Shelf-life of chilled foods

19 Shelf-life of chilled foods C. M. D. Man, London South Bank University, UK

19.1 Introduction All foods deteriorate during storage due to their perishable nature. Chilled foods in particular have relatively short shelf-lives, even when stored under refrigerated conditions. In the UK, chilled foods include a very wide variety of processed and prepared food and drinks ranging from raw prepared meats and fish and similar meal components, prepared fruit and vegetables, ready-to-eat salads, ready-to-eat dairy products, delicatessen meats, yellow fats and fat spreads, heat-and-eat snacks such as pizzas and savoury pastry products, ready meals, chilled soups and sauces, fruit juices and smoothies, to complete meals prepared for cooking (Anon., 2005). Until recently, the legal equivalent of the ‘shelf-life of food’ in the UK (and indeed within the EU) was the appropriate ‘minimum durability’ required by the Food Labelling Regulations 1996 (SI, 1996/1499), which have been amended by a number of subsequent regulations. This particular food labelling requirement applies to all food which is ready for delivery to the ultimate consumer or to a catering establishment, subject to certain exceptions. The requirement is fulfilled by a food business operator through the correct use of one of the following:

• In the case of a food which, from the microbiological point of view, is highly •

perishable and in consequence likely after a short period to constitute an immediate danger to health, a ‘use by’ date. In the case of a food other than one specified above, an indication of minimum durability, a ‘best before’ date.

In addition, the ‘best before’ date and the ‘use by’ date must be followed by any

574

Chilled foods

special storage conditions which need to be observed, such as ‘keep refrigerated at 0 °C to +5 °C’ or ‘keep in a cool, dry place’. Generally, storage conditions are important because in the EU’s original food labelling Directive 79/112/EEC (EEC, 1979), which the Food Labelling Regulations (HMSO, 1996) implement, the date of minimum durability is defined as the date until which the foodstuff retains its specific properties when properly stored. No food manufacturer can reasonably be held responsible for its food that does not live up to its declared shelf-life if it has been stored under conditions for which it is not intended. Recently, the term ‘shelflife’ has appeared in EU/UK food law (European Commission, 2005). The Commission Regulation (EC) 2073/2005 on microbiological criteria for foodstuffs, which came into force in the UK in January 2006, defines ‘shelf-life’ in Article 2 as either the period corresponding to the period preceding the ‘use by’ or the minimum durability date, as defined respectively in Articles 9 and 10 of Directive 2000/13/EC, the most recent food labelling Directive. Therefore, all food business operators have a legal obligation to determine, assign and maintain shelf-lives of their products, as this makes legal as well as commercial sense. What is less obvious from the labelling regulations is which foods should carry a ‘use by’ and which foods, a ‘best before’ date. In fact, it is the food manufacturer and food processor’s responsibility to decide which minimum durability indication is appropriate, either a ‘use by’ or a ‘best before’ date; after all, they are supposed to know their own products. Useful guidance, however, is available and it has been suggested that the following food groups, essentially all chilled foods, are likely to require a ‘use 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 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.

A comprehensive and perhaps more useful definition of the shelf-life of food has also been available for some time (IFST, 1993): It is the period of time during which the food product will (i) (ii)

remain safe; be certain to retain its desired sensory, chemical, physical, microbiological and functional characteristics; (iii) 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 food shelf-life, and safety must always take priority over quality, as food safety is both a fundamental and

Shelf-life of chilled foods

575

legal requirement. The aim of this chapter is to review these important aspects of shelf-life as they apply to chilled foods.

19.2 Safety of chilled foods In the UK, the Food Safety Act (HMSO, 1990) prohibits the sale of food that:

• • • • •

has been rendered injurious to health is unfit is so contaminated it would be unreasonable to expect it to be eaten is not of the nature or substance or quality demanded is falsely or misleadingly labelled.

It is well known that food safety hazards can be microbiological, chemical or physical in nature. While physical hazards like foreign matters that are sharp or can choke, and chemical hazards such as illegal food additives or agrochemical residues, pose serious food safety risks to all food categories, many of the known microbiological hazards that may be found in chilled foods tend to be specific to them as the prevailing environmental conditions select for the types of organisms that can survive and grow at refrigerated temperatures. Table 19.1 lists various pathogenic micro-organisms which may be associated with chilled foods, and with which chilled food manufacturers should be conversant. Today the most effective way to assure the safety of food, and to control all major types of food safety hazards, 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. These principles consist of the following (European Commission, 2004): (i) (ii)

(iii)

(iv) (v) (vi) (vii)

identifying any hazards that must be prevented, eliminated or reduced to acceptable levels; 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; establishing critical limits at CCPs which separate acceptability from unacceptability for the prevention, elimination or reduction of identified hazards; establishing and implementing effective monitoring procedures at CCPs; establishing corrective actions when monitoring indicates that a CCP is not under control; establishing procedures, which shall be carried out regularly, to verify that the measures outlined in (i) to (v) above are working effectively; and establishing documents and records commensurate with the nature and size of the food business to demonstrate the effective application of the measures outlined in (i) to (vi) above.

576

Chilled foods

Table 19.1 Minimum growth conditions for micro-organisms which may be associated with chilled foods (Betts et al., 2004; Voysey, 2007) Micro-organism

Minimum growth Minimum temp. (°C) pH for growth

Salmonella Staphylococcus aureus Bacillus cereus (spores/heat resistant) Clostridium botulinum (proteolytic A, B, F) Clostridium botulinum (non-proteolytic B, E, F) Listeria monocytogenes Escherichia coli Escherichia coli O157:H7 Clostridium perfringens Vibrio cholerae Vibrio parahaemolyticus Yersinia enterocolitica Aeromonas hydrophila

Minimum aw for growth

Anaerobic growth

4* 5.2* (10 for toxin) 4

3.8 4.0 (4.5 for toxin) 4.9

0.92 – 0.95 Yes 0.83 Yes (0.9 for toxin) 0.93 – 0.95 Yes

10

4.6

0.94

Yes

3* –0.4 7–8 6.5 12 10 5 –1.3 –0.1

4.7* 4.3 4.4 4.5 4.5 5 4.9 4.4 < 4.5

0.97 0.92 0.95 0.935 0.93 – 0.95 0.97 0.94 0.96 0.97

Yes Yes Yes Yes Yes Yes Yes Yes Yes

*when more than one figure is available, the lower one is given.

Earlier, Article 4 of the Regulation requires food business operators to adopt as appropriate a number of specific hygiene measures, which include compliance with microbiological criteria for foodstuffs, compliance with temperature control requirements for foodstuffs, maintenance of the cold chains, and sampling and analysis, all of which are applicable to chilled foods. Besides defining ‘shelf-life’ for the first time, Commission Regulation (EC) No. 2073/2005 on microbiological criteria for foodstuffs also 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. Food safety criteria given in the Regulation should therefore be used to assess the safety of a food product or batch of products within the framework of an effective HACCP system. These criteria will assist chilled food manufacturers with validating and verifying their food safety management systems while they are being developed and implemented. Some of the microbiological (food safety) criteria set out in Annex I of the Regulation which are relevant to chilled foods are given in Table 19.2. When the results of testing against these criteria are unsatisfactory, the food business operators are required to take the measures laid down in Article 7 of the Regulation, which include withdrawing unsafe food from the market, and maybe product recall. Food products which have been found to be unsafe, or whose safety has been called into question, effectively have lost their shelf-life. The importance of food safety in relation to an acceptable and meaningful shelflife cannot be overemphasised.

