Meat, chilled

Infobox on Meat, chilled
Example of Meat, chilled
Facts
Origin	This table shows only a selection of the most important countries of origin and should not be thought of as exhaustive: Europe: Sweden, Denmark, Germany, Netherlands, Belgium, France, Switzerland, Austria, Ireland, Romania, Bulgaria, Italy, England Africa Asia: China America: Argentina, Paraguay, Brazil, USA Australia: Australia, New Zealand
Stowage factor (in m3/t)	Approx. 1 m3/t (cartons) Approx. 2.80 - 3.56 m3/t (sides of pork)
Angle of repose	-
Humidity / moisture	Relative humidity: approx. 80 - 85% Water content: approx. 50 - 75% Maximum equilibrium moisture content: 80% Due to its high content of unfrozen water, chilled meat can only be considered for short transport operations as enzymatic and bacterial activity may continue slowly.
Oil content	-
Ventilation	Recommended ventilation conditions: circulating air, 6 circulations/hour without supply of fresh air in order to ensure uniform cooling of all parts of the cargo.
Risk factors	Chilled meat is very highly sensitive to temperature variations and foreign odors (see also text).

Meat, chilled

Description

When adequately chilled, meat retains the characteristics of fresh meat. The degree of redness of meat is determined by its myoglobin (muscle pigment) content, which depends upon the species, breed, age and other factors.

Meat is chemically composed of water (approx. 50-75%), protein (approx. 15-20%), fat (approx. 5-35%), mineral salts (1-2%) and carbohydrates (glycogen; 0.3-0.5%).

On account of its high water, protein and fat content, meat is very perishable. By maintaining low temperatures, deterioration is retarded.

The quality of meat is judged by its bacterial condition and appearance. Appearance criteria are primarily colour, percentage of fat and lean, and amount of drip exuding from the meat.

There are three types of microorganism that cause damage to food; bacteria, yeasts and moulds. There are occasions in food manufacture where the activity of microorganisms is exploited as part of the production process. However, during storage and transportation, their activity is almost always deleterious and their growth needs to be controlled.

Microorganisms are likely to be present on all foods and their presence should be assumed. The aim of food processing and storage is to limit the opportunities for microorganisms to grow to the point where they are able to cause food to spoil, or the point where they can cause food to be unsafe to eat. Note the distinction between food spoilage microorganisms (which cause food to become inedible, owing to the production of off-odours, taints, etc.) and food poisoning microorganisms (which cause illness and which may or may not be accompanied by signs of spoilage).

The ability of microorganisms to grow is affected by what are termed ‘intrinsic factors’ (i.e. those that relate to the properties of the food itself, such as pH and salt content of the food) and ‘extrinsic factors’ (i.e. those that are controlled by the environment in which the food is stored). In relation to transport, most attention needs to be paid to the extrinsic factors; these are temperature, humidity and the presence and concentration of particular gases in the environment.

Applications

Meat is an important source of nutrition.

Shipment / Storage

Chilled foods must be carried at temperatures between about -1.5°C and +5°C. For some products an upper maximum temperature of not more than 2°C may be specified, e.g. for chilled beef an upper limit of 0°C is recommended; delivery air of chilled meat should never be lower than -1.4°C, as otherwise there would be the risk of freezing injury (ice crystal formation).

The freezing temperatures of the moisture in beef cuts (formation of ice crystals) generally range around -1.7°C, although this is dependent on the chemical constitution of the meat.

The temperature of well vacuumed chilled beef can rise to +1°C (max. +2°C) before sustaining physical detriment (excessive blood-loss, discolouration, fat oxidation, enzymatic activity, microbial spoilage etc.). However, any temperature rise above -1.4°C (usual carrying temperature for chilled beef) resp. interruption of the cooling chain fundamentally deteriorates the Potential Storage Life of the product.

