Article pp.351-363 du Vol.28 n°4-5 (2008)

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INRA-UR370 Qualité des Produits Animaux – F-63122 Saint-Genès-Champanelle – Tél. : 04 73 62 44 92 – Fax : 04 73 62 40 89 – France.

Correspondence: [email protected]

FOCUS : JSMTV

Cooking of meat and meat-based product:

coupling with the reactions governing quality

A. Kondjoyan

RÉSUMÉ

La cuisson des viandes et produits carnés : les transferts et leurs couplages avec les réactions à l’origine de la qualité

Dans la société actuelle, les viandes et produits carnés sont presque toujours cuits avant d’être consommés. La cuisson sert à la fois à détruire les micro-orga- nismes pathogènes ou d’altération et à conférer à l’aliment une saveur, une cou- leur, une flaveur et une texture qui sont spécifiques du produit cuit. Le chauffage et la cuisson changent aussi les propriétés nutritionnelles du produit (variation de l’état et de la concentration en protéines, lipides et micronutriments). Même si les changements qui sont à l’origine de la variation de la qualité du produit sont globalement bien connus, les données disponibles restent fragmentaires, quali- tatives et souvent contradictoires. Une compréhension plus générique du sys- tème ne peut être obtenue qu’en séparant :

1) l’étude des transferts de chaleur et de matière et leurs conséquences en ter- mes d’hétérogénéités de l’environnement réactionnel dans le produit, et ; 2) l’étude des cinétiques réactionnelles en relation avec l’environnement phy- sico-chimique local.

Cette approche peut servir de base à une optimisation raisonnée des opérations de cuisson et de chauffage lorsqu’elle est combinée à la modélisation des phé- nomènes et des interactions.

Mots clés

cuisson, viande, modélisation, cinétique, qualité.

SUMMARY

Meat and meat-based products are almost always cooked before being eaten.

The cooking process not only destroys pathogenic or spoilage microorganisms but also develops food taste, colour, flavour and texture which are specific to the cooked product. Heating and cooking produce also changes of product’s nutritional properties (variation in the state and content of proteins, lipids and micronutrients). While the overall patterns of quality-related product changes during cooking are well understood, the scientific data available in literature reports

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352 Sci. Aliments 28(4-5), 2008 A. Kondjoyan

remains fragmented, qualitative and sometimes contradictory. A more generic understanding of the system can only be produced by segregating out:

1) heat and mass transfer-related effects and their consequences in terms of heterogeneity of the inside-sample reaction environment, and

2) reaction-driven kinetics in relation to this in-sample local environment. This approach, if combined with mathematical modelling of each individual phenome- non and their interactions, can serve as a reliable basis for properly optimizing heating and cooking treatments.

Keywords

cooking, meat, modelling, kinetic, quality.

1 – INTRODUCTION

In contemporary society, meat and meat-based products are almost always cooked before being eaten. The cooking process not only destroys pathogenic or spoilage microorganisms but also develops food taste and texture qualities that are specific to the cooked product. Cooking has an equally important impact on the toxico-nutritional properties of the meat product. The aim of this paper is to describe the physical phenomena brought into play when cooking meat-based products and their coupling with the biochemical reactions governing how taste, texture and toxico-nutritional factors develop in the food. The scope of this paper does not cover microbiology issues and their consequences in terms of health or food storage.

2 – COOKING TECHNOLOGY AND THE PHYSICAL PHENOMENA AT WORK

The technologies employed to cook or reheat meat and meat-based products can be split into groups based on primary energy input. These macro-groups differ- entiate processes based on immersion in liquid (boiling, deep-frying, etc.), contact (pan-frying, searing, etc.), convection, radiation (infrared or microwaving), phase change (steaming, etc.), and induction (ohmic heating). Each technology has its advantages and drawbacks.

Immersion-based techniques generate surface exchange that can be useful:

1) for treating bulky products, which are often vacuum-packed, or

2) alternatively, when products are free of packaging, for quickly heating and dehydrating them (deep-frying). Contact-based cooking has advantages in that it acts directly on the food surface, but since it also carries the drawback of low-quality heat contact, it is generally employed for quickly cooking thin- sliced meat. Microwaving is used to reheat or quickly cook a volume of product that will be predetermined by the microwave penetration depth. The cur- rent market trend is towards fast-heat technologies for cooking ready- packaged products presenting complex compositions (ready-made meat-

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based dishes). The process can be broken down into several different stages punctuated by periods of storage (industrial-scale pre-cooking steps followed by household cooking). This trend has led to the flourishing development of combination cooking technologies integrating several energy input systems (convection, phase change, infrared radiative heading, microwaving, etc.).

