The dynamics of aroma release during consumption of candies of different structures, and relationship with temporal perception

Food Chemistry 127 (2011) 1615–1624

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The dynamics of aroma release during consumption of candies of different structures, and relationship with temporal perception Isabelle Déléris a,⇑, Anne Saint-Eve a, Fanny Dakowski a, Etienne Sémon b, Jean-Luc Le Quéré b, Hervé Guillemin a, Isabelle Souchon a a b

UMR 782 INRA/AgroParisTech Génie et Microbiologie des Procédés Alimentaires, 1 avenue Lucien Brétigniéres, F-78850 Thiverval-Grignon, France UMR 1324 INRA/AgroSup Dijon/CNRS/Université de Bourgogne Centre des Sciences du Goût et de l’Alimentation (CSGA), 17 Rue Sully, F-21065 Dijon, France

a r t i c l e

i n f o

Article history: Received 3 November 2010 Received in revised form 3 January 2011 Accepted 7 February 2011 Available online 5 March 2011 Keywords: Aroma compounds Gels Structure Dynamic of release PTR-MS Perception over time Eating behaviour

a b s t r a c t We investigated the role of both candy texture and eating technique (melting or chewing) on the dynamics of aroma release. One novelty of this type of analysis was the simultaneous application of instrumental and sensory analysis. Four candy textures were established based on their storage modulus at 1 Hz by varying the gelatine content between 0 and 15% w/w. The in vivo release of three aroma compounds was monitored using Proton Transfer Reaction Mass Spectrometry and with a trained panel of testers. The gelatine content had no significant effect on the headspace/product partition and diffusion properties of the aroma compounds. The highest in vivo release for all aroma compounds was obtained with the 2% gelatine sample. Our findings indicated that aroma release was determined by interaction between the product properties and oral behaviour. Relations between the dynamics of release and perception (method of Temporal Dominance of Sensations) have been established on temporal parameters. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The role that food structure/texture can have on aroma release has been widely studied for different types of real and model foods. Such studies have benefited substantially from the development of rapid and sensitive instrumental techniques, particularly Proton Transfer Reaction Mass Spectrometry (PTR-MS) (Apréa, Biasioli, Gasperi, Märk, & Van Ruth, 2006; Boland, Delahunty, & Van Ruth, 2006; Mestres, Kieffer, & Buettner, 2006; Van Ruth, de Witte, & Uriarte, 2004) and Atmospheric Pressure Chemical Ionisation Mass Spectrometry (APCI-MS) (Gierczynski, Labouré, Sémon, & Guichard, 2007; Linforth, Baek, & Taylor, 1999; Ovejero-Lopez, Haahr, Van den Berg, & Bredie, 2004). The effects of product structure result from a combination of physicochemical (entrapment of aroma compounds in the product structure and/or obstruction of their mass transport) and physiological phenomena (types of oral behaviour) (Buettner & Beauchamp, 2010; Buettner, Beer, Hannig, Settles, & Schieberle, 2002; Pionnier et al.; 2004; Ruijschop, Burgering, Jacobs, & Boelrijk, 2009; Wright & Hills, 2003). In most cases, increasing product viscosity or firmness results in decreasing aroma release and perception (Baek, Linforth, Blake, & Taylor,

⇑ Corresponding author. Tel.: +33 0 1 30 81 54 39; fax: +33 0 1 30 81 55 97. E-mail address: [email protected] (I. Déléris). 0308-8146/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.02.028

1999; Boland et al., 2006; Hansson, Giannouli, & Van Ruth, 2003), although some contradictory results have been reported (Hollowood, Linforth, & Taylor, 2002; Mestres et al., 2006; Weel et al., 2002). The reasons for these divergent findings may include the use of different consumption protocols, the large variability between individual eating patterns and/or the existence of specific cognitive interactions. A better understanding of the possible relationships between sensory perception and the physicochemical properties of foods has long been an objective to allow better control of the sensory properties of food. There have been numerous studies addressing the relationship between in vivo aroma release and aroma perception (Biasioli et al., 2006; Gasperi et al., 2001; Heenan, Dufour, Hamid, Harvey, & Delahunty, 2009; Roberts, Pollien, Antille, Lindinger, & Yeretzian, 2003; Van Ruth et al., 2004). However, as far as we are aware, the dynamics of perception have not been considered. Also, few studies have focused on perception over time by applying the Time–Intensity sensory method; these few studies have nevertheless proposed some relations with the dynamics of in vivo aroma release (Baek et al., 1999; Lethuaut, Weel, Boelrijk, & Brossard, 2004; Linforth et al., 1999; Pionnier et al., 2004). The conclusions concerning relationship between aroma release and perception were diverse and largely depended on the system studied, highlighting the complexity and the variety of phenomena involved.

