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Encapsulation of Ethylene Gas into Granular ColdWater Soluble Starch: Structure and Release Kinetics Linfan Shi, Xiong Fu, Chin Ping Tan, Qiang Huang, and Bin Zhang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b05749 • Publication Date (Web): 20 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017
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Journal of Agricultural and Food Chemistry
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Encapsulation of Ethylene Gas into Granular
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Cold-Water Soluble Starch: Structure and Release
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Kinetics
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Linfan Shi a, Xiong Fu a,b, Chin Ping Tan c, Qiang Huang a,b,*, Bin Zhang a,b,*
6 7 a
8
School of Food Science and Engineering, South China University of Technology, Guangzhou 510640, PR China
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b
Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, Guangzhou 510640, PR China
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12 13
c
Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
14 15 16 17 18 19 20 21 22
* Corresponding authors
23
Tel.: +86 20 8711 3845; fax: +86 20 8711 3848.
24
E-mail address:
[email protected] (Q. Huang),
[email protected] (B. Zhang).
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ABSTRACT:
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Ethylene gas was introduced into granular cold-water soluble (GCWS) starches
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using a solid encapsulation method. The morphological and structural properties of
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the novel inclusion complexes (ICs) were characterized using scanning electron
30
microscopy, X-ray diffractometry, and Raman spectroscopy. The V-type single helix
31
of GCWS starches was formed through controlled gelatinization and ethanol
32
precipitation, and was approved to host ethylene gas. The controlled release
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characteristics of ICs were also investigated at various temperature and relative
34
humidity conditions. Avrami’s equation was fitted to understand the release kinetics
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and showed that the release of ethylene from the ICs was accelerated by increasing
36
temperature or RH, and was decelerated by increased degree of amylose
37
polymerization. The IC of Hylon-7 had the highest ethylene concentration (31.8%,
38
w/w) among the five starches, and the IC of normal potato starch showed the best
39
controlled release characteristics. As a renewable and inexpensive material, GCWS
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starch is a desirable solid encapsulation matrix, with potential in agriculture and food
41
applications.
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KEYWORDS: encapsulation, ethylene, granular cold-water soluble starch,
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controlled release
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INTRODUCTION
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Ethylene, which is a natural ripener for fruits and vegetables that is present in the
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tissues and organs of plants, is widely used in agriculture and food industry. It can
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also enhance the color development of fruits, induct adventitious roots and root hairs,
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and promote fruit flowering.1 As an active gas, ethylene is often stored in pressurized
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cylinders for commercial uses, which has the major disadvantages of leakage and
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explosion during the transport and storage. Encapsulating of gas into solid matrices
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with slow release properties can effectively avoid these defects.2 These solid matrices
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can be categorized into two groups using X-ray diffraction techniques:3 i) ordered
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structures with well-defined pore size, such as carbon nanotubes4 and zeolites5, and ii)
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disordered structures with a wide range of pore diameters and surface area, e.g.,
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activated carbons6 and cyclodextrins.7 These solid matrices with low gas
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encapsulation capability normally need to be prepared under harsh conditions, which
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is a major drawback for industrial use. For example, Vela et al. encapsulated CH4 into
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single-walled carbon nanotubes at a pressure of 35.62 MPa and 0 °C, with a low CH4
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concentration of 4.0% (w/w).8
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Among the reported solid matrices, only cyclodextrin (CD) can be used as food
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grade material.3 The hydrophobic interior of the α-CD toroids (0.47-0.53 nm) was
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reported to encapsulate gases with low molecular weight such as CH4, C2H4, C3H8, Kr,
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Cl2, and CO2.9,10 However, the primary commercial limitations of CD are its high cost
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and low encapsulated concentration.
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Amylose forms a left-handed single helix that is stabilized by hydrogen bonds, and
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has a hydrophilic outside surface and a hydrophobic inside helical cavity.11 Amylose
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single helices can pack together in a crystalline structure known as V-type crystalline
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structure.11 In previous studies, V-type complexes were used to prepare slowly
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digestible and resistant starches, to create a new delivery system to protect volatile
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and sensitive guest molecules, such as esters of vitamins, fatty acids and genistein.12,
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13
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fatty acids and alcohols with the aid of hydrophobic interactions.14 Chen and Jane
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prepared granular cold-water soluble (GCWS) starches through alcoholic-alkaline
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treatment, and achieved controlled release of atrazine, which was determined by the
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amylose content, particle size, and environmental temperature.15
V-type starch molecules are reported to host suitable guest molecules such as iodine,
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Although numerous attempts were made to entrap various gases in solid matrices,
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to the best of our knowledge no further details or discussions on encapsulated gases
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into starch have been reported.16,
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GCWS starches prepared from different botanical origins using a solid encapsulation
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method, and characterized the inclusion complexes (ICs) by scanning electron
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microscopy (SEM), X-ray diffractometry (XRD), and Raman spectroscopy. Avrami’s
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equation was used to study the controlled release kinetics of ethylene from ICs at
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various temperature and relative humidity conditions. Additionally, the mechanism of
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formation and stability of IC were discussed.
