Shelf-life prediction and testing

Shelf-life prediction and testing

12 Shelf-life prediction and testing P. J. Subramaniam, Leatherhead Food International, UK Abstract: This chapter covers the important considerations...

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12 Shelf-life prediction and testing P. J. Subramaniam, Leatherhead Food International, UK

Abstract: This chapter covers the important considerations that need to be taken into account when setting up shelf life and accelerated shelf-life tests and discusses specific shelf-life issues associated with chocolate confectionery products. The different tests used for assessing sensory changes are covered, together with specific methods found to be useful for praline, biscuit and wafer products. The importance of choosing testing regimes appropriate to products and considerations concerning sample handling are also covered. The sensory results of shelf life and accelerated shelf-life tests on chocolate products are also given. Key words: accelerated shelf life, biscuit, chocolate, confectionery, flavour, praline, sensory changes, shelf-life, staleness, texture, visual, wafer.

12.1 Introduction All product manufacturers and retailers know the importance of accurate measurement of shelf life. Without this information it is difficult to ensure that products are safe for consumption and are consumed at the highest quality possible to satisfy consumer expectations. Therefore, in determining the overall shelf life of a product, both the safety and quality aspects need to be considered. Shelf life has been defined in various ways but a useful definition is that given by the Institute of Food Science and Technology (IFST, 1993) as the period of time when the product:

• remains safe • is certain to retain desired sensory, chemical, physical and microbiological characteristics

• complies with any label declaration of nutritional data

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when stored under the recommended conditions. A more recent document (ASTM E 2454-05, 2005), the Standard Guide for Sensory Evaluation Methods to Determine the Sensory Shelf Life of Consumer Products defines shelf life in terms of sensory aspects as:

• the time period during which the product’s sensory characteristics and performance are as intended by the manufacturer;

• the product is consumable or usable during this period, providing the end-user with the intended sensory characteristics, performance, and benefits. Low-moisture foods such as chocolate and sugar confectionery are inherently stable at ambient conditions and therefore have relatively long shelf lives of 3–18 months (Subramaniam, 2000). Testing the shelf life of such products is a long and laborious task, which can delay the commercialisation of new products. The confectionery industry is therefore seeking ways of shortening the test period for ambient stable products and of identifying accelerated shelf-life test methods, including rapid instrumental methods, which can predict the shelf life within a short time. The most important quality parameter driving consumer acceptance, is of course, sensory quality, relating to how the product looks, feels and tastes. However, additional parameters also need to be satisfied, based on the claims made for the product. For fortified confectionery, it is important to ensure that the product composition at the end of shelf life still satisfies any claims and label requirements, for example vitamin and mineral levels. Therefore shelf life encompasses many different aspects of product quality and performance. The measurement of the rate of change of quality parameters as part of shelf-life tests can be carried out relatively easily, but deciding the cut-off points for specific quality parameters against which shelf life can be determined is more difficult, particularly for confectionery products where food safety is not compromised during shelf life. In these cases, the shelf life is set based on a combination of scientific data from shelf-life studies and the views of marketing personnel and based on company policy (Kilcast and Subramaniam, 2000). It is important that any decisions and recommendations made with regard to the shelf-life tests and quality control must have the support of all senior management (Thursby, 1974) This chapter focuses on the methods used to assess and predict shelf life, particularly of chocolate products. Although fat bloom is the main cause of deterioration of chocolate and filled chocolate products, the issues of fat migration, bloom development and control of fat bloom are covered in other chapters and therefore will not be covered here. Instead, the focus of this chapter will be the sensory, physical and chemical changes affecting the shelf life of chocolate confectionery products.

12.2 Shelf-life testing methods The measurement of shelf life requires first an understanding of all quality

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attributes that change during product storage. The key quality factors that limit the shelf life of the product need to be identified from this information and the critical levels of change of those different attributes causing product failure or significant quality loss must be determined. Generally, confectionery products, because of high levels of sugar and relatively low moisture contents, tend to be microbiologically stable, although a few osmophilic yeasts associated with fruits and nut meats can ferment syrup concentrations above 75% (Richardson, 1980). The shelf life therefore is based on the loss of specific sensory quality parameters. Quite often it can be difficult to define the end point values for specific sensory characteristics because a level of change that one consumer feels is unacceptable may still be regarded as acceptable by another. Shelf life is determined by scientific tests but the changes seen must always be related to consumers’ expectations. Converting the consumer expectations of quality to accurately measurable scientific parameters can be difficult. This is the aim of the instrumental, physical and chemical tests used as part of a shelf-life study. The cut-off limits of change, in particular quality attributes, are often therefore set based on a consensus of opinions and, in some companies, the views of only a small group of experienced personnel, rather than the result of large groups of taste panels (Thursby, 1974). The use of a scientific approach to shelf-life measurement is important in setting the test methods and the critical limits needed to reduce inaccuracies in shelf-life assessment. Shelf-life tests are carried out for different purposes. When used during product development, shelf-life tests can be started as soon as the very first samples are approved, even though further changes in formulation will be necessary (Barnett, 1980). They are often used for the purpose of assessing the success of formulation modifications and processing refinements carried out as part of a cost cutting exercise, or where a new market demands an extension of the shelf life of existing products. In these cases, a comparison of the behaviour of the old product (with a known shelf life) with the newly modified product under the same test conditions, perhaps even under accelerated conditions, will help to determine if the shelf life has been altered. Shelf-life tests must be carried out through direct methods where representative samples of products (ideally production samples) are subjected to realistic storage conditions that mimic the distribution cycle for the product. Table 12.1 gives the typical steps involved in setting up a sequential shelflife test. In such tests, samples placed in storage will be removed sequentially at set intervals and assessed for changes in quality using sensory tests and appropriate physical and chemical tests that measure specific quality parameters until a time when the product is deemed to be unacceptable. Other test designs can also be used, using staggered sampling techniques, which allow the direct comparison of the quality change in products stored for different lengths of time. The advantages and disadvantages of the different designs are discussed by Kilcast and Subramaniam (2000). Whatever the sampling design, the storage test conditions need to be chosen carefully and a good understanding of the storage, handling and climatic conditions of the markets in which the product will be sold is essential to the accuracy of shelf-life tests. The test conditions should be chosen based on the distribution cycle for the product. Commonly used shelf-life

