Comparative Analysis of Small-Molecule Diffusivity in Different Fat

Jan 5, 2018 - Based on these assumptions for disk-shaped bleached 2D diffusion, data could be normalized by the mean prebleach intensity within the RO...
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Comparative Analysis of Small-Molecule Diffusivity in Different Fat Crystal Network Xiuhang Chai, Zong Meng, Peirang Cao, Jiang Jiang, and Yuanfa Liu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b04677 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018

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Journal of Agricultural and Food Chemistry 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.

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Journal of Agricultural and Food Chemistry

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Comparative Analysis of Small-Molecule Diffusivity in Different

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Fat Crystal Network

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Xiu hang Chai , Zong Meng , Pei rang Cao ,Jiang Jiang , Yuan fa Liu *

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State Key Laboratory of Food Science and Technology, Synergetic Innovation Center of Food

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Safety and Nutrition, School of Food Science and Technology, Collaborative Innovation Center of

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Food Safety and Quality Control in Jiangsu Province, Jiangnan University, 1800 Lihu Road, Wuxi

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214122, Jiangsu, People’s Republic of China

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AUTHOR EMAIL ADDRESS: [email protected]

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*Corresponding

author

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[email protected].

[telephone

(086)510-85876799;

fax

(086)510-85876799;

e-mail

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ABSTRACT: Oil migration and fat recrystallization in fat-structured food materials can result in

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significant deterioration in food quality. Consequently, it is important to monitor and quantify the

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diffusivities of the migrants in fat crystal network. The diffusion coefficients of Nile red dye in

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liquid oils through fully hydrogenated palm kernel oil (FHPKO) / triolein (OOO) and fully

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hydrogenated soybean oil (FHSO) / triolein (OOO) systems were evaluated by the fluorescence

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recovery after photobleaching (FRAP) method. The effective diffusion coefficients (Deff) and

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mobile fraction (Mf) increased with the decrease of solid fat contents (SFC), with the changes of

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microstructure from more densely to slightly larger packed clusters for both FHPKO/OOO and

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FHSO/OOO systems. In addition, microstructural parameters of these systems were estimated by

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the image analysis. The results showed that the diffusion of dye and liquid oil was affected by the

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microstructure. The higher Deff was associated with lower fractal dimensions, lager crystal thickness,

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and larger average particle sizes. Finally, higher-permeability coefficients were calculated according

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to the Darcy’s Law and it was significantly correlated to the Deff.

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KEYWORDS: oil migration, fluorescence recovery after photobleaching, triacylglyceride crystal

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network, diffusion coefficients, fractal dimension

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Journal of Agricultural and Food Chemistry

INTRODUCTION

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The physical properties of fat-structured food materials, such as texture, sensory flavor and

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mouthfeel, are largely influenced by fat crystal network. The capability of a fat crystal network to

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entrap liquid oil is usually characterized by oil migration related to the interface function between the

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solid crystal and liquid oil, which leading to fat recrystallization to more stable polymorphs. Oil

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migration and fat recrystallization results in decline in food quality, which makes the products

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unacceptable 1. For example, oil migration and fat recrystallization in chocolates results in fat bloom

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on the surface 2. Chocolate is a complex mixture with solid particles distributed in liquid and solid

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triacylglyceride (TAG) phases 3. Therefore, part of these mobile components (minor lipids and

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specific TAGs) will be able to participate in transport phenomena in post-manufacture events such as

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oil diffusivity and fat recrystallization. The formulation, processing and storage conditions can

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impact the oil diffusivity and recrystallization through cocoa butter system

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reduction and sensory changes of products. Previous research has shown that TAG concentration

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gradients is one of factors leading to transport phenomena 7. Subsequently, more driving forces for

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transport phenomena have also been studied, such as structure, fat levels, solid particles 8, storage

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temperature

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dramatic breakthroughs through quantitatively analytical techniques 11, but the phenomenon of oil

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migration and fat recrystallization has not been understood well. Therefore, it is essential to make an

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in-depth understanding about the transport phenomena and influence mechanism in fat crystal

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network for industrial application.

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9, 10

4-6

, leading to quality

and so on. To date, studies on the mechanisms of oil migration have acquired some

In the recent decades, fluorescence recovery after photobleaching (FRAP) which is a

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microscopy-based technique has been widely applied in the dynamics research of various systems,

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especially in biological environments

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quantitatively analyze the translational mobility of labeled molecules with dye

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FRAP can be used to evaluate the small-molecule diffusion capacity by monitoring the fluorescence

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recovery of the bleached region. In addition, the FRAP experiment can be performed on the confocal

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scanning microscope (CLSM) for rapid analysis with high spatial resolution and minimum sample

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preparation. However, the FRAP technique has been sparingly applied to measure the diffusion in

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food system 17. Perry et al.

