Agricultural and Food Science in Finland 259 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 9 (2000): 259–268. © Agricultural and Food Science in Finland Manuscript received July 2000 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 9 (2000): 259–268. Equilibrium moisture content of flax/linseed and fibre hemp straw fractions Hanna-Riitta Kymäläinen and Antti Pasila Department of Agricultural Engineering and Household Technology, PO Box 27 (Viikki F), FIN-00014 University of Helsinki, Finland, e-mail: hanna-riitta.kymalainen@helsinki.fi This preliminary study of the equilibrium moisture content (EMC) of bast fibre plants (Linum usita- tissimum L. and Cannabis sativa L.) examines three fractions, fibre, fine shive and coarse shive. The plants were harvested at two times, the first in autumn and the second in spring. The autumn harvest yielded unretted, green material, while the frost-retted material harvested in spring may be classified as overretted. Interesting differences in EMC were found in the dampest air between the two harvest times irrespective of plant species: green fractions were faster to mould at the beginning and lost more weight in the 2-week test period than did the frost-retted samples. The green samples also attained higher EMCs before beginning to mould. Key words: fibre plants, fibres, separation, humidity, wetting, equilibrium moisture content, Linum usitatissimum, Cannabis sativa Introduction Traditionally bast fibre plants have been culti- vated for seeds or long spinning fibres. Only these, the most valuable parts have been used, and typically only one of them at a time. Recent changes in Finnish agriculture have prompted farmers to look for new approaches enabling the old cultivation plants to be applied in new ways. The cost factor and introduction of new applica- tion areas have led to a demand for total exploi- tation of the bast fibre plants, which in turn re- quires new fractioning techniques and knowl- edge of the behaviour of the fractions (Pehko- nen et al. 2000). In the North, where there is a huge variation between outdoor temperature and the relative hu- midity of air during the year, the moisture con- tent of the stems or of the raw material produced from the plants is one of the most critical fac- tors in achieving materials of an acceptable qual- ity. The moisture content of the raw material is therefore of special importance, as it is one of the properties that determine the selection of the raw material for a specific end-use product. The other important factor in moisture behaviour lies in the harvest, storage and processing chain be- mailto:hanna-riitta.kymalainen@helsinki.fi 260 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kymäläinen, H.-R. & Pasila, A. EMC of Linum usitatissimum and Cannabis sativa fractions fore the production of raw material. This pro- duction chain includes several phases and delay periods during which the material may be sub- jected to fluctuating conditions. Before the de- sired processed fraction is achieved, the straw or processed straw will have several opportuni- ties to alter its properties due to humidity. Equilibrium humidity is understood as a hy- groscopic equilibrium humidity, that is, the amount of water a material binds from the air at a certain temperature and relative humidity (RH) (Leppävuori et al. 1991). If a dry material is sub- mitted to a constant temperature and RH, then an equilibrium, a balance, will form between the humidity of the material and that of its surround- ings. An adsorption/desorption curve with sev- eral equilibriums at different relative air humid- i t i e s i s c a l l e d a ( a d / d e ) s o r p t i o n i s o t h e r m (Nevander and Elmarsson 1994). If products are subjected to fluctuating atmospheric conditions, the equilibrium moisture content (EMC) changes constantly (Skaar 1988). According to Skaar (1988), EMC has two phases: 1. monolayer sorption: the attraction between the sorbent (vapour) and sorbate is greater than that between the sorbent molecules themselves in the liquid state; 2. multilayer sorption: the attraction of the sorbed molecules to the sorbate is minimal. In addition, Nevander and Elmarsson (1994) have described a third phase, in which the hu- midity in the material is high, and menisci begin to form. This leads to