Dear tripathi,
In order to get a better understanding of thermal processing you should read this chapter (starting from page 73). PRINCIPLES OF THERMAL PROCESSES
There it states that:
"In order to determine the extent of the heat treatment, several factors must be known:
1) Type and heat resistance of the target microorganism, spore or enzyme present in the food
2) pH of the food
3) Heating conditions
4) Thermophysical properties of the food and container shape and size
5) Storage conditions following the process"
After time and temperature data for a given product in a given can size have been obtained by heat penetration studies, these data may be analysed by either of 2 methods:
a) The General of Graphical Method of Bigelow et al.b) The Formula Method of BallThe
General Method is used when it is desired to measure the exact sterilizing value of a process, when such conditions as come-up time (which is the time required in order for the retort temperature to reach the highest value), cooling water temperature, or the holding time after processing but before water cooling are different from normal retorting procedures. This method is also adapted to conditions when the heat penetration curve cannot be represented by one or two straight lines within the lethal temperature range on semi-logarithmic paper. Is it not readly adapted to the calculation of processes when the retort temperature and/or initial temperature are different from those under which the heating data were obtained. Time and temperature data during the cooling cycle as well as the heating cycle must be recorded in order to use the Graphical Method.
The
Formula Method is used when the heat penetration curve can be represented by not more than two straight lines on semi-logarithmic paper. The formula permits evaluationg processes for retort and initial temperature conditions differing from those under which the heating data were obtained. In the case of heating curbes represented by one line only on semi-log paper, the heat penetration factors can be converted to different can sizes.
The
Nomogram Method is merely a graphical solution of the previously used equations. No additional reference to charts, log tables or slide rules is needed; only a straight edge (preferably transparent) is required. Different nomograms are required for the two types of problems represented by a single straight line heating curve and a broken two-slope heating curve.
You can find the above methods in the links that I have provided you.
Here is also another good start which includes also the basics like steady and unsteady state heat transfer:
THERMAL PROCESSING OF FOODS STANDARDSIt is first necessary to set up a standard sterilizing value. The greatest interest in processing canned foods is with low acid products. With such foods, 250 F (121,1 C), has been generally established as the reference temperature and the lethal heat expressed in terms of minutes at 250 F.
F, D and z values
(The above link concerns pharmaceuticals but the terms used are the same in the food industry. You can find out more if you look in the links that I have provided you).
So according to the food item (and its pH) you have, you have to find by microbial analysis and by literature the extact D and z values of the most thermostable pathogenic microorganism (and/or enzyme that is undesirable) in it.
Foods can be devided into classes according to their pH:
- Low pH foods (pH >~ 4,5): meat, fish, milk, poultry, eggs, vegetables etc
- Acidic foods (pH = 4,0 - 4,49): tomato, pineapple, pear
- High acidic foods (pH < 4,0): fruitsSPOILAGE (AND SOME PATHOGENIC) MICROORGANISMS:I. LOW pH FOODS
A. Thermophilic microorganisms1. B. stearothermophilus. Causes flat sour due to the production of lactic and formic acid which are volatile acids so the product gains a distinctive smell but the can doesnt get deformed cause no gases are produced. D121,1 = 4-5 min.
2. C. thermosaccharolyticum. Causes anaerobic spoilage due to the production of butyric, lactic and formic acid, carbon dioxide and hydrogen so the product gains a distinctive smell and the can gets deformed. D121,1 = 3-4 min.
3. C. nigrificans. Causes sulphuric spoilage. The hydrogen sulphide produced affects the sulphur containing proteins like cysteine and cystine so the product gains a distinctive smell. D121,1 = 2-3 min.
B. Mesophilic microorganisms
1. C. botulinum. Causes rotting. There is also production of carbon dioxide, hydrogen sulphide, ammonia etc so the product gains a distinctive smell and the can gets deformed. D121,1 = 0,1-0,2 min.
2. C. sporogenes. The same as above. D121,1 = 0,8-1,5 min.
II. ACIDIC FOODS
Mesophilic mircoorganisms1. B. coagulans. Causes flat sour due to the production of lactic and acetic acid. D100 = 1,29 - 9,04 min. D121,1 = 0,01 - 0,07 min.
2. C. butyricum. Causes butyric fermentation. Butyric and acetic acid, carbon dioxide and hydrogen are produced. D100 = 0,1-0,5 min.
3. C. pasteurianum. The same. D100 = 0,1-0,5 min.
III. HIGH ACIDITY FOODSMesophilic microorganisms
1. Byssochlamys and Paecilomyces. They cause softening of the fruits due to the production of pectinolytic enzymes. D90 = 1-2 min.
2. Lactobacillus and Leuconostoc. They cause lactic fermentation. Lactic and acetic acid are produced among with carbon dioxide. D65 = 0,5 - 1 min.
