Shelf Life Assessment

Before You Begin

How do assessments work?

The assessments include a series of questions which ask you about your shelf life establishment. You will follow through and answer a series of multiple choice questions, with detailed information provided at the end.

All of the advice given has been written by food scientists, and is backed up by published research. You will see links to this research if you wish to know more about any particular issues. You can also find links to published guidance, information and relevant parts of the legislation.

The assessment is anonymous.

Although we will not monitor individual businesses, we will look at the overall results of the assessments across the UK. This will help us to monitor general microbiological risk and give advice to businesses on a UK basis.


Click or tap on words with dashed underline (Example: biofilm) to learn more.


Click or tap on words to learn more

Ambient temperature

Ambient is the natural temperature of the air in a room that has not been heated or cooled.


Aw stands for water activity. In food safety, aw is a measure of how much water within a food is available to micro-organisms. Aw values depend on the salt and the sugar content of the food (these can prevent water being available), the amount of water that would exist within the food and the amount that would exist as vapour if the food was placed in a sealed container. A lot of factors can change Aw, including temperature. The range of values for water activity is always between 0 and 1. A value of 1 is pure water. In general, bacteria need a high water activity value (at least 0.9) to multiply in food.

Bacterial typing

Bacterial typing is a microbiological method that allows laboratories to compare different strains of bacteria to see if they are the same. The process is similar to DNA fingerprinting in humans, which allows scientists to determine if DNA has come from the same or different people. If Listeria monocytogenes is repeatedly isolated from a plant environment over a long periods of time, strain typing can tell processors whether all the strains are the same or different. If the strains are all or mostly the same, that is good evidence that the plant has been persistently colonised by the same strain of Listeria monocytogenes.


A biofilm is a collection of microorganisms, frequently of different species, growing on top of a surface within a mesh of exopolysaccharide. Biofilms are resistant to a variety of environmental stresses such as desiccation and cleaning and sanitation chemicals.


Biotypes are closely related strains of bacteria. Typically, the strains originally came from a common ancestor. Bacteria evolve to become better suited to their environment and the evolutionary changes will differ in different places, resulting in a series of closely related strains or biotypes.


CFU stands for colony forming unit. A CFU is a colony of micro-organisms which usually grows from the multiplication of a single cell. A CFU is a measure of the number of micro-organisms in a sample of food.

Challenge testing

Challenge testing is when a sample of food is deliberately contaminated with a lab-grown bacterium such as L. monocytogenes (or spores from relevant sporeformer organisms) to see what will happen to the numbers of the pathogen over time. It can be used to help establish shelf lives for food.


Correlation describes the relationship between two sets of numbers. If the numbers change in a predictable way, that is as one set increases the other tends to increase then they are positively correlated. On the other hand, if one set tends to decrease as the other increases, then they are negatively correlated. If there is no discernible relationship then the two sets of numbers are not correlated.


Criterion is a standard set of conditions that are applied to foods in order for the product to be deemed acceptable for consumption. For example, for cold smoked salmon, it is important that the fish are cured properly so that the product tastes good. The criterion for effective curing could be laboratory testing of the fish to find out how much salt it contained. Only the fish with above a certain level of salt would pass the criterion.

Critical control point (CCP)

A point or a stage in a food production process where an identified hazard can either be eliminated or reduced to an acceptably low risk.

DNA fingerprinting method

DNA fingerprinting involves the examination of an organism’s genetic material to produce a reproducible genomic pattern that is unique to the organism.

D-values (decimal reduction times)

A D-value is a measurement of time. D-values are the time the that it takes to reduce a bacterial population by 90% (i. e. it is decimal reduction). D values, (in combination with Z values) are commonly used to assess how sensitive bacteria are to bactericidal treatments such as an exposure to heat. An example of a D-value is, if it took 1 minute to reduce a population of 1000 L. monocytogenes cells to a population of 100 cells at 74°C, D=1.


Fomites are inanimate objects that have become contaminated with micro-organisms and further spread contamination to people and objects that come into contact with them.


The number of occurrences of an event over a period of time. For example a clock that chimes on the half hour and the hour has a frequency of two chimes per hour.


An F-value is a measurement of time. F-values are the length of time it takes to completely destroy an entire population of bacterial cells. F-values are commonly calculated as 12 x a D-value. i. e. the amount of time required for a 12-log reduction in bacterial numbers.


GHP is an acronym for good hygienic practice. It is a particular way of handling and processing food so that it does not become contaminated. GHP can be applied to a wide range of activities such as the adequacy of the refrigeration of food; the use of gloves, aprons, and hair nets; the regular cleaning of the plant environment and the control of insects and vermin within and around the processing environment.

Gill gas-exchange surfaces

The gill gas-exchange surfaces are the part of the gills that removes oxygen from water.

