Meat is a highly perishable food when raw. Various factors during processing and storage play a role in how and how quickly raw meat can spoil. Even during slaughter, microorganisms on the skin of the slaughtered animal can introduce intestinal bacteria, for example, which can lead to spoilage due to microbiological contamination. These include pathogenic enterobacteriaceae such as salmonella and E. coli, as well as lactobacilli and streptococci. Certain microorganisms find the best growth conditions to multiply on raw meat, such as a high water content (aw value) and a favorable pH value. Cold-loving microorganisms can multiply better at refrigerator temperatures, while heat-loving microorganisms can grow if the meat is not stored at the correct temperature. Spores can also survive at freezer temperatures and multiply again when thawed.
The larger the surface area, for example by grinding the meat more, as is the case with minced meat or chopped meat, the more surface area is available for microorganisms and atmospheric oxygen, so that the shelf life of fresh, raw muscle meat drops to a maximum of 2 days. If the meat is marbled, i.e. slightly streaked with fatty tissue, this has a positive effect on the taste but can lead to rancid smells and flavors due to decomposition processes as storage progresses. The decomposition of meat proteins also leads to sensory changes such as dull and foul odors.
When proteins in meat are broken down by fermentation and conversion by microorganisms, biogenic amines such as putrescine, cadaverine or histamine are produced, which can have health consequences if ingested in high concentrations. Fermentation is used in the production of salami, for example, and serves to preserve the product.
Depending on the environmental conditions, there are many causes of spoilage in raw meat, and the changes often manifest themselves as unpleasant sensory impressions, e.g. sour, rotten, dull or rancid.
The course of the spoilage processes varies depending on the animal species and the associated composition of the fatty tissue, for example. In poultry, the fat consists of fatty acids that are more susceptible to oxidation processes than in beef. Depending on the degree of processing and comminution, the susceptibility to spoilage and rancidity due to oxidation is also greater and has an influence on the development of sensory impressions.
By adhering to good hygiene and manufacturing practices and maintaining the cold chain, it is possible to influence the shelf life.
Fresh beef, even color (left) and 10 days later with browning and mold (right).
Dry aged meat
In the “dry age” process, the maturing process is used under certain conditions to extend the shelf life. The meat, usually beef, is dry-aged on the bone under cool conditions. Maturing takes place over a period of 3-8 weeks, during which the meat hangs in the air in special maturing chambers and thus matures dry. Due to the loss of liquid and thus a reduction in the water content on the surface of the meat, the aroma is also intensified and described as nutty and buttery. As the meat matures during the hanging process, the glycogen still present in the muscle cells is converted into lactic acid with the help of oxygen. This creates the initially acidic sensory impression, and enzymes that break down the rigid structures of the muscles again after rigor mortis ensure that the meat becomes mellow and tender, but also milder again in terms of the acidic impression
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Monosodium glutamate is also formed by the degradation processes of enzymes and contributes to a more intense meat flavor. Due to the drying process and the cool temperatures, mold cultures, for example, are less able to multiply. For dry aged beef, muscle parts covered in fat are used wherever possible to avoid a large contact surface between the muscle cells and the oxygen, which forms a dark dry edge. These dried areas must also be removed for hygienic and sensory reasons, and so it is not a sustainable maturing process due to the longer energy input during cooling and the many losses through the cuts, but it has nevertheless become more important to restaurateurs and retailers due to its taste value.
The Interreg project PRECISE is investigating when meat starts to spoil to a certain extent and the extent to which it can be kept for longer in order to minimize food waste.