Directory UMM :Data Elmu:jurnal:A:Animal Feed Science and Technology:Vol88.Issue1-2.Nov2000:
Animal Feed Science and Technology
88 (2000) 1±12
Effect of methods of analysis and heat treatment
on viscosity of wheat, barley and oats
B. Svihusa,*, D.H. Edvardsena, M.R. Bedfordb, M. Gullordc
Department of Animal Science, Agricultural University of Norway, PO Box 5025, N-1432 AÊs, Norway
b
Finnfeeds International Ltd., Marlborough, Wiltshire SN8 1XN, UK
c
Norsk Kornforedling, Bjùrke ForsùksgaÊrd, N-2344 Ilseng, Norway
a
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
*
Corresponding author. Tel.: 47-6494-8012; fax: 47-6494-7960.
E-mail address: birger.svihus@ihf.nlh.no (B. Svihus).
0377-8401/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 0 0 ) 0 0 2 1 3 - 3
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B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
poultry and other non-ruminants. Soluble ®bres such as mixed-linked b-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
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
3
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
1000 5 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 40 3 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
5841g did 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 1000 5 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 2596g for 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.
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B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
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
AvicheckTM procedure 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 ((ln hr)/C) was used for all viscosity values in this study. The measured
viscosity of the different extraction liquids used in this study were all 1 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
5
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 1
Effect of sample:liquid ratio on transformed viscositya
IDVc,
IDVc,
IDVc,
IDVc,
IDVc,
IDVc,
1:2d
1:4d
1:5.3d
1:6.7d
1:8d
1:10d
Wheat 1b
Wheat 2b
Oats 1b
Oats 2b
Barley 1b
Barley 2b
3.9
4.3
5.3
5.4
±e
±e
5.5
6.6
7.6
6.3
±e
±e
±e
±e
27.8
28.7
28.8
30.3
±e
12.1
12.6
11.7
13.0
±e
±e
±e
23.6
23.5
23.6
25.8
±e
±e
29.6
29.1
30.1
31.5
(7.0)
(2.9)
(2.7)
(2.2)
(15.6)
(5.2)
(4.2)
(2.6)
(183.6)
(74.1)
(36.6)
(20.7)
(20.6)
(10.6)
(5.8)
(5.1)
(83.5)
(33.9)
(19.1)
(13.2)
WEVf,
WEVf,
WEVf,
WEVf,
WEVf,
1:1d
1:2d
1:3d
1:4d
1:5d
2.4
1.7
2.0
1.9
±e
±e
3.6
3.4
3.0
3.1
±e
4.7
5.2
5.0
5.3
±e
5.7
6.3
6.0
5.8
±e
11.1
12.0
11.8
11.9
±e
9.6
10.3
10.1
10.3
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
1:2d
1:3d
1:4d
1:5d
1:6d
1:7d
1:8d
1:9d
2.8
2.8
3.2
3.1
±e
±e
±e
±e
5.0
5.2
5.0
5.1
±e
±e
±e
±e
±e
±e
±e
±e
35.7
39.8
37.2
28.1
±e
±e
±e
±e
35.8
30.1
33.2
32.1
±e
±e
±e
16.2
17.7
16.7
17.9
±e
±e
±e
±e
30.5
34.1
32.9
36.4
±e
(257.2)
(78.6)
(43.1)
(23.3)
a
Figures in parentheses are viscosity values as they appear before transformation.
Grain sample used varied with the different extraction methods.
c
In vitro digestion viscosity.
d
Sample (g):liquid (ml) ratio.
e
The viscosity using these ratios was not measured.
f
Water extract viscosity.
g
Acid extract viscosity.
b
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).
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B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 2
Characteristics of different wheat varieties collected at Apelsvoll, Norway
1000 grain weight (g)
Hectolitre weight (kg)
Insoluble dietary ®bre (g/kg DM)
Soluble dietary ®bre (g/kg DM)
Soluble complex (g/kg DM)
Soluble complex after heating (g/kg DM)
WEVa
WEVa after heating
AEVb
IDVc
IDVc after heating
FFVd
n
Mean
Minimum
Maximum
Standard
deviation
20
20
10
10
10
10
20
20
20
20
10
20
40.40
84.79
106
16
51
47
2.7
3.5
4.1
5.6
6.1
4.3
32.38
79.41
85
11
43
44
1.6
2.2
2.8
4.3
4.8
3.4
47.48
86.91
114
21
58
53
3.7
4.9
5.6
7.2
7.7
5.5
4.900
1.758
9.0
4.0
4.0
3.0
0.61
0.85
0.85
0.96
0.90
0.73
a
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
b
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 3
Characteristics of different oats varieties collected at Apelsvoll, Norway
1000 grain weight (g)
Hectolitre weight (kg)
Insoluble dietary ®bre (g/kg DM)
Soluble dietary ®bre (g/kg DM)
Soluble complex (g/kg DM)
Soluble complex after heating (g/kg DM)
WEVa
WEVa after heating
AEVb
IDVc
IDVc after heating
FFVd
a
n
Mean
Minimum
Maximum
Standard
deviation
20
20
10
10
10
10
20
20
20
20
10
14e
32.54
57.43
262
28
79
82
6.9
18.6
37.4
28.7
35.9
8.9
24.46
53.56
108
22
64
65
4.0
6.6
26.8
12.1
17.9
6.0
39.56
61.97
321
37
96
118
9.8
33.4
47.7
51.1
47.7
11.9
3.916
2.058
61.0
5.0
10.0
15.0
1.55
7.52
6.07
12.05
9.00
1.95
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
e
Values for six varieties too high to be detected with this method.
