Food and Beverage Stability and Shelf Life || Food storage trials: an introduction

10.1 Introduction

Because of their perishable nature, all foods (taken to mean food and beverages)

deteriorate throughout storage, albeit at different rates. For this and other related

reasons, the behaviour of foods during storage is of immense interest to the

consuming public as well as to all those who make, prepare, handle, buy, sell

and distribute foods. The period during which a food remains safe and enjoyable

to eat has been called its shelf life, which is now a legal term within the EU.

Regulation (EC) No. 852/2004 of the European Parliament and of the Council

on the hygiene of foodstuffs, implemented in England along with other asso-

ciated regulations as the Food Hygiene (England) Regulations 2006, requires

food business operators to adopt as appropriate a number of specific hygiene

measures (Article 4(3(a))), which include `compliance with microbiological

criteria for foodstuffs' as laid down in Commission Regulation (EC) No. 2073/

2005 on microbiological criteria for foodstuffs. The latter regulation defines

`shelf life' as èither the period corresponding to the period preceding the ùse

10

Food storage trials: an introductionC. M. D. Man, London South Bank University, UK

Abstract: This chapter reviews definitions of shelf life and key conceptssuch as `best before' and ùse by' dates. It reviews key factors affecting foodspoilage and deterioration. It then goes on to discuss the principles andpractices of shelf life testing and storage trials to establish accurate shelf lifedates which manufacturers can use in food labelling.

Key words: shelf life, stability, food spoilage, best before dates, use bydates, storage trials.

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by' or the minimum durability date, as defined respectively in Articles 9 and 10

of Directive 2000/13/EC, the most recent European labelling Directive. A much

more useful and informative definition of shelf life of food has been available

for some time (IFST, 1993): it is the period of time during which the food will

· remain safe;

· be certain to retain its desired sensory, chemical, physical, microbiological

and functional characteristics;

· where appropriate, comply with any label declaration of nutrition data, when

stored under the recommended conditions.

Clearly, safety and quality are the two main aspects of shelf life of food, and

food safety must always take priority over quality as it is both a fundamental and

a legal requirement. According to Article 14 of the General Food Law

Regulation (EC) 178/2002, food must not be placed on the market if it is unsafe.

Food is deemed to be ùnsafe' if it is considered:

· injurious to health;

· unfit for human consumption.

Food can be injurious to health by virtue of the presence of a hazard, which may

be microbiological, chemical or physical in nature. Article 14(4) of the

Regulation goes on to say:

in determining whether any food is injurious to health, regard shall be

had:

(a) not only to the probable immediate and/or short-term and/or long-

term effects of that food on the health of a person consuming it,

but also on subsequent generations;

(b) to the probable cumulative toxic effects;

(c) to the particular health sensitivities of a specific category of

consumers where the food is intended for that category of

consumers.

In terms of ùnfitness' for human consumption the central concept is

unacceptability. Food can become unfit for human consumption because of

contamination, whether by foreign objects (e.g., glass, plastics), by chemicals

(e.g., cleaning chemicals, agrichemical residues) or by microbes causing

putrefaction, decomposition or decay. Consequently, there should be little doubt

in the mind of a food business operator as to what safe food means. Quality, on

the other hand, is not usually regulated by law unless it has to do with

compositional/marketing standards. British Standard BS EN ISO 9000:2005

(Quality management systems ± Fundamentals and vocabulary) defines quality

as `degree to which a set of inherent characteristics fulfils requirements'.

In practice, therefore, it is the job of every food business operator to establish

as fully as possible the requirements of its target consumer and to ensure that the

characteristics of its food product in question reflect and fulfil those require-

326 Food and beverage stability and shelf life


ments consistently. The need to provide an acceptable, reliable and consistent

product shelf life represents an obligation as well as a challenge to every food

business operator. In the UK the legal responsibility to assign an acceptable

shelf life is contained in the Food Labelling Regulations 1996 as amended, such

that pre-packed foods that are required to carry a durability indication (i.e., most

foods) must indicate either:

· `Best before' followed by the date up to and including which the food can

reasonably be expected to retain its specific properties if properly stored, or

· Ùse by', for foods which are, from the microbiological point of view, highly

perishable and in consequence are likely after a short period to constitute an

immediate danger to health, followed by the date up to and including which

the food, if properly stored, is recommended for use.

All such declarations must by followed by an indication of any storage

conditions that need to be observed if the unopened food is to retain its specific

properties up to the date indicated. This is understandable as all foods are

perishable, they will naturally deteriorate in an unexpected manner, or faster or

both, if they are stored under harsher conditions (usually warmer and more

humid) for which they are not intended. The decision as to whether a food

requires a ùse by' date is one for those who manufacture, pack and therefore

mark it in the first place. Useful guidance, however, is available and it has been

suggested that the following food groups, essentially all chilled foods, are likely

to require a ùse by' date (Crawford, 1998):

· dairy products, e.g. fresh cream-filled desserts

· cooked products, e.g. ready-to-heat meat dishes

· smoked or cured ready-to-eat meat or fish, e.g. hams, smoked salmon fillets

· prepared ready-to-eat foods, e.g. sandwiches, vegetable salads such as

coleslaw

· uncooked or partly cooked savoury pastry and dough products, e.g. pizzas,

sausage rolls

· raw ready-to-cook products, e.g. uncooked products comprising or containing

either meat, poultry or fish, with or without raw prepared vegetables

· vacuum or modified atmosphere packs, e.g. raw ready-to-cook duck breast

packed in modified atmosphere

In order to arrive at an acceptable, reliable and reproducible shelf life, a food

business operator will need to have answers for the following questions:

1. Is my product safe to eat throughout its intended shelf life? (Essentially, an

unsafe food product has no useful/meaningful shelf life.)

2. How long will my product last for before it becomes unacceptable to the

target consumer?

In order to answer these separate and yet related questions satisfactorily, a food

business operator needs to have sufficient knowledge about its product, in

Food storage trials: an introduction 327


particular, a thorough understanding of the shelf life limiting mechanism of

deterioration and factors that influence it. Typically, shelf life is determined by

conducting shelf life studies commonly but not exclusively by experimentation

using storage trials under defined conditions. The final shelf life that is

ultimately assigned may be decided dependent upon commercial considerations

such as product category and associated image perceptions, and the margin of

safety required. This shelf life is then expressed legally either as a ùse by' or

`best before' date. Such is the importance of the legal requirement to set

appropriate and accurate date marks that the UK Food Standards Agency

launched a consultation on 25 March 2010 on the latest revision of its existing

guidance on the application of date marks to food (FSA, 2010).

