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Thermal death time is how long it takes to kill a specific
bacterium Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the ...
at a specific
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
. It was originally developed for
food Food is any substance consumed by an organism for Nutrient, nutritional support. Food is usually of plant, animal, or Fungus, fungal origin and contains essential nutrients such as carbohydrates, fats, protein (nutrient), proteins, vitamins, ...
canning Canning is a method of food preservation in which food is processed and sealed in an airtight container (jars like Mason jars, and steel and tin cans). Canning provides a shelf life that typically ranges from one to five years, although under ...
and has found applications in
cosmetics Cosmetics are substances that are intended for application to the body for cleansing, beautifying, promoting attractiveness, or altering appearance. They are mixtures of chemical compounds derived from either Natural product, natural source ...
, producing salmonella-free feeds for animals (e.g. poultry) and
pharmaceuticals Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy ( pharmacotherapy) is an important part of the ...
.


History

In 1895, William Lyman Underwood of the Underwood Canning Company, a food company founded in 1822 at
Boston, Massachusetts Boston is the capital and most populous city in the Commonwealth (U.S. state), Commonwealth of Massachusetts in the United States. The city serves as the cultural and Financial centre, financial center of New England, a region of the Northeas ...
and later relocated to
Watertown, Massachusetts Watertown is a city in Middlesex County, Massachusetts, United States, part of Greater Boston. The population was 35,329 in the 2020 United States census, 2020 census. Its neighborhoods include Bemis, Coolidge Square, East Watertown, Watertown Sq ...
, approached William Thompson Sedgwick, chair of the
biology Biology is the scientific study of life and living organisms. It is a broad natural science that encompasses a wide range of fields and unifying principles that explain the structure, function, growth, History of life, origin, evolution, and ...
department at the
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of moder ...
, about losses his company was suffering due to swollen and burst cans despite the newest
retort In a chemistry laboratory, a retort is a device used for distillation or dry distillation of substances. It consists of a sphere, spherical vessel with a long downward-pointing neck. The liquid to be distilled is placed in the vessel and heat ...
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 Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...
. They first discovered that the clams contained heat-resistant bacterial
spore In biology, a spore is a unit of sexual reproduction, sexual (in fungi) or asexual reproduction that may be adapted for biological dispersal, dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores fo ...
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 Lobsters are Malacostraca, malacostracans Decapoda, decapod crustaceans of the family (biology), family Nephropidae or its Synonym (taxonomy), synonym Homaridae. They have long bodies with muscular tails and live in crevices or burrows on th ...
, sardines,
pea Pea (''pisum'' in Latin) is a pulse or fodder crop, but the word often refers to the seed or sometimes the pod of this flowering plant species. Peas are eaten as a vegetable. Carl Linnaeus gave the species the scientific name ''Pisum sativum' ...
s,
tomato The tomato (, ), ''Solanum lycopersicum'', is a plant whose fruit is an edible Berry (botany), berry that is eaten as a vegetable. The tomato is a member of the nightshade family that includes tobacco, potato, and chili peppers. It originate ...
es,
corn Maize (; ''Zea mays''), also known as corn in North American English, is a tall stout Poaceae, grass that produces cereal grain. It was domesticated by indigenous peoples of Mexico, indigenous peoples in southern Mexico about 9,000 years ago ...
, and
spinach Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to Central Asia, Central and Western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common vegetable consumed eit ...
. 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 is a branch of food science that addresses the production, preservation, quality control and research and development of food products. It may also be understood as the science of ensuring that a society is food secure and ha ...
, 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 The Food Products Association (formerly the National Food Processors Association or NFPA) was the principal U.S. scientific and technical trade association representing the Food industry, food processing industry until 2007. FPA was headquartered ...
(NCA). Bigelow and Ball's research focused on the thermal death time of ''
Clostridium botulinum ''Clostridium botulinum'' is a Gram-positive bacteria, gram-positive, Bacillus (shape), rod-shaped, Anaerobic organism, anaerobic, endospore, spore-forming, Motility, motile bacterium with the ability to produce botulinum toxin, which is a neurot ...
'' (''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 . This is designated as ''F''0. 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 (Celsius) or F18250 = 10 minutes (Fahrenheit). A lethal ratio (''L'') is also a sterilizing effect at 1 minute at other temperatures with (''T''). :L = 10^ where ''T''Ref is the reference temperature, usually ; ''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
spreadsheet A spreadsheet is a computer application for computation, organization, analysis and storage of data in tabular form. Spreadsheets were developed as computerized analogs of paper accounting worksheets. The program operates on data entered in c ...
programs. The
time Time is the continuous progression of existence that occurs in an apparently irreversible process, irreversible succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequ ...
would be shown on the x-axis while the
temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
would be shown on the ''y''-axis. This simple heating curve can also determine the lag factor (''j'') and the slope (''f''''h''). It also measures the product temperature rather than the can temperature. : j = 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: :\log g = \log jI - where ''g'' is the number of degrees below the retort temperature on a simple heating curve at the end of the heating period, ''B''''B'' is the time in minutes from the beginning of the process to the end of the heating period, and ''f''''h'' 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 number of
microbes A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
in products to ensure proper
food safety Food safety (or food hygiene) is used as a scientific method/discipline describing handling, food processing, preparation, and food storage, storage of food in ways that prevent foodborne illness. The occurrence of two or more cases of a simi ...
. 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 . ''z'' or z-value is used to determine the time values with different ''D''-values at different temperatures with its equation shown below: :z = \frac{\log D_1 - \log D_2} 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 ''D''-value at an unknown temperature can be calculate

