Physicochemical interactions

'Salting out' effects

Non-volatile compounds, which can be responsible for taste, can play directly or indirectly a role in the volatility of aroma compoimds. The effect of these nonvolatile compounds on the aroma compoimds in the vapour phase has been extensively studied (Nelson and Hoff 1968, Voilley et al. 1977, Dubois et al. 1995). Salt and acid were found to increase the headspace concentration of polar volatile compounds (Nawar 1971). Jennings (1965) showed that radioactivity due to the quantity of C14-labelled ethyl acetate increased in headspace in accordance with sodium chloride concentration. Schinneler et al. (1972) observed an increase of headspace concentration of octanol in presence of monophosphate inositol in the liquid phase. Voilley et al. (1977) reported three types of behaviour of model volatiles with sucrose, calcium chloride and citric acid, three non-volatile solutes, in aqueous solutions. The activity coefficients increased, decreased or remained the same. In a general manner, the volatility of aroma compounds increases with the presence of salts in the media. This 'salting-out' phenomenon is due to the mobilisation of solvation water molecules by salt molecules, leading to the repulsion of volatile molecules. Therefore, their volatility increase compared to a media without salt. The importance of volatile compound release depends on its nature and on the nature and salt concentration in the media (Dubois et al. 1995).

Apart from salts, several organic small molecules are found to be able to modify volatility of volatile compounds. Intense sweeteners such as aspartame and neohesperidine dihydrochalcone interact with volatile compounds and modify the intensity of flavour attributes (Lindley et al. 1993, Nahon et al. 1998). This phenomenon can lead to a selective release of these aroma compounds during consumption and it changes the flavour quality of a drink for example.

As already described for salts, mono- and disaccharides also affect the volatility by altering the activity coefficients of volatile compoimds (Land 1978).

Fig. 16.2 Main factors influencing flavour perception.

At a sufficient concentration, they lower the amount of bulk water by structuring water, increasing their effective concentration and consequently enhancing their volatility (Nawar 1971). It was also shown that chemical reactions may occur between sweeteners and some volatile compounds (Hussein et al. 1984). For example, Le Quere et al. (1994) found a decrease in some aldehydes concentration in diet orange soft drinks containing aspartame and a formation of new volatile compounds in diet orange drinks containing cyclamate. Larger carbohydrates such as polysaccharides often contribute to the increase in viscosity of a beverage, which influences the diffusion of small molecules.

Dufour and Bayonove (1999) reported interaction between wine polyphenols and aroma compounds. In a hydroalcoholic solution, isoamyl acetate, ethyl hexanoate, and benzaldehyde appeared to be more retained than limonene at low catechin concentrations (0-5 g/1). The tannin fraction induced a slight decrease of benzaldehyde volatility, a salting-out effect on limonene and had no effect on the two esters. On the basis of investigations at the molecular level, they hypothesised a hydrophobic driving force between catechins and benzaldahyde. Concerning ethanol, Conner et al. (1998) reported a decrease in activity coefficients of ethyl esters above 17% ethanol, corresponding to a progressive aggregation of alcohol molecules reducing the hydrophobic hydration of the alkyl chains.

A more systematic study on these effects was reported on a complex model food product, considering all the small molecules identified in the product. Delahunty and Piggott (1995) and Solms (1986) suggested that interaction between small water-soluble molecules and aroma compounds may occur in cheese. Studying a model solution representing a water extract of Camembert cheese, Pionnier et al. (2002) reported, beside classic salting-out effects mainly due to the presence of sodium chloride, the effect of some organic compounds. These components such as peptides had an effect on the retention or release of volatile compounds which can either decrease or increase headspace concentration of aroma. In particular, the presence of water-soluble extract compounds increase the headspace concentration of 2-heptanone, l-octen-3-ol and 3-methylbutanol. For other volatiles, no effect was globally observed but omission tests made on the model water solution containing soluble fraction components showed that mineral salts lead to a release of 2-nonanol while peptides lead to its retention in the solution.

