The antimicrobial action in MAP foods can be produced by changes in the redox potential (Eh) and CO2 concentrations based on the methods used. Aerobes and anaerobes, depending on the microbial species and reducing or oxidizing state of food, have different Eh requirements for growth, whereas facultative anaerobes grow over a wide Eh range. Vacuum packaging and gas flushing. Especially with CO2 or N2 or their mixture and no O2, discourage growth of aerobes but encourage growth of facultative anaerobes and anaerobes. However, even under these conditions of packaging, tissue oxygen and dissolved and trapped oxygen can allow initial growth of aerobes to produce CO2, even by the protein-rich foods, and ascorbic acid and reducing sugars in fruit and vegetable products, can alter the Eh of a food and encourage growth of anaerobes and facultative anaerobes. Thus, it is not possible to control microbial growth just by changing the Eh. Other methods, on addition to modification of environment, are environment, are necessary for effective preservation of foods. However, by controlling the growth of aerobic bacteria, many of which have short generation times, the shelf life of a product is greatly extended.