A Comparative Study on Emulsifying Properties of a Homologous

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Functional Structure/Activity Relationships

A Comparative Study on Emulsifying Properties of a Homologous Series of Long-chain 6#-O-Acylmaltose Esters Ya-Ru Ma, Martin G Banwell, Rian Yan, and Ping Lan J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b02391 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Journal of Agricultural and Food Chemistry

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A Comparative Study of the Emulsifying Properties of a

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Homologous Series of Long-chain 6ʹ-O-Acylmaltose Esters

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Ya-Ru Ma†, Martin G. Banwell‡,§, Rian Yan*†,, and Ping Lan*†,‡

6

†

7

China

8

‡

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519070, China

Department of Food Science and Engineering, Jinan University, Guangzhou, 510632,

Institute for Advanced and Applied Chemical Synthesis, Jinan University, Zhuhai,

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§

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National University, Canberra, ACT 2601, Australia

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*Corresponding author (Telephone: +86-20-85221367. Fax: +86-20-85224766.

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E-mail: [email protected])

Research School of Chemistry, Institute of Advanced Studies, The Australian

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ACS Paragon Plus Environment


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ABSTRACT Emulsifiers derived from renewable resources such as sucrose and

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fatty acids are high volume commodity chemicals and currently produced by

17

traditional chemical synthesis techniques that lack the capacity to form the most

18

desirable mono-esters (of sucrose) in a selective and efficient fashion. The

19

development of new emulsifiers (surfactants) from alternate, structurally simpler but

20

nevertheless abundant disaccharides such as maltose represents a possible solution to

21

this problem. Herein, we report the facile enzymatic preparation of a homologous

22

series of 6ʹ-O-acylmaltose esters and an in-depth evaluation of them revealing that

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their surfactant properties and thermal stabilities are largely determined by the length

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of the fatty acid chain. In the first such comparison, we show that the foaming and

25

emulsifying effects of certain of these maltose mono-esters are superior to those of

26

their sucrose-derived and commercially exploited counterparts. As such, maltose

27

esters have considerable potential as emulsifiers for use in, for example, the food

28

industry.

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KEYWORDS: 6ʹ-O-acylmaltose esters, surfactant properties, foaming properties,

30

emulsifying properties, thermal stabilities, practical applications


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INTRODUCTION Emulsifiers

(surfactants)

derived

from

renewable

resources

such

as

33

carbohydrates and fatty acids have been used extensively in the food, cosmetic,

34

detergent, and pharmaceutical industries, not least because they are odorless, tasteless,

35

non-ionic and biodegradable as well as being non-toxic. Such carbohydrate-derived

36

esters compare well in overall performance (i.e., foaming power, emulsification

37

effects, detergency, wettability) with petroleum-derived surface-active products. They

38

are also sustainably produced and have relatively low adverse impacts on health.1-4

39

Furthermore, certain carbohydrate esters display significant antimicrobial activity and

40

cytotoxicities5-9 while the specific properties of these esters, such as their hydrophilic

41

lipophilic balances (HLBs), can be “dialed up” over a wide range through the

42

attachment of varying numbers and types of fatty acyl moieties to different saccharide

43

cores.10 From an industrial perspective, only a few carbohydrates are suitably

44

functionalized, well priced and sufficiently readily available to serve as useful raw

45

materials.11 The few major types of carbohydrate-derived emulsifiers being produced

46

at industrial scale at the present time are sucrose esters, sorbitan (span) esters, alkyl

47

polyglycosides and fatty acid glucamides.11

48

Sucrose mono-esters are the most commonly encountered disaccharide-derived

49

surfactants on the market. While they are of particular utility in the food industry,

50

there are fundamental challenges associated with their manufacture that arise from the

51

presence of eight distinct hydroxyl groups within the carbohydrate core and the

52

ensuing potential for formation of complex product mixtures that include various 3



53

possible mono-, di-, tri- and even higher-order esters.12,13 Most of the sucrose esters

54

on the market are currently manufactured by very traditional chemical techniques that

55

involve reactions conducted under harsh conditions and subsequent, tedious

56

purification.14 The enzymatic synthesis of sucrose esters has garnered considerable

57

interest because of the potential for establishing environmentally benign processes

58

that proceed with high selectivity and so simplifying downstream purification.15-19

59

Nevertheless, the extensive studies20-26 conducted thus far have revealed some major

60

drawbacks associated with this approach, most notably the low activity of the

61

enzymes in the polar organic solvents required to dissolve the carbohydrates, the

62

relatively long reaction times involved (from 6 h to several days) as well as the low

63

concentrations of the substrate that must be employed (and thus restricting

64

throughput).

65

The demand for sugar-based mono-esters that possess appropriate emulsifying

66

properties is expected to increase substantially in the near future, especially if the

67

production of high purity forms can be realized.11 The development of surfactants

68

from disaccharides less complex than sucrose offers a means of doing so.

