Konjac gel fat analogue for use in meat products: Comparison with pork fats

Food Hydrocolloids 26 (2012) 63e72

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Konjac gel fat analogue for use in meat products: Comparison with pork fats F. Jiménez-Colmenero*, S. Cofrades, A.M. Herrero, F. Fernández-Martín, L. Rodríguez-Salas, C. Ruiz-Capillas Instituto de Ciencia y Tecnología de Alimentos y Nutrición (ICTAN-CSIC) (Formerly Instituto del Frío), Ciudad Universitaria, 28040 Madrid, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 November 2010 Accepted 11 April 2011

The paper reports a study of the characteristics of konjac gel fat analogue as compared to types of pork fat for use in fat reduction strategies for meat products. Various characteristicsdcolour, mechanical/ rheological behaviour and thermal propertiesdof pork fats (backfat-PBF and trimmed fat-PTF) and konjac gel (KFG) with different physical structures (intact or ground to 4 and 8 mm) were studied. Pork fat melting processes were evaluated by differential scanning calorimetry (DSC) at above- and belowzero temperature ranges with PBF and PTF always showing similar net results. KFG did not show any thermal event in the range from
40 to 100  C except the freezing/melting of its constitutional water. While water and fat binding properties of pork fats were affected by fat type and structural disintegration (ranging between 0 and 77%), in all cases KFG presented excellent thermal water binding (<1%). As compared to KFG, PBF showed greater (P < 0.05) hardness, chewiness, penetration force, gel strength, extrusion force and work of extrusion. These differences were minimized after grinding. Kramer shear values in KFG were greater (P < 0.05) than in PBF when this was ground to 4 mm, but lower (P < 0.05) at 8 mm. The highest Kramer shear values (P < 0.05) were recorded in PTF irrespective of the degree of disintegration. Rheological analyses indicate that the behaviour of KFG, which is thermally stable, is predominantly elastic during heating and exhibits rheological thermal behaviour (at over 40  C) similar to that of pork backfat. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Fat analogue Konjac gel Pork fats Mechanical properties Rheology DSC Fat melting-solidification

1. Introduction Dietary fat is needed as a metabolic energy source and a supplier of essential nutrients, but it must be consumed in moderation for reasons of human health. There is growing evidence associating dietary fat (quantity and type of fat) with chronic disorders such as ischaemic heart disease, some types of cancer and obesity (WHO, 2003). Fat reduction has generally been seen as an important strategy to improve the fat content of foods and produce healthier products. This aspect is especially relevant to the meat industry since some meat products contain high proportions of fat, and fat from meats has often been assumed to be a risk factor in consumer health (Ferguson, 2010; McAfee et al., 2010; Williamson, Foster, Stanner, & Buttriss, 2005) because of its relatively high contribution to fat intake. In industrialized countries, approximately 36e40% of the total calories in the food supply come from fat, nearly half of which is from meat intake (Byers, Turner, & Cross, 1993; Sheard, Wood, Nute, & Ball, 1998). Fat reduction in meat products is usually based on two main criteria: use of leaner meat raw materials and reduction of fat density (dilution) by adding

* Corresponding author. Tel.: þ34 91 549 23 00; fax: þ34 91 549 36 27. E-mail address: [email protected] (F. Jiménez-Colmenero). 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.04.007

water and other ingredients (Jiménez-Colmenero, 1996). These ingredients should assure a low-calorie content and help give the product the desired characteristics. A wide variety of ingredients has been proposed for this purpose, based on the use of proteins, carbohydrates or lipids (Jiménez-Colmenero, 1996; Keeton, 1994, among others). These include konjac (glucomannan)-based fat analogues, which open up interesting possibilities. Konjac glucomannan is a neutral polysaccharide produced by Amorphophallus konjac, a plant native to East Asia, where it has been used since ancient times. It presents notable physiological effects and exceptional technological properties which offer great potential for application in food technology. Its use as a food additive is authorized in Europe (E-425), and it is classified as GRAS by the FDA. Konjac flour is considered a low-calorie ingredient which, given its non-digestible fibre content, presents numerous physiological effects and therapeutic applications (González Canga et al., 2004; Tye, 1991; Zhang et al., 2001). Although Konjac can be used for different purposes on account of its technological properties, it forms gels which, combined with other ingredients (starch, carrageenates, gellan gum), can be used as ‘fat analogues’ in the formulation of reduced/low-fat meat products. Konjac (added in different ways and concentrations) has been used to reduce fat in products such as frankfurters (Jiménez-Colmenero et al., 2010; Kao & Lin, 2006; Lin & Huang, 2003; Osburn & Keeton, 2004), bologna

