Effect of chemical conditioning on the milling of high-tannin sorghum

Journal of the Science of Food and Agriculture
J Sci Food Agric 80:2216±2222 (online: 2000)
Effect of chemical conditioning on the milling of
high-tannin sorghum
Trust Beta,1 Lloyd W Rooney2 and John RN Taylor3*
1Department of Food Science and Nutrition, University of Zimbabwe, PO Box MP 167, Mount Pleasant, Harare, Zimbabwe
2Cereal Quality Laboratory, Department of Soil and Crop Science, Texas A&M University, College Station, TX 77843, USA
3Department of Food Science, University of Pretoria, Pretoria 0002, South Africa
Abstract: Suitable methods are required for milling tannin-containing sorghums. Sorghum varieties
SV2 (tannin-free), Chirimaugute (medium-tannin) and DC-75 (high-tannin) were milled using a
simple roller mill and a multi-sample tangential abrasive decortication device. Grain was conditioned
up to 20% moisture prior to milling using HCl (0.9%, v/v), formaldehyde (HCHO; 0.05%, v/v), NaOH
(0.3%, w/v) and water as control. Abrasive decortication and roller milling reduced levels of the
polyphenols. Polyphenol reduction was affected signi®cantly by variety, chemical treatment and
conditioning moisture (P`0.001). NaOH and HCHO treatments gave lower polyphenol content in the
meal and offal after roller milling. Enzyme inhibition (EI) by polyphenols was reduced by 52% after
decortication or roller milling. Chemical treatment did not signi®cantly affect EI levels after
decortication. NaOH and HCHO treatments gave the lowest EI when Chirimaugute was roller milled
at 20% moisture. The yield of product obtained after decortication was high at 12% moisture for SV2
and Chirimaugute and at 16% conditioning moisture for DC-75. Conditioning to 20% moisture prior to
milling did not improve product yield. Roller milling and decortication resulted in products that were
lighter in colour than the grain. Both abrasive decortication and roller milling plus NaOH and HCHO
reduced tannin content of offal. It is concluded that conditioning using NaOH could be advantageous in
roller milling but not in abrasive decortication of tannin-containing sorghums.
# 2000 Society of Chemical Industry
Keywords: sorghum; tannins; chemical treatments; milling
INTRODUCTION
lenging, as antinutritional tannins contained in the
Sorghum industrial milling technology is still evolving,
testa layer have to be removed or the tannins extracted
unlike those for wheat, rice and maize, where
or transformed. Tannin-containing sorghums tend to
specialised milling technologies have been developed
be soft,8 making high endosperm recovery dif®cult.
to give ®nished products of high acceptance. Simple
Mwasaru et al9 advocated development of harder,
roller mills with two pairs of rolls are gaining wide
rounder grain before commercial milling of tannin-
acceptance for small-scale maize milling in southern
containing sorghum can become economically
Africa.1 Conditioning sorghum using the same
feasible. However, the available sorghum varieties,
moisture levels applied to wheat results in a less clean
for example in Zimbabwe,10 lack ideal characteristics
fractionation, as the sorghum bran is much more
for processing; that is, a thin, white pericarp and
brittle.2 Other researchers have revealed that roller
corneous endosperm.11 Chemical treatments have
milling results may be improved by steeping at high
been used to detoxify tannin-containing sor-
moisture contents under semi-wet conditions.1,3 An
ghums.12,13 The objective of this study was to
alternative approach to that of roller milling is
determine the effect of combining chemical treatment
decortication. The decortication principle applied to
with conditioning on the milling of high-tannin
rice and barley dehullers, decorticators and polishers
sorghums.
has been used for sorghum.4,5 Abrasive decortication
operates on the principle of progressively rubbing off
the outer layers of the kernel.6 Traditionally, this is a
MATERIALS AND METHODS
dry technique. Decortication followed by hammer
Grains
milling is the most common industrial sorghum
Sorghum (Sorghum bicolor (L) Moench) grain of
milling process in southern Africa.7
varieties SV2 and Chirimaugute (Chiri) and hybrid
Milling high-tannin sorghums is particularly chal-
variety DC-75 were used. DC-75 (high-tannin) and
* Correspondence to: John RN Taylor, Department of Food Science, University of Pretoria, Pretoria 0002, South Africa
E-mail: jrnt@scientia.up.ac.za
Contract/grant sponsor: McKnight Foundation Collaborative
Contract/grant sponsor: Crop Research Programme
(Received 4 February 2000; revised version received 13 June 2000; accepted 11 August 2000)
# 2000 Society of Chemical Industry. J Sci Food Agric 0022±5142/2000/$30.00
2216

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Chemical treatments in sorghum milling
SV2 (tannin-free) have relatively corneous endo-
speci®c for condensed tannins.19 Sample extracts for
sperms, as determined visually according to the
the assays were obtained by shaking ground grain
method of Rooney and Miller,14 while Chirimaugute
(0.2g) in methanol (10ml) for 20min at 5min
(medium-tannin) has a ¯oury endosperm.10 The
intervals on a vortex mixer at room temperature. The
pericarp is thick in DC-75, medium in Chirimaugute
supernatant was obtained by centrifuging for 10min at
and thin in SV2 kernels.10 The sorghums were grown
1200Âg. For the anthocyanin production assay, 6ml
in the 1996/97 season at Matopos, Zimbabwe in ®eld
of acid butanol (50ml HCl (32%)dmÀ3) was added to
conditions under normal agronomic practices.
