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Konjac glucommanan compound gel: Effect of cyclodextrins
Accepted Manuscript Gelation of κ-carrageenan/Konjac glucommanan compound gel: Effect of cyclodextrins Chao Yuan, Dongyan Xu, Bo Cui, Yanli Wang PII:
S0268-005X(18)30864-6
DOI:
10.1016/j.foodhyd.2018.07.037
Reference:
FOOHYD 4566
To appear in:
Food Hydrocolloids
Received Date: 10 May 2018 Revised Date:
20 July 2018
Accepted Date: 22 July 2018
Please cite this article as: Yuan, C., Xu, D., Cui, B., Wang, Y., Gelation of κ-carrageenan/Konjac glucommanan compound gel: Effect of cyclodextrins, Food Hydrocolloids (2018), doi: 10.1016/ j.foodhyd.2018.07.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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κ-Carrageenan
Heating
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XRD
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Stirring
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KCl solution ( )
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Konjac glucomannan
SEM
CDs ( )
SEM
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Gelation of κ-Carrageenan/Konjac Glucommanan Compound Gel: Effect of
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Cyclodextrins
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Chao Yuana, b, *, Dongyan Xua, b, Bo Cuia, b, **, Yanli Wanga, b
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a
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of Technology, Shandong Academy of Sciences, Jinan, 250353, China
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b
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Academy of Sciences, Jinan 250353, China
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State Key Laboratory of Biobased Material and Green Papermaking, Qilu University
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School of Food Science and Engineering, Qilu University of Technology, Shandong
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* Corresponding author: [email protected] (C. Yuan).
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** Corresponding author: [email protected] (B. Cui).
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ABSTRACT The impact of cyclodextrins (CDs) on physical properties of the compound gel of
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κ-carrageenan/konjac glucommanan (KC/KGM) was investigated by texture profile
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analysis (TPA), compression modulus test, freeze-thaw stability test, scanning
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electron microscopy (SEM) and X-ray diffraction (XRD). TPA results showed that
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CDs had remarkable influence on mechanical properties of the compound gel. In
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concentration range of 0.5% - 1.5% (w/w), hardness was strengthened with the
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increase of CDs concentrations, then gradually decreased at higher CDs
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concentrations. Cohesiveness, gumminess and chewiness showed similar tendency as
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the hardness. However, influence of CDs on gel springiness was slight. Compressive
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experiment indicated that the CDs increased the compressive elastic modulus (S) of
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the compound gel and improved the deformation resistance of the gel network. In the
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freeze-thaw stability test, stability of the compound gels was obviously improved after
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adding CDs, about 10% more water was preserved after the first freeze-thaw cycle.
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SEM results showed that the microstructures of the compound gels containing CDs
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were much denser with smaller voids. XRD results indicated that the crystallization
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peaks of the compound gels decreased after CDs were added. The exclusion of CDs
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from the polysaccharides chains in the sol state led the KC/KGM compound solution
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to form a denser network after cooling and benefited the texture and freeze-thaw
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stability of the formed gel. CDs improved the synergistic interaction between KC and
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KGM polysaccharides. The influence of methyl-β-cyclodextrin (M-β-CD) on the
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formation and characteristics of KC/KGM compound gel was the most significant
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among all the selected CDs.
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Keywords: Konjac glucommanan; κ-Carrageenan; Gelation; Cyclodextrins; Physical
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properties
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1. Introduction There is considerable interest which has been focused on the role of food
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texturants in the development of food industry. Texturants are important food additive,
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since they act as a gelling, thickening and water retention agent, and they are also
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used as a lubricant (Kenwright et al., 2014; Rao & Kenny, 1975). In recent years, the
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food texturants have gradually grown as the hot spot of food colloid research, which
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is also a vital trend of hydrocolloids production and development in the future
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(Harding, Smith, Lawson, Gahler, & Wood, 2011; Tom Brennera, 2013). Among them,
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carrageenan and konjac glucomannan (KGM) are two polysaccharides which were the
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most widely used in the food industry. Carrageenan is a sulphated linear
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polysaccharide of D-galactose and 3, 6-anhydro-D-galactose join by α-1,3 and
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β-1,4-glycosidic linkage, which is obtained by extraction of certain red seaweeds of
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the Rhodophyceae class (J. Necas, 2013; Li, Ni, Shao, & Mao, 2014). It is classified
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into various types such as λ, κ, ι, ε and µ, mainly according to the quantities and
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position of sulphate esters. One of the most widely applied member of the
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carrageenan family is κ-carrageenan (KC), it is employed as a gelling agent, as well as
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a thickener and stabiliser. Nevertheless, KC has several defects, such as brittleness,
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low flexibility and dissolving out water seriously while KC form gel alone (J. Necas,
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2013; Loret, Ribelles, & Lundin, 2009a; Wei, Wang, & He, 2011a). KGM is a neutral
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heteropolysaccharide, with the main chain composed of β-(1,4)-glycosidic bond
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linked D-glucose and D-mannose residues in a molar ratio of 1:1.6 (Jin et al., 2015;
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Joanna Kruka, 2017). Under alkaline conditions, KGM will deacetylate, aggregate in
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Nondeacetylated KGM but at concentrations of 7% and higher can also form gel
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(Kohyama, Iida, & Nishinari, 1993; Nishinari, Williams, & Phillips, 1992; Penroj,
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Mitchell, Hill, & Ganjanagunchorn, 2005). KGM shows a synergistic interaction with
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KC under the condition of non-deacetylation, inducing the formation of a promising
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compound hydrocolloid gel. The KC/KGM compound gel formed in certain
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conditions is less brittleness, good flexibility and water retention ability for the
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synergistic effect between the two polysaccharides.
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Cyclodextrins (CDs) are cyclic oligosaccharides consisting of (α-1,4)-linked six
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(α-CD), seven (β-CD), eight (γ-CD) or more α-D-glucopyranose units with a
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hydrophobic internal cavity and a hydrophilic exterior (Del Valle, 2004; Yuan, Du,
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Zhang, Jin, & Liu, 2016). By random substitution of the hydroxy groups, even by
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lipophilic groups, gives amorphous mixtures of high water-soluble CD derivatives,
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including hydroxypropyl-β-cyclodextrin (HP-β-CD) and methyl-β-cyclodextrin
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(M-β-CD), which have wide application in food industry, (Inoue, 1998; Szente &
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Szejtli, 2004). Due to the particular characteristic of inner hydrophobic and outer
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hydrophilic, CDs and their derivatives can form water-soluble inclusion complexes
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with liposoluble components. The inclusion complexes have many beneficial
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functions, including water solubility, higher bioavailability and stability. The inclusion
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complexes using CDs as the wall material have many influences on the food system,
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such as texture, rheological properties and freeze-thaw stability (Inoue, Takahashi,
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Okada, Iwasaki, Murata, & Kanamoto, 2013; Loftsson & Brewster, 2010; Tian, Xu, Li,
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Jin, Chen, & Wang, 2009). It has long been known that gelation of polysaccharide hydrocolloids is strongly
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affected by cosolvents, such cosolvents, sugars and polyols. Therefore the effect has
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been investigated widely to enhance polysaccharide gelation and gel stability (Loret,
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Ribelles, & Lundin, 2009b; Oakenfull, 2000). Different from common single or oligo
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sugars, the particular circular truncated cone structure of CDs may generate special
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effect on the gelation of polysaccharide hydrocolloids. In recent years, researchers did
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find the effects of CDs on different food hydrocolloids. Tian et al. (Tian et al., 2009)
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reported that CDs could effectively affect the orderly rearrangement of starch, thereby
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inhibiting the retrogradation of starch. In our previous study, it was found that CDs
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had obvious effects on the texture properties and storage stability of KC (Yuan et al.,
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2016). Furthermore, the complex forming ability make CDs widely applied in gelled
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food. Nevertheless, to our knowledge, the influence of CDs on the KC/KGM
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compound gel was not clear and has not been studied before, even they often contain
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in application systems simultaneously, particularly in gelled foods. Although it was
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known that synergistic interaction happens between KC and KGM alone.
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In the present study, we explored the effects of 5 common CDs and CD
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derivatives on gelation and crystallization of KC/KGM compound gel through TPA,
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compression elastic modulus analysis, syneresis analysis, SEM and XRD methods.
