Utilisation of edible oils and GLA production by Mucor in the presence of acetate

English: Surveys launched across South Africa indicate that many frying establishments abuse their frying oils and fats during the frying process, resulting in degradation and concomitant production of potentially toxic oxidation products. Some of these compounds have been shown to be toxic to animals and in human in vitro studies. Consequently, strict regulations under the Foodstuffs, Cosmetics and Disinfectants Act, 1972 (Act 54 of 1972) were published on 16 August 1996. It is now an offense to use or sell used cooking oil or fat for human consumption containing high levels of these degradation products. Since frying establishments are not allowed to discard their used oils and fats by selling to the public for consumption or dumping into municipal drainage systems, it is important that these oils and fats are collected for re-use in another form. Consequently, the aim of this study was the biotransformation of used oil wastes (containing no toxic substances) to high value lipids containing gamma-linolenic acid (GLA). This polyunsaturated fatty acid is prescribed for the treatment of eczema. In order to achieve this, Mucor circinelloides f. circinelloides CBS 108.16 was first grown on 40 gil unused sunflower oil and, as expected, produced neutral lipids (NL) similar in fatty acyl composition to the original oil. The apparent repression of the Á6 fatty acid desaturation was partially reversed when cells were grown on oil (30 g/l) and sodium acetate (10 gil) as mixed substrates resulting in an increase in GLA content. Furthermore, a three-fold increase in oil substrate utilisation and doubling of biomass production to 19.1 gil occurred when sodium acetate was added to the oil substrate. When sodium acetate (10 gil) was added to a growth medium containing used cooking oil (UCO) similar results were obtained. This experimental procedure was repeated for seven additional Mucor strains and again the stimulatory effect of sodium acetate in combination with UCO was obvious. Next, the effect of different UCO concentrations in the presence of 10 g sodium acetatell on biomass and lipid production was investigated in Mucor circinelloides CBS 108.16. According to our results, a maximum biomass concentration of 48 gIl consisting of 82 % oil yielding about 35 g NUl and up to 900 mg GLNI was achieved. The addition of 30 g UCO/I in combination with 10 g sodium acetate/l proved to be the optimum UCO concentration in order to obtain maximum GLA yield. Similar results with this strain were obtained when UCO was replaced with fresh unused cooking oil. When these experiments were repeated with linseed oil and sodium acetate as sole carbon sources, much less GLA was produced (351 mg GLNI). According to bioreactor studies, the effects of sodium acetate addition can be attributed to the change in pH of the medium during cell growth in the presence and absence of acetate. In the absence of sodium acetate the pH decreased to 2.2, whereas in its presence it increased to about pH 8.0. During metabolism of sunflower oil in the presence of sodium acetate, the percentage of saturated fatty acids in the extracellular lipids increased, suggesting a higher specificity of the fungal lipase for unsaturated fatty acids. When the sodium acetate was omitted from the medium and the pH gradually increased according to a pH profile mimicking the natural increase in pH found in the medium containing sodium acetate, similar results as in the presence of sodium acetate were obtained. This observation indicated that the pH increase alone during cultivation was responsible for the increased sunflower oil utilisation, biomass and GLA production.