Rheological characterization of egg yolk custard developed by isostatic high pressure

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1 Rheological characterization of egg yolk custard developed by isostatic high pressure V.M. GÉLVEZ 2 and R. PLA 1 * 1 Planta de Tecnologia dels Aliments (CeRTA; XIT), Departament de Ciència Animal i dels Aliments, Facultat de Veterinària, Universitat Autònoma de Barcelona Bellaterra, Barcelona, Spain 2 Facultad de Ingenierías y Arquitectura, Universidad de Pamplona (N.S) Colombia *reyes.pla@uab.es Keywords: egg yolk; high pressure; food innovations Summary In recent years high pressure technology has received considerable attention as a food preservation method, but many studies have shown that most proteins undergo denaturation and may reform into a modified conformation. There is evidence that these changes modify the functional properties of egg components. The egg yolk proteins undergo aggregation and changes in structure following pressure treatment results in new rheological properties. From this point of view, the application of pressure to egg proteins may be a contribution for innovation and research of new foods. The aim of this study was to characterize rheologically the egg yolk custard developed by application of high pressure (500, 600 and 700 MPa at 20ºC during 20 min) and to compare it with that treated by heat (80ºC/20 min). The formulation (30% egg yolk, sugar, powdered milk, ι-carrageen and water) at ph 7.0 showed a viscoelastic behaviour that was measured by oscillatory, creep and recovery tests also flow behaviour was analysed. The frequency sweep test revelled that samples displayed true gel-like behaviour, such that G is higher then G but this value no increased when pressure increased, a similar trend was found for yield stress. The creep and recovery analysis showed that samples treated to 600 MPa showed the lowest values for Compliance and those treated at 500 or 700 MPa had the highest ones. The flow curve was analysed by Herschel-Buckley model. The samples showed shear-thinning behaviour and changes in the consistency. In conclusion, egg yolk custards with similar rheological characteristics to custards processed by heat can be obtained by means of high pressure processing. The rheological characterization showed that all the samples treated by pressure had true gel behaviour, and the custard treated at 600 MPa was very similar to the heated one. Introduction Hen s egg yolk can be found in a number of food products such as mayonnaise and salad dressings, bakery products and custards, etc., being an ingredient of high nutritional value with unique sensory characteristics and excellent functionality. Thus, the preparation and stability of yolk-based products depend on a large extent on yolk constituents functionality, or to interact, following a heat treatment, and form network structures exhibiting viscoelasticity in products such as creams and omelettes. Various researchers have reported the applications of isostatic high pressure (IHP) in various areas of food processing like inactivation of microorganisms and enzymes, denaturation and functionality of proteins and structure change of food materials. The proteins can be denatured, coagulated or gelled, depending on various factors like ph, ionic strength, type of protein, temperature and pressure applied. Knowledge of the rheological properties of food products is essential for the product development, quality control, sensory evaluation and design and evaluation of the processing equipment. The flow behaviour of a specific material can vary from Newtonian to time dependent non-newtonian in nature depending on its origin, composition and structure behaviour and previous history (Rao, 1986). Several researchers have studied the rheological characteristics of eggs and reported Newtonian as well as time-dependent non Newtonian flow behaviour of egg (Cornford et al., 1969; Ibarz et al., 1999). More recently, Lee et al. (1999) studied the effect of IHP pressure on whole egg rheology in terms of excess work of structure breakdown using steady shear analysis. Ponce et al. (2000) studied the texture profile analysis of egg white and liquid whole egg gels treated by IHP. Ahmed et al. (2003) 20

