Effect of methods of analysis and heat treatment
on viscosity of wheat, barley and oats
B. Svihus
a,*, D.H. Edvardsen
a, M.R. Bedford
b, M. Gullord
c aDepartment of Animal Science, Agricultural University of Norway, PO Box 5025, N-1432 AÊ s, NorwaybFinnfeeds International Ltd., Marlborough, Wiltshire SN8 1XN, UK cNorsk Kornforedling, Bjùrke ForsùksgaÊrd, N-2344 Ilseng, Norway
Received 13 July 1999; received in revised form 13 March 2000; accepted 12 September 2000
Abstract
The purpose of this work was to study variation in viscosity of different grain samples using different extraction methods, and to study effect of heat treatment on viscosity. A total of 80 samples of wheat, oats and barley harvested at two locations were analysed for physical appearance, ®bre content, and in vitro viscosity using water (WEV), acidic buffer (AEV), HCl/NaHCO3 buffer with pepsin and
pancreatin added (IDV) or the AvicheckTM method (FFV). A transformation where the natural
logarithm of the relative viscosity value was divided by the concentration of the sample in liquid was shown to give stable values over a wide range of sample concentrations in liquid for WEV and IDV. Barley and oats had similar viscosities although there was considerable variation between samples, while viscosity was much lower and with less variation between samples for wheat. Heat treatment in an autoclave at 1008C for 5 min increased WEV for all samples, but IDV increased for oats only as a consequence of heat treatment. Although WEV was much lower than IDV, the correlation between these two viscosities was high (0.95,P<0:05) for barley and moderate (0.68 and 0.69,P<0:05) for wheat and oats. A moderate to high signi®cant correlation existed between viscosity and soluble ®bre content for barley.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Fibres; Viscosity; Grain; Wheat; Barley; Oats
1. Introduction
One of the earliest suggestions for that viscosity is inversely related to nutritional value came from the work by Burnett (1966). At present, viscosity is well established as a factor of great importance for the nutritional value of grains like wheat, rye and barley for
88 (2000) 1±12
*Corresponding author. Tel.:47-6494-8012; fax:47-6494-7960.
E-mail address: birger.svihus@ihf.nlh.no (B. Svihus).
poultry and other non-ruminants. Soluble ®bres such as mixed-linkedb-glucans in barley (Almirall et al., 1995) and arabinoxylans in rye (Bedford and Classen, 1992) or wheat (Choct et al., 1996) interfere with the absorption of nutrients, particularly fats, due to the increased viscosity of the intestinal contents. Correspondingly, these ®bres lower blood cholesterol (Bengtsson et al., 1990; Svihus et al., 1997). The reduced fat absorption and blood cholesterol content is probably partly caused by binding or trapping of bile salts in the gut due to high viscosity (Levrat et al., 1996; Favier et al., 1997; Moundras et al., 1997), and/or a reduced emulsi®cation and lipolysis of the fat (Pasquier et al., 1996). In addition, it has been shown that diets with a high viscosity may increase the microbial activity in the digestive tract (Carre et al., 1995; Choct et al., 1996). An increased microbial activity may deconjugate bile salts and alter gut health.
Several authors have found high correlations between viscosity of grains measured in vitro and nutritional value (Campbell et al., 1989; Rotter et al., 1989; Dusel et al., 1997). However, no standardised method exists for in vitro viscosity analysis. One of the earliest proposals was the acid extraction method of Greenberg and Whitmore (1974). Although this method is simple and gives relatively high viscosities due to a low pH that inhibits endogenous enzymes, it has not been extensively used. The method of Bedford and Classen (1993), where the conditions in the intestine are imitated, has been more extensively used. This method is, however, relatively laborious, and some nutritionists have employed water extraction (Dusel et al., 1997) or a weak acid buffer (Carre and Melcion, 1995) to determine viscosity. In addition to the different buffers used in various methods, different sample/liquid ratios are also used. This further complicates any comparison of viscosities between studies, since a logarithmic relationship exists between concentration of sample in liquid and viscosity. Viscosity may be strongly affected by a number of factors including temperature (Vranjes, 1995), pH (Moore and Hoseney, 1990), oxidative agents such as hydrogen peroxide (Moore et al., 1990), different acids (Moore et al., 1990), ions (Smidsrùd and Draget, 1996) and proteins (Doublier et al., 1995). These factors could also introduce differences between extraction methods. In addition, Fry (1998) has shown that ascorbate in combination with Cu can reduce viscosity through scission of b-glucosidic polysaccharide bonds by hydroxyl radicals produced. The effect of temperature on viscosity is particularly interesting, since poultry feed may be exposed to high temperatures in the manufacturing process.