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Table 12.1 Steps in setting up shelf-life tests Step Shelf-life testing considerations 1 2 3 4 5 6 7

8 9 10 11 12

Consider all changes occurring in the product based on past experience or the behaviour of similar products on the market. Identify critical changes to be measured. Identify storage test conditions based on product cycle. Identify sensory, chemical and physical techniques for measuring level of attributes. Decide packaging for the samples to be tested. Decide the test intervals based on estimated or target shelf life (typically 2-week intervals for 3–4 month shelf life; one month intervals for 6–12 month shelf life; 2month intervals for 12–18 month shelf life). Prepare representative samples of the products for testing. Products may need to be stabilised for 1–2 weeks before putting through storage tests. The number produced should be considerably more than the calculated number of samples required for testing at every interval set. Set up storage unit(s) and monitor conditions and adjust until unit is stable. Place samples in the storage unit and assess samples for time 0 point. Remove samples from storage and assess for quality changes at each interval until end of test. Compare data collected at each interval against cut-off limits for consumer acceptance. Determine shelf life for product based on a discussion of the times to reach the cutoff points for specific quality attributes.

testing conditions which mimic ambient market conditions are 38–40 °C/80–90% relative humidity (RH) for tropical climates and 20–25 °C/50–70% RH for temperate conditions. Products with an expected shelf life of 12 to 18 months are sampled at 1–2 month intervals to measure shelf life.

12.2.1 Shelf-life limiting factors In order to determine the shelf life of a product, it is important to understand the relationship between different factors affecting its shelf life. The main factors affecting shelf life are product composition/structure, packaging and the distribution and storage conditions. Product factors such as composition, raw material quality, product structure, moisture content, water activity, fat content, liquid fat content, pH and sensitivity to oxygen are all important intrinsic factors affecting shelf life of confectionery (Subramaniam, 2000). The processing conditions used will also influence these product characteristics. Product structure, such as the surface smoothness and porosity of the product, plays important roles in determining product stability in both single and multi-component confectionery products, where fat and moisture migration are important mechanisms of deterioration. In products such as chocolate-coated biscuits, structural changes have been found to be important in determining the rate of fat migration. Although fat migration to a large extent is determined by the type of fat and the fat content of the biscuit and chocolate components, factors such as biscuit density and the surface texture also

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influence the result. The greater the density, and the rougher the surface of a biscuit, the greater the tendency for fat migration. Although high storage temperatures can increase the rate of fat migration, the direction of the migration may be different based on the storage temperature. Packaging has a significant role in determining the shelf life of confectionery. Good barrier properties, particularly to moisture, can be used to improve shelf life (Beehler, 1982). The maximum moisture loss or gain and oxygen ingress which cause a quality change, can be used to determine the barrier properties required for a product. The permitted moisture gain is 5% and the maximum oxygen ingress is 5–15ppm for nuts and snacks according to Robertson (1991). The permeability characteristics of packaging used to influence shelf life of products will play an integral role in shelf-life trials. As far as possible shelf-life tests should be carried out in the final packaging format to be used. Stability tests can be carried out on products packaged in perforated packaging, if the direct effects of humidity or oxygen on product stability need to be determined. Perforated packaging is normally used in accelerated shelf-life tests rather than conventional shelf-life tests, to accelerate moisture movement between product and environment or vice versa, whilst protecting the samples from dust and so on. Therefore the importance of choice of packaging for shelf-life tests cannot be overemphasized. The use of storage conditions representative of market environments is important for shelf-life study. Environmental factors such as temperature, humidity, oxygen and light will affect the shelf life. The relative effects of these factors will vary depending on the product and packaging characteristics. A common problem is that shelf-life studies are often carried out under carefully controlled environmental conditions that do not reflect reality, especially once the product leaves the factory and goes into a retail environment and then into homes. It is therefore important that manufacturers have a good understanding of the behaviour of products under realistic environmental conditions. The storage conditions chosen for shelf-life testing should not be the ideal for the product, but allow for some level of abuse seen during the product life cycle.

12.2.2 Sensory test methods Sensory tests are the most important tests to be carried out during a shelf-life study to assess the changes in perceived product attributes during storage. The changes in the attributes will relate directly to the stability of the product. If the sensory changes are not caused by microbial growth and spoilage, as in the case of most confectionery, the degree of change in sensory characteristics has to be directly related to product acceptability. Appropriate sensory tests need to be used to assess different aspects of shelf life. The ASTM has produced a guide that is useful for sensory shelf-life testing (ASTM, 2005). Both analytical and hedonic sensory tests can be used to gain knowledge about product stability. Analytical tests (such as difference tests and descriptive tests ) are useful to identify and measure changes in a product. Difference tests (e.g. paired comparison, duo-trio and triangle tests) are used to compare two products for sensory