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In this study, we encapsulated ethylene into
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MATERIALS AND METHODS
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Materials. Normal maize (NMS) and potato (NPS) starches were purchased from
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Dacheng Company (Changchun, China). High-amylose starch (Hylon-7, HA7 and
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Hylon-5, HA5) was purchased from Ingredion (Shanghai, China), and waxy maize
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starch (WMS) was a gift from Lihua Co., Ltd. (Qinhuangdao, China). Other
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chemicals were commercial products of analytical reagent grade.
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CLDs of Starches. Starch debranching was performed as described by Wang et
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al.18 The SEC weight distribution of debranched starch was analyzed in duplicate
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using an Agilent 1100 series SEC system (Agilent Technologies, Waldbronn,
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Germany) equipped with a refractive index detector (Shimadzu RID-10A, Shimadzu,
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Kyoto, Japan). A series of SEC columns (GRAM precolumn and GRAM 100 and
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GRAM 1000 columns, Polymer Standard Services, PSS, Mainz, Germany) placed in
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an oven at 80 °C were used to separate the debranched starch molecules. The mobile
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phase was a DMSO/LiBr solution (0.6 mL/min) and pullulan standards were used for
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calibration to convert the SEC elution volume into the hydrodynamic volume Vh or
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the corresponding radius Rh using the Mark-Houwink equation.19 The molecular size
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distribution data were plotted as SEC weight distribution, w (log Vh), against the
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hydrodynamic radius (Rh/nm). The amylose content (AC) was calculated as the ratio
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of the area under the curve (AUC) of the debranched SEC distribution curves for the
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larger branches to the total AUC for all branches.20
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Preparation of GCWS starches. GCWS starches were prepared following a
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reported method with minor modification.21 The HA7, HA5, NMS and NPS samples 5
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(10.0 g, dry starch basis, dsb) were suspended in 70.0 g of an aqueous ethanol
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solution (40%, w/w), and NaOH (3 M) was added at 4 g/min. The mixture was stirred
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at 100 rpm for 30 min at 35 °C, and the additional ethanol solution (20 g, 40%, w/w)
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was slowly added. The mixture was centrifuged at 1800×g for 10 min and washed
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twice with ethanol solution (40%, w/w). The recovered starch was resuspended in
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ethanol solution (40%, w/w), and neutralized with HCl (3 M in absolute ethanol). The
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mixture was washed with ethanol solution (80%, w/w) followed by absolute ethanol,
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and oven-dried at 80 °C for 3 h. The dry material was gently crushed in a hand agate
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mortar and passed through a 150-mesh nylon sieve. The preparation of the GCWS
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starch from WMS was performed following the above steps, expect for the dispersion
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process (ethanol solution, 80%, w/w) and the washing step (ethanol solution, 95%,
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w/w). The mass ratios of alkali/starch (HA7, HA5, NMS, NPS and WMS, dsb) were
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1:8, 1:5, 1:4, 1:3.5, and 1:3, respectively. Further increasing the amount of NaOH led
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to starch gelation (data not shown).
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Encapsulation of Ethylene Gas. The encapsulation of ethylene into GCWS starch
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was investigated by a solid encapsulation method at a pressure of 1.2 MPa for 24 h at
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25 °C. The GCWS starch (10.0 g, dsb) was weighed into a 250 mL quartz container,
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and placed into a pressure vessel chamber (Yanzheng Biological Technology Co., Ltd,
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Beijing, China). After vacuum treatment, the vessel chamber was flushed with
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ethylene until the desired pressure level was reached. After 24 h of reaction, the
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residual ethylene in the vessel chamber was absorbed by bromine solution (3%, w/w),
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and the ICs were transferred to a -80 °C freezer for further analysis. The properties of 6
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the ICs were determined within 8 h.
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Encapsulation Capacity. The ethylene encapsulation capacity of GCWS starches
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was determined using headspace gas chromatography (HS-GC). ICs (20 mg) and
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distilled water (1 mL) were added into an amber vial, and the cap was sealed tightly
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immediately. The ICs were magnetically stirred at 600 rpm for 5 min to dissolve the
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ICs thoroughly.
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A GC system (Agilent, 7890A, CA, USA) and an automatic headspace sampler
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(DANI, HS 86.50, Cologno Monzese, Italy) were used for HS-GC measurement. The
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GC system was equipped with a thermal flame ionization detector (Agilent, CA,
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USA), and a DB-5 capillary column (30 m × 0.25 mm × 0.1 µm, J & W Scientific,
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USA). The carrier gas was nitrogen at 40 mL/min, and the column temperature was
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35 °C. The temperature of the oven and injector was set at 60 °C and 250 °C,
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respectively. The peak area of the sampled ethylene was recorded and calculated as
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ethylene concentration based on the ethylene standard (o2si company, SC, USA).
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Because some ethylene dissolved in water, the ethylene concentrations in the ICs were
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calculated as the sum of two measurements: the ethylene in headspace and the
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dissolved ethylene in water. Dissolved ethylene in water was estimated based on the
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theoretical values from Henry’s Law22 as follows:
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Cw = 0.119 × Ch
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where Cw and Ch are the level of ethylene (cm3/m3) in water and headspace,
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respectively. The concentration ratio of ethylene in the ICs was presented as the
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quality ratio.
