EFFECT OF EXTRUSION ON ESSENTIAL AMINO ACIDS PROFILE AND COLOR OF WHOLE-GRAIN FLOURS OF QUALITY PROTEIN MAIZE (QPM) AND NORMAL MAIZE CULTIVARS

EFFECT OF EXTRUSION ON ESSENTIAL AMINO ACIDS PROFILE AND COLOR
OF WHOLE-GRAIN FLOURS OF QUALITY PROTEIN MAIZE (QPM) AND
NORMAL MAIZE CULTIVARS
MARIA CRISTINA DIAS PAES1, JOSEPH MAGA2
1Food Scientist, Embrapa Milho e Sorgo. Caixa Postal 151, CEP. 35701-970 Sete Lagoas, MG. E-mail:
mcdpaes@cnpms.embrapa.br (autora para correspondência).
2Emeritus Professor, Department of Food Science and Human Nutrition. Colorado State University, Fort Collins, CO,
80523, USA.
Revista Brasileira de Milho e Sorgo, v.3, n.1, p.10-20, 2004
ABSTRACT - Whole-grain flours of Quality Protein Maize (QPM) and normal maize were
extruded under controlled conditions in order to evaluate the effect of extrusion on the essential
amino acids profile and color of the raw material used in the production of maize based
extrudates. Flours were conditioned to 150g/kg moisture and processed in a single screw
extruder at a screw compression ratio of 3:1, screw speed of 80 rpm, and die head temperature
of 1300C, using two different die nozzle diameters (3 and 5 mm). Extrusion caused a
diminishment in the contents of the essential amino acids isoleucine, leucine, lysine, threonine
and valine when compared to their original flours (P<0.05). But, the contents of histidine,
methionine, phenylalanine and tryptophan were not different for flours and extrudates of the
same source (P>0.05). QPM samples, either raw or extruded, were significantly higher in
lysine, methionine and tryptophan compared to samples of normal maize (P<0.05). Extrudates
produced with yellow QPM flours were lighter than their correspondent raw material (P<0.05),
different from that of yellow normal maize. This trend was also observed for redness (a values)
in extrudates. On the other hand, white and yellow extrudates presented higher b values
(yellowness) than their correspond raw flour. Despite the adverse effect of extrusion in the
amino acid retention, the use of QPM flours in replacement of normal maize flours can provide
maize extrudates with superior protein quality.
Key words: maize, quality protein maize, extrusion, essential amino acids, color.
EFEITO DA EXTRUSÃO NO PERFIL DE AMINOÁCIDOS ESSENCIAIS E
COR DE FARINHAS INTEGRAIS DE CULTIVARES DE MILHO NORMAL E
DE ALTA QUALIDADE PROTÉICA (QPM)
RESUMO - Farinhas integrais de milhos de alta qualidade protéica (QPM) e normal foram
extrusadas sob condições controladas, com o objetivo de se avaliar o efeito da extrusão no
perfil de aminoácidos essenciais e cor da matéria-prima utilizada na fabricação de extrusados
de milho. As farinhas foram acondicionadas a 150g/kg de umidade e extrusadas em um extrusor
de rosca simples com razão de compressão da rosca de 3:1, velocidade de rotação da rosca
de 80 rpm e temperatura na cabeça da matriz de 130oC, utilizando-se duas matrizes com
diâmetros de abertura distintos (3 e 5 mm). A extrusão promoveu redução no conteúdo dos
aminoácidos essenciais isoleucina, leucina, lisina, treonina e valina das farinhas de milho (P<0.05),
entretanto, os conteúdos de histidina, metionina, fenilalanina, e triptofano não diferiram para as

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Effect of extrusion on essential amino acids ...
11
amostras de farinhas e extrusados de mesma origem (P>0.05). As farinhas e os extrusados de
milho QPM apresentaram teores de lisina, metionina e triptofano superiores àqueles de milho
normal (P<0.05). Os extrusados produzidos com farinhas de milho QPM amarelas apresenta-
ram-se mais claros do que as farinhas que lhes deram origem (P<0.05), diferentemente das
amostras de milho normal. Resultado semelhante foi observado para o índice de cor vermelha
(valores a) para os extrusados. No entanto, extrusados brancos ou amarelos apresentaram
maiores valores b (amarelo) do que as farinhas correspondentes. Apesar do efeito adverso da
extrusão sobre a retenção de alguns aminoácidos essenciais, o uso de farinhas de milho QPM
em substituição à farinha de milho normal pode conferir melhor qualidade protéica a extrusados
de milho.
