226 MADRAS FISHERIES BULLETIN VOL. XIII, 



the water, especially in the presence of these impurities and of 

 light and warmth, rapidly hydrolyses (decomposes) the oil and 

 sets free some of its fatty acids which are not only malodorous in 

 themselves, e.g., clupadonic acid, but further decompose and cause 

 rancidity ; hence the bad odour and taste of crude, unpurified fish 

 oils. The first purifying process then, is that of removing impuri- 

 ties. By receiving the crude oil from the separating tanks in deep 

 cisterns the solid impurities slowly fall to the bottom, but to pre- 

 vent decomposition and to save time— since the lighter suspended 

 and fluid impurities take very long to settle this precipitation must 

 be hastened. In the ordinary practice of the small factories it is 

 usual to pass the crude oil through a straining cloth of gunny into 

 a boiling pan, usually a vertical cylindrical pan set over an open 

 fire ; the larger impurities are retained in the cloth and the oil is then 

 heated ; this heating coagulates the albuminous matter, drives off 

 the suspended moisture, and sterilises the whole mass of oil which 

 is then poured off into casks or into a mere open storage pit with- 

 out further straining, still less filtering. Since the oil is sterilised 

 and most of the water removed, the oil is to a certain extent guard- 

 ed against chemical action, especially if drawn off at once into 

 tight casks and thus secured from air and light. But the coagulat- 

 ed and other matters still remain in the oil of which they form an 

 appreciable amount ; moreover if stored in open pits, merely 

 covered by planks or mats, air has considerable access to the oil, 

 and since a good deal of the water and impurities still remain in 

 the oil, hydrolysis begins and after some months of storage, may 

 result in a very large percentage — even above 50 per cent (para- 

 graph 94) — of free acids (acidity) and in serious rancidity, with the 

 malodorous and other results above mentioned ; hence a low price 

 for the oil. Moreover, by the open fire method of heating, the oil 

 is often badly scorched which adds a farther unpleasant odour 

 and a dark or even black colour which cannot possibly be removed 

 since it is due to carbonization of part of the oil ; such carboniza- 

 tion" also means loss of so much oil as has been carbonized. 



The Tanur Yard (see paragraphs 64, I17, etc., supra) conse- 

 quently washes its oil immediately after separation : the oil is good 

 to start with, being from fresh steam-boiled fish, and much of the 

 oil is skimmed from the boiling mass and is but slightly contamina- 

 ted with the albuminous matter found abundantly in oil from the 

 presses. If the oil, usually poured into tall separators, is treated 



