ANALYTICAL APPROACH
Identification of the Musty Component From an Off-Odor Packaging Film Robert J. McGorrin Thomas R. Pofahl William R. Croasmun Kraft, Inc., Technology Center 801 Waukegan Rd. Glenview, III. 60025
The use of plastic films for food packaging on rare occasion causes off-flavor problems, which are a result of residual additives and solvents in improperly made films. If present, these residual volatiles can migrate into the food product and contribute sensory defects because of their low flavor thresholds. Recently, a supplier's plastic packaging film was rejected by our plant personnel because of a musty off-odor defect. The odor intensity of the film was such that a distinguishable off-flavor would have been imparted to the finished food product. Because the supplier was unaware of the source of the odor defect and the reasons for its occurrence, a sample was submitted to the Basic Flavor group for analysis and identification of the off-odor.
pie using capillary gas chromatography (GC) and to locate the specific component(s) responsible for the musty odor defect. A sample of the film was heated at 100 °C in a closed jar equipped with a septum for gas sampling. Two mL of the headspace were injected onto a 50 m X 0.32 mm OV-101 fused silica capillary column. Figure 1 shows the flame ionization profile from a musty film headspace. Using simultaneous odor appraisal, only one GC peak, eluting at 19.77 min (Kovats Index 850), had a musty odor. The largest packaging volatile, eluting at 23.66 min, had
no detectable odor. Neither peak was found in the headspace of a control film. T h e large odorless peak subsequently was identified as 2-methyl-2,4pentanediol by capillary GC/MS and GC/IR. Because the musty component was present in the headspace sample at relatively high concentration, GC/MS with headspace sampling was used to obtain mass spectra of the unknown. The electron impact mass spectrum yielded limited structural information. Ions at m/z 31, 45, and 59 suggested an oxygenated species, but no molecular
Analysis of the film sample The odor was evidently quite volatile; its intensity greatly diminished after the film was unrolled and left standing on the bench for an hour at room temperature. Hence the first step in the analytical approach was to obtain a volatile headspace profile of the film sam0003-2700/87/A359-1109/$01.50/0 © 1987 American Chemical Society
Figure 1. Capillary GC profile of musty film headspace. ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 · 1109 A
P O R T R A I T SI N C H R O M A T O G R A P H Y Non-derivatized Sample Chiral Separation HPLC 9H
15 min.
OCHJCHCH 3 NHCH(CHJ)J
^ ^ ^
Propranolol
UV 254 mn 05 ml/min hexane: IPA : DEA (8 : 2 : 0.01)
a = 151
Racemic propranolol separation on Chiralcel' OP
1
Figure 2. Methane chemical ionization mass spectrum of the musty component. H0CO-CH,-CH(0H)COOH Malic Acid UV 254 nm ao ml/min 50"t 1mM CuSO„, pH 55 a = 156 15 min.
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ion could be detected. A methane chemical ionization (CI) mass spec trum (Figure 2) provided further struc tural clues. The M + 1 ion at 131 indi cated a molecular weight of 130 and suggested that a nitrogen compound was unlikely. The ion at 113 corre sponds to a loss of water, suggesting a secondary alcohol or cyclic ether struc ture. Finally, an ion at 101 implied a loss of formaldehyde from the molecu lar ion. To further narrow the structural possibilities, we turned to GC/MS/MS. In this tandem mass spectrometry technique, ions exiting the first mass analyzer collide with an inert gas and undergo further fragmentation. The
fragment ions are analyzed in the sec ond mass spectrometer. By setting the first mass analyzer to pass a single mass, and by scanning the second ana lyzer, we obtain a daughter ion spec trum displaying all the ions resulting from the fragmentation of a particular parent mass. Conversely, by scanning the first analyzer while passing a fixed mass through the second, we obtain a parent ion spectrum displaying all the ions that can produce a particular daughter ion upon fragmentation. Diagnostic ions from GC/MS/MS experiments are summarized as fol lows. Using electron impact ionization, daughter ions of 100 appear at 67 and 56. These are consistent with loss of a
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Figure 3. QC/FT-IR vapor-phase spectrum of the musty component. CIRCLE 24 ON READER SERVICE CARD
1110 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15. 1987
Table 1. Comparison of GC retention index with odor evaluation for synthesized isomers Synthetic isomer ?H ?H
V
t
L Χ.
