Effect of addition of plant extracts on the durability and sensory properties of oat flake cookies
Journal of Thermal Analysis and Calorimetry
Effect of addition of plant extracts on the durability and sensory properties of oat flake cookies
Anna Zbikowska 0 1 2 3 4
Mariola Kozlowska 0 1 2 3 4
Andrzej Poltorak 0 1 2 3 4
Malgorzata Kowalska 0 1 2 3 4
Jaroslawa Rutkowska 0 1 2 3 4
Milena Kupiec 0 1 2 3 4
0 Faculty of Food Sciences, Warsaw University of Life Sciences - SGGW , Nowoursynowska 159C, 02-787 Warsaw , Poland
1 & Malgorzata Kowalska
2 & Anna Zbikowska
3 Faculty of Material Science, Technology and Design, Kazimierz Pulaski University of Technology and Humanities in Radom , Chrobrego 27, 26-600 Radom , Poland
4 Faculty of Human Nutrition and Consumer Sciences, Warsaw University of Life Sciences -SGGW , Nowoursynowska 159C, 02-787 Warsaw , Poland
Fragile cookies are one of the most popular high-fat cereal products. Products of this type with added oat flakes may be labeled with a nutrition claim and may be included in functional foods. Fat from oat flakes and which is a recipe ingredient of cookies undergoes degradation during storage which leads to the formation of harmful oxidation products and sensory changes in products. The aim of this study was to determine the effect of green tea extracts (GT), blackcurrant (BC) seeds and nettles (N) on the rate of fat degradation in cookies with oat flakes and their sensory properties. In this work, the antioxidant effect of natural extracts antioxidant on oxidative stability of fat extracted from cookies was evaluated by differential scanning calorimetry (DSC) and by peroxide value, anisidine value and specific UV extinctions (K value). Moreover, cookies were subjected to sensory analysis and to instrumental measurements of color changes. The extracts used in the study had an influence on the rate of fat oxidation changes in cookies after baking and also during 3 months of storage. The most effective fat degradation inhibitor was the extract with GT in 1% addition, while the most unfavorable change was observed in fat with the lowest addition of N extract (0.5%). It was observed that the applied plant extracts did not significantly reduce the sensory properties of the products. The most desirable products were the cookies with 1% GT extract and those with the highest proportion of blackcurrant seed extract (1.5%).
Oat flake cookies; Plant antioxidants; Oxidative stability
Scientists, consumers and food manufacturers are
increasingly aware of the importance of food products in
maintenance of good health (called functional or therapeutic
]. By adding to typical cookies biologically active
ingredients (nutraceutics) rich in undesirable dietary
saturated fatty acids (SFA) as well as trans isomers [
], such as
dietary fiber (DF), one can improve their nutritional value,
so that they become included within functional foods. DF is
a very important food ingredient, because its high
proportion in the diet plays a major role in prevention of
civilization diseases such as diabetes, hypertension,
cardiovascular diseases, obesity, certain cancers and
A rich source of DF is oats, which contain at the same
time much more fat, rich in polyunsaturated fatty acids,
than other cereals [
]. Such fat is unstable, because of the
rapid oxidation process. Thus, oat products, such as oat
flakes, made into cereal products such as cookies, suffer
reduced durability [
The effect of lipid oxidation is formation of unwanted
flavors and odors that shorten the shelf life of food. At the
same time, the nutritional value and safety of food products
decrease as a consequence of the formation of harmful
oxidation products, which are associated with a number of
degenerative processes and disorders, such as
inflammation, cardiovascular diseases and cancer [
antioxidants to the food is an effective preventive element
for the lipid oxidation process. Concerns about the effects
of synthetic additives generate the need to find antioxidants
of natural origin. However, their applicability must be
checked beforehand, including their impact on the final
product, particularly on its sensory characteristics [
Green tea, nettles or black currant seeds, for example, may
have protective effect on fats in bakery products. The
literature includes reports of their antioxidant properties. The
antioxidant activity of green tea in food is associated with
the presence of catechins [
]. Flavonoids, especially
phenolic compounds, are present in nettles [
blackcurrant seeds (waste from juice production) can be a
source of natural antioxidants, and their extracts have
antioxidant effects in oil [
The aim of this study was to determine the effect of
plant extracts obtained from green tea, blackcurrant seeds
and nettles on the rate of fat degradation in cookies with
oat flakes and on their sensory properties.