the domination of capil- lary condensation. The EMC curve rises steep- ly. In the last period the practical difficulty arises of measuring the equilibrium in the last few per cent up to 100% RH. The last period is nev- ertheless very important. Equilibrium humidity has been found to have importance in the food, textile, and wood sec- tors. Moisture content may be expressed on a dry basis (d.b.) or a wet basis (w.b.), depending on the context in which the term is used. In food products, the systems’ equilibrium relative humidity is said to be closely related to the chemical, physical and biological properties of the products, and hence to their quality and stability, which in turn affect packaging prac- tices and storage conditions (Rockland and Stew- art 1978). In this respect, the term water activity is also often used. Water activity refers to the availability of water in a food medium, and there is a direct correlation between microbial growth on a substrate and the thermodynamic water ac- tivity of the substrate (Scott 1953). In textiles, moisture also has several other effects, for example: – the more hydrophilic a material, the easier dyeing and other processes in acqueous so- lutions will be – a very hydrophilic material is slow to dry – dimensional and tensile properties are de- pendent on moisture content (Needles 1986). Moisture regains (d.b.) are usually deter- mined for textile fibres in air at 21±1ºC and 65% RH (Needles 1986). The moisture regain for flax and hemp fibre is 12%, and for cotton, which is very nearly pure cellulose, 7–9% (Mauersberg- er 1948). For example, in textile laundering the moisture content is expressed on w.b. For cot- ton a 25–30% moisture absorbency (w.b.) has been measured at 100% RH of air (Needles 1986). Also for wood, moisture content is usually expressed on d.b. (Skaar 1988). The EMC be- haviour of flax/linseed and hemp shive contain- ing 23–30% of lignin (Sharma 1992, Koslowski et al. 1997) may be closer to that of wood than of fibre, because a woody cell wall has a lignin content of 20–35% (Siau 1995). When wood adsorbs water, the cell walls swell until they become water-saturated. This moisture content is called the wood’s fibre saturation point (Parham and Gray 1984). According to Kirk and Cowling (1984), wood will not decay as long as it is kept below its fibre saturation point, which is ~ 27% of its dry weight. It has been noticed in practice that humidity affects the stability of bast plants and fractions. Food consists of biological matter which, how- ever, differs from that of non-food fibrous ma- 261 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 9 (2000): 259–268. terials. Therefore it is essential to examine the stability of bast fibre plants from the practical point of view of the production chain. EMC also plays a role in the ease with which stems are processed in and after harvest (Pasila et al. 1998). The higher the equilibrium humidity, the higher is the friction coefficient between the stem and processing equipment and thus the more diffi- cult the stems are to process. This has a marked effect on the operation of the equipment, espe- cially in humid circumstances. The present study sought to provide insight into the adsorption EMCs of fractioned flax, lin- seed and hemp straw at two harvesting times, the first in autumn (green straw) and the second in spring (overretted straw). Material and methods Plants and harvesting time In this study two plants, flax (Linum usitatissi- mum L.) and hemp (Cannbis sativa L.) were in- vestigated. Two types of Linum usitatissimum were used: flax (varieties Viola and Viking) and linseed (variety Helmi). The hemp varieties were Felina 34 and Kompolti hybrid. Below, the terms flax, linseed and hemp are used. The plant sam- ples were picked at various farms in southern Finland in 1997. The plants were cultivated ac- cording to common farming practices in Finland (Hakala and Hongisto 1994), and were harvest- ed in autumn when the straw was still green and in spring using the dry-line method (Pasila 1999). The focus here is on both the green (unretted) and frost-retted (overretted) straw of each of the three plant types. Harvested plants were mechan- ically separated into three fractions: fibre, coarse shive and fine shive. Detailed information on the plants, separation