FNH 200 Study Guide - Final Guide: Thermal Death Time, Clostridium Botulinum, Aseptic Processing
13 Dec 2016
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Lesson 6: Thermal Preservation of Foods
- The safety and storage life of many perishable foods can be enhanced by the use of high
temperatures to inactive undesirable disease and spoilage-causing microorganisms and to inactivate
enzymes in food that can cause spoilage
Three categories of thermal preservation of foods are:
- Blanching
- Pasteurization
- Commercial Sterilization
Blanching: a form of thermal processing applied mainly to vegetables/ some fruits by exposing them to
heated or boiling water or even culinary steam for a short period of time
Blanching is a food processing operation designed to:
1. Inactivate enzymes in plant tissues so that enzymatic degradation does not occur in the interval
between packaging and thermal processing or during frozen storage or in the early stages of food
dehydration and after constitution of dehydrated plant foods
2. Wilt vegetable products to enable packing of the products into containers so that proper fill weights
can be achieved
3. Drive off intern and intracellular oxygen and other gases from plant tissues so that containers are
not deformed by excessively high internal pressures due to expanding gases within the container
and to permit formation of a vacuum in the container after thermal processing
Pasteurization: a thermal process that involves using temperatures of at least 72-degrees Celsius for 15
seconds, prior to packaging (high temperature short time or HTST process)
- Basis of preservation by pasteurization is to inactive pathogenic (disease-ausig ateia’s ad
viruses in low acid food products (E.g. milk)
- Acid food products mainly pasteurized to inactivate spoilage-causing microorganisms
- Pathogenic microorganisms cannot grow and do not survive very well in acid foods (E.g. citrus/apple
juice)
- In low acid and acid foods, many spoilage-causing microorganisms can still survive typical
pasteurization process conditions
- Pasteurization does not kill all the psychotrophic spoilage-causing bacteria in milk → refrigeration
Commercial Sterilization (CS): thermal process involves heating the food with a minimum treatment of
121-degree Celsius moist heat for 15 minutes
➔ Canning: process involves pre-sealing the food in containers prior to heating
➔ UHT-Aseptic packaging: involve heating the food before it is aseptically packaged
- Basis of preservation by CS is to destroy both spoilage and disease causing microorganisms in low-
aid ad aid foods, edeig the food oeiall steile
- Commercially sterile: condition obtained in a food that has been processed by the application of
heat, alone or in combination with other treatments, to render the food free from viable forms of
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microorganisms, including spores, capable of growing in the food at temperatures at which the food
is designed normally to be held during distribution and storage
- If a can of food is being sterilized, each food particle must receive the heat treatment → different
depending on the size of the can, the time to achieve sterility could be several hours
- Most commercially sterile products have a shelf life of 2 years or more
Ultra-high temperature processing (UHT) and Aseptic packaging: basis of UHT and aseptic packaging is
the appliatio of ulta high tepeatue heat) to food before packaging, then filling the food into
pre-sterilized containers in a sterile atmosphere → renders food shelf stable (no refrigeration needed)
- UHT- Aseptic packaging relatively new development → food can be heated to 140-150 degrees
Celsius very rapidly by injection of steam, held at that temperature for a short period (4-6 seconds)
and then cooled → the higher temperature and the minimal come-up time and cool-down time
leads to a higher quality product
- UHT processed food is aseptically packages into pre-sterilized containers → usually made of
laminated plastic, aluminum and paper, which are chemically sterilized with a combination of
hydrogen peroxide and heat, and then filled in the same piece of equipment which is housed in a
sterile environment
- Tetra Pak
- UHT – aseptically packaged products have a shelf life of 6 months or more, without refrigeration
- Example of food products processed with UHT: milk, juices, cream, yogurt, wine, baby foods, tomato
products, soups
- UHT processed milk and juices do not contain added agents to provide the long storage life at
ambient temperature in the laminated cartons
- Not all products are necessarily aseptically packaged → gives them advantage of a longer shelf life
at refrigeration temperatures compared to conventional pasteurized products, but does not
produce a shelf-stable product at ambient temperatures due to the possibility to post-processing
recontamination
How are heat treatments selected? Factors are:
1. What is the objective or purpose?
2. Are there additional preservation steps? (is it combined with other preservation methods?)
3. What are the physical, chemical properties of the food?
4. What is the heat resistance of microorganisms in the food?
- Imperative that thermal preservation processes be designed so that the slowest heating portion of
the food commodity receives the specified time-temperature thermal treatment to minimize risks of
illness and/or post-processing spoilage
Clostridium botulinum: habitat can be soil, water, and mud → all foods of agricultural and fisheries
origin must be considered as being potentially contaminated with Clostridium botulinum spores
- Low acid foods which are packaged and stored under anaerobic condition, require an specifically
designed thermal processing treatment to ensure the destruction of any clostridium botulinum
spores → provide large margin of safety
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Thermal Death Curves: describes the rate of death of a particular microorganism under a specified set
of conditions
- Bacteria is not killed instantaneously
- Microbial death during the thermal processing follows a logarithmic order
- They are killed by heat at a rate that is nearly proportional to the number present in the system
being heated
- The survivor cure/thermal death rate curve depicts the logarithmic order of death
- The time taken to traverse one logarithmic cycle represents the time, at a constant temperature,
required to kill 90% of a microbial population
- The exposure of microorganisms to a specific temperature for a period of time required to kill 90%
of the population is defined as the decimal reduction time or D-value
What effects do different conditions have on the D-value?
- If the temperature were increased, the D-value would decrease because the rate of microbial death
would increase
- A thermal death time curve can be constructed from a number of thermal death rate curves by
exposing the microorganism to a variety of temperatures and determining the decimal reduction
time at each temperature
Z-value: the number of degrees required for a specific thermal death time curve to pass through one log
cycle → indicates the resistance of a microbial population to changing temperature
➔ Different microorganisms will have different z-values
➔ A given microorganism will have different z-values in different foods
F-value: a mathematically calculated number that describes the total lethal effect of the process at the
slowest heating point in a food container
- The standard reference temperature is generally selected as 121.1-degrees Celsius
- The relative time (in minutes) required to sterilize any selected organism at 121-degrees Celsius is
known as the F-value of that organism
- Measures the lethality or the capacity of the heat treatment to sterilize
Margin of Safety: refers to the probability that a container of food could still contain a viable spore of
clostridium botulinum after the completion of the thermal processing
- The goal is to ensure that the margin of safety is as large as possible which means that the
probability of survival of spore of clostridium botulinum after thermal process is as low as possible
without causing undue heat damage to the quality factors and nutrient value of the food
- As the spore population in a food system is increased, the total time required at a particular
temperature to kill ALL the spores increases
- For low acid foods → a margin of safety of 12D is applied (subjected to a thermal process so that the
slowest heating portion of the food is exposed to an amount of thermal energy such that the
microbial spores present in the good will experience the equivalent of 12 successive decimal
reduction times)
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Document Summary
The safety and storage life of many perishable foods can be enhanced by the use of high temperatures to inactive undesirable disease and spoilage-causing microorganisms and to inactivate enzymes in food that can cause spoilage. Three categories of thermal preservation of foods are: Blanching: a form of thermal processing applied mainly to vegetables/ some fruits by exposing them to heated or boiling water or even culinary steam for a short period of time. Blanching is a food processing operation designed to: Pasteurization: a thermal process that involves using temperatures of at least 72-degrees celsius for 15 seconds, prior to packaging (high temperature short time or htst process) Basis of preservation by pasteurization is to inactive pathogenic (disease-(cid:272)ausi(cid:374)g(cid:895) (cid:271)a(cid:272)te(cid:396)ia"s a(cid:374)d viruses in low acid food products (e. g. milk) Acid food products mainly pasteurized to inactivate spoilage-causing microorganisms. Pathogenic microorganisms cannot grow and do not survive very well in acid foods (e. g. citrus/apple juice)
