Test Kit Technologies

Spectrophotometer vs Titration

Spectrophotometer vs. Buret

Michael M. Blumenthal, Ph.D.

Basic analysis of frying oils in food quality control laboratories requires rapid turn around, low cost, low maintenance, safe handling, and simplicity. The primary choice remains the titration for % Free Fatty Acids. The hydrolysis of heated oils is due to the introduction of materials from frying food including water & ionic salts, and also oxygen from the air. The triglyceride splits to form diglycerides, monoglycerides, glycerine and free fatty acids.

The acidity associated with fatty acid accumulation in the oil is tightly correlated with the shelf-life of the fried food, and loosely correlated with the heat transfer properties of the oil in the vat. Fatty acids readily oxidize in the oil to form rancid and other off-odors. The determination of smaller amounts of fatty acids is all that is necessary to estimate the potential for rancidity formation, and the more sensitive is the food, and the longer the distribution time, the lower the control limit on % FFA has to be set. The % FFA to control ranges from about 0.5% for potato chips, to about 2% for meat and poultry, and as high as 5% for re-fried food service items that will be consumed within minutes.

There are two commonly used methods to determine % FFA once the oil has been clarified by filtration or centrifugation.

The older method is to place a weighed amount (in grams) of the frying oil into a flask, dissolve the rapidly stirring oil in boiling alcohol solution, add phenolphthalein in alcohol indicator solution, and add in sodium hydroxide solution from a buret until a rapidly disappearing pink color is seen above the oil's reddish color, and that fugitive pink color remains for 30 seconds according to the eye of the analyst. Non-official variations on the method exist, including pre-prepared solutions of alkali in alcohol being dumped into hot oil to get close to the end point, and then finishing the titration with a few drops of alkali from a dropper according to some scheme. In general, under pressure, and with poor establishment of rigorous methodology, a technician's titration of blind duplicates in a series of samples from different production lines varies between 5% and 20% of the true value. An equation is used to convert milliliters of alkali to % FFA. The alcohol and fat solution then has to be discarded in a way not harmful to the environment.

The newer method is the VERI-FRY® FFA tests. Heated oil is added to the fill line in a pre-measured, preheated, screw-cap reagent gel vial. The vial is capped, placed back into a dry bath until the reagent gel resettles to the bottom of the tube to form a blue to green colored layer. The absorbance of light at a chosen wavelength is then determined in a spectrophotometer relative to a blank. Absorbance is read off a chart to convert color of the gel to % FFA. Error in the blind duplicates test is typically less than 10%. The non-toxic, non-hazardous test is disposed of in ordinary trash.

The spectrophotometer can also be used with other VERI-FRY® tests just as simply to determine WET (water emulsion titratables = water + surfactant materials), and also TPM (total polar materials = polymer, silt, and polar organic compounds responsible for off tastes).

If the VERI-FRY® tests had been available before the invention of the titration method do you think the titration method would ever have been proposed? The miniaturization and other advantages of the spectrophotometric method for determining % FFA and other chemistries are so overwhelmingly helpful in the quality control laboratory, that once used, the burets disappear from the bench and the spectrophotometer becomes the tool of choice.

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