process and basic properties of the fractions is presented by Kymäläinen et al. (unpublished). Approximately 1–5 kg amount o f s t r a w y i e l d e d f r a c t i o n s o f t h e s i z e o f 0.08–1 kg. EMC determination The experiment was conducted with a completely random design. Three randomly picked replicate samples from each treatment were tested for equilibrium humidity. All samples were divided into six parcels, one parcel containing 12 ran- domly chosen samples, and one parcel being in- vestigated at a time. One sample consisted of 1– 1.5 g of room-dry fibre or 2.5–3 g of room-dry shive. The sample was weighed into a weighed metal bowl (height 25 mm, diameter of bottom 45 mm and diameter of top 60 mm) without spe- cific density equalization. An analytical balance was used for weighing. The moisture content of the room-dry material was measured on a sepa- rate sample taken at the same time. Twelve metal sample bowls (holding sam- ples) were put in a plastic container placed in a conditioning room (t = 20 ± 2ºC, RH = 65 ± 2%). In the conditioning ISO 139 (1973) was followed without pre-conditioning. The operating princi- ple of the equipment is shown in Figure 1. The relative humidity of the air in the test containers was 15%, 76% and 97%, respectively. These points were selected to reflect the shape of a typ- ical EMC curve, which is relatively flat at inter- mediate values of relative humidities of air, but has a gradually steeper slope towards high val- ues of RH. The highest RHs of air are interest- ing due to the possible moulding of the biologi- cal matter typical in those conditions. The tem- perature was 20±1ºC. The same samples were put under different conditions to achieve equi- librium humidity in the order 15% (RH of air) → 76% → 97%, which were obtained using the following salts or water, respectively (NT BUILD 130, 1990): LiCl (salt + saturated salt liquid), NaCl (salt + saturated salt liquid), and H 2 O (distilled water). The humidity and temper- ature of the container were measured with a Rotronic Hygromer HP 100A probe. A set of 12 samples was kept under the cho- sen conditions until each of them reached the EMC. Samples were weighed once a day in the conditioning room. For weighing the sample 262 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kymäläinen, H.-R. & Pasila, A. EMC of Linum usitatissimum and Cannabis sativa fractions basket was lifted out of the container and cov- ered with a lid. Interesting differences between harvest times were observed, and therefore the data were sub- mitted to analysis of variance with a single cri- terion of classification (Steel and Torrie 1980) to test the difference in the EMCs and begin- ning of mass loss between green (harvested in autumn) and frost-retted samples (harvested in spring). Each plant was tested separately using harvest time (autumn/spring) as a criterion of classification. A significance level of 0.05 was used. Results At 15% and 76% RH of air, the EMC values of the green and frost-retted fractions were close to each other when one fraction was looked at a time (Fig. 2). EMC results calculated on d.b. are presented in Table 1. The most interesting dif- ferences in the EMC values were found in the dampest air: at 97% RH of air all unretted sam- ples had clearly higher EMCs than had the frost- retted ones. Also in the variance analysis, P-val- ues were low (maximally 0.007) in the dampest air; at 15% RH of air the P-values were as high as 0.949 (Table 2). P-values at 15 % and 76 % RH of air were in almost all cases clearly higher than they were in the dampest air. Due to the small number of measurement points, sorption isotherms were not drawn. How- ever, the present points were found to be con- sistent with the shape of the sorption isotherm c u r v e o f f o r e x a m p l e w o o d ( S k a a r 1 9 8 8 , Nevander and Elmarsson 1994). The three examined plants behaved similar- ly. At some cases one plant seemed to differ from the other two, for example all frost-retted hemp fine shive samples had the highest EMC values at 97 % RH of air (Fig. 2). However, statistical deviations on the whole do not give right to in- terpret real differences between the plants. In the dampest air, nearly all the samples began to lose weight due to moulding. The EMC values in the dampest air (97% RH) are in fact the highest moisture contents that the samples reached before losing weight (Fig. 2). Frost-ret- ted samples were on average, a little slower at starting to mould (Fig. 3), but the variation be- tween the three replicates was considerable, from 0 (green linseed fibre, green hemp fine shive, green coarse shive samples) to 6 days (frost-ret- ted linseed fine shive). Variance analysis showed the biggest differences for flax and linseed at the beginning of moulding: The P-values between unretted and frost-retted samples for flax were 0.015 (fibre), 0.049 (fine shive), and < 0.000 (coarse shive); for linseed 0.038, 0.065 and < 0.000 (respectively), and for hemp 0.065, 0.184 and 0.238. The unretted samples attained higher EMCs before losing weight than did the frost- retted samples (Fig. 2). Fig. 1. Equipment used in the Equilibrium moisture content measurement. 263 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 9 (2000): 259–268. Fig. 2. Water sorption Equilibrium moisture content (EMC) of fibre, fine shive and coarse shive (three replicates). RH = relative humidity. 264 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kymäläinen, H.-R. & Pasila, A. EMC of Linum usitatissimum and Cannabis sativa fractions Table 1. Equilibrium moisture contents of samples, dry basis (equals moisture regains). sample replicate moisture content % (d.b.) At 76% RH of air at 97% RH of air fibre fine shive coarse shive fibre fine shive coarse shive green 1 26.6 16.8 15.0 36.2 46.7 36.2 flax 2 18.3 15.6 15.7 26.0 43.6 26.0 3 15.8 15.8 13.2 35.6 43.3 35.6 green 1 25.2 18.3 15.6 36.7 49.2 36.7 linseed 2 17.9 16.7 15.5 35.9 44.2 35.9 3 19.4 18.0 15.6 36.2 48.2 36.2 green 1 17.5 17.4 16.0 27.4 50.4 27.4 hemp 2 14.7 17.2 15.9 36.7 43.2 36.7 3 15.2 17.4 15.8 35.3 46.4 35.3 spring 1 13.8 13.0 14.0 28.9 29.3 28.9 flax 2 11.6 14.6 14.2 30.2 31.3 30.2 3 13.5 14.6 14.3 30.3 31.3 30.3 spring 1 13.1 14.5 15.2 29.4 29.5 29.4 linseed 2 13.2 14.6 15.5 30.8 30.9 30.8 3 16.7 14.7 15.5 30.7 31.1 30.7 spring 1 19.6 14.6 15.3 32.0 35.5 32.0 hemp 2 20.4 14.6 14.0 29.5 33.2 29.5 3 18.0 15.4 14.4 31.4 34.7 31.4 RH = relative humidity Table 2. Differences between Equilibrium moisture contents (EMC) of green and frost-retted samples. Difference, calculat- ed from means of three replicates = ((EMC green – EMC frost ) / EMC green ) * 100%. Plant Fraction 15% RH of air 76% RH of air 97% RH of air Difference P-value Difference P-value Difference P-value % % % flax fibre 21.6 0.144 21.1 0.015 30.4 0.001 linseed " 17.5 0.360 17.0 0.088 22.5 0.007 hemp " –1.7 0.949 6.6 0.553 21.3 0.011 flax fine shive 9.1 0.445 10.8 0.041 24.0 <0.001 linseed " 29.0 0.007 15.0 0.004 27.3 <0.001 hemp " 20.9 0.014 12.4 0.001 17.8 0.003 flax coarse shive –10.5 0.513 2.6 0.605 13.5 <0.001 linseed " –6.8 0.252 1.0 0.148 12.4 <0.001 hemp " 11.0 0.190 7.3 0.028 12.1 0.002 RH = relative humidity Mass loss during the 14 day research period depended on the speed with which the fractions began to lose weight: the sooner a sample began to lose its mass, the bigger was the mass loss at the end of the test period (Fig. 4). 265 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 9 (2000): 259–268. Fig. 4. Mass loss of samples during 2-week research period (three replicates). Fig. 3. Time (d) during which samples began to lose weight at > 95% relative humidity due to moulding (three repli- cates). Discussion Clear differences were found between all green and frost-retted fractions at the highest RH of air, which was found to be the most interesting condition. When the results of EMC and capil- larity studies (Kymäläinen et al. unpublished) were compared, differences were noted in the behaviour of the samples. In the capillary test, a tube filled with the fraction to be investigated was brought into direct contact with water. Wa- ter was then sucked into the material by capil- lary forces. The frost-retted hemp fibres showed 266 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Kymäläinen, H.