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November 2008
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INFLUENCE OF KNOTWOOD EXTRACTS ON FORMATION OF HEXANAL IN PAPER
Jessica Holmback, Andrey Pranovich, Markku Auer and Bjarne Holmbom
Åbo Akademi University, Process Chemistry Centre, c/o Laboratory of Wood and Paper Chemistry, Porthansgatan 3, FI-20500 Turku/Åbo, Finland

ABSTRACT
The effects of natural antioxidants found in knotwood of Picea abies on the formation of hexanal in paper containing wood extractives during heat treatment was studied. Paper sheets of cotton cellulose were used. Lipophilic extractives were isolated from thermomechanical pulp (TMP) with hexane and added to the paper to reach the same extractive content as in wood-containing paper. Natural antioxidants were isolated from knotwood and alpha-tocopherol was used as a commercial antioxidant.

The papers with added extractives and antioxidants were placed in sealed vials and heat treated at 60°C to simulate ageing. Solid-Phase Micro Extraction (SPME) combined with gas chromatography was used to determine the amount of hexanal formed. Both natural antioxidants and alpha-tocopherol decreased the formation of hexanal. Knotwood extract (KWE) was effective at short heat treatment times. An addition of 1 kg/ton KWE could, however, not suppress the formation of hexanal at longer heat treatment times. The addition of 3 kg/ton KWE decreased the formation of hexanal even at heat treatment for 6 days to the same level as alpha-tocopherol, a well known antioxidant.

Keywords: Wood extractives, Oxidation, Hexanal, Antioxidants, Knotwood, Picea abies, Solid -Phase Micro Extraction

INTRODUCTION
The use of TMP in food packaging materials has increased during recent years. The driving forces for this development are both economical and environmental: the consumers consider these materials more environmentally friendly than packaging materials made from woodfree fibres.

Paper is never completely free from odour, for which there are several reasons. Oxidation of wood extractives is regarded as the main source for odour in paper grades that contain a lot of wood extractives. Residues of wood extractives in these paper grades are oxidized when exposed to air and heat during production, usage and storage. It is these oxidized products that can cause odour and off-flavour problems. Hexanal is the predominant oxidation product, but also other aldehydes, ketones and alcohols are formed and can contribute to odour.
Hexanal, together with other alkanals, are most likely formed by oxidative degradation of natural lipids among the wood extractives. Linoleic acid, a diunsaturated fatty acid, is a major component in the free fatty acids and triglycerides in wood extractives of Norway spruce (Picea abies). Radical-induced oxidation by oxygen of linoleic acid and its esters yields hexanal as the major volatile compound (Figure 1).

knot_1a
Figure 1. Formation of hexanal by oxidation of linoleic acid

The reactions can be catalyzed by enzymes or occur through auto-oxidation1. During production and storage of paper it is most likely that the formation of hexanal occurs through auto-oxidation. The main purpose of packaging materials is to protect and maintain the quality of the food. Contamination of the food with odorous compounds from the packaging material is therefore a big problem2. Also with other paper products, such as catalogues or books, unpleasant odour is not acceptable.

SPME together with gas chromatography is a convenient, sensitive, fast, solvent-free method for determination of volatile organic compounds in e.g. pulp and paper samples.

SPME comprises two processes. In the first process, the coated fibre is exposed to the sample and the target analytes are extracted from the sample matrix into the coating of the fibre. Desorption is the second process, where the fibre with concentrated analytes is transferred to an instrument, e.g. a gas chromatograph, for desorption followed by separation and quantification.

Knots in trees, i.e. the branch stubs encased in the tree stem, contain exceptionally large amounts of polyphenols. In recent studies, knots of several Picea species have been shown to be exceptionally rich sources of polyphenols3. For instance; the amount of extractable polyphenols in the knotwood of Picea abies is typically around 10%. These compounds are strong antioxidants and may become available on an industrial scale.

knot_2
Figure 2. Amount of hexanal (%) formed during the heat treatment. * Is the reference sample; amount of hexanal formed without addition of antioxidant.

