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The major concern of the meat safety is microbial spoilage. The sources of microbial contamination are possibly coming from the slaughter animals themselves, the process workers, and the processing environment(1). Moreover, other factors, e.g. moisture loss, color change, and oxidative rancidity will also affect the meat quality. To improve meat product hygiene and safety, packaging must control pathogenic microorganisms outgrowth, but also necessary to consider moisture loss, product appearance, gaseous exchange between the package and ambient atmospheres(2). Modifying the atmosphere surrounding a product by packaging is commonly used in practice. e.g. vacuum packing, modified-atmosphere packing(3). Interest in the use of active and intelligent packaging systems for meat and meat products has increased recent years(4).
Vacuum packing has been used for so many years for primary cuts of red meats, cooked meats, fish, cured meat (2, 3). Cured meat, are often vacuum packed for display since pigment nitrosomyoglobin is protected from oxidation(3). Packaging material should have low permeability to oxygen (5). For primary cuts of meat, three layered co-extrusions of ethyl vinyl acetate / polyvinylidene chloride / ethyl vinyl acetate are commonly used(5).
Exclusion of oxygen in vacuum packages can inhibit the aerobic bacteria growth (6, 7), e.g. Gram negative psychrotrophic Pseudomonas, which commonly spoilage the aerobically refrigerated stored meat, can be effectively controlled by vacuum packaging(8). Whereas, anaerobic bacteria in vacuum packaging tend to be higher than CO2-enriched packaging(6).
Listeria monocytogenes is Gram positive, facultative anaerobic bacteria. It poses a safety risk contamination of cooked, red-to-eat processed meat products (9), or fish products(10). It can survive and multiply in the refrigerated storage condition. It has been reported even can grow at -1.5°C (10). The results showed that vacuum packing cannot greatly inhibit this bacteria, therefore it predominate in refrigerated meat or meat products (11).
In vacuum packaging, Gram-positives, Lactic acid bacteria are commonly increased and dominant in meat products during storage. e.g. genera Lactobacillus, Carnobacterium and Leuconostoc(3). From safety point of view, Lactic acid bacteria can protect the meat products from pathogenic spoilage, such as used as a protective culture and may implemented in the commercial production of cooked meat products(12-14). Such as some lactic acid bacteria (LAB) strain naturally present in the vacuum package can produce bacteriocin to preserve the meat products(15). Buddy and coauthor (2003) discovered predominant bacteriocin-producing strain Leuconostoc carnosum 4010 from vacuum packed meat products. By addition of 107cfu/g Leuconostoc carnosum 4010 to vacuum packed meat sausage, the number of viable Listeria monocytogenes cells were immediately reduced to a level below the detection limit and no increase of Listeria. monocytogenes was observed during storage at 5°C for 21 days(15). Bredholt et al., (2001) did the similar research, indigenous lactic acid flora (Lactobacillus sakei) which indigenous grow in the ham was isolated and effectively inhibit growth of both L.monocytogenes, and E.coli O157:H7 and Y.enterocolitica O:3 in vacuum packed sausages at 8°C for 4 weeks(14, 16). Lee's study (1985) showed that the increased number of Lactobacillus spp. can also diminish the psychorotrophs consisted primarily of Pseudomonas spp. initially dominated in the vacuum packed fresh pork(12).
Brochothrix thermosphacta is a common Gram-positive, facultative anaerobic rod, which can spoilage chilled raw meat. Under the vacuum condition, the growth of Brochothrix thermosphacta significantly slower during meat storage at 6°C(13).
Enterobacteriaceae family primary present in the sliced meat surface were remain low and essentially unchanged by vacuum storage at 6°C (13). However, the dominant flora in vacuum packaging also depends on pH, high pH meat (e.g. turkey and chicken breast) (pH>6.0), psychortrophic Enterobacteriaceae with cannot grown on normal meat pH can grow, produce abundant H2S and causing greening of meat(3, 17). Shewanella putrefaciens is another case which can outgrowth by increased pH, causing spoilage of meat(3, 17)
Aeromonas hydrophila is Gram-negative, rod shaped bacterium. Growth has been detected in a variety of vacuum packaged products stored between -2 and 10°C such as beef(18), roast beef(10) and pork kept at 1°C (19).
Modified atmosphere packaging (MAP)
Modified atmosphere packaging (MAP) is mainly for the retail display of meat (3). Consumers prefer the bright color of these kinds of meat, which is coming from the oxymyoglobin. Therefore, vacuum package is not suitable to use, since myoglobin retain in its unoxygenated form (Deoxymyoglobin) with undesired purple color(20). In modified atmosphere packing (MAP), a bulk or retail pack is flushed through with a gas mixture usually containing combination of carbon dioxide (CO2), oxygen(O2) or nitrogen N2(3). Recent year, carbon monoxide (CO) was also used for stabilize the bright color of meat. However, CO is lack of inhibition of pathogen growth may pose a food safety risk(21). CO2 is included for its inhibitory effect. Nitrogen is non-inhibitory but has low water solubility and can prevent pack collapse when high concentration of CO2 is used(22). Reduced oxygen concentration (typically 60-80%) and optimized CO2 composition (around 30%) can delay the development of oxidative rancidity and slow down the growth of aerobes rather than suppressed entirely. Typically, fresh red meat are stored in MAP contain O2 80% and CO2 20%(22), while cooked meat are stored in 70% N2 30% CO2(23). The atmosphere of MAP may change during storage due to reactions between components of the atmosphere and the product and /or due to transmission of gases in or out of the pack though the packaging film(24).
