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Lipid oxidation is a major cause of quality deterioration in food emulsions. Polysaccharides used to improve emulsion stability and texture may also affect lipid oxidation. In the present study, the oxidative stability of olive oil–lemon juice salad dressings, stabilized with gum arabic or propylene glycol alginate in admixture with xanthan, was investigated. Oil-in-water emulsions (50:50, v/v) were prepared with lemon juice and extra virgin olive oil and then homogenized at various homogenization rates to form different particle sizes. Keepability was followed by storing at room temperature for 6–8 months and measuring the formation of primary and secondary oxidation products. The shelf life was compared to that of the bulk olive oil. It was shown that the polysaccharides had the ability to inhibit lipid oxidation, probably due to their amphiphilic character (gum arabic and propylene glycol alginate) as well as their ability to induce viscosity increase. Olive oil–lemon juice emulsions were also assessed for consumer acceptance. The panellists were asked to smell the samples and rate them according to rancidity using a four-point (1 = no perception, 4 = extreme) intensity scale. The results were in accordance to those of chemical analysis. Lipid oxidation was not affected by the oil droplet size, as demonstrated by peroxide value measurements and sensory evaluation.
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Oxidative stability of olive oil–lemon juice salad dressings
stabilized with polysaccharides
D. Paraskevopoulou, D. Boskou, A. Paraskevopoulou
*
Laboratory of Food Chemistry and Technology, School of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
Received 26 September 2005; received in revised form 16 January 2006; accepted 13 March 2006
Abstract
Lipid oxidation is a major cause of quality deterioration in food emulsions. Polysaccharides used to improve emulsion stability and
texture may also affect lipid oxidation. In the present study, the oxidative stability of olive oil–lemon juice salad dressings, stabilized with
gum arabic or propylene glycol alginate in admixture with xanthan, was investigated. Oil-in-water emulsions (50:50, v/v) were prepared
with lemon juice and extra virgin olive oil and then homogenized at various homogenization rates to form different particle sizes. Kee-
pability was followed by storing at room temperature for 6–8 months and measuring the formation of primary and secondary oxidation
products. The shelf life was compared to that of the bulk olive oil. It was shown that the polysaccharides had the ability to inhibit lipid
oxidation, probably due to their amphiphilic character (gum arabic and propylene glycol alginate) as well as their ability to induce vis-
cosity increase. Olive oil–lemon juice emulsions were also assessed for consumer acceptance. The panellists were asked to smell the sam-
ples and rate them according to rancidity using a four-point (1 = no perception, 4 = extreme) intensity scale. The results were in
accordance to those of chemical analysis. Lipid oxidation was not affected by the oil droplet size, as demonstrated by peroxide value
measurements and sensory evaluation.
Ó2006 Elsevier Ltd. All rights reserved.
Keywords: Salad dressing; Olive oil; Lemon juice; Gum arabic; Propylene glycol alginate; Xanthan; Oxidation; Keepability; Droplet size
1. Introduction
Emulsions are thermodynamically unstable systems
and they tend to separate into two layers over time
through a number of mechanisms (Dickinson, 1992;
McClements, 1999). In order to make emulsions kineti-
cally stable for a reasonable period of time, emulsifiers
and/or stabilizers (e.g. proteins, phospholipids, polysac-
charides) must be added. Polysaccharides are usually
added to o/w emulsions to enhance viscosity of the aque-
ous phase, thus preserving desirable textural characteris-
tics and stabilizing oil droplets against creaming
(Dickinson, 1992; Paraskevopoulou et al., 2003; Paraskev-
opoulou, Boskou, & Kiosseoglou, 2005; Paraskevopou-
lou, Kiosseoglou, Alevisopoulos, & Kasapis, 1997).
Amphiphilic polysaccharides, such as gum arabic and
propylene glycol alginate, act both as emulsifiers and sta-
bilizers. Because of their surface-active properties they can
adsorb onto the surface of freshly formed droplets during
homogenization and prevent them from aggregation by
steric and/or electrostatic forces. Studies have shown that
polysaccharides are also capable of retarding lipid oxida-
tion in o/w emulsions (Matsumura et al., 2003; Shimada,
Fujikawa, Yahara, & Nakamura, 1992; Shimada et al.,
1994; Shimada, Okada, Matsuo, & Yoshioka, 1996).
The polysaccharide-induced viscosity increase of the aque-
ous phase inhibits oxygen diffusion and slows down the
movement of oil droplets, thus reducing their collision
probability. Additionally, the metal ion-chelating ability
of polysaccharides has been proposed to account for their
ability to inhibit lipid oxidation, while tragacanth gum
has been found to act as a radical chain-breaker because
of its ability to donate hydrogen (Shimada et al., 1992).
