Sensory perception of creaminess and its relationship with food structure

Food Quality and Preference 13 (2002) 609–623 www.elsevier.com/locate/foodqual

Sensory perception of creaminess and its relationship with food structure§ David Kilcast*, Stuart Clegg Leatherhead Food RA, Randalls Rd, Leatherhead, Surrey KT22 7RY, UK Received 1 April 2002; received in revised form 13 May 2002; accepted 20 June 2002

Abstract Practical difficulties are frequently encountered in understanding the meaning of creaminess as used as a sensory attribute, and in the compositional and physical characteristics of foods that give rise to creaminess. Selected factors have been investigated in this project. The size of dispersed particles (solid, liquid and gaseous) on the perceived creaminess was of particular interest during the course of the work, and this was varied using appropriate changes in processing, during production of samples for analysis. Additional factors investigated included the presence of an added cream flavour in artificial creams, fat content in artificial creams and air content in chocolate mousses. Sensory characterisation of samples was carried out using sensory profiling, while physical characterisation of the samples was by means of particle size measurements, microscopy and rheology. The results show that perceived creaminess is a complex attribute in multi-phase food systems, being dependent on both flavour and textural characteristics of products, with different dependencies on these characteristics, depending on the product type concerned. The project demonstrated the value of manipulating processing and formulation variables as a means of generating a range of food structures for the investigation of creaminess. It is evident, however, that the concept of creaminess might differ between different structural types, limiting opportunities for devising general rules governing the factors controlling creaminess. Further studies should focus on specific structural types that are open to manipulation, and further research is in progress to investigate systematic changes to a mousse structure. # 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction The term creaminess, an important indicator of richness and high quality, is a descriptor that is often used when describing the sensory properties of food products, especially those containing fat. In the case of many product categories, reduction of fat in a given matrix has been considered as a challenge to mimic the rheological effects of the fat through the use of a fat-replacing system (Jones, 1996). With the wide range of hydrocolloids, fat mimetics and texture modifiers currently available, this approach is a relatively easy route. However, the matter of rheological matching cannot be viewed in isolation and needs to be related to the perceived sensory characteristics of a product, in particular creaminess. The most appropriate terminology and sensory attributes to describe the textural contributions of fat in § Paper presented at the 4th Pangborn Sensory Science Symposium, Dijon, July 2001. * Corresponding author. Tel.: +44-1372-376761; fax: +44-1372386228. E-mail address: [email protected] (D. Kilcast).

food products are far from resolved despite a great number of research investigations in this area (Cussler, Kokini, Weinheimer, & Moskowitz, 1979; Drewnowski, 1987; Jowitt, 1974; Kokini, Kadane, & Cussler, 1977; Szczesniak, 1979; Szczesniak, Brandt, & Friedman, 1963). Kokini (1987), in a review of efforts to relate liquid and semi-solid texture to rheological and frictional properties, claimed that creaminess could be predicted from scores of thickness and smoothness. In an early study on the mouthfeel of liquids (Kokini et al., 1977), a texture space consisting of 10 words was reduced to three key words (thick, smooth and slippery) and each of these three descriptors was shown to be related to frictional forces and/or viscous forces in the mouth. Creaminess was found to depend largely on smoothness and thickness but it was pointed out (Kokini et al., 1977; Cussler et al., 1979) that creaminess was not well predicted in the regression analysis. In an extension of the work on the liquid systems to a group of foods exhibiting various degrees of creaminess, Cussler et al. (1979) found that the attributes thick and smooth were unrelated, and could not be used to predict assessments of

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creaminess accurately. They concluded that creaminess might be more than a combination of smoothness and thickness. However, a subsequent study on predicting the texture of liquid and melting semi-solid foods (Kokini & Cussler, 1983), in which 14 commercial products were investigated, indicated that an estimate of creaminess could be obtained from thickness and smoothness, with regression analysis of the data resulting in the following equation: ðcreaminessÞ
ðthicknessÞ0:54 ðsmoothnessÞ0:84 The exponent for thickness from this experiment was identical to that obtained in the study on model liquid systems (Kokini et al., 1977), while that obtained for smoothness was only marginally higher than in the previous study. Thickness and smoothness were found to be largely independent of each other, consistent with earlier findings (Cussler et al., 1979) and with Szczesniak’s (1979) findings that viscosity-related terms (such as thick or viscous) and terms associated with feel on soft tissue surfaces (such as smooth or soft) were grouped into two distinctive categories. In a similar study into the textural attributes of fluid and semi-solid foods, in which 15 textural terms (generated by a trained sensory panel for 27 commercial products) were subjected to regression analysis (Kokini, Poole, Mason, Miller, & Stier, 1984), it was found that three attributes—slippery, thick and soft—gave the highest R2 values in a search with three independent variables. Equations were generated that provided a set of regression parameters that could be used to predict 12 texture attributes (including creaminess) from scores obtained for thick, soft and slippery. The equation for creamy was as follows: log creamy ¼ 0:52 log thick þ 1:56 log soft  0:32 log slippery It was noted by Kokini et al. (1984) that the results were consistent with those obtained previously (Kokini et al., 1977), with thickness and slipperiness being variables in both studies and softness being related to smoothness. In the above studies, thickness and smoothness were considered to be related to the viscous and frictional properties experienced in the mouth, thus allowing the identification of measurable physical properties that could be used to predict sensory texture and mouthfeel attributes. The difficulties in identifying the physical nature of creaminess often impact on sensory studies. Additionally, the attribute creaminess has a stronger influence on hedonic response than many other attributes. This hedonic aspect of creaminess frequently results in non-linear relationships between the attribute and other measurable attributes or rheological parameters (Daget, Joerg, &