10.2 Food deterioration and spoilage

Changes in the characteristics of food inevitably occur during its storage. With

very few exceptions such as cheeses and wines, these changes result in

deterioration and spoilage of the food to the point when it is no longer

acceptable to the target consumer and are usually classified as:

· microbiological

· non-microbiological

± biochemical

± chemical

± physical

± temperature-related.

When they happen these changes effectively constitute the underlying mech-

anism(s) of deterioration and spoilage of the food in question, which if allowed

to continue, will either singly or in combination cause the food to be rejected by

the consumer. Figure 10.1 provides a picture of the progression of these changes,

which cause the food to deteriorate and spoil during storage. In practice, a

number of such changes can take place simultaneously or in sequence; in many

cases, though, a particular type of change is likely to be the predominant one,

which turns out to be the shelf life limiting change. The primary aim of a shelf

life experiment, therefore, is to learn about these changes as they impact on the

behaviour of the food during storage. As time goes on, a point is eventually

reached when the food becomes unacceptable to the consumer, which marks the

end of its shelf life and which has to be determined. A brief review of the

different types of changes that can occur in food is given in the following

sections.

10.2.1 Microbiological changes

Besides the initial load or level of contamination, microbial growth depends on a

number of well-known factors, which have been summarised by Mossel (1971):



· intrinsic properties of the food (e.g., nutrients, pH, total acidity, water

activity, structure, presence of preservatives and/or natural antimicrobials,

redox potential)

· extrinsic factors (e.g., environmental temperature, relative humidity, gaseous

atmosphere)

· processing factors (e.g., heat destruction, freezing, packaging)

· implicit factors (e.g., physiological attributes such as specific growth rate of

the micro-organisms and microbial interactions).

Micro-organisms, be they pathogenic or spoilage, share the same factors for

growth. However, the growth of pathogenic organisms in food such as

Salmonella species and Listeria monocytogenes is not necessarily accompanied

by changes in appearance, smell, and even taste or texture that human senses can

detect, posing serious health concerns. On the other hand, growth of spoilage

organisms in food is often associated with signs that can readily be recognised as

changes in sensory properties, for example, visual mould growth and production

of objectionable odours and flavours. Examples of some common food spoilage

organisms and changes they cause in food are given in Table 10.1.

10.2.2 Biochemical and chemical changes

Raw materials from which practically all food products are manufactured are

biological in origin, and it may be unappreciated by the average consumer that

food is composed of chemicals. Some biochemical and/or chemical changes in

food are therefore inevitable. These changes can occur arising from reactions

within the food or from reactions between food components and external

species or factors such as oxygen or light respectively. In packaged food,

interactions, many of which are chemical in nature, can occur between

packaging and the food. With a few exceptions such as maturing of wines and

cheeses, and post-harvest ripening of fruits, most biochemical and chemical

changes in food are undesirable, deteriorative and effectively shelf life limiting.

Fig. 10.1 A picture of changes in food during storage.



Examples of some biochemical and chemical changes in food are given in

Table 10.2.

10.2.3 Physical changes

Significant transfer of moisture (or water vapour) and/or other substances in or

out of food can often cause deteriorative changes in food. These changes are

very common and can affect short-, medium- as well as long-life products. Most

of these changes are important from a product quality point of view while a few

can have food safety implications such as in the case of migration of chemical

components from the packaging material into food, particularly when the latter

has a long shelf life. In the EU, both overall and specific migrations of chemical

components from packaging materials into food are controlled by Regulation

(EC) No. 1935/2004 on Materials and Articles intended to come into contact

with Food. Examples of some physical changes in food are given in Table 10.3.

10.2.4 Temperature-related changes

Temperature, arguably the most important environmental factor, affects all of

the above changes and not always in the same way. Micro-organisms, pathogens

and spoilage organisms, exhibit a range of minimum growth temperatures below

which they cannot grow. For instance, temperature selects for the types of

organisms that can survive and grow at refrigerated temperatures. Table 10.4

Table 10.1 Examples of food spoilage organisms and changes they cause in food(adapted from Huis in't Veld, 1996)

Food spoilage organisms Changes in food

Gram-negative rod-shaped bacteria,e.g. Pseudomonas spp.

Production of off-flavours, visible slimeand pigmented growth in red meat, fish,poultry, milk and dairy products

Gram-positive spore-forming bacteriae.g. Bacillus and Clostridium spp.

`Sweet curdling' and `bitty cream' inmilk (Bacillus cereus)Gas production ± `late blowing' of hardcheeses (Clostridium spp.)

Other Gram-positive bacteria,e.g. Brochothrix thermosphacta

Off-flavour development in MAP and VPmeat products

Lactic acid bacteria, e.g. Lactobacillus,Streptococcus, Leuconostoc andPediococcus spp.

Slime formation, generation of CO2,production of lactic acid, causing a dropin pH and off-flavour development insome dairy products

Yeasts and moulds Production of soft rot in fruit, pigmentedgrowth in baked goods, production ofacid, gas or alcohol in some soft drinksand jams, development of off-odours inbeer



Table 10.2 Some biochemical and chemical changes in food (adapted from Man, 2002)

Biochemical/chemical reactions Changes in food

Oxidative rancidity, e.g. oxidation of fatsand oils involving a catalyst such ascopper ions; oxidation of fats and oilsinitiated by light in the presence of aphoto-sensitizer such as myoglobin;oxidation of fats and oils catalysed by theenzyme lipoxygenase

Rancidity (off-flavour development) infatty food and food products

Oxidation-reduction reactions withatmospheric oxygen

Degradation and loss of vitamins C, B1,A and E

Hydrolysis of aspartame (sweetener) Reduction in sweetness of calorie-free/low-calorie soft drinks

Non-enzymic browning (Maillardreaction)

Browning (discoloration) in dehydratedfruits and vegetables, instant potatopowder, dried egg white and dried milkproducts

