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Research Paper 44 33 C To F

Temperature classification
Note: The difference between fever and hyperthermia is the underlying mechanism. Different sources have different cut-offs for fever, hyperthermia and hyperpyrexia.

Normal human body temperature, also known as normothermia or euthermia, is the typical temperature range found in humans. The normal human body temperature range is typically stated as 36.5–37.5 °C (97.7–99.5 °F).[8]

Individual body temperature depends upon the age, exertion, infection, sex, and reproductive status of the subject, the time of day, the place in the body at which the measurement is made, and the subject's state of consciousness (waking, sleeping or sedated), activity level, and emotional state. It is typically maintained within this range by thermoregulation.

Human body temperature is of interest in medical practice, human reproduction, and athletics.

Methods of measurement[edit]

Taking a person's temperature is an initial part of a full clinical examination. There are various types of medical thermometers, as well as sites used for measurement, including:

  • In the rectum (rectal temperature)
  • In the mouth (oral temperature)
  • Under the arm (axillary temperature)
  • In the ear (tympanic temperature)
  • in the nose
  • In the vagina (vaginal temperature)
  • In the bladder
  • On the skin of the forehead over the temporal artery


Temperature control (thermoregulation) is part of a homeostatic mechanism that keeps the organism at optimum operating temperature, as the temperature affects the rate of chemical reactions. In humans, the average internal temperature is 37.0 °C (98.6 °F), though it varies among individuals. However, no person always has exactly the same temperature at every moment of the day. Temperatures cycle regularly up and down through the day, as controlled by the person's circadian rhythm. The lowest temperature occurs about two hours before the person normally wakes up. Additionally, temperatures change according to activities and external factors.[9][unreliable medical source?]

In addition to varying throughout the day, normal body temperature may also differ as much as 0.5 °C (0.9 °F) from one day to the next, so that the highest or lowest temperatures on one day will not always exactly match the highest or lowest temperatures on the next day.

Normal human body temperature varies slightly from person to person and by the time of day. Consequently, each type of measurement has a range of normal temperatures. The range for normal human body temperatures, taken orally, is 7002309950000000000♠36.8±0.5 °C (7002309927777777777♠98.2±0.9 °F).[10] This means that any oral temperature between 36.3 and 37.3 °C (97.3 and 99.1 °F) is likely to be normal.

The normal human body temperature is often stated as 36.5–37.5 °C (97.7–99.5 °F).[8] In adults a review of the literature has found a wider range of 33.2–38.2 °C (91.8–100.8 °F) for normal temperatures, depending on the gender and location measured.[11]

Reported values vary depending on how it is measured: oral (under the tongue): 7002309950000000000♠36.8±0.4 °C (7002309927777777777♠98.2±0.72 °F),[12] internal (rectal, vaginal): 37.0 °C (98.6 °F).[12] A rectal or vaginal measurement taken directly inside the body cavity is typically slightly higher than oral measurement, and oral measurement is somewhat higher than skin measurement. Other places, such as under the arm or in the ear, produce different typical temperatures.[12] While some people think of these averages as representing normal or ideal measurements, a wide range of temperatures has been found in healthy people.[5] The body temperature of a healthy person varies during the day by about 0.5 °C (0.9 °F) with lower temperatures in the morning and higher temperatures in the late afternoon and evening, as the body's needs and activities change.[12] Other circumstances also affect the body's temperature. The core body temperature of an individual tends to have the lowest value in the second half of the sleep cycle; the lowest point, called the nadir, is one of the primary markers for circadian rhythms. The body temperature also changes when a person is hungry, sleepy, sick, or cold.

Natural rhythms[edit]

Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest in the late afternoon, between 4:00 and 6:00 p.m. (assuming the person sleeps at night and stays awake during the day).[12][10] Therefore, an oral temperature of 37.3 °C (99.1 °F) would, strictly speaking, be a normal, healthy temperature in the afternoon but not in the early morning.[12] An individual's body temperature typically changes by about 0.5 °C (0.9 °F) between its highest and lowest points each day.[12][10]

Body temperature is sensitive to many hormones, so women have a temperature rhythm that varies with the menstrual cycle, called a circamensal rhythm.[9] A woman's basal body temperature rises sharply after ovulation, as estrogen production decreases and progesterone increases. Fertility awareness programs use this change to identify when a woman has ovulated in order to achieve or avoid pregnancy. During the luteal phase of the menstrual cycle, both the lowest and the average temperatures are slightly higher than during other parts of the cycle. However, the amount that the temperature rises during each day is slightly lower than typical, so the highest temperature of the day is not very much higher than usual.[13][unreliable medical source?]Hormonal contraceptives both suppress the circamensal rhythm and raise the typical body temperature by about 0.6 °C (1.1 °F).[9]

Temperature also varies with the change of seasons during each year. This pattern is called a circannual rhythm.[13] Studies of seasonal variations have produced inconsistent results. People living in different climates may have different seasonal patterns.