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We therefore aimed to evaluate the consequences of product structure, in the case of candies, on the dynamics of in vivo aroma release and to identify the relationships with temporal sensory perception (TDS measurements). We exploited instrumental and sensory methods in parallel as part of an original approach to describe aroma release and perception and to identify the relationships between these two phenomena. 2. Material and methods 2.1. Materials Aroma compounds (ethyl hexanoate, diacetyl, (Z)-hex-3-en-1ol and c-Decalactone) were purchased from Sigma Aldrich (France) and were at food grade quality (Table 1). These molecules were chosen because they present a wide range of physicochemical properties, notably in terms of volatility and hydrophobicity, and because their sensory properties were in agreement with the nature of the product (candies). A concentrated stock solution was prepared in polypropylene glycol (Sigma Aldrich, France) and used throughout the study to flavour candies (4 months). Four candies were prepared with different structures based on different gelatine contents (200PS30, Rousselot, Angoulême, France: 0%, 2%, 5% and 15% gelatine (w/w)) as described previously (Saint Eve et al., submitted for publication). The samples thus had similar compositions but different structures, from liquid (without gelatine) to elastic gels of different strengths (Saint Eve et al., submitted for publication). The concentrations of the other constituents were the same in all four preparations (glucose syrup: 25%w/w; saccharose: 25%w/w; citric acid: 1%w/w; red dye: 0.25%w/w). Products were flavoured with 0.4% (w/w) of the concentrated aroma solution: aroma compounds were added to the preparations at concentrations of 89.76 mg/kg for ethyl hexanoate, 17.37 mg/kg for diacetyl, 94.37 mg/kg for (Z)-hex-3-en-1-ol and 10.06 mg/kg for cDecalactone. The final concentration of aroma compounds in the products was verified by gas chromatography analysis after an extraction step with hexane (results not shown). There was no significant difference between the four products concerning final diacetyl, (Z)-hex-3-en-1-ol and cDecalactone concentrations. However, the final concentration of ethyl hexanoate was 3.7-fold higher in the 15% gelatine preparation than in the other three preparations. No aroma loss was detected during the flavouring step (which was at 50 °C). Ethyl hexanoate was thus presumed to have been lost during product storage and manipulation from all but the 15% gelatine preparation (due to the retention effect of the structure). All the data concerning the 15% gelatine product have consequently been corrected to take the difference in ethyl hexanoate concentration between products into account. 2.2. Experimental determination of the headspace/product partition coefficients (KH/P) of aroma compounds The headspace/product partition coefficients (KH/P) of aroma compounds were determined at 25 °C by the Phase Ratio Variation method (PRV) (Ettre, Welter, & Kolb, 1993). Once gelled, known amounts of product were cut into small pieces, placed in closed vials (22.4 ml, Chromacol, France) and incubated at 25 °C overnight to allow equilibration. Then, 2 ml aliquots of the headspace above the product were sampled with an automatic headspace CombiPal sampler (CTC Analytics, Switzerland) and injected into a gas chromatograph (GC-FID HP6890, Agilent Technologies, Germany) equipped with an HP-INNOWax polyethylene glycol semi-capillary column (30 m  0.53 mm, with a 1 lm-thick film) and a flame ionisation detector. The temperatures of the gas chromatograph injector and detector were set at 250 °C. The oven program was

37.3 min long, starting at 50 °C, with 4 °C/min up to 70 °C, then 5 °C/min up to 170 °C, 8 °C/min up to 220 °C and then 6 min at 220 °C. The carrier gas was helium (average velocity of 57 cm/s at 50 °C). Peak areas were measured using the Hewlett–Packard Chemstation integration software. A non-linear regression was applied to determine the headspace/product partition coefficients accurately (Atlan, Tréléa, Saint-Eve, Souchon, & Latrille, 2006). All experiments were performed in triplicate to validate the repeatability of the measurements. 2.3. Characterisation of in vitro and in vivo aroma release by PTR-MS measurements In vitro and in vivo release kinetics was measured using a HighSensitivity Proton Transfer Reaction-Mass Spectrometer (PTR-MS) (Ionicon Analytik, Innsbruck, Austria). The PTR-MS instrument drift tube was thermally controlled (60 °C) and operated with a voltage set at 600.1 (±0.4) V. Measurements were performed using the Multiple Ion Detection mode on 7 or 8 specific masses with a dwell time of 0.1 s per mass. Mass/charge ratios m/z 21 (signal for H3O+) and m/z 37 (signal for water clusters H2O–H3O+) were monitored to check instrument performances and cluster ion formation. A summary of the main values of the setting parameters is presented in Table 2. From the fragmentation patterns of individual compounds (Table 1), the molecules studied were monitored with m/z 83 (Z3-hexen-1-ol), m/z 87 (diacetyl), m/z 93 (c-Decalactone), m/z 145 (ethyl hexanoate) and m/z 171 (c-Decalactone). With this selection, no fragment overlapping was observed. Transmission factors for the measured m/z were considered as specified by Ionicon for the PTR-MS used in this study (transmission factors of 0.982, 0.948, 0.873, 0.323 and 0.252 for m/z 83, 87, 93, 145 and 171, respectively). Signal-to-noise ratios for all ions, except for m/z 93 and 171, were between 2.0 and 107.9 (depending on the m/z measured and the operating conditions), meaning that the responses for the compounds studied sufficiently exceeded the baseline. c-Decalactone was initially included in the flavouring mixture for its peach-like aroma, however, as no consistent results have been obtained (as concerns either sensory (Saint Eve et al., submitted for publication) or flavour release aspects) it was not further considered in this study. 2.4. Measurement of in vitro release kinetics of aroma compound from gels using the VASK method The release kinetics of aroma compounds was determined experimentally by the Volatile Aroma Stripping Kinetic (VASK) method (Lauverjat, de Loubens, Déléris, Tréléa, & Souchon, 2009). Flavoured products (25 g) were gelled in 0.25 l flasks (Schott, France) and placed in a thermostated vault at 25 °C for 12 h to allow a thermodynamic equilibrium to be established between the product and the headspace. Valves on the vial caps allowed purging of the headspace for 10 min with a constant air flow (fixed between 34 and 64 ml/min, Brooks 5860S mass flowmeter, Brooks Instrument, USA), 15 ml/min of which was injected into the PTR-MS reaction chamber. Three replicates were performed for each product. 2.5. Determination of apparent diffusion coefficient A description of the main mass transfer phenomena occurring in the flask allowed the establishment of a mechanistic model composed of a set of differential equations. The main assumptions were mass balances for each phase, local thermodynamic equilibrium at the interfaces and mass flux conservation through the interfaces at all times (Lauverjat et al., 2009). The apparent diffusion coefficient Dapp was determined by numerically fitting

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the mechanistic model to the experimental in vitro release data using the Levenberg–Marquardt algorithm (least squares curve fitting). Numeric calculations were performed using MatlabÒ 7 software (The Mathworks, Natick, MA) and the associated statistical toolbox.