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(Chin, Keeton, Longnecker, & Lamkey, 1998a, 1998b; Chin, Keeton, Miller, Longnecker, & Lamkey, 2000), fresh sausages (Osburn & Keeton, 1994) and pork nuggets (Berry & Bigner, 1996). It is well known that mixtures of different animal fatty tissues are usually used in the formulation of meat derivatives. These fat raw materials come essentially from subcutaneous fatty tissues (mainly backfat) and the inter-muscular and (to a lesser extent) intra-muscular fat adhering in varying proportions to the meat used to formulate the products (Hugo & Roodt, 2007). These types of fat source differ in terms of animal tissue structure, physical state of the lipid, composition (fatty acid profile and presence of the other components including collagen, water, etc.) and properties such as texture, thermal stability, colour, rheological behaviour, etc. (Barreto, Carballo, Fernandez-Martin, & Jiménez-Colmenero, 1996; Hugo & Roodt, 2007; Ospina-E, Cruz, Pérez-Alvarez, & FernándezLópez, 2010; Raes, De Smet, & Demeyer, 2004). As structural and functional components, lipids are involved in most food properties and have a decisive effect on meat product quality even when their concentration is low (Goutenfongea & Dumont, 1990). Since the amount and source of fat present affects the nature of the meat matrix and meat product characteristics (Barreto et al., 1996; Goutenfongea & Dumont, 1990; Ospina-E et al., 2010), differences in fat fractions used in meat product reformulation should be considered. These considerations are especially important in fat reduction strategies since low-fat products are generally made increasing the proportion of leaner meat raw materials and decreasing the presence of fattier meat raw materials (backfat). Changes in the relative proportions of different types of fat in fat reduction procedures necessarily entail changes in the relative contribution of their technological properties to meat product characteristics. In these cases, the technological contribution of backfat to the product characteristics is generally outweighed by that of the other types of lipid materials present. A great deal of work has been done in recent decades to characterize the quality of adipose tissue in an objective way in order to select pork fat batches according to their suitability for the manufacture of specific meat products (Glaser, Wenk, & Scheeder, 2004; Hugo & Roodt, 2007). For meat product formulation, fat analogues are ground down to varying particle sizes (visible or not to the human eye depending on requirements) in order to achieve the appearance and the technological, rheological and sensorial properties required for use as raw materials to replace animal fats (Tye, 1991). Fat analogues are added to meat products as alternatives to animal fat, and therefore their physicochemical characteristics need to be considered since they affect quality properties of reformulated products. Hence, the suitability of an analogue depends on how well it reproduces the characteristics of the requisite fat type (ground/comminuted to a desired particle size depending on the type of meat product) in the meat matrix, including colour, textural properties or rheological behaviour. KGM has been added to various meat product formulations in different forms, including as konjac gels (JiménezColmenero et al., 2010; Lin & Huang, 2003, 2008; Osburn & Keeton, 2004). When konjac flour is dissolved in an alkaline coagulant (such as calcium hydroxide), deacetylation occurs and a thermally stable gel is formed (Lin & Huang, 2003). Gelification is also affected by temperature. Konjac gels made without heating (e.g. cold-set gels) have been used as fat replacers in the formulation of reduced/low-fat meat products with different ground/ comminuted levels such as lamb sausages (Osburn & Keeton, 2004), pork meat batters (Fernández-Martín, López-López, Cofrades, & Jiménez-Colmenero, 2009), frankfurters (Jiménez-Colmenero et al., 2010) and fresh pork sausages (awaiting publication). As far as the authors are aware, there are no references in the literature to studies of the characteristics of these konjac gels used for animal fat replacement (Jiménez-Colmenero et al., 2010) as compared with

the different fat types and particle sizes used for the production of meat derivatives. A fuller knowledge of analogue characteristics (in relation to fat types) will therefore facilitate their use, help to elucidate their role in the protein matrix structure and help to improve the quality of healthy meat-based food systems to which they are added. The object of this research was therefore to study the physicochemical characteristics of konjac gel (previously stablished as fat replacer) and pork fats (backfat and trimmed fat) as affected by their physical structure (intact and ground in different conditions). Although the experimental conditions were not the same as when they are added to meat products, the information derived may be useful in assessing their behaviour in a complex matrix and hence may contribute to the development and use of more suitable fat analogues in the manufacture of meat products. 2. Materials and methods 2.1. Materials and konjac gels preparation Two types of fats were used as reference models representing the different types used industrially for the production of meat derivatives. One consisted of whole (block) pieces of pork backfat (PBF, 87.6% fat and 1.3% protein) and the other consisted of trimmed pork fat (PTF 68.5% fat and 5.1% protein content) obtained by trimming visible fat from the raw meat materials (hamtrimming, shouldertrimming and bellytrimming). Both types of raw material were purchased at a local market and stored at 2  C until analysis (within 48 h). Three different lots of each were analysed. Konjac gel (KFG) was made with konjac flour (glucomannan 83%, 120 mesh) from Trades S.A. (Barcelona, Spain), pre-gelled cornstarch (Amigel, Julio Criado, S.A., Madrid, Spain) from Julio Criado S.L. (Madrid, Spain), i-carrageenan (Hispanagar S.A., Burgos, Spain) and Ca(OH)2 (Panreac Química S.A., Barcelona, Spain). KFG preparation was based on that of Osburn and Keeton (2004) with modifications (Jiménez-Colmenero et al., 2010). Briefly, (for each 900 g of konjac gel preparation) 45 g of konjac flour was homogenized (Stephan Universal Machine UM5, Stephan Machinery GmbH and Co., Hameln, Germany) at 2850 rpm with water (583.2 mL) for 3 min, left to rest for 5 min then homogenized at 1400 rpm for a further 3 min. The i-carrageenan (9 g) was then added and the mixture homogenized again at 2850 rpm for 3 min. The pre-gelled cornstarch powder (27 g) was dispersed in 145.8 mL of water and homogenized at 2850 rpm with a mixture of konjac flour and icarrageenan, left to rest for 5 min then homogenized at 1400 rpm for a further 3 min. The mixture was cooled to 10  C, then 90 mL of Ca(OH)2 solution (1%) was added with gentle stirring at room temperature. The konjac gel was placed in suitable containers (so as to form blocks resembling native pork backfat), covered, manually overpressured to eliminate air and stored at 2  2  C until used (within 24 h of preparation). KFG was prepared in triplicate. 2.2. Experimental design In order to determine fat analogue characteristics (as compared with fat types) and thus help to elucidate their role in the protein matrix structure, this experiment entailed an analysis of the two materials in different structural conditions of relevance for meat derivative production. PBF and KFG were analysed in three forms: intact/native, and (since meat processing involves structural disintegration) ground in different conditions (4 and 8 mm hole diameter) using a Kenwood Mincer (MG300, Hampshire, UK). Since it is a trimmed product, PTF cannot be studied as an intact structure (as in the case of PBF); comparison with the other samples was therefore performed after grinding in all cases (pork fats and KFG)

