Knowledge base

Packing in MAP
Extending the shelf life of food products using a modified atmosphere.

The method of packaging and the type of packaging material are factors that largely determine the quality of packaged food, which includes health safety and long durability. The packaging methods that allowed for the extension of storage periods and became a permanent part of national production practice include: vacuum packaging and packaging in a modified (protective) atmosphere.

WHAT IS MAP?
Modified atmosphere packaging (MAP) means replacing the air in the packaging with various gas mixtures in which the proportion of individual ingredients is constant at the time of their introduction. Due to dissolution and penetration through the walls and closure, the composition of the mixture may slowly change while the product is stored in the packaging [1].

The simplest form of atmosphere modification is vacuum packaging. It involves placing the product in a foil package with low gas permeability, removing air from it and closing it hermetically [3].

HISTORY OF RESEARCH ON VACUUM PACKING
In Poland, activities related to the use of vacuum packaging and modified (protective) atmosphere packaging were initiated in the early 1990s and the Central Laboratory of Refrigeration from Łódź participated in them from the very beginning. The first works concerned the use of vacuum technology for packaging cold meats, especially sliced ​​and rennet-ripened cheeses, in unit packages made of multi-layer foils. The research was aimed at determining the technological suitability of packaging made of various types of foil and the durability of the packaged products.

For cold meats, depending on their type and parameters of packaging materials, these periods for storage temperatures of 0°C-4°C were:
from 7 days (sliced ​​canned ham, sausages)
up to 21 days (sliced ​​cooked ham, sliced ​​Toruń sausage),
up to 30 days – for cheese

The test criteria included the results of organoleptic assessment and the microbiological condition of products at the end of their storage period, included in the relevant subject standards. Studies on the durability of vacuum-packed products also included ready-made culinary dishes [5].
 
Example results are provided in Table 1.
 
Product		        Shelf life [days]
Packed without vacuum Vacuum packed
 (aw= 0,985)
Beef and pork burgers		  
3		 >7
 (aw = 0,979)
Beef burgers I		 21
 (aw= 0,953}
Beef burgers II		 28
Baked Chicken Legs 2 14-28
Fried beef skewers 3 7-14
Boiled pork shank 2 14
Roasted pork shank   28
Old Polish hunter’s stew 4 21
Forszmak 1 28
Dumplings stuffed with meat 2 14
Potato dumplings 2 16
The shelf life depends on the type of processing technology and packaging foil used
Table 1: Shelf life of selected refrigerated delicatessen and culinary products at a temperature of 0°C^4°C, stored

Durability, as can be seen from the data compiled, depends on the physicochemical characteristics of the product (especially aw), as well as the type of thermal treatment used. In the case of vacuum-packed hamburgers, as their water activity decreases, their shelf life increases. For beef and pork hamburgers with aw = 0.985, the durability does not exceed seven days, while for beef hamburgers with aw = 0.953 it reaches 28 days.

The example of pork knuckle shows the impact of the thermal treatment on the durability of the vacuum-packed product. The cooking process allows it to be stored in refrigerated conditions for up to 14 days, while baking allows it to be stored for up to 28 days.

Research on the durability of vacuum-packed products, continued recently, largely concerned raw meat, including poultry. The established storage periods at + 2°C^ +4°C were ten (for beef and veal) to fourteen days (for boneless turkey cuts). They were determined on the basis of the results of organoleptic and microbiological assessment. The criteria for this state were the requirements contained in the Regulation of the Minister of Agriculture and Rural Development of May 25, 2004 (Journal of Laws No. 132 of June 9, 2004, item 1419, Annex No. 3, item C). Starting from 1993, research was focused on modified atmosphere packaging, which was a consequence of the intensive development of this technique in leading countries in this field and the possibility of transferring their experiences to Poland. The use of the MAP technique was first recorded in 1927, when it was used to extend the storage period of apples by placing them in an atmosphere with reduced oxygen and increased carbon dioxide. In the 1930s, this technique was used to transport fruit in ship holds, as well as beef carcasses transported over long distances in an atmosphere of carbon dioxide, which made it possible to almost double their shelf life. However, it was not until the early 1970s that the technique was used in Europe in a commercial setting for retail packaging. In the UK in 1979, Marks and Spenser used the MAP technique for meat packaging. The success of this product led to the introduction of MAP two years later for packaging bacon, fish, portioned cooked meat and cooked shellfish. More and more manufacturers and supermarket chains have started using gas mixtures to meet the expectations of customers demanding extended storage periods and reduced use of preservatives.

The MAP technique is currently used in a wide range of fresh, chilled foods, including raw and cooked meat and poultry, fish products, fruits and vegetables, and recently coffee, tea and bakery products [2, 10].

TYPES OF GASES USED IN MAP PACKING
Gases that are components of gas mixtures used in modified atmosphere packaging
Carbon dioxide (CO 2) is a gas that is highly soluble in both water and fats. Not being a bactericidal or fungicidal agent, it has bacteriostatic and fungistatic properties. Although the bacteriostatic effect of CO2 has been known for many years, its mechanism has not been fully explained. The theories considered regarding the impact of CO2 on bacterial cells concern:
• changes in the functions of cell membranes in terms of nutrient uptake and absorption,
• direct inhibition of enzymes or reduction of their reactivity,
• penetration into bacterial cell membranes, which leads to a change in pH inside the cells,
• direct changes in the physicochemical properties of proteins.

Overall, the effect comes down to extending the lag phase of bacterial growth and reducing growth in the logarithmic phase. The effect of lagphase prolongation is more significant. When bacteria transition from lagphase to logarithmic phase, the effect of CO2 on bacterial growth is limited. This explains the importance of placing the product in a package filled with a protective atmosphere as quickly as possible. The bacteriostatic effect of CO2 depends on its concentration, partial pressure, gas volume, type of microorganism, degree of its development and initial number of cells, phase of their growth, storage temperature, acidity, water activity and the type of packaged product. Carbon dioxide is particularly effective for products at risk of spoilage due to the action of aerobic, gram-negative psychotrophic bacteria.

Nitrogen (N2) is an inert, tasteless and odorless gas. Its presence in packaging containing gas mixtures with a significant share of CO2 prevents the formation of a vacuum effect, which is a result of its poor solubility in water and fats. Additionally, it can delay rancidification processes and inhibit the growth of aerobic microorganisms.

Oxygen (O2) is used for packaging fresh meat to maintain the characteristic red color of the oxidized form of myoglobin, which is oxymyoglobin, as well as for packaging fruit and vegetables so as not to completely stop their respiration processes and maintain their color. Moreover, oxygen can inhibit the growth of anaerobic bacteria, which is of practical importance when packing white fish. In other cases, attempts are made to limit its presence inside the packaging because it enables the development of aerobic microorganisms and takes part in enzymatic reactions and transformations of food ingredients such as vitamins and flavor compounds. Examples of recommended compositions of gas mixtures used for modified atmosphere packaging are listed in Table 2:
 
Product		 Composition of gas mixtures
Oxygen Carbon dioxide Nitrogen
Red meat 60-85 15-40 -
Cooked/preserved meat - 20-35

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