Lesson 4. Thermal treatment & Microorganisms associatedwith canned foods

 Some terminologies

• Thermal Death Time (TDT) is the

time required to kill a given number

of microorganisms at a specific

temperature.

• The D value is the decimal

reduction time or the time required

to kill 1 log (90%) of microorganisms

in a medium. When D value is

determined at 121ºC, it is often

expressed as Dr

 Some terminologies

• The D-value gives the time needed at a certain temperature to kill an

organism while the Z-value relates the resistance of an organism to

differing temperatures.

• Z-value is a term used in microbial thermal death time calculations. It is

the number of degrees the temperature has to be increased to achieve a

tenfold (i.e. 1 log10) reduction in the D-value.

• The F value for a process is the number of minutes at 121ºC required to kill

a known population of microorganisms (spores or vegetative cells) in a

given food under specified conditions.

 Some terminologies

• This F value is usually set at 12 D values to give a theoretical 12 log

cycle reduction of the most heat-resistant species of mesophilic

spores in a can of food.

• The integrated lethal value of heat received by all points in a

container during processing is designated Fs or F0

• F0 = Dr (log a − log b),

• where a is the number of cells in the initial population and b is the

number of cells in the final population.

 Canned foods

Definition:

Foods that have been preserved by heat in hermetically sealed

containers; are typically packaged in cans, glass or flexible

packages; are processed to attain commercial sterility.

 Canned foods

Commercial sterility is defined as the condition achieved by

1) The application of heat with/without the control of water activity

which renders the food free of:

a) microorganisms capable of reproducing in the food under normal non-

refrigerated conditions of storage and distribution

b) viable microorganisms (including spores) of public health significance

 Microbial considerations of canned

foods

A) Microbiological considerations of canned foods

• They are thermal processed products

• Are hermetically sealed – no entry of microbes

• They may contain viable or pathogenic microorganisms which cannot

grow

• Their thermal processes depend on many factors including,

• The normal pH of product,

• Consistency

• Size of can

• Type of processing equipment used

• Mostly no preservatives are added

• Acids may be added to ensure stability.

 Description of canned foods based

on pH

• Based on pH, commercially processed foods are described as

• Low-acid foods – pH >4.6 and water activity > 0.85

• Acid (acidified) foods - ≤4.6

• The acidity, of foods determines how they must be processed for canning.

• C. botulinum spores can survive boiling water and germinate into vegetative cells

in sealed food cans during storage to produce the deadly botulinum toxin. They

must therefore be killed at sterilization temperature under pressure to make low

acid canned foods safe.

• That is why low acid canned foods are processed to maintain commercial sterility

 Spoilage of canned foods

Indications of microbiological spoilage of canned foods

• Abnormal odour

• Appearance and/or pH of the product

• Container may be swollen

Causes of spoilage

• Insufficient processing

• Inadequate cooling after processing or storage and distribution at elevated

temperatures

• Post-process contamination

 Spoilage of canned foods

• Other conditions that may cause slight to severe swelling and or

abnormal product appearance

• Incipient spoilage (occurs before product is thermally processed)

• Hydrogen swells

• Non-enzymatic browning

• Product formulation error and/or mishandling

• Enzymatic changes

 Microbiological groups associated

with canned foods

A) Low-acid foods

– Bacterial spores, particularly of Clostridium and Bacillus spp.

– Extremely heat resistant spore-forming thermophiles – causes

• Flat sour spoilage caused by B. stearothermophilus

• Thermophilic anaerobic spoilage, caused by obligately

thermophilic, sporeforming anaerobes, e.g. Clostridium

thermosaccharolyticum.

• Product usually has a cheesy odour

• Sulfide stinker spoilage - Caused by sporeforming anaerobic,

obligatory thermophilic Desulfotomaculum nigrificans, which

produces hydrogen sulphide.

 Microbial spoilage of low acid

canned foods

Spoilage of low acid canned foods cont’d

– LAB, yeast and moulds (through container leakages)

B) Acid or acidified foods

• Butyric acid anaerobes, e.g. Clostridium pasteurianum

• Aciduric flat sour Bacillus coagulans, Alicyclobacillus spp. (in fruit

juices)

• Heat resistant moulds, e.g. Byssochlamys fulva, Neosartorya

fisheri

• Yeast and asporogenous bacteria.