Bourne, 1987; Daget & Joerg, 1991; Wood, 1974), whereas terms that have a lesser effect on hedonic response tend to show more linear relationships. Generally, creaminess shows good correlation with the amount of fat in a system. Drewnowski (1987) suggested that fat content might be better monitored and perceived through the use of more abstract terms related to caloric density (e.g. oily, greasy). Most food systems exhibiting creamy characteristics are oil-in-water food emulsions, and have consequently received most attention. In an investigation of the perception of fat in model oil-in-water emulsions with different fat contents (Mela, Langley, & Martin, 1994), it was shown that increasing oil content in model oil-inwater emulsions gave rise to a logarithmic increase in viscosity (measured at a shear rate of 48 s1), and that this viscosity increase was a dominant factor affecting perceived fat content as assessed by sensory panels. However, statistical analyses of the data showed that fat content made independent contributions beyond viscosity alone to the perceived fat content, supporting earlier work on perceived fat content and creaminess in thickened milk (Mela, 1988). Other authors have recognised the importance of two rheological parameters in relation to fat perception (i.e. viscosity and flow behaviour index). For example, Wood (1974), in a study on soups thickened with a range of thickeners, established that maximum perception of creaminess was at a viscosity of between 50 and 80 mPas and a flow behaviour index (n) of about 0.5. A similar study carried out more recently (Daget & Joerg, 1991) on model cream soups formulated with different hydrocolloid thickeners found that the relationship between flow behaviour index and apparent viscosity was different for each thickener. The maximum creaminess values corresponded to viscosities between 90 and 325 mPas and a flow behaviour index of 0.12– 0.42. Interestingly, in this study, rheological optima for liking consistency were found always to be lower than those for creaminess, while perceived thickness was found to be linearly related to the logarithm of viscosity for all thickeners. In an earlier study investigating perceived creaminess in desserts (creams with xanthan), Daget et al. (1987) found that maximum creaminess was at a viscosity of between 880 and 7500 mPas and a corresponding flow behaviour index of 0.15 and 0.04 for 3.5 and 30% fat contents, respectively. The last two studies clearly illustrate the product dependency of perceived creaminess in terms of viscosity measurements. The results discussed above demonstrate the importance of two rheological parameters in perceived creaminess and acceptability, but the authors also established that other unknown aspects affected acceptability ratings. Indeed, Jones (1996) suggested that it was not possible to define an attribute such as creaminess in

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purely rheological terms and postulated that, in any given product, apart from viscous and smoothness characteristics, perceived creaminess would also be affected by intensity of flavour attributes present in the system. Two recent studies have investigated further the contributions of creaminess to consumer acceptability. Elmore, Heymann, Johnson, and Hewett (1999) investigated the sensations underlying the acceptance of textural creaminess in vanilla puddings with formulations giving a range of thicknesses, fat content and smoothness. They found that, whilst there was a strong relationship between acceptability and textural characteristics such as thickness and smoothness, there was evidence for possible contributions from flavour and visual characteristics. A study of liquid dairy products by RichardsonHarman et al. (2000) showed that the main dimension underlying consumer ratings of creaminess and acceptability was related to fat content, although viscosity had little effect, and with a contribution from dairy flavour. In addition to the basic rheological characteristics of oil-in-water food emulsion systems, the importance of the size and number of fat droplets has also received some attention. This reflects one of the major approaches to reducing the level of fat in food emulsions, which has been to replace some or all of the disperse oil/ fat phase in the higher-fat product with an alternative ingredient that can physically mimic the fat droplet (in terms of both size and possibly shape). The results of Mela et al. (1994) showed no apparent pattern in oil droplet size as affected by fat content in emulsions containing 0–48% fat. Using different homogenisation pressures to produce oil droplets of different sizes resulted in emulsions with slightly different viscosities (smaller oil droplets resulted in higher viscosities) and these differences in viscosity were significantly reflected in terms of differences in perceived sensory fat contents for systems containing liquid oil. The perceived and actual fat contents of emulsions with the same viscosity but differing in oil droplet size were higher for the samples with the larger oil droplets and this was interpreted as indicating an independent contribution of fat level to perceived fat content over and above that caused through viscosity increase. However, no effect of oil droplet size on perceived sensory fat contents was found for systems containing a predominantly solid fat, and this was attributed to the relatively smaller effects on viscosity with increasing fat content for the solid fat system compared with that of the liquid oil system. Richardson, Booth, and Stanley (1993) studied the effects of oil droplet size (homogenised and non-homogenised milk) and fat content on oral perception in 3.5 and 4.8% fat milks, with and without a thickener (carboxymethyl cellulose). The results showed no effect of fat content or homogenisation on viscosity (measured at 50 rads1) for either the unthickened milks or the milks thickened to the viscosity of double cream. For the

