CyTA – Journal of Food Vol. 9, No. 3, November 2011, 243–249
The effect of drying temperature on mechanical properties of pig skin gelatin films El efecto de la temperatura de secado sobre las propiedades meca´nicas de pelı´ culas de gelatina de cerdo G. Aguirre-Alvareza*, D.J. Pimentel-Gonza´leza, R.G. Campos-Montiela, T. Fosterb and S.E. Hillb a Instituto de Ciencias Agropecuarias, Universidad Auto´noma del Estado de Hidalgo, Rancho Universitario Km.1, C.P. 43600, Tulancingo, Hidalgo, Me´xico; bDivision of Food Sciences, School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, UK
(Received 2 May 2010; final version received 4 September 2010) Films from three different bloom strength were prepared and dried at three different temperatures (20 8C, 40 8C and 60 8C) to investigate whether the differences in ordering and aggregation during film formation, under controlled drying conditions, reflect changes in their mechanical properties. Results showed that Young’s modulus (E) was not significantly (p 0.05) influenced by the drying regime because it remained around 3.0 GPa. Film A-270 dried at 20 8C showed higher fracture properties when compared with the same sample dried at 40 8C and 60 8C. Brittleness followed: 60 8C 4 40 8C 4 20 8C. A-100 was sensitive to 60 8C because there was no formation of a continuous network. Films with higher bloom index showed improved fracture properties, it followed A-270 4 A-160 4 A100. Films reflected lower degree of crystallinity (*0%) as the drying temperature increased; the higher the crystallinity (*33%), the better the fracture properties. Films formed at 40 8C and 60 8C showed mostly an amorphous structure. Keywords: gelatin; film; fracture properties; Young’s modulus; drying temperature; crystallinity Pelı´ culas con tres diferentes grados de firmeza de gel fueron elaborados y secados a tres diferentes temperaturas (20 8C, 40 8C y 60 8C) para investigar si las diferencias en reorganizacio´n y agregacio´n durante la formacio´n de pelı´ cula bajo condiciones de secado controladas refleja cambios en sus propiedades meca´nicas. Los resultados mostraron que el mo´dulo de Young (E) de las pelı´ culas no fue influenciado significativamente (p 0,05) por el re´gimen de secado debido a que e´ste permanecio´ alrededor de 3,0 GPa. La pelı´ cula A-270 secado a 20 8C mostro´ mejores propiedades de fractura en comparacio´n a la misma muestra pero secada a 40 8C y 608. La rigidez en pelı´ culas siguio´ la tendencia: 60 8C 4 40 8C 4 20 8C. A-100 fue sensitiva a 60 8C porque no hubo formacio´n de una red continua. Las pelı´ culas con alto nivel de firmeza de gel mostro´ mejores propiedades de fractura, esta tendencia mostro´ A-2704A-1604A-100. Las pelı´ culas reflejaron bajo grado de cristalinidad (*0%) mientras que la temperatura de secado se incrementaba, cuanto ma´s alta la cristalinidad (*33%) mejores fueron las propiedades de fractura. Pelı´ culas formadas a 40 8C y 60 8C mostraron una estructura amorfa. Palabras clave: gelatina; pelı´ cula; propiedades de fractura; Mo´dulo de Young; temperatura de secado; cristalinidad
Introduction Gelatin has been used for a long time as a biological adhesive. In 4000 BC, Egyptians used a glue produced from collagen as an adhesive to glue their items of furniture together (Koepff, 1985). The bone and hide of animals were boiled to obtain this glue. The resultant product from boiling had the ability to solidify when cooled to produce the original form of edible gelatin (Keenan, 1997). In the late seventeenth century, the commercial gelatin manufacturing began. However, at the beginning of the nineteenth century, the gelatin
*Corresponding author. Email:
[email protected] ISSN 1947-6337 print/ISSN 1947-6345 online Ó 2011 Taylor & Francis DOI: 10.1080/19476337.2010.523902 http://www.informaworld.com
industry improved commercial methods and achieved high molecular weight collagen extracts that could form good quality gels (Bogue, 1922; Smith, 1929). Today, gelatin is one of the most popular biopolymers for the manufacture of a range of products. It is widely used in the pharmaceutical industry for the manufacture of hard and soft gelatin capsules, coating tablets and microcapsules (Hussain & Maji, 2008; Reich, 1995), in the photography industry, where high-quality gelatin is used for coating photographic paper (Attridge, 1992; Kuge, Arisawa, Aoki, & Hasegawa,
