Physico-chemical characteristics of ground meat relevant for patty forming and end product quality

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1 Physico-chemical characteristics of ground meat relevant for patty forming and end product quality A. JURGENS 1 *, J.D. DE MOOIJ 2, H. LOGTENBERG 1 and T.J. VERKLEIJ 1 1 TNO Quality of Life, P.O. Box 360, 3700 AJ Zeist, the Netherlands 2 Stork Titan B.V., P.O. Box 233, 5830 AE Boxmeer, the Netherlands *jurgens@voeding.tno.nl Keywords: moulding; rheology; thermal analysis; texture; meat dough Summary Relevant final product quality factors for chicken meat burgers after frying and during consumption are for instance shape, shrinkage and bite. These parameters are not only determined by meat composition but also by the different processing steps. The interacting effects of process conditions and meat composition are extremely important for defining the final product properties. In the present work the relationship between patty formation, physical characteristics of chicken meat dough and final product quality will be demonstrated. The release of free protein during chopping was determined by Near Infra Red (NIR) spectroscopy and related to meat dough elasticity. Thermal and mechanical properties of the meat dough were examined by respectively Differential Scanning Calorimetry (DSC) and rheological characterization, performed in the same temperature ranges as applied during patty forming (varying from -6 C till +5 C). The effects of processing on ground meat structure were examined by light microscopy. Final quality of oven-heated products was examined by Texture Profile Analysis (TPA). TPA and microscopy results showed recovery of the dough structure within the first hours after forming. However, part of the deterioration that occurred after low temperature (-8 C) moulding appeared irreversible. A strong relation between temperature and final texture due to moulding was found. This can be explained by the thermal profile during melting (DSC) in combination with the dough mechanical characteristics. The results show, that a critical temperature range exists, where small variations in temperature may lead to a significant variation in hardness and cohesiveness. Products heated from meat doughs formed at lower temperatures (-8 C) showed a lower hardness and cohesiveness. During dough rest the mechanical behaviour of the dough, formed at relatively low temperatures, recovered. However, cohesiveness of heated products after forming at relatively low temperatures was always significantly lower than that of products being formed at relatively higher temperatures. Introduction Control of product characteristics and quality, and efficient adaptation to changes in desired quality through dedicated processing are important issues for the industry. Product quality is finally judged by the consumer. Understanding of consumer liking may often be obscure. However, major aspects of consumer appreciation comprise visual appearance like colour and shape, textural aspects like bite and chewing properties, taste and smell. These characteristics depend on composition as well as on processing conditions. Composition of the burgers comprises the type of meat selected as the raw material. This results in a preferred combination of different proteins, fat and water and additives like e.g. salts and hydrocolloids. Processing involves steps like chopping, mixing, grinding, moulding, meat dough rest, forming, second rest and (pre-) heating, cooling, packaging and storage. An often neglected aspect in process optimization is the interrelation between raw material properties and processing with regard to quality. An extensive overview was given by Mikkelsen (1993). However, most studies in this context have been published on the relation of texture, cooking loss and microbial safety with the heating process, e.g. Erdogu et al. (2005) and Zorrilla et al. (2000). The relation between the forming process and product did not receive much attention, however. The present work relates, in more detail, to the relation between the physico-chemical properties of the meat dough, the thermal conditions during the forming process and dough rest after forming on final physical texture of poultry meat burgers after heating. The aim is to understand the different factors that dominate the quality aspects mentioned. 151

