Effectiveness of dimethylpolysiloxane during deep frying

Abstract

In the last decades, silicones and particularly dimethylpolysiloxane (DMPS) added to oils and fats at very low concentrations (1–5 mg/kg) have been widely applied as inhibitors of thermoxidative reactions in deep frying. In this review, information available on the mechanisms of DMPS action during the frying process is summarised. Studies on samples submitted to high temperature in the absence of foods, as well as those on continuous and discontinuous frying, are commented upon. It is concluded that the different effects reported can be attributed to the high number of variables interacting during frying. The maximum effectiveness of DMPS was obtained in discontinuous frying operations when the oil surface becomes unprotected against oxidation due to the absence of the food. On the contrary, in industrial continuous frying, the addition of DMPS was found to be ineffective due to the protection of the surface by steam originating from the food.

Full text

Gloria Márquez-Ruiz
Joaquín Velasco
M. Carmen Dobarganes
Instituto de la Grasa (CSIC),
41012 Sevilla, Spain
Effectiveness of dimethylpolysiloxane during
deep frying
In the last decades, silicones and particularly dimethylpolysiloxane (DMPS) added to
oils and fats at very low concentrations (1–5 mg/kg) have been widely applied as in-
hibitors of thermoxidative reactions in deep frying. In this review, information available
on the mechanisms of DMPS action during the frying process is summarised. Studies
on samples submitted to high temperature in the absence of foods, as well as those on
continuous and discontinuous frying, are commented upon. It is concluded that the
different effects reported can be attributed to the high number of variables interacting
during frying. The maximum effectiveness of DMPS was obtained in discontinuous
frying operations when the oil surface becomes unprotected against oxidation due to
the absence of the food. On the contrary, in industrial continuousfrying, the addition of
DMPS was found to be ineffective due to the protection of the surface by steam origi-
nating from the food.
Keywords: Continuous frying, dimethylpolysiloxane, discontinuous frying, polar com-
pounds, thermoxidation, tocopherols.
1 Introduction
At the high temperatures during frying, fats and oils
become highly susceptible to degradation and tend to
develop undesirable foam due to oxidative polymerisa-
tion. The use of silicones as antifoaming agents in aque-
ous systems is well known, and its action can be
explained by the surface chemical nature of the polymer
[1]. The addition of silicones, especially dimethylpolysi-
loxane (DMPS), to oils at very low concentrations has
been demonstrated to be useful to increase oil stability at
the high temperatures of the frying process. Under these
circumstances, the antifoaming effects found might also
be an indirect result of the lower incidence of oxidation [2].
In general, the extent of the effect found in the literature is
very variable, reaching from a drastic decrease of the
alteration [3–6] to small or non-significant differences be-
tween oils with and without DMPS added at the usual
concentrations [7–9]. The differences between the results
obtained may be due to the complexity of the frying pro-
cess, since oil degradation is influenced by a high number
of variables and parameters. In this respect, changes
from antioxidant to prooxidant activity of DMPS,
depending on the type of heating (either in an oven or on
plates), have been reported [10], although other authors
have found a positive effect in all the experiments, with
oils heated on plates deteriorating more rapidly [11]. Also,
the influence of DMPS has been reported to be depend-
ent on the temperature [12] and the oil unsaturation [13].
On the other hand, the analytical methods used to evalu-
ate fat degradation, i.e., carbonyl value [11, 13, 14], iodine
value [4, 15], viscosity [13–15], volatiles [16], room odour
and flavour scores [15, 17], triacylglycerol oxidation [3,
18], polar compounds [4, 5, 8, 9], and others, may also
have contributed significantly to the variability found, as
deduced from studies wherein several analytical methods
have been applied. These results would suggest interac-
tions between the variables of the frying process and de-
pendence on the analytical method applied.
In this paper, the mechanisms attributed to DMPS action
are reviewed. Justifications for the differences found on
the effect of added DMPS are presented, considering the
variables acting under the different conditions applied,
i.e., thermoxidation in the absence of food, discontinuous
frying or continuous frying operations.
