Li, Y.

Sopade, P. A.

University of Queensland eSpace

Kinetics of protein digestion and molecular weight profiles in a model 
sorghum-barley blend as affected by extrusion conditions 
Y. Li1 and P. A. Sopade1, 2, 3  
 
1School of Agriculture & Food Sciences, 2Centre for Nutrition & Food Sciences, University of Queensland, St. Lucia, QLD 4072 
3Correspondent (E-mail: p.sopade@uq.edu.au; Phone: 07 334 67653; Fax: 07 336 51177)  
Introduction 
Extruded cereals and ready-to-eat foods (RTE) are an integral 
part of the global diets (Ainsworth et al., 2007), and noted 
for their carbohydrate contents. However, their proteins are 
of interest too, particularly their digestibility. Sorghum, with 
its bioactives, is becoming popular as a non-gluten wheat 
substitute (Awika et al., 2005), and barley, with its β-glucans, 
offers nutritional and health benefits. Both cereals can be 
combined in composite products. Studies abound (Fapojuwo 
et al., 1987; Mahasukhonthachat et al., 2010) on the effects 
of extrusion on in vitro starch and protein digestion, but most 
concentrated on single cereals. Studies on cereal mixtures are 
relatively non-existent. In this study, the protein digestion of 
twin-screw extruded sorghum-barley was investigated from 
enzymatic and molecular standpoints. 
Research Objectives 
To investigate how extrusion moisture, screw speed, 
temperature, and feed rate affected protein digestibility, and 
to examine the molecular weight distributions of enzyme-
digested and non-digested extrudates. 
Result and Discussion 
In-vitro protein digestion 
• The extrudates and non-extrudates exhibited 
monophasic protein digestograms. A first order 
kinetic model {pHt = pHo + pH∞-o (1 – exp [-K t]); 
where pHt, pHo and pH∞ = pH at times t = t, t = 0 
and t→∞; K = rate of digestion} suitably 
described (r2>0.92; p<0.001) the digestograms.  
• The change in pH at infinite digestion (ΔpH∞ = 
pH∞-o) increased with extrusion moisture, and 
reduced with extrusion screw speed, while the 
rate of protein digestion essentially increased 
with these variables (Fig. 2). 
• In-vitro protein digestibility (IVPD) changed 
with extrusion conditions as in Fig. 3. 
• The per cent change in the MW of the 
largest proteins in the digested and non-
digested samples appeared to correlate 
(r2 = 0.41; p = 0.06)  with a measure 
(ΔpH∞ = pH∞-o) of the protein digestion 
(Fig. 5). The more the drop in pH as 
more proteins were digested, the higher 
was the change in MW. 
Conclusions 
Extrusion affected the protein digestibility of 
the 60:40 sorghum-barley blend. Changes in 
the MW of the proteins appeared to correlate 
with an index of protein digestion, but further 
studies are required to substantiate this. 
References 
 Ainsworth P, & others. 2007. J. Food Eng. 81, 702. 
 Awika JM, & others. 2005. J. Agric. Food Chem. 53, 6230. 
 Fapojuwo OO, & others. 1987. J. Food Sci. 52, 218. 
 Koa SS, & others. 2012. 62nd Aust. Cereal Chem. Conf. 26 – 
29 August. Gold Coast, QLD 4217. 
 Laemmli. U.K. & Favre. M. 1973. J. Mol. Bio. 80, 575. 
 Mahasukhonthachat K, & others. 2010. J. Cereal Sci. 51, 392. 
 Tinus T, & others. 2012. J. Food Eng. 113, 254. 
Fig. 1: Flow chart  
of the experiments 
Materials and Methods 
A mixture of 60% sorghum and 40% 
barley was extruded (Koa et al., 
2012) at different levels of moisture 
(20-40%), screw speed (150-300 
rpm), feed rate (2-5 kg/h), and 
temperature (110-160oC). Trypsin 
(T0303), chymotrypsin (C4129), 
protease (P5147), and casein (C7078) 
were obtained from Sigma-Aldrich, 
Australia, and used for the pH-drop 
method of in-vitro protein digestion 
(Tinus et al., 2012). Non-digested 
solids were freeze-dried (Fig. 1) and 
analysed for molecular weight 
distributions using the SDS-PAGE 
technique (Laemmli & Favre, 1973). 
EXTRUSION 
 
 
IN-VITRO PROTEIN 
DIGESTION 
 
 
CENTRIFUGATION 
 
 
FREEZE-DRY 
 
 
SDS-PAGE Molecular analysis of protein digestion 
• Extruded, non-extruded and digested samples 
revealed proteins (Fig. 4) with molecular weights 
from 2.3-116.5 kDa. 
Fig. 2: Effects of extrusion on pH∞-o and K 
p
H
∞
-o
 
K
 (
m
in
-1
) 
a 
a 
b A 
B B 
a 
a 
b 
A 
AB 
B 
Fig. 3. Effects of extrusion on IVPD 
Fig. 4: SDS-PAGE of typical samples 
STD        R          D-R          E           D-E 
STD: Standard MW 
R: Non-extruded  
D-R: Digested non-extruded 
E: Extruded 
D-E: Digested extruded 
Fig. 5: Molecular weight and protein digestion 
-ΔpH∞  = -0.009ΔMW + 0.67 
r² = 0.41 
0.44 
0.47 
0.50 
0.53 
0.56 
0.59 
0.62 
6 8 10 12 14 16 18 
-Δ
p
H
∞
 
ΔMW (%) 


Kinetics of protein digestion and molecular weight profiles in a model sorghum-barley blend as affected by extrusion conditions

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Kinetics of protein digestion and molecular weight profiles in a model sorghum-barley blend as affected by extrusion conditions

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