Table 2 summary of the main PTR-MS setting parameters for in vitro and in vivo measurements.

H316O+a

Signal (count.s1) Intra-measurement variations (%) Day-to-day variations (%) H2O–H3O+ signal (m/z 37) (% of H316O+ signal) Pdrift (mb) E/N (Td)b Signal-to-noise ratio

2.6. Measurements of in vivo aroma compound release kinetics and sensory analysis Twelve panellists (4 men/8 women, all members of the laboratory, 22–45 years old) were recruited for the study. They were instructed not to smoke, eat, drink, or use any persistent-flavoured product for at least one hour before the session. They were specifically trained to evaluate product aroma perception and to perform sensory analyses in parallel to the in vivo measurements. Nose space air was sampled via two inlets of a stainless nosepiece placed in both nostrils of the assessors. This system was fixed onto glasses so that the panellists could eat relatively normally. The inlet of the PTR-MS instrument was connected to the sampling device via a 1/1600 PEEK™ tube maintained at 60 °C. In addition to the ions monitored as described above, ion m/z 59 (derived from acetone) was monitored to trace panellist breath (Weel et al., 2002). For the liquid product (0% gelatine), an aliquot of 20 ml was presented in a 70 ml hermetically sealed cup. For the gelled products, 4 g pieces were presented in Petri dishes. Samples were coded with three-digit random numbers and presented in a monadic way. Each assay lasted 6–7 min. Four sessions of 30 min were organised to obtain four replicates for each product and each subject. During a given session, subjects had to eat four samples served at ambient temperature (20 °C), according to a defined procedure to reduce the inter-individual variability. The room air was first analysed for 10 s. Then, after positioning the sampling device in the two nostrils, panellists were asked to breathe regularly for 30 s (breath analysis). For liquid product, panellists were asked to sip some product through a straw and to consume it as they would normally. At the end of the session, cups were weighted to determine the exact amount of product that had been consumed. For gelled samples, panellists were instructed to put them in their mouth and let them melt slowly (protocol 1) or chew them as they would normally (protocol 2). This second protocol was used to evaluate the consequences of chewing behaviour on aroma release. During all measurements, the panellists were asked to keep their mouth closed and to only breathe through the nosepiece. The time of the first swallow was recorded. Release data were acquired for the 3 min after the beginning of the trial. Between each sample, panellists were asked to clean their mouth by eating plain crackers and drinking mineral water. Panellist breath was retested before

In vitro measurement

In vivo measurement

8.0 106 ± 0.3 106 8.4

8.1 106 ± 0.8 106 4.1

– 0.9–1.2

9.0 0.8–3.6

1.8 (±0.1) 178.3 (±4.0) 23.4–107.9

1.9 (±0.06) 164.8 (±5.5) 2.0–13.3

a

H316O+signal = H318O+signal (m/z 21)  500. E is the electric field and N is the number density of the gas in the drift tube, 1 Td = 1017 cm2 V1 s1. b

each new measurement. All the measurements were performed within a 16-day period. For data handling, release curves were divided into three main periods: (i) the phase before the product was put in the mouth (phase 0); (ii) the oral phase of consumption (phase 1); and (iii) the phase after swallowing (phase 2). For each sample, the mean PTR-MS signal measured during phase 0 was subtracted from the PTR-MS signals obtained during the phases of product consumption. The following release variables were extracted from each individual release curve and for each phase of product consumption: maximal intensities (Imax1 and Imax2), times at which Imax occurred (tmax1 and tmax2), and areas under the curve (AUC1 and AUC2). Variables were also calculated for the whole kinetic: maximal intensity (Imax), time at which Imax occurred (tmax) and total area under curve (AUC). As the objective was to compare the extent of aroma release between products, the use of arbitrary units for aroma release data was sufficient for the analysis of intensity differences. All the data were corrected for the amount of product that was actually consumed. The time at which products were put in-mouth (i.e. 42 s after the beginning of data acquisition) was used as the reference for data comparison. Sensory analysis using the method of Temporal Dominance of Sensations (Labbe, Schlich, Pineau, Gilbert, & Martin, 2009) was performed simultaneously with nose-space measurements. The protocol and results were as described in Saint Eve et al. submitted for publication.

2.7. Statistical analysis Non-parametric descriptive analysis was carried out using in vitro data (median, quartiles). The effect of product composition on

Table 1 Physicochemical properties of aroma compounds used in this study. Aroma compound

a

Chemical formula

Chemical structure

Molecular weight (g/mol)

Log P

a

Air/water partition cœfficient (25 °C)

Sensory attribute

Fragmentation pattern by PTR-MS: main peaks and relative abundance (into brackets)

a

Diacetyl

C4H6O2

86.09

-1.34

0.55  103

Buttery/caramel

87 (100); 59 (11); 88 (5)

(Z)-hex-3-en-1-ol

C6H12O

100.16

1.61

0.21  103

Green

55 (100); 83 (39); 81 (22)

3

Ethyl hexanoate

C8H16O2

144.21

2.83

25.6  10

Strawberry/pineapple

145 (100); 43 (18); 117 (10) .71 (9); 146 (9)

c-Decalactone

C10H18O2

170.25

2.72

9.79  103

Peach

171 (100); 55 (60); 93 (75); 107 (35); 81 (15)

Estimation with EPI SuiteTM program.