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in the same conditions (4 and 8 mm hole diameter). These sample structural conditions made it possible to assess their mechanical and rheological properties as reported further below. 2.3. Proximate analysis and pH Sample moisture and ash contents were determined (AOAC, 2000) in triplicate in all samples. Protein content was measured in quadruplicate in PBF and PTF with a LECO FP-2000 Nitrogen Determinator (Leco Corporation, St Joseph, MI, USA). Fat content was evaluated in triplicate according to Bligh and Dyer (1959). The pH was determined on a Radiometer model PHM 93 pHmeter (Meterlab, Copenhagen, Denmark) at room temperature on homogenates of samples in distilled water in a ratio 1:10 (w/v). Three replicates were performed for each formulation. 2.4. Water and fat binding properties Water and fat binding properties were determined (in quintuplicate) by measuring water and fat loss during heating (as a measure of thermal stability) in intact samples and samples ground to 8 mm (Jiménez-Colmenero, Carballo, & Solas, 1995). The samples (around 25 g) were stuffed (as pieces measuring 6  2.5  2.5 cm) into tubes and hermetically closed, then heated in a waterbath for 30 min at 70  C. They were then opened and left to stand upside down (for 50 min) to release the separated fat and/or water onto a plate. Heating (total) loss (water and fat loss) was measured as % of initial sample weight. Water loss was determined as % weight loss after heating the total released fluid (fat and water) for 16 h on a stove at 100  C. Fat loss was calculated as the difference between total loss (measured as % of initial sample weight) and water loss. 2.5. Colour measurement Objective colour CIE-LAB tristimulus values, L* (lightness); a* (redegreen axis), and b* (yelloweblue axis) parameters of the different pork fats and konjac gel were evaluated on a Chroma Meter CR-400 (Konica Minolta Business Technologies, Inc., Tokyo, Japan). Fifteen determinations were performed from each sample. 2.6. Mechanical properties Different textural analyses were carried out on the samples depending on their physical structure (intact and ground samples). Texture Profile Analysis (TPA), puncture test and compression/ extrusion test analysis were performed directly only on PBF and KFG samples, since the specific probes used to measure them could not be obtained from trimmed fat (PTF). The characteristics of three samples, ground at 4 and 8 mm of diameter, were evaluated by Kramer shear test. All analyses were performed at room temperature (22  C). TPA was performed as described by Bourne (1978). Ten cores (diam. ¼ 20 mm, height ¼ 20 mm) were axially compressed to 30% of their original height. A time of 5 s was allowed to elapse between the two compression cycles. Load cells (49 or 150 N) were used at a crosshead speed of 1 mm/s using a TA.XT2i Stable Micro Systems Texture Analyser (Stable Microsystems Ltd., Surrey, England). Attributes were calculated as follows: hardness (Hd) ¼ peak force (N) required for first compression; cohesiveness (Ch) ¼ ratio of active work done under the second compression curve to that done under the first compression curve (dimensionless); springiness (Sp) ¼ distance (mm) the sample recovers after the first compression; chewiness (Cw) ¼ Hd  Ch  Sp (N mm). Measurement was carried out ten times.

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Puncture test was performed on samples cut approximately 6 cm long, 5 cm wide and 2 cm thick. These were placed on a support with clamps to hold the sample firmly and an orifice in the centre to accommodate a cylindrical stainless steel plunger of 3 mm diameter used for the penetration test. A 49 N load cell was used. The penetration force was exerted at 15 mm using a TA.XT2i Stable Micro Systems Texture Analyser (Stable Microsystems Ltd., Surrey, England). The corresponding force-penetration curves were obtained at 0.8 mm/s crosshead speed and analysed. Penetration force (N) was the maximum force during penetration and gel strength (J) was defined as the area enclosed by the force-deformation curve at the rupture point. Determinations were carried out six times. Compression/extrusion tests were carried out using a miniature Kramer shear/Ottawa cell. A compression plate was used to perform the compression/extrusion analysis. Pieces of sample were cut approximately 2.5 cm long, 2.5 cm wide and 1 cm thick. This sample amount was enough to fill the cell to approximately 50% of its capacity. A 150 N load cell was used. The force was exerted at 50% deformation at 2 mm/s crosshead speed using a TA.HD.plus Stable Micro Systems Texture Analyser (Stable Microsystems Ltd., Surrey, England). Maximum force (extrusion force, N) and work of extrusion (N s) as total area under the curve values provide an indication of sample consistence. Determinations were carried out six times. Kramer shear test was performed using a miniature Kramer shear/Ottawa cell. A 5-bladed head was used to perform a shearing test. Kramer shear tests were carried out on ground samples (4 and 8 mm). The sample amount (5 g) was enough to fill the cell to approximately 50% of its capacity. A 49 N load cell was used. The force was exerted to a compression distance of 28 mm at 2 mm/s crosshead speed using a TA.XT2i Stable Micro Systems Texture Analyser (Stable Microsystems Ltd., Surrey, England). Kramer shear resistance values were calculated as the maximum force per g of sample (N/g). Measurements were carried out six times. All textural analyses were carried out using Texture Exponent software version 4.0.9.0. (Stable Microsystems Ltd., Surrey, England). 2.7. Dynamic rheological properties Dynamic rheological experiments were conducted using a controlled-stress Bohlin CVO-100 rheometer (Bohlin Instruments Ltd., Gloucestershire, UK). The measurement system was a circular plate geometry PP20 (20 mm diameter) with a 1 mm gap in all samples. A thin film of vaseline oil (Codex purissimum) was gently applied to the edge of each exposed sample in order to prevent moisture losses. Samples were allowed to relax for 5 min before conducting rheological measurements such as equilibration time after loading the sample on the sensor system. Temperature control was carried out with a Peltier Plate system (
40 to þ180  C; Bohlin Instruments, Gloucestershire, UK). The linear viscoelastic region was determined for each sample through stress sweeps at 0.16 Hz. After that, a dynamic frequency sweep was conducted by applying a constant pre-determined stress within the linear region, over a frequency range between 0.1 and 10 Hz for each sample. The measurements were made at 22  C. A temperature sweep was conducted from 10 to 90  C at a heating rate of 1  C/min using a frequency of 0.1 Hz and a constant strain of 0.5%. Parameters including complex modulus (G*), storage modulus (G0 ), loss modulus (G00 ), and the phase angle (d) were obtained using the analysis program software. Results reported were averages of at least three measurements. These rheological analyses were conducted on PBF and KFG. Trimmed pork fat (TPF) structural characteristics make it impossible to obtain a representative sample to properly evaluate rheological behaviour in the experimental conditions.