b
7
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 4
Characteristics of different barley varieties collected at Apelsvoll, Norway
1000 grain weight (g)
Hectolitre weight (kg)
Insoluble dietary ®bre (g/kg DM)
Soluble dietary ®bre (g/kg DM)
Soluble complex (g/kg DM)
Soluble complex after heating (g/kg DM)
WEVa
WEVa after heating
AEVb
IDVc
IDVc after heating
FFVd
n
Mean
Minimum
Maximum
Standard
deviation
20
20
10
10
10
10
20
20
20
20
10
10e
41.59
72.61
156
38
86
69
8.7
17.7
25.5
27.8
28.9
11.3
33.93
69.94
134
32
60
58
4.7
9.0
16.2
16.8
22.7
8.6
49.49
76.03
176
51
105
81
15.0
31.5
41.0
45.7
43.2
13.1
3.541
1.937
12.0
10.0
38.0
9.0
2.74
6.58
6.02
6.51
6.70
1.49
a
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
e
Values for 10 varieties too high to be detected with this method.
b
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.
8
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 5
Correlation between different measures in wheat (n 20), oats (n 26) and barley (n 34)
1000 grain
weight
Hectolitre WEVa
weight
WEVa after
heat treatment
AEVb
IDVc
0.83
0.85
0.95
0.92
NSe
NSe
0.78
0.76
0.77
0.81
0.92
0.93
0.85
NSe
0.69
0.68
0.69
0.95
0.93
NSe
0.57
0.86
0.72
0.86
FFVd
Wheat
Oats
Barley
NSe
NSe
NSe
NSe
0.72
0.79
WEVa
Wheat
Oats
Barley
ÿ0.59
NSe
NSe
NSe
0.75
0.71
WEVa after heat treatment
Wheat
Oats
Barley
NSe
NSe
NSe
NSe
0.79
0.78
AEVb
Wheat
Oats
Barley
NSe
NSe
NSe
NSe
NSe
NSe
IDVc
Wheat
Oats
Barley
NSe
0.40
NSe
NSe
0.50
0.62
0.81
0.89
0.92
0.86
NSe
0.80
a
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
e
Not signi®cant (P > 0:05).
b
Table 6
Correlation between ®bres and viscosity in wheat (n 10), oats (n 10) and barley (n 10)
Insoluble
dietary fibre
Soluble
dietary fibre
Soluble
complex
Soluble complex
after heat treatment
WEVa
Wheat
Oats
Barley
NSc
±0.65
NSc
NSc
NSc
0.66
NSc
0.75
0.94
NSc
0.82
0.80
WEVa after heat treatment
Wheat
Oats
Barley
NSc
ÿ0.75
NSc
NSc
NSc
0.64
NSc
0.69
0.89
NSc
0.89
0.69
IDVb
Wheat
Oats
Barley
NSc
NSc
NSc
NSc
NSc
0.71
NSc
NSc
0.92
NSc
NSc
0.78
IDVb after heat treatment
Wheat
Oats
Barley
NSc
NSc
NSc
NSc
NSc
0.66
NSc
NSc
0.97
0.76
NSc
0.87
a
Water extract viscosity.
In vitro digestion viscosity.
c
Not signi®cant (P > 0:05).
b
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
9
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 of ÿ0.83 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
10
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
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 ®bredegrading 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 endogenous b-glucanases in barley reduced amount and molecular
size of b-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 noncovalent 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.
References
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Favier, M.-L., Bost, P.-E., Guittard, C., DemigneÂ, C., ReÂmeÂsy, C., 1997. The cholesterol-lowering effect of guar
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various treatments. Cereal Chem. 76, 92±95.
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Ð enzymatic±gravimetric method, MES±Tris buffer: collaborative study. J. AOAC Int. 75, 395±416.
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starch, compared to guar gum, in depressing plasma cholesterol and enhancing fecal steroid excretion. Lipids
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(Eds.), Dietary Fibre Ð A Component of Food. Springer, London, pp. 41±56.
