The effect of Digestarom® on poultry meat quality
Consumers are demanding greater quality and safety from their meat, and they are also more concerned than ever about animal welfare and environmental pollution. To satisfy these demands, poultry producers must consider every factor that may affect the final quality of the meat they produce. These factors must be identified and accommodated at every stage, from the parent stock to the fertilized egg, through hatchery operations and broiler farms, right down to processing and delivery to the end consumer.
Bird muscle is highly specialized tissue, providing structural support to the bird; around 40 to 50% of a chicken's body weight is skeletal muscle. Broiler muscle tissue contains 74.6% water, 12.1% protein, 11.1% lipid, 1.2% carbohydrate and 1% ash. After slaughter, muscle undergoes certain changes to become meat. During this process, the muscle's energy resources (glycogen, ATP and CP) are exhausted, causing major changes in the pH of the meat. The structure of the muscle also changes dramatically and it becomes a relatively rigid and inextensible structure, a process known as rigor mortis. As time passes, more physiological changes occur, affecting the quality of the end product, the meat.
Factors that affect the progression of rigor mortis can be either premortem or postmortem. Bird nutrition and health status, and the level of stress or exhaustion before slaughter are some premortem factors. Bleeding, scalding, plucking, eviscerating, cooling, and storage temperature are postmortem factors. All of these affect the rate of rigor mortis development, which in turn affects the sensory and functional properties of the meat and meat products.
Rapid acidification (falling pH), particularly at the prevailing body temperature, causes pale, soft and exudative (PSE) meat. PSE is generally described as a major defect, affecting color (paler than it should be), texture (softer than normal) and water holding capacity (WHC). In chicken, muscle pH of around 6.0 or less and muscle temperature of around 35°C or higher approximately 20 minutes after bleeding is known to produce PSE meat. Although the high temperature/low pH theory is well understood in the muscles of most animals, the nature of this muscle denaturation and the biochemical events that accelerate postmortem muscular glycolysis in poultry are still unclear.
If muscle pH does not decrease due to low glycogen reserves in the muscle, the meat appears dark, firm and dry (DFD). This occurs when the bird experiences exhaustion before slaughter. In this case, the muscle contains a very small amount of glycogen, and lactic acid production is very low, so pH does not decrease completely (remains above 6.2). Figure 1 shows a typical postmortem pH reduction in normal, PSE and DFD meats. DFD meat with a final pH>6.2 is microbiologically unstable and prone to microbial contamination, even when initial microbial contamination is low.
Figure 1. Typical postmortem pH reduction in normal, PSE, and DFD meats [Adapted from Guerrero-Legarreta, I., Hui, Y.H. (2010)]
What is the exact definition of meat quality and why is it so important? Ingr (1989) gives a good definition of meat quality: “Meat quality is a term used to describe the overall meat characteristics including its physical, chemical, morphological, biochemical, microbial, sensory, technological, hygienic, nutritional and culinary properties”. Therefore, the quality of a meat is determined by more than just one or two characteristics. Almost all meat characteristics are closely interrelated, so an increase or decrease in one aspect may significantly alter the other aspects of meat quality.
The most important and perceptible quality attributes of meat that can affect the consumer's initial and final judgment, before and after buying the meat, are color, texture, odor, flavor, tenderness, and juiciness. Some other properties of the meat are also crucial to slaughterhouses and meat processors. These include quantifiable properties, such as shear force, drip loss, WHC, cook loss, protein solubility, fat binding capacity, final pH and shelf life.
Fluid loss from fresh meat through passive exudation is referred to as muscle exudate or drip (Bowker and Zhuang, 2013). As WHC (the ability of the uncooked meat to retain its inherent or added water during postmortem processing and storage) decreases, the drip loss of the meat increases. Meat with low WHC and high drip loss is considered to be of a poor quality by processors and consumers. Unnecessary weight loss is not the only problem for this type of meat, as it usually gets tougher and drier after cooking. Postmortem pH development in the muscle has a significant effect on a number of quality attributes, including WHC and color development in the meat. Meat with relatively high pH (DFD meat) has a very high WHC, but it is not tender enough to be acceptable to consumers. Conversely, meat with relatively low final pH (PES meat) has a greater drip loss than meat with a normal final pH (5.6–5.8) and controlling drip loss is arguably one of the greatest challenges facing any poultry processor.