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2004). Within the food industry, it has a wide range of applications such as: gelatin desserts (Baziwane & He, 2003; Zhou & Regenstein, 2007), edible films (LopezCarballo, Hernandez-Munoz, Gavara, & Ocio, 2008), confectionery (DeMars & Ziegler, 2001), clarification agent (Cosme, Ricardo-Da-Silva, & Laureano, 2007), emulsifier (Surh, Decker, & McClements, 2006) and stabiliser (Alakali, Okonkwo, & Iordye, 2008). The manufacture of gelatin involves several factors that define its structure and properties. For example, the source of collagen from where gelatin was extracted reflects differences in the thermal stability of gelatin (melting temperature). Another important factor that influences changes on the chemical composition of gelatin is the method of extraction, which could be acid or alkaline (Hinterwaldner, 1977). This action results in partial cleavage of the collagen cross-linked structure. The hydrolysis is commonly known as ‘the conditioning process’ (Schrieber & Gareis, 2007). It defines the characteristics of gelatin during conversion of collagen to gelatin. In this way, gelatin characteristics are dependent on different parameters such as acid/alkali extraction, source and thermal treatment parameters (Veis, 1964). The analytical measure of gelling power is the bloom value. This value represents the weight in grams required for a standard half-inch plunger to depress the surface of a gelatin gel (6.67%) to a depth of 4 mm. The gel is aged 17 h at 10 8C prior to measurement. The bloom value of commercial gelatin types is within the range 50–300 bloom (Schrieber & Gareis, 2007). Transitions during network formation in gelatin films In the preparation of gelatin films, the gelatin undergoes different transitions during the formation of the solution and the film. The gelatin is initially solubilised in distilled water at 20 8C at 4% concentration. Although care is taken to hydrate and get all the material into a true solution, micro gels, small agglomerates with a high amount of order would still be present (Ledward, 1986; Normand, Muller, Ravey, & Parker, 2000). The solution is then dried to form films by reduction of moisture. Depending on the reaction temperature, vitrification process takes place leading to the formation of films in the glassy state (Williams, 1998). The formation of gelatin films dried at 20 8C (cold films) starts with the gelation phenomenon. The gelatin molecules are able to form helices and these helices could associate to form aggregates. The mechanism of gelatin gelation in water involves a typical polymer crystallisation process from steps such as nucleation, propagation and maturation (Roos, 1995; Slade & Levine, 1987). Therefore, an ordered gelatin structure could be defined as the structure in which the crystallisation
process took place towards the conformation of native collagen structure. The drying period of the samples dried at 20 8C is around 72 h, while samples dried at 40 8C and 60 8C had a shorter period (12 h). Therefore, higher order is assumed to occur in slow drying samples (JohnstonBanks, 1990; Ledward, 1986; Veis, 1964). Upon drying, the reorganisation of the structure increase and the gelatin molecules became more aligned (Supplementary Figure 1). Once water is lost and the sample nears the glass transition temperature (Tg), little changes in the structure are possible. The sample will approach Tg slowly and hence retain flexibility and reorganisation until the glass is reached. The sample is then finally dried when it is fully inside the glassy region where little reorganisation is possible for the vitrified sample state. However, the manufacture of films dried at 40 8C and 60 8C is expected to follow a different mechanism of network formation. Supplementary Figure 2 exhibits that the vitrification process can start from the liquid form. The higher temperature drying allows the gelatin solution to remain fluid until it becomes highly concentrated. Gel formation is avoided because the rate of drying is faster than the rate of nucleation (Johnston-Banks, 1990). Drying temperatures of 40 8C and 60 8C are close to the gelatin melting temperature (Tm). At the end of drying, the film formation is created from a disordered structure, characterised by entangled and closely packed chains (Sperling, 2006). This contrasts with the junction zones and helical order conformations when films are dried at 20 8C. Once the hot film enters the glassy state, its nonequilibrium state is determined by its thermal history. Holding the drying film at temperatures close to the Tg would encourage its relaxation to equilibrium (Williams, 1998). There is a wide range in the types of foods and the types of textural and rheological properties that they exhibit, and also a wide variety of methods used to measure these properties. A conventional tensile test assumes that the sample fractures almost instantaneously in a perpendicular plane of the applied force. However, some foods subjected to tension do not fail suddenly; fracture begins with a small crack that slowly spreads across the sample over a period of time. The basic foundation underlying fracture mechanics is that all solids contain inhomogeneities, which exist in the form of flaws or cracks. These cracks may or may not be perpendicular to the plane of the applied tension (Bourne, 2002; Kilcast, 2004). Dry gelatin films are commercially an important product in food and pharmaceutical industries. Their flexibility and resistance to breakage are key factors in the quality of the film.