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unthickened milks, the ratings for creaminess and fat content were correlated, with samples of unthickened, non-homogenised milk scoring significantly higher for these attributes than samples of unthickened homogenised milk. No significant differences were found in creaminess or perceived fat content between the milks differing in actual fat contents. Interestingly, in the case of the thickened milks, the responses obtained for creaminess and perceived fat content were again similar for all four milk samples but, in this instance, the homogenised milk containing 4.8% fat scored significantly higher than the other thickened samples. Despite the fact that the authors do not give any data on oil droplet size and distribution, they concluded that a high density of evenly sized particles (i.e. produced during homogenisation), together with an adequately high viscosity, results in a realistic sense of creaminess. Richardson and Booth (1993) discussed further the effect of viscosity, globule size and size distribution, and inter-globule distance in terms of the creamy texture of milks and creams. Here they considered the possibility that wide variation in globule size might contribute to a lack of smoothness in dairy emulsions, which they consider reasonable if, for example, larger and/or smaller-sized globules tend to group together in regions between the tongue and palate. Overall, they concluded that varying the amount and dispersion of fat independently of the viscosity of the emulsion provided further support for the more general hypothesis that emulsified fat contributed something to creaminess in addition to its effect on viscosity. In a model study investigating the effects of oil droplets on physical and sensory properties of oil-in-water emulsion agar gels (Kim, Gohtani, & Yamano, 1996), it was found that, on sensory evaluation, samples of gel containing small oil droplets were perceived to be less oily than those containing larger oil droplets. However, the gels with small oil droplets were perceived as being harder than those with the larger oil droplets and this was reflected to some extent in the results of physical compressive properties. The effect of solid particle size, shape and nature on perceived textural characteristics has also received attention in a number of studies. The confectionery literature views the minimum particle size that can be detected by the palate to be in the region of 25 mm (Hinton, 1970) and, if particles of a chocolate are all or nearly all reduced below this size, the texture is considered to have reached optimum smoothness for chocolate (Minifie, 1980). However, even smaller particle sizes enhance and ‘round out’ the flavour of chocolate. In contrast to chocolate particles, which are irregular in shape but do not possess sharp edges, particles of alumina (used in toothpastes for their abrasive effect) are hard and possess sharp edges. Such particles produce a gritty sensation in the mouth even when the particle sizes are as low as 10 mm.

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Tyle (1993) found that oral texture and consistency perception and preference depended on the size, shape and hardness of particles in aqueous suspensions. Assessment of suspensions of model particles of garnet, polyethylene and mica showed that fairly large particles could be palatable if shape and hardness constraints were taken into account. Size differences were found to matter more if particles were hard and sharp than if they were soft and rounded. In another investigation into the oral perception of grittiness using a range of microcrystalline cellulose particles dispersed in aqueous suspensions, low and high viscous suspensions, and soft and hard gels, Imai, Hatae, and Shimada (1995) found that the proportion of people who perceived grittiness increased with increasing particle size and with increasing particle concentration. For each aqueous phase type investigated, multiple regression analysis showed a high correlation between the people who perceived grittiness and the logarithm of (particle sizeparticle concentration). For the particles and concentration ranges investigated, the factor contributing most to grittiness was concentration, followed by type of aqueous phase and then particle size. As particle size is reduced, there is obviously a point at which grittiness no longer becomes detectable and gives way to a smoother mouthfeel (possibly related to creaminess). Indeed, if it is assumed that the creaminess associated with emulsified fat in oil-in-water food emulsion systems stems to some extent from the particle size and/or particle size distribution of the fat, clearly one approach to improving the perceived creaminess of low-fat products is to mimic the effect of the fat droplet. This approach was explored in the development of the fat replacer Simplesse, whereby, through heat coagulation under continuous shear conditions, protein was encouraged to form spherical, insoluble microparticulates of 0.1–3.0 mm in diameter. It has been claimed that this particle size range is optimal for providing creamy characteristics (Singer, 1996; Singer, Yamamato, & Latella, 1988) and such claims, although not supported by many published experimental investigations, seem reasonable when it is considered that emulsified fat globules in food products are usually in the size range 0.1–3 mm (Jones, 1996). The use of micron-sized particulates has been exploited in a number of other commercial developments of fat mimetics (e.g. LITA, Trailblazer, Stellar), in which microparticulation has been used to obtain non-fat particles of similar size to fat droplets in emulsions. However, the extent to which it is necessary to mimic perfectly the size and shape of oil droplets is somewhat questionable, particularly as different approaches to fat replacement utilise a range of particle sizes beyond the 0.1–3 mm considered optimal, and shapes that range from distinct spheres through irregular particles to fibrous structures (Jones, 1996).

2. Study rationale The work reported here details the results of investigations that have been aimed at clarifying some of the unresolved issues concerning the perception of creaminess described above. In particular, the aims of the work were to investigate and clarify the relationships between particle size, shape and nature and their resulting effect on sensory properties, with particular reference to creaminess. Solid particles (calcium carbonate and alumina), liquid particles (oil or fat droplets) and gas particles (i.e. air bubbles) of different sizes were investigated in model systems, chocolate mousses and artificial creams. Sensory evaluation of all the products produced has been carried out with emphasis on identifying the perceived creaminess of the samples and other attributes that might be related to this, supported by rheological and microscopic characterisation of the products to enable inter-relationships between sensory characteristics and measurable physical parameters to be established.

3. Preparation procedures 3.1. Model systems containing solid particles Model systems containing five different types of solid particles were assessed. Three calcium carbonate types with different particle size distributions were supplied by Rhoˆne-Poulenc Chemicals, Birmingham, UK (Calopake F, Sturcal F and Sturcal H, coded CK, CS and CH, respectively). Two white fused alumina types with different size distributions (F1200 and F800, coded F12 and F8, respectively) were supplied by Universal Abrasives Ltd., Salford, UK. Each of the particles was investigated at three levels (1.5, 3 and 6%), giving a total of 16 samples (including a control with no particles added) for assessment. The particles were dispersed in a standard 0.3% solution of xanthan gum (Keltrol T, The NutraSweet Kelco Company, Tadworth, UK) and sucrose (3%). The xanthan was used to stabilise the particles against sedimentation and the sugar to make the systems more palatable for sensory evaluation. One-kilogram batches of the particle dispersions at the desired incorporation level were then prepared using the standard xanthan and sugar solution. The particles were added over 2 min to the xanthan/sugar solution while shearing with a Silverson mixer and then sheared for a further 4 min to ensure complete dispersion. These dispersions were then stored at 5  C prior to assessment. 3.2. Chocolate mousses Six samples of chocolate mousse were prepared using a single formulation that contained 7% butterfat; the

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mousses differed only as a result of the processing conditions used during their aeration. Samples were prepared at two different overruns and with three different beater speeds (i.e. six samples). The basic formulation used for the chocolate mousses is given in Table 1, while Table 2 gives the processing conditions used during aeration. Homogenisation was carried out in an APV-Gaulin two-stage laboratory homogeniser and aeration carried out in a Mini-Mondo continuous aerator. Following collection, the mousse samples were blast-chilled to 5  C and then stored at this temperature prior to assessment.

formulation, samples were collected under three sets of processing conditions, which were designed to give three different droplet sizes in the final product. The processing conditions used in preparation of each of the cream samples are given in Table 4. Samples were prepared in an APV-Rannie homogeniser, and following collection the samples were stored at 5  C prior to assessment. Full details of all preparation procedures are available on request.

