DETERIORATIVE CHANGES IN MEAT AND MEAT PRODUCTS

DETERIORATIVE CHANGES IN MEAT

Muscle tissue of living animals is free from microorganisms. Body surfaces with wool, hair and feather and visceral contents like urine, dung or any purulent discharge from an infected part contaminate it during slaughtering and dressing operations. Stored meat undergoes various physico-chemical and microbial changes because of its perishable nature. Meat contamination starts from farm level and progresses during slaughtering and dressing of carcass, during storage, transportation till it reaches the final consumer. The spoilage of meat is usually depicted by color change (change in myoglobin), slime formation (microbial colonies), whiskers (fungus) and putrefied odor (proteolysis, lipolysis, fat oxidation) which makes it unfit for human consumption. In short, proteolysis, lipid oxidation and microbial spoilage are the predominant changes.

            There are several methods adopted to detect deteriorative changes of meat and meat products.

1. Physical examination of meat
2. Estimation of pH
3. Estimation of Water Holding Capacity (WHC)
4. Estimation of Extract Release Volume (ERV)
5. Estimation of Thiobarbituric Acid Reactive Substances (TBRAS) value or TBA value.
6. Estimation of Microbial load

1. Physical examination of meat

The visual physical examination is done for any change in appearance and color, flavor or odor, physical damage etc.

a). Appearance and color: The raw fresh meat is bright red (bloom) in color while as cured meat has a typical pink color. Any change in color may occur accidentally during dressing (bile pigments), absorbed color from the paint of packaging, erosion color or due to microbial contamination.

b). Flavor or odor: The change in typical meat flavor may occur due to putrefactive changes or absorbed odor from foreign material. Any change in odor is a clear indication of spoilage of meat. The major cause of boar taint in pork is due to the presence of sex steroid namely 5α-androst-16-ene-3-one, commonly called androstenone. Sheep and goat odor active compounds include 4-ethyloctanoic acid, 4-methyloctanoic acid and 4-methylynonanoic acid.

c). Physical state: The slime formation or whiskers due to microbial contamination indicate spoilage of meat and meat products.

2. Estimation of pH of meat

The normal physiological pH of muscular tissue ranges between 7.0-7.2. The ultimate pH of meat reaches to 5.4-5.6 in about 18-24 hours post mortem and remains constant for sometime. On storage, there is change in pH of meat and meat products mainly due to metabolites of bacterial action. Generally, the pH slowly increases due to autolysis and bacterial growth giving suspicion of incipient spoilage. When the pH reaches 6.8 or more, objective signs of decomposition and changes in color and texture become apparent. The deviation in pH is considered as indicator of quality of meat sample.

pH of meat is measured by different methods:

1. Using digital pH meter or probe pH meter
2. Nitrazine yellow indicator test
3. pH paper strip

Estimation of pH using digital pH meter:

Requirements: Meat sample, weighing balance, double distilled water, tissue homogenizer or pestle and mortar, digital pH meter and glass beakers

Procedure:

1. Weigh 10 g of minced meat sample in a glass beaker.
2. Add 100 ml of double distilled water.
3. Homogenize the sample with the help of tissue homogenizer or triturate with the   help of pestle and mortar.
4. Calibrate the pH meter with standard solutions (pH 4 and 7) and making adjustments for temperature of the sample homogenates.
5. Immerse the electrode of digital pH meter into the meat homogenate.
6. Let the reading get stable and note the pH of the sample.

Precautions:

• The muscular tissue sample should be free from bone, grits or fascia.
• Try to use freshly prepared chemicals for pH estimation and pH meter calibration.