Materials and experiments
The study material consisted of oat flake cookies and
extracted fat from them. Based on preliminary studies and
information in the scientific literature on the antioxidant
efficacy of green tea extract [
], oat flake cookies with 1%
green tea extract were accepted as a control sample in this
Dough ingredients: oat flakes—33% (Kupiec sp. z. o.o.,
Poland), shortening—15% (AarhusKarlshamns, Sweden);
also wheat flour (17%), sugar (15%), milk powder (5%)
and eggs (15%), were purchased locally.
Aqueous plant extracts in the form of lyophilisate
(Goldmann HSH Sp. z o.o.) were used in the following
doses: green tea 1%, blackcurrant seeds and nettles in the
amounts 0.5, 1.0 and 1.5%. The formula for preparing
cookies contains as much as 33% of oat flakes (with a fiber
content of 9.5%), which allows them to be classified as
functional food. In accordance with the provisions in force,
fiber-rich foods (at least 3 g/100 g serving) may include
nutrition and health claims [
Cookie preparation method
The ingredients (shortening, sugar, milk powder and eggs)
were stirred for 5 min using the Braun Multiquick kitchen
processor (type 4644) to obtain a creamy texture. Then, the
flour, the plant extracts and oat flakes were added. A
square-shaped piece of dough with 55 mm sides and a
thickness of 4 mm was baked at 170 C for 8 min in the
Unox convection steam oven, type XBC (Vigodarzere,
Italy). Cookies were stored for 1, 2 and 3 months in carton
packs at 21 ± 2 C.
Methods of fat extraction
The lipids were extracted from cookies and from oat flakes
and extraction was carried out by Folch et al.’s method [
using a chloroform–methanol (2:1 v/v), at room
temperature. The extract was dried over Na2SO4.
Fatty acid composition of the shortening and fat extracted
from oat flakes was determined by gas chromatography
according to the ISO 5508:2000 standard. The fatty acids
were esterified according to ISO 5509: 2000 and applied to
the column (Agilent 6890 GC System); a flame ionization
detector and capillary column (60 m 9 0.25 mm ID SGE
BPX 70) were used. The oven temperature was
160–210 C, increasing at a rate of 2.5 C min-1. The
carrier gas was helium, air flow rate 30 mL min-1;
injector: split-splitless 240 C; detector: FID 250 C; software:
HP Chemstation v. 3.11. The composition of FA was
expressed as the peak area percentage of total fatty acids.
The peroxide value (PV), anisidine value (AV) and
specific UV extinctions (K values) were determined in
accordance with ISO standard methods (3960:2012;
6885:2008; and 3656:2011, respectively). All analyses
were carried out in three replicates.
DSC measurements were taken with a DSC 820 from
Mettler Toledo (Schwerzenbach, Switzerland) with air flow
of 60 mL min-1. The fat samples of 3.6–4.0 mg were
placed into aluminum pans and closed with lids with a hole
drilled in the center in order to allow the samples to be in
contact with the air stream. The aluminum reference pan as
identical as possible to the sample pan was left empty. The
sample and reference pans were heated linearly to 300 C
with the rates of 4, 7.5, 10, 12.5 and 15 C min-1. The
onset oxidation temperatures (tON, C) were determined as
the intersection of the extrapolated baseline and the tangent
line of the recorded exotherms. The tON experimental
values as a function of heating rates (b) were directly
measured and recalculated as absolute onset temperatures
(TON, K). Using the Ozawa–Flynn–Wall method and the
Arrhenius equation, the kinetic parameters of the oxidation
process (activation energy Ea, pre-exponential factor Z, and
induction time s) were calculated. The calculation
procedures for kinetic treatment were given in a recent
The stability of the lipid fraction in the cookies was
monitored during their storage for 3 months by
determining: PV and AV, coupling dienes and trienes, using
differential scanning calorimetry (DSC).
