## 2 Instrumental Measurement of Red Meat Sensory Quality  
### Appearance/Colour  
The appearance of red meat at retail is important as the first step in shaping consumers' perceptions of its quality [17]. Colour is a key indicator of quality and the primary aspect consumers assess [18]. Meat colour is influenced by both pigment-related chromatic components and achromatic elements coming from the muscle's structural features [19]. Once meat is removed from the carcass, a variety of related occurrences can affect the colour of the meat, which can result in notable visual alterations and ultimately affect how customers perceive the meat quality and freshness [20]. In this regard, temperature, pigment oxygenation status, visible fat content and oxidation of lipids during ageing and customer exposure are the primary factors affecting the colour of fresh meat [21]. Myoglobin, a heme-containing macromolecule, storing and delivering the oxygen necessary for aerobic energy production, significantly contributes to the colour of meat [22]. The amount of myoglobin present in meat, reflects the animal's oxygen storage and delivery needs during its lifetime, and this can vary based on factors such as the muscle type, species, age of the animal, diet, and environmental conditions [23]. The structure of myoglobin consists of two main components: an apoprotein and a heme group, which provides the red colour of meat/muscle [24]. Two key characteristics that affect its impact on meat are its concentration and its stability [25]. Myoglobin influences colour via its biochemical condition, particularly influenced by the oxygenation or reduction status [22]. The marbling effect resulting from intramuscular fat (IMF) not only influences the flavour, juiciness, and tenderness of meat but also contributes to its visual appeal [26], with marbling being viewed sometimes positively and sometimes negatively by consumers. The amount of IMF present in meat is influenced by various factors, such as the animal's breed, slaughter weight, feeding strategy, and rate of growth [26]. The process of adipogenesis,  
Figure 2: Meat sensory quality attributes assessed for prediction via optical sensing techniques. or the formation of fat cells, typically begins in the visceral fat deposit in ruminants, followed by intermuscular and subcutaneous deposits, with intramuscular fat development occurring last, whereas in monogastric animals, the subcutaneous fat deposit precedes that of intermuscular fat [27]. Genetic, nutritional, and environmental factors play important roles in controlling the adipogenesis process in skeletal muscle. IMF accumulation is largely determined by genetics and is positively correlated with the overall body fatness in animals [26]. Marbling is an essential aspect in the evaluation of meat quality by consumers, with higher marbling generally associated with better quality and flavour [28]. Finally, meat colour is also influenced by the muscle pH, with high pH levels in beef muscle resulting in a condition known as dark-firm-dry (DFD) meat. Depleted glycogen reserves in animals under stress before slaughter can lead to limited lactic acid formation post-slaughter, resulting in insufficient pH decline [29]. High pH levels cause less protein denaturation, tightly bound water, and minimal exudate formation, as the myofilament lattice does not shrink and the differences in the refractive index decrease between myofibrils and sarcoplasm. This leads to reduced light absorption, making the meat appear darker [30]. Exposing animals to acute stress just before, or during, slaughter can result in pale-soft-exudative (PSE) meat. Post-slaughter, muscles undergo acidification as glycogen breaks down into lactic acid [31]. In PSE meat, this acidification process occurs more rapidly than usual, leading to lower pH levels in the muscle when the carcass temperature is still high [32]. The combination of low pH and elevated temperature in PSE meat causes certain muscle proteins to denature, reducing their ability to hold water [33].