Thermal death time
Encyclopedia
Thermal death time is a concept used to determine how long it takes to kill a specific bacteria
at a specific temperature
. It was developed for food
canning
and has found applications in cosmetics
and pharmaceuticals.
of the Underwood Canning Company, a food company founded in 1822 at Boston, Massachusetts and later relocated to Watertown, Massachusetts
, approached William Thompson Sedgwick
, chair of the biology
department at the Massachusetts Institute of Technology
, about losses his company was suffering due to swollen and burst cans despite the newest retort
technology available. Sedgwick gave his assistant, Samuel Cate Prescott
, a detailed assignment on what needed to be done. Prescott and Underwood worked on the problem every afternoon from late 1895 to late 1896, focusing on canned clams
. They first discovered that the clams contained heat-resistant bacterial spore
s that were able to survive the processing; then that these spores' presence depended on the clams' living environment; and finally that these spores would be killed if processed at 250 ˚F (121 ˚C) for ten minutes in a retort.
These studies prompted the similar research of canned lobster
, sardines, peas
, tomatoes, corn
, and spinach
. Prescott and Underwood's work was first published in late 1896, with further papers appearing from 1897 to 1926. This research, though important to the growth of food technology
, was never patented. It would pave the way for thermal death time research that was pioneered by Bigelow and C. Olin Ball
from 1921 to 1936 at the National Canners Association
(NCA).
Bigelow and Ball's research focused on the thermal death time of Clostridium botulinum
(C. botulinum) that was determined in the early 1920s. Research continued with inoculated canning pack studies that were published by the NCA in 1968.
A lethal ratio (L) is also a sterilizing effect at 1 minute at other temperatures with (T).
where TRef is the reference temperature, usually 250 °F or 121 °C; z is the z-value, and T is the slowest heat point of the product temperature.
programs. The time
would be shown on the x-axis while the temperature
would be shown on the y-axis. This simple heating curve can also determine the lag factor (j) and the slope (fh). It also measures the product temperature rather than the can temperature.
where I = RT (Retort Temperature) − IT (Initial Temperature) and where j is constant for a given product.
It is also determined in the equation shown below:
where g is the number of degrees below the retort temperature on a simple heating curve at the end of the heating period, BB is the time in minutes from the beginning of the process to the end of the heating period, and fh is the time in minutes required for the straight-line portion of the heating curve plotted semilogarithmically on paper or a computer spreadsheet to pass through a log cycle.
A broken heating curve is also used in this method when dealing with different products in the same process such as chicken noodle soup in having to dealing with the meat and the noodles having different cooking times as an example. It is more complex than the simple heating curve for processing.
. This is usually done by thermal processing and finding ways to reduce the number of bacteria in the product. Time-temperature measurements of bacterial reduction is determined by a D-value, meaning how long it would take to reduce the bacterial population by 90% or one log10 at a given temperature. This D-value reference (DR) point is 250 °F or 121 °C.
z or z-value
is used to determine the time values with different D-values at different temperatures with its equation shown below:
where T is temperature in °F or °C.
This D-value is affected by pH of the product where low pH has faster D values on various foods.
The target of reduction in canning is the 12-D reduction of C. botulinum, which means that processing time will reduce the amount of this bacteria by 1012 bacteria per gram or milliliter. The DR for C. botulinum is 0.21 minute (12.6 seconds). A 12-D reduction will take 2.52 minutes (151 seconds).
This is taught in university
courses in food science
and microbiology
and is applicable to cosmetic and pharmaceutical manufacturing.
In 2001, the Purdue University
Computer Integrated Food Manufacturing Center and Pilot Plant put Ball's formula online for use.
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
at a specific temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
. It was developed for food
Food
Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...
canning
Canning
Canning is a method of preserving food in which the food contents are processed and sealed in an airtight container. Canning provides a typical shelf life ranging from one to five years, although under specific circumstances a freeze-dried canned product, such as canned, dried lentils, can last as...
and has found applications in cosmetics
Cosmetics
Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...
and pharmaceuticals.
