J. Agric. Food Chem.

2009, 57, 9793–9800 9793

DOI:10.1021/jf902199x

Emulsifying and Foaming Properties of Commercial Yellow

Pea (Pisum sativum L.) Seed Flours
ROTIMI E. ALUKO,*,†,‡ OLAWUNMI A. MOFOLASAYO,† AND BEVERLEY M. WATTS†
†
Department of Human Nutritional Sciences, University of Manitoba, Winnipeg, Manitoba,
Canada R3T 2N2, and ‡The Richardson Centre for Functional Foods and Nutraceuticals,
University of Manitoba, Winnipeg, Manitoba, Canada R3T 6C5

Commercial yellow pea seed flours prepared by a patented wet-milling process and pea protein
isolate (PPI) were analyzed for emulsifying and foaming properties at pH 3.0, 5.0, and 7.0 and
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compared to soybean protein isolate (SPI). PPI and SPI formed emulsions with significantly smaller
(p < 0.05) oil droplet sizes, 16-30 and 23-54 μm, respectively, than flours that primarily contained
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fiber such as Centara III and IV, or those that consisted mainly of starch: Centu-tex, Uptake 80 and
Accu-gel. PPI was a better emulsifier than SPI at pH 7.0, and a better foaming agent at pH 3.0 and
pH 7.0, although foaming capacity varied with sample concentration. Centu-tex and Uptake 80 have
exactly the same chemical composition, but the latter has a much smaller flour particle size range,
and had significantly smaller (p < 0.05) emulsion oil droplets. Incorporation of pea starch into SPI
emulsions produced a synergistic effect that led to significant increases (p < 0.05) in emulsification
capacity (reduced emulsion oil droplet size) when compared to SPI or starch alone. These results
showed that PPI had generally significantly higher (p < 0.05) emulsion and foam forming properties
than SPI, and that pea starch could be used to improve the quality of SPI-stabilized food emulsions.

KEYWORDS: Pea; seed flour; soybean; protein isolate; starch; emulsion; foaming

INTRODUCTION processed by wet milling to obtain purified fractions of fiber,

Yellow field pea seeds (Pisum sativum L.) are the main raw starch, and protein. These purified flours have better function-
materials in the commercial production of various food-grade ality for some types of food applications than the enriched air
flour fractions that are predominantly protein, starch, fiber or classified flours (4).
starch and fiber combinations. There has been increased interest Emulsification and foaming are two of the most important
in plant-derived food ingredients such as those from pea seeds functionalities that proteins and other amphoteric molecules
because of consumers’ demand for cholesterol-free and low-fat contribute to in the development of traditional or novel foods (6).
food products. Soybean seeds remain the primary source of plant- During emulsification these ingredients facilitate stable oil dro-
based food ingredients, but yellow field pea products can be used plet formation through development of interfacial membranes.
to give similar nutritive and functional properties to food and The membranes prevent coalescence of droplets, and so enhance
beverage formulations. For example, under appropriate condi- droplet dispersion in the immiscible phase of the emulsion (7-9).
tions, commercial or laboratory-prepared pea protein isolates Carbohydrates such as starch and fiber may also enhance emul-
were able to form good protein gels though they had less gel sion stability by acting as bulky barriers between the oil droplets,
strength when compared to similar soybean protein products (1-3). preventing or slowing down the rate of oil droplet coalescence.
However, additional functional properties of pea seed flours need Foam formation and stability are also dependent on interfacial
to be properly elucidated to facilitate their application in the membrane formation or barrier establishment to prevent coales-
formulation of either new or traditional foods. cence of air bubbles (8, 9). Therefore, apart from the use of
Typically, yellow field peas have up to 25% protein content, products that consist mostly of proteins or starch, it may be
55-68% starch and 6.5% fiber (4, 5). The pea seed proteins possible to form and stabilize emulsions and foams with suitable
consist of mostly the 11S (legumin), 7S (vicillin) and 2S (albumins); combinations of both products.
however, the protein isolates are composed mainly of the 11S and Though various pea flour products are currently produced in
7S storage proteins as evident from gel electrophoresis (1). Pea commercial quantities, utilization in food formulation is limited
seeds can be separated into the hulls, which are used to produce due to a lack of information on their functional properties.
high fiber flours, and the dehulled seeds, which can be processed In order to enhance acceptability of pea seed flours and increase
by pin milling and air classification to give protein-enriched and usage for the formulation of foods, there is the need to provide
starch-enriched flour fractions (4). The dehulled seeds can also be some basic information on their ability to act as emulsifiers
and foaming agents. The objectives of this work were to deter-
*Author to whom correspondence should be addressed (tel, þ1-204- mine the emulsifying and foaming properties of yellow pea
474-9555; fax, þ1 204-474-7593; e-mail, alukor@cc.umanitoba.ca). protein isolate, and of high starch and fiber flours, and to

