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Food and Beverage Chemistry/Biochemistry

Preparation and characterization of acylcaramel Yichao Geng, Yulin Ning, Shao Qiang, Yaozhong Lv, Xianfu Wei, Yujie Dai, Shiru Jia, Cheng Zhong, Shuli Man, Liming Zhang, and Xiuli Zhang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b07148 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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Journal of Agricultural and Food Chemistry

Preparation and characterization of acylcaramel

1 2

Yichao Genga, Yulin Ninga, Qiang Shaoa,Yaozhong Lva, Xianfu Weia, Yujie Dai*a, Shiru Jia*a,

3

Cheng Zhonga, Shuli Mana, Liming Zhanga, Xiuli Zhangb,

4

aState

Key Laboratory of Food Nutrition and Safety, Tianjin University of Science and Technology, Tianjin 300457, PR

5

China

6

bDepartment

of Biochemistry, University of Missouri, Columbia, MO 65211, USA

7

ABSTRACT

8

Caramel is a widely used water-soluble food pigment. The acylation of caramel was conducted by

9

aliphatic acyl chlorides with different chain lengths. Acetyl, butyryl, octyl, lauryl, palmityl and

10

stearyl caramels were prepared with the ratio of acyl chloride to caramel of 6. The formation of

11

acylated caramel was confirmed by FT-IR spectra and the acyl mass fraction in acylcaramel was

12

determined by potentiometric titration. Thermal analysis showed that the weight-loss of acylated

13

caramel was higher than that of the raw caramel. The SEM analysis showed that the morphology of

14

acylated caramel was significantly different from that of raw material. The acyl mass fraction of

15

acylated caramel increased with the increase of acyl chain lengths. Meanwhile, the lipo-hydro

16

partition coefficient, the solubility in corn oil, color, red and yellow indexes increased with the

17

increase of mass fraction of acyl in acylcaramel. It was found that stearyl caramel has the highest

18

lipid solubility of 5.73 mg/mL in corn oil, however, the color, red and yellow indexes of palmityl

19

caramel reached 25818.60, 1.149 and 1.757 respectively. This study provides a method to improve

20

the solubility of caramel in lipid phase and expand the application range of caramel.

21

Key words: acylcaramel; preparation; characterization; lipo-hydro partition coefficient.

ACS Paragon Plus Environment


22

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INTRODUCTION

23

Caramel refers to reddish-brown to brown-black viscous liquids or hygroscopic powders used to

24

impart color to various foods and beverages, varying in hue from light yellowish brown to dark

25

brown. Caramel has been as a commodity since one hundred years ago. It may be one of the most

26

widely used food colorants in industry and daily life. It is used as a pigment in a wide range of food

27

products such as baked goods, ham, pickles, ice creams and frozen desserts, noodles, steamed bread,

28

sauces, milk, beers, cola beverages and confectionery etc(1, 2). There are some earlier introduction

29

about the manufacture and applications of caramels reported by Greenshields and Macgillivray(3),

30

Greenshields(4) and Tomasik(5).

31

Caramel was originally made by heating sugar sources to temperatures up to about 200°C until to 15% of the initial weight was lost(1, 6). Cane sugar(7), beet sugar, honey(8),

32

about 10%

33

molasses and their mixtures were generally used as sugar sources, but glucose and fructose are most

34

readily used(9). Apart from the sugar sources, some inorganic salts such as ammonium sulfate and

35

sodium carbonate were also used(10, 11). The composition of caramels varies with the reactants used

36

and the manufacturing processes. The effects of types and amounts of reactants as well as the process

37

methods selected on the composition and properties of caramels are summarized in detail by Myers

38

and Howell (12).

39

Caramels, as a kind of macromolecular colloidal mixture, their composition and structure

40

attracted many scientists’ concerns. The composition includes volatile and nonvolatile components.

