Thermal and nonthermal sonication: Extraction of bioactive compounds from beet leaves and microbiological quality of extracts

Abstract

This work aimed to optimize a nonthermal (NT) ultrasound-assisted aqueous extraction (aq-UAE) to maximize the recovery of polyphenols, betaxanthins, and betacyanins. Extraction yields, thermal profiles and microbiological quality of the extract were compared with a previoulsy optimized thermal aq-UAE. Extraction yields resulted 68%–121% higher in NT-aqUAE. Polyphenols and betaxanthins were the most affected by high temperatures of thermal process showing decreses after 8 min of thermosonication at temperatures >50°C. Temperature during NT-aqUAE waslower than 30ºC avoiding the detrimental effects of heat. Microbial counts decreased 2-log cycles after thermosonication and the remaining microbial load did not resist the storage conditions. Conversely, no significant decreases were detected after NTaq-UAE. However, sublethal damage was observed as microbial counts presented a higher lag phase than control during refrigerated storage. Although NTaq-UAE process allows for higher phytochemical extraction yields than thermal aq-UAE, it does not contribute to controlling the microbial load of extracts. Practical application: Nonthermal ultrasound-assisted aqueous extraction of bioactive compounds from beet leaves was optimized in this work. This process improved the extraction yield of polyphenols and betalains compared with the thermal counterpart as thermolabile compounds degradation was avoided with the limitation on thermal increments. The developed process resulted simple and reasonably efficient for producing extracts rich in polyphenols and betalain pigments that hold potential to be used as natural additives. Additionally, evaluation of microbiological quality of extracts obtained by thermal and nonthermal optimized UAE processes allow to establish that thermal aq-UAE improved microbiological quality and stability of extracts while nonthermal one would require an additional treatment of the extract to improve its microbiological quality. Developed processes are effective postharvest strategies to the valorisation of horticultural by products such as beet leaves and increase biomass utilization improving the sustainability of the agri-food chain.