The total solids and fat contents of nine whole liquid egg products were assessed by means of electrical and thermal conductivity measurements. Linear correlations between conductivity values and total solids and fat contents were obtained with R2 values up to 0.995 and 0.990 and maximum errors of predic- 46 TABLE 6 - Linear regression models for the total solids (TS, %) and fat (FC, %) contents estimation from electrical (Ec, mS/cm) and thermal (Tc, W/mK) conductivity measurements. SE: Standard Error. Linear regression equation R2 p-level SE (%) TS=-6.857*Ec+65.373 0.995 0.000 0.27 FC=-4.993*Ec+40.070 0.985 0.000 0.33 TS=-172.967*Tc+109.605 0.992 0.000 0.33 FC=-126.449*Tc+72.521 0.990 0.000 0.27 tion up to 0.41% and 0.42%, respectively. The electrical conductivity of the albumen, yolk and egg mixtures linearly increased with temperature; at 20°C, temperature coefficients of 2.1%/°C, 2.1%/°C and 1.9 %/°C were respectively calculated. On the contrary, the thermal conductivity of the analysed products did not appear to be substantially influenced by the temperature of the liquid

Adriano Guarnieri

Annachiara Berardinelli

Chiara Cevoli

Luigi Ragni

Journal of Agricultural Engineering

English

The total solids and fat contents of nine whole liquid egg products were assessed by means of electrical and thermal conductivity measurements. Linear correlations between conductivity values and total solids and fat contents were obtained with R2 values up to 0.995 and 0.990 and maximum errors of prediction up to 0.41% and 0.42%, respectively. The electrical conductivity of the albumen, yolk and egg mixtures linearly increased with temperature; at 20°C, temperature coefficients of 2.1%/°C, 2.1%/°C and 1.9 %/°C were respectively calculated. On the contrary, the thermal conductivity of the analysed products did not appear to be substantially influenced
by the temperature of the liquid

BERARDINELLI, ANNACHIARA

RAGNI, LUIGI

CEVOLI, CHIARA

GUARNIERI, ADRIANO

Archivio istituzionale della ricerca - Alma Mater Studiorum Università di Bologna

