Chemistry | Chemical Analysis | Concentration determination of natural dyes
The first thing we perceive about a product is its color. Especially in relation to food, it raises certain expectations about taste, and influences our choice of purchase. Food colorants are often used to satisfy color expectations of products, make a food-typical taste recognizable, or to achieve a consistent, standardized color in products made from raw materials with varying quality and color intensity. Depending on their origin, we distinguish between natural, nature-identical and artificial (synthetic) colorants.
Due to an increasing consumer awareness of a healthy diet, the use of natural food colorants has gained significance in the last decades. Natural colorants are perceived as more authentic and of higher quality. For the coloring of jams, canned fruit or ice cream for example, plant-based colorants such as anthraquinones, anthocyanins, betalains, carotenoids, chlorophylls and flavonoids are therefore often preferentially added.
Annato seeds, from which the vegetable dye carotenoid is obtained
Natural colorants, such as the red-purple coloring anthocyanins, can be found in almost all higher plants, mostly in blossoms and fruits, but also in the leaves and roots. However, the exact amount of colorant content in the plant can vary. Therefore, during further processing of the isolated dye into color extracts and food coloring, the intensity of the berry dye, such as anthocyanin, must be measured and the final color value accurately determined. The reference method for this measurement is based on photometry, but the berry juice concentrate to be measured has a very high optical density and cannot be used directly as a sample for the measurement. Thus, when processing the plant colorant anthocyanin, juice samples are first diluted with water and then with a buffer solution for intermediate and final control so that only the desired red color will be determined. This procedure of sample collection, preparation and analysis requires considerable labor and material input.
In the production of food coloring from natural dyes, the color pigments are extracted from the raw materials used. For this purpose, the berries are squeezed and the juice is thickened by means of ultrafiltration. It is then further processed into essence in a highly concentrated form or even spray-dried and sold as powder or paste.
schematic illustration of an ATR measurement
Easier and more efficient than the sampling method in the laboratory is the determination of the dye concentration inline directly in the process using a spectroscopic immersion probe. This type of measurement enables precise and efficient determination of the dye content of the berry juice concentrate without prior dilution of the sample. A laboratory measurement to confirm the color values determined with the probe is thus only necessary once, shortly before the end of the ultrafiltration.
However, due to the high optical density and strong absorption of the undiluted dye, the use of conventional UV/VIS absorption measurements is out of question for this application. The spectrometer would only register noise without recognizable spectra.One way to overcome this limitation is to use a measurement method based on attenuated total reflection (ATR).
In an ATR measurement, the medium flows around the prism of the probe and, depending on the optical density of the medium, the measurement signal is attenuated at each reflection at the boundary layer between the medium and the prism (evanescent field). This method allows the measurement of strongly absorbing samples where measurements in transmission would not be possible because total absorption would occur.
For ATR measurements requiring extremely high measurement precision, Hellma has developed the "Katana XP" ATR probe. It features a monolithic, high-precision sapphire measuring head that guarantees accuracy and reproducibility of the measurement results. Due to its hygienic design and compact construction, it can be easily and flexibly integrated directly in the reaction vessel.
Hellma ATR Probe »Katana XP«
Through spectroscopic inline color value measurements, measurement times are shortened and the risk of errors is minimized by eliminating manual sampling and sample preparation. In the long term, material and personnel costs are reduced because intermediate tests involving laboratory analysis are no longer necessary as a result of process optimization.
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