Chemical and Microbiological evaluation of Set yoghurt during shelf life By SalahEldein Hussein Mohammed B.Sc. (Honour) Natural Resources and Environmental Studies University of Kordofan (2005) A thesis Submitted in Partial Fullilment for the Requirements of the Degree of M.Sc in Dairy Production and Technology Supervisor Dr. Mohamed Osman Mohamed Abdalla Department of Dairy Production Faculty of Animal Production University of Khartoum November 2008 Dedication To the soul of my mother To my dear father, brothers and sisters To my friends and colleagues ftÄt{xÄwx|Ç i ACKNOWLEDGMENTS Praise be to Allah I wish to express my sincere thanks and gratitude to Dr. Mohamed Osman Mohamed Abdalla for his helpful supervision, constructive criticism and valuable guidance throughout this work. My special thanks are due to Dr. Ibtisam Elyas Mohammed Elzubeeir, head Department of the Dairy Production for her continuous support, encouragement and kindness. My appreciation is due to Dr. Mohammed Khair Abdalla for his cooperation in choosing the suitable experimental design and analysis of the data and unlimited help freely offered to me. My thanks are due to Mr. Moawia Albirer and Best factory family for their help during collection of samples. Also my thanks are extended to my colleagues in Dairy Production and Technology, Course. Special thanks are due my friends, Mortada Mohammed, Ibrahim Homeda and Rania Abd el‐Gader, I wish to express special thanks to my family. ii LIST OF CONTENTS Page DETECTION i ACKNOWLEDGMENT ii LIST OF CONTENTS iii LIST OFTABLES v ABSTRACT vi ARABIC ABSTRACT vii CHAPTER ONE: Introduction 1 CHAPTER TWO: LITRETURE REVIEW 3 2.1 Fermentation and fermented milk products 3 2.2 Starter cultures 4 2.3 Types of yoghurt 5 2.4 Manufacture of yoghurt 7 2.5 Preparation of the basic mix 9 2.6 Homogenization 9 2.7 Heat treatment of milk 9 2.8 Production of set yoghurt 10 2.9 Cooling of yoghurt 10 2.10 Packaging of yoghurt 11 2.11 Yoghurt properties 11 2.12 Fermentation and microbiological aspects 13 2.13 Shelf life of yoghurt 13 2.14 Defects of yoghurt 14 2.15 Nutritional value of yoghurt 15 CHAPTER THREEMATERIALS AND METHODS 16 3.1 Collection of samples 16 3.2 Chemical analysis of yoghurt 16 3.2.1 Determination of total solids content 16 3.2.2 Determination of titratable acidity 16 iii 3.2.3 Determination of fat content 17 3.2.4 Determination of protein content 17 3.3 Determination of wheying‐off 18 3.4 Microbiological examination 18 3.4.1 Sterilization of equipment 18 3.4.2 Preparation of media 18 3.4.2.1 Violet Red Bile Agar 18 3.4.2.2 Lactobacillus MRS (Man Rogosa and Sharpe) Agar 19 3.4.2.3 Acidified potato dextrose agar (P.D.A) 19 3.5 Culture Methods 19 3.6 Statistical analysis 20 CHAPTER FOUR RESULTS AND DISCUSSION 21 4.1 Chemical composition of yoghurt 21 4.2 microbiological properties of set yoghurt 22 Table (1) fat contents and protein contents of set yoghurt samples during shelf life. 23 Table (2) Total solids contents and Acidity percentage of set yoghurt samples during shelf life. 24 Table (3) Wheying‐off percentage separated during shelf life 25 Table (4) Coliform count of Set yoghurt samples during shelf life 27 Table (5) Yeasts and Molds count of set yoghurt samples during shelf life 28 Table (6) Lactobacillus bulgaricus set yoghurt samples during shelf life 29 CHAPTER SIX CONCLUSION AND COMMENDATIONS 30 Conclusion 30 Recommendations 30 REFERENCES 31 iv LIST OF TABLES Table page page 1 Fat and protein contents of set yoghurt samples during shelf life . 