Vol. 19, No. 1: 24–30
Czech J. Food Sci.
Changes during the Extrusion of Semolina in Mixture with Sugars
AMR FAROUK MANSOUR, FRANTI?EK PUDIL, VÁCLAV JANDA and JAN POKORNÝ
Institute of Chemical Technology ? Department of Food Chemistry and Analysis, Prague, Czech Republic
FAROUK MANSOUR A., PUDIL F., JANDA V., POKORNÝ J. (2001): Changes during the extrusion of semolina in mixture
with sugars. Czech J. Food Sci., 19: 24?30.
Wheat semolina and its mixtures with 5% glucose, fructose of sucrose were processed in a sigle screw extruder at the maximum
temperature of 140°C and the processing time of 30 s. The nonenzymic browning was only moderate, but it was substantially
more intensive in mixtures with glucose or fructose than in the case of wheat semolina or its mixture with sucrose. Red and
yellow pigments were mainly formed. The odour acceptability was affected by the presence of sugars almost negligibly, but the
intensities were different, higher in extruded mixtures with glucose and fructose than in wheat semolina or its mixture with
sucrose. Small differences were observed in the sensory profile. Extrusion of semolina with sugars produced more sensory active
volatiles (52–69 identified compounds) than in extruded semolina (41 compounds). Pyrazines, furans and pyrans were the most
important sensory active compounds. Their amounts increased by the addition of sugars to semolina; the mixture of semolina
with glucose was particularly rich in active compounds. The formation of pyrazines was more enhanced by the addition of
fructose than of other sugars. Maltol, butyrolactone and acetic acid were present in large amounts. Even if sensory characteristics
were improved by addition of sugars to semolina, the difference was not very pronounced.
Keywords: browning; extrusion; furans; fructose; glucose; pyrazines; semolina; sensory value; sucrose; wheat
Extrusion cooking has become a favourite alternative
1992), in order to improve sensory and functional prop-
to baking, particularly for the technological processing
erties of the product. Extruded rice flakes are fully ac-
of cereals and other starchy foods into a variety of food
ceptable for breakfast cereals (VISSESURAKARN et al.
products (HUBER 1991). The main advantage is short pro-
cessing time, low energy input, and lower costs, compared
A disadvantage of extruded products is their weaker
aroma and flavour intensity than that of bread crust or of
Naturally, wheat flour, semolina or grits are, the most
other traditional bakery products. Therefore, the enhance-
widely used cereal raw material, and various extruded
ment of reactions resulting in the formation of sensory
products may be obtained with good reproducibility as
active substances is very important. Pyrolysis of proteins,
the process is continuous (LUNDGREN et al. 1991/2). Trit-
caramelisation of sugars, Maillard and related reactions
icale was also found advantageous for extruded products
should be taken into account (OSNABRUGGE 1981). Fu-
because of interesting flavour notes formed, compared
ran derivatives and maltol originate from sugar degrada-
with wheat (PFANNHAUSER 1993). The process was op-
tion during extrusion cooking (RIHA & HO 1996), which
timised for whole triticale kernels (LORENZ et al. 1974).
are similar to those found in bread aroma (ROTHE 1974).
Uncooked or pregelled starches (or starch-rich wheat frac-
They are present in large amounts, but nitrogen-contain-
tions obtained by air classification) can be also used in-
ing heterocycles are more important because of their very
stead of flour, especially for snack foods (FELDBERG
low perception thresholds (BOELENS & HEYDEL 1973).
Pyrazines were isolated from bread crust (ROTHE 1974)
Another important raw material for extrusion cooking
and bread crumb (SCHIEBERLE & GROSCH 1984). They
is corn meal, which is transformed into extrudates of in-
are also important in other extruded cereal products, such
teresting flavour character (CHEN et al. 1991). However,
as corn tortillas (KARAHADIAN & JOHNSON 1993; BUT-
the main advantage of corn processing is time, energy and
TERY & LING 1995). Their most probable pathway of
cost saving, similarly as in wheat products (MIDDEN
formation are interactions of Maillard and Strecker deg-
1989). Corn meal can be mixed with other ingredients,
radation products (RIHA & HO 1996; PFANNHAUSER
such as amaranth or rice (JOTOVALLOTOVAL & SEIBEL
1993; YOO & HO 1997). Their precursors are mainly free
Czech J. Food Sci.
