Thema der Doktorarbeit:

Influence of dielectric heating on quality of rape and sunflower seeds

Influence of dielectric heating on quality of rape and sunflower seeds

This study was done to investigate the influence of dielectric heating using microwaves and radio frequencies at different levels of duration and temperature on the moisture content, qualitative characteristics, antioxidant properties and proteins of rapeseeds and two varieties of sunflower seeds with different fatty acid composition. The aim was to find the optimal dielectric heating conditions in order to obtain high quality of oil-seeds and more oxidative stability. Three experiments were conducted using two experimental microwave applicators and one experimental radio frequency applicator. Seeds of two sunflower varieties, i.e. high linoleic and high oleic acid varieties, were used and one cultivar of rape-seed, which served as standard. Different quality indicators were analysed. Untreated seeds were used as control.

In the first experiment, the influence of temperature of microwave heating on quality parameter of rape and sunflower seeds was investigated. For part of this experiment linoleic sunflower seeds were dehulled to study the effect of dehulling on the influence of microwave heating on the quality of the oilseeds and their extracted oils as well as oil content, in comparison to hulled seeds.

The obtained results revealed a significant influence of microwave heating on some quality indicators of oilseeds. As temperature increased, the moisture content of seeds was significantly reduced. In comparison to hulled seeds, dehulling of linoleic sunflower seeds led to the reducing of the moisture content at temperatures 90 and 110 °C without burning of the seeds. The increase of acid and peroxide values of extracted oils from high oleic and linoleic sunflower seeds, which are two of the main indicators of the occurrence of rancidity of oilseeds, was significantly prevented until 70 °C, indicating a high oxidative stability of the heated seeds. The extracted oils from dehulled linoleic sunflower seeds had lower acid and peroxide values in comparison to hulled seeds. The extracted oils from rapeseeds had lower oxidative stability as shown by their acid and peroxide values. Microwave heating at temperatures of 90 and 110 °C led to higher acid and peroxide values of extracted oils from all studied seeds but their phospholipids contents were not significant changed under these conditions. Slight increase of the seed oil content was obtained with increased temperature. Moreover, dehulling of linoleic sunflower seeds facilitated significantly the extractability of the oil in comparison to hulled seeds. No significant influence on the activity of LOX, whereas the activity of PLD was significantly declined. The influence of microwave heating on the antioxidant properties of heated seeds was also investigated. Microwave heating had a significant influence on the activity of APX, which was enhanced with increased temperature in each of high oleic, hulled linoleic and dehulled linoleic sunflower seeds. POX is one of the most important indicators of lipid oxidation because of its heat stability. Increased temperature significantly reduced POX activity of the seeds. The total antioxidant capacity of the heated seeds did not changed significantly, although dehulling of linoleic sunflower seeds resulted in a significant increase of the antioxidant capacity compared to hulled seeds. Protein solubility of the studied seeds was increased significantly until 70 °C, and then it was reduced gradually at higher temperatures.

In the second experiment, the influence of duration of microwave heating as well as the increase of internal temperature depending on the duration of microwave heating on quality of rape and sunflower seeds was investigated. Increased duration had less influence on the quality of studied seeds compared to the temperature. Acid and peroxide values of high oleic and linoleic sunflower oils increased as expected with increasing duration. In rapeseed oil, acid and peroxide values were reduced. The activities of LOX and PLD of the seeds fluctuated without clear trend. No change in protein solubility of sunflower seeds was detected, whereas it was increased in rapeseeds. The antioxidant properties, mainly the activities of seed SOD and APX, were not significantly changed whereas POX activity was significantly increased. Moreover, no significant change in oil content of heated seeds was obtained but phospholipids content of extracted oils changed significantly.

In the third experiment, the influence of temperature of radio frequency heating on quality of rape and sunflower seeds was investigated.

In comparison to microwave heating, radio frequency temperatures of 90 and 110 °C did not caused any over drying of the seeds. Moreover, an increase of acid and peroxide values in the extracted oils from linoleic and high oleic sunflower seeds were prevented at these temperatures indicating a higher oxidative stability of the heated seeds. That might be related to deactivation of the lipid degrading enzymes, mainly lipase, LOX and POX. In contrast, rapeseeds oils had lower oxidative stability and their acid and peroxide values were increased. Furthermore, radio frequency heating led to a significant reduction of POX activity of the seeds. No significant change was obtained on LOX and PLD activities of the seeds, except a significant reduction of PLD activity of rapeseeds. A slight increase of oil content of the seeds was obtained, whereas phospholipids content of their extracted oils was not changed. No significant influence on protein solubility of high oleic and linoleic sunflower seeds was ascertained, whereas it was significantly reduced in rapeseeds. Moreover, radio frequency did not affect the total antioxidant capacity as well as the activities of SOD and APX in the studied seeds.

Finally, the obtained results revealed a significant influence of dielectric heating using microwave and radio frequencies on quality indicators of oilseeds. Some of the chemical and biochemical as well as the antioxidant properties of rape and sunflower seeds, the oxidative stability, protein solubility, oil content and quality of extracted oils were improved and the activities of oxidative and lipid degrading enzymes were inhibited, when the seeds were heated until 70 °C as well as when heated for five minutes at almost 70 °C. Through radio frequency treatment, the seeds could be heated at 90 and 110 °C without over drying.