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NF2016N_Food_Preservation_1ab

Presentation on heat treatment for food preservation
by

Richard Marshall

on 30 March 2012

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Transcript of NF2016N_Food_Preservation_1ab

Food Preservation 1ab Principles, technology of heat treatments
Basic principles - inactivation of microorgansisms
Rating of heat treatments
D values, z values, F values
Effectiveness of heat treatments
Thermal Death Time
Lethality Heat treatments
Application of heat to inactivate food spoilage mechansisms
Batch
Continuous

Time/temperature combinations What is the minium temperature required? 65 degC Above this temperature, enzymes inactivated, NAs break down, cells burst, starch gelatinised
Not all MOs are destroyed
Spores are heat resistant
Some MOs can withstand heat (thermophiles) Basic principles Inactivation of microorganisms How can we rate heat treatments? Normally use 'D' values
D = decimal reduction time

Time taken at a particular temperature to reduce numbers of a specific MO by one log cycle, or time to reduce numbers by 90%
eg 1,000,000 to 100,000 Example of D value Start with 100,000,000 cells (1 x 10 ) in 1g of food
Heat food to (eg) 90 deg C
Takes 5 minutes to reduce number of cells to 1 x 10 , ie a decimal reduction or one log cycle
How many minutes to reduce number to 1?
5 x 8 = 40 minutes
Each MO has unique D values 8 7 z values For each temperature, there is different D value
Higher temperature = lower D
log D value against Temperature gives curve with slope = z value
Shows how raising temperature increases killing rate So far.... We know how easily we can destroy a MO at a given temperature (D value)
Know effect of raising temperature on D value (z value)
Now need to know how effective a process is Effectiveness of a heating process Value called F value
F value is time in minutes at a specific temperature required to destroy a specific number of cells of a given z value

Can add up F values for each part of heat process F = F + F + F total heating holding cooling Making F usable Usually relate F to z and to a reference temperature, T

For C botulinum, z = 10, T = 121.1 C

Gives F , designated F ref


ref o 10

121.1 0 How to use F Process time is measured in minutes
= 'unit of process' at 121.1 C

If process has F value of 4, means that it has same effect as 4 minutes at 121.1 C

BUT only if process temperature is achieved INSTANTLY and cooling is INSTANT too

Allows different processes to be compared o o Which z value? In canning, Clostridium botulinum is most important, z = 10 C

In milk pasteurization, Coxelliae burnetii is reference organism, z = 4 C, 12 log cycle reduction o o How to work out the effectiveness of a heating process Measure time/temperature profile
Plot graph
Calculate Thermal Death Time (TDT) TDT calculation example For a process at 115 C, where C botulinum is most heat resistant organism, z value = 10 C

TDT = 10 = 10 = 3.98 minutes o o (121 - 115)/10 0.6 Try this one.....

Process at 98 C, z = 10 C, what is the TDT? Answer

TDT = 10 = 10 = 199.5 minutes (121 - 98)/10 2.3 Lethal rate Lethal rate, L, is ratio of D at particular temperature T to D at T

L is reciprocal of TDT

From time/temperature data, use tables of L against T to find L ref http://weblearn.londonmet.ac.uk/webct/urw/lc4130011.tp0/cobaltMainFrame.dowebct Use Lethal Rate to determine time/temp Calculate are under curve
Count squares, use integrator, use spreadsheet

Compare total heating given with that required Types of products Low acid - pH 5 and above
meat products, marine products, some soups, most vegetables
Medium acid - pH 4.6-4.9
meat/vegetable mixtures, pasta, soup, pears
Acid - pH 3.7-4.5
tomatoes, pears, figs, pineapple, other fruits
High acid - pH 3.6 or less
pickles, grapefruit, citrus juices Practical processing requirements Low acid - Botulinum cook, 121.1 C, 3 minutes

Acid/high acid <pH4.5, 100 C, 15 minutes o o Heat transfer Depends on nature of product
Density, free liquid etc

Convection
When liquid present that can circulate
Conduction
When solid food present, no circulation Can use spreadsheet for rapid calculations
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