GRAS (generally recognised as safe)

GRAS is an acronym for the phrase Generally Recognised As Safe. It is an approach to the safety of food additives used by the USA. A food additive is allowed for use by the US-FDA if it is generally recognised, “among qualified experts, as having been adequately shown to be safe under the conditions of its intended use, or unless the use of the substance is otherwise excepted from the definition of a food additive”.

High care area (HCA)

A HCA is broadly defined as an area designed to a high standard of hygiene where practices relating to personnel, ingredients, equipment and environment are actively managed to minimise the microbiological contamination of a RTE or ready-to-reheat product that contains uncooked ingredients. If best practices are followed, cold smoked and other uncooked foods should be produced in a HCA, separated from the raw materials and final packaged product.

High risk area (HRA)

A HRA is defined as areas designed to high standards of hygiene where practices relating to personnel, ingredients, equipment and environment are managed to minimise microbiological contamination of a ready-to-eat or ready-to-reheat product comprising only cooked ingredients. Good manufacturing practice (GMP) dictates that CSM should be manufactured in a HRA, again separated from raw materials and final packaged product.


An unconfirmed explanation for an observation or scientific problem that can be confirmed by undertaking experimental investigation.

Impaction sampler

An impaction sampler is a piece of equipment that sucks in a measured volume of air and measures the numbers of bacteria contained within that air.

Indigenous micro-organisms

Indigenous micro-organisms are those which are present in a place (e. g. soil, water) naturally.


Inoculation is the act of deliberately transferring bacterial cells from one place to another. For example, the transfer of L. monocytogenes cells from one nutrient source that has nearly run out to another nutrient source to keep the bacteria alive.


A modification or additional treatment introduced into a food manufacture process that helps to control a food safety hazard.


In food microbiology, an isolation is when food is tested and found to contain a particular micro-organism. For example, if fish was tested to see if it contained Salmonella, and the result was that it did, the laboratory would report an isolation and the Salmonella would be called the isolate.

L. monocytogenes

Listeria monocytogenes is a bacterium that causes an infection named listeriosis in humans. The elderly, pregnant women and immuno compromised people are particularly at risk from L. monocytogenes infections. The symptoms are very similar to meningitis and L. monocytogenes is routinely isolated from seafoods that have not been cooked.

Lab-cultured cocktail

A mixture of different species or strains of micro-organisms grown together in sterile water containing nutrients.

Lag time

When bacteria encounter a new environment, it takes them time to determine what nutrients are available to them. Once that information is available, the cells switch on what they require to make use of the nutrients and grow in the new place. Although the cells are busy, it looks like nothing is happening and thus the delay is called the lag phase before growth.


Lipid is a general name for a group of closely related molecules, which include fats, waxes and sterols such as cholesterol.

Log reduction

A log reduction is a 10 times reduction. For example, a log reduction of 1000 is 100.


Lysozymes are natural chemicals found in egg whites that can damage and kill some types of bacteria, including L. monocytogenes.

Make consideration

Consideration is when a problem is carefully thought about over a period of time before deciding on the best solution.

Multivariate analyses

Multivariate analyses is specialist branch of mathematics/statistics. In essence it is a way of trying to find out how things interact and are related to each other. For example, if lots of different food processing premises were surveyed and data was collected describing their manufacturing processes (including microbiological testing), it would be possible to use multivariate analyses to identify what stages of each process were beneficial in terms of food safety and how each of the different stages of a single process interacted to produce the final bacterial populations on food.


Food contaminants are any harmful substances unintentionally added to food. These substances include micro-organisms and chemicals, some of which can be from natural sources.


Niches are places where groups of bacteria can become established and colonise. Examples of niches are floor drains, cracks in equipment casings, and the inside of chiller door seals.

Operational prerequisite programme (oPRP), control point (CP) or secondary control point (SCP)

An operational prerequisite programme (oPRP), control point (CP) or secondary control point (SCP) are three terms that mean broadly the same thing and are used interchangeably by a number of food safety schemes. The term is usually described as a processing practice (e. g. the use of hot water sterilisers for gutting knives) or a supply condition (e. g. raw fish supplied should only be ocean caught) that does not critically control a hazard such as L. monocytogenes but reduces the likelihood of the hazard occurring. A process that has a number of SCPs is the classic example of a multiple hurdles approach to food safety.


A method of assessing the quality of meat or fish by examination using the senses. Typical properties are organoleptically assessed using taste, touch, colour, firmness and odour.

Persistent strains

Persistent strains of bacteria are those that have an ability to colonise food processing plant environments (e. g. floor drains), to resist cleaning and sanitation and to continuously contaminate final product, albeit at a low level.


pH is a measurement of the amount of acid that a substance (e.g. food) contains. pH is a chemical property of a substance. pH is measured on a scale with values between 1 and 14.