Red meat and poultry are very perishable raw materials. If stored under ambient conditions at 16°/30°C, the shelf life of both can be measured in tens of hours to a few days. Under the best conditions of chilled storage, close to the initial freezing point of the material, the storage life can be extended to approaching six weeks for some red meat. Even with the best commercial practice (strictly hygienic slaughtering, rapid cooling, vacuum packing and storage at super chill; -1 ± 0.5°C), the maximum life achievable in red meat is approximately 20 weeks. Freezing will extend the storage life of meat to a number of years.

The most important single factor governing microbial growth is temperature. The higher the temperature the greater is the rate of growth. Many meat micro-organisms will grow to some extent at all temperatures from below 0°C to above 65°C, but for a given organism, vigorous growth occurs in a more limited temperature range. It is customary to classify meat spoilage organisms in three categories. Psychrophiles have temperature optima between -2°C and 7°C, mesophiles between 10°C and 40°C and thermophiles from 43°C to 66°C. The distinction is by no means absolute, however, as certain gram-negative rods, which are generally regarded as mesophiles, will grow at -1.5°C.

In general, a reduction in the number of micro-organisms occurs when meat is frozen; but yeasts and moulds will grow at -5°C although not at -10°C. Carefully controlled experiments have failed to substantiate the prevalent view that thawed meat is intrinsically more perishable than meat which has not been frozen. Even so, under commercial handling conditions, the moister surface of thawed meat would tend to pick up greater numbers of bacteria and hence be potentially more liable to spoil. Even with storage at 0°C spoilage could occur through the activities of psychrophiles, e.g. Ps. Fluorescens.

The effect of temperature on microbial growth may differ according to the nature of the nutrients available. It is important to appreciate that, if there has been heavy microbial growth before freezing, a high concentration of microbial enzymes may have been produced. Thus, even if microbial growth is arrested by the process of freezing, the enzymes may continue to produce deleterious quality changes even down to about -30°C.

After temperature, the availability of moisture is perhaps the most important requirement for microbial growth on meat, although some types of bacteria may remain dormant for lengthy periods at low moisture levels; and spores resist destruction by dry heat more than by moist heat.

Given adequate cooling, the deterioration of fresh chilled meat is due to surface changes. The natural surface consists of fat and connective tissue; and during cooling the consistency of the latter changes, so that further loss of water by evaporation is restricted. On the other hand, muscle surfaces continue to lose water at a fairly fast rate; and this desiccation leads to an increased concentration of salts at the surface which causes oxidation of the muscle pigment to brown or greyish metmyoglobin and a darkening of colour due to optical changes in the tissue. Different muscles show differing susceptibility to such browning due to differing tendencies to desiccation. If surfaces are more moist, moulds of various colours will tend to grow, and these may affect the fat, causing rancidity and off-odours due to other changes. If the surfaces are moist still, bacteria can grow and, in sufficient numbers, produce off-odours and aggregate in visible colonies (slime). Apart from moisture, these features are a function of time and temperature.

When meat is removed from chill storage, moisture tends to condense on the cool surfaces, especially when the relative humidity of the atmosphere is high. This phenomenon is known as ‘sweating’. Apart from its potential effect in encouraging microbial growth, it causes the collagen fibres of connective tissue to swell and become white and opaque. This change is reversible, however, and there is no evidence that ‘sweating’ causes a permanent loss of ‘bloom’, the term given by the trade to a pleasing superficial appearance of the meat. On the one hand, a high relative humidity in the storage chambers will prevent desiccation and loss of bloom; on the other hand, it will encourage microbial growth. A balance has to be established between these two extremes over storage times. Although some degree of desiccation is desirable, excessive drying, of course, must be avoided, especially because of its effect on the layer of connective tissue separating the muscles from the exterior. Although this layer is very thin, it imparts a pleasing, translucent appearance to the surface of the carcase, even when there is little subcutaneous fat. When the layer becomes desiccated, the superficial appearance deteriorates remarkably.

