Subscriber access provided by University of South Dakota
Food and Beverage Chemistry/Biochemistry
Characterization and mechanisms of novel emulsions and nano-emulsion gels stabilized by edible cyclodextrin-based MOFs and glycyrrhizic acid Chao Qiu, Jinpeng Wang, Yang Qin, Xueming Xu, and Zhengyu Jin J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b03065 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 13, 2018
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 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 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.
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 34
Journal of Agricultural and Food Chemistry
1
Characterization and mechanisms of novel emulsions and
2
nano-emulsion gels stabilized by edible cyclodextrin-based MOFs
3
and glycyrrhizic acid
4
Chao Qiua,b,c, Jinpeng Wanga,b,c, Yang Qina,b, Xueming Xua,b,c, Zhengyu Jina,b,c,*
5 6
a. State Key Laboratory of Food Science and Technology, Jiangnan University, Wuxi,
7
Jiangsu 214122, China
8
b. School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu
9
214122, China
10
c. Synergetic Innovation Center of Food Safety and Nutrition, Jiangnan University,
11
Wuxi 214122, China
12
* Corresponding author: Zhengyu Jin (Tel:/Fax: 86-51085913299; Email:
13
[email protected] (Z.
Jin)).
1
ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
Page 2 of 34
14
ABSTRACT: In this study, a novel emulsion stabilized by nano cyclodextrin-based
15
MOFs and glycyrrhizic acid (CD-MOF/GA) was successfully fabricated, exhibiting
16
long-term storage stability. The characterization and mechanisms for the emulsion
17
formation with CD-MOF/GA were studied. The phase change of the emulsions from
18
sol to gel could be controlled using different oil fractions and mass ratios of CD-MOF
19
and GA. The rheological results showed that the emulsions were transformed from
20
liquid emulsions to emulsion gels when the oil fractions were higher than 0.3 and the
21
mass ratio of CD-MOF and GA was 1:3. The low-field nuclear magnetic resonance
22
results revealed that the T22 relaxation time of emulsions decreased from 403.702 to
23
231.013 ms when the oil fractions increased from 0.1 to 0.6, indicating the movable
24
water was converted to constructral water. The emulsions showed good stability even
25
in high-alkaline pH and high-temperature conditions.
26
KEYWORDS:
emulsions,
emulsion
gels,
stability,
rheology,
LF-NMR
2
ACS Paragon Plus Environment
Page 3 of 34
Journal of Agricultural and Food Chemistry
27
INTRODUCTION
28
Emulsion-based delivery systems, which encapsulate and deliver bioactive
29
compounds in a way that protects them against chemical degradation, thereby
30
enhancing bioavailability and release control, have gained significant interest for
31
many applications, including food products, pharmaceuticals, and cosmetics.1-4
32
Various types of emulsifiers can be used to kinetically stabilize food emulsions, but
33
conventional surfactants have a known risk of sample contamination.5,6 In recent
34
years, the emulsions stabilized with colloidal particles have attracted extensive
35
interests due to their advantages over other types of emulsions, such as being
36
surfactant-free and low-cost and providing long-term stabilization against coalescence
37
and Ostwald ripening.7-12 Various particles have been reported to stabilize emulsions,
38
such as silica,13 graphene oxide,14 and calcium carbonate.15 However, due to
39
environmental and food safety problems, these inorganic particles are not wise
40
choices and have limited applicability in the food industry.16 Therefore, a promising
41
solution was to develop natural, environmentally friendly, biodegradable, food-grade
42
colloidal particle-stabilized emulsions. The applications for emulsions using nano- or
43
microsized particles derived from renewable resources, including proteins, lipids,
44
phytosterol, cellulose, and chitin, have been studied.17-22 However, the stability of
45
these protein, lipid or polysaccharide stabilized emulsions was limited by the inherent
46
hydrophilic feature of the emulsifiers.
47
Metal organic frameworks (MOFs) are a new class of crystalline nanoporous
48
compounds that have gained significant interest recently due to their tunable size,
3
ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
Page 4 of 34
49
large surface area, and highly porous topology.23-29 MOFs are amphiphilic and
50
surface-active materials that can emulsify liquid droplets to form emulsions. Zhang et
51
al.14 reported the formation of a Pickering emulsion stabilized by a Zr-based MOF and
52
graphene oxide. Sabouni and Gomaa30 reported the successful preparation of
53
Pickering emulsions stabilized by MOFs MIL-101 and ZIF-8 nanoparticles. However,
54
MOFs based on transition metals or non-food-grade organic linkers considered to be
55
toxic are not acceptable for biological, food, or medical applications. Herein, we
56
report a strategy to overcome this problem using edible cyclodextrin-based MOFs
57
(CD-MOFs) composed of less toxic alkaline earth metals and food-grade
58
oligosaccharide γ-CD.
59
However, the CD-MOF-stabilized emulsions still have great challenges in
60
maintaining long-term stability. Glycyrrhizic acid (GA) is the major triterpene
61
glycoside contained in licorice root and is mainly recognized for its remarkable
62
medicinal properties, such as anti-inflammatory and anti-cancer activities.31 GA has
63
recently been the subject of renewed interest, due to its ability to supramolecular
64
self-assembly in water. For the first time, Saha et al.,32 demonstrated the GA
65
self-assembled in water into long fibrils with 2.5 nm diameter, and increasd the fibril
66
concentration leads to the formation of fibrillar hydrogel. Recently, Wan et al.,33,34
67
successfully prepared the structured emulsions based on the fibrillar self-assembly of
68
GA. The GA molecule, with its open-chain amphiphilic structure that contains both
69
hydrophilic (glucuronic acid) and hydrophobic (glycyrrhetic acid) fragments, has the
70
ability to decrease interfacial tension and stabilize oil-water interface.33,35 The
4
ACS Paragon Plus Environment
Page 5 of 34
Journal of Agricultural and Food Chemistry
71
obtained emulsions were further self-organized into the emulsion gel by applying a
72
subsequent cooling to trigger the gel network formation.
