To combat insect infestation of maize and maize products during storage without using chemical fumigants, a possible physical treatment method, microwave technology was investigated. Through its selective heating between cereals and insects, microwave technology is a possible physical treatment for eradication of insects and their eggs. Eradication of five insect species, namely Sitophilus zeamais, Rhizopertha dominica, Ephestia cautella, Cryptolestes ferrugineus and Tribolium Confusum was studied. Different microwave parameters such as power dosage, microwave mode, length of microwave cavity, maize exposure method and exposure time were investigated. The effective microwave treatment conditions were then selected, and used to treat maize kernels at laboratory scale. The effect of microwave treatment on the physicochemical properties of maize kernels was investigated. Microwave single exposures did not result in total insect mortality when maize was dropped through the microwave cavity (free falling) as the exposure times were too short. But utilising a pulley system, total insect mortality was achieved in a single exposure of 9 sec. A long microwave cavity (728 mm) resulted in maize kernel damage in terms of swelling, popping and discolouration. Redesigning the cavity by shortening it appeared to reduce these effects. The pulsed microwave mode was found to be better than continuous mode. The selected treatment that eradicated all five insect species with no visible kernel damage was pulsed mode at 2450 MHz frequency, using a 483 mm long microwave cavity, at a power level of 1.5 kW, with an exposure time of 9 sec. The selected conditions (normal treatment) and a more harsh treatment (2 kW power dosage, 18 sec exposure time) were applied to 4 kg samples of white and yellow maize kernels. The normal microwave treatment significantly decreased the moisture content and kernel weight of maize kernels but had no significant effect on test weight, stress cracks, germination and translucency. The harsh microwave treatment also had significant adverse effects on test weight, translucency, germinability, hardness and stress cracks. Additionally, reduced extractability of certain proteins was observed by 2D PAGE with the harsh microwave treatment. Both normal (power dosage of 1.5 kW, for 9 sec exposure time) and harsh (2 kW, 18 sec exposure time) treatment conditions eradicated adult insects and their eggs, but only the former maintains maize quality. The use of microwave technology has potential to be used as an insect control measure of maize products prior to packaging of the products. It is recommended that the effect of pulsed microwave disinfection on nutritional quality (starch and protein digestibility of maize products) be studied. Heat transfer phenomena should be studied and improved if possible to reduce the power usage and possibly shorten the exposure time from 9 sec. / Dissertation (MSc)--University of Pretoria, 2009. / Food Science / unrestricted

This thesis presents an investigation of carbonyl sulphide as a new fumigant and related methodology studies. The first part involved the investigation of a new fumigant - carbonyl sulphide, which has the potential to replace methyl bromide. Its biological response or activity was investigated, e.g. toxicity to target organisms and phytotoxicity, environmental and worker safety considerations. In the second investigation, analytical methods were developed for the determination of fumigant movement through timber and fumigant residues in grains as well as a method of chemical fractionation to determine the fate of carbonyl sulphide. A comprehensive literature review of 161 references in these two areas is reported. Carbonyl sulphide was highly toxic to adults of three coleopteran species tested, namely Rhyzopertha dominica (F.), Tribolium confusum du Val, and Sitophilus oryzae (L.), the most sensitive species was R. dominica. For 6 hr exposure at 25�C, the L(CxT)95 value for R. dominica, S. oryzae and T. confusum were, respectively, 36.48, 99.82 and 113.0mg h L-1. Carbonyl sulphide inhibited 100% of mould in wet wheat and more than 90% of mould on dry wheat at lOOmg L-1. Both carbonyl sulphide and hydrogen cyanide were low in phytotoxicity without affecting germination of wheat, at levels needed to control insects. Unlike hydrogen cyanide, carbonyl sulphide can be used at minimum levels without decreasing plumule length of wheat. Chemical data on the sorption of carbonyl sulphide are compared with data from methyl bromide. The levels of carbonyl sulphide in the headspace of five commodities (wheat, barley, paddy, sorghum and peanut) and timbers (hardwood and softwood) decay more slowly than do levels of methyl bromide. Carbonyl sulphide was blown through a column of wheat as easily as was phosphine and more easily than was methyl bromide, and its front was blown out faster than phosphine and methyl bromide. Movement of two fumigants (methyl bromide and carbonyl sulphide) through, and sorption on, softwood and hardwood were studied. Each fumigant was sorbed less on softwood than on hardwood and penetrated softwood better than hardwood. Carbonyl sulphide penetrated timber better than did methyl bromide, and was less sorbed on timber. A rapid method of solvent extraction was developed to enable rapid estimation of the amount on intact fumigant sorbed in wood. This procedure enabled near quantitative recovery of methyl bromide as either intact fumigant or as bromide ion. Carbonyl sulphide residue in unfumigated wheat was found to be around 25- SOppb. Carbonyl sulphide left little residue on fumigated grains. Desorption of carbonyl sulphide from the wheat was extremely fast, 85% of it was released after one day aeration which was very much greater than that of methyl bromide and carbon disulphide. After 6 days aeration the incorporation of 14COS on mungbean, wheat, paddy, rice and safflower was lower than 7Oppb (calculated as COS equivalent). Food value or nutritional quality of foodstuffs is not harmed by carbonyl sulphide fumigation. This result was assessed by identifying any nonreversible change or combined residues in biochemical fractions of commodities including lipids, protein, amino acids, carbohydrate, etc., and no irreversible reaction between carbonyl sulphide and any constituent such as B vitamin, atocopherol, lysine, maltose and starch. Fumigants did not affect lipids, although each fumigant was applied to wheat at exaggerated concentrations, nor wheat germ oil and canola oil treated with extremely high concentration of fumigants. Factors which affect analysis of fumigants including stability of chemicals in extraction solvent and partitioning of fumigant between solvent and air, were examined. The partition ratio, defined as the fumigant concentration in extraction solvent to that in the headspace, varied with fumigant. Methods for multi-fumigant analysis were developed or modified and gave high recoveries and efficiency. The procedure of Daft of solvent extraction followed by partitioning was modified by being performed in sealed flasks. This raised the recovery of carbonyl sulphide, methyl bromide, phosphine and carbon disulphide. Recoveries were near quantitative at levels down to 6-16ppb (w/w) for tested fumigants. Thus the modified Daft method can be adapted to enable determination of the main fumigants used on staple foodstuffs. Microwave irradiation method give higher efficiency of removal of fumigants from grains. Limits of quantification were < 0.2ng g-1 (ppb w/w) for each tested fumigant. The detection limit of COS was calculated, as natural levels of the fumigant were detected in commodities. These are feasible, simple and rapid (< 2 min.) to be use to analyse fumigant residue in grains. Carbonyl sulphide has potential as a fumigant for grain and timber and may replace methyl bromide in some uses, subject to further investigation in commercial situations.

