Unraveling the Wide Variation in the Thermal Behavior of Crystalline

Subscriber access provided by READING UNIV

Article

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 Schmidt Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01526 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 20, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Crystal Growth & Design is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

1 2 3

Unraveling the wide variation in the thermal behavior of crystalline sucrose using an enhanced laboratory recrystallization method

4

Yingshuang Lua, Danielle L. Grayb, Leilei Yinc, Leonard C. Thomas and Shelly Schmidte*

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

a

Department of Food Science and Human Nutrition University of Illinois at Urbana-Champaign 399A Bevier Hall 905 S Goodwin Ave Urbana, IL 61801 Phone: 217-974-5101 [email protected] b

George L. Clark X-Ray Facility & 3M Materials Laboratory University of Illinois at Urbana-Champaign 70 Noyes Laboratory 505 South Mathews Ave. Urbana, IL 61801 Phone: 217-244-1708 [email protected] c

Beckman Institute, Imaging Technology Group University of Illinois at Urbana-Champaign B604A Beckman Institute 405 North Mathews Urbana, IL 61801 Phone: 217-265-0875 [email protected] d

DSC Solutions 27 East Braeburn Drive Smyrna, DE 19977 Phone: 302 528-2838 [email protected] e

Department of Food Science and Human Nutrition University of Illinois at Urbana-Champaign 367 Bevier Hall 905 S Goodwin Ave Urbana, IL 61801 Phone: 217-333-6369 [email protected] *Corresponding author

ACS Paragon Plus Environment

1

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 49

43

ABSTRACT

44

Recently, we found that sucrose from beet sources exhibited only one large endothermic DSC

45

peak; whereas, sucrose from most cane sources exhibited two peaks. Thus, our objective was to

46

unravel the cause of this wide variation in thermal behavior by investigating both commercial

47

and recrystallized sucrose samples, using a variety of analytical techniques, including DSC, HPLC,

48

SXRD, Micro-CT. With the aid of recrystallization method enhancements and compositional

49

changes, sucrose crystals were intentionally altered to produce a variety of thermal behaviors,

50

including DSC curves exhibiting one or two endothermic peaks or a single peak with either a low

51

(144°C) or a high (190°C) Tmonset value. SXRD results for all sucrose crystals studied were

52

consistent with the known structure of sucrose. Thus, polymorphism is not the cause of thermal

53

behavior variation, but rather, the variation is attributed to the influence of occlusion

54

composition and chemistry on thermal decomposition. Micro-CT supported this assertion by

55

revealing the development of large cavities within the sucrose crystal during heat treatment

56

when occlusion composition and chemistry was conducive to thermal decomposition (e.g., low

57

ash content and pH), but showed impeded cavity formation when occlusions contained

58

inhibitory attributes (e.g., high ash content, sulfite or water removal via grinding).


2

Page 3 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

59


1. INTRODUCTION

60

The crystallization and melting behaviors of sucrose have been studied over a long period

61

of time.1-3 During our investigation of a large number of sucrose samples (18 beet and 31 cane

62

samples), a distinct difference was observed between the differential scanning calorimetry (DSC)

63

thermal curves of beet and cane sucrose sources at a 10°C/min heating rate.4 In general,

64

sucrose from beet sources exhibited only one large endothermic DSC peak with an average

65

onset temperature (Tmonset) of 188.45(0.43)°C; whereas, sucrose from most cane sources

66

exhibited two endothermic DSC peaks, one small and one large peak, yielding average Tmonset

67

values of 153.6(6) and 187.3(1.7)°C, respectively.4 Example DSC curves with labeled Tmonset

68

values of each sugar source are given in Figure 1. Previous studies have also revealed that the

69

appearance of the small endothermic DSC peak is associated with the formation of thermal

70

decomposition components in cane sucrose sources, which lead to the more general assertion

71

that the varied thermal behavior of sucrose is connected to the composition and chemistry of

72

the mother liquor occlusions trapped within the sucrose crystals, which are formed during the

73

crystallization process.5-6

74

In the literature, the terms “occlusions” and “inclusions” are often used interchangeably;

75

however, Harvey provides the following definitions of these terms: inclusions form by the

76

potential interfering ions whose size and charge are similar to a lattice ion and may substitute

77

into the lattice structure by chemical adsorption, provided that the interferent precipitates with

78

the same crystal structure; whereas, occlusions form when rapid precipitation traps a pocket of

79

solution within the growing precipitate.7 Based on these definitions, we will use the term

80

occlusion herein for our research findings, but when describing the findings of other


3


81

researchers we will use the terminology presented in their original article(s). For the interested

82

reader, an animation of the entrapment of mother liquor in growing sugar crystals is available

83

on the Nordic Sugar and YouTube websites.8

Page 4 of 49

84

The presence and significance of mother liquor occlusions within crystals was

85

recognized as far back as 1903 by Richards: “substances crystallizing from a solution enclose

86

within their crystals small quantities of the mother-liquor” and that this entrapment was

87

exceedingly common, “It is no careless exaggeration to state that in all my chemical experience

88

I have never yet obtained crystals from any kind of solution entirely free from accidentally

89

included mother-liquor; and, moreover, I have never found reason to believe that anyone else

90

ever has”.9 In the specific case of sucrose, the presence of water inside the sucrose crystal,

91

observed by light microscopy, was reported by Powers.3, 10 Powers stated that the included

92

water was “due to the ‘growing in’ of the mother syrup by the layers during the growth of the

93

crystal.”10 Powers also linked the amount of water in the crystal to the size of the crystal, with

94

large crystals (approaching an inch in length) containing more water (0.1 to 0.4%) compared to

95

smaller crystals (0.01 to 0.04%).10 It is interesting to note that Powers explained the wide

96

variation in specific gravity and melting point values for sucrose given in the literature, 1.58 to

97

1.60 g/cm3 and 160 to 186°C, respectively, to the presence of these water inclusions.10 Over the

98

years, Powers continued to study various aspects of “included water” in sucrose crystals,

99

including the mechanism of inclusion formation, identity of non-sucrose constituents held in

100

the syrup inclusions, gaseous inclusions, as well as crystal growth and structure (Powers 1959,

101

1960, 1962, 1963, and 1970).11-15 In addition to Powers, a number of others researchers have


4

Page 5 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


102

studied mother liquor occlusions in sucrose, including Thomas and Williams,16 Mackintosh and

103

White,17 Eastmond,18 Guo and White,19 Grimsey and Herrington,20 and Vaccari.21

104

Thomas and Williams studied the defects, or lattice imperfections, of crystalline sucrose

105

using optical and electron microscopy.16 They found that water was present in dislocation cores

106

within the sucrose crystal structure, which could be liberated upon heating and by mechanical

107

means. Thomas and Williams demonstrated that prolonged heating at 120°C under vacuum

108

gave rise to “decomposition volcanoes” on the surface of the sucrose crystal, again likely

109

situated at dislocation sites.16 They also noted that regions of higher imperfection density

110

underwent preferential caramelization when the sucrose crystals were heated. Eastmond

111

reviewing the work of Thomas and Williams, stated that results such as those of Thomas and

112

Williams demonstrate that lattice imperfections are important as reaction sites.18

113

Vaccari investigated factors affecting the growth and resultant crystal structure of sucrose,

114

including the presence of mother liquor occlusions.21 Trapping of mother liquor solution is

115

related to the instability of the surface structure, which is due to the high growth rate of the

116

various crystal faces. Rapid growth rates can be achieved through specific conditions of

117

supersaturation, temperature, and stirring. In addition, boiling of the sugar solution during the

118

crystallization process can also cause disturbances of the surface of the crystal. Boiling is usually

119

used in traditional sugar refining to retain the conditions of supersaturation for high yield, but

120

has adverse consequences on the quality of the resultant crystals. During boiling, vapor bubbles

121

tend to form mainly on the crystal surface, thus promoting inclusion of mother liquor. This is

122

explained by the rapid evaporation of the solution on the surface of the crystals resulting in a

123

local increase in supersaturation, as well as a local increase of the growth rate and an increase


5


Page 6 of 49

124

of the surface instability. The formation of cavities promotes the trapping of solution. To avoid

125

this phenomenon, it would be necessary to crystallize at low supersaturation (with very long

126

times) and without boiling (cooling crystallization) conditions, which are in contrast with normal

127

practices utilized in the sugar refining industry.

