Aroma barrier testing
The aroma of a food product is the whole of the volatile compounds that may be perceived by the olfactory system at extremely low levels, which implies that a reduced loss or sorption (adsorption and/or absorption) will be detected by the consumer (1). Losses of aroma compounds can be selective (affecting one or few components in a complex mixture) and result in changes of the aromatic profile. The food industry has long depended upon reliable, impermeable packaging materials such as glass and metal. Both suppliers and food manufacturers focus research efforts into lighter-weight, flexible and semirigid packages, which are typical qualities of plastics. While parameters such as functionality, recyclability, and cost are critical characteristics, the lack of complete impermeability and inertness in these polymer materials can have important effects. Due to their size (molecular weighto400 g mol1) and nature (very little polar until apolar and hydrophobic), the aroma compounds will interact with packaging materials often consisting of lipophilic hydrocarbons (2). Aroma compounds are able to interact with the polymer matrix, leading to polymer structural changes (3). Plastic packages are made up of polymers that form a matrix of crystalline and amorphous regions (which contain submicroscopic voids). Aromas permeate through packaging by first being adsorbed onto the package’s surfaces, diffusing through the voids (absorption), and, without a barrier material, desorbed to the package’s exterior. A sorption–diffusion mechanism is thus applied (4, 5). The mass transfer phenomenon, commonly described by the sorption, the migration, and the permeation can be determined by three parameters: S, the solubility coefficient; D, the diffusion coefficient; and P, the permeability coefficient (6). When diffusion is Fickian and sorption follows Henry’s low, the relationship P=DS can be used. Literature and knowledge on mass transfer of aroma compounds are few and no standard procedure is recommended. Methods developed for aroma compounds permeability measurements are commonly approached by isostatic or quasiisostatic methods and depend on the physical state (vapor or liquid) of the aroma compounds (6, 7).
The issue is how to obtain results of aroma permeability of packaging films in a reasonable time-frame. Depending on the static or dynamic conditions of permeation measurement, the detection systems and the environmental conditions (temperature, pressure, flows, physical state of the aromas, etc.), numerous apparatus have been designed, to different degrees of success, to obtain this information (4, 7):
Systems can be classified as a function of:
- the vapor or liquid state of the flavored medium in contact with the packaging film;
- the static, quasi-isostatic or dynamic methods;
- or the detection or analysis systems.
The aroma compound in contact with the film can be pure or dilute in a simple solution or in the food, in a vapor state, or in a liquid state at the inner surface of the film. When the flavored medium or pure aroma compound are static (no stirring, no sweeping), the technique is qualified as static and employs an accumulation process. These methods correspond to integral permeation processes (8). In dynamic (or quasi-isostatic) systems, the flavored medium and/or medium collected for analysis are stirred or swept, and the method deals with a differential permeation in which the instantaneous flow rate through the films is measured. When the two sides of the film are exposed to the same total pressure, but with different partial vapor pressures of the aroma, the technique is qualified as isobaric.
AROMA VAPOR PERMEABILITY MEASUREMENT
One primary concern when dealing with aroma barrier testing is generating an accurate test vapor of aroma. Accurate data require precise control of the test permeant concentration in the vapor phase. Since aromas are a complex mixture, the use of an aroma as a test permeant is typically difficult. Aroma generation is usually obtained by bubbling the carrier gas through pure liquid aroma and diluting the aroma saturated carrier gas with another flow of aroma-free carrier gas or with a second flow containing an other aroma compound.
The Isostatic and Isobaric Methods
The gravimetric method is probably the most simple but one of the less accurate. It requires us to work with pure aroma compounds and then at saturated concentration of aroma in the vapor phase in contact with the film. Only a partial vapor pressure differential of the flavor compound is applied between the two faces of the film; that is, the same total pressure is applied on both faces of the film. The method consists in storing pure aroma compound in a permeation cell, usually in glass or metal, sealed by the filmto study which is fixed between two Teflon O-rings. The permeation cell is stored in a ventilated room where the air is permanently renewed to maintain the lower concentration of flavor in the outside environment of the cell. The cell is periodically weighted and the cumulative mass of aroma loss as a function of time is plotted. This method is also called the integral permeation method (8). Figure 1 gives an example of the permeation cell and the kinetic obtained. This method, even when coupled with a very sensitive microbalance or a DVS system (Dynamic Vapor Sorption, Surface Measurement Systems Ltd, UK), such as done by Zhou et al. (9), is one of the less accurate (except when coupled with gas chromatography analysis) and permits us to measure permeability to only one pure compound.