1ml of sample extract in a test tube. The test tubes
were placed in a forced-air oven at 100°C for 50min.
Abrasive decortication
Absorbance was read at 550nm against a reagent
A multi-sample tangential abrasive dehulling device
blank, and no standard was used. The results were
(TADD model 4E-230, Venables Machine Work Ltd,
reported as absorbance units at 550nmgÀ1. Catechin
Saskatoon, Canada) consisting of an eight-cup dehul-
(Sigma Chemicals) was used as a standard in the
ling plate was used for removing successive layers from
vanillin assay, and sample blanks were included. The
sorghum grains abrasively at 1750rpm following the
results were expressed in catechin equivalents (mg CE
technique developed by Oomah et al6 and Reichert et
per 100mg sample).
al. 15 The moisture content of the grain was deter-
mined. The amount of solution required to condition
Enzyme inhibition
the grain was calculated according to AACC method
Enzyme inhibition in sorghum ¯our was determined
26±95.16 Grain (20g) was conditioned in plastic
following the method of Daiber.20 An enzyme extract
containers to 12, 16 or 20% moisture using solutions
was obtained by milling malt (5g) in distilled water
of HCl (0.9%, v/v), HCHO (0.05%, v/v), NaOH
(100ml) in an Ultra Turrax model T25 (Janke and
(0.3%, w/v) and water as control. Grains and the
Kunkel IKA-Labortechnik, Staufeni, Br, Germany)
solution were thoroughly mixed twice. The samples
for 5min and centrifuging the sample for 5min. To
were left to equilibrate overnight in a cold store at 4°C
determine residual activity after enzyme inhibition, the
according to the semi-wet milling procedure described
extract (12ml) was added to sorghum ¯our (120mg).
by Gomez1 and Cecil.3 The conditioned grains were
The mixture was shaken while incubating for 30min at
milled for 2min in the TADD, and the yield of
30°C. A sample containing no sorghum ¯our was
decorticated grain was recorded and calculated on a
included to determine original activity. After incu-
uniform moisture basis.
bation, samples were centrifuged for 5min and the
supernatants were used for diastasis. Diastatic power
Roller milling
(amylase activity) was determined and percentage
A double-roll roller mill with a single-pass 1.5mm
enzyme inhibition calculated as 100Â(original acti-
vibrating screen (Maximill, Kroonstaad, South Africa)
vityÀresidual activity)/(original activity).
was used to mill conditioned grain. The roll gaps on
the mill were set at 0.15 and 0.10mm for the top and
Colour values
bottom rolls respectively. The top and bottom rolls
Hunter L, a and b values were obtained using a Hunter
were corrugated to 7 and 22 ¯utes per inch respec-
Colour Quest 45/0 (Hunter Associates Laboratory,
tively. Roll speeds were set at a differential of 1.25:1.
Inc, Reston, VA, USA). An average of three readings
Grain (5kg) was weighed into plastic bags. Condition-
per sample were taken.
ing was carried out prior to milling using solutions of
HCl (0.9%, v/v), HCHO (0.05%, v/v), NaOH (0.3%,
Statistical analyses
w/v) and water as control. The bag was sealed and the
The general linear model procedure of SAS version
contents were mixed thoroughly. The sample was left
6.12 (SAS Institute, Cary, NC, USA) was used.
to equilibrate at 4°C overnight according to the semi-
Analysis of variance was used to determine the effect of
wet milling procedure.1,3 The yield of meal and offal
variety, treatment and moisture level prior to milling
obtained after roller milling was recorded and calcu-
on polyphenol content, yield and quality of product.
lated on a uniform moisture basis.
Means were separated using the least squares differ-
ence (LSD) at P `0.05.
Analyses
For the analyses the products obtained after roller and
abrasive milling were ground to pass through an
RESULTS AND DISCUSSION
800mm sieve.