Alpha-tocopherol, also known as vitamin E, was selected as a reference due to its function as a powerful biological antioxidant. Some work on the influence of antioxidants on formation of hexanal in pulp products has previously been reported4. The new aspect of this study is the use of polyphenols that are found naturally in trees and to some extent even in the pulp.

MATERIALS AND METHODS
Antioxidants
Two different antioxidants were studied; alphatocopherol (Sigma-Aldrich), which is a commercially available antioxidant and natural antioxidants extracted from Picea abies
knotwood.

Paper samples
Commercial paper sheets (blotting paper) made of cotton cellulose with a grammage of 250 g/m2, were used. Lipophilic extractives were isolated from TMP by extraction with hexane. About 1 mg/g of these extractives were added to the paper sheets to reach the same extractive content as in wood-containing paper.

Addition of antioxidants to papers
The antioxidants and TMP extractives, were applied to the paper sheets by dipping. Ethanol solutions, containing antioxidants and TMP extractives, were prepared. The amounts of antioxidants added to the paper sheets were 0,1 and 3 mg/g of pulp. The repeatability of adding extractives and antioxidants to paper by dipping showed a standard deviation of only 3%.

Accelerated ageing
Heat treatment was performed to simulate accelerated ageing. The samples were placed in an oven with circulated air at 60°C ± 2°C for 0, 1, 8, 24, 96 (4 days) and 144 hours (6 days) . 144 hours (6 days) corresponds to an ageing at room temperature for 3 to 12 months5. Care was taken to avoid contamination.

Solid Phase Micro Extraction – Gas Chromatography
From each paper sample, small pieces (0.5 x 0.5 cm) were cut and placed in a 4-mL glass vial suitable for Solid Phase Micro Extraction (SPME). The vial was sealed with a silicone septum. After the accelerated ageing at 60°C for the required time, the vials were thermostat-controlled for 30 min at 55°C, after which the SPME fibre was exposed to the air inside the vial in order to absorb the analytes.

The fibre used for SPME was a Supleco StableFlexTM SPME Fibre with 65 µm PDMSDVB Coating (pink). After 30 min of exposure, the SPME fibre was manually introduced into the Varian 3400 model gas chromatograph.

Desorption time was 5 min at 250°C. The volatile compounds were subsequently separated on a HP-5 capillary column (length = 30 m, inner diameter = 0.32 mm, film thickness = 0.25 µm) using a temperature programme from 50°C to 250°C at 8°C/min with initial hold for 3min and hydrogen as carrier gas (1 mL/min) and detected with a flame ionized detector (FID).

The amount of hexanal was calculated from the area of the hexanal peak compared to the amount of paper sample in the vial. The amount of hexanal is reported as a %-value, where 100% is the amount of hexanal released from paper with no antioxidant added. Two SPME analyses were performed in parallel for each paper sample.

Brightness measurements
Measurement of brightness of the thermally treated paper samples was performed with an Elrepho Datacolor 2000 according to ISO 2470 specifications.

RESULTS AND DISCUSSION
Hexanal, being the predominant oxidation product is often used as an indicator when looking at oxidation of unsaturated fatty acids in pulp and paper products.

However, one antioxidant may work well for one type of wood extractive or pulp but may not work in another case. Also the processes and surrounding conditions, such as handling and storage, will most likely affect the results. This study has been done under controlled conditions and with well known materials to give as good comparability as possible. Paper sheets of cotton cellulose with addition of wood extractives were chosen for this study, since paper sheets made from TMP naturally contains lignans and would have had an influence on the dosages of antioxidants.

Formation of hexanal
The formation of hexanal in papers containing extractives and a natural antioxidant, KWE, or a commercial available, well-known antioxidant, alpha-tocopherol, was examined. Alpha -tocopherol has been shown to be one of the most effective compounds to decrease the formation of hexanal in paper4.

