Microbial Contamination Of Yoghurt

Project and Seminar Material for Microbiology

Microbial Contamination Of Yoghurt


An assessment of the microbial contamination of selected yoghurt sold in Onitsha Market was determined in this study using standard microbiological procedures. The physical parameters of the brands at time of purchase were also determined. The results showed that the pH values of the samples ranged from 4.29-4.56, while their temperature readings were between 8 and 170c. The mean total count of samples on Brain Heart Infusion (BHI) and De Mann Rogosa Sharpe (MRS) agar media ranged from 2.0×107 to 6.0×108 and 1.0×108 to 5.4 × 108 cfu/ml respectively.

The yoghurt isolates were identified as Streptococcus and Lactobacillus species; these isolates were resistant to commonly used antibiotics and inhibited the growth of Staphylococcus aureus and Pseudomonas aeruginosa from clinical samples. No viable growth of isolates was observed in simulated gastric fluid of pH 1.5 to 2.5. Slight decrease in viable count of Lactobacillus spp. from 4.0×107 to 3.0×107 cfu/ml and Streptococcus spp. from 3.0×108 to 2.0×108 cfu/ml was observed in bile of pH 8.28 to 8.30. The isolates were recovered from faecal samples two weeks after ingestion with mean count ranging from no growth (zero) to 5.8×108 cfu/ml on MRS agar media.

The result of this study therefore indicated poor Microbiological standards of commercial yoghurts sold in Onitsha market at the time of this research. The isolates were found to exhibit some probiotic potentials and no pathogen was isolated from samples. It is recommended that strains of microorganisms that can deliver full probiotic potentials to consumers be used in commercial yoghurt production. This result underlines the need for improved hygienic measures in the processing, storage and distribution of these products to avert public health challenges.

Table Of Content

Preliminary Page(s)

  • Title page
  • Certification page
  • Dedication
  • Acknowledgement
  • Abstract
  • Table of content

Chapter One

1.0 Introduction

  • 1.1 Background of the Study
  • 1.2 Statement of Problem
  • 1.3 Aims and Objectives
  • 1.4 Significance of Study

Chapter Two

2.0 Literature Review

  • 2.1 Conceptual Review
  • 2.2 Yoghurt Culture Bacteria
  • 2.3 Yoghurt
  • 2.4 Health Benefits of Yoghurt
  • 2.5 Yoghurt Appearance
  • 2.6 Fermented Milk Products
  • 2.7 Milk Fermentation and Biochemical Changes

Chapter Three

3.0 Materials and Methods

  • 3.1 Materials
  • 3.2 Methodology
  • 3.2.1 Study Area
  • 3.2.2 Sample Collection
  • 3.3 Microbiological Analysis
  • 3.3.1. Preparation of Materials
  • 3.3.2. Preparation of Serial Dilutions
  • 3.3.3. Enumeration of Total Aerobic Bacteria (TEB)
  • 3.3.4. Enumeration of Coliform Bacteria
  • 3.3.5. Enumeration of Yeast and Moulds
  • 3.4. Isolation And Identification Of Microorganisms
  • 3.4.1. Gram Reaction
  • 3.5. Biochemical Test
  • 3.5.1. Catalase Test
  • 3.5.2. Indole Test
  • 3.5.3. Oxidase Test
  • 3.5.4. Motility Test
  • 3.5.5. Coagulase Test
  • 3.5.6. Citrate Utilization Test
  • 3.5.7. Methyl Red Test
  • 3.5.8. Voges Proskauer
  • 3.5.9. Carbohydrate Fermentation Test
  • 3.6 Data Analysis

Chapter Four

4.0 Results and Discussion

  • 4.1 Results
  • 4.2 Discussion

Chapter Five

5.0 Summary and Conclusion

  • 5.1 Summary
  • 5.2 Conclusion
  • References

Chapter One

1.0 Introduction

1.1 Background of the Study

Yoghurt is a cultured dairy product produced by the lactic acid fermentation of milk using a combination of bacteria such as Lactobacillus bulgaricus and Streptococcus thermophilus in the ratio of 1:1(Hui, 1992). Yoghurt is made from skimmed milk usually from cows, sometimes from other animals such as goat or sheep (Miller et al., 1964). Milk from which yoghurt is made is an excellent source of protein, vitamins and minerals like calcium and some antibacterial substances such as lysozyme and lactoperoxidase, as well as large amount of lactose sugar, peptone, phosphate and nitrogen-based enzymes (Alfa-Lawal, 1984).

Although, it is a traditional beverage in the Balkans and Middle East (Ghandge et al., 2008), yoghurt is consumed by all people of all nations. Yoghurt is produced by symbiotic actions of two lactic acid bacteria, namely Streptococcus thermophilus and Lactobacillus bulgaricus which ferment lactose to lactic acid, which gives it, its sour taste (Steinkraus, 1997; Tamine and Robinson, 2004; Kumar and Mishra, 2004; WDC, 2014). Yoghurt can serve as food and plays an important role in human nutrition, health maintaining, therapeutic and dietetic functions (Younus et al., 2002; Khan et al., 2008).