The choice of chemical treatments for milling was
based on previous reports using alkali,13,21 HCl,13
Polyphenol analysis
HCHO12 and high-moisture reconstitution13,22 to
The polyphenols in milled sorghum products were
improve the nutritional value of high-tannin sor-
measured using the butanol±HCl17 and vanillin±
ghums. The selected concentrations of HCHO and
HCl18 methods. By using a blank subtraction with
NaOH were based on ®ndings by Daiber12 and Dewar
the vanillin±HCl assay, ¯avonoid components of the
et al23 respectively. We used the information from our
grain were eliminated and the reaction became more
preliminary experiments on steep water uptake using
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T Beta, LW Rooney, JRN Taylor
various concentrations of HCl as the basis for selecting
20% moisture resulted in lower yields for all varieties.
the concentration of the acid. Conditioning moisture
Thus conditioning prior to decortication was not an
for roller milling was selected following ®ndings by
advantage for the purpose of increasing product yield.
Gomez.1 Conditioning during decortication is prac-
Eggum et al24 demonstrated that only 59% (compared
tised in the traditional mortar and pestle method up to
to 79% in wheat) of lysine remains in hand-dissected
about 20%,24 but machine decortication in an abrasive
sorghum endosperm and hence, in milling sorghum, a
mill is normally a dry milling technique.
high extraction rate of 85% is desirable. Extraction
rates were reasonably high at the 16% conditioning
Product yield
moisture with all three varieties. However, yields were
Yield of decorticated grain (87±96%) was highest for
slightly better at 16% moisture for DC-75. Meal yield
SV2, the tannin-free variety (Table 1). The yield falls
was decreased by up to 8, 6 and 7% on average when
within the range of extraction rates (71±98%) pre-
SV2, DC-75 and Chirimaugute respectively were
viously reported for 23 white sorghum varieties.25
conditioned to 20% moisture. High-tannin sorghum
DC-75 gave slightly better yields than Chirimaugute
lines that could be abrasively decorticated and yield at
(84 and 81% respectively) at 12% moisture. Video
least 70% of product have previously been identi®ed.27
image analysis of the endosperm texture of these grains
Subramanian et al28 found a correlation coef®cient of
indicated an increasing ¯oury endosperm from SV2,
0.89 (P `0.01) when the traditional mortar and pestle
DC-75 and Chirimaugute.10 From visual observation
method and the TADD abrasion equipment were used
it was clear that the lower yields were due to loss of
for sorghum milling. However, the reported yields
endosperm fractions into the offal. Thus yield was
could be slightly lowered under commercial opera-
more related to the ¯oury nature of the grains, as
tions, as in most laboratory decorticators, such as the
previously reported by Chibber et al8 and Reichert,5
modi®ed Udy Cyclone mill by Shepherd29 and the
than their polyphenol content. The importance of a
TADD abrasion mill, the frictional forces between the
relatively corneous endosperm was emphasised by
grains are minimised so that grains are decorticated in
Munck et al26 when they observed that introduced
a gentler way compared to commercial decortica-
Tanzanian sorghum with 31% soft endosperm was
tors.30
giving lower yields of 50% compared to local hard
cultivars (3.9±13.6% soft endosperm) that gave
Polyphenol content (butanol–HCl assay)
recoveries of 72±86%. However, yield of product was
The polyphenol content of the meal was signi®cantly
relatively higher than values previously reported for
affected (P `0.001) by variety, chemical treatment
brown sorghums.3,8 The ®ndings con®rm the report
and conditioning moisture prior to abrasive decortica-
by Gomez1 that roller milling of sorghum under semi-
tion or roller milling (Table 2). Reduction in poly-
wet conditions (16% moisture) is superior to dry
phenols after milling was due to loss of grain pericarp
milling.
and testa layers as offal. Polyphenol content was
Chemical treatment did not signi®cantly affect yield
markedly lower when abrasive decortication was used.