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2. Materials and methods
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2.1. Materials
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α-, β-, γ-CD (purity > 98%), and KC, KGM power (food-grade) were purchased
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(MW: 1310) were purchased from Sigma-Aldrich (Shanghai, China). KCl was
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purchased from Damao Chemical Reagent Company (Tianjin, China).
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2.2. Methods
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2.2.1. The KC/KGM compound gels preparation
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The total polysaccharide (gelling powder) content was 0.15 wt%. KC and KGM
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were blended and grinded at 5.5 : 4.5 (Wei, Wang, & He, 2011b) equably with a
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mortar, then screened by a 100 mesh sieve. A 0.15 wt% stock solution was prepared
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by directly dissolving the KC/KGM mixed-powder in potassium chloride solution
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(0.2%, w/w), which were stirred under 40 °C for 20 min. After that the hot solution
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was poured into 6 beakers (50 mL), 0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5% (w/w)
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CDs were added in under stirring, respectively. Keep on stirring 30 min at 80 °C,
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followed by standing 5 min to allow escape of air bubbles. The mixed solutions were
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put in a 25 °C incubator to lead formation of gels and keeping 6 h before test.
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2.2.2. Texture analysis
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Texture parameters were determined by a Texture Analyzer TA-XTplus/30
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(Stable Micro Systems, Surrey, UK). Samples were penetrated with a 12 mm diameter
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cylinder probe. In all the experiments, the samples were tested at a speed of 5 mm/s
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with control force of 5 g and compression depth of 50% (Ju & Kilara, 1998).
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2.2.3. Compression measure
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Compression properties of the compound gel systems were also performed with
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the Texture Analyzer TA-XTplus/30 but equipped with a parallel plate (36 mm
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were cut into cylinders (Ø36 mm × 10 mm). The samples were placed on the
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platform of the texture analyzer and compressed by the 36 mm parallel plate at a
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speed of 0.1 mm/s to a depth of 5 mm and load cell of 5N sensibility were set in the
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uniaxial compression experiments, the relationship between stress and stain was
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recorded. Under low strain range, the uniaxial compression results can be fitted by
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Neo-Hookean model, therefore the compressive elastic modulus (S) was obtained by
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the slope of the fitted curve.
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2.2.4. Freeze-thaw stability analysis
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15 mg/L mixture solution of KC/KGM (5.5 : 4.5) was heated at 40 °C for 20 min
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under stirring. After that the solution was weighted and transferred to 6 beakers (50
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mL), 1.5% (w/w) different types of CDs, namely, α-, β-, γ-CD, M-β-CD and HP-β-CD
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were added under stirring, separately. The one free with CDs was for control. The
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solutions were then heated at 80 °C for 30 min, each sample was evenly transferred
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into 2 centrifuge tubes. The tubes were cooled at 25 °C for 6 h to promote formation
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of gels. The gels were frozen at -18 °C for 24 h, then thawed at 25 °C for 6 h as a
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freeze-thaw cycle, the whole experiment repeatedly up to 5 freeze-thaw cycles. After
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each thawing cycles, samples were centrifuged at 1700g for 15 min, then removed the
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separated water and weighed. Divided the weight of the water decanted by the total
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weight of the gel before centrifugation, the percentage of syneresis was obtained.
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2.2.5. Scanning electron microscopy (SEM) observation
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The microstructures of the compound gels were investigated by SEM. The
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ACCEPTED MANUSCRIPT prepared compound gels were rapidly quenched with liquid nitrogen, then cut into
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cubes about 5 mm, placed in an ultra-low temperature refrigerator at -80 °C for 2 h,
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and then freeze dried to obtain the SEM samples. Before observation, the gels were
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coated with gold-palladium under argon atmosphere using a gold sputter module in a
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high vacuum evaporator. The coating time was about 30 s at an acceleration voltage
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of 20 kV. Images with magnification of 500 were recorded.
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2.2.6. X-ray diffraction (XRD) analysis
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The compound gels were characterized by a Bruker D8-Advance XRD
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instrument (Bruker AXS Inc., Germany). The parameters were performed in reflection
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mode at 40 kV, 40 mA with graphite-filtered Cu-Kα radiation. Measurement was
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taken between 5 - 60° (2θ) with a step size of 0.03° (Liu, Wang, Kang, Cui, & Yu,
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2018).
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2.2.7. Statistics
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TPA, compression test and freeze-thaw stability test were replicated in triplicate
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and data were reported as mean ± standard deviation. Statistical significance was
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assessed by Duncan’s new multiple range test with the level of significance set at p
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≤ 0.05.
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3. Results and discussion
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3.1. Effect of CDs on texture of the KC/KGM compound gels
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One of the most common instrumental imitative measurements of texture is
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texture profile analysis (TPA) (Wilkinsony, And, & Minekusy, 2000). The main
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mechanical parameters of TPA include hardness, adhesiveness, springiness, 9
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influence of CDs on the KC/KGM compound gel texture behavior were investigated.
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Hardness, springiness, gumminess, cohesiveness and chewiness were selected as
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study parameters according to the properties of gel system.
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Hardness refers to the force required for compressing material between dentes
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(in solids state) or between tongue and palate (in semisolid state) (Yuan et al., 2016).
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Figure 1a shows the influence of CDs on hardness of the KC/KGM compound gel. In
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the low CDs concentrations range (0.5% - 1.5%, w/w), the hardness added gradually
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as the CDs concentration increased. Then hardness shows a downward trend after the
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concentrations of 2.0%. It was reported that the exclusion of sugars during gel
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formation was one of the driving forces for better gel. Under low concentrations, the
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exclusion of CDs from polysaccharide surfaces in the sol state. The exclusion of the
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CDs reduced water population which weakened polysaccharide-water interactions and
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inducing polysaccharide strands to be close to each other, then getting a denser
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network structure with higher gel strength (Stenner, Matubayasi, & Shimizu, 2016).
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However, excessive CDs could cause difficulty in clustering of polysaccharide strands
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and hinder the formation of the KC/KGM compound network, thus, the hardness
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dropped along with the further increase of CDs concentrations. For the three natural
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CDs, the influence on gel hardness follows the order of β-CD > α-CD > γ-CD. Among
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them, β-CD molecule itself can form intermolecular hydrogen bonds which make it
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has the least of free hydroxyls. This could contribute to the uniform distribution of
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polysaccharide molecules. The size of α-CD is the smallest, the smaller molecular size
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of gel added with α-CD is relatively large. γ-CD has the largest size and the largest
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number of free hydroxyl groups, so it is easy to bind to polysaccharide molecules,
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which hinders the migration of polysaccharide molecules and is not conducive to the
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uniform distribution of polysaccharide molecules. For two modified CDs, HP-β-CD is
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hydroxyl substituted by hydroxypropyl groups on the basis of β-CD. The molecule is
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more extensible and the intermolecular hydrogen bonds is opened after substitution. It
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is easier to form hydrogen bond with KC/KGM polysaccharide molecules, and the
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larger molecule size prevents the polysaccharide molecules from approach, thus its
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gel hardness is the lowest. Compared with other CDs, M-β-CD showed the greatest
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influence on the hardness of the gels. M-β-CD is a derivative of β-CD which hydroxyl
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groups are partly replaced by methyl groups. The least free hydroxyl groups leading it
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the most difficult to form hydrogen bonds with the polysaccharide molecules.
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Therefore, it is more conducive to the formation of uniform gel system and gain the
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highest gel hardness.
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Springiness is the ability that material can be stretched and returns to its original
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length. The springiness curves of the KC/KGM compound gels with adding different
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concentrations of CDs are displayed in Fig. 1b. From the diagram it can be drawn that
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the addition of CDs have little effect on the springiness of the KC/KGM compound
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gel. The initial springiness of the compound gel is low for the gel is brittle. Compared
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with the two polysaccharide molecules, the size of CDs are far smaller and easier to
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dissolve in the water in the gel network, consequently, they play a slight role in the
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springiness of formed compound gels with a water content of about 98%. Cohesiveness is defined as the degree of compression between the teeth before
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the food breaks. Fig. 1c shows that the impacts of the natural and modified CDs on
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the cohesiveness of the KC/KGM compound gels were similar. The skeleton structure
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can affect the break of the compound gels, the crosslinking of the KC/KGM
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polysaccharide strands affects the strength of the skeleton. CDs weaken the
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cohesiveness of the gels because of CDs molecules hinders approach of the double
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helix which blocks the extension of the network wall. However, the cohesiveness of
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gel containing β-CD increases at high concentration, for the solubility of β-CD is low
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(< 1.8%), at higher concentration, β-CD precipitates easily, resulting in adhesion of
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β-CD to the network surface, which increased the toughness of the compound gel.
