Zbornik gozdarstva m lesarstva, 41, 1993, s. 173 - 183 
GDK 892. 19 
COMPOSITION OF HYDROL YSATES OF SOME HARDWOODS 
OCCURRING IN THE PROCESS OF THEIR HYDROTHERMAL 
TREATMENT BY COOKING 
Rastislav SOLAR*, Vesna TIŠLER** and František KAČIK*** 
Abstract 
Oak wood samples were hydrothermally treated at 80 and 120° C. The times of 
treatment varied from 2 to 32 hours according to the temperature. Obtained 
hydrolysates were analysed for the contents of acetic acid, 2-furaldehyde and 
monosaccharides prior to and after the inversion. Also the specific gravity, pH, 
contents of reducing substances and the weights of dry residua were determined. 
Key words: hydrothermal treatment, oak wood, hydrolysates, acetic acid, 2-furaldehyde, 
saccharides 
ZGRADBA HIDROLIZATOV NEKATERIH LISTAVCEV 
DOBLJENIH S HIDROTERMIČNO OBDELAVO PRI KUHANJU 
Rastislav SOLAR*, Vesna TIŠLER** and František KAČIK*** 
Izvleček 
Vzorce hrastovega lesa smo hidrote.rmično obdelali pri 80° C in 120° C, časi 
obdelave so se glede na temperaturo spreminjali od 2-32 ur. Dobljeni hidrolizati so 
vsebovali ocetno kislino, 2-furaldehid in monosaharide pred in po inverZIJI. 
Hidrolizatom smo določili njihovo gostoto, pH, količino reducirajočih substanc in 
vsebnosti suhe snovi. 
Ključne besede: hidrotermična obdelava, hrastov le~; hidrolizati, ocetna kislina, 2-
furaldehid, saharidi 
* Department of Chemistry and chemical Techologies of technical University in Zvolen, Slovakia 
** Department of Wood Science and Technology, University of Ljubljana, SLO 
*** Department of Chemistry and chemical Techologies of technical University in Zvolen, 
Slovakia 
* Department of Chemistry and chemical Techologies of technical University in Zvolen, Slovakia 
•• Department of Wood Science and Technology, University of Ljubljana, SLO 
*** Department of Chemistry and chemical Techologies of technical University in Zvolen, 
Slovakia 
174 
Zbornik gozdarstva in lesarstva, 41 
CONTENTS · 
1 INTRODUCTION ................................................................... 175 
2 EXPERIMENTAL ................................................................... ·176 
3 DISCUSSION ........................................................................... 177 
4 CONCLUSIONS ...................................................................... 181 
POVZETEK ............................................................................. 182 
REFERENCES ........................................................................ 182 
1 INTRODUCTION 
175 
Solar, R.: Composition of Hydrolysates of some Hardwoods 
Part I 
(oak wood hydrolysates) 
(hidrolizati hrastovega lesa) 
Steaming and modifying the wood properties by immersion into hot water 
(boiling) are the basic methods of wood plastification. Steaming is 
preferentially used prior to bending, and boiling is often applied when 
cutting and slicing the wood. There are also other methods of wood 
plastification (so called "chemical") based on the wood penetration with 
ammonia or with its derivatives. The practical utilisation of these is 
negligible because of the odour, toxicity as well as the costs of used 
chemicals. 
The plasticizing effect of steaming and boiling is based predominantly on 
the softening the middle lamela lignin and on the relaxation of the lignin-
polysaccharidic matrix of the wood (1). Depolymerisation of wood polyoses 
and the moisture content of the plasticized wood above FSP also contribute 
to wood plasticity (2) 
The principle of the plasticizing effect of amines is due mainly to the 
rupture of hydrogen bonds in the wood substance, in the cleavage of less 
stable covalent bonds between lignin and polysaccharides, as well as among 
polysacharides themselvs (3, 4). 
The hydrolysis of ester bonds between acetyls and xylans (in the case of 
hardwood species) plays an important role in both types of plastification 
thus enhancing hydrophillic nature of the wood (5). 
It can be concluded that the fina] properties of plasticized wood are the 
reflection of great number of various physico-chemical, chemical and 
structural alterations of this substance. 
Despite of the fact that the hydrothermal plastification of wood is a 
technological process which is not proceeding with the application of 
chemicals it produces some emitants (gaseous compounds, condensates and 
hydrolysates, respectively) which are potentially harmful to the environment. 