Vol. 19, No. 1: 24–30
amino acids (HWANG et al. 1995), but they are also pro-
Table 1. Conditions of extrusion cooking
duced by interactions of saccharides with protein, for in-
stance in the extrusion cooking of potato flakes (MAGA
& SIZER 1979). Another pathway is probably based on
the formation of dihydropyrazines, and their subsequent
Single screw collet
oxidation to pyrazines via the respective free radicals (MI-
extruder VUMPP 83
LIÆ et al. 1980).
Type of screw
The aroma intensity of extruded products may be in-
Distance between flights
creased by the fortification of the material to be extruded
with reducing sugars and amino acids, which are the most
important precursors of Maillard reactions. Honey belongs
to highly appreciated sources of reducing sugars. Addi-
only in the feed zone
tion of honey to whole meal increased the sensory value of
extruded breakfast cereals (NEUMANN & CHAMBERS
Maximum extrusion temperature
1993). Malt, which contains low molecular-weight hydro-
lytic products of starch, is another suitable ingredient, in-
creasing the formation of pyrazines (FORS 1987). Maltose,
glucose or fructose syrups would be a good choice, too.
In this paper we compare the effect of glucose, fructose
and sucrose on the flavour enhancement of extruded wheat
perature: 220°C; column: Supelco 60 m × 0.32 mm, coat-
MATERIAL AND METHODS
ed with Supelcowax 10, film thickness: 0.25 µm (Supel-
co, Bellefonte, USA); column temperature programming:
Material: Wheat semolina type T 600 (following the
50°C for 2 min, isothermal, then heating by 2 K/min to
Czechoslovak standard – ÈSN) was produced in agrement
220°C, and isothermally for 30 min at 220°C; carrier gas:
with the respective national standard; it contained 12.4%
helium; initial pressure 100 kPa; the inject:split ratio is
moisture, 0.4% ash, 8.6% protein and 0.3% fat. Sucrose,
1:25; FID detector; detector temperature 220°C.
D-glucose and D-fructose were purchased from Sigma-
Mass Spectrometric Detection: For GC with mass spec-
Aldrich (St. Louis, MN, USA). Sugars were ground sep-
trometric detection (GC/MS), the Fisons MSD 8000 mass
arately in a Moulinex mill (Moulinex, Paris, France), and
spectrometer was used; the Manual SPME Injection Meth-
5.0 kg of wheat semolina was then blended with 250 g of
od was applied; the ionizing energy was 70 eV. Identifi-
finely ground saccharides in a drum-type two-roller mill
cations were based on the comparison with the MS
(Romil, Brno, CR), provided with grooving rolls. The
Computer Library (NIST-Masslab-Software Package,
material was air dried (20°C, 50% relative humidity) be-
Fisons, Milan, Italy), and on the respective retention in-
fore the extrusion.
Extrusion Process: The material (5.0 kg of semolina
Determination of Browning Degree: Samples of ex-
and 0.25 kg of the respective saccharide) were thorough-
trudates were finely ground for 30 s in a Moulinex pro-
ly mixed, and then fed into the extruder. The samples were
cessor (Moulinex, Paris, France). The colour hue of the
extruded under the conditions summarized in Table 1, in
ground samples was measured using a CCD Fiber Optic
a pilot plant-scale equipment, manufactured by the Re-
Spectrometer S 200 (manufactured by Ocean Optic, Ltd.,
search Institute of Milling and Baking Industry, Machin-
Dunedin, FL, USA). The reflected light was measured at
ery Department (Prague, CR). The extruded samples were
the constant luminance (L*) of 50%. The colour hue was
crushed immediately, and stored at room temperature in
expressed in the CIE-LAB space, using a modular spec-
ground glass bottles.