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Gumminess is a secondary TPA parameter, which is obtained by multiplying
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hardness by cohesiveness. The results of gumminess are shown in Fig. 1d. The effect
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tendency of adding CDs on the gumminess of the compound gels are consistent with
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the influence on cohesiveness. The main reason is also attributed to the existence of
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CDs molecules hinders the approach of the KC/KGM polysaccharide strands and
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blocks the extension of the network wall.
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Chewiness is the mouth feel sensation of labored chewing due to sustained,
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elastic resistance from the food, which is also a secondary TPA parameter. Chewiness
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is calculated through multiplying gumminess by springiness. Figure 1e shows the
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effects of different CDs on chewiness of the compound gels. The chewiness shows a
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decreasing trend with the increase of CDs concentration. Among all CDs, γ-CD has
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the least effect on the chewiness of the compound gels.
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3.2. Compressive test analysis In mechanics, the mechanical properties of a material are usually expressed in
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terms of compressive elastic modulus (S). Compressive elastic modulus refers to the
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ratio of the vertical compressive stress to the total vertical strain of the sample. The
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higher the compressive modulus, the stronger the resistance of the material to
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deformation. Compressive elastic modulus of the pure KC/KGM compound gel and
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the KC/KGM compound gels containing CDs are shown in Table 1.
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Compressive modulus of the compound gel containing M-β-CD is the highest,
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reaching 9.08 kPa, followed by β-CD, α-CD, γ-CD, HP-β-CD, and finally the pure
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KC/KGM compound gel. The results show that the addition of CDs increases the
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anti-compression ability of the KC/KGM compound gel. The reason behind the
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change of compressive elastic modulus can be explained by that CDs enhanced the
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hydrophilic gel network. Similar results have been reported by Evingür and Peckan
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(2013; 2015). They studied the effect of KC on the mechanical properties of
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polyacrylamide-KC composite by the compressive elastic measure method. The
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results show that the composite’s compressive elastic modulus is found to be strongly
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dependent on the KC content and temperature, and it is proved that the addition of KC
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in low content can make the PAAm-KC composite form a superelastic gel network
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from the mechanical point of view. In present study, the addition of CDs makes the
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KC/KGM compound gel network denser. It is because that the addition of CDs
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promotes the helical aggregation of polysaccharides to form a short-distance gel
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structure can be intuitively represented by the increase of compression elastic
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modulus. Among tested CDs, M-β-CD has the best effect, which is consistent with the
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results of texture analysis.
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3.3. Effect of CDs on Freeze-thaw stability of the KC/KGM compound gels
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Freeze-thaw stability test plays a vital role in frozen food industry. In cold chain
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storage, the synergistic effect of thermal fluctuation and water phase change is the
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main factor of refrigerated food deterioration, especially gelled foods, which may
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affect the texture, viscosity or the functional characteristics of gel behavior (Jobling,
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Westcott, Tayal, Jeffcoat, & Schwall, 2002; Muadklay & Charoenrein, 2008; Tao et al.,
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2018; Zhang, Bai, Pan, Li, Cheng, & Chen, 2018). Although there is no standard test
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for freeze-thaw stability, the method employed in present study is a widely applied and
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particularly stringent test (Stephen A. Jobling, 2002). In the test, the freeze-thew
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stability of the compound gels with or without CDs additions were evaluated by
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determining the percentage of syneresis following 1 to 5 cycles of freezing-thawing
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and centrifugation. The syneresis results are shown in Table 2.
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Generally, the freeze-thaw stability of gels was poor for vast of water contained
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in the network. In this study, the syneresis of the KC/KGM compound gel is 90.45%
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after the first freeze-thew cycle and 96.70% after all 5 freeze-thew cycles. The result
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indicates that the formation of ice crystals can easily destroy the network structure of
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the compound gels which lead to high syneresis during thawing (Pongsawatmanit,
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Temsiripong, Ikeda, & Nishinari, 2006). The freeze-thaw stability of the compound
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CDs, the percentages of syneresis were reduced about 10% after the first freeze-thaw
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cycle. The compound gel with M-β-CD has the best water retention ability. The
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effects of CDs on the freeze-thaw stability of the compound gel are in agreement with
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the results of texture analysis. After 5 freeze-thaw cycles, syneresis of KC/KGM
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compound gels containing CDs are slightly lower than the KC/KGM compound gel
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without adding CDs. CDs can effectively improve the freeze-thaw stability of the
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compound gel for the most foods are frozen and thawed only 1 - 2 times in general.
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3.4. Effect of CDs on microstructure of the KC/KGM compound gels
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SEM is proved to be a method for direct observation of microstructures. The
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effect of addition of CDs on the microstructures of compound gels was examined at
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CDs concentration of 1.5%, the results are shown in Fig 2 (a - f). The profile
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microstructure of the pure KC/KGM compound gel is showed in Fig. 2a. It can be
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seen that the gel structure is similar to honeycomb. The wall layer of the gel network
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is thin, rich folds and there are holes in the wall. This may be responsible for the low
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hardness and high syneresis of the gel.
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Fig. 2 (b - f) are the SEM images of compound gels with addition of CDs. It can
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be found that the microstructures of the CDs added gels are significantly different
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from that without CDs. The structures of all the CDs added gels are denser and have
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smaller voids, it is, the gels have more junction zones and shorter distance walls.
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Moreover, some particles are observed which adhering on the wall of the gel networks.
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CDs molecules are attached to the network walls and form intermolecular hydrogen
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The similar result has been reported in the effect of protein on properties of gelatin gel
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(Pang, Deeth, Sopade, Sharma, & Bansal, 2014). The walls of the compound gels are
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smoother after CDs added, and the attachment of CDs to the surface of the network
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makes the walls thicker and stronger. Comparatively, M-β-CD added gel is
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outstanding and shows the highest density and the toughest wall. Among three natural
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CDs, β-CD is the best. CDs plays network supporting and water-retaining roles in the
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KC/KGM compound gel.
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3.5. XRD analysis
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The X-ray diffraction of the compound gels with or without CDs additions are
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analyzed by a Bruker D8-Advance XRD instrument. The three natural CDs display
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typical crystalline peaks around 2θ = 19° (Fig 3a - c). A typical noncrystalline broad
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peak around 2θ = 19° is exhibited by modified CDs, M-β-CD and HP-β-CD, in Fig.
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3d and 3e. However, the crystalline peaks around 2θ = 19° disappear in the compound
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gels containing CDs, which indicates that CDs are scattered in the gels network and
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unable to approach and form complete crystals. The curve of the KC/KGM compound
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gel shows three sharp crystalline peaks at 28°, 40° and 50° (Fig. 3A), indicating the
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existence of crystalline peaks with long rang order. The generated peaks of the
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KC/KGM compound gels illustrate that the molecular microstructures transform into
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regulated arrangement after the gel formation through the aggregation between
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polysaccharide molecules and synergistic reaction between KC and KGM.
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However, these peaks of the compound gel at 28°, 40° and 50° (2θ) were
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ACCEPTED MANUSCRIPT weakened after CDs were added. As can be seen from Fig 3d′, the addition of
327
M-β-CD has the greatest influence on the crystalline of the compound gel. It has the
328
least free hydroxyl groups among tested CDs which causes the difficulty to form
329
hydrogen bonds with the polysaccharide molecules. It is favorable for KC/KGM
330
polysaccharide molecules to form uniform mixture in water and easily form network
331
structure with many junction zones and short distance wall. The formation of long
332
distance wall in network structure is prevented by convenient intermolecular approach
333
of polysaccharides. Increase of the number of junction zones and short average length
334
of the wall lead the decrease of crystallinity. The formation of dense network structure
335
develope a better texture characteristics and higher water-retaining ability for the
336
compound gels. The influence of CDs on crystallinity of the KC/KGM compound
337
gels follows the order: M-β-CD ＞ α-CD ＞ γ-CD ≈ HP-β-CD ＞ β-CD.
