Importance of the pH in the shelf life and meat quality

By definition, pH is the negative logarithm of the hydrogen ion concentration, and indicates the acidic, neutral or alkaline character of a product on a scale that varies from zero to 14. A pH=7  is considered the value for neutrality,  whereas acid products are those with a pH below 7, being more acidic as the pH value approaches zero; while alkaline or basic products have pH values greater than 7, in a greater degree as the pH get close to 14. Fresh meat is considered a slightly acidic product as its ultimate pH value range from 5.5. to 5.8, depending on the animal species and slaughter conditions. However, the pH is an important indicator to evaluate the meat quality, as it is directly related with its color, texture, juiciness, and flavor. Furthermore, pH  is also related with the meat shelf life as this parameter determines the microbial community and their growth rate.

In the first place, the transformation from muscle to meat involves a series of biochemical changes that result in the gradual decrease in pH, as well as in  the establishment and subsequent resolution of rigor mortis. Interruption of the oxygen supply during slaughter induces the anaerobic conversion  of the muscle glucose (glycogen) into lactic acid and ATP called glycolysis.  In a healthy and rested animal there will be an adequate reserve of glycogen and its postmortem glycolysis will be accompanied by the gradual drop of the pH to values below 5.8, due to the accumulation of lactic acid, as shown in the Figure 1.

 

Figure 1. Representation of the pH postmortem variation for normal meat and DFD (dark, firm, and dry) and PSE (pale, soft and exudative) conditions.

 

However, improper handling of animals before and during slaughter can modify the final pH and the rate at which this parameter decreases.  For instance, fasting and prolonged stress reduce glycogen muscle concentration, that results in a low of lactic acid production and final pH values higher than the normal; thus, generating the dark, firm, and dry condition called DFD meat. This meat will be harder and more susceptible to microbial alteration since most spoilage bacteria have an optimal growth at pH from 6 to 7.5. On the other hand, antemortem stress causes a rapid conversion of glucose into lactic acid and a rapid decrease in pH, that is associated with the condition of pale, soft and exudative or PSE meat. The rapid acidification, when the temperature of the carcass is still close to 37° C, induces the denaturation of proteins that decreases their ability to retain water and other water-soluble elements; so that the meat will tend to lose water and other nutrients generating a surface exudate and weight losses, besides the presence of exudate also favor the growth of spoilage bacteria and reduces consumer acceptability.

As already stated, fresh meat is a slightly acidic product with a final pH between 5.5. to 5.8, with variations between animal species. However, the pH of meat can gradually increase during its storage derived from the accumulation of basic compounds such as ammonia and other volatile nitrogenous compounds, which are produced by the deamination of proteins, amino acids, and nucleotides because of endogenous enzymatic degradation, as well as from the growth of spoilage bacteria mainly of the genera Pseudomonas and Enterobacteria. Similarly, pH affects the microbial growth and survival by affecting the bioavailability of nutrients and the microbial metabolism. Accordingly with the pH of maximum growth,  microorganisms are classified into three groups: acidophiles that growth best at pH <5, neutrophils with optimal growth at pH close to 7.0, and alkaliphiles capable to growth at pH above 9. For instance, most E coli, Salmonella spp and  Staplylococcus strains are neutrophils, while Vibrio cholerae grows best at pH between 8 to 10. But most meat spoilage related microorganism growth within a pH range of 6-7.5. Nonetheless, meat can also be acidified during storage due to the accumulation of acids such as lactic, acetic, propionic, carbonic or others, due to the growth of acidic microbiota specially under vacuum or modified atmosphere packaging.  Therefore, pH has a great relevance to evaluate the meat quality along the process of slaughter, packaging, and distribution.

This topic was authored by  the AMEXITEC (Mexican Association of Meat Science and Technology) was founded in 2008, with the aim to generate and promote scientific knowledge on issues related to the meat science and technology. AMEXITEC is integrated by a network of researchers, academics, students, industrialists, and professionals related to the meat industry.

Be part of AMEXITEC by visiting our website and attend our 4^th Virtual Forum on October 26-27 https://amexitec.org/index.php/4to-foro-amexitec

 

Dr Edith Ponce-Alquicira