Microbiological Aspects of Food Preservation and Safety ... · Commercial sterilization is a heat treatment which uses moist heat and temperatures usually above 100°C. The main objective

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WHITEPAPER

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Published by Safefood 360, Inc. Part of Our Professional Whitepapers Series

Microbiological Aspects of Food Preservation

and Safety Methods

This whitepaper addresses the

microbiological aspects of food

preservation. The most commonly used

preservation methods found in the food

industry are explained and are clearly

defined, including mode of action,

preservation factors and unit processes to

achieve safety.

This is an important area of food safety

management given that many CCP’s are

covered by these methods. This paper is

intended for use by those directly

responsible for food safety systems and

programs within a food processing plant

and should underpin existing experience

and knowledge.

CONTENTS

1 Introduction

2 Heat Treatment & Preservation

2.1 Time / temperature preservation

2.2 Method of heat treatment and preservation

2.3 Pasteurization

2.4 Commercial sterilization

2.5 Calculation of heat processing time

2.6 D value—the decimal reduction time

3 Food Irradiation

3.1 Ultraviolet light (UV)

3.2 Microwaves

3.3 Ionizing radiation

3.4 Food irradiation

4 High Pressure Food Treatment

5 Preservation by Use of Low Temperature

6 Antimicrobial Effects of Food Dehydration

7 Chemical Preservation

Basic Microbiology for Quality Managers 2

Safefood 360º Whitepaper (September, 2013)

Microbiological hazards are one of the most significant causes of food poisoning. An understand-

ing of these hazards is crucial to understanding how suitable controls may be applied. Modern

food safety has its roots in food preservation methods. Initially these methods were applied to ex-

tend the shelf life of foods, and over time an understanding emerged that many of these methods

had the effect of making food safer for human consumption. Today these methods of preservation

and control are used widely in the global food sector as part of HACCP plans to consistently pro-

duce food for a mass consumption with high quality and safety.

In this whitepaper we will classify the main factors of food preservation and safety, and drill down

into the specific requirements for achieving safe food products. We will look closely at unit process

operations such as heat, irradiation, high pressure, low temperature, freezing, dehydration, modi-

fied packaging and chemicals. A variety of preservation and safety factors and modes of action are

used in modern food production. These are summarized in the following table. Their use and ap-

plication depends on a number of factors including the food product, hazards, legislation, consum-

er and customer demands.

Inactivation of microbial cells includes methods which kill significant numbers if not all microor-

ganisms within the food product and is usually irreversible. Examples of such processes include

heat preservation, radiation and high pressure processing. Inhibition of microbial cells does not

usually affect a lethal kill of all microorganisms but rather inhibits the growth of these microbes.

Restriction refers to the numerous pre-requisites of safe food production that are designed to

maintain microbial hazards at a safe level within the production environment and to prevent their

entry.



Heat has been used widely in food processing to preserve foods and render them safe for con-

sumption. However, the heat resistance of microbial cells can vary widely and an understanding of

the microbial hazards commonly present in the food is essential. Generally speaking, a time and

temperature profile of 60°C for 10-15 minutes is sufficient to kill yeasts and moulds. Bacterial veg-

etative cells are usually more resistant but are unlikely to survive temperatures greater than 90°C.

Bacterial spores can vary in their resistance, and anything from 1 minute to 20 hours may be re-

quired at a temperature of 100°C. The following table provides an indication of this variation.

The above table provides an excellent insight into the variation that may exist when it comes to

using heat treatment for addressing the hazard posed by spores. Ensuring your food safety control

plan or HACCP system is effective requires a robust hazard identification and analysis which clear-

ly characterises the specific pathogens.

When it comes to heat treatment processes, two factors in combination should be defined, i.e. time

and temperature. The general rule is the higher the temperature (To), the shorter the time neces-

sary to achieve a specific effect, e.g. destruction. If we take the example of Clostridium botulinum

in low acid foods, the following table illustrates this.

In the food industry there are two main methods of heat preservation and treatment:

Pasteurization

Commercial sterilization (appertization)

These methods are usually followed by or incorporating a packaging stage. Food may be heated in

the packaging or heated prior to packing after which follows aseptic packaging because of the sen-

sitivity of food. The product can be packaged hot or cold but preferably hot as in ‘hot fill’ or ‘hot-

pack’ processes.