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new findings about structural and kinetic information of starch. Subsequently, the diffusion of mobile

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molecules in model cheeses

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also used in the dairy systems to evaluate the influence of microstructure on the diffusion of solutes

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within dairy products

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mobility of the solute molecules inside the oleogel was observed using FRAP experiment 24. Overall,

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FRAP technique has being a fast and accurate method to study the structural and kinetic information

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in food systems. In fat-contained materials, assuming that the dye and oil move in tandem, the rate of

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intensity recovery is the same as the rate of molecular diffusion. Marty et al. 25 investigated molecule

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diffusion through polycrystalline triacylglyceride networks and proved that FRAP technique was a

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powerful tool to monitor the diffusion through the complex matrices. Similarly, Nicole et al.

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analyzed the diffusion coefficients of different types of liquid oil through fat crystal networks, and

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pointed out that the differences of diffusion coefficients were ascribed to the changes of crystal

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cluster structures. However, the relationship between the diffusion and parameters of crystal

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microstructures involving in average particles size of crystal and fractal dimension in different

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12-14

. FRAP is also an important and versatile technique to 15, 16

. Consequently,

determined the diffusion coefficients in starch solution, and got some

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has been studied by FRAP technique. And the FRAP method was

and the diffusion coefficient of pepsin in dairy gel

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. More recently, the

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Journal of Agricultural and Food Chemistry

crystal networks has not been taken into account seriously.

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Therefore, based on the FRAP technique coupled with the CSLM, the objective of the paper

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was to study the structural basis of small molecule diffusion through triacylglyceride crystal

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networks. Moreover, the combined effects of SFC and microstructures on the dye and liquid oil

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migration through the crystalline fat matrix in FHSO/OOO and FHPKO/OOO systems were also

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investigated.

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MATERIALS AND METHODS

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Sample preparation. Mixtures of triolein (OOO, Sigma-Aldrich, Shanghai, China) and fully

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hydrogenated palm kernel oil (FHPKO, Kerry specialty fats Ltd., Shanghai, China), and blends of

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OOO with fully hydrogenated soybean oil (FHSO, Kerry specialty fats Ltd., Shanghai, China) were

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prepared at the temperature of 60 °C. Blends ranged from 20 to 100% hard stock with mass ratios

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(w/w). Then, the blends were crystallized at 20 °C for 24 h for further analysis. The lipophilic

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fluorescent dye of Nile red (Sigma-Aldrich, Shanghai, China) was dissolved in the mixtures to get a

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final concentration of 1500 µmol/L 25. Mixed samples were held at 110 °C for 30 min and vortexed

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throughout to make Nile red complete dissolution. Once mixed, samples (about 15 µL) were placed

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on a preheated glass slide using a preheated capillary tube, and then a preheated cover slip glass was

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placed over the sample to produce a film with uniform thickness with no air bubbles. Finally, sample

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slides were crystallized at 20 °C in incubator for 24 h before FRAP experiments.

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Confocal imaging and FRAP. FRAP experiments were conducted on the confocal laser

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scanning microscope of Zeiss LSM510 (Zeiss Inc., Shanghai, China) equipped with an Ar/ KrAr

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laser. The Nile red fluorescence was excited with 514 nm laser (40 mW) and the fluorescence signals

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between 600 and 750 nm were recorded. A 40× objective lens with a numerical aperture (NA) of

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1.30 was applied to capture images at a resolution of 512 × 512 pixels at successive time intervals. In

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addition, the polarized light images were observed using a Zeiss Axiovert-200M inverted light

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microscope (Zeiss Inc., Shanghai, China) with a 20× (0.5 NA) objective with a Leica DFC450 video

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camera attached (Leica, Germany). All experiments were taken in a thermostated stage at 20 °C in

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air-conditioned room.

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FRAP experiments consisted of three steps: pre-bleaching, bleaching, and post-bleaching 27. In

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FRAP experiments, five reference images were acquired with the laser power intensity of 10% at the

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pre-bleaching step, then the selected areas (region of interest, ROI) with a radius r0= 22 µm were

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bleached at a 100% intensity with a bleach pulse of 30 image scans. Ultimately, the recovery images

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were taken at an interval of 20 s at 10% of the maximum laser intensity until full recovery, and the

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recovery time depended on the sample characteristics during the post-bleaching step. Three series of

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FRAP experiments were performed on each of the replicates, giving a total of nine series for each

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sample.

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FRAP analysis. Fluorescence recovery analysis in post-bleach images was carried out in Image

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J software. In order to exclude edge effects, the smaller circular area was used to determine oil

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diffusion instead of whole ROI (diameter of 7.25 µm) in order to avoid edge effects. According to

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previous reports

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effective dye diffusion coefficients can be calculated using the classical diffusion equation

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based on the Fick’s second law of diffusion.