CyTA – Journal of Food Mechanical properties of films have been studied in correlation with amount of water, (Kellaway, Marriott, & Robinson, 1978; Yakimets, et al., 2005), drying temperature and relative humidity (Bradbury & Martin, 1952; Reich, 1995), amount of triple-helix content (Bigi, Panzavolta, & Rubini, 2004; Dai & Liu, 2006), addition of cross linkers (Bigi et al., 2000; Cao, Fu, & He, 2007), addition of plasticisers (Veiga-Santos, Oliveira, Cereda, & Scamparini, 2007) and blending of hydrophilic biopolymers (Villagomez-Zavala et al., 2008). The effect of the preparation temperature and relative humidity on the mechanical properties and structure of gelatin films has been of interest for a long period (Bradbury & Martin, 1952); however, differences in bloom strength and its relationship with the mechanical properties of gelatin films and structure conformation due to drying temperature conditions are not fully understood. This work describes the effect of drying temperature on mechanical properties of films prepared with acid extracted gelatin from three different bloom strength values at a fixed relative humidity. Materials and methods Gelatin Pig skin gelatin (A-) with 100, 160 and 270 bloom strength values were obtained as powders with 10% water content (db) from Sigma Chemicals (UK). Their isoelectric point values varied from 6.5 to 8.8. Preparation of films A 4% aqueous gelatin solution was prepared by adding 4 g of gelatin into 100 ml of distilled water. Swelling of gelatin was carried out for 30 min at room temperature. The solution was placed in a 65 8C water bath for 30 min to melt the gelatin with gentle stirring. After this, the aqueous gelatin solution was used to prepare films by weighing 56.2 g into a 13.6 cm diameter polystyrene Petri dish. Sets of gelatin films were dried at 20 8C, 40 8C and 60 8C. These temperatures were chosen because they are below (20 8C) and above (40 8C and 60 8C) the glass transition temperature (Tg) and melting temperature (Tm) of the gelatin gel (Haug, Draget, & Smidsrod, 2004). Drying times of films exposed at 40 8C and 60 8C were about 12 h, and 72 h for those films dried at 20 8C. After drying, films were peeled from the substrate and cut into strips with known dimensions (10 mm 6 100 mm). Average thickness was 0.1 + 0.005 mm. Samples were stored for 7 days over P2O5 (0% R.H.) for the maximum removal of water. After this storage time, samples reported 2.3 + 0.1% db of water content. Then, strips were stored over salt solution (K2CO3 ¼ 44% R.H.) in a sealed box for 7 days to equilibrate the water content
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at 15% db. Just before the mechanical testing, hydrated samples were coated with silicon oil to avoid water content changes during tensile test. Tensile strength performance test Gelatin films were clamped in the texture analyser machine TA.XT Plus. The distance between tensile grips was 50 mm. This distance was considered as the original length of the sample for calculation of strain. Settings for the machine were set up as follows; test mode ¼ tension; test speed ¼ 0.13 mm/sec; distance ¼ 30 mm. The performance of the mechanical properties of films was assessed by stretching the sample from its original length, until the sample was fractured. The applied force, distance and slope of the curve were recorded with Exponent Micro System software v 2.060. Calculations The resistance of the films to elongation at small deformation is called Young’s modulus (E) (Sperling, 2006). This was calculated from the linear region of the curve at 0.5 mm of strain as follows: E¼
ðGradient DLÞ=ðt wÞ 1 109
where, E ¼ Young’s modulus (GPa); gradient ¼ slope of the curve at 0.5 mm of strain (N/mm); DL ¼ increment of length (mm); t ¼ thickness (m); w ¼ width of the film (m). The stress at break (sb) and strain at break (Eb) were obtained from the maximum force and distance of the tensile strength curve. In most of the cases, fracture occurred at the clamp grips. Differential scanning calorimetry (DSC) measurements A Perkin Elmer Pyris Diamond DSC machine with an intracooler 2P was used for this work. It was calibrated with indium (Tm,onset ¼ 156.6 8C, DH ¼ 28.45 J/g) and cyclohexane (Tm,onset ¼ 6.5 8C). An average of 45 mg of gelatin film with known water content (15% db) were packed and sealed in a 50 mL stainless steel pan. An empty pan with the same characteristics was utilised as a reference. Both heating and cooling scan rates were performed at 10 8C/min. Two heating scans were performed from 25 8C to 120 8C. Statistics A randomised design experiment and an analysis of variance (ANOVA) were applied to the experimental data which included a Tukey test at a level p 0.05.