(1)
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Scanning Electron Microcopy. Scanning electron micrographs were obtained with
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an EVO18 scanning electron microscope (Zeiss, Oberkochen, Germany). Starch
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samples were mounted on an aluminum stub using double-sided tape, coated with a
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thin film of gold (10 nm), and examined at an accelerating voltage of 10 kV.
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Raman Spectroscopy. Raman spectroscopy was recorded using a laser confocal
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micro-Raman scattering spectrometer (LabRAM Aramis, HJY Company, France) in
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the range of 500-4000 cm-1. The wavelength was 532 nm and the resolution was 2
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cm-1.
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X-ray Diffractometry. The X-ray diffractometer (D8 Advance, Bruker, Germany)
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was operated at 40 kV and 40 mA with Cu Kα radiation (λ = 0.154 nm). The
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measurements were scanned at a scan rate of 0.5°/min (angular angle 2θ = 4-35°)
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with a 0.02° step interval. The crystallinity of the starch was calculated using a MDI
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JADE software (version 6.5, Materials Data Inc., Livermore, California, USA).
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Crystallinity of the starch was calculated using the following equation: 23
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Crystallinity (%) = 100% × Ac / (Ac + Aa)
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where Ac is the crystalline area on the X-ray diffractogram and Aa is the amorphous
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area.
(2)
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Release Kinetics. Release kinetics of ethylene gas from ICs was investigated under
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different temperature or relative humidity (RH) conditions. The storage and
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transportation of fruits were usually carried out below room temperature. Therefore,
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ICs (1.0 g, dsb, moisture content ~ 9.0%) were weighed into a sealing bag and stored
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at four selected temperatures (-80, -20, 4 and 25 °C) over prolonged time periods. RH 8
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was controlled at 52.9% and 75.5% using a saturated magnesium nitrate or sodium
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chloride solution at 25 °C, respectively.24 ICs were analyzed for ethylene
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concentration using HS-GC. The ratio of ethylene release (X) was defined as in Eq.
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(3):
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X = (Ci - Ct) / Ci
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where Ci is the initial concentration of ethylene in the ICs and Ct is the concentration
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of ethylene in the ICs at time t (h).
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(3)
Avrami’s equation is a mathematic model used extensively to describe release
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kinetics, and can be described as follows:25
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X = 1 - exp (-kt n)
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where X is the ratio of active release at time t (h). k is the release rate constant and n is
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the Avrami parameter or release mechanism. Both k and n express the magnitude of
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release and are empirically determined. Eq. (4) can be rewritten using a double
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logarithm to obtain Eq. (5):
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In(-In(1-X))= lnk + nlnt
192 193 194
(4)
(5)
A linear plot of In(-In(1-X)) versus lnt in Eq. (4) allows the release rate constant k (intercept) and release parameter n (slope) to be determined. Activation energy Ea are widely applied to evaluate temperature effects on the
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release kinetics from ICs, and it can be expressed as follows:26
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k = k0 exp (-Ea/RT)
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where, k is the release rate constant and k0 is the pre-exponential factor related to k. Ea
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is the activation energy parameter, T is the absolute temperature (K) and R is the
(6)
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universal gas constant (8.314 × 10-3 kJ mol-1 K-1).
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Eq. (6) can be linearized as follows:
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lnk = lnk0 - Ea/RT
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(7)
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Ea can be determined by a linear plot of lnk versus 1/T and the slope equals (Ea/R).
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Statistical Analysis. The results are recorded as the means ± standard deviation
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(SD), and the significance of the differences between groups was tested using t-test
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analysis. The significance level was set at p < 0.05. Statistical analysis was conducted
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using SPSS version 19.0 software (SPSS, Inc., Chicago, IL).
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RESULTS AND DISCUSSION
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CLDs of Starches. The SEC weight CLDs profiles of five native starches are
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presented in Figure 1. The debranched SEC weight distribution can be empirically
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divided into two regions representing amylopectin (AP) chains (DP ~ 100).18 The SEC weight CLDs of all debranched
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starches showed three typical peaks. The first peak (denoted by AP1) included the
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shorter AP branches (peak Rh ~ 1.5 nm or DP ~ 12), which are usually confined to
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single lamella. The second peak (denoted by AP2) represented the longer AP branches
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(peak Rh ~ 2.5 nm or DP ~ 50), which are normally crossed through more than one
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crystalline lamella. The third peak (denoted by AM) represented the AM branches
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with a board distribution (Rh ~ 3.5-80 nm or DP ~ 100-30000). Some mutants of
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starches, e.g. high-amylose maize starches, particularly HA7, consist of a large
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amount of intermediate materials, of which the branch chain length (Figure 1, Rh ~ 3 10
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nm or DP ~ 55-80) and the density of branching points27 are between AM and AP
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molecules. To compare the fine structure between starches, a set of empirical
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parameters was defined, such as the DP at the maximum of each peak (DPAP1, DPAP2
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and DPAM), and the relative height of each peak to AP1 (hAP2/AP1 and hAM/AP1). The DP
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at the maximum of each peak represents the relative chain length in each group of
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branches, and the height ratio of each peak maximum to AP1 reflects the relative
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amount of chains in each group of branches.18 The structural parameters of starches
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are summarized in Table 1. The DPAM values followed the order NPS > NMS > HA5 >
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HA7, whereas the opposite trend was observed for hAM/AP1 values and AC. Cereal
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starches normally have a considerably lower DPAM values than those from roots and
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tubers.28 The DPAP1, DPAP2 and hAP2/AP1 values of HA7 were highest, followed by
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HA5 and NPS (in decreasing order), showing that HA7 has longer branch chains of
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AP (peak Rh ~ 1.5-2.5 nm or DP ~12-50). The DPAP1, DPAP2 and hAP2/AP1 values of
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WMS did not have much statistically significant differences (p > 0.05) compared to
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those of NMS, suggesting that the AP structure of NMS was similar to WMS.
