Cold smoking is normally seen as smoking where the core temperature will remain below 35 deg C. We use hot smoking where the core temperature riches > 35 deg C but < 45 deg C. Smoking and thermal treatment are therefore considered jointly. Temperature effects product taste, meat toughness, binding, coulour, and moisture loss.
During reddening, the temperature is increased, extraction flaps in the smokehouse closed to maintain humidity, and sulfhydryl groups are released which is a reducing substance in meat and important in proper cured colour formation. Fraczak and Padjdowski (1955) indicated that 80°C is the critical temperature for the decomposition of sulfhydryl groups in meat.” (Cole, 1961) (Reaction sequence)
During heating and smoking, there are several changes in the meat that has a direct effect on the colour development. The nitrosating species that is more dominant than NOCl is smoke due to the presence of phenolic compounds. In addition to the heat release of sulfhydryl groups, the pH is reduced in the meat. Randall and Bratzler (1970) noticed an increase in the myofibrillar protein nitrogen fraction, pH and free sulfhydryl groups of pork samples that were only heated, and a decrease of these values in the samples that were subjected to heat and smoke. “Results of this study indicated that smoke constituents react with the functional groups of meat proteins.” (Randall, 1970) These results seem to support a reddening step before smoke is applied due to the fact that heating would release the sulfhydryl groups and during the smoke steps, the pH will be reduced. (Reaction sequence)
DENATURING VS COAGULATION
With our consideration of smoking on meat, we have also entered the discussion of the effect of heat on meat. Before considering the effect of heat on the protein lets first see how the heat gets to it.
Mechanism of heat transfer
Heat is transferred during cooking through conduction, convection, and radiation. “Spakovszky and Greitzer (2002) defined conduction as ‘transfer of heat occurring through intervening matter without bulk motion of the matter,’ convection as heat transfer due to a flowing fluid, either a gas or a liquid, and radiation as ‘transmission of energy through space without the necessary presence of matter.’ Radiation can also be important in situations in which an intervening medium is present, such as heat transfer from a fire or from a glowing piece of metal (Spakovszky and Greitzer 2002).” (Yu, T.Y., et al, 2017)
“Meat cooking usually involves more than 1 mode of heat transfer (Bejerholm and others 2014).” During cooking in a smokehouse, heat treatment is achieved through dry heat surrounding the meat, but during reddening and smoking the air is or become moist and moist-heat (hydrothermal) thermal processing uses hot steam. Smoke House thermal treatment, including smoking, is, in reality, a combination of dry heat and moist heat. (Yu, T.Y., et al, 2017)
“Conventional cooking of meat results in heterogeneous heat treatment of the product on account of steep temperature gradients (Tornberg 2013). Emerging mild cooking techniques such as ohmic cooking can achieve a more homogeneous heating by heating the entire volume of meat at the same time (Tornberg 2013).” (Yu, T.Y., et al, 2017) THis is an important point for consideration in a continuous, fully automated system.
This is important in considering the effect of heat on the grid system with holes. The present role to steel ratio is 1:1,8. The exposed meat area is therefore approximately half (take the edging to be approximately 0.02 to give the total ratio of 1:2). This amplifies the effect of heating, but by what factor? This needs to be determined experimentally between different smokehouses. I have determined a variety of different options in smokehouse settings over the years.