69

Corn-derived maltose is an inexpensive and sustainably produced disaccharide

70

comprised of two 1′,4-linked glucose residues and so embodying just two primary

71

hydroxyl groups (there are three in sucrose). A previous study27 suggests that esters

72

derived from reducing sugars such as maltose cannot be readily obtained by

73

conventional chemical methods. Accordingly, most efforts to prepare maltose fatty

74

acid esters have focused on enzymatic approaches. The first regioselective 4


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monoacylations of maltose, which employed the protease subtilisin28 or the lipase

76

derived from Candida Antarctica,29 were rather inefficient. An improved process has

77

been reported involving diatomaceous earth-immobilized Humicola lanuginose in a

78

tert-amyl alcohol/DMSO solvent system and wherein fatty acid vinyl esters served as

79

the

80

6′-O-acylmaltose esters were obtained in yields of up to 80%.30 In further efforts to

81

realize practical syntheses of 6ʹ-O-acylmaltose esters, a range of commercially

82

available and immobilized lipases, most notably Novozym435 and Lipozyme TLIM,

83

has been used to catalyze the regioselective acylation of maltose with various series of

84

fatty acids or their derivatives.9,31-34 However, most of these protocols remain less

85

than practical because the necessary anhydrous conditions are achieved using

86

molecular sieves, polar solvents such as DMSO must be used, yields are modest

87

(99.0%), vinyl n-octanoate (>99.0%), vinyl

99

decanoate (>99.0%), vinyl laurate (>99.0%), vinyl myristate (>99.0%), vinyl

100

palmitate (>96.0%) and vinyl stearate (>95.0%) were purchased from the Tokyo

101

Chemical Industry Co. Ltd. (Tokyo, Japan) while Novozym435 was supplied by

102

Sigma-Aldrich (St. Louis, MO). Pure canola oil, full cream milk, and fresh coconuts

103

(for the preparation of coconut milk) were purchased from a local grocery store while

104

commercial sucrose stearic esters S-1170 and S-1570 were obtained from the

105

Mitsubishi-Chemical Foods Corporation (Tokyo, Japan). Monopalmitin (MP) was

106

purchased from Guangzhou Cardlo Biochemical Technology Co., Ltd. (Guangdong,

107

China).

108

General Protocols. Proton (1H) and carbon (13C) NMR spectra were recorded at

109

25 °C on a Bruker spectrometer (Billerica, MA) operating at 600 MHz for proton and

110

150 MHz for carbon nuclei. For 1H NMR spectra, the residual protio-solvent signals

111

were used as internal standards. Low-resolution ESI mass spectra were recorded on an

112

Agilent single-quadrupole liquid chromatograph-mass spectrometer (Santa Clara,

113

CA).

114

Synthesis of the 6ʹ-O-Acylmaltose Esters. A solution of D-maltose (1.00 g,

115

2.92 mmol) in anhydrous THF/pyridine (20 mL, 7:3 v/v) was treated with

116

Novozym435 (200 mg, 20% w/w loading) and the relevant fatty acid vinyl ester (3.0

117

equiv., 8.76 mmol). The ensuing mixture was stirred magnetically at 45 °C for 36 h.

118

The residual enzyme was then removed by filtration of the cooled reaction mixture 6


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119

and the solids thus retained washed with methanol (2 × 10 mL). The combined

120

filtrates were dried (Na2SO4), filtered then concentrated under reduced pressure to

121

afford a yellow solid, trituration of which (with 100 mL of diethyl ether) followed by

122

suction filtration gave a pale-yellow solid. This was subjected to flash

123

chromatography (silica, 1:9 v/v methanol/dichloromethane elution) to afford, after

124

concentration of the relevant fractions (Rf = 0.2), the corresponding 6ʹ-O-acylmaltose

125

ester 2-8 (Figure 1) as an amorphous, white solid.

126

6ʹ-O-Hexanoylmaltose, 2. Yield: 72% of a 2.2:1 mixture of α- and β-anomers.

127

1

128

Hz, 1H, H-6ʹa), 4.18 (dd, J = 11.8, 6.4 Hz, 1H, H-6ʹb), 3.89 (m, 2H, H-5ʹ, H-5), 3.83

129

(m, 2H, H-6), 3.62 (m, 1H, H-3ʹ), 3.45 (m, 3H, H-4, H-2ʹ, H-2), 3.28 (t, J = 9.5 Hz,

130

1H, H-4ʹ), 2.39 (t, J = 7.5 Hz, 2H, –CH2–CO–), 1.65 (m, 2H, –CH2–CH2–CO–), 1.29

131

(m, 4H, –CH2– hexanoyl backbone), 0.94 (t, J = 6.9 Hz, 3H, –CH3) (signals due to

132

two protons obscured or overlapping).

133

175.5 (C=O), 103.1 (C1ʹ), 93.8 (C1), 82.6 (C4), 75.0 (C3ʹ), 74.6 (C3), 74.3 (C2ʹ), 73.4

134

(C2), 72.3 (C5), 71.7 (C5ʹ), 71.7 (C4ʹ), 64.9 (C6ʹ), 62.4 (C6), 34.9 (–CH2–CO–), 32.4,

135

25.7, 23.4 (–CH2–CH3), 14.3 (–CH3). MS (ESI, +ve): m/z 458 ([M+NH4]+, 100%),

136

463 ([M+Na]+, 20).

