Bleaching techniques impact on some quality parameters in two different cold-pressed oils obtained at farm scale

The consumption of cold-pressed oil, such as linseed (Linum usitatissimum L.) and hemp (Cannabis sativa L.) oil has increased recently. In these oils obtained on the farm, a large number of components able to affect oil quality (mainly chlorophylls) are present. Given this, an experiment was conducted with the following aims: (1) to compare different bleaching methods by applying ultrasound waves and earths in cold-pressed hempseed and linseed oils and (2) to evaluate the effect of different types (acid-activated and natural) and amount (2% and 4%) of earths with different processing temperatures (60 and 80 degrees C) in hempseed oil, utilizing a bleaching method with earths alone. Regarding the linseed oil, any bleaching methodologies adopted must not include ultrasound treatment, as the high temperatures reached determine high levels of oxidation to the detriment of the oil. Concerning the hemp oil, the natural earths, especially at farm level, should be preferred, because in addition to removing chlorophyll satisfactorily, when used at 4% even at the lowest temperature (60 degrees C), an improvement of the stability and nutritional value (Vit. E) of the oil was observed. Practical applications The use of ultrasound, applied together with bleaching earths, is considered an interesting and efficient bleaching method, in order to remove several pigments able to affect quality of many vegetable oils, with the exception of linseed oil. In fact, the use of ultrasound during the bleaching process determines a high oil temperature (120 degrees C) and this, in an oil such as that of linseed characterized by a very high percentage (> 55%) of a-linolenic acid, negatively affects the oil stability by the formation of high quantity of primary oxidation products, evidenced by very high peroxide values. In the bleaching activity carried out at farm level in particular, natural earths should be preferred to the acid-activated ones. The latter, despite demonstrating an effective bleaching efficiency for few specific pigments, could increase the level of free fatty acids, reduce the antioxidant compounds (tocopherols) naturally present, and release undesirable contaminants in the oil.