Enzymic browning Browning in pre-cut vegetables and freshfruit salads

Chemical breakdown caused by light Colour fading

Electrochemical reactions between foodsand tinplate cans

Gas production, discoloration of food,etc., depending on the food and type ofmetal can

Table 10.3 Examples of physical changes in food (adapted from Man, 2002)

Product Quality change Underlying mechanism

Fresh vegetables Wilting Moisture loss

Biscuits Softening, loss of Moisture gaincrunchiness

Carbonated soft drinks Loss of fizziness Loss of carbonation (CO2) tothe environment

Orange juice Reduction in citrus Sorption of limonene andflavour intensity other aroma compounds by

the packaging material

Dressed salads, e.g. Changes in texture of Moisture migration fromcoleslaw vegetables, changes in vegetables to dressing

consistency of dressing

Chilled composite desserts, Gradual loss of distinctive Bleeding of colours,e.g. trifle layers migration of moisture/syrup



gives a list of pathogenic micro-organisms and their minimum growth tempera-

tures that are known to be associated with chilled foods. In consequence,

compliance with the relevant temperature control requirements, i.e., a maximum

storage temperature of 8 ëC, of the current food hygiene regulations (TSO, 2006)

is essential in assuring the microbiological safety and stability of chilled foods.

The effect of elevating temperature on many chemical reactions, and hence

potential adverse chemical changes in food during storage, is well known;

increasing the temperature generally increases the rate of chemical reactions by

a factor of 10. This empirical relation between the rate of reaction (k) and

temperature was first proposed by Svante Arrhenius in 1889:

k � A exp ÿ Ea

RT

� �where A � the frequency factor (or pre-exponential factor), Ea � the activation

energy, R � the universal gas constant (0.001987 kcal molÿ1 Kÿ1 or

8.31 Jmolÿ1 Kÿ1), and T � the absolute temperature in K (kelvin).

Converting this relationship to logarithmic form, the following is obtained:

log10k � log10AÿEa

2:303RT

or

ln k � lnAÿ Ea

RT

In theory, a plot of lnk versus the reciprocal of absolute temperature should

give a straight line, the slope of which is the activation energy divided by the gas

constant (Ea/R). A graph of ln k against 1/T is called an Arrhenius plot; many

chemical reactions have been found to show Arrhenius behaviour, i.e. their

Arrhenius plots show a straight line. Thus, by studying a reaction and measuring

k at two or three different temperatures, one could extrapolate with a straight

line to a lower temperature and predict the rate at this temperature. This is the

Table 10.4 Some pathogenic micro-organisms known to be associated with chilledfoods (Betts et al., 2004; Voysey, 2007)

Micro-organism Minimum growth temperature (ëC)

Salmonella 4Staphylococcus aureus 5.2 (10 for toxin)Bacillus cereus (spores/heat resistant) 4Clostridium botulinum (non-proteolytic B, E, F) 3Listeria monocytogenes ÿ0.4Escherichia coli 7±8Escherichia coli (O157:H7) 6.5Vibrio parahaemolyticus 5Yersinia enterocolitica ÿ1.3Aeromonas hydrophila ÿ0.1



basis of accelerated storage trials for shelf life at an elevated temperature. Often

though, reactions in real food systems are far more complex than can be easily

modelled by the Arrhenius equation. For certain non-microbiological changes in

food, lower temperatures do not automatically mean lower rates of change or

insignificant changes. For instance, bread stales fastest at refrigerated

temperatures; increased temperatures can slow the development of bread

staling, which is thought to be due to re-distribution of moisture and retro-

gradation of starch molecules. Fluctuating temperatures can cause ice crystal

formation in frozen foods such as ice cream, and changes in fat crystallinity are

promoted by fluctuating storage temperature, which encourage bloom to develop

in chocolate.

10.2.5 A summary

The changes that bring about deterioration and spoilage in food as outlined in

the previous sections can be summarised as in Fig. 10.2. Microbiological and

non-microbiological changes can take place in parallel or in sequence. More

than one type of change can take place at the same time, and changes in many

foods can be complex. Nevertheless, a number of well-known mechanisms

broadly classified as microbiological and non-microbiological changes can be

used to explain deterioration, spoilage and subsequent loss of shelf life in many

food products (Man, 2004):

· microbiological changes

· non-microbiological changes

± biochemical and chemical changes including light-induced changes

± moisture and/or water vapour transfer leading to gain or loss

± physical transfer of substances such as oxygen, odours or flavours other

than moisture and/or water vapour

± other mechanisms or changes such as loss of pack integrity.

The question as to which of the above changes and indeed what predominant

change will take place in a food, will depend on many shelf life influencing

Fig. 10.2 A basic model for food deterioration and spoilage (Ellis and Man, 2000).



factors, which can be categorised into product and external factors (IFST, 1993).

Product factors are related to the composition, make-up and properties of the

final product. They include the following:

· raw materials (their microbiology and biochemistry)

· product composition and formulation (e.g., use of preservatives)

· food structure (i.e., homogeneous versus heterogeneous)

· product assembly (i.e., composite, multi-component product)

· pH value, and total acidity including type of acid

· water activity (aw)

· redox potential (Eh)

· oxygen availability.

External factors are those that the final product is subject to or comes into

contact with as it moves through the food chain up to the point of consumption.

They include the following:

· hygienic conditions during preparing, processing, storage and distribution

· type and extent of processing (e.g., time-temperature combination of heat

treatment)

· conditions within packaging (i.e., composition and pressure of atmosphere)

· packaging materials and system

· exposure to light (UV and IR) during processing, storage and distribution

· temperature control throughout the food chain

· relative humidity during processing, storage and distribution

· consumer handling, preparation and use.

10.3 Storage trials

The most common and direct way of determining shelf life is to carry out

experimentally storage trials of the product in question under conditions that

simulate those it is likely to encounter during storage, distribution, retail display

and consumer use. The aims of all storage trials of food are the same, which are,

as indicated earlier in this chapter:

· to establish the safety of the food throughout its intended shelf life, whatever

its length, and

· to arrive at a period of time during which the food will be certain to retain its

sensory, chemical, physical, microbiological and functional characteristics

that meet the target consumer requirements, and where appropriate, comply

with any label declaration of nutrition data, when stored under the

recommended conditions.