Aroma-matrices interactions (proteins, lipids, polysaccharides) The chemical composition of the matrix, and consequently its structure, influences release and perception of flavour. The main components of food matrix are primarily lipids, proteins, carbohydrate and water. In food, lipids are mainly triglycerides that increase solubility of aroma substances (Allaneau 1979). The volatility of most compounds is lower in lipids than in water (Le Thanh et al. 1992) and, consequently, flavour threshold concentrations determined in oil are generally higher than in water (Jo and Ahn 1999). A decrease of the concentration of volatile compounds in the vapour phase was observed when the quantity of lipids increased in the food matrix. For example, Lubbers et al. (1994) showed that the retention of 28% of beta-ionone on yeast walls, in a model wine, was due to endogenous lipids. The quantity of sorbed compounds depends on the length of triglyceride fatty acid chains and retention is more important for a higher unsaturation degree of lipids (Maier 1975). The addition of a small amount of fat in an aqueous medium results in a partition equilibrium between the two in favour of the lipid phase due to its hydrophobicity (Bakker 1995). Changes in fat affect significantly the flavour release, according to volatile compound lipophilicity and lipid type (Roberts et al. 2003). Brauss et al. (1999) showed that low-fat yoghurts released volatiles more quickly and at a higher intensity but with less persistence than yoghurts containing higher fat proportion. More information about lipid-flavour and emulsion-flavour interactions can be found in Chapters 7 and 8 written by Ollivon and Dumont, respectively.

As macromolecules, proteins and carbohydrates are well known to interact with aroma compounds. These aspects will not be developed in this chapter as Chapters 9 and 10 written by Tromelin et al. and Delarue and Giampaoli, respectively, are mainly dedicated to protein-flavour and carbohydrate-flavour interactions. The existence of interactions between aroma compounds and proteins or carbohydrates were mainly studied in rather simple model systems. However, when fat is added in the system, the presence of proteins or saccharides no longer influences flavour release (Roberts and Pollien 2000).

Flavour release in mouth conditions

Flavour release in the mouth is affected by several parameters such as the structure of the food matrix, which is indirectly responsible for texture, and oral parameters: efficiency, duration and strength of chewing, mastication rate, salivary flow and composition, swallowing rate, temperature of the food when placed in the oral cavity. These factors are determinant for the food concentration of taste and aroma compounds released in the mouth during the chewing process. These compounds then reach odour receptors at the same time to produce flavour perception, including perceptual interaction effects.

Saliva is a complex aqueous medium containing several inorganic salts and organic compounds such as organic acids, sugars, glycoproteins (mucin), alpha-amylase, several other enzymes, antibodies The functions of saliva are multiple: dilution, hydration of food, lubrication of the oral mucus. Salivary components can influence volatile partitioning from solutions. Saliva components may interact with flavour components of food. Friel and Taylor (2001) showed that volatile component partition between aqueous and gaseous phases is unequally affected by artificial saliva according to their physical and chemical properties. They noted three types of behaviour; compounds not affected by the presence of mucin, compounds showing a decrease of partition in presence of mucin and compounds showing also a decrease with mucin that was modulated by the presence of salivary salts and sugars. In this last case, some competitions of binding were observed, as the final headspace concentration was dependent on the order of incorporation of mixture ingredients. In the same way, van Ruth et al. (2001) showed different types of interactions of salt and protein of artificial saliva with some aroma compounds, depending on saliva composition and saliva/water and oil/saliva ratio of the medium. In particular, saliva proteins lowered retention of highly volatile compounds such as dimethyl sulfide, 1-propanol, diacetyl, 2-butanone and ethyl acetate and increased retention of less volatile hydrophobic compounds. With oil/saliva systems, the volatile compounds were also found affected by the saliva composition. In particular, proteins of saliva were shown to increase salting-out effect for the more polar compounds which were distributed in a large proportion in the water phase. Concerning the ratio level, for the water/saliva system, the effect on the volatile compounds was limited to an increase of the salting-out effect observed with the higher saliva ratios for only eight compounds. For the oil/saliva system, all compounds but 1-butanol were significantly influenced by the ratio, due to the difference of solubility and affinity of volatile compounds for oil and aqueous phases. Most of the tested compounds were more retained in the system with high oil ratio, except for 1-propanol and diacetyl, which were less retained.

