Flavouring ratios and partition coefficients
for isoamyl acetate in various starch-based food matrices
Nathalie CAYOT1 *, Claude TAISANT1, Gaëlle ARVISENET1, Jean-Marie MEUNIER1, Andrée VOILLEY2
RÉSUMÉ Taux d’aromatisation et coefficients de partage de l’acétate d’isoamyle dans différentes matrices alimentaires amylacées.
L’étude traite des interactions physicochimiques entre différents amidons et l’acétate d’isoamyle dans une matrice alimentaire complexe. Quatre amidons différents ont été testés : un amidon de pomme de terre, un amidon de maïs, un amidon de maïs cireux et un amidon de maïs cireux réticulé et stabilisé. Le bilan d’aromatisation des matrices a été déterminé à partir d’extractions du composé d’arôme par l’appareil de Likens-Nickerson. L’intensité des interac- tions a été évaluée par analyse de l’espace de tête. La contribution de chaque ingrédient a été observée. L’aromatisation est plus importante dans les pro- duits à base d’amidons natifs que dans les produits à base d’amidon modifié.
Aucune relation n’a été établie entre la teneur en amylose des différents ami- dons testés et le taux d’aromatisation des produits. La molécule aromatisante a été retrouvée à la fois dans la phase aqueuse du produit et au sein du réseau gélifié. En fonction du type d’amidon utilisé, le taux d’aromatisation et le coeffi- cient de partage apparent gaz / liquide sont modifiés par la présence de sac- charose et/ou de lait.
Mots clés : aromatisation, libération d’arôme, amidon, espace de tête, interac- tions.
SUMMARY
The present work deals with the physicochemical interactions between diffe- rent starches and isoamyl acetate in a complex food matrix. Four different starches were tested: potato starch, corn starch, waxy corn starch, and cross- linked and stabilized waxy corn starch. The flavouring balance was established through flavour extractions with the Likens-Nickerson apparatus. Headspace analyses were performed to evaluate the intensity of interactions. The contribu- tion of each ingredient was evaluated. Flavouring is more pronounced for the
1. Laboratoire de biochimie alimentaire, Établissement national d’enseignement supérieur agronomique de Dijon, 21 Bd Olivier de Serres, 21800 Quetigny, France.
2. Laboratoire de Génie de procédés agroalimentaires et biotechnologiques, École nationale supérieure de biologie appliquée à la nutrition et à l’alimentation, 1, Esplanade Erasme, 21000 Dijon, France.
* Correspondance n.cayot@enesad.fr
native starch based products than for the modified starch based products. No relation was found between the amylose content of the different starches and the flavouring ratio of the products. The flavouring molecule was located both in the aqueous phase and in the gel network. Depending on the starch used, the flavouring ratio and the apparent vapour/liquid partition coefficient were modified by sucrose and/or by milk.
Key-words: flavouring, aroma release, starch, headspace, interactions.
1 - INTRODUCTION
In the field of food flavouring, starch is known to have a predominant influence on flavour release (GODSHALLand SOLMS, 1992). As starches are often used as thickeners and stabilizers, the rheology of the food products is of great importance to explain aroma retention. GODSHALL (1997) reported that for concentrated polysaccharide solutions, at concentrations greater than the coil overlap parameter (C*), the perceived flavour decreases as the perceived thick- ness increases. But starches are also known to be good flavour carriers because of physicochemical interactions between the aroma compounds and the polysaccharide macromolecules (GODSHALL and SOLMS, 1992). The reten- tion of aroma compounds in starchy matrices can be attributed to several mechanisms of non-chemical binding. These physicochemical interactions can be hydrogen interactions or hydrophobic interactions or inclusion complexation (FRIEDMAN, 1995). The establishment of interactions is a function of the process, the nature of the starch, the aroma compound and also depends on the water content. For example, the aroma-starch interactions occurring during a freeze- drying process were studied by MAIERet al. (1987). They used native starches from different botanical origins (potato, normal corn, waxy corn and tapioca).