41

The volatile fraction was reasonably well characterized (13, 14). However, in spite of much effort,

42

the composition and structure of the major portion of the nonvolatile fraction of caramels, little

43

progress was made due to lack of suitable analytical techniques to provide sufficient insight into the


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44

extraordinarily complex product in early times. In 1989, Tschiersky and Baltes revealed the presence

45

of 1,6-anhydro-P-D-glucopyranose and certain disaccharides such as cellobiose, maltose, isomaltose,

46

and gentiobiose with the GLC-EI and -CIMS analysis of permethylated samples(15). In 1991,

47

Defaye analyzed the oligosaccharide composition of caramel, proving the presence of a significant

48

proportion (18%) of difructose dianhydrides, and of an almost equivalent amount of glucosylated

49

difructose dianhydride derivatives, besides the previously identified glucobioses. In 2012, Golon and

50

Kuhnert characterized the caramel formed by heating from glucose, fructose, and saccharose using

51

high-resolution mass spectrometry (MS), followed by targeted liquid chromatography−tandem MS

52

experiments. They found caramel is composed from several thousand compounds formed by a small

53

number of unselective and chemoselective reactions. Caramelization products include oligomers

54

with up to six carbohydrate units formed through unselective glycosidic bond formation, dehydration

55

products of oligomers losing up to a maximum of eight water molecules, hydration products of sugar

56

oligomers, disproportionation products, and colored aromatic products.

57

When prepared with some alkaline nitrogenous additives, Melanospermine was formed in the

58

caramels. As an important product of Maillard reaction, it plays an important role in browning

59

reaction. Besides, because of single electrons are existed in the caramels, therefore they have the

60

relevant characteristics of free radicals. As can be seen from the results of the present study, caramel

61

colors contain the sugar and furan ring skeletons, as well as a large number of hydroxyl, carboxyl

62

and aldehyde groups (12, 16), which made them with good hydrophilicity. On the other hand, most

63

caramels hardly dissolve in oil phase or grease products, which confines their application in oil

64

containing foods or oil products.

65

Chemical modification is an important means to change or improve the performance of some



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66

materials(17). Many substances，such as starch and cellulose，can form a variety of products with

67

different properties through esterification, alkylation and other reactions(18, 19). However, up to

68

now, the composition and properties of caramel products are mainly adjusted by changing the raw

69

materials and the preparation processes. There are nearly no reports about improving caramel

70

properties by chemical modification. In this study, esterification of caramel with fatty acids was

71

adopted to prepare caramel acylation products, and also their properties were characterized.

72

MATERIALS AND METHODS

73

Materials. Commercial fatty acyl chlorides such as acetyl chloride, butyryl chloride, octyl

74

chloride, lauryl chloride, palmitoyl chloride and stearyl chloride, as well as raw caramel materials,

75

which were used to synthesize fatty acid acylated caramels, were purchased from Energy Chemical

76

Co., Ltd. Company (Shanghai, China). Stearic acid (analytical purity) was phased from Tianjin

77

Solomon Biotechnology Co., Ltd. (Tianjin China). Other reagents, unless otherwise specified, are

78

analytical pure. The water used was distilled water.


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79

Synthesis of Fatty Acid Acylated Caramels. The fatty acid acylated caramels (acylcaramels) were

80

synthesized through the acylation (esterification) reaction of caramel with fatty acyl chlorides under

81

different mass ratio of acyl chloride to caramel (Rac) based on some references(20, 21). The reaction

82

can be expressed by Scheme 1. O n Cl

+ n Cl

OH

O

O OH OH

O

HOOC HO

OH

O

O OH

HO

O n Cl

83

OH

n

O

HOOC HO

Fatty acid acyl chloride n=(2 to 18)

OH

O HO

COOH CHO

OH

O

OH

n

O

O HO

Caramel

O COOH CHO

OH

O

O

n

Acylcaramel

84

Scheme 1. Reaction for the acylation of caramel.

85

Around 0.5 g of raw caramel (preheated at 100-105 ºC for 2h and cooled to room temperature)

86

was accurately weighed and added into a newly dried 50 mL round bottom flask. 5 mL of anhydrous

87

dimethylformamide (DMF) was supplemented as the solvent and 1mL anhydrous pyridine as the

88

acid-binding regent. Then, the mixture was being stirred until raw caramel was fully dissolved. The

89

required amount of fat acyl chloride according to Mas was added slowly with a dropping funnel. The

90

above system was stirred at room temperature for 6h, and the reaction was quenched by adding a

91

small amount of distilled water. Subsequently, the mixture was filtered, and the filter cake was

92

washed with 15 mL distilled water and a large amount of hot anhydrous ethanol sequentially until the

93

filtrate was clear. The filter cake was dried at 50 °C in the vacuum drying oven to constant weight

94

and finally the reddish-brown solid powder of acylcaramel was obtained.



