ISSN
ISSN online
0717-3644
0718-221X
Maderas. Ciencia y tecnología 7(2): 79-85, 2005
MEASUREMENT OF COLOUR DEVELOPMENT IN PINUS RADIATA
♣
SAPWOOD BOARDS DURING DRYING AT VARIOUS SCHEDULES♣
Murray McCurdy1, Shusheng Pang1, Roger Keey1
ABSTRACT
Colour changes, such as kiln brown stain, that develop in Pinus radiata boards during kiln drying
can reduce the value of the final wood products and result in significant losses due to downgrade or
waste of dried wood by removing the darkened surfaces. This study measured how colour developed
in Pinus radiata sapwood boards under different drying schedules.
The boards used in these experiments were 40x100x800mm, cut from the same log and were endand edge-matched. Boards were dried at eight different schedules using temperatures from 50°C to
120°C and relative humidities from 14% to 67%. Separate boards were dried for 5 equal intervals
through each schedule and colour profiles measured through the boards using a surface reflectance
spectrophotometer. Lightness, L*, on a greyscale was used as an indication of colour change (darkening).
The results show that there is generally a greater decrease in lightness with higher temperature
schedules and also with slower, higher relative humidity, schedules. This suggests that both temperature
and drying time are significant factors in the formation of colour during drying. The most significant
changes in colour occurred near the board surfaces, indicating kiln brown stain.
Keywords: drying schedules, kiln brown stain, Pinus radiata, wood colour
INTRODUCTION
In kiln drying of softwood timber such as Pinus radiata sapwood, wood colour commonly changes,
which can reduce the quality of the final product and result in loss of value due to downgrade and
waste by removing the darkened surfaces.
The main discolouration affecting the quality of kiln dried Pinus radiata is kiln brown stain. This
is an irregular brown coloration that occurs 1 to 2 mm near the surface of drying boards and is
considerably darker than the surrounding wood. The very surface of the board is not stained and this
layer is consistent with the thin dry layer formed in the kiln drying of softwood timber (Pang et al.,
1994).
1
Wood Technology Research Centre, Chemical and Process Engineering Department, University of Canterbury, Private Bag 4800, Christchurch,
New Zealand.
Corresponding author: shusheng.pang@canterbury.ac.nz
Received: 21.06.2005. Accepted: 01.08.2005.
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Maderas. Ciencia y tecnología 7(2): 79-85, 2005
Univ er sidad del Bío - Bío
Studies by McDonald et al (2000) have shown that kiln brown stain in Pinus radiata is most likely
caused by a Maillard reaction between sugars and amino acids in the wood sap. Terziev (1995) and
Terziev et al. (1993) have measured the low molecular weight sugar distribution across the board
thickness, in a closely related species, P. sylvestris during drying and confirmed that the sugar contents
near the board surfaces are higher than the core during and after kiln drying. They also found that the
sugar gradients between the surface and the core are much severer with fast-drying schedules.
Studies by Kreber and Haslett (1997) have confirmed the above findings and found that high drying
temperatures intensify the formation of the stain. These studies also showed, surprisingly, that lowhumidity drying schedules (lower wet-bulb temperature) intensify the stain formation. In these studies,
visual inspection was used to determine the level of the stain. More recently Ledig and Seyfarth
(2001) have used a spectrophotometer to measure surface colour in European beech and have
successfully characterised the wood colour using the CIELab system.
Other studies by Boutelje (1990) observed that the nitrogen content at the board surface is also
higher than in the core, while Dieste (2002) found the nitrogen content is strongly correlated to the
colour changes at the board surfaces.
The overall objectives of this research were to investigate further the fundamental causes of the
kiln brown stain formation and to develop optimised drying schedules to produce light-coloured Pinus
radiata wood with acceptable drying time. This paper will describe the work on wood-colour
measurement and colour changes during drying. The aim of this study was to measure the effect of
different drying schedules on the development of colour using a spectrophotometer.