3.3. Artificial creams

4.1. Sensory assessment

Nine artificial creams were produced and assessed in the course of the work. Three formulations were processed (Table 3) differing in fat content (10 or 20%) and the presence or absence of an added flavour. For each

A trained sensory panel comprising 14 females was used to assess the sensory characteristics of the model systems and products. Sensory profiling was carried out in individual booths, with temperature conditions of 22  C, and under green light illumination to mask colour differences. Attributes were recorded using a computerised data acquisition system on unstructured line scales with appropriate anchor points. The extreme ends of these line scales were scored as 0 (left) and 100 (right). Still mineral water and apple pieces were available as palate cleansers during tasting. Fresh carrot sticks and sparkling mineral water were available between tasting sessions. For each product category, an initial discussion session was held using samples with the extremes of the sensory characteristics, and the panellists were asked to characterise the sensory properties of the samples individually. From the group discussion, the main descriptive attributes were then derived and their definition and anchor points agreed. Training sessions were then performed using the agreed terms, in which samples were presented randomly to panellists, coded with three-digit random numbers. In the experiment, the samples were presented in a balanced presentation sequence and coded with threedigit random numbers. The order of samples to panellists

Table 1 Formulation used in production of chocolate mousse samples Ingredient

%

Water Skimmed milk powder Sugar Cocoa powder Mono-diglyceride Unsalted butter Gelatin (200 Bloom)

62.71 9.00 12.00 5.50 0.70 8.54 1.55

Table 2 Processing conditions used for the aeration of chocolate mousse samples Sample

Desired overrun (%)

Flow speed

Manometer reading

Back pressure

Beater speed (rev/min)

Measured overrun (%)

1 2 3 4 5 6

130 130 130 65 65 65

350 350 350 350 350 350

3.6 3.6 3.6 1.6 1.6 1.6

3.0 3.0 3.0 3.0 3.0 3.0

200 600 1000 200 600 1000

134 131 133 64 61 61

4. Assessment procedures

Table 4 Formulation and processing conditions used for the preparation of artificial creams Table 3 Formulation used in production of artificial creams Ingredient

Formulation 1 Formulation 2 Formulation 3 (%) (%) (%)

Water 77.35 Fat blend (HPKO) 10.0 Sodium stearoyl-2-lactylate 0.3 Distilled monoglyceride 0.3 Sodium caseinate 2.0 Carrageenan 0.05 Maltodextrin (24 DE) 10.0 Cream flavour –

77.31 10.0 0.3 0.3 2.0 0.05 10.0 0.04

67.35 20.0 0.3 0.3 2.0 0.05 10.0 –

Temperature 2nd stage Cream 1st stage Sample Fat content flavour homogenisation homogenisation of collection ( C) pressure (bar) pressure (bar) (y/n) (%) 1 2 3 4 5 6 7 8 9

10.0 10.0 10.0 10.0 10.0 10.0 20.0 20.0 20.0

n n n y y y n n n

0 0 300 0 0 300 0 0 300

0 50 50 0 50 50 0 50 50

4 4 4 7 7 5 4 4 4

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was also randomised across sessions. Samples were assessed in triplicate, with the exception of the model systems containing solid particles, which were assessed in duplicate. Data were analysed by averaging attribute scores over all panellists. Analysis of variance (ANOVA) and multiple comparison testing in the form of least significant differences (LSD) were also performed to identify statistical differences between samples. Principal component analysis (PCA) was performed on the experimental data from the different product types in order to give an overview of the samples and attributes relationships. For the model systems containing solid particles, only one attribute, creaminess, was quantified, as the system was not formulated to represent a true food system. The samples were stored at 5  C prior to assessment by the panellists. An initial discussion session was carried out using eight of the samples, and panellists defined creamy characteristics as being associated with consistency (thin and thick texture), smoothness and a smooth mouthcoating effect. Scoring of creaminess for the various samples was performed according to this definition. However, it was noted that the perception gradually changed with time in the mouth, introducing other texture characteristics such as chalkiness and graininess. It was, therefore, agreed that the creaminess of the samples would be assessed when first introduced into the mouth. The samples were presented coded with threedigit numbers in sets of four in a semi-balanced design, and samples were assessed in duplicate. Samples of chocolate mousse were prepared 1 day prior to tasting and were stored at 5  C prior to assessment by the panellists at 22  C. In order to avoid the risk of introducing bias, no consensus was sought on the definition of overall creaminess, each panellist being given a free choice of meaning. Artificial cream samples were prepared the same week as assessment and were stored at 5  C prior to assessment by the panellists at 22  C. Discussion sessions were carried out using six of the creams to characterise flavour and texture attributes. In the experiment, the nine artificial cream samples were coded with random threedigit numbers and presented in sets of three, in a balanced incomplete block design. Samples were assessed in triplicate. 4.2. Physicochemical characterisation Rheological measurements were made using a CarriMed controlled stress rheometer (model CSL2100, TA Instruments, Leatherhead, UK). A sample was loaded onto the plate of the rheometer and oil was applied around the boundary of the cone to prevent sample evaporation. Measurements on the model systems and artificial creams were carried out at both 5 and 37  C to assess any differences between serving and body