128

The morphology of sucrose grown in aqueous solution has been studied by a number of

129

researchers.21-32 When sucrose crystals are grown in a pure aqueous solution, there are 15

130

possible faces, 8 of which are considered the most important faces.33 The absence of some

131

faces is because the faster growing faces become smaller and smaller until they disappear;

132

whereas the slower growing faces gradually become larger and larger and the final external

133

morphology is only composed of the slower growing faces.21 Thus, typically 8 faces of sucrose

134

are seen because the other 7 faces are fast growing faces, which eventually disappear.

135

The morphology of pure crystalline sucrose should always be the same regardless of plant

136

source, since the molecular structure of the sucrose crystal is determined by physical

137

constraints.24 However, crystalline sucrose obtained via the standard refining process is never

138

totally pure, since other compounds, in addition to sucrose, are unintentionally extracted from

139

the plant source during the refining process. In addition, compounds are also intentionally

140

added to aid in the refining process, which are not able to be completely removed. These non-

141

sucrose compounds (impurities) can be incorporated into the crystal lattice, affecting its

142

morphology; they can also be incorporated into the mother liquor occlusions within the crystal

143

lattice, affecting its composition and chemistry; and they can remain on the surface of the

144

crystals after removal of excess mother liquor by centrifugation. For example, raffinose, a

145

trisaccharide present in sugarbeet, is an example of a plant source compound known to affect


6

Page 7 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


146

crystal morphology. The presence of raffinose, even in rather low concentrations, results in

147

dramatic elongation of the b axis, yielding a crystal that appears long and thin (crystal image

148

shown in Vaccari,21 Figure 4 therein). For the interested reader, Vaccari provides a detailed

149

discussion regarding the effects of a variety of impurities on sucrose morphology.21

150

A major difference in the refining of beet and cane sucrose sources is that beet sugar

151

processing routinely includes a sulfitation step, whereas cane sugar refining usually does not.34-

152

35

153

the United States, sulfitation has rarely been used in cane raw sugar factories since the

154

1950's.36 However, in China, cane sugar refineries routinely include sulfitation steps for juice

155

clarification.37 The use of sulfitation in the refining of sugarbeets results in the formation of

156

sulfite and sulfate ions in the sugar juice,38 which, in turn, can be incorporated in the mother

157

liquor occlusions within the beet sugar crystals. Thus, the composition and chemistry of the

158

mother liquor occlusions are directly influenced by the plant source and sugar refining process

159

employed.

160

Among sugar cane processors worldwide, there is mixed interest in the use of sulfitation. In

In the research herein, we hypothesized that the presence of the small endothermic peak

161

in most “as is” crystalline cane sucrose DSC curves is associated with the onset of thermal

162

decomposition of sucrose within mother liquor occlusions, initiated by hydrolysis and mediated

163

by the composition and chemistry of the sucrose crystal.6 Thus, the objective of this study was

164

to unravel the wide variation in the thermal behavior of crystalline sucrose by characterizing

165

and comparing the physicochemical properties of the sucrose crystals from commercially

166

available white refined beet and cane sources, as well as from our own enhanced laboratory

167

recrystallization method. A variety of analytical techniques were applied to approach this


7


168

research objective, including Differential Scanning Calorimetry (DSC), High Performance Liquid

169

Chromatography (HPLC), Single Crystal X-ray Diffraction (SXRD), and X-ray Micro Computed

170

Tomography (Micro-CT) analyses.

171

2. MATERIALS AND METHODS

172

2.1. Materials

173

Page 8 of 49

Analytical grade crystalline sucrose (#S0389; ≥ 99.5%) was purchased from Sigma-

174

Aldrich Co. (St. Louis, MO). White refined beet (US beet) and white refined cane (US cane)

175

samples were obtained directly from U.S. Sugar Corporation (Clewiston, FL). These

176

commercially available sugars were tested “as is” without further purification. Potassium sulfite

177

(K2SO3; ≥ 97%), potassium sulfate (K2SO4; ≥99%), potassium iodide (KI anhydrate; ≥ 99%), and

178

the sugars (sucrose, glucose, and fructose) used as standards for HPLC analysis were purchased

179

from Sigma-Aldrich Co. (St. Louis, MO). HPLC grade water (Fisher Scientific Inc., Pittsburgh, PA)

180

was used for the preparation of standard and sample solutions.

181

2.2. Methods

182

2.2.1. General literature recrystallization method. Preliminary sucrose recrystallization

183

experiments were carried out based on the general method reported in the literature, where a

184

saturated solution is heated, crystallization is initiated by agitation, and the sample is cooled to

185

room temperature.39-40 Saturated solution components, Sigma sucrose (100 g), 1 wt% of K2SO4

186

(dry mass of impurity to sucrose), and HPLC grade water (25 mL), were mixed and heated on a

187

hot plate in a beaker to 128°C. After this temperature was reached, the solution was removed

188

from the heating source. At this point, the solution was vigorously hand stirred for 30 sec using

189

a spatula to initiate crystallization. The temperature of the solution was allowed to drop to


8

Page 9 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


190

room temperature. The resultant crystal mass was placed over P2O5 and dried overnight before

191

use. The appearance of the recrystallized sample was recorded using a Canon PowerShot Digital

192

Camera. Samples were subjected to DSC analysis in both the unground and ground state.

193

Grinding was carried out using a mortar and pestle. The ground crystals used passed through a

194

No. 100 U.S.A. standard testing sieve with 100 mesh and 150 µm opening size.

195

2.2.2. Enhanced laboratory recrystallization method. In order to produce large, single

196

crystals of enhanced quality (less surface defects) a slow cooling crystallization method was

197

employed.41 The enhancements to the method included a lower sucrose concentration, lower

198

final heating temperature (85°C), no agitation, a closed system, and the use of centrifugation to

199

remove surface mother liquor. The yield was much lower than in the literature recrystallization

200

method; however, single crystals with greatly improved appearance were obtained.

201

2.2.2a. Preparation of saturated sucrose-HPLC grade water solution. The reported

202

saturation concentration values at 70°C and 75°C of sucrose are approximately 76g and 77.5g

203

per 100 g of solution, respectively.42 Saturated sucrose solutions were prepared by adding 19 g

204

of analytical grade Sigma cane sucrose and 6 g of HPLC grade water into a 50mL centrifuge tube.

