Within 24 hours after death (1) glycogen ——-> lactic acid (2) muscle pH: 7.0 ——-> 5.6 (because of lactic acid) (3) muscle color: purple changes to bright red or pink (pH 7.0 ——-> 5.6)
Pale soft exudative (PSE) meat is the term used to describe a defective type of meat, seen predominantly in pork, but also in poultry. Unfortunately it is high-value cuts, such as loin and leg meat the borderline between PSE and non-PSE meat is not clearly defined. The combination of two factors, namely a low pH value shortly after slaughtering and a high temperature within meat (above 37 °C) at the same time, are the cause of the PSE condition. 1. Pale soft exudative meat (PSE) اللحوم الشاحبة اللينه, red soft exudative meat and dry firm dark meat اللحوم الجافه المعتمه
PSE meat is usually of pale colour, wet in appearance and very soft in texture and the PSE condition is caused by partially denatured proteins. Denatured proteins cannot hold, or bind, muscular water as well as fully native proteins. More specifically, the length of the myosin filament is reduced by around 8–10% during this process of denaturation and the WHC of meat (the capacity of meat itself to retain, or hold, its own tissue water) is greatly reduced as a result. As the muscle tissue develops acidity (decrease of pH) the water holding capacity decreases.
The light, or lighter, colour of PSE pork is explained by the small myofibrillar volume in the muscle tissue. Muscle tissue with a small myofibrillar volume has a high light- scattering ability; so light is reflected differently in PSE meat from normal pork.
Red soft exudative (RSE) meat is another term used to describe meat that has a quality defect. RSE pork has the same characteristics as PSE pork, except that it preserves the natural red colour of meat better than PSE pork, possibly because the carcass was chilled quickly after slaughter. Dry from dark (DFD) meat is the term used for another type of defective meat; DFD meat is also known as ‘dark-cutting meat’. DFD characteristics in meat can be seen predominantly in beef as well as lamb; however, some pigs nowadays also exhibit DFD character.
In poultry, both PSE and DFD character can be found. Poultry generally enter rigor mortis very quickly and post-mortem glycolysis seems to take place more rapidly in white muscles, such as breast, compared with red muscles from the leg.
From a technological view point, DFD meat has the advantage of high protein solubility, as acidification during rigor mortis never really took place and an actomyosin complex was only obtained to a small degree (Table 1). The WHC of DFD meat is also excellent; the high pH value correlates with a high WHC as the pH value of DFD meat is a long way from the IEP (pH value of 5.2).
scle color Glycogen at death Glycogen at 24 hr Lactate production Ultimate muscle pH Normal1.0%0.1%high5.6 Dark0.3%0.1%low6.0 to 6.5 Pale0.6%0.1%very high5.1
The terms ‘mechanically deboned meat’ (MDM) and ‘mechanically separated meat’ (MSM) basically describe the same type of meat. When discussing MDM and MSM, a distinction has to be made between ‘hard’ and ‘soft’ MDM and MSM. Hard MDM and MSM are generally produced from meaty bones of pork and beef, from which it would be quite labour intensive to clear all meat off the bone. Hard MDM from poultry is predominantly obtained from carcasses of chicken or turkey, with the valuable parts such as wings, legs and breast removed. Bones or carcasses are exposed to high pressure in a kind of ‘pressure chamber’, which has small holes in it. 2. Mechanically deboned meat and mechanically separated meatMechanically deboned meat mechanically separated meat
Through the application of high pressure, carcass- or bone-attached meat, fat and skin separates from the bones and those materials pass through the barrel sieve (around 0.5–0.8 mm in diameter) whilst the bone part remains inside the barrel and is discharged separately. Hard MDM and MSM should not contain bone particles within the material obtained and the amount of bones, or small fragments of bone, should not exceed 0.3%.
Hard MDM from pork or chicken demonstrates a significantly shorter shelf life, also under freezing conditions, than beef MDM. This is due to a significantly higher level of unsaturated fatty acids present in the fat fraction of chicken or pork MDM fat, than in beef fat, and rancidity develops quickly within such material.
MDM has a large surface area and is therefore susceptible to high levels of bacterial growth. The bacteria count in hard MDM should not be higher than in ordinary mincedمفروم muscle meat and should not exceed 10 5 –10 6 colony-forming units (cfu)/g. Hard MDM contains between 12 and 15% protein. The 12–15% protein represents around 60–70% of the protein found in muscle meat. hard MDM shows a high pH value of around 6.2–6.4
Soft MDM is obtained from meat trimmings, which are high in connective tissue and cartilage such as shank meat. These materials are put through a machine which separates meat from connective tissue based on the different degrees of firmness and textures of those materials. The meat material obtained from this separation process is high in protein and an excellent material for all types of sausage and coarse-minced salami.
Soft MDM contains around 15–17% protein, about 70–80% of the protein in lean muscle meat (around 21% protein). Therefore, the WBC and fat emulsification capacity of soft MDM is around 70–80% of that of lean muscle meat and all protein within soft MDM is still functional as it does not become damaged during processing. This material is rich in connective tissue and can be perfectly utilized in cooked sausages as the high content of connective tissue.
6- pH value The pH value has a significant impact on colour, shelf life, taste, microbiological stability, yield and texture of meat and meat products and is therefore one of the most important parameters within the production of meat products and meat itself. The pH value is referred to as the ‘acidity of meat’, which is only partly correct as the pH scale ranges from 0 to 14 and covers not only the sour range (Fig. 6). The pH values of meat and meat products lie generally between 4.6 (raw fermented salami) and 6.4. At a pH value of around 6.4, meat is spoiled owing to enzyme activity, which produces a large amount of metabolic byproducts as well as ammonia. Sliminess, bad smell and discolouration can be seen at this point as well.
A pH value of 0.1 is extremely sour, 7 is neutral whilst 14 is extremely alkaline. pH > 7 corresponds to an alkaline solution, pH = 7 to a neutral solution and pH < 7 to an acid solution.
Redox (reduction–oxidation) reactions (Fig. 7) are one of the most commonly occurring reactions in cells and are mostly catalyzed by enzymes. Reduction and oxidation refer to the exchange of electrons from a donor on to an acceptor where the donor is oxidized and the acceptor is reduced at the same Time. 8- E h value (redox potential)
The Eh value depends largely on the chemical composition and the O2 pressure within food as well as the pH value. A reduced Eh value lowers the ability for aerobic bacteria to grow whilst a high Eh value reduces the growth of anaerobic bacteria. A decrease in the Eh value can be achieved by the addition of reducing agents such as ascorbic acid or erythorbate or the application of vacuum during the manufacture of a product.
Freezer burn is a condition that occurs when frozen food has been damaged by dehydration and oxidation, due to air reaching the food. It is generally induced by substandard (non- airtight) packaging. Meat suffering from freezer burning exhibits on its outside layers a dry and fibrous structure owing to severe dehydration. Changes in colour within those layers can be observed as well and the original red colour changes into a lighter, sometimes even slight yellow–green, colour. Rancidity is also speeded up in those dry outside layers as a reduced water content favours the development of rancidity. 5- Freezer burning
Freezer burning can be largely avoided:- 1- If the product is packed or covered properly, the packaging material does not allow the gas to evaporate. 2- The packaging material should be of low water permeability and 3- little space as possible should be present between meat and the packaging material owing to possible oxidation in those areas.