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95

FT-IR Analysis. The sample was mixed with KBr powder in a ratio of 1:20. After being ground

96

evenly, the sample was pressed evenly in a tablet with 0.5 mm in thickness and 13 mm in diameter

97

for Fourier transform infrared spectrum (FT-IR) analyses(22). The FT-IR analyses were carried out

98

using FT-IR Spectrometer (Bruker Tensor-27, Bruker Daltonics Inc., Germany) with the scanning

99

wave number range of 500 cm-1-4000 cm-1, 16 times scan at 4 cm-1 resolution.

100

Determination of the Mass Fraction of Acyl in Acylcaramel. Because the caramel is a

101

complex mixture with unknown molecular weight and molecular structure, the number of hydroxyl

102

groups that can be esterified by fatty acids is also unknown. Therefore, in this study, the mass

103

fraction of fat acyl groups (Mf) was employed to express the acylation degree of the acylcaramel. It

104

was defined as the mass percentage of fat acyl groups in the total acylated caramel and can be

105

determined by the acid base titration method(23).

106

0.1 g sample was accurately weighted and put into 250 mL flask. 0.25 mol/L NaOH solution

107

was supplemented. The mixture was stirred with an electromagnetic stirrer at 60 °C for 2 h so that

108

the sample was saponified sufficiently. When it was cooled to room temperature, the remaining

109

sodium hydroxide after saponification was titrated with 0.1mol/L of hydrochloric acid, the end point

110

was determined with EasyPlusTM ET38 potentiometric titrator (METTLER-TOLEDO Co, ZURICH,

111

Switzerland.). Mf can be calculated by the following equation:

112

𝑀𝑓(%) =

𝑀𝐴𝐶𝐻𝐶𝑙(𝑉0 ― 𝑉) 1000 × 𝑚𝑠

(1)

113

Where 𝑀𝐴 is the formula weight of acyl group. 𝐶HCl is the molar concentration of

114

hydrochloric acid. ms is the mass of the sample weighed for the titration reaction. V0 is the volume

115

of hydrochloric acid solution consumed by the blank and V is the volume of hydrochloric acid

116

solution consumed by corresponding sample


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117

Thermogravimetric Analysis. Around 5 mg sample was accurately weighed and placed in a

118

platinum crucible for the thermogravimetric analysis. The thermogravimetric analysis (TGA) of

119

acylcaramel samples was carried out using a TGA-Q50 (TA Instruments-Waters LLC, US) thermal

120

analyzer in the range from room temperature to 800 °C in nitrogen atmosphere, at the heating rate of

121

10°C/min. The volume flow of nitrogen was 20 mL/min(24).

122

X-ray Powder Diffraction Analysis X-ray powder diffraction (XRD) analyses were carried out

123

on D/max-2500 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) with Cu Ka

124

radiation(1.542 Å) at 40 kV voltage, 2 theta scanning range of 3 °-35 °, scan rate is 1 ° / min(24).

125

Determination of Lipid-Water Partition Coefficient. Lipid-water partition coefficient is

126

usually denoted by P. It refers to the ratio of the concentrations of a compound in the lipid phase

127

(n-caprylic alcohol) and water phases when the two phase reach thermodynamic equilibrium at a

128

given temperature. It is usually computed by the following equation(25): P=CO/Cw

129 130 131

(2)

Where CO represents the concentration of the solute in the organic phase (n-caprylic alcohol) and Cw represents that in the water phase.

132

Although acylcaramel is a mixture, we use P to express its lipophilicity. The equilibrium

133

concentrations of acylcaramel in n-octanol and water were determined by UV-1800, UV-visible

134

spectrophotometry (Shimadzu Scientific Instruments, Japan). The maximum absorption wavelength

135

of 350 nm was selected as the detection wavelength for the measurement of the acylcaramel

136

concentration(26).