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Table 3 Median values of headspace/product partition coefficients (KH/P) of aroma compounds at 25 °C and associated quartiles. Determination using the Phase Ratio Variation method. Diacetyl T = 25 °C

Ethyl hexanoate 3

0% gelatine 2% gelatine 5% gelatine 15% gelatine

KH/P  10

(Q1; Q3)  10

1.72 1.70 2.09 2.66

(1.63; (1.61; (1.87; (2.15;

3

KH/P  10

1.74) 1.76) 2.21) 3.54)

3

15.8 22.2 17.1 33.6

(Z)-hex-3-en-1-ol 3

(Q1; Q3)  10

KH/P  103

(Q1; Q3)  103

(14.6; (18.8; (15.4; (30.0;

1.15 1.03 1.16 1.25

(0.98; (0.90; (0.95; (1.13;

18.1) 22.5) 18.7) 37.7)

1.46) 1.15) 1.30) 1.40)

Table 4 Median values of apparent diffusion properties (Dapp) of aroma compounds at 25 °C and associated quartiles. Determination using the volatile air stripping kinetic method (PTRMS measurements). T = 25 °C 0% Gelatine 2% Gelatine 5% Gelatine 15% Gelatine

Diacetyl

Ethyl hexanoate

(Z)-hex-3-en-1-ol

Dapp (1010 m2/s)

(Q1; Q3) (1010 m2/s)

Dapp (1010 m2/s)

(Q1; Q3) (1010 m2/s)

Dapp (1010 m2/s)

(Q1; Q3) (1010 m2/s)

3.50 1.59 1.11 2.35

(3.30; (1.50; (1.00; (2.30;

0.35 0.57 0.48 0.43

(0.28; 0.53) (0.48 ; 0.67) (0.39; 0.56) (0.39; 0.47)

4.92 1.53 1.26 1.39

(4.30; 5.30) – (1.00; 1.50) -

3.70) 1.70) 1.20) 2.50)

physicochemical parameters was evaluated with the Kruskal and Wallis test. Statistical analysis of variance and Student’s t-test were carried out with in vivo release data, using the GLM (general linear model) module and the t-test module of the SASÒ software package. A Student–Newman–Keuls (SNK) full comparison test was also used to screen for significant differences between individual instances of sample type. The level of significance was set at p < 0.05.

3. Results and discussion 3.1. Determination of the physicochemical properties of aroma compounds Results for the three compounds (c-Decalactone was excluded) are summarised in Table 3 for headspace/product partition properties and in Table 4 for apparent diffusion properties.

3.1.2. Apparent diffusion properties The apparent diffusion coefficients of aroma compounds in the tested products at 25 °C were between 0.35  1010 m2/s (ethyl hexanoate, 0% gelatine) and 4.92  1010 m2/s ((Z)-hex-3-en-1-ol, 0% gelatine). These experimental values were of the same order of magnitude as data reported in the literature, despite experimental conditions not being exactly identical (Déléris et al., 2008; Landy, Rogacheva, Lorient, & Voilley, 1998; Savary, Guichard, Doublier, Cayot, & Moreau, 2006). Ethyl hexanoate had the lowest diffusion coefficient, while diacetyl and (Z)-hex-3-en-1-ol had similar diffusion properties. Diffusion properties appeared to depend on molecule size, but this is probably not the only physicochemical characteristic determining the diffusion results. The apparent diffusion properties did not differ significantly between preparations, except for diacetyl between candies with 0% and 5% gelatine; this may have been due to the formation of a network structured by the gelatine protein at high gelatine concentration. 3.2. In vivo release kinetics

3.1.1. Headspace/product partition properties Experimental values were in agreement with data found in the literature (in presence of sugar: Déléris, Atlan, Souchon, Marin, & Tréléa, 2008; Friel, Linforth, & Taylor, 2000; Nahon, Harrison, & Roozen, 2000; in presence of gelatine: Bakker, Boudaud, & Harrison, 1998; Boland et al., 2006). At 25 °C, ethyl hexanoate appeared to be the most volatile compound from these products, regardless of gelatine content. Diacetyl and (Z)-hex-3-en-1-ol had similar headspace/product partition properties. The presence of sugar and citric acid seemed to increase the headspace/product partition properties of diacetyl and (Z)-hex-3-en-1-ol, but to decrease those of ethyl hexanoate (comparison with headspace/ water partition coefficients, Table 1). The headspace/product partition properties of diacetyl and ethyl hexanoate increased slightly with increasing gelatine content, but the differences were only significant for diacetyl, and only between 2% and 15% gelatine. The lower retention of this hydrophilic aroma compound in the product at the higher gelatine content may have been due to the lower availability of water molecules. Neither product composition nor structure had any significant effect on (Z)-hex-3-en-1-ol headspace/product partition properties. These findings provide no evidence of specific physicochemical interactions between these aroma compounds and gelatine.

Firstly, the extent of aroma release differed between subjects, for both the amount of aroma compounds which were released (Imax and AUC) and the shape of the release kinetics (Imax and tmax) (not shown). Similar inter-individual variability in aroma release profiles has been reported previously (Buettner et al., 2002; Pionnier et al., 2004); as such it may be a consequence of physiological differences in timing as well as in the performance of mastication and swallowing. In spite of this variability, subjects can be characterised according to the extent of aroma release, independent of the type of food (subjects who show a relative high aroma release when consuming a liquid/semi-liquid food product also show a relative high release for solid food products, Ruijschop et al., 2009). 3.2.1. Effect of product structure on in vivo release kinetics To illustrate the effect of gelatine content on in vivo release of the three ions during the melting protocol (protocol 1), release kinetics from all panellists were averaged (all replicates) and are presented on Fig. 1. All three ions exhibited similar behaviours, with a slight increase in the amount released between products with 0% and 2% of gelatine, a similar shape for release kinetics for products with 2% and 5% of gelatine, and a clear decrease in

