Aerobic plate counts provide a means of measuring the composite microbiological population in a food. The aerobic plate count is generally considered not to be a good predictor of the presence of pathogenic organisms since the pathogens are poor competitors and are usually overgrown by the non-pathogenic. However, when the total plate counts are used correctly, they can indicate the effect of harvesting, storage, processing, preservation and packaging, and in some cases should reflect the manner in which the food is to be used.
The microbiological spoilage point varies with different classes of foods but generally counts must be in the millions of microorganisms per gram before organoleptic deterioration occurs (Corlett, 1974). Since bacterial cells occur not only singly, but also in pairs, chains, clusters, and clumps; aerobic plate counts should be reported as colony counts per unit or colony forming units per unit instead of viable cell counts. Mixing the initial dilution in a mechanical blender provides better breakage of the clumps but does not ensure that the organisms are distributed as single cells in the dilutions (Speck, 1976).
Interpretation of total plate counts is not straightforward. Certain microbial grounds within the flora of a food are more apt to cause organoleptic spoilage than others. If these groups are given a selective growth advantage due to processing, packaging, or storage, then spoilage may occur at relatively low aerobic plate count levels. Research on vacuum-packed and non-vacuum-packed ham demonstrated the effect of packaging. Non-vaccuum-packed sliced ham had "borderline" sensory scores after 7 days of storage at 38F(3C) with a total plate count of 500,000 per gram; plate incubation temperature 86F (30C). The selective effect of oxygen which favored growth of spoilage-producing pseudomonads was thought to be responsible. High counts of lactobacilli in the anaerobic samples were not found to result in appreciable organoleptic spoilage (Corlett, 1974).
Choice of incubation temperature differs depending upon the type of microbial flora to be enumerated. Mesophilic organisms, both pathogenic and saprophytic species, multiply best at incubation temperatures of 86-98.6F (30-37C). The International Committee on Microbial Specifications for Foods (1978) stated four reasons for the importance of aerobic, mesophilic counts: (1) High counts in shelf stable foods often indicate contaminated raw materials or unsanitary processing; in perishable foods, improper time-temperature storage conditions may be indicated. large numbers occurring at these human body incubation temperatures mean that conditions may have existed which would favor the multiplication of pathogens. (2) Some strains of common mesophiles such as Proteus, enterococci, and pseudomonada have been reported to cause illness with high numbers in foods. (3) All known foodborne pathogenic bacteria are mesophilic and usually contribute to total plate counts. (4) When food spoilage is due to microorganisms, high counts are to be expected. Limitations of mesophilic counts include the failure to reflect numbers of dead bacteria in processed foods and high counts in curtain fermented and ripened foods that represent normal, non-objectionable microflora.
Psychrotrophic viable counts, determined by incubation at 32-50F(0-10C), are valuable in predicting shelf life of foods held in refrigerated storage. The term psychrotrophic is applied to organisms that exhibit growth at commercial refrigeration temperatures without reference to optimum growth temperature (Herbert, 1981). Species of Pseudomonas, Achromobacter, Flavobacterium, Alcigenes are often included in the psychrotrophic category along with certain molds such as Geotrichum and Botrytis(Mossel and Zwart, 1960). In refrigerated foods, the majority of psychotropic bacteria responsible for quality losses and subsequent spoilage are Gram negative rods. Most psychotropic bacteria are destroyed by pasteurization temperatures, so usually their presence in heat-processed foods implies post-processing contamination.
The presence of synchrotrons in large numbers indicates a high potential for spoilage during extended storage. Their growth rate is very temperature dependent, becoming increasingly slower with reduced temperatures (Speck, 1976). Low counts in foods to be stored under refrigeration for extended periods of time may not be good indicators of shelf life, as populations of 10 to the 6th or 10 to the 8th per gram with accompanying organoleptic changes may result in a few days or weeks, particularly at marginal refrigeration temperatures.