Chapter 11 control of microbial growth and death

isms, many items that are free of microorganisms are kept sterile by wrapping them in paper, plastic, or foil. Such wrappings are impermeable to microor­ganisms and prevent recontamination. For example, foods are packaged in cans, jars, or other containers that prevent contamination with microorganisms that could contaminate the foods, causing them to spoil and rendering them a threat to human health. Similarly, syringes, needles, scalpels, and other med­ical instruments are sealed in sterile packages, as are Petri dishes, pipettes, and many other items used in the microbiology laboratory. Surgical gloves and gowns help prevent microorganisms from passing from one person to another, especially during surgi­cal procedures or where a hospital worker comes in contact with a patient that has a contagious disease.

High Temperatures

Temperature is one of the most important environ­mental factors affecting the rates of microbial growth and death. Temperature influences the rates of chem­ical reactions by altering the three-dimensional shapes of proteins, thereby affecting the rates of en­zymatic activities. Heat can kill microorganisms by denaturing their enzymes. Consequently, high tem­peratures can be used to kill microorganisms in order to control their proliferation. At temperatures ex­ceeding the maximal growth temperature, the death rate exceeds the growth rate. The higher the temper­ature above the maximal growth temperature, the higher the death rate for that microorganism.

Temperature influences the rates of chemical reac­tions by affecting the three-dimensional configura­tion of proteins and thus enzymatic activity; high temperatures kill microorganisms by denaturing their enzymes.

High temperatures can be used to reduce the numbers of microorganisms or to eliminate all viable microorganisms. When using heat to kill microorganisms, the degree of the microorganism's heat resis­tance must be considered so that the correct exposure times and temperatures are used. Heat resistance varies among different microorganisms; these differences can be expressed through the concept of thermal death point. Thermal death point (TDP) is the lowest temperature required to kill all of the microorganisms in a liquid suspension in 10 minutes. Heat is the most widely applicable and effective agent for killing microorganisms and also the most economical and easily controlled.

The heat killing of microorganisms can be described by the decimal reduction time (D value) (FIG. 11-1). D is defined as the time required for a tenfold reduction in the number of viable cells at a given

FIG. 11-1 The D value is the time in minutes needed to duce the number of viable microorganisms by a factor ten (one log unit).

temperature, that is, the time required for a log г duction in the number of microorganisms. This is time required to bring about a 90% reduction in numbers of viable microorganisms. As the temper, ture is increased above the maximal growth tempe ature for a microorganism, the decimal reductio time is shortened. The decimal reduction time varu for different microorganisms. In the food industr the decimal reduction time is important in establising appropriate processing times for sterilizing food products.

The D value describes the rate of death at a given temperature.

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