2 Materials and methods Experimental work was based on meat doughs using simple basic formulations containing poultry meat, starch, water and salt according to compositions presented in Table 1. Table 1 Meat compositions "5% fat" "15% fat" (%) (%) Breast fillet Thigh meat Skin Water Starch Salt The formulations according to the Table varied in fat content between 5 and 15%. MEAT PREPARATION, STORAGE AND PATTY PREPARATION Meat doughs were prepared on a 40 kg scale. Chicken breast fillet and thigh meat were ground over 13 mm plates, while chicken skin was ground over 2 mm plates at a temperature of 4 C. Salt was added to the meat mixture, followed by the formulation water (0 C) while mixing in a cutter for 5 minutes. Next the skin and starch were added following another 10 minutes of mixing, while the temperature was kept at 4 C. The meat was divided and stored frozen at temperatures between 0 and -8 C for at least 1 day to maximum 4 days. Within each test series storage time and temperature were always kept constant. A number of additional tests were performed with varying cutter time, from 1 till 13 minutes to examine free protein release in relation to dough elasticity. Patties were formed at temperatures of 0, -2.5, -6 and -8.5 C by use of a hydraulic press and a mould of 100 mm in diameter. Patties were formed with different heights depending on the subsequent tests. Patties prepared for heating and end product testing were 12 mm high. Those prepared for rheological testing of the meat dough were 5 mm high, whereas those intended for texture analysis of the doughs had a height of 15 mm. Effective pressure on the dough during forming was 4 bars during 5 seconds. Household foil was used to avoid sticking to the mould. HEATING A standardised method was used for reproducible heating of the burger samples. Each patty was treated outside with soy oil and packed in aluminium foil. Heating took place in a Fessman hot air oven, set at 90 C and an RH of 100% during 30 minutes. Under these conditions a core temperature of about 70 C was reached. PHYSICAL TESTING Physical dough characteristics were determined by differential scanning calorimetry (DSC), dynamic rheological testing and texture analysis. DSC was performed with a TA Instruments 2920 calorimeter by scanning from -40 C up to +20 C with a scanning rate of 2 C/min. Meat dough samples were homogenised by milling under cryogenic conditions using liquid nitrogen to keep the meat completely frozen. About 15 mg samples were taken to determine the heat flow during heating. Dynamic rheological testing on meat dough was performed by using a Carri-Med CSL type stress controlled rheometer. This rheometer was equipped with serrated parallel flat plate geometry with a diameter of 40 mm and an ETM type temperature control unit using liquid nitrogen. The gap setting was 5 mm and an oscillatory shear was performed with a fixed amplitude of deformation of 10-3 (-) and a frequency of 1 Hz. The amplitude of deformation was selected well within the linear region which extends up till an amplitude of deformation of A temperature sweep was recorded from - 10 till +5 C at a rate of 1 C/min., during which the storage-(g ) and loss-(g ) moduli were recorded. Samples were applied at a preset temperature of the plates of -10 C. Free protein in meat dough was 152

3 determined with a Technicon Infralyser 400 near infrared spectrometer using a calibration method analogous to that of Oh and Groβklaus (1994). Heated burgers were tested using a Stable Micro Systems type TA-XT21 type texture analyser using a flat disc steel probe with a diameter 100 mm. A texture profile analysis (TPA) test was performed, which comprises two subsequent compressions. Probe speed was 1 mm/sec and a maximum deformation of 40%. The TPA test is well known in texture analysis and a number of parameters can be derived for which the reader is referred to Szczesniak (1973). As relevant parameters hardness, i.e. the maximum force (Pa) during the first compression, and cohesiveness were selected. Cohesiveness is the ratio of the compression energy during the second compression over the energy during the first compression. At each process condition, i.e. temperature during forming, TPA tests on heated samples were performed on 15 products in a first test series determining the effect of temperature of forming, and 8 products in the test series determining the effect of dough rest on texture. The hamburger structure was studied using an Olympus SZX 9 binocular microscope. Results and discussion MEAT DOUGH CHARACTERISTICS Figure 1 shows the release of free protein in doughs, containing 15% fat, prepared with different cutter times as determined by NIR and the resulting elastic modulus (G ) in relation to the free protein measured. Free Protein (%) G' (Pa) Cutter time (min) Free Protein (%) Figure 1 Free Protein determined by NIR as a function of cutter time (left) and resulting meat dough elasticity at 20 C (right). The results clearly show the effect of cutter time with protein release and its effect on the elasticity of the dough. Dough elasticity was found to vary by 40% depending on the cutter time as a result of protein release. The main effect found on final product quality with increasing free protein concentration was an increasing shrinkage. Shrinkage is expressed here as the difference in diameter of the heated hamburger (d burger ) with the diameter of the dough piece (d patty ) direct after forming: d=(d burger -d patty )/d patty *100 (%). We found the following relationship between shrinkage and free protein concentration: d=-0.28*c protein (%), in which the protein concentration is expressed in percentage free protein as found by NIR. In the following tests the cutter time kept constant at 5 minutes, resulting in a free protein concentration of 11.2±2.0%. The mechanical behaviour of the meat doughs, prepared with a standard cutter time of 5 minutes, as a function of temperature is presented in Figure 2. This Figure shows the storage (elastic) moduli and the ratio of the loss (viscous) modulus tan(δ) of the doughs during heating in the rheometer. These doughs contained 5 and 15% fat and had been stored at -6 C. The storage- or elastic modulus is a measure of the elastic rigidity of the dough at very small deformation. The results of the doughs stored at -2 C (not shown) were not significantly different, indicating that storage temperature over the short storage time applied, did not affect the structure. The general profiles of elasticity and tan( δ) vs. temperature of all products were independent of fat concentration and dough storage temperature. 153