The objectives of the current work were: (1) to study the variation in viscosity of wheat, oats and barley, (2) to compare different viscosity measurement methods, and (3) to study the effect of heat treatment of the grain on viscosity.
2. Materials and methods
Eighty samples (approximately 100 g) of wheat, barley and oats from the 1997 harvest were used. Twenty varieties from each grain species were grown at Apelsvoll, situated 120 km north of Oslo, Norway, while 14 of the barley varieties and six of the oats varieties in addition were grown at Rùd, which is 70 km south of Oslo. The grain was harvested at combine ripeness and was dried using forced heated air. Kernel weight was determined by thoroughly mixing the grain sample and then randomly selecting 50 whole
and undamaged kernels. This was done in duplicate, and counting and weighing was repeated if the difference between duplicate measurements was higher than 10%. Speci®c weight was determined by pouring whole grain into a 100 ml volumetric ¯ask to well above the 100 ml mark, and thereafter compressing the grain in the ¯ask by thrusting the ¯ask towards the table surface with a constant force 20 times. The ¯ask was then re®lled to the mark and the net weight of the grain was determined.
For further analyses, grain samples were ground in a Retsch centrifugal mill (Model ZM 100, F. Kurt Retsch GmbH, Haan, Germany) through a 0.5 mm sieve.
Heat treatment of the grain was performed in a Tomy autoclave (Model SS 325, Tomo Seiko, Tokyo, Japan). Centrifuge tubes containing 1 g of the sample were placed in the autoclave that was preheated to approximately 508C. The autoclave heated up to 1008C, and was kept at this temperature for 5 min, where after the pressure (0.5±0.7 kg/cm3) was released and the sample taken out. With this treatment, the total time in the autoclave was approximately 12 min.
2.1. Viscosity measurements
The water extract viscosity (WEV) measurements were based on the method described by Dusel et al. (1997). Two to ®ve millilitres of distilled water was mixed with 10005 mg grain in a 10 ml centrifuge tube with screw caps, with the highest volume for the most viscous samples. The tube was incubated in a water bath at 403
C for a constant time period with occasional stirring, followed by centrifugation for 10 min. Viscosity of the supernatant was measured on a Brook®eld digital viscometer (Model DV-II, Brook®eld Engineering Laboratories, Stoughton, MA 02172, USA) ®tted with a C-40 cone and plate. The temperature of the supernatant at the time of measurement was the same as room temperature (20±258C). The shear rate was 60 sÿ1
, or, for samples with a high viscosity, the maximum shear rate possible (lowest shear rate used was 1.5 sÿ1
). To avoid a time effect on viscosity, only six samples in duplicate were incubated and measured at a time, and the viscosity was measured in such a way that the average time from incubation to measurement for the duplicates was the same for all samples. To determine the optimal incubation time, two samples of wheat, barley and oats were incubated for 5, 10, 15, 20, 30, 45 and 60 min, with stirring two times or every 10 min during the incubation. WEV increased by increasing the incubation time to 15 min, followed by a sharp decrease in viscosity which stabilised after 20 min. Thus, a 30 min incubation time was selected for this analysis. Centrifugation varying from 1460g to 5841gdid not affect viscosity. Thus, a centrifugation force of 2596g was chosen.
The method for acid extract viscosity (AEV) measurement was based on the method described by Greenberg and Whitmore (1974). An acidic (pH 1.5) buffer was made by mixing 82.8 ml 1 M HCl and 7.4596 g KCl with 1 l of distilled water. The buffer was mixed with 10005 mg of sample in a centrifuge tube, and the tube was incubated at 408C for 1 h with occasional stirring. The tube was centrifuged at 2596gfor 10 min, and viscosity of the supernatant was measured as described for WEV. For the wheat samples, 3 ml of buffer was added, while this amount gave too high viscosities in many of the oats and barley samples. Therefore, 7 ml of the buffer was added to 1 g of sample for these grains.
