Food-grade paperboard: Odour profiling with mass spec-based electronic nose

Recycled paperboard odour

Used paperboard is commonly recycled as packaging material for items like fast food trays and boxes but it is more susceptible to off-odours than virgin material. If these odours are transmitted to the food or noticed by the consumers when the food is unpacked and prepared, it can lead to an unpleasant eating experience.

Analytical labs typically use GC/MS methods to test the odour profiles of foods and their packaging but some consider them to be time consuming for a quality control lab. An alternative method that is still based on mass spectrometry has been proposed by scientists in Belgium working at the Catholic University College of Ghent. Tim Van Caelenberg, Isabelle Van Leuven and Patrick Dirinck have evaluated the performance of a mass spectrometer employed as an electronic nose.

Packaging odours come from a variety of sources, including unsaturated fatty acids that are present in wood pulp resins, printing inks, varnishes and bacterial activity but their volatility guarantees that they can be detected. Other e-nose sensors have been used for testing packaging odours but they can suffer from interference from dust, water and carbon dioxide. However, their worst drawback is the inability to generate any chemical information. Mass spectrometry can overcome this.

Odour assessment

Four recycled paperboards were selected from different parts of the production process to test the method. They originated from two paper mills that used open or closed process water circuits and allowed the influence of bacterial growth in the process water to be considered.

An initial sensory analysis was carried out by a trained panel of 12 assessors based on four preselected attributes: odour intensity, musty/putrid notes, fatty/green notes and paperboard odour. This was followed by a headspace SPME-GC/MS analysis of the odour profiles which identified the volatile compounds emitted from the boards and provided target m/z values for the subsequent e-nose analysis.

The e-nose system consisted of a second GC/MS setup in which the column was maintained at a high temperature so that no separation of the volatile compounds was effected. The SPME fibre was inserted into the heated injector port and the total ion chromatogram was measured from m/z 40-180. The total mass spectrum of each paperboard was imported into modelling software.

Some m/z values were not considered for identification of the volatile odour compounds as they correspond, at least in part, to ions originating from the GC column, the SPME fibre, carbon dioxide, and mineral oils from the printing inks.

e-Nose profiling

The e-nose results were visualised in a principal components analysis plot based on 131 m/z values, which explained 95% of all of the data. It was based on the m/z values of the odour compounds that were selected from the GC/MS run. They included four aldehydes, four alcohols, three phenyl derivatives, three fatty acids, one furan and one ester.

Some key odour components were not included in the evaluation because their major ions were excluded from consideration due to their presence from other sources. So, hexanal with its grassy, green notes, and 1-octen-3-ol which has a musty, mushroom-like smell were excluded. However, the remaining ions were able to distinguish between the odour profiles of the four boards.

The first paperboard, taken during regular production in a mill employing a closed process water circuit, was characterised by m/z values at 77, 105 and 106 which were attributed to benzaldehyde and/or acetophenone. Both compounds are known to be breakdown products in recycled paperboard pulp from solvents associated with inks and varnishes. Additional discriminatory ions were attributed to a plasticiser and to 1-heptanol and 1-octanol which also originated from solvents.

Two other paperboards from closed water circuit production were taken during start-up after maintenance periods in which the process water was recycled or not. These boards were characterised by ions from acetic and butanoic acid, the latter being the main component of the volatiles. It is probably produced by anaerobic fermentation during the maintenance period.

The fourth board was produced by the open process water method. This one was distinguished by ions corresponding to 2-pentylfuran, which is formed by the autoxidation of unsaturated fatty acids from wood pulp or printing inks. Unsaturated aldehydes were also prominent, as were ions from styrene which is present in coatings added to improve printability to glossy leaflets.

This preliminary study suggests that headspace SPME coupled with the e-nose mass spectrometer can achieve odour analysis of recycled paperboards. Further studies with a wider range of paperboards and an analysis of virgin paperboard would help to establish the credentials of the technique.