Note 1: Determination of Off-Odors and Other Volatile Organics In Food Packaging Films By Direct Thermal Analysis-GC-MS

by Thomas Hartman, CAFT, Rutgers University, New Brunswick, NJ

INTRODUCTION

This investigation was conducted to determine the compounds responsible for an off-odor in a polypropylene food packaging film manufactured from a recycled resin feedstock. The off-odor was not present in films made under identical conditions using new resin. Therefore, it was our intention to obtain a profile of the volatile organic compounds from both film samples and look for differences, which may be the cause of the off-odor complaint.

Aliquots of both packaging films were subjected to direct thermal analysis-gas chromatography-mass spectrometry (TA-GC-MS) using a Scientific Instrument Services (SIS) Model TD-1 Short Path Thermal Desorber accessory. Both films were found to contain over 175 volatile organic compounds. However, the film made from recycled resin was found to contain a cluster of approximately 25 peaks in the low boiling region of its gas chromatogram which were absent in the film made from virgin resin. These peaks were identified as a mixture of C-7 to C-9 aliphatic and olefinic hydrocarbons with branched, linear and cyclic species present. Perhaps even more significant from an organoleptic viewpoint was the presence of N,N-dimethylformamide. The aromatic compound toluene was also detected. Based on this data, it was judged that these compounds were the cause of the off-odor in the recycled films.

Additional compounds found in both films included a range of phthalate ester plasticizers, phenolic antioxidants, as butylatedhydroxytoluene (BHT), paraffinic hydrocarbons used as mold release agents and a series of sesquiterpenoid compounds, as alpha-copaenes and alpha & beta-ylangenes.

DISCUSSION

Plastic films made for food packaging applications are primarily composed of high molecular weight polymers which are hence nonvolatile at temperatures below those which induce pyrolysis reactions. However, a multitude of low molecular weight volatile compounds are added to films as manufacturing aids and to impart functional properties into the products. Plasticizers and elastomers are used to promote flexibility and inhibit brittleness. Common plasticizers and elastomers include phthalates, adipates, other esters of dicarboxylic acids and fatty acid amide derivatives. Antioxidants are added to prevent oxidation of the film itself as well as to protect the foods stored within the films. Hindered phenols such as BHT, BHA and di-t-butylphenols are commonly employed for this purpose. We have also detected mono, di and triesters of these phenols with phosphoric acid as well as triphenylphosphites and trisnonylphenylphosphites. Ultra Violet (UV) stabilizers are added to prevent yellowing of the films upon exposure to light. Compounds such as diphenylketones and various methoxylated derivatives are commonly used for this purpose. During the manufacture of these films various processing aids are employed. These include accelerators and cross-linking agents to promote polymerization and polymer strength and mold release agents such as paraffins or organosilicones which are used to prevent sticking to the rollers as the films are extruded. Still, other additives may be added to prevent static build-up and fragrance compounds used to mask resulting off-odors in the films. In some heavily plasticized films, as polyvinylchloride (PVC), the total volatile organic fraction can be as high as 40 per cent w/w. Even non-plasticized films, as polyethylene and polypropylene, contain an appreciable volatile organic content which may range from parts per million levels (ppm) to several per cent w/w.

Given the plethora of volatile organic compounds present in food packaging films, it is not surprising that they readily migrate into foods stored therein, often causing off-odors. In addition, there is presently much concern for the potential health risks associated with consuming food products with high levels of packaging borne migrants. The problems have recently become exacerbated by the increased use of packaging films, as food wraps during microwave cooking. In this instance, the worst possible scenario for promoting the migration of volatile packaging film additives into foods exists. The high temperatures (at or near the boiling point of water) encountered in microwave food preparation serve to volatilize the organics in the films. The steam generated in the cooking process is an ideal heat transfer medium. The volatile organics thus liberated from the films permeate and diffuse into the food product, typically migrating into the hydrophobic lipid portion of the food. All of this is usually occurring with a relatively large surface area of film exposed in direct contact to the food in a semi-airtight environment, thereby trapping much of the volatile species within.

