Applications of MAP in the food industry Nonrespiring products

Non-respiring food products do not consume any oxygen during further storage. When such food products are packaged in a modified atmosphere, the aim is to retain the introduced atmosphere during the storage period. Therefore, high barrier films are used which are most often composed out of different layers of materials. Typical O2 and CO2 barrier materials are PA (polyamide), PVDC (polyvinylidenechloride) and EVOH (ethylenevinyl alcohol). Depending on the intended storage time, the O2-permeability of the applied films should be <2ml O2/m2.24h.atm determined at 75% relative humidity at 23 °C for products with a long shelf life and <10 ml O2/m2.24h.atm determined at the same conditions for products with a limited shelf life (<1 week).

One of the bottlenecks in modified atmosphere packaging lies in defining the optimal gas atmosphere for a food product in a specific packaging design. This optimal atmosphere depends on the intrinsic parameters of the food product (pH, water activity, fat content, type of fat) and the gas/product volume ratio in the chosen package type. The intrinsic parameters determine the sensitivity of the product for specific microbial, chemical and enzymatic degradation reactions. Products that are susceptible to microbial spoilage due to the development of Gram-negative bacteria (e.g. fresh meat and fish) and yeasts (salads) should be packaged in a CO2 enriched atmosphere because the growth of those microorganisms is significantly retarded by CO2. In general, oxygen is excluded from the gas mixture. For prolonging the shelf life of products which are spoiled by mould growth (e.g. hard cheeses) or by oxidation, it is essential to package in O2 free atmospheres. In some cases, O2 will be included for the reasons previously mentioned in section 16.3.

The use of CO2 is however limited due to its solubility in water and fat. This

Modified atmosphere packaging (MAP) Table 16.2 Recommended gas regimes for MAP of various non-respiring foods

Food type

Gas composition (%)

Purpose

Food type

Gas composition (%)

Purpose

Fresh meat

retail

15-40

0

60-85

^ Gram organisms (CO:

20

10

70

Colour (O2)

industrial packages

50-100

0-50

0

^ Gram- organisms

Poultry

70

20

10

^ Gram-, colour

Fish

lean, marine

50-60

0-20

30-40

^ Gram-, ^ TMA produ

fatty or fresh water

40-65

35-60

0

^ Gram-, ^ oxidation

Meat and fish products

aw > 0.94

50-70

30-50

0

^ Gram+

aw < 0.94

10-20

80-90

0

^ Yeasts and moulds

Shrimps

35

65

^ Gram- & Gram+

Cheese

hard

0-70

0-30

0

0

100

0

^ Moulds, ^ oxidation

soft

0

100

0

Bakery products

20-70

30-80

0

^ Yeasts & moulds

Dry products (aw < 0.60)

0

100

0

^ Oxidation

high solubility can cause collapsing of the package when the concentrations of CO2 are too high. This will especially be the case for food products containing high amounts of unsaturated fat such as smoked salmon and salads that contain mayonnaise. The influence of pH, temperature, fat content, water activity and gas/product ratio on the CO2 solubility has been quantified by Devlieghere et al (1998). Moreover, too high CO2 concentrations in the atmosphere can lead to an increased drip loss during storage. This can be explained by the pH drop induced by CO2 dissolving in the water phase of the product, causing a decrease in the water binding capacity of the proteins. Table 16.2 gives an overview of the recommended gas regimes for different non-respiring food products and the specific purpose of the gas mixture.

16.4.2 Respiring products (Equilibrium Modified Atmosphere Packaging)

In contrast to other types of food, fruits and vegetables continue to respire actively after harvesting. A packaging technology, used for prolonging the shelf life of respiring products, is Equilibrium Modified Atmosphere Packaging (EMAP). The air around the commodity is replaced by a gas combination of 1-5% O2 and 3-10% CO2 with the balance made up of N2. Inside the package, an equilibrium becomes established, when the O2 transmission rate (OTR) of the packaging film is matched by the O2 consumption rate of the packaged commodity. The respiration of the living plant tissue also results in the production of CO2, which diffuses through the packaging film, depending on the film's CO2 transmission rate

(CO2TR). The type of packaging film selected is based on the film OTR and CO2TR, which is required to obtain a desirable equilibrium modified atmosphere. For packaging fruits, the film also needs to have a certain permeability for ethylene (C2H4), which prevents an accumulation of the ripening hormone and prolongs fruit shelf life (Kader et al, 1989).