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Smidsrùd, O., Draget, K.I., 1996. Chemistry and physical properties of alginates. Carbohydr. Eur. 14, 6±13.
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poultry: effect of dietary enzyme addition. Poult. Sci. 76, 834±840.
Vranjes, M.V., 1995. Application of Trichoderma viride enzyme complex in diets for broiler chickens and laying
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88 (2000) 1±12
Effect of methods of analysis and heat treatment
on viscosity of wheat, barley and oats
B. Svihusa,*, D.H. Edvardsena, M.R. Bedfordb, M. Gullordc
Department of Animal Science, Agricultural University of Norway, PO Box 5025, N-1432 AÊs, Norway
b
Finnfeeds International Ltd., Marlborough, Wiltshire SN8 1XN, UK
c
Norsk Kornforedling, Bjùrke ForsùksgaÊrd, N-2344 Ilseng, Norway
a
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
*
Corresponding author. Tel.: 47-6494-8012; fax: 47-6494-7960.
E-mail address: birger.svihus@ihf.nlh.no (B. Svihus).
0377-8401/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 7 - 8 4 0 1 ( 0 0 ) 0 0 2 1 3 - 3
2
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
poultry and other non-ruminants. Soluble ®bres such as mixed-linked b-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
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
3
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
1000 5 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 40 3 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
5841g did 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 1000 5 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 2596g for 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.
4
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
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
AvicheckTM procedure 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 ((ln hr)/C) was used for all viscosity values in this study. The measured
viscosity of the different extraction liquids used in this study were all 1 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
5
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 1
Effect of sample:liquid ratio on transformed viscositya
IDVc,
IDVc,
IDVc,
IDVc,
IDVc,
IDVc,
1:2d
1:4d
1:5.3d
1:6.7d
1:8d
1:10d
Wheat 1b
Wheat 2b
Oats 1b
Oats 2b
Barley 1b
Barley 2b
3.9
4.3
5.3
5.4
±e
±e
5.5
6.6
7.6
6.3
±e
±e
±e
±e
27.8
28.7
28.8
30.3
±e
12.1
12.6
11.7
13.0
±e
±e
±e
23.6
23.5
23.6
25.8
±e
±e
29.6
29.1
30.1
31.5
(7.0)
(2.9)
(2.7)
(2.2)
(15.6)
(5.2)
(4.2)
(2.6)
(183.6)
(74.1)
(36.6)
(20.7)
(20.6)
(10.6)
(5.8)
(5.1)
(83.5)
(33.9)
(19.1)
(13.2)
WEVf,
WEVf,
WEVf,
WEVf,
WEVf,
1:1d
1:2d
1:3d
1:4d
1:5d
2.4
1.7
2.0
1.9
±e
±e
3.6
3.4
3.0
3.1
±e
4.7
5.2
5.0
5.3
±e
5.7
6.3
6.0
5.8
±e
11.1
12.0
11.8
11.9
±e
9.6
10.3
10.1
10.3
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
AEVg,
1:2d
1:3d
1:4d
1:5d
1:6d
1:7d
1:8d
1:9d
2.8
2.8
3.2
3.1
±e
±e
±e
±e
5.0
5.2
5.0
5.1
±e
±e
±e
±e
±e
±e
±e
±e
35.7
39.8
37.2
28.1
±e
±e
±e
±e
35.8
30.1
33.2
32.1
±e
±e
±e
16.2
17.7
16.7
17.9
±e
±e
±e
±e
30.5
34.1
32.9
36.4
±e
(257.2)
(78.6)
(43.1)
(23.3)
a
Figures in parentheses are viscosity values as they appear before transformation.
Grain sample used varied with the different extraction methods.
c
In vitro digestion viscosity.
d
Sample (g):liquid (ml) ratio.
e
The viscosity using these ratios was not measured.
f
Water extract viscosity.
g
Acid extract viscosity.
b
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).
