1/2006 - Potravinárstvo

INTRODUCTION
Flaxseed and walnut oils are recognized and accepted as a
healthful, edible oils with an outstanding content of αlinolenic
acid (ALA). This fatty acid, belonging to a type
of n-3 fatty acid, as well as its metabolites (EPA and
DHA) may protect against heat disease and their illnesses.
Oils high in n-3 fatty acid should be processed at the lower
temperatures possible to maintain high quality and storage
life. Consequently such oils are screw-pressed with little or
no pre-treatment and little or no external heating to press.
Furthermore, the oil undergoes no refining beyond
sedimentation or filtration. Fresh, unrefined flaxseed and
walnut oils from good seed or nut have an attractive
golden colour, a pleasant, nut-like flavour and mild odour
(Hossenian at al. 2004, Oomah 2009; Sabudak at al.
2007; Tzang at al. 2009; Wiesenborn, 2005).
Preparation of different food products – also containing
polyunsaturated fatty acids, often involves heating or
frying. When foods are heated or fried in oil occur many
complex chemical reactions resulting in the production of
degradation products. These new products influence
functional, sensory and nutritional quality of the oil. One
of the easy to observe changes in heated oil is colour
darkening (Maskan 2003). The changes in colour of
heated oils are the signs of oil deterioration. The colour
may darken as heating proceeds. This is mainly the result
of oxidative reactions. Therefore, colour has been widely
used as an index of oil quality and it can be monitored
using single or multiple wavelengths with a
spectrophotometer, using colour standards, Hunter L,a,b
values or using a Lovibond Tintometer (red, yellow and
blue) (Maskan 2003).
Potravinárstvo
VPLYV TEPLOTY NA FAREBNÚ A OXIDAČNÚ STABILITU OLEJOV
BOHATÝCH NA OBSAH POLYNENASÝTENÝCH MASTNÝCH KYSELÍN
THE EFFECT OF TEMPERATURE ON COLOUR OF OILS RICH IN
POLYUNSATURATED FATTY ACIDS
Kita Agnieszka, Płuciennik Ewa
ABSTRACT
The aim of this work was to study the influence of heating temperature and time on physico-chemical properties of coldpressed
oils rich in polyunsaturated fatty acids. The materials used for investigation were: two flaxseed oils (high linolenic
and low linolenic – Linola) and walnut oil. All oils were heated at three temperatures: 80, 140 and 180 ºC for maximum120
minutes. In fresh oils, as well as after heating for 15, 30, 60 and 120 min, colour (L,a,b) and oxidative changes (PV, AnV,
oxidative stability – Rancimat test) were analysed. It has been stated that heating affected colour of all examined oils. The
changes, observed as colour darkening, increased with heating time and temperature. During heating of oils L and b values
were decreased and a value was increased. The biggest colour changes were observed for walnut oil, while the most stable
colour was observed in low-linolenic flaxseed oil. Under the same conditions (180ºC/120min) ∆E value of walnut oil was 3
times higher than that of Linola oil. The colour changes were correlated with oxidative degradation of heated oils. Oxidative
stability decreased with increasing of heating temperature and the time. At highest heating temperature the oxidative
stability of high-linolenic flaxseed and walnut oils decreased by 75% and 60%, while low-linolenic flaxseed oil by 30%.
The best properties, independent of heating temperature and time, were exhibited by low-linolenic flaxseed oil (Linola).
Keywords: flaxseed oils, walnut oil, temperature, colour, oxidative stability
The increasing popularity of cold-pressed oils (especially
flaxseed oils) cause that they are used also in thermal short
processes however there is a luck of information about
their stability during heating (Wroniak at al. 2006).
Because of that the aim of this work was to study the
influence of heating temperature and time on physicochemical
properties of cold-pressed oils rich in
polyunsaturated fatty acids.
MATERIAL AND METHODS
Material
The material used for investigation were three kind of
cold-pressed oils: flaxseed oil, linola oil (low-linolenic
flaxseed oil) and walnut oil. All oils were obtained from
producing plant in Lower Silesia (Poland) as fresh oils
(after pressing and purification by filtration).
Characteristics of used oils is presented in Table 1.
Thermal processing procedures
Household fryers were used for controlled heating of oils
under laboratory conditions. The heating was
simultaneously conducted in three fryers to check the
repeatability of the results. 1000 cm 3 of oils were heated
at three temperatures: 80ºC, 140ºC and 180ºC for
maximum 120 minutes. Samples of oils were taken after
15, 30, 60 and 120 minutes of heating at each
temperature. The experiment was carried out in three
replications. The data are the mean results obtained in the
3 replications.
ročník 4 29 1/2010