Palavras-chaves: milho, alta qualidade protéica, extrusão, aminoácidos essenciais, cor.
QPM (Quality Protein Maize), a specialty
acids, mainly that of lysine and a reducing sugar
maize with increased levels of the essential amino
(Maga, 1989). The reaction results in the formation
acids lysine and tryptophan (Paes & Bicudo, 1995),
of colorful compounds, which confer either desirable
has been produced in developing countries as a
or undesirable color properties to foods (Friedman,
strategy to improve protein quality of maize-based
1996). The rate of the reaction is highly dependent
products and help reduce protein-energy
on reducing sugar content, free amino groups, and
malnutrition. Among different processing techniques
also on temperature and water activity (Harper,
used to process shelf-stable and desirable maize
1989). Low moisture content, high temperatures and
products, extrusion is of major importance. Ready–
high shear during extrusion have been shown to
to-eat breakfast cereals, snack foods, dry pet food,
promote starch degradation with sugar production
pre-gelatinized flours and instant maize flours are
that may favor the Maillard reaction (Gomez &
examples of maize based products manufactured
Aguilera, 1983; Gomez & Aguilera, 1984). Despite
through extrusion (Riaz, 1997). However, studies
the importance of extrusion on protein quality of raw
previously carried out to evaluate the effect of this
materials, there is a lack of information in the extrusion
technique on nutritive value of foods have shown that
effect on essential amino acids present in Quality
extrusion can adversely affect the retention of amino
Protein Maize (QPM). Therefore, this research
acids, therefore, reducing protein quality (Dahlin,
aimed: 1) to assess the effect of extrusion on amino
1991). Noguchi et al. (1982) registered losses
acid content of whole-grain flours of QPM and nor-
varying from 0 to 40% of reactive lysine in extruded
mal maize extruded at the same processing conditions,
soy protein-enriched starch biscuits. Also, Bjorck
and 2) to compare the essential amino acid contents
and coworkers (1993) observed significant reduction
and color of extruded QPM and normal maize whole-
of lysine content when a mixture of wheat/maize
grain flours.
(130g/kg moisture) was extruded at 170oC and 80
RPM. Similar to these findings, Martinez et al.
Materials and Methods
(1996) reported decrease in lysine content in maize
Sample identification
grits due to extrusion. Reduction in lysine upon
Kernels of the two Brazilian open-pollinated
extrusion has been attributed to nonenzymatic
semi-flint QPM maize cultivars, the white BR 451
browning or the Maillard reaction, which involves
and the yellow BR 473, and the QPM semi-flint
free amino groups of protein, peptides or amino
maize hybrid, the yellow BR 2121, were obtained
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12
Paes & Maga
from the Brazilian Agricultural Research Corporation,
mesh screen by using a UDY cyclone laboratory mill
Embrapa Maize and Sorghum, Sete Lagoas, Minas
(UDY Corp., Fort Collins, CO. Tryptophan content
Gerais State, Brazil. Grains were produced during
was determined after protein hydrolysis with 4.2N
the 1996 growing season on experimental plots
NaOH followed by separation in the Pico•TAG?
located in Sete Lagoas, MG, Brazil. The yellow dent
system with detection at 280nm according to the
normal maize, Pioneer 3779, was provided by the
procedure described by Allred and MacDonald
Colorado State University Soil and Crop Sciences
(1988). Methionine was determined according to
Department. Whole grain flours were produced
MacDonald et al. (1985) through performic acid
through milling of the whole kernels using a Thomas-
oxidation and acid hydrolysis. The modified Pico
Willey Laboratory Mill, model 4 (Arthur M. Thomas
•TAG? method (Cohen et al. 1989) was used to
Co., Philadelphia, PA) coupled with a 2mm-mesh
analyze the content of the additional amino acids.
screen.