205

Sample tubes were then warmed to 85°C using a water bath for about 1 h and occasionally

206

shaken by hand to help dissolve the sucrose until no crystals remained. The temperature of the

207

saturated solution was then allowed to drop spontaneously and continually in a 25°C incubator

208

to allow nucleation to occur in a closed system. Care was taken to avoid any shaking or

209

unnecessary movement during crystallization to encourage homogeneous nucleation. After

210

approximately 24 to 48 hours, the newly-grown, single crystals reached a desirable size (1 to 3.5

211

mm) and were ready to be harvested.


9


212

Page 10 of 49

2.2.2b. Addition of impurities. Sucrose recrystallization experiments were also carried out

213

with the addition of impurities. In this study, either potassium sulfate (K2SO4) at 1 wt% (dry

214

mass of impurities to sucrose) or potassium sulfite (K2SO3) at 0.5% was added to the Sigma cane

215

sucrose prior to being dissolved in the HPLC grade water. It is important to note that these

216

impurity levels were used so as to ensure adequate incorporation of sulfate or sulfite into the

217

sucrose crystals during the crystallization process, not as a reflection of the levels used in

218

processing of commercial sucrose. The saturated sucrose solution preparation and

219

recrystallization procedures outlined in 2.2.2a were then carried out.

220

2.2.2c. Harvest crystals using centrifugal filtration. The newly-grown, single crystals, with

221

the desired morphology, were carefully transferred from the 50 mL centrifuge tube using

222

tweezers into the Vivaspin® 20ml centrifugal concentrator tube (Vivaproducts, Inc. Littleton,

223

MA) and filtered, using centrifugal filtration (Eppendorf Centrifuge 5810R, Hamburg, Germany)

224

at 3600 rpm for 25 min, to remove mother liquor from the surface of the crystals. No wash

225

water was used during centrifugal filtration. The harvested crystals with minimal mother liquor

226

solution at the surfaces were then placed into a petri dish, covered, and conditioned under

227

ambient environmental conditions (20 to 30 %RH, 20 to 25°C) for 48 h. The laboratory

228

recrystallized samples were then transferred to 15mL glass vials, capped, and further sealed

229

with parafilm for storage until analysis. The morphology information for commercial and

230

laboratory recrystallized sucrose samples was recorded using a Leica M205C Microsystem (Leica,

231

Heidelberg Germany), equipped with polarized light. The mass of a single crystal ranged from

232

approximately 3 to 9 mg.

233

2.2.3. DSC analysis and HPLC sample preparation


10

Page 11 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

234


Thermal analysis, as well as sample preparation for HPLC analysis, was carried out using

235

a TA Instruments Q2000 DSC (New Castle, DE), equipped with a refrigerated cooling system

236

(RCS 90). The DSC was calibrated for enthalpy and temperature using a standard indium sample

237

(Tmonset of 156.6°C, ΔH of 28.71 J/g, TA Instruments, New Castle, DE) prior to sample scanning.

238

Hermetic aluminum Tzero pans and lids (TA Instruments, New Castle, DE) were used for all

239

calibration and sample measurements, including an empty pan as the reference. Dry nitrogen,

240

at a flow rate of 50 mL/min, was used as the purge gas.

241

For thermal analysis experiments, sucrose samples were equilibrated at 25°C and then

242

heated at a rate of 10°C/min to 220°C. A 3 mg sample mass was used for the commercial

243

samples; whereas, for the laboratory recrystallized Sigma samples, a single crystal was used,

244

due to the relatively large mass of the crystals (3 to 9 mg). An end temperature of 220°C was

245

selected so as to ensure coverage of the entire endothermic peak for all samples tested.

246

Thermal analysis experiments were conducted in duplicate.

247

For sample preparation for HPLC analysis, selected sucrose samples (approximately 5.0

248

mg) were heated to predetermined target temperatures unique to each sucrose sample, based

249

on previous research by Lu et al.5 HPLC analysis was also carried out on samples that received

250

no heating (25°C). The selected sucrose samples, with target temperatures list in parentheses

251

after each sucrose sample, were “as is” analytical grade Sigma cane (150, 160, 200°C), “as is” US

252

beet (190, 200°C), Sigma cane recrystallized in pure HPLC water using the enhanced laboratory

253

recrystallization method (140, 150, 160°C) , and Sigma cane recrystallized with 0.5% K2SO3 in

254

HPLC water using the enhanced laboratory recrystallization method (160, 170, 180, 190, 200°C).

255

Samples were equilibrated at 25°C and then heated at a rate of 10°C/min in the DSC. After


11


256

reaching each target temperature (actual temperatures were approximately 1.5°C lower than

257

target temperatures due to thermal lag), the system was quickly equilibrated back to room

258

temperature at a cooling rate of approximately 35°C/min.

259

2.2.4. HPLC analysis

260

Page 12 of 49

Approximately 5 mg of each sucrose sample (with [prepared in the DSC cell as described in

261

section 2.2.3] and without heating) was dissolved into 100 mL of HPLC water and then

262

transferred to a 2mL screw thread robovial with silicone septa caps before injection (Fisher

263

Scientific Inc., Pittsburgh, PA). Detection of sucrose and two initial thermal decomposition

264

indicator components (glucose and fructose) was carried out based on AOAC Official Method.43

265

Carbohydrates were separated by anion exchange chromatography and detected by pulsed

266

amperometric detection at a gold working electrode. A Dionex IC3000 HPLC equipped with a

267

gradient pump, Dionex Carbopac PA1 guard, and analytical columns, as well as an

268

electrochemical detector with disposable carbohydrate-certified gold electrodes was used. A

269

150mM solution of sodium hydroxide was used as the eluent at a flow rate of 1.0 mL/min. The

270

temperature of the column was set at 30°C. The flow rate was 1 mL/min with 10%

271

acetonitrile/0.1% acidified water solution. The water was acidified with 85% phosphoric acid.

272

The limit of detection (LOD) for sucrose, glucose, and fructose was 0.5 ppm. The temperature at

273

which the initial thermal decomposition component (glucose) was detected (Donset) was

274

labeled on the corresponding DSC curve. HPLC analysis was carried out in duplicates for all

275

samples.

276

2.2.5. Single Crystal X-ray Diffraction measurements


12

Page 13 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

277


For selected sucrose samples, unit cell parameters were determined for an individual,

278

representative crystal both before and after heat treatment. This was done by collecting a

279

short series of omega scans at room temperature using a Bruker D8 Venture Duo system (MoKα

280

radiation), equipped with a four-circle kappa-axis diffractometer and motorized Photon 100

281

CMOS detector. Data were harvested and the unit cells were indexed and refined using APEX II

282

software (Bruker AXS, Inc., Madison, WI). The unit cell parameters were then compared to

283

sucrose parameters contained in the Cambridge Crystallographic Data Centre (CCDC),

284

specifically Brown and Levy (refcode SUCROS).44 Each crystal used for SXRD unit cell collection

285

was selected using a Leica M205C Microsystem (Leica, Heidelberg Germany) under polarized

286

light and morphology information for each sample was recorded.