Physicochemical relates to the physical or chemical properties of a substance. Examples of physical properties are the weight of an object, its hardness, and its size. Chemical properties relate to things like the amount of salt, sugar, or fat that a food contains.

Predictions validated against real world observations

When mathematicians try to describe a system such as a food manufacture process, they check the validity of their work by comparing what happens in real life with the results forecast by their models.

Predictive microbiological (mathematical) modelling, critical survival, and growth characteristics

Modelling is a term used by specialist mathematicians to predict how things will behave. In the food industry, mathematical modelling is used to predict how bacteria will behave within a food. The process considers the properties of the food – for example, the salt, sugar and fat content of the food – and tries to predict if bacteria will survive, grow or die off if they contaminate a particular batch of food. Critical survival is a point where a food’s formulation allows bacterial survival in food. Growth is a point when bacteria are able to multiply in food.

Prerequisite programme (PRP)

A prerequisite programme (PRP) is defined as the basic supply and processing practices that are necessary to maintain a hygienic environment throughout the food chain. A PRP describes the conditions that are required for the manufacture and packaging of food that is safe for human consumption. Processor PRPs can extend both into the supply chain and their customer’s distribution chains.


Prevalence is the percentage of a population that have a particular trait. For example, if the number of people with red hair was 5 within 100 people tested, the prevalence of red hair would be 5%. In the food industry the prevalence is used to describe the degree of contamination of foods by potential pathogens such as L. monocytogenes.


Ribotyping is a form of fingerprinting for biological organisms, particularly bacteria. Ribotyping uses restriction (selective cutting) of amplified parts of the bacterial chromosomal DNA that encodes the RNA components used by cells to manufacture proteins. These components, which are called ribosomal RNA, are preserved enough to allow different, but closely related, bacterial strains to be differentiated.

Robust replication

In order for the results of an experiment to stand up to scrutiny, scientists repeat an experiment several times. Replication refers to repeatedly doing the same experiment over and over, using the same experimental conditions. Robust replication is when the number of repeats reaches a point where the result is considered reliable and reproducible.

Routine surveillance

Periodically, the government run routine checks of the food offered for sale in retail outlets to ensure it is safe. The process of visiting stores, buying food and testing it in a laboratory is called routine surveillance.

Salt content

The salt content is a measure of the amount of salt contained within a food.

Sensitive predictor

A sensitive predictor is information (e. g. a set of data) that reliably forecasts the status of another set of data. For example, being male, obese, and older than 50 is a sensitive predictor for a patient having diabetes.

Settle plates

Settle plates are small dishes containing nutrients in jelly that are placed to catch and count micro-organisms that settle out of the air.


A sporadic event is one that occurs unpredictably and rarely.


A strain is a genetic variant or a subtype of a bacteria. For example, each bacterium such as L. monocytogenes has lots of variants that are all slightly different to each other but evolved from a common ancestor and share the vast majority of their genes.


A sub-lethal injury is one that almost, but does not quite, kill a micro-organism. Cells that are sub-lethally injured can recover given favourable conditions.


The provision of evidence that supports a viewpoint or opinion.

Total aerobic mesophiles count

TAMC is the numbers of bacteria that can grow in an air atmosphere at temperatures around 30°C.


VBNC is an acronym for viable but not culturable. VBNC is a state that some microorganisms enter as a survival mechanism under conditions of extreme stress. VBNC lose their ability to grow and multiply using microbiological culturing methods. In essence, this means VBNC cells are undetectable using classic media-based detections. One reason VBNC are important is because although undetectable, passage through a suitable host environment such as a mammalian GI tract can still cause infection, with the environment taking the cells out of the VBNC state.

Vegetative bacteria

Vegetative bacteria are healthy, metabolically active bacteria that are growing and multiplying. The term is used to differentiate bacteria that are able to form a resting body that resists environmental stresses called a spore. Spores are also a viable form for some bacteria, although they are metabolically inactive and do not consume nutrients or grow.


Verification is the process of establishing the validity or truth of something. In the food industry, the term usually applies to a check that something has happened the way that it should. For example, after cleaning, a visual inspection could be carried out to make sure there was no food residues remaining on equipment. That would mean the cleaning was verified.

Whole genome sequencing

WGS refers to a methodology that determines the entire DNA sequence of an organism. A typical approach is called shotgun sequencing and fragments an entire genome into smaller pieces using shear or other process that cuts randomly. The smaller pieces are sequenced and overlapping sections are assembled using computer software to recreate an entire genome.


A Z-value is a measurement of temperature. Z-values are the increase (or decrease) in temperature that is required for a one decimal change in a D-value. Z-values are a measure of rate of change of death with a change in temperature. An example of a Z-value is, if it took 60 seconds to reduce a population of 1000 L. monocytogenes cells to a population of 100 cells at 74°C, and it took 6 seconds to cause the same reduction at 82°C, Z would be 8°C.