Methods Of Atmosphere Modification

Methods of atmosphere modification within a packaged food product may be subdivided into two main categories:

passive modification and active modification. In commodity-generated or passive modification, the product is packaged in a film with the correct gas permeability characteristics, and the atmosphere within the packaged product is modified as a result of the consumption of 02 and generation of C02 through respiration of the product, plus the permeation of gases through the package materials and structure. Passive modification is commonly used to modify the gas atmosphere of fresh respiring fruits and vegetables. However, to maintain the correct gas mixture within the packaged product, the gas permeabilities of the packaging films must be selected to allow 02 to enter the package at a rate similar to its consumption by the product. Similarly, C02 must be vented from the package to offset the production of C02 by the product. Failure to achieve this gas balance will result in a depletion of 02 and a buildup of C02, resulting in adverse changes in products. Depletion of oxygen to near 0 leads to anaerobic respiration or fermentation and the production of adversely flavored compounds. Most fresh vegetables and fruits are vulnerable to this type of injury under anoxic conditions.

Several methods can be used to actively modify the gas atmosphere within the packaged product. These include vacuum packaging, vacuum followed by gas, and injection of or sweep by gas mixtures.

Vacuum packaging is used extensively by the meat industry to extend the shelf life and the keeping quality of primal cut or wholesale cuts of fresh red meat. The product is placed in a package structure fabricated from film of low oxygen permeability, air is removed under vacuum, and the package is heat sealed. Under conditions of a good vacuum, headspace 02 is reduced to <1%. C02, produced from tissue plus microbiological respiration, may eventually increase to 10 to 20% within the package headspace. These conditions, that is, low 02 and/or elevated C02 levels, extend the shelf life of meat by inhibiting the growth of aerobic meat-spoilage microorganisms, particularly Pseudomonas and Alternaria species.

A novel method of active modification is through the use of oxygen absorbents after mechanical oxygen removal. These consist of sachets that are placed inside the packaged product. Alternately, the oxygen scavenger may be incorporated into the film. Another method of active modification is the use of ethanol vapor generators that modify the gas atmosphere by producing ethanol vapor within the package headspace to suppress mold growth. Both oxygen absorbents and ethanol vapor generators have been used for shelf-life extension of food.

Active modification may also be achieved by gas packaging. Gas packaging is simply an extension of vacuum packaging technology and involves the evacuation of air followed by the injection of the appropriate gas mixture.

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