137

For the preparation of acylcaramel n-octanol stock solution, around 10 mg acylcaramel samples

138

were taken into five 250 mL conical flasks respectively, and 70 mL water saturated n-octyl alcohol



Page 8 of 28

139

was supplemented to each flask. The flasks were being shaken at the constant temperature of 25 °C

140

with a water bath shaking table (THZ-82 ，Changzhou Champion Instrument Manufacturing Co., Ltd.,

141

China) for overnight (If some samples couldn’t be completely dissolved, the mixture of each flask

142

was filtered using a Buchner funnel filter covered with 0.22μ cellulose acetate film) as the stock

143

solution.

144

The concentration of the stock solution was determined by solute weighing method. 5 mL

145

accurate measured stock solution was added to a weighted dry watch glass, then the watch glass was

146

taken into the drying oven to be dried at 50 °C to constant weight. The watch glass was weighted

147

again. The weight of acylcaramel dissolved in n-octyl alcohol solution was obtained by the

148

difference of the weight of the watch glass before and after the sample was added and dried. The

149

concentration of the stock solution (Cs, w/v %) can be calculated according to the following

150

equation:

Cs 

151

WS  W0 V

152

Cs is the concentration of the stock solution; W0 is the weight of the blank watch glass and Ws is

153

the weight of watch glass after the stock solution was added and dried. V is the volume of the stock

154

solution.

155

In order to obtain the working curve for the determination of caramel concentration in n-octanol

156

by spectrophotometry，accurately measured 2 mL, 4 mL, 6 mL, 8 mL and 10 mL of each sample

157

stock solution were taken into a 10 mL volumetric flask and the water-saturated n-octyl alcohol was

158

supplemented

159

concentrations. Subsequently, the absorbance of each solution was measured at 350 nm using

160

UV-1800, UV-Visuable spectrophotometer (Shimadzu Scientific Instruments, Japan) with water

to

the specified

volume

to

form

the


solutions

with

different

sample

Page 9 of 28


161

saturated n-octyl alcohol as the blank. The relation between the UV absorbance and concentration of

162

each acylcaramel was regressed by Microsoft EXCEL 2003.

163

For the measurement of the P values, 4 mL of stock solution and 4.0 mL of the phosphate buffer

164

solution saturated with n-octyl alcohol at pH 7.0 for each of the five sample compounds were

165

transferred into a 10 mL pluged centrifugal tube respectively. Subsequently, the tube was well

166

shaken at 25 °C using a same water bath shaking table as stated above at 120 r/min for 4 h, so that

167

the two phases reached equilibrium. After that, the shaking was stopped for 1h and the concentration

168

of the upper layer n-octanol solution was determined with UV-visible spectrophotometry. The P

169

value can be calculated by the following equation:

170 171 172

𝑃=

𝐶O 𝐶𝑤

=

𝐶o 𝐶s ― 𝐶o

Where Cs is the concentration of the stock solution; Co is the acylcaramel concentration in n-octanol.

173

Determination of the Color, Red and Yellow Indexes of Acylcaramels in Edible Oil. The

174

color index (European Brewery Convention, EBC), red index (RI) and yellow index (YI) were

175

employed to evaluate the coloring properties of raw and acylated caramel samples. For the

176

determination of color indexes of acylated caramels, the corn oil was used as the solvent, sample

177

solutions were prepared separately at 30 °C. The absorbances of the solutions at 610 nm (A610) were

178

measured respectively with UV-1800, UV-VIS Spectrophotometer. And the measurements were

179

repeated 3 times to take the average values. The color index (CI) in EBC unit (European Brewery

180

Convention unit) was calculated by the following formula(27):

181

CI(EBC unit) =

𝐴610 × 20000 0.076


(1)


182

Where A610 is the absorbance of 0.1 %(w/v) sample solution at 610 nm. 𝐴510

RI=10 × log (𝐴610)

183 184 185 186

(2)

Where the A510 and A610 are the absorbances of 0.1 % sample solution at 510 nm and 610 nm respectively. 𝐴460

YI=10 × log (𝐴610)

187 188

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Where the A460 is the absorbance of 0.1% sample solution at 460 nm.