H NMR (600 MHz, CD3OD) δ (α-anomer) 5.13 (m, 1H, H-1ʹ), 4.41 (dd, J = 11.8, 3.1

13

C NMR (150 MHz, CD3OD) δ (α-anomer)

137

6ʹ-O-Octanoylmaltose, 3. Yield: 75% of a 2:1 mixture of α- and β-anomers. 1H

138

NMR (600 MHz, CD3OD) δ (α-anomer) 5.11 (m, 1H, H-1ʹ), 4.39 (dd, J = 12.0, 2.1 Hz,

139

1H, H-6ʹa), 4.16 (dd, J = 12.0, 6.4 Hz, 1H, H-6ʹb), 3.89 (m, 2H, H-5ʹ, H-5), 3.83 (m,

140

2H, H-6), 3.62 (m, 1H, H-3ʹ), 3.45 (m, 3H, H-4, H-2ʹ, H-2), 3.26 (t, J = 9.5 Hz, 1H, 7



141

H-4ʹ), 2.37 (t, J = 7.5 Hz, 2H, –CH2–CO–), 1.62 (m, 2H, –CH2–CH2–CO–), 1.29 (m,

142

8H, –CH2– octanoyl backbone), 0.90 (t, J = 6.9 Hz, 3H, –CH3) (signals due to two

143

protons obscured or overlapping). 13C NMR (150 MHz, CD3OD) δ (α-anomer) 175.5

144

(C=O), 103.1 (C1ʹ), 93.8 (C1), 82.5 (C4), 75.0 (C3ʹ), 74.6 (C3), 74.3 (C2ʹ), 73.4 (C2),

145

72.3 (C5), 71.7 (C5ʹ), 71.6 (C4ʹ), 64.9 (C6ʹ), 62.5 (C6), 34.9 (–CH2–CO–), 32.9, 30.2,

146

30.0, 26.0, 23.7 (–CH2–CH3), 14.4 (–CH3). MS (ESI, +ve): m/z 486 ([M+NH4]+,

147

100%), 491 ([M+Na]+, 40).

148

6ʹ-O-Decanoylmaltose, 4. Yield: 77% of a 3:1 mixture of α- and β-anomers. 1H

149


150


151

2H, H-6), 3.62 (m, 1H, H-3ʹ), 3.45 (m, 3H, H-4, H-2ʹ, H-2), 3.26 (t, J = 9.5 Hz, 1H,

152


153

12H, –CH2– decanoyl backbone), 0.90 (t, J = 6.9 Hz, 3H, –CH3) (signals due to two

154


155

(C=O), 103.1 (C1ʹ), 93.8 (C1), 82.6 (C4), 75.0 (C3ʹ), 74.6 (C3), 74.3 (C2ʹ), 73.4 (C2),

156

72.3 (C5), 71.7 (C5ʹ), 71.6 (C4ʹ), 65.0 (C6ʹ), 62.4 (C6), 34.9 (–CH2–CO–), 33.0, 30.6,

157

30.4(3), 30.4(0), 30.2, 26.0, 23.7 (–CH2–CH3), 14.4 (–CH3). MS (ESI, +ve): m/z 514

158

([M+NH4]+, 100%), 519 ([M+Na]+, 40).

159

6ʹ-O-Lauroylmaltose, 5. Yield: 80% of a 4:1 mixture of α- and β-anomers. 1H

160


161


162



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163


164

16H, –CH2– lauroyl backbone), 0.90 (t, J = 6.9 Hz, 3H, –CH3) (signals due to two

165


166

(C=O), 103.1 (C1ʹ), 93.8 (C1), 82.6 (C4), 75.0 (C3ʹ), 74.6 (C3), 74.3 (C2ʹ), 73.4 (C2),

167

72.3 (C5), 71.7 (C5ʹ), 71.6 (C4ʹ), 65.0 (C6ʹ), 62.4 (C6), 34.9 (–CH2–CO–), 33.1, 30.7

168

(×2), 30.6, 30.5, 30.4, 30.2, 26.0, 23.7 (–CH2–CH3), 14.4 (–CH3). MS (ESI, +ve): m/z

169

552 ([M+NH4]+, 100%), 547 ([M+Na]+, 50).

170

6ʹ-O-Myristoylmaltose, 6. Yield: 79% of a 4:1 mixture of α- and β-anomers. 1H

171


172


173


174


175

20H, –CH2– myristoyl backbone), 0.90 (t, J = 6.9 Hz, 3H, –CH3) (signals due to two

176


177

(C=O), 103.1 (C1ʹ), 93.8 (C1), 82.6 (C4), 75.0 (C3ʹ), 74.6 (C3), 74.3 (C2ʹ), 73.4 (C2),

178

72.3 (C5), 71.7 (C5ʹ), 71.6 (C4ʹ), 65.0 (C6ʹ), 62.4 (C6), 34.9 (–CH2–CO–), 33.1, 30.8,

179

30.7(7), 30.7(6), 30.7(3), 30.6, 30.7, 30.4, 30.2, 26.0, 23.7 (–CH2–CH3), 14.4 (–CH3).