The microflora dominant in MAP depends the type of meat, its storage temperature, whether previous vacuum packed or aerobically stored(3). Heterofermentative lactic acid bacteria can be more numerous due to the stimulate effect of oxygen on their growth(25). Gram-negative psychrotrophic organisms Pseudomonas can be effectively inhibited by 20% or more CO2 enriched atmosphere(8). Aeromonas hydrophila can be detected in chilled meat products under vacuum condition (10, 18, 19) or aerobically packed(26). However, CO2-enriched modified atmosphere packaging can inhibit Aeromonas hydrophila growth (18, 19, 27). The lag phase and generation time of Aeromonas hydrophila are both prolonged(26). CO2 have minor effect on inhibit Yersini enterocolitica growth. Runner et al., (1994) shows that bacterium appear to be relatively resistant to CO2 (28). In Zee et al., (1984) reported CO2 concentration should above 40% to inhibit the growth of Yersinia (29). Modified atmosphere packaging on inhibition of Listeria monocytogenes is contradictory and confusing. The growth depends on the atmosphere composition. It seems CO2 have no inhibitory effect on Listeria monocytogenes. Wilkinson and co-author (2006) determined the effect of CO2-MAP + 0.4% CO, vs. 100% CO2-MAP, on retail-ready fresh pork stored for 8 weeks in a master-package system. Total plate counts were not affected and Listeria was present on meat from both treatments(21). In generally, the effects of modified atmosphere on controlling the meat spoilage are summarized in Table 1.
Table 1. Effects of modified atmosphere packaging on meat pathogens
Interest in the use of active packaging systems for meat and meat products has increased recent years(4). Active packaging incorporate additives to the packaging system(4). The active functions and technologies include moisture control, O2-permeable films, O2 scavengers or absorbers, O2 generators, CO2 controllers, odor controllers, flavor enhancement, ethylene removal, antimicrobial agents, and microwave susceptors, etc. (30).
To prevent the development and spread of spoilage and pathogenic microorganisms via meat stuff, active packaging which incorporate antimicrobial agent in food packaging material could be a potential alternative solution. Instead of mixing antimicrobial compounds directly with food, incorporating them in films allows the functional effect at the food surface - where the microbial growth is mostly found - to be localized(31). Four types of antimicrobial films are commonly used, e.g. incorporation of the antimicrobial substance into sachet connect to package; direct incorporation of the antimicrobial compounds into packaging film; coating the package with a material that acts as a carrier for additive; or utilizing antimicrobial macromolecules with film forming properties or edible matrices (31, 32). The antimicrobial agents used include organic acid(33), chlorophyllins(34), bacteriocins(35), tocopherol(36), natural extracts (oregano extract(37), rosemary extracts(37)), etc.
Organic acid are commonly used as the antimicrobial agents incorporated into the film. e.g. Ouattara and the coauthors (2000) have determined the incorporation of acetic or propionic acid into a chitosan matrix, and applied to the meat products. propionic acid released faster and more complete than acetic acid from the chitosan matrix. Moreover, Lactic acid bacteria were not affected by antimicrobial film. Instead, Enterobacteriaceae and S.liquefaciens was delayed or completely inhibited as a result of film application(33).
Lopez-carballo et al., (2008) incorporated chlorophyllins into the gelatin film-forming solution. The results demonstrated that water soluble sodium magnesium chlorophyllin (E-140) and water soluble sodium copper chlorophyllin (E-141) reduced the growth of Staphylococcus aureus and Listeria monocytogenes by 5 log and 4 log respectively(34)..
Scannell and coauthor (2000) immobilized bacteriocins e.g. nisin and lacticin 3147 to cellulose-based bioactive inserts and antimicrobial polyethylene/polyamide pouches.
Adsorption of lacticin 3147 to plastic film was unsuccessful. Nisin bound well and the resulting film maintained its activity for 3-month period, both at room temperature and under refrigeration. Nisin-adsorbed bioactive inserts reduced levels of Listeria innocua by â‰¥2 log units in ham, and Staphylococcus aureus ~2.8 log units in ham(35).
Sensory properties of packaging
Packaging except having function of control the microbial growth, also contribute greatly to the meat texture, juiciness, color, and volatile.
