Thermal Death Time Module- 16 Lec- 16 Dr. Shishir Sinha Dept. of Chemical Engineering IIT Roorkee

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1 Thermal Death Time Module- 16 Lec- 16 Dr. Shishir Sinha Dept. of Chemical Engineering IIT Roorkee

2 Thermal death time 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. Survivorr curve When the microbial populations as a function of time are presented on semilogarithmic coordinates, a linear decrease in microbial population with time is observed. It is the survivor curve. It is emphasized that these relationships occur when the environment around the population is at a constantt temperaturee throughoutut the period of exposure. Survivorr Curves at Different Temp.

3 The image part with relationship ID rid6 was not found in the file. Definition of Thermal Death Time (F) This is by definition the time required for the complete kill of all organisms in a given suspension. The F value is simply the thermal death time, the time required to kill all organisms in a population, measured at 121 C. Thermal Death Time Curve o Definition of Decimal Reduction Time On survivor curve, the time required for a one log cycle reduction in microbial population is the decimal reduction time (D). Thermal death time Thermal death time can be determined one of two ways: 1) By using graphs or 2) By using mathematical formulas. Graphical method

4 This is usually expressed in minutes at the temperature of 250 F or 121 C. This is designated as F. Each 18 F or 10 C change results in a time change by a factor of This would be shown either as F English) = 10 minutes (SI) or F = 10 minutes (American A lethal ratio (L) is also a sterilizing effect at 1 minute at other temperatures with (T). L=10 ( T- T where T is the reference temperature, usually 250 F or 121 C; z is the z-value, and T Ref is the slowest heat point of the product temperature. Ref ) Formula method Prior to the advent of computers, this was plotted on semi logarithmic paper though it can also be done on spreadsheet 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 (f ). It also measures the product temperature rather than the h can temperature. j=ji/i 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: BB log g log ji f h

5 Where g is the number of degrees below the retort temperature on a simple heating curve at the end of the heating period, B is the time in minutes from the beginning of the process to B the end of the heating period, and f is the time in minutes required for the straight-line h portion of the heating curve plotted semi logarithmically 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. Thermal Resistance Curve A plot of decimal reduction time as a function of temperature on semilog coordinates results in a linear relationship. This linear relationship is the thermal resistance curve for a given microbial population. Thermal Resistance Curve of microbial population

6 Applications Of Thermal Death Time In the food industry, it is important to reduce the amount of microbes in products to ensure proper food safety. 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 log (D R ) point is 250 F or 121 C. 10 at a given temperature. This D-value reference z is used to determine the time values with different D-valuess at different temperatures with its equation shown below: T Z log D 2 1 T1 log D 2 where T is temperature in F or C.

7 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 10 bacteria per gram or milliliter. The D for C. botulinum is 0.21 minute (12.6 seconds). A 12-D reduction will R take 2.52 minutes (151 seconds). 12 Thermal Death-point Test It is sometimes necessary to determine the death-point of a bacterium by exposure to moist heat. This is most accurately performed by the aid of Stern berg's glass bulbs. A twenty-four-hour-old broth culture of the given bacterium, prepared beforehand, is to be poured out into a sterile Petri dish, then having taken a bulb and sterilised the point and broken it off with sterile forceps, The bulbous end is to be rapidly passed through the flame of a Bunsen burner four or five times to expel some of the air, and the sterile point of the shank is to be dipped into the fluid in the dish, and as the bulb cools the fluid runs slowly up the shank and falls into the bulb below. It is well not to fill the bulb more than one-quarter, thermal death-point test. As a great bulk of fluid is to be avoided, interfering as it does with the delicacy of the test. Removing the bulb from the fluid, its point is carefully sealed in the flame and it is then deposited in a small galvanised sheet-iron box perforated with many small holes, or into a stout, finely meshed wire box; both bulb and box are then to be placed in a water bath with enough water in it to submerge the box to the depth of at least one inch, and kept for the required time at a constant temperature. In testing vegetative forms of bacteria, it is recommended to begin with an exposure of five minutes at 50 C., then ten minutes at 50 C., and so on, for every five succeeding degrees up to 65. Spores are tested in boiling water with exposures varying from one minute up to twenty, or more. After conditions of time and temperature have been fulfilled, the bulb is removed, the shank wiped dry, the point broken off by forceps under sterile precautions, and the shank

8 grasped by the forceps near the bulb, which is now held uppermost so as to permit of the ready dish charge of the contents. This step is accomplished by introducing the shank of the bulb into a tube of previously melted agar, whose temperature is 42 C., and, bringing the upper empty end of the bulb near to the lowermost portion of Bunsen flame, expansion of the air at once drives the contents into the agar, When they are to be welll mixed and poured into a sterile Petri dish, and incubated for 72 hours, and examined for evidences of growth. Caution must be observed in expelling the contents of the bulb, lest the flame come into direct contact and vitiate the experiment.

9 Thermal death time curve Purple dots indicate the values plotted in above fig.

10 References &ved=0cdsqfjac&url=http%3a%2f%2ffshn.ifas.ufl.edu%2fseafood%2fsst% 2FAnnPdf%2F8th_109.pdf&ei=PB3zUvLnEYjLkwWf1IHwDw&usg=AFQjCNGw Yv_ksM1TLYIvNsnweMYkpdcz8Q&sig2=JBff0Md7QH7x34obClzhmw &ved=0cewqfjae&url=http%3a%2f%2fwww.gmaonline.org%2fuploadfiles %2F19203B000001D9.filename.Carrie_Ferstl_Technology_April_4.pdf&ei=PB3zUv LnEYjLkwWf1IHwDw&usg=AFQjCNGRFPc9qKp8g3JtaWBOVL1hD9ADmg&si g2=bj4ploch5ymieixipjwecq &ved=0cgeqfjag&url=http%3a%2f%2fseward.co.uk%2fwpcontent%2fuploads%2fsites%2f3%2f2013%2f03%2ffood-microbiology-heatresistance-salmonella-chicken-broth-stomacher- 400.pdf&ei=PB3zUvLnEYjLkwWf1IHwDw&usg=AFQjCNHtO5ItZBR_1ZeMelBT skd_pozjoa&sig2=1upb9r3vffflftmo55luxq

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