0308-8146/$ - see front matter Ó2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodchem.2006.03.022
*
Corresponding author. Tel.: +30 231 997832; fax: +30 231 997779.
E-mail address: adparask@chem.auth.gr (A. Paraskevopoulou).
www.elsevier.com/locate/foodchem
Food Chemistry 101 (2007) 1197–1204
Food
Chemistry
Lipid oxidation is considered to be the predominant
cause of quality deterioration of oils, fats and various
fat-containing foods during storage. It involves the interac-
tion between unsaturated lipids and oxygen-active species.
It can cause alterations in the quality characteristics of
foods, such as appearance, taste, texture, shelf life and
nutritional profile and lead to the development of undesir-
able off-flavour (Min & Boff, 2002). Numerous studies have
been carried out dealing with oxidation mechanisms in
bulk oils although, in most processed foods, lipids are
found as o/w or w/o emulsions. The oxidation of emulsified
lipid is mechanistically different from that of bulk oils. In
such products, the organization of the lipid molecules
within the system and their interactions with other food
components influences their susceptibility to lipid oxida-
tion. A great number of studies have highlighted the impact
of various factors in determining the oxidative stability of
an oil-in-water emulsion, including chemical structure of
lipids, oxygen concentration, antioxidants, interfacial char-
acteristics, droplet characteristics (concentration, size),
interactions with aqueous phase components (salts, sugars,
polysaccharides, proteins.) and the presence of prooxidants
(e.g. transition metal impurities) (McClements & Decker,
2000).
Salad dressings are very popular oil-in-water emulsion
products, which may vary in fat content (20–65%) and vis-
cosity (Dickinson & Stainsby, 1982). ‘‘Greek salad dress-
ings’’ are, by definition, mixtures of virgin olive oil and
lemon juice instantly prepared before use (Paraskevopou-
lou et al., 2005). Due to their composition, they are a good
source of biophenols (Boskou & Visioli, 2003) as well as
lipid-soluble and water-soluble vitamins (tocopherols, b-
carotene, ascorbic acid). Besides, virgin olive oil, thanks
to its balanced fatty acid composition, has highly appreci-
ated nutritional characteristics, known for a long time to
the people of the Mediterranean region, who use it daily
for a variety of culinary purposes (Garcia Mesa, Jimenez-
Marquez, Beltran-Maza, & Friaz-Ruiz, 1998; Romero,
Guesta, & Sanchez-Muniz, 1998). Its consumption has also
increased in non-Mediterranean areas because of the grow-
ing interest in the Mediterranean diet and the belief that it
prevents certain diseases (Boskou & Visioli, 2003; Tricho-
poulou, 1995).
The development of a ‘‘Greek salad dressing’’ contain-
ing olive oil and lemon juice that would exhibit reasonable
physicochemical stability over prolonged storage has been
the subject of a previous work (Paraskevopoulou et al.,
2005). Different combinations of xanthan gum with gum
arabic or propylene glycol alginate exhibited a positive
effect on the creaming and rheological behaviour, rate of
oil droplet coalescence and sensory properties of the salad
dressings. Gum arabic (gum Acacia), a hydrocolloid pro-
duced by the natural exudation of acacia trees, is a complex
mixture of six main carbohydrate components and a small
but functionally important amount of protein (2% w/w),
which is found as an integral part of the structure (Ware-
ing, 1999). Propylene glycol alginate, an esterified form of
alginic acid, is appreciated as a particularly effective thick-
ener in several acidic food applications, such as salad dress-
ings and lactic drinks (Onsøyen, 1999). Xanthan gum is an
extracellular polysaccharide made by the bacterium Xan-
thomonas campestris (Urlacher & Noble, 1999).
The favourable effect of the above-mentioned polysac-
charides, as well as the role of the oil droplet size on the
keepability of ‘‘Greek salad dressings’’, was the objective
of this work. By modifying the emulsification energy, emul-
sions with different droplet sizes were prepared. Keepabil-
ity was measured by periodically measuring peroxide
value, acidity and specific extinction coefficient K
232
. Fur-
thermore, the sensory perception of lipid oxidation prod-
ucts of the emulsions by the consumers was assessed.
