Food safety is a very broad topic. Pesticides, herbicides, chemical additives, and spoilage are all of concern, but food scientists, food processors, and consumers focus most on microbiological quality. Microorganisms pose a challenge to the food industry and most food processes are designed with microbial quality in mind. Microorganisms are often too small to be seen with the unaided eye and have the ability to reproduce rapidly. Many of them produce toxins and can cause infections. For all of these reasons, the microbiological quality of the food we eat is scrutinized closely.
Centuries ago, Genghis Khan was able to rule vast stretches of land through the mobility of his army. With very little food, he was able to engage in swift attacks over long periods of time. As the story goes, each horseman carried two leather bags. The larger one held dry milk produced by drying fluid milk in the sun during periods of rest. The smaller bag was used to rehydrate some of the milk powder with water, which was consumed during an offensive. The lightly equipped army of Khan thus could cover long distances in weeks, and eventually controlled most of the Asian continent. Yet, one has to wonder how many people suffered food-borne illness in those days.
Today, food-borne illness is of serious concern. Its frequency is not known because a great majority of the cases go unreported. Reporting food-borne diseases to public health authorities is not required in the United States. Estimates claim as many as 200 million cases in the U.S. per year. Only a small percentage of these are hospitalized. Most are passed off as traveler's diarrhea, 24 hour flu, or upset stomach. Salmonellosis, one of the more serious food-borne diseases, is said to be reported only about 1% of the time. About 42,000 cases of salmonellosis are reported in the U.S. annually, with about 150 deaths. So, there are potentially 4.2 million cases of Salmonella food poisoning annually despite the fact that the U.S. food supply is considered very safe and processed under the best conditions available.
Testing the foods we consume for the presence of pathogenic microorganisms is very important. Although 100% of the food cannot be tested, it can be deemed "safe" through proper audit of the food supply. In many instances, the pathogenic microorganisms are present in very small numbers, but for many of these pathogens, small numbers are all that are necessary to transmit disease or illness. For that reason, the presence of other microorganisms is monitored. These microorganisms provide an index of the sanitary quality of the product and may serve as an indicator of potential for the presence of pathogenic species. Escherichia coli (E. coli) is commonly employed as an indicator microorganism. Because E. coli is a coliform bacteria common to the intestinal tract of humans and animals, its relationship to intestinal food-borne pathogens is high.
Total counts of microorganisms are also an indication of the sanitary quality of a food. Referred to as the Standard Plate Count (SPC), this total count of viable microbes reflects the handling history, state of decomposition or degree of freshness of the food. Total counts may be taken to indicate the type of sanitary control exercised in the production, transport, and storage of the food. Most foods have standards or limits for total counts. This is especially true for milk.
It must be remembered that a low SPC does not always represent a safe product. It is possible to have low-count foods in which toxin-producing organisms have grown. These organisms produce toxins that remain stable under conditions that may not favor the survival of the microbial cell.
In adopting microbiological standards to milk, the first concern is product safety, followed by shelf-life. The following bacterial counts are standards for milk as recommended by the U.S. Public Health Service:
Grade A raw milk for pasteurization Not to exceed 100,000 bacteria per milliliter (ml) prior to commingling with other produced milk; and not exceeding 300,000 per ml as commingled milk prior to pasteurization.
Grade A pasteurized milk Not over 20,000 bacteria per ml, and not over 10 coliforms per ml.
The objective of pasteurization is to reduce the total microbial load, or SPC. In addition, pasteurization must destroy all pathogens that may be carried in the milk from the cow, particularly undulant fever, tuberculosis, Q-fever, and other diseases transmittable to humans. This is accomplished by setting the time and temperature of the heat treatment so that certain heat-resistant pathogens, specifically Mycobacterium tuberculosis and Coxiella burnetii (causative agents of Q-fever and tuberculosis, respectively) would be destroyed if present. Milk pasteurization temperatures are sufficient to destroy all yeasts, mold, and many of the spoilage bacteria.