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(b) Ion 87 800

700

700

600

600

0%-gelatine 2%-gelatine 5%-gelatine 15%-gelatine

500

400

Intensity (cps)

Intensity (cps)

(a) Ion 83 800

300

0%-gelatine 2%-gelatine 5%-gelatine 15%-gelatine

500

400

300

200

200

100

100

0

0 0

50

100

150

200

250

300

350

400

0

50

100

150

Time (s)

200

250

300

350

400

Time (s)

(c) Ion 145 800

700

Intensity (cps)

600

0%-gelatine 2%-gelatine 5%-gelatine 15%-gelatine

500

400

300

200

100

0 0

50

100

150

200

250

300

350

400

Time (s) Fig. 1. Mean release kinetics (all panellists and all replicates) obtained by in vivo measurement with PTR-MS during the consumption of products with protocol 1 (melting) for (a) ion m/z 83 ((Z)-hex-3-en-1-ol), (b) ion m/z 87 (diacetyl) and (c) ion m/z 145 (ethyl hexanoate). Vertical arrows indicate the mean time of swallowing for each product (all panellists). Time t0 refers to the moment at which the product was put in the mouth, i.e. 42 s after the beginning of data acquisition.

the amount released for the 15% gelatine product. The initial release rate also seemed to be affected by gelatine content: initial release decreased as the gelatine content increased, consistent with a previous report (Boland et al., 2006). Irrespective of the product, ion m/z 87 (diacetyl) seemed to be the most released, followed by ion m/z 145 (ethyl hexanoate) and ion m/z 83 ((Z)hex-3-en-1-ol) was the least released. The amounts released did not correlate directly with the initial aroma compound concentration in the products. Therefore, in vivo release was presumably determined by interactions between physicochemical and physiological phenomena. For all ions, the mean value of tmax increased (from 10 to 150 s) as the gelatine content increased, the largest difference being between products with 5% and 15% of gelatine (Fig. 2). This can be attributed to the longer product melting time at 37 °C (tmelting 37 °C) at high gelatine contents (Fig. 2) delaying aroma compound release (Guinard & Marty, 1995; Wilson & Brown, 1997). To exploit the data further and to identify the respective roles of physicochemical properties and of physiology, the mean values of

variables extracted from individual release kinetics (Imax1, Imax2, tmax1, tmax2, AUC1 and AUC2) were used for statistical analysis (Table 5). For all ions, the values of Imax and AUC were generally lower or similar before the first swallow than afterwards (except for AUC values in the case of the 15% gelatine product). This is in agreement with previous reports which extensively demonstrate the large contribution of swallowing on aroma release during food consumption (Buettner et al., 2002; Hodgson, Linforth, & Taylor, 2003; Tréléa et al., 2008). The particular behaviour of the 15% gelatine product may be a consequence of both product characteristics (gelatine content, melting time at 37 °C, gel strength) and consumption protocol (melting): all these factors were strongly correlated and imply a much longer residence time in the mouth for this product than for the others (tswallow, melting: 153 s compared to 13, 26 and 47 s for products with 0%, 2% or 5% of gelatine, respectively) (Fig. 2). The difference in residence time in the mouth between products (largely determined by product texture) may also explain the

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400

104

G'1Hz, 20°C (Pa)

103

tswallow, chewing (s)

300

102

t melting 37°C (s)

250

350

tswallow, melting (s)

101

200

100

150

10-1

100

10-2

50

10-3

tswallow , tmelting 37°C (s)

G'1Hz, 20°C (Pa)

105

0 0

2

5

15

Gelatine content (%) Fig. 2. Effect of product gelatine content on storage moduli G’, on product melting time at 37 °C tmelting 37° C and on swallowing times tswallow, melting and tswallow, chewing (mean values for the panel) obtained for the two eating protocols (melting or chewing). Data concerning rheological characteristics of products are from Saint Eve et al. (submitted for publication). Swallowing times were calculated from the moment at which the product was put in the mouth, i.e. 42 s after the beginning of data acquisition.

increase in both tmax1 and tmax2 values with gelatine content: regardless of ion, the products were clearly discriminated for these variables. Before the first swallowing event, for ions m/z 83 ((Z)-hex-3-en1-ol) and 87 (diacetyl), the most intense release (Imax1) was with the 2%, 5% and 15% gelatine samples, where the 15% gelatine product released the largest amount (AUC1) of aroma compounds. Concerning ion m/z 145 (ethyl hexanoate), Imax1 was not significantly different between products, whereas both the 5% and 15% gelatine samples released more of the aroma compound (higher AUC1) than the other two products. After the first swallow, the pattern of aroma release kinetics for the three ions remained similar, with a more intense and greater release of aroma compounds from products with 2% and 5% gelatine than from those with 0% or 15% gelatine. Signal persistence for each ion was evaluated by comparing peak widths for a fixed level of intensity (20% of the minimal value of Imax for the four products). The signal durations for ions m/z 83 ((Z)-hex-3-en-1-ol) and m/z 87 (diacetyl) were similar for 0%, 2% and 5% gelatine products, but were longer (1.7-fold and 1.4-fold, respectively) for 15% gelatine. Concerning ion m/z 145 (ethyl hexanoate), products with 0% and 2% gelatine were the least persistent: the peak width with 5% or 15% gelatine was 2-fold or 4.3fold larger, respectively.