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In former researches, it has reported that branched limited dextrin and spring
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dextrin had significant effect on the retrogradation of starch gel (Xu et al., 2013; Xu et
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al., 2014). In starch gel system, the dextrins inhibited starch fraction order by
341
controlling the formation of hydrogen bonds and affected the kinetics or the extent of
342
starch retrogradation. As a bucket-like dextrin, CDs have a beneficial effect on the
343
physical properties of the KC/KGM compound gels. On the basis of previous theories
344
and research results, the effect is mainly due to the preferential exclusion of CDs from
345
the polysaccharides chains in the sol state, resulting in a more uniform distribution of
346
polysaccharides. During gelation, the presence of CDs among the polysaccharides
347
prevents the formation of long distance wall in network structure, and after the gel
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phase and formation of a dense network structure. The texture, freeze-thaw stability,
350
microstructure and crystallinity results of this study is agreement with the theory
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research reported by Stenner, Matubayasi and Shimizu (2016). CDs effectively
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improved the synergistic interaction between KC and KGM polysaccharides.
353
4. Conclusions
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This work demonstrated that CDs have significant impact on texture
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characteristics, freeze-thaw stability, microstructure and crystallinity of the KC/KGM
356
compound gel. In TPA test, the hardness strengthened with the increase of CDs
357
concentrations in the range of 0.5% to 1.5%. After that the hardness decreased. The
358
addition of CDs had little effect on the springiness of compound gels. Cohesiveness of
359
the gels were significantly weakened at high CDs concentrations. In addition
360
gumminess and chewness showed the similar changes as cohesiveness. The
361
compressive experiment showed that the addition of CDs increase the compressive
362
elastic modulus and enhance the resistance to deformation of the compound gels,
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M-β-CD is the most significant. The freeze-thaw stability results indicated that CDs
364
could reduce the synthesis of the compound gels, the influence follows the order:
365
M-β-CD ＞ α-CD ＞ γ-CD ＞ β-CD ＞ HP-β-CD. The SEM results showed that
366
the microstructures of the compound gels with adding CDs were significantly
367
different from that without CDs. The microstructures of compound gels became dense
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with small voids after adding CDs. In XRD test, CDs made the crystallization peaks
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of the compound gel become smaller, which indicated the decrease of crystallinity.
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ACCEPTED MANUSCRIPT Generally, the physical properties of the compound gels containing CDs are better
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than those without CDs. In the sol state, the presence of CDs resulted a more uniform
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distribution of the polysaccharides. During gelation, the exclusion of CDs from the
373
polysaccharide surfaces could weaken polysaccharide-water interactions and cause
374
the polysaccharide strands to aggregate with neighbouring strands,. In the compound
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gel, CDs bound to the polysaccharide molecules through hydrogen bonds, and
376
adhered to the surface of the network to make the gel network dense. The influences
377
of M-β-CD on the physical properties of the KC/KGM compound gel were the most
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significant among the CDs for it had the least free hydroxyl groups.
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This study demonstrated that CDs improved the texture and stability of the
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KC/KGM compound gel, the conclusion may contribute to the development of health
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gel foods considering CDs can carry lipophilic functional components into the system.
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Acknowledgments
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This work was supported by the National Natural Science Foundation of China
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(Grant no. 31571881), Taishan Scholars Program of Shandong Province, China and
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Ten Thousand Talent Program of China.
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Conflict of interest statement
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We declare that we have no financial and personal relationships with other
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people or organizations that can inappropriately influence our work, there is no
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professional or other personal interest of any nature or kind in any product, service
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and/or company that could be construed as influencing the position presented in, or
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the review of, the manuscript entitled.
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Tian, Y., Xu, X., Li, Y., Jin, Z., Chen, H., & Wang, H. (2009). Effect of β-cyclodextrin on the
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Tom Brennera, Z. W. P. A. (2013). Rheology and synergy of κ-carrageenan/locust bean gum/konjac glucomannan gels. Carbohydrate Polymers, 98(1), 754-760.
Wei, Y., Wang, Y. L., & He, X. J. (2011a). Gel Properties of k-Carrageenan-Konjac Gum Mixed Gel and their Influence Factors. Advanced Materials Research, 396-398, 1389-1393. Wei, Y., Wang, Y. L., & He, X. J. (2011b). Gel Properties of k-Carrageenan-Konjac Gum Mixed Gel and their Influence Factors. Advanced Materials Research, 396-398, 1389-1393.
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Xu, J., Fan, X., Ning, Y., Wang, P., Jin, Z., Lv, H., Xu, B., & Xu, X. (2013). Effect of spring dextrin on retrogradation of wheat and corn starch gels. Food Hydrocolloids, 33(2), 361-367. Xu, J., Wang, Q., Bashari, M., Chen, F., Wang, P., Cui, L., Yuan, J., Xu, X., & Fan, X. (2014).
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Branched limit dextrin impact on wheat and waxy starch gels retrogradation. Food
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Hydrocolloids, 39(39), 136-143.
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Yuan, C., Du, L., Zhang, G., Jin, Z., & Liu, H. (2016). Influence of cyclodextrins on texture behavior and freeze-thaw stability of kappa-carrageenan gel. Food Chemistry, 210, 600-605.
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Zhang, B., Bai, B., Pan, Y., Li, X., Cheng, J., & Chen, H. (2018). Effects of pectin with different
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molecular weight on gelatinization behavior, textural properties, retrogradation and in vitro
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digestibility of corn starch. Food Chemistry, 264, 58-63.
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Table 1 Compression elastic modulous (S) of the KC/KGM compound gels with or
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without CD additions. Compression modulous (kPa)
KC/KGM
6.62±0.036c
α-CD/KC/KGM
7.60±0.12b
β-CD/KC/KGM
8.57±0.44a
γ-CD/KC/KGM
7.47±0.04b
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Sample (1.5%)
9.08±0.50a
M-β-CD/KC/KGM
7.45±0.10b
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Mean ± SD values in each column with different superscript letter are significantly
494
different (p ≤ 0.05).
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Table 2 Syneresis (%) of the KC/KGM compound gels with or without CDs additions
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in each cycle.
Sample
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Syneresis (%) Cycle 2
Cycle 3
Cycle 4
Cycle 5
κ-CA/KGM
90.46 ± 0.12a
94.37 ± 0.28a
94.99 ± 0.12a
95.24 ±0.21a
96.67 ± 0.40a
α-CD/KC/KGM
78.55 ± 1.90bc
86.38 ± 0.94c
90.28 ± 0.46bc
91.21 ± 0.34b
93.05 ± 0.06c
β-CD/KC/KGM
85.30 ± 3.73b
92.88 ± 1.34a
94.13 ± 1.08a
94.55 ± 1.01a
95.02 ± 1.23b
γ-CD/KC/KGM
78.88 ± 1.52c
87.70 ± 0.11bc
89.61 ± 0.49c
90.54 ± 0.82b
90.93 ± 0.57d
M-β-CD/KC/KGM
77.20 ± 0.40c
89.33 ± 2.08b
90.85 ± 0.61bc
91.88 ± 0.21b
93.31 ± 0.13c
HP-β-CD/KC/KGM
88.17 ± 0.03ab
90.18 ± 0.38b
91.17 ± 0.44b
91.79 ± 0.37b
93.05 ± 0.064c
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Mean ± SD values in each column with different superscript letter are significantly different (p ≤
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Fig. 1. Influence of CDs on texture parameters of the KC/KGM compound gels (a:
503
hardness; b: springiness; c: cohesiveness; d: gumminess; e: chewiness).
504
Fig. 2. SEM images of KC/KGM compound gel and the compound gels containing
505
CDs (a: KGM/KC, b: KGM/KC/α-CD, c: KGM/KC/β-CD, d: KGM/KC/γ-CD, e:
506
KGM/KC/M-β-CD, f: KGM/KC/HP-β-CD). Magnification of the images are all 500.
507
Fig. 3. XRD curves of CDs, KC/KGM compound gel and the compound gels
508
containing CDs (a: α-CD, b: β-CD, c: γ-CD, d: M-β-CD, e: HP-β-CD, A: KC/KGM, a':
509
KC/KGM/α-CD, b': KC/KGM/β-CD, c': KC/KGM/γ-CD, d': KC/KGM/M-β-CD, e':
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KC/KGM/HP-β-CD).
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Fig. 1. Influence of CDs on texture parameters of the KC/KGM compound gels (a:
514
hardness; b: springiness; c: cohesiveness; d: gumminess; e: chewiness).