1. IntroductionAccording to Italian industry demands and foodformulation, whole liquid egg products used as ingre-dients are characterized by a total solids (TS) contentranging between 20% and 26%. The whole liquid eggproducts are generally obtained from the breakageand the subsequent homogenization of the egg con-stituents. As the content of solids in whole egg is af-fected by the yolk to white ratio and by the yolk andwhite chemical compositions [Washburn 1979], thehomogenized product should be corrected in order toproduce ingredients characterized by a certain totalsolids content value. The yolk to white ratio is in turnrelated to the egg size that increases with the age ofthe hen [Fletcher 1983] and both the yolk and whitetotal solids content can be influenced by the hen’sstrain [Ahn 1997].Since the albumen and yolk are composed of reallydifferent percentages of water and chemical compo-nents [Stadelman 1995], electrical [Żywica 2005] andthermal [Ahmed 2008] conductivity measurementscould be used to assess, rapidly and economically, thequality of whole liquid egg products in terms of totalsolids and fat contents. Electrical conductivity is a measure of the trans-mission of an electric current through a material;foods containing charged electrolytes are able totransmit an electric current in relation to their electri-cal resistance R value [Figura 2007]. In addition tothe concentration and type of the ions, electrical con-ductivity is strongly dependent on temperature; con-ductivity readings often refer to a specific tempera-ture, typically 20°C or 25°C, obtained by calculatinga temperature coefficient (θ) expressed as a conduc-tivity variation in %/°C [Barron 2007]. Furthermore,the cell characteristics and the applied current affectthe polarization at the electrodes, an accumulation ofionic species near the electrode surfaces, which maylead to erroneous readings [Radiometer AnalyticalSAS, 2004]. To date, electrical conductivity is widely used forwater control [ISO 7888:1985] and in the dairy indus-try for quality analyses of milk [Henningsson 2005]and cheese fermentation processes [Paquet 2000];electrical property measurements are also importantduring food processes which are based on Ohmicheating [Fryer 1993] and pulsed electric field treat-ments used for microbial inactivation of liquids suchas juice-milk based beverages [Sampedro 2007] andliquid egg products [Amiali 2006]. In particular, Ami-ali 2006, showed a positive linear correlation betweenelectrical conductivity and temperature for differentliquid egg products and for albumen and yolk.Similarly, thermal conductivity is a food propertynecessary for the setting-up of thermal treatment suchas refrigeration, freezing, pasteurization and heatingprocesses in order to avoid possible product damage[Ahmed 2008]. As regards the influence of tempera-ture, discordant data have been reported in the litera-ture: with the increase of the temperature, slight de-creases [Gut 2005; Coimbra 2006] and increases[Fontana 1999] of thermal conductivity were ob-served.Based on electrical and thermal conductivity meas-urements, the present research intends to assess the to-tal solids and fat contents of whole liquid egg productscharacterized by different percentages of albumen andyolk. The effect of the solution temperature on theelectrical and thermal properties was also studied.2. Materials and methodsA total of 60 brown eggs were collected from a lo-cal poultry farm one day after laying and the mainquality indices were assessed: mass (g), diameter(mm), height (mm), air cell height (mm), thick albu-men height (mm) and yolk index. The air cell heightwas obtained by averaging four measurements carriedJ. of Ag. Eng. - Riv. di Ing. Agr. (2011), 1, 41-47ASSESSMENT OF THE TOTAL SOLIDS AND FAT CONTENTS IN WHOLE LIQUID EGG PRODUCTS BY ELECTRICALAND THERMAL CONDUCTIVITY MEASUREMENTSAnnachiara Berardinelli, Luigi Ragni, Chiara Cevoli, Adriano Guarnieri___________Paper received 03.06.2010; accepted 20.10.2010ANNACHIARA BERARDINELLI PhD; LUIGI RAGNI, researcher; CHIARACEVOLI, PhD; ADRIANO GUARNIERI, full professor - Department ofAgricultural Economics and Engineering DEIAgra, University of Bo-logna, P.zza Goidanich, 60, Cesena FC. e-mail: annachi.berardinel-li@unibo.it  Tel. 0039.0547636005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 41out from the base of the shell to three equidistantpoints on the circumference described by the contactline of the membrane to the shell and in the middlepoint of the same membrane. The thick albumen wascalculated by averaging three measurements taken atdifferent points of the thick albumen at a distance of10 mm from the yolk. The yolk index was obtainedby dividing the height by the diameter of the yolk[Funk 1948].The albumen and yolk content coming from eachegg were respectively divided in two batches; eachbatch was then manually mixed very slowly and 9whole egg liquid products (300 ml each), character-ized by different percentages (in mass) of the twocomponents (Tab.1), were prepared using an analyti-cal balance (Sartorius, BL 120 S, Sartorius mecha-tronics, Italy) and homogenised (Ultra-Turrax, IKA,Germany). The percentages of white and yolk in themixtures were selected in order to cover, as much aspossible, the range of the total solids (TS) contentcharacterizing the commercial whole liquid egg prod-ucts prepared for the Italian food industry. Measure-ments were carried out both on the whole liquid eggproducts and the albumen and yolk samples.Electrical conductivity (mS/cm) was measured asan average of three replicates by means of a 4-poleplatinum cell (SPT06, DeltaOHM, Italy) containing atemperature sensor (Pt100), connected to a portableconductivity meter (HD2306.0, DeltaOHM, Italy).Before measurements, the device was calibrated bymeans of both standard solutions of 0.01 mol/l and0.1 mol/l of KCL, characterized, at 25°C, by electricalconductivity values of 1.413 mS/cm and 12.88mS/cm.Since the conductivity of egg solutions of elec-trolytes increases as the temperature increases, theconductivity values were obtained from 2 to 28°C(with steps of about 4°C) in order to assess the tem-perature coefficient (θ) usually expressed as a con-ductivity variation in %/°C. This coefficient was cal-culated at the reference temperature of 20°C. Thermal conductivity (W/mK) was determined as amean of three replicates using a specific needle probenamed KD2 (Decagon Devices, Inc., Pullman, WA,USA). This consists of a hand-held reader and a sin-gle-needle sensor that can be inserted into the mediumunder test; a short duration thermal pulse is applied tothe heater and the temperature of the thermocouple re-sponse is used to simultaneously determine the ther-mal conductivity and thermal diffusivity. The wholeliquid egg products and the albumen and the yolksamples were put into 11 graduated cylinders (50 ml)and the thermal conductivity was directly measuredby the probe at two temperatures, 0 and 20°C respec-tively. It was important to wait for about 5 minutesbetween successive readings, to allow thermal equi-librium.The total solids (TS) and the fat contents of the liq-uid egg products were assessed in triplicates accord-ing to the AOAC Official Method No. 925.30 [1990]and Folch [1957], respectively. Regression models for the prediction of the totalsolids (TS) and the fat contents were obtained fromboth electrical and thermal conductivity measure-ments conducted for the whole egg liquid products atthe reference temperature of 20°C (SPSS, version 15,USA).3. Results and discussionAveraged values of the analysed quality indices -mass (g), diameter (mm), height (mm), air cell height(mm), thick albumen height (mm) and yolk index -are shown in Table 2.The analysed samples were characterized by meanvalues of the total solids (TS) content of 11.9 % and50.3% respectively for 100% of albumen and 100% ofyolk (Tab. 3); the TS values for the mixtures rangedbetween 21.3% and 31.3% (mean step of 1.3%). Thefat content of the mixtures ranged between 8.4% and15.4% (mean step of 0.9%) and, it was 0.03% and30.4% for the albumen and yolk samples, respectively. Figure 1 shows the mean values of the electricalconductivity (mS/cm) of the whole liquid egg prod-ucts in the temperature range of 2°C - 28°C. As ex-pected, the highest electrical conductivity was meas-ured for the albumen (8.1 mS/cm at 20°C) and the42TABLE 1 - Percentage of egg white and yolk in the 9 analysed whole egg liquid products.1 2 3 4 5 6 7 8 9Albumen (%) 75 72 69 66 62 59 56 53 50Yolk (%) 25 28 31 34 38 41 44 47 50Whole liquid egg productsEgg component005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 42lowest for the yolk (2.6 mS/cm at 20°C). This behav-iour is mainly due to high levels of moisture contentand mineral concentration characterizing the eggwhite and to the yolk fat content that acts as an insula-tor [Amiali 2006]. The electrical conductivity of theanalysed whole liquid egg products (from 75% to50% of albumen) ranged, at 20°C, from 5 mS/cm to6.5 mS/cm. Differences between the electrical con-ductivity values of the egg mixtures increased withthe temperature; at 20°C a mean conductivity differ-ence of 0.18 mS/cm was observed between the ninewhole liquid egg products. The relationship between electrical conductivityand temperature follows a linear trend. The model re-gressions, the coefficient of determination R2 (up to0.9999) and the temperature coefficient θ20°C (%/°C)are shown in Table 4. According to the linear depend-ence, the temperature coefficient (θ) was calculatedby using the linear correction:where T and 20°C are the temperatures at whichelectrical conductivities EcT and Ec20°C were respec-tively measured. The θ20°C values reported reflect thebasic behaviour of the albumen, yolk and whole liq-uid egg products [Barron 2007].The results of the thermal conductivity measure-ments conducted at 0°C and 20°C are reported inTable 5. Also for this parameter, the moisture contentjustifies the higher values observed for the albumen[Ahmed 2008]. As reported in the literature [Gut43TABLE 3 - Total solids (TS) and fat contents (%) of the analysed wholeliquid egg products. *see table 1.TABLE 2 - Main quality parameters of the eggs used to prepare the wholeliquid egg products.