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2.8. Differential scanning calorimetry (DSC) The melting/solidification process of the two fats (PBF and PTF) was monitored by DSC using a previously-calibrated differential scanning calorimeter (Q1000 Differential Scanning Calorimeter, TA Instruments, New Castle, DE, USA). Two temperature ranges were used trying to display the thermal behaviour of the fats after either refrigeration (temperature range from 0 to 55  C), or complete solidification (from
40 to 55  C). The fats were encapsulated in hermetic pans in amounts of about 10e15 mg (0.002, by double weighting) using an electronic balance (Sartorious ME235S, Goettingen, Germany). Three samples per fat were used in each type of simulation experiment. A single scanning range from
40 to 100  C was used for konjac gel (KFG) analysis, also by triplicate. The samples were cooled (10  C/min) to their respective starting temperatures (0 or
40  C), held there for 5 min for crystallization and then heated (10  C/min) to the final temperature. Thermal properties of the fats were studied in first and second heating scans. Enthalpy data were corrected for respective water-freezing effects (ice melting endotherm subtraction) to refer results to fat content (ΔH, J/gfat). Temperatures (T,  C) and melting enthalpies (ΔH, J/g) were generally recorded within 0.5  C and 5% respectively. 2.9. Statistical analysis The normal distribution of samples was checked using the ShapiroeWilks test. When samples fitted the normal distribution, one-way ANOVA analysis was performed. When samples did not fit the normal distribution, the KruskaleWallis test was used to test the null hypothesis that the medians of variable within each of the levels of samples were the same. Statistical analysis was carried out using Statgraphics Plus version 5.0. 3. Results and discussion 3.1. Water and fat binding properties and pH Both pork fats and the fat analogue are used to formulate cooked meat products, and therefore water and fat binding properties during thermal treatment are of interest in meat processing. Heating loss (water and fat loss) in native pork backfat (PBF) was less than 1%, while in ground PBF (8 mm) it was as high as 76.7%, much higher than in PTF (27.6%) in comparable conditions. The behaviour of KFG was different, in that irrespective of the state (intact or ground) heating loss (water and fat loss) was less than 1% . The fat analogue thus presented excellent thermal water binding properties despite containing very high proportions of water (around 90%). Lin and Huang (2008) also reported elevated waterholding capacity in varying molecular-weight konjac gels (heat-set gel). This means that addition of KFG to meat products entails no reduction in processing yield. However, the effect of substituting konjac gel for animal fat (pork backfat or lamb fat) was dependent on the proportion of substitution (Jiménez-Colmenero et al., 2010; Kao & Lin, 2006; Osburn & Keeton, 2004). This behaviour is attributed both to the nature of the meat matrix that is formed (Jiménez-Colmenero et al., 2010) and to the premature melting of the hydrocolloids and the release of bound water from the konjac