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Fig. 2. SEM images of KC/KGM compound gel and the compound gels containing
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CDs (a: KGM/KC, b: KGM/KC/α-CD, c: KGM/KC/β-CD, d: KGM/KC/γ-CD, e:
519
KGM/KC/M-β-CD, f: KGM/KC/HP-β-CD). Magnification of the images are all 500.
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Fig. 3. XRD curves of CDs, KC/KGM compound gel and the compound gels
523
containing CDs (a: α-CD, b: β-CD, c: γ-CD, d: M-β-CD, e: HP-β-CD, A: KC/KGM, a':
524
KC/KGM/α-CD, b': KC/KGM/β-CD, c': KC/KGM/γ-CD, d': KC/KGM/M-β-CD, e':
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KC/KGM/HP-β-CD).
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Highlights CDs remarkably enhanced the texture and freeze-thaw stability of the compound gel.
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SEM results indicated that CDs lead the compound gel to form a denser gel network.
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XRD results showed that CDs could reduce the crystallinity of gel network.
S0268-005X(18)30864-6
DOI:
10.1016/j.foodhyd.2018.07.037
Reference:
FOOHYD 4566
To appear in:
Food Hydrocolloids
Received Date: 10 May 2018 Revised Date:
20 July 2018
Accepted Date: 22 July 2018
Please cite this article as: Yuan, C., Xu, D., Cui, B., Wang, Y., Gelation of κ-carrageenan/Konjac glucommanan compound gel: Effect of cyclodextrins, Food Hydrocolloids (2018), doi: 10.1016/ j.foodhyd.2018.07.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
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κ-Carrageenan
Heating
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Stirring
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KCl solution ( )
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Konjac glucomannan
SEM
CDs ( )
SEM
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Gelation of κ-Carrageenan/Konjac Glucommanan Compound Gel: Effect of
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Cyclodextrins
3
Chao Yuana, b, *, Dongyan Xua, b, Bo Cuia, b, **, Yanli Wanga, b
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a
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of Technology, Shandong Academy of Sciences, Jinan, 250353, China
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b
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Academy of Sciences, Jinan 250353, China
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State Key Laboratory of Biobased Material and Green Papermaking, Qilu University
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School of Food Science and Engineering, Qilu University of Technology, Shandong
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* Corresponding author: [email protected] (C. Yuan).
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** Corresponding author: [email protected] (B. Cui).
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ABSTRACT The impact of cyclodextrins (CDs) on physical properties of the compound gel of
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κ-carrageenan/konjac glucommanan (KC/KGM) was investigated by texture profile
17
analysis (TPA), compression modulus test, freeze-thaw stability test, scanning
18
electron microscopy (SEM) and X-ray diffraction (XRD). TPA results showed that
19
CDs had remarkable influence on mechanical properties of the compound gel. In
20
concentration range of 0.5% - 1.5% (w/w), hardness was strengthened with the
21
increase of CDs concentrations, then gradually decreased at higher CDs
22
concentrations. Cohesiveness, gumminess and chewiness showed similar tendency as
23
the hardness. However, influence of CDs on gel springiness was slight. Compressive
24
experiment indicated that the CDs increased the compressive elastic modulus (S) of
25
the compound gel and improved the deformation resistance of the gel network. In the
26
freeze-thaw stability test, stability of the compound gels was obviously improved after
27
adding CDs, about 10% more water was preserved after the first freeze-thaw cycle.
28
SEM results showed that the microstructures of the compound gels containing CDs
29
were much denser with smaller voids. XRD results indicated that the crystallization
30
peaks of the compound gels decreased after CDs were added. The exclusion of CDs
31
from the polysaccharides chains in the sol state led the KC/KGM compound solution
32
to form a denser network after cooling and benefited the texture and freeze-thaw
33
stability of the formed gel. CDs improved the synergistic interaction between KC and
34
KGM polysaccharides. The influence of methyl-β-cyclodextrin (M-β-CD) on the
35
formation and characteristics of KC/KGM compound gel was the most significant
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among all the selected CDs.
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Keywords: Konjac glucommanan; κ-Carrageenan; Gelation; Cyclodextrins; Physical
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properties
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1. Introduction There is considerable interest which has been focused on the role of food
42
texturants in the development of food industry. Texturants are important food additive,
43
since they act as a gelling, thickening and water retention agent, and they are also
44
used as a lubricant (Kenwright et al., 2014; Rao & Kenny, 1975). In recent years, the
45
food texturants have gradually grown as the hot spot of food colloid research, which
46
is also a vital trend of hydrocolloids production and development in the future
47
(Harding, Smith, Lawson, Gahler, & Wood, 2011; Tom Brennera, 2013). Among them,
48
carrageenan and konjac glucomannan (KGM) are two polysaccharides which were the
49
most widely used in the food industry. Carrageenan is a sulphated linear
50
polysaccharide of D-galactose and 3, 6-anhydro-D-galactose join by α-1,3 and
51
β-1,4-glycosidic linkage, which is obtained by extraction of certain red seaweeds of
52
the Rhodophyceae class (J. Necas, 2013; Li, Ni, Shao, & Mao, 2014). It is classified
53
into various types such as λ, κ, ι, ε and µ, mainly according to the quantities and
54
position of sulphate esters. One of the most widely applied member of the
55
carrageenan family is κ-carrageenan (KC), it is employed as a gelling agent, as well as
56
a thickener and stabiliser. Nevertheless, KC has several defects, such as brittleness,
57
low flexibility and dissolving out water seriously while KC form gel alone (J. Necas,
58
2013; Loret, Ribelles, & Lundin, 2009a; Wei, Wang, & He, 2011a). KGM is a neutral
59
heteropolysaccharide, with the main chain composed of β-(1,4)-glycosidic bond
60
linked D-glucose and D-mannose residues in a molar ratio of 1:1.6 (Jin et al., 2015;
61
Joanna Kruka, 2017). Under alkaline conditions, KGM will deacetylate, aggregate in
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63
Nondeacetylated KGM but at concentrations of 7% and higher can also form gel
64
(Kohyama, Iida, & Nishinari, 1993; Nishinari, Williams, & Phillips, 1992; Penroj,
65
Mitchell, Hill, & Ganjanagunchorn, 2005). KGM shows a synergistic interaction with
66
KC under the condition of non-deacetylation, inducing the formation of a promising
67
compound hydrocolloid gel. The KC/KGM compound gel formed in certain
68
conditions is less brittleness, good flexibility and water retention ability for the
69
synergistic effect between the two polysaccharides.
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Cyclodextrins (CDs) are cyclic oligosaccharides consisting of (α-1,4)-linked six
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(α-CD), seven (β-CD), eight (γ-CD) or more α-D-glucopyranose units with a
72
hydrophobic internal cavity and a hydrophilic exterior (Del Valle, 2004; Yuan, Du,
73
Zhang, Jin, & Liu, 2016). By random substitution of the hydroxy groups, even by
74
lipophilic groups, gives amorphous mixtures of high water-soluble CD derivatives,
75
including hydroxypropyl-β-cyclodextrin (HP-β-CD) and methyl-β-cyclodextrin
76
(M-β-CD), which have wide application in food industry, (Inoue, 1998; Szente &
77
Szejtli, 2004). Due to the particular characteristic of inner hydrophobic and outer
78
hydrophilic, CDs and their derivatives can form water-soluble inclusion complexes
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with liposoluble components. The inclusion complexes have many beneficial
80
functions, including water solubility, higher bioavailability and stability. The inclusion
81
complexes using CDs as the wall material have many influences on the food system,
82
such as texture, rheological properties and freeze-thaw stability (Inoue, Takahashi,
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Okada, Iwasaki, Murata, & Kanamoto, 2013; Loftsson & Brewster, 2010; Tian, Xu, Li,
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Jin, Chen, & Wang, 2009). It has long been known that gelation of polysaccharide hydrocolloids is strongly
86
affected by cosolvents, such cosolvents, sugars and polyols. Therefore the effect has
87
been investigated widely to enhance polysaccharide gelation and gel stability (Loret,
88
Ribelles, & Lundin, 2009b; Oakenfull, 2000). Different from common single or oligo
89
sugars, the particular circular truncated cone structure of CDs may generate special
90
effect on the gelation of polysaccharide hydrocolloids. In recent years, researchers did
91
find the effects of CDs on different food hydrocolloids. Tian et al. (Tian et al., 2009)
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reported that CDs could effectively affect the orderly rearrangement of starch, thereby
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inhibiting the retrogradation of starch. In our previous study, it was found that CDs
94
had obvious effects on the texture properties and storage stability of KC (Yuan et al.,
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2016). Furthermore, the complex forming ability make CDs widely applied in gelled
96
food. Nevertheless, to our knowledge, the influence of CDs on the KC/KGM
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compound gel was not clear and has not been studied before, even they often contain
98
in application systems simultaneously, particularly in gelled foods. Although it was
99
known that synergistic interaction happens between KC and KGM alone.