Table 19.2 Some food safety criteria that are relevant to chilled foods, and applicable to products placed on the market during their shelf life (taken from Regulation (EC) No 2073/2005 – Annex 1, Chapter 1 and CFA, 2005) Sampling plan+ Limits n C m M

Criterion Micro-organism and food category

Examples of chilled foods

1.1

Listeria monocytogenes Ready-to-eat foods intended for infants and ready-to-eat foods for special medical purposes

Ready-to-eat baby foods 10 Ready-to-eat foods intended for infants less than 12 months old Ready-to-eat dietary food for special medical purposes for infants less than 6 months old

0

Absence in 25 g

EN/ISO 11290-1

1.2

Listeria monocytogenes Ready-to-eat foods able to support the growth of L. monocytogenes, other than those in 1.1

Chilled ready-to-eat products with more than 5 days’ life Pre-packed delicatessen products Pre-packed sliced cooked meat Smoked salmon Pate Soft cheese

5

0

100 cfu/g

EN/ISO 11290-2

5

0

*Absence in 25 g EN/ISO 11290-1

Yoghurt Hard cheese Products with a pH less than 4.4, e.g. coleslaw Products with shelf-life less than 5 days, e.g. sandwiches

5

0

100 cfu/g

Listeria monocytogenes Ready-to-eat foods unable to support the growth of L. monocytogenes, other than those in 1.1

EN/ISO 11290-2

Shelf-life of chilled foods

1.3

Analytical reference method

577

578

Table 19.2 continued Examples of chilled foods

1.4

Salmonella Minced meat and meat preparations intended to be eaten raw

Steak tartare

5

0

Absence in 25 g

EN/ISO 6579

1.8

Salmonella Meat products intended to be eaten raw, excluding products where the manufacturing process or the composition of the product will eliminate the salmonella risk

Salami Parma ham Cold smoked duck

5

0

Absence in 25 g

EN/ISO 6579

1.11

Salmonella Cheeses, butter and cream made from raw milk or milk that has undergone a lower heat treatment than pasteurisation

Roquefort, Brie de Meaux

5

0

Absence in 25 g

EN/ISO 6579

1.15

Salmonella Ready-to-eat foods containing raw egg, excluding products where the manufacturing process or the composition of the product will eliminate the salmonella risk

Mayonnaise and meringues made with unpasteurised egg

5

0

Absence in 25 g

EN/ISO 6579

1.16

Salmonella Cooked crustaceans and molluscan shellfish

Mussels, prawns, shrimp, lobster, crab 5

0

Absence in 25 g

EN/ISO 6579

Analytical reference method

Chilled foods

Sampling plan+ Limits n C m M

Criterion Micro-organism and food category

Salmonella Sprouted seeds (ready-to-eat)

Cress, salad/ready-to-eat bean sprouts 5

0

Absence in 25 g

EN/ISO 6579

1.19

Salmonella Pre-cut fruit and vegetables (ready-to-eat)

Ready-to-eat undressed vegetable 5 salads, green salads, mixed cut lettuce salads; prepared ready-to-eat mixed fruit salads, exotic fruit salads, prepared ready-to-eat fruit

0

Absence in 25 g

EN/ISO 6579

1.20

Salmonella Unpasteurised fruit and vegetable juices (ready-to-eat)

Freshly squeezed unpasteurised fruit juices, mixed fruit juices; smoothies; vegetable juices

5

0

Absence in 25 g

EN/ISO 6579

1.21

Staphylococcal enterotoxins Cheeses, milk powder and whey powder, as referred to in the coagulase-positive staphylococci criteria in Chapter 2.2 of Annex I of the Regulation

Cheeses, excluding processed cheese and non-fermented cheese

5

0

Not detected in 25 g

European screening method of the CRL for milk

1.24

E. coli Live bivalve molluscs and live echinoderms, tunicates and gastropods

Oysters, clams, sea urchins, winkles and welks

1

0

230 MPN/100 g ISO TS 16649-3 of flesh and intravalvular liquid

1.25

Histamine Fishery products from fish species associated with a high amount of histine

Tuna, mackerel, sardines, mahi

9

2

100 mg/kg

*applies to food before it has left the immediate control of the food business operator who has produced it. + all, with the exception of that for histamine, are essentially two-class attribute sampling plans.

200 mg/kg HPLC

Shelf-life of chilled foods

1.18

579

580

Chilled foods

A process hygiene criterion on the other hand is a criterion that indicates the acceptable functioning of the production process. It is not applicable to products placed on the market, and should not be used to assess the safety of a food product or batch of products. It sets an indicative contamination value, again within the framework of an effective HACCP system, above which corrective actions are required in order to maintain the hygiene of the process in compliance with food law. For example, the Regulation stipulates the enumeration of E. coli as an indicator for the level of hygiene in the manufacture of ready-to-eat pre-cut fruit and vegetables, and requires improvements in production hygiene and selection of raw materials should the limit of 1000 cfu/g (M) be exceeded (sampling plan: n = 5, c = 2).

19.3 Product factors affecting shelf-life In order to determine and assign a shelf-life that is acceptable to themselves and their customers, chilled food manufacturers must have a satisfactory understanding of how their products deteriorate and spoil during storage, as well as an adequate knowledge of the main factors that influence these changes. In principle, and in practice, a food can deteriorate microbiologically, chemically, biochemically, physically and organoleptically. Indeed, all these deteriorative changes can take place at the same time, albeit at different rates. In the end, it is the most damaging change, be it microbiological or organoleptical, which will ultimately decide how and when a chilled food becomes unacceptable, because of either safety or quality issues. Knowing the relevant mechanisms and kinetics of food deterioration and spoilage, and the important factors that influence them, is a prerequisite to a successful determination and assurance of the shelf-life of food. Over the years, much knowledge about food deterioration and spoilage has been accumulated, and a number of well-known mechanisms can be used to explain the loss of shelf-life in many foods (Man, 2004):

• moisture and/or vapour transfer leading to gain or loss • physical transfer of substances such as oxygen, odours or flavours other than moisture and/or water vapour

• light-induced changes, i.e. changes caused and/or initiated by exposure to daylight or artificial light

• chemical or biochemical changes • microbiological changes • other mechanisms or changes such as a loss of an important functional property. Table 19.3 shows some examples of food deterioration and spoilage applicable to chilled foods. By definition, chilled foods have the shortest shelf-lives of processed foods, and consumers do generally perceive them as ‘fresh’. Nevertheless, knowing the factors that can influence the main deteriorative and spoilage mechanism in a chilled food does help the manufacturer in preventing premature product failure, in mitigating the adverse effects of the change, and in assigning a realistic

Shelf-life of chilled foods Table 19.3

581

Some examples of food deterioration and spoilage in chilled foods

Product

Quality description

Cut lettuce, apple Discolouration (brown cut edges) Waldorf salad Discolouration of mayonnaise Cut vegetables Bioyoghurt Sandwiches

Limpness (loss of turgidity) Loss of function (bioactivity) Bread staleness

Fresh meat Bad/objectionable odour Freshly squeezed Separation, bad/objectionable orange juice odour and taste

Mechanism Enzymatic browning Transfer of brown colour from walnut into mayonnaise Moisture/water vapour loss Death of bioactive culture Moisture re-distribution (starch retrogradation) Microbial spoilage Microbial spoilage (yeast fermentation)

and acceptable shelf-life. Many different factors are known to affect product shelflife and they are briefly described below.