The modified atmosphere not only reduces the respiration rate and the ripening behaviour of fruit, but it also maintains the general structure and turgidity of the plant tissue for a much longer period, which results in better protection against microbial invasion. This atmosphere is also thought to inhibit the growth of spoilage microorganisms (Farber, 1991), which is mostly due to the low O2 concentration, because the elevated CO2 concentration (<10%) inside the package is not sufficiently high enough to act as an antimicrobial (Bennik et al, 1998). The shelf life is also prolonged by the suppression of the enzymatic browning reactions on cut surfaces (Kader et al, 1989, Jacxsens et al, 1999a).

Regarding the relatively short shelf life of fruits, raw vegetables, and fresh-cut vegetables, an active modification of the atmosphere is preferred, compared to a passive modification, which is caused by the produce respiring. Form-Fill-Seal (FFS) machines are used with a flushing system to obtain the optimal modified atmosphere for packaging this type of product.

The attained EMAs are influenced by produce respiration (which in turn is affected by product type, temperature, variety, size, maturity, and processing method), packaging film permeability (OTR, CO2TR, and C2H4TR), package dimensions, and fill weight. Consequently, it is a very complex procedure to establish an optimal EMA for different items of produce. The current knowledge of EMAP of fruits and vegetables is mainly empirical, but a systematic approach for designing optimal EMA packages for minimally processed fruits and vegetables is proposed by a number of different authors (Exama et al, 1993; Peppelenbos, 1996; Jacxsens et al, 1999b; Jacxsens et al, 2000). Several mathematical models have been published that predict the OTR and CO2TR of the packaging film, which is necessary to obtain the desired equilibrium gas atmosphere (Mannaperuma and Singh, 1994; Solomos, 1994; and Talasila et al, 1995). However, in these models an unrealistic constant storage temperature is assumed. Two important parameters in EMAP of fresh-cut produce, respiration rate and permeability of the packaging film are temperature dependent. The respiration rate is less affected by the temperature change (Qi0R = 2-3) than is the permeability of the packaging film (Qj0P = 1-2) (Exama et al, 1993; Jacxsens et al, 2000), as is illustrated in Fig. 16.1.

When temperature increases, a larger volume of O2 will be consumed by the fresh-cut produce than is diffused through the packaging film, resulting in a shift of the EMA towards an anaerobic atmosphere (<1% O2 and >10% CO2). Anaerobic atmospheres must be avoided in EMAP of respiring products because the shift towards anaerobic respiration will cause the formation of ethanol, acetal-dehyde, off-flavours, and off-odours. At lower temperatures, the O2 level will increase (>5%) in the EMA package and the benefits of EMA are lost. Changing temperatures during the transport, distribution, or storage of EMA packages will

5000

4500 J=

3000 O

1000 <B

500 E

Temperature (°C)

Fig. 16.1 Temperature dependence of the oxygen permeability and the respiration rate of shredded chicory. (Devlieghere et al, 2000c)

result in an equilibrium O2 level inside the packages that differs from the optimal 3%. A lack of OTR and CO2TR of commercial films adapted to the needs of middle and high respiring products can result in undesirable anaerobic atmospheres. When both gas fluxes cannot be matched, the O2 flux should take priority because it is the limiting factor in EMA packaging. A decreased O2 content is more effective in inhibiting respiration rate and decay than is a decreased CO2 concentration (Kader et al, 1989; Bennik et al, 1995). New types of packaging films, with an OTR that is adaptable to the needs of fresh cut packaged produce, offer new possibilities in replacing OPP (oriented polypropylene), BOPP (biaxi-ally oriented polypropylene), or LDPE (low density polyethylene) that are currently used in the industry and from which the OTR is not high enough for packaging products with medium or high respiration rates (Exama et al, 1993).