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Ten replicates per treatment were considered in this experiment.
of order in the films they were assessed by DSC measurements.
Results and discussion Mechanical testing
Thermal properties of gelatin films dried at different temperatures
The drying conditions carried out in this work prevented some materials from forming films suitable for mechanical testing. Films produced with A-100 gelatin were sensitive to 60 8C temperature in such a way that there was no formation of a continuous network. Supplementary Figure 3(a) shows the data for Young’s modulus of gelatin films dried at different temperatures. It suggests that the brittle behaviour of gelatin films remained at the same level, no matter what the bloom strength and drying conditions were, as there were no statistical differences (p 0.05). The Young’s modulus of the films did not seem to be greatly influenced by the drying temperature and bloom strength, but the strain and stress at break (fracture properties) of gelatin films were reduced as the drying temperature increased (Gennadios, 2002). Sample A-100 at 20 8C and 40 8C had the lowest force applied to fracture its structure; however, as the bloom strength increased, the force required to break the film increased too (Supplementary Figure 3(b)). Large deformation properties of films were affected in the same fashion as the stress at break, as can be seen in Supplementary Figure 3(c). It was assumed that the degree of cross linking of films dried at 40 8C and 60 8C was attributed to the entanglement of chains rather than ordered aggregates/ point junction zones. The limitation of short chain gelatins such as gelatin A-100 to form a continuous network when exposed at 60 8C indicates that the strong hydrolysis of collagen during gelatin manufacture (Veis, 1964) caused the degree of entanglement in these amorphous structures to be limited. The case of junction zone formation would also be molecular weight dependent. It would be expected that the high molecular weight chains would form more order and at a faster rate than the smaller chains. Weakness in the film when in the glassy state may be caused by presence of cracking and crazing in the films. These defects result from inhomogeneity caused by densification due to large junction points or due to molecules’ compaction (enthalpy relaxation). These films may contain cracks which then cause major failure in the films when stretched. Hence, although it appears that junction zones are important for films of high break strength, reduction of inhomogeneity might equally be important. Films produced at high temperature formed a more brittle material. This brittleness followed: 60 8C 4 40 8C 4 20 8C. To understand the amount
Supplementary Table 1 shows the thermal properties of pig skin gelatin films dried at different temperatures and conditioned at 15% water content (db), as measured by DSC. In Supplementary Figure 4, the sample A-270 was taken as an example to show the typical DSC curves of the first and second scan. The first scan (thermal history) exhibited two peak temperatures. The lower peak temperature was correlated with the glass transition temperature (Tg), while the second peak was associated with melting temperature of the film (Tm). Looking at the sample A-270, the first scan of films dried at 20 8C shows a massive amount of crystallinity (36.28% around 98.13 8C) compared to those films dried at 40 8C (0.74% around 109 8C) and 60 8C (0.18% around 112 8C). The second scan shows DCp values around zero which suggest that the time for reorganisation of gelatin structure was not sufficient and remaining chains in a coil conformation. The first peak temperature reflected an enthalpy relaxation (DH) event in the Tg region of films. The size of this peak depended on the thermal history of the film. A-160 showed that the enthalpy increased from 0.74 J/g to 2.98 J/g as the drying temperature increased. This phenomenon was accompanied with a shifting of peak temperature to higher values from 59.7 8C to 68.87 8C indicating better thermal stability. The same behaviour was observed for the rest of the samples. The second peak was correlated with the degree of reorganisation of the gelatin structure. The enthalpy was calculated as the area under this peak, and the peak temperature was taken as the Tm. The peak temperature was shifted to higher values as the drying temperature increased. Keeping A-160 as an example, a shift from 96 8C to 107.74 8C occurred as the drying temperature of the film changed from 20 8C to 60 8C. All the films were measured at 15% water content (db). However, the reordering could have occurred over a range of water content as film dried. At high temperature, the nucleation, which is known as the first stage of helix formation (Engel & Ba¨chinger, 2005), resulted difficult as the melting of the sample occurred. Hence, only very well organised structure with high melting temperature would be formed. Busnel, Morris, & Rossmurphy (1989) proposed that the intermolecular helix formation increased with the molecular weight. This increment is via biomolecular process, involving intermolecular b-turn and another gelatin macromolecule (bimolecular nucleation stage). Considering the enthalpy (DH) as the energy required to