19.3.1 Raw materials In general, the quality of a finished product is largely a reflection of the quality of its raw materials, and chilled foods are no exception. Not all the quality characteristics and parameters of a raw material will have an impact on product shelf-life but those that do will need to be identified and controlled, and their effect on shelf-life established. Produce offers a good example: some produce, e.g. cabbage and apples, which are used in many chilled foods such as coleslaw and fruit pies respectively, are often laid down in controlled atmosphere storage so as to maintain a continuity of supply outside their seasons. Freshly harvested cabbage tends to have a low yeast count, whereas cabbage from cold storage has a higher yeast count (Betts and Everis, 2000). Use of the latter results in coleslaw with a markedly reduced shelf-life for parts of the year, owing to the higher starting levels of yeast introduced via the raw material.

19.3.2 Product composition and formulation The composition and formulation of a food product can be a most important shelflife determining factor in many chilled foods. Butter and margarine are water-in-oil emulsions, and by law have a high fat content (minimum 80%) that limits the growth of most micro-organisms, including pathogens and spoilage organisms (Delamarre and Batt, 1999). Although rare today, they are still prone to oxidation. The development of fat spreads with a reduced fat content, ranging from 59% through 38% (light), to 23% (diet), has seen a shift from oxidation as a potentially predominant deterioration mechanism to others, such as emulsion instability and microbiological spoilage. Consequently, emulsifiers (e.g. mono- and diglycerides and lecithin) and preservatives (e.g. potassium sorbate) are normally added to increase the shelf-lives of these products. In products where there is a real risk of

582

Chilled foods

oxidation, vitamin E is usually added in preference to the less consumer-friendly artificial anti-oxidants. 19.3.3 Food structure Many solid and semisolid food products (e.g. sausages, mayonnaise, margarine) do not have a truly homogeneous and uniform structure. As a result, the chemical and physical conditions relevant to microbial growth and/or chemical and biochemical changes can vary with position in the food microstructure. Electron microscopy studies (Katsaras and Leistner, 1991) have revealed that the natural flora and the added starter cultures in fermented sausages are not evenly distributed, but are arrested in tiny cavities of the product; the ripening flora only grow in nests. These nests are 100–5000 µm apart, and large volumes of the sausage must be influenced by enzymes and metabolites (e.g. nitrate reductase, catalase (enzymes), lactic acid, bacteriocins (metabolites)) accumulated in such nests or cavities. The distribution and activity of these microbes in the microstructure of the sausage have a major influence on the ripening process, and hence the microbiological safety and quality of the product. Indeed, food structure is a most likely reason why in some foods real-life observations differ significantly from predictions obtained from predictive microbiological models based in liquid culture (Dens and Van Impe, 2001; Wilson et al., 2002). 19.3.4 Product assembly Product assembly or make-up may be viewed as the macrostructure of the food product. In complex and multi-component products, contact between components often results in migration of moisture, enzymes, colours, flavourings or oil from one component to another, as long as a concentration gradient exists for the substance concerned. When this happens, shelf-life can be affected if it leads to significant deterioration in quality. For instance, in fruit pies, migration of moisture from the filling to the pastry leads to a gradual loss of the desired texture, unless an effective barrier exists between the two components. And in multilayered trifles, the visual appearance is impaired by the migration of colouring components from one layer to another, in particular the sponge layer. Similarly, combining components of different microbiological status such as diced cheese or ham and coleslaw can result in a shorter shelf-life for the end product. The inclusion of a protein component in the form of diced cheese or ham creates additional interfaces with the mayonnaise and the raw vegetables in coleslaw, which allow the migration and re-distribution of acetic acid, enzymes, colour, moisture and so on, thus limiting the shelf-life of the product (Brocklehurst, 1994).

19.4 Intrinsic product properties affecting shelf-life 19.4.1 pH The pH value of a food product varies according to its composition, formulation

Shelf-life of chilled foods

583

and processing (e.g. fermentation processes), and it needs to be controlled where acidity has a major influence on the safety and quality of the product. The pH of a system is related to the concentration of hydrogen ions which, in the case of food, come from ‘acid’ ingredients, either indigenous or added, which dissociate in water, releasing them in the process. It is well known that the growth of microorganisms becomes more inhibited as the pH of the environment in which they live decreases. Minimum pH values for growth for some pathogenic micro-organisms that may be associated with chilled foods are given in Table 19.1. The inside of a bacterial cell is at a pH close to neutral and needs to be maintained at this level for the organism to grow, develop and reproduce. In acidic and therefore hostile environments, micro-organisms have to use an extra amount of energy to maintain their internal state of homeostasis and, in consequence, their growth is slowed or prevented where energy sources are limited; they may not necessarily die, although they may be injured by intracellular acidification, particularly at chill temperatures. Certain organic and inorganic acid ingredients (Table 19.4) have specific antimicrobial effects of their own, besides their pH-lowering property. In the case of the organic acids, their preserving effect is attributed to their undissociated forms, which can freely enter the cell and thereby reduce its internal pH. Charged molecules from the dissociated forms (e.g. protons and anions) are unable to cross the bacterial membranes and enter the cell. The antimicrobial effectiveness of weak organic acids generally increases in the order acetic, propionic, sorbic and benzoic, and is dependent on the concentration of undissociated acid. The proportion of the acid that is in the undissociated form is determined by its dissociation constant, the pKa, the pH value at which half of the acid is in its undissociated form, and the pH of the food. Therefore, the effectiveness of weak organic acids as preservatives is increased in acidic foods and decreased in foods with neutral or alkaline pH values. pH also affects many chemical and biochemical changes in food, such as enzymatic and non-enzymatic browning, degradation of aspartame, and the shade of some colours. These, in turn, can have an impact on shelf-life.

