Impact of chilling rates on beef quality; the potential of DEXA to predict fat content; and the genomic breed traceability of certified Angus beef sold in Brazil

The objectives of this study were 1) to evaluate the temperature/pH decline response and meat quality parameters of Bos indicus beef carcasses subjected to conventional and dynamic chilling environments; 2) to assess dual-energy X-ray absorptiometry (DEXA) precision and accuracy in predicting longissimus steaks intramuscular fat (IMF) content; and 3) to determine the extent to which the Nellore breed contributes to the genetic composition of certified Angus beef and evaluate its relationship with the longissimus steaks composition from four different brands. In the first study, forty carcasses were used in each treatment (conventional and dynamic) to evaluate the effects of chilling decline rates on meat quality. Temperature and pH were recorded throughout the first 24h of chilling. After this period, longissimus thoracis was sampled, and meat quality analyses were performed. As results, carcass shrink percentage was higher in the conventional than in the dynamic chilling environment (P = 0.049). The pH values were significantly higher in the dynamic chilling environment at 2, 4, 6, and 12 hours after slaughter (P < 0.05). Also, there was a tendency (P = 0.059) for higher ultimate pH (24h) in the dynamic chilling environment. Temperature values were significantly lower in the dynamic treatment from 4 to 24 hours postmortem (P < 0.001) compared to the conventional treatment. Meat samples from the conventional chilling rate had higher mean values for color parameters a*, b*, oxymyoglobin, and chroma (P ≤ 0.006). In contrast, the deoxymyoglobin value was higher in the dynamic chilling treatment (P = 0.002). The variation in chilling rate evaluated in this study impacted meat pH, color, and tenderness, as the dynamic chilling treatment resulted in a tendency towards higher ultimate pH levels, less intense red color with and higher shear force value. The second study was performed with 72 Angus and 72 Nellore steaks to assess the capability of DEXA to predict IMF content of beef longissimus against chemical IMF as the gold standard. DEXA performance of fat% prediction was assessed using a leave-one-out cross validation method among Angus and Nellore steaks, which generated a chemical fat% range of 14.05-36.82% and 2.46-7.84%, respectively, and using the pooled data. There was a significant positive association between DEXA predicted fat and chemical fat content. However, higher precision was found for pooled data (R2 = 0.95, RMSECV = 1.95) and Angus (R2 = 0.75, RMSECV = 2.39) than Nellore (R2 = 0.15, RMSECV = 1.22) group. Accuracy also had the same response with average slope values close to 1 for pooled data and Angus and a lower value (0.42) for Nellore group. DEXA precisely predicts IMF content across a wide range of fat content. However, its precision and accuracy of prediction within low-fat content samples are lower than in high-fat content. Finally, to conduct the third study, DNA genotyping and chemical analysis were evaluated in 96 samples of Angus-certified meat, obtained from different brands, to determine the extent to which Nellore breed contributes to the genetic composition of certified Angus beef and evaluate the tissue composition of longissimus steaks from four different brands. On average, meat samples presented 53% of Angus genome and 47% of Nellore genome. There was a tendency for a negative correlation between Nellore proportion and fat content (P = 0.065), indicating that an increase in Nellore genetics may lead to lower fat content. Evaluating the chemical analysis among the four brands, Brand 2 presented the highest collagen (2,06%) and fat (7.97%) content values compared to Brand 3, which presented the least amount of collagen (1.69%) and fat content (5.14%). In addition, Brand 1 differed from Brand 2 on protein content (P = 0.047), in which Brand 1 presented a higher value (21.39%). On DEXA composition, Brand 3 presented the least fat content (12.58%) compared to the other brands. In sum, Nellore genetics contributed up to 49% of the genome profile of Angus-certified beef evaluated in this study. Also, there are differences in tissue composition between brands, with a tendency for leaner meat as the Nellore genetics increase. Keywords: Beef carcass. Genotyping. Grading. Marbling. Meat quality. Tenderness. Temperature