The composition and the quantity of wood degradation products in the 
above mentioned emitants are dependent on the conditions (temperature, 
tirne, method applied and possibly the wood species processed) of 
176 
Zbornik gozdarstva in lesarstva, 41 
treatment. The aim of the presented paper was to assess the influence of 
tirne and temperature of hydrothermal treatment on the composition of oak 
wood hydrolysates. 
2 EXPERIMENT AL 
A series of comparable oak wood specimens with dimensions 2 x 2 x 30 
cm ( the longer dimension parallel with the fibres) were hydrothermally 
treated at 80° and 120° C. The tirne of treatment was 8, 16, 24 and 32 
hours at the lower temperature and 2, 4, 8 and 16 hours, respectively, at 
120° C. 
The experiments were carried out in a stainless steel autoclave with an 
automatic temperature control. The wood to distilled water ratio was m 
each case 1 to 4. The initial pH values of the medium were 4, 75 + O, 1. 
The treatments were performed in the absence of the air, which was 
removed from the autoclave by the vacuum, prior to cooking. 
The obtained treated wood specimens were desintegrated 
various analyses. The analytical data are presented 
hydrolysates were subjected to the following analyses: 
and submitted to 
in (6, 7). The 
- pH values of hydrolysates were measured with a digital MV 781 pH-
meter. 
- the specific gravity was determined by a pycnometer 
- the dry residua of hydrolysates were determined by drying in a dryer 
with the forced air circulation at the temperature of 98° C 
- amounts of 2, 4-dinitrophenyl hydrazones (2, 4 DNPH-ones) were 
determined following the method in (8) 
- the content of acetic acid in examined hydrolysates was determined by 
GLC (9) 
- the amounts of 2-furaldehyde were estimated by GLC (9) 
- the inversion of oligosaccharides present in hydrolysates was performed by 
the addition of H2SO4 so that the fina! concentration of acid was 3 %. 
Time of hydrolysis was 3 h, and the temperature 98° C. 
- contents of monosaccharides in the investigated samples were determined 
by the GLC of their aldonitrileacetates (10) - the reducing substances 
were determined according to the method of Bertrand (11) 
177 
Solar, R.: Composition of Hydrolysates of some Hardwoods 
3 DISCUSSION 
Data in Tab. 1 and 2 provide the basic information about the low and 
high temperature oak wood hydrolysates. As it follows from displayed data 
the pH value of resulting hydrolysates are more temperature than tirne 
dependent. The drop in pH values of examined hydrolysates is a result of 
the hydrolysis of acetyl as well as of the formyl esters (these are present 
in negligible amounts as reported in the literature (12) of hemiceUuloses 
(xylans) in treated material. 
Tab. 1 
Analysis of hydrolysates at 8()0 C 
Preglednica. 1 
Analiza hidrolizatov pri 8()0 C 
pH 
Density (g. cm-3) 
Reducing substances (g. dm-3) 
Dry residua (g. dm-3) 
DNPH-ones (g. dm-3) 
CH3COOH (g. dm-
3) 
2-furaldehyde (g. dm-3) 
Tab. 2 
8h 
3, 59 
1, 0020 
1, 40 
2, 15 
o, 80 
o, 69 
o, 02 
Analysis of hydrolysates at 12()0 C 
Preglednica. 2 
Analiza hidrolizatov pri 12()0 C 
2h 
pH 3, 10 
Density (g. cm-3) 1, 0020 
Reducing substances (g. dm-3) 
Dry residua (g. dm-3) 2, 95. 
DNPH-ones (g. dm-3) 5, 17 
CH3COOH (g. dm-
3) 1, 38 
2-furaldehyde (g. dm-3) o, 90 
o, 25 
16h 
3, 45 
1, 0016 
2, 45 
3, 66 
1, 09 
o, 80 
O, 03 
4h 
2, 94 
1, 0044 
5, 61 
9, 96 
2, 68 
1, 62 
o, 44 
24h 32h 
3, 40 3, 26 
1, 0017 1, 0033 
2, 62 4, 13 
3, 88 6, 70 
1, 36 2, 13 
1, 02 1, 33 
o, 05 o, 09 
8h 16h 
2, 82 2, 82 
1, 0078 1, 0119 
5, 48 20, 48 
20, 30 26, 41 
4, 51 6, 13 
3, 45 5, 00 
1, 02 1, 86 
The extremely low pH of hyrolysates after 8 and 16 hours of hydrothermal 
treatment maintained at 120° C might be explained by the liberation or 
formation of some extra formic acid or some other acid with the K dis 
ranging within values from 10-2 to 10-3. The formation of such acidic 
compounds might be a result of saccharidic moieties decomposition. We are 
178 
Zbornik gozdarstva in lesarstva., 41 
also of the opinion that it might be a conequence of certain cation-
exchange activity of uronides as well as of acidic groups of lignin (though 
these are known to be present in lignin in the minor quantities only) 
which may promote the liberation of some additional acids ( oxalic, 
hydrochloric, phosphoric etc. - by origin from the salts present in the 
wood), too. We do not consider any strong influence of tannins on tbe pH 
as their acidity (K dis) is lower than that of acetic acid. 