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Ethylene Gas Concentration in the ICs. To confirm whether the ICs were able to
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release ethylene completely in water, IC powder was dissolved in distilled water, and
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freeze-dried for further ethylene detection. We found no ethylene peak was detected
239
by HS-GC analysis (data not shown), indicating that all ethylene was released from
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the ICs.
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The ethylene concentrations in ICs are shown in Figure 2. The concentrations in
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ICs followed the order of their botanical origins: HA7 > HA5 > NMS > NPS > WMS. 11
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Notably, the highest ethylene concentration in HA7-IC was 31.8% (w/w). The
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NMS-IC sample had a similar ethylene concentration to NPS-IC, and WMS-IC
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showed the lowest values (2.3%, w/w). The ethylene concentrations in ICs of five
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starches had differed significantly (p < 0.5), indicating that the ethylene
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concentrations in ICs was positively dependent on the specific variety of starches. The
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amylose content of HA7, HA5, NMS, NPS and WMS was 69.8%, 48.6%, 26.5%,
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19.5% and 0.2%, respectively (Table 1). Thus, ethylene concentrations in ICs are
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associated with the AC of GCWS starches (correlation coefficient was 0.997 in Table
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2).
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Morphology. The SEM images of the native, GCWS and IC samples are shown in
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Figure 3. HA7 and HA5 (Fig. 3A1 and B1) showed round to elongate shape with 7-10
254
µm in size, and NMS and WMS (Fig. 3C1 and E1) exhibited polygonal granules with
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diameters of 10-20 µm. NPS (Fig. 3D1) appeared ellipsoidal with a size of 15-75 µm,
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in agreement with a previous report.29 The GCWS starches had characteristic
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microscopic features that depended on their botanical origins. The size of GCWS
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starches prepared through alcoholic-alkaline treatment was approximately 2-4 times
259
larger than that of the native counterparts. The majority of starch granules were still
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intact with a slightly wrinkled surface, and minor granules bursting from the surface
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(Fig. 3D2).30 The AM can maintain the integrity of the starch granules, and the
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swelling is primarily a property of AP. WMS (Fig. 3E2), which has a high AP content
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( >~ 99%), suffered higher swelling expansion and more severe deformation. The
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expansion of NPS (Fig. 3D2) was due to an appreciable amount of phosphate 12
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monoester groups, which are negatively charged and linked to AP molecules.31 The
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resulting charge repulsion effect helps untangle the individual branches, and increases
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the swelling of granules.32 Additionally, it is noticeable that ethylene encapsulation
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did not appear to change their morphological structure, indicating that the
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macromolecular architecture of GCWS starches does not play a major role in the ICs.
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Raman Spectroscopy Analysis. To confirm the formation of the starch-ethylene
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complex, HA7-IC was chosen as a representative. Compared with uncomplexed
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GCWS starch, the characteristic peaks at 1630-1600 cm-1 and 1500-1200 cm-1 were
273
higher in intensity for IC (Figure 4). The vibration at 1615 cm-1 in the Raman
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spectrum has been reported to be associated with the C=C bond in ethylene.33 In
275
addition, the band at 1623 cm-l was assigned to the coordinated C=C stretching
276
mode.34 Therefore, Raman spectroscopy changes at 1630-1600 cm-1 indicate the
277
ethylene adsorption in the HA7-IC sample, in line with the ethylene concentration
278
results (Fig. 2). Additionally, the peaks were sharper and higher in intensity for the IC
279
compared with the uncomplexed GCWS starch at 1500-1200 cm-1, which was
280
assigned to C-H and H-C-H bending. This is in line with a previous study suggesting
281
that the bands at 1340 cm-1 and 1440 cm-1 represented the deformation vibration for
282
=C-H in ethylene gas.35
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Crystalline Structure. Figure 5 shows the X-ray diffraction profiles of native,
284
GCWS and IC samples. The native NMS and WMS samples clearly showed the
285
A-type polymorph with major peaks at ~15°, 17°, 18° and 23° 2θ, whereas the native
286
HA7, HA5 and NPS samples showed a typical B-type diffraction pattern with 13
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characteristic peaks at ~5°, 17°, 22° and 24° 2θ.36 The crystallinity of native HA7,
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HA5, NMS, NPS and WMS was 24.1%, 26.1%, 33.5%, 44.6% and 48.0%,
289
respectively, consistent with a previous report.37 All GCWS and IC samples except for
290
WMS had a clear V-type crystalline structure with major peaks at ~7°, 13° and 20°
291
2θ.38 When starch molecules were placed in a strong alkaline solution, the protons of
292
the -OH group were dissociated, leaving negative charges on the starch molecules.
