Ziyi Zhan, Yu Chen, Paulina Ongkowijoyo, Yawen Wang, Hao Feng, and Nicki J. Engeseth

Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign

Introduction:
Sonication has the potential to be utilized for exciting applications in the food industry. Literature reports indicate the utilization of physical stimuli to enhance production of secondary metabolites in plant cells, as a defense response. Previous research efforts have focused on postharvest exposure of Romaine lettuce to sonication, resulting in increasing phenolic compounds, antioxidant capacity and Phenylalanine Ammonia Lyase (PAL) activity during storage. The goal of the current research was to extend this concept to the optimization of application of sonication to leafy green vegetables grown hydroponically, thus providing a strategy to improve quality of vegetables grown under highly controlled conditions such as vertical farming.

Method:
Burpee lettuce (Giant Caesar) was grown hydroponically and at 7 weeks after germination sonication was applied to intact plants in a batch sonication unit (25kHz, 26 W/L as 100% power). Control plants were placed in a similar bath with no sonication. After treatment plants were returned to hydroponic solution and at selected days they were freeze dried and frozen for analysis of total phenolics, antioxidant capacity (ORAC assay) and PAL activity. Various parameters have been investigated, including ultrasound treatment time, acoustic field density, number of sonication treatments and recovery time.

Results:
Greatest enhancement of total phenolics (14.96%) and antioxidant capacity (9.14%) compared to controls occurred at 100% power and 2 minutes of sonication. Lower power density treatments had an opposite effect; 30% power treatment for 30 seconds resulted in a 15.57% decrease in total phenolics and 100% power for 30 seconds showed a slight decrease (3.46%) in antioxidant capacity. PAL activity was not affected by these treatments. The best enhancement of total phenolics (21.49%) was with two exposures to 100% power for 30 seconds, with a one week interval between treatments. The optimal recovery time was 72 hours.

Significance:
This research demonstrates the potential application of minimal plant stress, such as sonication, to the enhancement of secondary metabolite production in hydroponic lettuce. This optimization could ultimately be tailored to optimization of quality of leafy green production in urban gardening and vertical farming scenarios.