From a product quality standpoint, the source of energy input itself has little impact; what is important is the consequences in terms of temperature gradient, water concentration and reagent content patterns. These changing profiles have to be determined locally, since treatments in solid products are never homogeneous.

Traditional cooking methods (such as convection, contact, immersion, and infrared radiation) lead to heterogeneities between product surface and product core. Micro- wave cooking/reheating leads to more complex patterns of heterogeneity, related to either the geometric shape of the product (overheating corners and angles, or in cylindrical products, overheating the product core) or to its composition (content of water, fat, connective tissue, bone, etc.). Heat and mass transfers during cooking occur mainly by conduction-diffusion until the temperature of the meat reaches 50°C. As the 50°C temperature threshold is passed, the myofibrillar proteins start to undergo denaturation. At 55°C-60°C, this denaturation is accompanied by fast-rate collagen contraction. The protein denaturation and collagen contraction force out the juice under mechanical stress, as well as causing deformation in the product’s geometry and shrinkage. These phenomena are dependent both on how the process is led and the characteristics of the raw meat (chemical composition, pH, initial sarcomere length, etc.). Water losses recorded in beef under conventional cooking conditions are in the 25%-40% bracket (Laroche, 1982, 1988). The overall level of water loss is generally tied to the final cooking temperature, but it is equally clear that the pattern of water loss varies with treatment time. Juice transfer in the product, together with local variations in water content and variations in product volume and geometric deformation combine to impact on the heat transfers occurring, which in turn has a knock-on effect on the kinetics of the protein denaturation and collagen shrinkage. These phenomena are therefore highly coupled. If unwrapped meat is exposed to air, surface evaporation occurs, increasing strongly as soon as the product temperature rises above 100°C. This leads to the formation of a crust that acts as a barrier, affecting both heat and mass transfers (Skjöldebrand, 1980).

Published studies usually tally these product changes in terms of averages, and the thermal conditions to which the food is subjected are only covered in very gen- eral terms. More often than not, the only specific data reported are the average temperature inside the equipment or its power output, and the temperature kinetics recorded at the core of the product. It is impossible to properly compare the results obtained in different equipments based on these data alone. This means that exist- ing equipments cannot be optimized and new equipments cannot be designed with- out going through full-scale experimental trialling, which has to be reiterated on a case-by-case basis.

To go one step further, we need to be able to:

1) quantify the transfers occurring within the equipments, and

2) measure temperature and water concentration patterns at multiple points within the product, which is no easy task in solids that undergo dramatic geometric shape changes during cooking. This is where heat transfer modelling combined with water and solute transfer modelling represents a powerful conceptual tool for quantifying the phenomena at work, downscaling the number of trials to be run, and predicting product changes under non-measu- rable conditions. The models need to be rigorously built and their results vali- dated through in-depth analysis. However, published validations are often

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rushed through by simply measuring weight loss and temperature at the product core (Huang & Mittal, 1995; Chen et al. 1999).

The main difficulty with water-based cooking is to account for the complexity of product geometry and compositional variation (Siripon et al., 2007). A number of other studies have analyzed and modelled grilling or pan frying of beefburgers. The published models rarely take into account mass transfers inside the meat which, among other things, are dependent on the burger’s fat content and porosity (Orosz- vari, 2004). These models settle with describing heat transfers within the matrix, only accounting for mass transfer in the mathematical expression of surface evaporation (Shilton et al., 2002). The temperature patterns will be different depending on whether the beefburger is contact-cooked on one side (Ikedia et al., 1996; Ou & Mit- tal, 2007) or both sides (Dagerskog, 1979; Pan et al., 2000; Ou & Mittal, 2006). When the contact is only established on one side, the models need to account for the time-variation of boundary conditions, particularly when the product is flipped on the hotplate (figure 1). The validation of the models remains limited by difficulties in measurement and their potentialities constrained by:

1) the fact that inside product mass transfer is not taken into account, or at best considered purely diffusive;

2) the crust is either not modelled or described in extremely over-simplistic terms (with thickness either invariable or only varying at a pre-set temperature threshold);

3) variation in contact resistance is not taken into account.