293
The repulsion between negative charges resulted in swelling of starch granules (Fig.
294
3A2-E2).21 The swelling power exerted a tension on neighboring crystallites of starch
295
molecule.39 Further swelling led to the uncoiling or dissociation of the double helix
296
structure, and the order of crystallites was destroyed (Fig. 5).40 Ethanol restricts the
297
swelling of granules due to dehydration effect, and also forms complexes with AM
298
and longer branch chains of AP molecules to generate the V-type single helix structure
299
(Fig. 5). The V-type crystallinity of GCWS starches followed the order HA7 > HA5 >
300
NMS > NPS. It was noted that the V-type crystallinity increased with increasing of
301
AC, and decreased with increasing DPAM values. Pearson’s correlations showed that
302
the ethylene concentrations in ICs were also positively and significantly correlated
303
with AC (Table 2). This is because AM changed to V-type single helices during the
304
alcoholic-alkaline treatment, and hosted ethylene gas to form the ICs. In addition,
305
long branch chains of AP also contributed to the content of V-type crystalline structure
306
during ethanol precipitation,38 supported by the fact that ICs from NPS and NMS with
307
different ACs (19.5% cf. 26.5%, Table 1) had similar ethylene concentrations. There
308
was no obvious change in the characteristic XRD peaks between GCWS starches and 14
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ICs. The crystallinity slightly increased, likely due to the formation of ethylene
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complexes.
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Release Kinetics. The time course of ethylene release from ICs at different
312
temperature and RH conditions is shown in Figure 6. The release ratio expressed the
313
relative amount of ethylene released from the ICs at different time intervals. At -80 °C
314
and -20 °C, the ethylene release ratio was below 0.15 after 120 h of storage (Fig. 6A
315
and B), probably due to the low kinetic energy of ethylene gas. After 120 h of storage
316
at 4 °C (Fig. 6C), the encapsulated ethylene gas in the ICs of HA7, HA5, NMS and
317
WMS almost completely released (release ratio > 0.99), and the release ratio of
318
NPS-IC was only 0.75. Ethylene gas showed a significantly faster release rate at
319
25 °C (Fig. 6D) among the four temperature conditions. A high temperature promotes
320
the molecule's speed (kinetic energy), thus, ethylene molecules will migrate faster
321
inside the cavities of GCWS starches resulting in an increase in diffusion rate.41
322
Similar temperature effects on release have been reported for CD encapsulated
323
eugenol from ICs.42
324
The release of ethylene gas from the ICs accelerated markedly with increasing RH.
325
After 4 hours of storage, a release ratio of 0.99 was found for all ICs at 52.9% RH
326
(Fig. 6E), and the same release ratio was monitored after 1.2 h at 75.5% RH (Fig. 6F).
327
Rising RH induced high moisture absorption into GCWS starches during storage,
328
resulting in the marked dissolution of single helices of GCWS starches. The release of
329
ethylene was related to the presence and concentration of water molecules
330
surrounding the ICs, in line with a previous study.43 15
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Avrami’s equations were used to analyze release kinetics, and fitted the ethylene
332
release kinetics very well (r2 > 0.9, Table 3). The ethylene release rate constant k and
333
release parameter n from the ICs are summarized in Table 3. The release parameter n
334
fell within the range of 0.30-0.90, indicating that the release of ethylene corresponded
335
to the diffusion limited mode for different temperatures. Similarly, the release
336
parameter n for various RH values ranged between 0.90 and 2.00, showing a
337
first-order kinetics mode. That means the major physical process was ethylene gas
338
diffusion from the solid phase into the gas phase at various temperatures, whereas RH
339
had a marked effect on the release kinetics of ethylene. The release rate constant k can
340
be an important index to indicate the rate of ethylene gas liberate from ICs depending
341
on storage temperature or RH. It should be noted that kHA7 ˃ kHA5 ˃ kNMS ˃ kNPS for
342
same the temperature and RH, likely due to different DPAM values for GCWS starches.
343
The linear AM chains contribute to the V-type crystalline structure formation in
344
GCWS starches, whereas the single helices of V-type crystalline structure are
345
unstable.44 Longer AM chains, impart greater stability to complexes. Starch with high
346
DPAM values can generate ICs with good organization and stability.45 The melting
347
temperature, stability and crystalline structure of amylose-lipid complexes increased
348
generally with DPAM values.46 NPS-IC showed better thermal stability and controlled
349
release characteristics than other starches, and released ethylene gas continuously at
350
4 °C for 192 h, likely due to its highest DPAM value (3156). Activation energy can be
351
interpreted as the relative index of the binding strength of granular cold-water soluble
352
starch with ethylene.47 The activation energy of ethylene release from ICs followed 16
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353
the order: NPS > NMS > HA5 > HA7 (Table 3), indicating that the binding strength
354
between ethylene and GCWS starch prepared from NPS was stronger than other
355
starches.