gel during cooking at 68  C (Osburn & Keeton, 2004). In our experiment, however, the thermal water binding stability of the konjac gel was not indicative of developments of that kind. No differences (P > 0.05) were observed in the pH values of PBF (6.67  0.36) or PTF (6.60  0.03). Konjac gel showed higher (P < 0.05) pH values (9.01  0.10) than pork fats, within the ranges reported for konjac/gellan gum mixed gels (Huang & Lin, 2004). 3.2. Mechanical properties The mechanical properties of foods are important to how consumers perceive their textural characteristics (Bourne, 1982). PBF showed lower (P < 0.05) springiness and cohesiveness than KFG (Table 1). In addition, PBF presented considerably higher consistency, as evidenced by TPA (hardness and chewiness), puncture test (penetration force and gel strength) and extrusion test (extrusion force and work of extrusion) (Tables 1 and 2). The observed differences between intact backfat and fat analogue were generally smaller after grinding (Table 3), suggesting that PBF suffers a major reduction in the mechanical properties upon grinding. In ground samples, Kramer shear values of KFG were greater (P < 0.05) than in PBF at 4 mm, but lower (P < 0.05) than in PBF at 8 mm. These values were highest (P < 0.05) in PTF irrespective of the degree of grinding. Differences in the consistency of pork fats have been attributed, among other factors, to their fatty acid composition (Davenel, Riaublanc, Marchal, & Gandemer, 1999; Glaser et al., 2004), since fats with higher unsaturated fatty acid contents generally have softer consistencies (Hugo & Roodt, 2007). It has been reported that inter- and intra-muscular fat contains smaller proportions of monounsaturated fatty acids (MUFAs) and higher proportions of PUFAs than removable fat deposits (e.g. backfat) (Raes et al., 2004). In our experiment since PTF showed higher Kramer shear values than PBF, the differences in the mechanical properties can be related mainly with other adipose tissue constituents, such as protein (the highest in PTF). The levels of structural disintegration of fats differ depending on the type of meat product, but in many cases pork fat properties in terms of suitability for use in meat products have been evaluated on intact backfat, using a puncture test to determine the consistency of the adipose tissue (Davenel et al., 1999; Glaser et al., 2004). This test takes into account all the components of adipose tissue including fat (fatty acid composition), collagen and water (Davenel et al., 1999; Glaser et al., 2004). Fat consistency has also been measured by compression test (TPA); showing a pronounced effect of dietary fat on backfat hardness and chewiness (López-Bote, Isabel, & Daza, 2002). The penetration force of native pork backfat measured in this study (Table 2) was comparable with the results of other studies (Glaser et al., 2004), but our results (Tables 1e3) also show that mechanical properties depended considerably on the type of fat and the degree of disintegration. This suggests that better procedures are needed for assessment of the technological suitability of fats (quality), especially for new formulations (reducing or replacing animal fat) in meat processing. The data available on the mechanical properties of konjac gels used as fat analogues in meat product formulation are very limited, including information of the kind analysed in this paper. There have been reports on the influence of several factors (pH, added gums,

Table 1 Textural profile analysis (TPA) of pork backfat (PBF) and konjac gel (KFG). Samples

Hardness (N)

Springiness (mm)

Cohesiveness (dimensionless)

Chewiness (N mm)

KFG PBF

2.38  0.41a 34.41  2.99b

0.96  0.01a 0.44  0.05b

0.90  0.01a 0.35  0.04b

2.05  0.37a 5.31  0.64b

Means  standard deviation. Different letters in the same column indicate significant differences (P < 0.05).

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Table 2 Penetration force, gel strength and extrusion test parameters of pork backfat (PBF) and konjac gel (KFG). Samples

Penetration force (N)

Gel strength (J)

Extrusion force (N)

Work of extrusion (J)

KFG PBF

1.29  0.16a 7.72  0.08b

7.21  1.07a 59.88  3.39b

76.33  6.91a 480.44  70.06b

151.24  12.53a 1274.57  79.45b

Means  standard deviation. Different letters in the same column indicate significant differences (P < 0.05).

etc.) on textural properties of konjac flour gels (Huang & Lin, 2004; Lin & Huang, 2008), but the findings are not comparable with those of the present experiment owing to differences in the formulation and preparation conditions of the gels (heat-set gels). Nevertheless, it is useful to determine the characteristics of these kinds of gels given that the effect of replacing pork backfat with KFGM generally varies according to the way it is added (konjac blend or gel, prehydrated, dry) and the proportion of fat replaced (Chin et al., 1998a; Jiménez-Colmenero et al., 2010; Kao & Lin, 2006; Lin & Huang, 2003; Osburn & Keeton, 2004). KFG has been used to replace pork backfat (reducing the fat content by over 15%) without noticeable changes in sensory quality of frankfurters (JiménezColmenero et al., 2010). Similarly, Osburn and Keeton (2004) reported that this type of konjac gel can be used (10 and 20%) in low-fat (8%) lamb sausage with only minor changes in product characteristics, including textural attributes. These findings contrast with the differences observed between the mechanical properties of pork fats and fat analogue (Tables 1e3), where values of PBF and konjac gel were closer to one another than to PTF.

3.4. Rheological analysis

3.3. Colour The colour of intact PBF had higher (P < 0.05) L*, a* and b* values than intact KFG (Table 4). Except for b* value, the colour parameters (L* and a*) were similar (P > 0.05) in intact and ground KFG (Table 4). No differences (P > 0.05) in colour parameters were observed as affected by the grinding process (4 and 8 mm), which is why Table 4 shows only a single data (average of the two grinding levels). Both types of ground pork fats had similar (P > 0.05) L* and a* values, while yellowness values were higher (P < 0.05) in PFT. The colour differences between ground pork fats were small despite the differences in their composition (with higher protein content in PFT than in PBF). As in the case of KFG, except for b* value, colour parameters (L* and a*) were similar (P > 0.05) in intact and ground pork fats (Table 4). Fat analogues were generally darker and less red than pork fats (Table 4). It has generally been reported that addition of konjac gel to reduced-fat frankfurters has little influence on colour perception (Kao & Lin, 2006; Lin & Huang, 2003). However, when compared with full-fat frankfurters, in reduced-fat products L* and b* generally tended to decrease and a* to increase with the addition of konjac gel (Kao & Lin, 2006; Lin & Huang, 2003). These findings are consistent with those reported in this experiment (Table 4). The contribution of pork fats (and the fat analogues used as fat replacers) to meat product colour varies depending on the

Table 3 Kramer shear resistance of pork backfat (PBF), pork trimmed fat (PTF) and konjac gel (KFG) as affected by structural disintegration. Samples

KFG PBF PTF

product’s characteristics. Whereas in coarse-ground products fat and lean can be distinguished by the naked eye, the further the meat structure is broken down, the more difficult this becomes, until the distinction is replaced by a more general perception of colour (Jiménez-Colmenero, 1996). The nature of PBF and PTF is such that the way in which can contribute technologically to meat product formulation and colour differs. For instance, the possible range of particle sizes (from coarsely ground to finely comminuted) is more limited in the case of PTF. Fat levels also affect meat product colour parameters, so that when fat content is reduced, the product is darker and redder than high-fat product (Carballo, Fernández, Barreto, Solas, & Jiménez Colmenero, 1996). What this means is that among the factors that influence the contribution of fat to the appearance of meat products (one of the prime factors motivating consumer choice), it is necessary also to consider those associated with changes in the relative proportions of the different types of fat. This can help to gain a clearer understanding of the consequences of reformulation processes and to formulate better fat analogues for use in low-fat meat processing.