temperature. Measurements on the chocolate mousses were carried out at 20  C. Firmness of the chocolate mousses was measured at 5  C using a Stable Micro Systems TAXT2 Texture Analyser (Stable Micro Systems, Godalming, UK), using a 35-mm diameter cylindrical probe set to penetrate to a depth of 8 mm at a rate of 1 mm/s and then return to the start point, again at a rate of 1 mm/s. Results were averaged over three replicate measurements. Particle size analysis of the model particle systems and determination of the oil droplet size distribution in the artificial creams were performed using a Malvern Mastersizer (Malvern Instruments Ltd, Malvern, UK). The air bubble size of the chocolate mousses was measured using an Optimas Image Analysis System (Optimas Corporation, Washington, USA). Images obtained from light and confocal microscopy were analysed to obtain the required bubble size information. Full details of all characterisation procedures are available on request.

5. Results 5.1. Model systems containing solid particles A key finding from these experiments demonstrated that particle concentration was a major influence on perceived textural characteristics, as also shown by Imai et al. (1995), and that a critical particle size turning point exists. Table 5 shows the sensory scores for creaminess, together with selected measures of particle size and measure viscosity, and the creaminess scores relative to the control are shown diagrammatically in

Table 5 Sensory and physicochemical data on model systems Formulation

Particle sizea Viscosity Coding Mean creaminess volume mean, (mPas) (100 s1, 37  C) score D(4,3)

Control 1.5% Calopake F 1.5% Sturcal F 1.5% Sturcal H 1.5% F1200 1.5% F800 3% Calopake F 3% Sturcal F 3% Sturcal H 3% F1200 3% F800 6% Calopake F 6% Sturcal F 6% Sturcal H 6% F1200 6% F800

Control 1.5CF 1.5CS 1.5CS 1.5A12 1.5A8 3CF 3CS 3CH 3F12 3F8 6CF 6CS 6CH 6A12 6A8

a

41.4 51.9 37.5 40.0 49.5 44.6 51.0 34.8 32.0 51.6 42.8 49.3 29.0 28.9 48.4 38.4 (LSD 10.8)

Measured on 1.5% dispersion only.

– 3.20 7.08 9.44 3.76 7.93 3.20 7.08 9.44 3.76 7.93 3.20 7.08 9.44 3.76 7.93

33.9 34.6 34.5 34.7 33.9 33.8 35.7 35.7 36.3 33.8 34.4 36.1 37.9 37.1 34.4 34.8

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Fig. 1. Perceived creaminess of solid dispersions, relative to that of the control. For sample codings, see Table 5.

Fig. 1. The control was assigned a level of creaminess that the panellists defined in terms of its mouthfeel and sweetness, and which was outside the context of classically creamy foods such as dairy products. The most creamy samples were those containing Calopake F and F1200. The samples perceived as being least creamy were those containing Sturcal F and Sturcal H at concentrations of 3 and 6%. Further, the samples containing 6% of Sturcal H and Sturcal F were significantly less creamy than the control containing no particles. With the exception of the system containing the F1200 particles, the perceived creaminess of any specific particle system decreased with increasing particle concentration. Fig. 2 shows the relationship between perceived creaminess and the size of the particles in terms of their D(4,3) values; two groupings can be seen. The samples with the smaller particles (Calopake F and F1200) are more creamy than the control, while the samples with the larger particles (Sturcal F and Sturcal H) are less creamy than the control. The samples containing the F800 particles, although having a particle size between those of the Sturcal F and Sturcal H systems, were more creamy than either of the Sturcal systems and, generally, very similar to the control in terms of their perceived creaminess. Fig. 2 also illustrates the general trend of decrease in creaminess with increase in particle concentration. These results, therefore, indicate that particle size does play a significant role in determining the perceived creaminess of these particle systems. A turning

point in particle size in the region of 4–7 mm seems to exist, below which there is an increased creamy perception and above which the creamy perception is reduced. These results therefore lend support to the work discussed by Singer (1996) regarding discontinuities in the oral sensing of particulates. Indeed, the particle sizes found here, above which creamy perception is replaced by a gritty perception, are very close to those quoted by Singer et al. (1988). Fig. 3 shows the relationship between perceived creaminess and viscosity. The creaminess decreases with increasing viscosity for the Sturcal F, Sturcal H and F800 particles, and is independent of viscosity for the Calopake F and F1200 particle systems. These results were somewhat surprising as it was expected that creaminess would increase with the amount of structure or viscosity in the system. Examination of the relationship between particle size and viscosity showed a positive relationship for the three calcium carbonate types, but not for the F800 and F1200 samples. This suggests that in situations where the Sturcal F and Sturcal H particles increase viscosity, the presence of the larger particles might be detected as a grittiness that suppresses creaminess. 5.2. Chocolate mousse system The key findings from this experiment showed that the size of the air bubbles is of primary importance in determining the overall perceived creaminess of chocolate mousses, with smaller bubbles resulting in creamier

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Fig. 2. Relationship between perceived creaminess and particle size for the solid dispersions. For sample codings, see Table 5.

Fig. 3. Relationship between perceived creaminess and viscosity for the solid dispersions. For sample codings, see Table 5.