Increased physical fitness increases the amount of daily variation in temperature.[13]

With increased age, both average body temperature and the amount of daily variability in the body temperature tend to decrease.[13] Elderly patients may have a decreased ability to generate body heat during a fever, so even a somewhat elevated temperature can indicate a serious underlying cause in geriatrics.

Measurement methods[edit]

Oral33.2–38.1 °C (91.8–100.6 °F)35.7–37.7 °C (96.3–99.9 °F)
Rectal36.8–37.1 °C (98.2–98.8 °F)36.7–37.5 °C (98.1–99.5 °F)
Tympanic35.7–37.5 °C (96.3–99.5 °F)35.5–37.5 °C (95.9–99.5 °F)

Different methods used for measuring temperature produce different results. The temperature reading depends on which part of the body is being measured. The typical daytime temperatures among healthy adults are as follows:

  • Temperature in the anus (rectum/rectal), vagina, or in the ear (otic) is about 37.5 °C (99.5 °F)[15][medical citation needed]
  • Temperature in the mouth (oral) is about 36.8 °C (98.2 °F)[10]
  • Temperature under the arm (axillary) is about 36.5 °C (97.7 °F)[15][medical citation needed]

Generally, oral, rectal, gut, and core body temperatures, although slightly different, are well-correlated, with oral temperature being the lowest of the four. Oral temperatures are generally about 0.4 °C (0.7 °F) lower than rectal temperatures.[12]

Oral temperatures are influenced by drinking, chewing, smoking, and breathing with the mouth open. Mouth breathing, cold drinks or food reduce oral temperatures; hot drinks, hot food, chewing, and smoking raise oral temperatures.[9]

Each measurement method also has different normal ranges depending on sex.[citation needed]


Many outside factors affect the measured temperature as well. "Normal" values are generally given for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature, 22.7 to 24.4 °C (73 to 76 °F), during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes, as the temperature of the food, drink, or smoke can dramatically affect the reading.

Temperature is increased after eating or drinking anything with calories. Caloric restriction, as for a weight-loss diet, decreases overall body temperature.[9] Drinking alcohol decreases the amount of daily change, slightly lowering daytime temperatures and noticeably raising nighttime temperatures.[9]

Exercise raises body temperatures. In adults, a noticeable increase usually requires strenuous exercise or exercise sustained over a significant time. Children develop higher temperatures with milder activities, like playing.

Psychological factors also influence body temperature: a very excited person often has an elevated temperature.

Wearing more clothing slows daily temperature changes and raises body temperature.[9] Similarly, sleeping with an electric blanket raises the body temperature at night.[9]

Sleep disturbances also affect temperatures. Normally, body temperature drops significantly at a person's normal bedtime and throughout the night. Short-term sleep deprivation produces a higher temperature at night than normal, but long-term sleep deprivation appears to reduce temperatures.[9]Insomnia and poor sleep quality are associated with smaller and later drops in body temperature.[9] Similarly, waking up unusually early, sleeping in, jet lag and changes to shift work schedules may affect body temperature.[9]



Main article: Fever

A temperature setpoint is the level at which the body attempts to maintain its temperature. When the setpoint is raised, the result is a fever. Most fevers are caused by infectious disease and can be lowered, if desired, with antipyretic medications.

An early morning temperature higher than 37.2 °C (99.0 °F) or a late afternoon temperature higher than 37.7 °C (99.9 °F) is normally considered a fever, assuming that the temperature is elevated due to a change in the hypothalamus's setpoint.[12] Lower thresholds are sometimes appropriate for elderly people.[12] The normal daily temperature variation is typically 0.5 °C (0.90 °F), but can be greater among people recovering from a fever.[12]

An organism at optimum temperature is considered afebrile or apyrexic, meaning "without fever". If temperature is raised, but the setpoint is not raised, then the result is hyperthermia.