Another viscosity measurement performed was the in vitro digestion viscosity (IDV) method described by Bedford and Classen (1993), where the sample is incubated in a HCl buffer with pepsin added, followed by incubation in a NaHCO3 solution containing
pancreatin. After incubation, the sample was centrifuged (10 min, 2596 G) and analysed for viscosity as described for WEV. Initial experimentation revealed that the amount of liquid added would need to be higher than that recommended by Bedford and Classen (1993). Therefore, the total volume of the two solutions was increased to 2.4 ml (1.8 ml HCl solution and 0.6 ml NaHCO3 solution). For some samples, a further increase in
volume (3.2, 4.0 and 4.8 ml) was needed in order to avoid too great a viscosity. Samples were also sent to the laboratory of Finnfeeds International to be analysed using the AvicheckTMprocedure developed by that company for determining viscosity (FFV). The principle for this procedure is the same as for IDV, although precise details are con®dential. Since it was necessary to vary the volume of the liquid according to the viscosity of the sample and according to the method used, a volume correction was needed in order to be able to compare the viscosity values between samples and methods. The transformation proposed by Carre and Melcion (1995) was selected for this purpose. The measured viscosity value was transformed to a relative viscosity value (hr) by dividing the measured
viscosity with the viscosity of the liquid used for extraction, and then the natural logarithm of
hr was divided by the concentration of sample in liquid (C) expressed as g/ml. The
transformation ((lnhr)/C) was used for all viscosity values in this study. The measured
viscosity of the different extraction liquids used in this study were all1 at a shear rate of 300 sÿ1. To validate the transformation method for the WEV, AEV and IDV methods for viscosity measurement, two samples of wheat, oats and barley varying in viscosity values were selected, and the six samples were incubated with four different volumes of liquid.
2.2. Dietary ®bre measurement
Ten samples of each grain species were selected for analysis of ®bre content. Soluble and insoluble dietary ®bres were measured using the enzymatic±gravimetric method of Lee et al. (1992). In addition, the amount of soluble complex matter after in vitro digestion was measured by adding 10 ml of 96% ethanol with a temperature of 608C to 2.5 ml of the supernatant. After 1 h the tubes were centrifuged (10 min, 2596 G), the supernatant was carefully poured off, and the precipitate was dried at 1058C overnight.
2.3. Statistical analysis
A simple Pearson correlation analysis was performed using the CORR-procedure of the Statistical Analysis System (SAS, 1987).
3. Results
3.1. Validation of method used for transforming viscosity data
Table 1 shows WEV, IDV and AEV, respectively, after incubation in different sample/ liquid concentrations. To illustrate the effect of sample:liquid ratios, viscosity values as
they appear before transformation have been included for IDV. The viscosity value was stable over different sample/liquid concentrations for WEV and IDV, while the values varied over different sample/liquid concentrations for AEV when viscosity of the samples was high.
3.2. Viscosity values for the grain samples
The wheat samples had considerably lower viscosities than the barley and oats samples, which had similar average viscosities. Viscosity varied between wheat varieties grown at Apelsvoll, and the most viscous variety generally had a WEV and AEV which was double the viscosity of the least viscous variety (Table 2). Viscosity values for the oats varieties also varied considerably, being 4±5 times higher in the varieties with the highest WEV and IDV than in the samples with lowest viscosity (Table 3). In the barley samples, the most extreme variety had a WEV, AEV and IDV that was 2±3 times higher than the variety with lowest viscosity (Table 4).