Strawberry production in California is limited by plant diseases such as Macrophomina root rot (caused by Macrophomina phaseolina) and Botrytis fruit rot (BFR) (caused by Botrytis cinerea). Current disease management strategies are compromised due to fumigant regulations or ineffective disease management practices. This thesis investigated methods to potentially improve the management of these two diseases. Host plant resistance evaluations for Macrophomina root rot were conducted for the 2020-2021 and 2021-2022 growing seasons. Fifty-one strawberry genotypes were screened in two field experiments where plants were inoculated artificially with Macrophomina phaseolina in both seasons. A wide range of plant resistance to Macrophomina root rot was observed. The three most resistant genotypes based on final plant mortality were ‘17C721P606’, ‘Yunuen’, and ‘Xareni’ in 2020-2021; ‘UCD Mojo’, ‘Mariposa’, and ‘Dayana’ in 2021-2022. A summary of similar experiments done in the previous four years showed ‘Osceola’ as highly resistant. Disease severity varied among years for specific genotypes as well as the average final mortality for all genotypes in the experiments. Strong positive associations were found for soil temperature during the first month after planting (R2= 0.79, P2= 0.79, P A survey of BFR levels in commercial strawberry fields with and without fungicide applications was conducted in Santa Maria, CA in 2021 and 2022. Weather stations were installed at each field to collect leaf wetness duration and temperature data and calculate the BFR risk factor based on the Strawberry Advisory System (StAS) developed at the University of Florida. There were no statistically significant differences between fungicide and no-fungicide treatments for both in-field and postharvest BFR incidence in 2021 and in-field BFR incidence in 2022, while no-fungicide treatment showed higher postharvest BFR incidence in 2022. BFR levels were low in both years. In 2021, average in-field BFR incidence for fungicide and no-fungicide treatments were 2.6 ± 0.3% and 2.5 ± 0.4%, respectively. Average postharvest BFR incidence for fungicide and no-fungicide treatments were 1.8 ± 0.2% and 2.0 ± 0.3%, respectively. In 2022, average in-field BFR incidence for fungicide and no-fungicide treatments were 3.0 ± 0.4% and 3.7 ± 0.4%, respectively. Average postharvest BFR incidence for fungicide and no-fungicide treatments were 0.6 ± 0.1% and 1.5 ± 0.2%, respectively. Risk factor from StAS was significantly associated with BFR incidence in 2021, but not in 2022. Screening new strawberry genotypes against Macrophomina root rot should be ongoing as part of a standard process for determining the susceptibility of currently grown and potentially new cultivars. Additional research under more diverse weather conditions is necessary to verify the impacts of reducing fungicide use in BFR management and to validate the use of StAS in making fungicide use decisions in California fields.

I argue that although the abusive conditions experienced by farmworkers have complicated causes, they have persisted and will continue to persist as long as farmworkers are stripped of virtually any political and economic power. The chapters build upon each other logically, beginning with the second chapter, which uses farmworker testimony to establish that a combination of economic and political circumstances have kept farmworkers from protesting not only methyl bromide, but every other dangerous condition they face in the fields. In the third chapter, I argue that despite commonly held assumptions, growers are virtually powerless to change the circumstances of farm workers because competition they face in the strawberry market precludes any single grower from paying their workers more than the going rate. I will conclude by arguing that to begin to improve the working conditions of farm workers, consumers will need to engage with the issue on both political and economic levels. The conclusion builds on the arguments established in the second and third chapters; namely, given that neither growers nor farmworkers will be able to leverage change within the current political and economic context, consumers are the only remaining actors with both the incentives and power to influence both the political and economic arenas. Just as scholarship that focuses on only one set of actors (i.e. only growers or only regulators) will necessarily fail to provide practical solutions because such papers tend to discount the pressures faced by and produced by other actors, so too will change be impossible without consumers who advocate that farmworkers both receive a just share of political voice and fair wages.