19.4.2 Water activity Water activity is a term used to describe the amount of free or unbound water within a system that is available and can be used by micro-organisms. The level of available water can be lowered by physical means such as dehydration or concentration, but more usually in chilled foods, by the addition of solutes known commonly as humectants such as salt and sugar, which form chemical bonds with water and prevent it from being used. Micro-organisms, like all living organisms, require water to survive. Any reduction in water activity in their environment will cause micro-organisms to dehydrate and reduce the diffusion of metabolites to and from cells, and will ultimately inhibit or prevent their growth. Water activity (aw) is defined as the ratio of the partial pressure of water in the atmosphere in equilibrium with the substrate (e.g. a food) to that of the atmosphere in equilibrium

584

Chilled foods

Table 19.4

Common organic and inorganic antimicrobial acids (Gould, 1996)

Acids

Examples of foods in which used

Weak organic acid and ester preservatives Propionate Sorbate Benzoate Benzoate esters (parabens)

Bread, cakes, cheeses, grain Cheeses, syrups, cakes, dressings Pickles, soft drinks, dressings Marinated fish products

Organic acid acidulants Lactic, citric, malic, acetic, etc.

Low pH sauces, mayonnaise, dressings, salads, drinks, yoghurts, fruit juices and concentrates

Inorganic acid preservatives Sulphite Nitrite

Fruit pieces, dried fruit, wine, meat sausages Cured meat products

Mineral acid acidulants Phosphoric, hydrochloric

Soft drinks

with pure water at the same temperature, and is expressed on a scale of 0 to 1 where 1 is for pure water. There is no reported microbial growth below an aw of 0.6 and most bacteria (except Staphylococcus aureus) do not generally grow below an aw of 0.9. The minimum aw for growth for some pathogenic micro-organisms which may be associated with chilled foods are given in Table 19.1. In general, the water activity of a product can be considered in three main categories (Betts et al., 2004):

• Low aw (< 0.85). The significant spoilage organisms are yeasts and moulds; most pathogenic organisms should not grow at these aw levels.

• Intermediate aw (0.85–0.92). Within this range, aerobic spore-forming bacte-

•

ria, e.g. Bacillus licheniformis, may be able to grow, in addition to yeasts and moulds. Most pathogenic bacteria should be inhibited, with the exception of Staphylococcus aureus, which may be able to grow and produce toxin at aw close to or lower than 0.90. Temperature control below 8 °C should be sufficient to prevent this from occurring. High aw (> 0.92). Most pathogenic bacteria should be able to grow at this aw and above such that it is no longer a major antimicrobial factor in its own right.

Besides microbial growth, water activity values have been widely used to indicate the stability of foods with respect to the potential for chemical and biochemical changes, and physical transfer such as moisture migration. The latter is particularly relevant to multi-component or layered chilled foods.

19.4.3 Redox potential (Eh) Oxidation-reduction (redox) reactions involve a transfer of electrons between atoms or molecules, and as an atom or compound loses electrons, it is oxidised.

Shelf-life of chilled foods

585

Oxidation of course takes places when a compound reacts with oxygen. The tendency of a medium to accept or surrender electrons, to oxidise or to reduce, is termed its redox potential (Eh). The redox potential that can be measured in food is the result of several factors (Adams and Moss, 2000):

• • • • • •

redox couples present ratio of oxidant and reductant pH poising capacity, i.e. the capacity to resist a change in the food’s redox potential availability of oxygen microbial activity.

An oxidising or reducing environment results, depending on the overall balance in the redox potential of the food. Also, as redox potential is a function of oxygen availability, it is closely linked with the extrinsic factor of storage atmosphere; the exclusion of air in vacuum packaging or canning reduces the Eh. In general, microbial growth in a food reduces its Eh, which exerts an important selective effect on the microflora of a food such that, for instance, aerobes, e.g. Bacillus spp., are capable of growth at full oxygen tensions, and obligate anaerobes, e.g. Clostridium perfringens, cannot survive in the presence of oxygen. Leistner, who pioneered the concept of ‘hurdle technology’, identified redox potential, besides competitive flora, as one of the major preservation ‘hurdles’ in assuring the safety and consistent quality of fermented (raw) meat sausages (Leistner, 2000).

19.5 Extrinsic factors affecting shelf-life 19.5.1 Hygiene Good hygiene, an integral part of good manufacturing practice (GMP), is fundamental to the manufacture of safe and wholesome food products. Hygienic design and operation requirements distinguish food from non-food manufacture, and can be viewed as part of the prerequisite programmes, fundamental to the effective application of HACCP principles. Poor hygiene leads to contamination, which may be physical, chemical or microbiological in nature, and which can have a major impact on the safety and quality of foods. Microbiologically, poor hygiene control may result in high levels of unwanted micro-organisms being introduced, which may have adverse effects on the safety and stability of the product and hence its shelf-life. Research has shown that poor slicing hygiene caused the spoilage (and reduced shelf-life) of some chilled vacuum-packed cured meats (Holley, 1997) and that the in-house flora had a definite impact on the microbiological quality and shelf-life of cold-smoked salmon (Hansen et al., 1998). While the Commission Regulation (EC) No. 2073/2005 on microbiological criteria for foodstuffs does not impose a general requirement for increased end product microbiological testing or positive release, it does highlight and re-enforce the following basic hygiene requirements:

586

Chilled foods

• monitoring of processing areas and equipment for Listeria monocytogenes in •

•

the manufacture of ready-to-eat foods, which may pose a Listeria monocytogenes risk for public health (Article 5); monitoring of processing areas and equipment for Enterobacteriaceae in the manufacture of dried infant formulae or dried foods for special medical purposes intended for infants below 6 months which pose an Enterobacter sakazakii risk (Article 5); analysing trends in the test results (against process hygiene criteria), and taking appropriate actions without undue delay to remedy the situation in order to prevent the occurrence of microbiological risks when a trend towards unsatisfactory results is observed (Article 9).

19.5.2 Processing Processing covers a wide range of operations, which may be applied to food for a variety of purposes. It can exert a considerable effect on the microflora, physical, chemical, biochemical, nutritional and sensory properties of a food product, and hence its shelf-life. Milk is a classic example. Fresh milk that has been pasteurised usually has a few days’ shelf-life at refrigeration temperature. The same material that has been pasteurised and undergone microfiltration can last for a few weeks at chill temperature. And, if it has been given an ultra-high temperature (UHT) treatment and packaged aseptically, it can have a shelf-life of months under ambient condition. Whatever the type of processing, it must not be considered in isolation. This is particularly true with chilled foods as the processing employed (e.g. pasteurisation) is unlikely to provide adequate preservation on its own, given the current trend towards mild processing technology. As heat treatment is widely used in food manufacture and processing, it is worth summarising the three main categories of heat treatment used to stabilise foods (Betts, 2006; CCFRA, 2006):

• Mild pasteurisation to inactivate vegetative micro-organisms. Typically, this is