These findings demonstrate the large contribution of product structure. Before the first swallow, the amount of aroma released was higher for the product with 15% gelatine than for the other products, for all the three ions. This may have been because of the much longer time of product residence in the mouth for the sample with the highest gelatine content (longest melting time). The aroma release was more intense for products with intermediate gelatine contents (2% and 5% gelatine). Although the in-mouth residence times of these products were shorter than for the product with 15% gelatine, the aroma compounds were probably more easily and more rapidly released from these less strong gels. For the liquid sample, the combination of a short in-mouth residence time and a limited retention of aroma compound (no threedimensional network) resulted in the fastest release. After the first swallow, aroma release appeared to be less intense for 0% and 15% gelatine products than for the two others, probably due to product structure. One possible explanation is that the more liquid the sample, the less thick the residual deposit in the pharynx cavity, leading to less aroma release (Normand, Avison, & Parker, 2004; Tréléa et al., 2008). For the 15% gelatine product, the long residence time in the mouth before swallowing could be associated with greater overall aroma release during this period and less aroma compound available for release after swallowing, in spite of the thickest deposit on mucosa. The persistence was longest for 15% gelatine, presumably due to the residual product layer being the thickest (highest viscosity of the melted product), contributing to aroma release after swallowing. The physicochemical properties (headspace/product partition and diffusion properties) identified above cannot completely explain the differences observed in aroma release. In spite of their different physicochemical properties (notably hydrophobicity and volatility), the three aroma compounds showed similar temporal behaviours concerning in vivo release. Intensity ratios between ions at each time point of the mean release kinetics indicated that the main difference was the level of release, which could not be explained solely by the initial concentration of aroma compounds in the products. Diacetyl (represented by ion m/z 87) was released to a similar extent as ethyl hexanoate (represented by ion m/z 145) (intensity ratio close to 1) despite having the lowest initial concentration and headspace/product partition properties. This can be explained by the hydrophilic behaviour of diacetyl, for which the release is probably favoured by product melting and mixing with saliva during consumption. Some small differences were observed for the 0% gelatine product when melted and for the 5% and 15% gelatine products when

Table 5 Mean values (and associated standard deviations) of the variables extracted from in vivo aroma release kinetics (PTR-MS measurements) obtained during the consumption of products with protocol 1 (melting) for ions m/z 83 ((Z)-hex-3-en-1-ol), m/z 87 (diacetyl) and m/z 145 (ethyl hexanoate). Data were corrected for the amount of product that was actually consumed and for the true aroma compound concentration in products. The values of tmax1 and tmax2 were calculated from the time at which the product was put in the mouth, i.e. 42 s after the beginning of data acquisition. All factors had a p-value <0.001 concerning product effect (ANOVA). Letters a–d indicate means that significantly differ at p < 0.05 (SNK test). Before the 1st swallow

After the 1st swallow

Product

Imax1 (cps)

tmax1 (s)

AUC1 (a.u.)

Imax2 (cps)

Ion 83 ((Z)-hex-3-en-1-ol)

0% gelatine 2% gelatine 5% gelatine 15% gelatine

54 (60) b 333 (526) a 301 (399) a 326 (284) a

10 (5) c 21 (12) c 37 (21) b 117 (61) a

198 (312) b 2266 (5854) b 3482 (7012) b 11,431 (17,502) a

300 564 454 347

Ion 87 (diacetyl)

0% gelatine 2% gelatine 5% gelatine 15% gelatine

902 (896) b 1615 (1640) a 1421 (1341) a 1224 (795) ab

8 (5) d 21(9) c 38 (20) b 127 (54) a

2015 (1947) c 8116 (10,622) b 11,418 (15,668) b 27,428 (28,984) a

1261 1879 1557 1156

Ion 145 (ethyl hexanoate)

0% gelatine 2% gelatine 5% gelatine 15% gelatine

685 794 982 613

7 (5) d 21 (10) c 38 (20) b 134 (60) a

2780 (3198) b 5987 (10,994) b 13,129 (21,705) a 15,823 (21,468) a

662 (715) b 972 (769) a 1169 (958) a 567 (506) b

(853) a (1061) a (1122) a (607) a

tmax2 (s)

AUC2 (a.u.)

c a b c

29 (13) d 40 (16) c 60 (22) b 170 (51) a

6613 (6958) b 12,220 (9271) a 11,385 (12,403) a 7935 (6767) b

(1052) b (1222) a (1007) ab (920) b

17 (5) d 32 (10) c 54 (20) b 166 (52) a

10,498 17,609 16,219 11,410

18 (5) d 32 (12) c 54 (21) b 167 (50) a

5985 (5749) c 9984 (8350) b 14,258 (13,868) a 6802 (6181) bc

(371) (485) (404) (238)

(8339) b (9473) a (12,072) a (11,966) b

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1100

(a) Ion 83

1000

5%-gelatine_melt 5%-gelatine_chew 15%-gelatine_melt 15%-gelatine_chew

800 700

900

5%-gelatine_melt 5%-gelatine_chew 15%-gelatine_melt 15%-gelatine_chew

800

Intensity (cps)

900

Intensity (cps)

(b) Ion 87

1000

600 500 400

700 600 500 400

300

300

200

200

100

100 0

0 0

50

100

150

200

250

300

350

0

400

50

100

150

200

250

300

350

400

Time (s)

Time (s)

1100

(c) Ion 145 1000 900

5%-gelatine_melt 5%-gelatine_chew 15%-gelatine_melt 15%-gelatine_chew

Intensity (cps)

800 700 600 500 400 300 200 100 0 0

50

100

150

200

250

300

350

400

Time (s) Fig. 3. Mean release kinetics (all panellists and all replicates) obtained by in vivo measurements with PTR-MS during the consumption of 5% or 15% gelatine products with protocol 1 (melting) or protocol 2 (chewing) for (a) ion m/z 83 ((Z)-hex-3-en-1-ol), (b) ion m/z 87 (diacetyl) and (c) ion m/z 145 (ethyl hexanoate). Vertical arrows represent the mean time of the first swallow for each product (all panellists). Time t0 refers to the moment at which the product was put in the mouth, i.e. 42 s after the beginning of data acquisition.