2. Materials and methods
2.1. Materials
Gum arabic (GA) and xanthan gum (X) were purchased
from Sigma Chemical Co. (USA). Propylene glycol algi-
nate (PGA) (degree of esterification 40–60%) was kindly
supplied by FMC Biopolymer (Brussels, Belgium). Extra
virgin olive oil was bought from the local market. Lemon
juice was a commercial sample purchased in a local super-
market. Egg yolk (EY) was obtained by first breaking fresh
hen’s eggs and, following complete removal of the adhering
white by rolling the intact yolks on tissue paper, the vitel-
line membrane was punctured and the liquid yolks of a
number of eggs were collected. Benzoic acid was obtained
from Riedel de Hae
¨n (Germany).
2.2. Preparation of salad dressings
Oil-in-water emulsions (50% v/v) were prepared as fol-
lows: a lemon juice polysaccharide solution was first
prepared by slowly dispersing 1% w/v gum arabic (or
1% w/v PGA) and 0.5% w/v xanthan gum, with stirring
for at least 6 h, to ensure complete dissolution. The
emulsions were prepared by adding dropwise, while mix-
ing with a propeller-type mechanical stirrer, 50 ml of
virgin olive oil to 50 ml of the polysaccharide solution.
The droplet size of the resulting crude emulsion was then
reduced further using an Ultra-Turrax T25 homogenizer
(IKA Instruments, Germany), equipped with an
S25KG-25F dispersing tool. By modification of the
energy of emulsification three gum arabic/xanthan-stabi-
lized (GA/X 9500 rpm, GA/X 13,500 rpm, GA/X
24,000 rpm) and three propylene glycol alginate/xan-
than-stabilized (PGA/X 8000, PGA/X 9500, PGA/X
20,500 rpm) emulsions with different oil droplet sizes,
were produced. Thus, it was possible to test the effect
of droplet size and the effect of stabilizing agent on the
oxidative stability of the produced Greek-type salad
dressings. Benzoic acid was added at a concentration of
1&w/v in the continuous phase. The pH values of the
final samples ranged between 3.4 and 3.6.
1198 D. Paraskevopoulou et al. / Food Chemistry 101 (2007) 1197–1204
For comparison, another olive oil-in-lemon juice salad
dressing (50:50 v/v) containing 1% w/v egg yolk solids, in
the place of GA or PGA, was prepared by the same emul-
sification procedure, following homogenization at
13,500 rpm for 2 min.
Olive oil samples and salad dressings (100 ml) were
stored in screw-capped glass containers (100 ml) prior to
analysis.
2.3. Droplet size determination
The droplet size distribution of the salad dressings over
the storage period was measured using a laser light scatter-
ing instrument (Malvern Instruments, UK). Measurements
are either reported as the full particle size distribution or as
the surface-volume mean diameter d3=2¼Pnid3
i=Pnid2
i,
where n
i
is the number of droplets of radius d
i
. To prevent
multiple scattering effects, the emulsions were diluted with
deionized water prior to analysis so that the droplet con-
centration was less than about 0.02% v/v. The dilute emul-
sions were placed directly into the measurement cell of the
instrument and stirred slowly during the measurement.
Each sample was analyzed 4 times and the data are pre-
sented as averages. Droplet sizes were checked periodically
to monitor emulsion stability.
2.4. Viscosity measurements
The viscosities of the salad dressings were determined at
25 °C at various shear rates (from 0.1 to 20.4 s
1
) with the
aid of a Brookfield DV-II, LV viscometer (USA), equipped
with concentric cylinder geometry. The flow curves giving
viscosity g(MPa s) as a function of shear rate _
c(s
1
) were
characteristic of shear-thinning behaviour.
2.5. Oxidative stability evaluation
2.5.1. Experimental design
Twenty containers of each emulsion were stored in
conditions similar to those in consumers’ sales points:
inside a chamber at 23 °C and placed in shelf units,
2.5 m high with illumination for 12 h/day. Containers
were sampled every three or four weeks for complete
3.5–6 months storage.
Furthermore, 100 ml of virgin olive oil were stored
under the same conditions and sampled in a similar way.
2.5.2. Analytical determinations
Free acidity, peroxide value (PV) and specific extinction
coefficient K
232
were determined according to the analytical
methods described in Regulation EC/2568/91 of the Com-
mission of the European Union (EU, 1991). Oil was sepa-
rated from the salad dressings by repeated freeze–thaw
cycles, followed by centrifugation (Jacobsen et al., 1999).
Two containers of each sample were independently ana-
lyzed in each sampling and each parameter was measured
twice. The results are expressed as mean values.
Free acidity, given as % oleic acid, was determined by
titration of a solution of oil in ethanol/ether (1:1) with eth-
anolic potash.