356
ethylene from NPS-IC. The release of ethylene from WMS-IC did not obey any rule,
357
due to the low ethylene concentration in IC. Therefore, the activation energy of
358
WMS-IC based on the release rate constant is not calculated in Table 3). However, the
359
improvement of IC stability and potential loss of ethylene gas during longer-term
360
storage are under study and will be reported in the future.
These data were consistent with the controlled release characteristics of
361
In conclusion, GCWS starch showed its capacity to encapsulate ethylene gas. The
362
ethylene concentrations in ICs followed the order HA7 > HA5 > NMS > NPS > WMS,
363
and the maximum ethylene concentration in HA7-IC was up to 31.8% (w/w). The
364
results from XRD, HS-GS and Raman spectroscopy confirmed that the double helices
365
of AM and longer AP branch chains of GCWS starches changed to single helices that
366
can host ethylene gas. Avrami’s equation was used to analyze the release kinetics of
367
ethylene gas from ICs, and the results showed that all treatments showed diffusion
368
release mechanisms in temperature experiments, whereas the release mechanism in
369
RH experiments resembled first-order kinetics. The release rate of ethylene gas from
370
ICs accelerated with rising temperature or RH, and decreased with increasing DPAM.
371
NPS-IC showed better controlled release characteristics than other starches, and
372
released the ethylene gas continuously at 4 °C for 192 h. GCWS starch is a potential
373
solid matrix for gas encapsulation in agriculture and food industry, with applications
374
such as fruit ripening and food shelf-life regulation. 17
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375
AUTHOR INFORMATION
376
Corresponding Author
377
*(Q.Huang) Telephone: +86-20-8711-3845. Fax: +86-20-8711-3848. E-mail:
378
[email protected] 379
*(B.Zhang) Telephone: +86-20-8711-3845. Fax: +86-20-8711-3848. E-mail:
380
[email protected] 381
Notes
382
The authors declare no competing financial interest.
383
ABBREVIATIONS USED
384
AM, amylose; AP, amylopectin; AUC, area under the curve; CD, cyclodextrin; CLD,
385
chain length distribution; DMSO, dimethyl sulfoxide; DP, degree of polymerization;
386
GCWS, granular cold-water soluble, HA5, Hylon-5; HA7, hylon-7; HS-GC,
387
headspace gas chromatography; IC, inclusion complex; NMS, normal maize starch;
388
NPS, normal potato starch; RH, relative humidity; SD, standard deviation; SEM,
389
scanning electron microscopy; SEC, size exclusion chromatography; SPSS, statistical
390
product and service solutions; WMS, waxy maize starch; XRD, X-ray diffractometry.
391
ACKNOWLEDGMENTS
392
The authors thank the financial support received from NSFC-Guangdong Joint
393
Foundation Key Project (U1501214), Natural Science Foundation of Guangdong
394
Province, China (2014A030313236), and the Fundamental Research Funds for the
395
Central Universities of China (2015ZZ107).
396 18
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for calculating starch crystallinity and its correlation with double helix content: a
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496 497
(38) Jane, J., Craig, S. A. S., Seib, P. A., Hoseney, R. C. Characterization of granular cold water-soluble starch. Starch 1986, 38, 258-263.
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(39) Chan, Y. C., Braun, P. J., French, D., Robyt, J. F. Porcine pancreatic
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alpha-amylase hydrolysis of hydroxyethylated amylose and specificity of subsite
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binding. Biochemistry 1984, 23, 5795-5800.
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(40) Jane, J.; Craig, S.; Seib, P.; Hoseney, R. Characterization of granular cold wate-soluble starch. Starch 1986, 38, 258-263.
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(41) Loganathan, S.; Tikmani, M.; Edubilli, S.; Mishra, A.; Ghoshal, K. A. CO2
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adsorption kinetics on mesoporous silica under wide range of pressure and
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temperature. Chem. Eng. J. 2014, 256, 1-8.
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(42) Seo, E. J., Min, S. G.; Choi, M. J. Release characteristics of freeze-dried eugenol 23
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encapsulated with beta-cyclodextrin by molecular inclusion method. J.
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Microencapsul. 2010, 27, 496-505.
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(43) Li, X.; Jin, Z.; Wang, J. Complexation of allyl isothiocyanate by α-and
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β-cyclodextrin and its controlled release characteristics. Food Chem. 2007, 103,
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(44) Dries, D. M.; Gomand, S. V.; Goderis, B.; Delcour, J. A. Structural and thermal
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transitions during the conversion from native to granular cold-water swelling
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maize starch. Carbohydr. Polym. 2014, 114, 196-205.
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(45) Godet, M. C.; Bizot, H., Buléon, A. Crystallization of amylose-fatty acid
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complexes prepared with different amylose chain lengths. Carbohydr. Polym.