Table 1
Effect of sample:liquid ratio on transformed viscositya
Wheat 1b Wheat 2b Oats 1b Oats 2b Barley 1b Barley 2b
IDVc, 1:2d 3.9 (7.0) 5.5 (15.6) ±e ±e ±e ±e
IDVc, 1:4d 4.3 (2.9) 6.6 (5.2) ±e 12.1 (20.6) ±e ±e
IDVc, 1:5.3d 5.3 (2.7) 7.6 (4.2) 27.8 (183.6) 12.6 (10.6) 23.6 (83.5) 29.6 (257.2)
IDVc, 1:6.7d 5.4 (2.2) 6.3 (2.6) 28.7 (74.1) 11.7 (5.8) 23.5 (33.9) 29.1 (78.6)
IDVc, 1:8d ±e ±e 28.8 (36.6) 13.0 (5.1) 23.6 (19.1) 30.1 (43.1)
IDVc, 1:10d ±e ±e 30.3 (20.7) ±e 25.8 (13.2) 31.5 (23.3)
WEVf, 1:1d 2.4 ±e ±e ±e ±e ±e
WEVf, 1:2d 1.7 3.6 4.7 5.7 11.1 9.6
WEVf, 1:3d 2.0 3.4 5.2 6.3 12.0 10.3
WEVf, 1:4d 1.9 3.0 5.0 6.0 11.8 10.1
WEVf, 1:5d ±e 3.1 5.3 5.8 11.9 10.3
AEVg, 1:2d 2.8 5.0 ±e ±e ±e ±e
AEVg, 1:3d 2.8 5.2 ±e ±e ±e ±e
AEVg, 1:4d 3.2 5.0 ±e ±e ±e ±e
AEVg, 1:5d 3.1 5.1 ±e ±e 16.2 30.5
AEVg, 1:6d ±e ±e 35.7 35.8 17.7 34.1
AEVg, 1:7d ±e ±e 39.8 30.1 16.7 32.9
AEVg, 1:8d ±e ±e 37.2 33.2 17.9 36.4
AEVg, 1:9d ±e ±e 28.1 32.1 ±e ±e
aFigures in parentheses are viscosity values as they appear before transformation.
bGrain sample used varied with the different extraction methods.
cIn vitro digestion viscosity.
dSample (g):liquid (ml) ratio.
eThe viscosity using these ratios was not measured.
fWater extract viscosity. gAcid extract viscosity.
3.3. Effect of heat treatment on viscosity
WEV increased after heat treatment. The increase was consistent but small for wheat (Table 2), but of considerable magnitude for oats and barley (Tables 3 and 4), where the viscosity was more than tripled after heat treatment for some oats samples and more than
Table 2
Characteristics of different wheat varieties collected at Apelsvoll, Norway
n Mean Minimum Maximum Standard
deviation
1000 grain weight (g) 20 40.40 32.38 47.48 4.900
Hectolitre weight (kg) 20 84.79 79.41 86.91 1.758
Insoluble dietary ®bre (g/kg DM) 10 106 85 114 9.0
Soluble dietary ®bre (g/kg DM) 10 16 11 21 4.0
Soluble complex (g/kg DM) 10 51 43 58 4.0
Soluble complex after heating (g/kg DM) 10 47 44 53 3.0
WEVa 20 2.7 1.6 3.7 0.61
WEVaafter heating 20 3.5 2.2 4.9 0.85
AEVb 20 4.1 2.8 5.6 0.85
IDVc 20 5.6 4.3 7.2 0.96
IDVcafter heating 10 6.1 4.8 7.7 0.90
FFVd 20 4.3 3.4 5.5 0.73
aWater extract viscosity.
bAcid extract viscosity.
cIn vitro digestion viscosity.
dAvicheckTMviscosity.
Table 3
Characteristics of different oats varieties collected at Apelsvoll, Norway
n Mean Minimum Maximum Standard
deviation
1000 grain weight (g) 20 32.54 24.46 39.56 3.916
Hectolitre weight (kg) 20 57.43 53.56 61.97 2.058
Insoluble dietary ®bre (g/kg DM) 10 262 108 321 61.0
Soluble dietary ®bre (g/kg DM) 10 28 22 37 5.0
Soluble complex (g/kg DM) 10 79 64 96 10.0
Soluble complex after heating (g/kg DM) 10 82 65 118 15.0
WEVa 20 6.9 4.0 9.8 1.55
WEVaafter heating 20 18.6 6.6 33.4 7.52
AEVb 20 37.4 26.8 47.7 6.07
IDVc 20 28.7 12.1 51.1 12.05
IDVcafter heating 10 35.9 17.9 47.7 9.00
FFVd 14e 8.9 6.0 11.9 1.95
aWater extract viscosity.
bAcid extract viscosity.
cIn vitro digestion viscosity.
dAvicheckTMviscosity.
eValues for six varieties too high to be detected with this method.
doubled for some barley samples. For the 10 samples from each species where IDV was measured after heat treatment, a consistent increase in IDV after heat treatment was only seen for oats.