of roller-milled product (Table 1). Conditioning to
Roller-milled ¯our apparently had more offal con-
Treatment
12/AD
16/AD
20/AD
16/RM
20/RM
16/RM(O)
20/RM(O)
SV2
Water
95.0b
94.7ab
87.8a
90.9
79.8
6.9
13.1
HCl
95.3ab
94.9a
87.8a
90.5
85.2
6.4
9.1
HCHO
95.3ab
94.6bc
86.7b
91.8
84.7
6.6
9.2
NaOH
95.6a
94.1c
87.8a
89.5
83.9
7.4
9.8
Mean
95.3
94.6
87.5
90.7
83.4
6.8
10.3
DC-75
Water
83.6c
86.8ab
84.6ab
88.5
7.4
HCl
84.1bc
87.0a
84.4ab
87.5
8.0
HCHO
84.4ab
86.9ab
84.8a
88.1
7.0
NaOH
85.0a
86.3b
84.3b
89.9
6.7
Mean
84.3
86.8
84.5
88.5
7.3
Chiri
Table 1. Effect of conditioning sorghum
Water
81.2a
80.3bc
74.6b
91.0
82.2
6.4
10.6
varieties SV2, DC-75 and Chiri to 12,
HCl
81.3a
80.0c
75.0b
90.3
82.0
6.3
11.3
16 and 20% moisture content using
HCHO
80.8ab
81.1a
75.9a
89.9
81.8
5.8
10.4
water (control), HCl (0.9%, v/v), HCHO
(0.05%, v/v) and NaOH (0.3%, w/v),
NaOH
80.5b
80.4ab
75.7a
89.4
82.9
6.2
10.9
prior to abrasive decortication (AD) or
Mean
81.0
80.5
75.3
90.2
82.2
6.2
10.8
roller milling (RM), on the meal and offal
(O) yield
Values with same letter in same column for each variety are not statistically different (P `0.05).
2218
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Chemical treatments in sorghum milling
DC-75
Chiri
Table 2. Effect of conditioning sorghum
varieties DC-75 and Chiri to 12, 16 and
Treatment 12/AD 16/AD
20/AD
16/RM 12/AD 16/AD 20/AD 16/RM 20/RM
20% moisture content using water
(control), HCl (0.9%, v/v), HCHO
Water
10.6b
18.3b
20.8a
32.3a
6.5a
8.2a
8.0a
22.1a
6.3b
(0.05%, v/v) and NaOH (0.3%, w/v),
HCl
13.2a
15.7c
17.5bc
32.6b
6.2a
7.9a
7.9a
17.6b
9.3a
prior to abrasive decortication (AD) or
HCHO
13.1a
18.0b
15.2c
30.2c
5.4b
6.8b
5.3b
14.2c
5.0c
roller milling (RM), on the tannin
NaOH
13.4a
22.9a
17.9b
29.6c
6.3a
7.7a
4.8b
15.3c
5.8bc
content of the meal as measured by the
butanol–HCl assay (absorbance units
Tannin content of raw grain: DC-75, 45.9; Chiri, 29.3.
at 550 nm gÀ1, dry basis)
Values with same letter in same column are not statistically different (P `0.05).
tamination. Conditioning to 12% moisture followed
ing 55% was not achieved, as yield of product was kept
by decortication, a technique equivalent to the
relatively high.
traditional dry abrasion practice, reduced polyphenol
Roller milling DC-75 after conditioning to 16%
levels by 71 and 81% in DC-75 and Chirimaugute
moisture gave ¯our products that had higher poly-
respectively. Polyphenol reduction was greatest at
phenol content than those obtained by abrasive
12% moisture, and no further advantage was obtained
decortication at 12%, giving further evidence that
by conditioning to higher moisture prior to decortica-
polyphenols were largely removed by the abrading
tion. It was presumed that polyphenol reduction was
action. However, polyphenol content was markedly
largely due to the abrading action. At 16 and 20%
reduced by 56±71% after roller milling Chirimaugute
moisture, DC-75 ¯our had higher polyphenol content
at 20% moisture. Polyphenol content was lower at 20
than at 12% (Table 2). Polyphenol content was
than 16% moisture in Chirimaugute roller-milled
29±36% less when DC-75 grain was conditioned to
meal.
16% moisture and roller milled.
At 20% moisture, HCHO and NaOH gave low
With roller milling, NaOH and HCHO gave rela-
polyphenol content in Chirimaugute roller-milled
tively lower polyphenols than water and HCl for both
meal. Polyphenol reduction possibly involved forma-
varieties, as we have observed in steeping experiments
tion of high±molecular±weight polymers33 that were
for malting (Beta et al, unpublished). NaOH and
cross-linked and insoluble. The ®ndings con®rm the
HCHO treatments of high-tannin sorghums for feed
work of Reichert et al13 where tannins were reduced by
have also been found to improve the weight gain of
97, 83 and 39% when sorghum grains imbibed 0.8M
birds.31
NaOH, 0.8M HCl and water respectively at 25% by
weight at 25°C with an equilibration period of 2 days.
