The Influence of Pre-Treatment, Preservation Method, and Storage Time on Carotenoid and Chlorophyll Profile in Nettle Juices and its Relation to Colour Changes

One of the most valuable health-promoting plants used in the production of functional, green drinks is nettle. Acidic environments, oxygen, high temperature, and enzymes can negatively affect the colour of green juices, which is an important aspect for consumers. Therefore, the study analyzed the effect of pre-treatment, preservation method (freeze-drying, freezing, sterilization), and storage on the profile of carotenoids and chlorophylls (HPLC) in nettles juices and its correlation with colour changes (CIELAB). All the examined parameters impacted the profile of both groups of compounds and their colour. The most significant unfavorable changes were observed after sterilization. Blanching positively affected the level of coloured compounds in canned juices, with a greater enhancement being observed after a longer pre-treatment time (90 s). After storage, in frozen and freeze-dried juices, and similarly in unpreserved juices, lutein, violaxanthin and chlorophyll a predominated. However, in sterilized juices, neoxanthin and violaxanthin were completely degraded, and β-carotene and chlorophyll b were found at a significant level. During the storage of canned and frozen products, chlorophyll a was degraded to a greater extent, while in lyophilizates chlorophyll b. The transformation of neoxanthin, violaxanthin and chlorophyll a in nettle juices was determined via analysis of a* colour parameter.