They observed that the amounts of trapped aroma decreased respectively in the following order: potato, waxy corn, normal corn and tapioca. The results showed that the decrease was independent of the amylose content of the tes- ted starches. Nevertheless, it is often reported that the main mechanism for starch to trap flavour is to form inclusion complexes between amylose and aroma compounds (SOLMS and GUGGENBUEHL, 1990). But it has been reported that amylopectin, to a lesser extent than amylose, can also be involved in this type of interaction (GODSHALL, 1997). RUTSCHMANN and SOLMS (1990 a-f) did extensive research on the formation of inclusion complexes between potato starch and different aroma compounds. These authors reported the formation of inclusion complexes with menthone, decanal, 1-naphtol, and limonene. The analysis of these complexes by X-ray diffraction showed typical V-amylose spectra. In fact the linear chain of amylose adopts a helical conformation that forms a hydrophobic cavity able to receive some ligands. The helical conforma- tion varies in dimension as a function of the physicochemical characteristics of the ligand (steric hindrance, hydrophobicity, length of the carbon chain). No clear relationship has been established between the chemical structure of a molecule and its ability to form inclusion complexes. Following the complexa- tion of various ligands by potato starch by determining the viscoelastic proper- ties of low concentration starch dispersions, NUESSLI (1998) observed that at a
certain temperature, which was characteristic for each ligand, turbidity occur- red, indicating the formation of insoluble complexes. What occurred was either gelation (decanal, octanal, fenchone), precipitation (geraniol, carvone) or no change of the rheological properties (thymol, menthone, menthol, (+)-camphor, α-naphthol). This depended on time, on ligand properties and on the type of aggregates. NUESSLI (1998) proposed a model that linked the complexation of various ligands by starch and the evolution of the structure of low concentration starch dispersions. Some ligands could be adsorbed externally to the helix of amylose. Inter- and intrahelical interactions determined the different behaviours observed. RUTSCHMANNet al. (1989) had previously established that there was a cooperative effect when inclusion complexes were formed and that several interaction sites could co-exist. They indicated that under certain conditions of concentrations, the ligand could be adsorbed on the outer face of the helix.
The vast majority of studies concerning aroma-starch interactions were conducted on very simple model systems and it is often impossible to apply the results to “real” products. Our aim in the present study was to estimate these interactions in an almost “real” food product. This implies that the amount of each component is realistic. This food product is composed of milk and sucrose, plus one of the selected starches and is flavoured with isoamyl acetate to simulate a gelified dairy dessert. To encompass the broad diversity of structure and composition of starches, we chose to work with four different starches: a native potato starch (PS), a native normal maize starch (CS), a native waxy maize starch (WS) and a modified (crosslinked and stabilized) waxy maize starch (MWS). This work attempts to characterize the flavouring efficiency of a food product as a function of its composition, the type of starch used and the process. By measuring the gas-food partition coefficients for the different products, we sought to learn more about aroma-starch interactions.
These results are a first step to explore aroma retention in such products through sensory analyses.
2 - MATERIAL AND METHODS
2.1 Preparation of the food product The food matrix was composed of:
– 70 g starch;
– 100 g commercial sucrose;
– 100 g skimmed milk powder rehydrated with Evian water up to 1000 mL.
Evian water, a drinking water, was chosen since it is neutral from a sensory point of view.
Four starches were tested, all of them provided by Roquette Frères (Les- trem, France). Isoamyl acetate (Aldrich, purity 97%, CAS nr 123-92-2) was used as the ligand. This volatile molecule has the following characteristics: molecular weight = 130.18 g·mol–1; boiling point = 142°C at 756 mm Hg; hydrophobicity log P = 2.1 (log P is the logarithm of the partition coefficient between octanol
and water, estimated by the Rekker method (NAUTA and REKKER, 1977)). Two hundred µL of isoamyl acetate were used to aromatize a quantity of food pro- duct corresponding to one liter of reconstituted milk. The amount of the added flavouring substance was determined by previous experiments to be compatible with both sensory and instrumental analyses.