troscopy system in the reflectance mode. A standard light
Extraction of Volatiles: The Solid Phase MicroExtrac-
source D 65 after Comité International de l’Eclairage (CIE)
tion (SPME) procedure was used. A 65 µm Carbowax™
was used, corresponding to the sun surface (6500 K). The
divinylbenzene fibre for Manual Holder (Red label) was
reflection assembly probe consisted of 7 fibres in a fer-
produced by Supelco (Bellefonte, USA). The extraction
rule. The results were processed by the software Spec-
time was 1 h at 85°C, and the desorption time was 2 min
trawin Version 3.1, and the following CIE-LAB parameters
at 220°C. The fibre was cleaned for 30 min at 220°C be-
were determined: the a* value – redness (positive) to green-
fore extracting the next sample.
ness (negative); the b* value – yellowness (positive) to
Gas Chromatography: For gas chromatography (GC),
blueness (negative); the hue angle: h* = tan–1 b*/a*;
an apparatus GC 8000 (Fisons Instruments, Milan, Italy)
the chroma: c* = (a*2 + b*2)0.5; the hue difference ?E =
was equipped with an autosampler HS 8000; injection tem-
(?a*2 + ?b*2 + ?L*2)0.5.
Vol. 19, No. 1: 24–30
Czech J. Food Sci.
Sensory Analysis: Conditions for the sensory analysis
Table 2. Colour parameters of extruded samples
were in agreement with specifications of the international
standard (ISO 6658-1985: Sensory analysis – Methodol-
Characteristic Sample A Sample B Sample C Sample D
ogy – General guidance); test rooms (ISO 8589-1989:
Sensory analysis – General guidance for the design of test
rooms); selected, trained and monitored assessors (ISO
intensity [mm] 17.2
8586-1991: Sensory analysis – General guidance for the
selection, training and monitoring of assessors, Part 1:
Selected assessors); unstructured graphical scales (ISO
4121-1978: Sensory analysis – Grading of food products
by methods using scale categories) were represented by
straight lines 70 mm long, provided with descriptions on
either end for assessor’s orientation (intensity of brown
colouration: 0 mm = very weak, 70 mm very strong; odour
acceptability: 0 mm = very disagreeable, 70 mm = very
to non-reducing sugars, and may react only after hydro-
agreeable; odour intensity: 0 mm = very weak, 70 mm =
lytical cleavage into a mixture of glucose and fructose.
very strong; texture acceptability, as assessed by pressing
The browning of mixtures of semolina with reducing sug-
small amount of coarse extruded grits between fingers:
ars, such as glucose or fructose (samples B and C, re-
0 mm = pasty, 70 mm = very crispy). The sensory profile
spectively) was substantially more intensive. The same
(ISO 6564-1995: Sensory analysis – Flavour profile) was
applies of different CIE-LAB characteristics. The pro-
based on free choice profiling, and 32 descriptors were
nounced increase was observed after the addition of glu-
originally collected, which were compressed into 10 more
cose or fructose to semolina, slightly more intensive in
complex descriptors: 1 = roasted, bread crust, roasted pea-
mixture with glucose than in a mixture with fructose. It
nuts, gingerbread with honey; 2 = burnt, caramel, bitter;
may be due to differences in reaction rates of browning
3 = woody, bark, resins; 4 = pasty, stored flour, stale, bread
intermediates between Amadori (POKORNÝ et al. 1988)
crumb; 5 = spicy, onion, garlic, sulphuric; 6 = sharp, pun-
and Heyns (PILKOVÁ et al. 1990) rearrangements. The
gent, burning; 7 = fatty, oily, buttery; 8 = earthy, musty,
increase of a* and b* corresponds to the increase in red-
mouldy, sweat, wet dog; 8 = malty, cocoa, sweet; 9 = sol-
ness and yellowness, respectively, which occurs in the be-
vents, synthetic, chemicals; 10 = others – specify, which;
ginning of nonenzymic browning. Products with more
in the odour profile evaluation, unstructured graphical
intensive green and blue colour notes are formed only
scales were again used: 0 mm = odour note absent, im-
later, as a result of secondary reactions. They cannot oc-
perceptible, 70 mm = very strong. The colour intensity
cur in course of very short extrusion time of 30 s, and at
(degree of browning) was assessed under the standard light
relatively low temperatures (120–140°C).
source C (specified by CIE, corresponding to the spectrum
Odour (Aroma) Evaluation of Extruded Products: The
of sun surface = 6500 K). Odour profiles were tested by
odour acceptability was not significantly affected by the
sniffing from a ground wide-neck glass bottle, 250 ml vol-
presence of sugars in the extruded mixture, added before
ume. The results are based on 20 responses (standard devi-
ations of the means were 3–6% of the graphical scale).
