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peroxide value grows with an increasing rate (up to Lpa1 point), and then, with a decreasing rate (up to Lpa2

point). From the second inflexion point, the peroxide value simply drops. For oils heated at 120 0 C, curves

denoted as b were obtained. They are characterized by an increasing rate of the rise in the peroxide value from

the initial step up to the LNpb point which is an inflexion point. After this point, the rate begins to fall

continuously. At 140 0 C and 170 0 C (temperature at which potato chips were fried), curves denoted as c were

achieved. They are increasing convex curves between the initial value and the inflexion point LNpc. In this case

the growth rate of the measured parameter gradually decreases, and as soon as the inflexion point LNpc is

achieved and exceeded, the peroxide value regularly drops.

The curves reflecting the dependence between changes in the flavor and time (at 140 0 C) are convex and

decreasing (Fig.2). It was assumed that the critical value of flavor equals 2 points; if this oil parameter drops to

the level of 2 points, it is no longer consumable and cannot be used owing to its flavor.

Fig.1: Curves of changes in peroxide value LN(t) Fig.2: Curves of changes in flavor at 140 o C

Kinetic analysis and conclusions


0 0,5 1,0 1,5 2,0 2,5 3,0 t h

The sets of experimental data were kinetically analysed. The initial curve parts were analysed in the range

from the initial peroxide value to a value remaining slightly above 10.

Kinetic models were applied to analyse the results obtained. The models applied enable the

determination of the process order and rate constant in the investigating process as well as of the

durability periods of the oils used.

For interpretation, kinetic models were used; they enabled the quantitative assessment of changes in the

investigated parameter that relatively quickly reached its critical level. This critical level was considered as a

basis for the determination of oil durability.

Depending upon the time period and temperatures of the heating process, the mechanism of oil oxidation

changes and this phenomenon is evidenced by the different process order values obtained. It was found that

formally mechanism changes in flavor are similar in the same conditions of heated oils, and this fact was

proved by the same process orders as stated for various oils heated at the investigated temperatures.

The calculated parameters (on the basis changes of peroxide values) were applied to evaluate the durability

periods. They decrease with the temperature of the process and amount to: 6.8 to 8.4 hours at a temperature of

90 0 C, 1.3 to 2.7h (120 0 C), 0.3 to 0.9 (140 0 C). The durability periods of oils used to the fry chips reached 0.6 to

1.2 hours.

For comparison, the durability periods were calculated on the basis of changes of the flavor values. They are

as follows: 4.5 to 5.6 hours at a temperature of 120 0 C, 2.7 to 3.2 (140 0 C). It was found that flavor is a less

reliable parameter than peroxide value to calculate the durability periods for various oils heated at the applied


* Lidia Ostasz, Bronislaw Buczek, Elzbieta Kondratowicz Pietruszka, Cracow University of Economics, Faculty of Commodity

Science, Departament of Chemistry and Process Kinetics, 5 Sienkiewicza St., 30-033 Cracow, Poland, Tel.: +48-12-2945353

ext. 47, Fax: +48-12-2945357, E-mail:

S(t) 140







soybean oil sunflower oil rape oil

FORUM WARE 32 (2004) NR. 1 - 4

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