Liquid refreshments, or beverages, consist of a broad group of consumable liquids—ranging from carbonated soft drinks, fruit/vegetable juices, and milk to coffee, tea, and hot chocolate, as well as spirits. These beverages are a key factor in human health and nutrition, and selection of these products is based not only on taste but often on color as it relates to taste memory.
Color and appearance measurements for the beverage industry are used to ensure that the overall product appearance is the same from lot to lot. Production, storage, and ingredient changes can alter the base color of a beverage resulting in the perception that the product is different or of lower quality.
When color is different than what a consumer expects, their minds tell them that the taste is different too. The goal of course is to make sure that the customer doesn’t see objectionable differences or have color negatively influence a buying decision.
Beverages can be opaque, translucent, or transparent, and each form requires different instrumentation and techniques for successful color and appearance measurement. In developing a method for evaluation of a beverage, it is important to note that consistency in measurement is critical to comparing results from sample to sample or lot to lot.
Opaque liquids usually have a high solids content, therefore reflect light instead of allowing it to pass and are usually characterized by a high Brix value. These type samples are best measured using directional 45/0 degrees reflectance instrumentation. This is the geometry that most closely matches how the human eye “sees” color. Samples are typically placed in a 50 millimeter (mm) cell and then the cell is placed at the instrument port.
Translucent liquids possess a medium level of solids content, along with a lower Brix value and allow light to pass through diffusely. Both reflective and transmittance measurement modes may work well depending on the translucency of the sample. As a rule of thumb, if you can see slight details of your thumb or finger through the liquid, then transmittance is the preferred measurement method. If you cannot see slight details, then reflectance measurement using directional 45/0 degrees is preferred, though it is also possible to use diffuse d/8 degree sphere geometry. Regardless, translucent samples require a fixed path length and a constant background to allow measurement comparison.
Transparent liquids have a very low, or zero, solids content and allow light to pass through with little or no distortion so that objects on the other side can be clearly seen. These liquids can only be measured using transmission instrumentation. Examples of transparent liquids include soft drinks, sport health drinks, and coffees.
In addition to the color of finished products, the quality measurement of ingredient dyes, pigments, or other substances is valuable. There are seven colors approved by the FDA as GRAS (Generally Recognized as Safe), see Table 1. Dye concentration is one of the raw materials measurements that can be determined using % absorbance measurements at certain wavelengths in transmittance.
Using an instrument with a diffuse d/8 degree geometry, transmission measurements for a wide range of transparent liquids or food colors/additives can be accomplished. As color saturation increases, a shorter path length transmittance cell is used. As color saturation decreases, a wider path length cell is used. An overview of path length selection is presented in Table 2.