3.4. Fibre content and correlations
The dietary ®bre values were within a normal range for wheat and barley (Tables 2 and 4). One of the oats samples was a naked variety, and thus had a very low total dietary ®bre content (Table 3). The amount of soluble complex matter precipitated in 80% ethanol after in vitro digestion was higher than the soluble dietary ®bre content, and the amount did not increase after heat treatment.
The correlations between WEV, IDV and FFV for wheat, oats and barley were signi®cant and varied between 0.68 and 0.95 (Table 5). The correlations between AEV and the other viscosity measurements were lower and not always signi®cant. While the correlations between viscosity values and 1000 grain weight were low, a signi®cant positive correlation existed between hectolitre weight and WEV, IDV and FFV for barley and oats.
For wheat, no signi®cant correlation existed between viscosity and ®bre content, except for a positive correlation between IDV and soluble complex after heat treatment (Table 6). For oats, there was a signi®cant positive correlation between WEV and soluble complex, and a signi®cant negative correlation between WEV and insoluble dietary ®bre. For barley, a signi®cant positive correlation existed between all viscosity measurements and the soluble ®bre content, but with a higher correlation between viscosity and soluble complex than between viscosity and soluble dietary ®bre.
Table 4
Characteristics of different barley varieties collected at Apelsvoll, Norway
n Mean Minimum Maximum Standard
deviation
1000 grain weight (g) 20 41.59 33.93 49.49 3.541
Hectolitre weight (kg) 20 72.61 69.94 76.03 1.937
Insoluble dietary ®bre (g/kg DM) 10 156 134 176 12.0
Soluble dietary ®bre (g/kg DM) 10 38 32 51 10.0
Soluble complex (g/kg DM) 10 86 60 105 38.0
Soluble complex after heating (g/kg DM) 10 69 58 81 9.0
WEVa 20 8.7 4.7 15.0 2.74
WEVaafter heating 20 17.7 9.0 31.5 6.58
AEVb 20 25.5 16.2 41.0 6.02
IDVc 20 27.8 16.8 45.7 6.51
IDVcafter heating 10 28.9 22.7 43.2 6.70
FFVd 10e 11.3 8.6 13.1 1.49
aWater extract viscosity.
bAcid extract viscosity.
cIn vitro digestion viscosity.
dAvicheckTMviscosity.
eValues for 10 varieties too high to be detected with this method.
Table 5
Correlation between different measures in wheat (n20), oats (n26) and barley (n34)
1000 grain
Barley NSe 0.71 0.93 0.69 0.95
WEVaafter heat treatment Wheat NSe NSe 0.93 0.86
Oats NSe 0.79 NSe 0.72
Barley NSe 0.78 0.57 0.86
AEVb Wheat NSe NSe 0.86
Barley NSc 0.66 0.94 0.80
WEVaafter heat treatment Wheat NSc NSc NSc NSc
Oats ÿ0.75 NSc 0.69 0.89
Barley NSc 0.64 0.89 0.69
IDVb Wheat NSc NSc NSc NSc
Oats NSc NSc NSc NSc
Barley NSc 0.71 0.92 0.78
IDVbafter heat treatment Wheat NSc NSc NSc 0.76
Oats NSc NSc NSc NSc
Barley NSc 0.66 0.97 0.87
aWater extract viscosity.
bIn vitro digestion viscosity.
cNot signi®cant (P>0:05).
4. Discussion
Since the correlations between the different viscosity measurement methods varied a lot depending on grain species, differences in correlations to nutritive value between the different measures may occur. Rotter et al. (1989) found a correlation between WEV (120 min incubation time) in barley and weight grain in Leghorn chicks of ÿ0.74 to ÿ0.78. Similarly, Dusel et al. (1997) found a correlation ofÿ0.83 between WEV in wheat and AME determined using broiler chicks.
Incubation at different sample weight/liquid volume ratios showed that the transformation method proposed by Carre et al. (1994) gave very similar values independent of sample weight/liquid volume ratios for the WEV and IDV methods. This method could be of potentially great signi®cant for comparison of viscosity values obtained using different sample weight/volume ratios and different extraction liquids, as is commonly seen in literature. In addition, the range in viscosity value is reduced by this correction, which makes it easier to compare low viscosity and high viscosity values. A logarithmic transformed viscosity value is also often better correlated to production results than the value itself, at least for high viscosity grains where the range in viscosity can be extremely large and not normally distributed (Bedford and Classen, 1992). This transformation method could also be used to determine in vivo intestinal viscosity in cases where a dilution of the chyme is necessary in order to obtain enough supernatant after centrifugation, and it could be used to compare intestinal viscosities between birds with different water contents in the chyme.