For these reasons, it is obvious why there is a need for analytical methodology to study the volatile organic composition of packaging films. These investigations serve to define the additive composition of particular films and to assess the potential for migration of these species into foods. In addition, these studies are useful for general QA/QC testing, packaging compatibility studies or for comparing films from different vendors. The Scientific Instrument Services (SIS) Model TD-1 Short Path Thermal Desorber GC accessory is an ideal instrument which can quickly and easily analyze small quantities of films to yield both qualitative and quantitative data on the volatile organic composition. Films are simply weighed out, placed in a glass lined stainless steel desorber tube and fitted to the instrument and heated under controlled conditions whilst the thermally desorbed components are sparged from the product into the gas chromatograph injection port. A typical sample size ranges from 1.0 to 100 mg depending on the plasticizer content; analysis times range from 30 to 60 minutes per sample.

The objective of this analytical investigation was to determine the chemical composition of the species responsible for an off-odor in a polypropylene film made from recycled resin tailings. However, the total profile of volatile organic compounds present in the films was also characterized.

INSTRUMENTATION

All experiments were conducted using an SIS Model TD-1 Short Path Thermal Desorber accessory connected to a Varian 3400 gas chromatograph. The GC was directly interfaced to a Finnigan MAT 8230 high resolution magnetic sector mass spectrometer. Data were acquired and processed using a Finnigan MAT SS300 data system.

EXPERIMENTAL

Portions (100 mg) of the new and recycled films were cut and weighed out in preparation for analysis. Care was taken to avoid the deposition of fingerprint residues on the films while handling them. To accomplish this, portions of the films from the interior of the rolls were sampled; handling was carried out using clean lint free nylon gloves. The films were then tightly folded and placed into SIS preconditioned 4.0 mm i.d. glass lined stainless steel desorption tubes. An empty desorption tube was included with the two polypropylene sample tubes to serve as a negative control. The tubes were then fitted with SIS syringe adapters and connected to the TD-1 thermal desorber.

THERMAL DESORPTION -GC-MS ANALYSIS

The tubes were thermally desorbed for 10 minutes at a temperature of 100 degrees C while sparging with helium carrier gas at a flow rate of 10 ml per minute directly into the GC injection port. The GC injector was maintained at 250 degrees C with a split ratio of 10:1. The GC column was a 60 meter x 0.32 mm i.d. DB-1 capillary column containing a 0.25 µm film thickness with a flow rate of 1.0 ml per minute (He). The column was temperature programmed from -40 degrees C (held for 10 min. to cryofocus during thermal desorption interval) to 280 degrees C at a rate of 10 degrees C per minute. The mass spectrometer was operated in electron ionization mode (70 EV) scanning masses 35-350 once each second with a 0.8 second interscan time.

RESULTS AND DISCUSSION

Approximately 175 and 200 individual volatile organic compounds were observed in the new and recycled polypropylene films, respectively. The negative control (empty desorption tube) experiment was found to be free of interferences, with the exception of a minor peak for an organosilicone compound from the injection port septum bleed. Figure # 1 is a comparison of the total ion current chromatograms obtained from the new and recycled films. It is readily apparent that the chromatogram from the recycled film contains a cluster of approximately 25 peaks (retention times 9 through 17 min.) which are absent in the chromatogram of the film made from new resin.

Figure #1 (Top) Total ion chromatogram from polypropylene film with no off-odor made from new resin.

Figure #1 (Bottom) Total ion chromatogram from polypropylene film off-odor made from recycled resin.

Figure # 2 is a close-up view of this region in the two chromatograms. These compounds were identified as a mixture of branched, linear and cyclic aliphatic and olefinic hydrocarbons ranging from C-7 to C-9. The only exceptions were small peaks from N,N-dimethylformamide and toluene.

Figure #2 (Top) Closeup view of chromatogram from polypropylene with no off-odor made from new resin.

Figure #2 (Bottom) Closeup view of chromatogram from polypropylene with off-odor made from recycled resin.

The identities of these compounds are summarized in Table # 1. It is assumed that these components are responsible for the off-odor present in the film.

