Survival of Listeria monocytogenes on Blueberries (Vaccinium corymbosum) and Shelf Life Determination under Controlled Atmosphere Storage
Concha-Meyer, Anibal Andres
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Listeria monocytogenes represents a high risk for consumers, because it causes severe illness. This work studied in-vitro growth and survival of L. monocytogenes in media acidified with malic acid, lactic acid or blueberry extract. The growth of L. monocytogenes and shelf life extension of fresh blueberries were evaluated after storage at 4 deg C or 12 deg C under different controlled atmosphere conditions, including air (control); 5% O2 and 15% CO2, 80% N2 (CAS); or ozone gas (O3) 4ppm at 4 deg C or 2.5ppm at 12 deg C, at high relative humidity (90-95%) for a total of 10 days. L. monocytogenes growth in tryptic soy broth with yeast extract (TSB+YE) mixed with different acid solutions (malic acid, lactic acid and blueberry extract) and incubated at 25 deg C for 24h, was calculated measuring optical density. Complete inhibition occurred in the presence of treatments including malic acid pH 2.0 and pH 3.0; lactic acid pH 2.0, pH 3.0 and pH 4.0; and with blueberry extract pH 2.0 in the mixture. After 6h, there were significant differences among growing treatments. At 18h, there were no significant differences in turbidity among media mixed with blueberry extract at pH 3.0, 4.0 and 5.0, and their optical density values were higher than treatments including media mixed with malic acid 5.0 or lactic acid 5.0. Blueberry extract was not an effective acidifying media and acid adapted L. monocytogenes grew in acidified media. Fresh blueberries inoculated with L. monocytogenes were stored at 4 deg C or 12 deg C under different controlled atmosphere conditions (Air, CAS or O3 4ppm at 4 deg C or 2.5ppm at 12 deg C) and sampled on day 0, 1, 4, 7 and 10 for bacterial growth, weight loss, firmness and yeast and molds counts. CAS did not delay or inhibit Listeria monocytogenes, yeast, or molds by day 10. Storage at 4 deg C showed lower weight loss values compared to 12 deg C. Ozone controlled weight loss and firmness loss. Moreover, gaseous ozone achieved 3 and 2 log reductions when compared with air at 4 deg C and 12 deg C, respectively.
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