180

MS (ESI, +ve): m/z 570 ([M+NH4]+, 100%), 575 ([M+Na]+, 40).

181

6ʹ-O-Palmitoylmaltose, 7. Yield: 76% of a 2:1 mixture of α- and β-anomers. 1H

182


183


184




185


186

24H, –CH2– palmitoyl backbone), 0.90 (t, J = 6.9 Hz, 3H, –CH3) (signals due to two

187


188

(C=O), 103.1 (C1ʹ), 93.8 (C1), 82.6 (C4), 75.0 (C3ʹ), 74.6 (C3), 74.3 (C2ʹ), 73.4 (C2),

189

72.3 (C5), 71.7 (C5ʹ), 71.6 (C4ʹ), 65.0 (C6ʹ), 62.4 (C6), 34.9 (–CH2–CO–), 33.1, 30.8

190

(×2), 30.7(9), 30.7(8), 30.7(6), 30.7(4), 30.6, 30.5, 30.4, 30.2, 26.0, 23.7 (–CH2–CH3),

191

14.4 (–CH3). MS (ESI, +ve): m/z 598 ([M+NH4]+, 100%), 603 ([M+Na]+, 80).

192

6ʹ-O-Stearoylmaltose, 8. Yield: 65% of a 2:1 mixture of α- and β-anomers. 1H

193


194


195


196


197

28H, –CH2– stearoyl backbone), 0.90 (t, J = 6.9 Hz, 3H, –CH3) (signals due to two

198

proton obscured or overlapping).

199

(C=O), 101.7 (C1ʹ), 92.4 (C1), 81.2 (C4), 73.7 (C3ʹ), 73.2 (C3), 72.9 (C2ʹ), 72.0 (C2),

200

70.9 (C5), 70.3 (C5ʹ), 70.2 (C4ʹ), 63.6 (C6ʹ), 61.0 (C6), 33.7 (–CH2–CO–), 31.7, 29.4

201

(×3), 29.3(8) (×2), 29.3(6) (×2), 29.3(2), 29.2, 29.1, 29.0, 28.9, 24.8, 22.3 (–CH2–

202

CH3), 13.1 (–CH3). MS (ESI, +ve): m/z 626 ([M+NH4]+, 100%), 631 ([M+Na]+, 20).

13

C NMR (150 MHz, CD3OD) δ (α-anomer) 174.1

203

Determination of Hydrophilic-Lipophilic Balance (HLB) Values. The water

204

number method36 was employed to determine the HLB values of esters 2-8. Thus, 1.0

205

g of each compound was dissolved in DMF/benzene (25 mL, 100:5 v/v) and the

206

resulting solution titrated with distilled water while being maintained at 25 ±1 °C until 10


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207

a permanent turbidity was obtained. All results were reproducible to within 0.5 units

208

of HLB. A plot of consumed water number vs HLB was then established using

209

emulsifiers with known HLB values, including Span 80 (HLB = 4.3), Span 60 (HLB

210

= 4.8), Span 40 (HLB = 6.7), Span 20 (HLB = 8.6), Tween 80 (HLB = 15.6) and

211

Tween 20 (HLB = 16.7), for calibration purposes. The HLB values of compounds 2-8,

212

along with the commercial sucrose esters S-1170 and S-1570 were then read from this

213

plot.

214

Determination of Critical Micelle Concentration (CMC). The surface tensions

215

of esters 2-8 were measured using the Wilhelmy plate method.35 Thus, a series of

216

variably concentrated aqueous solutions of the esters were prepared then maintained

217

at 28 °C. The equilibrium surface tensions of each solution were then measured using

218

a Krüss tensiometer (Hamburg, Germany). The CMCs of each sample were then

219

calculated from the break in the surface tension values versus log10 concentration and

220

are expressed in μM. γCMC is the surface tension corresponding to the CMC.

221

Measurement of Oil-Water Interfacial Tension. Canola oil-water interfacial

222

tensions were determined by the Du NoüY ring method37 using a JYW-200B

223

automatic interfacial tensiometer (Chengde, China). A series of aqueous solutions of

224

the test samples at concentrations above the corresponding CMC were mixed with

225

equal volumes of canola oil. The oil-water interfacial tension was measured with a

226

horizontal hanging platinum ring of known geometry (platinum circle radius = 9.55

227

mm, platinum wire radius = 0.3 mm). The ring was dipped into the solutions

228

maintained at 28 °C then pulled out again. The maximum force, in mN/m, required to 11



229

pull the ring through the interface is defined as the oil-water interfacial tension or

230

γO/W.

