In modified atmosphere packaging (MAP), the air inside the package is replaced with a single gas or a mixture of gases. The choice of gas atmosphere depends on the product to be packed, and for bread it usually consists of pure nitrogen (N2), pure carbon dioxide (CO2), or a combination of these. Other gases are only seldom used. N2 is an inert, tasteless gas with low solubility in water and lipids. It has no antimicrobial effect but may be used to displace oxygen from the packaging atmosphere, thereby eliminating the basis for growth of aerobic microorganisms. CO2 is mainly used in MAP due to is antimicrobial effect; however, it has to be used in relatively high concentrations (over 20%) to be effective (70,71). CO2 is highly soluble in both water and lipid and the solubility increased with decreased temperature. The concentration of dissolved CO2 will increase with increased partial pressure of CO2 (concentration and pressure). When used in high concentration, it may therefore result in collapse of the package. This is especially the case in products with high moisture stored at decreased temperature (72). N2 is often mixed into the gas atmosphere to avoid collapse of the package. The antimicrobial effect of CO2 can be linked to the concentration of dissolved CO2 in the product rather than the concentration over the product (73). This fact may be the main reason for the increased antimicrobial effect at reduced temperature that was reported for wheat bread (74). In order to secure a sufficient concentration of CO2 in the product after packaging and to avoid collapse, it is important that the ratio between gas and product is not less than 2:1.
The antimicrobial effect of CO2 is complex and four activity mechanisms have been identified (72): (a) alteration of cell membrane function, including effects on nutrient uptake and absorption, (b) direct inhibition ofenzymes or decreases in the rate ofenzyme reactions, (c) penetration of cell membranes, leading to intracellular pH changes, and (d) direct changes in the physicochemical properties of proteins.
The best performance in modified atmosphere packaging is obtained by the deep draw technique or similar methods that involves an evacuation of the package before it is flushed with the desired gas; however, this has a much lower throughput than flow pack (71). Flow pack results in a much higher oxygen level in the final pack because it is just forcing out the air surrounding the product by blowing the package gas into the package just before it is sealed off. The spongy structure of bread thus results in residual oxygen content of 3-5%, which is insufficient to retard fungal growth (75). At this O2 level, no effect of increased carbon dioxide concentration (25%) was seen on P. roqueforti, whereas it was somewhat inhibitory to P. commune (76).
Growth of one of the most important bread spoilage fungi (P. roqueforti) on the laboratory substrate CYA (69), packaged in modified atmosphere is plotted in Fig. 2. This shows that even at very low oxygen levels, this organism is able to reach a substantial size within 2 weeks. Carbon dioxide inhibits growth, but only by applying an oxygen scavenger could growth be completely inhibited. The yeast Endomyces fibuliger is even more resistant to low oxygen levels. It reached an average of 1.5 mm when cultured for 1 week in a package with an oxygen absorber, and 4 mm at 0.02 and 0.1% oxygen. Relative to the control, which reached 11 mm in one week, the diameter was 14, 36, and 36%, respectively (69). The effect of increased carbon dioxide was not tested for this fungus. In general, molds do not grow in packages with oxygen absorbers but at 0.02-0.03% residual oxygen in either pure nitrogen or 50% nitrogen and 50% carbon dioxide, their colony diameter reached 18-29% of the control. In pure carbon dioxide, the molds were strongly inhibited, reaching only 1-6% at 0.02-0.03% oxygen and 4-11% at 1.0% residual oxygen (69). Black and coworkers also found that oxygen levels lower than 0.2% were required to hinder fungal spoilage of pita bread (77). In MAP sponge cake, it was shown that growth of Eurotium was unaffected by changes in the residual oxygen level (0.02-0.5%) when the CO2 level was high. At decreased CO2 levels, however, the growth rate increased with increasing oxygen level (78). Several authors have discussed whether MAP will speed up the staling of wheat bread or not have any effect at all (79). In a recent study, Rasmussen and Hansen found that neither packaging in pure CO2 nor in a mixture of CO2 and N2 resulted in a significantly different staling rate than storage in air (79). They found that loss of water was the most important factor besides the crystallization behavior of starch in the bread. Thus, MAP could be used to extend the microbial shelf life of wheat bread without risk of speeding up staling (79).
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