•

•

a process of 70 °C for 2 minutes or equivalent (z value of 7.5C°). This is primarily aimed at achieving a 6 log reduction in Listeria monocytogenes and other vegetative pathogens; it is also sufficient to inactivate most Enterobacteriaceae, Pseudomonas and yeasts that could spoil chilled foods. Severe pasteurisation to inactivate psychrotrophic or acid-tolerant spore-formers. Typically, this is a process of 90 °C for 10 minutes or equivalent (z value of 9C°) that may be given to chilled food products that are vacuum packed or modified atmosphere packed and have a shelf-life of greater than 10 days. The process is designed to achieve a 6 log reduction of psychrotrophic strains of Clostridium botulinum; it will also inactivate vegetative spoilage organisms. A similar process of 95° C for 5 minutes or 95° C for 10 minutes or equivalent (z value of 8.3C°) may be given to acidic ambient stable products, designed to inactivate acid-tolerant spore-formers that could grow and spoil the product if present after heat treatment. Sterilisation to achieve commercial sterility in low-acid canned foods. This well-known heat treatment equivalent to 121.1 °C for at least 3 minutes (Fo3)

Shelf-life of chilled foods

587

based on a z value of 10C° is given to low-acid canned foods to achieve a 12 log reduction of mesophilic Clostridium botulinum. The process will also inactivate all vegetative micro-organisms and the majority of spore-forming organisms capable of causing spoilage in temperate zones. Chilled foods are not usually given such a harsh treatment, which could result in inferior organoleptic quality.

19.5.3 Packaging materials and systems Packaging and packaging systems are integral parts of modern food processing and preservation. Packaging materials protect and preserve by virtue of their many different properties. Primary packaging protects food against physical damage and in many cases attack by pests, and prevents contamination during transport, storage and distribution. Suitable packaging materials offer a barrier against light, gaseous exchange and/or moisture vapour transfer, protecting the food from many of the deteriorative changes that can be shelf-life limiting. In many cases, packaging has become part of a food preservation system such as in aseptic processing and packaging, and modified atmosphere packaging (MAP). Modified atmosphere packaging extends the shelf-life of fresh red meat where the bright red colour of the meat is maintained by a high level of oxygen, e.g. 70%; excluding oxygen from airpacked chilled foods such as fresh pasta products, many of which also have a reduced aw, delays spoilage by Pseudomonas spp. and extends their shelf-lives significantly. The safety, with respect to Clostridium botulinum, of chilled foods that have been mildly pasteurised in hermetically sealed packages or heated and packed without recontamination (i.e. sous vide foods) has been evaluated by a European Chilled Food Federation ‘Botulinum Working Party’. It was concluded that for such foods, safety can be assured by a minimum heat process and strict limitation of chill shelf-life (less than about 5 days) or, for longer life products (more than about 5 days), by storage below 3 °C, by heat treatment sufficient to deliver a 6 log reduction in numbers of spores of psychrotrophic strains of Clostridium botulinum and storage below 10 °C, or by intrinsic preservation factors shown to be effective in modelling or inoculated pack/challenge tests (Gould, 1999).

19.5.4 Storage, distribution and retail display Conditions, which include temperature, humidity and lighting, experienced by chilled foods during their storage, distribution and retail display can greatly influence their shelf-lives. The current temperature control regulations in the UK stipulate a maximum of 8 °C during distribution and retail display of chilled foods. Compliance with temperature control requirements for foodstuffs, and maintenance of the cold chains, are two of the specific legal requirements contained in the EU Regulation (EC) No 852/2004 on the hygiene of foodstuffs. For chilled foods, many of which are high-risk products, controlled storage at the correct temperature and maintenance of the cold chain are therefore essential in the assurance of their microbiological safety and stability. It has been shown that temperature abuse

588

Chilled foods

during storage could initiate yeast growth in fruit yoghurt resulting in excessive gas formation, off flavours and discolouration (Viljoen et al., 2003) whereas under proper storage conditions, this product can be expected to last up to 30 days at chill temperatures.

19.5.5 Consumer handling and use Consumer handling and use, over which the manufacturers often have little control apart from the provision of clear storage and/or use instructions on the label, can be an important shelf-life influencing factor. The little information that is available about chilled foods in the UK (Evans et al., 1991; Evans, 1998) seems to suggest that shopping and carrying-home patterns, as well as their duration and conditions, are very variable. Furthermore, in a separate piece of research, some consumers were found to use potentially unsafe practices such as transporting and storing chilled foods at the wrong temperature, holding cooked food at ambient temperature for prolonged periods, and using inadequate re-heating (Worsfold and Griffith, 1997). In the long run, widespread and sustained education of the consumers is probably the only effective remedy, which in the UK appears to have been taken seriously by the Food Standards Agency (see www.food.gov.uk/).

19.6 Interaction between intrinsic factors and extrinsic product factors In recent years, trends towards the use of fewer preservatives, lower levels of salt and, in general, milder forms of heat treatment have meant that many chilled food products do not rely exclusively on a single factor for preservation but instead depend on the interaction of a number of preservation factors such as salt, pasteurisation, chill temperature and so on, working collectively and synergistically to assure microbiological safety and stability, and give the product its character. This approach to food preservation, as mentioned earlier, is referred to as hurdle technology (Leistner, 2000). In practice, it is therefore important that all relevant hurdles are correct and preservation factors meet their target levels consistently in production. If any of the hurdles is incorrect or the target levels not met, then food safety and quality may be compromised. Thus, all should be considered in the HACCP plan, and if necessary, controlled by appropriate CCPs.

19.7 Determining product shelf-life The most common and direct way of determining shelf-life is to conduct storage trials of the product in question under conditions that mimic those it is likely to experience during storage, distribution, retail display and consumer use. For most chilled foods, this direct approach to shelf-life determination is the most appropriate approach in practice. As food safety and consistent quality are the two main

Shelf-life of chilled foods

589

aspects of shelf-life, and both have to be designed into a food product, it is hardly surprising that shelf-life determination features prominently during the development of a new product. At least four types of shelf-life determinations can be distinguished for chilled foods, each serving slightly different purposes (IFST, 1993):

• Initial shelf-life study. This is normally conducted during the concept product

•

•

•

development stage when neither the actual production process nor the product or packaging format has been finalised. Safety (especially microbiological) of the product has either been evaluated or is evaluated alongside this study, based on HACCP principles. This initial study provides an indication of the probable mechanism by which the product is likely to deteriorate and spoil, and the main factors that affect it. It is essential to stress that concept product samples must not be offered for taste tests until their safety has been established. Preliminary shelf-life determination. This is the first detailed determination. It is normally carried out during the latter part of the kitchen/pilot development stage or when successful plant/factory trials have been completed. Information and data obtained are used to assign a provisional shelf-life, which may be included in the draft product, process and packaging specifications. Confirmatory shelf-life determination. This is normally carried out towards the end of the product development process, using product samples made under factory conditions and to a set of provisional specifications. By then, a complete HACCP study will also have been completed and a plan validated. Information and data obtained are intended to confirm or revise the provisional shelf-life previously established, and to help finalise the provisional specifications in preparation for product launch. Routine shelf-life determination. This is carried out in support of on-going production post product launch. It should include verification of the HACCP plan, assessment of the variability of product shelf-life and, if required, a revision of the assigned shelf-life and/or the production process. In certain types of products such as fresh fruits and vegetables, because of their variable nature, routine shelf-life determination is usually an integral part of the daily packing operations. Here, shelf-life test results are used to forewarn packers and retailers of potential quality problems, inform management regarding any shelf-life adjustment, and reveal temporal patterns in quality that can be used to trigger a change in the source of supply (Aked, 2000).