231

Evaluation of Foamability and Foaming Stability. Evaluations of the

232

foamability and the foaming stability of esters 2-8 together with four controls were

233

undertaken using a previously described method.38 Thus, 10 mL of 0.2% and 0.5%

234

w/w aqueous dispersions of each sample were placed in a 50 mL, flat-bottomed

235

measuring cylinder and the height of solution (H0) was measured before being

236

homogenized, using a blender, at 3000 rpm for 2 min. The foam height (H2) and the

237

total height (H1) were determined immediately. Following resting periods of 10 min,

238

the new foam heights (H3) were then recorded. The foamability and the foaming

239

stabilities of esters 2-8 together with those of the controls S-1170, S-1570,

240

monopalmitin (MP), and Tween 80 were calculated using the following equations:

241

Foamability (%) = (H1-H0)/ H0 × 100%

242

Foaming stability (%) = H3/ H2 × 100%

243

A more end-use oriented evaluation of ester 6 and control S-1570 was conducted

244

by measuring their foamabilities and their foaming stabilities in a milk system. Thus,

245

250 mL of 0.2% w/w dispersions of each of ester 6 and S-1570 in full cream milk

246

were placed in a BodumTM milk-frother. After being processed for 5 min, the initial

247

foam height was measured and then again after a resting period of 10 min. The thus

248

determined foaming properties of the two samples were then compared (see below).

249

Determination of Emulsion Stability Index. The emulsifying stability indices

250

of esters 2-8 together with four controls were determined using minor modifications 12


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251

of known39 protocols. Specifically, 10.0 g of 0.01%, 0.2% and 0.5% w/w aqueous

252

solutions of the test samples were each mixed with 1.0 g of canola oil while a solution

253

lacking a test material was used as the blank control. The ensuing mixtures were

254

homogenized for 2 min at 10,000 rpm using a blender and a 20 µL sample of the

255

resulting emulsion was diluted with 2 mL deionized water before being drawn into a

256

clean spectrometric cuvette. The absorbance of the emulsion at 500 nm was read at 0

257

min and 20 min. The emulsion stability indices (ESIs) were then calculated using the

258

following equation:

259

ESI (%) = A0 × 20/[A0 – A20] × 100%

260

where A0 and A20 are the absorbances obtained at 0 min and 20 min.

261

Analysis of Thermal Stability. The thermal stabilities of esters 2-8 as well as

262

two sucrose ester controls, viz. S-1170 and S-1570, were evaluated by a

263

thermogravimetric analysis. Specifically, 10 mg of each sample was placed on a

264

Thermo Gravimetric Analyzer (TGA, TG209F3-ASC, Germany) with the testing

265

temperature range being from 50 to 350 °C while a heating rate of 5 °C/min was

266

employed. The samples were held in the Al2O3 crucible while being maintained under

267

nitrogen atmosphere. The reductions in mass of each sample as a function of

268

temperature were then recorded.

269

Measurement of Particle Size Distributions of Coconut Milk Emulsions. The

270

flesh (100 g) from fresh coconuts was mixed with water (33 mL) in a juicer for 5 min

271

and the ensuing blend filtered through cheesecloth to obtain fresh coconut milk.40 10.0

272

g of 0.3% w/w solutions of ester 6, S-1570 and Tween 80 in fresh coconut milk were 13



273

homogenized for 2 min at 10,000 rpm using a blender. One drop of each emulsion was

274

transferred to a microscope slide and a cover slip placed over the sample. The samples

275

were then analyzed using an Olympus model BX61 optical microscope (Tokyo, Japan)

276

fitted with a camera and objective lens of 200× and capable of 1024 × 720 resolution.

277

Image Products 6.0 software (Olympus, JP Ltd.) was then used to determine the

278

distribution and size of the particles in each emulsion.41

279

Statistical Analyses. Each evaluation experiment was performed three times.

280

The results are presented as the mean values ± standard deviation (SD). Statistical

281

analyses were performed using SPSS software (SPSS, Inc., Chicago, IL) to identify

282

any significant differences. All values were assessed by one-way analysis of variance

283

(ANOVA) in conjunction with Duncan's new multiple-range test (MRT).

284

RESULTS AND DISCUSSION

285

Syntheses and Characterization of Products. A fundamental challenge

286

associated with developing an enzymatic procedure for the selective acylation of

287

maltose with long fatty acids arises through the de-activation of the enzyme by the

288

polar solvents (DMSO, DMF, DMA) normally used to dissolve disaccharides. To

289

circumvent such difficulties, most recent protocols employ co-solvent systems in

290

which such highly polar solvents are doped with alcoholic ones such as 2-butanol or

291

t-amyl alcohol that serve as adjuvants.9,31-34 In this study the THF/pyridine system that

292

showed high efficiency in the acylation of glucose41 was used and the volume ratio

293

optimized (to 7:3) in order to achieve the highest yields. With this system, the

294

concentration of maltose in the reaction medium could be as high as 0.14 M and such 14


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295

that molecular sieves did not need to be used. Employing the commercially available

296

Novozym435 in combination of vinyl esters (as the acyl donors) has proven

297

advantageous since no activation of the enzyme is required prior to conducting the

298

reaction. The protocol described here is robust and offers the capacity to generate a

299

signiﬁcant number of maltose esters that incorporate the carboxylic acid residue

300

exclusively at the C-6ʹ position.

