Ultimate pH as a muscle-quality indicator trait is becoming more widely used by packers/processors and seedstock suppliers. The pork harvesting and processing industries want to know more about environmental factors that can improve ultimate pH and other indicator traits so that more products can be sold as premium products. In turn, the breeding stock industry is placing emphasis on traits through indexes or direct-breeding-value selection in order to supply the food chain with high quality pork.
pH is a challenging pork quality trait because pork meat does not have a pH value. Its biological component is hydrogen ion concentration. Before computers were widely used, hydrogen ion concentrations were converted to a more usable scale – pH – using a mathematical conversion. The objective was to make the extremely small hydrogen ion concentration values easier to evaluate – pH offers a larger scale. This conversion creates an interesting situation outlined in the following example:
A National Pork Board check-off funded project was created to better understand the statistical properties of pH and hydrogen ion concentration. Using data from the National Barrow Show Progeny Test, where ultimate pH was measured on loin samples, researchers evaluated the effects of using pH or hydrogen ion concentration on genetic evaluations of environmental effects such as day of harvest, breed and others. Original ultimate pH values were converted to hydrogen ion concentration to make these evaluations.
Numerical changes in the values for various environmental factors occurred, and in some cases it would change the declared differences among subclasses. For example, differences among breeds surfaced, depending on whether pH or hydrogen ion concentration was used. Ranking of sires by genetic merit for pH differed slightly from that observed for hydrogen ion concentration. The genetic effects are best illustrated graphically in Figure 1 (on page 32).
The graph shows selection based on a 5 percent selection rate for both ultimate pH and hydrogen ion concentration. Notice that selection based on ultimate pH rather than hydrogen ion concentration would encourage you to select animals from the upper left and right quadrant of the graph. If based on hydrogen ion concentration, you would select animals in the upper and lower left quadrants. Either method would cause you to select animals having values in the upper left quadrant. But there are other potential animals that you would miss.
The confusion comes in to play with the upper right (green) and lower left (red) quadrants. Using ultimate pH, you would select the animals in the upper right (green) quadrant. If you used the more accurate hydrogen ion concentration, those animals would not be selected. Instead, you would choose animals in the lower left (red) quadrant.
This is termed selection error. Heritability estimates were approximately 0.10 higher for hydrogen ion concentration when compared with ultimate pH (0.62 versus 0.52). In both cases, the heritability estimates would be considered high. However, you could expect more rapid genetic improvement if selection is based on hydrogen ion concentration rather than pH. Genetic correlations between ultimate pH and/or hydrogen ion concentration and other pork-quality traits like Minolta color evaluation, lipid content and mechanical tenderness were nearly identical. (See table below.) This suggests that selection based on either ultimate pH or hydrogen ion concentration would result in similar changes in the other quality traits.
Hydrogen ion concentration may be the better choice for genetic improvement because it is the biologically active component. That is, living cells and meat experience hydrogen ion concentrations, not pH values. Processors attempting to isolate environmental factors that influence pork quality like day, line, plant and such, could expect a similar response. Those processors may find that mean ultimate pH and mean hydrogen ion concentration (converted back to pH scale after analysis) are different. The greater heritability suggests that breeders should make selections based on hydrogen ion concentration analysis rather than ultimate pH. The differences in estimating effects such as breed, plant or other environmental factors seem to illustrate that pork processors should do the same.
Accomplishing this would be quite simple. Participants throughout the chain should be using current tools to measure pH, and recording the data. You should be sure that pork processors evaluating ultimate pH on your market hogs convert the pH values back to hydrogen ion concentration values before data is statistically evaluated. This simple, inexpensive change could result in better decisions for you, the pork processors and breeders. The end results will be more rapid pork quality gains, improved consumer eating experiences, and increased customer demand.