Effect of Modified Atmospheres on Microflora and Respiration of California Prunes
J. L. ZETTLER1* AND S. NAVARRO²
¹Horticultural Crops Research Laboratory, USDA-ARS, Fresno, CA 93727, USA
[*e-mail: lzettler@qnis.net]
²Dept. of Stored Products, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel
Abstract: We investigated the possibility that California prunes stored in modified atmospheres (MAs) will tolerate higher water activity (aw) levels than those required at normal atmospheres and that the naturally occurring microfloral infection level on the prunes may generate the MAs that inhibit mold activity in airtight conditions. Dry prunes were moisturized to a range from 16 to 38% moisture content (m.c.) to acquire samples with 0.575 to 0.858 aw. In unsealed conditions at 35°C, there was a level of naturally occurring microorganisms (aerobic plate count, yeast and mold count) that generally increased above 0.70 aw. However, incubating these samples in a sealed container for 35 days at 35°C indicated that levels of naturally occurring microorganisms remained unchanged throughout the range of water activities. Microfloral respiration as a function of temperature was determined by incubating similarly moisturized prune samples at 25, 30 and 35°C and then measuring the declining O₂ concentrations through time. Results showed that the higher both the water activity and temperature, the more intense the O₂ consumption by the product. A nearly linear relationship was observed between O₂ depletion and time. Anaerobic conditions were reached in less than 2 days at 25°C and 0.858 aw, the highest water activity tested. At higher temperatures, anaerobic conditions were achieved at water activities of 0.824 and above. These results indicate that under aerobic conditions microorganisms can flourish on and cause deterioration of prunes if the water activity of the fruit is above 0.7 (m.c. of 24.6%). However, under sealed conditions, prunes can tolerate a higher water activity without microorganism growth or deterioration of the fruit.
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