Comprehensive information and guidance on the evaluation of product shelf-life of chilled foods are available (Betts et al., 2004) and will not be covered in detail in this chapter. The following aspects of direct storage trials deserve some consideration.

19.7.1 Objective of the storage trial The objective of the storage trial is to determine how the trial should be designed, planned and undertaken, and how the results should be interpreted. The same

590

Chilled foods

chilled food product destined for both retail sale and for food service from a delicatessen counter, where portions of the product are expected to be sold over a period of time and during which the container is open for most of the day, would require two different experimental designs to reflect the two applications. Likewise, the design of the storage trial should take into account whether the food is intended to be consumed in one sitting or for multiple uses. Shelf-life studies, including a review of the HACCP plan, are recommended in the following circumstances (CFA, 2006):

• • • • • • • •

new product development, modification or extension new process development or modification new packaging development range extensions any change of ingredient/s or packaging to an existing product shelf-life extensions on existing products change of production site change or movement of production equipment that could influence the site plan.

19.7.2 Storage conditions These are governed by the existing temperature control regulations applicable to chilled foods (more often by the capabilities of the logistics and retail chains, which operate at lower temperatures), and by the conditions the product being studied is likely to encounter during distribution, storage and display. A baseline storage protocol, which is intended to be representative of manufacture and distribution of most chilled foods in the UK, has been recommended (Betts et al., 2004), and is given in Table 19.5. Individual chilled food manufacturers can of course modify this protocol according to their own requirements. There is, however, little point in adopting a protocol that subjects chilled foods to excessive and unrealistic abuse in respect of temperature, or time, or both, as such conditions will invariably lead to premature deterioration and spoilage, which are unlikely to be characteristic of their actual behaviour when the products are stored properly under the recommended storage conditions. Chilled food manufacturers, however, may wish to distinguish and employ optimum, typical (average) and worse-case Table 19.5

Recommended storage protocol for chilled foods (Betts et al., 2004)

Manufacturing stage

Storage temperature

Storage time

Under commercial control In-house storage at manufacturer Distribution vehicles and storage depot Retail display

5 °C or 7 °C 5 °C or 7 °C 5 °C or 7 °C

To be defined by manufacturer and/or retailer

Outside commercial control Consumer purchase Consumer storage

22 °C 7 °C

2 hours Remainder of shelf-life

Shelf-life of chilled foods

591

storage conditions provided they have the confidence about these conditions, and have the capabilities to simulate them.

19.7.3 The end of shelf-life The main task of a shelf-life study 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. Regarding microbiological safety and stability, the following should be of help in fixing an end-point for chilled foods: (i)

relevant food legislation, e.g. Commission Regulation (EC) 2073/2005 on microbiological criteria for foodstuffs; (ii) 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); (iii) guides on microbiological criteria for foods provided by independent professional bodies such as the UK Institute of Food Science and Technology (IFST, 1999); (iv) guides on microbiological criteria for foods produced by independent food research associations such as Campden and Chorleywood Food Research Association (Voysey, 2007); (v) 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 (Knorr, 1998; Holzapfel et al., 1998; Shortt, 1999; Birollo et al., 2000); (vi) predictive models (e.g. ComBase) as outlined in Section 19.7.5. Non-microbiological criteria that are used to set shelf-life are much more productspecific. 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, 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 assured, sensory evaluation is the most popular means by which the end of shelf-life is determined. This is important because safe food does not necessarily mean organoleptically acceptable food to the consumer. In the absence of a trained panel of sensory assessors, consumer tests give a useful measure of liking that can be used more directly to estimate shelf-life (Kilcast, 2000). The most common procedure is to ask consumers representative of the target population to scale acceptability on a nine-point hedonic scale, anchored from ‘like extremely’ to ‘dislike extremely’. A minimum of 50 consumers should be recruited for the test, although lower numbers (32–40) have been used. Some examples of sensory characteristics which may change

592

Chilled foods

Table 19.6 Examples of sensory characteristics which may change during storage (adopted from Betts et al., 2004) Descriptor Appearance Browning Discoloration Colour loss Darkening Thinning Sogginess

Sensory characteristics Cut surfaces turning brown, e.g. apple, lettuce Development of different colour, e.g. greying of cooked potato, pinking of cooked poultry meat;over-cooked products (nonenzymatic browning) Disappearance of the usual colour, e.g. loss/fading of red colour of cooked cured meat Colour becoming darker, e.g. tomato products Reduction in consistency of sauces, gravies, etc. Appearance of having absorbed moisture or liquid, e.g. pastry products, sponge cakes Loss of turgidity, product looking tired, e.g. vegetable crudités

Limpness Odour/flavour Flavour loss Loss of characteristic/typical flavour, e.g. fruit and vegetables Sourness/cheesiness Acid, lactic flavour of old milk Mouldy Odour/flavour associated with mould growth, e.g. mouldy sandwiches Alcoholic Resembling wine, found in flavoured milk products and some modified atmosphere packs; fermenting yoghurts Ammoniacal Pseudomonas on spoiling meat and poultry products Cardboard/stale Odour/flavour associated with old/rancid fat products ‘Off’ notes Associated with specific organisms and/or specific foods, e.g. putrid, rotting meat dishes Texture Firmness High resistance to deformation, e.g. carrot Crispness Tendency to yield suddenly, e.g. celery Crunchiness Making a crunching noise when bitten, e.g. apple Chewiness Tough and fibrous texture requiring much chewing, e.g. tough meat, citrus fruit with tough membranes Sogginess Associated with uptake of moisture/liquid, e.g. pastry Dryness Lack of moisture, e.g. stale old bread

during storage, and their corresponding descriptors, are given in Table 19.6. Another more powerful and informative technique for shelf-life assessment is quantitative descriptive analysis (QDA), which requires the use of a small panel of highly trained (6–12) assessors, and the use of training samples that illustrate quality changes that occur during storage. The technique involves three main steps: development of a standardised vocabulary; quantification of selected sensory characteristics that can be used to describe critical quality changes during storage; and analysis of the results using parametric statistical methods. The results may be presented as spider diagrams aiding interpretation (Lewis and Dale, 1994). These requirements together with the need for a dedicated sensory evaluation facility often preclude small chilled foods manufacturers from employing QDA to study shelf-life. Given that many of the raw materials used in food processing and manufacture

Shelf-life of chilled foods

593

are biological in origin, and microbial responses are naturally variable even under a defined set of conditions, it is vitally important that shelf-life storage trials are replicated a number of times to provide sufficient confidence in the assigned shelflife. And as a precaution, it is prudent to build into the assigned shelf-life a generous margin of safety, which can always be revised in light of further experience. Relevant information, for instance obtained via customer complaints and/or a consumer ‘help line’, may be used to inform a management decision should it be necessary to revise product shelf-life.

