Influence of the temperature in the extraction of nuts oils by means of screw press

The extraction of vegetable oils by means of screw presses is considered as a cold extraction. However, in many cases this type of machinery needs a pre-heating of the nozzle in which the separation of the oil takes place so that it is produced in a satisfactory way. In this case, the extraction of almond, pistachio and walnut oil has been evaluated in a screw press, with heating ring for the nozzle, and screw speed variator. To evaluate the influence of the temperature on the oil extraction, 5 points of the press were selected and the temperature was recorded during the extraction process in thermocouples. The temperature of the heater ring varied between ambient temperature and 200 degrees C. At low processing temperatures, an increase in oil temperature was observed due to the friction of the raw material, which reached values around 60 degrees C in almond and pistachio, being somewhat smaller in walnut oil. At higher temperatures (between 100 and 200 degrees C), the oil temperature was not increased above 80 degrees C by the cooling produced by the continuous supply of the fresh raw material. The screw speed also played an important role in the extraction process. Slow speeds increase yields, but increase oil temperature. On the other hand, when the screw speed is increased, the continuous supply of fresh raw material in a faster way contributes to the cooling of the system, avoiding an excessive increase of the temperature. The temperature influenced greatly the physicochemical parameters of the regulated quality of the oils, because in these cases in which the increase of the temperature was higher it observed higher values of acidity, peroxide index and measurements in the ultraviolet (K-232 and K-270). On the other hand, elevated temperatures may originate the appearance of Maillard reaction compounds with antioxidant capacity that may contribute to increase the oxidative stability of the oils. It is therefore important to consider that in the elaboration of vegetable oils by means of the screw press, the temperature can affect the characteristics of the obtained oils, even when the extraction is done at room temperature.