287

To additionally confirm that the small differences in unit cell parameters from sucrose

288

source to sucrose source had little effect on the known bulk structure, a full crystal structure

289

determination was performed on a representative crystal of each sucrose source prior to heat

290

treatment. Intensity data for the full crystal structures were collected on either a Bruker

291

Siemens Apex II platform diffractometer (used for analytical grade Sigma cane, US Beet, and US

292

Cane samples) or a Burker D8 Venture Duo system (used for Sigma recrystallized in HPLC water

293

(Lab Method) and Sigma recrystallized with 0.5% K2SO3 (Lab Method) samples). The collection,

294

cell refinement, and integration of intensity data were carried out with the APEX2 software

295

(Bruker AXS, Inc., Madison, Wisconsin, USA). Multiscan absorption corrections were performed

296

numerically with SADABS.45 All structures were refined with the full-matrix least-squares

297

SHELXL program.46

298

2.2.6. Micro-CT measurements


13


299

Page 14 of 49

X-ray Micro Computed Tomography (Micro-CT) was used to produce 3D images to non-

300

destructively and non-invasively reveal the internal structure of the crystal samples.47 The X-ray

301

microscope takes multiple projection images at different viewing angles to provide the original

302

2D images. A computer then utilizes these 2D projection images to reconstruct 3D volumetric

303

data to reveal the internal structure. The Xradia Bio Micro-CT (MicroXCT-400), utilized in this

304

study, is a high-resolution 3D X-ray imaging system, which is optimized for non-destructive

305

imaging of complex internal structures. Voxel size and optical magnification of CT scans were

306

selected based on the crystal size. X-ray voltage was set at 40 kV and a total of 901 projection

307

images were collected over 360 degree angle for each sample scan. The number of actual

308

sample images, however, may vary depending on the original size and geometry of the crystal.

309

Preliminary experiments, using the MicroXCT-400 were carried out to collect images of

310

recrystallized analytical grade Sigma cane sucrose grown in saturated sucrose solution, in order

311

to differentiate the solid crystalline phase from the surrounding saturated sucrose solution. In

312

addition, in order to visualize the mother liquor occlusions entrapped in the sucrose crystal, 10%

313

KI (weight of dry matter), which has much higher x-ray attenuation compared to crystalline

314

sucrose and serves as a contrast agent for Micro-CT scan, was recrystallized with the analytical

315

grade Sigma cane sucrose using our enhanced laboratory recrystallization protocol outlined

316

previously. For Micro-CT analysis, “as is” Sigma cane, “as is” US beet, “as is” US cane, Sigma

317

cane recrystallized in HPLC grade water (Lab Method), and Sigma cane recrystallized with 0.5%

318

K2SO3 (Lab Method) crystals were scanned prior to heat treatment, using Micro-CT. Then each

319

crystal was heated to 165°C at 10°C/min using the DSC, except for the Sigma cane recrystallized

320

in HPLC grade water sample, which was heated to a 140°C end temperature, due to its lower


14

Page 15 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


321

DSC Tmonset value. After heating (approximately 1.5°C lower than target temperature), the

322

system was quick cooled equilibrated back to room temperature at a cooling rate of

323

approximated 35°C/min. Then the same crystal was rescanned using Micro-CT under the same

324

experimental conditions, in order to observe any changes occurring inside the crystal due to the

325

heating process.

326

Image analysis and reconstruction were carried out using FEI Avizo visualization and

327

analysis software (version 9.0.1, Visualization Sciences Group, Mérignac Cedex, France). The

328

percent porosity (porosity%) values were obtained by analyzing the 2D images of each sucrose

329

sample, both before and after heat treatment. The porosity% value, which is the ratio of the

330

pore volume to the total volume of sample, was then calculated using Equation 1:

331

Porosity% = (Vp / Vt) x 100%

Equation 1

332

where, in the study herein, Vp is the volume of void space (gas filled cavities) within the crystal

333

and Vt is the total volume of crystal, including the void space. In addition, for enhanced

334

visualization, the 3D structure of each sucrose crystal, before and after heating treatment, was

335

reconstructed from the 2D images using the Avizo software volume rendering functions. In the

336

3D images, blue coloring was used for the matrix, which represents the bulk portion of the

337

sucrose crystal lattice, and red coloring was used for the void space (gas filled cavities) observed

338

in sucrose samples, both before and after heat treatment.

339

3. RESULTS AND DISCUSSION

340

3.1. Sucrose recrystallization

341 342

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


15


Page 16 of 49

343

harvesting a single crystal exhibiting classic morphology as discussed by Vavrinecz,33 the

344

literature recrystallization method produced large masses of agglomerated crystals (Figure 2a).

345

As a result, the literature recrystallized sample needed to be cut into small pieces, in order to

346

seal into the DSC pans for thermal analysis and HLPC sample preparation. In contrast, Beckett et

347

al. ground their recrystallized samples,39 instead of cutting them, before placing them into the

348

DSC pan for analysis, the implications of which are discussed in detail in section 3.2.

349

The appearance of the crystals produced using our enhanced laboratory crystallization

350

method, analytical grade Sigma cane recrystallized in HPLC water and analytical grade Sigma

351

cane recrystallized with 0.5% of K2SO3, are shown in Figures 2b and 2c, respectively. By applying

352

our enhanced laboratory recrystallization method – lower sucrose concentration, lower final

353

heating temperature (85°C compared to 128°C), no agitation, a closed system, and the use of

354

centrifugation to remove surface mother liquor – large size (approximately 1 to 3.5 mm), single

355

crystals with less surface defects were obtained.

356

The appearance of three commercially available sucrose samples (“as is” analytical grade

357

Sigma cane, “as is” US beet, and “as is” US cane) are shown in Figures 2d to 2f, respectively. As

358

can be observed, the analytical grade Sigma cane (Figure 2d) is somewhat larger in size (0.9 to

359

1.2 mm) compared to the US beet (Figure 2e) and US cane (Figure 2f) sucrose crystals, which

360

are screened during commercial processing to yield an average particle size of approximately

361

0.40 to 0.50 mm for regular granulated sugar (also called table sugar). In addition, more surface

362

defects (cracks, twinning) were observed in the commercial sucrose crystals as compared to our

363

laboratory recrystallized sucrose crystals. Another interesting observation is that the white

364

refined beet sugar sample was consistently shinier, when visually examined, than the white


16

Page 17 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


365

refined cane sucrose samples. Generally, a dull appearance relates to defects within the

366

crystalline structure of a material.48 Though our laboratory recrystallized sucrose were single, large crystals with fewer

367 368

apparent defects, the growth rate was slow (24 to 48 hours) and the yield was low. Generally, a

369

faster growth rate results in rougher crystal surfaces and deeper cavities.21 A low yield was

370

predictable based on the enhancements employed. The degree of supersaturation was low,

371

which means the driving force for crystallization is low. A closed container was used, forcing

372

crystallization to occur without water evaporation, thus, the maximum amount of crystals that

373

could grow was dependent on the difference in solubility between the starting and ending

374

temperatures of crystallization.30 However, by applying our enhanced laboratory

375

recrystallization method, we did not obtain the large mass of agglomerated crystals as was

376

obtained when using the general literature recrystallization method,39-40 nor did we harvest the

377

large conglomerates, which represent a collection of crystals joined together randomly as the

378

recrystallized sucrose, obtained by Lee and Chang.49 Instead, we were able to grow and harvest

379

large, single crystals with fewer surface defects. In addition to the growing conditions, the enhanced morphology and quality of our

380 381

laboratory recrystallized sucrose samples was also dependent on the centrifuge conditions

382

applied in this study. It is known that in both beet and cane sugar refining, the crystals in

383

massecuite are separated from the surrounding syrup or molasses by centrifugal machines.34, 38,

384

50

385

rinse the crystals, improving sugar quality.38 In order to mimic the procedure used in sugar

386

refineries, the newly-grown crystals, in this study, were transferred into centrifugal

In sugar refineries, water or dilute mother liquor is sometimes used during centrifugation to


17


Page 18 of 49

387

concentrator tubes and filtered, using centrifugal filtration, at 3600 rpm for 25 min. However,

