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REV. CHIM. (Bucharest) 64 No. 8 2013 http://www.revistadechimie.ro 909

Rheology of Gelatin starch SystemsI. Influence of system composition

BOGDANA SIMIONESCU1,2, SORIN ALEXANDRU IBANESCU1, MARICEL DANU1, IULIAN ROTARU3, CONSTANTA IBANESCU1,3*

1Petru Poni Institute of Macromolecular Chemistry Iasi, 41A Aleea Grigore Ghica Vod, 700487, Iasi, Romania2Costin D. Nenitescu Organic Chemistry Centre, 202B Splaiul Independentei, 71141, Bucharest, Romania3Gheorghe Asachi Technical University of Iasi, 73 Prof. dr. docent Dimitrie Mangeron Str., 700050, Iasi, Romania

Texture and sensory properties of foods are often correlated with the rheological behaviour of the system. Inbakery products gluten through its main components gliadin and glutenin is responsible for the viscoelasticbehaviour, essential for the final quality and consumers acceptance of the product. Replacing gluten indietary products is not an easy task and attempts have been performed with various proteins. A series ofstarch gelatin composite gels have been prepared and rheologically characterized to check the suitabilityof gelatin as gluten replacement. Amplitude sweeps, frequency sweeps and temperature tests were carriedout to evidence the influence of starch:gelatin ratio on the overall rheological properties of the final products.

Keywords: dynamic moduli, gelatin, starch, temperature sweep

The interest for gluten free dietary products hasincreased in the last period due to the negative effect ofgluten in different gastrointestinal diseases [1-5]. Whenthe structural and textural properties of different foods areconsidered it is worth to mention the large variety ofproducts is mainly based on two types of macromolecules:proteins and polysaccharides [6]. Each type of biopolymerhas a specific influence on the overall properties of foodsand the different kinds of achievable structures and texturesare the result of the vast diversity of possible intra- andintermolecular bonds. In flour based foods gluten isessential for the global viscoelastic behaviour. The mainprotein fractions in gluten are glutenins responsible for

the elastic properties and resistance to extension of thesystem, and gliadins conferring the viscous behaviourand the dough cohesiveness [7, 8]. The role of starch andgluten as well as of the other components of the wheatflour dough in the overall rheological behaviour of thesystem and in the quality of foods has been discussed in aprevious paper [9]. Replacing gluten is not an easy taskwithout affecting the global rheological properties of thefinal product and its texture and acceptance by theconsumer. Considering the general theory of compositebiopolymer gels, attempts have been made to find the beststarch proteins combination able to maintain the desiredproperties for consumers satisfaction [1-7]. Hydrocolloidslike xanthan gum, carboxymethylcellulose, alginate andcarrageenan [5, 10] as well as pectin, agarose, and oat -glucan [10] have been used in various combinations withnative or modified starch to mimic the properties of gluten.Experiments have been performed, as well, to obtain andcharacterize different types of gelatin based networksuseful both in food and biomedical applications [11-15].The idea of developing gelatin starch networks for foodapplications is not new, but some aspects concerning thegel structure and the influence of the two components onthe rheological properties of composite gels are stillunclear.

Rheological characterization of food products can offeressential information regarding the structure of the

material, as well as about its processability, sensoryperception, handling capacity, shape stability.

* email: [email protected]

This paper aims to get insight in the structure of differentgelatin starch gels and to establish correlations betweenthe rheological properties and macroscopic behaviour ofthe systems gaining relationship between the evolution ofdynamic moduli helpful in interpreting the protein-polysaccharide interactions within each system.

Experimental partMaterials and methods

Type B gelatin (G9382; 100-115 millimoles of freecarboxyl groups per 100 g of protein and a pI of 4.7-5.2; thepH of a 1.5 % solution at 25 C is 5.0-7.5 and has a mediumBloom number) and native wheat starch (S-5127) (


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system. The polymer samples were heated using a Peltiersystem, with direct detection of sample temperature. Athin layer of low viscosity silicon oil was used to avoidmoisture loss. To check the reproducibility of the results

three specimens of each sample were analyzed. Allrheological characteristics were determined in dynamicoscillation mode (DSO) [9, 17].