Starch, sucrose and reconstituted milk were stirred together for 3 min and the mixture was left to rehydrate for 12 min. Then, the mixture was poured into a reactor vessel and the aroma compound was added. The reactor vessel (IKA LR 2000 V reactor, 2 liters useful volume) was sealed. The reactor was heated and the temperature of the product was regulated (± 0.1°C precision) by means of a laboratory thermostat via a double jacket (Lauda C6CS - temperature sen- sor PT 100). During cooking, the mixture was stirred by an anchor stirring paddle that rotated at 50 rpm. At the beginning of cooking, the temperature of the product was 25°C, the heating speed was 3°C·min–1. The temperature set point varied from one starch to another. Once the product reached this set point, the temperature was maintained during a period called “cooking time”.
The cooking time and temperature for each starch are reported in table 1. These operating conditions led to well-swollen starch granules, corresponding to the highest viscosity of the starch dispersion.
Immediately after cooking, the food product was partitioned and conditio- ned, either into “headspace” glass vessels for headspace analyses (sample volume / total volume = 1/3) or into polypropylene boxes for other analyses.
The samples were stored, hermetically sealed, at least 24 h at 6°C before ana- lyses.
To study the contribution of each ingredient to the interactions between the isoamyl acetate and the food matrix, simplified matrices were prepared with or without milk or sucrose.
2.2 Separation of the different phases of the starchy gels
For each product, a 10 g aliquot was dispersed in 35 mL ultrapure water.
This suspension was stirred and then centrifuged at 3000 g for 15 min in sealed tubes. Isoamyl acetate was extracted from the supernatant and from the sedi- ment with the Likens-Nickerson (LIKENSand NICKERSON, 1964) apparatus. The dry matter was determined both in the supernatant and in the sediment. The amylose content was measured in the supernatant.
2.3 Determination of constituants contents
2.3.1 Determination of amylose content
The amylose content was determined in the supernatants by a spectropho- tometric method measuring the amylose-iodine complexes at six wavelengths as described by JARVISand WALKER(1993).
2.3.2 Determination of protein content
Protein content of the supernatants and the sediments were determined by the Kjeldahl method (AOAC, 1970) using a nitrogen factor of 6.25.
Table 1 Characteristics of the tested starches and operating conditions for the cooking treatment StarchAmyloseExtractableProteinTemperatureCookingLoss of aromaInitialFinalFinal Scontentlipidsset pointtimecompound duewater contentwater content (% w/w)(% w/w)(% w/w)(°C)(min)to cooking process(% w/w)(% w/w)pH (%) Potato starch21—0.18001278.077.76.6 PS Maize starch260.10.49034278.077.36.6 CS Waxy maize10.10.48501878.077.36.8 starch WS Modified1—0.3598207978.076.36.5 waxy maize starch MWS Standard (with milk and0——9034282.481.56.6 sucrose, without starch)
2.3.3 Determination of sucrose content
The determination of sucrose content was performed with an enzymatic test-combination sucrose/D-glucose (Boehringer Mannheim), in both superna- tant and sediment.
2.3.4 Determination of water content
The determination of water content was done by weighting exactly approxi- mately 2 g of product, before and after drying at 104°C during 24 h.
2.4 Extraction of isoamyl acetate by the Likens-Nickerson apparatus From the whole food matrix and from the centrifugation sediment
10 g aliquots were dispersed in 100 mL of ultrapure water saturated with NaCl (360 g·L–1). Methyl heptanoate (2.5 mg) was added as a standard.
From the centrifugation supernatant
The supernatant was diluted with ultrapure water up to 100 ml. This solution was saturated with NaCl (360 g·L–1) and 2.5 mg of methyl heptanoate were added.
In the presence of milk, 100 µL of antifoam (active polymers of silicone, Sigma) were added before extraction.
For each composition, the cooking procedure was done in triplicate and, for each sample, extraction was replicated four times.
2.5 Equilibrium headspace analyses
The samples in headspace glass vessels were left to equilibrate 72 h at 6°C and 1 h at 20°C just before analysis. A sample of 1 mL of the headspace was taken with a gas syringe. These analyses were done in triplicate for two cooking batches.
