Masters Theses

Date of Award

8-1998

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

David A. Golden

Committee Members

Ann F. Draughon, John Mount

Abstract

This research was conducted as two separate studies. In the first study, the suitability of PALCAM and modified Oxford (MOX) agars for recovering sublethally heat- and lactic acid-injured Listeria monocytogenes was investigated. L. monocytogenes LMIOIM, LM103M (meat isolates), and Scott A were suspended in tryptose phosphate broth (TPB), heated for up to 40 min at 54°C, and surface plated onto tryptose phosphate agar(TPA), TPA + 4% NaCl (TPAS), PALCAM, and MOX. TPA and TPAS were used to determine total viable and sublethally injured populations, respectively. Heat-injured LM103M was recovered in the highest numbers on all media, followed by Scott A and LMlOlM (P<0.01). TPA allowed best recovery of all test strains, followed by PALCAM and MOX, which were not different, and TP AS (P<0.01). For acid-injury studies, uninjured and heat-injured (54°C for 20 min) test strains were suspended in phosphate-buffered TPB + 0.85% lactic acid (bTPBLA) at 25°C for up to 24 h and plated as described above. Uninjured and heat-injured L monocytogenes were recovered better from bTPBLA on MOX than on PALCAM (P<0.05). Heat injured L. monocytogenes LM103M was recovered better than LMIOIM but similar to Scott A on MOX and PALCAM (P<0.05), whereas Scott A was recovered similarly to LMIOIM and LM103M on MOX and PALCAM (P>0.05). Acid-injury of L. monocytogenes LM103M was enhanced by prior heat stress. In the second study, the effect of package atmosphere on survival of uninjured and sublethally heat-injured L. monocytogenes, inoculated onto tryptose phosphate agar containing 0.85% lactic acid and 2% NaCl (TPALAS) was investigated. Inoculated TPALAS plates were packaged in air, 100% N₂ (N₂), 30% CO₂/70% N₂ (CO₂/N₂), and vacuum and stored at 4 and 20°C for up to 31 days. Recovery of monocytogenes from TPALAS was influenced by the injury status (i.e., injured and uninjured) of the inoculum, storage atmosphere (air, N₂, CO₂/N₂, and vacuum), storage temperature (4 and 20°C), and recovery media (tryptose phosphate agar [TPA] and modified Oxford agar [MOX]) (P<0.05). Storage in CO₂/N₂ atmosphere was more inhibitory to uninjured L. monocytogenes stored at 20°C than air or vacuum, but less than N₂ (P<0.05). When sublethally heat-injured L. monocytogenes was stored at 20°C, CO₂/N₂ atmosphere allowed better recovery than N₂ (P<0.05) but was not different than air (P>0.05). At 4°C, uninjured and sublethally heat-injured L. monocytogenes were recovered in highest numbers from samples packaged in N₂ followed by CO₂/N₂ (P>0.05). Overall, non-selective TPA allowed greater recovery of L. monocytogenes than the selective MOX (P<0.05). Uninjured cells stored at 4°C were recovered better than sublethally heat-injured cells on TPA (P<0.05). On TPA, L. monocytogenes stored under air was recovered better than L. monocytogenes stored under N₂ or CO₂/N₂ (P<0.05). However, recovery on MOX was best when L. monocytogenes was stored under N₂ (P<0.05)

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