Many different time and temperature combinations can

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Unformatted text preview: an be used for pasteurization. The LTLT (low-temperature/long-time) process involves bringing milk to a temperature of 63°C for 30 minutes followed by quick cooling to 4°C. In contrast, the HTST (hightemperature/short-time, also called flash pasteurization) method brings the milk to a temperature of 72°C for only 15 seconds. Pasteurization can not be relied upon to inactivate heat stable exotoxins and it does not kill endospores. FOOD-294 Sterilization temperatures Obviously the objective of sterilization is to kill vegetative microbes of all types and kill bacterial endospores. To accomplish this, the temperature raised to >100oC using elevated steam pressure. It is used for foods that can withstand such a treatment without "significant" loss of palatability and nutritional value (eg. canned and bottled foods). This being said, canning does cause some loss of food value, particularly that of labile biochemicals such as vitamins. Historically, the standard conditions for steam sterilization are 121°C at 15 psi for 20 minutes, a set of conditions rarely experienced by microbes living at sea level; even endospores quickly die under these conditions. This combination of pressure and temperature is the principle behind sterilization using the steam autoclave and the home pressure cooker. Moist heat is a much more effective killer than dry heat, thanks to the ability of water to penetrate cells. Many bacteria, for instance, easily withstand 100°C dry heat but not 100°C in boiling water. We humans are not so different, finding it easier to endure temperature of 32°C in dry climates than in humid ones. Life in a pressure cooker Recall (Chapter 6) that an archaeon Geogemma barossii (formerly known just as “Strain 121”) was discovered in 2003 living near a hydrothermal vent in Juan de Fuca Ridge just off the West Coast of North America. This prokaryote has shattered the commonly held belief that autoclave conditions kills all life. In fact, this organism doubles every 24 h at 121°C and 15 psi. Further a temperature of 130oC has only bacteriostatic effect on this organism meaning that although growth is halted, the archaeon remains viable and will reproduce if it is transferred to cooler temperatures. For Geogemma barossii, a cool temperature is 100oC! Number of viable cells or endospores Time (min) Figure 9-6: A microbial death curve FOOD-295 How microbes die Exposure of microbes to lethal chemical or physical agents does not instantly kill all of them. Microbes die according to a negative exponential curve, where cell numbers are reduced in equal fractions at constant intervals (Figure 9-7). The exponential equation describing the death of a cell population is Nt = No 10 -kt/2.303 where t = exposure time N t = the number of surviving cells at time t No = the initial number of viable cells present k = death rate constant for the population (units time-1) Decimal reduction time The efficacy of a given killing condition is measured as decimal reduction time (D-value), which is he length of time it takes that agent (or condition) to kill 90% of the population (a drop of 1 log unit, or a drop to 10% of the original value). In other words, D = t when...
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This note was uploaded on 10/25/2013 for the course MICB 201 taught by Professor Davidturner during the Fall '12 term at The University of British Columbia.

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