) 103.01(2) 103.01(2) 103.05(3) 102.839(16) 103.046(15)

90 90 90 90 90 90 90 90 90 90 90

715.04 714.6(9) 715.3(1.1) 713.3(14) 711.9(7) 710.1(9) 713.3(1.0) 707.90(15) 708.6(1.5) 715.0(3) 714.7(2)

P21 P21 P21 P21 P21 P21 P21 P21 P21 P21 P21

22.5 ± 1.5 23.5 23.5 23.5 23.5 23.5 23.5 23.5 23.5 23.2 23.2

a

Each parameter was reported as an average value (standard deviation). Recrystallized (rec.) samples were obtained using the enhanced laboratory method. bUnit cell parameters of sucrose reported by Brown and Levy44 and recorded in the Cambridge Crystallographic Data Centre. cHeat treated by scanning in the DSC at 10 °C/min to target temperatures (either 165 or 140 °C).

Table 2. Crystallographic and Metrical Details of Full Structure Refinements Crystal CCDC Accession Code Empirical formula Formula weight Temperature Wavelength Space group Unit Cell Dimensions

Volume Z Density (calculated) Absorption coefficient F(000) Theta range for data collection Index ranges

Reflections collected Independent reflections Completeness to theta Absorption correction Max. and min transmission Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole

Sigma cane “as is”

US Beet “as is”

US Cane “as is”

Sigma rec. in HPLC water

Sigma rec. w/0.5% K2SO3

1473968 C12H22O11 342.30 182(2) K 0.71073 Å P21 a = 7.7277(10) Å b = 8.6776(11) Å c = 10.8341(13) Å β = 102.9640(10)° 707.99(15) Å3 2 1.606 Mg/m3 0.144 mm−1 364 1.93 to 26.32°





−9 ≤ h ≤ 9 −10 ≤ k ≤ 10 −13 ≤ l ≤ 13 8132 2882 [R(int) = 0.0319] 26.32°, 99.9% Integration

−9 ≤ h ≤ 9 −10 ≤ k ≤ 10 −13 ≤ l ≤ 13 2964 2954 [R(int) = twinned]

−11 ≤ h ≤ 11 −12 ≤ k ≤ 12 −15 ≤ l ≤ 15 19351 4314 [R(int) = 0.0330] 30.52°, 99.7% Integration

−12 ≤ h ≤ 12 −14 ≤ k ≤ 14 −18 ≤ l ≤ 18 43698 6845 [R(int) = 0.0254]

−10 ≤ h ≤ 10 −11 ≤ k ≤ 11 −14 ≤ l ≤ 14 21920 3512 [R(int) = 0.0330]

25.242°, 99.8% Semiempirical from equivalents 0.7471 and 0.7120 6845/1/274 1.105 R1 = 0.0226 wR2 = 0.0608 R1 = 0.0228 wR2 = 0.0610 N/A 0.298 and −0.368 e.Å−3

25.242°, 99.9% Semiempirical from equivalents 1.0000 and 0.9624 3512/1/275 1.078 R1 = 0.0261 wR2 = 0.0603 R1 = 0.0277 wR2 = 0.0615 0.103(6) 0.302 and −0.222 e.Å−3

0.9985 and 0.9038 2882/1/274 1.039 R1 = 0.0290 wR2 = 0.0681 R1 = 0.0306 wR2 = 0.0690 N/A 0.164 and −0.175 e.Å−3

26.52°, 99.6% Semiempirical from equivalents 0.7454 and 0.3588 2954/1/274 0.918 R1 = 0.0343 wR2 = 0.0603 R1 = 0.0435 wR2 = 0.0631 N/A 0.174 and −0.196 e.Å−3

0.9796 and 0.9647 4314/1/233 1.029 R1 = 0.0292 wR2 = 0.0758 R1 = 0.0313 wR2 = 0.0771 N/A 0.342 and −0.199 e.Å−3

Supporting Information. Comparison of the thermal decomposition HPLC data for analytical grade Sigma cane, US cane, and US beet was previously reported by Lu et al.5 This variation in thermal behavior of each sucrose sample can be attributed to the difference in the composition and chemistry of the mother liquor occlusions within the sucrose

temperature at which the initial thermal decomposition component (glucose) was detected (Donset) for each of these sucrose samples was close to its own DSC Tmonset value, 151, 188, 144, and 190 °C, respectively (as listed above and shown in Figure 4). All HPLC data, including % sucrose, % glucose, and % fructose, are provided in Table S1 in the 1076



