ABSTRACT
The study investigated the effect of addition of soy protein isolates on the nutrient composition and rheological properties of soy yoghurt. Soybean seeds (1kg) were processed into liquid soy milk using conventional method. Soy protein isolates (procured from Jumia Nigeria) was mixed with the soy milk at 0% (SY2), 2% (SY3), 4% (SY4), 6% (SY5), 8% (SY6) and 10% (SY7); and the mixture processed into soy yoghurt using a commercial starter culture (Streptococcus thermopthilus and Lactobacilus bulgaricus). The yoghurts were analysed for quality characteristics. There were decrease in moisture content, fat, and carbohydrate contents as the soy protein isolate increased in the yoghurt samples. Moisture content ranged from 79.86 to 84.42%. Conversely, protein content increased and ranged from 4.81 in SY2 (soy yoghurt with 0% protein isolate) to 12.43 in SY7 (soy yoghurt with 10% soy protein isolates), but was 3.59 in SY1 (cow’s milk yoghurt, the control). Addition of soy protein isolate decreased syneresis from 42.75 in SY2 to 28.25 in SY7, pH from 4.58 in SY2 to 4.41 in SY7. But, increased viscosity from 406.36cP in SY2 to 501.97cP in SY7. Total solids (g/100g) and density (kg/cm3) improved. Total bacterial count (cfu/ml) varied indiscriminately at below acceptable minimum standard. But, no coliform was detected in the yoghurt samples. The control (cow milk yoghurt, SY1) had similar value of microbial load with the soy milk yoghurt. All the soy yoghurt samples were acceptable as the scores for all the sensory attributes ranged from 4.65 to 7.25, which were within dislike slightly to like moderately in the hedonic scale. The sensory scores were at most in SY4 (4% soy protein addition) but, decreased progressively at higher level of addition. However, the cow’s milk yoghurt had significantly higher sensory scores in acceptability than soy yoghurt samples.
TABLE OF CONTENTS
Title Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table of Contents vi
List of Tables ix
List of Figures x
List of Plates xi
Abstract xii
CHAPTER 1: INTRODUCTION
1.1 Background of Study 1
1.2 Statements of the Problem 4
1.3 Justification of the Study 4
1.4 Objective of the Study 5
CHAPTER 2: LITERATURE REVIEW
2.1 Description of Soybean 6
2.1.1 Nutritional Importance of Soybean 7
2.1.2 Health Significance 10
2.1.2.1 Effects on Cancer 10
2.1.2.2 Effect on Hypercholesterolemia and Cardiovascular Diseases 11
2.1.2.3 Effect on Osteoporosis and Menopause 12
2.1.2.4 Effects on Hypotensive Activity 12
2.1.2.5 Effect on Blood Pressure and Endothelial Function 12
2.1.3 Anti-nutritional Factors in Soybean 13
2.1.4 Uses Soybean 14
2.2 Protein Isolates 17
2.2.1 Extraction Methods 18
2.2.1.1 Isoelectric Precipitation 18
2.2.1.2 Alkaline Extraction 18
2.2.1.3 Ultra-Filtration Method 19
2.2.2 Comparism between Protein Isolates and Protein Concentrates 20
2.2.3 Functional Properties of Protein Isolate 22
2.2.3.1 Bulk Density 22
2.2.3.2 Protein Solubility 22
2.2.3.3 Foaming Capacity and Stability 22
2.2.3.4Water/oil Absorption 23
2.2.3.5 Emulsifying Activity and Stability 23
2.2.3.6 Least Gelation Concentration 23
2.2.4 Antinutritional Factor in Protein Isolate 24
2.2.5 Application of Protein Isolates In Food 25
2.3 Yoghurt 26
2.3.1 Nutritional Benefits of Yoghurt 27
2.3.2 Health Benefits of Yogurt 29
2.3.3 Types of Yoghurt 31
2.3.4 Rheological and Physical Properties of Yoghurt 33
CHAPTER 3: MATERIALS AND METHODS
3.1 Sources of Raw Material 37
3.2 Processing of Raw Material 37
3.2.1 Soy Milk Processing 38
3.2.2 Soy Yoghurt Processing 39
3.3 Proximate Analysis 40
3.3.1 Moisture Content Determination 40
3.3.2 Total Ash Content Determination 41
3.3.3 Crude Protein Determination 41
3.3.4 Fat Content Determination 42
3.3.5 Crude Fibre Determination 43
3.3.6 Carbohydrate Content Determination 44
3.3.7 Energy Value 44
3.3.8 Dry Matter (DM) 44
3.4 Physicochemical Properties Analysis 44
3.4.1 Determination of Viscosity 44
3.4.2 Determination of Syneresis 45
3.4.3 Determination of pH 45
3.4.4 Determination of Density 45
3.4.5 Total Titratable Acidity (Free Acidity) 46
3.4.6 Total Solids 46
3.5 Microbiological Analysis 46
3.5.1 Determination of Bacterial Count 47
3.5.2 Determination of Fungal Count 47
3.5.3 Determination of Coliform Count 47
3.6 Sensory Evaluation 48
3.7 Statistical Analysis 48
CHAPTER 4: RESULTS AND DISCUSSION
4.1 Production of Soy Yoghurt with Different Levels Of Added Soy Protein Isolate 49