Figure 2 illustrates the effect of thermal contact resistance on simulated temperatures during contact-based cooking of a 4 cm-thick beef sample on a hotplate heated to 180°C. The thermal contact resistance introduced into the model takes the successive values 0, 0.001, 0.01, and 0.03, where 0 is perfect contact, 0.001 is a

Figure 1

Temperatures measured and simulated when frying a 9.5 mm-thick beefburger on a 160°C contact surface. Temp1, Temp5, and Temp10 are the temperatures simulated

for side 1, the core and side 2 of the sample, respectively. Symbols correspond to measurements actually recorded at these same points (taken from Ou & Mittal, 2007).

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1 mm thickness of juice, and 0.01 and 0.03 represent 0.3 mm and 1.0 mm of air between hotplate and meat surface. Perfect contact is an extreme scenario where the surface temperature virtually instantaneously reaches hotplate temperature, whereas the 1 mm air cushion corresponds to a contact-free zone created by meat deformation. Nevertheless, even if these extreme scenarios are eliminated, the uncertainty over the TCR (Thermal Contact Resistance) value leads to around 30°C in simulated surface temperature variance, making it crucial in both study configura- tions to develop models and experimental methods to account for crust formation and better characterize the boundary conditions. INRA-QuaPA is conducting inverse numerical method-based research designed to resolve these problems.

Although the literature contains papers on the modelling of certain ovens (Mistry et al., 2006), there are few studies that can be used to predict how heating affects temperature and water content kinetics inside the product. Two situations have been modelled in order to describe how beef cooks in a oven. The first case involved only forced convection, which was combined with intense drying and rapid crust formation on the meat surface (Skjöldebrand, 1980; Holtz & Skjöldebrand, 1986). In the second case, the food was heated by a combination of radiation and forced convection, with less evaporation at the beginning of the treatment. Furthermore, as the studied oven was assumed be relatively small, evaporation was limited by moisture increase in the oven cavity (Singh et al., 1983; Obuz et al., 2002). The first case was modelled by splitting the product into two components, i.e. crust and core (Holtz &

Skjöldebrand, 1986). Crust was defined as the zone where temperature was strictly greater than 100°C. Heat transfer coefficient, evaporation rate and time-to-crust were adjusted in the model according to measured data. In the second case, moisture increase in the oven cavity was assumed to lead to a wet-bulb temperature of 100°C at the end of the cooking period. Surface water activity was thus assumed to stay at 1.0 through to the end of the treatment (Singh et al., 1983; Obuz et al., 2002), but this hypothesis, which is actually dependent on the out-leakage rate of the oven, was not verified. Furthermore, none of the models takes into account water migration inside the product and product shrinkage. However, shrinkage leads to positional variation in the measurement points during cooking, which can completely change how the results are interpreted, especially for points nearer the product surface.

Figures 3a and 3b give the temperature measured in an oven under a priori iden- tical positions, and using a laboratory set-up. With the exception of results at the product core, the data recorded were completely different. The differences stemmed

Surface temperature (°C)

0 50 100 150 200

0 200 400 600

Time (s) 1 2

3

4

Figure 2

Simulated surface temperatures during contact-based cooking of a 4 cm-thick beef sample on a hotplate heated to 180°C. Thermal contact resistance,

TCR, is: (1) 0, (2) 0.001, (3) 0.01, (4) 0.03 W^-1 m² K, respectively.

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from the probe positions varying considerably during cooking due to meat shrinkage. This example illustrates the difficulties involved in obtaining correct measurements for validating these models.

- 10 10 30 50 70 90 110 130 150

0 2000 4000 6000 8000

Température (°C)

Temps (s) 1

2 3 4

Figure 3a

Temperatures measured inside a piece of meat placed in a forced-convection oven with an air temperature of 190°C. The underside section of meat was thermally isolated.

The temperature sensors were initially positioned at sensor-to -surface distances of:

(1) 2 mm, (2) 4 mm, (3) 6 mm and (4) 10 mm.

150 130 110 90 70

30 50

10

– 10 0 2000 4000 6000 8000

Temps (s) 1 Température (°C)

2 34

Figure 3b

Temperatures measured inside a piece of meat placed in a forced-convection oven under the same conditions as for figure 3a. The initial temperature sensor positions

are a priori the same as for figure 3a.

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3 – DEVELOPMENT OF PRODUCT QUALITY

Cooking causes time-course changes in the taste and texture qualities of meat and meat-based products. It can also lead to changes in their nutritional value, and generate compounds that are considered toxic. This section describes the time- course developments in taste and texture qualities, dietary lipid content and vitamin content, as well as the formation of potentially carcinogenic substances. This section does not, however, describe the effects of cooking on protein and amino acid oxidation and the aggregation reactions that can affect their digestibility, and the fate of bioactive peptides, all of which is covered in Rémond et al. 2008.