Polyphenol content (vanillin–HCl assay)
The differences in tannin reduction could be due to
Polyphenol reduction (range 12±53%) was less pro-
higher solution concentrations used in their experi-
nounced in decorticated samples with the vanillin
ments. Tannin reduction levels of 24±52% were also
(Table 3) than with the butanol±HCl assay. The
reported when 10% by weight of sorghum grain of 0.5
vanillin method tends to underestimate the weight of
or 0.2M NaOH was used for treatment of high-tannin
condensed tannins owing to their reduced reactivity.32
sorghum that was then stored for 3 and 11 days.34
Polyphenol content was reduced by 41% for DC-75
The results of the vanillin±HCl assay with blank
and 49% for Chirimaugute after decortication at 12%
subtraction (Table 4) were essentially the same as
moisture. Reduction in polyphenol content was due to
above, only the magnitude was different. Thus the
loss of the grain outer layers by abrasion. Chibber et al8
tannin content of roller-milled meal was reduced by
and Mwasaru et al9 reduced polyphenol content by up
15±20 and 18±44% when DC-75 and Chirimaugute
to 91% using a similar dry abrasion technique, but at
respectively were conditioned to 16% moisture
lower extraction rates. Polyphenol reduction exceed-
(Table 4).
Table 3. Effect of conditioning sorghum varieties DC-75 and Chiri to 12, 16 and 20% moisture content using water (control), HCl (0.9%, v/v),
HCHO (0.05%, v/v) and NaOH (0.3%, w/v), prior to abrasive decortication (AD) or roller milling (RM), on the polyphenol content of the meal and
offal(O)as measured by the vanillin–HCl assay in catechin equivalents (mg CE per 100 mg sample, dry basis)
DC-75
Chiri
Treatment 12/AD 16/AD 20/AD 16/RM 16/RM(O) 12/AD 16/AD 20/AD 16/RM 20/RM 16/RM(O) 20/RM(O)
Water
3.8a
4.7b
4.8c
5.0a
12.9b
1.8c
2.3b
2.3b
2.4d
1.3b
5.1b
7.4c
HCl
3.6b
4.6c
5.6a
4.6c
13.7a
1.9b
2.4a
2.6a
3.0a
1.7a
5.7a
9.6a
HCHO
3.6b
4.5d
4.8c
4.7b
11.0c
1.9b
2.1c
2.0d
2.5c
1.1d
4.8c
6.1d
NaOH
3.8a
4.9a
5.0b
4.7b
10.7d
2.1a
2.3b
2.1c
2.6b
1.2c
4.6d
8.0b
Polyphenol content of raw grain: DC-75, 6.3; Chiri, 3.8.
Values with same letter in same column are not statistically different (P `0.05).
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T Beta, LW Rooney, JRN Taylor
Table 4. Effect of conditioning sorghum varieties DC-75 and Chiri to 12, 16 and 20% moisture content using water (control), HCl (0.9%, v/v),
HCHO (0.05%, v/v) and NaOH (0.3%, w/v), prior to abrasive decortication (AD) or roller milling (RM), on the tannin content of the meal and
offal(O) as measured by the vanillin–HCl assay with blank subtraction in catechin equivalents (mg CE per 100 mg sample, dry basis)
DC-75
Chiri
Treatment 12/AD 16/AD 20/AD 16/RM 16/RM(O) 12/AD 16/AD 20/AD 16/RM 20/RM 16/RM(O) 20/RM(O)
Water
3.4b
4.3b
4.3c
4.7a
11.6b
1.2c
1.6b
1.6b
1.7d
0.8b
3.4b
5.6c
HCl
3.2d
4.3b
5.1a
4.4b
12.4a
1.3b
1.7a
1.8a
2.5a
1.2a
4.0a
7.2a
HCHO
3.3c
4.1c
4.3c
4.4b
9.8c
1.2c
1.4c
1.3d
2.1b
0.7c
3.0d
4.6d
NaOH
3.5a
4.5a
4.5b
4.4b
9.4d
1.4a
1.6b
1.5c
2.0c
0.7c
3.1c
6.2b
Tannin content of raw grain: DC-75, 5.5; Chiri, 3.1.
Values with same letter in same column are not statistically different (P `0.05).
The tannin content of offal differed signi®cantly
abrasive decortication for the purpose of reducing the
(P `0.001) among treatments (Table 4). Offal poly-
enzyme inhibitory power of polyphenols.
phenol content was much higher than that of meal,
The control (water) gave higher inhibition that
because offal contained most of the pericarp and testa
differed signi®cantly from HCl, HCHO and NaOH
layers. Tannin content of offal was 72±126% greater
treatment in roller milling. EI in Chirimaugute meal
than in DC-75 grain at 16% moisture. However,
was higher at 16 than 20% moisture (31 and 24%
Chirimaugute had up to 30% more tannin in its offal at
respectively). NaOH and HCHO gave the lowest
the same moisture, presumably owing to higher
enzyme inhibition when Chirimaugute was condi-
endosperm contamination. Tannin content in the
tioned to 20% moisture prior to roller milling. Thus
offal increased by 49±133% when Chirimaugute was
roller milling at high moisture showed the effectiveness
conditioned to 20% moisture prior to roller milling.
of NaOH and HCHO on polyphenol reduction.