knowing the ''D''-value at a given temperature provided the ''Z''-value is known. 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 a factor of 1012. 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 A university () is an educational institution, institution of tertiary education and research which awards academic degrees in several Discipline (academia), academic disciplines. ''University'' is derived from the Latin phrase , which roughly ...
courses in
food science Food science (or bromatology) is the basic science and applied science of food; its scope starts at overlap with agricultural science and nutritional science and leads through the scientific aspects of food safety and food processing, informing ...
and
microbiology Microbiology () is the branches of science, scientific study of microorganisms, those being of unicellular organism, unicellular (single-celled), multicellular organism, multicellular (consisting of complex cells), or non-cellular life, acellula ...
and is applicable to cosmetic and pharmaceutical manufacturing. In 2001, the
Purdue University Purdue University is a Public university#United States, public Land-grant university, land-grant research university in West Lafayette, Indiana, United States, and the flagship campus of the Purdue University system. The university was founded ...
br>Computer Integrated Food Manufacturing Center and Pilot Plant
put Ball's formula online for use.


References

*Downing, D.L. (1996). ''A Complete Course In Canning - Book II: Microbiology, Packaging, HACCP & Ingredients, 13th Edition.'' Timonium, MD: CTI Publications, Inc. pp. 62–3, 71-5, 93-6.

- Accessed November 5, 2006. *Goldblith, S.A. (1993). ''Pioneers in Food Science, Volume 1: Samuel Cate Prescott - M.I.T. Dean and Pioneer Food Technologist.'' Trumball, CT: Food & Nutrition Press. pp 22–28.
History about Underwood Canning Company
- Accessed October 28, 2006. *Jay, J.M. (1992). ''Modern Food Microbiology, 4th Edition.'' New York: Chapman & Hall. pp. 342–6. *Juneja, V.K. and L. Huang. (2003). "Thermal Death Time." In ''Encyclopedia of Agricultural, Food, and Biological Engineering.'' D.R. Heldman, Ed. New York: Marcel Dekker, Inc. pp. 1011–1013. *Powers, J.J. (2000). "The Food Industry Contribution: Preeminence in Science and in Application." ''A Century of Food Science.'' Institute of Food Technologists: Chicago. pp. 17–18. *Prescott, L.M., J.P. Harley, & D.A. Klien. (1993). ''Microbiology, 2nd Edition.'' Dubuque, IA: William C. Brown Publishers. p. 314. Food science Microbiology terms