“Heat may cause proteins to lose their native conformation (denature) by providing the polypeptides with kinetic energy, increasing their “thermal motion,” and thus rupturing the weak intramolecular forces (such as nonpolar interaction, various kinds of electrostatic interaction, and disulfide bonds) that hold the proteins together (Davis and Williams 1998). As the temperature increases, a protein starts to unfold. When almost all the tertiary and secondary structures are lost, the unfolded protein may aggregate, have its disulfide bonds scrambled, undergo side-chain modifications (Davis and Williams 1998), and cross-link with other polypeptides. Aggregation is the consequence of nonpolar interaction between heat-denatured proteins whose hydrophobic groups have turned outward into the surrounding water, in order to adopt a lower energy state (Davis and Williams 1998). A variety of side-chain modifications, such as those induced by oxidation or the Maillard reaction, have been characterized in proteins following heat treatment.” As heat increases, the 3-dimensional structure of meat proteins change. These changes manifest in a change in colour and gelation. (Yu, T.Y., et al, 2017)
DEVELOPMENT OF NITROSYLMYOCHROMOGEN
“Upon thermal processing, globin denatures and detaches itself from the iron atom, and surrounds the hem moiety. Nitrosylmyochromogen or nitrosylprotoheme is the pigment formed upon cooking and it confers the characteristic pink colour to cooked cured meats.” (Pegg, R. B. and Shahidi, F; 2000: 42)
We also need to review the main muscle proteins found in the body.
Skeletal muscles are bundles of muscle cells (also known as muscle fibers) embedded in connective tissue. (Yu, T.Y., et al, 2017) These muscle proteins “are grouped into three general classifications: (1) myofibrillar, (2) stromal, and (3) sarcoplasmic. Each class of proteins differs as to the functional properties it contributes.” (www.meatscience.org)
-> Myofibrillar Proteins
The first very important protein to take note off is the myofibrillar protein for the purpose of water binding and binding meat pieces together. These muscle fibers are muscle cells, grouped into muscle bundles. The structural backbone of the myofibrils is actin and myosin. (Toldra, 2002) They are the most abundant proteins in muscle and are directly involved in the ability of muscle to contract and to relax. (www.meatscience.org) Myofibrils also include tropomyosin and troponin, regulatory proteins associated with muscle contraction. Parallel to the long axis of the myofibril, are two very large proteins called titin and nebulin. (Toldra, 2002)
Myosin is a protein which is described as the motor, and the structural protein, actin’s filaments are the tracks along which myosin moves, and ATP is the fuel that powers movement. (Lodish, 2000) Myosin “converts chemical energy in the form of ATP to mechanical energy, thus generating force and movement.” (Cooper. 2000)
“Together, actin and myosin make up about 55-60% of the total muscle protein of vertebrate skeletal muscle, with the thicker myosin myofilaments yielding about twice as much protein as the thinner actin myofilaments. Actin alone does not have binding properties, but in the presence of myosin, acto-myosin is formed, which enhances the binding effect of myosin.” (Patterson, The Salt Cured Pig) In meat processing, it is important to note that it is the myofibrillar proteins which are soluble in high ionic strength buffers. (Toldra, 2002)
“Texture, moisture retention, and tenderness of processed muscle foods are influenced by the functionality of myofibrillar protein.” (Xiong, Y. L.;1994) The pork muscle that contains the most myosin is the longissimus dorsi or the eye-muscle or longissimus muscle on the loin. “The muscle fiber bundles of the longissimus dorsi are arranged at an acute angle to the vertebral column. The cross-sectional area of the longissimus dorsi increases towards the posterior part of the ribcage, but it has an approximately constant cross-sectional area through the loin.” (Animal Biosciences)
-> Sarcoplasmic Proteins
“The sarcoplasmic proteins include hemoglobin and myoglobin pigments and a wide variety of enzymes. Pigments from hemoglobin and myoglobin help to contribute the red colour to muscle.” (www.meatscience.org) These proteins are water soluble. Besides myoglobin and hemoglobin, this class of proteins also includes metabolic enzymes (mitochondrial, lysosomal, microsomal, nucleus or free in the cytosol). (Toldra, 2002)
Very important to remember for the purpose of meat processing is that myoglobin is the protein pigment responsible for the red colour in meat. The redness of meat is largely dependant on the concentration of myoglobin. Myoglobin is the storehouse for oxygen in the muscle. Because different muscles need different oxygen levels, the concentration of myoglobin will differ between muscles. The loin muscles in pigs are for example used for support and posture and therefore contains low levels of myoglobin. Myoglobin levels are further influenced by species, breed, sex, age (older animals generally have more myoglobin), training or exercise (this is why free-range pigs have more myoglobin than stall-fed animals), and nutrition. (Pegg and Shahidi, 2000)
-> Stromal Proteins
“Connective tissue is composed of a watery substance into which is dispersed, a matrix of stromal- protein fibrils; these stromal proteins are collagen, elastin, and reticulin.