(3)

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189

Determination of the Solubility of Acyl Caramel in Edible Oil. 0.01 g of each of the acylated

190

caramel samples with different fatty acid acyl groups was weighted and added into 20 mL accurately

191

weighted centrifuge tube containing 10 g corn oil, then the mixture was shaken in the THZ-82 water

192

bath shaking table at 30 °C for 10h. After that ， the tube was centrifuged using a Lg10-2.4a high

193

speed centrifuge (Beijing Jingli Centrifuge Co., Ltd, China) at 6000 r/min for 10 min. The

194

supernatant was then removed and the tube with the undissolved sample was put into a vacuum

195

thermostatic drying oven at 80 °C to be dried to a constant weight. The solubility could be obtained

196

by calculating the percentage of the weight of the dissolved sample in the weight of the oil.

197

Scanning Electron Microscope Analysis. Analysis of the morphology of the samples was

198

collected by a Hitachi SU1510 scanning electron microscope (SEM). The dry samples were placed

199

onto a copper holder with conductive adhesive and coating with 10 nm of sputtered gold using a

200

sputter-coater (Hitachi E-1010) before observation. SEM images were obtained using a Hitachi

201

SU-1510 SEM (Hitachi, Tokyo, Japan) at an acceleration voltage of 10 kV and the vacuum pressure

202

was maintained below 1 × 10−5 torr(28). For the direct comparison of the surface morphology, the

203

same magnification of 500× was selected for all the implants.

204

RESULTS AND DISCUSSION

205

Preparation of Acylcaramel with Different. Mf. By changing the ratio of acyl chloride to

206

caramel (Rac) from 0.5, 1, 2, 4 to 6, the acylcaramels with different mass fraction Mfs were prepared.

207

The Mfs of acylcaramels prepared with the acylation reaction of caramel by stearyl chloride with

208

different Racs are shown in Figure 1.

209



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90 80

Mf

70 60 50 40 30 20 0

1

2

3

4

5

6

Rac

210 211

Figure 1 The relation between Mf and Rac of stearyl caramel.

212

As shown in the Figure 1, the Mf of stearyl caramel increased with the increase of Rac. When Rac

213

reached higher than 2, the increase Mf became slow and when Rac was 6, Mf reached the highest.

214

Thus, the acylation of caramel with other fatty acyl chloride of different lengths of alkyl chains were

215

conducted with 6 of Rac.

216

Table 1 listed the Mfs of acylated caramels produced with different fatty acyl chlorides. It can

217

be seen that Mf increased with the length of the alkyl chain, which attributes to the mass of fatty acyl

218

increased when the chain length of fatty acyl increased.


Page 13 of 28


219

Lipo-Hydro Partition Coefficient. The lipid-water partition coefficient (P) and its pair value

220

(logP) are important indexes for the hydrophilic property of a compound. The higher the P and logP,

221

the more hydrophobicity values are, the higher hydrophilicity or oil solubility of the compound. The

222

P and logP values of acylcaramels with the different aryl chain lengths were shown in table 1. It can

223

be found that as the acyl chain grows, the P and logP values increase, indicating that the acylated

224

caramels are more soluble in lipid phase as the acyl chain grows(29), and stearyl sucrose has the

225

highest P and logP values, while the values of raw material and acetyl caramel couldn’t be measured

226

because they are almost insoluble in water-saturated n-octyl alcohol.

227

In addition, the effect of the Mf on P and logP values of stearyl caramel was also determined. As

228

shown in the table 1, it was found that the P and logP values of stearyl caramel increased with the

229

increase of Mf, indicating that their lipid solubility also increased.

230

Table 1. the mass fraction of acyls (Mf) and lipo-hydro partition coefficient (P) and logP values of

231

raw and acylated caramels.*

232

Samples

Mf (%)

P

logP

Raw caramel

0

-

-

Acetyl caramel

14.96±0.17

-

-

Butyryl caramel

23.12±0.07

0.38

-0.42

Capryloyl caramel

42.02±0.25

0.53

-0.28

Lauroyl caramel

61.18±0.18

1.53

0.19

Palmityl caramel

82.20±0.71

2.66

0.42

Stearyl caramel

82.00±0.26

4.97

0.70

* “- ” indicates that the value was not obtained.