2 Mechanism of DMPS action
According to previous studies, DMPS has a very low fat
solubility (,1 mg/kg) and tends to accumulate at the oil
surface, protecting the bulk oil against oxidation [19]. The
minimum amount that exerts a protective action would
correspond to that forming a monolayer on the oil surface,
as stated by decreasing the amount of DMPS until its
Correspondence:M. Carmen Dobarganes, Instituto de la Grasa
(CSIC), Avda. Padre Garcia Tejero 4, 41012 Sevilla, Spain. Phone:
+34-95-4611550, Fax: +34-95-4616790; e-mail: cdobar@cica.es
752 DOI 10.1002/ejlt.200400999 Eur. J. Lipid Sci. Technol. 106 (2004) 752–758
2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.ejlst.de
Review Article

Eur. J. Lipid Sci. Technol. 106 (2004) 752–758 Effectiveness of dimethylpolysiloxane in frying 753
action disappeared [3, 20]. It has been found that con-
centrations as low as 0.06 mg/kg were enough to provide
an effective protection to the oils. This concentration
depended on the surface-to-oil volume ratio and was be-
tween 0.05–0.06 mg/cm2oil-to-air surface [3].
Apart from the excellent research work carried out by
Freeman et al. [3], the systematic series of studies under-
gone in Japan to elucidate the mechanism of action of
silicone oil in thermoxidation stands out. Conclusions
drawn from these experiments can be summarised as
follows:
(1) The protective effect of DMPS was detected only when
the silicon oil was rather insoluble in fat. Fat soluble sili-
cone oils, such as higher-fatty acid-modified silicone oils,
showed no action. Among the silicone oils tested, those
with viscosity values of 20 and 100 centistokes were
active, while those with viscosity values of 0.65, 1 and 5
centistokes had no protective effect when added at a
concentration of 1 mg/kg, in experiments carried out at
180 7C for 4, 6 and 8 h [21].
(2) Under nitrogen protection, the effect of DMPS dis-
appeared in all the oils assayed [11].
(3) When one drop of soybean oil coloured with Sudan III
was added to oils with and without DMPS, the measure-
ment of the time between the addition and disappearance
of the pigment was longer in oils with DMPS added [22].
(4) DMPS showed a protective effect in all non-con-
jugated unsaturated oils, but not in conjugated unsatu-
rated oils like tung oil [21].
(5) The water and oxygen dissolved in oils with and with-
out DMPS did not account for the ability of DMPS pro-
tection [23].
(6) DMPS seemed to decrease dissolution of iron into fats
and oils [24].
Although some of the results were not clear, overall results
indicated that the protective effect exerted by silicon oils
depended on their solubility and viscosity and confirmed
those found in previous reports suggesting that DMPS
inhibits the convection currents contributing to the
entrance of oxygen into the oil.
Other alternative and/or complementary ways of DMPS
action suggested could be either the formation of a
physical barrier preventing the penetration of oxygen into
the oil, its antioxidant activity by an ionic mechanism
inhibiting the oxidative propagation reactions, or the
exposure to the atmosphere of an inert surface delaying
the action of oxygen [3].
To sum up, although the precise mode of action has not
been conclusively verified, inhibition of the convection
currents contributing to oxidation seems to play an
important role in DMPS action. On the contrary, con-
sideration of DMPS as a true antioxidant is not probable,
as it has been consistently shown that the DMPS effect
takes place only at high temperature and no differences
were found between samples with and without DMPS
during storage or in accelerated oxidation studies [25–
27].
In recent years, the action of DMPS addition has been re-
evaluated, considering the previous studies, to define the
possibilities of using high linoleic and high oleic sunflower
oils in industrial frying as an alternative to palm olein and
hydrogenated oils. The main results of the extensive pro-
ject funded by the European Commission were published
in a special issue of Grasas y Aceites. Results obtained for
DMPS action in thermoxidation experiments in the
absence of foods, in laboratory assays of discontinuous
and continuous frying and in industrial continuous frying
of crisps are summarised in the next paragraphs. Tab. 1
lists the composition of the sunflower oils used for all the
experiments commented where the only important differ-
ence between the oils was fatty acid composition.
Tab. 1. Chemical evaluation of sunflower oils{.