Peroxide value, expressed as milliequivalents of active
oxygen per kilogramme of oil (meq/kg), was determined
as follows: a mixture of oil and chloroform/acetic acid
was left to react with a solution of potassium iodide in
darkness; the free iodine was then titrated with a sodium
thiosulfate solution.
K
232
extinction coefficient was calculated from the
adsorption of a solution of the oil in iso-octane at
232 nm, using a UV spectrophotometer (Hitachi, Japan)
and a path length of 1 cm.
2.6. Organoleptic assessment
Organoleptic evaluation was performed for salad dress-
ings and extra virgin olive oil, following their storage time,
simultaneously with oxidative stability measurements. The
analytical panel, consisting of 20 untrained members, was
asked to make its evaluation on the basis of rancidity.
The panellists were students and members of the staff of
the Laboratory and were asked to score the samples by
smelling them. Rancidity grading was based upon a four-
point intensity scale: 1 = no perception; 2 = weak; 3 =
medium; 4 = extreme (Jellinek, 1984). The dressings and
olive oil samples were served in glass beakers (20 ml in
each beaker) at room temperature.
2.7. Statistical analysis
All experiments were performed on quadruplicate sam-
ples. Statistical analyses were conducted with the one-way
ANOVA software package. Significant differences among
means (p< 0.05) were determined by LSD test.
3. Results and discussion
3.1. Properties of the salad dressings
Prepared salad dressings were monomodal with the drop-
let size distributions overlapping one another, as shown in
Fig. 1. The emulsions prepared with a mixture of GA/X
were physicochemically stable over the storage period as
no coalescence and no droplet size or viscosity variations
were observed over the time of experiments (Table 1,
Fig. 2). On the other hand, the droplet size of the rest of
the emulsions (prepared with a mixture of PGA/X or EY/
X) increased significantly (p< 0.05) during storage. From
the droplet size data obtained by the laser diffraction, it
was observed that GA/X-stabilized emulsions had the low-
est droplet sizes, likely due to the fact that gum arabic is
more surface-active than PGA. The larger droplet size of
EY/X-stabilized emulsions could be attributed to their rela-
tively low egg yolk content (1% w/v egg yolk solids).
Finally, their viscosity remained stable throughout the
storage period (Fig. 2).
D. Paraskevopoulou et al. / Food Chemistry 101 (2007) 1197–1204 1199
3.2. Acidity of the lipid phase
The salad dressings showed an increase of acidity
with the storage time that became significant (p< 0.05)
at the end of the storage period (Fig. 3). This increase
was attributed to the migration of acids/acidic com-
pounds from lemon juice to olive oil droplets during their
preparation.
3.3. Oxidative stability of salad dressings
3.3.1. Quality characteristics of olive oil sample
The quality characteristics that influence the storage
stability of olive oil were determined before the preparation
of salad dressings and are shown in Table 2. Acidity, per-
oxide value and coefficients of specific extinction (K
232
,
K
270
) were determined. All values were lower than the lim-
its set by the EU Regulation 2568/91 for extra virgin olive
oil.
0
2
4
6
8
10
12
0.1 1 10 100 1000 10000
Droplet size(
µ
m)
Volume (%)
Fig. 1. Droplet size distribution of olive oil–lemon juice emulsions
prepared at various energies of emulsification and stabilized with GA/X
(j,m,d), PGA/X (h,n,s) and EY/X ( ) mixtures 24 h after prepara-
tion. Emulsions prepared with: low energy input (j,h); medium energy
input (m,n); high energy input (d,s).
Table 1
Effect of energy of emulsification on the mean droplet diameter (d
32
) increase with time of olive oil–lemon juice salad dressings stabilized with mixtures of
PGA/X, GA/X or EY/X
t(days) Propylene glycol alginate/xanthan Gum arabic/xanthan Egg yolk/xanthan
8000 rpm 9500 rpm 20,500 rpm 9500 rpm 13,500 rpm 24,000 rpm 13,500 rpm
0 15.9
a
12.4
a
7.2
a
6.3
a
4.5
a
4.5
a
15.1
a
30 16.3
a
12.6
a
7.2
a
6.5
a
4.6
a
4.6
a
17.0
a
45 16.5
a
13.5
a
7.2
a
–––
60 16.6
a
14.2
b
7.5
a
6.0
a
4.1
a
4.9
a
21.2
a,b
90 21.9
b
16.8
b
8.0
a
6.1
a
4.5
a
5.0
a
22.0
b
160 – 6.1
a
4.3
a
5.1
a
25.3
b
For each column, different superscripts indicate significant differences (p< 0.05) among mean droplet diameters.
