Meat processors need to be aware of the pH of meat and the significance of it; because pH greatly affects the functionality of meat during processing.
What is pH? It is a measurement of acidity. The range for measuring pH is zero to fourteen; the lower the pH the more acidic a solution is, the higher the pH the more alkaline. In a normal living muscle the pH is approximately 7. Glycogen which is the source of energy for the muscle in the living animal is produced and stored in the liver. When energy is needed for muscle movement glycogen is converted to ATP. For this conversion to occur oxygen needs to be present. When the animal is slaughtered, oxygen is no longer present due to the removal of blood and the glycogen is instead broken down to lactic acid as the muscle turns into meat. The pH typically is in the range of 5.4 to 5.7 although sometimes factors come into play
which result in muscle pH being outside this range.
The drop in pH will influence the overall quality of the final meat product. The pH will affect the water holding capacity of meat and it may affect the color of the meat. Both the rate and extent of post-mortem pH fall will influence meat quality characteristics. A phenomenon called PSE in pork (which stands for Pale, Soft and Exudative) commonly results from a rapid breakdown of glycogen into lactic acid after slaughter. Some breeds are more susceptible to this rapid fall than others. PSE pork is at the lower end of the pH range.
PSE Pork DFD Beef
Environment also influences the extent of pH fall. Longer times of feed withdrawal, before transport for instance, will decrease the amount of glycogen present in the muscle at slaughter; thereby, resulting in a higher ultimate pH than normal. Meat that has a very high ultimate pH (i.e. greater than 6.3) appears relatively dry. This Dark, Firm and Dry (DFD) product has a relatively high water holding capacity although the color of the fresh meat is not appealing. The product also has a very firm texture and is also produced when animals experience long term pre-slaughter stress and are slaughtered before they can rest and replenish the glycogen. Since glycogen is the substrate for lactic acid production in muscle, the less glycogen that is present at harvest, the less lactic acid is produced after harvest and subsequently the less the pH decline in postmortem muscle. In cattle that are rested and not exposed to stress the DFD product will typically not occur.
With regards, to water holding capacity (WHC), the concept or term to be familiar with is isoelectric point (pl). This is the pH where positive charges are equal to negative charges. The isoelectric point of meat is approximately 5.2. As the pH of meat decreases due to the buildup of lactic acid, the pH approaches the isoelectric point. As the pH approaches the isoelectric point, the difference in positive and negative charges becomes less and less. When positive and negative charges are equal, we find the water holding capacity to be at its lowest level. This is important to keep in mind because normal meat pH of 5.4 to 5.7, as previously mentioned, is near the worst case in regards to WHC.
As the above graph indicates, to improve WHC in meat the processor needs to modify the pH so that it is further away from the isoelectric point. Examples of techniques that can accomplish this are minimizing stress of animals prior to slaughter, use of pre-rigor meat commonly know as "hot boned" and the addition of sodium phosphates during comminution.