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D. Kilcast, S. Clegg / Food Quality and Preference 13 (2002) 609–623 Table 6 Definitions and anchor points used in the chocolate mousse profile Attribute

Definitions

Anchor points

Overall creaminess Cocoa flavour Sweet Creamy flavour Smoothness/creamy texture

A personal assessment of the overall creaminess of the sample Degree of perceived cocoa powder flavour Degree of perceived sweetness associated with sucrose Degree of perceived flavour associated with dairy milk or dairy cream Degree of smoothness/creaminess perceived in the mouth associated with the absence (smooth) or presence (not smooth) of big air bubbles Amount of air perceived in the mouth Degree of density/firmness perceived in the mouth, Perceived size of air bubbles in the mouth Degree of perceived jelly texture associated with gelatine Degree of powderiness/flouriness perceived in the mouth associated with cocoa powder The speed at which the sample melts down in the mouth Amount of fat perceived in the mouth

Low–High Low–High Not–Very Not–Very Not–Very

Airy Heaviness Bubble size Jelly Powdery Meltdown rate Fatty

Not–Very Light–Heavy Small–Big Not–Very Not–Very Fast–Slow Low–High

Table 7 Sensory and physical data on chocolate mousses Sensory attributes

Sample number 1 a

2

3

4

5

6

LSD

Overall creaminess Cocoaa Sweet Cream flavour

44.1 66.5 33.9 34.4

51.9 57.9 37.0 36.1

55.2 53.7 35.3 41.3

41.8 65.0 30.9 29.9

47.2 64.2 36.2 33.0

51.3 63.3 32.9 38.6

10.7 9.3 10.7 12.7

Creaminess/smoothness (texture)a Airya Heavinessa Bubble sizea Jellya Powdery Meltdown ratea Fatty

40.6 74.0 24.3 63.5 2.6 21.9 19.7 15.0

57.6 53.8 26.0 37.0 2.8 25.0 23.0 15.8

58.4 49.9 27.3 27.5 1.3 23.1 20.6 14.6

51.7 39.4 35.1 25.8 9.4 30.0 30.5 16.4

60.5 29.0 41.3 15.0 12.3 26.3 37.1 19.1

64.7 30.5 37.9 16.8 6.1 24.3 32.1 18.8

10.1 8.6 9.2 7.9 6.1 11.1 8.3 9.5

134 58.8 66.0

131 46.9 75.7

133 28.6 86.3

64 63.4 69.5

61 38.2 71.5

61 35.0 81.2

Physical measures Overrun (%) Bubble size (mm) Viscosity (Pas, 20  C, 1 s1) a

Indicates those sensory attributes that differ at the 5% level. If the difference between the sample means is greater than the LSD value, then the sample means differ at the 5% level of significance.

samples. An increase in the amount of air in the chocolate mousses also contributed to a higher perceived overall creaminess, particularly when the increased amount of air was combined with small air bubble sizes. The sensory profile definitions are shown in Table 6, and sensory profile data together with the overrun, bubble size and viscosity data are shown in Table 7. During panellist discussion sessions, the smoothness of the samples was identified as being related to the creamy texture of the samples. Consequently, the smoothness attribute was taken as a measure of creamy texture. A clear trend of increasing overall creaminess with increasing beater speed used during aeration (decreasing bubble size) was found for the mousses at both the high (samples 1, 2 and 3) and low (samples 4, 5 and 6) overruns investigated (Fig. 4a). There is some evidence (not

statistically significant) that increasing the air content reduces overall creaminess but increases creamy texture, suggesting that some factor other than creamy texture contributes to overall creaminess. The cocoa flavour attribute shows evidence for an interaction between air content and bubble size. At low air contents, cocoa flavour is independent of bubble size, whilst at high air contents cocoa flavour decreases with decreasing air bubble size. This might be related to different flavour release characteristics, or to a contrast effect arising from the increasing creamy attributes associated with decreasing bubble size. However, as the same creaminess attribute trends are also seen at low air contents, this supports a flavour release mechanism. Overall creaminess showed an increasing relationship with increasing viscosity (Fig. 4b), which is relatively

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Fig. 4. a,b. Relationship between overall creaminess with air bubble size (Fig. 4a) and viscosity (Fig. 4b). The LSD value 5% level for the creaminess scores is 10.7. Sample codings are: 1-high overrun, low bubble size; 2-high overrun, medium size; 3-high overrun, high size; 4-low overrun, low size; 5-low overrun, medium size; 6-low overrun, high size.

Fig. 5. Principal component analysis of the sensory and physical data for the chocolate mousse samples. Sample codings are: 1-high overrun, low bubble size; 2-high overrun, medium size; 3-high overrun, high size; 4-low overrun, low size; 5-low overrun, medium size; 6-low overrun, high size. The codings for the physical attributes are : Force, load (g) at 8-mm penetration using a 35-mm diameter cylindrical probe at 1mm/s; Speed, flow speed during aeration; V1, viscosity (Pas) at 20  C at shear rate of 1 s1; G1(10), storage modulus, G0 (Pa) measured at 10 Hz; Yield, yield value (Pa); Size, average bubble size (mm). Other attributes ate sensory attributes shown in Table 7.

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overruns (samples 4, 5 and 6) are on the left-hand side characterised by heavy textures. The positive PC2 direction positions the samples according to the low bubble sizes generated by the high beater speed used during the whipping process, both for the samples with the high overruns (samples 1, 2 and 3) and for the samples with the low overruns (samples 4, 5 and 6).

unaffected by air content. However, as there was a strong negative relationship between bubble size and viscosity, it is difficult to ascribe the overall creaminess to either bubble size or viscosity with any confidence. A principal component analysis of the sensory and physical data is shown in Fig. 5. Eighty-three per cent of the variance in the data was accounted for in the first two components (PC1 44% and PC2 39%). PC1 is essentially a light–heavy dimension, and PC2 a not creamy–creamy dimension, this interpretation being supported by examination of the PCA plot for the sensory attributes alone (not shown). The six samples are well separated from each other on both PC1 and PC2. The samples with the higher overrun (samples 1, 2 and 3) are located on the right-hand side of the PCA plot characterised by light textures, and those with the lower