4 The differences between the two fat concentrations was relatively small compared to the effect of temperature during defrost. Within the range of -10 till about -7 C the structures were too rigid to determine the modules. From -6 C till -4 C the storage modulus weakly decreases, while the loss angle, presented as tan(δ) =G /G, slowly increases from 0.1 up to 0.2. The loss angle is a measure of the ratio of elastic and viscous behaviour of the sample. The lower the value of tan(δ), the higher the elastic behaviour. Values below 1 indicate that elastic behaviour dominates over viscous loss. Below -4 C the dough behaves as an elastic solid with highly dominating elastic behaviour. Above -4 C the elastic modulus strongly decreases until a temperature of about 2 C is reached, Between -4 C and -3 C tan(δ) strongly increases up to values of about This sudden increase in tan(δ) indicates a strong change in structural behaviour with more viscous loss. Above 2 C the elastic modulus decreases very slowly. 1.E+08 G' (Pa) tan(delta) = G"/G' E+07 G' 5% fat' G' 15% fat tan(delta) 5% fat tan(delta) 15% fat E E E E temperature ( C) Figure 2 Storage (elastic) moduli and ratio tan(δ), of 5 and 15% fat containing meat doughs, stored at -6 C, during a temperature sweep from -6 till +5 C. Figure 3 presents the heat flow during heating of a meat dough sample stored containing 5% fat in the temperature range from -15 till +10 C. Included in this Figure is the elasticity (G ) of the same meat dough heat flow (mw) G G' (Pa) 1.00E heat flow 1.00E E E E temperature ( C) Figure 3 Heat flow determined by DSC during melting of a meat dough sample with 5% fat and the elasticity of the sample dough determined by dynamic rheology. 154

5 Dominating in the thermogram is the melting of ice. Frequently only the peak temperature of the melting is recorded, but the results show that the complete melting profile is relevant to the elasticity of the dough (see Figure 3), final product texture, discussed below, and therefore for optimal processing. Note that melting already starts at very low temperatures and the absolute onset of melting is hard to detect since the deviation of the horizontal baseline is difficult to identify. This melting behaviour can be understood on the basis of the freezing process and the concept of freeze concentration. When cooling down to below 0 C the initial freezing point will be determined by the amount of solutes in the water phase causing a freezing point depression. Once this freezing point is reached ice may be formed, though in practice some delay in freezing will occur, known as super cooling. Ice will be formed from pure water molecules with the result that the remaining unfrozen mass is higher in solids content than the original composition. This causes a further freezing point depression of the remaining mass. Upon further cooling this will continue and the last bit of ice formation will occur far below the initial freezing point. Upon heating some melting will occur already at very low temperatures. Some of the water is physically bound, mostly to hydrophilic sites of the starch and protein. Integration of the melting peak results in the total heat of melting. Combining with the literature value for the heat of melting of ice, 334 J/g, provides the possibility to calculate the amount of frozen water in the sample. Comparison with the total water content in each sample delivers the amount of bound or unfreezable water. In two types of meat doughs, one containing 5% and the other 15% fat the concentration of unfreezable water found in this way was respectively 13.2 and 13.4%, while the amount of free (ice forming) water was respectively 54.8 and 59.6%. Freezing characteristics, as presented here, dominate product quality and are therefore considered relevant to other type of formulations. HEATED PRODUCT TEXTURE Hardness- and cohesiveness values obtained from the TPA tests of the heated burgers (15% fat) formed at different temperatures are presented in Figure 4. Hardness (Pa) Cohesiveness (-) temperature during forming ( C) temperature during forming ( C) Figure 4 Hardness and cohesiveness of heated meat burgers prepared from patties formed at different temperatures. Error bars indicate the (95%) standard error obtained from 15 repeats. From the results it is obvious that the internal product structure is more weakened by forming at lower temperatures. This can be understood from the rheological and thermal characteristics of the doughs presented above. At lower temperatures those products contain more ice so deformation during forming therefore requires much more stress (energy) compared to forming at higher temperatures. Higher stresses will cause more deterioration of internal bonds which apparently do not recover within the timeframe of dough rest between forming and heating. 155

