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ABSTRACT
The study examined the identification of fungi associated with spoilage of sweet potato. Microbial attacks during storage are one of the primary causes of product deterioration, and can limit the process of prolonging the shelf-life of harvested food. In this study, sweet potatoes were stored at temperatures of 13, 21, and 29 °C for 4 weeks. Samples were collected during storage and plated on potato dextrose agar, from which axenic mold cultures were obtained and identified using 26S rRNA gene sequences. Physiological changes of potato tubers were assessed with respect to pathogenicity, enzyme activity, and atmospheric storage conditions. Six fungal species were identified, namely Penicillium chrysogenum (P. rubens), P. brevicompactum, Mucor circinelloides, Cladosporium cladosporiodes, P. expansum, and P. crustosum. The following fungal isolates, namely P. expansum, P. brevicompactum, and Rhizopus oryzae, were recovered from the re-infected samples and selected according to their levels of enzyme activity. This study revealed high levels of activity for cellulase and pectinase, which were most notable during the initial three days of testing, and were followed by a steady decrease (P<0.05). Polygalacturonase activity was prominent with values ranging from 12.64 to 56.79 U/mg (P. expansum) and 18.36 to 79.01 U/mg (P. brevicompactum). Spoilage was obvious in the control group, which had a 100% decay at the end of the experimental period compared with samples treated with iprodione and sodium hypochlorite, in which the decay rates were 5% and 55%, respectively. The data for the iprodione- and sodium hypochlorite-treated samples at the end of the 3-month storage period showed that they were significantly different (P=0.041), with the sodium hypochlorite-treated samples producing twice the rate of infection compared to the iprodione-treated samples. The comparative rate of the progression of decay in the treated samples can be expressed as iprodione<sodium hypochlorite<control. This study demonstrates that sweet potato tissue damage is due to the activities of microbial enzymes and, in particular, the pectinases of the organisms isolated from the infected potato tissues, and suggests the advantages of utilizing iprodione as a curing agent for potato tubers before storage.
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND OF STUDY
Sweet potatoes (Ipomea batatas Lam.) are grown in over one hundred different countries in tropical and sub-tropical regions. The production of sweet potatoes across the world is estimated to be approximately 140 903 000 t/a and 92% of these are reportedly produced in Asia and the Pacific Islands (FAO, 2002). The tubers vary greatly in size, shape, color, and quality of taste, depending on the variety. When the roots are stored they are sensitive to changes in soil temperature, which is largely dependent on their stage of root development (Williams et al., 1980). Storage of sweet potato tubers after harvest is imperative, as this practice may prevent a surfeit of potatoes entering the food markets at any given time and prolong the period of fresh tuber availability, especially when the crop is not in season or when the economic circumstances and/or regional climates in a particular area of production dictate its production during the year.
Onwueme and Charles (1994) and Hall (1993) postulated divergent cropping methods and increased crop cultivation as a necessity to facilitate the required long-term storage of roots, thereby meeting world food requirements during intervening seasons. Rapid deterioration in the condition of the produce is a result of exposure to environmental conditions and is exacerbated by mechanical damage during handling and transport (Rees et al., 2003).
Fungal diseases which can afflict sweet potatoes include: surface rot and root rot caused by various species of the genus Fusarium; soft rot caused by the fungus Rhizopus stolonifer; Java black rot caused by the fungus Diplodia gossypina; scurf disease caused by the fungus Monilochaetes infuscans; black rot of the tuber which is caused by the species Ceratocystis fimbriata, among others. The activities of the spoilage-causing organisms are usually supported by microbial enzymes, which are secreted into the sweet potato tubers. These enzymes constitute the main agents of deterioration, and it is also worth noting that enzymes of microbial origin have been exploited in food and medical research, paper and textile industries, as well as in waste utilization and biotechnologies (Lim et al., 1985).
The shelf-life of sweet potato roots varies from a few days to a few weeks according to the cultivar, as well as to the conditions prevailing at the time of harvest and during storage (Kurup and Balagopalan, 1991; Acedo et al., 1996; Cabanilla, 1996; Rees et al., 1998; Mtunda et al., 2001).
Many methods have been used in the post-harvest preservation of sweet potatoes with the singular aim of preventing deterioration and thereby prolonging shelf-life. These methods include, but are not restricted to, curing under controlled temperature and relative humidity, the use of disinfectants and cold storage. These methods are often used in isolation or in limited combinations in order to impede the progress of the spoilage agents. However, it is evident that these approaches have not achieved the anticipated or desired results in terms of preventing post-harvest spoilage of sweet potatoes or prolonging their shelf-life. In an attempt to suggest an alternative approach, this study investigates the activities of enzymes produced by filamentous fungi that have been isolated from infected sweet potato tubers.
1.2 STATEMENT OF PROBLEM
In the face of the present food situation in Nigeria, one of the major obstacle to food production is that of storage. During harvesting seasons tubers crops are very much available at cheap prices but this availability is usually short lived due to storage problems. Tropical crops suffer tremendously due to inadequate storage practices. Sweet potato as crop suffer most and tuber crops. Nigeria farmers under their local practices have the problem of storing the tuber over a period of time after harvest. These problems are those originating from:
- Attack by micro-organizing which gain entry through harvest bruises, insect and rodents their by causing rotting
- Attack by rodents and arthropods especially insects
- Weight loss due to high temperature which causes high rate of evaporation leading to high loss to water
- Sprouting which renders the tubers unpalatable.
1.3 AIM AND OBJECTIVES OF THE STUDY
The aim of this work is to investigate and identify the fungi associated with spoilage of sweet potato tubers sold in Bori market.
The objectives were:
- To determine out the microorganisms associated with the spoilage of sweet potato tubers and the degree of damage they cause
- To determine the condition necessary for the storage of sweet potato tubers in order to ensure that healthy tubers of good quality are available.
1.4 SIGNIFICANCE OF THE STUDY
This study will be of significant importance to farmers as it will guide local farmers in Nigeria in the effort to minimize too much loss of their harvested tubers.
The still will also be significant to students and researchers as it will serve as a reference material for further or related research.
1.5 SCOPE OF THE STUDY
This work involves identification and enumeration of fungi from sample of sweet potato sold in Bori market. It also looks at the microorganisms associated with it, mode of infection, epidemiology and ways to treat and prevent it.Complete
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