The values of specific gravity of examined hydrolysates change with the 
tirne very little in the case of oak wood treatment at 8° C. The increase 
in the treatment temperature to 120° C results in an ascending specific 
gravity of hydrolysates, especially after 8 hours of treatment. 
The amounts of dry residua of hydrolysates are both tirne and teperature 
dependent. Treatment at 120° C causes a deeper wood decomposition and 
more intensive diffusion of wood degradation products into the cooking 
medium. This processes are reflected in increased amounts of soluble 
hydro)ysis products in hydrolysates arising in the case of cooking the wood 
at 120° C. At this temperature, however some condensation of newly 
formed products in both wood matrix and cookig medium cannot be 
avoided. This leads to the formation of resin like materials precipitated on 
the surfaces of autoclave and treated wood specimens (probably in the 
inner spaces of the wood substance also ) in later stages of cooking. 
As for the amounts of precipitated carbonyl containing compounds (incl. 
saccharides, unfortunately) in the form of their 2, 4 DNPH-ones, these are 
also the tirne and temperature dependent, and ali gravimetric data may be 
influenced by the secondary reactions of saccharides yielding insoluble 
osazones. The higher tirne and temperature of treatment the higher 
amounts of 2, 4 DNPH-ones were determined. It is difficult to explain the 
discrepance between higher total amounts of reducing compounds in the 
hydrolysates (their amounts being expressed as D- glucose in accordance 
with the Bertrand method applied) and lower amounts of dinitrophenyl 
hydrazones precipitated (probably most of the reducing compounds are of 
lower relative molecular weight than D-glucose and there is also a 
possibility of incomplete condensation reaction or of formation of soluble 
intermediates). 
The concentration of liberated acetic acid in the low temperature (80° C) 
oak wood hydrolysates is gradually rising with the tirne of treatment, but 
the fina) concentration after 32 hour lasting experiment was very low - 1,33 
g.J-1. The increase in the treatment temperature to 120° C is reflected in 
179 
Solar, R.: Composition of Hydrolysates of some Hardwoods 
the gradually ascending concentration of this acid up to many times higher 
values as were detected in the case of hydrolysates of the low temperature 
wood treatment. 
Relatively high acetic acid concentrations in the hydrolysates ansmg in the 
treatment at a temperature of 120° C are sufficient to cause pronounced 
hydrolytic reactions of polyosesand condensation reactions (known to 
proceed even under the mild acidic conditions at elevated temperatures (13, 
14) ofthe lignin macromolecules. The 2-furaldehyde formation in the 
hydrothermal treatment is strongly dependent on the temperature and tirne 
of oak wood cooking. 
At lower temperature (80° C) the formation of this moiety is nearly 
negligible, thus indicating a very low rate of pentosan hydrolysis and further 
conversion of liberated pentoses. The formation of 2-furaldehyde is much 
more intensive when experiments are carried out at 120° C. This refers to 
a deep decomposition of hemicelluloses, particularly of pentosans. 