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differences. These tests are sensitive but will only provide a limited amount of information on the sensory changes occurring during storage. Descriptive tests, for example product profiling, measure changes in the individual attributes of products. Results from such tests can be correlated with instrumental methods that measure the same or a similar attribute (e.g. colour or texture characteristics) allowing quantification of the sensory changes. The quantitative profile methods use trained sensory panels to measure the level of intensity of individual attributes and are therefore able to give more information than difference tests. Quantitative descriptive analysis (QDA) is the most commonly used method where the data produced can be statistically analysed and presented in a visual form (Kilcast, 2000). Example of glossaries established for shelf-life testing of milk chocolate and chocolate-coated praline are shown in Tables 12.2 and 12.3, respectively. these glossaries are derived by trained sensory panels after assessing a range of similar products of different ages and discussing the perceived attributes. The attributes are scored on scales (e.g. an unstructured line corresponding to a scale of 0–100 could be used). The orientation of the anchors (end points) for individual attribute scales reflect the anticipated direction of change in the attribute on storage. Samples can be presented as identified controls at the beginning of the Table 12.2 Glossary of terms for milk chocolate Attribute

Appearance Colour Gloss

Definition

Milk chocolate colour, assessed on upper surface Amount of shine, or gloss, assessed on upper surface

Anchorsa

Position of control on scale 0–100b

Light-to-Dark

20

Not-to-Very

20

Texture Hardness 1st bite Amount of force required to break Not-to-Very sample Crumbliness Way in which sample breaks into Not-to-Very small pieces Smoothness Feeling on tongue and palate, where a Not-to-Very powdery sample is not smooth Waxy Slippery sensation on teeth Not-to-Very Cloying Way in which the sample adheres to Not-to-Very the teeth and the mouth Meltdown rate Speed of meltdown in the mouth Slow-to-Fast Flavour Flavour impact Milk chocolate Stale a

Speed of flavour development Flavour of milk chocolate Flavour of old chocolate

Slow-to-Fast Not-to-Very Not-to-Very

20 10 30 20 20 30 20 30 10

The orientation of the scale reflects the direction of the anticipated change on storage and an increase in attribute relates to an increased score. b This number indicates the fixed position of the control sample decided in discussion sessions prior to the test.

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Table 12.3 Glossary of terms for chocolate-coated pralines Attribute

Appearance Chocolate colour Gloss Texture Density Crumbliness Smoothness Meltdown rate Flavour Chocolate Nutty Stale Other off

Anchorsa

Position of control on scale 0–100b

Milk chocolate colour, assessed on upper surface Amount of shine, or gloss, assessed on upper surface

Light-to-Dark

20

Not-to-Very

30

Solid state of sample where a highly dense sample has a texture similar to fudge The extent to which the sample readily breaks into small pieces Feeling on tongue and palate, where a powdery sample is not smooth Speed of meltdown of sample

Low-to-High

10

Not-to-Very

10

Not-to-Very

30

Slow-to-Fast

30

Flavour of milk chocolate Flavour of mixed ground nuts Old flavours not associated with fresh pralines Other off flavours e.g. chemical

Not-to-Very Not-to-Very Not-to-Very

30 30 0

Not-to-Very

0

Definition

a

The orientation of the scale reflects the direction of the anticipated change on storage and an increase in attribute relates to an increased score. This number indicates the fixed position of the control sample decided in discussion sessions prior to the test.

b

session. In this case, the position of the control is marked by the sensory assessors, by way of consensus, on the scale so that test samples can be scored relative to the control as shown in Tables 12.2 and 12.3. The hedonic tests (such as preference tests and acceptability tests) measure the level of consumer liking and perception of quality. These tests can be useful in determining the end points for shelf life, when consumer acceptability falls sharply. QDA tests on the same products can then be used to measure the level of change in particular attribute(s) to identify the important changes which make the product unacceptable to the consumer. A correlation of both types of tests is often required to measure and then decide the shelf life of products. The final decision on the end point and shelf life will often be a commercial one (Kilcast and Subramaniam, 2000). Hedonic tests use consumers to carry out the assessments, with tests at a central location, at home in the form of home usage tests or anywhere where the consumer can access the internet. Products are commonly scored on a category scale as in the example below: 1. like extremely 2. like very much 3. like moderately

240 4. 5. 6. 7. 8. 9.

Enrobed and filled chocolate, confectionery and bakery products like slightly neither like nor dislike dislike slightly dislike moderately dislike very much dislike extremely

Hedonic tests provide a direct measure of consumer acceptance but are expensive as they are carried out with many consumers to represent the market.

12.2.3 Other methods for measuring physical and chemical changes Physical and chemical tests are useful in measuring the primary factors that cause deterioration of a product under normal storage conditions. The results of instrumental tests can be correlated with sensory results and are often used to set critical limits for product deterioration. Some commonly used tests are described. Moisture content The texture of all confectionery products containing water is mainly determined by the moisture content (Mansvelt, 1973). The moisture content of products can be determined by appropriate tests such as oven and vacuum-oven drying and Karl Fischer titration. In a shelf-life study, the moisture content of the product can be correlated with the level of specific sensory attributes (e.g. texture in wafer and biscuit products) to set the critical limits for moisture gain or loss during storage. The rate of moisture loss/gain leading to significant changes in the sensory characteristics can be used as the basis for shelf-life prediction. The critical moisture content for wafers can be as low as 1% (Subramaniam et al., 1997). In soft nougat centres with 8–10% moisture, an increase or decrease in moisture of 1% is said to make a significant difference to texture (Mansvelt, 1973). The changes in water content also induce crystallisation of some of the sugar present, which can then compromise the microbial stability of the product. In the case of multicomponent products, the individual components need to be analysed separately in order to study moisture movement between components. In certain multi-component products, it is interesting to note that changes in texture can occur without the loss or gain of water by the product. Water activity Water activity (Aw) is an important property of foods and relates to the equilibrium relative humidity (ERH) of products (Aw × 100 = ERH). The ERH of a product is the humidity at which a product will neither gain nor lose moisture. The difference between the equilibrium relative humidity (ERH) of the product and the relative humidity of the storage environment is the driving force for moisture movement between the product and the environment. Products with a higher ERH than the RH of the environment (e.g. fondants, marzipan) will dry out during storage. However products with an ERH lower than the RH of the environment (e.g. sugar glass,