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In the present study, we explored the effects of 5 common CDs and CD
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derivatives on gelation and crystallization of KC/KGM compound gel through TPA,
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compression elastic modulus analysis, syneresis analysis, SEM and XRD methods.
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2. Materials and methods
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2.1. Materials
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α-, β-, γ-CD (purity > 98%), and KC, KGM power (food-grade) were purchased
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(MW: 1310) were purchased from Sigma-Aldrich (Shanghai, China). KCl was
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purchased from Damao Chemical Reagent Company (Tianjin, China).
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2.2. Methods
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2.2.1. The KC/KGM compound gels preparation
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The total polysaccharide (gelling powder) content was 0.15 wt%. KC and KGM
112
were blended and grinded at 5.5 : 4.5 (Wei, Wang, & He, 2011b) equably with a
113
mortar, then screened by a 100 mesh sieve. A 0.15 wt% stock solution was prepared
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by directly dissolving the KC/KGM mixed-powder in potassium chloride solution
115
(0.2%, w/w), which were stirred under 40 °C for 20 min. After that the hot solution
116
was poured into 6 beakers (50 mL), 0%, 0.5%, 1.0%, 1.5%, 2.0% and 2.5% (w/w)
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CDs were added in under stirring, respectively. Keep on stirring 30 min at 80 °C,
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followed by standing 5 min to allow escape of air bubbles. The mixed solutions were
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put in a 25 °C incubator to lead formation of gels and keeping 6 h before test.
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2.2.2. Texture analysis
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Texture parameters were determined by a Texture Analyzer TA-XTplus/30
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(Stable Micro Systems, Surrey, UK). Samples were penetrated with a 12 mm diameter
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cylinder probe. In all the experiments, the samples were tested at a speed of 5 mm/s
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with control force of 5 g and compression depth of 50% (Ju & Kilara, 1998).
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2.2.3. Compression measure
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Compression properties of the compound gel systems were also performed with
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the Texture Analyzer TA-XTplus/30 but equipped with a parallel plate (36 mm
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were cut into cylinders (Ø36 mm × 10 mm). The samples were placed on the
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platform of the texture analyzer and compressed by the 36 mm parallel plate at a
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speed of 0.1 mm/s to a depth of 5 mm and load cell of 5N sensibility were set in the
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uniaxial compression experiments, the relationship between stress and stain was
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recorded. Under low strain range, the uniaxial compression results can be fitted by
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Neo-Hookean model, therefore the compressive elastic modulus (S) was obtained by
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the slope of the fitted curve.
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2.2.4. Freeze-thaw stability analysis
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15 mg/L mixture solution of KC/KGM (5.5 : 4.5) was heated at 40 °C for 20 min
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under stirring. After that the solution was weighted and transferred to 6 beakers (50
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mL), 1.5% (w/w) different types of CDs, namely, α-, β-, γ-CD, M-β-CD and HP-β-CD
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were added under stirring, separately. The one free with CDs was for control. The
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solutions were then heated at 80 °C for 30 min, each sample was evenly transferred
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into 2 centrifuge tubes. The tubes were cooled at 25 °C for 6 h to promote formation
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of gels. The gels were frozen at -18 °C for 24 h, then thawed at 25 °C for 6 h as a
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freeze-thaw cycle, the whole experiment repeatedly up to 5 freeze-thaw cycles. After
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each thawing cycles, samples were centrifuged at 1700g for 15 min, then removed the
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separated water and weighed. Divided the weight of the water decanted by the total
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weight of the gel before centrifugation, the percentage of syneresis was obtained.
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2.2.5. Scanning electron microscopy (SEM) observation
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The microstructures of the compound gels were investigated by SEM. The
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cubes about 5 mm, placed in an ultra-low temperature refrigerator at -80 °C for 2 h,
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and then freeze dried to obtain the SEM samples. Before observation, the gels were
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coated with gold-palladium under argon atmosphere using a gold sputter module in a
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high vacuum evaporator. The coating time was about 30 s at an acceleration voltage
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of 20 kV. Images with magnification of 500 were recorded.
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2.2.6. X-ray diffraction (XRD) analysis
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The compound gels were characterized by a Bruker D8-Advance XRD
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instrument (Bruker AXS Inc., Germany). The parameters were performed in reflection
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mode at 40 kV, 40 mA with graphite-filtered Cu-Kα radiation. Measurement was
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taken between 5 - 60° (2θ) with a step size of 0.03° (Liu, Wang, Kang, Cui, & Yu,
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2018).
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2.2.7. Statistics
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TPA, compression test and freeze-thaw stability test were replicated in triplicate
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and data were reported as mean ± standard deviation. Statistical significance was
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assessed by Duncan’s new multiple range test with the level of significance set at p
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≤ 0.05.
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3. Results and discussion
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3.1. Effect of CDs on texture of the KC/KGM compound gels
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One of the most common instrumental imitative measurements of texture is
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texture profile analysis (TPA) (Wilkinsony, And, & Minekusy, 2000). The main
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mechanical parameters of TPA include hardness, adhesiveness, springiness, 9
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influence of CDs on the KC/KGM compound gel texture behavior were investigated.
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Hardness, springiness, gumminess, cohesiveness and chewiness were selected as
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study parameters according to the properties of gel system.
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Hardness refers to the force required for compressing material between dentes
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(in solids state) or between tongue and palate (in semisolid state) (Yuan et al., 2016).
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Figure 1a shows the influence of CDs on hardness of the KC/KGM compound gel. In
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the low CDs concentrations range (0.5% - 1.5%, w/w), the hardness added gradually
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as the CDs concentration increased. Then hardness shows a downward trend after the
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concentrations of 2.0%. It was reported that the exclusion of sugars during gel
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formation was one of the driving forces for better gel. Under low concentrations, the
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exclusion of CDs from polysaccharide surfaces in the sol state. The exclusion of the
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CDs reduced water population which weakened polysaccharide-water interactions and
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inducing polysaccharide strands to be close to each other, then getting a denser
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network structure with higher gel strength (Stenner, Matubayasi, & Shimizu, 2016).
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However, excessive CDs could cause difficulty in clustering of polysaccharide strands
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and hinder the formation of the KC/KGM compound network, thus, the hardness
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dropped along with the further increase of CDs concentrations. For the three natural
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CDs, the influence on gel hardness follows the order of β-CD > α-CD > γ-CD. Among
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them, β-CD molecule itself can form intermolecular hydrogen bonds which make it
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has the least of free hydroxyls. This could contribute to the uniform distribution of
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polysaccharide molecules. The size of α-CD is the smallest, the smaller molecular size
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of gel added with α-CD is relatively large. γ-CD has the largest size and the largest
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number of free hydroxyl groups, so it is easy to bind to polysaccharide molecules,
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which hinders the migration of polysaccharide molecules and is not conducive to the
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uniform distribution of polysaccharide molecules. For two modified CDs, HP-β-CD is
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hydroxyl substituted by hydroxypropyl groups on the basis of β-CD. The molecule is
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more extensible and the intermolecular hydrogen bonds is opened after substitution. It
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is easier to form hydrogen bond with KC/KGM polysaccharide molecules, and the
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larger molecule size prevents the polysaccharide molecules from approach, thus its
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gel hardness is the lowest. Compared with other CDs, M-β-CD showed the greatest
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influence on the hardness of the gels. M-β-CD is a derivative of β-CD which hydroxyl
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groups are partly replaced by methyl groups. The least free hydroxyl groups leading it
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the most difficult to form hydrogen bonds with the polysaccharide molecules.
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Therefore, it is more conducive to the formation of uniform gel system and gain the
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highest gel hardness.