History
In 1895, William Lyman UnderwoodWilliam Lyman Underwood
William Lyman Underwood was an American photographer who was also involved in the research of time-temperature canning research at the Massachusetts Institute of Technology during 1895-96.-Biography:...
of the Underwood Canning Company, a food company founded in 1822 at Boston, Massachusetts and later relocated to Watertown, Massachusetts
Watertown, Massachusetts
The Town of Watertown is a city in Middlesex County, Massachusetts, United States. The population was 31,915 at the 2010 census.- History :Archeological evidence suggests that Watertown was inhabited for thousands of years before the arrival of settlers from England...
, approached William Thompson Sedgwick
William Thompson Sedgwick
William Thompson Sedgwick was a key figure in shaping public health in the United States.William T...
, chair of the biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
department at the Massachusetts Institute of Technology
Massachusetts Institute of Technology
The Massachusetts Institute of Technology is a private research university located in Cambridge, Massachusetts. MIT has five schools and one college, containing a total of 32 academic departments, with a strong emphasis on scientific and technological education and research.Founded in 1861 in...
, about losses his company was suffering due to swollen and burst cans despite the newest retort
Pressure cooking
Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. Because the boiling point of water increases as the pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a higher...
technology available. Sedgwick gave his assistant, Samuel Cate Prescott
Samuel Cate Prescott
Samuel Cate Prescott was an American food scientist and microbiologist who was involved in the development of food safety, food science, public health, and industrial microbiology.-Early life:...
, a detailed assignment on what needed to be done. Prescott and Underwood worked on the problem every afternoon from late 1895 to late 1896, focusing on canned clams
CLaMS
CLaMS is a modular chemistry transport model system developed at Forschungszentrum Jülich, Germany. CLaMS was first described by McKenna et al. and was expanded into three dimensions by Konopka et al....
. They first discovered that the clams contained heat-resistant bacterial spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s that were able to survive the processing; then that these spores' presence depended on the clams' living environment; and finally that these spores would be killed if processed at 250 ˚F (121 ˚C) for ten minutes in a retort.
These studies prompted the similar research of canned lobster
Lobster
Clawed lobsters comprise a family of large marine crustaceans. Highly prized as seafood, lobsters are economically important, and are often one of the most profitable commodities in coastal areas they populate.Though several groups of crustaceans are known as lobsters, the clawed lobsters are most...
, sardines, peas
PEAS
P.E.A.S. is an acronym in artificial intelligence that stands for Performance, Environment, Actuators, Sensors.-Performance:Performance is a function that measures the quality of the actions the agent did....
, tomatoes, corn
Maize
Maize known in many English-speaking countries as corn or mielie/mealie, is a grain domesticated by indigenous peoples in Mesoamerica in prehistoric times. The leafy stalk produces ears which contain seeds called kernels. Though technically a grain, maize kernels are used in cooking as a vegetable...
, and spinach
Spinach
Spinach is an edible flowering plant in the family of Amaranthaceae. It is native to central and southwestern Asia. It is an annual plant , which grows to a height of up to 30 cm. Spinach may survive over winter in temperate regions...
. Prescott and Underwood's work was first published in late 1896, with further papers appearing from 1897 to 1926. This research, though important to the growth of food technology
Food technology
Food technology, is a branch of food science which deals with the actual production processes to make foods.-Early history of food technology:...
, was never patented. It would pave the way for thermal death time research that was pioneered by Bigelow and C. Olin Ball
C. Olin Ball
Charles Olin Ball was an American food scientist and inventor who was involved in the thermal death time studies in the food canning industry during the early 1920s. This research would be used as standard by the United States Food and Drug Administration for calculating thermal processes in canning...
from 1921 to 1936 at the National Canners Association
Food Products Association
The Food Products Association or FPA was the principal scientific and technical U.S. trade association representing the food products industry...
(NCA).
Bigelow and Ball's research focused on the thermal death time of Clostridium botulinum
Clostridium botulinum
Clostridium botulinum is a Gram-positive, rod-shaped bacterium that produces several toxins. The best known are its neurotoxins, subdivided in types A-G, that cause the flaccid muscular paralysis seen in botulism. It is also the main paralytic agent in botox. C. botulinum is an anaerobic...