The sensory method is sometimes more accurate than the gravimetric ones, and it only needs selected and trained panelists and does not required any specific equipment. First, the concentration level (threshold) at which the panelist perceived the flavor has to be measured and quantified by sensory experiments (or has to be found in the literature). Second, permeation cells having two compartments, one containing the flavor solution or flavored food and the other empty, separated by the tested film were stored in standard conditions [Figure 2 (10)]. The panelists smelt at different times. When the flavor is detected in the empty compartment, permeability could be calculated. The results obtained by this method were comparable to those obtained by the analytical method which used gas chromatography quantification. The advantage of this method is also that it uses either (a) low flavor concentrations and aroma compound mixtures because the nose is very accurate and is able to detect several flavor compounds at concentrations lower than parts per billion or (b) real food products instead of pure aroma compounds. Indeed, the nose is very accurate and is able to detect simultaneously several flavor compounds at concentrations lower than the ppb. Numerous studies demonstrate the performances (detection level and identification of aroma) of the panelists and concluded that the technical instrument’s (electronic nose) capability for detecting the aroma differences was fairly comparable with the sensory human detector.
The Gas Chromatography method uses the same type of cell that is used for the sensory method. The cell is made either of glass or stainless steel, but the two chambers of the permeation cell are equipped with a sampling port having PTFE septa. The gas phase is periodically collected with a gas-tight syringe in both chambers and injected in gas chromatograph for analysis (11).
Quasi-isostatic and Isobaric Method
In this method, the cell consists of an upper and lower chamber separated by the test packaging film (Figure 3). Complete separation and closure of each chamber were accomplished with Viton or PTFE O-rings between which the film is placed. The cell is accurately maintained at the testing temperature. The pure aroma compound, or a dilute aroma solution or a real flavored product, was placed in a glass dish at the bottom of the lower cell as used by Rubino et al. (12). The RH conditions could also be controlled. A smaller dish filled with a saturated salt solution controlling the water activity was placed in the center of the aroma dish. Aroma vapor that diffused through the film was purged by a stream of carrier gas (nitrogen, helium, or argon) that flowed through the upper cell at a constant rate and that carried the permeated vapor to the gas chromatograph for measurement as displayed in Figure 3.
Isostatic with Dynamic (or Continuous) Flow and Isobaric Methods
The isostatic technique is very similar to that of quasiisostatic and is the most used technique for measuring aroma vapor permeabilities. The material sample was also mounted in a two-chamber permeability cell, but both chambers of the permeation cell are continuously swept by the flow of gas: on one side the aroma-enriched carrier gas and on the other side the permeated low aroma concentration carrier gas (Figure 4a). As the permeant diffuses across the membrane, headspace samples are automatically collected and quantified using gas chromatography. This principle corresponds to the automatic apparatus sold for aroma permeability measurement, such as the Aromatrans (Mocon, Minneapolis, MN, USA) or the MAS2000s (Mas Technologies Inc., Zumbrota, MN, USA). Instead of FID-equipped gas chromatography analysis or detection, some authors used more specific detectors such as PDS or mass spectrometer coupled with the gas chromatograph, or UV–VIS spectrometer, FTIR or ATR–FTIR spectrometer directly coupled at the vent of the permeation cell (13).
In this method, permeant flow values as a function of time is recorded during the experiment. Initially the permeant flow is zero. After some time, permeant flow starts to increase during the transition state until it reaches a constant value. At this time, the system is in a stationary state and the experiment can be stopped. From the flow FN at the stationary state, the permeability can be calculated according differential permeation equations (6, 8, 14). Figure 4b shows the data obtained for 1-hexanol through a polypropylene film (6).
To improve the sensitivity and accuracy of the isostatic method, a cold trap (liquid nitrogen) or adsorbent trap (tenax, active charcoal, etc.) can be placed at the vent of the cell to concentrate the permeated aroma compounds. After definite times, the trap is desorbed by heating and injected in a gas chromatograph as shown in Figure 5 (15– 17). This is necessary for obtaining a suitable method allowing the permeability measurement of high-barrier polymer films at low permeant vapor pressures.
Static and Manometric Methods
Basically, the manometricmethodsmeasure the quantity of aroma vapor that has permeated through a test specimen in a given time, as a change in pressure and volume (18). The test specimen forms a barrier between two chambers in a permeation cell. A constant pressure of the aroma vapor is maintained in one chamber and a low pressure, usually vacuum, is initially established in the other chamber. A manometer is coupled to the low-pressure chamber and is used to measure the change in pressure and volume over a specified length of time. In order that the quantity of vapor measured be equal to that entering the polymer film, steady-state conditions must exist. This requires a period of time to lapse so that a constant concentration gradient is obtained across the film.
In the permeation device developed by Okuno et al. (19) and described in Figure 6, the permeability is determined in static conditions and for a pressure differential lower than 1 atm. This method allows us to strongly improve the sensitivity of the Lomax (18) manometric method previously described. First, the liquid (aroma compound or volatile organic compound) in the vessel was degassed as follows: After closing the valve V1, the liquid in the liquid vessel was frozen at liquid nitrogen temperature (1961C). Then the liquid vessel was evacuated by opening the valve V1, and the frozen liquid was melted at ambient temperature. This procedure was repeated several times. The valves V1, V2, V5, and V6 were closed, followed by heating the liquid vessel at a certain temperature. The vapor pressure supplied to the membrane was controlled by the liquid temperature in the vessel. The permeation measurement started by opening the valve V1.