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Springiness is the ability that material can be stretched and returns to its original
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length. The springiness curves of the KC/KGM compound gels with adding different
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concentrations of CDs are displayed in Fig. 1b. From the diagram it can be drawn that
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the addition of CDs have little effect on the springiness of the KC/KGM compound
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gel. The initial springiness of the compound gel is low for the gel is brittle. Compared
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with the two polysaccharide molecules, the size of CDs are far smaller and easier to
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dissolve in the water in the gel network, consequently, they play a slight role in the
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springiness of formed compound gels with a water content of about 98%. Cohesiveness is defined as the degree of compression between the teeth before
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the food breaks. Fig. 1c shows that the impacts of the natural and modified CDs on
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the cohesiveness of the KC/KGM compound gels were similar. The skeleton structure
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can affect the break of the compound gels, the crosslinking of the KC/KGM
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polysaccharide strands affects the strength of the skeleton. CDs weaken the
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cohesiveness of the gels because of CDs molecules hinders approach of the double
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helix which blocks the extension of the network wall. However, the cohesiveness of
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gel containing β-CD increases at high concentration, for the solubility of β-CD is low
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(< 1.8%), at higher concentration, β-CD precipitates easily, resulting in adhesion of
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β-CD to the network surface, which increased the toughness of the compound gel.
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Gumminess is a secondary TPA parameter, which is obtained by multiplying
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hardness by cohesiveness. The results of gumminess are shown in Fig. 1d. The effect
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tendency of adding CDs on the gumminess of the compound gels are consistent with
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the influence on cohesiveness. The main reason is also attributed to the existence of
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CDs molecules hinders the approach of the KC/KGM polysaccharide strands and
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blocks the extension of the network wall.
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Chewiness is the mouth feel sensation of labored chewing due to sustained,
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elastic resistance from the food, which is also a secondary TPA parameter. Chewiness
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is calculated through multiplying gumminess by springiness. Figure 1e shows the
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effects of different CDs on chewiness of the compound gels. The chewiness shows a
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decreasing trend with the increase of CDs concentration. Among all CDs, γ-CD has
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the least effect on the chewiness of the compound gels.
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3.2. Compressive test analysis In mechanics, the mechanical properties of a material are usually expressed in
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terms of compressive elastic modulus (S). Compressive elastic modulus refers to the
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ratio of the vertical compressive stress to the total vertical strain of the sample. The
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higher the compressive modulus, the stronger the resistance of the material to
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deformation. Compressive elastic modulus of the pure KC/KGM compound gel and
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the KC/KGM compound gels containing CDs are shown in Table 1.
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Compressive modulus of the compound gel containing M-β-CD is the highest,
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reaching 9.08 kPa, followed by β-CD, α-CD, γ-CD, HP-β-CD, and finally the pure
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KC/KGM compound gel. The results show that the addition of CDs increases the
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anti-compression ability of the KC/KGM compound gel. The reason behind the
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change of compressive elastic modulus can be explained by that CDs enhanced the
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hydrophilic gel network. Similar results have been reported by Evingür and Peckan
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(2013; 2015). They studied the effect of KC on the mechanical properties of
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polyacrylamide-KC composite by the compressive elastic measure method. The
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results show that the composite’s compressive elastic modulus is found to be strongly
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dependent on the KC content and temperature, and it is proved that the addition of KC
256
in low content can make the PAAm-KC composite form a superelastic gel network
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from the mechanical point of view. In present study, the addition of CDs makes the
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KC/KGM compound gel network denser. It is because that the addition of CDs
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promotes the helical aggregation of polysaccharides to form a short-distance gel
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structure can be intuitively represented by the increase of compression elastic
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modulus. Among tested CDs, M-β-CD has the best effect, which is consistent with the
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results of texture analysis.
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3.3. Effect of CDs on Freeze-thaw stability of the KC/KGM compound gels
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Freeze-thaw stability test plays a vital role in frozen food industry. In cold chain
266
storage, the synergistic effect of thermal fluctuation and water phase change is the
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main factor of refrigerated food deterioration, especially gelled foods, which may
268
affect the texture, viscosity or the functional characteristics of gel behavior (Jobling,
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Westcott, Tayal, Jeffcoat, & Schwall, 2002; Muadklay & Charoenrein, 2008; Tao et al.,
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2018; Zhang, Bai, Pan, Li, Cheng, & Chen, 2018). Although there is no standard test
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for freeze-thaw stability, the method employed in present study is a widely applied and
272
particularly stringent test (Stephen A. Jobling, 2002). In the test, the freeze-thew
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stability of the compound gels with or without CDs additions were evaluated by
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determining the percentage of syneresis following 1 to 5 cycles of freezing-thawing
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and centrifugation. The syneresis results are shown in Table 2.
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Generally, the freeze-thaw stability of gels was poor for vast of water contained
277
in the network. In this study, the syneresis of the KC/KGM compound gel is 90.45%
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after the first freeze-thew cycle and 96.70% after all 5 freeze-thew cycles. The result
279
indicates that the formation of ice crystals can easily destroy the network structure of
280
the compound gels which lead to high syneresis during thawing (Pongsawatmanit,
281
Temsiripong, Ikeda, & Nishinari, 2006). The freeze-thaw stability of the compound
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283
CDs, the percentages of syneresis were reduced about 10% after the first freeze-thaw
284
cycle. The compound gel with M-β-CD has the best water retention ability. The
285
effects of CDs on the freeze-thaw stability of the compound gel are in agreement with
286
the results of texture analysis. After 5 freeze-thaw cycles, syneresis of KC/KGM
287
compound gels containing CDs are slightly lower than the KC/KGM compound gel
288
without adding CDs. CDs can effectively improve the freeze-thaw stability of the
289
compound gel for the most foods are frozen and thawed only 1 - 2 times in general.
290
3.4. Effect of CDs on microstructure of the KC/KGM compound gels
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SEM is proved to be a method for direct observation of microstructures. The
292
effect of addition of CDs on the microstructures of compound gels was examined at
293
CDs concentration of 1.5%, the results are shown in Fig 2 (a - f). The profile
294
microstructure of the pure KC/KGM compound gel is showed in Fig. 2a. It can be
295
seen that the gel structure is similar to honeycomb. The wall layer of the gel network
296
is thin, rich folds and there are holes in the wall. This may be responsible for the low
297
hardness and high syneresis of the gel.
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Fig. 2 (b - f) are the SEM images of compound gels with addition of CDs. It can
299
be found that the microstructures of the CDs added gels are significantly different
300
from that without CDs. The structures of all the CDs added gels are denser and have
301
smaller voids, it is, the gels have more junction zones and shorter distance walls.
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Moreover, some particles are observed which adhering on the wall of the gel networks.
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CDs molecules are attached to the network walls and form intermolecular hydrogen
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The similar result has been reported in the effect of protein on properties of gelatin gel
306
(Pang, Deeth, Sopade, Sharma, & Bansal, 2014). The walls of the compound gels are
307
smoother after CDs added, and the attachment of CDs to the surface of the network
308
makes the walls thicker and stronger. Comparatively, M-β-CD added gel is
309
outstanding and shows the highest density and the toughest wall. Among three natural
310
CDs, β-CD is the best. CDs plays network supporting and water-retaining roles in the
311
KC/KGM compound gel.
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3.5. XRD analysis
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The X-ray diffraction of the compound gels with or without CDs additions are
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analyzed by a Bruker D8-Advance XRD instrument. The three natural CDs display
315
typical crystalline peaks around 2θ = 19° (Fig 3a - c). A typical noncrystalline broad
316
peak around 2θ = 19° is exhibited by modified CDs, M-β-CD and HP-β-CD, in Fig.
317
3d and 3e. However, the crystalline peaks around 2θ = 19° disappear in the compound
318
gels containing CDs, which indicates that CDs are scattered in the gels network and
319
unable to approach and form complete crystals. The curve of the KC/KGM compound
320
gel shows three sharp crystalline peaks at 28°, 40° and 50° (Fig. 3A), indicating the
321
existence of crystalline peaks with long rang order. The generated peaks of the
322
KC/KGM compound gels illustrate that the molecular microstructures transform into
323
regulated arrangement after the gel formation through the aggregation between
324
polysaccharide molecules and synergistic reaction between KC and KGM.