Page 14 of 28

233 234 235

Analysis of FT-IR Spectra. The FT-IR spectra of raw caramel, acylcaramels and the mixture of raw caramel with stearic acid are shown in Figure 2.

a b

Transmittance(a. u.)

c d e 1741 C=O stretch

f g

2920℃ 2850 CH2

3430 O-H stretch h

4000

1469 C-H stretch

1109 1163 C-O stretch

720 C-H stretch

1707 C=O stretch

3500

3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

236 237

Figure 2 FT-IR spectra of raw caramel and acylcaramels.

238

a:Raw caramel b: Acetyl caramel c: Butyryl caramel d: Octyl caramel e: Lauryl caramel

239

f: Palmityl caramel g: Stearyl caramel h: Mixture of raw caramel with stearic acid.

240

The wide absorption peak at 3430 cm-1 is the hydroxyl stretching vibration. By comparing the

241

hydroxyl absorption peaks of the six acylated caramels with the raw caramel, it can be found that

242

after the acylation of caramel, the hydroxyl absorptions were greatly reduced, suggesting that a large

243

number of hydroxyl groups in caramel were consumed. The absorption peaks at 2920 cm-1, 2850

244

cm-1 and 1469 cm-1 were all the absorption peaks of -CH2, and their areas increased with the increase

245

of fatty acid chain length(30). In addition, when n > 4 for the chain of - (CH2) n-, an absorption peak

246

was appeared at 720 cm-1, and as the chain length increased, the absorption peak moved towards

247

lower wave numbers, but its strength increased. The absorption peaks at 1741 cm-1, 1109 cm-1 and


Page 15 of 28


248

1163 cm-1 are the absorption peaks generated by carbonyl in saturated linear fatty acid ester(31), and

249

the absorption intensity of the three absorption peaks increases with the growth of the fatty acid

250

chain length. As a comparison, the carbonyl stretch vibration of COOH appear at 1707cm-1 (32)and

251

no peak at 1109cm-1 for the mixture of raw caramel with stearic acid.

252

Based on the above FT-IR data and analysis, we can deduce that the hydroxyl group in the

253

caramel raw material has esterified with the corresponding fatty acyl chloride, and the corresponding

254

caramel acylation products have been obtained. Thermogravimetric

255

Analysis.

The

TG

curves

and

the

corresponding

derivative

256

thermogravimetry (DTG) curves of raw caramel and fatty acid acylated caramels are shown in Figure

257

3.

258

100

A

90 80

Weight %

70 a

60

b

50

c

40 f

30

d

g

e

20 10 0

0

100

200

300

400

500

600

T(℃ )

259


700

800


Page 16 of 28

0.8

B

deriviate wight℃ %/min℃

0.7 0.6

g

0.5

f

0.4

e d c

0.3

b a

0.2 0.1 0.0 0

100

200

300

400

500

600

700

800

T(℃ )

260 261 262 263

Figure 3 TG (A) and DTG (B) curves of raw and acylated caramels a:Raw caramel b: Acetyl caramel c: Butyryl caramel d: Octyl caramel e: Lauryl caramel f: Palmityl caramel g: Stearyl caramel.

264

It can be seen from the TG curves in Figure 3A that the thermal weight loss process of the

265

samples was mainly divided into three stages. The first stage occurred under the temperature range

266

from room temperature to 151 °C. This stage is the dehydration process of samples, which was

267

mainly caused by the evaporation of free water and the dehydration reaction of the combined water

268

in acylcaramels(33). The weight-loss is 5.30% for the raw caramel from 25 °C to 151°C. For

269

acylated caramel products, the weight loss rates for acetyl caramel, butyl caramel, octyl caramel,

270

lauroyl caramel, palm caramel and stearyl caramel were 5.05%, 4.98%, 4.04%,1.36% and 1.33%

271

respectively. It can be seen that the weight-losses of the products decreased with the increase of fatty

272

acid chain length at this stage, which may be attributed to that the hydrophobicity of the products

273

increased and the ability to bind water decreased and with the increase of fatty acid chain length..