(EcT – Ec20°C) × 100θ20°CEc20°C (T – 20°C)Mass (g)Diameter (mm)Height (mm)Thick albumen height (mm)Air cell height (mm)yolk indexQuality index Average SD6745604.94.40.457231.30.50.1112345678921.322.623.925.226.527.629.130.131.311.9 0.40 0.03 0.0050.020.030.090.020.090.230.060.120.060.030.040.200.020.050.010.080.130.0950.3 0.06 30.4 0.118.48.59.810.911.712.613.514.615.4YolkWhole liquidegg products*AlbumenAnalysed samplesTotal solids content(%)Fat content(%)Average SD Average SD005_Berardinelli(586)_41  28-03-2011  14:26  Pagina 432004; Coimbra 2006], the thermal conductivity valuesof the analysed samples were not greatly influencedby the temperature: a mean decrement of about 1%was measured passing from 0°C to 20°C. At 20°C, amean conductivity difference of 0.007 W/mK was ob-served between the nine whole liquid egg products,characterized by a range of 0.510 - 0.455 W/mK.The correlations of the total solids and fat contentswith the electrical and thermal conductivity valuesobtained at 20°C for the whole liquid egg products are44Fig. 1 - Electrical conductivity (mS/cm) of albumen, yolk and whole liquid egg products (*, see table 1) versus temperature (°C).TABLE 4 - Linear regression models and temperature coefficients for albumen, yolk andwhole liquid egg products. *See table 1; Ec: electrical conductivity (mS/cm); T: tempera-ture (°C); θ20°C: temperature coefficient (%/°C).Ec= 0.1644T+4.7561 0.9994 2.11 Ec= 0.137T+3.7277 0.9994 2.12 Ec= 0.1319T+3.6221 0.9990 2.13 Ec= 0.1263T+3.5116 0.9995 2.14 Ec= 0.1215T+3.3846 0.9998 2.15 Ec= 0.1201T+3.2139 0.9995 2.16 Ec= 0.1157T+3.1414 0.9997 2.17 Ec= 0.1102T+3.0912 0.9999 2.18 Ec= 0.1081T+2.993 0.9994 2.19 Ec= 0.1058T+2.8685 0.9998 2.1Ec= 0.0483T+1.6139 0.9923 1.9AlbumenWhole liquid egg  products*YolkRegression equation R2????C (%/¡C)Analysed samplesθ20°C( /°C)005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 44indicated in Table 6. This dependence was linear forboth dependent variables and conductivity measure-ments. For both electrical and thermal conductivities,the best models were linear with R2 values of 0.995and 0.992 for the total solids content, respectively; theR2 values of 0.985 and 0.990 for the prediction of thefat content, respectively, were slightly lower. The val-ues predicted by the models versus the observed val-ues of the two parameters for electrical and thermalconductivity are reported in Figure 2.According to the results in Table 6 and in Figure 2,the total solids content can be predicted with a maxi-mum error of 0.41% and 0.43% for electrical andthermal conductivity values, respectively; for the fatcontent, the maximum errors were 0.56% and 0.42%,respectively.4. ConclusionsThe results of the present research showed that theconductivity measurements carried out on the wholeliquid egg products are able to predict the total solidsand the fat contents with an acceptable error. Bothquality parameters can be estimated with linear mod-45TABLE 5 - Thermal conductivity (W/mK) of albumen, yolkand whole liquid egg products (*, see table 1) at 0°C and 20°C.0°C 20°C0.556 0.5491 0.518 0.5102 0.510 0.5053 0.505 0.4954 0.495 0.4885 0.485 0.4786 0.478 0.4727 0.472 0.4678 0.461 0.4599 0.457 0.4550.370 0.369Thermal conductivity (W/mK)Analysed samplesAlbumenWhole liquid egg  products*YolkFig. 2 - The values predicted by the models versus the observed values of the total solids and fat contents (%) from electrical and thermal conduc-tivity measurements.005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 45els characterized by R2 values ranging from 0.985 to0.995; the best model was obtained for the assessmentof the total solids content from the electrical conduc-tivity. A simple device based on these properties couldrepresent a useful and economic tool for the rapidquality evaluation of whole liquid egg products dur-ing their preparation and subsequent use by the foodindustry.As reported in the literature, the electrical conduc-tivity was greatly influenced by the temperature of theproduct, with a mean percentage increase in conduc-tivity of 2.1 (%) for a temperature change of 1°C; onthe contrary, the thermal conductivity did not appearto be substantially influenced by the temperature ofthe solution. The present research represents a first attempt tomeasure total solids and fat content of egg mixtures bymeans of electrical and thermal properties. Further in-vestigations could be carried out in order to improvethe predictive power of the models, by studying the ef-fect of different oscillating frequencies and voltagesthat affect the dissociation of the conductive solutions.ReferencesAhmed J., Rahman M.S., Thermal Conductivity Measure-ment of Foods in Food Properties Handbook, SecondEdition. 