chewed: the release of ion m/z 145 was slightly higher and earlier than that of ion m/z 87. The physicochemical characteristics of (Z)hex-3-en-1-ol probably explained its release being lower than those of the two other molecules (low headspace product/partition properties and hydrophobic character), regardless of product or eating protocol. 3.2.2. Effect of consumption protocol on in vivo release kinetics Product structure can influence aroma release by modifying not only the physicochemical properties and interactions between aroma molecules and product constituents, but also by affecting oral behaviour. To assess the role of eating behaviour and notably mastication, the 5% and 15% gelatine products were eaten with an alternative protocol: panellists were asked to chew samples as normally as possible. Release kinetics are illustrated on Fig. 3. The times of residence in-mouth were completely different between the two protocols, so the analysis of release kinetics per period (before and after swallowing) was not pertinent and we therefore compared whole release curves (Imax, tmax, and AUC). The mean val-

ues of the variable extracted from individual in vivo release kinetics are summarised in Table 6. The product-protocol interaction was significant: thus, statistical analysis of the effect of the eating protocol was performed separately for the 5% gelatine product and the 15% gelatine product. The tmax values were shorter and release rates higher with the chewing protocol than with the melting protocol, for all ions and both products. The effect was much more pronounced for the 15% gelatine product (59%-decrease in tmax value) than for the 5% gelatine product (37%-decrease in tmax value). This can be correlated to the mean residence times of products in the mouth: they were 53% and 75% shorter for the 5% and 15% gelatine products, respectively, when chewed than when melted (Fig. 2). For both products and all ions, chewing resulted in a 1.1 to 3.3fold increase in the initial release rate (Fig. 3). The impact of oral behaviour was nevertheless limited in the case of the 5% gelatine product sample: chewing increased Imax values for ions m/z 87 and 145 but decreased signal persistence for ion m/z 83 (12%) and ion m/z 87 (29%) (not shown); the other variables extracted

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from the release kinetics were similar for the two protocols for 5% gelatine product. For the 15% gelatine sample, Imax for ion m/z 87 was significantly higher, and both AUC values (total released amounts) and in signal persistence (not shown) for the three ions were significantly lower when chewed than when melted. The larger differences observed for the 15% gelatine product were presumably the consequence of chewing on the in-mouth residence time, which was twice as long for the 15% gelatine as for the 5% gelatine (Fig. 2). By breaking the product into small pieces, chewing increases the area of contact with air and favours product mixing with saliva (and thus its dissolution). This also leads to the shortest time of residence in-mouth and highest Imax. In the case of the 5% gelatine product, these effects explain the more intense release, but were insufficient to increase the total amount of aroma released. In the case of the 15% gelatine product, although the contact area between air and the product was increased by chewing, the in-mouth residence time was 4-fold longer when the sample was melted, and this remained the main determinant of aroma release. Our various findings show that in addition to the important contribution of product structure on the dynamics of aroma release, the phenomena involved are complex (combination of interactions between physicochemical and physiological mechanisms). 3.3. Relation with sensory data We conducted a sensory analysis (method of temporal dynamics of sensations, TDS) in parallel with the instrumental measurement of in vivo aroma release to evaluate the temporal dimension of perceptions (Saint Eve et al., submitted for publication). The sensory results showed that the temporal profiles for the dominant perception were dependent on product structure. The overall duration of the dominance period (all sensory attributes taken together) increased linearly with gelatine content, from 43 s for 0% gelatine to 140 s for 15% gelatine. Chewing decreased the dominance duration, and this decrease was larger for the 15% gelatine product than for the 5% gelatine product. The dominant sensation for the liquid product was the ‘‘strawberry’’ note. For other products, the temporal characteristics of perceptions were more complex: the ‘‘strawberry’’ note was also scored as dominant at various consumption times but was preceded by the ‘‘green’’ note for the 2% gelatine sample and by the ‘‘butter’’ note for the 5% gelatine and the 15% gelatine samples. Mean in vivo aroma release kinetics and mean sensory dominances are represented together for each product and each consumption protocol in Fig. 4. Establishing such relationships between release and sensory data can be a difficult task, notably because the extracted measures do not necessarily represent the same phenomena. We assumed that the most pertinent variables for comparing the two data sets were the dominance duration and the sequences of sensory attributes for sensory data, and the tmax value for each ion and the comparison between signal levels (intensity ratio) for each ion for the instrumental data. We identified various relationships between sensory and instrumental data. The ‘‘strawberry’’ attribute, mainly associated with ethyl hexanoate (ion m/z 145), was cited as being dominant in all products and for both eating protocols. This can be attributed to both the high value of headspace/product partition properties of this aroma compound and to the low value of its perception threshold (0.5 lg/l in water for retronasal perception, Rychlik, Schieberle, & Grosch, 1998). For the 0% gelatine sample, this attribute was the only one identified, probably because of the early release of the related aroma compound (ethyl hexanoate, m/z 145) and the very short in-mouth residence time of the product being insufficient for the perception of other attributes. The perception

Table 6 Mean values (and associated standard deviations) of the variables extracted from in vivo aroma release kinetics (PTR-MS measurements), obtained during the consumption of products with protocols 1 (melting) or 2 (chewing), for ion m/z 83 ((Z)-hex-3-en-1-ol), m/z 87 (diacetyl) and m/z 145 (ethyl hexanoate). Data were corrected for the amount of product that was actually consumed and for the true concentration of aroma compounds in products. The values of tmax were calculated from the moment at which the product was put in the mouth, i.e. 42 s after the beginning of data acquisition. Letters a and b indicate means that significantly differ at p < 0.05 (SNK test). Ion

Protocol

Imax (cps)

tmax (s)

AUC (a.u.)