388

to avoid possible dissolution effects of using a rinse, no rinsing step was included. The

389

centrifuge conditions used herein, selected to obtain better separation effects, were at a higher

390

speed and longer time than those typically used in the sugar industry (1000 rpm [standard

391

speed] or 1600 to 2200 rpm [high speed] for 10 minutes). In addition, our recrystallized sucrose

392

crystals were harvested once they grew to relatively large sizes, resulting in less surface area for

393

mother liquor contact as compared to smaller crystals. This resulted in purging the mother

394

liquor from the surface of the crystals with greater ease in the centrifugal apparatus and better

395

purging efficiency.50

396

3.2. DSC analysis

397

The DSC curves of “as is” analytical grade Sigma cane sucrose, recrystallized Sigma cane

398

sucrose with 1% K2SO4 using the general literature recrystallization method,39-40 unground and

399

ground, as well as recrystallized Sigma cane sucrose with 1% K2SO4 using our enhanced

400

laboratory recrystallization method are plotted in Figure 3. As can be observed from Figure 3,

401

the small endothermic DSC peak was not inhibited by addition of 1% K2SO4, as reported by

402

others,39-40 using either the literature method or our own laboratory recrystallized method,

403

unless a grinding step was added prior to scanning in the DSC. Beckett et al. reported using a

404

grinding step prior to DSC measurement,39 which likely explains their conclusion that addition

405

of 1% K2SO4 is responsible for the absence of the small endothermic DSC peak around 150°C. As

406

reported by Lu et al., physical grinding alone, without the addition of impurities (e.g., K2SO4),

407

was sufficient to cause the small endothermic DSC peak in analytical grade Sigma and white

408

refined US cane sucrose samples to disappear.6


18

Page 19 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

409


Beckett et al., mainly attributed the appearance of the peak at 150°C to impurities in the

410

sucrose, especially the mineral salt content;39 however, they did not consider the impact of

411

sample grinding and associated loss of water on the presence of the small peak. However,

412

grinding of the crystals before analysis extends beyond an inert sample preparation step, as

413

stated by Richards, “It is usually considered as a sufficient precaution to powder the material

414

finely and expose it to the air for a short time, in order to allow the undesirable included water

415

to evaporate.”9

416

By repeating the K2SO4 impurity recrystallization work done by Beckett,39 with and without

417

grinding the sample before DSC measurement, we were able to differentiate which aspect

418

(impurities versus grinding) had a significant impact on the presence of the small endothermic

419

DSC peak around 150°C. As shown in Figure 3, analytical grade Sigma cane sucrose

420

recrystallized with 1% K2SO4 (Lit. Method) without grinding resulted in an even larger small DSC

421

peak compared to “as is” analytical grade Sigma cane sucrose. However, after grinding, the

422

small DSC peak in analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 (Lit. Method)

423

was completely eliminated. This result is mainly attributable to the mechanical disruption of the

424

mother liquor occlusions distributed throughout the sucrose crystal, allowing the water to

425

evaporate and, thus, inhibiting the thermal induced hydrolysis process that would have

426

occurred during heating if the water were present.6 Based on these results, it was noted that

427

K2SO4 is less reactive than the related salt K2SO3. Thus, the laboratory recrystallization method

428

was also carried out using K2SO3.

429 430

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


19


Page 20 of 49

431

recrystallized with 0.5% K2SO3 (Lab method) are plotted in Figure 4. A somewhat surprising

432

result, shown in Figure 4, was that by recrystallizing analytical grade Sigma cane sucrose in

433

HPLC grade water using the enhanced laboratory recrystallization method, the large

434

endothermic peak typically observed in white refined cane sucrose (e.g., Sigma cane “as is”

435

curve in Figure 4, with a small peak Tmonset of approximately 151°C and a large peak Tmonset

436

of approximately 188°C) completely disappeared, leaving only one, comparably large,

437

endothermic DSC peak with a Tmonset of approximately 144°C. At the other extreme,

438

recrystallization of Sigma cane sucrose with the addition of 0.5% potassium sulfite (K2SO3),

439

using our enhanced laboratory recrystallized method, resulted in elimination of the small

440

endothermic DSC peak; thus, only the large endothermic DSC peak was present, with a Tmonset

441

of approximately 190°C. As expected, the “as is” US beet sucrose exhibited one large peak with

442

a Tmonset of approximately 188°C, in agreement with Lu et al.4, 6

443

In general, for most crystalline materials, the presence of even a small quantity of

444

impurities will lower the melting point by a few degrees and broaden the melting transition

445

temperature range. Because impurities cause defects in the crystalline lattice, it is easier to

446

overcome the intermolecular interactions between the molecules, and consequently, a lower

447

temperature is required for melting in the presence of impurities.48 Interestingly, in this study,

448

analytical grade Sigma cane sucrose recrystallized in very pure HPLC grade water, due to the

449

partitioning effect, the newly-grown crystals have less impurities as compared to the “as is”

450

analytical grade Sigma crystals, but exhibited the lowest Tmonset value (144°C); whereas Sigma

451

recrystallized with impurities (0.5% K2SO3) exhibited the highest Tmonset value (190°C).

452

3.3 HPLC analysis


20

Page 21 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

453


HPLC analysis indicates that the initial thermal decomposition component, glucose, was

454

first detected at 160°C for “as is” analytical grade Sigma cane, 200°C for “as is” US beet, 150°C

455

for recrystallized Sigma cane in HPLC water (Lab Method), and 200°C for recrystallized Sigma

456

cane with 0.5% K2SO3 (Lab Method), respectively, as prepared in the DSC at a heating rate of

457

10°C/min (Figure 4). Overall, the temperature at which the initial thermal decomposition

458

component (glucose) was detected (Donset) for each of these sucrose samples was close to its

459

own DSC Tmonset value, 151, 188, 144 and 190°C, respectively (as listed above and shown in

460

Figure 4). All HPLC data, including % sucrose, %glucose, and % fructose, are provided in Table S1

461

in the Supporting Information. Comparison of the thermal decomposition HPLC data for

462

analytical grade Sigma cane, US cane, and US beet was previously reported by Lu et al.5

463

This variation in thermal behavior of each sucrose sample can be attributed to the

464

difference in the composition and chemistry of the mother liquor occlusions within the sucrose

465

crystal structure.6 In the United States, an important difference between white refined beet

466

and cane sugar processing is that beet sugar processing routinely includes a sulfitation step,

467

whereas cane sugar processing does not.34-35 Sulfitation has rarely been used in cane sugar

468

factories since the 1950's.36 As mentioned in the introduction, the gaseous SO2, used in the

469

sulfitation step, is converted to sulfite or sulfate ions after dissolving in the aqueous sugarbeet

470

juice solution. The thermal decomposition resistance in commercial beet sugar (US beet) and

471

laboratory recrystallized sucrose with addition of 0.5% K2SO3 (Lab Method) is hypothesized to

472

be the result of the residual sulfite in the mother liquor occlusions, measured by Lu et al., using

473

a total sulfite assay, microplate format (Megazyme, Wicklow, Ireland), as 11.16 ± 4.85 ppm in

474

“as is” US beet and 12.93 ± 3.61 ppm in 0.5% K2SO3 (Lab Method) samples.6 One possible


21


Page 22 of 49

475

hypothesized mechanism to explain the influence of sulfite on the thermal behavior of sucrose

476

is that sulfite ions can react with the carbonyl groups in reducing sugars to form bisulfite

477

adducts, which, based on research carried out by Shi,51 can suppress the thermal

478

decomposition of monosaccharides. However, additional research is needed in order to fully

479

investigate the suppression mechanism. Thus, in the case of our Sigma cane recrystallized with

480

0.5% K2SO3 (Lab Method), the addition of sulfite helps to inhibit thermal induced hydrolysis in

481

the mother liquor occlusions; thus, enhancing the thermal stability of the sucrose crystal.