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Figure 5. Reconstructed 3D Micro-CT images of “as is” Sigma cane, “as is” US beet, “as is” US cane, analytical grade Sigma recrystallized with HPLC grade water (Lab Method), and analytical grade Sigma recrystallized with 0.5% K2SO3 (Lab Method) crystals, before and after heat treatment, along with their calculated percent porosity (porosity%) values. Heat treatment consisted of heating the individual crystals in the DSC at 10 °C/min to the target temperature (either 140 or 165 °C) and then quick cooling to room temperature (RT) at approximately 35 °C/min. The blue colored matrix represents the bulk portion of the sucrose crystal lattice and the red spots are the gas filled cavities observed in sucrose samples before and after heat 1077



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Figure 5. continued treatment. It is important to note that the images may have slightly different orientations for the same crystal before and after heat treatment; however, these differences do not affect the obtained porosity% values, since these values were obtained using the 2D images, as discussed in the Materials and Methods section.

crystal structure.6 In the United States, an important difference between white refined beet and cane sugar processing is that beet sugar processing routinely includes a sulfitation step, whereas cane sugar processing does not.34,35 Sulfitation has rarely been used in cane sugar factories since the 1950s.36 As mentioned in the Introduction, the gaseous SO2, used in the sulfitation step, is converted to sulfite or sulfate ions after dissolving in the aqueous sugar beet juice solution. The thermal decomposition resistance in commercial beet sugar (US beet) and laboratory recrystallized sucrose with addition of 0.5% K2SO3 (Lab Method) is hypothesized to be the result of the residual sulfite in the mother liquor occlusions, measured by Lu et al., using a total sulfite assay, microplate format (Megazyme, Wicklow, Ireland), as 11.16 ± 4.85 ppm in “as is” US beet and 12.93 ± 3.61 ppm in 0.5% K2SO3 (Lab Method) samples.6 One possible hypothesized mechanism to explain the influence of sulfite on the thermal behavior of sucrose is that sulfite ions can react with the carbonyl groups in reducing sugars to form bisulfite adducts, which, based on research carried out by Shi,51 can suppress the thermal decomposition of monosaccharides. However, additional research is needed in order to fully investigate the suppression mechanism. Thus, in the case of our Sigma cane recrystallized with 0.5% K2SO3 (Lab Method), the addition of sulfite helps to inhibit thermal induced hydrolysis in the mother liquor occlusions, thus enhancing the thermal stability of the sucrose crystal. 3.4. Single Crystal X-ray Diffraction. The room temperature unit cell parameters for all sucrose crystals studied herein (commercial, recrystallized with and without K2SO3, before, and after heat treatment), determined using SXRD, are provided in Table 1 and are consistent with the known unit cell parameters of sucrose reported by Brown and Levy in 1973.44 The full crystal structures of “as is” analytical grade Sigma cane, “as is” US beet, “as is” US cane, Sigma cane recrystallized in HPLC grade water (Lab Method), and Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) were collected using SXRD, refined, and deposited to the CCDC database (CCDC 1473968, 1473969, 1473970, 1578547, and 1578548, respectively). Table 2 contains the crystallographic and metrical details of the full structure refinements for the five sucrose samples prior to heat treatment. It is important to note that the unit cell parameters in Table 1 were determined using short SXRD data collection runs before and after heating of each specific crystal that was examined using Micro-CT to confirm that there was little to no crystallographic change in the bulk structure. Though there is a small endothermic peak present in both ”as is” Sigma cane and “as is” US cane DSC curves (as shown in Figure 1), the average structure of those samples, studied herein, show no evidence to support the previously proposed “metastable sucrose polymorphs” theory used to explain the presence of the small endothermic DSC peak.49 According to this theory, the “metastable sucrose polymorphs” (which melt around 150 to 160 °C, as compared to the high melting thermodynamically stable form, melting around 185 °C) are explained by the conformational disorder about the −CH2− OH functional groups of the fructofuranose ring that results in