© 2009 American Chemical Society Published on Web 10/01/2009 pubs.acs.org/JAFC

9794 J. Agric. Food Chem., Vol. 57, No. 20, 2009 Aluko et al.
Table 1. Percent Proximate Composition (Dry Weight Basis) of Commercial
Flours from Yellow Field Pea Seed and Soybean Seeds
protein
name (N  6.25) lipids fiber starch sugars ash

Accu-Gel 1.0 0.1 0.0 98.7 0.0 0.2

Centara III 6.0 0.5 90.0 0.1 0.5 2.0
Centara IV 3.0 0.5 93.0 0.0 1.5 2.0
Centu-Tex (425 μm) 10.0 0.5 35.0 50.0 2.5 2.0
Uptake 80 (175 μm) 10.0 0.5 35.0 50.0 2.5 2.0
Propulse (pea protein isolate) 82.0 0.3 0.0 0.7 11.7 4.0
soy protein isolate 90.0 4.0 0.0 0.0 <1.0 5.0

determine the influence of pea starch on the performance of the

protein isolates.

MATERIALS AND METHODS

Materials. Pea flour products (from nongenetically modified seeds)
were obtained as gifts from the Nutri-Pea Ltd. company (Portage la
Prairie, Manitoba, Canada). Details of production of the yellow field
pea seed ingredients are proprietary (10). In general the process involves
a similar procedure to the one described by Sosulski and McCurdy (4).
Briefly, the peas are dehulled and ground into flour, which is passed
through a screen to separate the coarse fiber particles. The flow-through
flour is then extracted with an acidic solution and centrifuged, and the
supernatant is used for isoelectric protein precipitation. This gives
a protein concentrate (about 85% protein) while the residue is used as
a source of starch, and a starch-fiber composite flour. The hulls are ground
separately to produce very high fiber flours. In this work we used two high
fiber (>90%) products, Centara III and Centara IV, and two starch-fiber
products, Centu-Tex and Uptake 80, as well as one pea protein isolate
(Propulse) and one high starch (>98%) product, Accu-Gel, as shown in
Table 1. Soybean protein isolate (SPI, PRO-FAM 974) that was used for
comparison purpose was a gift from Archer Daniels Midland Company
(ADM, Decatur, IL). Data on the proximate composition of all the flours
were provided by Nutri-Pea Ltd. (pea flours) or ADM (soy protein isolate)
and are shown in Table 1.
Emulsion Formation and Measurement. Oil-in-water emulsions
were prepared as previously described by Aluko and McIntosh (11) with
the following modifications. Using the different ingredients or mixtures of
protein and starch, slurries were prepared in 5 mL of 0.1 M phosphate Figure 1. Effect of pH on the emulsifying capacity (oil droplet size) of
buffer pH 3.0, 5.0, or 7.0 followed by addition of 0.5 mL of pure canola oil soybean protein isolate (SPI, 90% protein) and pea seed flours: Centara III
(10%, v/v). The oil/water mixture was homogenized at 20,000 rpm for and Centara IV (high fiber, >90%); Centu-Tex and Uptake 80 (starch, 35%-
1 min, stopped for 5 s and then homogenized again for another 1 min using fiber, 50%), Accu-Gel, high starch (>98%), pea protein isolate (PPI, 82%
the 20 mm nonfoaming shaft on a Polytron PT 3100 homogenizer protein). For each box plot, bars with different letters are significantly
(Kinematica AG, Lucerne, Switzerland). The oil droplet size (d3,2) of the different (p < 0.05).
emulsions was determined in a Mastersizer 2000 (Malvern Instruments
Ltd., Malvern, U.K.) with distilled water as dispersant. Emulsion sample RESULTS AND DISCUSSION
was added (under constant shearing) to about 100 mL of water contained Effect of Sample Concentration and pH on Emulsion Formation
in the small volume wet sample dispersion unit (Hydro 2000S) attached to and Stability. The effects of sample concentration and pH on the
the instrument until the required level of obscuration was attained. The
oil droplet size of emulsions stabilized by pea seed flours are shown
instrument was set to measure each emulsion in triplicate and to calculate
the mean value; each emulsion was prepared twice. Emulsions were left at in Figure 1. Increased sample concentration, from 10 to 50 mg/mL,