3.1 Development texture, flavor, color and nutritional qualities Cooking affects tenderness, juiciness, flavour and colour of the food. Juiciness is directly related to juice losses during cooking, and is often perceived as sharing overlap with tenderness. The effects of cooking on tenderness is linked to several factors, including myofibrillar denaturation, collagen shrinkage, collagen solubiliza- tion, and weight-loss of the sample, among others. The effect is of course also dependent on the characteristics which affect the quality of raw meat: animal species, breed, age, sex, muscle type, and ageing conditions (cold-shrinkage, etc.). It is also dependent on the cooking conditions. Warner-Bratzler shear force measurements show that beef and rabbit muscle resistance is directly related to cooking temperature and sample weight loss. Shear force resistance increases as the product is heated from raw to 50°C, then drops during the shift from 50°C to 55°C-60°C before increasing again thereafter (Lepetit et al., 2000, Combes et al., 2003). Peak toughness occurs in the 77°C to 80°C temperature bracket, and does not appear to vary between 80°C to 90°C. The increasing toughness during the temperature gradient up to 50°C is interpreted as being related to the collagen spirals of the perimy- sium stretching until they get denatured between 50°C and 55°C-60°C. The increase in toughness post-60°C appears to be related to interactions between myofibrils and collagen fibers. The measurements, which have been incorporated into a model (Lepetit, 2007), were produced in water-bath-based experiments conducted at temperatures under 90°C and for cooking times of over 15 minutes. They cannot therefore be suitably transposed to searing or pan-frying steaks (McKenna et al., 2004) or oven-based spit-roasting, where time and temperature conditions have both a significant effect on tenderness (King et al., 2003).

The Maillard reactions induced by heating animal products produce a great many compounds, some of which are precursors of the development of colour and flavour in the cooked end-product (Laroche, 1988; Labuza et al., 1994). As meat is heated, it undergoes colour changes initially due to myoglobin denaturation, shifting from deep red to pink and then on to a greyish colour before finishing in a light brown. These shifts have conventionally marked the temperature range shifts from 60°C to 60°C-70°C to 70°C-80°C, respectively (Lawrie, 1985). Beyond the 85°C threshold, Maillard molecules form along with the melanoid pigments associated with grilled-meat colouring.

The time-variation of flavour is linked to both Maillard reactions and the oxidation of lipid compounds. From a nutrition standpoint, meat is generally considered too rich in saturated fatty acids and is touted as a risk factor for cardiovascular disease.

This is despite the fact that raw European meat has a fairly low intramuscular lipid content, generally ranging from 1.5% to 4.0% in pork and beef, and peaking at 5%-

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7% in fat animals (Culioli, 2003). Intramuscular lipids can be split into two catego- ries: phospholipids (also known as structural lipids) and triglycerides (also known as storage lipids), which show strong between-animal variation in content and composition. Dietary adjustments can change the degree of fatty acid saturation, especially in monogastric species, whereas the effect is attenuated in ruminants. Cooking has a variable effect on lipid concentrations depending on cooking method and cooking time. Under immersion-based cooking, 10% to 20% of the initial lipid content can be transferred to the cooking juices (Culioli, 2003). Under quicker cooking methods, the lipid content remains relatively stable in terms of % dry matter (Bauchart, 2004).

Mottram (1998) reviewed the formation of volatile compounds created through Mail- lard reactions. The complex, branched reaction pathways result in an extremely broad range of compounds. Pentoses, and particularly ribose (formed from nucle- otide degradation), and sulphur amino acids like cysteine are major components in the formation of the cyclic compounds detected in the volatile fraction of cooked meat. The aldehydes and carbonyls formed via lipid oxidation react with the compounds produced through Maillard reactions to confer meat its characteristic cooked smell. The thermal degradation of lipids is associated with the cooked meat smell specific to each animal species (cattle, sheep, goat and pigs). Cooking tends to promote lipid oxidation, most strongly with higher proportions of unsaturated fatty acids. Lipid oxidation in beefs kicks in at temperatures above 100°C, and is proba- bly highly dependent on the activity of water molecules. Cooking-induced lipid oxidation can also result in the formation of warmed over flavours when the product is subsequently placed in storage (St Angelo et al., 1988, 1992; Byrne et al., 2002).

