Heat Sterilisation 1

8/16/2019 Heat Sterilisation 1

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Heat Sterilisation

•Microbial Heat Resistance & Survivor Curves;

•

Thermal Death times & Spoilage probability;•Process Calculations

•Processing Systems

Dr. Ian Thompson


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Preservation Processes Definition: Processing steps required to reduce or eliminate the

potential for food borne illnesses and spoilage.

Pasteurisation (traditional):

◦ Increase in product temperature to inactivate specific pathogenic

micro-organisms;

◦

Is product Shelf stable?◦ Typically a few weeks with refrigeration (< 5 oC);

Commercial Sterilisation:

◦ More intense thermal process to reduce population of all

microorganisms in product;

◦ Is product shelf stable?

◦ Absolutely, for 12 months or more at room temperature

(does not require refrigeration).


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Microbial Survival Curves

The main issue relating to food preservation is themicrobial population;

An external agent (heat) is used to reduce the population

of microbes present;

Vegetative cells (eg. E. coli, Salmonella, Listeria monocytogenes) willdecrease;

Microbial spores will decrease in a similar manner – after a

lag period;


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Effect of Heat on Micro-organisms

Preservative Effect due to denaturation of Proteins:

◦ Destroys enzyme activity & enzyme controlled metabolism in

microbes;

Rate of destruction is a first order reaction

◦ When food is heated to a given temperature, the same % die in agiven time interval – regardless of the initial numbers present.

◦ This gives rise to a logarithmic order of death survival curve

(death curve);

dN/dt = - k.N n , n = 1 for first order;

N = microbial population, t = time, k = first order rate constant;


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Microbial Survivor Curves


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Decimal reduction time (D):◦ Time needed to destroy 90% of the microorganisms – i.e. to

reduce the load by a factor of 10;

◦ Higher D values means higher thermal resistance

◦ Slope of death rate curve (log N vs. time) for 1 log reduction;

Consider:

At t = 0, initial population = No,

After time t, population = Nt.Then: log No – log Nt α t (log No / log Nt) = c. tBy definition,

When (log No / log Nt) = 1, then t = tD & c = 1/D

(log No / log Nt) = t / D [Eqn. 1]

(N/No) = 10 – t/D


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Problem 1

The following data were obtained from a thermalresistance experiment conducted on a spore suspension

at 112 oC:

Time No. of survivors

0 10 6

4 1.1 x 10 5

8 1.2 x 10 4

12 1.2 x 10 3

Determine the D value for the organism.

(Ans: 4.1 minutes)


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First Order Kinetics &

Decimal Reduction Time First Order Kinetics:

dN/dt = - k.N n , n = 1 for first order;

(N/No) = e – kt

From Decimal Reduction time:

(N/No) = 10 – t/D

On condition of a first order survivor curve:

10 – t/D = e – kt 2.303/D = k [Eqn. 2]First order rate constant (k) is inversely proportional to

decimal reduction time (D).


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Commercial ApplicationApproaches to establish thermal process

Time/temperature combination dependent of m/b load.

◦ Higher numbers of microbes will require longer time variation in

microbial load of raw material will require recalculating process for

each batch;

OR

Specific time/temperature combination used for every

batch – irrespective of microbial load

◦ Adequate preparation procedures are used to ensure that rawmaterial has acceptable and uniform quality (specifications);


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Microbial Destruction occurs logarithmically

Is it possible to destroy all microorganisms?◦ By heating for an infinite time – since population will only approach

zero;

From survivor curve equation:

N = N0

x 10-(t/D) N0 only if t [Eqn. 3]

An infinite time will be required for the destruction of all viable microorganisms.

OR

Reduce microbial load by predetermined amount or factor

Basis for concept of commercial sterility.

The probability of survival of a single micro-organism after heattreatment can be predicted – based on microbial loads, heatresistance, temperature and time of heating.

Commercial Application


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◦ The thermal resistance

constant (z) describes theinfluence of temperature (T)

on the decimal reduction

time (D);

z = change in T (T2 – T1) [Eqn. 4]

log D1 – logD2

Hence, the D value (time)and z value (temperature) is

used to characterize the heat

resistance of microbes;

Thermal Death Time Curve


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Problem 2

The decimal reduction times D for a spore suspensionwere measured at several temperatures, as follows:

Temp (oC) D (min)

104 27.5

107 14.5

110 7.5

113 4.0

116 2.2

Determine the thermal resistance constant z for the spores.

(Ans. = 11 oC )


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Heat Processing data


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Factors affecting the Heat

Resistance of Microbes

Type of Microbes◦ Different species/strains show wide variation in heat resistance;

◦ Spores are more heat resistant than vegetative cells’

Incubation conditions (during cell growth or spore formation)

◦ Temperature – spores produced at higher temperature are moreheat resistant;

◦ Age – of vegetative cells;

◦ Medium of growth – (fatty acids influence heat resistance);

Conditions during heat treatment◦ pH of food (pathogenic/spoilage bacteria more heat resistant at

neutral pH


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Conditions during heat treatment◦ pH of food (pathogenic/spoilage bacteria more heat resistant at

neutral pH: yeast and fungi less heat resistant than bacteria)

◦ Water activity – aw ◦ Moist heat - more effective than dry heat;

◦ Composition of food (protein/fat/sucrose cause an increase);

Growth media and incubation conditions

◦ Used to assess recovery (survivors) of microbes in heat

resistance studies;

Factors affecting the Heat

Resistance of Microbes


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Microbes vs Enzymes

Most enzymes have D & z values within a similar rangeto microbes;

Some enzymes are very heat resistant and may not be

denatured by relatively short heat treatments

The heat resistance of enzymes and microbes found inspecific foods is used to calculate the heating conditions

needed;

In practice, the most heat resistant species

(microbe/enzyme) is used a basis for calculating theprocess time/temperature conditions.