room temperature for 30 min, after which the oil droplet size measurement produced beneficial effects on emulsification (reduced droplet size)
was repeated. Emulsion stability was determined as the percentage ratio of at all pH values for Centu-tex, Uptake 80, Centara III and IV and
initial to the 30 min d3,2 values. Accu-gel. These flours all have low protein content (e10%). There
Foam Formation and Measurement. Foams were formed as pre- were no significant concentration effects (p < 0.05) on emulsifica-
viously described (11) with the following modifications. Slurries were tion ability of the soy protein isolate (SPI) and the pea protein
prepared in 10 mL of 0.1 M phosphate buffer pH 3.0, 5.0, or 7.0 followed isolate (PPI) at pH 5.0 and pH 7.0, but droplet size was reduced
by homogenization at 20,000 rpm for 1 min using a 20 mm foaming shaft with the higher concentrations at pH 3.0. At pH 3.0, 5.0, and 7.0,
on the Polytron PT 3100 homogenizer. The foam was formed in a 50 mL the two isolates produced emulsions with smaller droplets (16-
graduated centrifuge tube, which enabled determination of foam volume 54 μm) than did the flours that contained lower amounts of protein
(mL). Each sample was analyzed in triplicate, and the mean value is
(e10%), which had droplet sizes of 30-100 μm. Within this group
reported. The volume of foam remaining after 30 min at room temperature
was expressed as a percent value of original foam volume to obtain foam of higher starch flours, Centu-tex and Uptake 80 (10% protein)
stability. produced emulsions that had smaller droplets (34-50 μm), and
Statistical Analysis. Analysis of variance and Duncan’s multiple therefore better quality, than Centara III and IV and Accu-gel
range tests were performed to determine significant differences between (1-6% protein) that had larger droplet sizes of 50-100 μm.
mean values within each group using the Statistical Analysis Systems These results are consistent with previous studies which
(SAS) desktop software, version 9.1. have demonstrated better emulsifying ability of proteins when
Article J. Agric. Food Chem., Vol. 57, No. 20, 2009 9795
compared to carbohydrates (12, 13). Emulsifying activity of
defatted macadamia flours has also been shown to be dependent
on protein content (14). The importance of proteins is further
reflected in the fact that Accu-gel, which is 99% starch, produced
emulsions with significantly higher (p < 0.05) droplet sizes at
pH 5.0 and pH 7.0 than the flours with 3-10% protein content.
Previous work has also shown that flours with lower ratios of
nonprotein to protein components have better emulsion forming
ability (15). It is possible that at pH 3.0 and coupled with the high
rate of shear (20,000 rpm) used for emulsion formation a slight
acid-induced disruption of starch granule structure could have
released modified (hydrolyzed) starch polymeric molecules,
which enhanced the emulsifying ability of Accu-gel when com-
pared to its emulsifying ability at pH 5.0 and 7.0. This is similar to
a previous report that showed enhanced emulsion forming
capacity of hydrolyzed starch when compared to unhydrolyzed
starch (16).
The emulsifying capability of PPI was significantly higher (p<
0.05) at all concentrations than that of SPI when emulsions were
formed at pH 5.0 and 7.0. The oil droplet size range was 14-
16 μm for PPI and 38-42 μm for SPI. However, at a pH of
3.0 there was a higher emulsifying effect only at the lowest
concentration of 10 mg/mL. In a previous work, it was shown
that PPI had better emulsion forming ability than SPI at
pH 5.0, but not at pH 7.0 (17). PPI has higher levels of sugars
than SPI, which may contribute to increased solubility of the pea
proteins and better emulsification capacity. The result is in
agreement with a previous report that showed a positive relation-
ship between protein solubility of pea or soybean proteins and