274

The second stage for the weight-loss of the samples was taken place in the range of 151 °C


Page 17 of 28


275

-410 °C. This stage is the rapid pyrolysis of the samples and the TG curve dropped sharply.

276

Correspondingly, the DTG curves in Figure 3B show significant peaks indicating significant

277

weight-loss rate in this range. The internal structure of the sample underwent pyrolysis and

278

volatilization at high temperature and the degree of pyrolysis was mainly affected by the sample

279

structure and composition. At this stage, the heights of weight-loss rate peaks of acylcaramels are

280

higher than that of the raw material and the weight-loss ratios are greater than that of raw material

281

(Figure 3A and Table 2), indicating the pyrolysis was more intense at this stage when caramel was

282

acylated. With the increase of molecular weight of acyl groups, the weight-losses of acylcaramels

283

increased gradually. Compared with acylcaramels，the raw material has the lowest weight-loss and

284

its weight-loss process was irregular and had many small weight loss peaks.

285 286

Table 2. The weight loss ratios of the raw and acylated caramels at different heating temperature ranges (%). Samples

Stage 1(%) Stage 2 (%)

Stage 3(%)

Total(%)

(25-150)°C (151-410) °C (410-800)°C Raw caramel

5.30

24.20 Figure

17.79

57.30

Acetyl caramel

5.05

42.04

21.20

68.29

Butyryl caramel

4.63

46.12

14.81

64.95

Capryloyl caramel 4.70

52.63

20.80

78.13

Lauroyl caramel

2.47

58.92

14.87

76.25

Palmityl caramel

1.33

71.17

11.20

83.70

Stearyl caramel

1.21

71.5

7.85

80.56

287



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288

The third stage of the thermal decomposition was occurred in the temperature range of 410°C

289

-800°C. This stage is mainly attributed to the bond-cracking and carbonization of the sample

290

residues. At this stage, the weight-loss processes of raw caramel and acylcaramels were relatively

291

smooth. The difference is that two peaks appeared at 500°C and 787 °C in the weight-loss rate curve

292

of raw caramel(Figure 3B). The weight-losses of all acylated caramels were greater than that of the

293

raw material, which may be attributed to the volatilization of the acyl groups generated from the

294

thermal decomposition of acylated caramels.

295 296

X-ray Diffraction Analysis. X-ray diffraction patterns of raw caramel and fatty acid acylated caramels are shown in Figure 4.

297 298

Figure. 4 X-ray diffraction patterns of the raw caramel and fatty acid acylated caramels

299

a: acetyl caramel c: butyl acyl caramel d: simba acyl caramel e: lauroyl caramel f: palmitoyl caramel

300

g: stearyl caramel h: Hexadecanoyl caramel i: Octadecanoyl caramel.

301

As can be seen from Figure 4, the diffraction peaks of all samples are very weak, indicating that

302

they are in low crystallization degree or in disordered amorphous structure. The raw caramel has a

303

distinct broad gentle peak at 20º of 2θ. However, the X-ray diffraction curves of acylcaramels


Page 19 of 28


304

showed some regular changes. The diffraction curve of acetyl caramel is nearly flat, however, the

305

height of diffraction peak near 20ºof 2θ increased with the chain length of acyl groups and that of

306

stearyl caramel has the highest peak height. These phenomena indicated that the crystallization

307

degree of acylated caramel increased with the increase of the length of the acyl chain.

308

SEM Analysis. In order to investigate the morphology changes of caramel after acylation of the

309

raw material by fatty acyl chlorides, SEM analysis was carried out on various samples. The SEM

310

images of samples at 500x magnification were shown in Figure 5.

311

312 313 314

Figure 5. The SEM images of the raw and acylated caramels at 500x magnification a:raw caramel; b: butyryl caramel; c: lauryl caramel d: stearyl caramel.

315

It can be seen that the raw caramel shows a pile of small globules with smooth surface and

316

different sizes (Figure 5A). However, the caramel morphology changed greatly after acylation.

317

Acylated caramels such as butyryl, lauryl and stearyl caramels formed irregular porous particles

318

(shown in Figure 5B, C, D) ， which may be due to the changes in the hydrophilic properties of

319

caramel after acylation.