Jacxsens et al (2000) proposed an integrated model in which the design of an optimal EMA package for fresh-cut produce and fruits is possible, taking into consideration the changing temperatures and O2/CO2 concentrations inside the package. A prediction of the equilibrium O2 concentration inside the packages, designed to obtain 3% O2 at 7°C, could be conducted between a temperature range of 2 to 15°C. These packages (3% O2 at 7°C) had acceptable O2 concentrations between 2 and 10 °C. However, above 10 °C an increase in the growth of spoilage microorganisms and a sharp decrease in sensorial quality were noticed.

The application of high O2 concentrations (i.e. >70% O2) could overcome the disadvantages of low O2 modified atmosphere packaging (EMA) for some ready-to-eat vegetables. High O2 was found to be particularly effective in inhibiting enzymatic discolouration, preventing anaerobic fermentation reactions and inhibiting microbial growth (Day, 1996; Day, 2000; Day, 2001). Amanatidou et al (1999) screened microorganisms associated with the spoilage and safety of minimally processed vegetables. In general, exposure to high oxygen alone (80 to 90% O2, balance N2) did not inhibit microbial growth strongly and was highly variable. A prolongation of the lag phase was more pronounced at higher O2 concentrations. Amanatidou et al, (1999) as well as Kader and Ben-Yehoshua (2000) suggested that these high O2-levels could lead to intracellular generation of reactive oxygen species (ROS, O2-, H2O2, OH*), damaging vital cell components and thereby reducing cell viability when oxidative stresses overwhelm cellular protection systems. Combined with an increased CO2 concentration (10 to 20%), a more effective inhibitory effect on the growth of all microorganisms was noticed in comparison with the individual gases alone (Gonzalez Roncero and Day, 1998; Amanatidou et al, 1999; Amanatidou et al, 2000). Wszelaki and Mitcham (1999) found that 80-100% O2 inhibited the in vivo growth of Botrytis cinerea on strawberries. Based on practical trials (best benefits on sensory quality and antimicrobial effects), the recommended gas levels immediately after packaging are 80-95% O2 and 5-20% N2. Carbon dioxide level increases naturally due to product respiration (Day, 2001; Jacxsens et al, 2001a). Exposure to high O2 levels may stimulate, have no effect on or reduce rates of respiration of produce depending on the commodity, maturity and ripeness stage, concentrations of O2, CO2 and C2 H4 and time and temperature of storage (Kader and Ben-Yehoshua, 2000). Respiration intensity is directly correlated to the shelf life of produce (Kader et al, 1989). Therefore, the quantification of the effect of high O2 levels on the respiratory activity is necessary (Jacxsens et al, 2001a). To maximise the benefits of a high O2 atmosphere, it is desirable to maintain levels of >40% O2 in the head-space and to build up CO2 levels to 10-25%, depending on the type of packaged produce. These conditions can be obtained by altering packaging parameters such as storage temperature, selected permeability for O2 and CO2 of the packaging film and reducing or increasing gas/product ratio (Day, 2001).

High O2 MAP of vegetables is only commercialised in some specific cases, probably because of the lack of understanding of the basic biological mechanisms involved in inhibiting microbial growth, enzymatic browning and concerns about possible safety implications. Concentrations higher than 25% O2 are considered to be explosive and special precautions have to be taken on the work floor (BCGA, 1998). In order to keep the high oxygen inside the package, it is advised to apply barrier films or low permeable OPP films (Day, 2001). However, for high respiring products, such as strawberries or raspberries, it is better to combine high O2 atmospheres with a permeable film for O2 and CO2, as applied in EMA packaging, in order to prevent a too high accumulation of CO2 (Jacxsens et al, 2001b).

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