Pasteurization is widely used across the food industry for the following reasons:

To kill all pathogens (milk)

To decrease the microbial load in a heat sensitive food (milk)

To kill the main spoilage organisms which are not very heat resistant (beverages)

To kill competing microorganisms (fermented foods)

Figure 1: Plate Heater Exchanger Pasteurizer

In order to maintain the preservation effects additional preservation methods may be required such

as packaging and refrigeration. Milk, for example, requires a high temperature, short time (HTST)

combination of 71°C/15 seconds. More rigorous treatments are actually employed industrially. An

alternative profile or holding method of 62.8°C/30 minutes can be applied. Temperature reduction

to <6°C is applied immediately in all cases. For ice-cream a HTST profile is also applied at 82.2°

C/16-20 seconds. For the holding method, a combination of 71.1°C/30 minutes may be used. Pas-

teurization is also used for wines, beers, fruit juices and dried fruits. In regard to bottle beers, tem-

peratures are limited by the boiling point of alcohol which is 78°C.

Commercial sterilization is a heat treatment which uses moist heat and temperatures usually above

100°C. The main objective is to produce a product stable at ambient temperature for long periods

of time. Small numbers of resistant spores may survive but cannot grow under normal storage con-

ditions for such products. Packaging materials employed in this method include cans (where the

food may be heat treated directly in the can), glass, thermoplastics or tetrapak where food is heated



pre-packaging. The final shelf life of the product depends on the packaging material and may be

years for some canned products.

The choice of actual processing temperature and time is a

balance between ensuring the food is appertized

(microorganisms and enzymes are inactivated) and the col-

our, flavour, texture and nutritional quality of the food being

maintained. The rate of heat penetration into the food must

be known. The portion that heats most slowly is known as

the cold point and presents the most difficulty in terms of

heat treatment.

Figure 2: Commercial Sterilization Unit

The calculation of heat processing time for canned products requires:

Thermal death time curve for organisms likely to cause problems, e.g.

Clostridium botulinum

Knowledge of heat penetration rates in the can, especially the ‘cold point’

The rate of heat penetration into the container depends on:

Container material

Size and shape of container

Initial To of food

Retort To

Rotation and agitation of food in retort

Consistency of contents (liquid, semi-liquid, puree, solid)

Convection heating is more efficient than conduction heating. This is achieved by natural convec-

tion or forced convection (movement of cans). Methods include hydrostatic cookers and coolers

which can be continuous feed retorts.

Thermal death time (TDT) should be calculated and represents the number of minutes required to

destroy a stated number of microorganisms at a specific temperature. The higher the initial number

of cells or spores the longer the time required to reduce the number of survivors to 1 per gram.

The following table indicates typical TDT’s for spores of Clostridium botulinum.

As the temperature increases, the rate of reduction is logarithmic. Therefore, the highest tempera-

ture which will not decrease the organoleptic quality of the food is normally employed. Certain

heating allow the use of temperatures of 130°C without causing destruction of the food, e.g. UHT

milk treated at 135°C for 1-2 seconds.



The D value is the time required to kill 90% of the microbial population at a specific temperature.

For example, the D 121.1°C for Clostridium botulinum spores is 0.21 minutes. As a safety precau-

tion a 12-D heat treatment is used for Clos. botulinum in low acid foods, i.e. sufficient heat to re-

duce 1012 spores to 1, or 1 spore per can to 1 per 1012 cans. For a 12-D cook you take 12 x 0.21

minutes = 2.52 minutes. For additional safety this is rounded up to 3 minutes which is known as

the ‘Minimum Botulinal Cook’ (3 minutes at 121°C).

Other values encountered in heat processing include the Z value. This is the number of degrees of

temperature required to allow a ten-fold reduction in the time required. It is calculated by estimat-

ing the D values of an organism at a number of temperatures. This data then allows calculation of

heat resistance over a broader range of temperatures.

The F0 Value is the sterilization value and equals D121.1°C (log a – log b) minutes where a = ini-

tial no. of cells and b = final no. of cells:

e.g. F0 value for Clos. botulinum 0.21 (log 1 – log 10-12)

= 0.21 x 12

= 2.52 minutes

F0 value is a measure of the capacity of a heat process to eliminate microorganisms from all points

in a container of food.

This method employs radiation from a particular source. When microorganisms are irradiated con-

stantly the number of survivors declines exponentially with time.

Figure 3: Irradiation Plant



This may be used to decontaminate:

Air (most efficient use)

Liquids in films, e.g. water (long exposure time, expensive, complex)

Surfaces (long exposure time, limited use)

Packaging where heat is inappropriate, material must be transparent

to UV or pack must be open

Solid foods in very thin layers, e.g. sugar

There are a number of disadvantages with using UV. It has very low penetration, and it may cause

rancidity of lipid and its effects may be irreversible.

