STUDIES ON MICROORGANISMS ASSOCIATED WITH DETERIORATED DIOSCOREA ROTUNDATA L. (WHITE YAM)

ABSTRACT

Microorganisms associated with Deteriorated dioscorea rotundata L. (white yam) was studied. Discorea rotundata (white yam) with no symptoms of infection were procured locally from different yam sellers in Umuahia main market, Ubani, Abia state. The yam samples were stored using different methods which include; Ambient temperature, Polyethylene bag at 40oC, Unprotected from rain, Unprotected from rodent attack (27oC), dark cupboard (30oC) and Ambient temperature at 27oC and were labeled samples A, B, C, D and E respectively. The yam samples were stored for four (4) weeks in which the spoilage progress was monitored after which bacteria and fungi were isolated and identified using standard methods of analysis. A total of 9 isolates (four bacteria and 5 fungi) were isolated after some physiological, morphological and biochemical test were carried out. The study revealed that the yam sample stored at ambient temperature (27oC) (sample E) showed no sign of spoilage up to day 18 while rodent attack, dark cupboard and rain all facilitates spoilage of white yam. The bacteria isolated were Erwinia sp., Pseudomonas specie, Staphylococcus specie and E. coli. The fungi isolated were Aspergillus niger, Aspergillus flavus, Rhizopus stolonifer, Fusarium oxysporium and Mucor spp. The incidence of occurrence of these organisms revealed that Aspergillus niger had the highest occurrence (31.25%) for fungi and occurred more in sample stored with polyethylene bag while Mucor spp had the least occurrence (12.5%). Erwinia sp and E. coli had the highest occurrence (29.41%) for bacteria and was found in the sample stored with polyethylene and ambient temperature. While Staphylococcus specie had the lowest occurrence (17.65%). The research has isolated and identified the microorganisms associated with the spoilage of white yam.

TABLE OF CONTENTS

Title page i

Declaration ii

Certification iii

Dedication iv

Acknowledgement v

Table of contents vi

List of Tables ix

Abstract x

CHAPTER 1: INTRODUCTION

1.1 Introduction 1

1.4 Aim and Objectives 3

CHAPTER 2: LITERATURE REVIEW

2.1 Botany of yam and its importance 4

2.2 Harvesting of yam tuber 5

2.3 Agronomy of Yam and Storage process 6

2.4 Methods of traditional storage of yams in Nigeria 7

2.5 Factors affecting the shelf life of yams 8

2.6 Storage pathogens 10

2.6.1 Pathogens of yam rot diseases 10

2.6.2 Control of yam rot diseases 12

2.7 Nutrients in Yam 16

2.8 Emergence of post-harvest pathogen in yam 18

2.8.1 Wet rot 19

2.8.2 Dry rot 19

2.8.3 Soft rot 20

2.9 Organisms involve in Post-harvest spoilage of yam 20

2.9.1 Management of post-harvest pathogens of stored yam 21

CHAPTER 3: MATERIALS AND METHODS

1.1 Source of raw material 23

3.2 Sample Preparation 23

3.3 Processing of materials and media 24

3.3.1 Sterilization of materials 24

3.4 Isolation of fungi associated with yam spoilage 24

3.5 Identification of isolated fungi pathogens 24

3.6 Isolation of bacteria pathogens 25

3.7 Identification of isolated bacteria pathogens 25

3.8 Biochemical test 26

3.8.1 Gram staining 26

3.8.2 Oxidase test 26

3.8.3 Coagulase tests 27

3.8.4 Catalase test 27

3.8.5 Indole test 27

3.8.6 Methyl Red test 27

3.8.7 Motility Test 28

CHAPTER 4: RESULTS

4.1 Shelf life determination of the white yam samples 29

4.2 Morphological Characteristics of the Isolates 34

4.3 Biochemical Identification of Isolates 36

4.4 Incidence of occurrence (bacteria) 38

4.5 Total plate count 40

4.8 Total plate count of the white yam samples (cfu/ml) 43

4.6 Morphological Characteristics of fungi Isolates 42

4.7 Incidence of occurrence (fungi) 44

CHAPTER 5: DISCUSSION, CONCLUSION AND RECOMMENDATIONS

5.1 Discussion 46

5.2 Conclusion and Recommendation 49

REFERENCES 50

LIST OF TABLES

Table 4.1: Spoilage progress based on the various storage conditions for week 1 30

Table 4.2: Spoilage progress based on the various storage conditions for week 2 31

Table 4.3: Spoilage progress based on the various storage conditions for week 3 32