4.2 Proximate Composition of the Yoghurt Samples 50
4.3 Rheological and Physicochemical Composition of the Yoghurt Samples 54
4.4 Microbial Quality of the Yoghurt Samples 59
4.5 Sensory properties of the Yoghurt Samples 61
CHAPTER 5: CONCLUSION AND RECOMMENDATIONS
5.1 Conclusion 64
5.2 Recommendation 64
REFERENCES 66
LIST OF TABLES
Table 2.1: Anti-nutritional factors in soybeans 13
Table 2.2: Protein content of some protein isolates 20
Table 2.3: Comparism between isolates and concentrates 21
Table 2.4: Functional Properties of Protein Isolate 24
Table 3.1: Formulation for the Soy-Milk-Protein Isolates 39
Table 4.1: Proximate Composition of Soy Yoghurt Samples 52
Table 4.2: Rheological and Physicochemical Properties of Soy Yoghurt Samples 56
Table 4.3: Microbial Qualities of the Yoghurt Samples 60
Table 4.4: Sensory Properties of Soy Yoghurt Samples 62
LIST OF FIGURES
Figure 2.1: Systematic flow chart for legume protein Isolates recovery 19
Figure 3.1: Flow diagram for the production of soymilk. 38
Figure 3.2: Flow diagram for the production of soy yoghurt 40
LIST OF PLATES
Plate 3.1 Physical appearance of soy bean seeds 37
Plate 4.1 Physical appearance of Yoghurt samples 49
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND OF STUDY
Interest in functional food has continued to increase recently among consumers, due to greater concern on health and nutrition and the need to prevent diseases. Also due to increasing health-promoting effect of fermented foods, they have become significant part of many indigenous diets. This has led to exploration of fermented vegetable proteins. Fermented vegetable milk in comparison with dairy milk has lower saturated fat and total fat content, low carbohydrate content, no cholesterol and free of lactose (Trindada et al., 2001; Xunjie, 2000).
Soybeans (Glycine max), a legume, native of eastern Asia is well-known for its health-promoting effects (Grieshop et al., 2003). Soybean has been an important food in Asian countries, and is also accepted as a healthy ingredient in Europe and in the United States of America. It serves as an excellent source of vegetable protein especially in the diet of vegetarians. Soybean is the world’s most important seed legume, which contributes to 25% of the global edible oil which is rich in polyunsaturated fatty acids and about two-thirds of the world’s protein concentrates for livestock feeding (Liu, 1997). Soybean contains approximately 40% protein and 20% on an average dry matter base, thus has been an important source of protein in countries where its consumption is high. It has a great potential to be developed further as a source of protein for vegetarians and for people who don’t consume animal proteins for ethical, religious or environmental reasons (Lee et al., 2016).
Soybean is also a source of carbohydrates, lipids, vitamins, minerals and contains antioxidants and phytochemicals that are linked to various health benefits, such as antitumor activities, antihypertension, and cholesterol lowering effects, inhibiting bone resorption and stimulating bone formation, and prevention of breast and prostate cancer (Messiral and Lopronzi, 2001). In spite of its high nutritional value, soybean has not been accepted by some consumers due to its beany flavour and some of the ingredient causing flatulence.
Soybean can yield 6-8L of soymilk per kg. Soymilk is a healthy drink and it is considered very important for people who are allergic to animal milk protein (people with lactose intolerance) (Jana, 2001). Soymilk is a good alternative to dairy milk due to its high quality protein and its low cost.
In spite of the high nutritional value of soybean in its raw state, it should be heated before use in other to (1) deactivate physiological harmful substances, such as trypsin inhibitor which reduces the proteolysis, and hemagglutinin which causes red blood cells to agglutinate, (2) induce the denaturation of soybean protein to enhance the digestion, (3) soften the tissues of soybean for further processing, (4) remove or reduce the raw soybean odour to improve the palatability, and (5) to sterilize it (Nishinari et al., 2014).
Protein isolates are the most refined form of protein containing the greatest amount of protein with greater digestibility. Unlike flour and concentrates, it contains no dietary fiber (Jay and Michael, 2004).