23 2 Total solids contents and Acidity percentage of samples during shelf life . 24 3 Wheying‐off percentage separated during shelf life 25 4 Coliform count of Set yoghurt samples during shelf life 27 5 Yeasts and Molds count of set yoghurt samples during shelf life . 28 6 Lactobacillus bulgaricus set yoghurt samples during shelf life 29 LIST OF TABLES v ABSTRACT A study was performed to determine the effect of shelf life on chemical and microbiological characteristics of set yoghurt manufactured in Khartoum State. Samples were collected and kept in the laboratory at refrigerator temperature. The chemical analysis was carried out to determine the contents protein, fat and total solids contents. The physiochemical analysis included titratable of acidity and wheying-off. Whereas should be analyzed and microbiological examination (coliform, lactobacillus bulgaricus and yeasts and molds counts) were performed at 1, 3, 6 and 9 day intervals. Results revealed that protein, fat, total solids and acidity and whey-off did not show any significant difference (P>0.05) effects throughout the storage period. Results of microbiological examination showed non significant variation (P>0.05) in the count of coliform bacteria, yeasts and molds and Lactobacillus bulgaricus throughout the shelf life of 9 days. vi اﻟﻤﺴﺘﺨﻠﺺ أﺟﺮﻳﺖ هﺬﻩ اﻟﺪراﺳﺔ ﻟﺘﻘﻴﻢ اﻟﺘﺄﺛﻴﺮ اﻟﻜﻴﻤﻴﺎﺋﻲ واﻟﻤﻴﻜﺮوﺑﻲ ﻟﻠﺰﺑﺎدي اﻟﻤﺘﻤﺎﺳﻚ اﻟﺨﺜﺮة ﺧﻼل ﻓﺘﺮة اﻟﺼﻼﺣﻴﺔ . ﺟﻤﻌﺖ ﻋﻴﻨﺎت اﻟﺰﺑﺎدي ﻣﻦ ﻣﺼﻨﻊ ﻟﺘﺼﻨﻴﻊ اﻟﻠﺒﻦ وﻣﻨﺘﺠﺎﺗﻪ ﺑﻮﻻﻳﺔ اﻟﺨﺮﻃﻮم ، و ﺣﻔﻈﺖ ﻋﻠﻰ درﺟﺔ ﺣﺮارة اﻟﺜﻼﺟﺔ. أﺷﺘﻤﻠﺖ اﻟﺪراﺳﺔ ﻋﻠﻰ اﻟﺘﺤﻠﻴﻞ اﻟﻜﻴﻤﺎﺋﻰ ﻟﻤﻌﺮﻓﺔ ( ﻧﺴﺒﺔ اﻟﺒﺮوﺗﻴﻦ، اﻟﺪهﻦ واﻟﺠﻮاﻣﺪ اﻟﻜﻠﻴﺔ ). و اﻟﺘﺤﻠﻴﻞ اﻟﻔﻴﺰﻳﺎﺋﻲ ﻟﺘﺤﺪﻳﺪ ﻧﺴﺒﺔ اﻟﺤﻤﻮﺿﻪ وﻧﺴﺒﺔ اﻟﺸﺮش ، آﻤﺎ ﺗﻢ ﺗﺤﻠﻴﻞ اﻟﻌﻴﻨﺎت ﻟﻤﻌﺮﻓﺔ اﻟﺘﺄﺛﻴﺮ اﻟﻤﻴﻜﺮوﺑﻴﻮﻟﻮﺟﻲ ﺑﻮاﺳﻄﺔ اﻟﻌﺪ اﻟﻜﻠﻰ ﻟﻜﻞ ﻣﻦ اﻟﻜﻠﻮروﻓﻮرم ﺑﻜﺘﻴﺮﻳﺎ ء، ﻻآﺘﻮﺑﺎﺳﻠﺲ ﺑﻠﻘﺎرﻳﻜﺲ واﻟﺨﻤﺎﺋﺮ واﻟﻔﻄﺮﻳﺎت، ﺧﻼل ﻓﺘﺮة اﻟﺼﻼﺣﻴﺔ (ﻋﺸﺮة أﻳﺎم). أوﺿﺤﺖ اﻟﻨﺘﺎﺋﺞ أن ﻧﺴﺒﺔ اﻟﺒﺮوﺗﻴﻦ، اﻟﺪهﻦ، اﻟﺠﻮاﻣﺪ اﻟﻜﻠﻴﺔ ، اﻟﺤﻤﻮﺿﺔ وﻧﺴﺒﺔ اﻟﺸﺮش ﻟﻢ ﺗﺘﺄﺛﺮ ﻣﻌﻨﻮﻳﺎ ﺧﻼل ﻓﺘﺮة اﻟﺼﻼﺣﻴﺔ (P>0.05) آﻤﺎ أوﺿﺤﺖ اﻟﻨﺘﺎﺋﺞ أن اﻟﻌﺪد اﻟﻜﻠﻰ ﻟﻠﺒﻜﺘﺮﻳﺎ واﻟﺒﻜﺘﺮﻳﺎ اﻟﻤﻨﺘﺠﺔ ﻟﺤﻤﺾ اﻟﻼآﺘﻴﻚ واﻟﺨﻤﺎﺋﺮ واﻟﻔﻄﺮﻳﺎت ﻟﻢ ﺗﺘﺄﺛﺮ ﻣﻌﻨﻮﻳﺎ ﺧﻼل ﻓﺘﺮة اﻟﺼﻼﺣﻴﺔ .(P>0.05) vii CHAPTER ONE Introduction Yoghurt is a fermented milk product obtained by fermentation of lactose to lactic acid by the action of two types of lactic acid bacteria Streptococcus thermophilus (S. salivarius-ssp. thremophilus) and Lactobacillus bulgaricus (L. delbrueckii ssp. bulgaricus) and these two microorganisms must live in large numbers in the final product (Tayfour, 1994). Yoghurt is an extremely popular fermented milk food in Europe, Asia and Africa. It is known by quite different names in different parts of the world: Leben or Raib (Egypt- Saudia Arabia) Madzoon or Matzoon (Armania), Naja (Bulgaria) and Dalia (India) (Kosikowski, 1966). In many developing countries of Asia and Africa, yoghurt is more likely to be produced as naturally soured milk and consumed by the adult people more than fresh whole milk. It is generally considered a safe product and its unique flavor appeals to so many people. That consideration is incorporate in expensive nutrients make it an almost complete food in these areas .In particular, consumers in the Americans are now more aware of the fine