How to Heat-Sanitize Food: Pasteurization, Sterilization, and Thermal Death Curves"

Pasteurization and Stertilization

The oldest method for heat-sanitizing food is known as pasteurization, a technique named after the renowned French scientist Louis Pasteur. This pioneering chemist and microbiologist began developing the pasteurization method shortly before publishing his groundbreaking studies on the role of bacteria in disease, which are credited with providing the first convincing evidence for the germ theory of disease.

Many individuals associate pasteurization solely with the heating of dairy products to eliminate pathogenic bacteria, but the term is used in various other contexts. In fact, Pasteur initially devised this technique for the preservation of wine and beer.

Another frequently used term is sterilization. While sterilization techniques are often thought to eradicate all microorganisms, this is not typically the case. More commonly, the heat treatment is designed to eliminate the most dangerous pathogens while sparing other types of bacteria.

Time and Temperature to kill them all...well most of them

The combination of temperature and time required to eliminate 90% of a specific bacterial population (representing a 1D reduction) depends on several factors, with the most significant being the type of bacteria. Heat tolerance varies widely among different species. Additionally, the pH level and the presence of salt, sodium nitrite, or other additives can have a significant impact, as can the presence of specific proteins or fats. Fats can either provide protection to bacteria from heat or make them more sensitive to elevated temperatures.

The Thermal Death Curve

Since the rate at which bacteria perish increases with temperature, it may take 15 minutes to kill 90% of them at 54°C (130°F), whereas only a few seconds are needed at 100°C (212°F). A graphical representation, referred to as a thermal death curve, outlines the time and temperature combinations necessary to achieve a specific reduction in the bacterial count. The shape of these curves varies according to bacterial species and environmental conditions, such as pH and the type of food. Typically, specialists model a thermal death curve mathematically as an exponential function, and when plotted, it forms a straight line that facilitates extrapolation to higher or lower temperatures.

The precise positioning of a thermal death curve is not as critical as the principle that various combinations of time and temperature can achieve the same level of food safety by eliminating the same proportion of bacteria. This means there's usually a choice between cooking at high heat for a short time or cooking at low heat for a longer period, and it won't affect the bacteria or the safety of the food. However different choices of time and temperature can significantly impact the appearance and flavor of the dish.

Food microbiologists determine thermal death curves by cultivating bacteria under various conditions, subjecting them to heat, and then monitoring how many survive or perish over time. These findings are regularly documented in scientific journals like the International Journal of Food Microbiology.

In the commercial food industry, various methods are employed to eliminate bacteria, including ultrahigh pressure, gamma-ray irradiation, strong electric fields, and ultraviolet light. While these approaches have potential applications in the culinary world because they can eradicate bacteria without affecting flavor and texture, further research is necessary before they can transition from laboratory and industrial settings to smaller kitchens.

If you are aware of the time (D1) needed to achieve a 1D reduction at a reference temperature (Tr), you can utilize an equation to compute the 1D cooking time (t) at any other temperature (T) above the minimum lethal temperature (Tmin).

The equation is

T kill =D 10 ^(T-T ref)/z where Tmin < T < Tmax

A typical value for the parameter Z is 10°C (18°F), meaning that a 10°C (18°F) change in temperature results in the time required to achieve the same reduction in bacteria increasing or decreasing by a factor of 10.