Samples were hydrolyzed with 6N HCl for 24 hr
Proximate composition
under nitrogen atmosphere at 110oC followed by
Unprocessed maize flours were analyzed for
derivatization with methanol, triethylamine and
moisture, crude fat, ash, protein (N x 6.25) and total
phenylisothiocyanate (PITC). Resulting
dietary fiber using standard procedures described
phenylthiocarbamate (PTC) amino acids were
by AACC (1995). Total carbohydrate was
vacuum-dried and re-suspended with Pico •TAG?
calculated by difference. Results were expressed on
diluent. Aliquots (20µl) of the diluted samples were
a dry basis.
injected in the system. Detection was done at 254nm.
The standard sample used for calibration and analysis
Sample preparation for extrusion
was treated the same as the samples, except that
Following initial proximate analysis, two
only 8µl was injected. The mobile phase was Pico
1,000 g replicates of each maize flour were adjusted
•TAG? eluents A and B added to (Na) EDTA. The
to 150g/kg moisture by adding the appropriate
2
column was set at 35oC.
calculated volume of distilled water at 20oC.
Uniformity was obtained by mixing the material
Extrusion conditions
thoroughly for 15 minutes. Samples were allowed to
A Brabender Plasticorder single-screw
equilibrate for 24 hours at 10oC in sealed plastic bags.
extruder, Model PL-V500 (C.W. Brabender
Moisture content was determined prior to processing
Instruments, Inc., South Hackensack, NJ) with a
19.05mm barrel diameter, a 20:1 length to diameter
to ensure adequate moisture content.
ratio and eight 0.79x 3.18mm longitudinal grooves
Amino acid determination
was used for processing. A screw compression ratio
Whole flours of various maize genotypes and
of 3:1 along with die nozzles diameters of 3mm and
their extrudates were analyzed for the amino acids
5mm, were also applied. Runs were performed at
histidine, leucine, isoleucine, lysine, methionine,
130oC die head temperature, while keeping the screw
phenylalanine, threonine, tryptophan, valine, tyrosine
speed at 80 rpm. Temperature was maintained with
and tryptophan using a reverse phase high
electrically heated, compressed air-cooled collars
performance liquid chromatograph Pico •TAG?
around the barrel. The extruder was fed full and the
amino acid analysis system (Waters Corporation,
order of processing was chosen at random. Each
Milford, MA). Prior to the analyses samples of raw
extrusion run was brought to steady state as indicated
and extruded samples were ground through a 0.5mm
by constant torque before sampling and data
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Effect of extrusion on essential amino acids ...
13
collection. Resulting extrudates were dried for 48
and BR 451 flours having the highest and lowest
hours at room temperature (~20-210c and 30%
values, respectively. Carbohydrate content was
relative humidity) before being evaluated for amino
significantly lower for BR 2121 (736.4 g/Kg)
acids and color.
compared to the other QPM samples and the nor-
Color
mal maize flours. In other QPM samples,
Color was determined using a Chroma Me-
carbohydrate content was significantly different from
ter model CR 310 (Minolta) set on the Hunter L, a,
that for normal maize flour and ranged from 756.5
b values scale. The equipment was standardized with
to 787.9 g/Kg. Pioneer 3779 flour had the lowest
a white square, which had standards of L = 97.67,
protein concentration (69.9 g/Kg). BR 451 (92.0 g/
a = +0.03 and b = +1.63. For raw material samples,
Kg) and BR 2121 (90.6 g/Kg) did not differ in
twenty grams of flour were placed into a 0.5cm
protein content, but they were significantly higher
diameter translucent plastic petri dish that was
compared to BR 473 (86.6 g/Kg) and Pioneer 3779.
covered and coupled to the equipment in such a way
Similar to protein content, percent crude fat was
that no external light was allowed. For extrudates,
significantly higher for the QPM flours than for Pioneer
color was determined by using small dry pieces of
3779 flour. Significant differences for crude fat were
each sample placed into the whole internal surface
not observed between BR 451 (46.3 g/Kg) and BR
of a plastic petri dish. Three readings were taken for
2121 (52.9 g/Kg) flours, where values were lower
each replicate sample by rotating the petri dish and
than that obtained for the flour of BR 473 (61.6g/
the average used for means comparisons.