0
2000
4000
6000
8000
10000
0 50 100 150 200
t
(days)
(mPa.s)
Fig. 2. Shear viscosity (at 2 s
1
,25°C) of olive oil–lemon juice emulsions
prepared with GA (j,m,d), PGA (h,n,s)orEY( ) 1% (w/v) (on a dry
basis) and xanthan 0.45% (w/v) as a function of storage time. Emulsions
prepared with: low energy input (j,h); medium energy input (m,n); high
energy input (d,s).
0.4
0.6
0.8
1.0
1.2
1.4
0 50 100 150 200
t
(days)
Acidity
(% oleic acid)
Fig. 3. Changes in acidity over storage time at 23 °C in virgin olive oil
sample and olive oil–lemon juice emulsions prepared with GA (j,m,d),
PGA (h,n,s)orEY( ) 1% (w/v) (on a dry basis) and xanthan 0.45%
(w/v) as a function of storage time. Emulsions prepared with: low energy
input (j,h); medium energy input (m,n); high energy input (d,s).
1200 D. Paraskevopoulou et al. / Food Chemistry 101 (2007) 1197–1204
3.3.2. Primary oxidation
Lipid oxidation in oil-in-water emulsions is taking place
at the surface of the oil droplets. Many studies suggest that
the interaction between lipid hydroperoxides, the first
products formed by oxidation, located at the droplet sur-
face and transition metals, from the aqueous phase, is the
most common cause of oxidative instability (Mei, McCle-
ments, & Decker, 1998a; Mei, McClements, & Decker,
1998b). The most likely mechanism is the decomposition
of lipid hydroperoxides (ROOH) by the pro-oxidants into
highly reactive peroxyl (ROO
) and alkoxyl (RO
) radicals,
which react with unsaturated lipids within the droplets or
at the oil–water interface, leading to the formation of lipid
radicals. The lipid oxidation chain reaction propagates as
these lipid radicals react with other lipids in their immedi-
ate vicinity.
Fig. 4 shows the storage stability of virgin olive oil and
GA/X-, PGA/X- and EY/X-stabilized olive oil/lemon juice
salad dressings. Oxidation was more rapid in olive oil sam-
ple than in salad dressings. The rate of hydroperoxide for-
mation increased sharply in olive oil after an induction
period of thirty (30) days. The polysaccharide- and egg
yolk-stabilized emulsions exhibited a significantly lower
peroxide value (PV) (p< 0.05) than that of the oil through-
out the storage period, which indicated the effectiveness of
the above emulsifiers/stabilizers in the keepability of the
salad dressings. At the time (100 days) olive oil reached a
peroxide value of 75 meq O
2
/Kg oil, GA/X- and EY/X-sta-
bilized salad dressings had seven times lower peroxide val-
ues while PGA/X-stabilized emulsions four times.
3.3.3. Viscosity
The viscosity increase of the continuous phase (lemon
juice) and hence the slowing down of the movement of the
reactants present in it may account for their ability to retard
lipid oxidation. The effect of polysaccharide addition on the
viscosity of lemon juice/olive oil emulsions has already been
examined (Paraskevopoulou et al., 2005). Measurement of
viscosity of the salad dressings at various stages of storage
indicates that increased viscosity contributes to an increased
stability to lipid oxidation (Fig. 2).
3.3.4. Droplet surface
Additionally, the presence of GA and PGA at the oil
droplet surface, due to their surface-active parts in their
molecules, may also contribute to the oxidative stability
of the salad dressings. Lipid hydroperoxides are surface-
active and therefore migrate to and concentrate at the
surface of the emulsion droplets, being susceptible to inter-
actions with aqueous phase oxidation catalysts, such as
iron (Nuchi, Hernandez, McClements, & Decker, 2002).
Proteins, as well as polysaccharides, are often used in food
emulsions to stabilize droplets against flocculation or coa-
lescence and to improve their textural properties (Dickin-
son, 1992; McClements, 1999). According to McClements
and Decker (2000), adsorbed emulsifiers are likely to be
particularly effective at retarding lipid oxidation because
of the ability of the interfacial membrane to act as a phys-
ical barrier that separates lipid substrates from pro-oxi-
dants in the aqueous phase. Additionally, certain types of
emulsifier molecules, containing either sugar or amino acid
moieties (e.g. gum arabic, soluble soybean polysaccharide
and proteins), may act as a chemical barrier to lipid oxida-
tion by scavenging free radicals (Matsumura, Satake,
Egami, & Mori, 2000; McClements & Decker, 2000). The
inhibiting effect of peptide-bound polysaccharide GA
against lipid oxidation in methyl oleate or methyl linoleate
emulsions emulsified by b-casein or other surfactants
(sugar ester S-1670, Tween 20) has been reported by Mat-
sumura et al. (2000, 2003). Another mechanism that may
account for the capability of the three polysaccharides to
inhibit lipid oxidation is metal ion chelation. Xanthan
has been shown to suppress oil peroxidation by inactiva-
tion of Fe
2+
ions, present in the system (Karadjova,
Zachariadis, Boskou, & Stratis, 1998), due to its ability
to chelate metal ions at negatively charged pyruvate sites
(Shimada et al., 1992, 1994).