HLSO HOSO
Fatty acid composition [wt-% of oil]:
C16:0 6.8 4.5
C18:0 4.9 4.3
C18:1 21.4 72.4
C18:2 65.1 16.9
C18:3 0.1 0.1
Others 1.7 1.8
Polar compounds [wt-% of oil]:
Total 3.2 2.9
oxTGM 0.9 0.7
TGD 0.6 0.3
TGP ,0.1 ,0.1
DG 1.1 1.3
FFA 0.6 0.6
Minor components:
a-tocopherol [mg/kg] 603 631
g-tocopherol [mg/kg] 28 13
Fe [mg/kg] ,4.4 ,4.4
Cu [mg/kg] ,1.3 ,1.3
{Abbreviations: HLSO – high linoleic sunflower oil,
HOSO – high oleic sunflower oil, oxTGM – oxidized
triacylglycerol monomers, TGD – triacylglycerol
dimers, TGP – triacylglycerol polymers, DG – dia-
cylglycerols, FFA – free fatty acids.
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754 G. Márquez-Ruiz et al. Eur. J. Lipid Sci. Technol. 106 (2004) 752–758
3 Thermoxidation in the absence of food
The main objective of this study was to clarify the effect of
DMPS and its dependence on the main known variables
influencing the frying process, i.e., temperature, surface-
to-oil volume ratio, type of heating, unsaturation degree,
and heating period [5]. Analyses included polar com-
pound determination by silica column chromatography,
the method widely used for evaluation of total degrada-
tion compounds in used frying oils [28], as well as further
quantification of oxidised and polymeric compounds by
high-performance size exclusion chromatography [29].
Samples of 25 or 50 mL oil (high linoleic sunflower oil
(HLSO), high oleic sunflower oil (HOSO), and a 50% mix-
ture of both of them), without or with 2 mg/kg DMPS, were
heated in 150-mL standard beakers, thus providing an
excellent control of the surface-to-oil volume ratio vari-
able, 1 and 0.5 cm21, respectively. Two temperatures, i.e.,
170 7C and 180 7C, were selected. Heating of the samples
was carried out either in an oven or on plates. The main
difference between the two treatments consisted in the
occurrence of a temperature gradient when the sample
was heated on plates, while the bulk of the sample was at
the same temperature in the oven. Finally, depending on
the values of the main variables, oils were heated for 5,
10, and 20 h, even though 5-h heating samples were pre-
pared for all the combinations.
Analysis of variance demonstrated that two-way interac-
tions between type of heating/DMPS addition, type of
heating/oil unsaturation, surface-to-oil volume ratio/
DMPS addition and surface-to-oil volume ratio/oil unsat-
uration were significant, thus indicating the complexity of
the frying process.
Fig. 1 shows selected representative results for DMPS
action. As can be observed, strong differences were
obtained when samples were prepared on plates, while
no clear differences were found for samples heated in an
Fig. 1. Influence of the type of heating on DMPS effec-
tiveness: Polar compounds formation after thermoxida-
tion for 10 h (void bars: oils without DMPS; filled bars: oils
12 mg/kg DMPS). HLSO, high linoleic sunflower oil;
HOSO, high oleic sunflower oil.
oven, showing the strong interaction between the two
variables. Values of polar compounds obtained on plates
were between 3- and 4-fold lower than those found in the
oven, clearly suggesting that DMPS addition had a dras-
tic positive effect on frying fat alteration when a tempera-
ture gradient was established. The differences found
would demonstrate the existence of a strong protection of
DMPS against oxidation during heating on plates, as
other reactions contributing to an increase in polar com-
pound levels under frying conditions, like thermal poly-
merisation or cyclisation (due to the high temperature),
are expected to be of the same order, while hydrolysis
was avoided due to the absence of food moisture.
Interestingly, the significantly higher degradation of con-
trol samples when heating on plates would indicate that,
under these circumstances, the entrance of oxygen is
favoured, possibly due to the higher absorption of oxygen
at the lower surface temperature on plates. On the con-
trary, the degradation level of samples with DMPS heated
in an oven was higher than that found on plates, thus
indicating that DMPS had no protective action when there
was no temperature gradient.
As shown in Fig. 2, very low concentrations of DMPS were
necessary to obtain a positive effect during thermox-
idation on plates. The figure corresponds to samples
heated on plates at a surface-to-oil volume ratio (1 cm21)
much higher than that applied in discontinuous frying,
normally ranging from 0.1 to 0.3 cm21in food outlets and
domestic fryers. As can be observed, polar compounds
decreased considerably in samples supplemented with
only 0.06 mg/kg DMPS. Thus, considering that the active
DMPS concentration in oil would correspond to that nec-
essary to form a monolayer on the surface, even lower
concentrations may be active for real surface-to-oil vol-
ume ratios.