Kg). The whole-grain maize flours did not differ in
ash composition. However, dietary fiber
Experimental design and data analysis
concentration was significantly different among
The experimental design was a completely
samples, respectively, 83.2 g/Kg, 91.6 g/Kg, 98.5g/
randomized design with treatments organized as a
Kg and 106.1g/Kg for BR 473, BR 451, Pioneer
two by two factorial. The two factors consisted of
3779, and BR 2121 flours. Although QPM cultivars
four levels for maize cultivars (BR 451, BR 473, BR
were semi-flint maize types, their chemical
2121 and Pioneer 3779) and two levels for die
composition most closely resembled the data
diameter (3 and 5mm). The experimental unit was
previously published for normal dent maize (Watson,
the extrusion at fixed moisture, die head temperature
1987).
and screw speed. Dependent variables consisted of
essential amino acids and L, a and b Hunter values.
Extrusion effect on essential amino acids
Data were analyzed applying ANOVA, using the
Extrusion conditions significantly affected the
Statistical Analysis System (SAS) 6.12 (SAS
essential amino acid content of the whole-grain maize
Institute, Cary, NC). Actual means separation was
flours. However, the interaction of treatment (raw/
performed using the Least Significant Differences
extrusion through 3mm die/extrusion through 5mm
(LSD) test.
die) with cultivar had no significance in amino acid
content (P>0.05), which implies that essential amino
Results and Discussion
acid content of the whole-grain flours of various
Proximate composition
cultivars was affected by extrusion through different
The proximate composition for whole-grain
dies in a similar way. Essential amino acids least
flours is presented in Table 1. Moisture content
square means for combined raw maize flours and
ranged from 91.5 to 118.8 g/Kg, with Pioneer 3779
their extrudates are shown in Table 2. Extrusion
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Paes & Maga
TABLE 1. Proximate composition of raw whole-grain flours of normal and QPM maize cultivars (g/Kg).
*db= dry basis; ** Values are the average of two replicates; *** Means with same superscripts in a column within the
same group are not significantly different (P<0.05).
1calculated by difference.
TABLE 2. Essential amino acid content (mg/g protein) in raw whole-grain flours and their extrudates combined
for maize cultivars.
*Means followed by the same superscript in a row are not significantly different (P<0.05).
adversely affected the contents of the amino acids
that the low shear rate and low residence time
isoleucine, leucine, lysine, threonine and valine in
resulting from the usage of a 5mm die played a
maize flours (P<0.05), but it did not cause any
protective role in leucine destruction. The extrusion
alteration in the contents of methionine, phenylalanine,
effect on amino acid retention found in this study is in
tryptophan and histidine (P>0.05). When the effect
agreement with other findings reported for potato
of die diameter on essential amino acid content was
flakes (Maga and Sizer, 1979), extruded wheat flour
evaluated, only leucine was significantly affected. Its
(Bjorck et al., 1984) and extruded maize meal
content was lower in extrudates obtained with a 3mm
(Beaufrand et al., 1978). Among essential amino
die (120.9 mg aa/g protein) compared to the 5mm
acids which presented losses, lysine was the most
die (122.8 mg aa/g protein) (P<0.05). It is possible
affected. Moreover, valine also underwent a high
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Effect of extrusion on essential amino acids ...
15
percent loss. Lysine loss was in accordance with
Considering the higher a and b values observed for
results reported by other authors for different cere-
the extrudates as compared to raw flours (Tables 5
al-based products. Bjorck et al. (1984) reported a
and 6), it might be possible that nonenzymatic
lysine loss of 25 and 37g/Kg for wheat flour extruded
browning reactions occurred during the extrusion of
respectively at 150 and 200 rpm, applying 171oC
whole-grain maize flours, affecting the retention of
and 15g/Kg moisture in a twin screw-extruder. Also,
lysine in the extrudates. Lorenz et al. (1974) reported
Bjorck et al. (1985) observed a 13g/Kg lysine loss
a correlation between color measurements and lysine
when a maize starch biscuit dough was extruded in a
content in extruded triticale.
twin-screw extruder at 170oC/13g/Kg moisture/
Cultivar was a statistically significant factor
80rpm. In agreement with the previous results,
influencing essential amino acid composition of maize
Lasekan et al. (1996) described a 10g/Kg lysine
(P<0.01), even though the interaction treatment
loss in whole-grain maize flour extruded in a twin-
versus cultivar had no significance (P>0.05).
