PS9-5. Effects of High-Pressure Processing Holding Time on Oxygen Consumption and Metmyoglobin Reducing Activity of Dark-Cutting Steaks.

Abstract Dark-cutting beef is discounted during grading due to a lack of bright red color. Previous studies noted that 300 megapascals (MPa) pressure applied for 90 seconds reversed the dark color of dark-cutting steaks to normal-pH bright red. However, the mechanistic basis of this color improvement remains unclear. The objective of this study was to determine the effects of high-pressure processing holding time on oxygen consumption, metmyoglobin reducing activity, and histological changes of dark-cutting steaks. No-roll dark-cutting beef longissimus lumborum loins were purchased from a commercial processor 3 days postmortem (n = 6 loins). Each loin was divided into four equal sections, vacuum packaged, and randomly assigned to control, 300 MPa (90 s), 300 MPa (120 s), or 300 MPa (150 s) treatments. A Hiperbaric 55 machine with circulating chilled water was used to apply pressure. Following pressure application, three 1.91 cm thick steaks from each loin section were assigned to retail display, oxygen consumption, metmyoglobin reducing activity, pH, or histological analysis. Retail display steaks were placed on Styrofoam trays wrapped with polyvinyl chloride film and displayed for 5 days. A HunterLab MiniScan spectrophotometer was used to measure surface color. Reflectance-based approaches were used to determine oxygen consumption and nitrite-induced metmyoglobin reducing activity on day 0 of retail display. The experimental design was a randomized block design (n = 6 replicates), and data were analyzed using PROC GLIMMIX of SAS. There were no effects (P > 0.05) of holding time at 300 MPa on pH. A treatment × storage time interaction was observed for retail color (a* and L* values). All three holding times at 300 MPa improved (P < 0.05) redness (a*) compared with untreated dark-cutting steaks on days 0 to 3; however, no differences (P > 0.05) among holding time treatments were observed during retail display. All three holding times improved lightness (L*; P < 0.05) compared with control dark-cutting steaks throughout retail display, with no differences (P > 0.05) among holding time treatments. Control dark-cutting steaks had greater (P < 0.05) oxygen consumption than steaks treated at 90 and 150 s. Metmyoglobin reducing activity did not differ (P > 0.05) between control and 90 s treatments; however, 120 and 150 s pressure applications resulted in lower (P < 0.05) metmyoglobin reducing activity than control steaks. Microscopic examination of hematoxylin- and eosin-stained sections revealed increased spacing between muscle bundles in pressure-treated steaks compared with controls. These results suggest that structural alterations and decreased oxygen consumption improve redness of dark-cutting steaks treated with 300 MPa. Increased spacing between muscle bundles allows greater oxygen penetration, facilitating conversion from dark red to bright red color. Pressure application may also damage mitochondria, reduce oxygen consumption, and promote bloom development in dark-cutting steaks.