Analysis of plant extracts
The total phenolic content was determined using the Folin–
Ciocalteu reagent method described by Singleton and Rossi
] with a slight modification. Briefly, 1 mL powdered
plant extract (1 mg mL-1) was transferred into a flask and
mixed with deionised water (10 mL). Then,
Folin–Ciocalteu reagent (0.5 mL) was added to the mixture. After
3 min, 5 mL of Na2CO3 (20%, w/v) solution was also
added. After standing for 1 h at room temperature, the
absorbance was measured at 765 nm. Gallic acid was used
to construct the standard curve. The total phenolic content
was expressed as mg gallic acid equivalents per gram of
Free radical scavenging activity of the powdered plant
extracts was determined by the method of Gow-Chin and
] with some modification. One milliliter of
0.3 mM freshly prepared DPPH methanol solution was
mixed with 0.2 mL (1 mg mL-1) of the extract and 3.8 mL
of methanol. After standing for 10 min at room
temperature, the absorbance at 517 nm was measured. Trolox was
used for constructing the standard curve. The antioxidant
capacity based on the DPPH-free radical scavenging ability
of the extract was expressed as mM Trolox equivalents per
gram of dry matter of extract.
The 2,20-azinobis (3-ethylbenzothiazoline-6-sulfonic
acid) diammonium salt (ABTS)-free radical scavenging
activity was assayed by the method developed by Re et al.
]. Briefly, ABTS was produced by reacting ABTS stock
solution (14 mM) with potassium persulfate (4.9 mM) and
was kept for 12–16 h in the dark at room temperature. For
the analysis, the solution was diluted in water to the
absorbance of 0.7 ± 0.02 at 734 nm. Then, 40 lL of plant
extract and 4 mL of ABTS•? working solution were
mixed. After 6 min, the absorbance of samples at 734 nm
was recorded and compared to that of the calibrated Trolox
standard. Results were expressed as mM Trolox
equivalents per gram of dry matter of extract.
Color analysis of cookies
The color of oat flake cookies was measured using a
tristimulus reflectance colorimeter (Minolta CM-3600d,
Konica Minolta Sensing, Inc., Tokyo, Japan). The intensity
of the color was determined by its three elements: L *
(whiteness, zero blackness), a * (positive value—red,
negative value—green), b * (positive value—yellow,
negative value—blue). The final result was the arithmetic
mean of 12 measurements.
Sensory characteristic of fragile cookies
Sensory evaluation was performed by a trained team of 20
persons, after 24 h of baking.
Detailed quantitative descriptive analysis (QDA) was
performed using the analytical procedure described in ISO
13299:2003. The following characteristics were specified:
appearance (color, cracks on the surface of the cookies),
aroma (herbal, tea, fruity, desirable), texture (hardness,
fragility), taste (sweet, herbal, tea, fruity, desirable) and
overall quality of samples. The obtained values were
converted into numerical values on a scale of 0–10.
The Statgraphics plus 4.0 package (Statistical Graphics
Corp., USA) was used for analysis. Interpretation of results
was based on Tukey’s test with a significance level of
p B 0.05.
Quality of the fatty material, fat from oat flakes and antioxidant properties of the extracts
Fat oxidation stability of fat, its technological suitability
and nutritional value depend on the composition of fatty
]. The high total content of saturated FAs and FA
trans isomers (77.72%) in the applied shortening indicates
a high fat oxidation resistance. The dominating FA in the
shortening was palmitic acid (44.0%) and in oat flakes was
lauric acid (41.9%) and oleic acid (34.3%) (Table 1).
The content of primary oxidation products in the fatty
material expressed in peroxide value (PV) was
0.82 mEqO kg-1 fat. On the other hand, the PV of fat
extracted from oat flakes was higher (2.2 mEqO kg-1 fat)
but also did not exceed the limit (3 mEqO kg-1 fat)
determining good bakery fat quality [
]. The result of the
number of secondary products of oxidation measured by
AV in fats used for baking was 1.2, which, being below 3,
proved their good quality [
]. Similarly, low values of AV
(mean 2.05) were obtained in the lipids extracted from oat
The results of the total phenolic content determination
and antioxidant activity of the examined plant extracts are
presented in Table 1. The highest content of phenolic
compounds was recorded in green tea extract followed by
blackcurrant seeds and nettle extracts. In the DPPH and
ABTS assays, also green tea extract demonstrated the
highest antioxidant capacity while nettle extract exhibited
the lowest antioxidant activity. Green tea contains phenolic
compounds with antioxidant properties. The most effective
antioxidant compounds are catechins that show the ability
to scavenge-free radicals and to chelate metal ions. In
addition, green tea extracts contain compounds such as
terpenes, sesquiterpenes and organic acids. Catechin
molecules are characterized by presence of various
hydroxyl groups on A- and B-rings and hydroxyl group on
carbon 3 of dihydropyran heterocycle (the C-ring). Their
number and position in the molecule are the factors
determining the antioxidant activity of flavonoids [
However, blackcurrant seed extracts contain rutin,
isoquercetin and taxifolin. Compounds belonging to two
major groups were detected in nettle extracts: phenolic
acids and flavonoids. The presence of phenolic compounds,
especially flavonoids and caffeic acid derivatives, in the
nettle extract may also pronounce on its antioxidant effect
although it is significantly less than plants from the
Lamiaceae family [
]. The nettle extract had the lowest
total phenolic content (23.74 mg GA g-1 of the extract)
and presented the lowest value of antioxidant activity in all
Quality of fat from cookies after baking and during storage
Baking caused the formation of primary and secondary
lipid oxidation products, irrespective of variant (Fig. 1a, b).
