Ultrasonication as a strategy for enhancement of antioxidant capacity and overall quality in hydroponically grown lettuce

Zhan, Ziyi

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https://hdl.handle.net/2142/125778 Copy

Description

Title

Ultrasonication as a strategy for enhancement of antioxidant capacity and overall quality in hydroponically grown lettuce

Author(s)

Zhan, Ziyi

Issue Date

2024-07-09

Director of Research (if dissertation) or Advisor (if thesis)

Engeseth, Nicki

Doctoral Committee Chair(s)

Feng, Hao

Committee Member(s)

• Cadwallader, Keith
• Helferich, William

Department of Study

Food Science & Human Nutrition

Discipline

Food Science & Human Nutrition

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Lettuce quality
• Hydroponic growth
• Antioxidant capacity
• Ultrasonication
• Abiotic stress
• Phenolic acids
• Flavonoids
• HPLC
• Enzyme activity

Abstract

Consumers continue to be interested in the health benefits gained by eating lean, nutritious foods. Lettuce, a good source of nutrients, is widely consumed around the world. Leafy green vegetable consumption is somewhat limited based upon inherent problems such as seasonality and lack of accessibility in urban areas. As a result, there has been enhanced interest in alternative growing environments, including hydroponics and vertical/indoor farming. However, hydroponic lettuce has potential issues over soil grown lettuce, including lesser eating quality, higher moisture content, and different levels of metabolites. Previous research indicated that the abiotic stress, ultrasonication, could enhance the antioxidant capacity and texture of soil grown lettuce, based on the accumulation of secondary metabolites in plants. However, soil grown lettuce must be removed from its environment for ultrasonic treatment, exposing it to numerous environmental stresses and making recovery difficult. In contrast, hydroponic systems allow for a more effective integration of ultrasound. For the first time, efforts will be made to explore strategies to enhance the quality of hydroponically grown lettuce through the stresses generated by ultrasound. The overall goal of this dissertation was to study the effect of ultrasonication as an abiotic stress on secondary metabolites, antioxidant capacity, and texture of hydroponically grown lettuce, and investigate mechanistically the impact of ultrasonication on the biosynthetic pathways of major secondary metabolites. The general approaches were (1) examine the effectiveness of ultrasonication as a strategy for enhancement of antioxidant capacity and texture in hydroponically grown lettuce, (2) identify groups of bioactive components that are most impacted by the ultrasonication, and (3) explore the effect on biosynthetic pathways of major secondary metabolites associated with antioxidant capacity. The major findings of this dissertation were: (1) After 5 minutes of exposure to 26 W/L acoustic power density (APD) ultrasound and a subsequent 72-hour recovery period, there was a significant increase in the antioxidant capacity (Oxygen Radical Absorbance Capacity, ORAC) in lettuce, along with a significant increase in antioxidant-related substances such as polyphenols and ascorbic acid. Furthermore, this enhancement could be further intensified in a second ultrasound treatment after one week. (2) The content of chlorophyll a and b was not affected by the ultrasound treatment. The content of lignin and the texture of the leaves were significantly affected by the ultrasound. Results showed that lignin content began to increase 96 hours after the ultrasound treatment and stabilized around 120 hours. The hardness of the outer and middle leaves, as well as the midribs, significantly increased in the 96-hour and 120-hour groups, consistent with the trend in lignin content changes. However, the texture of the inner new leaves did not show significant changes. (3) High-Performance Liquid Chromatography (HPLC) profiling of polyphenols and flavonoids in lettuce revealed that the three most abundant polyphenols were chicoric acid (12.94 mg/g), chlorogenic acid (12.37 mg/g), and coumaric acid (6.74 mg/g). Flavonoid content, represented by quercetin at 5.49 mg/g, was lower compared to phenolic acid. Ultrasound treatment had a significant impact on these polyphenols, with their concentrations starting to increase significantly after a 72-hour recovery period, peaking at 96 hours, and then declining after 120 hours. The ratios of these three polyphenolic substances did not change significantly throughout the process. In contrast, the content of the flavonoid quercetin did not change significantly after ultrasound treatment. (4) Analysis of key enzymes in the polyphenol synthesis pathway (i.e., Phenylalanine ammonia lyase (PAL) and Cinnamic acid 4-hydroxylase (C4H)) and in the flavonoid synthesis pathway (i.e., Chalcone isomerase (CHI)) showed that the activities of PAL and C4H started to increase significantly after a 72-hour recovery period, peaking at 96 hours and declining at 120 hours, which paralleled the trends in polyphenol content. However, CHI activity did not change significantly. These enzymatic activity results are consistent with the HPLC findings, confirming that ultrasound treatment stimulated the polyphenol synthesis pathway, thereby significantly increasing the content of various polyphenols, but this factor did not stimulate the flavonoid synthesis pathway. Ultrasonication has the potential to be applied in industrialized hydroponics and enhance the eating quality of hydroponically grown leafy vegetables. This study provided more fundamental knowledge of lettuce responses to ultrasonic stimulation. Determination of the impact of ultrasonication on secondary plant metabolites and their biosynthetic pathways will provide a theoretical basis for designing future applications of ultrasonication in hydroponics.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125778

Copyright and License Information

Copyright 2024 Ziyi Zhan

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