301

The structures of products 2-8 were confirmed by undertaking appropriate MS

302

spectrometric and NMR spectroscopic analyses including, in the latter case, those

303

arising from the outcomes of HMBC experiments. In all instances, two sets of

304

chemical shifts were observed in the NMR spectra, reflecting the presence of both the

305

α- and β-anomeric forms of the product disaccharide.30 In all cases, the former

306

anomer predominated when the 1H NMR spectra were recorded in CD3OD but when

307

DMSO-d6 was used as solvent then the two isomers were present in a 1:1 ratio. Key

308

elements of the HMBC correlation spectra derived from 6-Oʹ-lauroylmaltose are

309

shown in Figure 2. These are representative of the series as a whole. As revealed by

310

such means, the methylene protons at C-6ʹ interact with the carbonyl carbon of the

311

ester moiety and thus establishing the site of esterification. Other key interactions are

312

shown in Figure 2 and, taken together, these allowed for the assignment of the

313

resonances due to the non-hydroxylic protons associated with the maltose and the

314

side-chain substructures.

315

Hydrophile-Lipophile Balance (HLB) Values. The HLB values of each

316

maltose mono-ester, as determined by the water number method, are shown in Table 1 15



317

and revealed, unexpectedly, an inverted V-shaped profile. Thus, ester 2, possessing the

318

shortest hydrophobic side-chain, displayed a low HLB value (11.21) while those

319

esters 2-5 having increasingly long side-chains displayed concomitantly higher HLB

320

values (rising from 11.21 to 15.21 through the series). These trends are contrary to

321

those predicted,9 a possible explanation being the poor solubilities of esters 2-4 in

322

benzene/DMF system employed in the water number method. This would result in

323

less water being required to form a permanent turbidity (and thus leading to lower

324

HLB values). Esters 5–8 displayed decreasing HLB values (down to 10.34), a trend

325

attributed to the more hydrophobic effects of the longer side-chains. Significantly,

326

esters 2, 3 and 7 showed similar HLB values to commercial sucrose stearoyl ester

327

S-1170 that contains 64% sucrose mono-esters, while the HLB value of ester 5 is

328

close to that of S-1570 containing 74% sucrose mono-esters. Since the potential utility

329

of emulsifiers is often defined using the HLB system,43 these values suggest that, like

330

the S-1170 and S-1570, the 6ʹ-O-acylmaltose mono-esters could have similar practical

331

applications.

332

Critical Micelle Concentration (CMC). The CMC is the concentration of

333

surfactants above which micelles start forming. In general, CMC values are correlated

334

with the length of the hydrophobic moiety.35 As shown in Table 1 for maltose esters

335

2-6, their CMCs decreased from 6578 to 9 µM on moving from 6ʹ-O-hexanoylmaltose

336

to 6ʹ-O-palmitoylmaltose (concomitantly, the γCMC values increase with the length of

337

the ester side-chains). However, it is noteworthy that ester 8, possessing longest side

338

chain, showed a slightly higher CMC value than congener 7. A similar trend has been 16


Page 16 of 39

Page 17 of 39


339

found for other sugar (sucrose, fructose) esters.37 Ferrer et al. suggested this might be

340

due to coiling of the large hydrocarbon chain that results in a significant shorter

341

hydrocarbon tail and, therefore, an unexpectedly higher CMC value.35 Regardless of

342

the origins of them, esters 5-8 display relatively low CMC values, making such

343

compounds very attractive nonionic surfactants.

344

Oil-Water Interfacial Tension (γO/W). To be an effective emulsifier in the food

345

industry, a compound must possess good surface activity and provide a low interfacial

346

tension within the system in which it will be used.35 The oil-water interfacial tension

347

of each maltose ester above their respective CMCs is shown in Table 1 and indicate

348

that they are effective emulsifiers. While the hydrophobic side chain plays an

349

important role in the interfacial tension value no clear trends were observed.

350

Nevertheless, the γO/W data derived from this study demonstrate that the maltose esters

351

have a tendency to migrate to the oil-water interface and so act as potent emulsifiers.

352

Foaming Properties. Foams and emulsions have wide practical applications in

353

the detergent, food and cosmetics industries.44 Foams, which are usually formed with

354

a systematic hexagonal configuration, are thermodynamically unstable in an emulsion

355

system. That is, given sufficiently long-standing times, the foams derived from

356

immiscible mixtures will separate into two distinct phases.44,45 Surfactants stabilize

357

the foams by diminishing all destabilizing mechanisms including liquid drainage and

358

disproportionation, collapse by lamellar cleavage, and Ostwald ripening.46,47 The

359

foaming properties of surfactants are normally evaluated in terms of their “foamability”

360

(the capacity to form foams) and “foaming stability” (FS), the latter being defined as 17



361

the ability to maintain foams. As shown in Figure 3A, ester 2 had the weakest

362

foamability at all concentrations. When the length of the fatty residue increased from

363

7 to 13 carbons, as seen in the homologous series 3-6, the foamabilities of these esters

364

increased from 47.5% to 193.1% at 0.5% w/w. As such, esters 4-6 had dramatically

365

superior foamabilities relative to all the controls at the same concentrations, with the

366

value of that for ester 6 being five times greater than those of S-1170 and S-1570.