5.3. Artificial cream system The definitions and anchor points used in the sensory profile are shown in Table 8, and sensory profile data together with the fat droplet sizes and viscosities are shown in Table 9. Sample 7 (containing 20% fat and homogenised to the least extent) had the highest overall creaminess and

Table 8 Definitions and anchor points used in the artificial cream profile Attributes

Definitions

Anchor points

Overall creaminess Vanilla Milky/creamy Sweet Nutty Oily flavour Water flavour Creamy texture Thickness Smoothness Greasy Powdery Mouthcoating

A personal assessment of the overall creaminess (flavour+texture) of the sample Degree of perceived artificial vanilla flavour as in vanilla ice cream Degree of perceived milk/cream flavour as in the top of the milk Degree of perceived sweetness associated with sucrose Degree of perceived nut flavour associated with mixed nuts Degree of perceived vegetable oil flavour Degree of perceived water flavour Overall creamy texture perceived in the cream range Degree of perceived thickness when sample first put in the mouth Degree of smoothness perceived in the mouth Amount of fat perceived in the mouth Degree of powderiness/flouriness perceived in the mouth Degree of perceived mouthcoating effect in the mouth

Low–High Not–Very Not–Very Not–Very Not–Very Not–Very Not–Very Not–Very Thin–Thick Not–Very Not–Very Not–Very Not–Very

Table 9 Sensory and physical data on artificial creams Sensory attribute

Sample number 1

2

3

4

5

6

7

8

9

LSD

Overall creaminessa Vanillaa Milk/creamy flavoura Sweet Nuttya Oily flavour Watery flavoura Overall creamy texturea Thicknessa Smooth Greasya Powderya Mouthcoatinga

47.9 10.4 44.7 27.8 27.7 9.8 17.0 45.1 43.4 64.2 20.1 6.7 37.9

46.2 9.7 42.1 29.5 26.2 6.0 17.6 42.2 40.5 63.0 15.5 3.3 30.7

42.1 11.6 41.0 27.4 28.5 4.2 18.1 39.8 36.6 65.2 11.5 4.1 28.0

50.4 59.7 39.1 39.8 8.9 9.6 15.9 47.0 46.7 64.3 18.2 5.7 41.4

41.5 62.6 36.4 46.0 10.5 5.3 19.9 41.9 39.5 64.7 13.7 6.0 33.3

41.9 59.7 34.1 44.5 8.0 6.1 17.7 39.1 38.5 64.6 11.2 4.6 31.3

60.1 11.4 50.6 28.1 18.0 13.4 8.7 60.9 61.2 66.0 26.3 7.0 49.4

50.6 9.5 49.3 27.4 21.5 9.1 12.7 51.1 49.3 63.1 21.0 2.8 40.4

48.4 6.5 52.8 25.9 28.6 8.1 11.9 49.0 48.7 65.7 19.0 2.4 36.3

11.2 10.4 12.6 11.6 12.6 9.3 7.7 11.2 11.0 8.0 8.4 4.8 10.3

Physical measures Oil droplet size, D(4,3) Viscosity (mPas, 5  C, 50 s1)

2.8 49.6

1.1 29.2

0.5 26.6

2.8 44.5

1.2 36.1

0.5 30.4

2.6 89.0

1.0 52.0

0.5 43.0

a

Indicates those attributes that differ at the 5% level. If the difference between the sample means is greater than the LSD value, then the sample means differ at the 5% level of significance.

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was significantly different from all other samples for this attribute, except for samples 4 and 8. The samples with 20% fat (7, 8 and 9) scored higher than the samples with 10% fat (although not statistically significant), when samples homogenised to the same extent were compared. A strong relationship between overall creaminess and creamy texture scores is apparent in Fig. 6, in which

the samples with added creamy flavour (4, 5 and 6) show the same relationship as those with no added creamy flavour, suggesting that the addition of cream flavour had little influence on overall creaminess. The effects of oil droplet size and fat content are shown in Fig. 7a and b. Fig. 7a shows clear relationships between creamy texture and oil droplet size which

Fig. 6. Relationship between overall creaminess and creamy texture for the artificial creams. Sample codings are: samples 1, 2, 3—10% fat, no added flavour; samples 4, 5, 6—10% fat, added flavour; samples 7, 8, 9—20% fat, no added flavour. Samples 1, 4, 7—low homogenisation level (high droplet size); samples 2, 5, 8—medium homogenisation level (medium droplet size); samples 3, 6,9—high homogenisation level (low droplet size).

Fig. 7. Relationship creamy texture and oil droplet size (Fig. 7a) and between overall creaminess and oil droplet size (Fig. 7b) for the artificial creams. Sample codings are: samples 1, 2, 3—10% fat, no added flavour; samples 4, 5, 6—10% fat, added flavour; samples 7, 8, 9—20% fat, no added flavour. Samples 1, 4, 7—low homogenisation level (high droplet size); samples 2, 5, 8—medium homogenisation level (medium droplet size); samples 3, 6, 9—high homogenisation level (low droplet size).