The vapor permeated through the membrane was collected in one the U-tube at liquid nitrogen temperature. The collected permeants were vaporized in the U-tube disconnected from the system by closing the valve V4. The valve V5 was opened and then the pressure of the permeated aroma vapor in a known volume is measured by the pressure gauge 5. During the above pressure measurement, another U-tube was used to collect the permeated vapors. The steady state of permeability of the vapor was determined by repeating this procedure. If several compounds and mixed and studied simultaneously, the composition of the permeate has to be analyzed by gas chromatography. The main limit of this method is that the permeability value is strongly dependent of the vapor pressure applied at the inner surface of the membrane, and of the efficiency of the cold trap, as shown in Figure 7 where the permeability of a PVC sheet to ethanol varies twice according the ethanol vapor pressure used.
AROMA LIQUID PERMEABILITY MEASUREMENT
Quasi-isostatic Isobaric Method
The permeability of packaging films in liquid medium is simply determined using a two-compartment glass diffusion cell (20). The two compartments were separated by the film and were continuously stirred with a magnetic stirrer to ensure homogeneity of the solutions on both sides of the film. The permeate concentration can be measured by gas chromatography analysis or, in the case of Figure 8, by fluorescence spectrometry.
This method was also adapted for measuring the permeability of a complete package, taking into account the sealing zone, and using a continuous measurement of the permeated aroma concentration by UV–VIS spectrophotometer such as done by Gotz and Weisser (21) and shown in Figure 9.
Static High-Pressure Method
Gotz and and Weisser (21) designed a permeation device for measuring permeation of packaging in liquid submitted to high pressure. A two-compartment permeation cell was integrated in a high-pressure autoclave (Figure 10).
The bottom of a compartment of the cell is moving like a piston to apply high pressure on the liquid in contact with the film. The pressure p0 of the hydraulic fluid (water/ glycol) is applied by a piston to the lower region of the autoclave filled with water. Another piston separates the water from the aroma–solvent (water/ethanol) solution. After the pressure is built up, the cell is rinsed again in order to obtained the amount of aroma that permeated at pmax = p0 through the film. The end of the rinsing defines the beginning of the holding time. After the holding time, the cell is rinsed again with the solvent out of the reservoir. The amount permeated and then the liquid permeability is determined by GC analysis of the solvent after holding time.
Dynamic Isobaric Method
The rotating diffusion cell is designed hydrodynamically so that stationary diffusion layers of known thickness are created on each side of the film (Figure 11). The flux of aroma compound from the inner compartment across the film was measured by periodically sampling with a micro syringe from the solution in the outer compartment and analyzed by GC. The rotating diffusion cell method enables us to carry out a study on the mass transfer of solutes from a liquid phase to another liquid. The fundamental theory of the Levich model (22) provides the means of aroma compound transfer from the inner to the outer compartment by the overall permeability of aromas but also the aroma diffusivity within the film packaging and the interfacial resistances.
SORPTION METHODS
There are many works focused on the determination of aroma mass barrier which are based on sorption experiments. The retention or the release of substances is monitored during the experiment. This evolution can be followed by gravimetry (DVS, electrobalances, spring balances), by manometry, by GC or HPLC, and so on. Figure 12 (24) presents aroma compound sorption kinetics.
The quantity of aroma compounds adsorbed by the film during transient state of the mass transfer was often obtained using a modified microatmosphere method. Dried films cut into small pieces were exposed to atmospheres saturated with pure or dilute aroma compound. This atmosphere was usually conditioned at 0% relative humidity and continuously swept with a carrier gas (helium) containing a known vapor concentration of aroma compounds. The atmosphere inside the flask containing the film was kept at a constant aroma concentration. The total amount of volatile compound sorbed at a given time until constant Q was determined after solvent extraction such as nhexane (for which extraction yield is 97%) of a film sample and by injection of the resulting aroma solution in a gas– liquid chromatograph (GLC). The quantities of aroma compounds adsorbed in the films are expressed as mg mL1 of dry film. The aroma flux was defined as the ratio of the weight of permeated vapors (g) to the product of exposed area (m2) and time (s). The flux was expressed as g m−2 s−1. In practice, permeability P and solubility coefficients S are calculated from the following equations:
where F is the transfer rate (µg m−2 s−1), Dp is the vapor partial pressure gradient, e is the film thickness (m), and Q is the quantity of volatile compound sorbed in the film (mg mL1).
The diffusion coefficient can be calculated from the half-time method of sorption experiment (Figure 13), designating the time t1/2 at which the transfer rate is equal to half of the transfer rate at the steady state obtained by a differential permeation method (5, 8, 25) or from numerical solution of Fick equations. The diffusion coefficient (D) was calculated using the equations
and
where Mt is the aroma quantity sorbed at the time t,MN is the aroma quantity sorbed at the equilibrium, and M0 is the initial aroma quantity in the film sample.
Themeasurementsmethods of sorption are often used to search the affinity properties of the aroma compounds. These methods can also be used to determine diffusion and then the permeability from the sorption kinetics. But permeability can be only estimated from the sorption method when the Henry and Fick’s laws are obeyed. This calculation applies only in the absence of strong interactions between the volatile compound and the polymer– that is for a constant diffusivity and a linear sorption isotherm.