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

CHAPTER ONE

2.0 INTRODUCTION

The demand for probiotic foods is growing rapidly due to increased consumer awareness regarding the health benefits probiotic foods (Siro et al., 2008; Tripathi and Giri, 2014). Yoghurt is a well-known, popular food that contains probiotics (Tamime and Robinson, 1995; Shiby and Mishra, 2013; Najgebauer-Lejko, 2014). Probiotics such as Lactobacillus acidophilus are commonly incorporated into yoghurts worldwide. The possible benefits of consuming yoghurt containing L. acidophilus include controlling diarrhoea and urogenital infections, alleviating lactose intolerance, preventing gastrointestinal diseases, lowering serum cholesterol levels, providing anticarcinogenic activity, reducing allergens and improving disease resistance (FAO/WHO, 2001; Sanders et al., 2013; Najgebauer- Lejko, 2014; Mishra et al., 2015). In addition, probiotics also provide antioxidative properties and stimulate the immune system, which are strain-dependent effects that have been the subject of many recent studies (Mishra et al., 2015; Santiago-Lopez et al., 2015).

Yoghurt is a dairy product produced by bacterial fermentation of milk. The bacteria used to make it are known as yoghurt cultures. The starter culture contains Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus  (Whitt and Salyers, 2002). In addition, other Lactobacilli and Bifidobacteria are sometimes added. Lactose is usually fermented by these bacteria to produce lactic acid, carbondioxide, acetic acid, diacetyl, acetaldehyde and several other components. The lactic acid produced contributes to the sour taste of yoghurt by decreasing the pH. It also gives the product its characteristic texture by acting on the milk proteins  (Lucey et al.,2009).

Yoghurt can be made from the milk of a variety of mammals but most often from cow, buffalo or goat milk. Plain yoghurt is generally made from cow milk and is unflavoured and unsweetened.

Flavoured yoghurt generally has fruits or flavourings added along with plenty of sugar. Yoghurt is an excellent source of proteins, vitamins, calcium, lysozyme,  lactose, peptone, phosphate and nitrogen-based enzymes. Yoghurt is a means of protein intake for healthy living (Isleten and Karagul-Yuceer, 2006). This fermented milk prolongs the shelf life of some foods and milk preparations (Lee and Lucey, 2010). It has traditionally been considered a probiotic- carrier food with health promoting effects (Salvador and Fiszman, 2004). It is also reported to be involved in minimizing lactose intolerance and lowering of serum cholesterol level (Sofu and Ekinici, 2007). Several lactobacilli have antitumor compounds in their cell walls and foods such as yoghurt containing lactic acid bacteria especially Lactobacillus acidophilus may contribute to the control of cancer (Cabellero, 2003). Other health promoting properties of live lactic acid bacteria in yoghurt include improving the immune response and better assimilation of protein, calcium and iron (Muir and Banks, 2000).

Yoghurt due to its high nutritive value is susceptible to contamination by pathogenic microorganisms thereby resulting in spoilage (Yabaya and Idris, 2012). Yeasts and molds are the primary contaminants of yoghurts produced in Nigeria. They utilize some acids and produce a corresponding decrease in acidity which may favour the growth of putrefactive bacteria (Vanous, 2011). Yoghurts are also a selective environment for the growth of yeasts. The introduction of fruits and sugar has amplified the risk of spoilage by yeasts by providing additional sources of contamination and fermentable substrates.

1.1 AIMS AND OBJECTIVES

The aims and objectives are;

1. To isolate and identify microorganisms associated with laboratory produced and commercially produced Yoghurt.

2. To determine the shelf life of the yoghurt samples.

3. To determine and compare the effects of different preservation methods on the shelf life of yoghurt.

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