Using the Gardner Scale to Enhance Quality and Create Color Consistency in Edible Oils


Posted on April 24, 2018

To a gourmet chef, a fine bottle of high-quality extra virgin olive oil is a thing of beauty. Many of these high-end oils are deep gold in color, with an appearance that’s rich, thick and almost honey-like. But this isn’t the only color that olive oil can be; some oils are the color of pale straw, while others are so dark brown that they appear nearly purple.1 The color of olive oil differs significantly depending on the type of olives the manufacturer uses, how the product is handled during manufacturing, and how the final product is stored before it reaches customers. In the case of olive oil, extra virgin varieties are almost always much darker in appearance compared to more heavily-processed oils, like pure, refined, or pomace products.

Because the color of the oil can tell customers a great deal about how it was processed and stored, color consistency in edible oil products is an essential part of your manufacturing procedure. However, you can’t always use the same color classification for oil as you would for clear, transparent liquids like water or certain polymers. Oils used in cooking products are often naturally yellow to brown in color, and as such, they may require you to use the Gardner scale in order to accurately analyze and classify your product’s color consistency. Unlike the APHA/Pt-Co/Hazen color scale (which measures the purity of colorless or nearly-colorless liquids), the Gardner scale is designed specifically to measure color consistency in yellow or brown transparent samples. There is some overlap between the APHA and Gardner scales at the lowest end of the Gardner scale (below Gardner 2). However, if your sample is darker than Gardner 2, then the Gardner scale may be the best classification method for your edible oil products. Using this scale, you can attain a more accurate understanding of your edible oil products while ensuring that every batch of product is consistent and aesthetically-appealing.

What is the Gardner Color Scale?

The Gardner color scale is a well-established method for identifying and categorizing the color of transparent liquid products that fall naturally in the yellow to brown color range, such as cooking oils, resins, fatty acids, and wood varnishes. When the Gardner scale was originally developed in the 1920s, it consisted of 18 different liquid color standards against which manufacturers could compare their own products. Each color standard was contained inside of clear glass tubes, and the 18 different liquid colors were made from a mix of potassium chloroplatinate, potassium dichromate, ferric chloride and cobaltous chloride.2 Each tube represented a slightly different color, ranging from pale yellow (Gardner 1) to very dark brown (Gardner 18).

However, the earliest forms of the Gardner scale were imprecise and vulnerable to human error. To use the scale, you had to perform a visual comparison of the product sample against its matching Gardner liquid. Because the human eye can be biased when it comes to accurately detecting colors, these versions of the Gardner scale were not completely reliable. Moreover, the liquid inside of the glass vials was prone to natural changes in color over time, and manufacturers of Gardner scale vials couldn’t always replicate the liquid colors perfectly.  

Today, the Gardner scale has been integrated into more advanced color measurement instruments, solving many of these past inconsistencies and challenges. To use the scale, you no longer have to rely on the subjective human eye to test for color consistency between products. Rather, the Gardner scale has been converted into precise digital color data that can be included in a spectrophotometer’s software. Using spectral analysis and a 10 mm or 20mm path length transmission cell, modern spectrophotometers can triangulate chromaticity coordinates for a given sample, then compare the data to the corresponding color on the Gardner scale. If the sample falls outside of the desired scale placement, the instrument will alert the operator to the problem right away. This can help you identify inconsistencies in your manufacturing process or in the raw oil material you use to create your products. In short, the Gardner color scale can work alongside the most advanced spectrophotometers to detect even the slightest color variation for yellow or brown edible oil products.

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