DETERMINATION OF FREE FATTY ACIDS.
Since the free fatty acid content of a fat or oil represents a loss of glycerine, the greater the percentage of free fatty acid, the less glycerine is contained in the fat or oil, it is advisable to purchase a fat or oil with the lower free acid, other properties and the price being the same.
While the mean molecular weight of the mixed free fatty acids varies with the same and different oils or fats and should be determined for any particular analysis for accuracy, the free fatty acid is usually expressed as oleic acid, which has a molecular weight of 282.
To carry out the analysis 5 to 20 grams of the fat are weighed out into an Erlenmeyer flask and 50 cubic centimeters of carefully neutralized alcohol are added. In order to neutralize the alcohol add a few drops of phenolphthalein solution to same and add a weak caustic soda solution drop by drop until a very faint pink color is obtained upon shaking or stirring the alcohol thoroughly. The mixture of fat and neutralized alcohol is then heated to boiling and titrated with tenth normal alkali solution, using phenolphthalein as an indicator. As only the free fatty acids are readily soluble in the alcohol and the fat itself only slightly mixes with it, the flask should be well agitated toward the end of the titration. When a faint pink color remains after thoroughly agitating the flask the end point is reached. In order to calculate the percentage of free fatty acid as oleic acid, multiply the number of cubic centimeters of tenth normal alkali used as read on the burette by 0.0282 and divide by the number of grams of fat taken for the determination and multiply by 100.
When dark colored oils or fats are being titrated it is often difficult to obtain a good end point with phenolphthalein. In such cases about 2 cubic centimeters of a 2 per cent. alcoholic solution of Alkali Blue 6 B is recommended.
Another method of directly determining the free fatty acid content of tallow or grease upon which this determination is most often made is to weigh out into an Erlenmeyer flask exactly 5.645 grams of a sample of tallow or grease. Add about 75 cubic centimeters of neutralized alcohol. Heat until it boils, then titrate with tenth normal alkali and divide the reading by 2, which gives the percentage of free fatty acid as oleic. If a fifth normal caustic solution is used, the reading on the burette gives the percentage of free fatty acid directly. This method, while it eliminates the necessity of calculation, is troublesome in that it is difficult to obtain the exact weight of fat.
MOISTURE.
To calculate the amount of moisture contained in a fat or oil 5 to 10 grams are weighed into a flat bottom dish, together with a known amount of clean, dry sand, if it is so desired. The dish is then heated over a water bath, or at a temperature of 100-110 degs. C., until it no longer loses weight upon drying and reweighing the dish. One hour should elapse between the time the dish is put on the water bath and the time it is taken off to reweigh. The difference between the weight of the dish is put on the water bath and the time it is taken off when it reaches a constant weight is moisture. This difference divided by the original weight of the fat or oil × 100 gives the percentage of moisture.
When highly unsaturated fats or oils are being analyzed for moisture, an error may be introduced either by the absorption of oxygen, which is accelerated at higher temperature, or by the formation of volatile fatty acids. The former causes an increase in weight, the latter causes a decrease. To obviate this, the above operation of drying should be carried out in the presence of some inert gas like hydrogen, carbon dioxide, or nitrogen.