Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Agricultural and Biosystems Engineering


Industrial and Agricultural Technology

First Advisor



Agriculture is the backbone of the Tanzanian economy. It accounts for about one-third of the gross domestic product (GDP), provides 85 percent of all exports and serves as a livelihood to over 80 percent of the total population. Maize or corn (Zea mays L.) is the primary staple crop; it’s grown in nearly all agro-ecological zones in the country. Tanzania is a major maize producer in Sub-Saharan Africa. In the last four decades, Tanzania has ranked among the top 25 maize producing countries in the world. Despite the steady production of maize over the past three decades, post-harvest losses of maize remained significantly high, especially for small-holder farmers. Post-harvest handling, poor infrastructure, and weather variability, bio-deterioration brought about by pest organisms such as insects, molds, and fungi, rodent, bacteria, pathogens, and viruses often aggravate such losses.

In tropical countries, a large proportion of the maize is harvested and stored under humid and warm climatic conditions, which subsequently results in rapid deterioration of the grains, mainly because of growth of molds and pests. Deterioration of maize is mainly affected by moisture content, temperature (grain and air), relative humidity, storage conditions, fungal growth, and insect pests. Fungal growth, especially Aspergillus flavus and Fusarium sp in maize, facilitated by hot and humid conditions, poses a major health risk through the production of mycotoxins. Mycotoxins are toxic secondary metabolites of fungi that frequently contaminate the maize in the field and/or during storage. The most important mycotoxins in maize are the aflatoxins, Fumonisins, deoxynivalenol, and ochratoxin. In order to maintain high-quality maize for both short- and long-term storage, maize must be protected from weather, the growth of microorganisms, and insect pests.

Stored product pests such as Sitophilus zeamais (Motschulsky), the maize weevil, are serious pests of economic importance in stored products in tropical and subtropical countries. Infestation often starts in the field, but serious damage is done during maize storage. This study determined the resistance of flint corn and dent corn to infestation by S. zeamais. Improved King Philip hybrid flint corn and Fontanelle 6T-510 hybrid dent corn were used. Two temperature conditions (10 and 27ºC) and two storage times (15 and 30 days) were used. Results showed flint corn was more resistant to insect damage than dent corn at 27ºC and 30 days storage time. After 30 d storage time and 27ºC, the death rate of the weevls was significantly higher in flint corn (R2 = 0.945) compared to dent corn (R2 = 0.634). Likewise, the damaged seed was 10% higher in dent corn than in flint corn at 27ºC and 30 days. However, no significant difference was observed for seed weight loss between flint corn and dent corn at the same storage conditions.

Further, the study evaluated S. zeamais infestation on seven varieties of maize. Seven commercial maize varieties (white dent, yellow dent, orange flint, Indian flint, white and yellow popcorn, and sweet corn), two temperature conditions (10 and 27 °C) and three storage times (30, 60, and 90 days) were used. The moisture contents of all maize samples were adjusted to 15.5 ± 0.5% (wet basis) prior to initiating storage trials. Numbers of live weevils, seed damage, weight loss, and weight of powder produced were assessed at the end of each storage time. As expected, severe damage was observed at 27ºC and 90 d for all maize varieties. Exponential growth rates of S. zeamais were observed in almost all maize varieties. Among seven varieties evaluated, orange flint corn, yellow, and white popcorn show resistance to S. zeamais. Sweet and dent corn were most susceptible to maize weevil infestation. Higher numbers of live S. zeamais were observed on Indian flint corn and sweet corn. Consequently, there was a higher seed weight damage and weight loss. In addition, seed damaged, percentage seed weight loss and weight of powder produced was significantly and positively correlated with a number of live S. zeamais (r = 0.91, P<0.05), (r = 0.88, P<0.05), and (r = 0.89, P<0.05) respectively. Thus, some varieties of flint corn and popcorn can be considered as potential maize varieties to be used to reduce the postharvest loss of maize in tropical countries due to their natural resistance to S. zeamais infestation.