73
To date, there has been no study on CD-MOF and GA complex particle-stabilized
74
emulsions. Emulsions transformed from liquid emulsions to emulsion gels have
75
become an emerging trend, as they provide superior stabilization and better delivery
76
for bioactives.36 Therefore, we hypothesize that the long-term stable emulsions could
77
be achieved by using new bioparticles MOFs and GA as stabilizers. Furthermore, the
78
droplet size and phase change of the emulsions from sol to gel could be controlled
79
using different oil fractions and mass ratios of stabilizers.
80
MATERIALS AND METHODS
81
Materials. GA and γ-CD were purchased from Sinopharm Chemical Reagent Co.,
82
Ltd. (Shanghai, China). Waxy cornstarch was purchased from Gaofeng Starch
83
Technologies Co., Ltd. (Suzhou, China). Arawan soybean oil was purchased at a local
84
supermarket (Wuxi, China). All other chemicals were reagent grade.
85
Preparation of the emulsion and emulsion gels. The CD-MOFs were green
86
synthesized by the seed-mediated method described by Qiu et al.37 The incubation
87
time was changed to 1 h. For the preparation of emulsions stabilized by CD-MOF and
88
GA, different concentrations of GA solutions were prepared by dissolving GA in
89
deionized water and boiling a water bath for two minutes to completely dissolve the
90
solution. Then, CD-MOF (0.4% w/v) was added to the different concentrations of GA
91
solutions. The emulsions were immediately prepared using a high-speed homogenizer
92
(Ultra-Turrax Digital D-500, Wiggens, Germany) at 12,000 rpm for three minutes
5
ACS Paragon Plus Environment
Journal of Agricultural and Food Chemistry
Page 6 of 34
93
before cooling. The emulsions or emulsion gels with different mass ratios
94
(CD-MOF:GA 2:1, 2:3, 1:2, 2:5, 1:3, 1:4, w/w) at 0.7 oil fractions (termed as
95
MOF-GA 2:1, 2:3, 1:2, 2:5, 1:3, and 1:4, respectively), and different oil fractions (0.1,
96
0.3, 0.5, 0.6, 0.7, 0.9; termed as MOF-GA 0.1, 0.3, 0.5, 0.6, 0.7, 0.9, respectively) at
97
mass ratios of 1:3 were cooled overnight at room temperature before further
98
experiments. The final pH value of all the obtained emulsions was around 5.0.
99
Rheological tests. The steady flow properties and dynamic viscoelastic
100
characteristics of samples were determined using a AR rheometer (DHR-3, TA
101
Instrument Inc., USA). The samples were measured using a parallel plate geometry
102
(40 mm) at a gap of 1 mm. The tests commenced according to the following
103
parameters: (1) Steady flow tests were performed at 25 °C over the shear rate range of
104
0.1–100 s−1 to measure the apparent viscosity. (2) Frequency sweep tests were
105
performed at 25 °C over the angular frequency range of 1–200 rad/s and a constant
106
deformation (1% strain) within the linear viscoelastic range. The changes of storage
107
modulus (G'), loss modulus (G"), and loss tangent (tanδ = G"/ G') as a function of
108
frequency (ω) were obtained.
109
Droplet size of emulsions. The average droplet size and polydispersity index
110
(PDI) were measured by dynamic light scattering (DLS) using a Malvern Zetasizer
111
Nano (Malvern, Worcestershire, UK). Measurements were taken at 25 °C and at a 90°
112
scattering angle.
113
Transmission electron microscopy (TEM). The TEM morphology images of
114
the CD-MOFs were taken with a Hitachi 7650 TEM with an acceleration voltage of
6
ACS Paragon Plus Environment
Page 7 of 34
Journal of Agricultural and Food Chemistry
115
80 kV. A drop of the CD-MOFs dispersed in anhydrous methanol solution was placed
116
on a copper grid and lyophilized.
117
Low-field
nuclear
magnetic
resonance
(LF-NMR).
The
LF-NMR
118
measurements of the emulsions were performed using a 23 MHz NMR analyzer
119
(NMI20-015V-I, Niumag Co., Ltd., Suzhou, China) with a sample tube of 25 mm in
120
diameter, following the method of Chen et al.38 The temperature of the LF-NMR
121
instrument was maintained at 32 °C during the spin-spin relaxation time (T2)
122
measurements,
123
Carr-Purcell-Meiboom-Gill (CPMG) sequence.
which
were
performed
using
a
sequence
based
on
the
124
Stability of emulsions. The stability of emulsions was evaluated following the
125
method of Dai et al.39 The fresh emulsions with 0.7 oil fractions at 1:3 (CD-MOF:GA)
126
mass ratios were adjusted to pH 2, 4, 7, 9, and 11 by adding NaOH or HCl (0.1 M).
127
To test thermal stability of the emulsions, the emulsions were heated at temperatures
128
ranging from 30 to 80 °C for 30 minutes in a water bath and then cooled down to
129
room temperature.
130
Statistical analysis. All experiments were conducted at least thrice, and the mean
131
values and standard deviations were determined. The experimental data were
132
analyzed using analysis of variance (ANOVA) and were expressed as mean values ±
133
standard deviations. Differences were considered at a significance level of 95% (P