2 evaluated the effect of IHP in the apparent viscosity and thixotropy of liquid whole egg, egg albumen and egg yolk. Gelvez et al. (2003) analyzed the rheology of Crema Catalan- like custard induced by IHP (500 and 600 MPa) to different ph (5.5 and 7.0). They observed that this gels showed non Newtonian and tixotropic behaviour and shear-thinning properties. The aim of this study was to characterize rheologically the egg yolk custard developed by application of high pressure. Material and Methods RAW MATERIAL AND PREPARATION OF SAMPLES Skimmed milk powder, sugar and fresh eggs were purchased in a local supermarket. The egg yolk and chalazas were separated by hand. Formulations were prepared with 30% of egg yolk and 6% skimmed milk powder, 1.5% sugar and 0.6% ι-carrageenan (CP Kelco, Terrasa, Spain) in water. All ingredients were mixed with a Heidolph mixer (Germany). The ph was adjusted to 7.0 with NaOH (O.5 N) and/or HCl (3 N). COMPOSITION AND PH ANALYSIS The dry matter and ash content of samples were determined in concordance with AOAC (1990) methodology, protein content (N*6.25) by Kjeldahl s method, fat content by Soxhlet s method with petroleum ether as solvent (after hydrolyzing the sample with a 2 M HCl solution) and the carbohydrates were calculating by difference. The ph was measured with a Crisson ph meter (Crison Instruments, S.A, Alella, España) HEAT AND PRESSURE TREATMENTS Samples were put inside a non-edible cellulose casing (Nojax; Viskase, Bagnolet, France) of 2.2 cm diameter. The heat treatment induce was performed in a water bath at 80ºC for 20 min. Pressure treatments (500, 600 and 700 MPa for 20 min) were performed in a discontinuous isostatic press (Stansted Fluid Power; LTD, Stansted, UK) at 40ºC. All treatments were done in triplicate and samples were kept at 4ºC until analysis the day after the treatment. RHEOLOGICAL STUDY Pressurized and heated samples were allowed to temperate at room temperature, then they were carefully taken out of the cellulose casing and were cut into disks of 0.5 cm in height. Rheological measurements were made with a Rheometer RSI (Thermo Haake, Germany) equipped with a PP35 Serrated (two parallel plates) measuring head, the distance between plates was 1.00 mm. Three replicates from each sample were analyzed. The following curves were obtained: Flow curves: with controlled shear rate (CSR) in the range s -1 at 20 ± 1 C. The experimental flow curves were modelled using the Herschel-Bulkley model: (τ = τ c +k.y n ), τ c = yield value (Pa), k = viscosity coefficient (Pa.s n ), Á= shear rate (s -1 ) and n = flow index (n < 1 corresponds to a shear thinning behaviour, n > 1 corresponds to a Newtonian behaviour). Dynamic rheological properties: two types of tests were conducted. Stress sweep test: was first conducted at frequency of 1 Hz to determine the linear viscoelastic region in the range Pa. Two sets of data are derived: these are the storage modulus G, which is the elastic portion of the sample and the loss modulus ( G ) which is the viscous portion. G > G : The viscous deformation behaviour predominates over the elastic behaviour. The test sample shows a fluid-like character also in a state at rest. G < G : The elastic deformation behaviour predominates over the viscous behaviour. The structure shows certain stiffness. Frequency sweep test: was conducted to characterize the degree of viscoelasticity of the samples. The rheological properties (G and G ) are measured as a function of frequency (1.0 and 10 Hz) at constant stress amplitude (8 Pa). 21

3 Creep and recovery test: were made at stress 8 Pa during 120 s. The obtained creep compliance J (t) was analyzed by the Maxwell-Voigt model using the following equation: J (t) = 1/E H + 1/E V (1 exp(-t/τ)) + t/η N (τ = η V / E V ) Where E H is the elastic modulus of a Hookean body (Pa); E V is the elastic modulus of the Voigt body (Pa), τ is retardation time(s), η V is viscosity of the Voigt body (Pa.s) and η n is the viscosity of the Newtonian body (Pa.s) STATISTICAL ANALYSIS Analysis of variance was performed using the General Linear Models procedure of SAS (SAS Institute, Cary, NC). LSMEANS test was used to obtain comparisons among sample means. Significance level was set at p< Results and discussion COMPOSITION AND PH The composition and ph of samples are shown in the following Table. Parameters Value Moisture % ± 0.05 Protein % 7.28 ± 0.15 Fat % 9.74 ± 0.14 Ash % 0.93 ± 0.03 Carbohydrates (by difference) 4.35 ph 6.63 FLOW BEHAVIOUR Flow curve Flow curves (Figure 1) of heat and pressure treated samples were established by expressing Log shear stress (Pa) vs. Log shear rate (1/s). Flow curves were analyzed with the Herschel-Buckley model (τ = τ c +k.y n ), the different parameters are given in Table 2. This model can be used when n 1 and τ c >0 (Anton et al., 2001, Barbosa-Cánovas et al., 1983). The regression delivers a correlation r 1 or a very small value for ch 2 in all cases indicating that the equation fitted adequately well Log [Pa] LogÁ [1/s] 500 MPa 600 MPa 700 MPa Herschel- Buckley Figure 1 Flow curve analyzed by the Herschel-Buckley model. 22