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formation, identity and physico-chemical properties. J. Cereal Sci. 2010, 51,
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(47) Agbenin, J. O.; Raij, B. V. Kinetics and energetics of phosphate release from
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65, 1108-1114.
524
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List of figure captions
526
Figure 1 SEC weight CLDs of debranched starches extracted from various starches.
527
Figure 2 Ethylene concentrations in inclusion complexes.
528
Figure 3 SEM photos of native (1), granular cold-water soluble (2) and inclusion
529
complex (3) samples. HA7 (A), HA5 (B), NMS (C), NPS (D) and WMS (E).
530
Figure 4 Raman spectroscopy of HA7 sample. (a) GCWS starch prepared from HA7,
531
(b) IC.
532
Figure 5 X-ray patterns of native (a), granular cold-water soluble (b) and inclusion
533
complex (c) samples. HA7 (A), HA5 (B), NMS (C), NPS (D) and WMS (E).
534
Figure 6 Release of ethylene from complexes at various temperature and RH
535
conditions. -80 °C (A), -20 °C (B), 4 °C (C), 25 °C (D), 52.9% RH (E) and 75.5% RH
536
(F).
25
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Table 1 Structural parameters obtained from SEC weight CLDs of various debranched starches a
537
Samplesb
AC(%)
HA7 HA5 NMS NPS WMS
69.8 ± 0.3 a 48.6 ± 0.9 b 26.5 ± 0.2 c 19.5 ± 0.3 d 0.2 ± 0.1 e
DP of peak maximum in SEC weight molecular size distribution of debrached starch DPAP1 DPAP2 DPAM a a 22.2 ± 0 48.3 ± 0.1 448 ± 2 a b b 20.7 ± 0.3 45.1 ± 0.1 539 ± 17 ab 16.2± 0.1 c 39.7 ± 0.1 c 821 ± 10 ac 19.3 ± 0.1 d 43.7 ± 0.1 d 3156 ± 212 d c c 16.4 ± 0.1 39.9 ± 0.2 -
Height of peak maximum in SEC weight molecular size distribution of debranched starch as a ratio to AP1 peak height hAP2/AP1 hAM/AP1 a 1.448 ± 0.006 1.816 ± 0.023 a b 1.163 ± 0.018 0.636 ± 0.325 b 0.580 ± 0.003 c 0.146 ± 0.001 c 0.918 ± 0.001 d 0.144 ± 0.004 d c 0.599 ± 0.006 -
538
a
539
small and/or broad to determine the local maximum, which appears as a shoulder of the adjacent larger peak. SEC: size exclusion
540
chromatography, CLDs: chain length distributions.
541
b
542
content, DP: degree of polymerization, AM: amylose, AP: amylopectin.
Mean ± standard deviation is calculated from duplicate measurements. "-" means the parameters cannot be identified, because the peak is too
Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS: normal potato starch, WMS: waxy maize starch, AC: amylose
26
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Table 2 Correlation coefficients between starch structures and ethylene concentrations in inclusion complexes (Y) a
543
Y AC DPAP1 DPAP2 DPAM hAP2/AP1 hAM/AP1 544
*
545
a
Y 1 0.997** 0.881* 0.890* -0.091 0.910* 0.900*
AC 1 0.858 0.872 -0.167 0.898* 0.919*
DPAP1
DPAP2
DPAM
hAP2/AP1
hAM/AP1
1 0.993** 0.091 0.992** 0.841
1 0.073 0.995** 0.886*
1 -0.014 -0.266
1 0.901*
1
Correlation is significant at the 0.05 level. **Correlation is significant at the 0.01 level.
Sample code: AC: amylose content, DP: degree of polymerization, AM: amylose, AP: amylopectin.
27
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Page 28 of 35
546
Table 3 Release rate constants k, release parameters n, fitting degree r2 and activation energy Ea of ethylene from ICs based on Avrami’s equation
547
at various temperature and RH conditions a Temperature (°C)
RH (%)
Ea
b
Samples
-80
548 549 550
-20
4
25
(kJ/mol)
52.9
k×10-3 (h-1) 10.05 28.52 262.90 612.01 1650.37 HA7 n 0.52 0.35 0.51 0.68 18.24 1.12 2 r 0.981 0.988 0.984 0.994 0.997 k×10-3 (h-1) 7.43 23.03 158.98 546.62 1368.89 HA5 n 0.41 0.32 0.61 0.55 18.42 0.98 2 r 0.990 0.984 0.930 0.997 0.994 5.76 207.63 1004.01 k×10-3 (h-1) 13.31 123.94 NMS 20.49 n 0.45 0.48 0.63 0.87 1.26 2 r 0.948 0.995 0.957 0.986 0.992 -3 -1 k×10 (h ) 5.52 11.39 38.47 192.63 818.73 NPS n 0.39 0.41 0.76 0.61 36.97 1.51 2 r 0.938 0.990 0.980 0.992 0.979 -3 -1 8.70 14.12 122.21 214.17 k×10 (h ) 1236.15 N. A.c WMS n 0.44 0.41 0.66 0.83 0.95 2 r 0.997 0.945 0.995 0.999 0.979 a ICs: inclusion complexes, RH: relative humidity. b Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS: normal potato starch, WMS: waxy maize starch. c N.A.: not analyzed. 28
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75.5 4763.58 1.01 0.994 4241.85 0.96 0.999 2989.18 0.91 0.993 2418.14 1.10 0.950 2462.06 0.99 0.993
Page 29 of 35
Journal of Agricultural and Food Chemistry
DP
w(logVh) [arb. units]
5 10 25
552
AM
2
AP2 AP1
10000 HA7 HA5 NMS NPS WMS
1
0 551
1000
100
1
10 Rh (nm)
100
Figure 1 SEC weight CLDs of debranched starches extracted from various starches.
