Masters Theses

Date of Award

5-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Food Science and Technology

Major Professor

P. Michael Davidson

Committee Members

Riëtte van Laack, David A. Golden

Abstract

Ultimate pH (the pH of meat 24 hr post-mortem) is one of the most important contributors to defects and reduced sensory and microbial shelflife of pork. The objectives of this study were to determine: (1) the effect of ultimate pH (pHu) of pork loin on its shelflife at the wholesale and retail level and (2) its influence on the growth of Salmonella Typhimurium DT104 in ground pork. Thirty-six pork loins (pH range = 5.56- 6.57) were collected at a commercial slaughter facility 24 hr post-slaughter. All pigs were from the same genetic line. Aerobic (APC), psychrotrophic (PPC), Enterobacteriaceae (EC) and lactic acid bacteria (LAB) counts were obtained for vacuum-packaged (VP) pork loins stored for 0, 6, 14, 24, and 34 days at 4/C. For APC, log CFU/cm2 of loins at pHu< 5.85 were significantly lower than loins at pHu> 5.85 (p< 0.05). Loins at pHu< 5.7 were consistently 1.5-2.5 logs lower than pHu> 5.85 and time to reach 7.0 log CFU/cm2 was approximately 12 days longer than at pHu> 5.85. For PPC, pHu< 5.7 was also significantly lower than pHu> 5.85 over the storage period and was consistently 1.5-2 logs lower than pHu> 5.85 from day 14 to 34. For EC, pHu< 5.95 were lower than pHu> 5.95. Loins of pHu< 5.7 were consistently 1.5-3 logs lower than at pHu> 5.85 for 34 days. EC of pHu< 5.7 at 34 days was no different than pHu> 6.16 at day 14. LAB growth was not influenced by pHu. No off-odors were detected in samples until days 24 and 34. All samples with off-odors were between 7.4 and 9.2 log CFU/cm2 and most were of pHu> 6.16. Glucose and lactate concentrations were determined for VP pork loins stored for 0, 6, 14, 24, and 34 days at 4/C. Loins of pHu< 5.7 consistently had 5.6 to 9.4 :mole glucose/g more than pHu> 5.85. Similarly, lactate concentration of loins of pHu< 5.7 was greater than pHu> 5.85 through day 24. 2-thiobarbituric acid (TBA) values (oxidative rancidity) were determined in loins at days 0 and 34 of VP storage and were greater in loins of pHu< 5.7 compared to pHu> 6.16. Percent drip loss (purge) was determined in loins on days 6, 14, 24, and 34 VP storage. Loins of pHu< 5.7 displayed their greatest drip loss at days 24 and 34 and were greater than pHu> 5.7 (4.6 and 3.9% greater than pHu> 6.16 on those respective days). Chops cut from VP loins (wholesale) were displayed in a retail case and stored for 5 days aerobically at 5/C. Chops were grouped by pH as follows (group: pH range): A:5.41-5.65, B:5.66-5.80; C:5.81-6.95; D:6.96-6.10; E: > 6.10. APC, EC, lightness (L*), a*, and b* values were determined both initially and after 5 days of storage under retail display conditions. Hue/ and chroma were determined using the following equations: (arctan b*/a*) and (sqrt (a*2 + b *2 )), respectively. Initial APC between pHu groups was different only at day 14 VP storage with group A being 1.0 to 1.6 log CFU/cm2 less than groups C-E. No initial APC exceeded log 7.0 CFU/cm2. At day 5 retail storage, Group A APC was consistently 1.0 to 1.5 log CFU/cm2 lower than group E throughout the 34 day duration of the study and likewise did not exceed 7.0 log CFU/cm2 until 39 days (34 day VP samples). All other group APC did not exceed 7.0 log CFU/cm2 until 29 days (24 day VP samples). Initially, EC for group A was consistently 1.0 to 2.0 log CFU/cm2 lower than group E at days 14, 24, and 34 of VP storage. EC at day 5 retail display for group A was lower than group E and groups C-E after days 11 (day 6 VP samples) and 19 (day 14 VP samples), respectively. Groups A-E did not exceed 7.0 log CFU/cm2 until after 39 days (34 day VP samples). Chop pHu did not influence hue/ or chroma either initially or after 5 days of retail display. Initially, chops of pHu< 5.8 were lighter than pHu> 6 .11 throughout the 34 day storage period and similarly after 5 days of retail display in chops cut from loins vacuum-packaged for 14 and 24 days. After 34 days of vacuum-packaging and 5 days of retail display, pHu< 5.65 was lighter than pHu> 5.81. Growth of Salmonella enterica ssp. enterica serovar Typhimurium DT104 was determined in ground pork of pHu 5.75 and 6.52 stored aerobically and anaerobically (vacuum-packaged) at 10/C. Non-vacuum-packaged samples were analyzed for up to 8 days and VP samples up to15 days of storage. Growth of S. Typhimurium was slowest in VP ground pork of pHu at 5.75 than 6.52 throughout the study except at day 15. S. Typhimurium at pHu 5.75 displayed a lag phase of 3 to 6 days versus < 3 days for pHu at 6.52. Ground pork of pHu at 5.75 was 1.5 log CFU/g lower at day 3 than pHu at 6.52. At day 6, S. Typhimurium was 7.8 and 8.6 log CFU/g for pHu at 5.75 and 6.52, respectively and was stationary for the duration of storage except for pHu at 5.75 at day 15 which increased to 8.4 log CFU/g. Aerobic packaging of the inoculated ground pork resulted in a faster growth rate. S. Typhimurium in pHu at 5.75 pork was consistently 0.7 to 1.0 log CFU/g lower than pHu at 6.52 throughout the storage of 8 days. Growth of S. Typhimurium was significantly higher at each day of sampling throughout the duration of the study for both groups. Pork of pHu at 5.75 reached 8.6 log CFU/g in 8 days in aerobic packaging compared to 15 days in vacuum-packaging. Pork of pHu at 6.52 achieved 9.6 log CFU/g by day 8. Based on these results, a pork loin pHu of 5.8 to 5.9 is optimum to provide a shelflife of at least 24 days with minimal off-odors, less drip loss, oxidation and incidence of lighter color. Likewise, chops cut from these vacuum-packaged loins could be retail displayed for 5 days at 5/C and optimum shelflife would be achieved. Furthermore, a lower pH would help slow the growth of S. Typhimurium in VP ground pork.

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