3. Estimation of water holding capacity

Water holding capacity is defined as the ability of the meat to retain its inherent water under the application of external forces such as cutting, grinding, pressing, heating and centrifugation etc. On the other hand, water binding capacity is defined as the ability of the meat to bind extra water added to a product. Water is a dipolar molecule and as such is attracted to charged species like proteins. Water in muscle exists in threeforms: free, bound and immobilized form. By definition, bound water is water that exists in the vicinity of non-aqueous constituents (like proteins) and has reduced mobility i.e. does not easily move to other compartments. In immobilized form, the water molecules may be held either by stearic (space) effects and/or by attraction to the bound water. This water is held within the structure of the muscle but is not bound per se to protein. In early post-mortem tissue, this water does not flow freely from the tissue, yet it can be removed by drying, and can be easily converted to ice during freezing. Entrapped or immobilized water is most affected by the rigor process and the conversion of muscle to meat. Upon alteration of muscle cell structure and lowering of the pH, this water can also eventually escape as purge. Free water is water whose flow from the tissue is unimpeded. Weak surface forces mainly hold this fraction of water in meat. Free water is not readily seen in pre-rigor meat, but can develop as conditions change that allow the entrapped water to move from the structures where it is found.

       WHC of fresh meat is slightly lower than the WHC in vivo. During the process of conditioning or ageing, a beneficial effect on WHC is observed as it is either retained as in fresh meat or is slightly improved. However, when deteriorative changes are initiated in the meat and bacterial growth takes place, the WHC of meat is quite enhanced. The degraded muscle proteins having been changed to peptones, polypeptides and amino acids by bacterial action hold fast more water due to opening up of water binding sites in the molecules.

Significance:

1. Many physical properties of meat such as color, texture and firmness of raw meat and juiciness and tenderness of cooked meat are partially dependent on water holding capacity (WHC).
2. Higher the WHC, more is the yield of the product.
3. It prevents rendering out of fat from sausages, so lower weight loss of the product.

Factors affecting WHC of meat:

1. pH: The water holding capacity of meat depends upon its pH. As the pH of meat decreases, WHC also decreases and when it reaches to iso-electric point (5.3-5.4), the WHC is lowest due to no net charge on the proteins.

2. Net charge effect: During the conversion of muscle to meat, lactic acid builds up in the tissue leading to a reduction in pH of the meat. Once the pH has reached the isoelectric point (pI) of the major proteins, especially myosin (pI = 5.4), the net charge of the protein is zero, meaning the numbers of positive and negative charges on the proteins are essentially equal. These positive and negative groups within the protein are attracted to each other and result in a reduction in the amount of water that can be attracted and held by that protein. Additionally, since like charges repel, as the net charge of the proteins that make up the myofibril approaches zero (diminished net negative or positive charge), repulsion of structures within the myofibril is reduced, allowing those structures to pack more closely together. The end result of this is a reduction of space within the myofibril. Partial denaturation of the myosin head at low pH (especially if the temperature is still high) is also thought to be responsible for a large part of the shrinkage in myofibrillar lattice spacing.

3. Stearic effect: As muscle goes into rigor, cross-bridges form between the thick and thin filaments, thus reducing available space for water to reside.

Abnormal post mortem conditions affecting WHC:

1. Pale Soft Exudative (PSE) meat

PSE in meat is caused by severe, short-term stress just prior to slaughter in stress susceptible animals, for example during off-loading, handling, holding in pens and stunning. Here the animal is subjected to severe anxiety and fright caused by manhandling, fighting in the pens and bad stunning techniques. All this may result in biochemical processes in the muscle in particular in rapid breakdown of muscle glycogen and subsequent protein denaturation, leading to very pale meat with pronounced acidity (pH values of 5.4-5.6 immediately after slaughter), exudative and poor flavor. This type of meat is difficult to use or cannot be used at all by butchers or meat processors as it is associated with lowered processing yields, increasing cooking losses and reduced juiciness. PSE condition is generally restricted to the muscles of pigs. But can sometimes also be seen in beef, lamb and poultry. Allowing pigs to rest for one hour prior to slaughter and proper handling will considerably reduce the risk of PSE.

