Taint and odour neutrality
One purpose of a package is to protect its contents from damage. For many products, such protection also includes preservation of the product’s flavour. The package itself must not contribute to any unacceptable alterations by releasing or absorbing odorous substances which could affect susceptible products such as cigarettes and chocolate.
Odour from paperboard can arise from a number of sources such as wood resins from mechanical pulp or residual chemicals from chemical pulp. During the production of the paperboard, biological activity may produce odorous substances. Furthermore, the coating contains synthetic binders, and there is a risk that these impart an odour to the paperboard.
Mechanical and chemical pulps are selected to minimise odour and steps are taken within the mill to eliminate biological activity in the machine systems. The paperboard is tested on a regular basis to ensure that the risk of tainting of food is minimised. In addition, coating materials are subject to stringent specification and control to reduce tainting risks.
Experience has, however, shown that by far the greatest risk of tainting of sensitive products by cartons comes from the printing ink or varnish residues remaining in the paperboard after printing. Ink solvents and vehicles are often absorbed into the paperboard and may be absorbed by the fibres only to be released later. Paperboard may also absorb odorants during storage, and care should be taken to store the paperboard in an odour-free area prior to printing.
Different printing methods can cause odour problems to varying degrees. Classical offset ink, based on drying oils, develops large amounts of volatile substances when drying. It is essential that the printed sheets are well dried and well aired to prevent problems. Modern “odourless” offset inks, which reduce these risks, are available.
UV curing of offset inks is sometimes used to obtain a very high gloss. In case of insufficient or irregular curing this printing technique can cause odour problems. Gravure printing is often considered the safest method to avoid odour problems provided the solvents are carefully chosen and the drying is sufficient.
To detect and measure volatile and possibly odorous substances in paperboard or paperboard cartons, gas chromatography is often used. Ideally, each volatile substance in the sample is represented by a peak in the chromatogram and the peak area indicates the concentration of the substance.
The paperboard choice
Paperboard that will be used for the packaging of sensitive goods such as foodstuffs or tobacco should be tested to ensure that it meets required taint and odour standards. As mentioned earlier, the surface coating composition and pulp are some possible sources of odorous substances in the paperboard. Printing inks can also cause odours and different printing methods show major differences in odour contribution.
Both Folding Box Board (where mechanical and chemical pulps are used in combination) and Solid Bleached Board (where only pure chemical pulp is used) consist of primary fibres, which means that their contents are known. Chemical pulp offers the least contribution to taint and odour.
Click to enlarge.
Characteristics of taint and odour neutrality
A paperboard carton must be as free as possible of odorous substances which could originate from:
• the pulp used in the paperboard
• the coating of the paperboard
• extrusion coating
• printing, lamination or other conversion steps.
Assessment of taint and odour
An optimised paperboard package will not interact with its contents in such a way that their odour and flavour are changed. In order to be regarded as a good performer with regard to taint and odour, a paperboard must therefore have a very low concentration of odorous substances.
Different pulp and coating characteristics
Bleached chemical pulp contains cellulose and only traces of impurities. These are small amounts of fatty acids, resins and other impurities which could create odour problems. Fatty acids will oxidise in storage, if present, and develop a “woody” or even rancid odour.
Mechanical pulp contains large amounts of lignin (wood substance) and resins containing fatty acids. A paperboard based on this type of pulp could contribute to taint and odour risks, but these can be diminished with a proper manufacturing technique and screening.
Primary fibres can be fairly well controlled but secondary fibres are often of unknown origin and have undergone various converting stages such as printing before being reclaimed and reused. This means there is a considerable risk of inconsistency in the amount of contamination and impurities in the paperboard, leading to variations in the board’s taint and odour characteristics. A paperboard based on recycled fibres is therefore not recommended for the packaging of sensitive products.
Most high-class packaging boards have a pigment coating to ensure good printability. The binders in the coating are normally latices, which may contain various organic substances as impurities. Some of these could create odour problems and must be carefully controlled.
For paperboard and cartons to be taint and odour neutral, the following features play a crucial role:
• primary fibre
• the coating ingredients
• the plastic coating
• the printing method used.
The most sensitive instrument available to measure the odour and/or flavour of a substance is a human being. Only humans can describe an odour or a flavour. The members of trained panels assign numerical ratings and record their impression of tainting flavours or volatile odours experienced. By using no fewer than eight assessors, accurate and objective results may be achieved.
A number of sensory test methods are available. The choice of test method is dependent on factors such as the specific issue (e.g. type of products, type of questions) and how quickly the results are needed.
Instrumental techniques are valuable complements to the human assessments. Headspace sampling combined with gas chromatography (GC) is used to measure chemical compounds that are released from the products. To identify the compounds, this method is combined with mass spectrometry (MS).
In headspace sampling, the volatile substances that are released are collected in gas form. In GC the volatile substances are separated through differences in boiling points and absorption rates in the GC column. Their concentrations are recorded with a detector, normally a flame ionisation detector (FID). However, the instrument cannot differentiate between odorous and non-odorous substances. To solve this problem it is possible to split the gas stream after the separation and lead one part to the instrument. The other part is led to a person who sniffs the gas stream and notes whether there is a noticeable odour.
Chromatogram of unprinted paperboard.
Chromatogram of printed paperboard. Click to enlarge.
For quality control two methods may be used. The first uses a panel which compares the outturn sample (either finished product or raw material) with a reference. In the second a gas chromatogram is run to detect any new peaks indicating contamination or to note major changes in the concentration of known odorous substances.
When evaluating a gas chromatogram it is important to select compounds which are known as risk factors if they occur above the detection threshold. These compounds are of course different for different applications; for instance, cigarettes and chocolate are susceptible to different compounds.
Sampling and sample handling
To achieve proper measurements attention must be paid to the sample handling. Due to the sensitive nature of this type of analysis some important issues are to avoid perfume and perfumed soap before handling the samples and to use proper aluminium foil as protection for the samples. Please consult the Laboratory for
Sensory and Chemical Analyses for advice.
Iggesund Paperboard’s Laboratory for Sensory and Chemical Analyses is accredited (accreditation number 1740 ISO/IEC 17025) for the following analyses:
Gas cromathography principle. Click to enlarge images.
||Intensity of taint resulting from interaction material – test medium.
||Scores (0 – 4)
||The standard test medium
is ground chocolate but other media can be chosen.
||Odour intensity of the materials.
||Scores (0 – 4)
||The materials are put into glass flasks and subsequently the air of the glass flask is smelled.
|Difference testing Triangle test (ISO 4120) Duo-trio test (ISO 10399)
||Pair-wise comparison (mod. ISO 5495).
||The certainty of that there is a difference.
||Two samples are compared. Can be applied both to taint and odour.