Main article: Hyperthermia

Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate. It is usually caused by prolonged exposure to high temperatures. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to deal effectively with the heat, causing the body temperature to climb uncontrollably. Hyperthermia at or above about 40 °C (104 °F) is a life-threatening medical emergency that requires immediate treatment. Common symptoms include headache, confusion, and fatigue. If sweating has resulted in dehydration, then the affected person may have dry, red skin.

In a medical setting, mild hyperthermia is commonly called heat exhaustion or heat prostration; severe hyperthermia is called heat stroke. Heat stroke may come on suddenly, but it usually follows the untreated milder stages. Treatment involves cooling and rehydrating the body; fever-reducing drugs are useless for this condition. This may be done through moving out of direct sunlight to a cooler and shaded environment, drinking water, removing clothing that might keep heat close to the body, or sitting in front of a fan. Bathing in tepid or cool water, or even just washing the face and other exposed areas of the skin, can be helpful.

With fever, the body's core temperature rises to a higher temperature through the action of the part of the brain that controls the body temperature; with hyperthermia, the body temperature is raised without the influence of the heat control centers.


Main article: Hypothermia

In hypothermia, body temperature drops below that required for normal metabolism and bodily functions. In humans, this is usually due to excessive exposure to cold air or water, but it can be deliberately induced as a medical treatment. Symptoms usually appear when the body's core temperature drops by 1–2 °C (1.8–3.6 °F) below normal temperature.

Basal body temperature[edit]

Main article: Basal body temperature

Basal body temperature is the lowest temperature attained by the body during rest (usually during sleep). It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature. In women, temperature differs at various points in the menstrual cycle, and this can be used in the long-term to track ovulation both for the purpose of aiding conception or avoiding pregnancy. This process is called fertility awareness.

Core temperature[edit]

Core temperature, also called core body temperature, is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Significant core temperature elevation (hyperthermia) or depression (hypothermia) that is prolonged for more than a brief period of time is incompatible with human life.

Temperature examination in the rectum is the traditional gold standard measurement used to estimate core temperature (oral temperature is affected by hot or cold drinks and mouth-breathing). Rectal temperature is expected to be approximately one Fahrenheit degree higher than an oral temperature taken on the same person at the same time. Ear thermometers measure eardrum temperature using infrared sensors. The blood supply to the tympanic membrane is shared with the brain. However, this method of measuring body temperature is not as accurate as rectal measurement and has a low sensitivity for fevers, missing three or four out of every ten fevers in children.[16] Ear temperature measurement may be acceptable for observing trends in body temperature but is less useful in consistently identifying fevers.

Until recently, direct measurement of core body temperature required surgical insertion of a probe, so a variety of indirect methods have commonly been used. The rectal or vaginal temperature is generally considered to give the most accurate assessment of core body temperature, particularly in hypothermia. In the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement.[17]

Temperature variation[edit]


  • 44 °C (111.2 °F) or more – Almost certainly death will occur; however, people have been known to survive up to 46.5 °C (115.7 °F).[18][unreliable medical source?][19]
  • 43 °C (109.4 °F) – Normally death, or there may be serious brain damage, continuous convulsions and shock. Cardio-respiratory collapse will likely occur.
  • 42 °C (107.6 °F) – Subject may turn pale or remain flushed and red. They may become comatose, be in severe delirium, vomiting, and convulsions can occur. Blood pressure may be high or low and heart rate will be very fast.
  • 41 °C (105.8 °F) – (Medical emergency) – Fainting, vomiting, severe headache, dizziness, confusion, hallucinations, delirium and drowsiness can occur. There may also be palpitations and breathlessness.
  • 40 °C (104.0 °F) – Fainting, dehydration, weakness, vomiting, headache, breathlessness and dizziness may occur as well as profuse sweating. Starts to be life-threatening.
  • 39 °C (102.2 °F) – Severe sweating, flushed and red. Fast heart rate and breathlessness. There may be exhaustion accompanying this. Children and people with epilepsy may be very likely to get convulsions at this point.
  • 38 °C (100.4 °F) – (Classed as hyperthermia if not caused by a fever) – Feeling hot, sweating, feeling thirsty, feeling very uncomfortable, slightly hungry. If this is caused by fever, there may also be chills.