We calculated an apparent partition coefficient K through mass fractions.
Isoamyl acetate was considered to be at an infinite dilution in our starchy pro- ducts; its mass fraction was 1.4 ×10–4.
with
Yi: mass fraction of isoamyl acetate in headspace and
Xi: mass fraction of isoamyl acetate in food matrix, calculated from the aroma extractions.
2.6 GC analysis
The analyses were conducted on a HP 6890 gas chromatograph (Hewlett Packard) with a split/splitless injector (50% split, 230°C) and equipped with a flame ionization detector (250°C, H2: 40 mL·min–1, air: 240 mL·min–1). The vola-
K Y
i X
i i
∞ =
tile compounds were separated by a 25 m ×0.32 mm (i.d.) fused silica capillary column FFAP-CB (Chrompack) of film thickness 0.25 µm. The column oven was temperature programmed from 40°C to 250°C at 5°C·min–1 for the solutions extracted by the Likens-Nickerson apparatus and was isothermal at 100°C for headspace analyses. Ethyl decanoate was added as the external standard. The calibration procedure for headspace was the same as for liquid injection.
2.7 Statistical analysis
Statistical analyses of the values were performed with the Statbox Pro soft- ware.
The Anova procedure (two factors: “type of starch” and “formulation”) was performed, followed by a Newman-Keuls test. P values ≤0.05 were considered significant. For the flavouring ratios values, the two factors analysis of variance was followed by a one factor analysis of variance (formulation) for each type of starch.
3 - RESULTS
3.1 Flavouring ratios
The ratios between the quantity of isoamyl acetate measured by GC after extraction and the theoretical quantity of isoamyl acetate introduced before cooking were named flavouring ratios.
As each type of starch was cooked according to its optimum treatment, that is to say, under conditions of time and temperature that gave the greatest swel- ling of the starch granules, the cooking conditions were not equivalent for all the products. A loss of the aroma compound occured during the cooking process.
This process-induced loss was measured by extraction under all cooking condi- tions with an aqueous solution of the aroma. We then obtained a flavouring ratio of the aqueous solution for each cooking process (table 1). Then the values pre- sented here and discussed along the following lines are corrected values (i.e., we considered that the flavouring ratios of the aqueous solution were equal under all cooking conditions: measured flavouring ratios were divided by 100 minus the loss of aroma due to cooking process) so that the results may be compared. It was noticeable that the loss of water due to the cooking process was low. The most important loss of water was measured for MWS products — corresponding to the most drastic treatment — and was about 2% (table 1).
In the starchy matrices with native starches, the corrected flavouring ratios ranged from 37 to 73% depending on the composition of the products. But in the products made with modified starch, the corrected flavouring ratios ranged between 20 and 50%.
As shown in figure 1, the four tested starches gave different flavouring ratios.
There were statistically significant differences between their abilities to trap isoamyl acetate. The Newman-Keuls test (NEWMAN, 1939; KEULS, 1952) gave three homogeneous groups among these four starches. The CS-based product
showed the highest flavouring ratio, the MWS-based product the lowest one and PS and WS based products gave intermediate values.
The change of the flavouring ratios as a function of the composition can be seen in figure 1. For all starches, the flavouring ratios were significantly impro- ved by the addition of milk. Conversely, the flavouring ratios tended to decrease with the addition of sucrose except for CS. Finally, the addition of both milk and sucrose gave a significantly higher flavouring ratio only for MWS.
Figure 1
Influence of type of starch and of composition on the corrected flavouring ratios measured in various food matrices (values bearing the same letter on the bar
chart are not significantly different)
Corrected flavouring ratio = ratio of isoamyl acetate in food matrix at end of cooking and at start of cooking / (100 – loss of aroma due to cooking process)
3.2 Distribution of the aroma compound between the centrifugation supernatant and the sediment
The aim of the centrifugation of the diluted food matrices was to separate the aroma fraction located in the aqueous phase from the aroma fraction trap- ped in the gel network. The amylose contents were determined in the superna- tants. The amount of amylose in the supernatants ranged from 0.006 to
0.014 g·L–1. These results led us to consider that almost all of the starch was located in the sediment. Almost all the proteins (85% w/w) were located in the sediment. Sucrose was equally distributed in the supernatant and in the sedi- ment.