4 Industrial continuous frying of crisps
Palm olein, which is widely used in the manufacture of
fried products, was used as reference oil in industrial
continuous frying assays of crisps. Industrial frying con-
ditions as well as characteristics of the fryer and process
were reported in detail [30].
Fig. 3 summarises the results obtained for polar com-
pounds after a 10- and a 20-hfrying [8]. As can be observed,
DMPS was not effective as no differences in degradation
levels were found between oils with and without DMPS. It is
interesting to remark that no significant differences due to
DMPS addition were found, neither for any of the com-
pounds analysed, i.e., polymers, oxidised triacylglycerols,
and tocopherols [8], nor for sensory evaluation [31, 32].
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Eur. J. Lipid Sci. Technol. 106 (2004) 752–758 Effectiveness of dimethylpolysiloxane in frying 755
Fig. 2. Influence of DMPS concentration on
polar compounds formation in high linoleic
sunflower oil thermoxidized at 170 7C on plates
(void bars: 5-h heating; filled bars: 10-h heat-
ing).
Fig. 3. Effectiveness of DMPS addition (2 mg/
kg) in industrial continuous frying of crisps:
Polar compounds formation at 10 and 20 hours
(void bars: initial oil; striped bars: 10-h frying;
filled bars: 20-h frying). PO, palm olein; HLSO,
high linoleic sunflower oil; HOSO, high oleic
sunflower oil.
Due to the good manufacturing frying practices applied,
i.e., fryer design, high rate of production, low period of
turnover, and others, the alteration level of oilsafter 2 d of
frying (10 h/d) was very low, and analytical deviations could
explain that no significant differences were obtained.
Alternatively, the dynamic nature of the surface during fry-
ing might have contributed to the lack of DMPS action.
To explain these results and investigate further the action
of DMPS under industrial conditions, it was necessary to
simulate in the laboratory the main characteristic of con-
tinuous frying operations, which is the continuous pres-
ence of food in the fryer.
5 Laboratory continuous and
discontinuous frying
These experiments were of complementary aid to confirm
the results under practical conditions and to define more
clearly the circumstances under which DMPS would have
a positive action [9]. All experiments were carried out at
170 7C; initial surface-to-oil volume ratio was 0.3 cm21,
and the total period of heating was 6 h. Potatoes peeled
and cut in homogeneous strips were used, and batches of
200 g were fried for 10 min. Differences between both
types of frying procedures are summarised below.
5.1 Simulated continuous frying
The presence of potatoes in the fryer was maintained
during all the heating period. Thus, 32 batches of pota-
toes were fried in each oil, and a minimum initial heating
period of 20 min and a final one of 20 min completed the
6-h heating. It was necessary to add 250 mL oil after the
21st frying operation, to maintain a minimum amount of
oil. The surface-to-oil volume ratio changed from the
initial 0.3 cm21to 0.5 cm21after the 32nd frying operation.
5.2 Discontinuous frying
Ten batches of potatoes were fried in each oil, and inter-
vals of 20 min were established between frying opera-
tions. The initial heating period was 50 min, and the final
one was 30 min. No replenishments of oil were made. The
surface-to-oil volume ratio changed from the initial
0.3 cm21to 0.4 cm21after the tenth frying operation.
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756 G. Márquez-Ruiz et al. Eur. J. Lipid Sci. Technol. 106 (2004) 752–758
Fig. 4 illustrates the great differences found in DMPS
effectiveness between simulated continuous and dis-
continuous frying. Results obtained for simulated con-
tinuous frying were similar to those found in industrial fry-
ing; i.e., DMPS exerted no appreciable protective action.
Quite in contrast, DMPS was highly effective in dis-
continuous frying, supporting the notion that oil degrada-
tion occurred mainly when the food was not present and
the surface was unprotected from the penetration of oxy-
gen. The effect of DMPS was even stronger, considering
that higher alteration could have been expected in con-
tinuous frying because of the greater increase in surface-
to-oil volume ratio during frying.
Fig. 4. Influence of the type of frying process on DMPS
effectiveness: Polar compounds formation after a 6-h
frying: (void bars: oils without DMPS; filled bars: oils 1
2 mg/kg DMPS). HLSO, high linoleic sunflower oil; HOSO,
high oleic sunflower oil.
Contrarily to the results obtained in discontinuous frying,
the shift to a less unsaturated oil in continuous frying was
more beneficial to decrease alteration than adding DMPS
at low concentration, given that polar compound values
for HOSO were always lower than those obtained for
HLSO at any time of heating [9]. Oils extracted from fried
potatoes were also analysed, and results were very simi-
lar to those obtained for their counterpart frying oils [9].