3.3.5. Emulsifier/stabilizer type
As can be seen (Fig. 4), hydroperoxide formation in the
salad dressings was significantly affected by the emulsifier/
stabilizer type used for their preparation. During the first
days of storage all the emulsions appeared to be equally
Table 2
Quality characteristics of the virgin olive oil used in the experiments
Quality characteristics Value Quality limits
a
Acidity (percentage oleic acid) 0.49 61%
Peroxide value (meq O
2
/kg of oil) 9.65 620
K
232
[where K = absorbance/C (g/100 ml oil)] 1.72 62.50
K
270
[where K = absorbance/C (g/100 ml oil)] 0.20 60.20
a
Set by the EU Regulation 2568/91.
0
20
40
60
80
100
0 50 100 150 200
t
(days)
PV
(meq O2/kg oil)
Fig. 4. Changes in peroxide values (PVs) over storage time at 23 °Cin
virgin olive oil sample (r) and olive oil–lemon juice emulsions prepared
with GA (j,m,d), PGA (h,n,s)orEY( ) 1% (w/v) (on a dry basis)
and xanthan 0.45% (w/v) as a function of storage time. Emulsions
prepared with: low energy input (j,h); medium energy input (m,n); high
energy input (d,s).
D. Paraskevopoulou et al. / Food Chemistry 101 (2007) 1197–1204 1201
effective at stabilizing olive oil droplets. After 20 days of
storage, GA/X and EY/X were found to be more effective
than PGA/X since both are better emulsifiers than PGA.
Their ability to protect lipid hydroperoxides is likely due
to the ability of these molecules to adsorb at the oil droplet
surface, forming a film of high surface shear viscosity.
3.3.6. Oil droplet size
Oil droplet size influence on hydroperoxide formation
rates is also seen in Fig. 4. For a fixed oil droplet concen-
tration (50% in our experiments), an increased rate of oxi-
dation was expected as the droplet size decreased, because
of the increased surface area that was exposed to the aque-
ous phase. However, no dependence of the lipid oxidation
rate on droplet size of olive oil/lemon juice emulsions was
observed at any point of time during their storage. This
was explained by the limited amounts of lipid hydroperox-
ides (10 meq O
2
/kg oil on preparation day) that were
available in the emulsion systems and might have been
present at the droplet surface. Other researchers have
reported similar droplet effects on lipid oxidation (Osborn
& Akoh, 2004; Roozen, Frankel, & Kinsella, 1994). If there
was a high concentration of hydroperoxides in the systems,
then increasing the surface area by decreasing the oil drop-
let size might increased their concentration at the interface,
leading to the expected increase of lipid oxidation with
decreasing droplet size.
3.3.7. Oxidation products
During lipid oxidation, a number of decomposition reac-
tions occur simultaneously, that in turn result in the gener-
ation of a wide variety of different molecules, including
aldehydes, ketones, alcohols and hydrocarbons. These oxi-
dation products are responsible for the characteristic phys-
icochemical and sensory properties of oxidized oils, since
they are likely to be more surface-active than the initial lipid
and some of them are water-soluble. The measurement of
the K
232
specific extinction coefficient was used to determine
the level of conjugated dienes present in the emulsified oil. It
is known that the oxidation products of oils and fats, which
may result from their decomposition, display characteristic
spectra in the ultraviolet region and at about 232 nm.
Therefore, a determination of the absorbance at 232 nm is
an indication of the state of oxidation of a fat.
The K
232
coefficient of specific extinction showed a sig-
nificantly (p< 0.05) higher initial value for olive oil–lemon
juice salad dressings stabilized with both GA (3.2) and
PGA (4.0), in comparison to olive oil (1.75). This is
probably due to carbonyl compounds present in lemon
juice being different from those due to oil rancidity (Fig. 5).