  • 36.5–37.5 °C (97.7–99.5 °F) is a typically reported range for normal body temperature[8]


  • 36 °C (97 °F) – Feeling cold, mild to moderate shivering (body temperature may drop this low during sleep). May be a normal body temperature.
  • 35 °C (95 °F) – (Hypothermia is less than 35 °C (95 °F)) – Intense shivering, numbness and bluish/grayness of the skin. There is the possibility of heart irritability.
  • 34 °C (93 °F) – Severe shivering, loss of movement of fingers, blueness and confusion. Some behavioural changes may take place.
  • 33 °C (91 °F) – Moderate to severe confusion, sleepiness, depressed reflexes, progressive loss of shivering, slow heart beat, shallow breathing. Shivering may stop. Subject may be unresponsive to certain stimuli.
  • 32 °C (90 °F) – (Medical emergency) – Hallucinations, delirium, complete confusion, extreme sleepiness that is progressively becoming comatose. Shivering is absent (subject may even think they are hot). Reflex may be absent or very slight.
  • 31 °C (88 °F) – Comatose, very rarely conscious. No or slight reflexes. Very shallow breathing and slow heart rate. Possibility of serious heart rhythm problems.
  • 28 °C (82 °F) – Severe heart rhythm disturbances are likely and breathing may stop at any time. Patient may appear to be dead.
  • 24–26 °C (75–79 °F) or less – Death usually occurs due to irregular heart beat or respiratory arrest; however, some patients have been known to survive with body temperatures as low as 14.2 °C (57.6 °F).[20]

Historical understanding[edit]

In the 19th century, most books quoted "blood heat" as 98 °F, until a study published the mean (but not the variance) of a large sample as 36.88 °C (98.38 °F).[21] Subsequently that mean was widely quoted as "37 °C or 98.4 °F"[22][23] until editors realised 37 °C is closer to 98.6 °F than 98.4 °F. Dictionaries and other sources that quoted these averages did add the word "about" to show that there is some variance, but generally did not state how wide the variance is.


  1. ^Marx, John (2006). Rosen's emergency medicine: concepts and clinical practice. Mosby/Elsevier. p. 2239. ISBN 978-0-323-02845-5. 
  2. ^Karakitsos D, Karabinis A (September 2008). "Hypothermia therapy after traumatic brain injury in children". N. Engl. J. Med. 359 (11): 1179–80. doi:10.1056/NEJMc081418. PMID 18788094. 
  3. ^Pryor, Jennifer A.; Prasad, Ammani S. (2008). Physiotherapy for Respiratory and Cardiac Problems: Adults and Paediatrics. Elsevier Health Sciences. p. 8. ISBN 0702039748. 
  4. ^ abAxelrod YK, Diringer MN (May 2008). "Temperature management in acute neurologic disorders". Neurol. Clin. 26 (2): 585–603, xi. doi:10.1016/j.ncl.2008.02.005. PMID 18514828. 
  5. ^ abcLaupland KB (July 2009). "Fever in the critically ill medical patient". Crit. Care Med. 37 (7 Suppl): S273–8. doi:10.1097/CCM.0b013e3181aa6117. PMID 19535958. 
  6. ^Grunau BE, Wiens MO, Brubacher JR (September 2010). "Dantrolene in the treatment of MDMA-related hyperpyrexia: a systematic review". CJEM. 12 (5): 435–442. doi:10.1017/s1481803500012598. PMID 20880437.  
  7. ^Sharma HS, ed. (2007). Neurobiology of Hyperthermia (1st ed.). Elsevier. pp. 175–177, 485. ISBN 9780080549996. Retrieved 19 November 2016.  
  8. ^ abcKarakitsos D, Karabinis A (September 2008). "Hypothermia therapy after traumatic brain injury in children". N. Engl. J. Med. 359 (11): 1179–80. doi:10.1056/NEJMc081418. PMID 18788094. 
  9. ^ abcdefghijkKelly GS (March 2007). "Body temperature variability (Part 2): masking influences of body temperature variability and a review of body temperature variability in disease"(PDF). Altern Med Rev. 12 (1): 49–62. PMID 17397267. [unreliable medical source?]
  10. ^ abcdMackowiak, P. A.; S. S. Wasserman; M. M. Levine (1992-09-23). "A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich". JAMA. 268 (12): 1578–1580. doi:10.1001/jama.1992.03490120092034. PMID 1302471. 
  11. ^Sund-Levander M, Forsberg C, Wahren LK (June 2002). "Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review". Scand J Caring Sci. 16 (2): 122–8. doi:10.1046/j.1471-6712.2002.00069.x. PMID 12000664. 
  12. ^ abcdefghijkLongo, Dan L., ed. (2011). Harrison's principles of internal medicine (18th ed.). New York: McGraw-Hill. p. 142. ISBN 978-0-07-174889-6. 
  13. ^ abcdKelly G (December 2006). "Body temperature variability (Part 1): a review of the history of body temperature and its variability due to site selection, biological rhythms, fitness, and aging"(PDF). Altern Med Rev. 11 (4): 278–93. PMID 17176167. [unreliable medical source?]
  14. ^Sund-Levander M, Forsberg C, Wahren LK (2002). "Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review". Scand J Caring Sci. 16 (2): 122–8. doi:10.1046/j.1471-6712.2002.00069.x. PMID 12000664. 
  15. ^ abWong, Lena (2005). "Temperature of a Healthy Human (Body Temperature)". The Physics Factbook. Retrieved 2007-08-22. 
  16. ^Dodd SR, Lancaster GA, Craig JV, Smyth RL, Williamson PR (April 2006). "In a systematic review, infrared ear thermometry for fever diagnosis in children finds poor sensitivity". J Clin Epidemiol. 59 (4): 354–7. doi:10.1016/j.jclinepi.2005.10.004. PMID 16549256. 
  17. ^McKenzie, J.E.; Osgood, D.W. (2004). "Validation of a new telemetric core temperature monitor". Journal of Thermal Biology. 29 (7–8): 605. doi:10.1016/j.jtherbio.2004.08.020. 
  18. ^Chudler, Eric H. "Biological Rhythms". Neuroscience for Kids. faculty.washington.edu.
  19. ^"Man's temperature registered more than 115.7". Bulletin Journal. August 7, 1980.
  20. ^Excerpt: Humans, Body Extremes. Guinness World Records. 2004. 
  21. ^Inwit Publishing, Inc. and Inwit, LLC – Writings, Links and Software Demonstrations – A Fahrenheit–Celsius Activity. inwit.com.
  22. ^Oxford Dictionary of English, 2010 edition, entry on "blood heat"
  23. ^Collins English Dictionary, 1979 edition, entry on "blood heat"
Diurnal variation in body temperature, ranging from about 7002310650000000000♠37.5 °C from 10 a.m. to 6 p.m., and falling to about 7002309549999999999♠36.4 °C from 2 a.m. to 6 a.m.