2008, Taylor & Francis Group, LLC.Ahn D.U., Kim S.M., Shu H., Effect of egg size and strainand age of hen on the solids content of chicken eggs.Poultry Science, 1997, 76, 914-919.Amiali M., Ngadi M.O., Raghavan V.G.S., Nguyen D.H.,Electrical conductivities of liquid egg products and fruitjuices exposed to high pulsed electric fields. Internation-al Journal of Food Properties, 2006, 9 533-540.AOAC, Association of Official Analytical Chemists. Offi-cial method No. 925.30. Gaithersburg, 1990, MD20877-2417, USA.Barron J.J., Ashton C., The effect of temperature on con-ductivity measurements. Technical papers 2007,(http://www.reagecon.com/techpapers.shtml).Coimbra J.S.R., Gabas A.L., Minim L.A., Garcia RojasE.E., Telis V.R.N., Telis-Romero J., Density, heat capaci-ty and thermal conductivity of liquid egg products. Jour-nal of Food Engineering, 2006, 74, 186-190.Figura L.O., Teixeira A.A., Electrical properties. In FoodPhysics, 2007, Springer-Verlag Berlin Heidelberg.Fletcher D.L., Britton W.M., Pesti G.M., Rahn A.P., SavageS.I., The relationship of layer flock age and egg weighton egg component yields and solids content. PoultryScience, 1983, 62 1800-1805.Folch J., Lees M., Stanley G.H.S., A simple method for theisolation and purification of total lipids from animal tis-sues. Journal of Biological Chemistry, 1957, 226, 497-509.Fontana A.J., Varith J., Ikediala J., Reyes J., Wacker B.,Thermal properties of selected foods using a dual needleheat-pulse sensor. ASAE Meeting Presentation, 1999.Toronto, Ontario Canada.Fryer P.J., de Alwis A.A.P., Koury E., Stapley A.G.F.,Zhang L., Ohmic processing of solid-liquid mixtures:heat generation and convention effects. Journal of FoodEngineering, 1993, 18, 101-125.Funk E.M., The relation of the yolk index determined innatural position to the yolk index, as determined afterseparating the yolk from the albumen. Poultry Science,1948, 27, 367.Gut J.A.W., Pinto J.M., Gabas A.L., Telis-Romero J., Con-tinuous pasteurization of egg yolk: thermophysical prop-erties and process simulation. Journal of Food ProcessEngineering, 2005, 28, 181-203.Henningsson M., Östergren K., Dejmek P., The electricalconductivity of milk-the effect of dilution and tempera-ture. International Journal of Food Properties, 2005, 8,15-22.ISO 7888:1985 “Water Quality – Determination of Electri-cal Conductivity”.Paquet J., Lacroix C., Audet P., Thibault J., Electrical con-ductivity as a tool for analysing fermentation processesfor production of cheese starters. International DairyJournal, 2000, 10, 391-399.Radiometer Analytical SAS. Conductivity Theory and Prac-tice, 2004, D61M002 France.Sampedro F., Rivas A., Rodrigo A., Martínez A., RodrigoM., Pulsed electric fields inactivation of Lactobacillusplantarum in an orange juice-milk based beverage: ef-fect of process parameters. Journal of Food Engineering2007, 80, 931:938. Washburn K.W., Genetic variation in the chemical compo-sition of the egg. Poultry Science, 1979, 58, 529-535.Żywica R., Pierzynowska-Korniak G., Wójcik J., Applica-tion of food products electrical model parameters forevaluation of apple purée dilution. Journal of Food Engi-neering, 2005, 67, 413-418.SUMMARYThe total solids and fat contents of nine whole liq-uid egg products were assessed by means of electri-cal and thermal conductivity measurements. Linearcorrelations between conductivity values and totalsolids and fat contents were obtained with R2 valuesup to 0.995 and 0.990 and maximum errors of predic-46TABLE 6 - Linear regression models for the total solids(TS, %) and fat (FC, %) contents estimation from electrical(Ec, mS/cm) and thermal (Tc, W/mK) conductivity measure-ments. SE: Standard Error.Linear regression equation R2 p-level SE (%)TS=-6.857*Ec+65.373 0.995 0.000 0.27FC=-4.993*Ec+40.070 0.985 0.000 0.33TS=-172.967*Tc+109.605 0.992 0.000 0.33FC=-126.449*Tc+72.521 0.990 0.000 0.27005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 46tion up to 0.41% and 0.42%, respectively.The electrical conductivity of the albumen, yolkand egg mixtures linearly increased with temperature;at 20°C, temperature coefficients of 2.1%/°C, 2.1%/°Cand 1.9 %/°C were respectively calculated. On thecontrary, the thermal conductivity of the analysedproducts did not appear to be substantially influencedby the temperature of the liquid.  Keywords: whole liquid egg products, total solidscontent, fat content, electrical conductivity, thermalconductivity.47005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 47005_Berardinelli(586)_41  22-03-2011  19:12  Pagina 48

Assessment of the total solids and fat contents in whole liquid egg products by electrical and thermal conductivity measurements

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ASSESSMENT OF THE TOTAL SOLIDS AND FAT CONTENTS IN WHOLE LIQUID EGG PRODUCTS BY ELECTRICAL AND THERMAL CONDUCTIVITY MEASUREMENTS

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ASSESSMENT OF THE TOTAL SOLIDS AND FAT CONTENTS IN WHOLE LIQUID EGG PRODUCTS BY ELECTRICAL AND THERMAL CONDUCTIVITY MEASUREMENTS

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