# Intrinsic and Extrinsic Factors Affecting the Color of Fresh Beef Meat--Comprehensive Review
###### Abstract  
Meat color research from the last two decades suggests that a combination of different intrinsic (ultimate pH, age of the animals, muscle position, breed, slaughter weight, and sex) and extrinsic factors (production systems and feeding, pre-mortem stress, slaughter season, and chilling rates) might have a deep impact in the color of beef muscle and influence consumers' acceptance of fresh meat. Ultimate pH and muscle position were perceived as the most determinant intrinsic factors, whereas production systems, feeding, and ante-mortem stress were the extrinsic factors that more strongly influenced beef color attributes. From an industrial perspective, the extrinsic factors can be improved through the technological process at a higher ratio than the intrinsic ones. This review aims to evaluate the effect of each of those factors on myoglobin oxidation and beef color traits from a comprehensive standpoint. All the information discussed in this manuscript focuses on an industrial environment and offers possible solutions and recommendations for the global meat industry. ultimate pH; age of the animals; muscle position; slaughter weight; production systems; feeding; pre-mortem stress; slaughter season; chilling rates +
Footnote †: journal: Medical  
myoglobin, which can eventually be converted into one of its three oxidation states [5; 9]. Depending on the ligand bound to the sixth coordinate of myoglobin and on the redox status of the heme iron, three different forms of myoglobin can be produced: deoxymyoglobin (purplish-red color, ferrous iron, and no ligand attached), oxymyoglobin (bright cherry-red color, ferrous iron, and oxygen attached), and metmyoglobin (dull-brown color, ferric iron, and water attached) [10]. Myoglobin oxidation/reduction is dictated by oxygen amounts, antioxidant availability, and enzymatic activity [11]. Metmyoglobin reduction can be achieved either from enzymatic, non-enzymatic, or mitochondrial-mediated pathways [12]. All these three processes require electrons produced from NADH regeneration via lactate dehydrogenase (LDH) or glyceraldehyde 3-phosphate dehydrogenase (GAPDH) activity [9; 13]. Metmyoglobin enzymatic reduction that happens in the mitochondrial membrane through cytochrome b5 reductase is considered the most important cellular enzymatic process involved in meat color development because of its critical importance in cellular electron transport [14; 15]. The metmyoglobin formed during the oxidation phase is the compound that more strongly affects the meat's color and appearance, and it can ultimately determine the consumer's purchase willingness [16]. Oxidative changes that affect meat color have been traditionally quantified using three linear color coordinates, as detailed in Figure 1: L* (lightness), which ranges from 0 value for black and 100 for white; a* (redness), which ranges from green if negative to red if positive; and b* (yellowness), which ranges from blue if negative to yellow if positive. Other color scores such as \(\Delta\)E (total color difference), C* (chroma), and h* (hue angle) might also be included in some publications to describe differences of color between two stimuli, three-dimensional perception of color and the degree of vividness, respectively [16; 17; 18]. Meat color measurements can be performed using a spectrometer, which determines different myoglobin fractions at specific depressions in the light spectrum, or a colorimeter, which quantifies meat color by a mean calculation of different points scanned on the meat surface [16; 17]. Recently, a CVS (computer vision system) was implemented as a more accurate technique for color determination [18; 19] due to less light deviation through the meat matrix and a higher scope area of the sample [17; 19]. It was reported that the color intensity of fresh meat is influenced by an interaction of multiple intrinsic and extrinsic factors during muscle-to-meat conversion [20].