Semester

Fall

Date of Graduation

2019

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Division of Plant and Soil Sciences

Committee Chair

Kang-Mo Ku

Committee Co-Chair

Nicole Waterland

Committee Member

Kristen Matak

Committee Member

Xiaoli L. Etienne

Committee Member

Jonathan Cumming

Abstract

In the United States, Brassica vegetables, including broccoli (Brassica oleracea var. italica) and kale (Brassica oleracea var. acephala or Brassicae napus var. pabularia), are widely consumed and are easily accessible in farmer’s market or grocery stores with increasing interest of their health-promoting properties. For example, the consumption of broccoli has been associated with anti-cancer activity in in vitro and in vivo trails due to the high content of phytochemicals, minerals, vitamins, and fibers. Application of methyl jasmonate (MeJA) has been reported to enhance the potential health-promoting compounds in Brassica vegetables, glucosinolates (GS), especially indolyl GS neoglucobrassicin are induced by mimicking insect damage because MeJA activates herbivory defense system.

Although exogenous MeJA application has been recognized as a method to simulate insect herbivory and to invoke downstream defense mechanisms (inducing GS accumulation), only limited studies investigating the difference in metabolite changes or transcription level changes between two treatments in Brassica species. In Chapter 2, the metabolomic and transcriptomic changes between insect damage (4 days treatment of cabbage looper caterpillars) were compared. The primary result indicated levels of gene expression changes are slightly different between two treatments while the metabolite changes were similar but in different levels. For example, significantly increased indolyl GS was found in both treatments but higher in insect feeding groups.

GS loss during the cooking process has been intensively reported in Brassica vegetables; however, there is limited research on whether MeJA application affects GS retention after different cooking methods while MeJA application significantly increases inducible GS in Brassica vegetables. In Chapter 3, the phytochemical profile change after three different cooking methods (boiling, steaming, and microwaving) and two cooking times (2 and 5 minutes) on control and MeJA-treated broccoli. Among six cooking treatments (methods x times), 5 minutes boiling led to the most significant loss in total aliphatic (22%) and indole GS (62%) in control while it caused 47% total aliphatic and 54% indole GS loss in MeJA-treated broccoli; however, MeJA-treated broccoli contained 1.9-fold higher GS than the untreated broccoli after 5 min of boiling, which was considered the most drastic treatment in this study.

These changes by exogenous MeJA application, including GS level, GS hydrolysis products level, and primary metabolites, may alter the sensorial attributes of Brassica vegetables. Therefore, MeJA application may impact consumer acceptance and sensorial quality of Brassica vegetables, and the degree of impact may depend on level of myrosinase activation during cooking. In Chapter 4, an untrained consumer panel detected the differences between raw control and 250 µM MeJA treated broccoli; however, four minutes of steaming eliminated the detectable bitterness in MeJA-treated broccoli and the consumer panel could not detect the difference between control and MeJA-treated broccoli. Neoglucobrassicin-derived hydrolysis products, N-methoxyindole-3-carbinol, N-methoxyindolyl-3-carboxaldehyde, and N-methoxyindole-3-acetonitrile, were the most important metabolites in determining the overall liking and the taste of broccoli samples using partial least square regression model. After 4 min steaming, MeJA-treated broccoli still contained 7.8-fold more neoglucobrassicin and 50% more total GS than untreated broccoli

Despite all the positive results of exogenous MeJA application on Brassica vegetables, the applicability of this practice is limited without further evaluations on the feasibility from the consumers as well as from the growers. In Chapter 5, the consumer survey suggested that consumers who considered “anticancer broccoli” as the most attractive trait may be the potential customers who were willing to pay higher price ($1.00 or $1.50 more per broccoli head) for MeJA-treated broccoli; however, recruited growers from the local farmer’s market were concerned about the detectable bitterness in raw MeJA-treated broccoli would deter consumers from buying their broccoli in the future. Exogenous MeJA application may bring extra gross income for growers with small-scale broccoli production when we hypothesized a scenario from a very small farm (acre) when the elasticity of vegetable was set at -0.58 or-0.79.

Collectively, the results from this project implied that exogenous MeJA treatment (mimicking insect herbivory) on Brassica vegetables has its potential for use in the food industry and in the right market’s niche. For example, MeJA-treated broccoli can be used as an ingredient to boost the nutrition quality and/or as a value-added ingredient in precooked meals because cooked MeJA-treated broccoli contained a higher level of GS comparing to cooked untreated broccoli. Therefore, exogenous MeJA application may be more suitable to the farms providing Brassica vegetables directly to the food processing industry instead of the farms selling fresh produce directly to the consumers.

Embargo Reason

Publication Pending

Available for download on Thursday, October 29, 2020

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