CyTA – Journal of Food disorganise the helical structure of the gelatin film, it was possible to deduce that gelatin films formed at 20 8C reflected a more ordered structure, in comparison with those films formed at 40 8C and 60 8C. A massive reduction in DH was observed as the drying temperature increased. For example, A-270, shifted in enthalpy from 18.23 J/g to 0.09 J/g for films dried at 20 8C and 60 8C, respectively. The total amount of crystallinity in the films was calculated as follows: the enthalpy of films as obtained from DSC measurements was divided by the collagen enthalpy of 50.25 J/g, the result was multiplied by 100 and expressed as a percentage (Achet & He, 1995; Tanioka, Miyasaka, & Ishikawa, 1976). It was observed that preparation conditions played an important role in the final structure of films. Samples prepared at 20 8C possessed an ordered structure, while films formed at 40 8C and 60 8C showed mostly an amorphous structure (Coopes, 1976; Hermel et al., 1991). Enthalpy relaxation peaks were more obvious in the less-ordered films. During first scan, all the films were heated from 25 8C to 120 8C with the aim to delete the thermal history but with thecare of not exposing the sample to excessive thermal degradation. Samples were then cooled from 120 8C to 25 8C at a rate of 10 8C per minute. The second scan was performed in the same way as the first one. From this heating scan, the Tg was measured as the half value in the step change of heat capacity (Roos, 1995). The Tg values of the partial crystalline gelatin (first heat) was observed to be lower than that of the amorphous structure (second heat) because of increased plasticisation of water (Slade & Levine, 1987). Jolley (1970) discovered that crystalline regions of air dried gelatin contain more water than the amorphous region. Therefore, upon melting of the crystals and release of this water which is redistributed throughout the amorphous matrix, the overall water content of the glassy phase is increased and its Tg decreases (Marshall & Petrie, 1980). For samples that were prepared at 20 8C, the Tg appeared at lower temperature values compared to those films prepared at 40 8C and 60 8C. Relationship between molecular order and mechanical properties When drying films at different temperatures, it was established that different levels of order were created. Supplementary Figure 5 shows only the experimental data of treatments at 20 8C and 40 8C because values of treatments at 60 8C overlap with those obtained at 40 8C, then for reasons of clarity they were excluded from the plot. Supplementary Figure 5a shows that Young’s modulus remained around 3.0 GPa irrespective of the degree of crystallinity or bloom strength. However, fracture properties seemed to change
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slightly with the degree of crystallinity. The stress at break (Supplementary Figure 5(b)) and strain at break (Supplementary Figure 5(c)) increases as the crystallinity (amount of order) increases. Gelatin film A-270 dried at 20 8C showed higher fracture properties when compared with the same sample but dried at 40 8C. This trend is in good agreement with previous works conducted with gelatin films at different degrees of crystallinity and temperature of drying (Bradbury & Martin, 1952; Dai & Liu, 2006). These results suggested that the degree of order played a major role when the sample is stretched until breaking point, because fracture properties of gelatin films showed a dependence on the amount of order. However, this was not the dominant factor in the film’s behaviour. The bloom strength of gelatin films was also the key factor in the final film’s fracture properties because samples dried at 20 8C showed: the higher the bloom strength, the better the fracture properties. Same behaviour was observed for samples dried at 40 8C. Conclusions Gelatin films underwent major changes as they dried: helix formation, junction zone creation and the glass transition. However, film dried at 40 8C and 60 8C formed a more brittle structure than those films dried at 20 8C. Hence, the preparation conditions played an important role in the final structure of films. Despite these marked differences in structural organisation, the Young’s modulus remained at the same level irrespective of the bloom strength and drying temperature. The brittleness of films was observed to increase as the drying temperature increased. DSC results showed that films with lower degree of reorganisation (crystallinity) reflected lower values of enthalpy which was correlated with an amorphous structure. As the network structure of films dried at 40 8C and 608 showed little crystallinity, it was assumed that the degree of cross linking of these films was attributed to the entanglement of chains rather than ordered aggregates/junction zones, as is the case in films dried at 20 8C. The limitation of the short chain gelatins such as A100 to develop a continuous network structure when dried at 60 8C suggested that the low molecular weight chain samples were limited in their degree of entanglement during film formation. Drying temperature parameter seemed to affect more obviously the fracture properties of films because the lower bloom strength gelatin (A-100) could not form a continuous network at 60 8C. However, in films prepared at 20 8C, it was possible to see clearly the effect of bloom strength parameter; the higher the bloom value, the better the fracture properties.