Results and discussionGelatin and starch may be regarded as model systems

for the study and understanding of the protein-polysaccharide interactions in food products. Gelatin, ahydrocolloid derived from collagen, is commonly used infoods due to its gelling capacity and possibility to developcomposite gels with tunable and controllable properties.Starch is one of the most common and widely usedpolysaccharide in food, pharmaceutics, health care,

biodegradable plastics, and cosmetics. Wheat starchconsists of approximately 25 % linear amylose and 75 %branched amylopectin [18-20], both componentsinfluencing the gelatinization and thermal behaviour of theresulted gels. It is known the peptide bonds in gelatin areheat labile [21], thus inducing difficulties in thermalprocessing and affecting the structural stability of the finalproducts. By the generation of supplementary crosslinksbetween the gelatin chains introducing, for example, apolysaccharide in the system, the thermal stability andgelling capacity could be enhanced. Moreover, the resultingcomposite gels may serve as base for a series of gluten-free foods with a pleasant texture.

Rheological measurements carried out within the linear

viscoelastic region (LVR) are useful methods for the studyof gelatinization phenomena and the structurecharacterization of gels. Consequently, correlations can beestablished between the macroscopic behaviour of variousgels and their internal structure [22]. Our intention was toevidence how much the ratio between gelatin and starchinfluences the rheological behaviour of different compositegels and to correlate the mechanical properties with themicrostructure. It is well known starch swells but is notsoluble in cold water; however, different thermaltreatments can produce clear gels suitable to beincorporated into gelatin network. For this reason the firststep in our study was to investigate the rheological

behaviour of three starch samples: a 5 % starch dispersion(S1) and two samples obtained after 30 min (S2) and 8 h(S3) thermal treatment of the dispersion. Firstly, a strainsweep was carried out varying the amplitude of thedeformation applied to the sample between 0.01 and 100% at constant frequency (1 Hz). Thus, not only the limits ofthe linear viscoelastic region (LVR) could be established,but important information concerning the developmentand the mechanical stability of the network were obtained.Viscoelastic properties of the material were evaluated byrecording the evolution of the dynamic moduli, G and G.The evolution of the storage modulus (G) gives informationabout the solid-like (elastic) behaviour of the sample whilethe loss modulus (G) offers information about the liquid-

like (viscous) behaviour [9]. The tests were performed atconstant temperature (25oC) (fig. 1).Analyzing the evolution of G and G, a liquid-like

behaviour is obvious for sample S1, with the loss modulusmore than four orders of magnitude higher than the storage

modulus. It is clear that for this sample the starch granulesswell in water and form a stable dispersion as long as thesample is under strain. Moreover, an internal network offorces develops inside the material, increasing the systemstability at large deformations. At rest, in time, phaseseparation can be noticed. The deformation limit resultingfrom the amplitude sweep data for sample S1 is around0.1 %. In the case of samples S2 and S3, the rheologicalbehaviour is completely different. Heating the dispersion,the starch granules swell, but, in the mean time, theamylose and the amylopectin are released and becomepartially soluble and gelatinized. A network structure isdeveloped and it is easy to notice for both samples that Gexceeds G with almost one order of magnitude. The twosamples have an extended LVR, meaning the internalstructure is maintained almost unchanged up to largedeformation range. The extension of LVR can be explainedby the formation of amylose and amylopectin inter-penetrated network reinforced with swollen granules [23].The prolonged thermal treatment determines thematuration of the network and the orientation andorganization of the internal structure reflected in anincrease of mechanical stability but also in flexibility. Whilein the case of S2 sample a brittle fracture in the networkstructure appears once the LVR is exceeded this behaviour

is not anymore visible for S3 sample. The S3 structure isless sensitive to mechanical deformations. For bothsamples, a limiting value of deformation around 10% canbe considered for all subsequent oscillatory measurements.

The frequency sweep (fig. 2) is widely used as standardtest in polymer rheology. In this test a sinusoidal strain withconstant amplitude (depending on the LVR limits for eachsample) is applied, the oscillation frequency being varied(between 10-1 and 103 rad/s) [9, 17]. All the measurementswere carried out at 25 oC. The mechanical spectra of thethree starch samples evidenced the clear liquid-likebehaviour of the sample S1 and the preponderantly solid-like behavior of the thermal treated samples (S2 and S3)for which G > G over the entire applied frequency domain.For long relaxation times (small frequencies) the structureis completely developed and stable.