367

That said, foamability began to decrease for esters with alkyl side-chains

368

incorporating more than 13 carbons. Although esters 7 and 8 displayed properties

369

similar to those of commercial monopalmitin (MP), their foamabilities can hardly be

370

regarded as “satisfactory”. Similar trends were observed for esters 2–8 in terms of

371

their foaming stabilities (Figure 3B). So, once again, esters 5 and 6 had the best FS

372

values and these were superior to those of the four controls. However, in contrast to

373

the outcomes of the foamability study just described, there was no obvious (positive)

374

correlation between FS value and sample concentration. So, for example, in the case

375

of esters 2, 5 and 6, their FS values recorded at 0.2% w/w were equal to or higher than

376

the ones recorded at 0.5%. The clearly inverted V-type profiles revealed in Figures 3A

377

and 3B indicate that maltose mono-esters incorporating 11 and 13 alkyl side-chain

378

carbons show excellent foamability and FS properties, a feature that is attributed to

379

their low initial surface tension that thus allow them migrate to the interface

380

sufficiently rapidly to form bubbles with long half-lives.48

381

In order to further validate the outcomes described above and to compare the

382

foaming properties of maltose mono-esters with their commercially-deployed 18


Page 18 of 39

Page 19 of 39


383

sucrose-based counterparts, ester 6 and S-1570 were used to generate milk foam in

384

milk-frother (Figure 4). Unsurprisingly, given the data presented immediately above,

385

ester 6 exhibited better foamability than S-1570, the initial foam height of the former

386

being 10.1 cm while, under the same conditions, that of the latter was 9.6 cm.

387

Furthermore, the measured foam height after a standing period of 10 min was greater

388

for ester 6 than for congener S-1570.

389

Emulsion Stability. The emulsion stability indices (ESIs) of each of the maltose

390

mono-esters 2-8 as well as those of the four controls were determined at 0.01%, 0.2%

391

and 0.5% w/w concentrations and the outcomes of such measurements are shown in

392

Figure 5. In this instance, the ESIs correlate positively with concentration, while an

393

inverted V-type profile is observed in moving through the homologous series of esters.

394

So, for compounds 2-6 the ESIs increased from 297.4 through to 1696.3 at 0.5% w/w,

395

the latter value being the highest. The ESIs determined for esters 7 and 8 are

396

comparable with that for monopalmitin (MP) but significantly lower than the other

397

controls. The ESI values were found to be affected by the length of alkyl chain, but

398

further experiments should be conducted to understand why the esters with medium

399

length side chain showed the best emulsion stabilities. As an emulsifier in food

400

industry, one must display high stability of the emulsion obtained. According to the

401

results of the current work, maltose mono-esters 5 and 6 displayed better ESIs than

402

sucrose-based controls S-1170 and S-1570 and so indicating they could have greater

403

efficacy in stabilizing emulsions.

19



404

Thermal Stability. Maltose is a disaccharide formed from two 1′,4-linked

405

glucose units and, as such, is a reducing sugar capable of engaging in various

406

transformations not available to its sucrose-derived congeners.27 In seeking to probe

407

their reactivities, esters 2-8 as well as S-1170 and S-1570 were subject to

408

thermogravimetric analysis. The outcomes of the relevant experiments are shown in

409

Figure 6 and reveal that ester 8 incorporating the longest alkyl side chain is the most

410

heat-sensitive with the temperature at which a 10% loss of mass was observed being

411

132 °C while that for ester 2, incorporating the shortest side-chain, exceeded 230 °C.

412

In overall terms, then, thermal stability decreased as the length of side-chain increased.

413

Maltose esters 2-7 displayed similar thermal stabilities to the sucrose esters S-1170

414

and S-1570 at lower temperatures, but showed superior thermal stabilities in the

415

temperature range 250 to 350 °C. This result indicates that despite the reducing nature

416

of maltose, esters 2-7 exhibit thermal stabilities comparable to those of their

417

commercially-deployed sucrose congeners.

418

Particle Size Distributions of Coconut Milk Emulsions. The microstructures

419

of the samples of coconut milk emulsion formed using maltose ester 6, S-1570 and

420

Tween 80 were examined using an optical microscope. The number of particles and

421

their size distribution obtained from relevant experiments are shown in Figure 7

422

(notably, there were no significant variations of these distributions over time). These

423

data reveal the emulsifying effect of surfactants on the stability of coconut milk-based

424

emulsions.41 As shown in Figure 7A, the fresh (untreated) coconut milk emulsion was

425

comprised of variably-sized droplets up to 3000 µm in diameter that were 20


Page 20 of 39

Page 21 of 39


426

non-uniformly

427

treated/processed using maltose ester 6 then the droplets were now no more than 500

428

µm in diameter (Figure 7B) with the reduced particle size distribution diminishing the

429

probability of coalescence (of the droplets) and, therefore, enhancing the stability of

430

the associated emulsion.49 Figures 7C and D reveal the relatively larger particle size

431

distributions in the coconut milk-derived emulsions processed with S-1570 and Tween

432

80 and so resulting in slightly less stable emulsions (compared with those formed

433

using ester 2). Although other surfactants (i.e., SDS, Tween 20 and Tween 60) have

434

been applied to stabilize coconut milk emulsions in previous studies,50,51 this work

435

demonstrated, for the first time, that certain maltose mono-ester can reduce the

436

particle size distribution and enhance the emulsion stability.

dispersed.