MATERIALS AND METHODS
The wood used in this experiment was cut from a single log (4m long) that was grown on the West
Coast of the South Island of New Zealand. The log was selected to be free from compression wood
and to have centrally located pith. The sawmill was set to cut 40x100mm flat-sawn boards from the
log and the position of each board in the log was recorded during cutting. These boards were
subsequently cut into end- and edge-matched 800mm long sample boards, treated with anti-sap stain
chemicals and then stored, wrapped in plastic, at 4°C.
Initially boards were dried fully at eight different schedules with the drying conditions shown in
Table 1. These drying operations were carried out in a single board drying tunnel with an air speed of
5m/s over the board surfaces. There was a ninth ACT schedule planned but the test failed due to
equipment malfunction. The boards used for each dry-bulb temperature series were end-matched. For
each schedule a second edge-matched board was cut into four smaller boards that were dried for 20%,
40%, 60% and 80% of the total drying time to determine how moisture and colour profiles develop
during drying.
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Maderas. Ciencia y tecnología 7(2): 79-85, 2005
Measurements of Colour...: Mc Curdy et al.
Two 25mm long samples were cut from each board upon drying. The first of these was immediately
cut into a 25×25×100mm block and then this block was sliced from surface to centre into 20
approximately 1mm thick slices to determine the moisture-content profile. The second 25mm long
sample was vacuum-dried at 40°C for 2 days and then sliced in the same way as described above to
determine the colour profile. A Minolta CM-2500d surface reflectance spectrophotometer was used to
measure the colour of each slice. The colour was represented using the CIELAB colour space where
L* is lightness, a* is red-green share, and b* is blue-yellow share.
RESULTS AND DISCUSSION
The lightness profiles for the boards dried at HT are shown in Fig 1.This graph shows that later in
drying there is a significant reduction in lightness near the surface, indicating the formation of kiln
brown stain. The overall lightness throughout the boards has also decreased with increased drying
time.
The graph in Fig. 2. shows the lightness profiles for a selected ACT schedule. The other ACT
schedules show similar trends with the main differences being in the lightness values. Overall, these
schedules show trends similar to the HT schedule, though some of them do show some inconsistency,
with the partially dried samples being darker than the fully dried samples.
90
85
Lightness
80
75
2hr
4hr
6hr
8hr
full
70
65
60
0
2
4
6
8
10
12
14
16
18
20
Distance from Surface, mm
Fig. 1. Lightness profiles (L*) for end- and edge-matched Pinus radiata boards dried at HT
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Maderas. Ciencia y tecnología 7(2): 79-85, 2005
Univ er sidad del Bío - Bío
The graph in Fig 3. shows the lightness profiles for a selected CT schedule. The lightness changes
show considerable variation for these schedules especially at the surface. The same is true for the LT
schedule in Fig 4.
The results of the colour development in the eight schedules are summarised in Table 1. To get an
indication of the total colour change throughout drying the mean core lightness (slices 4-20) for the
first sample (20% drying time) was subtracted from the mean surface lightness (slices 1-3) for the fully
dried sample, for each schedule. This was repeated for the values of a* and b* and the quantity
∆E was then calculated from these values, using Equation (1) to give a measure of the total change in
colour.
∆E =
∆L2 + ∆a 2 + ∆b 2
(1)
85
Lightness
80
3.5hr
7hr
10hr
13.5hr
Full
75
70
65
0
2
4
6
8
10
12
14
16
18
20
Distance from Surface, mm
Fig. 2. Lightness profiles (L*) for end- and edge-matched Pinus radiata boards dried at ACT-LRH2
schedule.
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Maderas. Ciencia y tecnología 7(2): 79-85, 2005
Measurements of Colour...: Mc Curdy et al.
85
Lightness
80
75
10hr
20hr
30hr
40hr
full
70
65
0
2
4
6
8
10
12
14
16
18
20
Distance from Surface, mm
Fig. 3. Lightness profiles (L*) for end- and edge-matched Pinus radiata boards dried at CT-MRH
schedule.
85
Lightness
80
27 hr
54 hr
80 hr
106 hr
full
75
70
0
5
10
15
20
Distance from surface, mm
Fig. 4. Lightness profiles (L*) for end- and edge-matched Pinus radiata boards dried at LT schedule.
