MICROBIAL ANALYSIS AND SHELF LIFE OF LABORATORY AND COMMERCIALLY PRODUCED YOGHURT

ABSTRACT

The microbial quality and shelf life of laboratory and commercially produced yoghurt were investigated. Six (6) laboratory prepared yoghurt, unsweetened (420) sweetened (421), unsweetened with sodium benzoate (422),sweetened with sodium benzoate(423), unsweetened with ginger (424), sweetened with ginger (425) and three (3) commercially produced yoghurt, Raph yoghurt (426) Nutri yo yoghurt (427) and Hollandia yoghurt (428) were used for this study. The bacterial genera; Lactobacillus acidophilus, Lactobacillus bulgaricus and Streptococcus thermophilus and three fungal genera Saccharomyces cerevisae, Saccharomyces lactis and Pichia spp. were isolated. The Total Viable Count (TVC) ranged from 3.0 x 108 cfu/ml to 2.48109 cfu/ml. The Total Fungal Count (TFC) ranged from 4.0 x 107 cfu/ml to 2.6 x 108 cfu/ml while the Total Lactobacillus Count (TLC) ranged from 2.6 x 108 cfu/ml to 2.30 x 109 cfu/ml. A zero count was recorded for the Total Coliform Count (TCC) and the Total Staphylococcus Count (TSC). For the physiochemical parameters of the yoghurt samples the pH ranged from 4.02 ± 1.00 to 6.50 ± 0.48, Specific Gravity (SG) ranged from 0.6971 ± 1729 to 1.0751 ± 0.2051. The Total Titrable Acidity (TTA) ranged from 8.71 ± 2.96 to 17.80 ± 6.13, while the Total Soluble Solid (TSS) ranged from 5.70 ± 4.37 to 14.00 ± 3.93. The sensory gradually moved from liked extremely (1) on day 0 to neither liked nor disliked (5) on day 14, even though the general acceptability stopped at liked slightly (4).  The result of this study showed that refrigeration and deep freezing irrespective of the preservatives used prolonged the shelf life of yoghurt. 

TABLE OF CONTENTS

Title Page ﾿ i

Certification ﾿ ii

Dedication ﾿ iii

Acknowledgements ﾿ iv

Table of Contents ﾿ v

Lists of Tables ﾿ vii

List of Figures ﾿ viii

Abstract ﾿ ix

CHAPTER ONE

1.0 ﾿ Introduction ﾿ 1

1.1 ﾿ Aims and Objectives ﾿ 3

CHAPTER TWO

2.0  ﾿ Literature Review ﾿ 4

2.0.1 ﾿ Microorganisms Involved In Yoghurt ﾿ 6

2.1 ﾿ Varieties and Types of Yoghurt ﾿ 9

2.2 ﾿ Types of Yoghurt ﾿ 15

2.3 ﾿ Yoghurt Manufacturing Process ﾿ 17

2.3.1 ﾿ Milk Standardization ﾿ 17

2.3.2 ﾿ Homogenization ﾿ 18

2.3.3 ﾿ Heat treatment ﾿ 19

2.3.4 ﾿ Fermentation process ﾿ 21

2.3.5 ﾿ Cooling ﾿ 21

2.4 ﾿ Production of Yoghurt ﾿ 23

2.5 ﾿ Probiotic Benefits of Yoghurt ﾿ 25

2.6 ﾿ Application of Probiotic Microorganisms in Functional Foods ﾿ 25

2.7 ﾿ Yoghurt Spoilage Microorganisms ﾿ 27

2.7.1 ﾿ Psychrotrophs ﾿ 27

2.7.2 ﾿ Coliforms ﾿ 28

2.7.3 ﾿ Lactic Acid Bacteria ﾿ 28

2.7.4 ﾿ Fungi ﾿ 29

CHAPTER THREE

3.0 ﾿ Materials and Methods ﾿ 30

3.1 ﾿ Sample Collection ﾿ 30

3.2 ﾿ Media and Media Preparation ﾿ 30

3.3  ﾿ Sterilization ﾿ 31

3.4 ﾿ Experimental Design ﾿ 31

3.5 ﾿ Sample Preparation ﾿ 32

3.6 ﾿ Laboratory Production of Yoghurt ﾿ 33

3.7 ﾿ Identification of Bacterial Isolates ﾿ 34

3.7.1  ﾿ Gram Staining ﾿ 34

3.7.2 ﾿ Motility Test ﾿ 34

3.8 ﾿ Identification of Fungal Isolates ﾿ 35

3.9 ﾿ Biochemical Cultural Characteristics ﾿ 36

3.9.1  ﾿ Catalase Test ﾿ 36

3.9.2 ﾿ Coagulase Test ﾿ 36

3.9.3 ﾿ Citrate Test ﾿ 36

3.9. 4 ﾿ Indole Test ﾿ 37

3.9.5 ﾿ Urease Test ﾿ 37

3.9.6 ﾿ Triple Sugar Iron Test ﾿ 38

3.9.7 ﾿ Oxidase Test ﾿ 38

3.9.8  ﾿ Methyl Red/ Voges Proskauer Test ﾿ 38

3.9.9 ﾿ Sugar Fermentation ﾿ 39

3.10 ﾿ Organoleptic Properties ﾿ 39

CHAPTER FOUR

4.0 ﾿ RESULTS ﾿ 41

CHAPTER FIVE

5.0 ﾿ Discussion, Conclusion and Recommendation ﾿ 65 ﾿

5.1 ﾿ Discussion ﾿ 65

5.2 ﾿ Conclusion ﾿ 67

5.3 ﾿ Recommendation                                                                                                        68 ﾿

      ﾿ References                                                                                                                   69 ﾿

LIST OF TABLES

Table ﾿ Title ﾿ Page

2.1: ﾿ Manufacturing Process of Set- and Stirred-Yoghurt ﾿ 11 ﾿

2.2: ﾿ A Schematic Presentation of the Production of Yoghurt ﾿ 20

1: ﾿ Characterization and Identification of Microbial Isolates ﾿ 42

2: ﾿ Occurrence of Microbial Isolates along Evaluation Time (In Day) ﾿ 43

3: ﾿ Microbial Count of the Yogurt Samples at 10-6 Dilution ﾿ 46

Factor upon Storage at Different Temps. For 14 Days 

4: ﾿ Physiochemical Characteristics of the Yoghurt Samples ﾿ 54

5: ﾿ Sensory Evaluation of the yoghurt Samples ﾿ 57

LIST OF FIGURES 

Figure ﾿ Title ﾿ Page

1 ﾿ Manufacturing process of set and stirred-yoghurt ﾿ 22 ﾿

2 ﾿ A schematic presentation of the production of yoghurt ﾿ 24