the misalignment of intramolecular hydrogen bonds between the hydroxyl groups and the glucopyranose ring oxygen.49 This theory, however, does not explain the cause of the small endotherm DSC peak observed in most cane sucrose samples. It is known that XRD provides the best structural evidence for polymorphism. The sucrose crystals examined herein have unit cells consistent with the known unit cell of sucrose; therefore, the appearance of the small peak in the DSC cane sucrose curves, including the one for our Sigma cane recrystallized in HPLC water (Lab Method) sample, is not attributable to a new polymorph as suggested by Okuno et al.,52 Lee and Lin,53 and Lee and Chang.49 A search of the literature did yield a highpressure polymorph of sucrose, sucrose II, formed at a critical pressure of 4.80 GPa at 295 K.54 However, sucrose II is not stable at ambient conditions. It is important to note that the sucrose crystals grown and studied by Lee and Chang were prepared by adding different types of alcohols (methanol, furfuryl, or tetrahydrofuryl) at 60 °C all at once into saturated aqueous sucrose solutions,49 a different crystallization method than the one employed herein. 3.5. Micro-CT. A preliminary 2D Micro-CT image of newly grown Sigma sucrose crystals surrounded by saturated sucrose mother liquor solution is shown in Figure S1. Based on the lack of differences in gray scale color in Figure S1, the sucrose crystals can hardly be differentiated from the surrounding saturated mother liquor solution, reflective of similar material densities. Thus, without using a contrast agent, it is not readily feasible to distinguish the mother liquor occlusion from the crystal lattice using Micro-CT scanning. A trapped air bubble in the solution, however, is clearly distinguishable, as it exhibits a much darker color (lower X-ray attenuation, lower density) than the surrounding saturated sucrose solution and crystal lattice. In order to attempt to visualize the mother liquor occlusions entrapped within the sucrose crystals, 10% KI (weight of dry matter), which has much higher X-ray attenuation compared to crystalline sucrose, was added to the mother liquor as a contrast agent during Sigma sucrose recrystallization. Interestingly, based on the contrast differences, we were able to observe the high attenuation KI (bright spot in 2D image and yellow dot in 3D volume rendering) entrapped in the sucrose crystalline solid (Figure S2). Therefore, compared to the traditional visualizing method by addition of colored substances during sucrose crystallization,10,21 this study successfully developed a new method using a contrast agent (KI), during sucrose crystallization to further prove the existence of mother liquor occlusion within sucrose crystal by Micro-CT scanning. The reconstructed 3D Micro-CT images of each sucrose sample, before and after heat treatment, along with their calculated percent porosity (porosity%) values are shown in Figure 5. The blue matrix represents the bulk portion of the sucrose crystal lattice and the red spots are the gas filled cavities observed in sucrose samples before and after heat (165 or 140 °C) treatment. Based on the Micro-CT verification study demonstrated in Figure S1, the entrapped mother liquor occlusions (viscous liquid) could not be clearly differentiated from the crystal lattice (solid), due to their similar densities. 1078



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Information) and the region of CT scanning cannot be focused on exactly the same location after heating as before heating. It is also possible that heating the crystal below its melting point (without decomposition) resulted in annealing (also termed sintering), that is, healing of some of the crystal defects, thus resulting in a lower after heat treatment porosity. Another important point that needs to be discussed is the difference in the mechanism of cavity formation in sucrose crystals before (during crystal growth) and after heating. Powers was the first to report observing both mother liquor and gaseous inclusions in sucrose crystals under the microscope.3,10 By observing select crystal specimens in the act of dissolving, Powers reported that “when an inclusion of syrup is breached, the heavy syrup may be seen streaming downward, whereas when a gaseous inclusion is breached a bubble may be seen to strain like a balloon, and then to break away and rapidly rise to the surface”.10 As to the origin of the bubbles, Powers states: “The probable origin is that air dissolved in the original crystallizing syrup became supersaturated and formed as bubbles on the growing face. These were then overgrown by the layers”.10 Gas bubble incorporation in growing crystals was also studied by Wilcox and Kuo,55 who mentioned the work of Powers.10,11 The theory of cavity formation within crystals and its related trapping of mother solution has been discussed and illustrated by Vaccari.21 In general, a faster crystal growth rate results in rougher surfaces, higher growth steps, and deeper cavities, consequently resulting in the progressive closing of the cavities and entrapment of mother liquor. In contrast to cavity formation during crystal growth, heatgenerated cavity formation (e.g., in Sigma and US cane heating to 165 °C) is associated with the presence of the small DSC peak and is attributed to thermal induced hydrolysis and subsequent thermal decomposition processes, within the mother liquor occlusions. The presence of these internal, heat-generated cavities, captured noninvasively in intact Sigma and US cane crystals heated to 165 °C for the first time by Micro-CT (Figure 5), may be connected to the research of Thomas and Williams.16 As previously observed by Thomas and Williams, water present in dislocation cores within the sucrose crystal structure could be liberated upon heating and by mechanical means; where heating gave rise to “decomposition volcanoes”16 that we hypothesize to be similar to the heat generated cavity formation observed herein. Rescanning of our heat-treated crystals in the DSC at 10 °C/ min, resulted in the detection of a small (ΔCp value of 0.037 J/ g) glass transition at 64 °C (midpoint) for Sigma cane, but not for US beet.5 The observation of the presence of amorphous content supports the hydrolysis hypothesis in Sigma cane and US cane samples. However, the occlusions alone are not sufficient to explain the presence of the small peak, since the US beet and Sigma cane recrystallized with 0.5% K2SO3 also contain mother liquor occlusions, but do not exhibit the small peak or form large numbers of cavity areas after heating to 165 °C. This result can be explained by the relatively high amount of sulfite in beet sources and Sigma recrystallized cane with 0.5% K2SO3,6 which is attributable to the sulfitation step used during the beet sugar refining process or the addition of K2SO3 during recrystallization. It is known that sulfite can inhibit browning reactions caused by ascorbic acid, lipid, Maillard, and enzymatic browning reactions.56 In the literature, SO2 was reported to react with carbonyl groups in sugar molecule to form sugar bisulfite adduct, which suppressed the degradation of monosaccharides,51 and, thus, could inhibit the formation of