Kramer shear resistance values (N/g) 4 mm

8 mm

3.72  0.25c 2.33  0.04b 7.03  0.90d

5.17  0.56a 8.26  0.87b 52.22  17.92c

Means  standard deviation. Different letters in the same column indicate significant differences (P < 0.05).

It is essential to understand the rheological behaviour of ground/comminuted meat systems not only for the design of machines, processes and operations but also for the control of product characteristics and consumer acceptability (Rizvi, 1981). Therefore, it is very useful to have information on components and new ingredients. 3.4.1. Frequency sweep test for konjac gels and pork backfat Dynamic frequency tests were carried out within the limits of viscoelastic range to determine the frequency dependence of complex, storage and loss moduli (G*, G0 and G00 , respectively) of the PBF and KFG (Fig. 1). The rheological modulus values were modelled following power law as suggested by Friedrich and Heymann (1988): *

G* ¼ An un

(1)

0 G00 un

(2)

0

G ¼

00

G00 ¼ G000 un

(3)

Table 4 Colour parameters of pork backfat (PBF), pork trimmed fat (PTF) and konjac gel (KFG) as intact and after ground (at 4 and 8 mm). a*

b*

Samples (intact) KFG 61.71  0.44b PBF 75.05  1.27c

L*

-0.41  0.08a 2.57  0.06c

-3.53  0.18b 3.11  0.07c

Samples (ground) KFG 62.46  0.63b PBF 74.10  1.45c PTF 74.40  1.13c

-0.28  0.15a 3.11  0.41c 4.08  0.61c

-2.09  0.48a 1.83  0.28c 5.97  0.50d

Means  standard deviation. Different letters in the same column for each type of samples (intact and ground) indicate significant differences (P < 0.05). No differences (P > 0.05) in colour parameters were observed as affected by grinding process (4 and 8 mm), which is why this table shows only a single figure (average of the two grinding levels).

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100000

120000

10000

G´ PBF G´´ PBF G´ KFG G´´ KFG

1000

G´G´´ (Pa)

G´G´´ (Pa)

100000 80000

G´ PBF G´´ PBF G´ KFG G´´ KFG

60000 40000 20000 0

100 0,10

1,00

10,00

100,00

0

20

40

60

80

100

Temperature (°C)

ω (rad/s) Fig. 1. Variation of the storage modulus (G0 ) and loss modulus (G00 ) as a function of the angular frequency (u) as determined in the frequency sweep tests (at 25  C) in pork backfat (PBF) and konjac gel (KFG).

The model of Friedrich and Heymann (1988) was used to describe the three-dimensional structure characterizing a gel in terms of An, which is related to the overall stiffness or resistance to deformation within the linear viscoelastic region at an angular frequency (u) of 1 rad/s. This parameter can be used to assess the firmness of gels (Campo & Tovar, 2008). The power law exponent, n*, can be used as a convenient measure of the relative viscoelasticity of a gel; it should be near-zero for gels exhibiting an ideal elastic behaviour (i.e., no frequency dependence) and increase with increasing viscoelasticity (Zhou & Mulvaney, 1998). Table 5 shows the values of the gel stiffness parameter and exponent n* for the samples studied. As can be seen, An is much higher for PBF than for KFG; therefore PBF was firmer and stronger than KFG, which is consistent with their mechanical properties (Tables 1e3). The larger value of An is indicative of strong particle-particle interactions and/or the network type structure in a stabilized form (Carreau, Cotton, Citerne, & Moan, 2002; Hirashima, Takasashi, & Nishinari, 2004). A mechanical spectrum can be also expressed in terms of the storage modulus (G0 ) and loss modulus (G00 ) as a function of frequency (Eqs. (2) and (3)). With respect to G0 0 and G00 0, it can be identified with resistance to elastic and viscous deformation respectively at 1 Hz, and the phase angle (d), represented by the ratio of G00 and G0 , gives a relative measure of associated energy loss vs energy stored in a cyclic deformation. A phase angle of 90  C represents a fully viscous material, while a fully elastic material is characterized by a zero phase angle. Table 5 shows the fitted parameter values for Equations (2) and (3) and the phase angle for pork backfat and konjac gels. (G0 0
G00 0) can be used as a new measure of gel strength, consistent with the previous definition of An (Campo & Tovar, 2008). A good gel is known to exhibit the same proportional change in G0 and G00 with frequency over a wide range; in other words, n0 and n00 must be identical (Park, 2000). Based on the values of Table 5, the difference (n0
n00 ) was highest in PBF, indicating greater loss of structural stability with time and greater structural discontinuity but stronger (>G*, G0 0). This finding is consistent with the fact that the phase angle was greater in PBF than in KGF (Table 5), which would indicate less elasticity or greater

Fig. 2. Dynamic storage (G0 ) and loss (G00 ) modulus for pork backfat (PBF) and konjac gel (KFG) as determined in the temperature sweep tests.