482

3.4. Single Crystal X-ray Diffraction

483

The room temperature unit cell parameters for all sucrose crystals studied herein

484

(commercial, recrystallized with and without K2SO3, before, and after heat treatment),

485

determined using SXRD, are provided in Table 1 and are consistent with the known unit cell

486

parameters of sucrose reported by Brown and Levy in 1973.44 The full crystal structures of “as is”

487

analytical grade Sigma cane, “as is” US beet, “as is” US cane, Sigma cane recrystallized in HPLC

488

grade water (Lab Method), and Sigma cane recrystallized with 0.5% K2SO3 (Lab Method) were

489

collected using SXRD, refined, and deposited to the CCDC database (CCDC 1473968, 1473969,

490

1473970, 1578547, and 1578548, respectively). Table 2 contains the crystallographic and

491

metrical details of the full structure refinements for the five sucrose samples prior to heat

492

treatment. It is important to note that the unit cell parameters in Table 1 were determined

493

using short SXRD data collection runs before and after heating of each specific crystal that was

494

examined using Micro-CT to confirm that there was little to no crystallographic change in the

495

bulk structure.


22

Page 23 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

496


Though there is a small endothermic peak present in both ”as is” Sigma cane and “as is” US

497

cane DSC curves (as shown in Figure 1), the average structure of those samples, studied herein,

498

show no evidence to support the previously proposed “metastable sucrose polymorphs” theory

499

used to explain the presence of the small endothermic DSC peak.49 According to this theory, the

500

“metastable sucrose polymorphs” (which melt around 150 to 160°C, as compared to the high

501

melting thermodynamically stable form, melting around 185°C) are explained by the

502

conformational disorder about the -CH2-OH functional groups of the fructofuranose ring that

503

results in the misalignment of intramolecular hydrogen bonds between the hydroxyl groups and

504

the glucopyranose ring oxygen.49 This theory, however, does not explain the cause of the small

505

endotherm DSC peak observed in most cane sucrose samples. It is known that XRD provides the

506

best structural evidence for polymorphism. The sucrose crystals examined herein have unit cells

507

consistent with the known unit cell of sucrose, therefore, the appearance of the small peak in

508

the DSC cane sucrose curves, including the one for our Sigma cane recrystallized in HPLC water

509

(Lab Method) sample, is not attributable to a new polymorph as suggested by Okuno et al.,52

510

Lee and Lin,53 and Lee and Chang.49 A search of the literature did yield a high-pressure

511

polymorph of sucrose, sucrose II, formed at a critical pressure of 4.80 GPa at 295K.54 However,

512

sucrose II is not stable at ambient conditions. It is important to note that the sucrose crystals

513

grown and studied by Lee and Chang were prepared by adding different types of alcohols

514

(methanol, furfuryl, or tetrahydrofuryl) at 60°C all at once into saturated aqueous sucrose

515

solutions,49 a different crystallization method than the one employed herein.

516

3.5. Micro-CT


23


517

Page 24 of 49

A preliminary 2D Micro-CT image of newly-grown Sigma sucrose crystals surrounded by

518

saturated sucrose mother liquor solution is shown in Figure S1. Based on the lack of differences

519

in gray scale color in Figure S1, the sucrose crystals can hardly be differentiated from the

520

surrounding saturated mother liquor solution, reflective of similar material densities. Thus,

521

without using a contrast agent, it is not readily feasible to distinguish the mother liquor

522

occlusion from the crystal lattice using Micro-CT scanning. A trapped air bubble in the solution,

523

however, is clearly distinguishable, as it exhibits a much darker color (lower x-ray attenuation,

524

lower density) than the surrounding saturated sucrose solution and crystal lattice. In order to

525

attempt to visualize the mother liquor occlusions entrapped within the sucrose crystals, 10% KI

526

(weight of dry matter), which has much higher x-ray attenuation compared to crystalline

527

sucrose, was added to the mother liquor as a contrast agent during Sigma sucrose

528

recrystallization. Interestingly, based on the contrast differences, we were able to observe the

529

high attenuation KI (bright spot in 2D image and yellow dot in 3D volume rendering) entrapped

530

in the sucrose crystalline solid (Figure S2). Therefore, compared to the traditional visualizing

531

method by addition of colored substances during sucrose crystallization,10, 21 this study

532

successfully developed a new method using a contrast agent (KI), during sucrose crystallization

533

to further prove the existence of mother liquor occlusion within sucrose crystal by Micro-CT

534

scanning.

535

The reconstructed 3D Micro-CT images of each sucrose sample, before and after heat

536

treatment, along with their calculated percent porosity (porosity%) values are shown in Figure 5.

537

The blue matrix represents the bulk portion of the sucrose crystal lattice and the red spots are

538

the gas filled cavities observed in sucrose samples before and after heat (165°C and 140°C)


24

Page 25 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


539

treatment. Based on the Micro-CT verification study demonstrated in Figure S1, the entrapped

540

mother liquor occlusions (viscous liquid) could not be clearly differentiated from the crystal

541

lattice (solid), due to their similar densities. The small dark areas in the 2D images were,

542

therefore, identified as internal gas filled cavities and rendered with red color in the 3D images

543

for better visualization. The 3D Micro-CT images, before and after heat treatment, are shown in

544

Figure 5 and associated reconstructed 2D Micro-CT images in videos, SV1 through SV10, in the

545

Supporting Information).

546

Overall, “as is” analytical grade Sigma cane sucrose (before heat treatment) exhibited the

547

smallest number of internal gas filled cavities, with a small porosity of 0.00588 ± 0.00002%,

548

following by “as is” US beet (0.020 ± 0.0032%) and “as is” US cane (0.075 ± 0.0730%) (Figure 5

549

and Videos SV1, 2 and 3, respectively in the Supporting information). The before heat

550

treatment recrystallized (Lab Method) samples had somewhat higher porosity values compared

551

to the commercial samples, with analytical grade Sigma cane recrystallized with 0.5% K2SO3

552

(Lab Method) having a higher porosity (0.175 ± 0.0063%, Figure 5 and Video SV4 in the

553

Supporting information) than analytical grade Sigma cane recrystallized in HPLC grade water

554

sucrose (Lab Method) (0.106 ± 0.0515% Figure 5 and Video SV5 in the Supporting information).

555

After being heated to 165°C, a temperature 10°C higher than the onset of the small

556

endothermic DSC peak, the crystal was immediately cooled quickly back to room temperature

557

and re-scanned using Micro-CT. After heat treatment, analytical grade Sigma cane (2.67 ± 0.067%

558

porosity) and US cane (2.15 ± 0.115% porosity) exhibited the formation of numerous internal

559

cavities with large sizes (Figure 5 and Videos SV6, SV8 in the Supporting Information) compared

560

to heat treated US beet (0.049 ± 0.0046% porosity) and Sigma cane recrystallized with 0.5%


25


Page 26 of 49

561

K2SO3 (Lab Method) (0.169 ± 0.0154% porosity) sucrose crystals (Figure 5 and Videos SV7, SV9

562

in the Supporting Information). It is important to note that analytical grade Sigma cane and US

563

cane crystals maintained their original external morphology, despite the numerous formation of

564

cavities upon heating, similar to that of US beet and Sigma cane recrystallized with 0.5% K2SO3

565

(Lab Method), which did not form numerous cavities upon heating.