Frying can also have its impact on meat lipid content and composition (Pena, 1994;

Saghir, 2005; Thurner, 2007, Haak et al., 2007). Furthermore, meat loses relatively few lipids through escaping juices - especially in terms of polyunsaturated fatty acids. Hence, the main driver affecting the lipid profile of a sample (related to animal feed) is the penetration of cooking oil, and not the initial lipid composition of the raw meat. Trials carried out on pork fillet (LT) show that pan-fried meat absorbs a significant proportion of the cooking oil, although inter-trial conditions were highly variable. Olive oil appears to penetrate meat more than margarine or cooking oil rich in unsaturated lipids (Haak et al., 2007). Research into the effect of pan-frying on cho- lesterol oxidation products (COPs) shows that cooked samples contain significantly higher levels of COPS than raw samples and that the effect is strengthened further by higher cooking intensity, as well as by type of oil (Thurner et al., 2007), since more COPs are produced when cooking with olive oil than vegetable seed oils.

Meat also contains a number of micronutrients that play important roles in human health. Excluding liver, which is particularly rich in vitamin A, meat and meat- based products are most importantly rich in B vitamins. The mineral supply from meat essentially involves vitamins B1, B2, B3, B6 and B12. The highest concentrations of Vitamin B12 (adenosylcobalamin), which can only be organically synthesized by micro-organisms, are found in ruminants (particularly in the liver). Normal levels of meat consumption will cover 80% of the RDA of vitamin B12. Meat, and especially red meat, also contains three trace elements required for good human health: iron, zinc and selenium. Most of the iron provided in red meat is in heme iron form, which is 2.5-fold more assimilable than the non-heme iron found in plants. The same is true for selenium, which, in plants, is mainly found in chelated form. A daily meat intake of 100 g covers around 30% of the RDA of iron and 70% of the RDA of zinc and selenium (Chanson et al., 2003). Meat also contains vitamin E, which plays an important role due to its antioxidant properties and can also be routinely added to animal feeds (as is the case with pigs).

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Heat treatments can lead to changes in the structure or chemical bonding of the micronutrients. They can also escape in the cooking juice, massively so for iron, zinc and selenium. Furthermore, the temperature increase can break the iron-heme bond, thereby releasing the free iron. This free iron not only downgrades the nutritional value of the product (Culioli, 2003), it can even act as a source for the free radicals that trigger the oxidation and intestinal inflammation processes capable of eventually leading to tumour formation. Since B vitamins are water-soluble, they too can escape via the cooking juice. They are also temperature-sensitive, especially thi- amine (B1), although niacin (B3) is the most temperature-resistant. Cooking losses of a given class of B vitamins are essentially treatment-dependent, i.e. dependent on the pairing of time-temperature and ambient humidity (“dry” or “moist” cooking). An estimated 20% to 50% of the initial B vitamin content can be lost during the cooking process (Culioli, 2003). In roast pork, a further 15-20% are lost during the reheating and storage steps in contract catering or home-delivered food settings (Lassen et al., 2002). One study focused specifically on the fate of vitamin B12 during cooking.

Three types of beef (rib steak, rump steak and chuck steak) were cooked to a 55°C core temperature using different traditional processes: grilling or pan-frying for 1 minute, deep-fat frying at 170°C for 38 seconds, oven-roasting at 240°C for 50 minutes, or braising at 80°C for 2 hrs 15 mins (Ortigues-Marty et al., 2006). The results highlighted that vitamin B12 losses were less significant than cooking juice losses, and that total vitamin B12 losses were only statistically significant with deep- fat frying (5% losses) and braising (25% losses). Vitamin E is fat-soluble and does not escape in cooking juices to any great extent. A recent study (Bauchart, 2004) reported that vitamin E losses were only recorded in beef when slow-cooked (roasting for 50 min at 240°C, immersion-based cooking at 80°C for 2 hrs 15 mins).

3.2 Formation of products causing health problems

Cooking can generate compounds causing human health problems. In charcuterie products, adding nitrates or nitrites can lead to the in vivo formation of nitrosamines (potentially carcinogenic) in the strongly-acidic conditions of the stomach.

Nitrites carry the highest toxic potential. Cooking has the immediate effect of reduc- ing the number of nitrites by transforming them into nitrates (Honikel, 2008). Con- versely, nitrite reduction tends to be slower in cooked products that are kept in storage due to the fact that storage inactivates the bacteria that transform nitrites into nitrates. Furthermore, at very high temperatures, nitrosamines (nitrosopyrrolid- ine) can form directly in nitrite-cured meat. The reaction depends on the presence of secondary amino acids and on the fact that acidic pH or the presence of metal ions can allow NO+ to form. Nitrosamine synthesis is generally kept to a minimum due to the low nitrite content of raw meat. Nitrosamines have, however, been detected in fried bacon, grilled fermented sausages, and charcuterie-based pizza toppings.