-R. & Pasila, A. EMC of Linum usitatissimum and Cannabis sativa fractions good capillarity; frost-retted flax and linseed the poorest. In the dampest air of the EMC test, all frost-retted samples behaved similarly, differing from the unretted samples, and irrespective of the plant species. The direct water intake differs from one plant species to another, but also the moistening or water contact type affects the abil- ity of the fractions to absorb moisture. Moisture behaviour is related to the chemi- cal and morphological structure of plants. Pectins and hemicellulose absorb more moisture from the air than does cellulose (Focher et al. 1992). The amount of pectin decreases in fibre during retting, and thus frost-retted fibres were expect- ed to absorb less water than unretted ones, which was also observed in the dampest air as lower moisture contents before moulding. On the oth- er hand, lignin, which contains few free hydrox- yls and is thus less hydrophilic than cellulose or hemicellulose (Parham and Gray 1984), may have lowered the hygroscopicity of unretted fi- bres due to the higher shive content of those fi- bres (Kymäläinen et al. unpublished). Coarse shive could be expected to behave more like woody material than fine shive due to the other plant components of fine shive. Unret- ted and retted coarse shive samples showed con- formity in the capillarity test (Kymäläinen et al. unpublished), which can be explained by the sim- ilar compositions of unretted and retted shive (Akin et al. 1996). This explanation does not hold for the EMC test at 97% RH, because a differ- ence of about 5 percentage units was noticed in those EMCs of green and frost-retted shive irre- spective of plant species. The manner in which water passes into a fraction in direct water con- tact differs from moisture uptake from air, a fact of importance in the production chain and for the usage of products made of flax, linseed or hemp straw. Almost all dry weight moisture contents of samples at the highest RH of air exceed the fi- bre saturation point of wood, 27% (Table 1), and therefore it is not surprising that they began to decay (Kirk and Cowling 1984). In 76% RH of air, the dry weight moisture contents varied from 11.6% to 26.6% (Table 1), and no degradation occurred. Lignin is known to inhibit microbial degradation of plants (Akin 1989). Comparison of lignin-poor fibre and lignin-rich shive did not show any such inhibition here: The biggest dif- ferences in microbial degradation were found in green fractions, both fibre and shive, compared to frost-retted fractions. The research problem of this study arose from a national interest of getting advantage of the retting and drying process in winter and spring. This study is related to a harvest technology re- search focusing on the frost-retting, and there- fore no dew-retting or enzymatic retting was used. In Finland it is important to find alterna- tives for traditional retting procedures because of the high drying costs. In the future it would be valuable to compare the green and frost-ret- ted material with dew-retted straw, which is widely used in Europe. The present research was introductory in set- ting from the instrumentation point of view. If a greater range of RH conditions were used, a water sorption isotherm might be figured. The number of replicates was restricted by the ex- perimental procedure. The large number of sam- ples in a parcel increased the variation due to changes in the sample masses during weighing under RH conditions different from those in the container. 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EMC of Linum usitatissimum and Cannabis sativa fractions SELOSTUS Pellavan ja kuituhampun korren jakeiden tasapainokosteus Hanna-Riitta Kymäläinen ja Antti Pasila Helsingin yliopisto Tutkimuksessa kartoitettiin jakeistetun kuitupellavan, öljypellavan ja kuituhampun tasapainokosteuskäyt- täytymistä kolmessa eri ilmankosteudessa. Korret ja- keistettiin mekaanisesti kuiduksi sekä hienojakoiseksi ja karkeaksi päistäreeksi. Korjuu tapahtui syksyllä ja keväällä: syksyllä saatiin liottamattomia varsia, ke- väällä korjatut voitiin luokitella ylilionneiksi. Mie- lenkiintoisimmat erot korjuuaikojen välillä havaittiin korkeimmassa ilmankosteudessa kasvilajista riippu- matta: syksyllä korjatut vihreät jakeet alkoivat ho- mehtua nopeammin ja menettivät enemmän massas- taan kahden viikon tutkimusjakson aikana kuin ke- väällä korjatut jakeet. Vihreät jakeet saavuttivat ke- vätkorjattuja korkeamman kosteuspitoisuuden ennen kuin niiden massa alkoi homehtumisen vuoksi laskea. Title Introduction Material and methods Results Discussion References SELOSTUS