320



Page 20 of 28

321

The Solubility of Acyl Caramel in Edible Oil. General commercial caramels have good water

322

solubility and coloring ability, but their solubility is very low in edible oil, which severely limits its

323

application as a pigment in edible oil and its products. Through the acylation of caramel, the

324

hydroxyl groups of the caramel were esterified and the lipid solubility of acylcaramels was

325

improved. The solubility of different acylcaramels in vegetable oil (corn oil) was determined. The

326

results are shown in the table 3. It can be seen that as the acyl chain grow, its lipid solubility, color,

327

red and yellow indexes increased. Stearyl caramel has the highest values of lipid solubility of 5.73

328

mg/mL. However,

329

which reached, 25818.60, 1.149 and 1.757 respectively. As a sample, in contrast to corn oil without

330

caramel, the appearance of corn oil colored with palmityl caramel was shown in Figure 6.

lauroyl caramel has the highest color, red and yellow indexes in corn oil,

331

Table 3 Solubility, color, red and yellow indexes of edible oil with different acylcaramel samples

332

added. Samples

Solubility（mg/mL） CI（EBC unit）

RI

YI

Acetyl caramel

0.77

5809.98

0.803 1.713

Butyryl caramel

1.03

9964.23

0.847 1.550

Capryloyl caramel

1.84

11298.63

0.360 0.738

Lauroyl caramel

2.47

25818.60

1.149 1.757

Palmityl caramel

4.34

16250.30

0.396 0.623

Stearyl caramel

5.73

22978.48

0.400 0.609

333


Page 21 of 28


334 335

Figure 6 Comparison of the appearance of corn oil (left) and that with palmityl caramel solved in

336

corn oil.

337

In summary, acylated caramels were obtained by esterification of caramel hydroxyl groups with

338

aliphatic acyl chloride with different chain lengths. Acetyl, butyryl, octyl, lauryl, palmityl and stearyl

339

caramels were prepared with the ratio of acyl chloride and caramel of 6. The formation of acylated

340

caramel was confirmed by FT-IR spectra. Thermal analysis showed that the weight-loss of acylated

341

caramel was higher than that of the raw caramel. The SEM analysis showed that the morphology of

342

acylated caramel was significantly different from that of raw material. The acyl mass fraction of

343

acylated caramel increased with the increase of acyl chain length. Meanwhile, the lipo-hydro

344

partition coefficient, the solubility in corn oil, color, red and yellow indexes increased with the

345

increase of mass fraction of acyl. This study provides a method to improve the solubility of caramel

346

in lipid phase and expand the application range of caramel.

347

AUTHOR INFORMATION

348

Corresponding Author

349

1.Yujie Dai,



Page 22 of 28

350

State Key Laboratory of Food Nutrition and Safety, College of Biotechnology, Tianjin University of

351

Science and Technology.

352

Tel: +86 22 60601265. Fax: +86 22 60602298. E-mail: [email protected].

353

2. Shiru Jia

354

State Key Laboratory of Food Nutrition and Safety, College of Biotechnology, Tianjin University of

355

Science and Technology.

356

Tel: +86 22 60601598. Fax: +86 22 60602298. E-mail: [email protected].

357

Fundings

358

This work was supported by National Key R&D Program of China, Grant No.

359

2018YFD0400205, Tianjin science and technology plan project, Grant No. 18PTSYJC00140, and

360

National Natural Science Foundation of China (Grant No. 21272171).

361

Notes

362

The authors declare no competing financial interest.

363

REFERENCES

364

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TABLE OF CONTENTS GRAPHIC (TOC GRAPHIC) O n Cl

HO n Cl + O n Cl

OH

HOOC HO

O HO

OH

O

O

OH OH

Water soluble

n

O

Esterification COOH

CHO

OH O

OH

HOOC O HO n O

O

O

OH OH

Oil soluble O COOH

O HO

Fatty acid acyl chloride Caramel (n=2 to18)

OH

CHO OH O

O

n

Acylcaramel Vegitable oil

Vegitable oil Coloring

441



Scheme of reaction for the acylation of caramel 205x70mm (300 x 300 DPI)


Page 28 of 28

Preparation and characterization of acylcaramel - Journal of

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