# Intrinsic and Extrinsic Factors Affecting the Color of Fresh Beef Meat--Comprehensive Review
## 3 Results and Discussion  
### Intrinsic Factors  
#### 3.1.1 Ultimate pH  
Some authors considered that ultimate pH is one of the most significant and discriminating criteria in determining meat color [2; 23]. It is commonly agreed that depleted glycogen reserves in animals during stressful pre-mortem events might cause limited lactic acid formation in post-mortem muscle and insufficient pH decline [23; 24]. Since high-pH beef muscle fails to reach a bright-red color, this condition is referred to as dark-firm-dry (DFD) meat [13; 25]. DFD carcasses had lower glycolytic potential than those with normal pH values, resulting in less glycogen availability for lactic acid formation post-mortem [25]. Previous data also showed that muscle glycogen amounts lower than 50 umol/g at slaughter could be indicative of DFD meat [26]. The darker color in DFD meat also resulted in  
Figure 2: PRISMA flow chart for article selection between the years 2000 and 2022 in the influence of intrinsic and extrinsic factors in beef meat color [21]. a higher accumulation of myoglobin oxidation products and less light reflectance on the meat surface [23; 27]. DFD beef showed a higher percentage of oxidative type I muscle fibers and a lower percentage of glycolytic type II muscle fibers, indicating higher myoglobin and mitochondria contents than normal pH muscles [13]. Previous data showed that higher mitochondrial counts and mitochondrial activity in DFD meat resulted in a darker meat surface color due to higher tissue oxygen consumption, which restricted the capacity for myoglobin oxygenation during blooming [9; 13; 27]. Greater water-holding capacity in DFD meat resulted in more swollen fibers that increased light scattering from the myoglobin fractions and generated the typical darker superficial appearance of DFD muscle [28; 29]. Recently, it was suggested that higher amounts of tricarboxylic acid cycle (TCA) metabolites present in DFD meat could be further used for cellular respiration that increased mitochondrial oxygen consumption and deoxymyoglobin formation [13]. Higher myoglobin concentrations and inefficient mitochondria abundance revealed that animals predisposed to DFD-related issues had an oxidative metabolism that accelerated glycogen depletion as the degree of darkening in DFD meat increased [24; 25]. It was also suggested that lower phosphorylation degree of sarcoplasmic proteins in intermediate (5.7 < pH < 6.09) and high pH groups (pH > 6.09) could have influenced the activity of certain metabolic enzymes associated with the development of DFD meat color [30]. Additional proteome studies also showed higher metabolic enzymatic activity in DFD meat with eight overabundant metabolic enzymes compared to normal-pH meat. However, normal-pH meat showed increased LDH-D activity, thus improving meat color [29]. Due to variations in the management systems, handling of the animals, slaughterhouse policies, consumer preferences, and the pH thresholds that the local food authorities may assign to this defect, the incidence of problems involving DFD cattle varies across different countries. However, until now, early detection of animals predisposed to a DFD status has not been substantiated and results still have partial repercussions. The most common strategies have been related to the measurement of some enzymes or hormones in the bloodstream of animals before and during slaughtering, such as cortisol, creatine kinase (CK), and lactate dehydrogenase (LDH). Clear interactions were found between higher production of those substances and animal welfare, but, no relationships were found for ultimate pH or meat color [31; 32; 33]. Infrared eye temperature determination also showed contradictory results to predict the incidence of DFD problems in beef [34]. Reliable markers that can determine the relationships between pre-mortem processes and the ultimate pH in meat still need to be validated. DFD carcasses are usually reduced in price due to the meat's undesirable color.