The physical structure of roasted coffee beans is a complex composite of materials, containing high molecular weight fibrous molecules interspersed with amorphous and partially crystalline domains of a vast array of smaller organics. The extremely complex structure of both the roasted beans and grinding apparatus makes accurate first principles modeling a daunting prospect, and so fracturing is best studied experimentally (in line with previous studies of grinding other amorphous materials)28,29,30,31,32. That said, it could well be expected that the specific mix of chemicals that give different coffees their distinctive flavour may change the way in which the bean is fragmentised.

To investigate this, we elected to sample four speciality grade coffees. The selection spans the variables of origin, variety, processing method and roast profile, and is a representative cross section of contemporary speciality coffee. The four coffees described in Table 1 were ground at ambient conditions using the stipulated methods.

Here we are concerned with the deviations in grind profile as a function of coffee origin, although before embarking on these experiments it was unclear what the grind profile looked like. The EK 43 produces particles ranging from 0.1 μm to 1000 μm, and whilst we have elected to present most of the data on a logarithmic scale, the linear scale is shown for the Tanzanian coffee in the upper panel of Fig. 3. All grind profiles appear as a skewed-Gaussian shape. In this case, we present the particle number distribution in the shaded blue region, and the integral in grey. We can arbitrarily define the fine particulate cutoff, graphically represented as a purple dashed line = n where:

Here, n is a diameter in μm. From the upper panel of Fig. 3, the Tanzanian n = 70 μm (mode = 13.0 μm, where the mode is the most frequent size occurrence). Given the skewed nature of the distribution, the mode is helpful in assigning key features of the distribution. However, it is not only the number of particles that contributes to the extraction of coffee, but also the available surface area obtained from these particles.