Thus conditioning Chirimaugute grains to higher
moisture resulted in more tannins being extracted into
Product colour
offal or less endosperm contamination of offal. Offal
In sorghum milling, colour improvement is of primary
tannin content was low with NaOH and HCHO
importance for consumer acceptance. Variety, con-
treatment at 16% moisture. At 20% moisture the
ditioning treatment and moisture signi®cantly affected
tannin levels of Chirimaugute offal were markedly high
(P `0.001) product colour (Table 6). There was no
with HCl.
advantage in conditioning up to 20% moisture prior to
abrasive decortication for the purpose of improving
¯our colour, as L values were highest at 12%.
Enzyme inhibition by polyphenols in the meal
Chirimaugute produced the darkest ¯our although
Enzyme inhibition (EI) assays indicated signi®cant
its polyphenol content was signi®cantly less than that
differences between the two varieties (Table 5). The
of DC-75. Factors other than polyphenol content
mean EI for Chirimaugute and DC-75 ¯our was 16
alone are involved in determining the colour of
and 41% respectively. This was presumed to be due to
sorghum products.34 Hunter L values differed sig-
the high tannin content in DC-75 grains10 that could
ni®cantly among treatments, with HCl giving a slight
not be completely removed. Chemical treatment had
improvement in ¯our colour, possibly owing to
no effect on EI after abrasive decortication, con®rming
destruction of xanthophylls.
the ®nding that abrasion was mostly responsible for
SV2 had higher L values than DC-75 and Chir-
polyphenol reduction. EI was similar at 16 and 20%
imaugute after roller milling. SV2 is a tannin-free
(mean 30% EI) but differed signi®cantly from the 12%
variety, unlike the latter.10 Flour colour was improved
(mean 25% EI) conditioning moisture. Thus con-
more at 20% moisture than at 16% moisture for SV2
ditioning to higher moisture was not important in
and Chirimaugute. HCl gave slightly higher L values
DC-75
Chiri
Treatment 12/AD 16/AD 20/AD 16/RM 12/AD 16/AD 20/AD 16/RM 20/RM
Water
36b
46a
42c
54a
17a
12c
23a
19a
13a
HCl
24c
40b
51a
41b
12c
14bc
18b
17b
13a
Table 5. Effect of conditioning sorghum
HCHO
38b
41b
38d
38c
14b
17a
19b
14c
9b
varieties DC-75 and Chiri to 12, 16 and
NaOH
40a
46a
45b
43b
17a
16ab
11c
19a
9b
20% moisture content using water
(control), HCl (0.9%, v/v), HCHO
Meana
58
46
45
46
55
56
46
48
67
(0.05%, v/v) and NaOH (0.3%, w/v),
prior to abrasive decortication (AD) or
Enzyme inhibition of raw grain: DC-75, 80; Chiri, 33.
a
roller milling (RM), on enzyme inhibition
Percentage reduction in inhibition.
(%) by the polyphenols in the meal
Values with same letter in same column are not statistically different (P `0.05).
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Chemical treatments in sorghum milling
Table 6. Effect of conditioning sorghum varieties SV2, DC-75 and Chiri to 12, 16 and 20% moisture content using water (control), HCl (0.9%, v/v), HCHO (0.05%,
v/v) and NaOH (0.3%, w/v), prior to abrasive decortication (AD) or roller milling (RM), on meal colour as measured by the Hunter L value
SV2
DC-75
Chiri
Treatment 12/AD 16/AD 20/AD 16/RM 20/RM 12/AD 16/AD 20/AD 16/RM 12/AD 16/AD 20/AD 16/RM 20/RM
Water
76.7c
76.1b 76.0b
79.7a
80.4b 68.5d 66.5c 66.8b
67.1d 66.3a 65.6a
65.1d 65.6a
68.2b
HCl
75.0a
76.3a 76.5a
79.8a
80.5a
68.8b 67.6a 67.2a
69.0a
66.2b 65.4b 65.6a
63.8b
67.7c
HCHO
76.9b 75.9c 75.8c
78.8b
80.4b 69.0a
67.0b 66.2c
68.7b 66.2b 65.6a
65.4b 64.4d
68.5a
NaOH
76.5d 76.2a 75.7c
79.7a
80.1c
68.7c
66.3c 66.1c
67.7c
66.2b 65.3b 65.2c
63.7c
67.6d
Meana
3
2
2
7
8
10
10
10
11
11
10
10
8
14
L value of whole grain ¯our: SV2, 74.35; DC-75, 60.68; Chiri, 59.52.
a Percentage increase in L value.