Data presented in the in Tab. 3 and 4 are concerned with the contents of 
saccarides in hydrolysates investigated. From comparison of these data it 
follows that concentations of both mono- and oligosaccharides in 
hydrolysates are also strongly dependent on the temperature and the tirne 
of hydrolytic treatment to which the wood was exposed. The most sensitive 
polysaccharides of oak seem to be those containing arabinose, since 
concentration of this monosaccharide is unexpectedly high even in the case 
of the hydrolysates produced in low temperature treatments. The low 
stability of arabans is probably given by the furanose form of this 
compound incorporated in the chains of arabans as well as in the side 
chains of some other hemicelluloses. The arabinofuranose being generally 
the prevailing building unit in arabanes is reported to be liberated very 
easily in the hydrolytic treatments (15). The contents of L- arabinose in 
the low temperature hydrolysates do not increase fluently with the tirne of 
treatment. In the interval of 16 and 24 hours of treatment at 80° C the 
liberation of L-arabinose slows down and remaines nearly constant ( cca 
250 mg. )·1, see tab. 3), and afterwards increases rapidly. A very similar 
trend (with exception of D-glucose) can be observed in amounts of other 
monosaccharides, though in general their concentrations ere severa) times 
lower. The inversion of low temperature oak wood hydrolysates increased 
the total amounts of monosaccharides determined a little and with respect 
to the contents of individual monosaccharides also very irregularly. The 
effect of inversion on amounts of monosacharides in hydrolysates produced 
by treatment at a temperature of 80° C is less pronounced with the tirne 
180 
Zbornik gozdarstva in lesarstva, 41 
Tab. 3 
Contents of monosaccharides in hydrolysates obtained at the temperature of 800 
C, before and after inversion with 3% sulphuric acid (data in brackets) 
(mg.dm-3) 
Preglednica. 3 
Količine monosaharidov v hidrolizatih pred in po inverziji s 3 % žveplovo (VI) 
kislino (podatki v oklepajih) pri temperaturi 800 C (mg.dm-3) 
8h 16h 24h 32h 
L-rhamnose 12. 24 45. 56 42. 37 63. 75 
(23. 12) (46. 13) (72. 10) (65. 28) 
L-arabinose 143. 10 240. 62 255. 32 638. 12 
(221. 54) (328. 60) (502. 44) (722. 92) 
D-xylose 25. 87 74. 55 71. 39 187. 26 
(56. 67) (174. 48) (165. 25) (158. 69) 
D-mannose o o 8. 22 24. 66 
(O) (12. 33) (16. 45) (22. 20) 
D-galaktose 6. 93 27. 41 29, 91 82. 58 
(48. 51) (61. 23) (71. 20) (72. 07) 
D-glukose o 58. 80 129. 85 196. 00 
(24. 50) (139. 65) (166. 61) (178. 85) 
L 188. 14 446. 94 537. 06 1192. 77 
(374. 34) (762. 42) (994. 42) (1219. 19) 
Tab. 4 
Contents of monosaccharides in hydrolysates obtained at the temperature of 
1200 C, before and after inversion with 3% sulphuric acid (data in brackets) 
(mg.dm-3) 
Preglednica. 4 
Količina monosaharidov v hidrolizatih pred in po inverziji s 3 % žveplovo (VI) 
kislino (podatki v oklepajih) pri temperaturi pri 1200 C (mg.dm-3) 
8h 16h 24h 32h 
L-rhamnose 63. 11 122. 40 462. 05 92. 41 
(70. 72) (165. 92) (481. 44) (429. 75) 
L-arabinose 364. 64 542. 72 845. 58 223. 35 
(394. 32) (646. 60) (884. 04) (1026. 08) 
D-xylose 199. 58 1089. 09 7365. 07 2123. 00 
(857. 47) (3811. 81) (10762. 75) (13588. 96) 
D-mannose 8. 40 30. 80 123. 34 46. 99 
(14. 00) (49. 28) (81. 20) (156. 80) 
D-galaktose 72. 07 158. 000 1005. 21 231. 72 
(243. 94) (449. 89) (1302. 84) (1240. 77) 
D-glukose 14. 56 210. 71 272. 25 110. 96 
(129. 85) (308. 70) (262. 15) (436. 10) 
L 722. 36 2153. 71 10073. 50 2828. 43 
(1710. 30) (5432. 20) (13774. 42) (19706. 89) 
181 
Solar, R.: Composition of Hydrolysates of some Hardwoods 
of treatment. This indicates. the very low resistance and amounts of short 
chain hemicelluloses, or their oligomeric fragments, that passed from the 
treated wood into the cooking medium (distilled water) 
The data in Tab. 4 point out the rapid decomposition of wood polyoses at 
increased treatment temperatures (120° C). The concentration of 
monosaccharides in hydrolysates is tirne dependent. After first 8 hours of 
treatment the even increase in single monosaccharides concentration 
becomes reverse thus indicating the enhanced thermo-hydrolytic degradation 
of saccharidic moieties as compared to the speed of new saccharides 
liberation from wood. The most abundant monosaccharide (except of 
arabinose in the early stages of hydrothermal treatment at 120° C ) in 
examined hydrolysates is D-xylose. This fact is undoubtedly due to the high 
amount of xylans in the oak wood polyoses (6). Relatively low 
concentrations of other monosaccharides in the cooking media are a natura) 
reflection of their low representation in the treated wood. As for the 
presence of D-glucose in hydrolysates it is rather low and probably given 
by the very limited contents of wood starch and by the increased resistance 
of cellulose (except of some of its amorphous and short chain portions) 
towards hydrolysis under experimental conditions. The inversion of 
hydrolysates produced at 120° C results in an irregular increase in the 
amounts of monosaccharides determined. The comparison of amounts of 
monosaccharides determined prior and after inversion gives rise to the 
conclusion of the parallel diffusion of mono- and oligomeric saccharides 
from the oak wood into the cooking media. The ratio of monomeric units 
prior to inversion to monomeric units after inversion is variable within the 
range of 1 : 4 to 1 : 7, approximately. 
