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