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Fig. 8. a,b Relationship between viscosity and oil droplet size for the artificial creams, showing the effect of droplet agglomeration. Sample codings are: samples 1, 2, 3—10% fat, no added flavour; samples 4, 5, 6—10% fat, added flavour; samples 7, 8, 9—20% fat, no added flavour. Samples 1, 4, 7—low homogenisation level (high droplet size); samples 2, 5, 8—medium homogenisation level (medium droplet size); samples 3, 6, 9—high homogenisation level (low droplet size).

depend on the fat content. For each fat content, creamy texture increases with increasing droplet size, with the higher fat content giving higher creamy texture scores. Added flavour has no effect on creamy texture or overall creaminess, and there are indications that, at the intermediate oil droplet size (samples 2 and 5), added cream flavour decreases the overall creaminess. The data in Table 9 show that the main consequences in adding cream flavour are to increase the vanilla and sweet flavour attributes, and to decrease the nutty attribute. These effects are likely to relate to the flavour composition of this flavour type. The increased creaminess with increasing fat droplet size was unexpected, and opposite to the effect seen by Mela et al. (1994), who were working with model oil-inwater emulsions containing sunflower oil. Typically, it would be expected that the viscosity of a liquid/liquid oil-in-water emulsion would increase with a decrease in droplet size (i.e. an increase in homogenisation pressure). This is the reverse of the findings of this project, as can be seen in the plot of viscosity against droplet size in Fig. 8. However, the fat used in the present study was semi-solid and this can result in the fat droplets sticking together to form clusters. Such clusters result in increased viscosity, and an explanation for the higher

viscosities in the creams homogenised to the least extent could therefore be that, in these samples, a greater proportion of the fat droplets were clustered together.

6. General discussion In common with many other sensory attributes, it is highly likely that the perception of creaminess is highly context-sensitive. Consequently, each system investigated needs to be discussed separately. 6.1. Effect of particles The model system containing dispersed solid particles exhibited properties that clearly showed that the presence of particles can influence the perceived creaminess, as defined by this specific panel. In this simple system, the perception of creaminess in the control sample, with no dispersed solid, is likely to be associated with the perceived solution properties, primarily viscosity. Addition of small particles increased the creaminess, whereas addition of larger particles reduced creaminess. The latter effect is more easily rationalised, as discussions with the panellists showed that the larger particles

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were associated with a distinct grittiness, which acted to suppress any creaminess. The origin of the increased creaminess with small particles is more complex, as these dispersions also had higher viscosities; it is consequently difficult to associate the increased creaminess with particle size, particle concentration or viscosity. However, it can be seen from Fig. 3 that the 6% CF sample had a higher viscosity than the other samples of similar particle size, but with the same creaminess, providing some indication that the viscosity contribution is not relevant. The data from this experiment point to a particle size cut-off region between 4 and 7 mm, below which creaminess is enhanced and above which creaminess is suppressed, but with a dependence on the particle type. This region is close to that reported by Singer et al. (1988). The suppression is greater at higher particle contents, but surprisingly creaminess did not increase at higher contents of small particles. 6.2. Effect of aeration—mousse system The chocolate mousse system provided a convenient vehicle for studying the effects of the importance of aeration to creaminess perception. The system proved capable of accepting a range of process conditions to generate a wide range of structural variants that were stable and were appropriate for detailed sensory analysis. The experiment clearly identified air bubble size as the primary factor influencing creaminess, with air content a secondary factor, but it was not clear whether this was a direct effect resulting from the perception of the air bubbles or a secondary effect arising from viscosity changes. There was some evidence that the cocoa flavour attribute was influenced by different flavour release characteristics arising from different air contents and bubble sizes. However, in retrospect, the system would have been improved by eliminating the chocolate component, in order to secure a clearer focus on the creamy attributes. The attempt to examine the use of cream flavour additions to the artificial cream system produced ambiguous results, as the main effects of the cream flavour were to add vanilla flavour and sweetness, and indeed there were some indications that creamy flavour was suppressed. It is of course possible that the intended purpose of this commercial product was to enhance flavour characteristics that consumers might associate with familiar creamy products, but this experimental variable added little to the present study. 6.3. Artificial cream system The artificial cream study generated some valuable results in other respects. Overall creaminess was strongly correlated with the creamy texture, with little

contribution from creamy flavour. As anticipated, creaminess increased with increasing fat content, but more surprisingly creaminess also increased with increased fat droplet size. This was the converse of the anticipated effect that creaminess would increase with reduced particle size and increased viscosity, and was a consequence of agglomeration of the individual fat droplets into larger structures that increased viscosity. This was ascribed to the presence of solid fat in the chosen system. In practice, oil-in-water food emulsions can contain either liquid fat, which does not exhibit such clustering, or fats containing varying proportions of solid fat. It is be expected therefore that a wide range of effects should be observed in practice, and that generalised rules would be difficult to develop.

7. Conclusions This set of studies has focused on structural systems of direct relevance to the food industry, and on selected specific structural and compositional factors for investigation out of the wider range that might have been considered, such as dispersant type in the model system, gelling agent type in the mousse system and fat type in the artificial creams. A more detailed investigation of creaminess in a wider range of compositional and structural factors is not feasible across different product types, and closer, systematic studies on a specific food type depend on the ability to generate the range of products for testing. This present study encountered substantial difficulties in producing ranges of products that were sufficiently stable for sensory and physical testing. Indeed, investigations of two additional product types (whipped creams and oil-in-water dressings) were not reported as a consequence of the poor structural integrity of many of the formulations produced. In contrast, the mousse system proved to be flexible and adaptable to controlled change and should be suitable for more systematic experimentation; further work utilising a modified mousse formulation in being carried out at Leatherhead Food RA. Some elements of the contribution of flavour to texture perception were investigated, but this contribution remains poorly understood and requires further investigation. In particular, the potential negative impact of specific flavour contributions on creaminess has not been investigated at all, and it might be expected that attributes such as bitter and astringent might have a suppression effect. Further research is also needed into the contribution of appearance attributes, which are likely to have a strong influence on expectations, and into cross-cultural aspects of creaminess perception. Whilst it is unlikely that a general model of creaminess will ever be achievable, such studies will help to create a more complete picture.

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Acknowledgements The authors would like to thank the industrial members of consortium AY197 for their financial support for this project, and to staff in the Product Development and Sensory and Consumer Science departments for their contributions to the project.

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