The greatest problem consumers encounter with soy milk remains its very short shelf life due to microbial activity. The combination of different preservation methods may just be the solution to the problem. The synergistic effect of citric acid and sodium benzoate preservatives at various concentrations within their maximum permissible levels along with pasteurization, on the microbial keeping quality of home- made soy milk stored at ambient temperature was therefore, studied. This is with the intention of determining which combination is best for a prolonged shelf life. Fourteen soymilk samples were prepared by extracting the soymilk from whole, boiled and dehulled soybean seed, pasteurized, preserved and stored. All soy milk samples were analyzed for proximate composition (moisture %, ash %, protein %, fat and carbohydrate %) and organoleptic properties (color, texture, taste, flavor, aroma and overall acceptability). Standard microbiological techniques were employed in the enumeration of potential spoilage organisms in soy milk samples evaluated to determine the shelf-stability of the soymilk during the period of storage. pH analysis of the soymilk samples was conducted throughout the period of storage and it was revealed that the pH of the samples reduced significantly (p<0.05) as the storage time increased. Proximate analysis of the soymilk shows that averagely it has a water content of 87.67 %, 3.00 % ash, 2.30 % fat, 5.07 % protein and 1.93 % carbohydrate. The results of the sensory evaluation revealed that flavor with respect to taste and smell had significant influence (p<0.05) on overall acceptability of the soy milk. Mean sensory scores for flavour, colour, taste, texture and aroma ranged from 3.60 to 6.35, 3.65 to 6.25, 3.20 to 6.45, 3.45 to 5.45 and 4.40 to 6.45 (p<0.05) respectively. Though the acceptability range of all the soymilk samples was low, the soymilk sample (sample B) treated with 5g of citric acid and 0.4g of sodium benzoate had a higher acceptance value (6.20). The sensory properties (colour, taste, texture, flavor and general acceptability) were, however, observed to decrease significantly (p<0.05) with the increase in the fortification level of the soymilk samples. The results obtained showed that the fortification of soymilk with citric acid and sodium benzoate resulted in a more shelf stable beverage, but with a low level of acceptance by the consumers. The Total Viable Count (TVC) at day one (1) ranged from 0.5×104to 1.0×104 Cfu/ml, while the Total Coliform Count (TCC) ranged from <0.1×104 to 0.5×104 Cfu/ml, with the lowest value coming from sample H preserved with 12.5g of citric acid and 0.45g of sodium benzoate, thus making sample H microbiologically safer than the others. The TVC and TCC values at day 14 ranged from 2.0×104 to 5.0×104 Cfu/ml and 2.2x104 to 4.9x104 Cfu/ml respectively which was also accompanied with a decrease in the sensory properties of the soymilk samples. The isolated and identified bacteria includes Streptococcus sp, Pseudomonas sp, Proteus sp, Bacillus spp, Staphylococcus aureus, Klebsiella sp, Escherichia coli and Enterobacter species. The detection of E. coli reflected a poor hygienic standard in the production of the soymilk samples tested. However, Results obtained showed that soymilk can keep for up to 7 days at ambient temperature with its organoleptic properties still intact.
TABLE OF CONTENTS
LIST OF TABLES vii
LIST OF FIGURES ix
ABSTRACT x
INTRODUCTION 1
1.1 Background to the Study 1
1.2 Justification of Study 4
1.3 Aim and Objectives of research 5
1.4 Definition of Terms 5
1.5 Scope of the Study 6
1.6 Limitations of study 6
CHAPTER TWO 7
LITERATURE REVIEW 7
2.1 SOYBEAN 7
2.1.1 Composition and Nutritional Value of Soybean 8
2.1.2 Side Effects of Soy Consumption 11
2.1.3 SOY ISOFLAVONES AND THEIR HEALTH BENEFITS 12
2.1.4 SOYISOFLAVONE AND MENOPAUSE 15
2.1.5 SOYBEAN AND HEART DISEASES 15
2.2 SOYMILK 19
2.2.1 SOYMILK PRODUCTION 20
2.2.3 COMPOSITION AND NUTRITIONAL VALUE OF SOYMILK 24
2.2.4 SOYMILK SPOILAGE AND PRESERVATION 27
CHAPTER THREE 37
MATERIALS AND METHODS 37
3.1 Source of Raw Materials 37
3.2 Research Design 37
3.3 Sample Preparation 39
3.4 Proximate Analysis 40
3.4.1 Moisture Content Determination 40
3.4.2 Fat Content Determination 41
3.4.3 Protein Determination 42
3.4.4 Ash Content Determination 44
3.4.5 Carbohydrate Content Determination 44
3.5 pH Determination: 44
3.6 Sensory Evaluation 45
3.7 Microbial Analysis of Sample 46
3.7.1 Preparation of Media 46
3.7.2 Total Plate Count 48
3.7.3 Identification of Isolates 49
3.7.3.1 Bacteria Identification 49
3.7.3.2 Biochemical Tests 50
3.8 Statistical Analysis 56
CHAPTER FOUR 58
4.0 RESULTS AND DISCUSSIONS 58
4.1 Sensory Evaluation 58
4.2 Proximate Composition 61
4.3 Microbial Analysis 63
4.4 Bacterial Isolation 66
CHAPTER FIVE 68
5.1 Conclusion 68
5.2 Recommendations 71
REFERENCE 74
APPENDICES 81
