Mechanisms of Crust Development at the Surface of Beef Meat ...

Food Bioprocess Technol DOI 10.1007/s11947-014-1321-y

ORIGINAL PAPER

Mechanisms of Crust Development at the Surface of Beef Meat Subjected to Hot Air: An Experimental Study Stéphane Portanguen & Predrag Ikonic & Sylvie Clerjon & Alain Kondjoyan

Received: 6 February 2014 / Accepted: 15 April 2014 # Springer Science+Business Media New York 2014

Abstract When cooking meat, the development of a crust is an important element for consumer. It is the place of Maillard reactions and development of flavor. It is also where carcinogenic compounds can be formed. To our knowledge, no study has been devoted to the formation of the crust during cooking pieces of meat, so these mechanisms were studied under laboratory conditions. Temperatures were measured close to the surface using a special isolated device that corrects for movements due to meat thermal shrinkage. These temperature measurements were paired with microscopic measurements of the crust thickness and with the measurement of profiles of water content by magnetic resonance imaging to interpret crust formation and structure. Three areas were characterized in the crust, and the thickness of the colored area was proved to vary linearly with time. Images obtained by X-ray microtomography showed a great heterogeneity of the porosity in the crust of the heated samples. Keywords Meat crust . Hot air . Temperature profiles . Water content profiles . MRI . Porosity Abbreviations aw Water activity Cp Thermal capacity of the meat (J kg K−1) DM Dry matter HAAs Heterocyclic aromatic amines LD Longissimus dorsi LL Longissimus lomborum S. Portanguen (*) : S. Clerjon : A. Kondjoyan UR370 Qualité des Produits Animaux, INRA, 63122 Saint-Genès-Champanelle, France e-mail: [email protected] P. Ikonic Institute of Food Technology, University of Novi Sad, Bulevar cara Lazara 1, 21000 Novi Sad, Serbia

LT PD PEEK SM T TB Tb X λ ρ ΔE

Longissimus thoracis Proton density Polyether ether ketone Semimembranosus Temperature Triceps brachei Boiling temperature Sample average Thermal conductivity of the meat (W m−1 K−1) Density of the meat (kg m−3) √((L1 −L2)2 +(a1 −a2)2 +(b1 −b2)2)

Introduction Crust development at the surface of food products during grilling and roasting is an important element of consumer appetence for foods. Crust development stems from Maillard reactions that affect food color and flavor, but these same reactions can also lead to the formation of potentially carcinogenic compounds such as heterocyclic aromatic amines (HAAs; Kondjoyan et al. 2010a). Crust is very thin and thus difficult to study, which may explain why the mechanisms of crust formation are not yet fully elucidated. Most of the literature on crust formation and structure is on processed cereals, sometimes extending to meat products (Skjöldebrand and Olsson 1980), but little research has focused on crust formation at the surface of heated meat. This review on crust focuses on meat products. Results on nonfood products or cereals are only cited to illustrate phenomena that have not yet been described or measured on meat. The literature has defined crust in different ways: (1) as a dried evaporating area where the meat temperature is greater than the boiling water temperature (Feyissa et al. 2013), (2) as a set of mechanical properties (hardness, crustiness) (Barbut

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2013), or (3) as a color variation that leads to a brown area visually separated from the gray-pink “crumb”. Crust formation and transport phenomena at the food surface are couples, since heat–mass transfers lead to crust formation while crust structure/composition affects the transfer properties (Feyissa et al. 2013; Lalam et al. 2013). Crust is often considered as the area where temperature is above boiling point. In this case, the frontier between the crusted and non-crusted areas is the evaporation front. It has been shown in clay, gypsum, and brick that the evaporating front penetrates the porous material at constant velocity (Pel et al. 2002). As the area at the interface between evaporation front and product surface becomes dryer, the product’s mechanical properties change. Barbut (2013) characterized meat crust formation by measuring a set of mechanical properties such as the rapid increase in shear forces and force energy expenditure (amount of work) during meat slicing, and found that the values measured were two-fold higher on cooked product (chicken fillets fried at 70 °C at core) than on raw product. Microscopic observations also showed that during crust formation, muscle fibers became narrower, revealing strong water loss (muscle fibers torn along their longitudinal axis and removed along their transverse axis). Dehydrated meat loses its viscous elastic properties and becomes brittle. Its porosity increases, and the pores fill with gas. In cereal products (corn kernels), dehydration is accompanied by fractures in the material occurring at the peak of the stress force (Tremeac 2005). Crust formation affects meat permeability and thus water transport (Feyissa et al. 2013). Wählby and Skjoldebrand (2002) studied the effect of crust formation on heat–mass transfers in bread samples. The samples, which weighed 40 or 50 g, were cooked at 175 °C and then reheated at different temperatures with or without their crust. The authors concluded that crust had no effect on heat transfer by conduction (crust has no insulating effect) but that crust formation significantly affected mass transfer by slowing the rate of water evaporation from the crumb. Thus, the inside temperature of the samples “without crust” was lower than that of samples “with crust” because more energy was used for evaporation when the crust was removed. Sheridan and Shilton (2002) showed, in beef burger patties subjected to infrared treatment, as soon as crust is formed, the evaporation rate seems to be constant. Thermal exchange in crust is made all the more complex by the fact that the thermal properties are unknown. However, thermal conductivity (λ) is much lower in a porous material filled with gas than in the same material filled with water, density (ρ) and heat capacity (Cp) vary in the same way (Ozisik 1985). Crust thickness has been measured on bakery products using various techniques from optical microscopy (Wählby and Skjöldebrand 2002) and scanning electron microscopy (Gallagher et al. 2003) to color analysis to correlate ΔE with crust thickness (in the CIE L*a*b* color space; Mohd Jusoh