Collagen is the single most abundant protein found in the intact body of mammalian species, being present in horns, hooves, bone, skin, tendons, ligaments, fascia, cartilage and muscle. Collagen is a unique and specialised protein which serves a variety of functions. The primary functions of collagen are to provide strength and support and to help form an impervious membrane (as in skin). In meat, collagen is a major factor influencing the tenderness of the muscle after cooking. Collagen is not broken down easily by cooking except with moist—heat cookery methods. Collagen is white, thin and transparent. Microscopically, it appears in a coiled formation which softens and contracts to a short, thick mass when it is heated and helping give cooked meat a plump appearance. Collagen itself is tough; however, heating (to the appropriate temperature) converts collagen to gelatin which is tender. In the consideration of a TG mix, collagen is one of our most important considerations.
Elastin (often yellow in colour) is found in the walls of the circulatory system as well as in connective tissues throughout the animal body and provide elasticity to those tissues. Reticulin is present in much smaller amounts than either collagen or elastin. It is speculated that reticulin may be a precursor to either collagen and/or elastin as it is more prevalent in younger animals.” (www.meatscience.org)
It is interesting that collagen has been used for centuries to create strings to bind things and for strings on musical instruments. Catstring or catgut is made by twisting together strands of purified collagen taken from the serosal or submucosal layer of the small intestine of healthy ruminants (cattle, sheep, goats) or from beef tendon and has been in use for a long time 900’s AD. (Wray, 2006) Gut strings were being used as medical sutures as early as the 3rd century AD as Galen, a prominent Greek physician from the Roman Empire, is known to have used them. (Nutton, 2012)
Abū al-Qāsim Khalaf ibn al-‘Abbās al-Zahrāwī al-Ansari (Hamarneh, et al., 1963)(Arabic: أبو القاسم خلف بن العباس الزهراوي; 936–1013), popularly known as Al-Zahrawi (الزهراوي), Latinised as Abulcasis (from Arabic Abū al-Qāsim), was an Arab Muslim physician, surgeon and chemist who lived in Al-Andalus in the early 900’s CE. He is considered as the greatest surgeon of the Middle Ages (Meri, 2005), and has been described as the father of surgery. (Krebs, 2004). He became the first person to have used Catgut to stitch up a wound. He discovered the natural dissolvability of the Catgut when his monkey ate the strings of his musical instrument called an Oud. (Rooney, 2009)
Later, in 1818, the modern founder of surgery, Joseph Lister, and his former student William Macewen independently and quite remarkably, almost at the exact same time, reported on the advantages of a biodegradable stitch using “catgut”, prepared from the small intestine of a sheep. Over the ensuing years, countless innovations have extended the reach of collagen in the engineering and repair of soft tissue in medicine and numerous other industrial applications. (Chattopadhyay, 2014) The interesting point should not escape our notice that collagen is included in our TG mixes ta facilitate meat protein – TG – connective tissue – TG – meat protein binding structure. Collagen is surface-active and is capable of penetrating a lipid-free interface. (Chattopadhyay, 2014)
The other major constituent of meat is, of course, lipids or fat but I deal with this separately below.
During thermal processing, moisture loss will take place. Let us predict the optimal temperature range that will give us the right moisture loss and colour development in the shortest possible time. Countries such as Australia sell their bacon cooked but in the UK, New Zealand, Canada, the USA and South Africa, bacon is sold par-cooked. I, therefore, consider temperatures which will be considered par-cooked and fully cooked.