The K
232
coefficient in virgin olive oil increased with stor-
age time and the limit of 2.50 was exceeded after 100 days of
storage. In salad dressings, on the other hand, the K
232
coef-
ficient remained practically constant during the storage per-
iod, indicating a delay in the formation of oxidation
products. This was expected because of the delayed devel-
opment of hydroperoxides in the emulsions (Fig. 4). Emul-
sions stabilized with a mixture of PGA/X exhibited K
232
values (4.0) significantly higher than the corresponding
ones for GA/X-stabilized emulsions (3.5). Likewise, EY/
X-stabilized emulsions registered a constant value for K
232
(3.4) throughout the storage period. This was probably
due to the fact that egg yolk proteins are below their isoelec-
tric point in the pH (3.5) of the emulsion systems, thereby
producing positively charged emulsion droplets. As Osborn
and Akoh (2003) have pointed out, at this pH value, the
positively charged metal ions can not bind to the emulsion
droplets, which explains the decreased oxidation rates in the
EY/X-stabilized salad dressings.
3.4. Sensory studies
Sensory evaluations have been carried out to character-
ize the salad dressings and to find a correlation with chem-
ical analysis. The sensory evaluation of the salad dressings
appears to be necessary since the increased shelf life of
these products by the addition of polysaccharides, such
as GA and PGA, is futile if sensory properties are unac-
ceptable to consumers. For this reason, a questionnaire
was conducted to detect the acceptability of lemon juice–
olive oil emulsions (4-point intensity scale: 1 = no percep-
tion; 2 = weak; 3 = medium; 4 = extreme). The results
are presented in Table 3. Storage of virgin olive oil at
23 °C revealed the appearance of rancid odour (2.33 ±
1.07) in almost 9 weeks. This was also indicated by the per-
oxide value and K
232
measurements. Lower values for ran-
cidity were measured in all emulsions compared to olive oil
even after 33 weeks.
On the other hand, in the emulsions stabilized with
PGA/X, the intensity of rancidity was not changed signif-
icantly during storage (1.33 ± 0.65–1.58 ± 0.90 after 15
0.0
1.0
2.0
3.0
4.0
5.0
0 50 100 150 200
t
(days)
K
232
values
Fig. 5. Changes in specific extinction coefficient values (K
232
) over storage
time at 23 °C in virgin olive oil sample (r) and olive oil–lemon juice
emulsions prepared with GA (j,m,d), PGA (h,n,s)orEY( ) 1% (w/
v) (on a dry basis) and xanthan 0.45% (w/v) as a function of storage time.
Emulsions prepared with: low energy input (j,h); medium energy input
(m,n); high energy input (d,s).
1202 D. Paraskevopoulou et al. / Food Chemistry 101 (2007) 1197–1204
weeks), whereas in the EY/X or GA/X-stabilized emulsions
the panellists reported a weak perception of rancid flavour
only after 21 weeks (2.00 ± 0.93 and 1.63 ± 0.75–
2.25 ± 1.28, respectively). Finally, the statistical analysis
between salad dressings of similar initial droplet size, stabi-
lized with mixtures of GA/X or PGA/X and homogenized
at 20,500 rpm and 9500 rpm, respectively, revealed no sig-
nificant differences among them.
4. Conclusions
It can be concluded that the polysaccharides, gum ara-
bic and propylene glycol alginate, have the ability to inhibit
lipid oxidation in addition to their stabilizing/emulsifying
capacity. This is probably due to their amphiphilic charac-
ter and partly due to the presence of xanthan. Gum arabic,
in particular, more effectively suppressed the oxidation dur-
ing the storage period. This is attributed to its better sur-
face-active properties in comparison to propylene glycol
alginate. Lipid oxidation was not affected by the oil droplet
size, as demonstrated by peroxide value measurements and
sensory evaluation.
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09 04142133
1.42 ± 0.51
a
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b
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a
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a
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a
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a
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a,b
1.11 ± 0.32
a
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a,b,c
1.75 ± 0.46
b,c
2.00 ± 0.89
c
a–c: Different superscripts mean that the results in each row for each polysaccharide are significantly different (p< 0.05).
