1/2006 - Potravinárstvo

Colour measurements
Colour measurements were carried out using a Minolta
CR-200 colorimeter. The instrument was standarized each
time with a black and white (L=93,01, a=-1,11, b=1,3) tile.
The colour values were expressed as L (indicating
lightness), a (indicating hue on the green (-) to red (+)
axis) and b (indicating hue on the blue (-) to yellow (+)
axis). Next, colour difference (∆E) was calculated as ∆E=
[(L0-L) 2 + (a0-a) 2 + (b0-b) 2 ] 1/2 , where L0, a0 and b0 are the
colour parameters of the fresh (no heated) oil samples.
Oxidative stability
The oxidative stabilities of oil samples were determined by
the Rancimat 679 (Metrohm, Herisau, Switzerland). In
brief, 2,5g of oil were heated into reaction vessel in
triplicate and heated to 110°C with an air flow of 20L/h.
Volatile products released during the oxidation process
were collected in a flask containing distilled water due to
formation of volatile compounds. The oxidative stability
(OSI) is defined as the point of rapid change in the rate of
oxidation, and the results are expressed in hours (h) .
Statistical analysis
The data were analysed statistically using Statistica ver. 6
programme (2001). For comparison, the results obtained
were subjected to one-way analysis of variance with the
application of Duncan’s test (p≤0.05).
Results of work
Characteristics of oils used in experiment
All used in experiment cold-pressed oils exhibited typical
fatty acids content and physico-chemical properties (Table
1). The predominant in oils were polyunsaturated fatty
acids: linoleic acid (from 12,9% in flaxseed oil to 69,02%
in linola oil) and α-linolenic acid (from 2,59% in linola oil
to 65,77% in flaxseed oil). Other chemical parameters
showed good quality of oils.
Effect of heating parameters on colour of oil
Table 2 shows the change in Hunter parameters L, a, b
values of used oils before and after heating. The fresh oils
differed significantly in lightness. The most light colour
exhibited low-linolenic flaxseed oil (Linola) (L=43,18),
while the darkest - walnut oil (L=37,37). Heating
decreased lightness of all analysed oils. Significant
changes in lightness of flaxseed and walnut oils were
observed at 140 0 C, while in linola oil at 180 0 C. The time
of heating influenced lightness only in the case of walnut
oil. According with increasing heating time oil became
darker.
The a (redness and greenness) value for fresh oils
amounted from -0,67 (flaxseed oil) to 2,71 (walnut oil).
The heating parameters influenced a value of flaxseed and
walnut oils only. The a value increased with increasing
heating temperature and time. The b (yellowness) value of
examined oils before heating amounted from 21,76
(walnut oil) to 33,05 (linola oil). The heating temperature
significantly influenced on b value of all oils. Together
with increasing heating temperature b value decreased,
Potravinárstvo
while heating time only in few case decreased yellowness
of analysed oils. At 180 o C the biggest decrease of b value
was observed for walnut oil (about 59%), while for
flaxseed oil 27% and linola oil 13%.
The ∆E values for heated oils were shown on Figure 1.
The ∆E values for oils heated at lowest temperature (80 o C)
were very low (below 2) and similar for all oils. At higher
temperatures the biggest changes of ∆E values were
observed for walnut oil (especially at 180 o C when ∆E
increased to 15,9), while the most stable colour exhibited
linola oil (∆E=4,5).
Effect of heating parameters on oxidative stability of oils
The highest oxidative stability exhibited linola oil (1,67h)
while the lowest flaxseed oil (1,13h) (Figure 2). Heating
decreased this quality parameter of all analysed oils.
Heating of linola oil at 80 0 C decreased its oxidative
stability by 6%, but for flaxseed oil the decrease was on a
level of 60%. The biggest changes were observed at
highest heating temperature (180 o C), when oxidative
stability of linola oil decreased by 33%, walnut oil by 60
% and flaxseed oil by 75 %.
DISCUSSION
Effect of heating parameters on colour of oils
The colour of oils has been widely used as a subjective or
objective index to determine the quality of used oil. When
oils are heated, they rapidly change from a light yellow to
an orange brown colour. This is the combined result of
oxidation, polymerisation and other chemical changes. In
described experiment heating of oils influenced their
colour by darkening. The decrease of L and b values of
analysed oils were in agreement with the observations of
Choo at al. (2007) and Maskan (2003). The darkening
may be due to reduction of natural pigments (mostly
carotenoids) present in the oil resulting in oxidation or
decomposition during heating. Walnut oil exhibited the
lowest L value (as fresh oil) in comparison with two
flaxseed oils. The bigger amount of pigments as well as
different composition made it more sensitive for colour
changes. It could be also associated with decomposition of
fatty acids – especially polyunsaturated fatty acids. As was
observed oils with higher level of α-linolenic acid (ALA)
were more sensitive on colour changes than linola oil (with
reduced ALA content below 3%). The better stability of
flaxseed oil than walnut oil could be also explained by
presence of natural antioxidant present in this oil (as
lignans) (Abuzaytoun at al., 2006; Bozan at al. 2008).
However the temperature was the main factor influencing
colour changes of analysed oils, also the time was
significant parameter. Longer time prolonged bigger
colour changes. The influence of time on colour of coldpressed
oils could be observed also at room temperatures.
Sikorska et al. (2007) stored extra-virgin olive oil during
12 months and observed that if oil was stored in the light
the ∆E values increased to 16 (while in the darkness were
below 2).
ročník 4 30 1/2010