10.3.1 Safe shelf life

In order to establish food safety, the most effective way, which is also a legal

requirement within the EU/UK, is to use the internationally recognised system



based on the Hazard Analysis and Critical Control Points (HACCP) principles as

detailed in Article 5 of the EU Regulation (EC) No. 852/2004 on the hygiene of

foodstuffs. The principles consist of the following (European Commission,

2004):

(i) identifying any hazards that must be prevented, eliminated or

reduced to acceptable levels;

(ii) identifying the critical control points (CCPs) at the step or steps at

which control is essential to prevent or eliminate a hazard or to reduce

it to acceptable levels;

(iii) establishing critical limits at CCPs which separate acceptability

from unacceptability for the prevention, elimination or reduction of

identified hazards;

(iv) establishing and implementing effective monitoring procedures at

CCPs;

(v) establishing corrective actions when monitoring indicates that a

critical control point is not under control;

(vi) establishing procedures, which shall be carried out regularly, to

verify that the measures outlined in (i) to (v) above are working

effectively; and

(vii) establishing documents and records commensurate with the nature

and size of the food business to demonstrate the effective application of

the measures in (i) to (vi) above.

Earlier, Article 4 of the same Regulation requires food business operators to

adopt as appropriate a number of specific hygiene measures, which include

among others compliance with microbiological criteria for foodstuffs as set out

in Commission Regulation (EC) No. 2073/2005 on microbiological criteria for

foodstuffs. This Regulation establishes two types of microbiological criteria:

food safety criteria, and process hygiene criteria (FSA, 2005). A food safety

criterion is one that defines the acceptability of a product or a batch of foodstuff,

applicable to products placed on the market. Applicable food safety criteria in

Regulation No. 2073/2005 should therefore be used to establish safe micro-

biological shelf life during product development and to assess the micro-

biological safety of a food product or batch of products within the framework of

an effective HACCP-based food safety management system. Examples of the

microbiological (food safety) criteria set out in Annex I of Regulation No. 2073/

2005 are given in Table 10.5.

In an effort to assist food businesses of all levels of expertise to assign

appropriate and correct date marks, the Chilled Food Association in the UK

recently published a good practice guide on `Shelf life of ready to eat food in

relation to L. monocytogenes', which was produced by a stakeholder drafting

group chaired by the British Retail Consortium and which has been endorsed by

the FSA (CFA, 2010). Benefiting from a wide knowledge and experience base,

and taking advantage of collective wisdom, this guide effectively develops and

expands the requirement in Regulation No. 2073/2005 for food business



Table 10.5 Examples of food safety criteria applicable to products placed on the market during their shelf life (taken from FSA, 2005 ± Annex 1,Chapter 1)

Criterion Micro-organism and food Examples of foods Sampling plan Limits Analytical referencecategory method

n c m M

1.2 Listeria monocytogenes Chilled ready-to-eat products 5 0 100 cfu/g EN/ISO 11290-2Ready-to-eat foods able to with more than 5 days' lifesupport the growth of L. Pre-packed delicatessenmonocytogenes, other than productsthose intended for infants and Pre-packed sliced cooked meat 5 0 *Absence in 25 g EN/ISO 11290-1for special medical purposes Smoked salmon

PaÃteÂSoft cheese

1.3 Listeria monocytogenes YoghurtReady-to-eat foods unable to Hard cheese 5 0 100 cfu/g EN/ISO 11290-2support the growth of L. Products with a pH less thanmonocytogenes, other than 4.4, e.g. coleslawthose intended for infants and Products with shelf life lessfor special medical purposes than 5 days, e.g. sandwiches

1.4 Salmonella Steak tartare 5 0 Absence in 25 g EN/ISO 6579Minced meat and meatpreparations intended to beeaten raw

1.8 Salmonella SalamiMeat products intended to be Parma ham 5 0 Absence in 25 g EN/ISO 6579eaten raw, excluding products Cold smoked duckwhere the manufacturingprocess or the compositionof the product will eliminatethe salmonella risk

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1.20 Salmonella Freshly squeezed 5 0 Absence in 25 g EN/ISO 6579Unpasteurised fruit and unpasteurised fruit juices,vegetable juices (ready-to-eat) mixed fruit juices; smoothies;

vegetable juices

1.21 Staphylococcal enterotoxins Cheeses, excluding processed 5 0 Not detected European screeningCheeses, milk powder and cheese and non-fermented in 25 g method of the CRLwhey powder, as referred to cheese for milkin the coagulase-positivestaphylococci criteria inChapter 2.2 of Annex 1 ofRegulation No. 2073/2005

1.23 Enterobacter sakazakii Dried infant formulae and 30 0 Absence in 10 g ISO/DTS 22964Infant milk and dairy products, dried dietary foods foras referred to in the special medical purposesEnterobacteriaceae criterion in intended for infants belowChapter 2.2 of Annex 1 of six months of ageRegulation No. 2073/2005

1.24 E. coli Oysters, clams, sea 1 0 230 MPN/100 g ISO TS 16649-3Live bivalve molluscs and live urchins, winkles and welks of flesh andechinoderms, tunicates and intra-valvular liquidgastropods

1.25 Histamine Tuna, mackerel, sardines, 9 2 100 100 HPLCFishery products from fish mahi mg/kg mg/kgspecies associated with ahigh amount of histidine

* This criterion applies to products before they have left the immediate control of the producing food business operator, when he is not able to demonstrate, to thesatisfaction of the competent authority, that the product will not exceed the limit of 100 cfu/g throughout the shelf life.

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operators to `conduct studies' (Article 3(2)) as necessary to ensure `that the food

safety criteria applicable throughout the shelf life of the products can be met

under reasonably foreseeable conditions of distribution, storage and use' (Article

3 1(b)). Annex II of the same Regulation says

The studies referred to in Article 3(2) shall include:

· specifications for physico-chemical characteristics of the product,

such as pH, aw, salt content, concentration of preservatives and the type

of packaging system, taking into account the storage and processing

conditions, the possibilities for contamination and the foreseen shelf life,

and

· consultation of available scientific literature and research data

regarding the growth and survival characteristics of the micro-organisms

of concern.