Table 3. Sensory odour and texture evaluation of extruded sam-
ples (values are expressed in mm of the graphical scale)
RESULTS AND DISCUSSION
The following extruded samples were prepared, and
A = extruded semolina;
B = extruded mixture of semolina + 5 % D-glucose;
C = extruded mixture of semolina + 5 % D-fructose;
D = extruded mixture of semolina + 5 % sucrose.
Sensory odour profile
All samples were analysed within 2–3 weeks after prep-
Roasted, bread crust
aration, and were stored in a refrigerator in ground-glass
bottles in the meantime.
Pasty, bread crumb
Evaluation of Browning Degree: The effects of sugars
on the browning degree during the extrusion are evident
from Table 2. The degree of browning was small in the
case of semolina without sugar additions (sample A) or
with the addition of sucrose (sample D), which belongs
Sharp, sweat, pungent
Czech J. Food Sci.
Vol. 19, No. 1: 24–30
the extrusion, obviously because of similar roasted char-
Table 4. Number of substances identified in the volatile frac-
acter, which is perceived as the main, and very positive
sensory attribute (Table 3). The odour intensity was some-
what higher in extruded mixtures of semolina with reduc-
ing sugars, whereas no effect was observed in the case of
a mixture with sucrose.
The texture, expressed as crispness, varied only a little
so that insignificant differences were observed by senso-
ry analysis. It is an advantage for the comparison of sam-
ples, as low crispness has an adverse effect on the sensory
quality not only of texture, but also of aroma and flavour
(STREJÈEK et al. 1993).
The flavour profile depended on sugars added to the
extruded mixture. The presence of reducing sugars (sam-
ples B and C) increased the roasted character, typical for
bread crust, but sucrose (sample D) had no effect. All the
three sugars increased the intensity of caramel-like odour
note, which is similar to burnt odour (Table 3). At the
same time, the intensity of fatty, buttery odour note in-
creased, too. Solvent, chemical or malty odour notes were
ing. In all samples, furans and pyrazines belonged to the
so weak (less than 5 mm of the graphical scale in the av-
most numerous substances.
erage), that they have not been included in the Table 3.
The total number of compounds was higher in extruded
Large differences were observed in the intensity of a fla-
mixtures with sugars (samples B, C and D, Figs. 2–4, re-
vour note after stored flour, paste or bread crumb.
spectively) than in extruded semolina, including furans
Volatile Sensory Active Compounds in Extruded
and pyrazines. From this respect, the composition of vol-
Samples: Examples of gas chromatographic analysis of
atiles was similar to that of bread crust, even when the
volatiles from the samples A–D are shown in Figs. 1–4,
overall intensity was weaker. At very high temperatures
and higher content of precursors, the number of furan
The list of detected and identified compounds of the
derivatives was reported to be much higher (BALTES &
volatile fractions is shown in Table 4. Only 41 compounds
BOCHMANN 1987a). During bread baking, the tempera-
were identified in extruded semolina without any addi-
ture and duration of heating are substantially higher than
tions (sample A, Fig. 1); of course, much higher number
during extrusion cooking. The pyrrole and pyridine frac-
of compounds would be produced by thermolysis at higher
tions therefore become more important in bread crust. No
temperature (ANDREJS et al. 1995), such as during bak-
pyridines were detected in our extruded samples.
Fig. 1. Gas chromatogramme of volatiles isolated from extruded
Fig. 3. Gas chromatogramme of volatiles isolated from extruded
semolina (Sample A)
mixture of semolina with fructose (Sample C)
Axis y ? detector response [p. c. of the scale]; Axis x ? retention time [min]; the respective retention times are attached to individual peaks
Fig. 2. Gas chromatogramme of volatiles isolated from extruded
Fig. 4. Gas chromatogramme of volatiles isolated from extruded
mixture of semolina with glucose (Sample B)
mixture of semolina with sucrose (Sample D)
Vol. 19, No. 1: 24–30
Czech J. Food Sci.