emulsification capacity (17). Previous work has also shown that
commercial PPI has more polypeptide chains (2 legumin and
7 vicillin) when compared to commercial SPI (2 legumin and
2 β-conglycinin) as revealed by gel electrophoresis (1). Thus the
higher emulsifying capacity (lower oil droplet size) of the PPI may
be due to the wider variety of polypeptide chains available for oil
droplet formation. Our results differ from previous research
:: :: ::
reported by Tomoskozi et al. (18), which indicated that SPI
had better emulsion forming ability than PPI. However these
contradictory results could be due to the processing history of the Figure 2. Oil droplet size distribution of emulsions stabilized by soybean
protein isolates. In our work commercial samples were used, protein isolate (SPI, 90% protein) and pea seed flours: Centara III and
rather than protein isolates prepared in the laboratory. There Centara IV (high fiber, >90%); Centu-Tex and Uptake 80 (starch 35%, fiber
seem to have been very minimal changes in structural properties 50%); Accu-Gel, high starch (>98%); pea protein isolate (PPI, 82%
of the polypeptides present within the PPI in the pH range from protein).
3.0 to 7.0, as evident from the lack of any significant change (p <
0.05) in average emulsion droplet size. Therefore, the results particle size has detrimental effects, since these products have
suggest that the pea isolate proteins were more resistant to identical chemical composition. Smaller particles could enhance
changes in pH than the proteins of the soybean isolate. Our the dispersion of the emulsifying components thus increasing
results are in contrast with previous work which showed increased interactions with the oil-water interface and improving forma-
oil droplet coalescence and droplet size of soybean-stabilized tion of interfacial membranes. Larger flour particles would then
emulsions at pH 5.0 when compared to those at pH 3.0 and be expected to reduce dispersion of the emulsifying components,
7.0 (12). The differences in results could be due to the type of including the polypeptides that facilitate reduction of interfacial
emulsions used; we used higher concentrations of proteins and tension, and so resulting in poorer emulsion formation. Oil
lower homogenization pressure when compared to the work of droplet size distribution in the emulsions is shown in Figure 2.
Roudsari et al. (12), who used lower protein concentrations but The main difference is the narrow range of the size of oil droplets
very high homogenization pressure. In the present work, emul- (10-100 μm) formed by soybean proteins in comparison to the
sion stability was measured after a short duration of 30 min when wider range of size (2-1000 μm) observed for the other emul-
compared to 15 days for the previous work (12). Moreover the sions. Therefore, the SPI-stabilized emulsions had more uniform
mean particle size of emulsions in this work is higher than values size of oil droplets, which is an indication of the protein’s ability to
reported by Roudsari et al. (12). completely coat the oil droplets during homogenization and
Uptake 80 and Centu-tex have the same proximate composi- prevent their coalescence after homogenization (19). The results
tion but differ in the mean particle size of the flours, 175 and indicate that there is more complete interaction of SPI proteins
425 μm, respectively (Table 1). Emulsions formed by Uptake with the emulsified oil droplets during homogenization, which
80 had droplets ranging from 29 to 40 μm compared to ones of aids in dispersing their larger sized aggregates into smaller sized
37-51 μm for Centu-tex emulsions. These results suggest that particles to produce a narrow range of oil droplet size. Similar
smaller particle size enhances emulsion formation, while large results were obtained for SPI-stabilized emulsions at different oil
9796 J. Agric. Food Chem., Vol. 57, No. 20, 2009 Aluko et al.