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Table 12.4 Commonly used methods of instrumental texture measurement Test

Product

Attributes measured

Three-point bend

Snap Softening Staleness Crumbliness Crunchiness Firmness Firmness

Shear

Chocolate Biscuit Wafer Chocolate Biscuit Jelly products Praline Toffee Gums Toffee Cereal

Extrusion

Caramel/toffee

Compression

Cut test Incisor

Firmness Hardness on first bite Toughness Firmness Toughness Stickiness

biscuits, wafers) will pick up moisture during storage. An ERH difference of more than 2% between two components or between product and environment will cause moisture movement (Cakebread, 1976). Moisture can also move within the product if components of the product differ in ERH, as is often seen in multi-component products (e.g. chocolate-coated biscuits containing water-based fillings). The greater the difference in the ERH between adjacent components (such as the biscuit and the filling), the greater will be the tendency for moisture to migrate and the shorter the shelf life. The measurement of water activity used to be a long and laborious process involving storage of samples in sealed glass jars over different saturated salt solutions to create a range of humidities. The measurement can now be made using water activity meters that produce results within ten minutes. There are many practical considerations during ERH measurement and these are given in detail by Bell and Labuza (2000). Texture measurement Instrumental measurements are very useful in measuring changes in texture during storage of products. If the parameters of the texture measurement are chosen carefully, a high level of correlation can be obtained with the sensory results. Many different methods have been devised and tested to measure different aspects of sensory texture. These include the relatively simple force deformation techniques (Lu and Abbott, 2004) and acoustic/sound measurement techniques (Duizer, 2004) using texture analysers, to more complex techniques such as near infrared (NIR) diffuse reflectance (Millar, 2004) and nuclear magnetic reflectance (NMR) and magnetic resonance imaging (MRI) as described by Thybo et al. (2004). Many studies have been conducted to determine the best tests for measuring the texture of specific products. Table 12.4 lists commonly used instrumental texture methods to measure different texture attributes.

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Colour and gloss measurement The appearance characteristics of products are known to have a significant influence on the acceptability of foods. Attributes which affect overall appearance include colour and gloss, particularly in chocolate products (Subramaniam and Groves, 2001). Colour and surface appearance has been shown to affect the results of chocolate flavour in taste panel calibration tests (Musser, 1973). Loss of colour can occur during storage as a result of the effect of temperature and light. The latest trend is to substitute artificial colours in confectionery products with natural colours which are generally less stable. The loss of colour or development of darker colour (often because of Maillard browning reaction) can reduce acceptability and must be assessed as part of product acceptance during shelf-life testing. For example, in nougat and fondant product, the centre can change from a white colour to yellow/brown (Mansvelt, 1973) as a result of Maillard browning. Colour can be easily measured using colorimeters and the instrumental values correlated with sensory acceptability as part of shelf-life assessment. Gloss levels of products can be measured by determining the amount of light reflected from the surface using a gloss meter, which is said to measure gloss independent of colour (Smith, 1999). The nature of the reflection is dependent on the characteristics of the surface (Musser, 1973). A smooth surface causes a mirror or specular type of reflection and leads to a high level of gloss. However, rough surfaces scatter the reflected light, resulting in a lower level of total reflection (termed diffused reflection) causing the product to appear dull. Changes in the surface characteristics as the product ages, for example bloom development, can be a shelf-life limiting factor. In shelf-life tests, instrumental gloss and colour measurements have been found to correlate with bloom development (Subramaniam et al., 1997, 2005a). Shelf-life studies have raised many questions about how the numerical values relate to visual gloss. The perception of gloss and the levels considered by consumers to be the ideal for different chocolate products have not been fully investigated. Traditionally, high gloss was considered to be related to high quality. However, chocolate products with very high levels of gloss are sometimes considered to be less acceptable (Fillion et al., 2001). Therefore, the instrumental results of gloss measurements need to be interpreted carefully. Rancidity Oils and fats deteriorate through oxidative (reaction of unsaturated fatty acids with reactive molecules such as oxygen) and hydrolytic rancidity (chemical or enzymic hydrolysis liberating fatty acids from triglycerides) giving rise to off-flavours and off-odours. Hydrolytic rancidity gives rise to ‘soapy’ off-notes in systems containing lauric fats and promotes deterioration by direct oxidation (Kristott, 2000; Matz, 1976). Oxidative rancidity causes food spoilage by fat deterioration causing pungent or acrid odours and is the most important of the two with respect to product acceptability (Labuza, 1982). The quality assessment of fats and oils involves both sensory and chemical tests. The chemical tests are based on either quantifying the triglyceride and fatty acid decomposition products, or measuring volatile

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Colour 80

Stale

70

Gloss

60 50 40

Milk chocolate

Hardness on 1st bite

30 20 10 0

Flavour impact

Crumbliness

Meltdown rate

Smoothness

Cloying Week 0

Fig. 12.1

Waxy Week 16

Week 75

Effect of storing milk chocolate at 5 °C.

decomposition compounds in the headspace of closed oil containers. At least two different tests are required to assess freshness of oils and fats (Kristott, 2000). The most common tests are the measurement of peroxide value, anisidine value, free fatty acid content, total oxidation (Totox) value, thiobarbituric acid (TBA) and extinction at 230 and 270 nm (Kristott, 2000). Infrared spectroscopy is also suggested (Gordon, 2004). However, correlation of specific test values to the level of off-flavours and odours as assessed by a sensory panel is essential for individual products in order to relate instrumental results accurately to the level of rancidity and overall acceptability. More information on the influence of fats and the issues of fat deterioration are covered in greater detail in other chapters.