325
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However, these peaks of the compound gel at 28°, 40° and 50° (2θ) were
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ACCEPTED MANUSCRIPT weakened after CDs were added. As can be seen from Fig 3d′, the addition of
327
M-β-CD has the greatest influence on the crystalline of the compound gel. It has the
328
least free hydroxyl groups among tested CDs which causes the difficulty to form
329
hydrogen bonds with the polysaccharide molecules. It is favorable for KC/KGM
330
polysaccharide molecules to form uniform mixture in water and easily form network
331
structure with many junction zones and short distance wall. The formation of long
332
distance wall in network structure is prevented by convenient intermolecular approach
333
of polysaccharides. Increase of the number of junction zones and short average length
334
of the wall lead the decrease of crystallinity. The formation of dense network structure
335
develope a better texture characteristics and higher water-retaining ability for the
336
compound gels. The influence of CDs on crystallinity of the KC/KGM compound
337
gels follows the order: M-β-CD ＞ α-CD ＞ γ-CD ≈ HP-β-CD ＞ β-CD.
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In former researches, it has reported that branched limited dextrin and spring
339
dextrin had significant effect on the retrogradation of starch gel (Xu et al., 2013; Xu et
340
al., 2014). In starch gel system, the dextrins inhibited starch fraction order by
341
controlling the formation of hydrogen bonds and affected the kinetics or the extent of
342
starch retrogradation. As a bucket-like dextrin, CDs have a beneficial effect on the
343
physical properties of the KC/KGM compound gels. On the basis of previous theories
344
and research results, the effect is mainly due to the preferential exclusion of CDs from
345
the polysaccharides chains in the sol state, resulting in a more uniform distribution of
346
polysaccharides. During gelation, the presence of CDs among the polysaccharides
347
prevents the formation of long distance wall in network structure, and after the gel
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349
phase and formation of a dense network structure. The texture, freeze-thaw stability,
350
microstructure and crystallinity results of this study is agreement with the theory
351
research reported by Stenner, Matubayasi and Shimizu (2016). CDs effectively
352
improved the synergistic interaction between KC and KGM polysaccharides.
353
4. Conclusions
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This work demonstrated that CDs have significant impact on texture
355
characteristics, freeze-thaw stability, microstructure and crystallinity of the KC/KGM
356
compound gel. In TPA test, the hardness strengthened with the increase of CDs
357
concentrations in the range of 0.5% to 1.5%. After that the hardness decreased. The
358
addition of CDs had little effect on the springiness of compound gels. Cohesiveness of
359
the gels were significantly weakened at high CDs concentrations. In addition
360
gumminess and chewness showed the similar changes as cohesiveness. The
361
compressive experiment showed that the addition of CDs increase the compressive
362
elastic modulus and enhance the resistance to deformation of the compound gels,
363
M-β-CD is the most significant. The freeze-thaw stability results indicated that CDs
364
could reduce the synthesis of the compound gels, the influence follows the order:
365
M-β-CD ＞ α-CD ＞ γ-CD ＞ β-CD ＞ HP-β-CD. The SEM results showed that
366
the microstructures of the compound gels with adding CDs were significantly
367
different from that without CDs. The microstructures of compound gels became dense
368
with small voids after adding CDs. In XRD test, CDs made the crystallization peaks
369
of the compound gel become smaller, which indicated the decrease of crystallinity.
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ACCEPTED MANUSCRIPT Generally, the physical properties of the compound gels containing CDs are better
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than those without CDs. In the sol state, the presence of CDs resulted a more uniform
372
distribution of the polysaccharides. During gelation, the exclusion of CDs from the
373
polysaccharide surfaces could weaken polysaccharide-water interactions and cause
374
the polysaccharide strands to aggregate with neighbouring strands,. In the compound
375
gel, CDs bound to the polysaccharide molecules through hydrogen bonds, and
376
adhered to the surface of the network to make the gel network dense. The influences
377
of M-β-CD on the physical properties of the KC/KGM compound gel were the most
378
significant among the CDs for it had the least free hydroxyl groups.
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This study demonstrated that CDs improved the texture and stability of the
380
KC/KGM compound gel, the conclusion may contribute to the development of health
381
gel foods considering CDs can carry lipophilic functional components into the system.
382
Acknowledgments
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This work was supported by the National Natural Science Foundation of China
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(Grant no. 31571881), Taishan Scholars Program of Shandong Province, China and
385
Ten Thousand Talent Program of China.
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Conflict of interest statement
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We declare that we have no financial and personal relationships with other
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people or organizations that can inappropriately influence our work, there is no
389
professional or other personal interest of any nature or kind in any product, service
390
and/or company that could be construed as influencing the position presented in, or
391
the review of, the manuscript entitled.
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Table 1 Compression elastic modulous (S) of the KC/KGM compound gels with or
492
without CD additions. Compression modulous (kPa)
KC/KGM
6.62±0.036c
α-CD/KC/KGM
7.60±0.12b
β-CD/KC/KGM
8.57±0.44a
γ-CD/KC/KGM
7.47±0.04b
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Sample (1.5%)
9.08±0.50a
M-β-CD/KC/KGM
7.45±0.10b
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Mean ± SD values in each column with different superscript letter are significantly
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different (p ≤ 0.05).
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Table 2 Syneresis (%) of the KC/KGM compound gels with or without CDs additions
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in each cycle.
Sample
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Syneresis (%) Cycle 2
Cycle 3
Cycle 4
Cycle 5
κ-CA/KGM
90.46 ± 0.12a
94.37 ± 0.28a
94.99 ± 0.12a
95.24 ±0.21a
96.67 ± 0.40a
α-CD/KC/KGM
78.55 ± 1.90bc
86.38 ± 0.94c
90.28 ± 0.46bc
91.21 ± 0.34b
93.05 ± 0.06c
β-CD/KC/KGM
85.30 ± 3.73b
92.88 ± 1.34a
94.13 ± 1.08a
94.55 ± 1.01a
95.02 ± 1.23b
γ-CD/KC/KGM
78.88 ± 1.52c
87.70 ± 0.11bc
89.61 ± 0.49c
90.54 ± 0.82b
90.93 ± 0.57d
M-β-CD/KC/KGM
77.20 ± 0.40c
89.33 ± 2.08b
90.85 ± 0.61bc
91.88 ± 0.21b
93.31 ± 0.13c
HP-β-CD/KC/KGM
88.17 ± 0.03ab
90.18 ± 0.38b
91.17 ± 0.44b
91.79 ± 0.37b
93.05 ± 0.064c
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Mean ± SD values in each column with different superscript letter are significantly different (p ≤
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Fig. 1. Influence of CDs on texture parameters of the KC/KGM compound gels (a:
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hardness; b: springiness; c: cohesiveness; d: gumminess; e: chewiness).
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Fig. 2. SEM images of KC/KGM compound gel and the compound gels containing
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CDs (a: KGM/KC, b: KGM/KC/α-CD, c: KGM/KC/β-CD, d: KGM/KC/γ-CD, e:
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KGM/KC/M-β-CD, f: KGM/KC/HP-β-CD). Magnification of the images are all 500.
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Fig. 3. XRD curves of CDs, KC/KGM compound gel and the compound gels
508
containing CDs (a: α-CD, b: β-CD, c: γ-CD, d: M-β-CD, e: HP-β-CD, A: KC/KGM, a':
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KC/KGM/α-CD, b': KC/KGM/β-CD, c': KC/KGM/γ-CD, d': KC/KGM/M-β-CD, e':
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KC/KGM/HP-β-CD).
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Fig. 1. Influence of CDs on texture parameters of the KC/KGM compound gels (a:
514
hardness; b: springiness; c: cohesiveness; d: gumminess; e: chewiness).
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Fig. 2. SEM images of KC/KGM compound gel and the compound gels containing
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CDs (a: KGM/KC, b: KGM/KC/α-CD, c: KGM/KC/β-CD, d: KGM/KC/γ-CD, e:
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KGM/KC/M-β-CD, f: KGM/KC/HP-β-CD). Magnification of the images are all 500.
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Fig. 3. XRD curves of CDs, KC/KGM compound gel and the compound gels
523
containing CDs (a: α-CD, b: β-CD, c: γ-CD, d: M-β-CD, e: HP-β-CD, A: KC/KGM, a':
524
KC/KGM/α-CD, b': KC/KGM/β-CD, c': KC/KGM/γ-CD, d': KC/KGM/M-β-CD, e':
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KC/KGM/HP-β-CD).
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Highlights CDs remarkably enhanced the texture and freeze-thaw stability of the compound gel.
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SEM results indicated that CDs lead the compound gel to form a denser gel network.
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XRD results showed that CDs could reduce the crystallinity of gel network.