In food industry, shelf life is a frequently used term. This is a very important indicator, because it is the guarantee and commitment of manufacturers to the quality and efficacy of commodities during the circulation period.
Shelf life of food refers to the storage time when the quality of food remains acceptable to consumers from the perspective of sensory and food safety. Food, as the basic source of energy and nutrients necessary for people’s survival, not only meets people’s physiological needs and nutritional and health requirements, but also needs good quality.
Consumers’ requirement for food quality is rising higher and higher. Less additives and environmentally friendly packaging to obtain food with good sensory performance, high nutritional value, health, convenience and long shelf life has become the choice target of most consumers. But all foods will go through different degrees of deterioration during storage. Deterioration includes the degradation of sensory acceptance, reduced nutritional value and food safety.
Essentially, shelf life mainly depends on four factors: food composition, processing conditions, packaging and storage conditions. These factors have been incorporated into HACCP (Hazard Analysis Critical Control Point) which is a food safety and quality control system. Food is a complex system, in which biological, physical and chemical changes occur simultaneously. These reactions will lead to the decline of nutritional value of food and produce undesirable by-products, even toxins. Similar reactions continue to occur during storage after processing, at which time the reaction rate depends on the intrinsic nature of food, fresh meat packaging solutions, storage and transportation conditions, etc.
1. Causes of spoilage of chilled meat
The main causes of spoilage and deterioration of cold meat can be summarized as follows:
(1) The growth of microorganisms on the surface of the body results in the spoilage and deterioration of meat.
The main microorganisms are Pseudomonas spp. under aerobic conditions and Lactobacillus spp. and Enterobacter spp. under vacuum conditions. Bacteria mostly grow on the surface of the meat. Bacteria on the surface of meat can decompose proteins and other nutrients, which make the surface of the meaat sticky, produce odor and decay in a short time.
(2) The oxidative discoloration of myoglobin in meat affects the appearance color of meat.
The color change of meat is controlled by the chemical state of myoglobin and residual hemoglobin. During cold storage, muscle tissues absorb oxygen from the air, which changes the structure of body tissues. A few days before storage, myoglobin forms unstable oxymyoglobin, which makes fresh meat bright red, and then gradually forms stable high iron myoglobin, which makes fresh meat dark red (brown).
(3) Deterioration of meat quality caused by enzymes.
High temperature can significantly increase the activities of enzymes in fresh meat and microorganisms, and increase the activities of enzymes secreted by microorganisms around them. Therefore, it can facilitate various enzymatic reactions and deteriorate meat products.
1) Effect of Microorganisms on Meat Products
Microbial contamination of meat products is affected by many aspects, such as slaughter process, subsequent storage conditions, etc. The combination and interaction of internal and external factors determine the growth of microorganisms in meat products. Under normal conditions, the growth of these cold-adapted bacteria was inhibited to a certain extent because the chilled meat was always in a low temperature environment of 0-4 degrees Celsius. However, if the cold chain system is not perfect enough, improper control will occur during storage, transportation and sales, which will lead to the change or failure of the control system and increase the temperature. Microorganisms will rapidly proliferate, accelerate the deterioration of fresh meat, and even pose a potential threat to public health.
2) The influence of external environment on meat products
Temperature can cause the oxidation of fat and fat-soluble components, browning and quality degradation; humidity can cause food hygroscopicity or dehumidification, resulting in shrinkage and deformation of food morphology; light can promote chemical reactions and changes in color, aroma and taste of food; chemical reactions can be promoted in the presence of oxygen, resulting in changes in food quality; filling inert gases can inhibit such chemical reactions. Therefore, the purpose of inhibiting the decline of food quality is achieved.
3) The influence of intrinsic properties of meat products on it
When the water-soluble ingredients of food are higher or hygroscopically absorbed, browning, solubility and resilience decrease, and nutrient loss occur. When there are more fat-soluble components, the oxidation of fat will change color, fade, change of flavor, loss of vitamins and so on when oxygen and light exist. In foods with enzymes, high water content or moisture absorption of food easily lead to the activation of enzymes, resulting in deterioration of food color, aroma and taste.
2. Method of prolonging shelf life of meat
1) Vacuum packaging
①Introduction of Vacuum Packaging
Vacuum packaging is a kind of packaging method that packs products into airtight packaging containers and extracts air from the inside of the containers so that the sealed containers can reach a predetermined vacuum. Meat vacuum packaging can inhibit the growth of many spoilage microorganisms and slow down the oxidation of fat in meat by keeping the hypoxic environment in the packaging bag. At the same time, it can reduce the loss of water and keep the appearance clean, thus prolonging the shelf life of meat products.