The small dark areas in the 2D images were, therefore, identified as internal gas filled cavities and rendered with red color in the 3D images for better visualization. The 3D MicroCT images, before and after heat treatment, are shown in Figure 5 and associated reconstructed 2D Micro-CT images in videos, SV1 through SV10, in the Supporting Information). Overall, “as is” analytical grade Sigma cane sucrose (before heat treatment) exhibited the smallest number of internal gas filled cavities, with a small porosity of 0.00588 ± 0.00002%, followed by “as is” US beet (0.020 ± 0.0032%) and “as is” US cane (0.075 ± 0.0730%) (Figure 5 and Videos SV1, 2, and 3, respectively, in the Supporting Information). The before heat treatment recrystallized (Lab Method) samples had somewhat higher porosity values compared to the commercial samples, with analytical grade Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) having a higher porosity (0.175 ± 0.0063%, Figure 5 and Video SV4 in the Supporting Information) than analytical grade Sigma cane recrystallized in HPLC grade water sucrose (Lab Method) (0.106 ± 0.0515% Figure 5 and Video SV5 in the Supporting Information). After being heated to 165 °C, a temperature 10 °C higher than the onset of the small endothermic DSC peak, the crystal was immediately cooled quickly back to room temperature and rescanned using Micro-CT. After heat treatment, analytical grade Sigma cane (2.67 ± 0.067% porosity) and US cane (2.15 ± 0.115% porosity) exhibited the formation of numerous internal cavities with large sizes (Figure 5 and Videos SV6, SV8 in the Supporting Information) compared to heat treated US beet (0.049 ± 0.0046% porosity) and Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) (0.169 ± 0.0154% porosity) sucrose crystals (Figure 5 and Videos SV7, SV9 in the Supporting Information). It is important to note that analytical grade Sigma cane and US cane crystals maintained their original external morphology, despite the numerous formation of cavities upon heating, similar to that of US beet and Sigma cane recrystallized with 0.5% K2SO3 (Lab Method), which did not form numerous cavities upon heating. In the case of analytical grade Sigma cane recrystallized in HPLC grade water (Lab Method), after heat treatment to 140 °C, which is close to its own Tmonset value (144 °C), it exhibited only slightly more cavities (0.133 ± 0.0197%, Figure 5 and Video SV10 in the Supporting Information) compared to before its heat treatment (0.106 ± 0.0515%). This modest increase in the number of new cavities upon heating for the analytical grade Sigma cane recrystallized in HPLC grade water (Lab Method) sample can be explained by the fact that the heat treatment temperature of 140 °C was below the Tmonset value, whereas in the case of “as is” analytical grade Sigma cane, the 165 °C heat treatment temperature was selected to be above the small endothermic DSC peak Tmonset value. The heat treatment temperature of 140 °C was selected to be below the Tmonset since the analytical grade Sigma cane recrystallized in HPLC grade water (Lab Method) sample exhibited only one DSC peak at a low Tmonset value (144 °C). A slight, nonstatistically different, decrease in porosity for Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) crystal after heating to 165 °C was observed. This slight decrease could be due to the large size of the recrystallized crystal, for which in order to maintain the high resolution of the image, only a portion of the crystal was able to be scanned. Therefore, it appears as a cylindrical shape after 3D reconstruction (Figure 5 and Videos SV4 [before heat treatment], SV8 [after heat treatment] in the Supporting 1079