PVs of fat from cookies after baking were varied and
ranged from 0.84 (variant with 1% GT extract) to
1.22 mEqO kg-1 fat (variant with 0.5% nettle extract). AV
in cookie fat ranged from 1.64 for the sample with 1% GT
to 2.79 for cookie fat with 0.5% extract of N. The content
of conjugated polyunsaturated FA such as hydroxy
peroxides—dienes (K232)—in fat extracted from cookies after
baking was the highest in the control sample (2.98) and
sample with 0.5% addition of N (2.49), while the lowest
(2.23) was in the sample of fat from the variant with 1.5%
BC extract (Fig. 1c). The K268 indicator values were
small, unrelated to the variant and did not exceed 0.75
Changes in oxidation products (PV, LA, K232, K268)
were found in the fat of stored cookies. The first 2 months
of the test showed constant growth in the amount of
primary oxidation products in all variants. PVs ranged from
0.88 (variant with 1% GT) to 1.68 mEqO kg-1 fat (variant
with 0.5% N) and 1.70 mEqO kg-1 (control variant). After
3 months of storage, the PV (in relation to PV after
2 months) was reduced in cookies with GT, BC extracts in
samples with 1.5% extract of N. After the whole test, the
smallest PV characterized the fat from cookies with 1%
addition of GT extract (0.87 mEqO kg-1) and the highest
with 0.5% N extract (1.77 mEqO kg-1)—Fig. 1a.
Increasing time of storage was associated with constant
growth of secondary lipid oxidation products measured by
AV (Fig. 1b), irrespective of variant. Fat from 1% GT
cookies was characterized by the lowest content of
secondary oxidation products. (The AV after 3 months was
5.0.) On the other hand, the fastest increase in AV was
observed for nettle extract samples. (After 3 months, the
AV for fat from cookies with 0.5% N was 10.9 and for
control sample 11.01.)
Changes in the K232 and K268 ratios, as determined by
the fats stored in the cookies, were small. Fat extracted
from stored GT cookies was characterized by the lowest
content of both dienes and trienes (after 3 months,
respectively, 2.35 and 0.37). During storage, the content of
dienes decreased, regardless of variant. Such a change was
not observed for K268 (Fig. 1d).
The onset temperature (tON) is usually taken as a
suitable parameter for characterizing lipid oxidation under
non-isothermal condition. The sample with a higher tON is
more stable than the one for which tON values obtained at
the same heating rates are lower. The DSC curves of fat
samples extracted from cookies without the addition of
plant extracts and fortified with green tea, nettle and black
currant seed extracts at a concentration of 1%, analyzed at
a heating rate of 10 C min-1, are illustrated in Fig. 2.
Their shapes are similar, but they are shifted toward higher
temperatures depending on types of plant extracts added.