Table 4.4: Spoilage progress based on the various storage conditions for week 4 33

Table 4.5 Morphological Characteristics of the Isolates 35

Table 4.6: Biochemical Identification of Isolates 37

Table 4.7: Incidence of occurrence (bacteria) 39

Table 4.8: Total plate count of the white yam samples (cfu/ml) 41

Table 4.9: Morphological Characteristics of fungi Isolates 43

Table 4.10: Incidence of occurrence (fungi) 45

CHAPTER 1

INTRODUCTION

1.1 Background of the study

The crop yam (Dioscorea spp) is one of the common food crops in the tropics and plays vital roles in food security as a staple food in the regions where it is being cultivated (Maroya et al, 2014). Dioscorea spp occur in Asia, East Africa, the Caribbean, India and Tonga Kingdom, south pacific as well as West Africa (Okigbo, 2004). Maroya et al, 2014 estimated that yam consumption yearly is over 48 million tones globally. Out of the 48 million tons of yam (95% global supply) that are produced on 4 million hectares annually, Nigeria alone produces 67.7% of global yam supply (FAO, 2010). This makes Nigeria the highest consumer of this staple crop after Cassava and Maize. However, demand for this commodity is increasing; as incomes increase, consumers shift from substitutes to yam especially when the price of yam relative to price of its substitutes declines (Maroya et al, 2014).

Yam plays an outstanding function in social cultural lives of some producing regions like the celebrated Moon festival and also the popular Yam festival in West Africa, an act that is well observed. In Nigeria for instance, the meals offered to deity and associates consist mainly of mashed yam (Ogunleye, 2005). Yam storage in comparison with some other staple crops has relatively longer life span, so stored tuber symbolizes stored wealth, which can be sold throughout the year by the marketers. Also, tuber yam in West Africa particularly Nigeria can be converted into different staple transitional and end product forms (Okaka and Aajekwu, 1990; Okaka et al, 1991) which can be consumed by human beings and animals, used as the essential ingredient of snacks and flour that is now used in instant puree making.

Notwithstanding the importance of yam to both the national and regional economies, in comparison with other crops, little attention has been paid to the constraints that hinder its efficient production in Africa. In 1994, the National Agricultural Research Strategy Plan for Ghana identified yam as a priority crop for research and development and specifically identified the need to reduce post-harvest losses. These losses are mainly due to tuber dehydration, respiratory loss of carbohydrates, tuber sprouting and microbial decay.

Tuber yam, among many other staple foods, is susceptible to postharvest diseases caused by bacteria and fungi under poor storage conditions. In Nigeria, pathogens constitute the major factor to rot in yam production (Nari, 2004). Losses caused by pathogens attack vary from 20-30% generally in some crops (Park et al, 2008).

Lack of good storage and processing facilities cause a lot of wastage of agricultural products such as tubers, roots, pulses, fruits and vegetable crops. Specifically, storage is one of the critical problems limiting yam production. According to Okigbo (2004), an estimate of 56% of yam is lost to rot after six months of storage. These losses are due to pathological problems brought about by bacteria, fungi and nematodes (Okigbo, 2004).

Roots and tubers bruised or otherwise damaged during harvesting may undergo early infestation with moulds and viruses which may in turn lead to rotting. In most countries, several traditional methods of storage are in use. However, losses in these types of storage are very high and may be attributed to a number of factors such as microbial infestation of yam, rodents, insects, or physiological losses due to sprouting and respiration (FAO, 1985). An assessment of mycoflora and occurrence of aflatoxin B1 in dried yam chips by Bankole and Mabekoje (2004) showed that Aspergillus and Penicillium were the two prevalent genera of fungi, and that the number of colony forming units of the two genera in the yam chips studied exceeded the tolerance limits in foodstuffs specified by the International Commission on Microbiological Specifications for Foods (ICMSF, 1996).

In other for us to limit postharvest losses in yam and subsequently increase yam production and utilization, there is therefore the need to isolate and identify microorganisms associated with yam spoilage so as to proffer possible solution to their prevention.

1.2 Aim and Objectives

The main objective of this study is to isolate and identify microorganisms associated with the spoilage of yam.

The specific objectives include:

1. To determine the factors that facilitates the spoilage of white yam

2. To identify the parameters that hasten the spoilage of yam

3. To isolate the fungi associated with the spoilage of yam

4. To identify the isolated fungi from the yam samples

5. To isolate the bacteria associated with the spoilage of yam

6. To identify the isolated bacteria from the yam samples

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