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Thermal Death Time (F)

Total time required to accomplish a stated reduction in apopulation of vegetative cells or spores;

Expressed as a multiple of D-values (on a 1st order survivor

curve model);

Therefore, 4 D = 10 -4 reduction = 99.99% reduction

For commercially sterile, low acid shelf stable foods,

thermal death time (F) = 12D.

Reference thermal death times = Fo (sub-zero)

- where process

temperature and z values are specified;

Eg. For C. botulinum, temperature = 121.1 oC & z value =10oC;

- i.e. – F T or F12110z


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Spoilage Probability The spoilage probability is used to estimate the number of spoiled

containers within a total batch of processed product.

RECALL: log No - log N = t / D;

If No & N represents the initial & desired final microbial

population respectively - for a thermal death time F, then

log No - log N = F / D.

◦

If the total number of containers being processed = r, then thetotal microbial load at the beginning of the process =

r x No

and log (r. No) - log (r. N) = F / D

If the goal is to achieve a probability of one “survivor” from all

containers, then log (r. N) 0 and

log (r. No) = F / D r. No = 10F/D

the spoilage probability, 1/r = No / 10 F/D


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1/r = No / 10F/D [Eqn. 5]

1/r - represents the total number of containers processed

(r) resulting in 1 with spoilage;

The expression can be used to estimate F, given No & D.

Prevailing assumption:The survivor curve for the spoilage microorganism is 1st order.

Spoilage Probability


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Problem 3

Estimate the spoilage probability of a 50 minute process at113 oC – when D = 4 minutes, and the initial microbial

population is 10 4 per container.

Solution:

1/r = No / 10F/D

= 10 4 / 10 50/4 = 10 4 / 10 12.5

= 10 -8.5 = 3.16 x 10 -9.

r = 3.16 x 10 8 - i.e. the spoilage of 1 container in 3.16 x 10 8

containers (r) can be expected.


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General Method for

Process Calculations Based on classical paper by Bigelow (1920);

Major Assumption:

◦ The thermal death time, F, for the microbial population considered must

be known at all temperatures to which the product is exposed during the

preservation process.Recall: Thermal Death Time, D, decreases with increasing temperature

Method:

1. Draw sterility curve for process: Sterilization rate (F/t) vs. time2. Area under the curve (in time units) is the lethal effect of

the process (lethality) – i.e. – the integrated impact of

time and temperature on the microbial population.


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Recall: relationship with Thermal Death time (F)

log No - log N = F / D.

& Thermal resistance constant, z

◦ z = (T2 – T1)

log D1 – logD2

Derived to give:

log FR – log F = (T – TR) FR / F = 10(T- TR)/z [Eqn. 6]

z- where FR is the thermal death time known at a reference temp, TR

This eqn. can be used to compute the thermal death time, F – at any

temperature, T .

General Method for

Process Calculations


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Application to Pasteurisation

During batch pasteurisation, the food is heated to adefined temperature and held for a defined period – to

achieve a certain lethality.

Only the holding period is considered (not the heating

or cooling phases); Pasteurisation of milk is based on the reduction of a

microbial pathogen with:

◦ D 63 = 2.5 min, z = 4.1oC, & a thermal death time, F = 12 D.

◦Process ensures survival of pathogen is negligible.

◦ The traditional batch process is holding at 63oC for 30 min.

◦ Since the reference TR is 63oC, the lethality is 1.


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The continuous HTST process attains lethality during aholding period at a temperature close to the heating

medium;

The extent to which heating and cooling will contribute to

lethality depends on the rate of heating and cooling; For HTST, the milk product is heated to 71.5 oC and it will

only need to be held for 15 sec. to achieve the same

lethality (=1) as the batch process at TR.

If it were to be held for 30 minutes, the lethality would be120 (but the milk would be no good ).

Application to Pasteurisation


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Problem 4

A thermal process is accomplished by instantaneousheating to 138 oC – followed by a 4 sec hold and

instantaneous cooling.

Estimate the lethality (in sec) at 121 oC if the thermal

resistance of the microorganism is 8.5 oC. Hints: use eqn - FR / F = 10

(T - TR)/z

TR = 138oC & FR = 4 s

F121 = F138 x 10(138 – 121)/ 8.5

= 4 x 10 2 = 400 s


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Reference Material

Prescribed Text: Fellows, Peter J., (2009). Food Processing Technology:

Principles and Practice, 3rd edition. Woodhead: Cambridge,England. ISBN 1-4398- 0821X

Highly Recommended Reading:

Singh, R Paul & Heldman, Dennis R., (2009). Introduction toFood Engineering. 4th edition. Academic Press: Amsterdam.ISBN 0-1237-370900-4.

Online Resources

Earle, R.L. and Earle, M.D., Unit Operations in Food Processing(Web edition), New Zealand Institute of Food Science andTechnology. http://www.nzifst.org.nz/unitoperations/index.htm;
http://www.nzifst.org.nz/unitoperations/index.htmhttp://www.nzifst.org.nz/unitoperations/index.htm

Heat Sterilisation 1

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