Figure 3. Effect of pH on the emulsifying stability (percent increase in oil Figure 4. Emulsifying capacity (oil droplet size at time zero) and stability
droplet size) of soybean protein isolate (SPI, 90% protein) and pea seed (oil droplet size after 30 min) of pea seed starch/protein combinations at
flours: Centara III and Centara IV (high fiber, >90%); Centu-Tex and pH 7.0. For each box plot, bars with different letters are significantly
Uptake 80 (starch 35%, fiber 50%), Accu-Gel, high starch (>98%), pea different (p < 0.05).
protein isolate (PPI, 82% protein). For each box plot, bars with different
letters are significantly different (p < 0.05). Emulsion Quality of Starch-Protein Mixtures. Proteins are the
main emulsifying agents in many foods, but the presence of
concentrations where formation of narrow range of oil droplet carbohydrates within the food matrix can alter the emulsifying
size was attributed to increased SPI-oil droplet interactions (20). ability of the proteins and produce changes in food quality (19,23).
Formation of larger size droplets has also been attributed to Addition of starches to gluten-free products enhanced formation
reduction in the degree of hydrogen bond-mediated interactions of the appropriate protein-starch networks needed to produce
between the electric layer of ions on the oil droplets and surfactant fermented bakery products (24). Starch products may be incor-
(proteins or carbohydrates) molecules (21). It is possible that the porated into foods to increase or decrease emulsion capacity in
pea proteins and carbohydrates found in the pea flours had accordance with quality preferences of the manufacturer. Figure 4
weaker interactions with the oil droplet electric layer, which shows the effects of pea starch (Accu-gel) on the emulsifying
resulted in the larger sizes of the emulsion droplets when properties of SPI and PPI at pH 7.0 and at varying ratios of starch
compared to the soybean proteins. to protein. A pH of 7.0 was used because this is near to the
Emulsion stability is shown in Figure 3, and the results suggest pH values of many manufactured foods. Emulsions formed using
significant differences (p < 0.05) at pH 3.0 mainly at low con- PPI had smaller oil droplets (better quality) than emulsions
centration of 10 mg/mL. In contrast there was an increase in formed using SPI, a result similar to that obtained at pH 7.0
the number of significantly different (p < 0.05) results at pH 7.0. without the added starch (Figure 1). At pH 3.0, incorporation of
Generally, all the emulsions were very stable (>80%) at all starch significantly (p < 0.05) enhanced (lower oil droplet sizes)
pH values except those made with starch (Accu-gel) and fiber the emulsion formation by SPI, but had negative effects on
(Centara III and Centara IV) products, which had decreased emulsion formation by PPI. In fact at 10 mg/mL total sample
stability at pH 7.0. The high stability of these emulsions suggests concentration the ratio of 8 mg of starch to 2 mg of protein
that the pea seed flours may be suitable ingredients for the produced PPI and SPI emulsions with a difference of only about
formulation of food emulsions that have good short-term stabi- 5 μm in droplet size, as compared to 10-15 μm at lower
lity properties. concentrations of starch. The significant decrease (p < 0.05) in
Article J. Agric. Food Chem., Vol. 57, No. 20, 2009 9797
droplet size of SPI emulsions was similar at the three sample
concentrations, which suggests that the amount of sample used
to make the emulsion did not affect the nature of starch-
protein interactions. In research reported by Babiker et al. (25),
the conjugation of a polysaccharide to soybean proteins also
improved the emulsion forming ability of SPI.
It is important to emphasize that Accu-gel on its own produced
very poor emulsions. Therefore, the ability of Accu-gel to
improve emulsion forming ability of soybean proteins indicates
a synergistic effect that may be attributed to starch-protein
interactions. As discussed above, the poor emulsifying ability of
SPI at pH 7.0 may be due to increased charge density, which
prevents formation of strong interfacial protein membranes and
smaller droplets. Therefore, it is reasonable to suggest that
addition of pea starch to SPI may have led to a reduction in
charge density, possibly as a result of neutralization of protein
charges by oppositely charged starch residues. The emulsifying