Table 5. Contents of classes of sensory active volatiles (ex-
Table 6. Contents of individual pyrazines in extruded samples
pressed in p.c. of total peak areas)
(expressed in p.c. of peak areas of the total pyrazine fraction)
Volatile fatty acids
The quantitative distribution of different classes of com-
pounds is evident from Table 5. The content of pyrazines,
pyrroles and furans in the volatile fraction was increased
by the addition of sugars, which is not surprising in the
case of furans, as they are produced from sugars. The
content of other precursors, protein and free amino acids,
decreased following the addition of sugars, but the de-
crease was more than compensated by a higher concen-
tration of other precursors – namely the decomposition
products of sugars, so that the resulting fraction of pyra-
they were high molecular-weight compounds (10–12 car-
zines was much larger. Proteins, peptides and amino ac-
bon atoms). Fatty acids, mainly acetic acid, were also not
ids, present as natural components in semolina, were
substantial as flavour carriers. Hydrocarbons with 10–18
obviously sufficient for the formation of pyrazines. The
carbon atoms were present, but they cannot be consid-
amount of ammonia, necessary for the pyrazine forma-
ered as substances of major importance because of their
tion, can be liberated not only by the cleavage of the pep-
higher detection thresholds.
tide bond, but also by deamidation of glutamine and
In the discussion on the composition of volatiles it
asparagine bound in proteins (ZHANG 1994).
should be made clear that the results depend on the ex-
The amount of aldehydes and various dicarbonylic pre-
traction method. In the case of the SPME method it de-
cursors increased by decomposition of sugars, e.g. the
pends mainly on properties of the film on the surface of
addition of glucose enhanced the formation of 2,3-penta-
the fibre. The conclusions thus apply to the Carbowax
dione and 1-hydroxy-2-propanol, which became a major
fibre; the affinity of volatiles to smell receptors may be
product (6.4–7.6% total volatiles). Biacetyl was not de-
still different, anyway, so that the results should be al-
tected because of its high volatility, but 2,3-butandiol was
ways compared with sensory characteristics.
higher by 1–2% of total volatiles in extruded mixtures
The most sensory active group of extruded volatiles –
with sugars than in extruded semolina. The content of bu-
pyrazines, were influenced by sugar addition not only in
tyrolactone substantially increased, too. No maltol was
their total amount, but also in the composition. Relative
detected in extruded semolina (sample A), but it became
concentrations of individual pyrazines (expressed in p.c.
a major compound in extruded mixtures with sugars. Mal-
of peak area of total pyrazines) are shown in Table 6. The
tol is produced by thermolysis of Amadori compounds
composition of volatile pyrazines also depends on the in-
(YAYLAYAN et al. 1992). Naturally, there were much more
tensity of heating. Relatively low contents of trimethyl
abundant in extruded mixtures of semolina with sugars
pyrazine and 2,6-dimethyl-3-ethyl pyrazine were observed
(samples B–D). On the contrary, the content of benzalde-
in wheat bread crust, which was exposed to high temper-
hyde, which does not belong to Maillard products, was
atures over 200°C (SCHIEBERLE & GROSCH 1987),
nearly the same in all samples; it contributes to the mod-
whereas relatively high amounts were present in our ex-
erately bitter odour of extruded samples.
truded samples, heated to only 140°C. Pyrazines identi-
Alkanals detected among the volatiles could not have
fied in our volatiles mostly consisted of methyl and
much pronounced influence on the resulting flavour as
methyl-ethyl substituted derivatives. They were report-
Czech J. Food Sci.
Vol. 19, No. 1: 24–30
ed among pyrazines from crust of white American bread
FORS S.M. (1987): Isolation of pyrazines in extruded malt by
(SIZER et al. 1975). Traces of ethyl, vinyl and acetyl de-
stepwise extraction and distillation. Lebensm.-Wiss. Technol.,
rivatives were only detected in extruded samples contain-
ing sugars (samples B–D), whereas ethylmethyl and
FORS S.M., ERIKSSON C.E. (1986): Pyrazines in extruded malt.
methylacetyl derivatives were present even in extruded
J. Sci. Food Agric., 37: 991?1000.
semolina (sample A). It is in agreement with the forma-
HUBER G.R. (1991): Carbohydrates in extrusion processing.