Breast meat color can be measured by visual inspection (human eye) or by colorimeter. The most widely accepted method for measuring meat color is probably the CIELAB color space. The CIELAB, or CIE L* a* b*, is a 3-dimensional color expression where L* is the lightness component, ranging from 0 to 100 (from black to white), and the parameters a* (from green if negative to red if positive) and b* (from blue if negative to yellow if positive) are two chromatic components which range from -120 to +120. The values of lightness (L*) and yellowness (b*) are known to decrease, and the value of redness (a*) to increase as meat pH increases. This means that meat might be darker, less yellow and more red when pH is higher.
Tenderness is also considered a very important attribute of meat quality. Meat tenderness can be evaluated by measuring the force required to shear a meat sample across the direction of the muscle fibers. In this method, lower shear forces indicate more tender meat. The Allo-Kramer (AK) is a multi-blade device for measuring tenderness in poultry meat. Chicken can be considered suitably tender when its shear force is below 5.50 kgf/g
Bird nutrition is known to have a significant effect on health, performance and, of course, meat quality. Poultry nutritionists around the world are actively looking for ingredients that can improve bird performance, health and welfare. Plant extracts and essential oils can be added to feed as a powerful mechanism to increase bird performance and health, and improve subsequent meat quality, and our knowledge of plants has increased dramatically during the last decade or so.
Phytogenic feed additives (PFAs) provide a broad spectrum of biologically active substances, according to their source and chemical structure. PFAs can generally exert antioxidant, anti-inflammatory, antibacterial and other biological effects, supporting animal performance and product quality.
In a trial conducted on a commercial farm in Italy in 2016, adding the PFA Digestarom® to a broiler diet improved growth in Ross 308 broilers compared to the control birds. Feed conversion ratio was significantly better in the treatment group (585 birds with FCR of 1.784) than the control group (585 birds with FCR of 1.792), and daily weight gain was also greater in the Digestarom® group (58.0 g/d) than the control group (57.6 g/d).
In addition to these performance-boosting effects, including Digestarom® in the broiler diet improved carcass traits and meat quality parameters. The increased body weight in the birds fed Digestarom® was accompanied by higher breast meat yield (30.7%) than the control group (30.3%) (Figure 1). Digestarom® increases nutrient digestibility, especially that of crude protein and amino acids, favoring their absorption, so the improved utilization of amino acids leading to increased muscle growth is a logical result. This confirms previous results demonstrating that higher levels of digestible dietary amino acid lysine are associated with increased breast meat yield (Berri, 2008).
Figure 2. Breast meat yield after 41 days of feeding broilers a control diet or a Digestarom® diet.
Table 1 shows that the addition of the PFA Digestarom® to broiler diets improves meat quality parameters. Including Digestarom® in the diet increased the final pH of the meat and enhanced its redness. Berri et al. (2007) demonstrated that increased muscle fiber diameter (cross-sectional area), and therefore muscle weight, was associated with reduced postmortem glycolytic activity and lactate formation, resulting in a slower postmortem pH reduction and less breast meat drip loss.
Table 1. The effect of a control diet or a Digestarom® diet on chicken meat quality parameters
In addition to their beneficial effects on physical and chemical parameters, PFAs also improve the sensory properties of the meat. Lipid peroxidation plays a major role in meat deterioration during storage and processing, reducing quality and consumer acceptance. Inhibiting or delaying this oxidative degradation is therefore highly important (Estévez, 2015) and is the reason that the antioxidant capacity of Digestarom® in poultry plasma and meat was investigated (Greece 2009, Figure 3).
Figure 3. Effect of a control diet and Digestarom® diet on A) plasma total antioxidant capacity and B) meat total antioxidant capacity in broiler chickens (*p<0.05).
It is clear that including the PFA Digestarom® in a broiler diet significantly improved the oxidative status of plasma and meat, indicating strong protection against oxidative degradation. Sensory analysis of broiler thigh meat confirmed the above findings, revealing that the meat of broilers fed a diet that included Digestarom® created a superior general impression than the control birds (Figure 4).
Our findings highlight the great potential of phytogenic feed additives as a tool to improve poultry meat quality and meet consumer expectations. However, the huge variety and complexity of PFAs require further trials to be conducted to clarify their mode of action.
Figure 4. Effect of a control diet or Digestarom® diet on the sensory characteristics of broiler thigh meat (*p <0.10)
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