The nutritional quality of yoghurt has been reported and is known to contain high-quality protein, calcium and phosphorous. Its carbohydrate can be utilized easily by those intolerant to lactose (Younus et al., 2002; Alakali et al., 2008; Ghandge et al., 2008). It is also believed that yoghurt has valuable therapeutic properties and helps in curing gastrointestinal disorders (Athar, 1986; Wolinsky, 2000; Younus et al., 2002; Vasiljevic and Shah, 2008).
Yoghurt has been described as a nutritiously balanced food containing almost all the nutrients present in milk but in a more assailable form (Anthar, 1986).

Microorganisms present in fermented dairy products stabilize bowl microflora. The health claims associated with the consumption of yoghurt include alleviation of lactose intolerance, lowering of serum cholesterol level, and treatment of diarrhea and possibility of exhibiting anticancer activity (Nickerson, 1994). Yoghurt can be consumed as both food and thirst quenching beverage. Yoghurt due to its high nutritive value is susceptible to contamination by pathogenic microorganisms causing spoilage (Nickerson, and Sinskey, 1972).

Yoghurts may provide additional health benefits, for example it may reduce cholesterol levels (DiRienzo, 2000). Study suggested that certain diseases with gastrointestinal tract such as lactose intolerance, diarrhea, colon cancer and other bacterial infection were inhibited through high consumption of yoghurt (Dave and Shah, 1997). Molds and yeasts are the primary contaminants in yoghurt produced commercially in Nigeria. They are responsible for off-flavor, loss of texture quality due to gas production and package swelling and shrinkage (Suriyarachichi, and Fleet, 1981).

1.2 Statement of Problem

Food is one of the vehicle involved in the transmission of diseases. Microorganisms like all other living things need food to grow and reproduce. As a result of this, contaminating microorganism consume the chemical components of food and later replace them with metabolic products, which are capable of altering the texture, tenderness, flavor and color of food, or even cause loss nutritional value or become unpleasant and harmful to consumers. Millions of people die every year as a result of food borne diseases and about 5000 victims die every year (CDC, 2004).

Poor food handling is largely responsible for food contamination with bacteria, fungi, and protozoa being the major contaminants and their presence in food causes a wide range of diseases. The effect of these microorganisms has been mild, severe or fatal depending on the ineffective dose (CDC, 2004).

1.3 Aims and Objectives

The aims and objectives are:

  1. To investigate the microbiological contamination and qualities of yoghurt sold in Onitsha roundabout.
  2. To investigate the nutritional qualities of yoghurt sold in Onitsha roundabout.

1.4 Significance of Study

The research work will enable us to know the types of microorganisms associated with yoghurt sold in Onitsha roundabout. The result obtained will be used to create awareness and educate the public about the possibility of contamination of yoghurt produced.

Chapter Five

5.0 Summary and Conclusion

5.1 Summary

The study showed the trend of tolerance of yoghurt isolates to bile at 8.29 to 8.30. Here, all isolates of Lactobacillus and Streptococcus species survived treatment with bile at the incubation period of 37°C for 24h. The viable counts of the Lactobacillus specie and Streptococcus specie, however, decreased from 4.0×107 to 3.0×107 cfu /ml and from 3.0×108 to 2.0×108 cfu /ml respectively. Other studies on lactic acid bacteria corroborate with these findings (Haller et al., 2001). Bile production contributes to the antimicrobial arsenal developed by mammalian host to inhibit microbial colonization and potential infection; this could contribute to reduction in viable cell count as observed in this study. The bile resistant traits of intestinal yoghurt isolates are crucial for maintaining viability during GI transit and are desirable attributes of an orally administered probiotic (Charteris et al., 2000; Elina et al., 2001; Elkins and Lisa, 2004).

This study also showed that the yoghurt samples after being consumed by healthy volunteers, two weeks after the gastro intestinal tract sill had a good number of the yoghurt microflora. It could be explained that yoghurt microflora were able to adhere and colonize the intestinal mucosa, these characteristics are necessary for selecting probiotics for commercial use in foods and therapeutic, to ensure that they are retained inherently. The ability to adhere is also, to some extent likely to be connected to the ability to stimulate the immune defense (Ouwehand et al., 2009; Plant and Conway, 2002). However, permanent colonization by the probiotics is unwanted and remains unsolved whether colonization is critical for probiotics to have their effect at all (Ouwehand et al., 2009; Fedorak and Madsen, 2004). That the isolates were found/recovered from stool even though they did not survive the simulated gastric fluid after 24h incubation, calls for further investigation.

5.2 Conclusion

It can be concluded that some yoghurts circulating Onitsha community in Anambra state Nigeria, contain no contaminating gram negative microorganisms/coliforms and are thus acceptable. The starter culture used in the production possesses some desired attributes of a probiotic organism such as surviving production process, storage temperature, resistance to bile and some commonly used antibiotics, ability to inhibit the growth of some pathogenic organisms, and colonization/adherence to intestinal mucosa (recovery from faecal samples). The use of strains of probiotic organisms better adapted to resist gastric acid and more of the commonly used antibiotic is advocated, further work in this regard is also necessary.

Milk can harbor a number of microorganisms and can be a good source of food borne diseases. In most foods, the total bacterial count is often an indication for the sanitary quality, safety and utility of foods. It may reflect the conditions under which the product is manufactured such as contamination of raw materials and ingredients, the effectiveness of processing and the sanitary conditions of equipment and utensils at the processing plants. It is possible to have pathogen-free yoghurts through high and strict preventive measures.

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