H*
OK)
Τ "^ L i
A"• τ * A UÇ * γ - * OH
OH
Τ
1
M4
+
^ CT~^0
(CH2o)„iU o r
ι
Unknown component
second oxygen. (Recall that the 100 ion arises from a loss of CH2O from the molecular ion as indicated by the CI mass spectrum.) With methane chemi cal ionization, the 75 ion was highly diagnostic. The only significant parent of 75 occurs at 131, indicating that the 75 ion originates by a direct loss of a C4H7 hydrocarbon fragment from the protonated molecular ion. Daughter ions of 75 at 45 and 31 suggested losses of formaldehyde and acetaldehyde, re spectively. The 75 fragment has a mo lecular formula of C3H7O2. This sug gests that the unknown has two oxygen atoms in close proximity. In summary, combined MS/MS data require a mo lecular formula of C7H14O2, which in turn requires a double bond or cyclic To obtain information on the chemi cal functionality of the compound, we turned to capillary GC/IR, again using headspace sampling. The vapor-phase IR spectrum of the musty component is shown in Figure 3. Absence of a band in the 1680-1800-cm _1 region conclusive ly excluded a carbonyl functionality. Presence of a strong band in the 10401150-cm _1 region was consistent with one or more C—Ο bonds. Absence of an Ο—Η vibration at 3500-3600 cm" 1 im plied an alcohol was unlikely. Finally, absence of a distinct C = C stretch at 1630-1680 c m - 1 or an olefinic C—H stretch at 3000-3100 cm" 1 suggested that an olefin was unlikely and that a cyclic structure was necessary to satisfy the empirical formula. Together, the MS and IR data indicated a cyclic diether of molecular formula C7H14O2.
Kovats Index
Odor
«30
Sweet, camphor-like
734
Camphor-liniment
810
Musty, liniment
850
Musty
850
Musty
uation. Table I outlines four synthetic targets envisioned as likely possibili ties for the musty compound, along with the synthetic route used to pre pare them, their Kovats Index, and odor. Each has the molecular weight, molecular formula, and functionality suggested by the analytical results. From a chemical point of view, it ap peared likely that the musty compo nent arose by the addition of a diol to a carbonyl compound to form either a cyclic ketal or acetal. One possible carbonyl source was acetone, which is often present as a residual solvent in packaging materials. Reaction of ace tone with either 1,3-butanediol or 2,3butanediol produced compounds whose odors, GC retention indexes, and mass spectra failed to match the un known (Table I). Another possible re sidual solvent is methyl ethyl ketone, which must be reacted with a propane diol to yield a compound with the cor rect molecular weight. Accordingly, we
prepared the addition product of meth yl ethyl ketone and 1,2-propanediol. The corresponding cyclic ketal also failed to match the unknown. At this point we realized what should have been obvious much earlier. The largest volatile in the packaging headspace, 2methyl-2,4-pentanediol, was a poten tial cyclic ether precursor. Reaction of this diol with paraformaldehyde yield ed 4,4,6-trimethyl-l,3-dioxane. This compound eluted at Kovats Index 850 on an OV-101 capillary column. Its electron impact mass spectrum, CI mass spectrum, and IR spectrum matched those of the unknown. Final ly, the synthetic product had a distinct, musty odor that matched that of the unknown (Table I). Thus the musty component was conclusively identified as 4,4,6-trimethyl-l,3-dioxane.
Musty component origin How did this material get into the film? The 2-methyl-2,4-pentanediol is used as a solvent coating to help ink adhere to the film when the film is printed. During the production of this lot of film, residual coating material re mained behind. Apparently, during storage of the film, formaldehyde from an unknown source reacted with the diol to produce the musty component. To prevent recurrence of this problem, the identity of the musty component and its precursors were communicated to the packaging suppliers to assist them in quality control. We have briefly indicated how a combination of chromatographic, spec troscopic, synthetic, and organoleptic methods were collectively used to iden tify the voltatile component responsi ble for the musty odor. This analytical approach illustrates how an arsenal of sophisticated methods may be used to solve a real-world analytical problem.
Synthesis of musty candidates At this point we had dramatically nar rowed the field of possible structures, but we lacked an analytical method to make a final structure determination. We therefore turned to chemical syn thesis to prepare candidate compounds for GC, MS, IR, and organoleptic eval
The authors, who are members of the Kraft Basic Flavor Chemistry group, are (left to right): Thomas R. Pofahl (mass spectrometry), Robert J. McGorrin (group leader), and William R. Croasmun (analytical spectroscopy).
1112 A · ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