6 DOUGH REST In order to study the effect of dough rest in more detail, the rheology of doughs formed at -8 C and at 0 C were compared within 15 minutes after the forming process. Figure 5 shows an example of the change in tan(δ), i.e. ratio between viscous loss and elastic storage modulus, with time when measured at 12 C. Note that these experiments could not start earlier than 15 minutes after forming. This time was needed for the core of the samples to achieve the desired temperature. A higher tan(δ) indicates more viscous like behaviour, while a more elastic like behaviour is indicated by lower tan(δ) values. tan(δ) (0) C C time (min) Figure 5 Tan(δ) vs. dough rest time of doughs formed at 0 and -8 C. While the dough formed at 0 C hardly showed any change in the value of tan( δ) with time. Tan(δ) of the dough formed at -8 C decreased strongly within the first 20 minutes of measurement and more slowly thereafter until it became equal in value to the dough formed at 0 C. This occurs after about 100 minutes of measurement. This suggests a complete recovery of the structure of the dough formed at -8 C, at least comparable to that of the dough formed at 0 C after about 1 to 2 hours. At several dough rest times after forming, products were heated to study the effect of rest time on texture. Hardness and cohesiveness, determined with the TPA test, of heated products after different dough rest times is presented in Figure Hardness (Pa) 0.75 Cohesiveness (-) C C C C dough rest time (min) dough rest time (min) Figure 6 Hardness (left) and Cohesiveness (right) of heated products prepared from doughs formed at temperatures of 0 C and -8 C respectively and with various dough rest times before heating. Error bars indicate the (95%) standard error of 8 repeats. We note that the absolute values of hardness and cohesiveness differ from values obtained in an earlier experiment. Partly this could be due to batch to batch variations; the present results were obtained with another stock of meat. Nevertheless we observed lower hardness when patties were formed at lower temperatures, except after about 1000 minutes (about 17 hrs!) dough rest. The low 156

7 Ground meat product processing and physics: A. Jurgens et al. value of the 0 C product after 1000 minutes rest time might be a result of experimental error, since we did not expect any changes on the basis of the mechanical dough behaviour during rest and on stability of cohesiveness during dough rest time. Cohesiveness was also lower for the lower temperature at forming and showed to be independent of dough rest time. These results show that an irreversible structure breakdown occurs during forming at a low temperature of -8 C, which was a bit unexpected when this is compared to the recovery of the mechanical behaviour, tan(δ) presented in Figure 4. This is not yet fully understood. In Figure 7 shows two examples of the centre of burgers heated from doughs formed at 0 and -8 C respectively and with a dough rest of 4 hrs. 0 C -8 C 5 mm Figure 7 Images of heated products prepared from doughs formed at respectively 0 C and -8 C after 4 hrs dough rest. The product formed from dough at -8 C showed more inhomogeneity in structure than the product heated from dough formed at 0 C. Over all it was observed that products heated from doughs formed at -8 C showed a much more crumbly texture with inhomogeneities smaller than about 1 mm. Upon a long dough rest of 18 hrs the structure became smoother. Since fracture occurs more easily when inhomegeneity of the structure is larger, this supports the observation that products formed at the lower temperatures have a lower hardness. Conclusions The mechanical behaviour of the poultry meat doughs processed within the temperature range of 10 C till 5 C, and the texture of heated products was dominated by the melting profile. Three principal temperature ranges could be identified: 1. T<-4 C, in which the dough behaves as a rigid mass which is mechanically difficult to deform C<T<0 C, in which the consistency of the dough is subject to a rapid change, due to a strong change in melting. This is a critical range since fluctuations in temperature will lead to strong variations in final product texture. 3. T>0 C above which the mechanical dough characteristics vary much less. In this study, doughs became too soft to perform the forming process above about 0 C. Thermal analysis of the melting profile of the doughs tested showed to be in good relation to the mechanical dough properties and the forming process at the different temperature ranges. Variations in formulation will change the mechanical and thermal characteristics. We have shown that a variation from 5 to15% fat lowered the consistency slightly but did not change the main behaviour. Varying temperature during forming showed to have a significant effect on the final product texture after heating. This effect was most apparent in changes in cohesion in the product, i.e. the breakdown of structure during mechanical work as demonstrated by TPA analysis. It was shown that more breakdown of dough structure occurred at lower temperatures during forming. During dough rest the mechanical behaviour of the dough, formed at relatively low temperatures, recovered. However, cohesiveness of heated products after forming at relatively low temperatures was always significantly lower than that of products being formed at relatively higher temperatures. 157

8 References Erdogu, F., Zorilla, S.E., Singh, R.P. (2005), Effects of different objective functions on optimal decision variables: a study using modified complex method to optimize hamburger cooking., Food Sci. and Techn., 38, Mikkelsen, V.L., (1993), Hamburger patty technology: a literature review, Publ., Meat Ind.Res.Inst.of New Zealand, 932. Oh, E.K., Groβklaus, D. (1994) Measurement of components in meat patties by near infrared reflectance spectroscopy, Meat Sci., 41, Sczczesniak, A.S. (1973), Instrumental methods for texture measurements, in: Texture Measurements of Foods, Kramer, A and Szczesniak, A.S eds., D. Reidel publ. company, Dordrecht, Boston, pp Zorrilla, S.E., Rodevo, C.O., Singh, R.P. (2000), A new approach to correlate textural and cooking parameters with operating conditions during double-sided cooking of meat patties, J. Texture Studies, 31,