Other factors relative to saliva, during the time the food is in the mouth, may affect the concentration of aroma compounds. During ingestion in the mouth, aroma compounds can be adsorbed on the oral mucous membrane according to their chemical properties and released slowly in saliva for a longer time than the time residence of food in the oral cavity (Buettner and Welle 2004, Buettner 2004). This phenomenon is probably at the origin of persistence perception. Some compounds such as esters or sulphur compounds can also be degraded by enzymatic activities in saliva, thereby reducing persistence or generating other potent odorants (Buettner 2002a,b).

Beside salivary composition and volume, mastication is another important factor in flavour release in mouth (van Ruth and Roozen 2000), increasing the release of aroma compounds with progressive d├ęstructuration of the food matrix which is the first transformation process of food during feeding. Mastication involves the breakdown of a solid food into smaller particles, the incorporation of saliva, the agglomeration and shaping of the resulting mixture into a cohesive bolus, and finally the transport of the bolus to the pharynx. These functions correspond to a highly complex sensory-motor activity integrating various components of the masticatory system, such as teeth, jaw muscles, lips, cheeks, tongue and the production of salivary secretion. The resulting pattern of chewing is adjusted to the food bolus properties at any time during the chewing sequence according to a precise peripheral feedback. The chewing behaviour adapts itself to the initial structure of food and the evolution of texture during the in-mouth chewing process (Lucas et al. 2002, Bourne 2004, Mioche 2004). During this process, texture and released flavour are perceived, and these perception determinants have a large impact on food acceptability and choice. The process occurring from aroma compounds release to their passage through the retronasal airway where receptors are located are very complex. During liquid ingestion, it seems that aroma compounds are able to pass through the retronasal airway only during the swallowing phases (Linforth et al. 2002) because the mouth works as a closed system. It seems that the barrier between the mouth and the pharynx is opened intermittently only during these phases, leading aroma pulses to be carried by retronasal airflow in the throat to reach aroma receptors (Buettner et al. 2002). During mastication, the air is pumped out of the mouth into the throat with each chew and aroma release can be associated with the resulting pulses of air pumped from the mouth (Hodgson et al. 2003). So, in the case of consumption of solid foods, both mastication and swallowing contribute to aroma delivery to receptors. In the case of taste compounds, the process seems simpler. As they are generally water soluble, these compounds are extracted by saliva which allows them to be readily in contact with taste receptors located on the tongue.

Interindividual differences are very important in flavour release and perception patterns (Brown et al. 1996), that is, mainly related to different chewing, salivary and airflow parameters which vary between individuals (Mioche et al. 1999, Mathoniere et al. 2000, Pionnier et al. 2004a). Moreover, jaw muscle activity is known to change with age, leading to changes in several parameters such as bite force, number of chews and chewing duration (Mioche et al. 2004). Pionnier et al. (2004a,b,c) used these interindividual differences to show relationships between aroma and taste compound release, aroma and taste perception, and oral parameters. These authors, working on a flavoured model cheese, showed in particular that for aroma compounds the maximum concentration reached and the total amount released during chewing are related to respiratory and masticatory parameters. For the non-volatile taste compounds, a maximum concentration detected late and a low total taste compound release are related to long chewing time and low saliva flow rate, low chewing rates, low masticatory performances and low swallowing rates. Concerning the relationships with perception, they found that the time necessary to reach the maximal intensity of salty, sour and mouldy attributes of the model cheese is positively related to the time to reach the maximal concentration of sodium, citric acid and heptan-2-one and to physiological parameters. The role of saliva flow in the release and perception of salt was also studied by Neyraud et al. (2003). In particular, these authors showed the impact of salt released from a matrix and of the chewing action on saliva flow rate, pH and sodium chloride concentration. They also described the cyclic swallowing of saliva which is replaced progressively by newly secreted saliva of low salt content, leading to further extraction of salt from the gum matrix. For more information, see Chapters 12 and 13 written by Boelrijk, Smit and Weel, and Linforth and Taylor, respectively, which are dedicated to these aspects.

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