12.3 Sensory changes during storage of chocolate confectionery A number of studies have been conducted to determine the sensory changes occurring in both dark and milk chocolate products during storage under different conditions (Subramaniam et al., 1997, 2005a). The effect of storage conditions on milk chocolate characteristics is shown in Figs 12.1 to 12.3. The results show that storage at 5 °C for 75 weeks does not significantly change the sensory quality of milk chocolate and is therefore a suitable temperature to store chocolate products

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Enrobed and filled chocolate, confectionery and bakery products Colour 80

Stale

70

Gloss

60 50 40

Milk chocolate

Hardness on 1st bite

30 20 10 0

Flavour impact

Crumbliness

Meltdown rate

Smoothness

Cloying

Waxy

Week 0

Fig. 12.2

Week 16

Week 75

Effect of storing milk chocolate at 20 °C/50% RH. Colour 80

Stale

Gloss

70 60 50 40

Milk chocolate

Hardness on 1st bite

30 20 10 0

Flavour impact

Crumbliness

Meltdown rate

Smoothness

Cloying Week 0

Fig. 12.3

Waxy Week 10

Week 16

Effect of storing milk chocolate at 24 °C/70% RH.

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90 80

not–very

70 60 50 40 30 20 10 0 W0

W2

W4

Sample 5°C

Fig. 12.4

W6

W8

W10 W12 W16 W29 W37 W48 W57 W65 W75 Storage time (weeks)

Sample 20°C/50% RH

Sample 20°C/70% RH

x Sample 24°C/70% RH

Loss of milk chocolate flavour in a milk chocolate stored under different conditions.

to be used as reference samples during shelf-life tests. Storage at 20 °C/50% RH, which simulates normal ambient storage, for the same length of time significantly changes both flavour and texture characteristics. Storage at 24 °C/70% RH for 16 weeks produced the same trend in changes as at 20 °C/50% RH, suggesting that the former conditions can be used as an accelerated test for shelf-life prediction of chocolate. The following sections will focus more closely on changes in specific flavour and texture attributes of confectionery products. Both changes in texture and flavour are significant during storage. 12.3.1 Flavour changes Three main types of flavour changes are common during storage (Subramaniam, 2007): 1. flavour loss leading to weaker flavours in open textured and porous products e.g. fondant, compressed tablets and panned goods; 2. staleness development caused by oxidation of flavour components, especially essential oils, e.g. mints, citrus; 3. development of rancidity caused by fat oxidation. Changes in flavour attributes have been measured during the storage of milk chocolate under different storage conditions using trained sensory panels (Subramaniam et al., 2005a). The loss of chocolate flavour and the development of stale flavour were the significant changes seen and these changes were found to occur gradually throughout storage. The rate of loss of chocolate flavour and development of stale flavours in milk chocolate can be seen in Figs 12.4 and 12.5, respectively. The results show that the changes produced at 20 °C/50% RH over 75

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Fig. 12.5

Development of stale flavour in milk chocolates stored under different conditions.

weeks are similar to the flavour changes seen over 16 weeks at 24 °C/70% RH, demonstrating that the latter could be used to accelerate flavour changes in chocolate. The origin and development of stale flavours in chocolate has been the subject of many debates by chocolate manufacturers. Staleness appears to take different forms and has even been found to fluctuate depending on storage conditions. Moisture was thought to play a role in this process. Certain stale flavours in chocolate were thought to be reversible to some degree and able to be reduced or even eliminated by remelting and moulding chocolate, allowing the recycling of chocolate products. Little is still known about the origins of the stale notes and whether ingredients may be the source of the problem. One study focusing on staleness development in milk chocolate showed that stale or aged milk chocolates had a harder texture than fresh samples (Fig. 12.6.), which seemed to relate to changes in the microstructure (Subramaniam et al., 2005b). The fat continuous matrix in the fresh chocolate was thought to change to a sugar–protein continuous matrix during ageing. It is possible that the formation of such a matrix may be responsible for the change (loss) in the chocolate flavour noted in aged chocolates. Another possibility may be that flavours are masked by the formation of a rigid network, which then is perceived as a loss of flavour on ageing. Although fresh and stale milk chocolate samples were tested by gas chromatography–mass spectrometry (GC–MS), the study was not able conclusively to identify common markers for

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Force to penetrate 4 mm (g)

6000 5000 4000 Fresh Stale

3000 2000 1000 0 A

B

C

D

Chocolate

Fig. 12.6 Comparison of hardness of ‘fresh’ and ‘stale’ variants of four different milk chocolates showing that the fresh chocolates are in general softer than the stale chocolate.

staleness in products of different composition. However, the products of oxidation were found to be higher in some of the stale products.

12.3.2 Texture deterioration of chocolate, wafer, biscuit and praline The smoothness of chocolate is an important quality attribute. The progression of fat bloom is commonly associated with a loss of smoothness and an increase in crumbliness. The loss of smoothness of milk chocolate under different storage conditions is shown in Fig. 12.7. The results showed that storage at 5 °C retains milk chocolate texture in a smooth state but storage at 20 °C and 24 °C causes a loss of smoothness on storage, the loss being accelerated at the higher temperature. Wafer, biscuit and praline are common components in chocolate products. Texture is an important attribute for these products and can significantly influence shelf life. The textural changes occurring during the ageing of these components under different storage regimes have been measured (Kilcast and Subramaniam, 1998). In the case of wafer, the three-point bend/snap test measuring peak force, peak area and the break distance were useful indicators of textural quality. Break distance was found to relate to stale texture development, which caused the wafer to become less crispy and more pliable. The reduction of crispness in wafer as a result of the development of staleness was also reflected in the peak force and area results, which increased as a rubbery/chewy texture developed on storage. Increase in moisture content of wafer samples during storage correlated with an increase in the break distance and highlighted the importance of preventing moisture absorption. The biscuit product used in this study was an open structured, slightly crumbly textured oatmeal biscuit with a high fat content of 19.6%. Texture tests found that the number of fractures as measured during the hardness test was a good indicator