5% gelatine m/z 83

Melting

465 (404) a

60 (22) a

((Z)-hex-3-en-1-ol)

Chewing

384 (213) a

35 (11) b

m/z 87

Melting

(1255)

54 (21) a

(diacetyl)

Chewing

(1379)

15 (11) b

m/z 145

Melting

(1194)

54 (20) a

(ethyl hexanoate)

Chewing

1671 b 2057 a 1407 b 2080 a

(1646)

24 (6) b

14,867 a 10,046 a 27,557 a 26,862 a 27,476 a 31,486 a

15% gelatine m/z 83

Melting

393 (276) a

(Z)-hex-3-en-1-ol m/z 87

Chewing Melting

361 (225) a 1572 (932) b

(diacetyl)

Chewing

1665 (800) a

m/z 145

Melting

813 (640) a

(ethyl hexanoate)

Chewing

1078 (789) a

170 (51) a 50 (33) b 166 (53) a 40 (16) b 167 (50) a 42 (25) b

(18,318) (7758) (27,518) (24,763) (31,629) (30,381)

19,365 (21,863) a 9716 (9442) b 38,837 (31,868) a 25,067 (17,930) b 22,624 (22,709) a 12,149 (8894) b

of the ‘‘strawberry’’ note due to ethyl hexanoate perfectly coincided with swallowing. Perception became more complex as the gelatine content increased, and occurred increasingly earlier before swallowing, consistent with the kinetics of aroma release. With the melting protocol, as the gelatine content increased, the residence time in the mouth appeared to have an increasing effect on aroma release and perception, more than the swallowing event. In the case of the 2% gelatine sample, the apparition of the ‘‘green’’ attribute during the 17s immediately after swallowing can be directly related to the release of ion m/z 83 ((Z)-hex-3-en-1-ol), which was greater from this product than from the three other products (highest Imax2 and AUC2 values, Table 5). For the 5% and 15% gelatine samples, the ‘‘green’’ note was not perceived as dominant, consistent with the low amount of ion m/z 83 released and the greater release of ion m/z 87 (diacetyl) responsible for the ‘‘butter’’ note, favoured by an increase in residence time in the mouth. Although the release of ions m/z 87 and 145 was affected, product chewing mostly impacted on the residence time in the mouth. As a consequence, the release profiles became similar to the 2% gelatine product, except for ion m/z 83, for which the release remained low. These results may explain the dominance of the ‘‘strawberry’’ note for the 5% and 15% gelatine products with chewing. 4. Conclusion Various links could be established between sensory and release data, mostly concerning temporal variables. However, such conclusions were mainly based on a descriptive analysis of the data where the variety and the complexity of the phenomena involved made it difficult to establish clear relationships between the two sets of data. Further work is therefore needed to improve the characterisation of release and perception, and to identify the links between them, thereby generating a better understanding.

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(b) 2%-gelatine, melting

(a) 0%-gelatine, melting

1100

1100

mean swallowing time

mean swallowing time 1000

strawberry

900 800

strawberry

green

800

700 600 500 400

700 600 500 400

300

300

200

200

100

100 0

0 0

25

50

75

100

125

150

0

175

25

50

75

125

150

175

(d) 15%-gelatine, melting

(c) 5%-gelatine, melting

1100

1100

mean swallowing time

mean swallowing time 1000

1000

butter

butter

ion 83 ion 87 ion 145

900

strawberry

900

strawberry

ion 83 ion 87 ion 145

strawberry

800

Intensity (cps)

800

Intensity (cps)

100

Time (s)

Time (s)

700 600 500 400

700 600 500 400

300

300

200

200

100

100 0

0 0

25

50

75

100

125

150

0

175

25

50

75

100

125

150

175

200

225

250

Time (s)

Time (s)

(f) 15%-gelatine, chewing

(e) 5%-gelatine, chewing

1100

1100

mean swallowing time 1000

mean swallowing time 1000

strawberry

strawberry

900

900

ion 83 ion 87 ion 145

800

ion 83 ion 87 ion 145

800

700

Intensity (cps)

Intensity (cps)

ion 83 ion 87 ion 145

900

Intensity (cps)

Intensity (cps)

1000

ion 83 ion 87 ion 145

600 500 400

700 600 500 400

300

300

200

200

100

100 0

0 0

25

50

75

100

Time (s)

125

150

175

0

25

50

75

100

125

150

175

200

225

250

Time (s)

Fig. 4. Mean release kinetics (all panellists and all replicates) (symbols) for ion m/z 83 ((Z)-hex-3-en-1-ol), ion m/z 87 (diacetyl) and ion m/z 145 (ethyl hexanoate) obtained by in vivo measurements with PTR-MS, in addition to the characteristic and mean duration of the dominant perception measured by the DTS method (boxes) during the consumption of (a) 0% gelatine sample with protocol 1 (melting), (b) 2% gelatine sample with protocol 1 (melting), (c) 5% gelatine sample with protocol 1 (melting), (d) 5% gelatine sample with protocol 2 (chewing), (e) 15% gelatine sample with protocol 1 (melting) and (f) 15% gelatine sample with protocol (chewing). The vertical dashed line indicates the mean time at which the first swallow occurred. Time t0 refers to the moment at which the product was put in the mouth, i.e. 42 s after the beginning of data acquisition.

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Acknowledgments We gratefully acknowledge the panellists for their contribution to in vivo and sensory measurements. We also thank Alex Edelman of Alex Edelman & Associates for revising the English version of the manuscript.

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