Values with same letter in same column are not statistically different (P `0.05).
than other treatments after roller milling SV2 and DC-
5 Reichert RD, Sorghum dry-milling, in Proceedings of an Interna-
75 as found in abrasive milling. The mean increase in
tional Symposium on Sorghum Grain Quality, Ed by Rooney LW
L value was 7, 8 and 15% after roller milling SV2,
and Murty DS, International Crops Research Institute for the
Chirimaugute and DC-75 respectively.
Semi-Arid Tropics (ICRISAT), Patancheru, pp 547±563
(1982).
6 Oomah BD, Reichert RD and Youngs CD, A novel, multi-
sample, tangential abrasive dehulling device (TADD). Cereal
CONCLUSIONS
Chem 56:392±395 (1981).
Conditioning treatment and moisture prior to roller
7 Schmidt OG, An analysis of progress and achievements in
milling of high-tannin sorghums improved product
regional dehulling projects, in Utilisation of Sorghum and Millets,
yield and reduced polyphenol content of the meal.
Ed by Gomez MI, House LR, Rooney LW and Dendy DAV,
Conditioning grains prior to abrasive decortication
International Crops Research Institute for the Semi-Arid
Tropics (ICRISAT), Patancheru, pp 9±18 (1992).
neither gave signi®cant reduction in polyphenol
8 Chibber BAK, Mertz ET and Axtell JD, Effects of dehulling on
content nor improved colour and yield of decorticated
tannin content, protein distribution and quality of high and low
¯our. Enzyme inhibition was higher in products
tannin sorghum. J Agric Food Chem 26:679±683 (1978).
obtained from the high-tannin than the medium-
9 Mwasaru MA, Reichert RD and Mukuru SZ, Factors affecting
tannin variety. Polyphenol content was not related to
the abrasive dehulling ef®ciency of high-tannin sorghum.
yield and colour of the product after milling. The
Cereal Chem 65:171±174 (1988).
®ndings that abrasive and roller milling plus HCHO
10 Beta T, Rooney LW, Marovatsanga LT and Taylor JRN,
Phenolic compounds and kernel characteristics of Zimbab-
and NaOH reduced tannin content of offal could be
wean sorghums. J Sci Food Agric 79:1003±1010 (1999).
important if offal is used for animal feed.
11 Rooney LW and Walker HJ, Genetic and environmental factors
affecting the milling properties of sorghum for use in food
products. Cereal Foods World 23:485 (1978).
ACKNOWLEDGEMENTS
12 Daiber KH, Treatment of cereal grain. S Afr PaÈtent75/4957
The authors are greatly indebted to the McKnight
(1975).
Foundation Collaborative Crop Research Programme
13 Reichert RD, Fleming SE and Schwab DJ, Tannin deactivation
and nutritional improvement of sorghum by anaerobic storage
for sponsoring this study carried under the project
of H
entitled `Ecological biochemistry of proanthocyani-
2O-, HCl-, NaOH-treated grain. J Agric Food Chem
28:824±829 (1980).
dins and related ¯avonoids in the Zimbabwean small-
14 Rooney LW and Miller F, Variation in the structure and kernel
scale farming system'.
characteristics of sorghum, in Proceedings of an International
Symposium on Sorghum Grain Quality, Ed by Rooney LW and
Murty DS, International Crops Research Institute for the
Semi-Arid Tropics (ICRISAT), Patancheru, pp 143±162
REFERENCES
(1982).
1 Gomez MI, Comparative evaluation and optimization of a
15 Reichert RD, Tyler RT, York AE, Schwab DJ, Tatarynovich JE
milling system for small grains, in Cereal Science and
and Mwasaru MA, Description of a production model of the
TechnologyÐImpact on a Changing Africa: Proceedings of the
International Association for Cereal Science and Technology (ICC)
tangential abrasive dehulling device and its application to
International Symposium, Ed by Taylor JRN, Randall PG and
breeders' samples. Cereal Chem 63:201±205 (1986).
Viljoen JH, Council for Scienti®c and Industrial Research
16 American Association of Cereal Chemists (AACC), Approved
(CSIR), Pretoria, pp 463±474 (1993).
Methods of the American Association of Cereal Chemists, 8th edn.