A large number of investigators have shown that when O₂ permeable packaging is used, refrigerated fresh meats undergo gram-negative spoilage accompanied by increased pH and foul odours, with Pseudomonas spp. being the predominant organisms. It has been shown that the storage life of vacuum-packaged beef is inversely related to film permeability, with the longest shelf-life (>15 weeks) obtained with a ‘zero’ O₂ film and the shortest (2-4 weeks) with a highly permeable film. Growth of Pseudomonas spp. increases with increasing film permeability. On the other hand, if O₂ impermeable packaging is used, the growth of lactic acid bacteria and sometimes that of Brochothrix thermosphacta is favoured because of increased levels of CO₂. These organisms typically effect a decrease in pH and create an unfavourable environment for most food-borne pathogens and gram-negative bacteria. It should be noted that, because of the absence of air, chilled meat may show an abnormal discolouration and upon removal of the vacuum packaging can give off a characteristic and distinctive odour. On exposure to air, the colour of the meat reverts to normal and the distinctive odour will disappear. Thus no immediate conclusions should be drawn as to the condition of the meat after removal of the packaging. However, (CO₂) gas-bubbles in vacuum packs of meat are usually deemed evidence of incipient microbial spoilage. Much variability in the keeping quality of pre-packaged fresh meat can be reflected by such features as moisture content, salt concentration, pH, degree of exudation, the surviving activity of oxidizing enzymes, the degree of protein denaturation, the content of amino acids and other micronutrients and, thereby, microbial growth.

During life, the pH of muscle is close to neutral (pH = 7). Following death, enzymes in the muscle begin to convert glycogen (the energy storage carbohydrate of muscle) into lactic acid. This causes a drop in pH, typically to about 5.7 in normal meat. This is still above the isoelectric pH (about 5.5) of the meat proteins. If animals are starved in the period prior to transport to the abattoir, their muscles will contain very little glycogen at the time of death. Little lactic acid will therefore be produced, and the ultimate pH of the meat will be high, perhaps up to about 6.8. At this pH, the proteins have a very high water-binding capacity, so the meat will appear dry and firm; it is also dark in colour. Although the high water-binding capacity might appear to be an advantage, because of its high pH, this (dark, firm, dry) meat is prone to very rapid microbial spoilage.

Additional information on chilled meat

The main problem encountered with chilled meat is temperature variation. Evidence with bone in beef is usually blood drip from temperature rise. Stained wrappers and fats and discolouration of muscular tissue will also be detected. In more severe cases of prolonged variation surface slime formation and mould formation can be expected.

In the case of boneless cuts any blood drip will, of course, be retained within the vacuum bag. Beef cuts normally show some free blood within the bag and therefore attention should be paid to the amount of free blood which, if excessive, will indicate a problem. Such exudate is not anticipated from lamb cuts and therefore the presence of appreciable amounts of blood will indicate a problem. Damage to chilled meats can also occur from downward variations of temperature. This will be evidenced by the hard/firm feel of the meat.

Such damage is usually accompanied by the presence of intra-muscular ice crystal formation. Chilled lamb carcasses/cuts are being shipped from Australia/New Zealand to the E.U. in foil bags. The selected cuts or carcasses are firstly individually wrapped in absorbent white paper toweling and then placed, five cuts or two whole carcasses, into a foil barrier bag. The bag is flushed with food grade carbon dioxide and then evacuated to remove all traces of oxygen before finally being flushed and loaded with an excess of carbon dioxide. During these ‘gassing’ operations extreme care has to be taken throughout to ensure the integrity of the seal. The basic idea of ‘gas flushing’ produce is that over the storage/transit period this carbon dioxide gas is absorbed into the meat. This absorption of gas has an inhibitory effect on those bacteria considered undesirable whilst enhancing the growth of lactobacilli. The presence of such bacteria assists in maintaining product so that it is in good condition when made ready for use. Unfortunately, damage does not come to light until the bag is opened at final destination, i.e. the supermarket outlets or, in the case of lamb carcasses, when boning to individual cuts.

Temperature fluctuations will also cause deterioration similar to poor sealing. Considerable care has to be taken when differentiating between poor sealing and fluctuations.

Risk factors

• Odor
• Contamination
• Mechanical influences
• Toxicity / Hazards to health
• Shrinkage/Shortage
• Insect infestation / Diseases
• Bacterial spoilage