When necessary on the basis of the above-mentioned studies, the food

business operator shall conduct additional studies, which may include:

· Predictive mathematical modelling established for the food in

question, using critical growth or survival factors for the micro-

organisms of concern in the product,

· Tests to investigate the ability of the appropriately inoculated micro-

organism of concern to grow or survive in the product under different

reasonably foreseeable storage conditions.

· Studies to evaluate the growth or survival of the micro-organisms of

concern that may be present in the product during the shelf life under

reasonably foreseeable conditions of distribution, storage and use.

The above-mentioned studies shall take into account the inherent

variability linked to the product, the micro-organisms in question and

the processing and storage conditions.

In practice, therefore, in order to establish safe shelf life of ready-to-eat food in

relation to L. monocytogenes and indeed other pathogens, a food business

operator should use all or any suitable combination of the following:

· product characteristics and relevant scientific literature and research data

· historical data pertinent to the control of the pathogen of concern (i.e., in this

case L. monocytogenes)

· predictive microbiology, i.e. internet-based predictive microbiological

models e.g. ComBase (http://www.combase.cc)

· specific laboratory shelf life studies

± challenge testing

± storage trials under controlled conditions

· Collaboration between food businesses in conducting shelf life studies.

In view of the primary importance to assure microbiological safety of food,

further guides in relation to other pathogens are likely to be produced in future,

in particular, for ready-to-eat foods.



10.3.2 Challenge testing

In general, a challenge test is a laboratory investigation of the behaviour of a

product when subjected to a set of controlled experimental conditions. Chal-

lenge testing, in the context of shelf life studies, almost always refers to

microbiological challenge testing, the aim of which is to simulate what can

happen to a food product during processing, distribution and subsequent

handling, following inoculation with one or more micro-organisms of concern.

As such, it is a very useful tool for determining the ability of a food to support

the growth of pathogens or spoilage organisms. The main areas of application of

microbiological challenge testing include:

· determining microbiological safety and assessing the risk of food poisoning

after HACCP has identified the organisms likely to be the microbial hazards

for the product at some stage during production and distribution; for example,

this is useful in determining the safe shelf life of chilled foods (Uyttendaele et

al., 2004)

· establishing quality shelf life by inoculating the product with food spoilage

organisms known or likely to contaminate it; for example, this is useful in

evaluating the microbiological stability of emulsified and non-emulsified

condiment sauces intended for ambient distribution (Jones, 2000)

· studying the effects of different formulations of the food on a target

organism, i.e. either a pathogen or a spoilage organism, during product

development with a view to achieving an acceptable shelf life

· validating processes such as aseptic processing and packaging that are

intended to deliver some degree of lethality against a target organism or

group of target organisms.

In all situations, relevant expertise and skills together with the necessary

laboratory facility must be available to produce meaningful results from

challenge testing. When conducting a microbiological challenge test, a number

of factors need to be carefully considered:

· the selection of appropriate pathogens/spoilage organisms

· the level of challenge inoculum

· the inoculum preparation and method of inoculation

· duration of the study

· formulation factors and storage conditions

· sample examination

· data analysis and interpretation, and pass/fail criteria.

Useful and detailed guidelines for the design and planning of microbiological

challenge testing are available (Anon., 2003; 2010; Notermans et al., 1993).

10.3.3 Quality shelf life and storage trials

Ideally, storage trials aimed at establishing the quality shelf life of a food

product can begin once its safety has been established. In practice, and more



often than not, storage trials will be run in parallel to food safety evaluation

based on HACCP principles as required by law. While in principle shelf life

storage trials should employ conditions that mimic those the product in question

is expected to encounter during storage, distribution, retail display and consumer

use, in practice, and in many small and medium-sized companies, a fully

comprehensive storage trial is rarely possible as conditions during distribution

and retail display, for instance, are difficult and expensive to reproduce. Con-

sumer storage, handling and use, too, are often highly variable and unpredict-

able, and over which the manufacturer has little control. What the manufacturer

must be certain about is the objective of the storage trial, which, after all, is a

controlled experiment, and the manufacturer must be clear about what variables

he can control and what he cannot.

Storage conditions

Storage conditions can be fixed or cyclical, or a combination of both. For a

given set of storage conditions, the following variations should ideally be

available (Man, 2002):

· Optimum conditions: These are the most desirable conditions of temperature,

humidity, light and so on under which the most optimistic shelf life data

should be obtained.

· Typical or average conditions: These are the conditions that are expected to

be most commonly experienced by the product and under which shelf life

data that apply to the bulk of future production should be generated.

· Worst case conditions: These are the most extreme but not abuse conditions

that the product is expected to encounter and under which the most

conservative shelf life data should be obtained. The latter, if used to assign a

shelf life, should give it a margin of safety ensuring that product failures due

to insufficient shelf life are highly unlikely in practice.

For cost reasons, storage trials tend to employ fixed conditions, which, in the

absence of universal standards, commonly include:

· Frozen: ÿ18 ëC or lower (relative humidity is usually near 100%).

· Chilled: 0 to �5 ëC, with a maximum of �8 ëC (relative humidity is usually

very high: ~90%+).

· Temperate: 25 ëC, 75% relative humidity.

· Tropical: 38 ëC, 90% relative humidity.

· Control: control conditions (for storage of control samples) are usually the

optimum conditions, be they ambient, chilled or frozen.

Samples for storage trials

As outlined in Section 10.2.5, there are product (e.g., raw materials, product

composition) as well as external (e.g., packaging, processing) factors that can

influence shelf life. As such, they need to be known, controlled and standardised

across replicate storage trials or trials conducted during product development,



otherwise results from these trials could be misleading or even meaningless. A

corollary of this is that every time a significant change is made in any of these

factors, for instance, in the microbiological quality of a major raw material or in

the time-temperature combination of a thermal treatment, fresh storage trials

will have to be conducted. The number and size of samples to be laid down for

trials need to be carefully chosen. The type of product, its end-use application,

the anticipated or required shelf life and the tests planned for assessing changes

during storage are some of the factors that need to be taken into account. Should

there be great uncertainty about the shelf life, it is better to be generous with the

number of samples retained than to run out of samples before the storage trial

ends. Frozen storage at ÿ18 ëC or lower is often used as a means of keeping

control samples. However, if freezing and thawing are known to affect the

product adversely, facilities must be available for the preparation of fresh

reference/control samples that are identical to the test samples, at any time

during a storage trial.