② Advantages of meat Vacuum Packaging
The main advantages of vacuum packaging are good protection, reliable sealing, high vacuum and less residual air. It can effectively inhibit the proliferation of microorganisms such as bacteria and avoid oxidation, mildew and corruption of the contents.
For fresh meat, vacuum packaging has the following advantages:
(1) Prevent drying.
The packaging material is provided with steam shielding to prevent drying and keep the surface of fresh meat soft.
(2) Preventing oxidation.
When vacuum is pumped, oxygen and air are removed together, packaging materials and atmospheric shielding make no oxygen enter the packaging bag, oxidation is thoroughly prevented.
(3) Preventing the loss of meat fragrance.
Packaging materials can effectively prevent the spillover of volatile aromatic substances, but also prevent the taste of different products.
(4) Avoid freezing loss.
Packaging materials isolate the product from the outside world, so ice formation and air drying during freezing can minimize the loss.
③Defects of Vacuum Packaging
The principle of vacuum packaging is relatively simple: remove the oxygen in the packaging, prevent the change of content properties (such as food deterioration). Generally speaking, the oxygen concentration is less than 1%, the growth and reproduction speed of microorganisms will decline rapidly, and the shelf life of low-temperature meat products in vacuum packaging is generally more than one month. However, if contaminated microorganisms, especially viable rotting strains, will continue to grow and reproduce, resulting in product spoilage during shelf life, affecting product quality.
Vacuum packaging can not inhibit the growth of anaerobic bacteria and food deterioration and color caused by enzyme reaction. Lactobacillus is the main spoilage bacteria in vacuum packaging of low-temperature meat products. When low-temperature meat products are vacuum packaged, these spoilage bacteria can not be detected by cultivation at the early stage of storage (< 10cfu/g), but then the products are rapidly corrupted, which increases the spoilage rate of the products, causes great difficulties to quality management, and poses a serious threat to product safety. So sometimes it is necessary to combine other auxiliary methods, such as refrigeration, high temperature sterilization, pickling and so on.
2) MAP Modified atmosphere packaging
①Principle of Modified Atmosphere Packaging
The fresh-keeping mechanism of modified atmosphere packaging is to protect food with good barrier packaging materials. It means filling certain ideal gas components into the packaging, improving the environmental components in the packaging under certain temperature conditions, inhibiting the growth of microorganisms and preventing the spoilage caused by enzymes, so as to achieve the purpose of preservation and preservation, thus prolonging the shelf life of products. The gases used are usually carbon dioxide, oxygen and nitrogen or their combinations, and each gas has different effects on fresh meat. Generally, the shelf life of fresh meat packaged by inflatable packaging can reach 10-14 days.
(1) Carbon dioxide acts as a bacterial inhibitor.
Carbon dioxide is a stable compound, colorless and tasteless, which has a strong inhibitory effect on the reproduction of most aerobic bacteria and fungi. At the same time, it can also prolong the lag period of bacterial growth and reduce the speed of its logarithmic growth period.
The bacteriostasis of carbon dioxide is that carbon dioxide replaces oxygen in the packaging bag with high concentration of carbon dioxide, which inhibits the growth of a large number of aerobic microorganisms (such as Pseudomonas sp.). The second is that carbon dioxide can dissolve in water to form carbonic acid, reduce the pH value of the environment, and make some acid-resistant microorganisms lose the necessary conditions for survival. Permeation of cells.
At the same temperature and pressure, the solubility of carbon dioxide in water is 6 times that of oxygen, and the rate of infiltration into cells is 30 times that of oxygen. Because of the large amount of carbon dioxide infiltration, the structure of cell membrane will be affected, the permeability of membrane to ions will be increased, the balance of metabolism inside and outside the membrane will be changed, and the normal metabolism of cells will be disturbed, so that the production of bacteria will be inhibited. Carbon dioxide infiltration also stimulates mitochondrial ATPase activity, accelerates oxidative phosphorylation, reduces ATP, and reduces the energy required for metabolic growth.
(2) The effect of oxygen.
The use of oxygen in modified atmosphere packaging is mainly due to the combination of myoglobin and oxygen molecules in muscle, which turns into oxymyoglobin and is bright red. Therefore, in order to keep the meat bright red, oxygen must be in the packaging bag. Oxygen content in natural air is about 20.9%, so the surface of fresh cut meat is light red when exposed to air.
