Specific Energy: A New Approach to Ultrasound-assisted Extraction of Natural Colorants

The demand for natural colorants has boosted the search for innovative technologies to obtain them. Ultrasound-assisted extraction (UAE) is one of the new green techniques that has been studied due to its process advantages that include high yields, short extraction times and nonutilization of elevated temperatures. However, a nonstandardization of the UAE variables complicates comparisons and hinders progress in the studies of this topic. In this review, the focus is the verification and discussion of which UAE process conditions authors have used to obtain natural colorants. Thus, it is possible to confirm that some authors used ultrasonic systems that are not appropriate for performing a good extraction, that some used a great amount of solvent and a long extraction time, and that researchers did not express the main variables (nominal power, extraction time and sample mass) as a function of the specific energy applied to processing. Therefore, it is possible to conclude that some studies using UAE were not conducted to obtain the best results, and the expression of the variables as a function of specific energy can generate a standardization, which facilitates comparison among the results obtained by the scientific community.