Experimental design and sampling schedule

At present, there are few universally accepted protocols for storage trials for

shelf life testing, be it legal or industrial. A number of designs have been put

forward (Kilcast and Subramaniam, 2000), including some based on a statistical

approach (Gacula, 1975). All have advantages and disadvantages, as well as

varying implications on resources that include number of samples, storage

facilities, development and maintenance of a trained taste panel and the amount

of testing required. When conventional profiling is used to study sensory

changes during storage, difficulty can arise due to the taste panel generating

inconsistent responses over time, particularly if the storage time is long and test

frequency low. Difficulties such as this further underline the importance of

assuring the quality of stored samples, both test and control, if storage trials were

not to produce at best inconclusive and at worst incorrect shelf lives.

Nevertheless, the following are some possible protocols (Man, 2002):

· Short shelf life products: For chilled foods with shelf life of up to one week

(e.g., ready meals), samples can be taken off daily for testing.

· Medium shelf life products: For products with a shelf life of up to three weeks

(e.g., some ambient cakes and pastry), samples can be taken off on days 0, 7,

14, 19, 21 and 25.

· Long shelf life products: For products with a shelf life of up to one year (e.g.,

some breakfast cereals and heat-processed shelf-stable foods), samples can be

taken off at monthly intervals or at months 0, 1, 2, 3, 6, 12 and (perhaps) 18.

The exact frequency will depend on the product and on how much is already

known about its storage behaviour.

Accelerated storage trials

Sometimes, accelerated storage trials, mostly based on the Arrhenius equation

(see Section 10.2.4), may be used to shorten the time required to estimate a shelf



life, which otherwise can take an unrealistically long time to determine. In

principle, accelerated storage trials are applicable to any deterioration process,

biochemical, chemical, microbiological or physical, that has a valid kinetic

model. In practice, because of their obvious advantages over direct storage trials,

validated accelerated storage trials may be viewed as commercially sensitive

such that only a few are available in the literature. The latter include the

following:

· shelf life and safety of minimally processed CAP/MAP chilled foods over a

limited temperature range (Labuza et al., 1992)

· aspartame stability in commercially sterilised flavoured dairy beverages (Bell

and Labuza, 1994)

· accelerated storage of commercial orange juice in 1 litre TetraBrikTM

(Petersen et al., 1998)

· accelerated shelf life testing of whey-protein-coated peanuts (Lee et al.,

2003).

The limitations of accelerated storage tests are well known; they tend to be

product-specific and their results have to be interpreted with care based on

detailed product knowledge and sound scientific principles. Fuller accounts of

the limitations are available (IFST, 1993; Mizrahi, 2000). Accelerated tests must

not be mistaken for àbuse tests'. An accelerated test is only of value if the shelf

life limiting mechanism of deterioration under accelerated conditions is the

same as that under normal/ambient conditions, and the relationship between

changes under accelerated conditions and those under normal storage needs to

be confirmed and validated using food products of known quality.

An accelerated storage model that has enjoyed notable commercial success

and is widely used in the baking industry is called ERH CalcTM (Fig. 10.3). The

model is part of a computer-based `Cake Expert System' for the baking industry

originally developed by the UK Flour Milling and Baking Research Association

(now part of Campden BRI). ERH Calc allows users to run simulations on flour

confectionery formulations and rapidly calculate their theoretical equilibrium

relative humidities (ERHs) and estimate their mould-free shelf lives. The latter,

though, do not necessarily mean that the products themselves are organoleptically

acceptable.

Shelf life tests

As pointed out earlier, besides food safety, an acceptable shelf life is expected to

retain desired sensory, chemical, physical, functional or microbiological charac-

teristics of the product and which, where appropriate, should comply with any

label declaration of nutritional information throughout its shelf life. Thus, tests

employed to measure shelf life tend to be product-specific, reflecting the quality

characteristics of the product being studied. In a sense, the tests to be used are

informed by the knowledge and understanding of the ways the food product

deteriorates and spoils, including the mechanism of deterioration that is shelf life

limiting. A systematic and structured approach based on the HACCP principles



has been used to implement a control system designed to prevent rancidity in

confectionery and biscuits (Frampton, 1994). Essentially, this approach follows

the same principles of HACCP; here a hazard is taken to mean a micro-

biological, chemical or physical agent in, or condition of, the food with a

potential to cause it to deteriorate and spoil, terminating its shelf life. Applying

the principles systematically leads to the determination of the critical control

points at which control can be exercised and which are necessary to eliminate or

delay the shelf life limiting hazard, preventing it from ending the required shelf

life prematurely. Given the nature of the potential and possible hazards, the

following types of tests can be used individually or in combination to measure

the progress of shelf life:

· microbiological examination, including challenge testing

· chemical analysis

· physical/instrumental testing, measurement and analysis

· sensory evaluation.

Many shelf life studies together with the tests employed have been published in

both the primary and secondary literature. Table 10.6 gives some examples that

illustrate the specific tests used to measure shelf lives in light of the underlying

mechanisms of deterioration.

Fig. 10.3 Predicting mould-free shelf life of baked goods using ERH Calc (reproducedwith kind permission of Campden BRI).



Table 10.6 Examples of published shelf life studies and their tests

Product Storageconditions

Shelf life tests Approximateshelf life

Reference

Orange juice (11.2 ëBrix)reconstituted from concentrate,then exposed to thermosonicationand pulse electric fields

25 ëC Total bacterial countsConductivitySoluble solidspHColour attributes (tristimuluscolorimeter)on day 0, 14, 28, 84 and 168

168 days Walkling-Ribeiro et al.(2009)

Fresh Pacific salmon slices treatedwith salts of organic acids

1 ëC pHATP breakdown productsTotal volatile base nitrogen (TVB-N)and trimethylamine (TMA)Sensory analysis of cooked sliceson day 0, 3, 6, 9, 12 and 15

15 days Sallam (2007)

Fresh pork sausages packaged invarious modified atmospheres

2� 1 ëC Composition of gas mixturespH of meatColour instrumental measurement andmetmyoglobin percentageLipid oxidation analysisCounts of aerobic psychrotrophic floraSensory evaluationon day 0, 4, 8, 12, 16 and 20

20 days Martinez et al. (2006)

ßWoodhead

PublishingLim

ited,2011

Assigning shelf life

The main aim of a storage trial for shelf life is to find out as accurately as

possible, under specified storage conditions, the point in time at which the

product has become either unsafe or unacceptable to the target consumers, and if

the product meets its shelf life objectives. In terms of microbiological safety and

stability, the following should be useful in helping to fix an end-point for the

shelf life of the food being studied:

· Relevant food legislation, e.g. Commission Regulation (EC) 2073/2005 on

microbiological criteria for foodstuffs.