et al. 2009). To our knowledge, the evolution of colored crust thickness has never been measured on meat products, despite color being a potentially good indicator of crust formation. During cooking, meat color varies from red to white, gray, brown, and black. These color changes are associated with biochemical Maillard reactions, where this first step (complexing of proteins and carbohydrates) is reflected by browning, and the final step (synthesis of melanoids) is reflected by blackening. The chemical reaction rates and the corresponding color changes are affected by the water content of the meat (Garcia-Segovia et al. 2007). Pore distribution can affect mass transfers at the product surface. Feyissa et al. (2013) worked to the assumption that when beef meat is roasted, pore sizes are greater near the product surface than at the center, thus affecting moisture transfer and permeability of the product, but without actually measuring porosity. X-ray microtomography is a non-invasive technique used to study the microstructure of materials that carries the added advantage of operating in environments with low water content. X-ray microtomography investigation of food product porosity has proved that homogeneous tissues can only be found at the middle of some fresh products (Herremans et al. 2013), and that once a product matures or is processed, the microstructure rapidly evolves and tends to become disorganized. This is especially true during frying, grilling, and roasting, all of which involve intense heat and mass transfers. Most studies on crust structure have been performed on fried foods (potatoes, breaded meats) or processed cereal products. Ngadi et al. (2009) showed that in-crust pore development during deep-fat frying is linked to evaporation of moisture. Intense heat could spark explosive evaporation that would form large pores and crevasses, especially in a quick crust formation as the crust acts as a barrier to evaporation, with the pressure increase due to locally intense heat transfers forming the largest pores. Three types of structures were identified in porous solids: (1) interconnected pores, (2) isolated non-interconnected pores, and (3) “blind” pores (available only in one direction) (Ngadi et al. 2009). According to Ngadi et al. (2009) and Besbes et al. (2013), variation of initial water content, process conditions, and initial structural heterogeneity can dictate the heterogeneity of the crust structure. Van Dyck et al. (2014) separate three zones with different porosities in the crust developed at the surface of bread. Krokida et al. (2000) observe that during the cooking of fried potatoes, the increase in temperature causes a strong reduction in the volume of the food while increasing its porosity, which is dependent on intense shrinkage phenomena linked to the evolution of the temperature and to the cooking system used during the experiments. The aim of this paper is to analyze the mechanisms responsible for crust formation at the surface of beef meat samples subjected to a hot air jet. Different experimental approaches are used to get an overview of evolution in crust structure.


Materials and Methods Longissimus thoracis muscles were taken from carcasses of 18-month-old heifers immediately after slaughter. The muscles were cut into big pieces, aged for 12 days under vacuumpacked conditions, then frozen and stored at −20 °C. Crust development and structure were analyzed via four types of approach: temperature measurements, image analysis, water content measurements, and X-Ray microtomography. Thermal Treatment and Temperature Measurement An analysis of thermal exchanges in the crust area requires the measurement of surface and under surface temperatures. This is not easy when meat is cooked in an oven due to the problems of accessibility and to the probe movements generated by heat shrinkage of the meat sample (Kondjoyan et al. 2014). Thus, experiments were performed using (1) an open jet system which enables IR measurement of surface temperature and (2) a specific device that partially compensates for heat shrinkage of the sample. Figure 1 gives a schematic representation of the device designed for this study. A cylinder of meat 48 mm in diameter Fig. 1 Schematic illustration of the device (made of PEEK®) used to analyze thermal transfers during the heating of the cylindrical meat roast. A fine mesh grid and a spring are used to limit the effect of meat contraction on thermocouple positioning (dimensions in mm)