Addition of KWE to the paper samples decreased the formation of hexanal in the beginning of the heat treatment (Figure 2). The lower dosage (1 kg/ton) of KWE decreased the formation of hexanal to 70% (compared to the amount of hexanal without addition of antioxidant) after 8 hours and 41% after 24 hours. After additional heat treatment, the lower dosage of KWE was not sufficient to suppress the formation of hexanal. An increase in the dosage of KWE (3 kg/ton) gave a decrease in the formation of hexanal.

After 8 hours, the amount of hexanal formed was 36% and after 24 hours only 24%. After 6 days (144 hours) of heat treatment the amount of hexanal was 35%. This can be considered as a good result since 6 days of heat treatment equals storage for 3-12 months at room temperature.

Alpha-tocopherol could not suppress the formation of hexanal at short heat treatment times. Especially for the higher dosage (3kg/ton) was the formation of hexanal not lower after 8 hours without antioxidant. After additional heat treatment, the higher dosage of alpha-tocopherol kept the formation of hexanal low. Similar results have been found previously4.

For both antioxidants tested, the lower dosages (1 kg/ton) were not sufficient to keep the formation of hexanal low at longer heat treatment. At the lower dosages, the antioxidant is probably consumed after a certain duration of heat treatment. The higher dosages managed quite well to keep the amount of hexanal low even for longer periods of heat treatment.

Brightness
Brightness was measured for all paper samples. No significant changes in brightness could be observed for the heat-treated paper samples. Addition of antioxidants had no effect on brightness.

A small increase in yellowness could be observed for the paper samples when higher dosages of antioxidants were applied. This is most likely due to the addition of antioxidants and wood extractives that may contain some coloured compounds.

CONCLUSIONS
Knots in trees, i.e. the branch stubs encased in the tree stem, contain exceptionally large amounts of polyphenols. These compounds are strong antioxidants and may become available on an industrial scale.

We have shown that these polyphenols can reduce the formation of hexanal formed from wood extractives in paper. The formation of hexanal was low even after 6 days of heat treatment. These results show that these polyphenols can compete very well with known antioxidants used to decrease the formation of hexanal in paper.

ACKNOWLEDGEMENTS
The authors thank Jalle Hemming for all the help with gas chromatography. Financial aid was provided by the Finnish Funding Agency for Technology and Innovations (Tekes) and the industry partners Ciba Specialty Chemicals, Kemira, Metso, Mreal and UPM-Kymmene.

This work was part of the activities carried out at the Åbo Akademi Process Chemistry Centre within the Finnish Centre of Excellence Programme (2000-2011) by the Academy of Finland.

REFERENCES
1. Svedberg U, Högberg H-E, Högberg J and Galle B. Emission of hexanal and carbon monoxide from storage of wood pellets, a potential occupational and domestic health hazard. Am Occup Hyg 48:4 (2004) pp339-349.

2. Forsgren G, Frisell H and Ericsson B. Taint and odour related quality monitoring of two food packaging products using gas chromatography, gas sensors and sensory analysis. Nord Pulp Pap Res J 14:5 (1999).

3. Willför S, Hemming J, Reunanen M, Eckerman C and Holmbom B. Lignans and lipophilic extractives in Norway spruce knots and stemwood. Holzforschung 57:1 (2003) pp27-36.

4. Shils M, Olson J, Shike M and Ross A. Modern nutrition in health and disease. 10th edition Williams and Wilkins Baltimore (1999) pp347-362.

5. Fagerlund A, Shanks D, Sunnerheim K and Engman L. Protective effects of synthetic and naturally occurring antioxidants in pulp products, Nord Pulp Pap Res J 18:2 (2003) pp176-181.

6. Sundberg A, Weinholm M, Fardim P, Lassus A, Helander M and Holmbom B. Oxidation of extractives in newsprint paper. Proceeding of Eight European Workshop on Lignocellulosics and Pulp (2004) pp115-118. Institute of Wood Chemistry Riga.

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