DIFFERENCES IN MOISTURE LOSS
“Bendall and Restall (1983) systematically studied the physical changes occurring during heating of intact beef-derived single muscle cells, and also the very small myofiber bundles of 0.19 mm in diameter (containing 40 to 50 cells) at final temperatures between 40 and 90 °C. In addition, the authors also studied heating of larger bundles of 2 mm in diameter.” (Yu, T.Y., et al, 2017)
According to their work, the stewing process progresses as follows:
From 40 to 52.5 °C
Denaturation of sarcoplasmic (include hemoglobin and myoglobin) and myofibrillar proteins occurs. Related to colour development the denaturation will effect sarcoplasmic protein even though its denaturation probably occurs from at least 25 °C. Related to moisture and the range of 40 to 52.5 °C, a slow loss of fluid from the myofibers into the extra-myofiber spaces occurs without shortening. (Yu, T.Y., et al, 2017) The maximum activity observed for TG was at 40 °C for the commercial TG. At temperatures above 45 °C, TG suffered a rapid drop in its activity. (Ceresinoa, 2018)
Between 52.5 and 60 °C
At this temperature, there is “an increasingly rapid loss of fluid from the myofibers, reaching a maximum rate and extent at about 59 °C.” There is no overall shortening at this temperature mainly due to heat shrinkage of the basement membrane collagen (type IV and perhaps type V as well) at about 58 °C. (Yu, T.Y., et al, 2017)
Between 64 to 94 °C
“Considerable overall shortening and a decrease in cross-sectional area are noted, accompanied by increased cooking loss with heat shrinkage of the endomysial, perimysial, and epimysial collagen.” (Yu, T.Y., et al, 2017)
“Long periods of stewing causes partial or complete gelatinization of the epimysial collagen, followed by the peri- and endomysial collagen, resulting in the soft and tender feature of stews (Bendall and Restall 1983). It is worth mentioning that meat with a high pH (Zhang and others 2005) or fat content (Wood and others 1986; Jung and others 2016) has been shown to exhibit higher water-holding capacity.” (Yu, T.Y., et al, 2017)
The important aspect for us is the key temperature of < 52.5 where moisture loss becomes “rapid”. This gives us an important upper “meat temperature” limit above which rapid moisture loss occurs.
The following section confirms the conclusion of par-cooked bacon’s optimal thermal processing range of between 40 and 52 deg C. Due to inconsistencies in the smoke chamber, it is suggested that a maximum internal core temperature of 40 deg C is set.
KINETICS OF THERMAL DENATURATION
Kajitani, et al, (2011) studied the kinetics of thermal denaturation of protein in cured pork meat related to each of the three protein classes of meat proteins namely myosin (from myofibrillar proteins), sarcoplasmic proteins and collagen (from stromal proteins). Of great interest to us is the sarcoplasmic proteins which include the pigment containing myoglobin.
The first important consideration is that the “thermal denaturation of muscle proteins such as myosin, sarcoplasmic proteins and collagen, and actin, occurs at different temperatures. To describe those reactions during thermal processing, temperature dependency of the reaction rate constant is necessary.” As the level of NaCl in the meat increased, “the thermal-denaturation rate constant of each protein increased.” (Kajitani, et al, 2011)
Adding salt to the sarcoplasmic proteins means that it starts to denature at a temperature of around 50 deg C, reaching a peak at around 68 deg C. Adding Sodium Chloride moves the graph to the left.
Graph source: (Kajitani, et al, 2011)
Having now considered thermal treatment and smoke in some detail, we can move to a consideration of TG in particular, but we will broadly keep looking at colour development, binding strength, and water loss.
TG is mixed into solution before added to the meat. The TG mix contains connective proteins and the first important matter to take into account is the solubility of these proteins.
The maximum activity observed for TG was at 40 °C for the commercial TG. At temperatures above 45 °C, TG suffered a rapid drop in its activity. Optimal pH for commercial TG was found to be between pH 5.5 and 6.0. (Ceresinoa, 2018)
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