Microbial, Biochemical and Food Biotechnology
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Browsing Microbial, Biochemical and Food Biotechnology by Subject "Acetate"
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Item Open Access The production of citric acid by Yarrowia lipolytica when cultivated on edible and waste fats(University of the Free State, 2003-11) Venter, Tania; Kock, J. L. F.; Smit, M. S.; Hugo, A.English: Large amounts of edible fat waste (approx. 100 000 tons p.a.) are generated in South Africa when edible fats, mainly sunflower fat, are used in frying processes. When these fats are overexposed to heat a part of this fat waste becomes toxic to humans. Another part of these fat waste (approx. 50%) are however still fit for human consumption and has been discarded by frying establishments while within regulatory limits. Consequently these wastes have the potential to be processed to safe usable foodstuffs such as citric acid which is mainly imported to South Africa. Since it was shown that Yarrowia lipolytica can convert monounsaturated fats (such as canola fat) to citric acid, the aim of this dissertation became to explore the possibility of this yeast to convert the low cost waste product i.e. used edible fat waste (still fit for human consumption) to a more valuable product such as citric acid in the presence of acetate. In order to achieve this, eighteen strains of Yarrowia lipolytica were grown for 6 days on a medium consisting of 30 g/l sunflower fat in the presence and absence of 10 g/l acetate. It was discovered that the addition of acetate caused a drastic increase in citric acid production by many strains of Yarrowia lipolytica. Strikingly Yarrowia lipolytica UOFS Y-1701 produced increased amounts of citric acid in the presence of acetate i.e. 0.5 g/l in the absence of acetate to 18.7 g/l in the presence of acetate. Similarly, the ratio of citric acid : isocitric acid increased significantly from 1.7:1 in the absence of acetate to 3.7:1 in the presence of acetate after 240 h of growth. During the growth period the acetate as well as the 30 g/l fat was almost completely utilized (100% and 99.3% respectively). This same experiment was repeated, but this time with 30 g/l simulated sunflower fat waste (PTGs = 11%) together with 10 g/l acetate, and as control, 40 g/l simulated sunflower fat waste without acetate. Very low citric acid concentrations were obtained when Yarrowia lipolytica was cultivated in simulated sunflower fat waste in the presence (maximum of 1.0 g/l) and the absence (maximum of 0.3 g/l) of acetate. When the effects of increased levels of acetate and PTGs were calculated on the basis of a 22 factorial design and by interpreting responses in both Chapters 2 and 3, the following were found: (1) The addition of 10 g/l acetate showed a large increase of +4.7 units in citric acid production. This effect was cancelled (citric acid production dropped by –4.7 units) when the PTG levels increased by 10% (m/m). (2) The addition of 10 g/l acetate lead to an increase (+4.1 units) in biomass production while the increase in PTGs decreased its production by –2.3 units. (3) When 10 g/l acetate was added, a large increase in extracellular lipids (i.e. sunflower fat and waste) utilization occurred (+12.0 units) while the opposite was true (extracellular lipids decreased by –7.6 units) when the PTG level increased by 10% (m/m). (4) Interestingly, the intracellular lipids increased in cells when both acetate and PTG levels were increased (i.e. increased by +6.0 and +2.9 units respectively). (5) Furthermore, the pH increased when both acetate (by +0.7 units) and PTG levels (by +0.3 units) were raised. These results indicate that low cost used frying fat waste is probably not a good substrate for citric acid production.