The results listed in Table 2 show that the increase in the
heating rate from 4 to 15 C min-1 led to an increase in
tON values for all the tested fat samples, although they were
lower than their counterparts with extracts for the control
fat sample tested after baking without addition of plant
extracts. However, (raw) fat used to prepare cookies is
characterized by a higher onset oxidation temperature than
fat extracted from cookies after baking. Application of high
temperature during baking may influence stability of fat
obtained from cookies. It was also found that stability of
samples tested after baking and containing the plant
extracts was significantly improved in comparison with the
sample without plant material. Addition of 1% of green tea
extract demonstrated the highest tON values. The oxidation
onset temperature also increased for fat samples with the
addition of nettle and black currant seed extracts in
comparison with the control sample. In the case of fat samples
with the addition of nettle extract, the highest onset
oxidation temperatures were observed when nettle extract at a
concentration of 1% was used. A similar observation was
made for the samples enriched with black currant seed
extracts. The oxidation onset temperature also increased for
the fat samples with the addition of plant extracts after
3 months of their storage at room temperature compared to
the sample without the addition of plant extracts. The fat
extracted from cookies containing green tea extract was
characterized by a higher onset oxidation temperature at
each heating rates than their counterparts with nettle and
black currant seed extracts. It was also noticed that the
oxidation onset temperatures of samples with the addition
of nettle extract after 3 months of storage were slightly
higher than the samples after baking. However, the fat
samples containing blackcurrant seed extracts at the
concentrations of 0.5, 1.0 and 1.5%, respectively, exhibit
negligibly lower onset temperatures than their counterparts
after storage at room temperature.
The kinetic parameters, namely activation energy,
preexponential factor and induction time at 180 and 190 C,
which were computed from tON for each sample tested are
shown in Table 3. For fat samples tested after baking, the
activation energy (Ea) values varied from 98.89 to
152.42 kJ mol-1 but after 3 months of storage they ranged
from 85.79 to 140.96 kJ mol-1. The highest activation
energy values were observed for the samples containing
blackcurrant seed extracts among the tested fat samples.
The calculated activation energy values for the tested
samples depended on the type of used plant extracts and
their concentration. In the case of nettle extracts, the Ea
value after baking was the highest at the concentration of
1.5% and after storage at the level of 0.5%. For samples
containing blackcurrant seed extracts, this parameter was
the highest at the level of 1.0% after baking and 0.5% after
storage at room temperature. Based on the induction time
calculated at 180 C, we can see that the average
thermooxidative stability of samples studied after baking and
storage for 3 months was the highest for the fat fraction
with green tea extracts. Taking into account this parameter,
the rank of tested fat samples with the addition of plant
extracts resistant to thermo-oxidative decomposition after
3 months of storage increased in the following sequence: N
0.5% \ N 1.0% \ N 1.5% \ BC 1.5% \ BC 1.0% \ BC
0.5% \ GT 1%.
Effect of addition of plant extracts on cookies color and sensory properties
The type of applied additive significantly influenced the
color parameters (Table 4). The L value decreases as the
content of extracts increases. The highest percentage of red
was shown in cookies with GT extract (? 5.13) and 1.5%
BC extract (? 4.94), while the smallest value characterized
the sample with the lowest addition of N extract (? 3.55).
With the increase in the dose of BC extract, the proportion
of yellow color decreased. The smallest value of the b
parameter was found in the sample with the highest addition
of BC extract (? 19.82).
A statistically significant influence of applied extracts on
the sensory distinctions rating was found (Table 5). Most
of the products obtained high marks for color (above 7.0)—
they were golden brown. The addition of N extract resulted
in lightening of cookie color (variant with 1.5 N rated
lowest—6.8) and in the appearance of cracks on the
surface. In turn, the addition of BC extract caused darkening
and reduced the number of cracks on the surface of
GT extract caused the smell and taste of tea, and in
addition the aroma of herbs in the cookies was the least
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300
noticeable. The addition of blackcurrant extract resulted in
an intensification of odor and fruit flavor. Nettle extracts, in
turn, significantly increased the perception of the aroma
and taste of herbs in cookies. According to evaluators,
cookies with the most desirable smell were characterized
by the largest addition of BC extract, while the cookies
with 0.5% nettle extract had the least desirable odor. In
addition, the sweet taste was the most noticeable in cookies
with a nettle extract but the lowest sweetness was found in
products with BC and GT extract. The type and amount of
additives had the smallest effect on the texture of oat flake
General quality of the products was in the range from
6.4 (variant with 0.5% N) to 7.6 (variant with GT and
control sample). Cookies with green tea extract were the
Heating rate b/ C min-1
C control, GT green tea, BC blackcurrant seed, N nettle
a, b, c—mean values denoted by different letters (in lines) differ statistically significantly (p \ 0.05)
most desirable, while the least desirable were those that
were enriched with 0.5% nettle extract.