Unlike other forms of radiation, microwaves act indirectly on microorganisms by generation of

heat through oscillation of water molecules. Cold spots can remain making tempering essential.

Microwave radiation has limited industrial application.

Sources of ionizing radiation include α, β and λ rays, and may also include the use of electrons. An

ion is a charged particle which is not stable. They are highly lethal with varying penetration power.

Radiation source is usually Cobolt-60. λ rays are applied and have a half-life of 5 years with a 1%

loss per month. Units of radiation are measured in Grays (Gy); 1 Gray = absorption of 1 joule/kg.

1000 Grays = 1 kGy

Irradiation is used on a variety of foods and packaging with various objectives. The following table

summarises some of the applications in food.

There are a number of food irradiation processes which are classified in the following table:



In high pressure processing an equal pressure is applied throughout the food (isostatic). Profiles

for cold isostatic pressure at ambient To include 50 - 600 MPa. Treatment times can vary from 0.5

to 5 minutes. Sealed flexible packs are usually used. Vegetative cells are generally very sensitive to

the effects. Spores are variable but can be highly resistant under some conditions. Viruses have a

high tolerance to pressure.

The food industry uses a number of low temperature methods to achieve preservation of the per-

ishable foods:



Temperatures above freezing point generally result in metabolic rates of microbes to be slowed

down or stopped. Temperatures below freezing point generally result in metabolic activity being

stopped. Enzymatic reactions are temperature dependent. A rise in temperature (within limits) will

lead to an increased rate and lowering the temperature will decrease the rate. The change in the

reaction rate over a 10 degree change in temperature is known as the temperature coefficient (Q10)

Generally the Q10 of biological systems is 2.

Psychrophiles and psychrotophs are problematic when it comes to effects of low temperature. The

minimum temperature at which an organism has been found to grow is -34°C (a yeast species).

Growth at freezer temperatures if it occurs is extremely slow. Examples of minimum growth tem-

peratures:

Storage temperatures <4°C will generally prevent the growth of food pathogens except Listeria

and Yersinia.

Freezing of foods can cause initial mortality immediately on freezing and depends on the species.

Surviving cells die off gradually, and the rate of death is quickest at temperatures just below freez-

ing point, with the slowest at below -20°C. All cells rarely die off. Defrosting foods must be treated

as fresh products as regards microbiological activity. Endospores and toxins are not affected by

low temperatures. All frozen foods should be defrosted at 4°C to reduce or prevent microbial

growth. The rate of thawing also affects microbial cells – the faster they thaw, the greater the num-

ber of survivors. Repeating freezing and thawing disrupts both the food and microbial cells. It may

be a hazardous procedure if sufficient time is given for growth or survivors.

The typical methods employed include sun drying, mechanical drying and freeze-drying. Certain

food preparation methods of foods may have some antimicrobial effect, e.g. blanching, addition of

preservatives, cooking, fat removal and addition of sugar or other solutes. The moisture contents

of dried foods vary from 2% to 50%; Intermediate moisture foods from 20% to 50% or aw =0.60-

0.85. The drying process per se does not kill microorganisms. Some may be killed but most may be

recovered from dried foods if present prior to drying. Most bacteria and yeasts require aw > 0.90

to grow. Dried foods are not usually susceptible to spoilage. S. aureus is the most xerotolerant of

the pathogens, i.e. grows in aw of 0.86. Rehydration (i.e. the addition of water or adding to other

wet ingredients) enables microorganisms present to re-commence growth.



Freeze-drying has the least destructive effect on microorgan-

isms and depends on the age of the cells. Reduction of water

activity and subsequent antimicrobial effects also occur on ad-

dition of sugar or salt, but other inhibitory factors are also in-

volved, e.g. high chlorine levels. All microbial cells require wa-

ter to grow but not to survive.

Figure 4: Freeze-drying Unit

There are many substances capable of inhibiting, retarding or arresting the growth of microorgan-

isms or deterioration of food due to microorganisms. Chemical preservatives may also improve the

organoleptic quality of food. The effects of chemical preservation depend on the type of chemical,

concentration of use, food characteristics and the type, number and previous history of microor-

ganisms in the food.

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Documents

Microbiological Aspects of Food Preservation and Safety ... · Commercial sterilization is a heat treatment which uses moist heat and temperatures usually above 100°C. The main objective