screw extruder at 45g/Kg feed moisture/1350C
Therefore, means of raw and extruded whole-grain
temperature. All authors have attributed the Maillard
maize flours (small and 5mm die) for each amino acid
reaction as the main cause for lysine reduction.
were pooled and the resulting means were compared
Perhaps, extrusion at low moisture and high
among cultivars. The results are shown in Table 3.
temperature led to starch degradation, thus providing
Isoleucine, leucine, phenylalanine and valine contents
reducing sugars (Pham and Del Rosario, 1984) at
were not statistically different among cultivars
the same time that it modifies protein structure,
(P>0.05). However, all QPM exhibited higher lysine
exposing reactive sites, which favors browning
and tryptophan contents compared to the normal
reactions (Mauron, 1990). Since the lysine ?-amino maize, regardless of the extrusion effect on these
group has been referred to as a major reactant in the
essential amino acids. Lysine, the first reported limiting
Maillard reaction (Mauron, 1990), it might explain
amino acid in normal maize (Paes and Bicudo, 1995),
the extrusion effect on this particular amino acid.
was significantly lower in BR 473 compared to BR
TABLE 3. Essential amino acids in QPM and normal maize combined raw whole flours and extruded materials.
* Values are means of combined raw and extruded samples, which were the average of two replicates.
** Means followed by same letter in a row are not statistically different (P<0.05).
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Paes & Maga
2121, whose content of this amino acid did not differ
Extruded QPM samples were lighter in color
from BR 451. However, tryptophan, the second
compared to their raw flours (P<0.05), whereas the
reported limiting amino acid in normal maize, was
normal maize extrudate was darker than its
significantly higher in BR 2121 than in the other two
correspondent raw flour (P<0.05). Yellow QPM
QPM cultivars. Histidine was higher in Pioneer 3779
extrudates of BR 2121 and BR 473 were significantly
and BR 2121 compared to BR 473. But, the BR
lighter than the normal yellow maize extrudates
451 and BR 473 did not statistically differ in this
(P<0.05). The results were reversed for the raw
amino acid. BR 473 and BR 2121 presented more
materials (Table 4). Similar fading of color was
methionine compared to Pioneer 3779, which
observed on Hopi blue maize whole-grain flour
content did not differ from that of BR 451. Threonine
extruded under temperatures above 120oC, when
was greater in Pioneer 3779 and BR 451 compared
feed moisture content was 150 g/Kg (Maga and Liu,
to BR 473 and BR 2121.
1993). The fading was attributed to the partial
Color
destruction of pigments upon processing, resulting in
L (Lightness)
extrudates with higher L values. Perhaps, the
The term lightness or brightness refers to the
expansion of the dough upon exit the die has
relationship between reflected and absorbed light,
contributed to increase in the surface area and favored
regardless of specific wavelength. The values range
light dispersion, giving a lighter appearance to the
from 0 (black) to 100 (white), (Pomeranz and
puffed products.
Meloan, 1994). Extrusion processing adversely
a (Redness)
affected L values of QPM and normal maize whole-
The a value or redness in a Hunter scale
grain flours, but die size was not a significant factor
determines green, gray and red color of a product.
for this color property for all studied samples
Positive a value refers to red, negative to green, and
(P>0.05). Due to this fact, the pooled means for L
values near zero indicate gray (Pomeranz and
values were obtained, and statistically analyzed. The
Meloan, 1994). Both raw whole-grain maize flours
resulting least square means of each cultivar is
and their correspondent extrudates had positive a
presented in Table 4, as well as the L value means
values (Table 5). However, extrusion under 15g/Kg
obtained from the raw whole-grain maize flours.
moisture/130oC die head temperature, for any die
TABLE 4. Hunter L value (lightness) in different raw whole-grain flours of various maize cultivars and their
extrudates.
*Values are the average of two replicates and represent the extrusion condition 150g/Kg moisture/130oC
** Pooled least square means of 3mm and 5mm die diameters.
*** Means followed by the same lower case superscript in a column within a group are not significantly different P<0.05
**** Means followed by the same capital superscript in a row are not significantly different P<0.05.
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Effect of extrusion on essential amino acids ...