Pastry products are very popular because they are tasty and
easy to eat. Such products could be a great carrier of
substances increasing their nutritional value. A product
with addition of 33% oat flakes was created. It can be
considered as functional food for which a nutrition claim
can be made, because a portion would provide at least 3 g
of dietary fiber (DF) [
]. The appropriate level of DF in
the diet is very important, because it plays a major role in
prevention of civilization diseases [
]. At the same time,
a large amount of oat flakes (fat content of 9.5%) was
added to the analyzed products as labile fat (Table 1),
which can easily degrade at high temperatures and during
storage of biscuits. Zbikowska and Rutkowska  observed
that as the amount of oat flakes increased (10, 20 and 30%
in relation to flour), the rate of fat degradation in cookies
also increased. The authors stated that especially 30%
oatmeal additive reduces durability of cookies. The
dominant FA in oat flakes was polyunsaturated FA (76.8%—
Table 1), which easily undergoes degradation processes.
The green tea extract used in the study was
characterized by the highest antioxidant activity (Table 1). The
differences between the results obtained in the work and
the literature data may be due to the differences in
materials which were analyzed.
High temperature during baking had a negative
influence on fat quality [
]. The plant extracts affected the rate
of oxidation changes during baking. The least effective in
inhibiting fat oxidation in cookies was the 0.5% N extract.
Fat from control cookies and from cookies with 1.0% GT
What is more, a very good protective effect was shown by
the GT extract, which confirmed other researchers’ opinion
that green tea extracts can be a good source of antioxidants
added to food [
The extended storage time of cookies negatively affects
the quality of fat contained [
]. Oxidation processes are of
particular significance in cookies as they contain high
counts of fat. In our study, we observed a downgrade in PV
after 3 months at the end of cookie storage (variant with
additions of GT extracts, 1% BC and 1.5% N) compared to
PV after 2 months of storage. This was due to
decomposition of hydroperoxides leading to compounds such as
alcohols, aldehydes and ketones, i.e., to autoxidative
]. As explained by Aidos et al. [
], the peroxide
increases with time to a maximum level and then
decomposes rapidly to secondary products resulting in
decreased PV. This phenomenon was confirmed by high
AVs in cookies stored for 3 months (Fig. 1a, b). The
content of polyunsaturated fatty acids (PUFA) in cookies,
increasing with the addition of oat flakes (33%), which
contain as much as 9.5% of lipids, mainly PUFA (44%),
specifically linoleic acid (42.7%), induced intense
generation of oxidation products as shown by PVs after 3-month
As in the case of baking, the most effective antioxidant
activity was shown in the GT extract. Similarly,
MildnerSzkudlarz et al. [
] observed high antioxidant activity of
fresh green tea extract, obtained just before the study. The
authors found a significant increase in oxidative stability of
fat thermostated (60 C, for 20 days) in biscuit packs.
Table 4 Values of color parameters of cookies with oat flakes
depending on the type and concentration of plant extracts
During oxidation, under the influence of time,
temperature and air, linolenic and linoleic acids are oxidized in
hydroxy peroxides in which double bonds are conjugated—
]. This work shows the changes in the K 232 and
K 268 values. As reported by Del Caro et al. [
K 268 value of fat during storage is influenced by light
access. In our study, the cookies were stored without light,
so the changes in the contents of the trienes were small.