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Supplementary material The supplementary material for this article is available online at http://dx.doi.org/10.1080/19476337.2010. 523902 Acknowledgements The first author gratefully acknowledges the Universidad Auto´noma del Estado de Hidalgo and Programa para el mejoramiento del Profesorado (PROMEP) for their financial support.
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Supplementary Table 1. measured by DSC.
Thermal properties of pig skin gelatin films (15% water content d.b.) dried at different temperatures as
Tabla adicional 1. Propiedades termales de la gelatina de cerdo (15% de contenido de agua d.b.) secada a diferentes temperaturas segu´n las mediciones por DSC. First scan (Thermal history) Peak 1 (Tg) Sample ID
Peak Temp (8C)
Second scan
Peak 2 (Tm)
DH (J/g)
Peak Tem (8C)
Crystallinity (%)
Half Cp (8C)
DCp (J/g *8C)
16.79 16.20 18.23
33.40 32.25 36.28
54.81 NM NM
0.317 NM NM
0.28 0.10 0.37
0.56 0.19 0.74
64.55 66.00 66.00
0.290 0.249 0.265
NF 63.00 63.02
NF 0.294 0.293
DH (J/g) 208C
A-100 A-160 A-270
60.12 59.7 62.76
0.57 0.74 0.57
95.28 96.00 98.13 408C
A-100 A-160 A-270
71.42 68.92 71.17
0.81 2.68 1.42
101.25 101.61 109.48 608C
A-100 NF A-160 68.87 A-270 71.58 NF ¼ No film formation
NF 2.96 2.24
NF 107.74 112.27
NF NF 0.16 0.32 0.09 0.18 NM ¼ Not measurable
Note: Peak temperature ¼ maximum value within a specific temperature interval, due to reaction or transition in the sample; DH ¼ difference in enthalpy between reference state and the measured sample; crystallinity ¼ Fraction of crystalline material; half Cp ¼ temperature at which, in the region of a step, the difference between extrapolated initial baseline and measured curve is equal to the difference between measured curve and extrapolated final baseline; DCp ¼ change in heat capacity at the glass transition.
Supplementary Figure 1. (1990). Figura adicional 1. Banks (1990).
Transition state (gel-glass) of gelatin film during drying at 20 8C. Adapted from Johnston-Banks
Estado de transicio´n (gel-cristal) de pelı´ cula de gelatina durante secado a 20 8C. Adaptacio´n de Johnston-
Supplementary Figure 2. (2006).
Transition state (sol-glass) of gelatin film during drying at 40 8C and 60 8C. Adapted from Sperling
Figura adicional 2. Estado de transicio´n (solucio´n-cristal) de pelı´ cula de gelatina durante secados a 40 8C y 60 8C. Adapatacio´n de Sperling (2006).
3 Supplementary Figure 3. Mechanical properties against bloom strength in pig skin gelatin films dried at different temperatures. Different letters represent significant differences. Error bars represent +1 SD of 10 replicates. Figura adicional 3. Propiedades meca´nicas frente a grados de firmeza en pelı´ culas de gelatina de cerdo secadas a diferentes temperaturas. Diferentes letras representan diferencias significativas. Barras de error representan +1 DS de 10 re´plicas.
Supplementary Figure 4. DSC curves of pig skin gelatin film (nominal bloom strength 270) dried at three different temperatures and equilibrated at 15% water content db. Numbers 1 and 2 represent the first and second scan, respectively. Figura adicional 4. Curvas DSC de pelı´ cula de gelatina de cerdo (grado de firmeza nominal 270) secada a tres temperaturas diferentes y equilibarada a 15% de contenido de agua db. Nu´meros 1 y 2 representan los barridos primero y segundo, respectivamente.
Supplementary Figure 5. Mechanical properties against crystallinity in pig skin gelatin films with different bloom strength. Error bars represent +1 SD of 10 replicates. Figura adicional 5. Propiedades meca´nicas frente a cristalinidad en pelı´ culas de gelatina de cerdo con diferentes grados de firmeza. Barras de error representan +1 DS de 10 re´plicas.