(C. botulinum) that was determined in the early 1920s. Research continued with inoculated canning pack studies that were published by the NCA in 1968.
Mathematical formulas
Thermal death time can be determined one of two ways: 1) by using graphs or 2) by using mathematical formulas.Graphical method
This is usually expressed in minutes at the temperature of 250 °F or 121 °C. This is designated as F0. Each 18 °F or 10 °C change results in a time change by a factor of 10. This would be shown either as F10121 = 10 minutes (SI) or F18250 = 10 minutes (American English).A lethal ratio (L) is also a sterilizing effect at 1 minute at other temperatures with (T).
where TRef is the reference temperature, usually 250 °F or 121 °C; z is the z-value, and T is the slowest heat point of the product temperature.
Formula method
Prior to the advent of computers, this was plotted on semilogarithmic paper though it can also be done on spreadsheetSpreadsheet
A spreadsheet is a computer application that simulates a paper accounting worksheet. It displays multiple cells usually in a two-dimensional matrix or grid consisting of rows and columns. Each cell contains alphanumeric text, numeric values or formulas...
programs. The time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
would be shown on the x-axis while the temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
would be shown on the y-axis. This simple heating curve can also determine the lag factor (j) and the slope (fh). It also measures the product temperature rather than the can temperature.
where I = RT (Retort Temperature) − IT (Initial Temperature) and where j is constant for a given product.
It is also determined in the equation shown below:
where g is the number of degrees below the retort temperature on a simple heating curve at the end of the heating period, BB is the time in minutes from the beginning of the process to the end of the heating period, and fh is the time in minutes required for the straight-line portion of the heating curve plotted semilogarithmically on paper or a computer spreadsheet to pass through a log cycle.
A broken heating curve is also used in this method when dealing with different products in the same process such as chicken noodle soup in having to dealing with the meat and the noodles having different cooking times as an example. It is more complex than the simple heating curve for processing.
Applications
In the food industry, it is important to reduce the amount of microbes in products to ensure proper food safetyFood safety
Food safety is a scientific discipline describing handling, preparation, and storage of food in ways that prevent foodborne illness. This includes a number of routines that should be followed to avoid potentially severe health hazards....
. This is usually done by thermal processing and finding ways to reduce the number of bacteria in the product. Time-temperature measurements of bacterial reduction is determined by a D-value, meaning how long it would take to reduce the bacterial population by 90% or one log10 at a given temperature. This D-value reference (DR) point is 250 °F or 121 °C.
z or z-value
Z-value
Z-value is a term used in thermal death time calculations. The z-value of an organism is the temperature, in degrees Fahrenheit or Celsius, that is required for the thermal destruction curve to move one log cycle...
is used to determine the time values with different D-values at different temperatures with its equation shown below:
where T is temperature in °F or °C.
This D-value is affected by pH of the product where low pH has faster D values on various foods.
The target of reduction in canning is the 12-D reduction of C. botulinum, which means that processing time will reduce the amount of this bacteria by 1012 bacteria per gram or milliliter. The DR for C. botulinum is 0.21 minute (12.6 seconds). A 12-D reduction will take 2.52 minutes (151 seconds).
This is taught in university
University
A university is an institution of higher education and research, which grants academic degrees in a variety of subjects. A university is an organisation that provides both undergraduate education and postgraduate education...
courses in food science
Food science
Food science is a study concerned with all technical aspects of foods, beginning with harvesting or slaughtering, and ending with its cooking and consumption, an ideology commonly referred to as "from field to fork"...
and microbiology
Microbiology
Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...
and is applicable to cosmetic and pharmaceutical manufacturing.
In 2001, the Purdue University
Purdue University
Purdue University, located in West Lafayette, Indiana, U.S., is the flagship university of the six-campus Purdue University system. Purdue was founded on May 6, 1869, as a land-grant university when the Indiana General Assembly, taking advantage of the Morrill Act, accepted a donation of land and...
Computer Integrated Food Manufacturing Center and Pilot Plant put Ball's formula online for use.