During storage, the content of dienes in the final phase of
the test decreased. A similar trend was observed in studies
by Caponio et al. [
The green tea extract used in our study was
characterized by higher polyphenol content and antioxidant activity
than the extracts obtained from blackcurrant seeds and
nettle leaves. Total polyphenol content in green tea
samples measured by Chrpova et al. [
] varied between 63.7
and 112.8 mg equivalent of gallic acid per gram of dried
leaves. In turn, in the study by Stankovic et al. [
values for total phenolic content of green extracts ranged
from 16.02 to 233.68 mg GA g-1 and were dependent on
the solvent used for the extraction. Also, the origin and
type of tea affect the polyphenol content in extracts. The
polyphenols in green tea belong to various classes of
compounds including: catechins, flavones, their glucosides
and phenolic acids. The presence of catechins in green tea,
which may act as radical scavengers, might also influence
the antioxidant activity of extracts prepared from tea. The
results of our study on polyphenol contents in blackcurrant
seeds are slightly higher than data reported by
Kahkonen et al. [
]. However, the total phenolic content
in nettle extract and its antioxidant activity were lower than
the values obtained in studies by Ghaima et al. [
In conclusion, the applied plant extracts, especially
green tea extract, can be used to retard lipid oxidation and
to extend the shelf life of food products. Jaswir et al. [
using DSC analysis showed that addition of the plant
extracts to the oil reduces the oxidation as evidenced by
longer tON of antioxidant-treated samples. Also Shu-Yao
et al. [
] revealed that extract of Pleurotus citrinopileatus
combined with vitamin E was effective as natural
antioxidant in camelia oil. Similarly, Kozlowska et al. [
observed that oxidative stability of the lipid fraction
extracted from cookies after baking was improved when
spice extracts were used as antioxidants. Turan [
reported that addition of plant extracts increased the
induction period of canola oil. The autoxidation of fats is a
complicated, multistep and exothermic process that may be
studied using DSC technique. DSC analysis can be adopted
as a method with several advantages, such as a shorter
analysis time, small amounts of study samples required and
the possibility of continuous monitoring of the total
thermal effect of lipid oxidation [
]. It is also a good thermal
method for determining kinetic parameters of non-inhibited
and inhibited fats and oils oxidation. The first exothermal
process observed on DSC curves of non-isothermal
oxidation of fat is autoxidation, in which hydroperoxides are
formed and the starting temperature of that process can be
used to calculate activation energy, pre-exponential factor
and oxidation rate constant [
]. Higher tON values are
linked to better oxidative stability or longer shelf life of
studied fats and are useful to evaluate antioxidant activity
of natural or synthetic antioxidants added to samples.
Therefore, in our study fat extracted from cookies
containing green tea extract was found to be more stable in
terms of oxidation than fat obtained from cookies enriched
with other plant extracts and fat without the addition of
natural antioxidant. After 3-month storage of cookies, the
greatest changes in tON values were observed for the
sample without plant extracts, whereas the samples
supplemented with green tea and nettle extracts were more
stable. Application of high temperature during the baking
process promotes formation of Maillard reaction products
responsible for color and taste of food products [
turn, during storage of food samples rich in fat, their fatty
acid composition, presence of antioxidative and
prooxidative concomitant compounds, and interactions between
them are important [
Consumers pay special attention to the sensory
characteristics of fragile cookies, on which the quality of the fat
has a significant effect. According to other scientific work
], the possible positive effect of addition of plant
extracts is not only on nutritional value but also on the
taste. According to the authors, natural antioxidants found
in herbs and fruits increase the polygenic FA stability and
also may contribute to unique sensory properties as they
contain compounds with a specific taste and odor.
Similarly, in our research applied plant extracts influenced the
Cracks on the surface
a, b, c—mean values denoted by different letters (in column) differ statistically significantly (p \ 0.05)
sensory quality of oat flake cookies. Most evaluators
considered the taste and smell of the products as desirable.
They positively referred to the fruit and tea scent and taste
The type and amount of addition of plant extracts
influenced the rate of fat oxidation changes in oat flake cookies.
The most effective protection was shown by the green tea
extract and the weakest by the nettle extract (regardless of
the amount of the additive).
The addition of plant extracts to oat flake cookies,
regardless of the type or dose, influenced the sensory
properties of oat flake cookies. The most desirable products
were those with the green tea extract and the highest
proportion of blackcurrant extract. Other products were
evaluated as less desirable, but they were still of good quality
and it could be expected that such products would be
accepted by a large group of consumers. The results
obtained from non-isothermal DSC showed that oxidative
stability of fat extracted from cookies after baking was
improved when green tea extract as antioxidant at a
concentration of 1% was used. During storage of cookies for
3 months, it was observed that fat from cookies without
plant extracts was characterized by lower tON values than
samples with the addition of nettle and black currant seed
extracts. The best antioxidant activity was also shown by
the green tea extract. The green tea extract may be a
suitable natural additive to protect fats from oxidation.
Acknowledgements The authors declare no conflict of interest. The
research was carried out within the framework of the research task
financed by funds of the Faculty of Food Science of Warsaw
University of Life Sciences (WULS-SGGW).
Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License (http://creative
commons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
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