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Maderas. Ciencia y tecnología 7(2): 79-85, 2005
Univ er sidad del Bío - Bío
In general, the results show that in the early stages of drying only the surface (1mm thick) is
darkened but the colour difference from the core is less than 10%. In the early stages of drying, liquid
moves to and evaporates near the surface, but due to the fast drying rate the wood temperature would
be low and the elapsed time is short so the discoloration is not significant. Terziev (1995) found that
the concentration of low molecular weight sugars and nitrogen at the surface were increased in the
early stage of drying regardless of drying temperatures. This observation can be evidence to show that
Maillard reaction is not very active during this stage because the reaction consumes the sugars and
nitrogen, reducing their concentration.
In the late stages of drying, the colour changes are more pronounced and this is particularly significant
in the second sliced layer (1-2 mm) from the drying surface. This behaviour is consistent with the thin
dry layer formed in the kiln drying of sapwood softwood lumber. During drying the liquid flow to the
surface is inhibited by the pit aspiration during sawing process. In this way, more liquid will evaporate
beneath the thin layer of 0.5 to 1 mm, thus the discoloration precursors concentration will be higher in
this region, promoting the Maillard reaction.
Table 1. Drying schedules tested showing the drying conditions and the colour development.
Schedule
HT
ACT-HRH
ACT-MRH
ACT-LRH2
CT-HRH
CT-MRH
CT-LRH
LT
Dry Bulb
120¡C
90°C
90°C
90°C
70°C
70°C
70°C
50¡C
Wet Bulb
70°C
80°C
70°C
50°C
60°C
50°C
40°C
40°C
∆L
-13 (16%)
-11 (14%)
-6 (7%)
-8 (9%)
-4 (5%)
-4 (6%)
-3 (3%)
-1 (1%)
∆a*
3 (71%)
2 (42%)
1 (24%)
1 (20%)
0
0
0
0
∆b*
5 (23%)
7 (32%)
6 (29%)
3 (14%)
4 (17%)
2 (9%)
2 (9%)
7 (33%)
∆E
14.7
13.6
8.8
8.5
5.3
4.9
3.3
7.0
Note: In Table 1, HT = High temperature schedule; ACT = Accelerated conventional temperature schedule; CT =
Conventional temperature schedule; LT = Low temperature schedule; HRH = High relative humidity; MRH =
medium relative humidity; LRH = Low relative humidity.
These experiments have shown that higher drying temperatures produce more darkening of wood
colour, which is in agreement with the findings of Kreber and Haslett (1997). The development of this
darkening appears to be quite rapid for the HT schedule, but becomes slower and more erratic at lower
temperature schedules. This variability may indicate that wood properties have more effect on colour
development at lower temperatures than at high temperatures, when the temperature-dependent kinetics
is much slower.
These experiments have also shown that there is less darkening and stain formation on drying at
low relative humidity. This can be explained as the influence of drying time. The lower relative
humidity schedules dry faster and therefore the colour has less time to develop. The temperature at the
evaporative front is also lower due to the lower wet bulb temperature. However, this observation is not
in agreement with previous studies of Kreber and Haslett (1997).
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Measurements of Colour...: Mc Curdy et al.
Maderas. Ciencia y tecnología 7(2): 79-85, 2005
These experiments have also shown the value of using a spectrophotometer for measuring kiln
brown stain in Pinus radiata. This has enabled the measurement of magnitude of colour change
compared with qualitative measurements by eye and could be an important tool for quality control in
the future.
CONCLUSION
The surface colour development in Pinus radiata boards increases as the dry bulb temperature of
the drying schedule increases. The surface colour development decreases as the wet bulb depression
of the schedule increases. The latter effect is mostly due to the shortened drying time but is also
influenced by the lower surface temperature during the capillary drying phase.
ACKNOWLEDGEMENTS
This project is sponsored by New Zealand Foundation of Research, Science and Technology through
a subcontract from the Department of Physics, University of Otago. The authors thank Professor Gerry
Carrington of Otago University for his support.
NOTA
♣This paper was first presented at the 8IWDC, Brasov, Romania, and up-dated for MADERAS.
Ciencia y tecnología journal.
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