Figure 2
Influence of type of starch and of composition on the percentage of trapped isoamyl acetate measured in various food matrices (values bearing the same letter on the bar
chart are not significantly different)
The percentage of isoamyl acetate bound to starch was obtained from the ratio of the aroma quantity measured in the sediment to the aroma quantity mea- sured in the whole product after cooking. This percentage ranged from 23 to 68% depending on the composition of the product. As shown in figure 2, for the products composed only of starch, the percentage of starch-bound aroma had only a slight variation from 30 to 40%. The Newman-Keuls test indicated that the percentage of aroma trapped in WS-based product was significantly higher than for the others. The percentage of starch bound isoamyl acetate was significantly increased by the addition of milk in the medium for WS and tended to diminish for PS and MWS. The addition of sucrose had an increasing flavouring effect for CS and WS. Finally the addition of both sucrose and milk significantly improved the percentage of starch bound aroma only in the case of WS (see figure 2).
3.3 Apparent vapour-liquid partition coefficients
The modifications of the thermodynamic equilibrium of isoamyl acetate indu- ced by the interactions with the food matrices were followed by equilibrium
headspace analyses. The reported results are the apparent vapour-matrix parti- tion coefficients that we classed as vapour-liquid partition coefficients.
As shown in the table 2, the apparent vapour-liquid partition coefficients decreased in the presence of starch. K values were 5 to 8 times lower in the presence of starch. Interactions between starch and isoamyl acetate were clearly demonstrated, irrespective of the type of starch considered. In products without starch, both milk and sucrose increased the K values.
Table 2
Effect of the presence of starch on the apparent vapour-liquid partition coefficient (mass fraction) in various matrices
Composition Water Water + Water + Water +
of the matrix sucrose milk powder milk powder +
sucrose Without starch 14.8 ±1.6 16.4 ±2.3 21.7 ±4.5 18.0 ±1.6
With starch 2.4 ±1.4 3.4 ±2.0 2.6 ±1.7 3.4 ±1.8 N.B.: K values reported for the starch-based matrices are average values obtained with the four diffe- rent starches.
Apparent vapour- liquid partition coefficient of isoamyl acetate K
Figure 3
Influence of type of starch and composition on the apparent vapour-liquid partition coefficient measured in various food matrices (values bearing the same
letter on the bar chart are not significantly different)
As shown in figure 3, considering the products composed of starch only, it appeared that these coefficients decreased in the order PS, WS, CS and MWS.
From a statistical point of view, CS and WS were in the same homogeneous group.
The addition of sucrose, with or without milk, in these food matrices induced an significant increase in the partition coefficients, that is to say, less retention.
The effect of milk on the partition coefficients was less obvious.
4 - DISCUSSION
Isoamyl acetate has been added to different food matrices at a concentra- tion far lower than its solubility limit. ESPINOZA-DIAZ(1999) indicated a solubility in water at 25°C of 2.4 g·L–1for isoamyl acetate. We considered that the aroma compound was thus homogeneously dispersed in the product before gelation.
Some losses in the aroma compound were observed during the cooking process and they varied as a function of the [time, temperature] conditions.
Obviously, the different processing conditions implied some difficulties in inter- preting the results but resulted in starches with the same state of cooking (swel- ling) and the same availability to form interactions with the aroma compounds.
Comparing starches at various granular disintegration states was not relevant as long as interactions with aroma were involved.
Nevertheless, after the values of the flavouring ratio were corrected, there were still some differences between the flavouring ratios. The MWS-based pro- ducts behaved differently from the other products. The crosslinking of this starch induced a modification of the molecular network so that the water hol- ding capacity was modified. The aroma-starch interactions may therefore have been altered by the modification of water-starch interactions. The flavouring ratio obtained with MWS was lower than that obtained with the other starches but was increased in the presence of sucrose and milk and then became identi- cal to that obtained with PS and WS. Conversely, the fraction of isoamyl acetate found in the sediment was about the same for all the starches and the main part of the aroma compound was in the supernatant. Moreover, if we consider the partition coefficients, MWS presented the greatest intensity of interaction.
