Effect of Relative Humidity

HIMMELFARB, PHILIP (University of Massachusetts, Amherst), HAMED M. EL-BISI, R. B. READ, JR., AND WARREN LITSKY. Effect of relative humidity on the bactericidal activity of propylene oxide vapor. Appl. Microbiol. 10:431-435. 1962.-Because of the low toxicity of its breakdown product, propylene oxide (PO) vapor will play an increasingly important role in the preservation of foods. It is therefore necessary that the diversified variables which influence effectiveness of PO treatment be thoroughly investigated and understood prior to advocating its general use in industry. Accordingly, the present study was undertaken to determine the effect of relative humidity (RH) upon the bactericidal activity of PO sterilant atmospheres. Death rates were established at increasing RH values of < 1, 52, 65, 80, and 98 % and under constant conditions of concentration, pressure, and temperature. Test bacterial populations were preconditioned to corresponding moisture levels. Results indicate that gram-positive cocci were relatively insensitive to PO vapor at dry conditions but became progressively less resistant with the increase in RH up to a maximum of 65 to 70 %. Lactic acid bacteria and gram-negative rods were much more sensitive at dry conditions, showing much less dependency upon water vapor. Bacillus subtilis spores elicited the highest degree of resistance but the death rate substantially increased with the increase in RH. Among the gaseous compounds that have been utilized to fumigate and reduce bacterial numbers in or on food products, medicinals, and various solid objects, ethylene oxide and propylene oxide (PO) have proven to be effective. Propylene oxide vapor has recently received a great deal of attention as a spoilage retardant and sterilizing agent because of its relatively nontoxic glycol residue. Ark (1947) reported that PO vapor (0.3 to 0.5 cm3/liter) inactivated various phytopathogenic bacteria and fungi in 45 to 60 min. Although it was stated that a concentration of 1.0 cm3/liter of PO vapor was lethal to all test organisms, it must be emphasized that no reference was made to precise relative humidity (RH) requirements. ' Contribution No. 12 of the University of Massachusetts, College of Agriculture Experiment Station, Amherst. 2 Present address: Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio. Whelton, Phaff, and Mrak (1946) tested the fungicidal and bactericidal activity of PO vapor against organisms grown on agar slants, and found that molds and yeasts were killed in 3 hr at 73.4 F with a PO concentration of 0.3 cm3/liter; bacteria were reported to be more resistant. Here again, RH was neither indicated nor controlled. Mrak and Stadtman (1946) indicated that PO vapor decreased dried-fruit spoilage when it was introduced into the packages at the time of sealing. Approximately twice as much PO vapor was needed to kill bacteria than was required for molds and yeasts. It was also stated that sufficient moisture must be present inside the package for any preservation to occur. However, here again, no precise RH requirements were indicated. In a patent granted to Mrak et al. (1950) for the PO vapor preservation of dried fruit, it was indicated that at least 25 % water must be contained in the dried fruit for proper PO treatment. Recently, Bruch and Koesterer (1961) reported that powdered or flaked foods could be treated with PO vapor. In this report it was indicated that PO vapor was more capable of inactivating Bacillus subtilis spores at lower RH levels; the time for 90% kill was 0.65, 0.75, 1.05, and 1.2-1.3 hr for 22, 36, 86, and 100 % RH values, respectively. Although reported RH levels were reached by the evaporation of precalculated quantities of water from filter-paper strips, no attempt to measure the actual operating RH values was reported. To date, no data have been reported to correlate carefully measured RH levels with the antibacterial activity of PO vapor in an enclosed, temperature-controlled atmosphere. Accordingly, the object of this investigation was to determine the RH optima for kill of major groups of bacterial cells exposed to PO vapor at a constant temperature and concentration. MATERIALS AND METHODS An apparatus was constructed which consisted of eight 3-necked (vertical) 1,000-ml flasks suspended in a circulating water bath and connected in series to a vacuum pump. The flasks were of the internal ground-glass taper (45/50) type, fitted with 34/45 reducing tubes, and were utilized in the following manner. One of the side necks was fitted with a two-hole rubber stopper, and to this a glass T-tube was connected for vacuum and flushing in series. An inductor tube, sealed with a serum cap and 431 on O cber 3, 2017 by gest ht://aem .sm .rg/ D ow nladed fom