Many aroma compounds, used to flavor food products, are used in a solid state, after encapsulation. Carbohydrates are the most common matrices used to entrap these volatiles. It has been observed that depending on the aroma compound and the carrier, efficiency can vary. This article reviews the influence of physicochemical properties of both the volatiles and the carriers on retention. The latter depends on the functional groups of the aroma compound. Moreover, it increases with molecular weight and decreases with the polarity and relative volatility of the aroma compound. This behavior can be explained by the effect of these properties on the diffusion of the aroma compound through the matrix and on the ability of the volatile to form small drops. The physicochemical properties of the carrier are important too. Retention of the aroma compound increases with the molecular weight of the carrier until an optimum is reached and then decreases for very high polymerization degrees. Viscosity, solubility, and film forming properties improve the retention ability. The effect of these parameters is discussed according to their effect on the diffusion of the volatiles. The state of the carrier has to be taken into account too. The amorphous state provides the highest retention, collapse results in losses of aroma, and crystallization leads to the greatest losses of flavors.

This is an author-deposited version published in: http://oatao.univ-toulouse.fr/ Eprints ID: 7878 AbstractThe irreversible hydrolysis of 1-chlorobutane to 1-butanol and HCl by lyophilized cells of Rhodococcus erythropolis NCIMB 13064, using a solid-gas biofilter, is described as a model reaction. 1-Chlorobutane is hydrolyzed by the haloalkane dehalogenase from R. erythropolis. A critical water thermodynamic activity (a w ) of 0.4 is necessary for the enzyme to become active and optimal dehalogenase activity for the lyophilized cells is obtained for a a w of 0.9. A temperature of reaction of 40 • C represents the best compromise between stability and activity. The activation energy of the reaction was determined and found equal to 59.5 kJ/mol. The absence of internal diffusional limitation of substrates in the biofilter was observed. The apparent Michaelis-Menten constants K m and V max for the lyophilized cells of R. erythropolis were 0.011 (1-chlorobutane thermodynamic activity, a ClBut ) and 3.22 moles/min g of cell, respectively. The activity and stability of lyophilized cells were dependent on the quantity of HCl produced. Since possible modifications of local pH by the HCl product, pH control by the addition of volatile Lewis base (triethylamine) in the gaseous phase was employed. Triethylamine plays the role of a volatile buffer that controls the local pH and the ionization state of the dehalogenase and prevents inhibition by Cl − . Finally, cells broken by the action of the lysozyme, were more stable than intact cells and more active. An initial reaction rate equal to 4.5 moles/min g of cell was observed.

Biofiltration of air polluted by volatile organic compounds is now recognized by the industrial and research communities as an effective and viable alternative to standard environmental technologies. Whereas many studies have focused on solid/liquid/gas biofilters, there have been fewer reports on waste air treatment using other biological processes, especially in a solid/gas biofilter. In this study, a comparison was made of the hydrolysis of halogenated compounds (such as 1-chlorobutane) by lyophilized Rhodococcus erythropolis cells in a novel solid/gas biofilter and in the aqueous phase. We first determined the culture conditions for the production of R. erythropolis cells with a strong dehalogenase activity. Four different media were studied and the amount of 1-chlorobutane was optimized. Next, we report the possibility to use R. erythropolis cells in a solid/gas biofilter in order to transform halogenated compounds in corresponding alcohols. The effect of experimental parameters (total flow into the biofilter, thermodynamic activity of the substrates, temperature, carbon chain length of halogenated substrates) on the activity and stability of lyophilized cells in the gas phase was determined. A critical water thermodynamic activity (a(w)) of 0.4 is necessary for the enzyme to become active and optimal dehalogenase activity for the lyophilized cells is obtained for an a(w) of 0.9. A temperature of reaction of 40 degrees C represents the best compromise between stability and activity. Activation energy of the reaction was determined and found equal to 59.5 KJ/mol. The pH effect on the dehalogenase activity of R. erythropolis cells was also studied in the gas phase and in the aqueous phase. It was observed that pH 9.0 provided the best activity in both systems. We observed that in the aqueous phase R. erythropolis cells were less sensitive to the variation in pH than R. erythropolis cells in the gas phase. Finally, the addition of volatile Lewis base (triethylamine) in the gaseous phase and the action of the lysozyme in order to permeabilize the cells was found to be highly beneficial to the effectiveness of the biofilter.

Retention of six aroma compounds has been studied after dehydration of ternary mixtures of aroma water and beta-cyclodextrin. A maximal retention of a mole of aroma per mole of beta-cyclodextrin has been observed for five of the aroma compounds, whereas retention of benzyl alcohol can be twice as high. Retention of a mixture of aroma compounds has also been studied. It has been noted that when volatile compounds compete for the same binding sites on beta-cyclodextrin, ethyl hexanoate, 2-methylbutyric acid, and benzyl alcohol are, respectively, better retained than ethyl propionate, hexanoic acid, and hexanol. Preferential retention observed with esters can be simply explained by their difference of physicochemical properties, but for the acids and alcohols a study at the molecular scale has been necessary. The better retention of 2-methylbutyric acid can be explained by differences in the nature of interaction between the acids and their carrier. At least selectivity of retention noted for the alcohol could be due to a difference in the location of the guest and also a difference in the number of aroma molecules that can be bound per polysaccharide molecule.