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28-31

, during the homogenous medium and two-dimensional diffusion process, the

 

∁,  = ∇ ∁, 

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(1)

(1)

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Journal of Agricultural and Food Chemistry

Where, D is the lateral diffusion coefficient; ∁,  is the concentration of unbleached fluorophores at position r and time t. Based on these assumptions for disk-shaped bleached 2D diffusion, data could be normalized by the mean pre-bleach intensity within the ROI. The recovery curve  was written as:  

 = 

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(2)

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Where,  is fluorescence at time t,  is fluorescence of the first post-bleach image (time

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0),  is fluorescence at equilibrium. FRAP recovery curves were then fitted with the formula

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according to Axelrod’s theory: 

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Where, K0 is the bleaching efficiency parameter, A is a fitting parameter associated with the immobile fraction, and " is the radial diffusion time. And the effective diffusion coefficients (Deff) could be determined from the exponential fits of the data (the recovery half-time t1/2) based on the FRAP recovery formula.

#$$ =

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(3)

 =  1 − exp [− ] + A

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.&&×() *+/)

(4)

Where, Deff is effective diffusion coefficient, ω is bleached area radius, t1/2 is the recovery half-time. In contrast, mobile fractions (Mf) could be determined graphically according to normalization and data fitting procedures of Siggia’s model 33 as follows (Fig 1C):

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 =

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 -

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(5)

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Where, . is the average intensity of pre-bleached images.

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Nine series FRAP experiments were done on each sample and then the data averaged. Data

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analysis and fitting were conducted by use of Origin Pro 9.0 for Windows (OriginLab, Northampton,

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MA).

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Microstructural components analysis. The fractal dimension was analyzed with the

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particle-counting method. The particle-counting fractal dimensions (Df) were measured through a

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properly thresholded and inverted pre-bleached image using the Object Image 2.01 software. A

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log-log plot of the number of crystal reflections or “particles” (Np) observed within boxes and the

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length size (R) of theses boxes gives a line with a slope equivalent to Df 34, 35.

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The particle equivalent diameters were obtained by the use of the Fovea Pro Image Processing

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Tool Kit 4.0 plug-ins (Reindeer Graphics Inc., North Carolina, USA) in Adobe Photoshop 6.0

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(Adobe System Inc., San Jose, USA). The polarized light microscopes were thresholded and inverted

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to calculate the particle equivalent diameters according to the previous report

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Supplementary Figure 1, the program assumes a circular geometry and obtains the square root of the

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quotient of the area to π. This procedure was repeated in at least 9 images for each particle size.

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. As shown in

Solid fat content (SFC). SFC was measured by pulsed nuclear magnetic resonance (pNMR) 37

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with a Bruker PC120 series NMR analyzer (Bruker, Karlsruhe, Germany)

. The water bath was

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used to cool samples rapidly and offer accurate temperature control. The instrument was

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automatically calibrated by the use of three standards (supplied by Bruker) with the solid contents of

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0, 31.3, and 74.6%, respectively. Approximately 2.5 g of each sample was placed in a glass NMR

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tube for all pNMR experiments and was kept at 110 °C for 30 min to ensure complete melting and

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destroy any crystal memory, and then stored at the temperature of 20 °C in incubator to crystalize for

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24 h prior to monitoring SFC. Then the glass NMR tubes were put into the NMR analyzer and SFC

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readings were obtained on the computer. All measurements were performed in triplicate.

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Crystal polymorphism and crystalline domain size. X-ray diffraction (XRD) data were

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collected by use of D8 Advance XRD (Bruker, Karlsrube, Germany) equipped with Cu-Kα radiation

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and Ni filter

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divergence slit of 1.0 mm, scatter slit of 1.0 mm, and receiving slit of 0.3 mm, respectively. For the

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small-angle X-ray diffraction analysis (SAXD), samples were scanned from 1 to 10 deg at a rate of

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0.02°/min. The wide-angle X-ray diffraction analysis (WAXD) was obtained through scanning the

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samples from 11 to 30 deg at the rate of 1°/min. Peak Fit software (Seasolve, Framingham, MA,

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USA) was used to analyze the obtained data in both SAXD and WAXD patterns. The thickness of the

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nanoscale crystals was calculated by the well-known Scherrer formula 39.

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. The copper lamp (λ=1.54Å for copper) was set to 30kV and 10 mA with the

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ξ = 2345678

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(6)

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Where, k is the shape factor with a value of 0.9 for crystallites of unknown shape, θ is the

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diffraction angle, FWHM is the full width at half of the maximum peak height in radians

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corresponding to the first small angle reflection reflecting the (001) plane, and λ is the XRD

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wavelength with the value of 1.54 Å for copper 40.

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Statistical analysis. All experiments were determined at least in triplicate. All data were presented as the means

±

standard deviations (SD). Significant differences between samples were

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analyzed by the use of ANOVA with Duncan’s multiple-range test in statistical analysis system

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software of SPSS. The significance of differences among mean values was identified at a level of p