The dynamic moduli are strongly dependent onfrequency for sample S1. A slight dependence on frequency

Table 1

STARCH-GELATIN COMPOSITE GELS

Fig. 1. Dynamic moduli (G full symbols; G- open symbols)evolution recoded during the amplitude sweep of the three starch

samples


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can be also observed for samples S2 and S3. It is clear thata prolonged thermal treatment allows more amylose andamylopectin to be released and to participate in the network

formation and a more stable gel is formed.Dynamic temperature sweep tests were induced afterequilibration at the initial temperature (10oC). The sampleswere heated from 10 to 80 at 0.5 oC/min at a constantangular frequency of 1 Hz and a constant strain in the linearviscoelastic region (fig. 3). Analyzing the evolution of Gand G with temperature clear differences among thethree samples can be noticed. The liquid dispersion S1presents a first stage of slight increase of both dynamicmoduli corresponding to the release of amylose andamylopectin in the temperature range of 10-60 oC.

Analyzing the results of the rheological tests performedwith the three starch samples it was decided to use the S3sample as a base for the starch-gelatin composite gels(Table 1). The idea was to track the influence of differentratios between starch and gelatin on the mechanical andthermal stability of the resulted gels. In figure 4, theevolution of the storage and loss moduli for all the preparedsamples is presented as resulted from the amplitude sweeptests (constant frequency of 1 Hz and variable strain

between 0.01 and 100 %). The behaviour of all analyzedsystems varies from solid-like (G>G) (samples S3 to SG7)to liquid-like (G


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studied systems. Up to the point where the amount ofgelatin equals that of starch (SG5) the effect is beneficialdetermining an increase in the stability and the degree oforganization of the resultant gels. It is known gelatin gelshave a honeycomb, less packed structure as comparedwith starch gels with more space available for filler granulesand water [25].

and G is more than one order of magnitude higher thanG. Melted gelatin may act as a plasticizer of the wholenetwork. This behaviour, a consequence of increasinggelatin concentration, is a result of the competitivenessbetween the two biopolymers for the continuous phase ofthe composite system and the way in which the free wateris entrapped in a less or more densely packed network. Forintermediate composition interpenetrated starch andgelatin network are developed. For samples with highergelatin content a clear melting temperature of the gelatinphase could be evidenced.

ConclusionsThe differences between the mechanical and thermal

properties for a series of starch-gelatin systems have beenreveled using dynamic oscillatory rheological tests. Tuningthe starch:gelatin ratio, softer or harder gels with varioustextures were obtained suitable for preparation of different

kind of foods. It was important to notice that even forhigher gelatin contents, the gels were stable on a largetemperature domain, sustaining the idea of using gelatinas a protein alternative in gluten-free dietary products.Temperature sweeps demonstrated the competitionbetween the two polymers as building blocks for the basenetwork. It was concluded that starch gelatin networksbehaved like deformable particle filled system. The meltingtemperatures of the composite gels were very close to themelting temperatures of the gelatin gels. Theexperimentally determined helix-coil transitiontemperatures for gelatin are in good agreement withliterature data [25-27]. The temperature profiles of thecomposite gels closely resembled the temperature profilesof the gelatin or starch networks depending on thecomposition. Further studies are intended to comparestarch gelatin and starch gluten gels and to evidencesimilarities in their rheological behaviour as an argumentfor the use of gelatin as an alternative for gluten in foodproducts.

Acknowledgements: The authors acknowledge the financial supportof European Social Fund Cristofor I. Simionescu PostdoctoralFellowship Prog ramme (ID POSDRU/89/1.5 /S/55216), Sectori alOperational Programme Human Resources Development 2007 2013.

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Fig. 5. Frequency sweep results for composite starch-gelatin gels

(G full symbols; G- open symbols)

Added gelatin increases the flexibility and shape stabilityof the network contributing to a pleasant texture and a softsensation. Increasing more the gelatin content, the gelsbecome softer and have the tendency to melt undermechanical action, valuable attribute for the fabrication ofspecific deserts. It is important to notice the possibility oftuning the structure, the strength and, as a consequence,the texture and sensory perception of the product bymanipulating the starch:gelatin ratio. Depending on thegelatin content the difference in the storage modulus ofthe final gels can be higher than three orders of magnitude.

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Fig. 6. Dynamic temperature tests for composite starch-gelatin gels(G full symbols; G- open symbols)

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0.01

0.1

1

10

100

1000

10000

Temperature (oC)

Pa

G', G"


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Manuscript received: 20.03.2013

simionescu 8 13.pdf

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