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1204 D. Paraskevopoulou et al. / Food Chemistry 101 (2007) 1197–1204
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BACKGROUND The starch retrogradation and moisture migration of boiled wheat noodles (BWNs) result in quality deterioration and short shelf life. The objective of this research was to investigate whether konjac glucomannan (KGM) could improve the quality of BWNs and further establish the shelf-life prediction model. RESULTS The moisture distribution, recrystallization and thermal properties of BWNs during refrigerated or ambient temperature storage were determined. Low-field nuclear magnetic resonance data showed that KGM addition induced left-shifts of T21 and T22 values, indicating that KGM limited the mobility of bound and immobile water among noodle matrices. X-ray diffraction spectra revealed that KGM did not change the crystal patterns of BWNs but could inhibit the starch recrystallization after refrigerated storage. The Tp and ΔH values of retrograded samples notably (p<0.05) decreased with the increase of KGM addition, suggesting the hinderance of starch retrogradation behavior by KGM. The shelf life of BWNs was predicted by accelerated storage test combined with Arrhenius Equation. Present data displayed that the predicted shelf life of vacuum-packed and sterilized BWNs with 10 g∙Kg⁻¹ KGM at 25°C was 733 days, 2.4 folds of the control group. CONCLUSION The BWNs with KGM addition could inhibit starch retrogradation and improve the storage stability, consequently promoting the noodle quality. This article is protected by copyright. All rights reserved.
Book
Food Emulsions: Principles, Practice, and Techniques, Second Edition introduces the fundamentals of emulsion science and demonstrates how this knowledge can be applied to better understand and control the appearance, stability, and texture of many common and important emulsion-based foods. Revised and expanded to reflect recent developments, this second edition provides the most comprehensive and contemporary discussion of the field of food emulsions currently available. It contains practical information about the formulation, preparation, and characterization of food emulsions, as well as the fundamental knowledge needed to control and improve food emulsion properties. New features include updates of all chapters, a critical assessment of the major functional ingredients used in food emulsions, and reviews of recent advances in characterizing emulsion properties.
Chapter
The exudate gums were amongst the first thickening, emulsifying and stabilising agents used in food. Despite competition from other materials they continue to be used in large quantities. Indeed, in food, the quantity of gum arabic used exceeds any other Polysaccharide additive apart from starch and its derivatives.
Chapter
Alginate is one of the most significant of all the hydrocolloids used in food. In order to understand why this is so, this chapter describes those alginate characteristics which are crucial for food applications. Emphasis is placed on how the nature of brown algae used for alginate extraction determines the alginate chemistry and thereby the functional properties and applicability of alginates. Knowledge recently obtained from elaborate genetic engineering and biotechnological research (Ertesvåg, 1994) describes a possible way of controlling alginate chemistry by the use of ‘manmade’ alginate modifying enzymes.
Article
Xanthan gum was the first of a new generation of polysaccharides, produced by biotechnology. The polymer was discovered by the United States Drug Administration (USDA) and classified under the name B-1459 (xanthan gum). The gum, produced by Xanthomonas campestris NRRL B-1459, appeared to have valuable properties that would allow it to compete with natural gums. The production of xanthan gum started in the 1960s in the USA. Today there are four major suppliers worldwide and smaller manufacturers in Japan, Europe and the USA.
Chapter
Xanthan gum was the first of a new generation of Polysaccharides produced by biotechnology. The polymer was discovered by the US Drug Administration (USDA) and classified under the name B-1459 (xanthan gum). The gum, produced by Xanthomonas campestris NRRL B-1459, appeared to have valuable properties that would allow it to compete with natural gums. The production of xanthan gum started in the 1960s in the USA. Today there are four major suppliers world-wide and smaller manufacturers in Japan, Europe and the USA.
Article
The effect of emulsifier type, droplet size, and oil concentration on lipid oxidation was determined for caprylic acid/canola oil structured lipid-based emulsions. Oil-in-water emulsion samples were prepared with sucrose fatty acid ester or whey protein isolate and 10 or 30% oil, and then homogenized at 1000 or 10,000 psi to form different particle sizes. The peroxide values, anisidine values, and TOTOX values of emulsions stored at 50 °C were measured over time. Decreasing oil concentrations led to an increase in total oxidation. Whey protein isolate had a significant antioxidant effect on the oxidation rates compared to sucrose fatty acid esters. Particles size had no effect on lipid oxidation in structured lipid-based oil-in-water emulsions.
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The antioxidative effects of xanthan and other polysaccharides were evaluated by measuring the Fe2+ -induced consumption of oxygen dissolved in the oil/water emulsion. The degree of oxygen consumption was, from the lowest, in the order of xanthan < pectin <guar gum ≤ tragacanth gum, and was closely related to the Fe2+-binding ability of the polysaccharides. The oxygen consumption was also affected by the viscosity of the aqueous solution in the emulsion, but not by the oil-droplet sire. The antioxidative mechanism for xanthan can be accounted for primarily by its high metal-binding ability, and additionally by its viscous behavior.