Clarity is used to describe the clearness of a sample relative to water. Nephelometric Turbidity Units (NTU) is a visual standard used to grade the appearance of liquids, as is the Lovibond Color Filter Scale. The pharmaceutical industry and others employ 4 ranges of NTU levels when describing Clarity. Level 1 - which is 0 NTU to less than 3 NTU is considered Water clear, Level 2 - which is 3 NTU to less than 6 NTU is considered visually clear to a trained observer. At Level 3 - which is greater than 6 NTU to less than 18 NTU is where a typical observer notices clarity disappearing.
Please look at the poster to see color neutral representation of the Turbidity Levels I just described. These levels are constant for any hue, Red, Yellow, Blue and of course Neutral. Initially reporting the Turbidity will allow the recipient of a Certificate of Analysis to know the true clarity associated with the sample.
The primary purpose of method Cc13j-97 (reapproved 2022) is to measure the Color of a Fat or Oil automatically. Later we will show that Turbidity affects color, to prove this we want to now measure the color of the same sample we just used to determine Turbidity. We can do this because in 1972 the US National Bureau of Standards, now known as NIST repeated their spectrophotometric measurements of the Lovibond Color filters. (this supersedes earlier measurements done at the request of the Tintometer Co.) These measurements allow one to use a spectrophotometer to measure the transmittance of a solution then find the corresponding filter color. See Graph on poster for the NIST representation of the Lovibond Glass filters, which form the basis for the AOCS RY scale. A spectrophotometer is an instrument that breaks light into component wavelength bands, for color measurement typically 31 bands at equal 10nm intervals across the visible spectrum. This is the minimum resolution recommended by the International Commission on Color (CIE) to measure color. The graphs show how the AOCS R and Y filters plot in Human Color space. Spectrophotometers are calibrated independent of Colored Filters, instead using laser lines of elemental emission such as Mercury, Krypton and Argon. The measured data follows Lamberts Law which states that absorption is directly proportional to the thickness of the sample. This allows a user to measure using commonly available cells with 10mm or 20mm pathlengths, including disposable cuvettes. Other cell sizes such as 24mm diameter vials, 33mm or 50mm cells.
Method Cc13j-97 is based on the automatic determination using the 4 sets of Lovibond Colored filters or two AOCS filters when a sample is measured in a 5.25” or 1” glass cell. The AOCS instructs the user to first measure using a 5.25” cell, if the resulting color is too saturated then the user must measure again using a 1” cell. This needs to be done since filter colorimeters are limited to only be able to match the color of the solution using the 3 or 4 primary colors. If a spectrophotometer were used to measure the sample at any of the pathlength discussed earlier the result would be displayed first at 5.25”, if the user determined that the R + Y values were too large, there would be no need to remeasure the sample, the initial spectral reading could just be calculated to represent a 1” pathlength.
HunterLab has conducted studies similar to what is outlined in Method Cc13,-97 where identical samples were measured both in a colorimeter – the PFXi-995 – using a 5.25” cell and a spectrophotometer – the HunterLab Vista – using a 10mm cell. The p-values (results of a statistical T-test) were equal to 0.005 between the results. Statistically there is no significant difference between the results. This was done for both AOCS R and AOCS Y colors. Additionally similar results were achieved for Lovibond R and Y colors.
Next a study was done where the same sample was measured in the three most commonly used cell pathlengths, 10mm cuvette (cuvettes made of optical lens grade PMMA are inexpensive enough to be disposable) 20mm large format glass cell (50mm tall by 50mm wide by 20mm pathlength) and 24mm diameter vial (these cylinders are 50mm tall, have a threaded throat and come with a sealing cap. A filled and capped sample eliminates the chance for spillage, is easy to heat in a water bath then quickly transfer to the spectrophotometer [measurement time for the Vista is 5 seconds or less, this instrument does a calibration flash test before each measurement]. These vials are also suited for aging or stability studies as the sample never has to be removed from the sealed vial.
The results showed no appreciable difference through a range of 5 colors as proof that Lamberts Law applies to the measurement of oils and fats.
Conclusion
Turbidity can indicate that filtering is complete providing more consistent results. Reporting Turbidity with Color from the single sample preparation eliminates variations due to temperature. The edible oils shown below were initially visually determined to be free of turbidity. Instrumental analysis showed that oil was in Range 2. Further filtering moved oil to Range 1 and color measurement changed.
Recording both instrumental measurements of Turbidity and Color would be an improvement to the current Cc13j-97 (reapproved 2022) Color of Fats and Oils, Automated Method. The Precision statement and results for this method are now 26 years old and do not include modern instruments that employ a spectrophotometer to measure the color.
It is time to revisit the current AOCS method. By doing so, AOCS will facilitate the use of advancements in technology, meet the growing needs of edible oil manufacturers, help streamline their refinement processes, ease product transfer from producers to buyers, save them time, save them money, and help them maintain control of their bottom line.
AOCS is positioned to help edible oil producers, buyers, and resellers use the most efficient means available to determine the quality, color, turbidity, and full spectral curve of their products.
For customers, HunterLab is the right place to get all of those answers.