Most polysaccharide solutions exhibit non-Newtonian behaviour, i.e. viscosity falls with increasing shear rate. This is particularly true for solutions with viscosity above 10 cP, where individual polysaccharide chains in the solution become entangled (Morris, 1992). Under such circumstances, viscosity measurements at a standardised shear rate would have been an advantage. Since the Brook®eld viscometer has a limited ability to measure high viscosity solutions at a high shear rate, either all the samples should have been measured at a low shear rate, or the high viscosity samples should have been further diluted.
AEV was not consistent over different sample weight/volume ratios, at least not for samples with a high viscosity. An unstable viscosity value that initially increased during viscosity reading, followed by a steady decrease, was also observed during AEV analysis. In addition, the viscosity of different layers of the supernatant often varied considerably with this method. The correlation between AEV and other viscosity measurements were also variable for barley and oats. The inconsistent results could indicate that AEV may not be a recommendable method for viscosity measurements, at least not in solutions with a high viscosity. According to Carre and Melcion (1995), AEV is not recommended for use in high-protein samples due to a possible in¯uence of proteins on viscosity at a low pH. The results show that a considerable variation in viscosity exists among samples of oats and barley. Variability in viscosity of different barley varieties has also been described before (Villamide et al., 1997; Rotter et al., 1989), but literature on viscosity of oats has not been found. The variation in viscosity of wheat corresponds to variations obtained earlier (Dusel et al., 1997).
The fact that heat treatment increased WEV to a much higher extent than IDV, could indicate that elimination of endogenous ®bre-degrading enzymes due to the high
temperature is not the cause of increased WEV after heat treatment, since a similar increase in IDV would have been expected in that case. However, this hypothesis is only valid under the condition that the proteases added do not degrade endogenous ®bre-degrading enzymes. In such a case, the lack of response to heat treatment for IDV may be explained by the fact that enzymes are already destroyed, and that there is therefore no additional effect of heat treatment. Moore and Hoseney (1990) concluded that endogenous enzymes do not affect viscosity of wheat, while Knuckles and Chiu (1999) concluded that endogenousb-glucanases in barley reduced amount and molecular size ofb-glucans extracted in water for 1 h at 238C. A change in the solubility of ®bres and/or an increase in protein±®bre or ®bre±®bre interaction could be an alternative cause for increased WEV after heat treatment. The solubility and the tertiary and secondary structure of the ®bres and the proteins may change under heat treatment, and new non-covalent bonds between ®bres or between ®bres and proteins may occur. Interactions between different ®bres (Morris, 1995) or between ®bres and proteins (Doublier et al., 1995) are known to potentially increase viscosity dramatically. The much higher IDV than WEV for the same sample could similarly be caused by solubilisation of proteins and thus of ®bres attached to proteins by the proteases added during in vitro incubation. Thus, there may not be a potential for further changes in solubility and/or structure of ®bres and proteins due to heat treatment. The amount of soluble complex after in vitro incubation also remained the same after heat treatment.
Although the soluble complex fraction after in vitro digestion is less precisely de®ned since it is not corrected for protein and ash as is the case for the soluble dietary ®bre fraction, the higher correlation between viscosity and soluble complex than between viscosity and soluble dietary ®bre may indicate that the soluble complex fraction is more valuable when components of the grain are to be correlated to viscosity. However, the correlations between ®bres and viscosity were not high for wheat and oats. There are many reasons why viscosity may not be well correlated to soluble ®bre content. The soluble dietary ®bre fraction is extracted using different buffers than those used for viscosity determination. In addition, the temperature of the buffer (608C) used for extraction of soluble dietary ®bres is much higher than the temperature (408C) used under extraction of ®bres for viscosity measurements. The extraction temperature may affect solubility of the ®bres. In addition, the molecular size and the chemical composition of the ®bres may be a more important factor determining viscosity than quantity.
From the current study it can be concluded that viscosity of different samples of wheat, barley and oats vary considerably, and that the viscosity values are affected by extraction method and heat treatment. A transformation as proposed by Carre and Melcion (1995) may be recommended to reduce range in readings and for comparison of results obtained using different buffers and different sample/buffer ratios.
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