ability of soybean proteins has previously been found to be
enhanced by the presence of soybean seed cotyledon polysacchar-
ides (12) or through conjugation with dextran (7, 26). The
presence of the bulky starch molecules may also enhance forma-
tion of stable oil droplets by acting as physical barriers against oil
droplet coalescence, which complements the emulsion forming
ability of the proteins. The progressive nature of the improvement
in emulsion forming ability of SPI with increases in starch
concentration support our hypothesis that the protein-starch
interactions favored decreased charge density at the oil-water
interface and physical separation of the oil droplets. A decrease in
charge density will enhance interactions at the oil-water interface
and lead to the formation of strong interfacial membranes that
produce emulsions of reduced droplet size. Similarly the interac-
tions between SPI and pea starch could have improved the
amphipathic properties of starch, giving enhanced emulsion
forming ability when compared to starch alone. This type of
synergy may be exploited in the manufacture of high quality SPI
food emulsion products that incorporate optimal levels of pea
starch. Similarly, during manufacture of cereal-based products
such as protein enriched breakfast cereals, it has been shown that
protein-starch interactions contribute to texture and rheological
Figure 5. Effect of pH on the foaming capacity (foam volume) of soybean
properties of dough (27). Thus, addition of pea starch to cereal-
protein isolate (SPI, 90% protein) and pea seed flours: Centara III and
based ingredients could enhance incorporation of soybean pro-
Centara IV (high fiber, >90%); Centu-Tex and Uptake 80 (starch 35%, fiber
teins and produce high quality food products. In contrast, the
50%), Accu-Gel, high starch (>98%), pea protein isolate (PPI, 82%
results suggest that pea proteins did not interact with pea starch to
protein). For each box plot, bars with different letters are significantly
produce any substantial change in emulsion forming ability.
different (p < 0.05).
Therefore, we can deduce that the structural conformation of
pea proteins at pH 7.0 was not changed by addition of starch, foaming capacity, the effect of concentration was much less
especially with respect to the ability to form interfacial mem- (Figure 5). Overall, SPI and PPI produced higher volume foams
branes at the oil-water interface. than the flours with lower protein concentrations. The results
Figure 4 also shows that the emulsions containing combina- suggest that formation of interfacial protein membranes at the
tions of pea starch and proteins were highly stable because there air-water interface enhanced encapsulation of air bubbles.
was no significant difference (p > 0.05) in emulsion oil droplet Similar to the emulsion results, foam formation was largely
size between the t = 0 and t = 30 min measurements for each dependent on the protein content of the samples. The foaming
sample. The results are generally consistent with Figure 3 where ability of PPI and SPI at pH 3.0, and concentrations of up to
we have shown high levels of stability for emulsions made with the 50 mg/mL, increased as sample concentrations increased, while at
pea seed flours. Therefore, incorporation of starch into the a pH of 7.0 the foam volume decreased as concentrations
protein flours did not have any negative effect on the ability of increased. At the highest concentration (100 mg/mL) foaming
the proteins to stabilize oil-in-water emulsions. ability was significantly reduced for all pH values. The results
Effect of Sample Concentration and pH on Foam Formation and suggest that, at the high concentrations of surfactants used in this,
Stability. Foam formation is an important requirement in the there could have been limited solubility (dispersibility) in water
manufacture of foods such as ice cream, cakes and meringues. that enhances foam breakage rather than foam formation. Effects
Therefore, the ability of the pea seed flours to form foams could of pH and concentration on the foam foaming ability of PPI and
be essential to their application in the manufacture of nondairy SPI differed considerably. At pH 3.0 and 7.0 and up to 50 mg/mL
foods. For PPI and SPI, concentration significantly (p < 0.05) sample concentration, PPI had significantly higher (p < 0.05)