4 CONCLUSIONS 
Obtained analytical data allow to formulate the following conclusions: 
- the mild conditions of oak wood hydrothermal treatment are reflected in 
the minute concentrations of wood degradation products 
- L-arabinose is the most abundant saccharide in hydrolysates obtained in 
the mild conditions of treatment 
- severe conditions of hydrothermal treatment maintained at 120° C 
yielded increased concentrations of wood degradation products 
- relatively high concentrations of acetic acid, 2-furaldehyde and of 
saccharidic moieties in the high temperature hydrolysates ( could be much 
higher if the lower wood to water ratio and higher temperatures were 
182 
Zbornik gozdarstva in lesarstva, 41 
used) might be interesting from the view-point of their chemical or 
biochemical utilisation 
- the low pH values of oak wood hydrolysates together with the high 
amount of tannins in this wood species may cause the increased rate of 
metallic surface corrosion (16), which is much undesired possibility 
- experiments point out possible negative impact of produced hydrolysates 
on the environment in the proximity of wood processing plants applying 
the methods of hydrothermal treatment 
POVZETEK 
Iz dobljenih analiznih podatkov lahko izvedemo naslednje ugotovitve: 
- mili pogoji hidrotermične obdelave se odražajo v nizkih koncentracijah 
razgradnih produktov lesa 
- v hidrolizatih dobljenih pod milimi pogoji obdelave je izmed saharidov 
najpogosteje nastopala L-arabinoza 
- nekateri pogoji hidrotermične obdelave pri 120° C so vodili k povečanju 
koncentracij razgradnih produktov 
- relativno visoke koncentracije ocetne kisline, 2-furaldehida in saharidov v 
hidrolizatih, dobljenih pri višjih temperaturah, bi lahko bile zanimive s 
stališča kemijske in biokemijske uporabe. Te koncenracije bi lahko 
povezali z zmanjšanjem razmerja med količino vode in lesa. 
- nizke pH vrednosti hidrolizatov hrastovega lesa skupaj z veliko količino 
taninov v teh vzorcih lesa pospešijo korozijo kovinskih površin, kar je 
seveda nezaželeno. 
- poskusi tudi nakazujejo možne negativne vplive proizvedenih hidrolizatov 
na okolje kar se še posebej nanaša na lesne proizvodnje, kjer uporabljajo 
metode hidrotermične obdelave lesa. 
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kontrola), STL, Bratislava, s. 328 
BRABEC, J. / SZALAI, P., 1973. Papir a cel., 12, (5), s. 57 
GORING, A l., 1963. Pulp and Paper Mag. Can., 64, s. 517 
KLAUDITZ, W., 1957. Holzforsch., 11, s. 47 
KOLLMAN, F., 1964. Rheol. acta, s. 260 
KRATZL, K / SILBERNAGAL H., 1952. Monatsch. Chem., 83, s. 1022 
MAKOVINI, I. / SOLAR, R.; Reinprecht, L., 1992. Drevarsk:y vyskum, 134, s. 39 
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MELCER, I. et al., 1971. Vplyv teploty a doby na fyzikalnochemicke zmeny 
komponentov topoloveho dreva v kvapalnom mediu (Viskumna sprava), Zvolen, 
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SCHUERCH, C. 1964. Forest Prod. J., 14, (9), s. 377 
SOLAR, R. / MELCER, I., 1990. Zbornik vedeckych prac DF VŠLD vo Zvolene, 
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SOLAR, R. / MELCER, I., 1992. Drevarsky vyskum, 133, s. 11 
SOLAR, R. / KAČIK, F. / MELCER, I., 1990. Drevarsky viskum, 126, s. 65 
SOLAR, R., 1976. Thesis, Zvolen 
TIMMEL, T. E., 1957. Svensk Papperstidn., 60, s. 762 
VARMA, R. / VARMA, R. S. / WARDI, A. H., 1973. J. Cromatogr., 77, s. 222 
WALLIS, A. F. A. "Solvolysis by acids" in Sarkanen, K. V.; Ludwig C. H., 1971. 
Lignins ( occurence, formation, structure and reactions ), Willey-Interscience, New 
York, s. 345