Chapter 12 - Fungi and Bacteria

Abstract

Less than 1% [O2] is required to substantially inhibit the growth of obligate aerobic and microaerophilic bacteria, and air’s O2 content must be reduced below 1%, and often to <0.2% before growth of most fungi is strongly inhibited. Anoxia prevents or inhibits mold growth, but does not kill fungal spores, and vegetative cells are able to renew their growth when O2 is reintroduced. CA and MA usually cannot prevent decay by lowering [O2] because at atmospheric pressure <1–2% [O2] causes low-[O2] injury to most types of plant matter, and fungal growth inhibition seldom reaches 50% at the CO2 concentrations tolerated by most types of plant matter. A direct growth inhibition of disease organisms by MA packaging or with CO2 added during CA storage has only been demonstrable with strawberries, cherries, blueberries, and a few other horticultural commodities able to withstand more than 10–20% [CO2]. Development of many aerobic bacteria and molds is inhibited by ultra-low [CO2] and, and CO2 incubators are used in bacteriology laboratories to accelerate the growth of CO2-dependent bacteria. A low [CO2] condition arises during hypobaric storage because LP facilitates respiratory CO2 diffusion from plant matter, and the vacuum chamber is ventilated with rarified air changes in which the atmospheric CO2 content has decreased due to expansion during entry into the storage chamber. The resultant low CO2 concentration may augment the antifungal effect caused by a simultaneous reduction in [O2]. White blood cells generate HOCl from H2O2 and Cl− and the HOCl plays an important role in the host defense microbiocidal reactions of polymorphonuclear leukocytes (PMNs) after they engulf invading pathogens. The gross response to applied exogenous HOCl is similar to that occurring in phagocytosing PMNs and the cell-free MPOase-H2O2-Cl− system. HOCl vapor, generated from an alkaline sodium hypochlorite solution in response to atmospheric CO2 present in air passing through the solution, can be rapidly vacuum infiltrated into the intercellular system of stored plant matter present in a hypobaric chamber. Between 0.1 and 5.9ppm HOCl vapor is 100% effective in preventing decay from developing in various types of plant matter. The EPA considers HOCl to be nontoxic to human and animal tissues and nondeleterious to the environment, and the FDA has approved the use of high aqueous concentrations of HOCl in contact with fresh foods and cut flowers. In 2001, the FDA declared ozone a safe antimicrobial agent for the treatment, storage, and processing of fruits and vegetables. Because of ozone’s short half-life, it must be continuously generated in situ by exposing O2 to high-energy electrons formed during a corona discharge or to photon quantum energy contained in <200nm wavelength UV radiation. The most effective continuously applied aerial ozone concentration that consistently prevents bacterial and fungal decay without injuring plant matter usually is between 0.1 and 2.0ppm.