566

In the case of analytical grade Sigma cane recrystallized in HPLC grade water (Lab Method),

567

after heat treatment to 140°C, which is close to its own Tmonset value (144°C), it exhibited only

568

slightly more cavities (0.133 ± 0.0197%, Figure 5 and Video SV10 in the Supporting information)

569

compared to before its heat treatment (0.106 ± 0.0515%). This modest increase in the number

570

of new cavities upon heating for the analytical grade Sigma cane recrystallized in HPLC grade

571

water (Lab Method) sample can be explained by the fact that the heat treatment temperature

572

of 140°C was below the Tmonset value; whereas, in the case of “as is” analytical grade Sigma

573

cane, the 165°C heat treatment temperature was selected to be above the small endothermic

574

DSC peak Tmonset value. The heat treatment temperature of 140°C was selected to be below

575

the Tmonset since the analytical grade Sigma cane recrystallized in HPLC grade water (Lab

576

Method) sample exhibited only one DSC peak at a low Tmonset value (144°C).

577

A slight, non- statistically different, decrease in porosity for Sigma cane recrystallized with

578

0.5% K2SO3 (Lab Method) crystal after heating to 165°C was observed. This slight decrease

579

could be due to the large size of the recrystallized crystal, which in order to maintain the high

580

resolution of the image, only a portion of the crystal was able to be scanned. Therefore, it

581

appears as a cylindrical shape after 3D reconstruction (Figure 5 and Videos SV4 [before heat

582

treatment], SV8 [after heat treatment] in the Supporting Information) and the region of CT


26

Page 27 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


583

scanning cannot be focused on exactly the same location after heating as before heating. It is

584

also possible that heating the crystal below its melting point (without decomposition) resulted

585

in annealing (also termed sintering), that is, healing of some of the crystal defects; thus,

586

resulting in a lower after heat treatment porosity.

587

Another important point that needs to be discussed is the difference in the mechanism

588

of cavity formation in sucrose crystals before (during crystal growth) and after heating. Powers

589

was the first to report observing both mother liquor and gaseous inclusions in sucrose crystals

590

under the microscope.3, 10 By observing select crystal specimens in the act of dissolving, Powers

591

reported that “when an inclusion of syrup is breached, the heavy syrup may be seen streaming

592

downward, whereas when a gaseous inclusion is breached a bubble may be seen to strain like a

593

balloon, and then to break away and rapidly rise to the surface.”10 As to the origin of the

594

bubbles, Powers states: “The probable origin is that air dissolved in the original crystallizing

595

syrup became supersaturated and formed as bubbles on the growing face. These were then

596

overgrown by the layers.”10 Gas bubble incorporation in growing crystals was also studied by

597

Wilcox and Kuo,55 who mentioned the work of Powers.10-11 The theory of cavity formation

598

within crystals and its related trapping of mother solution has been discussed and illustrated by

599

Vaccari.21 In general, a faster crystal growth rate results in rougher surfaces, higher growth

600

steps, and deeper cavities, consequently resulting in the progressive closing of the cavities and

601

entrapment of mother liquor.

602

In contrast to cavity formation during crystal growth, heat-generated cavity formation

603

(e.g., in Sigma and US cane heating to 165°C) is associated with the presence of the small DSC

604

peak and is attributed to thermal induced hydrolysis and subsequent thermal decomposition


27


Page 28 of 49

605

processes, within the mother liquor occlusions. The presence of these internal, heat-generated

606

cavities, captured non-invasively in intact Sigma and US cane crystals heated to 165°C for the

607

first time by Micro-CT (Figure 5), may be connected to the research of Thomas and Williams.16

608

As previously observed by Thomas and Williams, water present in dislocation cores within the

609

sucrose crystal structure could be liberated upon heating and by mechanical means; where

610

heating gave rise to “decomposition volcanoes”16 that we hypothesis to be similar to the heat

611

generated cavity formation observed herein.

612

Rescanning of our heat-treated crystals in the DSC at 10°C/min, resulted in the detection

613

of a small (ΔCp value of 0.037 J/g) glass transition at 64°C (midpoint) for Sigma cane, but not for

614

US beet.5 The observation of the presence of amorphous content supports the hydrolysis

615

hypothesis in Sigma cane and US cane samples. However, the occlusions alone are not

616

sufficient to explain the presence of the small peak, since the US beet and Sigma cane

617

recrystallized with 0.5% K2SO3 also contain mother liquor occlusions, but do not exhibit the

618

small peak or form large numbers of cavity areas after heating to 165°C. This result can be

619

explained by the relatively high amount of sulfite in beet sources and Sigma recrystallized cane

620

with 0.5% K2SO3,6 which is attributable to the sulfitation step used during the beet sugar

621

refining process or the addition of K2SO3 during recrystallization. It is known that sulfite can

622

inhibit browning reactions caused by ascorbic acid, lipid, Maillard and enzymatic browning

623

reactions.56 In the literature, SO2 was reported to react with carbonyl groups in sugar molecule

624

to sugar bisulfite adduct, which suppressed the degradation of monosaccharides,51 and, thus,

625

could inhibit the formation of large cavity areas due to thermal induced hydrolysis in sugar beet

626

sources and recrystallized Sigma sucrose with addition of 0.5% K2SO3 as observed in our study .


28

Page 29 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

627 628


4. CONCLUSIONS Though sucrose is a very common crystalline material, its thermal behavior is quite

629

complex. With the aid of laboratory recrystallization method enhancements, compositional

630

alterations, and the use of various analytical techniques, including DSC, HPLC, SXRD, and Micro-

631

CT, we assert that the varied thermal behavior of crystalline sucrose is due to the influence of

632

mother liquor occlusion composition and chemistry on its thermal decomposition propensity,

633

rather than due to polymorphism. Mother liquor occlusions with a composition and chemistry

634

that are conducive to thermal decomposition (e.g., high purity [low ash/mineral content] and

635

low pH) result in a DSC curve with a low Tmonset value. Whereas, occlusions with a

636

composition and chemistry that contained inhibitory attributes (e.g., high ash/mineral content,

637

sulfite, or water removal via grinding) result in DSC curves with a high Tmonset value. In general,

638

crystalline materials are known to yield a constant melting temperature; however, this is not

639

the case with crystalline sucrose as its thermal behavior is actually a complex combination of

640

thermal decomposition, as influenced by mother liquor composition and chemistry, and melting.


29


641

■ AUTHOR INFORMATION

642

Corresponding Author

643

*Tel.: +01-217-333-6369. E-mail: [email protected]. Web: http://fshn.illinois.edu/directory/sjs

644

Present Address

645

†*University of Illinois at Urbana-Champaign, Department of Food Science and Human

646

Nutrition, 367 Bevier Hall, 905 S Goodwin Ave, Urbana, IL 61801, USA

647

Funding Sources

648 649

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

650

Notes

651

The authors declare no competing financial interest.

652

■ ACKNOWLEDGMENTS

653

The authors would like to acknowledge the single X-ray diffraction and the Micro-CT

654

instruments located at The George L. Clark X-ray Facility, School of Chemical Sciences and the

655

Imaging Technology Group at the Beckman Institute, respectively, at the University of Illinois

656

and Urbana-Champaign.