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Enrobed and filled chocolate, confectionery and bakery products

80 75 ✱

not–very

70 65 ✱

60



55





50



✱ ✱

45 40 W0

W2

W4

W6

W8

W10 W12 W16 W29 W37 W48 W57 W65 W75 Storage time (weeks)

Fig. 12.7

Loss of smoothness in milk chocolates stored under different conditions.

of crumbliness. A significant decrease in the number of fractures suggested a decrease in crumbliness as the biscuit aged. The break distance measurement in a snap test was not found to be a good measure of the textural changes occurring during the staling of biscuits. Moisture migration is the important mechanism causing sensory changes in dry products such as wafer and biscuit. In chocolatecoated variants, moisture would move by diffusion through the chocolate, but, if the coating is incomplete, moisture movement will be through the cracks and holes in the coating, accelerating moisture absorption and reducing shelf life. In the case of praline, a cut test was found to be useful in measuring the changes in texture during storage. The peak force was found to relate to the surface hardness of the samples and the peak area to the firmness of the bulk product. Praline samples stored at 5 °C and 20 °C showed some softening of the surface of the samples prior to further hardening on storage. However, at 28 °C/70% RH, praline samples developed a crust on the surface, perhaps owing to the absorption of moisture and showed a loss of smoothness. The sensory changes seen in a chocolate-coated praline during storage related to moisture and fat migration.

12.3.3 Visual changes Colour and gloss are the two main visual characteristics of chocolate. The factors that affect colour of chocolate include the cocoa characteristics (type and roasting temperature, alkalising); milk powder type and level; particle size and tempering. The factors important for gloss are tempering and cooling, moulding or enrobing and age (Voltz and Beckett, 1997). Changes in gloss and colour can be indicators of loss of quality. The onset of bloom development on chocolate can be monitored by measuring changes in colour and gloss. Subramaniam et al. (1997) found that gloss assessed by a trained sensory panel correlated reasonably well with

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260 õ

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 ¸

Gloss units

220

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Filling C

Fig. 12.8 Effect of level of hazelnut oil in filling on gloss during storage of filled chocolates at 24 °C (fillings A, B and C had 20%, 32% and 40% hazelnut oil, respectively).

instrumental measurements from a gloss meter. The measurement parameters used on the gloss meter can influence the results obtained and need to be chosen based on product dimensions. Instrumental gloss measurement is a useful way to study the progression of bloom and has been used to measure the performance of antibloom filling fats and the effect of hazelnut oil on migration-induced bloom on chocolate (Subramaniam and Groves, 2003). The surface gloss of samples containing fillings with 20%, 32% and 40% hazelnut oil (fillings A, B and C, respectively) stored under accelerated test conditions shows clearly how hazelnut oil accelerates bloom development (Fig. 12.8). Bloom was first noted on samples A, B and C after 7–8 weeks, 2–3 weeks and 1–2 weeks, respectively when stored at 24 °C. Gloss is measured in arbitary gloss units, a value of 275 units being associated with a polished black tile. It can be seen that a small change of about 20 units can be linked with early stages of bloom development. Interestingly, the gloss meter is able to capture how the gloss level increases for the high hazelnut oil product (sample C) at certain points in the storage because of migration of nut oil on to the chocolate surface. This shows the importance of visual assessment alongside the instrumental measurement of colour and gloss to interpret the causes of visual changes in stored samples.

12.4 Shelf-life prediction Shelf-life prediction is increasingly becoming an important part of any new product development and because of improved controlled storage testing facilities in more recent times it is seen as a real possibility by companies. Shelf-life prediction will never be able to replace shelf-life testing, but it is a useful tool to have available where commercial decisions have to be made within time con-

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straints for product launch. This section will cover the important aspects of accelerated shelf-life testing.

12.4.1 Accelerated shelf-life testing (ASLT) ASLT aims to accelerate the rate of deterioration of the product without altering the mechanisms or order of changes seen in the product under normal storage conditions. In any ASLT it is important that no new changes are brought about by the testing conditions which are designed to force the product to age quickly. ASLTs are particularly useful in predicting the shelf life of ambient-stable products such as confectionery which have long shelf lives. It should be noted that ASLTs cannot be used if microbial growth occurs in the products. These tests are designed solely to accelerate physicochemical changes. Products spoiling because of microbial growth have other appropriate tests for prediction of microbial growth. The principles of ASLT are covered in detail by Mizrahi (2000 and 2004). Although ASLTs are extremely useful and have their place, there are many limitations which need to be considered in using these tests (Robertson,1991). A good understanding of the product under study is needed before accelerated tests can be set up to make sure that tests are carried out as accurately as possible to predict shelf life. In their simplest form, ASLTs are useful as comparative studies in which the rate of change of specific characteristics of a new product is checked against that of an existing similar product of known shelf life. Accelerated shelf-life tests (ASLT) are used for many different purposes:

• • • • •

prediction of shelf life assessment of product stability in a short period of time abuse testing of products troubleshooting of instability problems formulation screening at initial stages of product development.