Fig. 5 includes results for the remaining contents of
a-tocopherol in frying oils after a 6-h frying. It is observed
that the loss of tocopherol from the initial oils (about
600 mg/kg) correlated with both the level of polar com-
pounds (Fig. 4) and oil unsaturation, as was also found in
the thermoxidative assays [5]. Interestingly, as can be
observed, the loss of tocopherols was more pronounced
in HOSO, which is of a lower degree of unsaturation. Later
studies in model systems [33] and in oils of different
unsaturation degree [34] have confirmed that oils were
exhausted of tocopherols at lower levels of polar com-
pounds as the degree of unsaturation decreased. On the
other hand, when samples with similar polar compound
levels from both frying systems were compared, the
Fig. 5. Influence of the type of frying process on DMPS
effectiveness: Remaining tocopherol levels after 6 h of
frying (void bars: oil without DMPS; filled bars: oil 12 mg/
kg DMPS). HLSO, high linoleic sunflower oil; HOSO, high
oleic sunflower oil.
tocopherol loss seemed to be more pronounced in con-
tinuous frying. Taking into account the differences be-
tween both frying procedures, the results would suggest a
higher volatilisation due to the steam water continuously
produced during simulated continuous frying. This higher
loss of natural antioxidants would consequently decrease
the stability of the fried potatoes [35]. In this respect, the
consequence of adding DMPS in discontinuous frying
was highly positive, since oils with DMPS practically kept
the initial levels of tocopherol. This was particularly
remarkable in the case of the less unsaturated oil (HOSO),
since samples without DMPS were virtually devoid of
antioxidants after a 6-h heating.
6 DMPS in fried foods
The last interesting point refers to the concentration of
DMPS in fried foods. Results determining the concentra-
tion of DMPS by atomic absorption both in oils and fried
foods have only been reported by Freeman et al. [3]. They
found that the concentration of DMPS in the oils extract-
ed from the fried chips increased exponentially as DMPS
concentration in the frying oils increased. Thus, the con-
centration was similar in frying oils and fried-chips lipids
for oils containing 1 mg/kg DMPS, double in the fried-
chips lipids for oils containing 2 mg/kg DMPS, and as high
as around 40 mg/kg in the fried-chips lipids for oils con-
taining 5 mg/kg DMPS. These differences were attributed
to the low solubility of DMPS and hence to the adherence
of the excess DMPS to any available surface, including
the walls of the vessel or the surface of the fried product.
The higher the DMPS concentration in the oil, the much
higher is its concentration on the surface of the chips,
since a significant part of excess DMPS would be
mechanically picked up by the fried food. The amount of
DMPS in dietary fried foods is not of toxicological con-
cern, since the upper estimate of acceptable daily intake
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Eur. J. Lipid Sci. Technol. 106 (2004) 752–758 Effectiveness of dimethylpolysiloxane in frying 757
in man is as high as 1.5 mg/kg body weight [36]. Given the
low concentration necessary for a protective effect during
frying, it is difficult that the oil becomes unprotected
under normal conditions of discontinuous frying. In fact,
in this experiment, DMPS concentration in oils after frying
remained between 0.5 and 1 mg/kg. Also, it has been
reported that DMPS continued to be active after 20 frying
operations in oils containing 2 mg DMPS/kg, without
adding fresh oil [2].
7 Conclusions
The most remarkable conclusions drawn from the studies
on the action of DMPS are the following:
(1) Addition of DMPS would not be necessary in con-
tinuous frying, since not only the frying equipment is
designed to avoid light-catalysed reactions and to reduce
air contact, but also the oil surface is well protected by
steam water from the fried food.
(2) Addition of DMPS at very low concentrations has a
positive effect in discontinuous frying.
(3) DMPS is specifically active for oil protection during the
period in which the oil is not protected by steam water
from the fried food.
(4) From the previous conclusions, it is clear that addition
of DMPS would be of particular interest in fried food
shops where the fryers usually remain without food during
significant periods of times, i.e., catering services, fast-
food outlets, restaurants, and others.
Acknowledgements
This study was supported by CICYT (Project AGL 2001-
0505) and Junta de Andalucía.
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[Received: May 4, 2004; accepted: July 28, 2004]
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