influenced foaming ability (foam volume), but for Centu-tex, foaming ability (15-22 mL) when compared to SPI (5-16 mL).
Uptake 80, Centara III and IV, and Accu-gel, which had lower The results suggest that PPI is a better foaming agent with a more
9798 J. Agric. Food Chem., Vol. 57, No. 20, 2009 Aluko et al.
flexible polypeptide conformation at pH 3.0 and 7.0 when
compared to SPI. The presence of higher levels of sugars
(∼12%) may have also enhanced foaming ability of the PPI
when compared to SPI that had <1% sugar content (Table 1).
Previous reports have also shown superior foaming properties of
pea protein isolate when compared to SPI (4, 5). However, our
:: :: ::
results are in contrast to those obtained by Tomoskozi et al. (18),
which showed poorer foaming ability of PPI when compared to
SPI. At the highest sample concentration of 100 mg/mL, foam
formation was decreased for PPI and SPI, suggesting excessive
protein-protein interactions that would have limited ability to
form flexible interfacial membranes that are required to encap-
sulate the air bubbles.
At pH 5.0, significant differences (p<0.05) between the foam-
ing abilities of PPI and SPI were observed and were concentration
dependent (Figure 5). The foaming ability of 10 and 25 mg/mL
concentrations of SPI was significantly higher (p<0.05) at pH 5.0
when compared to that at pH 3.0. Since pH 5.0 is near the
isoelectric point (pI) of soybean proteins, it is possible that the
reduction in net charge density enhanced protein-protein inter-
actions such that strong interfacial membranes are formed, which
facilitated better foaming ability. At pH 7.0, foaming ability
increased for PPI and SPI indicating better (compared to pH 3.0
and 5.0) structural conformation suitable for interfacial mem-
brane formation. The results suggest that as the pH increased
there were increases in the net charge density of PPI and SPI,
which enhanced protein unfolding and flexibility that contributed
to better foam formation. However, as the protein concentration
increased, foaming ability was decreased at pH 5.0 probably as a
result of increased protein-protein interactions or reduced solu-
bility that decreased flexibility and ability to form efficient
interfacial membranes. At pH 7.0, the increase in protein con-
centrations also led to significant decreases (p < 0.05) in foam
volume, which could be attributed to excessive charge density or
reduced solubility that prevented formation of interfacial mem-
branes at the level required for efficient encapsulation of air
bubbles. For SPI, there was an increase in foaming ability as the
pH increased from acidic values (pH 3.0 and 5.0) to neutral value
(pH 7.0), a result that is similar to those previously reported by Figure 6. Effect of pH on the foam stability (percent decrease in foam
Aluko et al. (15). volume) of soybean protein isolate (SPI, 90% protein) and pea seed flours:
Particle size also affected the foaming ability of Centu-tex and Centara III and Centara IV (high fiber, >90%); Centu-Tex and Uptake
Uptake 80, two samples with the same composition but different 80 (starch 35%, fiber 50%), Accu-Gel, high starch (>98%), pea protein
flour particle sizes of 425 and 175 μm, respectively. This is most isolate (PPI, 82% protein). For each box plot, bars with different letters are
noticeable at the highest sample concentration of 100 mg/mL significantly different (p < 0.05).
where Centu-tex (large particle size) was unable to form any foam
at the three pH values used in this work (Figure 5). In contrast, stable at all the pH values and sample concentrations used in this
Uptake 80 (smaller particle size) still produced some foams work. The results suggest that proteins are more important than
with 100 mg/mL sample concentration at the three pH values, nonprotein components with respect to foam stabilization. The
indicating availability of foaming agents. At a concentration of number of stable foams was higher at pH 7.0 when compared to
25 mg/mL, the foaming ability of Uptake 80 was significantly pH 3.0 and 5.0, which suggests increased formation of strong
increased (p<0.05) at pH 7.0 when compared to pH 3.0 and 5.0. interfacial membranes as acidity level of the environment was
Therefore, the foaming agents (especially proteins) were more reduced. At the highest sample concentration (100 mg/mL) used
available within the smaller particle size of Uptake 80 and in this work the number of stable foams (5) was higher at pH 5.0