Amides and derivatives of unsaturated fats can also react with nitrites producing compounds like alkyl nitrites, which have not been precisely quantified in meat.

In creatinine-containing products like most meats, the Maillard reaction can lead to the formation of heterocyclic aromatic amines (HAAs) once the product’s temperature goes over the 90°C-100°C threshold (Skog, 1998). Studies have demonstrated that HAAs are highly mutagenic and that they can lead to breast cancer, prostate cancer and colon cancer, sometimes even causing heart damage (Gaubatz, 1997;

Skog, 1998; Felton, 2004). The rate of HAA synthesis increases with temperature, reaching very high rates at between 150°C and 200°C, which are temperatures com- monly found when grilling or roasting meats. HAA synthesis tends to be promoted by low water activity but slowed by marination (Pais, 1999; Sinha, 1997). Results

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suggest that increased lipid content curbs the formation of HAAs (Hwang & Ngadi, 2002), an effect shared with certain antioxidants like vitamin E (Balogh et al., 2000).

Wine, garlic or other ingredients in the marinade could also have a similar effect (Buqsquet et al., 2006; Gibis, 2007). Phenol compounds in the frying oil (Person et al., 2003) or lipid oxidation compounds (Randel et al., 2007) also appear to be linked to lower quantities of HAAs. HAAs mainly form at the product surface, in the “crust”.

The quantity produced is directly dependent on cooking process and cooking equipment. However, measuring these elements remains a complex task due to the difficulty in measuring the temperature at the product surface (Knize et al. 1994, 1995; Mirkovic, 2000). Adjusting the frying process and changing the type of oil used can lead to a 10-fold drop in HAA production (Skog, 1998; Randel et al., 2007).

Epidemiological studies have widely reported an indirect link between quantity of HAAs produced and cooking stage, which is itself assessed through the colour of the cooked meat (Sinha et al., 1998, 1999; Rohrmann, 2001, 2002, 2007). Meat juices subjected to high temperatures and which contain short-chain peptides often show a high HAA content (Skog et al., 1997). Kinetics models have been developed in model systems (Murkovic, 2004) or meat juice models (Arvidsson et al., 1999) in order to understand the dynamics involved and predict HAA synthesis in relation to the time-temperature combinations employed in cooking the products. Results from the model systems appear to suggest that a high sugar content or a surplus of AA derivatives inhibits the formation of HAAs whereas an increase in creatinine content has the opposite effect. However, it remains debatable whether these results can be extrapolated to meat. Research conducted to assess the effect of creatinine content in pork on HAA synthesis (Pfau, Resenvold & Young, 2006) showed no visible effect.

Although cooked meat and meat juices are a significant source of HAAs, it remains difficult to reliably estimate the degree to which consumers are exposed (Skog, 2002; Murkovic, 2004). It is therefore crucial to scientifically analyze the effect of cooking conditions on the formation of HAAs in real-world products according to consumer practices (Alaejos, 2008).

The surface of smoked meat-based products is known to contain high quantities of smoke-delivered polycyclic aromatic hydrocarbons (PAHs), which are potentially carcinogenic. The dominant presence in these compounds is benzo[a]pyrene (BaP).

PAHs are found in grilled or smoked charcoal-barbecued and wood-barbecued meat. Since BaP content in the product depends on smoke temperature and dura- tion of exposure, well-done meats tend to contain higher BaP (Kazerouni et al., 2001).

4 – COUPLING BETWEEN TRANSFER PHENOMENA AND MEAT QUALITY

The previous chapter highlighted how heating and cooking animal products pro- duces wide-reaching texture changes and leads to specific sensorial qualities (colour, taste, flavour, tenderness, juiciness, etc.) and nutritional properties (variation in the state and content of proteins, lipids and micronutrients). While the overall patterns of quality-related product changes during cooking are well understood, the scientific data available in literature reports remains fragmented and qualitative. The results published to date are far from sufficient to control how cooking is managed or to quantitatively predict the effects of changes in product composition or operat- ing conditions on the time-course development of the targeted quality. There are

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three main drivers of this state of events. First is the fact that heat-mass transfers and biochemical reactions are, in practice, coupled. This means that effects related to variations in product shape, dimensions, structure and composition have wide- reaching impacts on the specific performance features of the equipment used. The end-result is that the conclusions drawn are dependent on indissociable combinations of product-equipment interplay. Second, the results reported in the literature are based on observations between two states –“raw” and “cooked”– meaning that we still have little understanding of time-course changes between the two. The third and final cause is the fact that the results reported are averages measured within a sample, whereas, as shown in the first section, the treatment transformations induce both physical and biochemical changes that are highly heterogeneous within a product sample like a piece of meat. Sharper analyses enabling a more generic understanding of the system can only be produced by segregating out:

1) heat and mass transfer-related effects and their consequences in terms of heterogeneity of the inside-sample reaction environment, and

2) reaction-driven kinetics in relation to this in-sample local environment. This approach, if combined with mathematical modelling of each individual pheno- menon and their interactions, could then serve as a reliable basis for properly optimizing treatments designed to impart one or more target qualities. Howe- ver, this kind of approach has rarely been used. A handful of pioneering studies has nevertheless been conducted, one aiming to predict the quantity of HAAs formed when pan-frying a piece of beef (Tran et al., 2002).

The INRA QuaPA laboratory is currently deploying this kinetics and mechanistic modelling approach. Heat and mass transfers are being analyzed on a meat matrix.

Preliminary research (CIFRE thesis, S. Oillic, INRA/ADIV) is geared towards modelling surface-level transfers during convection, radiative or contact-based cooking (crust formation) and identifying the temperature gradients during product’s deformation. A second research direction is focused on analyzing the mass transfer induced by the mechanical stress caused by cooking a piece of beef, and on the resulting product deformation. Reactions and transfers can only be de-linked by working with model samples and designing specific experimental systems for controlling the micro-environment conditions under which the reactions take place. The work is conducted on a thin and uniform slice of solid product, and not in a liquid- based environment, in order to take into account any matrix-related effects. Labora- tory-based systems have been designed and built in order to run treatments on these slices and track the progression of the reactions by bringing in hold-temperature steps. The transfer models are then combined with the kinetics models to predict the development of sensorial qualities and toxico-nutritional properties during meat cooking. This research is mainly frameworked under the European “Prosafe- beef” project and the National French Research Agency “Lipivimus” project (coordi- nated by the INRA’s herbivore sciences unit), in partnership with other INRA research units and national and international teams. Under Prosafebeef, water concentration and temperature modelling is applied to predict changes in product colour, tenderness and juiciness. The product’s sensorial qualities are cross-correlated against the predicted levels of HAAs formed in beef. Under Lipivimus, the scientific objective of the cooking research is to provide deeper insight into lipid oxidation changes in a heated meat matrix and the effects of these changes on the development of flavour and colour in the cooked end-product.

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REFERENCES

ALAEJOS M.S. GONZALEZ V. AFONSO A.

M., 2008. Food Add. Cont., 25, 2-24.

ANGELO A.J., SPANIER A.M., BETT K.L.,1992, Lipid oxidation in food. Am.

Chem. Soc., Washington DC.

ARVIDSON P., VAN BOEKEL M.A.J.S., SKOG K., SOLYAKOV A., JÄGERSTADT M., 1999. F. Sci., 64, 216-221.

BALOGH Z., GRAY J.I., GOMAA E.A., BOO- REN A.M., 2000. Food Chem. Toxico., 38, 395-401.

BAUCHART D., 2004, Interbev contract round-up report, 101 pp.

BUSQUETS R., PUIGNOU L., GALCERAN M.T., SKOG K., 2006. J. Agri. Food Chem., 54, 8376-8384.

BYRNE D.V., BREDIE W.L.P., MOTTRAM D.S., MARTENS M., 2002. Meat Sci. 61, 127-139.

CHANSON A., BRACHET P., GROLIER P., and ROCK E., 2003. Sci. Alim., 23, 47-55.

CHEN S.S., WRIGHT N.T., HUMPHREY J.D., 1997. Trans. ASME, 119, 372-373.

COMBES, S., LEPETIT, J., DARCHE, B., LEBAS, F., 2003. Meat Sci., 66, 91-96.

CULIOLI J., BERRI C., MOUROT J., 2003. Sci.

Alim., 23, 13-34.

DAGERSKOG M., 1979. L. W. T., 12, 217-224.

FELTON J.S., KNIZE M.G., WU R.W., COLVIN M.E., HATCH T., MALFATTI M.A., 2007.

Mut. Res. Fund., 616, 90-94.

GAUBATZ J.W., 1997. J. Nut. Bioch. 8, 490- 496.

GIBIS M., 2007. J. Agri. F. Chem., 55, 10240- 10247.

HAAK L., SIOEN I., RAES K., VAN CAMP J., DE SMET S., 2007. Food Chem., 102, 857-864.