viscosity of pork backfat, possibly a consequence of higher fat and lower water content. The behaviour of KFG is consistent with the mechanical spectra typical of polysaccharide gels, in which G0 is always much greater (around 10 times) than G00 and is practically independent of the frequency applied over a wide range (Copetti, Grassi, Lapasin, & Pricl, 1997). The differences in the viscoelastic behaviour of PBF and KFG samples could be due to the differences in composition noted earlier, and hence to different types of molecular interactions. For instance, pork backfat is considered a “hard” fat from the adipose tissue of the dorso-lumbar region of the animal. Its high consistency at 25  C is a consequence of all the components of the adipose tissue, above all its high SFA content (Ospina-E et al., 2010), and also to the level of collagen and the low water content. On the contrary, konjac gel composition is characterized by high water content (>90%) and lower polysaccharide content. Also, it has been reported that in the case of polysaccharide hydrogels like konjac gels, the crosslink involves aggregates of molecular chains in helix formation. The bonds involved are generally non-covalent, such as hydrogen bonds, hydrophobic or ionic bonds. In general, the good characteristics observed in konjac gel (multicomponent gel) appear to be the result of combination of a konjac glucomannan with other biopolymers such as iotacarrageenan and pre-gelled cornstarch, and the possible effect of these components (synergism in the case of iota-carrageenan) on the mechanical and rheological behaviour of the thermally stable gels formed after deacetylation (along with calcium hydroxide). 3.4.2. Temperature sweep test for konjac gel and pork backfat Comparative analysis of the rheological behaviour of fat analogues and pork backfat during thermal treatment is useful as a means of better understanding the behaviour of these ingredients in reformulation processes. Fig. 2 shows the temperature sweep for konjac gel and pork backfat in terms of the thermal behaviour of G0 and G00 modulus from 10 to 90  C. G0 values were higher than G00 values in KFG over the entire experimental range. There were practically no changes in the elastic or viscous moduli at temperatures under 50e60  C. The main rheological changes in KFG took place above 60  C, where

Table 5 Fitted values for the rheological parameters as determined in the frequency sweep test for pork backfat (PBF) and konjac gel (KFG). Samples

G*

G0 n

PBF KFG

*

An (kPa s )

n

741  0.23 11.2  0.081

0.061  0.001 0.052  0.001

Means  standard deviation.

0

G00 n0

0

G 0 (kPa s )

n

696  0.199 11.19  0.08

0.059  0.001 0.052  0.001

G

00

d ( ) 0

n00

(kPa s )

253  0.123 1.06  0.007

n

00

0.078  0.001 0.044  0.001

21.14 5.30

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there was a gradual and slight increase in G0 associated with the formation of a rather stiff elastic matrix structure. The slightness of these rheological changes is consistent with the fact that the konjac gels formed (upon addition of alkaline coagulant) were thermally stable (even up to temperatures in excess of 200  C) (Kohyama, Iida, & Nishinari, 1993; Nishinari, Williams, & Phillips, 1992; Thomas, 1997; Tye, 1991). However, an increase of konjac gel strength has been reported with rising temperature (Fig. 2) (Tye, 1991). The behaviour of PBF was completely different, with much higher elastic and viscous moduli than in KFG at temperatures below 30  C (Fig. 2). However, the thermal pattern of G0 and G00 from 30 to 90  C was very similar for both types of samples. Also, in the case of pork backfat the elastic modulus was much higher than the viscous modulus, and both presented the same behaviour throughout the temperature range. These differences in the thermal behaviour of PBF and KFG at less than 30e40  C are consistent with the differences found in their mechanical properties (Fig. 1, Table 5) and may be due to the differences in their structure and composition. For instance, while the pork backfat (adipose tissue) includes fat, collagen, protein and a small amount of water (Davenel et al., 1999; Glaser et al., 2004), KFG contains high proportions of water (>90%) and small amounts of polysaccharides. The drastic decrease of both moduli in pork backfat up to temperatures of around 30e40  C is consistent with the melting of PBF analysed below by DSC and shown graphically in Fig. 3. These results suggest that the behaviour of konjac gel, which was thermally stable in the experimental conditions (up to 90  C), was predominantly elastic during thermal treatment. Also, its behaviour was similar to that of the animal fat at temperatures commonly used in cooked meat processing. The characteristics of the konjac gel were such as to suggest that it could feasibly be used as a fat replacement in thermally-treated meat products.

3.5. Thermal properties The normalized DSC traces for fat content in the PBF and PTF melting process in the above-zero range are shown in Fig. 3a and c respectively. Obviously only glycerides with high melting points were visible. They presented continuous melting (solid lines) with a main peak centred at around 32  C (PBF) and 30  C (PTF), followed by a small peak at around 44  C in both cases. Fusion enthalpy values (ΔH) by straight baseline integration between 10 and 50  C were 58.5 and 54.8 J/gfat for PBF and PTF respectively (very close together). In both kinds of fat second scans (dash lines) produced somewhat less crystallization and smaller temperature (29 and 31  C) and melting intervals, as well as enthalpy values (52.3 and 50.8 J/gfat). The melting process is also shown in Fig. 3b and d, where the amounts of solid phase still present in the samples (Solid Fat Index, SFI, calculated by partial areas integration) are graphically represented as a function of the melting temperature. Like the DSC traces, the melting evolution was similar for both fats, with sigmoidal patterns and nearly 80% of solid phase reduction at around 35  C, declining thereafter up to final melting at around 50  C. In calorimetric terms, PBF (Fig. 3b, solid line) was somewhat “harder” than PTF (Fig. 3d, solid line). Second scans showed relatively “softer” samples (dash lines) in all cases, with very similar solid fat evolution on melting. These results compared well with those reported by Ospina-E et al. (2010) in a RMN study between 10 and 40  C on the melting characteristics of PBF with different fatty acid compositions. On the other hand, these results do not necessarily contradict corresponding Kramer shear resistance values (Table 3) since PBT had greater protein and residual contents than PBF. When fats were studied at sub-zero temperatures by solidifying at
40  C, the melting interval ranged from ca.
20 to 50  C (Fig. 4a for PBF and 4c for PTF; first scans, solid lines; second scans,

a

Heat Flow (W/gfat) Endo Down

Heat Flow (W/gfat) Endo Down

c

0.1

0

10

20

30

40

50

0.1

0

60

10

Temperature (ºC)

b

20

30

40

50

60

50

60

Temperature (ºC)

d

100

100

80

Solid Phase (%)