Yoghurt is a dairy product usually made from animal milk which is produced by fermenting milk with a bacteria culture, mostly Lactobacillus bulgaricus and Streptococcus thermophilus usually in the ratio of 1:1. It is acidic in flavor and has a smooth texture (Osundahunsi et al., 2007). Among various cultured dairy product, yoghurt is unique with presence of acetaldehyde which is relatively high in concentration and desirable as an essential flavour component (Osundahunsi et al., 2007). Health benefits range from protecting against osteoporosis to relieving irritable bowel diseases and aiding digestion.
Soy yoghurt, also called soybean yoghurt is a fermented soymilk produced from soymilk; sometimes added with bacteria namely Lactobacillus bulgaricus and Streptococcus thermophilus, and sweeteners such as fructose, glucose or sucrose (Jana, 2001). Due to the continuous increase in population and the inadequate supply of protein which has inadvertently increased the occurrence of malnutrition in developing countries, and to meet the protein demands in developing countries, where animal protein is also grossly inadequate and relatively expensive, research efforts have been geared towards finding alternative sources of protein from legume seeds (Osundahunsi et al., 2007). Soy yoghurt are also becoming more popular because of their low level of cholesterol and saturated fat, and the fact that they are lactose-free (Drake and Gerrard 2006; Sarka 2006; Xiao, 2011). Unsaturated fatty acid in the diet is recommended to reduce the incidence of cardiovascular diseases (Messina, 2010).
The main objections to soy yoghurt and other soybean products by some consumers as a good replacement for animal based yoghurt are the associated intrinsic flavours which have been described as beany or astringent, phenomenon of flatulence and its textural mouth-feel (Osundahunsi et al., 2007). The texture of yoghurt is important with regard to the quality of the product.
Fermentation has been found to reduce the beany or soy flavour in soymilk (Jimoh and Kolapo, 2007), and also reduce antinutritional factors. Reduction in the objectionable flavour and flatulent sugars namely starchyose and raffinose occur after fermentation by lactic acid bacteria (Osundahunsi et al., 2007).
However, it has been reported that acceptability of soy yoghurt has been enhanced by modification of processing methods. The increase in the consumption of soy yoghurt in the United States of America is attributed to increased viscosity which improved mouth feel, texture and overall acceptability as a good substitute to dairy yoghurt. The addition of soy protein isolates (SPI) in the yoghurt formulation modified their rheological, physicochemical and sensory properties (Akin and Ozcan, 2017).
1.2 STATEMENTS OF THE PROBLEM
The most typical defects of soy yoghurt are low viscosity and reduced firmness, syneresis or whey separation (Domagala et al., 2013). That is to say that yoghurts produced from soymilk often times lacks body and good yogurt texture and therefore most often do not fit in as the best alternative for dairy yoghurt.
Whey separation (wheying-off) is defined as the expulsion of whey from a gel network which then becomes visible as surface whey. Wheying-off negatively affects consumer perception of yogurt as consumers think there is something microbiologically wrong with the product.
Commercial yoghurt manufacturers use stabilizers such as pectin, gelatin and starch to try to prevent whey separation and syneresis. Another approach is to increase the total solids content of yogurt milk, especially the protein content, to reduce wheying-off (Lucey et al., 1998a).
Therefore, using soy protein isolate (a pure form of protein) to increase the total solid content of soymilk become necessary in other to prevent whey separation, improve the mouth feel and enhance the general acceptability of soy yoghurt.
1.3 JUSTIFICATION OF THE STUDY
The most quality attribute for consumer acceptability of soy yoghurt is its fine, viscous, smooth texture and mouth-feel. Therefore, improving texture and mouth-feel of soy yoghurt will enhance its consumer acceptability and add value to the product. Soy protein isolates form firm, hard, resilient gels, unlike soy flour and concentrates that form soft and fragile gels (Riaz, 2006). This characteristic will help to improve the age long problem of wheying off in soy yoghurt which hindered its wide consumer acceptability. Protein isolates are also acceptable as ingredients for dairy application due to their fine particle size and dispersibility.
Again, increasing consumption of soy yoghurt will improve health status of consumers since soybean is a rich and cheap source of protein (Osundahunsi et al., 2007). This will also reduce the incidence of lactose intolerance among yoghurt consumers.
1.4 OBJECTIVES OF THE STUDY
The main aim of this research work was to evaluate the effect of added soy protein isolates on rheological and sensory properties of soy yoghurt. The specific objectives were to:
i. produce soy milk from soybean
ii. produce soy yoghurt with different levels of added soy protein isolate
iii. determine proximate composition of soy yoghurt samples
iv. determine rheological and physicochemical properties of soy yoghurt samples
v. determine the quality of the yoghurt samples
vi. determine sensory properties of soy yoghurt samples.
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