4 Table 2 Modelling of the curves for a shear rate between 10 and 500 s -1. Treatment τ ο (Pa) k n R MPa c ± a ± MPa b ± d ± MPa b ± c ± C / 20 min a ± b ± *Means within a column with no common superscript differ significantly (p < 0.05) All samples showed a non-newtonian behaviour (n< 1). As pressure increases, the consistency coefficient (k) decreases and the yield stress (τ o ) increases. The yield stress value is higher in the heated sample, but the flow index is lower. The flow index increased as the pressure increased at 600 MPa but decreased slightly as the pressure increased at 700 MPa. Anton et al. (2001) observed that emulsions prepared with 0.5, 2.5 and 5.5% egg yolk at ph 7.0 presented a slightly lower flow index as the pressure increased (200 or 500 MPa) whatever protein concentration. Ahmed et al. (2003) found that egg yolk pressurized between 100 and 400 MPa had higher τ o and k (for a residence time of 30 min) for higher pressures. They assumed that the structural changes in high density lipoproteins could be one of the reasons. Viscoelastic properties Amplitude sweep In the oscillatory test all samples showed a shear-thinning behaviour, the elastic modulus (G ) is higher than the viscous one (G ), the material behaves more like a solid; that is, the deformations are essentially elastic or recoverable MPa 600MPa 700MPa G' [Pa],G" [Pa] [Pa] ThermoHaake RheoWin Pro 2.94 Figure 2 Determination of the Linear Viscoelastic region. For a constant frequency of 1 Hz and a stress between 0.05 to 300 Pa, the Linear Viscoelastic (LVE) region was determined, all samples showed LVE approximately for three decades (Figure 2), in this region the applied stresses and deformations are reversible. The heat-treated sample showed higher storage modulus than the pressure treated ones, and the samples treated at 600 MPa were the more elastic. 23

5 Frequency sweep test The Figure 3 illustrates the data obtained in the frequency sweep test, the storage and loss moduli exhibited a slight increase when frequency increased, which was similar to the previous test. In all samples, G is higher than G over the ω analyzed, and though both modulus (G and G ) were higher in heat samples than in the pressure ones, but the values for gels formed at 600 and 700 MPa were very close. These results are in agreement with Ibarz et al. (1999), they found that the egg yolk gels obtained by heat presented higher values of G and G compared with those obtained by pressure at 410, 480, 550, 620, 650 and 680 MPa, and treatment times from 1 to 30 min, furthermore the gels tended to collapse at higher pressure and longer treatment times due to denaturation of proteins. Based on the frequency sweep data, we found that the heat and pressure treated samples are true gels because a gel has a three dimensional network structure and shows over frequency range a higher elastic proportion; G is higher than G and is almost independent of frequency (ω). On the other hand, from a structural point of view, the linear regression of log ω versus log G and log G showed zero slope and independence on frequency for true gels: Table 3 contains the magnitudes of slopes (n and n ) and intercepts (k and k ) in the following equations G = k (ω) n G = k (ω) n Log G' [Pa],G" [Pa] MPa 600 MPa 700 MPa ThermoHaake RheoWin Pro 2.94 Log ω [rad/s] Figure 3 Plot of frequency sweep test. Table 3 Magnitudes of intercepts (k, k ) and slopes (n, n ) from linear regression of Log ω vs Log G y G. Treatment G (Pa) G (Pa) k n R k n R MPa a a MPa a a MPa a a C / 20 min a a 0.99 *Means within a column with no common superscript differ significantly (p < 0.05) We found that all samples presented a slight slope, which are statistically equal and therefore the heat induced gels and pressure induced gels are true gels. 24