12.4.2 Testing regimes The storage conditions used to accelerate the deteriorative process depend on the characteristics of the products. The test conditions for any accelerated tests must ensure that the changes induced are only those that are caused under the normal storage conditions. For dark chocolate, the temperature limit will be close to 30 °C and for milk chocolate 24 °C, as higher temperatures will cause the fat to melt inducing changes that are caused by abuse rather than because of ageing. The exact conditions used for ASLT should be chosen based on product stability characteristics, typical climatic conditions during distribution and retail storage. Table 12.5 gives some ASLT conditions used for confectionery products (adapted from Subramaniam, 2007). As discussed earlier in this chapter, packaging is an important factor to consider in any shelf-life test. This includes accelerated tests, where products will be often stored at high temperatures and humidity. It is important to undertake these tests in

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the final packaging format. Products in their secondary outers can be tested to ensure that they are to the required standard. A common test for sugar confectionery items packaged in outer cartons will be that they show no significant change for 16 weeks at 25 °C/60 % RH. Barnett (1980) suggested an accelerated test where packaged products are alternated daily between storage temperatures of 26.7 °C(80 °F) and 15.6 °C (60 °F) for 6–12 weeks. One week under these conditions was thought to equate to one month of normal storage for most fatbased confectionery products and those containing nuts.

12.4.3 Sample handling The same issues of sample handling in shelf-life tests apply also to ASLT. Sensory tests are vital for both shelf-life testing and ASLT. Sensory tests require that reference samples are retained for comparison with aged samples produced at each storage test interval. These reference samples should be equivalent to fresh samples of the product. How to maintain the freshness of the reference sample has been a long standing debate. Some believe that chocolate products can be frozen and presented after thawing at each point. However, others suggest that freezing induces changes that can be noticed by a sensory panel, in particular relating to flavour. In general it has been found that storage at 5 °C can be used to arrest changes occurring in chocolate under normal storage and has been used widely in many studies (Subramaniam et al., 1995, 2005a). However, chocolate samples have also been frozen successfully for chemical analysis at a later date. Table 12.5 Accelerated shelf-life testing conditions for confectionery (adapted from Subramaniam, 2007) Product

Deteriorative change

Typical ASLT conditions

Dark chocolate Milk chocolate Chocolate Chocolates with fatty filling Sugar glass Toffee Gums and jellies Sweets with natural colours Biscuits/wafers

Bloom Bloom Staleness Fat migration Moisture pick up Graining and cold flow Drying out Colour fading Staleness

24–28 °C 24 °C 24–28 °C/70% RH 24 °C 25 °C and 50% RH 25 °C and 70% RH 25 °C/50% RH 25°C + relevant humidity + light 28 °C/70% RH

12.5 Future trends Shelf-life testing will always be required to test and verify the safety and stability of products and sensory tests will always remain as a vital part of any study. However, manufacturers are under greater demands to release products on to the market more quickly before shelf-life tests are completed. This is particularly the

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case for confectionery products which show a high turnover of product formats and new developments. For this reason ASLTs are increasingly being used as part of the decision making on product stability. If these are to be used with a high level of confidence, further work is required to develop validated tests with proven relationships of stability under normal ambient and ASLT conditions. The use of rapid physical and chemical tests which can predict shelf life will be an important future development for the industry. One important aspect of this is the correlation of sensory data with chemical and physical data produced by different techniques. Unfortunately no instrument can be as all encompassing and sensitive as the human palate. This means that sensory testing and relating quality changes to consumer acceptability will always be expected to remain the core of any future shelf-life tests.

12.6 Sources of further information and advice A vast amount of published information exists on shelf-life assessment of different food products. However, many of these approach the subject in a theoretical manner rather than from a practical basis. All those who have set up shelf-life trials will understand that there are many practical considerations in carrying out such tests. Unfortunately very little data exists on this aspect of the subject, except in the much older publications which do offer some important information and advice not found in later publications. Much research is still required on ASLT of different aspects of product quality. The most useful published information is provided by Mizrahi (2000 and 2004) as given in the reference section. However, there are a number of comprehensive books on shelf-life testing which can be combined with confectionery specific books and these are listed as follows:

• Labuza, TP (1982). Shelf-life Dating of Foods, Food and Nutrition Press, Westport, CT.

• Charalambous, G (1993). Shelf-life Studies of Foods and Beverages – Chemical, Biological, Physical and Nutritional Aspects, Elsevier Science, Amsterdam.

• IFST (1993). Shelf-life of Foods – Guidelines for its determination and Prediction, Institute of Food Science & Technology, London.

• Man, CMD and Jones, AA (1995). Shelf-life Evaluation of Foods, Blackie Academic & Professional, an imprint of Chapman and Hall, London, UK.

• Kilcast, D and Subramaniam, PJ (2000). The Stability and Shelf-life of Food, Woodhead Publishing, Cambridge UK.

• Steele, R (2004). Understanding and Measuring the Shelf-life of Food, Woodhead Publishing, Cambridge UK.

• Beckett, ST (2009). Industrial Chocolate Manufacture and Use, 4th edition. Blackwell Science Ltd, London, UK.

• Minifie, BW (1980) Chocolate, Cocoa and Confectionery: Science and Technology, 2nd edition, AVI publishing, Westport, Conneticut.

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12.7 Acknowledgements Leatherhead Food International holds the copyright to all figures and tables used in this chapter and these should not be reproduced without permission.

