Date of Graduation

5-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Food Science (PhD)

Degree Level

Graduate

Department

Food Science

Advisor/Mentor

Atungulu, Griffiths G.

Committee Member

Howard, Luke R.

Second Committee Member

Sadaka, Sammy S.

Third Committee Member

Carbonero, Franck G.

Fourth Committee Member

Mauromoustakos, Andronikos T.

Fifth Committee Member

Bench, B. J.

Sixth Committee Member

Foley, Steven L.

Keywords

Food Processing Commons; Food Safety; Food Studies Commons; Grain Engineering; Rice Color; Rice Storage

Abstract

Economically, discolored rice kernels have less market value, which can seriously impact farm net profit. The reasons for rice discoloration during storage have not been studied extensively and many questions remain. Therefore, the primary goal of this study was to improve understanding of the role and contributions of storage practices on rice quality degradation and discoloration. In this study, three rice cultivars including XL753 (hybrid), Roy J (pureline), and Titan were assessed for changes in quality and microbial kinetics. The rice samples at a high moisture content (MC) of 21% w.b. and a low MC of 16% were stored at three temperature levels, 20°C, 30°C, and 40°C. Samples were analyzed every four weeks for up to 16 weeks of storage. Because fungi may be responsible for rice discoloration, the samples were also treated with antifungals (Sodium chloride and Natamycin) to compare rice discoloration differences between non-treated samples and the antifungal treated samples during storage. Electron beam (EB) irradiation was also used to silence microbes in high MC rice before they were stored at the same storage temperatures for up to eight weeks as indicated above. The discoloration development in rice, the fungal growth, and fungal diversity, chemical changes, quality attributes, and their associations were investigated. Rice discoloration was highly dependent on storage MC, temperature, and duration, increasing significantly as MC, temperature, and duration increased. The highest rice discoloration occurred in all rice cultivars when stored at 21% MC and 40°C for 16 weeks. However, discoloration was also profoundly higher for Roy J than for other cultivars when stored at 21% MC and 30°C for 16 weeks. The discoloration was significantly lower for the sodium chloride treatment, while natamycin was not effective compared to control. Hybrid rice had significantly less discoloration compared to pureline and medium-grain. Fungal counts were similar across all cultivars. Fungal growth followed a decreasing trend with higher storage temperature as discoloration increased. An inverse relation between fungi and discoloration suggests that discoloration particularly at higher temperatures was not explained by microbe activities. Metagenomics analysis confirmed that the rice had been infected before storage by a diverse group of fungi whose fungal abundance was found to vary considerably with storage conditions. Alternaria, Penicillium, Aspergillus, Nigrospora, and Fusarium were the five most abundant fungal genera among all identified fungi on initial pre-stored rice samples. Each of these fungi can produce pigment and change the color in rice samples depending on storage temperature and rice cultivars. In some cases, these fungi decreased more during storage with sodium chloride treatment. Rice discoloration was significantly induced in rice treated with EB irradiation. The induced discoloration in EB irradiated and non-irradiated samples was explained by chemical changes particularly at high storage temperature that discouraged fungal growth. Rice discoloration is an issue that is developed by microbial and chemical reactions during storage. Microbial and chemical involvement could explain most of the discoloration in rice stored at low and high temperatures, respectively. The results of this study provide valuable information to growers, processors, and industries on conditions necessary to maintain rice quality.

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