2 Perten HA, Practical experience in processing and use of millet
AACC, St Paul, MN (1983).
and sorghum in Senegal and Sudan. Cereal Foods World
17 Porter LJ, Hirstich LN and Chan BG, The conversion of
28:680±683 (1983).
procyanidins to prodelphinidins to cyanidin and delphinidin.
3 Cecil JE, Roller milling sorghum and millet grain using a semi-
Phytochemistry 25:223±230 (1986).
wet process. Report L74, Tropical Development and Research
18 Burns RE, Method for estimation of tannin in grain sorghum.
Institute, London (1986).
Agron J 63:511±512 (1971).
4 Hulse JH, Laing EM and Pearson DE, Sorghum and Millets: Their
19 Price ML, Van Scoyoc S and Butler LG, A critical examination
Composition and Nutritive Value. Academic Press, London
of the vanillin reaction as an assay for tannin in sorghum grain.
(1980).
J Agric Food Chem 26:1214±1218 (1978).
J Sci Food Agric 80:2216±2222 (online: 2000)
2221

---

T Beta, LW Rooney, JRN Taylor
20 Daiber KH, Enzyme inhibition by polyphenols of sorghum grain
lines with good dehulling characteristics. Cereal Chem 65:165±
and malt. J Sci Food Agric 26:1399±1411 (1975).
170 (1988).
21 Chavan JK, Kadam SS, Ghonsikar CP and Salunkhe DK,
28 Subramanian V, Suryaprakash S, Jambunathan R and Murty
Removal of tannins and improvement of in vitro protein
DS, Dehulling and milling characteristics of some sorghum
digestibility of sorghum seeds by soaking in alkali. J Food Sci
cultivars, in Proceedings of an International Workshop on Sorghum
44:1319±1321 (1979).
Grain Quality, Ed by Gomez MI, House LR, Rooney LW and
22 Mitaru BN, Reichert D and Blair R, Improvement of the
Dendy DAV, International Crops Research Institute for the
nutritive value of high tannin sorghums for broiler chickens by
Semi-Arid Tropics (ICRISAT), Patancheru, pp 126±140
high moisture storage (reconstitution). Poultry Sci 62:2065±
(1988).
2072 (1983).
29 Shepherd AD, Laboratory abrasive decortication mill for small
23 Dewar J, Orovan E and Taylor JRN, Effect of alkaline steeping
grains. Cereal Chem 56:517±519 (1979).
on water uptake and malt quality in sorghum. J Inst Brew
30 Munck L, New milling technologies and products: whole plant
103:283±285 (1997).
utilization by milling and separation of the botanical and
24 Eggum BO, Bach Knudsen KE, Munck L, Axtell JD and
chemical components, in Sorghum and Millets: Chemistry and
Mukuru SZ, Milling and nutritional value of sorghum in
Tanzania, in Proceedings of an International Symposium on
Technology, Ed by Dendy DAV, American Association of
Sorghum Grain Quality, Ed by Rooney LW and Murty DS,
Cereal Chemists, St Paul, MN, pp 223±281 (1995).
International Crops Research Institute for the Semi-Arid
31 Schutte GM and Smith GA, Untreated, sodium hydroxide- and
Tropics (ICRISAT), Patancheru, pp 211±225 (1982).
formaldehyde-treated birdproof grain sorghum as energy
25 Oomah BD, Reichert RD and Youngs CD, Milling characteris-
source in broiler diets. S Afr J Anim Sci 21:115±119 (1991).
tics of some sorghum varieties. Sorghum Newslett 26:97 (1983).
32 Goldstein JL and Swain T, Changes in tannins in ripening fruits.
26 Munck L, Bach Knudsen KE and Axtell JD, Industrial milling of
Phytochemistry 2:371±383 (1963).
sorghum for the 1980s, in Sorghum in the Eighties: Proceedings of
33 Porter LJ, Structure and chemical properties of the condensed
an International Symposium on Sorghum, Ed by House LR,
tannins, in Plant Polyphenols, Ed by Hemingway RW and Laks
Mughogho LK, Peacock JM and Mertin JV, International
PE, Plenum, New York, pp 245±258 (1992).
Crops Research Institute for the Semi-Arid Tropics (ICRI-
34 Price ML, Butler LG, Rogler JC and Featherston WR,
SAT), Patancheru, pp 565±570 (1982).
Overcoming the nutritionally harmful effects of tannin in
27 Reichert RD, Mwasaru MA and Mukuru SZ, Characterization
sorghum grain by treatment with inexpensive chemicals. J
of colored grain sorghum lines and identi®cation of high tannin
Agric Food Chem 27:441±445 (1979).
2222
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