553
Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS:
554
normal potato starch, WMS: waxy maize starch, DP: degree of polymerization, SEC:
555
size exclusion chromatography, CLDs: chain length distributions, AM: amylose, AP:
556
amylopectin.
29
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Journal of Agricultural and Food Chemistry
Ethylene concentration (%)
35
a
30 b
25 20
c
15
c
10 5
d
0 HA7
HA5
NMS
NPS
WMS
557 558
Figure 2 Ethylene concentrations in inclusion complexes.
559
Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS:
560
normal potato starch, WMS: waxy maize starch. Samples with different letters above
561
the columns are significantly different at p < 0.05.
30
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Journal of Agricultural and Food Chemistry
A1
A2
A3
10µm
10µm
10µm
562
B1
B3
B2
10µm
10µm
10µm
563
C1
C2
C3
10µm
10µm
10µm
564
D1
D2
D3
30µm
30µm
30µm
565
E1
E3
E2
10µm
10µm
10µm
566 567 568
Figure 3 SEM photos of native (1), granular cold-water soluble (2) and inclusion
569
complex (3) samples. HA7 (A), HA5 (B), NMS (C), NPS (D) and WMS (E).
570
Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS:
571
normal potato starch, WMS: waxy maize starch.
31
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Intensity [arb. units]
Journal of Agricultural and Food Chemistry
Page 32 of 35
b a
4000
3500
3000
2500
2000
1500
1000
500
-1
572 573
wavenumber(cm
574
Figure 4 Raman spectroscopy of HA7 sample. (a) GCWS starch prepared from HA7, (b) IC.
575
)
Sample code: HA7: Hylon-7, GCWS: granular cold-water soluble, IC: inclusion complex.
576
32
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B
a
24.1%
b
29.2%
c
31.5%
5
10
577
15
20 25 2 Theta (°)
30
Intensity [arb. units]
Intensity [arb. units]
A
b
23.8%
c
25.2%
10
15
20 25 2 Theta (°)
30
35
D
a
33.5%
b
16.1%
c
17.0%
10
15
20 25 2 Theta (°)
30
Intensity [arb. units]
Intensity [arb. units]
578 579
26.1%
5
35
C
5
a
a 44.6%
b
14.5%
c
15.2%
5
35
10
15
20 25 2 Theta (°)
30
35
Intensity [arb. units]
E a
48.0%
b
0.4%
c
0.5%
5
580
10
15
20 25 2 Theta (°)
30
35
581
Figure 5 X-ray patterns of native (a), granular cold-water soluble (b) and inclusion complex (c)
582
samples. HA7 (A), HA5 (B), NMS (C), NPS (D) and WMS (E).
583
Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS: normal potato starch,
584
WMS: waxy maize starch.
33
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A
B HA7 HA5 NMS NPS WMS
0.10
0.05
0.00
0.15 Ethylene release ratio
Ethylene release ratio
0.15
0.10
20
40
585
60 80 Time (h)
HA7 HA5 NMS NPS WMS
0.05
0.00 0
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100 120
0
20
40
60 80 Time (h)
100 120
D
C
1.0 Ethylene release fraction
Ethylene release ratio
1.0 0.8 0.6 HA7 HA5 NMS NPS WMS
0.4 0.2 0.0
0.8 0.6
0.2 0.0
0
50
586
100 150 Time (h)
HA7 HA5 NMS NPS WMS
0.4
200
0
20
40
60 80 Time (h)
100 120
F
E
1.0 Ethylene release ratio
Ethylene release ratio
1.0 0.8 0.6 HA7 HA5 NMS NPS WMS
0.4 0.2 0.0
0.8 0.6
0.2 0.0
0
587
1
2 3 Time (h)
4
HA7 HA5 NMS NPS WMS
0.4
0.0
0.2
0.4 0.6 0.8 Time (h)
1.0
1.2
588
Figure 6 Release of ethylene from ICs at various temperature and RH conditions.
589
-80 °C (A), -20 °C (B), 4 °C (C), 25 °C (D), 52.9% RH (E) and 75.5% RH (F).
590
Sample code: HA7: Hylon-7, HA5: Hylon-5, NMS: normal maize starch, NPS:
591
normal potato starch, WMS: waxy maize starch, RH: relative humidity, ICs: inclusion
592
complexes.
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Graphic abstract Encapsulation of Ethylene Gas into Granular Cold-Water Soluble Starch: Structure and Release Kinetics Linfan Shi, Xiong Fu, Chin Ping Tan, Qiang Huang, Bin Zhang
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