Moreover, the study also determined the techno-economic analysis (TEA) and life cycle analysis (LCA) of maize storage for middle-class farmers in developing countries. Maize is the most widely cultivated cereal crop worldwide. It is produced on a seasonal basis, usually harvested once per year. To maintain a constant supply throughout the year, maize should be properly stored. But this entails high cost and high-energy consumption, which can contribute significant amounts of greenhouse gas emissions. Three storage capacities (25,000 bu, 250,000 bu, and 2,500,000 bu) per year were evaluated for economic analysis and environmental impact. The result shows the total storage cost per kilogram decreased as storage capacity increased (3.69$/bu, 1.89$/bu, and 0.42$/bu). Likewise, energy consumption (electricity, diesel, and liquid propane) increased as storage capacity increased. Consequently, more greenhouse gas emissions (CO2, CH4, and NOX) were emitted to the environment. Thus, to obtain an optimal balance between economics and the environment, it is important for the farmers to understand the concepts of techno-economic analysis and life cycle assessment. Furthermore, the study also determined the measured and predicted temperature of maize under hermetic conditions. Three different storage conditions (room at 25°C, cooling at 4°C, and freezing at -20°C) were investigated. Yellow dent corn variety Blue River 571136 from Iowa, harvested in 2011 was used. Maize was stored in two hermetically sealed bins (50-cm diameter x 76-cm height). Five logger sensors were installed inside the bin to measure temperature and relative humidity of the air and maize grain. The sensors were located at the top, center, bottom, left and right at about 12 centimeters apart. After placing each barrel into storage, temperature and relative humidity values were measured every minute for 9 days throughout the duration of the experiment. Model validation was carried out by comparing predicted with measured maize grain temperature data in the radial and vertical directions. The temperature in the hermetically sealed cylindrical bins varied, mostly in the radial direction and very little in the axial vertical directions. No noticeable change in temperature was observed in the room condition. Moreover, the temperature in the grain changed more rapidly in the freezing conditions than in the room temperature and cooling conditions. Furthermore, the lag time between the center temperature and the side (right, left, top, and bottom) was greater in the radial direction compared to in the vertical direction. The maximum difference between predicted and measured temperature was ±1.5°C. The predicted and measured values of maize grain temperature at radial and vertical directions were found to be in good agreement. The model shows a good potential application to predict the temperature of maize grain stored at the room, cooling and freezing conditions under hermetic storage.

In addition, the study determined the impact of moisture content and S. zeamais on maize quality during hermetic and non-hermetic storage conditions. Commercially commingled maize kernels were conditioned to target moistures 14, 16, 18, and 20% moisture content (wet basis), and then three replications of 300 grams of maize grain were stored in glass jars or triple Ziploc® slider 66 μm (2.6-mil) polyethylene bags at four conditions: hermetic with weevils, hermetic no-weevils, non-hermetic with weevils, non-hermetic no-weevils. All jars and bags were stored in an environmental chamber at 27°C and 70% relative humidity for either 30 or 60 days. At the end of each storage period, jars and bags were assessed for visual mold growth, mycotoxin levels, CO2 and O2 concentrations, pH level, the numbers of live and dead S. zeamais, and maize moisture content. The maize stored in non-hermetic conditions with weevils at 18 and 20% exhibited high levels of mold growth and aflatoxin contamination (>150 ppb). Although mold growth was observed, there were no aflatoxins detected in maize stored in hermetic conditions. The CO2 and O2 concentrations were directly related to the maize moisture contents and storage times. In general, CO2 increased and O2 gradually decreased as storage time increased. No significant difference in pH was observed in any storage conditions (P<0.05). Total mortality (100%) of S. zeamais was observed in all hermetically stored samples at the end of 60 days storage. The number of S. zeamais linearly increased with storage time for maize stored in non-hermetic conditions. Moisture content for hermetically stored maize was relatively constant. Moreover, a positive correlation between moisture content and storage time was observed for maize stored in non-hermetic conditions with weevils (r = 0.96, P<0.05). The results indicate that moisture content and the number of S. zeamais play a significant role in maize storage, both under hermetic and non-hermetic conditions.

