Since foods are of animal or vegetable origin, it is interesting to consider the characteristics of animal and plant tissues that influence microbial growth.
The microbiological quality of the food is conditioned, first, to the quantity and type of microorganisms initially present (initial contamination) and then to the multiplication of these in the food. The quality of the raw materials and the hygiene (of environments, manipulators and surfaces) represent the initial contamination. The type of food and environmental conditions regulate multiplication.
The factors inherent to food may also be called intrinsic parameters, such as pH and water activity (Aa) and those inherent to the environment of extrinsic parameters, such as temperature, relative humidity (RH) and the presence of gases.
Such factors may be optimal or limiting, interfering greatly in the multiplication of microorganisms, including food-borne pathogens, which are primarily responsible for food-borne infections and intoxications.
The cell division of the bacterium, during bacterial growth, is preceded by an increase in the cell mass, by the synthesis of the proteins, polysaccharides, lipids and nucleic acids that constitute it. The synthesis of these compounds only occurs if the bacterium has access to nutrients that provide adequate basic elements.
Bacteria, molds and yeasts have a wide variety of nutritional requirements, but they generally find conditions conducive to their multiplication.
Under ideal conditions, the bacteria are the microorganisms with the highest growth rate, and can have a generation time (tg) in an average of 20 minutes. Therefore, even in those cases where the initial contamination of a food is small, high counts can be achieved in a short time. However, such velocity is not constant, with sharp variations, which will depend on the growth phase they are in and the environmental conditions. The intrinsic and extrinsic parameters, therefore, also determine the rate of multiplication.
Yeasts, in turn, have a generation time of 2 to 3 hours, therefore, higher than bacteria, and the molds (filamentous fungi) multiply more slowly than yeasts.
Thus, in a food that presents conditions for the development of the three groups of microorganisms, the bacteria will be dominant and, therefore, will be the cause of the deterioration. On the other hand, yeasts and molds will be important in the deterioration of those foods that do not provide conditions for the rapid growth of bacteria.