Date of Graduation

8-2023

Document Type

Thesis

Degree Name

Master of Science in Food Science (MS)

Degree Level

Graduate

Department

Food Science

Advisor/Mentor

Atungulu, Griffiths G.

Committee Member

Bluhm, Burton H.

Second Committee Member

Crandall, Philip G.

Keywords

Aflatoxin B1; Aspergillus flavus; Microwave treatment; Rice consumption; Rice fractions; Single-pass microbial inactivation

Abstract

Securing the microbial safety of rice is a rising priority within the food industry, especially when used as an ingredient to manufacture ready-to-eat, minimally-processed products. Mold contamination is typically the most problematic for rice because certain species of Aspergillus produce toxic secondary metabolites known as aflatoxins, rendering the grain unfit for human and animal consumption. The objectives of this study are to: (1) investigate the effectiveness of using high-power (915 MHz frequency), short-duration microwaves (MWs) to inactivate microbes on rough rice; (2) use a three-level screening design to determine which storage factors, such as temperature, relative humidity (RH), storage duration, rice moisture content (MC), and initial A. flavus spore concentration, significantly influence the growth kinetics of Aspergillus flavus, ergosterol, and aflatoxin B1 (AFBI) production in rice. A follow-up experiment was then conducted to assess the impact of significant factors on AFB1 and ergosterol production at extreme levels. Freshly harvested long-grain rough rice (RT 7321) with a MC wet basis (w.b.) of 21% and a bed thickness of 20 mm was exposed to MW powers of 16, 18, and 20 kW for varying durations (1, 2, and 3 minutes). The microbial load on treated and untreated samples was then determined using standard procedures. Rough, brown, and milled rice samples with different MCs (12%, 15%, and 18% w.b.) were used for the second objective. The rice samples, both autoclaved and non-autoclaved, were inoculated with different A. flavus spore concentrations (1 × 104 and 1 × 106 spores/mL) and incubated at different temperatures (20°C, 25°C, and 30°C) and RHs (75%, 85%, and 95%). After incubation period of 3, 9, and 15 days, A. flavus plate count, ergosterol and AFB1 production were measured. Throughout all storage durations, A. flavus growth was optimal at 30°C and 95% RH. AFB1 production was dependent on rice fraction and storage conditions, with brown rice having the highest concentration (9.198 μg/g) after 15 days of incubation at 30°C and 75% RH. However, during the latter part of the study, it was observed that AFB1 concentrations in brown rice inoculated with 1 × 106 spores/mL of A. flavus and stored at 30°C and 95% RH initially increased after 20 days and then decreased towards the end of the storage period (60 days). The results are expected to help understand the application of MW technology to mitigate toxicity-related problems associated with the presence of microbes in rice and identify proper storage conditions that can minimize the risk of aflatoxin contamination, thereby improving rice safety.

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