The study also determined whether there is a synergistic interaction between P. truncatus and S. zeamais during storage. The interaction between the two insects was evaluated in terms of the numbers of the live population, percent damaged grain, the weight of powder (flour) produced, and percentage seed weight loss. Higher damage was observed in non-hermetic storage with P. truncatus and in mixed treatments (P. truncatus and S. zeamais). A significant difference (P<0.05) and positive correlation were observed between the number of live population, percentage grain damage, the weight of powder produced, and percentage seed weight loss on infestation by P. truncates, S. zeamais, and mixed treatments. S. zeamais dominate populations in the early stage but were outnumbered by P. truncatus after 60 d of storage in the individual species as well as in mixed treatments. The high percentage grain damage was observed in non-hermetic storage after 60 days in P. truncatus (58%) and mixed treatments (54%). The weight of powder produced ranged from 0-30 grams per 250 grams of maize. Percentage seed weight loss decreased after 60 days for P. truncatus and mixed treatments, but increased onward for S. zeamais, a low synergistic interaction between P. truncatus and S. zeamais was observed. However, P. truncatus plays a significant role when two insects coexist and cause more severe damage than S. zeamais in maize under non-hermetic storage conditions.

Furthermore, the study determined the practicability of periodic physical disturbance on S. zeamais mortality and adaptation by smallholder farmers in developing countries. S. zeamais is the most widely occurring and important cosmopolitan postharvest insect pest of stored maize in tropic and sub-tropical regions. Preventing infestation of this pest without using chemicals remains a huge challenge for smallholder farmers in the developing countries. Physical control methods are effective and attractive alternative methods to prevent, and control stored product pests in grain handling and storage facilities. Physical techniques are based on the application of some kind of force to manipulate the storage environments. They can provide unfavorable conditions for insect pests to multiply or damage to the grain. In this experiment, disturbed and stationary/control treatments were arranged in a Completely Randomized Design (CRD) with three replications and three-storage times (30, 60, and 90 days) in three regions of Tanzania. A total of 108 clean 20L (L284 x W234 x H391) millimeter plastic containers were each loaded with 10 kilograms of fresh white dent corn and 0.50 kilograms of maize infested with S. zeamais. The initial numbers of S. zeamais were determined. For the turned treatment, containers were disturbed or turned twice a day, whereas, for the controls, the containers were not disturbed until the end of storage. The overall percent mortality after 30, 60, and 90 days of storage were 88, 96, and 98% respectively. A statistically significant difference (P<0.05) was observed for the number of live S. zeamais in the control treatments. While the number of live S. zeamais in the turned treatment significantly decreased as storage time increased. The study shows the potential of a feasible, simple, affordable, safe and effective method of protecting maize grain for small-holder farmers in developing countries without using chemicals.

Lastly, the study assessed the postharvest practices and awareness of mycotoxins contamination in maize grain. Maize is a major cereal crop in Tanzania and it is grown in diverse agro-ecological zones. Like other sub-Saharan countries, postharvest losses of maize during storage in Tanzania remain significantly high, especially for smallholder farmers. Unpredictable weather and poor postharvest practice contribute significantly to rapid deterioration of grain and mold contamination, and subsequent production of mycotoxins. The purpose of this study was to assess the postharvest practices and awareness and knowledge of mycotoxin contamination in maize grain in three agro-ecological zones (Eastern, Central, and Northern) of Tanzania between November 2015 and February 2016. A survey using semi-structured questionnaires was administered to farmers, traders, and consumers of maize. A total of 90 people (30 from each zone) were surveyed with a response rate of was 96% (87). In addition, several samples of maize were collected and analyzed for aflatoxin, fumonisin, and Zearalenone contamination to validate the awareness and knowledge of mycotoxin contamination of maize. The result shows a high level of postharvest losses of maize mainly through insect infestation. Moreover, over 80% of the farmers, traders, and consumers of maize were unaware of mycotoxins contamination. All maize samples collected contained detected levels of mycotoxins. The maximum concentration of aflatoxins, fumonisin, and Zearalenone in maize samples was 19.20 ppb, 7.60 ppm, and 189.90 ppb respectively. Education intervention is necessary to decrease the disconnect observed between actual mycotoxin contamination and the awareness and knowledge of farmers, traders, and consumers of maize in Tanzania. Enhancing awareness and knowledge provide the opportunity to educate on post-harvest practices that reduce postharvest losses of maize in Tanzania.

Copyright Owner

Rashid Ali Suleiman



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401 pages