· Guidelines for the microbiological quality of some ready-to-eat foods

(Gilbert et al., 2000) given by enforcement authorities or agencies in support

of their work, e.g. those given by the UK Health Protection Agency

(previously the UK Public Health Laboratory Service).

· Guides on microbiological criteria for foods produced by independent food

research associations such as Campden BRI (Voysey, 2007).

· Current industrial best practice as published in the primary literature, which

suggests probiotic functional foods and drinks should contain at least 107 live

and active bacteria per g or ml for their functional claims to be maintained

over the shelf life period (Birollo et al., 2000).

· Predictive models, e.g. ComBase.

Non-microbiological criteria that are used to set shelf life tend to be relatively

more product-specific. In an ideal situation, these criteria are either contained in the

original marketing brief or can be developed from it. Crucially, the criteria, be they

physical, chemical or sensory, need to be correlated to the quality attributes that are

critical to product acceptability/consumer requirements, and hence quality (as

opposed to safe) shelf life and, where appropriate, they should be agreed between

the manufacturer and its customer. Once product safety has been established,

sensory evaluation is the most popular means by which the end of shelf life is

determined. A detailed treatment of sensory evaluation to study shelf life, either

using a trained panel, or a sample of consumers, is beyond the scope of this chapter.

10.3.4 A summary

Success in determining the shelf life of a food product depends on the following

factors:

· confidence in assuring food safety

· ability to define the critical quality characteristics that determine product

acceptability and meet customer requirements

· knowledge and understanding of the pertinent mechanisms of deterioration

and spoilage including the shelf life limiting mechanism

· adequate capability, either in-house or external, in terms of both technical

know-how and appropriate resources (skilled staff, testing facility etc.), to

measure shelf life either directly through storage trials or indirectly through

prediction and estimation, or both.



Storage trials for shelf life determination are controlled experiments, to which

the basic principles of experimental design that include use of control, random-

isation and replication apply. An estimate of shelf life without an indication of

its variability is of little value. A safe food product of acceptable quality that

consistently pleases its consumers has its origin in good product design that must

include carefully planned and professionally executed shelf life testing.

Replication of the storage trial experiment on sufficient food samples of agreed

and consistent quality is essential for the setting of reliable and reproducible

shelf life.

10.4 Future trends

In the past two decades, as a result of major research efforts in a number of

countries, notably the US, UK and Australia, coupled with ever-increasing

power of personal computers, the use of Internet-based predictive micro-

biological models as an aid to HACCP and microbiological risk assessment has

had a significant and positive impact on the management of microbiological

safety of foods. Food safety, which includes chemical and microbiological

safety, is of fundamental importance and will always remain so.

Recent research has focused on sensory shelf life in an effort to maximise

consumer acceptance and minimise food waste due to inaccurate shelf life or

shelf life that is too conservative. Apart from catastrophic circumstances, food

products do not usually fail all at once such that for a given product there is a

distribution of shelf lives over time, and concomitantly, an increasing

proportion of the consumers are expected to reject the product over the same

period. Realisation of this has led researchers to use survival analysis statistics

to study sensory shelf life of foods (Hough et al., 2003). Since then, Bayesian

methods and the Arrhenius equation have been used separately to study

sensory shelf life of foods and to analyse data based on consumers' acceptance

or rejection of samples stored at different times and different temperatures,

respectively (Luz Calle et al., 2006; Hough et al., 2006). The number of

consumers necessary for shelf life estimations based on survival analysis

statistics has also been determined in a simulation study that assumes a

Weibull distribution for the data model (Hough et al., 2007). Advantages of

applying survival analysis statistics to sensory shelf life estimations include

relatively simple sensory work with say 50±100 consumers and that the

estimations are based directly on consumer data. The disadvantages are that the

underlying mechanism of deterioration that limits shelf life will not be

provided by the consumer data if it is unknown, and specialised statistical

software and expertise are required for the calculations and interpretation of

the results (Hough, 2006).

Even more recent research has begun to look at the possibility of integrating

the modelling of safety and quality of foods, taking a complex systems approach

to estimating shelf life (Martins et al., 2008). In the meantime, storage trials for



estimating shelf life remains a cornerstone of the shelf life determination of

foods with which all responsible food businesses should be conversant.

10.5 References

ANON. (2003) Microbiological Challenge Testing. Comprehensive Reviews in Food

Science and Food Safety, vol. 2 (Supplement), 46±49, IFT, Chicago, IL.

ANON. (2010) Challenge testing protocols for assessing the safety and quality of food and

drink. Guideline No. 63, Campden BRI, Chipping Campden, UK.

BELL L N and LABUZA T P (1994) Aspartame stability in commercially sterilised flavoured

dairy beverages. Journal of Dairy Science, 77, 34±38.

BETTS G D, BROWN H M and EVERIS L K (EDS) (2004) Evaluation of Product Shelf-life for

Chilled Foods. Guideline No. 46, Campden and Chorleywood Food Research

Association, Chipping Campden, UK.

BIROLLO G A, REINHEIMER J A and VINDEROLA C G (2000) Viability of lactic acid microflora

in different types of yoghurt. Food Research International, 33, 799±805.

CFA (2010) Shelf life of ready to eat food in relation to L. monocytogenes ± Guidance for

food business operators, 1st edn. Chilled Food Association, Kettering, UK.

CRAWFORD C (1998) The New QUID Regulations. Chandos Publishing, Oxford.

ELLIS M J and MAN CMD (2000) The methodology of shelf-life determination. In: Shelf-life

Evaluation of Foods, 2nd edn. Man D and Jones A (eds). Aspen Publishers,

Gaithersburg, MD, pp. 23±49.

