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As mentioned previously in the chapter 6, water represents the largest portion of muscle tissue (about 75%) and it is organized in layers located around polar molecules and among stratums of cellular materials. Several forces control meat water movements (Pearce et al., 2011).

WHC is one of the most important functional properties of meat and an important quality attribute for both processors and consumers. Water-holding capacity or juiciness is definite as the aptitude of meat to hold water (naturally present in meat or added) during the application of forces like heat and pressure. The ability of meat to hold water helps with tenderness, juiciness, firmness and appearance of the meat, leading to an improvement in quality and economic value. WHC of meat can be categorized as water binding potential

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(WBP), expressible moisture and free drip, each having different applications. For instance, WBP is defined as the ability of the muscle proteins to retain excess water and under the influence of external forces; therefore it represents the maximum amount of water that muscle proteins can retain under the established condition (Swatland, 1994). Expressible moisture represents the amount of water that can be expelled out of the meat by use of force, and free drip is the amount of water lost from the meat without any force other than gravity (Swatland, 1994), which is important for retail display and consumer acceptability of tray packed meat.

Most of the water (88 to 95%) inside the muscle is held within intracellular spaces between actin and myosin filaments, and only a small portion (5 to 12%) is located between the myofibrils (Offer and Knight, 1988); the portion of water that affects more the meat water-holding capacity (WHC) is located among those miofibrillar proteins (actin and myosin) where it is detained in intermolecular spaces by capillary force. Several factors such as pH, sarcomere length, ionic strength and osmotic pressure affect WHC (Offer and Knight, 1988) because they all influence the distance between myosin and actin/tropomyosin. This space can range from 320 Å to 570 Å and is preserved by electrostatic forces, which are active even for relatively long distances (Brewer, 2004; Pearce et al., 2011). Development of rigor mortis also influence WHC. After animal death, lactic acid is produced and pH declines causing a reduction in water binding ability of the meat due to protein denaturation, loss of protein solubility and therefore reduction of reactive groups available for water binding on muscle proteins (Offer and Knight, 1988). When pH decreases to values close to the isoelectric point of proteins (4.9-5.3), water binding ability of the proteins is impaired, thick and thin filaments move closer together, myofibrils shrink, and the volume of sarcoplasm increases. Eventually, muscle fibers deplete all their ATP, their membranes no longer confine the cell water, and fluid is lost from the muscle fiber that may contribute to the exudate lost from the meat (Swatland, 1994).

Proteins (for example actin and myosin) are molecules formed by amino acids jointed among themselves by peptide bonds in order to form an amino acidic chain, that represents the primary structure (sequential order of amino acids). The polypeptide chain is structured in order to form a three-dimensional molecule, which represents the second and the third structure. Finally proteins can show a quaternary structure that explains the geometric organization between different polypeptide chains usually bonded with each other through no covalent bounds. Amino acids present several side chains that are externally located respect the main protein filament. They can be charged in different ways (neutral, positively or negatively) according to the type of amino acid and environmental pH. As previously

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mentioned, during the transformation of muscle to meat, pH decreases as a result of the increase of the muscle lactic acid amount. This dramatic decreasing of pH value causes the reduction of the protein reactive charged groups, which represents the ones able to bond free water. This variation of pH determines a relevant decrease of WHC values it mainly depends of three factors:

a) Net charge effect. Definite as the total quantity of amino acidic charged groups able to bond water molecules. Meat pH, as a consequence of lactic acid accumulation, decreases until it reaches the isoelectric point (at this pH protein presents the same number of negatively and positively charged groups). In this way, for muscle proteins the pH is around 5.5. In this conditions, only a few groups of the side chain are able to interact with water (net charge effect). When pH is around 7.0 (live muscle pH), muscle WHC presents higher value than after rigor-mortis with pH around 5.6. This shows the possibility to obtain a better net charge at a higher pH and consequently proteins can held more molecules of water (Barbut, 2002; Huff-Lonergan and Lonergan, 2005).

b) Steric effect. Definite as the repulsion observable fact noticeable between different side chains charged with a analogous charge. It is well known that charged groups with the same charge repulse themselves. This represents a positive phenomenon, especially for the meat processor, because more molecules of water can be held if bigger spaces between fibre filaments are formed. It can be possible at pH lower or higher according to the isoelectric point, where an elevated quantity of negatively or positively charged groups are present, resulting in more repulsion. WHC of post mortem meat is abridged, for the reason that the pH is near to isoelectric point. Post mortem meat WHC can be increased with the variation of meat pH using alkaline (e.g. phosphates) or acid ingredients (Huff-Lonergan and Lonergan, 2005).

c) Ions exchange. Definite as the phenomenon that occurs when rigor mortis has been completed; during the process of aging, when ions are relocated after degradation of cell structure performed by enzymes located in myofibrillar proteins. Some divalent cations as Mg²⁺ or Ca²⁺ are replaced with monovalent cations like Na⁺ and K⁺, resulting in the creation of free side groups charged of protein, which increases the meat WHC. Calcium (Ca²⁺) is a divalent cation that is released during post mortem process. It has the ability to bind and consequently neutralize two negatively charged side groups. When calcium is

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substituted by monovalent ions, proteins increase the number of free binding site to bind water (Pearce et al., 2011; Barbut, 2002).

Every chemical, physical or enzymatic process that increases the space between protein filaments improve the quantity of water to be held and thus increasing the WHC. This is possible because the large part of water in muscle is located in the space between thick (myosin) and thin (actin) filaments that form myofibrils. This is the reason why salt (NaCl or KCl) and phosphates are used to improve water holding capacity in meat product. Usually salt and phosphate in water solution are included into raw meat by injection, marination or tumbling, resulting in a higher juiciness and cooking yield of the product.

In order to estimate WHC in raw meat and in meat products the following methods are usually used: (1) applying pressure (mild to severe; by compression or centrifugation); (2) monitoring sample performance during regular processing such as cooking or storage; (3) watching meat product microstructure; (4) applying special technique like nuclear magnetic resonance (NMR) to check water molecules state and position; (5) using optical sensors (Barbut, 2002).