As our foods are of plant and/or animal origin, it is worthwhile to consider those characteristics of plant and animal tissues that affect the growth of microorganisms. The plants and animals that serve as food sources have all evolved mechanisms of defense against the invasion and proliferation of microorganisms, and some of these remain in effect in fresh foods.
By taking these natural phenomena into account, one can make effective use of each or all in preventing or retarding the growth of pathogenic and spoilage organisms in the products that are derived from them.
It may be noted from Table 3–3 that most of the meats and seafoods have a final ultimate pH of about 5.6 and above. This makes these products susceptible to bacteria as well as to mold and yeast spoilage. Most vegetables have lower pH values than fruits, and, consequently, vegetables should be subject more to bacterial than fungal spoilage.
With respect to the keeping quality of meats, it is well established that meat from fatigued animals spoils faster than that from rested animals and that this is a direct consequence of final pH attained upon completion of rigor mortis.
Upon the death of a well-rested meat animal, the usual 1% glycogen is converted to lactic acid, which directly causes a depression in pH values from about 7.4 to about 5.6, depending on the type of animal. Callow11 found the lowest pH values for beef to be 5.1 and the highest 6.2 after rigor mortis.
The usual pH value attained upon completion of rigor mortis of beef is around 5.6.5 The lowest and highest values for lamb and pork were found by Callow to be 5.4 and 6.7, and 5.3 and 6.9, respectively. Briskey8 reported that the ultimate pH of pork may be as low as