The grind profiles for the four coffees examined here are shown in the middle and lower panels of Fig. 3. They are presented on a logarithmic scale to accommodate the surface area contribution from the large particles. The surface area is estimated using a spherical approximation for the particles33, and is shown by the dotted line. Here, the data appears distinctly bimodal because the fine particulates contribute to the majority of the accessible surface area (modes ii and v), but large particulates (one/two orders of magnitude larger in diameter, iii and vi) are also present. These have an influence even at low concentrations.

There are minor differences in the grind profiles: The profiles shown in black and purple share similar particle number modes (i), and have a fine particulate cutoff of 76.4 ± 3.5 μm. The profiles shown in red and blue produced a slightly finer particle distribution with a number mode (iv) 1.3 ± 0.7 μm) more fine than the black/purple coffees, and a fine particulates cutoff of 69.6 ± 3.1 μm. In summary, the coffees appear to produce a very similar grind distribution irrespective of the variables associated with bean production. Full ANOVA details are presented in Table S1. It should be noted that all of the beans considered here are roasted relatively ‘light’ compared to typical consumer grade coffee (although on the ‘Agtron Gourmet Scale’, these coffees all are catagorised as light-medium roast). We can only speculate how heavily decomposed beans (e.g. ‘dark’ or French roast) may deviate from these results; further experimentation is required to elucidate that effect.

For espresso, the coffee grinds can be thought of as a granular material, where the increase in pressure during tamping jams the materials34,35,36. The variability in particle size plays a significant role in the accessible surface area, but also in the vacuous space in which the water may flow through. From the work of Herman37, it is apparent that large particles install significant order of neighbouring small particles, which increases local density and therefore can result in inhomogeneous water flow through the espresso puck. However, given the subjectivity of coffee flavour and the preferences of practitioners working in the industry, it is not clear if there is an ideal particle size distribution: We only hope to shed light on the surprising consistencies between coffees.

Do Differences in the Roasted Bean Grind Temperature Affect the Final Grind? Temperature changes in amorphous materials can lead to well defined glass transitions, where the material changes from rubbery and flexible to being hard and brittle38. Some solids can also undergo shattering transitions, where there is an increased fragmentation rate as particle size decreases, resulting in production of greater numbers of fine particles39. This property is instigated by both temperature and crack velocity. It is understood that crystalline materials progress towards this shatter transition point with decreased temperature, because the strain on the lattice becomes proportionally larger with decreased lattice kinetics. However, roasted coffee is a complex material and glass or shattering transition points are unlikely to be constant across macroscopic regions of the bean, if present at all. Therefore, while it is reasonable to expect that a change in temperature will affect the grinding result, describing how and why this occurred is problematic. Experiment provides the simplest and most reliable route to assessing how temperature influences ground coffee particle size.

The lower the original bean temperature, the colder the produced particles will be at every stage of grinding. However colder bean fragments will absorb heat from their surroundings more quickly due to the larger temperature gradient, effectively reducing the indicated temperature difference between the samples. Therefore, the observed change in grind profile should be considered a lower limit on the effects of grinding at reduced temperatures. Given the inhomogeneous nature of the beans, it is likely that cooling the burrs (and hence further reducing the temperature of the particles as they are ground) would smoothly continue the trend observed in Fig. 4.

Some fraction of particles are produced in their final size from the initial fracturing of the whole bean (or large portion thereof), and so are truly produced at the stated temperature. However, experiments using a single impact event (i.e. hitting a cold bean with a mallet), show that only a small amount of small particles are produced on initial bean fracturing, so most particles do have some time for thermalisation before further fracturing occurs.

Even with some particle thermalisation due to room temperature burrs, the initial bean temperature has a significant effect on the modal particle size distribution (Fig. 4a) reducing the mode by 31% as the beans are cooled from room temperature to −200 °C, as shown in Fig. 4b. Additionally, the distribution generally becomes narrower as the beans are cooled (Fig. 4c) with the biggest change occurring between room temperature and −19 °C beans. The room temperature grind profile is also distinctly less Gaussian-like, with the development of a hip at approximately 9.5 μm. This detail could indicate that some components of the bean undergo a shattering transition between 20 °C and −19 °C, and studies are ongoing into the origin of this feature.

To probe the reversibility of this transition, we performed the same room temperature experiments with coffee beans that had been cooled to liquid nitrogen temperatures and then allowed to reheat to room temperature. It appears that if there is a transition, it is reversible as there were no notable differences between the two samples. This is not surprising given the very low water concentration in roasted coffee: The thermal contraction and re-expansion of coffee did not play a significant role in the grind profile obtained from either test set.

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