Variation in early postmortem pH of fresh pork loin affects quality attributes and degradation of key proteins

Abstract

Inconsistency in fresh pork quality is a persistent challenge faced by the United States pork industry. In order to maintain the demand for pork products globally, the mechanisms responsible for quality development must be investigated further to improve meat quality and consumer acceptance. The discovery of reliable molecular markers that can be measured non-destructively shortly postmortem to predict quality traits could aid in enhancing the consistency and value of fresh pork products. Therefore, this project aimed to contribute to the search for markers of quality by advancing the understanding of how early postmortem pH variation affects the quality, sensory, and biochemical attributes of fresh pork loin. Desmin was chosen as the target protein of interest in this study as the postmortem degradation of desmin, or lack thereof, has been shown in previous studies to influence important meat quality traits such as tenderness and water-holding capacity. To achieve the objective, fresh pork loins (N=62) were selected from the production line of a commercial facility at 1-day postmortem. Loins were selected based on the following pH criteria to ensure a population that was diverse in quality (pH ≤ 5.70; pH ≥ 5.90). Loins were then vacuum packaged and immediately transported on ice to the Iowa State University Meats Laboratory. The product was then sorted into distinct groups by 1-day postmortem pH: low/normal (n=25, 5.59-5.71, x̄= 5.67) and high/normal (n=25, 5.90-6.46, x̄=6.09), resulting in a refined population of (N=50) on which all analyses were performed. All loins were cut into 2.54cm or 0.635cm thick chops and trimmed of external fat and connective tissue. Subjective color score, marbling score, and Hunter L, a, and b values were determined on chops the same day as collection. Drip loss of chops at 1-day postmortem was measured over 24 hours. The remaining loin chops not utilized in these initial measurements were individually vacuum packaged per aging time, aged for a total of 1, 7, or 14 days (4°C), and frozen until analysis. Two chops from each loin at each time point were used for sensory analysis and to determine cook loss. Palatability characteristics consisting of off-flavor, pork flavor, juiciness, tenderness, and chewiness were evaluated by a trained panel of (n=3) on a 10-point categorical scale. Instrumental tenderness was performed on a pair of cooked 1, 7, and 14-day aged chops using an Instron fitted with a star probe attachment. Intramuscular lipid and moisture content were determined on frozen, homogenized longissimus thoracis (LT). Desmin degradation was analyzed using low ionic strength extracts from the LD from each aging period. The abundance of desmin degradation products within the soluble protein fraction was determined using western blotting analyses. Chops from the High/Normal pH group had approximately 79% less drip loss at day 1 and approximately 19% less cook loss than Low/Normal pH chops at all aging times. High/normal chops exhibited higher color scores than Low/Normal chops on day 1 (P<0.001) and day 14 (P<0.0001). Moreover, High/Normal chops exhibited a lower Hunter L value at day 1 (P<0.0001) and 14 (P<0.0001) compared to Low/Normal chops. Intramuscular lipid content was not different between the pH groups (P=0.36). Intramuscular moisture content was greater in High/Normal chops than in Low/Normal chops (74.11% vs. 73.53%) (P<0.05). High/Normal chops had significantly lower star probe values than Low/Normal chops at 1 (5.11kg vs. 5.88 kg), 7 (4.90 kg vs. 6.14), and 14 days of aging (4.07 kg vs. 5.18 kg). The trained panel rated 14- day aged High/Normal chops to be more juicy (P<0.001), tender (P<0.0001), and less chewy (P<0.05) while having greater pork flavor (P<0.0001) and less off-flavor (P<0.0001). The abundance of soluble desmin degradation products was significantly greater in low ionic strength extracts from High/Normal chops at 1, 7, and 14 days postmortem (P<0.0001). Variation in 1-day postmortem pH significantly impacted the quality, sensory, and biochemical attributes of fresh and aged pork loin. Higher pH at 1-day postmortem resulted in conditions that likely generated more proteolysis of desmin, which is proposed to result in improved quality of fresh pork. Furthermore, desmin degradation products were detected within the soluble protein fraction of postmortem muscle in this study, which have been identified by previous literature as a reliable indicator of aged pork tenderness; observations of the current study confirm this. Results from this study are also the first to detect the appearance of soluble desmin degradation products as early as 1 day postmortem and that differences in the abundance of soluble desmin persisted throughout 14 days of aging. Data from this study suggest that 1-day postmortem pH could be indicative of quality development in aged pork and that soluble desmin fragments could potentially be utilized as a viable predictor of value within fresh pork products.