responded to the increase in pH by becoming more flexible with when compared to pH 3.0 (2 foams) and pH 7.0 (3 foams).
increased capacity to encapsulate air bubbles. The large particle Therefore, high sample concentration could be used to remedy
size of Centu-tex flour may have imposed limitations to the poor foam stability properties of these flours at pH 5.0.
availability of foaming agents at high concentrations which Foaming Quality of Starch-Protein Mixtures. Protein-
prevented formation of interfacial membranes. The results con- polysaccharide interactions are also known to affect foaming
firm that small particles of flours contribute to better foaming properties since nonspecific interactions can lead to attractive and
properties, especially at high sample concentrations where clump- repulsive forces that induce complex formation or immiscibility
ing can occur to limit interaction with the air-water interface. of biopolymers (18). The effects of pea starch on foaming abilities
Foam stability was highly dependent on pH and sample of PPI and SPI are shown in Figure 7. When compared to the
concentration as shown in Figure 6. At 10 mg/mL concentration results shown in Figure 5, it can be seen that the initial incorpora-
only the high protein flours (SPI and PPI) produced stable foams tion of 20% pea starch actually enhanced foaming ability of SPI
at pH 5. Similarly only the foams produced by SPI and PPI were but not PPI. For example, foaming capacity of SPI at pH 7.0 and
Article J. Agric. Food Chem., Vol. 57, No. 20, 2009 9799
not enough protein/starch complexes to form sufficient interfacial
membranes that will encapsulate the air bubbles. But as the
sample concentrations increased, more interfacial membranes
could be formed, which enhanced foam formation. This trend
is especially noticeable for SPI which had a maximum of 13 mL of
foam at 10 mg/mL (Figure 7A) when compared to 23 mL of foam
at 100 mg/mL (Figure 7C) sample concentration. Just as observed
for emulsion formation, the presence of low levels of pea starch
led to improved foam formation by SPI when compared to the
amount of foam formed by SPI alone. However, unlike the trend
observed with emulsion formation, increased ratios of starch
to protein had significant (p < 0.05) negative effects on foam
formation. Foaming capacity of the pea starch/protein mixtures
was significantly reduced (p < 0.05) during short-term (30 min)
storage at room temperature as shown by the lower foam volumes
obtained after 30 min (Figure 7). Thus, unlike the emulsions, pea
starch did not improve foam stability of soybean and pea
proteins.
The present results showed that emulsion and foam formations
were dependent on protein levels in the pea flours with the
protein-deficient flours giving poor results. It is evident that
interfacial membrane formation at the oil-water and air-water
interfaces is highly dependent on protein-protein interactions to
provide good emulsion and foam forming abilities. A smaller
particle size enhanced the emulsion and foam forming abilities of
flours, which may be attributed to greater availability of inter-
facial pressure-lowering components of the flour. In contrast,
large particles limit availability of the interfacial pressure-low-
ering components, and lead to poor emulsion and foam forming
abilities. The improvement in the emulsion forming capability of
soybean protein with the addition of pea starch could be the result
of favorable protein-starch interactions. This could be exploited
to enhance the quality of soybean-based food emulsions. Overall,
the superior emulsion and foam forming abilities of PPI may be
exploited in the food industry as suitable replacement of tradi-
tional soybean-stabilized food emulsions, especially in the man-
Figure 7. Foaming capacity (foam volume at time zero) and stability
ufacture of hypoallergenic foods for people allergic to soybean
(foam volume after 30 min) of pea seed starch/protein combinations at proteins.
pH 7.0. For each box plot, bars with different letters are significantly
different (p < 0.05).
ACKNOWLEDGMENT

50 and 100 mg/mL was 12 and 10 mL (Figure 5C), respectively, We thank our industrial partner, Nutri-Pea Ltd (Portage la
when compared to 20 and 22 mL for 10:40 and 20:80 starch: Prairie, Manitoba), for supply of the yellow pea seed flours.
protein ratios (Figures 7B and 7C), respectively. The result for SPI
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