6
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 2
Characteristics of different wheat varieties collected at Apelsvoll, Norway
1000 grain weight (g)
Hectolitre weight (kg)
Insoluble dietary ®bre (g/kg DM)
Soluble dietary ®bre (g/kg DM)
Soluble complex (g/kg DM)
Soluble complex after heating (g/kg DM)
WEVa
WEVa after heating
AEVb
IDVc
IDVc after heating
FFVd
n
Mean
Minimum
Maximum
Standard
deviation
20
20
10
10
10
10
20
20
20
20
10
20
40.40
84.79
106
16
51
47
2.7
3.5
4.1
5.6
6.1
4.3
32.38
79.41
85
11
43
44
1.6
2.2
2.8
4.3
4.8
3.4
47.48
86.91
114
21
58
53
3.7
4.9
5.6
7.2
7.7
5.5
4.900
1.758
9.0
4.0
4.0
3.0
0.61
0.85
0.85
0.96
0.90
0.73
a
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
b
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 3
Characteristics of different oats varieties collected at Apelsvoll, Norway
1000 grain weight (g)
Hectolitre weight (kg)
Insoluble dietary ®bre (g/kg DM)
Soluble dietary ®bre (g/kg DM)
Soluble complex (g/kg DM)
Soluble complex after heating (g/kg DM)
WEVa
WEVa after heating
AEVb
IDVc
IDVc after heating
FFVd
a
n
Mean
Minimum
Maximum
Standard
deviation
20
20
10
10
10
10
20
20
20
20
10
14e
32.54
57.43
262
28
79
82
6.9
18.6
37.4
28.7
35.9
8.9
24.46
53.56
108
22
64
65
4.0
6.6
26.8
12.1
17.9
6.0
39.56
61.97
321
37
96
118
9.8
33.4
47.7
51.1
47.7
11.9
3.916
2.058
61.0
5.0
10.0
15.0
1.55
7.52
6.07
12.05
9.00
1.95
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
e
Values for six varieties too high to be detected with this method.
b
7
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 4
Characteristics of different barley varieties collected at Apelsvoll, Norway
1000 grain weight (g)
Hectolitre weight (kg)
Insoluble dietary ®bre (g/kg DM)
Soluble dietary ®bre (g/kg DM)
Soluble complex (g/kg DM)
Soluble complex after heating (g/kg DM)
WEVa
WEVa after heating
AEVb
IDVc
IDVc after heating
FFVd
n
Mean
Minimum
Maximum
Standard
deviation
20
20
10
10
10
10
20
20
20
20
10
10e
41.59
72.61
156
38
86
69
8.7
17.7
25.5
27.8
28.9
11.3
33.93
69.94
134
32
60
58
4.7
9.0
16.2
16.8
22.7
8.6
49.49
76.03
176
51
105
81
15.0
31.5
41.0
45.7
43.2
13.1
3.541
1.937
12.0
10.0
38.0
9.0
2.74
6.58
6.02
6.51
6.70
1.49
a
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
e
Values for 10 varieties too high to be detected with this method.
b
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.
8
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
Table 5
Correlation between different measures in wheat (n 20), oats (n 26) and barley (n 34)
1000 grain
weight
Hectolitre WEVa
weight
WEVa after
heat treatment
AEVb
IDVc
0.83
0.85
0.95
0.92
NSe
NSe
0.78
0.76
0.77
0.81
0.92
0.93
0.85
NSe
0.69
0.68
0.69
0.95
0.93
NSe
0.57
0.86
0.72
0.86
FFVd
Wheat
Oats
Barley
NSe
NSe
NSe
NSe
0.72
0.79
WEVa
Wheat
Oats
Barley
ÿ0.59
NSe
NSe
NSe
0.75
0.71
WEVa after heat treatment
Wheat
Oats
Barley
NSe
NSe
NSe
NSe
0.79
0.78
AEVb
Wheat
Oats
Barley
NSe
NSe
NSe
NSe
NSe
NSe
IDVc
Wheat
Oats
Barley
NSe
0.40
NSe
NSe
0.50
0.62
0.81
0.89
0.92
0.86
NSe
0.80
a
Water extract viscosity.
Acid extract viscosity.
c
In vitro digestion viscosity.
d
AvicheckTM viscosity.
e
Not signi®cant (P > 0:05).
b
Table 6
Correlation between ®bres and viscosity in wheat (n 10), oats (n 10) and barley (n 10)
Insoluble
dietary fibre
Soluble
dietary fibre
Soluble
complex
Soluble complex
after heat treatment
WEVa
Wheat
Oats
Barley
NSc
±0.65
NSc
NSc
NSc
0.66
NSc
0.75
0.94
NSc
0.82
0.80
WEVa after heat treatment
Wheat
Oats
Barley
NSc
ÿ0.75
NSc
NSc
NSc
0.64
NSc
0.69
0.89
NSc
0.89
0.69
IDVb
Wheat
Oats
Barley
NSc
NSc
NSc
NSc
NSc
0.71
NSc
NSc
0.92
NSc
NSc
0.78
IDVb after heat treatment
Wheat
Oats
Barley
NSc
NSc
NSc
NSc
NSc
0.66
NSc
NSc
0.97
0.76
NSc
0.87
a
Water extract viscosity.
In vitro digestion viscosity.
c
Not signi®cant (P > 0:05).
b
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
9
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 of ÿ0.83 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
10
B. Svihus et al. / Animal Feed Science and Technology 88 (2000) 1±12
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 ®bredegrading 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 endogenous b-glucanases in barley reduced amount and molecular
size of b-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 noncovalent 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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