30


Page 30 of 49

Page 31 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


657

■ ABBREVIATIONS

658

DSC, differential scanning calorimetry; Tmonset, onset melting temperature; ΔH, enthalpy of

659

melting; HPLC, high performance liquid chromatography; Donset, temperature at which the

660

initial thermal decomposition component (glucose) was detected; SXRD, single crystal x-ray

661

diffraction; CCDC, Cambridge Crystallographic Data Centre; Micro-CT, X-ray Micro Computed

662

Tomography; 2D, two-dimensional; 3D, three-dimensional

663 664

■ ACCESSION CODES

665

CCDC 1473968 (Analytical grade Sigma cane sucrose), 1473969 (US beet sucrose), 1473970 (US

666

cane sucrose), 1578547 (Analytical grade Sigma cane recrystallized in HPLC grade water [Lab

667

Method]), 1578548 (Analytical grade Sigma cane recrystallized with 0.5% of K2SO3 [Lab

668

Method]), contain the supplementary crystallographic data for this paper. These data can be

669

obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

670

[email protected], or by contacting The Cambridge Crystallographic Data Centre,

671

12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

672 673

■ SUPPORTING INFORMATION

674

Supporting information is available for this manuscript, including HPLC analysis for heat treated

675

and no heat treatment samples (Table S1), 2D Micro-CT image of newly-grown Sigma sucrose

676

crystals surrounded by saturated sucrose solution (Figure S1), Micro-CT scanned 2D image and

677

3D volume rendering of Sigma sucrose recrystallized with 10% KI (Figure S2), and videos of the

678

reconstructed 2D Micro-CT images of the sucrose crystals in Table 1 (Videos SV1 to SV10).

679 31



680

FIGURES AND TABLES

681 682 683 684

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.

32


Page 32 of 49

Page 33 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2a. Appearance of analytical grade Sigma cane sucrose recrystallized with 1% K2SO4 using the general literature recrystallization method.

33


Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 2b. Appearance of analytical grade Sigma cane sucrose recrystallized in HPLC water using the enhanced laboratory recrystallization method.

34


Page 34 of 49

Page 35 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 2c. Appearance of analytical grade Sigma cane sucrose recrystallized with 0.5% K2SO3 in HPLC water using the enhanced laboratory recrystallization method.

35



Figure 2d. Appearance of “as is” analytical grade Sigma cane sucrose.

36


Page 36 of 49

Page 37 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Figure 2e. Appearance of “as is” commercial US beet sucrose.

37



Figure 2f. Appearance of “as is” commercial US cane sucrose.

38


Page 38 of 49

Page 39 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


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.

39



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.

40


Page 40 of 49

Page 41 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Sigma cane “as is”

US beet “as is”

Before Heat Treatment

After Heat Treatment to 165°C and Quick Cooled to RT

Porosity%: 0.00588 ± 0.00002

Porosity%: 2.67 ± 0.067



Porosity%: 0.020 ± 0.0032

Porosity%: 0.049 ± 0.0046 41



US cane “as is”

Sigma cane rec. in HPLC grade water Lab Method



Porosity%: 0.075 ± 0.0730

Porosity%: 2.15 ± 0.115



Porosity%: 0.106 ± 0.0515

Porosity%: 0.133 ± 0.0197 42


Page 42 of 49

Page 43 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Sigma cane rec. with 0.5% K2SO3 Lab Method



Porosity%: 0.175 ± 0.0063

Porosity%: 0.169 ± 0.0154

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°C 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 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.

43



Page 44 of 49

Table 1. Unit cell parameters of selected beet and cane sucrose samples obtained using single crystal X-ray diffraction. Each parameter was reported as an average value (standard deviation). Recrystallized (rec.) samples were obtained using the enhanced laboratory method. Space Temp Volume group (°C) Sample ID a (Å) b (Å) c (Å) α (°) β (°) γ (°) (Å3) Sucrose Referencea

7.7585(4)

8.7050(4)

10.8633(5)

90

102.945

Sigma cane “as is”

7.763(3)

8.703(4)

10.858 (6)

90

Sigma cane 165°Cb

7.770(4)

8.689(5)

10.878(6)

US beet “as is”

7.766(6)

8.690(7)

US beet 165°Cb

7.752(3)

US cane “as is”

90

715.04

P21

22.5±1.5

103.042(19) 90

714.6(9)

P21

23.5

90

103.072(20) 90

715.3(1.1)

P21

23.5

10.848(8)

90

103.1(4)

713.3(14)

P21

23.5

8.692(3)

10.844(5)

90

103.019(17) 90

711.9(7)

P21

23.5

7.741 (3)

8.686(4)

10.834(5)

90

102.887(17) 90

710.1(9)

P21

23.5

US cane 165°Cb

7.749(4)

8.700(4)

10.861(6)

90

103.01(2)

90

713.3(1.0)

P21

23.5

Sigma rec. in HPLC water

7.740(6)

8.668(7)

10.829(8)

90

103.01(2)

90

707.90(15) P21

23.5

Sigma rec. in HPLC water 140°Cb

7.741(5)

8.685(6)

10.821(9)

90

103.05(3)

90

708.6(1.5)

P21

23.5

Sigma rec. w/ 0.5% K2SO3

7.7667(14)

8.7026(11) 10.850(2)

90

102.839(16) 90

715.0(3)

P21

23.2

90

Sigma rec. w/ 0.5% K2SO3 165°Cb 7.7595 (10) 8.7092(8) 10.856(2) 90 103.046(15) 90 714.7(2) P21 23.2 Unit cell parameters of sucrose reported by Brown and Levy (1973) and recorded in the Cambridge Crystallographic Data Centre. b Heat treated by scanning in the DSC at 10°C/min to target temperatures (either 165°C or 140°C). a

44


Page 45 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 2. Crystallographic and metrical details of full structure refinements. Crystal CCDC Accession Codes Empirical formula Formula weight Temperature Wavelength Space group Unit Cell Dimensions

Volume Z Density (calculated)

Sigma cane “as is” 1473968 C12 H22 O11 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

US Beet “as is” 1473969 C12 H22 O11 342.30 182(2) K 0.71073 Å P21 a = 7.741(2) Å b = 8.691(2) Å c = 10.853(3) Å β = 102.981(3)° 711.5(3) Å3

US Cane “as is” 1473970 C12 H22 O11 342.30 173(2) K 0.71073 Å P21 a = 7.7376(15) Å b = 8.6930(16) Å c = 10.833(2) Å β = 102.991(2)°. 710.0(2) Å3

Sigma rec. in HPLC water 1578547 C12 H22 O11 342.30 100(2) K 0.71073 Å P21 a = 7.7109(5) Å b = 8.6514(6) Å c = 10.7979(7) Å β = 103.013(2)° 701.83(8) Å3

Sigma rec. w/ 0.5% K2SO3 1578548 C12 H22 O11 342.30 100(2) K 0.71073 Å P21 a = 7.7173(2) Å b = 8.6642(2) Å c = 10.8143(3) Å β = 102.9785(10)° 704.62(3) Å3

2 1.606 Mg/m3 0.144 mm-1

2 1.598 Mg/m3 0.143 mm-1

2 1.601 Mg/m3 0.143 mm-1

2 1.620 Mg/m3 0.145 mm-1

2 1.613 Mg/m3 0.145 mm-1

Reflections collected Independent reflections

364 1.93 to 26.32° -9

Unraveling the Wide Variation in the Thermal Behavior of Crystalline

Recommend Documents