TABLE #1 COMPOUNDS UNIQUELY IDENTIFIED IN RECYCLED FILM

N,N-DIMETHYLFORMAMIDE

2-METHYLHEXANE

3-METHYLHEXANE

2-METHYL-4-HEXENE

3-METHYL-4-HEXENE

1,2-DIMETHYLCYCLOPENTANE

2,3-DIMETHYLHEXANE

HEPTANE

METHYLCYCLOHEXANE

ETHYLCYCLOPENTANE

BICYCLOHEPTANE

TOLUENE

2,3,5-TRIMETHYLHEXANE

2,3,4-TRIMETHYLHEXANE

3,3,5-TRIMETHYL-1-HEXENE

OTHER UNIDENTIFIED ISOMERS OF ABOVE COMPOUNDS 

Additional compounds which were found in both films include a series of plasticizers, antioxidants and mold release agents. Figure # 3 is a mass chromatogram for m/z 149 overlaid on the total ion chromatogram. This mass is the base peak of all of the phthalate ester plasticizers (except for dimethylphthalate) and is useful for rapidly locating these components in the otherwise complex chromatogram. The four major peaks listed in order of elution were identified as diethyl, diisobutyl, dibutyl and diisooctyl phthalates. Figure #3 Mass Chromatogram for m/z 149 which is the base peak characteristic of phthalate ester plasticizers.

Figure #3 - Mass Chromatogram for m/z 149 which is the base peak characteristic of phthalate ester plasticizers.

The antioxidants present in the film are similarly presented in the mass chromatograms shown in Figure # 4. The characteristic masses of m/z 220, 205, 191, 206 are the base peaks and molecular ion species for BHT and di-t-butylphenol respectively. Lower concentrations of other phenolic analogues of these two compounds are also present.

Figure #4 Mass chromatograms for the ions characteristic of the antioxidants BHT (m/z 220, 205) and di-t-butyl phenol (m/z 206, 191)

By far, the compounds found in the highest concentrations are the paraffinic hydrocarbons used as mold release agents in the films. Figure # 5 is a mass chromatogram plot for the characteristic fragment ions m/z 43 (C3H7), 57 (C4H9) and 71 (C5H11) from the paraffin series. These compounds ranged from C-9 to C-16 and were mostly branched and linear aliphatics, although low levels of olefins were also observed.

Figure #5 Mass chromatogram of fragment ions characteristic of paraffinic hydrocarbons used as mold release agents.

An interesting group of sesquiterpenoid type compounds were also observed in these films. They exhibited molecular ions at m/z 204 and contained base peaks at m/z 161. The mass chromatograms for these ions are shown in Figure # 6.

Figure #6 Mass chromatogram for 204 M.W. sesquiterpenoid compounds. Compounds present include alpha-copaene and alpha & beta-ylangenes.[

Library searches of these compounds indicated perfect matches with the compounds alpha-copaene and alpha & beta-ylangenes. The Merck Index lists these compounds as constituents of several plant derived essential oils used as fragrance materials. Other industrial applications are listed for use in varnishes, varnish cleaners and removers and in the manufacture of photographic paper. The authors are uncertain what their purpose is in these particular polypropylene films.

CONCLUSIONS

The SIS model TD-1 Short Path Thermal Desorber accessory, used in combination with GC-MS is an ideal instrument for the direct thermal analysis of volatile organic compounds in packaging films. Although the sample size is small, it provides for rapid analysis and no laborious sample preparation ,as solvent extraction, is required. In this instance, the apparatus was successfully employed for the determination of off-odor components, as well as other volatile additives including plasticizers, antioxidants and mold release agents. In the past, we have used this technique to quantify the residual vinyl chloride monomer in polyvinylchloride resin beads and polymer films. The methodology outlined in this manuscript may be useful for QA/QC testing, off-odor analysis, packaging compatibility studies and screening of products from various vendors.

ACKNOWLEDGMENT

We acknowledge the Center for Advanced Food Technology (CAFT) Mass Spectrometry laboratory for conducting this study. CAFT is an initiative of the New Jersey Commission of Science and Technology. This is CAFT/New Jersey Agricultural Experiment Station (NJAES) publication # E-10570-1-90.