This manuscript clarifies the relationship between flavor formation, lipid metabolism, and microorganisms in fermented fish products.

Mutton fat has a similar fatty acid profile with Cocoa Butter (CB), except that its degree of unsaturation of Triacylglycerol (TAG) at the Sn-2 position is considerably lower than CB and maybe increased by Sn-2 specific lipase to produce Cocoa Butter Equivalent (CBE), a healthy functional lipid. However, there is no commercially available Sn-2 specific lipase that can be used to convert mutton fat to CBE by improving its Unsaturated Fatty Acids (UFA) at Sn-2 position. Similar to plant, yeast fat contains higher UFAs at the Sn-2 position than animal fat. In this study, we investigated the conversion of mutton fat to CBE by fermentation of oleaginous yeast Yarrowia lipolytica which acts as a "Sn-2 specific lipase". The yeast was able to grow on mutton fat as the sole carbon source yielding a dry cell weight of 14.11 g L −1 and 33.1% lipid content after 3 days of cultivation. At optimal fermentation conditions, the degree of unsaturation of TAGs at the Sn-2 position increased from 61.5 (mutton fat) to 89.3% (cellar lipid, 72 h) while the amount of Saturated Fatty Acids (SFA) of the Total Fatty Acids (TFA) was decreased from 58.9 to 34.5%. In addition, the presence of methyl stearate as the cosubstrate in the medium improved the ratio of SFAs/TFAs. It was found that fatty acid profile of the yeast fat with 24.60% palmitic acid, 31.34% stearic acid, 34.29% oleic acid, 5.57% linoleic acid and degree of unsaturation at Sn-2 position in TAGs (84.66%) resembled that of CB when the yeast was grown on mutton fat/methyl stearate (with a ratio of 60/40) as carbon source. These results suggest that biotransformation or metabolism could be directed by using mixtures of inexpensive animal fats and saturated fatty acid or methyl as co-substrates, to produce functional lipids with predetermined composition, such as CBE.

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