KINETICS AND ENGINEERING OF

STERILIZATIONKINETICS: To study the calculation and velocity of reaction by which we can find out at which phase reaction takes place.

ENGINEERING: To study the design of media,batch,fed batch and continuous sterlization process.

KINETICS OF MEDIA STERILIZATION

Corbett(1985),described that media may be sterilized by filtration,radiation,ultrasonic treatment,chemical tretment and heat.However,for practical reason steam is used almost universally for the sterilization of fermentation media.

Thus destruction of micro-organisms by steam (moist heat) may be described as first order chemical reaction and thus may be represented by following equation:

-dN/dt =kN………………………….(1)where N is the number of viable organism present, t is the time of sterilization treatment, k is the reaction rate constant of the reaction or specific death rate.On integration of this equation following expression is obtaind:

Nt/N₀ = ₑ-kt…………………(2)Where N₀ is the number of viable organism

present at the start of sterilization treatment ,Nt is number of viable organism present after a treatment period ,t.

On taking natural logrithms equation (2) becomes In(Nt/N₀) = - kt ,so from this it may be

observed that number of viable organism declines exponentially over the treatment period.

THE DESIGN OF BATCH STERILIZATION PROCESSThe highest temperature for the batch

sterilizaton is 121 C so the procedure is ͦdesigned such that exposure of the medium to this temperature is kept to a minimum and avoid the destruction of nutrient.

This is achieved by taking into account the contribution made to sterilization by heating and cooling period of the batch treatment(Deindoerfer and Humphery (1959)).

Cont……………..The following information must be available

to design the batch sterilization process……1. A profile of decrease and increase in

temperature of the fermentation media during heating and cooling period of sterilization cycle.

2. The number of microorganism present originally in the medium.

3. The thermal death characteristics of the design organism(Bacillus stearothermophilus)

Conti…………………….Thus by knowing the original number of

microorganism present required Del factor may be calculated…

V̅= = In(₁₀11 /₁₀3) =In ₁₀14 =32.2 …where ₁₀11 and ₁₀-3 are number of viable

organism in unsterile broth and risk of contamination is 1 in 1000 respectively.

However destruction of microorganism during heating,cooling at 121 the overall Del ͦfactor may be calculated as

V̅=overall=V̅=heating+ V̅=holding+V̅=cooling.

D-Time: (decimal reduction time ) It is the time required to kill the 90% of microorganism or spores in a sample at specified temperature 121 9.A semilogrithmic plot of population remains versus time may be plotted to calculate the surviver remain.

Z- Value : is based on time-temperature relationship for survival of test

microorganism ,based on D-value response at various temperature. Thus increase in temperature needed to reduced the D-time to 10% of original value.

F-Value: is the time in minute at a specific temperature (250 f or 121 ) ͦ ͦ

needed to kill a population of cells or spores .Many food industries makes extensive use of D and Z value after a food has been canned to eliminate botulism arising from clostridium botulinum spore.

CALCULATION OF DEL FACTOR DURING HEATING AND COOLINGThe relationship between Del factor, and the

temperature and time is given by the equation. V̅= =A . t . ₑ-(E/RT)………….(1) However,during heating and cooling

temperature is not constant therefor calculation of V̅= require integration of equation (1) for the time –temperature regime observed.

Thus the value of specific death rate of B.stearothermophilus spores at each mid point temperature may be deduced from Arrhenius equation using thermal death characteristics published by Deinoderfer and Humphery(1959).

Conti……….The value of the Del factor corresponding to

each time increment may be then calculated from the eqution :

v= = k₁ t, v= = k₂ t, v= = k₃ t, etc.Thus value of Del factors for all the

increment will then equal the Del factor for the heating up period and cooling-down period may be calculated in similer fashion.

Calculation of holding time at constant temperature :

Holding time may be calculated by the difference :

V̅= holding = V̅= overall – V̅= heating - V̅= cooling

For example V̅= holding =32.2 – 9.8 -10.1 =12.2.But v==kt, the specific death rate of

B.stearothermophilus at 121 is 2.54 /min ͦ therefore, t = v=/k= 12.3/2.54 =4.84 min

Rechard? s rapid method for the design of sterilization cycle :Richards(1968) proposed a rapid method for

the design of sterilization cycle avoiding the time consuming graphical integration .

Thus according to this method all spore destruction occurs at the temperature above 100 C and those part of heating and cooling ͦabove 100 C are linear. ͦ

For example , if fermentation broth is heated from 100 to 12 C in 30 minutes and cooled ͦ 1ͦfrom 121 to 100 in 17 minutes , ͦ ͦ

Conti…….V̅ ̅ heating = (12.549 ₓ30)/21 =17.93V̅G cooling = (12.549ₓ 17)/21 =10.16

Thus by this method Del for heating and cooling periods the holding time at the constant temperature may be calculated.

The design of continuous sterilization processes :This may also approached in exactly the same

way as for batch sterilization system but major difference and advantage is that it include a time period and much higher temperature may be utilized, thus reducing the holding time and reducing the degree of nutrient degradation.

Thus required Del factor may be calculated by combination of temperature and holding time which gives an acceptably small degree of nutrient decay.

Conti…Therefore, Del factor equation obtained is :

In t = E /RT + In Q/A where Q = Nutrient quality A = Arrhenius constant

Conti….There are two type of continuous sterilizer

used to treatment of fermentation media :1 . Indirect heat exchanger :They are double spiral type which consist of

two sheets of high grade stainless steel which have been curved around a central axis to form a double spiral .

Ends of spiral are sealed by cover. To achieve sterilization temperature steam is passed through one spiral and medium through other in countercurrent stream .

Spiral heat exchanger are also used to cool the medium after passing through the holding coil .

Conti……Major advantage of spiral heat exchanger are

: The two streams of medium and steam are

separated by continuous stainless steel barrier with gasket seal being confined to the joints with the end plate so no chances of cross contamination .

Spiral route allows self cleaning ,sufficient clearances , reduces the rate of sedimentation , fouling and burning on .

Conti….2 . Direct heat exchanger ( steam injector ) : In this system steam is directly injects into the

unsterile broth then medium passed through holding coil ,expansion valve (cooling) , and then expansion chamber from here steam out to vaccum and sterile medium collected on the other side .

Advantage :V̅ery short heating up timesUsed for media containing suspended solidsLow capital costEasy cleaning and maintenanceHigh steam utilization efficiency

Conti …..Disadvantage:Foaming may occur during heatingRequire clean steam free from anticorrosion

additive to direct contact with medium ..

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