12.8 References ASTM E 2454-05 (2005). Standard Guide for Sensory Evaluation Methods to Determine the

Sensory Shelf Life of Consumer Products, USA. (1980). ‘New products-production’, Manufacturing Confectioner, 60(6), 87– 91. BEEHLER DC (1982). ‘The effect of packaging on candy bar shelf-life’, Candy Industry, 35– 8. BELL LN AND LABUZA TP (2000). Moisture Sorption – Practical Aspects of Isotherm Measurement and Use, 2nd edition. American Association of Cereal Chemists, St Paul, USA. CAKEBREAD SH (1976). ‘Ingredient migration in composite products’, Confectionery Production, 42(5), 26–37. DUIZER LM (2004). ‘Sound input techniques for measuring texture’ , in Texture in Food Volume 2: Solid Foods, Kilcast D (ed.), CRC Press, Boca Raton, pp146–66. FILLION L, ARAZI S, LAWSON S AND KILCAST D (2001). Gloss Perception and its Importance in Food Products: an exploratory study, LFRA Research Report, Leatherhead Food International. GORDON MH (2004). ‘Factors affecting lipid oxidation’, in Understanding and Measuring Shelf-life of Food, Steele R (ed.), Woodhead Publishing, Cambridge, UK, pp 128–41. IFST (1993). Shelf-life of foods: Guidelines for its Determination and Prediction, Institute of Food Science and Technology (UK), London. KILCAST D (2000). ‘Sensory evaluation methods for shelf-life assessment’, in The Stability and Shelf-life of Food, Kilcast D and Subramaniam PJ (eds), Woodhead Publishing, Cambridge, UK, pp 79–106. KILCAST D AND SUBRAMANIAM PJ (1998). Shelf-life Prediction of Composite Chocolatebased Low-Moisture Products: Stage 1: critical evaluation of characterisation methods. MAFF-LINK Project Report for Project No. AFQ1116. KILCAST D AND SUBRAMANIAM PJ (2000). ‘Introduction’ in The Stability and Shelf-life of Food, Kilcast D and Subramaniam PJ(eds), Woodhead Publishing, Cambridge, UK, pp 1–22. KRISTOTT J (2000). ‘Fats and oils’, in The Stability and Shelf-life of Food, Kilcast D and Subramaniam PJ (eds), Woodhead Publishing, Cambridge, UK, pp 279-310. LABUZA TP (1982). ‘Shelf-life of fried snack foods’, in Shelf-life Dating of Foods, Labuza TP (ed.), Food & Nutrition Press, Westport, Connecticut, pp 129–48. LU R AND ABBOTT JA (2004). ‘ Force/deformation techniques for measuring texture’, in Texture in Food Volume 2: Solid Foods, Kilcast D (ed.), CRC Press, Boca Raton, pp 109–45. MATZ SA (1976). Snack Food Technology, AVI publishing Company, Westport Connecticut. MANSVELT, JW (1973). ‘Shelf-life of sugar confectionery’, Confectionery Production, 10, 542–9. MILLAR S (2004). ‘Near infrared (NIR) diffuse reflectance in texture measurement’, in Texture in Food Volume 2: Solid Foods, Kilcast D (ed.), CRC Press, Boca Raton, pp167–83. MIZRAHI S (2000). ‘Accelerated shelf-life tests’, in The Stability and Shelf-life of Food, Kilcast D and Subramaniam PJ (eds), Woodhead Publishing, Cambridge, UK, pp 107–28. MIZRAHI S (2004). ‘Accelerated shelf-life tests’, in Understanding and Measuring Shelf-life of Food, Steele R (ed.), Woodhead Publishing, Cambridge, UK, pp 317–339. BARNETT CD

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MUSSER JC (1973). ‘Gloss on chocolate and confectionery coatings’, 27th PMCA Production

Conference, Pennsylvania Manufacturing Confectioners’ Association, Centre Valley PMCA, pp 46–50. RICHARDSON T (1980). ‘Confectionery production’, The Manufacturing Confectioner, 12, 52–5. ROBERTSON GL (1991). ‘Predicting the shelf-life of packaged foods’, Asian Food Journal, 6(2), 43–51. SMITH G (1999). ‘New emerging technology in gloss measurement’, Innovations in Food Technology, August, 44–5. SUBRAMANIAM PJ (2000). ‘Confectionery products’, in The Stability and Shelf-life of Food, EKilcast D and Subramaniam PJ (eds), Woodhead Publishing Limited, Cambridge, UK, pp 221–48. SUBRAMANIAM PJ (2007). ‘Determining shelf-life of confectionery products’, Manufacturing Confectioner, 87(6), 85–91. SUBRAMANIAM PJ AND GROVES K (2001). A study of Gloss Characteristics of Chocolate Coatings, Research Report 783, Leatherhead Food International. SUBRAMANIAM, PJ AND GROVES K (2003). A Study of Anti-bloom Fats for Delaying Migration-induced Bloom, Research Report 830, Leatherhead Food International. SUBRAMANIAM PJ, ROBERTS CA, KILCAST D AND JONES SA (1997). Accelerated Shelf-life Testing of Chocolate Products, Research Report No. 738, Leatherhead Food International. SUBRAMANIAM PJ, LAWSON S, EELES M AND GROVES KHM (2005a). An Investigation of Accelerated Shelf-life Testing Conditions for Milk Chocolate and Chocolate-coated Pralines. Research Report No, 882, Leatherhead Food International. SUBRAMANIAM PJ, PHELPS T, LAWSON S, GROVES KHM AND REID WJ (2005b). An Investigation of Staleness Development in Milk Chocolate, Research Report No. 878, Leatherhead Food International. THURSBY ML (1974). ‘Optimize the shelf-life of your products’, Candy and Snack Industry, 139(12), 34. THYBO AK, KARLSSON AH, BERTRAM HC, ANDERSEN HJ, SZCZYPINSKI P AND DONSTRUP S (2004) ‘Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) in texture measurement’, in Texture in Food Volume 2: Solid Foods, Kilcast D (ed.), CRC Press, Boca Raton, pp 184–204. VOLTZ M AND BECKETT ST (1997). ‘Sensory of chocolate’, The Manufacturing Confectioner, 2, 49–53.