3. FOOD ChEMISTRy & bIOTEChNOLOGy 3.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Food Chemistry & Biotechnology
P97 STuDy OF OxIDATION STAbILITy
OF SELECTED VEGETAGLE OILS
HAnA ŠTOUDKOVá, MOnIKA MAXOVá, JIří
KUČERíK and JAnA ZEMAnOVá
Brno University of Technology, Faculty of Chemistry, Purkynova
118, 61200 Brno, Czech Republic,
stoudkova@fch.vutbr.cz
Introduction
Vegetable oils are obtained from different parts of oilseed
plants by cold expression and subsequent extraction
and purification. Since long ago, oils belong to among basic
cosmetic preparations and use in pharmaceuticals, food industry
and other industrial purposes. They embody the whole
series of authenticated positive efects on human organism
and they are used as a part of food supplements 1 .
Food lipids undergo a chain of changes due to ripening,
harvesting, processing and storage. These changes are caused
by several factors including browning reactions, microbial
spoilage and lipid autoxidation. Lipid oxidation is a free-radical
chain reaction that causes a total change in the sensory
properties and nutritive value of food products 2 .
Oxidation processes influence quality of oils and fats.
Characteristic changes associated with oxidative deterioration
include development of unpleasant tastes and odours
as well as changes in color, viskosity, density or solubility 3 .
The others of the effects of lipid oxidation can be loss of vitamins,
and damage to proteins 2 .
Differential scanning calorimetry (DSC) is the technique
used for establishing the oxidative stability of oils and fats
and characterizion of their physical properties 4,5 . Oxidative
stability and deterioration of oils depend on initial composition,
concentration of minor compounds with antioxidant
or prooxidant characteristics, degree of processing, and storage
conditions 6 . Quality and stability of vegetable oils are
important factors that influence its acceptability and market
value.
The induction period (IP) is measured as the time
required to reach an endpoint of oxidation corresponding to
either a level of detectable rancidity or a sudden change in the
rate of oxidation 7 .
Experimental
S a m p l e s
Different vegetable oils from various plant origins were
used in this study. These were Grape Seed oil refined, Crude
Linseed oil, Castor oil, Almond oil refined, Soya Bean oil
refined, Avocado oil refined, Apricot Kernel oil refined, Corn
oil refined and Olive oil refined.
Samples were obtained from M + H, Míča and Harašta,
s.r.o. After opening the bottles with oils there were stored
at 4 °C.
s795
M e t h o d
Differential Scanning Calorimetry (DSC) of each sample
in oxygen atmosphere were performed several times until
reproducible results were obtained. For this purpose Shimadzu
DSC-60 (Kyoto, Japan) was used connected through
TA-60WS with computer, where the data were collected.
The furnace was calibrated by using transition temperatures
of fusion of indium and zinc (melting point: 156.6 °C
for indium, 419.6 °C for zinc).
Samples were measured out 2.5 µl (0.2,
0.5 °C min –1 ), 5.0 µl (1.0, 3.0, 5.0, 7.0 °C min –1 ) and 10 µl
(10.0, 15.0 °C min –1 ) and measured in open aluminum pan.
Flow rate of oxygen was set at 15 ml min –1 and rate of heating
0.2, 0.5, 1.0, 3.0, 5.0, 7.0, 10.0 and 15.0 °C min –1 from
room temperature (25 °C) to 300 °C was applied. Obtained
results were treated by means of enclosed software TA-60.
Results
For the using vegetable oils, the kinetic parameters
important to the determination of induction period were
obtained for non-isothermal DSC measurements. The onset
temperatures of oxidation for various using oils were measured
with scan rates 0.2, 0.5, 1.0, 3.0, 5.0, 7.0, 10.0 and
15.0 °C min –1 .
The parameters A and B were obtained by a comparison
of experimental and theoretical values of onset temperatures
of the oxidation peaks using the program TInD. The values
of kinetic parameters A and B are listed in the Table I.
Table I
Values of the kinetic parameters A and B
Oils A [min] B [K]
Castor 2.72 × 10 –13 1.38 × 10 4
Olive 6.62 × 10 –13 1.32 × 10 4
Soya Bean 1.54 × 10 –11 1.16 × 10 4
Avocado 4.69 × 10 –11 1.12 × 10 4
Grape Seed 4.43 × 10 –12 1.19 × 10 4
Almond 5.26 × 10 –11 1.10 × 10 4
Corn 1.00 × 10 –10 1.08 × 10 4
Apricot Kernel 1.29 × 10 –10 1.06 × 10 4
Crude Linseed 8.99 × 10 –12 1.12 × 10 4
The temperature dependence of the induction period can
be expressed according to the Equation (1):
t i = A exp (B/T) (1)
Providing that parameters A [min] and B [K] were obtained
by minimizing the sum of squares between experimental
and theoretical values of heating rate and T [K] is specific
temperature, induction period t i [min] can be determined.
Induction period values (t i ) of various using vegetable
oils calculated for temperature 25 °C and 100 °C are shown
in Table II.