6 Creep and recovery test Figure 4 show the creep and recovery test of heat and pressure treated samples; in the Table 4 are summarized the data for this test. Creep and recovery analysis confirmed the results of the dynamic test. The zero shear viscosity is a viscous response that indicates that the samples have Newtonian properties at low shear rates over a limited range. The heat sample presented higher zero shear viscosity value than the pressure treated samples and the 600 MPa sample showed the highest Newtonian properties. The compliance (J o ), that is connected with the reciprocal of the elastic modulus of a Hookean body (J 0 = 1/G 0 ), is lower when pressure increases but is equal for samples treated at 600 and 700 MPa. The gel formed at 500 MPa is more rigid and less fluid than other gels because a greater part of the structure is not recovered due to the bonds between structural units are broken MPa 600 MPa 700 MPa 0.05 Â [-] t [s] ThermoHaake RheoWin Pro 2.94 Figure 4 Curves of creep and recovery test. The gel formed by heat showed less deformation than the pressure ones. This is according with studies done by Hayashi (1989) who proved that egg yolk gels induced by pressure at 5000 to Kg/cm 2 were softer than gels induced by heat and resisted to break down. Okamoto et al. (1990) found significant differences in appearance and textural properties between pressure and heat induced gels of egg yolk, the gel formed at 4000 kg/cm 2 kept a solid shape. However, it was very soft and adhesive. Harder gels were obtained at 4500 kg/cm 2 and 5000 kg/cm 2 but they had a lower adhesiveness. Doumoulin et al. (1998) obtained firm gels of egg yolk treated at 500, 400 and 300 MPa at 10, 25 and 50 C for 30 min; all gels were unbreakable by compressing manually between fingers. Table 4 Results of creep and recovery test. Parameters 500 MPa 600 MPa 700 MPa 80 C / 20min Zero shear viscosity 5.17E+04 ± 757 d 1.06E+05 ± 3394 b 8.60E+04 ± 3783 c 2.23E+05 ± 9405 a Compliance J o 6.88E-03 ± 3.25E- 3.93E-03 ± 5.01E- 2.29E-03 ± 3.42E- (Pa 1 ) 04 a b 4.55E-03 ± 1.97E-04b c Elastic modulus (G 0 ) ± 6.8 c ± 33 b ± 10 b ± 66 a Elastic deformation (γ) 0.055± a 0.031± b 0.036± b 0.018± c *Means within a variable with no common superscript differ significantly (p < 0.05) 25

7 Conclusions It is possible to obtain a new food product by pressure (500, 600 and 700 MPa during 20 min at 40 C) with a high proportion of egg yolk, warranting the microbiological quality and a high nutritional value. The rheology of mixes (egg yolk 30%, skimmed milk powder 6%, sugar 1.5%, ι-carrageenan 0.6% and water, ph 7.0) showed non-newtonian behaviour and shear-thinning properties (n > 1) and all formed true gels. The samples treated at 600 and 700 MPa were not statistically different, and had similar rheological behaviour to the mix treated at 80ºC / 20 min. References AHMED, J., RAMASWAMY, H.S., ALLI, I. and NGADI, M. (2003) Effect of high pressure on rheological characteristics of liquid egg. Lebensmittel Wissen und Technologie 36: ANTON, M., CHAPLEAU N., BEAUMAL V., DELEPINE S. and DE LAMBALLERIE-ANTON M. (2001) Effect of high-pressure treatment on rheology of oil-in-water emulsions prepared with hen egg yolk. Innovative Food Science & Emerging Technologies 2: 9-21 BARBOSA-CÁNOVAS G. V. and PELEG M. (1983) Flow parameters of selected commercial semiliquid food products. Journal of Texture Studies 14: CORNFORD, S. J., PARKINSON, T. L. and ROBB, J. (1969) Rheological characteristics of processed whole egg. Journal of Food Technology 4: DUMOULIN, M., OZAWA, S. and HAYASHI, R. (1998) Textural properties of pressure-induced gels of food proteins obtained under different temperatures including subzero. Journal Food Science 63: GÉLVEZ V. M., MOR-MUR M., YUSTE J. and PLA; R. (2003) Development of a Catalan-like custard by high pressure processing. Quality of eggs and eggs products. III. Proceedings of X th European symposium on the quality of eggs and eggs products. Saint-Brieuc, France: HAYASHI, R. (1989) Application of high pressure to food processing and preservation: philosophy and development. Engineering and food, (Eds.) W. E. L. Spiess y H. Schubert.2 Elsevier Applied Science, London, United Kingdom: 815 IBARZ, A., SANGRONIS, E., BARBOSA-CANOVAS, G.V. and SWANSON, B.G. (1999) Textura de los geles de huevo obtenidos por alta presión. Food Science. and Technology International 5: LEE, D-U., HIENZ, V. and KNORR, D. (1999) Evaluation of processing criteria for the high pressure treatment of liquid whole egg: rheological study. Lebensmittel Wissen und Technologie 321: OKAMOTO, M., KAWAMURA, Y. and HAYASHI, R. (1990) Applications of high pressure to food processing textural comparison of pressure and heat induced gels of proteins. Agricultural Biological Chemistry 54 1: PONCE, E. (2000) Aplicación de alta presión hidrostática en ovoproductos. Tesis Doctoral. D. Ciencia animal y de los Alimentos. Universidad Autónoma de Barcelona. Bellaterra. 26