80

Solid Phase (%)

69

60

40

60

40

20

20

0

0 0

10

20

30

40

Temperature (ºC)

50

60

0

10

20

30

40

Temperature (ºC)

Fig. 3. Melting process at above-zero temperatures for pork backfat (PBF) and pork trimmed fat (PTF). PBF: a, normalized (W/gfat) typical DSC traces; b, solid phase (%) as a function of melting temperature ( C). PTF: c, normalized (W/gfat) typical DSC traces; d, solid phase (%) as a function of melting temperature ( C). First scans, solid lines; second scans, dash lines. Bars indicate ordinate scales.

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dash lines). In addition to the already known fusion zones of higher melting glycerides, in all cases there was a large area with maximum temperatures at around 0e1  C. Total enthalpy of fusion reached 115 and 100 J/gfat for PBF in first and second melting processes, which are nearly 15% higher than previously reported values for a previously dehydrated PBF sample (Barreto et al., 1996). The evolution of the solid phase coexisting with the melted phase is shown in Fig. 4b as a function of the melting temperature; with few differences between the two PBF processes. It is worth mentioning that the solid and dash curves in Fig. 4c displayed much larger and sharper endotherms in the lower temperature zones for PTF than for PBF. This was mainly due to the PTF much bigger ice melting peak reflecting the high water content of these encapsulated samples (25e30%) relative to the low content (w7%) of PBF samples. However, when PTF enthalpy data were corrected to factor in that effect, it was possible to

estimate a rough melting pattern (Fig. 4c, dot-dash line) for the PTF-water free sample, which compared satisfactorily with those of PBF in Fig. 4a (where the humidity was so low that there was no need of any correction within the experimental margin of error). Total enthalpy of fusion (ΔH, J/gfat) for the PTF fat alone then came close to the PBF values, and the anomalous SFI patterns in Fig. 4d were also corrected (dot-dash line) to approximate them to those of PBF fat (Fig. 4b). As expected, heat capacity data of PBF and PTF fats at both pre- and post-transitions temperature zones (flat traces) evolved linearly, around 2.1 and 3.2 J/g K respectively for PBF, and 1.9 and 2.8 J/g K respectively for PTF. These values were consistent with corresponding water contents of the fats. The upper flat traces can be safely extrapolated linearly up to 100  C, as evidenced experimentally (not shown), which assessed that both fats were thermally stable in the current working region of temperatures.

Fig. 4. Melting process from below-zero temperatures for pork backfat (PBF) and pork trimmed fat (PTF), and thermal data of konjac gel (KFG). PBF: a, normalized (W/gfat) typical DSC traces; b, solid phase (%) as a function of melting temperature ( C). PTF: c, normalized (W/gfat) typical DSC traces; d, solid phase (%) as a function of melting temperature ( C). First scans, solid lines; second scans, dash lines; fat-alone PTF, dot-dash lines. KFG: e, normalized (W/g) typical DSC trace between
40 and 100  C. Bars indicate ordinate scales.

F. Jiménez-Colmenero et al. / Food Hydrocolloids 26 (2012) 63e72

Thermal study of the formation process of the konjac gel was not feasible since the deacetylated konjac flour gelled too fast to allow analysis. DSC data of the finished konjac gel (KFG) is shown however in Fig. 4e. It can be seen a reversible (first and second scans) single endothermic transition corresponding to ice melting with ΔH w 316 J/g, consistent with a water content averaging w95% in the encapsulated samples. DSC traces at high temperatures were flat indicating that current KFG gel was thermally stable at least up to 100  C, and that its heat capacity evolved linearly at around 4.0 J/ g K, w20% higher than PBF value because of the much higher water content of KFG. All these thermal data indicated also that PBF or PTF fats substitution by KFG gel in meat formulations may likely be advantageous concerning processing energy consumptions.

4. Conclusions Reformulation process with modifications in the amounts and relative proportions of different type of fats (subcutaneous, interand intra-muscular) and addition of fat analogues are used in fat reduction strategies in meat product. These ingredients affect the characteristics of the meat matrix, and therefore a comparative study of their technological, mechanical and rheological properties will help to elucidate their role in the protein matrix structure and to improve the quality of healthy meat-based food systems to which they are added. Pork backfat and trimmed fat differ in properties such as texture, thermal stability, colour and rheological or melting behaviour. Since both types of fat sources are implicated in the quality attributes of meat products, it is important to take their characteristics into account when choosing a fat analogue. Technological properties and gel characteristics of konjac gel varied with the degree of structural disintegration, and the differences between konjac gel and pork backfat were smaller when samples were ground. This konjac gel is especially suitable as a pork backfat replacer in fat reduction processes.

Acknowledgment This research was supported under projects AGL2008-04892CO3-01 and the Consolider Ingenio CSD2007-00016, Ministerio de Ciencia y Tecnología.

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