2. Dark Firm and Dry (DFD) meat

This condition can be found in carcasses of cattle or sheep and sometimes pigs and turkeys soon after slaughter. The carcass meat is darker and drier than normal and has a much firmer texture. The muscle glycogen has been used up during the period of handling, transport and pre-slaughter and as a result, after slaughter, there is little lactic acid production, which results in DFD meat. This meat is of inferior quality as the less pronounced taste and the dark color is less acceptable to the consumer and has a shorter shelf life due to the abnormally high pH-value of the meat (6.4-6.8). DFD meat means that the carcass was from an animal that was stressed, injured or diseased before being slaughtered. DFD meat is dry or sticky in texture because of its excellent water binding capacity.

Methods for estimation of WHC:

1. Gravity method:

Requirements: Meat sample, Whatmann filter paper No. 4, glass beaker, measuring cylinder, tissue homogenizer/ pestle and mortar and funnel.

Procedure:

1. Take 25 g of minced meat and blend with 100 ml of chilled (4^oC) water in a tissue homogenizer.
2. Filter the homogenate through Whatmann filter paper No. 4 (speed=fast).
3. Filtration is carried out at room temperature for 30 minutes and filtrate collected in measuring cylinder.
4. Volume of filtrate in cylinder accumulated in 30 minutes is recorded and results estimated.
5. More The volume of filtrate, lesser the WHC and vice versa.

   2. Press method:

   Requirements: Meat sample, compensation polar planimeter, Whatmann No. 41 filter paper, glass slides, 100 g weight, balance, sharp pencil.

Procedure:

1. 300 mg of meat sample preferably from Longissimus dorsi muscle is taken and placed on a Whatman filter paper No. 41 (speed=fast).
2. The filter paper is then placed between two glass slides.
3. On top of the upper glass slide, a 100 g weight is placed for a period of 3 minutes so as to exert a downward force. This arrangement is kept on a hard top plate.
4. The released water from the meat sample is absorbed in the filter paper as an impression.
5. With a sharp pencil, the boundary of the impression is carefully demarcated and this area is measured with the help of compensation polar planimeter. The area measured is reported in square centimeters.
6. An increase in the area wetted by the meat sample is interpreted as a decrease in water holding capacity.

4. Estimation of Extract Release Volume (ERV):

ERV refers to the volume of aqueous extract released by meat homogenate (meat: extraction solution in 1:4 ratio) when it is passed through Whatmann filter paper No. 1 for a given period of time. As meat undergoes microbial spoilage, proteins get hydrolysed to peptones, polypeptides and amino acids and then opening the water binding sites in the molecule. The unfolding of molecules results in binding of water to these sites and thus, decreasing the extract release volume of meat. Therefore, good quality of meat has higher ERV value than poor quality meat.

The ERV is inversely related to WHC which is highly correlated with pH of sample. It is inversely proportional to the extent of spoilage of meat.

ERV decreases with progress of spoilage and low filtrate will be detected in putrid meat. ERV becomes zero when frank spoilage or putrefaction sets in. Most of the meat can be considered acceptable provided ERV is at least 17 ml and pH below 6. It has been found that the reduction in ERV from 40 ml to 0 ml increases the total bacterial count from 10⁶ to 10¹⁰ per gram over a period of 10-14 days.

Requirements: Meat sample, distilled water or normal saline, Whatmann filter paper No. 1, glass beaker, measuring cylinder, tissue homogenizer/ pestle and mortar and funnel.

Preparation of extraction reagent:

Take 50 ml of 0.2 Molar Potassium dihydrogen orthophosphate and 3.72 ml of 0.2 Molar Sodium hydroxide and dilute to 200 ml with distilled water. This compound of pH 5.8 is used as extraction reagent.

Procedure (Pearson method):

1. Take 15 g of minced meat and blend with 60 ml of extraction reagent (pH 5.8) for 2 minutes in a tissue homogeniser.
2. The blended contents are quantitatively transferred to a glass funnel (10 cm diameter) with a Whatmann filter paper No. 1 (18.5 cm diameter) folded thrice so as to make eight sections.
3. Filtration is carried out at room temperature for 15 minutes and filtrate collected in a 100 ml measuring cylinder.
4. Volume (ml) of filtrate in cylinder is recorded and results estimated.

Precautions:

• Properly homogenize the sample.
• Note the time correctly as results are interpreted on the basis of time.

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