19.7.4 Challenge testing As pointed out earlier, food safety is best assured by applying HACCP principles, which is also a legal requirement within the European Union. There are times, however, when it is necessary to find out if the product is likely to remain microbiologically safe and stable during its shelf-life should it become contaminated with undesirable micro-organisms (either pathogenic or spoilage) that may cause the product to be unsafe or unstable, or be exposed to higher temperatures than designed. This can be determined using microbiological challenge testing where product is deliberately inoculated with relevant pathogenic or spoilage organisms and evaluated for their potential to survive or grow within the product under normal (and abnormal) storage conditions. Results can allow the risk of food poisoning or microbial spoilage due to the organism(s) used to be evaluated if contamination or temperature abuse did occur. Clearly, challenge testing is nonroutine and requires specialist skills and expertise, which many small chilled foods manufacturers may not have. In this case, the use of an external accredited laboratory that has the necessary skills, experience and facilities could be a costeffective option. Further information on challenge testing is available (Rose, 1987; Notermans et al., 1993; Notermans and in’t Veld, 1994).

19.7.5 Predictive models for estimating microbiological shelf-life A lot has been published about predictive microbiology for almost two decades. In the first book on the subject written by McMeekin and co-authors, ‘predictive microbiology’ was defined as a quantitative science that enables users to evaluate objectively the effect of processing, distribution and storage operations on the microbiological safety and quality of foods (McMeekin et al., 1993). Since then, predictive microbiology and its applications have seen spectacular growth and development, principally in the UK and the US, but also in other countries like Australia. Today, an internet-based, common database called ‘ComBase’ (www.combase.cc), hosted by the Institute of Food Research (IFR), Norwich, UK, is publicly and freely available for research and training/education purposes, for food microbiologists, manufacturers, risk assessors, enforcement officers, and teachers and students of food-related courses. The database consists of data sets from two major software packages, namely the Food MicroModel (FMM), and the Pathogen Modeling Program (PMP) developed by a UK’s former Ministry of

594

Chilled foods

Agriculture, Fisheries and Food (MAFF) initiative and the Eastern Regional Research Center (ERRC) of the US Department of Agriculture – Agricultural Research Service (USDA-ARS) respectively, as well as data from other members of the ComBase consortium and those compiled from the scientific literature. ComBase represents a major advancement in the evolution of predictive modelling and its widespread application, as the data contained in it have been standardised and organised in a manner that permits efficient access and retrieval (Baranyi and Tamplin, 2004). As has been possible for some time, predictive models can be used to predict the growth rates of bacteria under various conditions, and obtain data on heat inactivation, survival and time to toxin production (where applicable), which can be applied to all stages of the manufacturing process. For instance:

• • • • • • • •

new product, process and packaging development product reformulation microbial hazard identification estimating microbiological shelf-life (safety and stability) setting microbiological specifications trouble shooting alternative to challenge testing aid to quantitative microbiological risk assessment.

As an illustration, the effect of changes in storage temperature on growth of Yersinia enterocolitica can be seen in Fig.19.1, which shows predicted growth of the organism at three different temperatures in a broth culture at pH 6.5, containing 2% salt with an initial inoculum level of 103 cfu/ml. The predictions were obtained using the PMP7 Release 1 (PMP, 2005: www.arserrc.gov/mfs/pathogen.htm). It remains a fact, however, that the knowledge, skills and experience of a food microbiologist are invaluable in interpreting the information and results from predictive microbiology.

19.8 Future trends In the UK, the chilled foods sector has undoubtedly been a success story. The total UK chilled prepared foods market has been estimated to be worth in excess of £8bn at retail selling prices by the end of 2004 (Anon., 2005). The sector has performed much better than the food sector as a whole for a number of years, and this is expected to continue. Shelf-life is about safety and consistent quality. Almost by definition, chilled foods have relatively short shelf-lives, and with this, there is a general perception by consumers that they are ‘fresh’, ‘wholesome’ and ‘healthy’. As has been explained in this chapter, and discussed elsewhere in this book, many chilled foods are high-risk products, and food safety is of paramount importance to the chilled foods sector as it is to the rest of the industry. Recent consumer-driven trends towards fewer artificial preservatives, lower levels of salt, milder forms of heat processing, and fortification of foods with minerals and vitamins have created a greater technological challenge for chilled foods manufacturers, not least in the

Shelf-life of chilled foods

595

Yersinia enterocolitica in broth culture – effect of temperature on growth

12 10

Log cfu/ml

8

7 °C 6 °C

6

5 °C

4 2 0 0

5

10

15

20

25

Days

Temperature °C

Days to 106 cfu/ml

7

5.5

6

6.5

5

8

Fig. 19.1 Predictions from Pathogen Modeling Program (PMP7 Release 1, 2005) (broth culture – pH: 6.5, salt: 2.0%, initial level of inoculum: 1000 cfu/ml).

assurance of food safety. To ensure continued success, more effort and investment, not only in research and development but also in training and education, will be needed by all concerned: suppliers, manufacturers, caterers, retailers, trade associations, professional bodies, enforcement agencies, etc. to maintain and raise the standard of safety of chilled foods, and to continue to provide consumers with enjoyable products of safe, consistent and acceptable shelf-lives.

19.9 Sources of further information and advice There are numerous papers and articles on the shelf-life of chilled foods and related areas in the primary literature. The following are a number of publications on shelflife which are particularly useful: ANON.

(2006) Extension of Product Shelf-life for the Food Processor. A strategic report compiled for the Food Processing Faraday by the Scientific and Technical Information Section, Leatherhead Food International, Leatherhead, UK.

596

Chilled foods

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. ESKIN, N A M AND ROBINSON, D S (eds) (2000) Food Shelf Life Stability: Chemical, Biochemical and Microbiological Changes. CRC Press, Boca Raton, USA. IFST (1993) Shelf Life of Foods – Guidelines for Its Determination and Prediction. Institute of Food Science and Technology, London, UK. KILCAST, D AND SUBRAMANIAM, P (eds) (2000) The Stability and Shelf-life of Food. Woodhead Publishing Limited, Cambridge, UK. MAN, D (2002) Shelf Life. Food Industry Briefing Series, Blackwell Science, Oxford, UK. MAN, C M D AND JONES, A A (eds) (2000) Shelf-life Evaluation of Foods, second edition, Aspen Publishers, Gaithersburg, Maryland, USA. STEELE, R (ed) (2004) Understanding and Measuring the Shelf-life of Food. Woodhead Publishing Limited, Cambridge, UK.