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Accession Codes

large cavity areas due to thermal induced hydrolysis in sugar beet sources and recrystallized Sigma sucrose with addition of 0.5% K2SO3 as observed in our study.

CCDC 1473968−1473970 and 1578547−1578548 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

4. CONCLUSIONS Though sucrose is a very common crystalline material, its thermal behavior is quite complex. With the aid of laboratory recrystallization method enhancements, compositional alterations, and the use of various analytical techniques, including DSC, HPLC, SXRD, and Micro-CT, we assert that the varied thermal behavior of crystalline sucrose is due to the influence of mother liquor occlusion composition and chemistry on its thermal decomposition propensity, rather than due to polymorphism. Mother liquor occlusions with a composition and chemistry that are conducive to thermal decomposition (e.g., high purity [low ash/mineral content] and low pH) result in a DSC curve with a low Tmonset value, whereas occlusions with a composition and chemistry that contained inhibitory attributes (e.g., high ash/mineral content, sulfite, or water removal via grinding) result in DSC curves with a high Tmonset value. In general, crystalline materials are known to yield a constant melting temperature; however, this is not the case with crystalline sucrose as its thermal behavior is actually a complex combination of thermal decomposition, as influenced by mother liquor composition and chemistry, and melting.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +01-217-333-6369. ORCID

Danielle L. Gray: 0000-0003-0059-2096 Shelly J. Schmidt: 0000-0001-6801-0668 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the single X-ray diffraction and the Micro-CT instruments located at The George L. Clark X-ray Facility, School of Chemical Sciences and the Imaging Technology Group at the Beckman Institute, respectively, at the University of Illinois and Urbana− Champaign. In addition, the authors would like to thank Dr. Pawan Takhar, Professor of Food Engineering at the University of Illinois, for assistance with the porosity% analysis of the Micro-CT images and use of the FEI Avizo visualization and analysis software in his laboratory.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01526. HPLC analysis for heat treated and no heat treatment samples (Table S1), 2D Micro-CT image of newly grown Sigma sucrose crystals surrounded by saturated sucrose solution (Figure S1), Micro-CT scanned 2D image and 3D volume rendering of Sigma sucrose recrystallized with 10% KI (Figure S2) (PDF) Video of “as is” analytical grade Sigma cane sucrose, Micro-CT video scale bar = 500 μm (AVI) Video of “as is” US beet, Micro-CT video scale bar = 500 μm (AVI) Video of “as is” US cane, Micro-CT video scale bar = 500 μm (AVI) Video of analytical grade Sigma cane recrystallized with 0.5% K2SO3 (Lab Method), Micro-CT video scale bar = 250 μm (AVI) Video of analytical grade Sigma cane recrystallized in HPLC grade water sucrose (Lab Method), Micro-CT video scale bar = 500 μm (AVI) Video of the analytical grade Sigma cane heat treated to 165 °C, Micro-CT video scale bar = 500 μm (AVI) Video of the US beet heat treated to 165 °C, Micro-CT video scale bar = 500 μm (AVI) Video of the US cane heat treated to 165 °C, Micro-CT video scale bar = 500 μm (AVI) Video of the analytical grade Sigma cane recryrstallizezd in HPLC grade water (Lab Method) heat treated to 140 °C, Micro-CT video scale bar = 500 μm (AVI) Video of the analytical grade Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) heat treated to 165 °C, Micro-CT video scale bar = 250 μm (AVI)

ABBREVIATIONS DSC differential scanning calorimetry Tmonset onset melting temperature ΔH enthalpy of melting HPLC high performance liquid chromatography Donset temperature at which the initial thermal decomposition component (glucose) was detected SXRD single crystal X-ray diffraction CCDC Cambridge Crystallographic Data Centre Micro-CT X-ray Micro Computed Tomography 2D two-dimensional 3D three-dimensional

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REFERENCES

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