418 M.A. Pozo-Bay ́ ́ on and G. Reineccius
is determined by the vapour-liquid equilibrium (VLE) of each individual odorant,
which is characterized by the partition coefficient (ki) between the vapour phase
and the liquid phase. This coefficient is expressed by the ratio between compound
concentrations in the gas phase vs the liquid phase in the sample at equilibrium:
ki=[C
gas
i in (mol or g)/m(^3) ]/Cliq
i in (mol or g)/m
(^3)
The partition coefficient can be expressed in molar fraction, also called absolute
volatility (Ki) or by the activity coefficient (γi). Both parameters are related as is
follows:
Ki=yi/xi=i∗P◦i(T)/PT (8F.2)
wherexiandyiare the molar fractions in the liquid and gas phases, respectively,
P◦i(T)is the vapour pressure of pure componentiat a given temperature T (Pa),
and PTis the total pressure (Pa). Volatile compounds in food and beverages are
usually near infinite dilution (concentrations lower than 10−^4 mole fraction), and
thus activity coefficient can be considered constant and is noted as∞i. The product
i×P◦i(T)is Henry’s constant.
Partitioning of volatile substances between the liquid and gas phases is mainly
governed by aroma compound volatility and solubility. These physicochemical
properties are expected to be influenced by wine constituents present in the medium,
for instance polysaccharides, polyphenols,proteins among others. Consideration of
the physicochemical interactions that occur between aroma compounds and wine
constituents is necessary to understandthe perception of wine aroma during con-
sumption. The binding that occurs at a molecular level reflects changes at a macro-
scopic level of the thermodynamic equilibrium, such as volatility and solubility, or
changes in kinetic phenomena. Thus, thermodynamic and dynamic approaches can
be used to study the behaviour of aroma compounds in simple (model) or complex
(foods) media.
8F.1.2 Methods to Measure Interactions Between Aroma
Compounds and Wine Macro-Components
The methods employed to measure the interactions that occur between aroma com-
pounds and other food or beverage constituents are frequently based on measuring
changes in the vapour-liquid equilibrium when different macromolecules are present
in the media. The determination of thegas-liquid partitioning with and without a
food macromolecule is widely employed.
There are different experimental methods for determining the gas-liquid partition
coefficients leading to the determination of activity coefficients at infinite dilution
∞i. The most frequently used methods are dynamic and static headspace methods.