tion of both 2-vinyl-5-methyl pyrazine and 2-vinyl-6-
Food Technol., 45: 160-161.
methyl pyrazine in extruded malt (FORS & ERICSSON
HWANG H.I., HARTMANN T.G., HO C.-T. (1995): Relative
1986), which also contains reducing sugars. Pyrrolopyra-
activities of amino acids in pyrazine formation. J. Agr. Food
zines and furanopyrazines were not found, although they
Chem., 43: 179?184.
were detected in roasted model samples (BALTES & BOCH-
JOTOVALLOTOVAL N.K., SEIBEL W. (1992): Studien über die
MANN 1987b), as in our extrusion experiments, the tem-
Herstellung von Teigwaren auf Reis- und Maisbasis. Getreide,
perature was too low and the content of free amino acids
Mehl Brot, 46: 149?154.
negligible. Cyclopentapyrazines were also not found from
KARAHADIAN C., JOHNSON K.A. (1993): Analysis of head-
the same reason.
space volatiles and sensory characteristics of fresh corn tor-
tillas made from fresh masa dough and spray-dried masa flour.
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LUNDGREN B., KARLSTRÖM B., TORRANG-LINDBOM G.,
and furans. However, the differences between extruded
ANDERSSON Y., CLAPPERTON J. (1991/2): Extruded wheat
semolina and extruded mixtures of semolina with sugars
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were not very pronounced.
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Acknowledgements: We are obliged to Mrs. HANA POSKO-
the extrusion of potato flakes. Lebensm.-Wiss. Technol., 12:
ÈILOVÁ, MSc., Goldim, Sobìslav, who enabled us to prepare
of extruded samples.
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Cereal Foods World, 34: 941?943.
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VISESSURAKARN S., DUANGMAL K., ITTHIWACHANA S.
Received for publication March 24, 2000
(1991): Use of broken rice for breakfast cereals. Food, 21:
Accepted for publication May 4, 2000
24?36; Abstract FSTA (12), M 070 (1991).
FAROUK MANSOUR A., PUDIL F., JANDA V., POKORNÝ J. (2001): Zmìny p?enièné krupice ve smìsi s cukry bìhem extruze.
Czech J. Food Sci., 19: 24?30.
P?enièná krupice a její smìsi s 5 % glukosy, fruktosy nebo sacharosy byly zpracovány na jedno?nekovém extrudéru pøi maximální
teplotì 140 °C a prodlevì 30 s. Za tìchto podmínek probìhlo neenzymové hnìdnutí v malé míøe u krupice nebo její smìsi se
sacharosou, znatelnìj?í bylo u smìsí s glukosou nebo fruktosou (vznikaly hlavnì ?lutì a? èervenì zbarvené produkty). Textura
byla u v?ech vzorkù podobná. Cukry mìly zanedbatelný vliv na pøíjemnost chuti, ale intenzita vùnì byla prùkaznì vy??í u smìsí
s glukosou nebo fruktosou ne? u smìsi se sacharosou nebo u krupice samotné. Byly také zji?tìny rozdíly v senzorických profilech
vùnì ? u extrudovaných produktù s cukry bylo prokázáno více tìkavých látek (52?69 identifikovaných) ne? u extrudované
krupice (41 látek). K senzoricky nejvýznamnìj?ím slouèeninám patøily pyraziny, furany a pyrany, jejich? poèet i mno?ství po
pøídavku cukrù vzrostly (po pøídavku glukosy hlavnì u furanù a fruktosy u pyrazinù). K dal?ím význaènì zastoupeným produktùm
patøil butyrolakton, maltol a kyselina octová. I pøes mírné zlep?ení senzorických charakteristik se vliv cukrù projevil málo
Klíèová slova: extruze; furany; fruktosa; glukosa; hnìdnutí; krupice; p?enice; pyraziny; sacharosa; senzorické vlastnosti
Prof. Ing. JAN POKORNÝ, DrSc., Vysoká ?kola chemicko-technologická, Ústav chemie a analýzy potravin, Technická 5,
166 28 Praha 6, Czech Republic; tel.: + 420 2 24 35 32 64; fax: + 420 2 311 99 90; e-mail: email@example.com