and 50–55 mm in height was accurately cut from big pieces of the Longissimus thoracis muscle following the procedure detailed in Kondjoyan et al. 2010a, b. The meat cylinder was placed in a support formed by a bigger hollow cylinder made in PEEK® (0.25 W m−1 K−1). The cylindrical meat sample was pushed by a spring fixed to a disc located at the bottom of the device onto a grid fixed by a lid at the top of the device. Spring contraction was adjusted using four screws located under the disc. Three 0.9-mm-diameter needlepoint thermocouples named Tc1, Tc2, and Tc3 were accurately placed at 2, 8, and 20 mm from the raw meat surface in an independent piece of PEEK®. These three thermocouples were checked for perfect straightness before each experiment and rectified if necessary. The small plastic piece was located in a pre-machined guide in the PEEK® cylinder and slid perpendicularly to its axis until the end of the thermocouple reached the axis. This ensured an accuracy of +/−0.5 mm on the distance of the three thermocouples to the meat surface. The support was then placed under the center of an open jet system. The functioning of this open air jet system is detailed in Kondjoyan et al. 2010b. During heat treatment, air temperature at the outlet was either 160, 190, 225, or 260 °C, and distance d between sample surface and pipe outlet was set at

Infra-red pyrometer HOT AIR Thermocouples K near the surface

Lid and grid located between meat surface and lid

Tc1 Tc2 Tc3 51 to 63

112

30

Meat cylinder 48 16

Spring 15 to 27 Height adjustment by screws

34 80

Thermocouples K (Tc1, Tc2, Tc3) at 2, 8 and 20 mm


36 mm (i.e., 124, 158, 192, and 210 °C at sample surface). Heating was stopped by sliding the support beneath a 45– 55 ms−1 jet flow of cold air (temperature 3–5 °C) produced by a Ranque–Hilsch vortex tube. Meat surface temperature was measured continuously throughout thermal treatment using a calibrated IR pyrometer (Kondjoyan et al. 2010a, b). During heating and cooling, the temperatures of the air jet impacting the meat were measured using a thermocouple fixed to the PEEK® cylinder and set 2 mm above the meat surface. Meat internal temperature was measured on the cylinder axis and at different distances from the surface using the abovementioned thermocouples. After cooling, the meat cylinder was removed from the support taking every precaution not to tear away any crust while withdrawing the surface grid. Thermocouples were taken out of the sample, and 30-mmlong and 0.9-mm-diameter needles were sunk into thermocouple holes to a depth of 10 mm to maintain these holes open. The meat cylinder was either kept whole for MRI measurements or cut into pieces for image analysis, dry matter (DM) and X-ray microtomography measurements (Fig. 2). Experimental design is detailed in Table 1.

taken by the standard and the binocular cameras were then used for image analysis to accurately determine thermocouple position. To measure the thickness of the colored crust, the top of one half of the meat sample was cut perpendicularly to the cylinder axis such that the thickness of the slice was equal to the thickness of the brown-colored crust plus a further 2– 3 mm of “non-brown-colored” crust. Binocular pictures of this area were taken and the thickness of the colored area was measured visually from the binocular images. Measurements were taken in at least five crust locations to obtain repeatable average and standard deviation values. In a first step, GIMP2.6 software was run on the image to accurately separate the brown-colored crust area from the non-brown-colored area following the irregular border. In a second step, a calibration of lengths was done using the pictures of the reference length taken in each image to avoid any difference in image resolution. In a third step, average crust thickness and its standard deviation were calculated from all the acquisitions obtained on different portions of crust (specific programs developed using Matlab 7.0® and its image processing toolbox).

Measurement of the Water Content Profiles Image Analysis The cylinder was cut into two halves. The needles were pushed deeper into the meat until the tip became visible. A millimeter scale was placed near the halves, and pictures of each half were taken by a 35-mm-lens digital camera. Another set of pictures of crust area was taken using a camera (×10 zoom) associated to binoculars (×1.6 or ×2 magnification) and a 100×0.1=10-mm scale (PYSER-SGI, UK). Pictures Fig. 2 Schematic representation of the different stages of the sample analysis

Water content profiles were determined either by DM measurements or using IRM analysis. For DM analysis, the half that still contained a crust at its top was sliced perpendicularly to the cylinder axis in five pieces whose top-to-bottom thicknesses were 5, 10, 10, 10, and 10 mm, respectively. Some meat samples had contracted so much that only four slices could be obtained. DM was measured on each slice to determine water content Stage II Meat slicing

Stage I Meat cutting

Image analysis:

Stage III Analysis Porosity measurement by X-ray microtomography

1) Determination of the crust thickness; 2) Determination of the real a ccurate location of sensors to interpret the temperature profile; 3) Determination of the water content profile by MRI. 2 halves

