Metabolomics and bioinformatic analyses to determine the effects of oxygen exposure within longissimus lumborum steak on beef discoloration.

ABSTRACT

Meat discoloration starts from the interior and spreads to oxymyoglobin layer on the surface. The effects of oxygen exposure within a steak on the metabolome have not been evaluated. Therefore, the objective of this study was to evaluate the impact of oxygen exposure on the metabolome of the longissimus lumborum muscle. Six United States Department of Agriculture (USDA) Low Choice beef strip loins were sliced into steaks (1.91-cm) and packaged in polyvinyl chloride overwrap trays for 3 or 6 d of retail display. The oxygen exposed (OE) surface was the display surface during retail, and the non-oxygen exposed (NOE) surface was the intact interior muscle. The instrumental color was evaluated using a HunterLab MiniScan spectrophotometer. To analyze the NOE surface on days 3 and 6, steaks were sliced parallel to the OE surface to expose the NOE surface. Metmyoglobin reducing ability (MRA) was determined by nitrite-induced metmyoglobin reduction. A gas chromatography-mass spectrometry was used to identify metabolites. The a* values of steaks decreased (P < 0.05) with display time. MRA was greater (P < 0.05) in the NOE surface compared with the OE surface on days 3 and 6. The KEGG pathway analysis indicated the tricarboxylic acid (TCA) cycle, pentose and glucuronate interconversions, phenylalanine, tyrosine, and tryptophan metabolism were influenced by the oxygen exposure. The decrease in abundance of succinate from days 0 to 6 during retail display aligned with a decline in redness during display. Furthermore, citric acid and gluconic acid were indicated as important metabolites affected by oxygen exposure and retail display based on the variable importance in the projection in the PLS-DA plot. Citric acid was lower in the NOE surface than the OE surface on day 6 of retail display, which could relate to the formation of succinate for extended oxidative stability. Greater alpha-tocopherol (P < 0.05) in the NOE surface supported less oxidative changes compared to the OE surface during retail display. These results indicate the presence of oxygen can influence metabolite profile and promote migration of the metmyoglobin layer from interior to surface.

In a retail setting, aerobic packaging allows beef steaks to form a bright-cherry red appearance. However, prolonged oxygen exposure can also lead to surface discoloration, which is negatively perceived by consumers. Surface discoloration results in either discounts or products being discarded in the grocery store resulting in approximately $3.7 billion loss annually in the United States. Hence, understanding the process of oxygen exposed discoloration would help to limit the economic loss and meat waste. The current study evaluated the impact of oxygen exposure on metabolites, oxygen consumption, and metmyoglobin reducing activity of beef strip loin steaks. The results indicated that oxygen exposure decreased metmyoglobin reducing activity. Furthermore, metabolites that could limit surface discoloration showed lower abundance in oxygen-exposed surface during retail display. Oxygen negatively impacts the color of beef loin steaks and the stability of the color during retail. Expanding our knowledge of processes involved in metmyoglobin formation could help to develop strategies to help limit economic loss associated with surface discoloration.