EUROPEAN COMMISSION (2004) Regulation (EC) No. 852/2004 of the European Parliament

and of the Council on the hygiene of foodstuffs. Official Journal of the European

Union, 25 June 2004, L 226/3± L226/21.

FRAMPTON A (1994) Prevention of rancidity in confectionery and biscuits ± a Hazard

Analysis Critical Control Point (HACCP) approach. In: Rancidity in Foods, 3rd

edn. Allen J C and Hamilton R J (eds). Blackie Academic & Professional, London,

pp. 161±178.

FSA (2005) General Guidance for Food Business Operators. EC Regulation No. 2073/

2005 on Microbiological Criteria for Foodstuffs. Food Standards Agency, UK

(www.food.gov.uk/).

FSA (2010) Food Standards Agency guidance on the application of date marks in food.

[Online]. Available from: http://www.food.gov.uk/consultations/consulteng/2010/

fsaguidanceappdatemarksfoodeng (accessed 1 April 2010).

GACULA M C (1975) The design of experiments for shelf-life study. Journal of Food

Science, 40, 399±403.

GILBERT R J, DE LOUVOIS J, DONOVAN T, LITTLE C, NYE K, BIBEIRO C D, RICHARDS J, ROBERTS D

and BOLTON F J (2000) Guidelines for the microbiological quality of some ready-to-

eat foods sampled at the point of sale. Communicable Disease and Public Health,

3(3), 163±167.

HOUGH G (2006) How does survival analysis help us in estimating the probability of a

consumer rejecting a stored product? In: Workshop summary: sensory shelf-life

testing. Food Quality and Preference, 17, 644±645.

HOUGH G, LANGOHR K, GOÂMEZ G and CURIA A (2003) Survival analysis applied to sensory

shelf life of foods. Journal of Food Science, 68, 359±362.

HOUGH G, GARITTA L and GOÂMEZ G (2006) Sensory shelf-life predictions by survival



analysis accelerated storage models. Food Quality and Preference, 17, 468±473.

HOUGH G, LUZ CALLE M, SERRAT C and CURIA A (2007) Number of consumers necessary for

shelf life estimations based on survival analysis statistics. Food Quality and

Preference, 18, 771±775.

HUIS IN'T VELD J H J (1996) Microbial and biochemical spoilage of foods: an overview.

International Journal of Food Microbiology, 33, 1±18.

IFST (1993) Shelf life of Foods ± Guidelines for its Determination and Prediction. Institute

of Food Science & Technology, London.

JONES A A (2000) Ambient-stable sauces and pickles. In: Shelf-life Evaluation of Foods,

2nd edn. Man D and Jones A (eds). Aspen Publishers, Gaithersburg, MD, pp. 211±

226.

KILCAST D and SUBRAMANIAM P (2000) Introduction. In: The Stability and Shelf-life of

Food, Kilcast D and Subramaniam P (eds). Woodhead Publishing, Cambridge, pp.

1±19.

LABUZA T P, FU B and TAOUKIS P S (1992) Prediction for shelf-life and safety of minimally

processed CAP/MAP chilled foods. Journal of Food Protection, 55, 741±750.

LEE S-Y, GUINARD J-X and KROCHTA J M (2003) Relating sensory and instrumental data to

conduct an accelerated shelf-life testing of whey-protein-coated peanuts. In:

Freshness and Shelf-life of Foods. Cadwallader K and Weenen H (eds). American

Chemical Society, Washington, DC, pp. 175±187.

LUZ CALLE M, HOUGH G, CURIA A and GOÂMEZ G (2006) Bayesian survival analysis

modelling applied to sensory shelf life of foods. Food Quality and Preference, 17,

307±312.

MAN C M D (2002) Shelf Life. Food Industry Briefing Series, Blackwell Science, Oxford.

MAN C M D (2004) Shelf-life testing. In: Understanding and Measuring the Shelf-life of

Food. Steele, R (ed.). Woodhead Publishing, Cambridge, pp. 340±356.

MARTINS R C, LOPES V V, VICENTE A A and TEIXEIRA J A (2008) Computational shelf-life

dating: complex systems approaches to food quality and safety. Food Bioprocess

Technol., 1, 207±222.

MARTINEZ L, DJENANE D, CILLA I, BELTRAÂ N J A and RONCALEÂS P (2006) Effect of varying

oxygen concentrations on the shelf-life of fresh pork sausages packaged in

modified atmosphere. Food Chemistry, 94, 219±225.

MIZRAHI S (2000) Accelerated shelf-life tests. In: The Stability and Shelf-life of Food,

Kilcast D and Subramaniam P (eds). Woodhead Publishing, Cambridge, pp. 107±

128.

MOSSEL D A A (1971) Physiological and metabolic attributes of microbial groups asso-

ciated with foods. Journal of Applied Bacteriology, 34, 95±118.

NOTERMANS S, IN'T VELD P, WIJTZES T and MEAD G C (1993) A user's guide to microbial

challenge testing for ensuring the safety and stability of food products. Food

Microbiology, 10, 145±157.

PETERSEN M A, TéNDER D and POLL L (1998) Comparison of normal and accelerated

storage of commercial orange juice ± changes in flavour and content of volatile

compounds. Food Quality and Preference, 9 (1/2), 43±51.

SALLAM K I (2007) Chemical, sensory and shelf life evaluation of sliced salmon treated

with salts of organic acids. Food Chemistry, 101, 592±600.

TSO (2006) The Food Hygiene (England) Regulations (SI 2006/14), The Stationary

Office, London.

UYTTENDAELE M, RAJKOVIC A, BENOS G, FRANCËOIS K, DEVLIEGHERE F and DEBEVERE J (2004)

Evaluation of a challenge testing protocol to assess the stability of ready-to-eat



cooked meat products against growth of Listeria monocytogenes. International

Journal of Food Microbiology, 90, 219±236.

VOYSEY P A (2007) Establishment and Use of Microbiological Criteria (Standards,

Specifications and Guidelines) for Foods. Guideline No. 52, Campden and

Chorleywood Food Research Association, Chipping Campden, UK.

WALKLING-RIBEIRO M, NOCI F, CRONIN D A, LYNG J G and MORGAN D J (2009) Shelf life and

sensory evaluation of orange juice after exposure to thermosonication and pulsed

electric fields. Food and Bioproducts Processing, 87, 102±107.



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