Elimination of oxygen is often used as a control measure for inhibiting the growth of molds. Exclusion of oxygen will not prevent growth of yeasts. Studies on bakery products have demonstrated that atmospheric O2 levels must be reduced to 0.1-1% to effectively inhibit growth of molds. In studies on toasted bread, Cerny (1979) demonstrated that visible mold would occur in 3 days in air; 5 days in 99% N2-1%O2; and > 100 days in 99.9% N2-0.1% O2, 99% C02-1.0% O2, 99.8% CO2-0.2% O2, and 100% CO2. This study demonstrates that although molds are considered to be aerobic organisms, certain species have the ability to grow at very low levels of O2 concentrations. Effectively controlling mold by simple gas flushing can be difficult in practice. Chemical oxygen scavenger can be used in place of or to supplement gas flushing (Farber 1991). Oxygen scavengers will also give protection against package leaks and infiltrations of O2 through the package.
Carbon dioxide exerts antifungal action that supplements simple exclusion of O2 and, thus is more effective than inert gas such as nitrogen. The gas probably exerts antifungal activity by altering intracellular pH levels (Gorris 1994).
Recent research has shown CO2 to have potential use with food. Carbon monoxide inhibits yeast and molds that causes postharvest decay in fruits and vegetables (Wagner and Moberg 1989). The potential toxicity of this compound to workers requires special handling procedures.
Sulfur dioxide is broadly effective against yeasts and molds. It is used extensively to control growth of undesirable microorganism in fruits, fruit drinks, wines, sausages, fresh shrimp, and pickles. The antimicrobial activity of SO2 is associated with the unionized form of the molecule. Therefore, it is most effective at pH values < 4.0, where this form predominates (Weidzicha 2000).
Ethylene oxide has been widely used to reduce microbial contamination and to kill insects in various dried foods. The gas has been used to treat gums, spice, dried fruits, corn, wheat, barley, dried egg, and gelatin (ICMSF 1980). Concern over the toxicity of residues has limited the use of this gas in recent years.
Propylene oxide has been less studied than ethylene oxide. However, it appears that its antifungal effects are similar (Wagner and Moberg 1989). Yeasts and molds are more sensitive to the gas than bacteria. Propylene oxide has been used as a fumigant for control of microorganisms and insects in bulk quantities of goods such as cocoa, gums, processed spice, starch, and processed nutmeats (ICMSF 1980).
Ozone (O3) is a strong antimicrobial agent with numerous applications in the food industry. It has been used for decades in many countries and was recently given GRAS status in the United States. Ozone in the aqueous or gaseous phase is active against a wide range of bacteria, molds, and yeasts. Most applications are targeted to decontamination of fruit and vegetable surfaces by washing in ozonated water (Xu 1999). A second application is fruit and vegetable storage. Barth et al. (1995) assessed ozone exposure on storage of blackberries stored at 2°C in air with 0.3 ppm ozone. Fungal development was suppressed while 20% of the control fruits showed decay. The effectiveness of ozone is influenced by the intrinsic factors of a food. It also oxidizes food surfaces when used at high levels. Further research may reduce some of these concerns so ozone can be used in broader food applications.
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