It is reported that when the temperature is 0 C and the relative humidity is 99.3%, the oxygen partial pressure is 800 +400 Pa (0.4% oxygen to 1.2% oxygen), the formation of methemoglobin is the most. Oxygen must be more than 5% in order to reduce the formation of methemoglobin. But according to Bartkowski (1982), oxygen must be more than 10% to maintain bright red, and more than 40% of oxygen can maintain a good color for 9 days. The formation of bright red oxymyoglobin is also related to whether the meat surface is wet or not. When the surface is wet, the dissolved oxygen is more, and it is easy to form bright red.
(3) The effect of nitrogen.
Nitrogen is an inert gas that generally does not work on packages and is not absorbed by food. Nitrogen has low air permeability to plastic packaging materials, so it can be used as a buffer or balance of mixed gases in packaging, and can prevent collapse caused by carbon dioxide filling.
② MAP modified atmosphere packaging Advantage
Modified atmosphere packaging is one of the most common and effective meat packaging solutions to prolong the shelf life of all kinds of food (including chilled meat). Reasonable modified atmosphere packaging can not only ensure the health quality of meat products, prolong shelf life, but also have a good impact on the sensory quality of chilled meat. The most commonly used gas components in modified atmosphere packaging are carbon dioxide, oxygen and nitrogen.
The use of this gas mixture is mainly from the control of microbial growth and sensory quality (such as color). The effect of modified atmosphere packaging on fresh meat mainly depends on the initial number of bacteria in raw meat, the concentration of carbon dioxide, the presence or absence of oxygen in packaging bags, the permeability of packaging materials, storage temperature and gas composition. It inhibits microorganisms by causing changes in the rate of bacterial synthase and enzyme reactions and by affecting carboxylation and deamination reactions. A certain proportion of oxygen is added to the mixture to maintain the color of fresh meat and inhibit the growth of pathogenic bacteria.
Modified atmosphere packaging system is a closed system. There are two processes in this system: one is the physiological serialization process of products, i.e. the respiratory process of metabolism; the other is the gas exchange process between products and packaging bags caused by the permeability of packaging materials. These two processes make the air conditioning system a dynamic system.
The principle of modified atmosphere packaging can be summarized as two points: 1. destroying the environment in which microorganisms grow and reproduce; 2. maintaining certain activity of cells in food and keeping a certain degree of freshness.
There are two main factors affecting food storage: aerobic bacteria and oxidation reaction, both of which require oxygen. Therefore, to prolong shelf life, it is necessary to reduce the oxygen content in the environment. Modified atmosphere packaging technology can make up for the deficiency of vacuum packaging to a certain extent and further inhibit microbial corruption.
③ Notices for Modified Atmosphere Packaging
(1) The treatment of fresh meat before packaging.
After slaughter, if the pigs were cooled for 24 hours at 0 14 C, the acid excretion process would occur after the ATP stopped its activity. The nutrition and taste of chilled meat treated in this way are much better than that of quick-frozen meat and hot fresh meat. In addition, in order to ensure the fresh-keeping effect of modified atmosphere packaging, it is necessary to control the hygienic index of fresh meat before packaging and prevent microbial contamination.
(2) Selection of packaging materials.
Modified atmosphere packaging should choose packaging materials with good barrier to prevent gas spillover in packaging, but also to prevent the infiltration of oxygen in the atmosphere. Usually, composite packaging films based on PET, PP, PA and PVDC are selected. In general, the smaller the permeability coefficient is, the better the barrier is. In addition, all packaging materials must have sufficient mechanical strength to withstand the pressure changes during vacuum pumping. At the same time, a certain stiffness of the material is required so that the packaging bag can be automatically opened and inflated.
(3) Control of product storage temperature.
The effect of temperature on freshness preservation comes from two aspects: one is that the temperature directly affects the activities of various microorganisms on the surface of the body; the other is that the barrier of packaging materials is closely related to temperature, and the higher the temperature, the lower the barrier of packaging materials. Therefore, it is necessary to realize temperature control in the whole process from product, storage, transportation to sales.
At present, vacuum packaging technology can inhibit the growth of microorganisms, prevent secondary pollution, slow down the speed of fat oxidation, make meat clean and prolong the shelf life of meat products. If the cold meat is packed in vacuum, it will have a certain effect on preservation, but its color will be purple red, which is not perfect.
Modified atmosphere packaging is an ideal method to prolong the shelf life of meat products. Modified atmosphere packaging technology can inhibit the growth of microorganisms and prolong the shelf life of meat products. Modified atmosphere packaging can not only make meat bright red, but also significantly inhibit the growth and reproduction of spoilage microorganisms, ensure the health quality of meat products. It can also maintain the good sensory quality of fresh meat products and significantly extend the shelf life.
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