Measurements of dry matter

Food Bioprocess Technol Table 1 Experimental design and tube, jet, and meat surface temperatures in each heating condition

Tube temperature (°C)

Average impacting jet temperature (°C)±standard deviation

Meat surface temperature at the end of the longest exp. (°C)±standard deviation

Heating time (min)

160 190 225 260

124±6 158±8 192±4 210±6

122±3 154±2 190±2 214±3

20, 40, 90 20, 40 20, 30, 60, 90 20, 40, 60, 90

profile as a function of distance from surface, measured at the center of the slice. MRI experiments were also performed to obtain more accurate water content profiles in the crust area. Eight samples were cooked on one face as described in Thermal treatment and temperature measurement. After cooling, each meat cylinder was placed in an MRI tube and the top of the tube was filled with gelatin. This gelatin consists in almost 50 % water content (by weight) of a commercial animal gelatin powder (pork skin, 60 bloom, Rousselot, L’Isle-sur-Sorgue, France) in de-ionized water. Gelatin molds the crust-air boundary, making it detectable and giving a water content reference for quantitative measurement. Experiments were performed at 400 MHz on an Avance DRX400 system (Bruker GmbH, Ettlingen, Germany) equipped with an actively shielded gradient coil for microimaging. Beef samples were placed in a birdcage radiofrequency coil used for both excitation and signal reception. Temperature was held at 25 °C in all MRI acquisitions. For each sample, we acquired a high-resolution spin-echo image in the central longitudinal slice at slice thickness 2 mm and image resolution 0.15×0.5 mm2 with the higher resolution in the crust water profile direction and eight multi-slice spin-echo images that each contained nine longitudinal slices at slice thickness of 1 mm and image resolution 0.15×1 mm2, with the higher resolution again in the crust water profile direction (direction of the cylinder axis). The eight multi-slice spin-echo images were performed at eight different echo times from 4 to 11 ms so as to calculate relaxation time T2 assuming a monoexponential behavior and, from there, to plot the proton density (PD; i.e., water content) images. TE1

PD was computed voxelwise according to PD ¼ DPI1˙ e T 2 where DPI1 is the first spin-echo image acquired with an echo time (TE1) at 4 ms. Water profiles were then extracted from PD images. The beginning of the crust was detected via the high contrast between gelatin and crust. All profiles corresponding to a given heat treatment (temperature/time) were averaged. Finally, gelatin gel area on PD images was used as a reference to calculate water content from measured PD, knowing that gelatin water content, which was determined in each case, was between 49.7 and 54.0 % depending on the experiment.

X-ray Microtomography Analysis The samples of crust meat heated under the conditions shown in lines 3 and 4 of Table 1 were analyzed by microCT scanning (Explore CT-120, GE Healthcare, INSERM ClermontFerrand, France) with 49.4 μm resolution images obtained at 70 kV and 50 mA. Image analysis was performed using Amide® 0.9.2. software and 300 to 500 2D images were taken to reconstruct the 3D structure of 14×22×2.5 mm samples.

Results and Discussion Sample Temperature Temperatures of the heating jets impacting meat samples are given in Table 1. These values are logically lower than the tube temperatures as the jets instantly mix with ambient air on release from the tube. Temperatures measured in the meat during the 124, 158, and the 210 °C treatments are reported in Fig. 3a, b. The measurement given in Fig. 3b at different distances from the surface corresponds to three different experiments. Thermocouples were closer to the meat surface at the end of the treatment than at the beginning due to the combined effect of meat contraction and spring relaxation. This distance may thus differ from one case to another. During treatments, the Tc1 or Tc2 temperature reached or surpassed the water boiling temperature whereas this was never the case for the Tc3 thermocouple initially located 20 mm under the surface. When the temperature had reached boiling point, it remained at this value either throughout the experiment or only for a while after which it again increased toward air temperature. Plateau duration at boiling point temperature depended both on the initial location of the thermocouples (Tc1 or Tc2) and on their movements under different air temperatures. It was always shorter for Tc1 than for TC2, and for a given thermocouple, it was all the more short than when the air temperature was high. For example, the plateau was barely existent at Tc1 when air temperature was 210 °C while its duration was longer than one hour when air temperature was 124 °C.


120 158°C, 0.8mm

124°C, 0.8 mm

3a

100 Temperature (°C)

Fig. 3 Temperatures measured using thermocouples Tc1 and Tc2 (Fig. 1) during heating at different distances from the sample surface. These distances are measured on the cooked meat after thermal contraction: a The meat cylinder is subjected to a 124 and 158 °C air jet. b At 210 °C, the temperature at the surface is measured using the calibrated IR pyrometer. Dotted circles represent the time at which Tc1 or Tc2 temperature begins to rise above the boiling point temperature, and where aw begins to decrease rapidly (