Redness of meat
Vacuum packed meat products generally have less redness compared with modified atmosphere packaging(38). High oxygen content is required to promote red color(39).Addition low amount of carbon monoxide (CO) is also commercially used to stabilize the redness compared with high-oxygen MAP (21, 40-42). To maintain the red color of the meet, Garcia-Esteban et al., (2004) has tried pretreated the beef steaks with 5% of carbon monoxide (CO) for 24h or 100% CO for 1 hour, and then pack in vacuum packaging, the red color can be stabilized for more than 21 days(43). Although oxygen is required for the redness of meat, higher oxygen MAP, e.g. above 80% may also introduce rapid a decrease in color stability during display(44).
High oxygen level within MAP also promote lipid oxidation, generally, the low oxygen content in the vacuum packaging can avoid the lipid oxidation(45). Temperature and time were also important factors for minimizing lipid oxidation(46). Moreover, the MAP gas compositions influence the lipid oxidation as well. Octanal and nonanal from auto-oxidation were significantly influenced by the atmosphere composition, with the highest values in samples packed at the lowest CO2 concentration(47). Carbon monoxide (CO) has been proved can prevent the rancidity Bornez et al., (2009) did research on lamb meat, which stunned and followed modified atmosphere packaging. The lipid oxidation significantly higher in package A (70% O2 + 30% CO2) than package B (69.3% N2, 30% CO2, 0.7% CO) at 7, 14 and 21 days post-packaging(48). Recently, USDA approved distribution of fresh meats in a master bag system using 0.4% carbon monoxide (CO) (41). Carbon dioxide concentration may also promote oxidation of both myoglobin and lipids, most probably due to its effect of lowering pH. Martinez et al., (2005) recommended using atmospheres containing low CO2 concentration 20% rather than high 60%(49).
Juiciness and toughness
Meat packaged in vacuum packaging experience more hardening compared to the MAP (50). The oxygen content in the modified atmosphere packaging will affects the meat juiciness and toughness. Zakrys et al., (2009) did the research with beef steak at different oxygen content. The consumer panel preference of 40% of oxygen is related to the decreased toughness and increased juiciness(51). However, high oxygen content e.g. 80% oxygen in MAP had negatively affects tenderness and juiciness scores(44) (52).
Organic acid treatments have been involved in the vacuum or modified atmosphere packaging(53, 54). For example, Sahoo & Anjaneyulu (1997) have demonstrated incorporated organic acids e.g 500ppm sodium ascorbate, 10 ppm Î±-tocopherol acetate and 0.5% sodium tripolyphosphate with vacuum packing for Buffalo meat nuggets and stored at 4°C, extended the shelf life from 10 to 30 days under refrigerated storage(55).
Zeitoun & coauthor (1991) investigated effectiveness of combining various concentrations (2%, 5% and 10% w/v) of lactic acid/sodium lactate buffer (pH 3.0) with modified atmosphere (MAP) packaging (90% CO2 and 10% O2) on Listeria monocytogenes Z7 serotype 1 and on the shelf life of chicken legs stored at 6°C.The antimicrobial effect of lactic acid buffer systems (pH 3.0) increased with increasing concentrations of lactic acid in the buffered system. The best results were obtained by the combined use of 10% acid/sodium lactate buffer (pH 3.0) and MAP(56).
Antioxidants can prevent discoloration in packaged meat (57-59). Nicolalde et al., (2006) has compared the pork carcasses exposure to antioxidant solutions (ascorbic acid, citric acid, propyl gallate or ascorbic/EDTA mix), packaged in modified atmosphere (80% O2 and 20% CO2), then displayed in a retail case at 4 °C for 8 days. After 2 days of display, samples treated with propyl gallate were visually redder, less discolored and less green/black than samples treated with other antioxidants(57). Essential oil used for MAP can extend the shelf life. In Karabagias's (2011) research, the result showed that essential oils TEO (thyme) 0.1% incorporate to MAP (80% CO2/20%N2) was more effective for lamb meat preservation. The microbial population were reduced up to 2.8 log cfu/g on day 9 of storage (60).
Some authors investigated the irradiation- vacuum packing combined effects on meat quality. Sulfur-containing compounds were responsible for most of the irradiation off-odor(61), such as dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide(62). Ahn et al., (2000) concluded the most vacuum packaging was better than aerobic packaging for irradiation and subsequent storage of meat because it minimized oxidative changes in patties and produced minimal amounts of volatile compounds that might be responsible for irradiation off-odor during storage(61). Moreover, the irradiation can incorporate to the modified atmosphere packaging (CO-MAP) to reduce microbial loads and extend the shelf life. Ramamoorthi et al., (2009) tested irradiation of fresh beef after CO-MAP packaging at 4°C for 28days, after day 0, no coliform were detected after irradiation at 1.5 or 2.0kGy(63).
The replace of current packaging material polyvinyl chloride (PVC) to biodegradable film can be environmental friendly solution for fresh meat packaging. Cannaris et al., (2005) investigated polymer biodegradable films by either starch-polyester combination, or by a mixture of three biodegradable polyesters on preservation the fresh cut meat, the results shows, no difference on microbiological counts compared with PVC (64).