On the other hand, the flavouring ratios obtained with CS were the highest.
But the amount of the aroma extracted from the sediment was identical to those obtained with PS and MWS. And the partition coefficients were approximately four times bigger than those of MWS.
To complete the range of maize starches, the results obtained for WS should be examined. WS showed intermediate behaviour as far as the flavouring ratio was concerned but the proportion of trapped isoamyl acetate was higher than in CS or MWS. It can thus be concluded that the amylose content is not a deci- sive characteristic for the starch to trap this aroma compound. If we consider the partition coefficient, we have similar results for CS and WS. Once more, the intensity of interactions is identical for the 1% amylose starch (WS) and the 26% amylose starch (CS).
PS was selected, as it is often reported as the best aroma trap among starches. This starch is known to be pure compared with the other starches presented here and because it has few pre-existing complexes with lipids, it was considered to be able to form more complexes with added ligands (NUESSLI et al., 1997). It appears from our results that PS-based products have intermediate behaviour for almost all the parameters followed in this study.
For all the tested starches, it was not surprising that the presence of milk increased the flavouring ratios. But this effect was not large. As a general rule, proteins are known to interact with aroma substances. Moreover their retention capacity is increased by thermal denaturation (BAKKER, 1995). More precisely, CHARLES et al. (1997) reported that β-lactoglobulin interacted significantly with isoamyl acetate. It was noticeable that the pHs of our food products were quite similar. They ranged from 6.5 for the MWS-based products to 6.8 for the native starch-based products. The modifications of milk proteins that might occur for this pH range are some rearrangement of disulfide bonds (CAYOT, 1998). Such modifications could perhaps modify the accessibility of some hydrophobic bin- ding sites. So the MWS-based products may have different behaviour compa- red to other milk products. Under these pH conditions no covalent binding was possible with an ester like isoamyl acetate. FISCHERand WIDDER(1997) indica- ted that hydrophobic interactions could reduce the headspace concentration of the flavour component. This was not the case in our study.
Sucrose, however, had a clear effect on the partition coefficients. For all the starches, the amount of aroma compound in the headspace was increased in the presence of sucrose. BAKKER(1995) indicated that several data showed this tendency. Water availability would be decreased by the presence of sucrose in the food matrix and so aroma molecules would be concentrated. Thus the aroma molecules would be better transferred to the vapour phase (VOILLEY et al., 1977).
5 - CONCLUSION
In real food systems, flavouring results from complex phenomena. Each component of the matrix can contribute to flavour retention. More work is also necessary to understand the interactions between the non volatile components.
For example, the blending order of the components might have an influence on aroma retention.
The best flavouring ratio was obtained with native maize starch, but the low flavouring ratio obtained with the modified waxy starch was partially compensa- ted by the presence of sucrose and milk. The highest percentage of isoamyl acetate trapped in the sediment was for the waxy maize starch, that is to say, in these products the aroma was less likely to reach the vapour phase. Finally the highest gas-matrix partition coefficient was obtained with potato starch. This means that the aroma compounds had a better chance of reaching the vapour phase and of affecting someone’s olfactory epithelium. As the modified waxy starches are often used in the food industry to stabilize the matrix and to avoid syneresis, flavourists and rheologists have to reach a compromise to obtain
good texture without too much aroma retention. The role of amylose in aroma retention seems to be less important. These results have to be validated by sen- sory analyses. Since other aroma compounds may lead to different interactions, this study needs to be continued and expanded.
ACKNOWLEDGMENTS
We thank Laurence CORREIAand Céline LAFARGEfor technical support, parti- cularly for aroma extractions. We thank Roquette Frères for providing us with starches and sound advice.
Received 21 February 2000, revised 12 July 2000, accepted 11 September 2000.
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