Proteomic and metabolomic profiles link lightness and eating quality in aged pork with normal pH

Meat lightness is an important trait influencing pork quality, with lighter pork often linked to inferior tenderness, juiciness, and flavor. Although the effects of extreme pH conditions on meat quality are well studied, the biochemical mechanisms underlying variation in pork lightness within normal ultimate pH remain unclear. Here, we used low-ionic-strength soluble proteomics and metabolomics to characterize approximately 2-wk aged pork loins that differed in lightness (Dark category, Hunter L*=45.4±1.2, n = 28; Intermediate category: Hunter L*=48.1±0.6, n = 29; Light category: Hunter L*=50.8±1.0, n = 29) but with normal 24 h pH (5.5 < pH < 6.0). Chops in the Light category had lower eating quality, water-holding capacity, and post-aging pH (P < 0.05) than those in the Intermediate and Dark categories. The Light category had higher sugar abundance than the other categories and lower amino acid abundance than the Dark category (P < 0.05). Proteins associated with pH-dependent denaturation, structural protein proteolysis, location-specific membrane degradation, and aminopeptidase activity differed in abundance between the Dark and Light categories (P < 0.05). The lower ultimate pH in the Light category may reflect greater glycolytic activity despite similar lactate production. It may also alter the solubility of sarcoplasmic proteins, potentially increasing lightness via light scattering and toughness via protein aggregation. Changes in structural protein solubility with increasing pork lightness may reflect protein-specific postmortem proteolysis. In the Light category, the protein patterns are consistent with less degradation of intracellular membranes but greater disruption of transverse tubule (plasma) membranes. This disruption could facilitate reduced sarcoplasmic protein concentration and allow intracellular fluid to move more rapidly into the extracellular space, increasing purge loss and reducing juiciness. It could also lower sarcoplasmic protein abundance and increase surface light scattering. Greater aminopeptidase abundance in the Dark category may help explain the observed accumulation of proteinogenic (protein-building) amino acids and their products, which could contribute to desirable flavor formation via higher-pH Maillard reaction while reducing off-flavors. Together, these findings suggest that postmortem energy metabolism, water-soluble protein abundances, proteolysis, and membrane integrity contribute to variation in pork lightness and eating quality within the normal ultimate pH ranges.