3. Escherichia coli, Aerobacter aerogenes, Mycobacterium tuberculosis, Salmonella, Brucella, Streptococcus, Staphylococcus. They cause food poisining and diseases. D65 ~< 0,6 min.
In products of low acidity (pH>4,5) it is very probable that
C. botulinum is present. It is the most thermoresistant, mesophilic, sporogenic, anaerobic, pathogenic bacterium. The tightly sealed cans offer an ideal enviroment for C. botulinum to grow and produce toxins. The persentage of death of individuals who consumed foods which contained those toxins is over 65%. Because C. botulinum is very common in nature and especially in soil/dirt, all food products can be concerned as infected with its spores. For this reason every product with a pH > 4,5 has to be sterilized in such a way that C. botulinum spores are destroyed unless special measures are taken which can inhibit their development.
For the ensuring of the safety of the consumers, in products of low acidity, and with C. botulinum as a reference, a thermal process equal with:
Fo = m D = 12 x 0,1 min to 12 x 0,3 min = 1,2 to 3,6 min ...should be applied because for C. botulinum m = 12 and D at 121,1 C is 0,1 to 0,3 min. Where m is the reduction exponent and is equal with:
m = log Ni / N, where Ni = initial count of microorganism and N = final count after the processing.
So for the calculation of the required time of the thermal process (F), the above type can be used. But in the specific case where z = 10 C and the reference temperature is 121,1 C, F should be symbolized as
Fo. Fo expresses the reduction of the microbial population of reference (with a z = 10 C) which the heating process at 121,1 C for 1 min induces. Heating at 121,1 C for 1 min means instant increasing of the product's temperature at 121,1 C, sustaining the product at this temperature for 1 min and then instantly reducing the temperature. Please do read more about it in the links I have provided you.
THERMAL STERILIZATION OF SOME PRODUCTS: FOOD / Fo (time required for the total destuction of microorganisms in minutes)
Beans / 7-15 min
Spinach / 4-5 min
Tomato / 0,7-1 min
Corn / 9-15 min
Mushroom / 6-10 min
Meat / 6-8 min
Fish / 2,5-8 min
Chicken / 6-8 min
Condensed milk / 5-6 min
FACTORS AFFECTING THE THERMAL RESISTANCE OF MICROORGANISMS:A. Factors relevant with the microorganism
1. Physical resistance. Generally bacteria are more resistant than moulds & yeasts. Also the bacterial spores are very resistant.
2. Initial concentration/microbial load. It is obvious that the smaller the initial micro load the smaller the thermal resistance.
3. The stage of development. Microorganisms are very heat sensitive at the
log phase of their development because in that phase they still haven't developed a resistant membrane.
B. Factors relevant with the natural enviroment of development
1. Temperature. Microorganisms have the maximal resistance when they are in their optimal temperature of development
2. Chemical composition. Microorganisms have the maximal resistance when they are in a enviroment with optimal chemical composition.
C. Factors relevant with the thermal destruction enviroment
1. Temperature. The highest the temperature the easiest the microorganism dies.
2. Chemical composition.
- pH. The highest the pH the more difficult to kill the microorganism
- Sugar concentration. The highest it is the more difficult to kill the microorganism because sugars create a protective "covering" around it.
- Fat concetration. The same with sugar.
- Water. It reduces the thermal resistance of the microorganism.
METHODS TO MEASURE THE THERMAL RESISTANCE OF MICROORGANISM:1. Tube method
- In pyrex tubes with a diameter of 7 mm and length of 130 mm we add 1-4 ml of product in each one. The concentration of the reference microorganism must be known.
- Closure of the tube next to fire.
- Heating in a specific temperature and different times.
- Cooling.
- Inoculation in appropriate substrate.2. Can method
- In cans (15 ml capacity) we add 10-15 g of product. The concentration of the reference microorganism must be known.
- Closure of the cans with mechanical vacuum (so the microorganism will survive).
- Heating in a specific temperature and different times.
- Cooling.
- Inoculation in appropriate substrate.The first method requires small quantities of product so that heat can penetrate it fast. The second method, even though is not instant/fast, it simulates "real conditions".
FACTORS AFFECTING THE VELOCITY OF THEMAL PENETRATION IN THE FOOD ITEM:
1. The texture. Velocity in a liquid product > Velocity in a solid product.
2. Packaging material. Velocity in metal > Velocity in glass.
3. Sugars. High concentration decrease the velocity.
4. Hydrocolloids (water soluble proteins, carrageennans, starch, pectins, agar etc). They reduce the velocity.
5. Size of the product. Big size reduces the velocity.
6. Movement of the product. Increases the velocity.
7. Temperature difference between retort and product. The highest it is the highest the velocity.
For a better insight and understanding of the thermal processing please do read the links I have provided you in the last post.
If you are going to use pouches check out this link:
Sterilization of Food in Retort PouchesBest Regards and lots of luck!
P.S. I hate thermal processing.