17
diameters, caused reduction in the redness values of
the increases in a value due to the possible shorter
the yellow QPM whole-grain flours (P<0.01).
residence time, less shear, and lower pressure
Conversely, the processing did not significantly alter
associated with faster dough flow of a larger outlet
the a value of the BR 451 white flour. A low a value
die. Low shear results in less energy dissipation and
might be caused by temperature and shear effect on
thus less heat generated within the barrel. Besides,
pigments present in yellow maize, since a value of
reduced residence time implies less exposure to
the white color BR 451 flour was not affected by
heating (Miller, 1990), thus reducing the chances for
extrusion (P>0.05). Maga (1989) mentioned that
Maillard reaction to occur.
heat applied in the raw material during extrusion might
b (Yellowness)
damage anthocyanins, the pigments naturally present
Extrusion significantly altered the yellowness
in maize kernels. Also, Marty and Berset (1986)
of the maize flours (P<0.05) (Table 6). For all
demonstrated that extrusion cooking severely
cultivars, the b values were significantly higher for
affected beta-carotene, the major carotene in maize.
the extrudates (P<0.05) than for the raw maize flours,
Moreover, die diameter significantly affected BR
which indicates more yellow products (Table 6). Die
2121 QPM extrudate redness. For the extruded
diameter was not a cause of variation in the
samples obtained with a 3mm die, the average a
yellowness (P>0.05) of same QPM flour. Extruded
value was significantly higher (2.26) than that
yellow QPM flours, BR 473 and BR 2121, within
observed (1.33) for extrudates resulting of a 5mm
the 3mm die group, were not significantly different
die (P<0.05) (Table 5). It is possible that extrusion
with regard to b value compared to the normal maize
of BR 2121 flour through a 3mm die opening, which
(Pioneer 3779) extrudate (P>0.05). In contrast, the
is related to high shear, high dough temperature and
BR 451 extrudate had a significantly lower b value
longer residence time (Miller, 1990), promoted
than the other three materials in the same die group.
formation of compounds, such as those originated
This result was expected since BR 451 QPM flour
by the Maillard reaction (Friedman, 1996), which
was white. Extrudates of QPM yellow cultivars
might have produced red color in the extrudate,
obtained through a larger die diameter had higher b
despite of possible destruction of the BR 2121 flour
values as compared to those of the normal maize
natural pigments. In contrast, extrusion of the BR
extrudates (P<0.05), which also had a higher b value
2121 flour through a 5mm die have protected against
(P<0.05) than BR 451.
TABLE 5. Hunter a value (redness) of raw whole-grain maize flours of various cultivars and their extrudates.
* Values are the average of two replicates
** Means followed by same superscript in the column within a group are not significant different P<0.05
*** Means followed by the same capital superscript in a row are not significant different P<0.01
1 150g/Kg moisture/130oC extrusion temperature, except as indicated.
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Paes & Maga
TABLE 6. Hunter b value (yellowness) of raw whole-grain maize flours of various cultivars and their extrudates.
* Values are the average of two replicates
** Means followed by same lower case superscript in a column within a group are not significantly different P<0.05
*** Means followed by the same capital superscript within a row are not significantly different P<0.05.
1 Values represent the extrusion conditions 150g/Kg moisture/130oC temperature.
Conclusion
procede de cuison extrusion sur la disponibillite des
Extrusion adversely affected the contents of
proteines. Annales de la Nutrition et de
essential amino acid present in QPM and normal
l’alimentation, Paris, v. 32, p. 365-375, 1978.
maize whole-grain flours, although the improved
BJORCK, I.; NOGUCHI, A.; ASP, N. G.;
essential amino acid profile of QPM is maintained
CHEFTEL, J. C.; DAHLQVIST, A. Protein
even after processing. Therefore, the use of whole-
nutritional value of biscuit processed by extrusion-
grain QPM flours in replacement of normal maize
cooking: Effects on available lysine. Journal of
flours can provide extrudates with superior protein
Agricultural and Food Chemistry, Washington, v.
quality. But, attention shall be given to color
31, p. 488-492, 1983.
modification of yellow maize flours processed through
extrusion since the process affects lightness, redness
BJORCK, I.; ASP, N. G.; DAHLQVIST, A. Protein
and yellowness.
nutritional value of extrusion-cooked wheat flours.
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INTERNATIONAL SYMPOSIUM ON
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