19.10 References ADAMS, M R AND MOSS, M O (2000) Food Microbiology. Second Edition. The Royal Society

of Chemistry, Cambridge, UK. AKED, J (2000) Fruits and Vegetables. In: The Stability and Shelf-life of Food. Kilcast, D and

Subramaniam, P (eds.), pp 249–278. Woodhead Publishing, Cambridge, UK. ANON. (2005), Chilled Foods, Key Note Limited, Middlesex UK. BARANYI, J AND TAMPLIN, M (2004) ComBase: A Common Database

on Microbial Responses to Food Environments. Journal of Food Protection, 67(9), 1967–1971. BETTS, G D (2006) Determining the stability and shelf-life of foods. In: Food Spoilage Microorganisms. de W Blackburn, C (ed.), pp 119–143. Woodhead Publishing, Cambridge, UK. 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. BETTS, G AND EVERIS, L (2000) Shelf-life determination and challenge testing. In: Chilled Foods – A Comprehensive Guide, 2nd edn., Stringer, M and Dennis, C (eds), pp 259–285. Woodhead Publishing, Cambridge, 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. BROCKLEHURST, T F (1994) Delicatessen salads and chilled prepared fruit and vegetable products. In: Shelf Life Evaluation of Foods, Man, C M D and Jones, A A (eds), pp 87– 126. Blackie Academic and Professional, London, UK. CCFRA (2006) Pasteurisation: A Food Industry Practical Guide. Guideline No. 51, 2nd edn., Campden and Chorleywood Food Research Association, Chipping Campden, UK. CHILLED FOOD ASSOCIATION (CFA) (2006) Best Practice Guidelines for the Production of Chilled Foods, 4th edn., TSO, Norwich, UK. CHILLED FOOD ASSOCIATION (CFA) (2005) Guidance on the Practical Implementation of the EC Regulation on Microbiological Criteria for Foodstuffs. Chilled Food Association, Kettering, UK. CRAWFORD, C (1998) The New QUID Regulations. Chandos Publishing, Oxford, UK. DELAMARRE, S AND BATT, C A (1999) The microbiology and historical safety of margarine. Food Microbiology, 16, 327–333. DENS, E J AND VAN IMPE, J F (2001) On the need for another type of predictive model in structured foods. International Journal of Food Microbiology, 64, 247–260. EEC (1979) Council Directive 79/112/EEC of 18 December 1978 on approximation of the laws of the Member States relating to the labelling, presentation and advertising of

Shelf-life of chilled foods

597

foodstuffs, Official Journal of the European Communities (L33) of 8 Febraury 1979, pp. 1–14. 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, 25th June 2004, L 226/3–L 226/21. EUROPEAN COMMISSION (2005) Regulation (EC) No 2073/2005 on microbiological criteria for foodstuffs. Official Journal of the European Union, 22nd December 2005, L 338/1– L338/26. EVANS, J A (1998) Consumer perceptions and practice in the handling of chilled foods. In: Sous Vide and Cook–Chill Processing for the Food Industry, Ghazala, S (ed), pp 312–360, Aspen Publishers, Gaithersburg, Maryland, USA. EVANS, J A, STANTON, J I, RUSSELL, S L AND JAMES, S J (1991) Consumer Handling of Chilled Foods: A Survey of Time and Temperature Conditions. MAFF Publications, London, UK. 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/) GILBERT, R J, DE LOUVOIS, J, DONOVAN, T, LITTLE, C, NYE, K, RIBEIRO, 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. GOULD, G W (1996) Methods of preservation and extension of shelf life. International Journal of Food Microbiology, 33, 51–64. GOULD, G W (1999) Sous vide foods: Conclusions of an ECFF Botulinum Working Party. Food Control, 10, 47–51. HANSEN, L T, RØNTVED, S D AND HUSS, H H (1998) Microbiological quality and shelf life of cold-smoked salmon from three different processing plants. Food Microbiology, 15, 137– 150. HMSO (1990) Food Safety Act. Her Majesty’s Stationery Office, London, UK. HMSO (1996) The Food Labelling Regulations (SI 1996/1499). Her Majesty’s Stationery Office, London, UK. HOLLEY, R A (1997) Impact of slicing hygiene upon shelf life and distribution of spoilage bacteria in vacuum packaged cured meats. Food Microbiology, 14, 201–211. HOLZAPFEL, W H, HABERER, P, SNEL, J, SCHILLINGER, U AND HUIS IN’T VELD, J H J (1998) Overview of gut flora and probiotics. International Journal of Food Microbiology, 41, 85–101. IFST (1993) Shelf Life of Foods – Guidelines for its Determination and Prediction. Institute of Food Science and Technology (UK), London. IFST (1999) Development and Use of Microbiological Criteria for Foods. Institute of Food Science and Technology (UK), London. KATSARAS, K AND LEISTNER, L (1991) Distribution and development of bacterial colonies in fermented sausages. Biofouling, 5, 115–124. KILCAST, D (2000) Sensory evaluation methods for shelf-life assessment. In: The Stability and Shelf-life of Food, Kilcast, D and Subramaniam, P (eds), pp 79–105. Woodhead Publishing Limited, Cambridge, UK. KNORR, D (1998) Technology aspects related to micro-organisms in functional foods. Trends in Food Science and Technology, 9, 295–306. LEISTNER, L (2000) Minimally processed, ready-to-eat, and ambient-stable meat products. In: Shelf-life Evaluation of Foods, 2nd, Man, C M D and Jones, A A (eds), pp 242–263. Aspen Publishers, Inc. Gaithersburg, Maryland, USA. LEWIS, M AND DALE, R H (1994) Chilled yoghurt and other dairy desserts. In: Shelf Life Evaluation of Foods, Man, C M D and Jones, A A (eds), pp 127–155. Blackie Academic and Professional, London, UK. MAN, C M D (2004) Shelf-life testing. In: Understanding and Measuring the Shelf-life of Food, Steele, R (ed), pp 340–356. Woodhead Publishing, Cambridge, UK.

598

Chilled foods

MCMEEKIN, T A, OLLEY, J N, ROSS, T AND RATKOWSKY, D A (1993) Predictive Microbiology:

Theory and Application. Research Studies Press, Somerset. (1994) Microbiological challenge testing for ensuring safety of food products. International Journal of Food Microbiology, 24, 33–39. 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. ROSE, S A (1987) Guidelines for Microbiological Challenge Testing. Technical Manual No. 20. Campden Food and Drink Research Association, Chipping Campden, Gloucestershire. SHORTT, C (1999) The probiotic century: Historical and current perspective. Trends in Food Science and Technology, 10, 411–417. VILJOEN, B C, LOURENS-HATTINGH, A, IKALAFENG, B AND PETER, G (2003) Temperature abuse initiating yeast growth in yoghurt. Food Research International, 36, 193–197. 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 WILSON, P D G, BROCKLEHURST, T F, ARINO, S, THUAULT, D, JAKOBSEN, M, LANGE, M, FARKAS, J, WIMPENNY, J W T AND VAN IMPE J F (2002) Modelling microbial growth in structured foods: Towards a unified approach. International Journal of Food Microbiology, 73, 275–289. WORSFOLD, D AND GRIFFITH, C (1997) Food safety behaviour in the home. British Food Journal, 99(3), 97–104. NOTERMANS, S AND IN’T VELD, P