HOLTZ E., SKJÖLDEBRAND C., 1986. J.

Food Proc. Eng., 5, 109-121.

HUANG E., MITTAL G.S., 1995. J. Food Eng.

24, 87-100.

HWANG D.K., NGADI M., 2002. L. W. T., 35, 600-606.

IKEDIALA J.N., CORREIA A., FENTON G.A., BEN-ABDALLAH N., 1996. J. Food Sci., 61, 796-802.

KAZEROUNI N., SINHA R., HSU C.H., Green- berg A., Rothman N., 2001. Food Chem.

Toxico., 39, 423-436.

KING D.A., DIKERMAN M.E., WHEELER T.L., KASTNER C.L., KOOHMARAIE M., 2003.

J. Anim. Sci, 81, 1473-1481.

KNIZE M.G., CUNNINGHAM P.L., AVILA J.R., JONES A.L., GRIFFIN A., FELTON J.S., 1994. Food Chem. Toxico., 32, 55-60.

LABUZA T.P., REINECCIUS G.A., MONNIER V.M., O'BRIEN J., BAYNES J.W., 1994.

Roy. Soc. Chem., Cambridge.

LAROCHE M., 1982. L.W.T., 15, 131-134.

LAROCHE M., 1988. In: Girard J.P. Technolo- gie de la viande et des produits carnés.

APRIA, INRA, Tec&Doc-Lavoisier, Paris.

LASSEN A., KALL M., HANSEN K., OVESEN L., 2002. Eur. Food Res. Tech., 215, 194- 199.

LAWRIE R.A., 1985, Meat Sci. Pergamon Press, Oxford.

LEPETIT J., GRAJALES A., FAVIER R., 2000.

Meat Sci., 54, 239-250.

LEPETIT J., 2007. Meat Sci., 76, 147-149.

MCKENNA D.R. et al., 2004. Meat Sci., 399- 406.

MOTTRAM D.S., 1998. Food Chem., 62, 415- 424.

MURKOVIC M., PFANNHAUSER W., 2000.

Fres. J. An. Chem., 366, 375-378.

OBUZ E., POWELL T.H., and DIKEMAN M.E., 2002. L.W.T., 35, 637-644.

OROSZVARI B. K., 2004, The mechanisms controlling heat and mass transfer on frying of beefburgers, PhD Lund University of Technology (Sweden), 180 pp.

ORTIGUES-MARTY I., THOMAS E., PREVE- RAUD D.P., GIRARD C.L., BAUCHART D., DURAND D., PEYRON A., 2006. Meat Sci., 73, 451-458.

OU D., MITTAL G.S., 2005. Food Res. Int., 39, 133-144.

OU D., MITTAL G.S., 2007. J. Food Eng., 80, 33-45.

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PAN Z., SINGH R.P., RUMSEY T.R., 2000. J.

Food Eng., 46, 9-19.

PENA G.M., SAMPERIO M.A.Q., 1994. Revis.

Clin. Espan., 194, 966-969.

PERSSON E., GRAZIANI G., FERRACANE R., FOGLIANO V., SKOG K., 2003. Food Chem. Toxico., 41, 1587-1597.

PFAU W., ROSENVOLD K., YOUNG J.F., 2006. Food Chem. Toxico., 44, 2086- 2091.

RANDEL G., BALZER M., GRUPE S., DRUSCH S., KAINA B., PLATT K.L., SCHWARZ K., 2007. Food Chem. Toxico., 45, 2245-2253.

ROHRMANN S., ZOLLER D., HERMANN S., LINSEISEN J., 2007. Brit. J. Nut., 98, 1112-1115.

SAGHIR S., WAGNER K.H., ELMADFA I., 2005. Meat Sci., 71, 440-445.

SHILTON N., MALLIKARJUNAN P., SHERI- DAN P., 2002. J. Food Eng., 55, 217-222.

SINGH N., AKINS R.G., ERICKSON L.E., 1984. J. Food Proc. Eng., 7, 205-220.

SIRIPON K., TANSAKUL A., MITTAL G.S., 2007. Food Res. Int., 40, 923-930.

SKJÖLDEBRAND C., OLSSON C., 1980.

L.W.T., 13, 148-151.

SKOG K.I., JOHANSSON M.A.E., JÄGERS- TAD M.I., 1998. Food Chem. Toxico., 36, 879-896.

THURNER K., RAZZAZI-FAZELI E., WAGNER K.H., ELMADFA I., LUF W., 2007. Eur.

Food Res. Tech., 224, 797-800.

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