When

the

same

coconut

milk

emulsion

was

437

Overall, then, the surfactant properties and thermal stabilities of the now readily

438

accessible and pure 6-Oʹ-acylmaltose mono-esters 2-8 described above are largely

439

determined by the length of the associated fatty acid chain (hydrophobic tail) with the

440

medium-chain variants (i.e. compounds 5 and 6) showing the best properties. In the

441

first practical evaluation of such pure maltose esters, it has been established that

442

certain of them display better foaming and emulsifying properties than their

443

commercially-deployed sucrose counterparts. Given the low cost of maltose and facile

444

preparation of corresponding mono-esters under the current protocol, maltose

445

mono-esters must now be regarded as having great potential, particularly as

446

emulsifiers, in the food industry.

447 21



Page 22 of 39

448

ASSOCIATED CONTENT

449

Supporting Information

450

1

451

via the internet at http://pubs.acs.org.

452

AUTHOR INFORMATION

453

Corresponding Author

454

*(R. Y.) E-mail: [email protected]

455

*(P. L.) Telephone: +86-20-85221367. Fax: +86-20-85224766.

456

E-mail: [email protected]

457

ORCID

458

Ya-Ru Ma: 0000-0001-7593-3348

459

Martin G. Banwell: 0000-0002-0582-475X

460

Ping Lan: 0000-0002-9285-3259

461

Funding

462

This study was supported by the Fundamental Research Funds for the Central

463

Universities (grant 21617327), the Chinese National Natural Science Foundation

464

(grant

465

2017A030313202), the Guangdong Province Science and Technology Program (grant

466

2016B020204002) and the Pearl River Scholar Program of Guangdong Province,

467

People’s Republic of China.

468

Notes

469

The authors declare no competing financial interest.

H and 13C NMR Spectra of compounds 2-8. This material is available free of charge

31700670),

the

Guangdong

Natural

Science

22


Foundation

(grant

Page 23 of 39


470

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619 620

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621

29



622

FIGURE CAPTIONS

623

Figure 1. Enzymatic syntheses of the 6ʹ-O-acylmaltose esters 2-8.

624

Figure 2. Key HMBC correlations for 6-Oʹ-lauroylmaltose 5.

625

Figure 3. Foamability (A) and foaming stability (B) over 10 min of compounds 2-8

626

and the commercial surfactants MP, S-1170, S1570 and Tween 80 at 0.2% and 0.5%

627

(w/w) concentrations (mean ± SD, n = 3).

628

Figure 4. Evaluation of foamability (A) and foaming stability (B) over 10 min of the

629

milk (blank) and the milk with ester 6 or S1570 at 0.1% w/w concentrations in a milk

630

frother.

631

Figure 5. The emulsion stability indices of compounds 2-8 and the commercial

632

surfactants MP, S-1170, S1570 and Tween 80 over 20 min at 0.01%, 0.1%, and 0.5%

633

w/w concentrations (mean ± SD, n = 3).

634

Figure 6. Thermal stabilities of compounds 2−8 and the commercial surfactants

635

S-1170 and S-1570.

636

Figure 7. Particle size distributions of the fresh coconut milk (A), and the fresh

637

coconut milk with compounds 6 (B), S-1570 (C) or Tween 80 (D).

638

30


Page 30 of 39

Page 31 of 39


Table1. HLB, CMC, γCMC and γO/W of 6ʹ-O-Acylmaltose Esters 2-8 HLB Sample

CMC

γCMC

γO/W

(µM)

(mN/m)

(mN/m)

2

11.21

6578

39.4

8.1

3

11.65

4103

38.9

6.5

4

12.79

2862

39.6

3.4

5

15.21

223

35.3

2.7

6

13.38

45

35.1

1.9

7

11.49

9

33.4

3.5

8

10.34

48

32.6

4.2

31



Figure 1.

32


Page 32 of 39

Page 33 of 39


Figure 2.

33



Page 34 of 39

200

A

0.2% (w/w) 0.5% (w/w)

Foamability(%)

150

100

50

0 2

3

4

5

6

7

8

MP

S1170

S1570

Tween 80

100

Foaming Stability (%) at 10 min

90

B

0.2% (w/w) 0.5% (w/w)

80 70 60 50 40 30 20 10 0 2

3

4

5

6

7

8

MP

Figure 3.

34


S1170

S1570

Tween 80

Page 35 of 39


Figure 4.

35



Page 36 of 39

0.01% (w/w) 2000

0.10% (w/w) 1800

0.50% (w/w)

Emulsion Stability Index

1600 1400 1200 1000 800 600 400 200 0 Blank

2

3

4

5

6

7

8

Figure 5.

36


MP

S1170

S1570

Tween 80

Page 37 of 39


2

3

4

5

6

7

8

S1170

S1570

100 90

Remaining Mass (%)

80 70 60 50 40 30 20 10 0 125

175

225

275

Temperature (℃)

Figure 6.

37


325

375


Figure 7.

38


Page 38 of 39

Page 39 of 39


TABLE OF CONTENTS GRAPHIC

39


A Comparative Study on Emulsifying Properties of a Homologous

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