properties of yoghurt, and its consumption is increasing particularly in large cities (Kosikowski,1966). It is produced in different forms such as whole milk yoghurt, skim milk yoghurt, cream yoghurt, fruit yoghurt and liquid yoghurt (Balasubramanyam and Kulkarnis, 1991). In 1973, Food and Agriculture Organization (FAO) and World Health Organization (WHO) of the United Nations put certain standards for types of yoghurt according to their fat content, and hence, yoghurt was classified into whole fat (above 3.0%), moderate fat (3.0-0.5%) and low fat yoghurt 1 (<0.5%), set yoghurt and liquid yoghurt (of low viscosity) (Tamime and Deeth, 1980) and these may be plain (natural) or they may contain additives like fruit or flavors, or it may be coloured (Tayfour, 1994). A major concern of yoghurt industry is the production and maintenance of a product with optimum consistency and stability. The factors known to improve consistency are increasing total solids, manipulation of processing variables and characteristics of starter culture. The objectives of study are: 1- Evaluation of chemical characteristics of set yoghurt during shelf life. 2- Evaluation of microbiological quality of set yoghurt during shelf life. 2 CHAPTER TWO LITRETURE REVIEW 2.1 Fermentation and fermented milk products Fermentation of milk was originated in the Near East and through Central and Eastern Europe. Fermentation is defined as any modification of chemical or physical properties of milk or dairy products, resulting from the activity of microorganisms or their enzymes which cause the main marked changes (Frank and Marth, 1988). Moreover, the earliest example of fermented milk was warm, raw milk from cows, sheep, goats, camels or horses of the nomads roaming the area (Elmardi, 1988). Fermentation was first used around ten thousand years ago when humans made the transition from food collectors to food producers and was a common method to extend the longevity of dairy products. Today, this process remains the same except that Streptococcus themophilus and Lactoobucillus bulgaricus can be used in the fermentation of yoghurt (Tamime and Robinson, 1991). Fermented milk products are cultured dairy products made from skim, whole or slightly concentrated milk that require specific lactic acid bacteria to develop their characteristic flavor and texture (Thapa, 2000). Fortunately, the dominant bacteria were lactic acid Streptococci and lactobacilli, which generally suppress the spoilage and pathogenic organisms very effectively (Kosikowski, 1982). Moreover, desired alternation of food microorganisms are referred to as fermentation regardless of the type of metabolism (Banwart, 1981). Similarly, fermentation is defined as anaerobic breakdown of an organic substance by enzyme in which the final hydrogen acceptor is an organic compound. These products include cultured butter milk, sour cream, yoghurt, acidophilus milk, kefir and concentrated fermented milk products 3 (Hargrove and Alford, 1972). Moreover, production of good fermented milk required a source of good flavored, low bacteria in milk, heat treatment of the milk, an active properly functioning appropriate starter, quick chilling of fermented product and high standard sanitation (Kosikowski, 1982). 2.2 Starter cultures The starter culture, Lactobacillus bulgaricus and Streptococcus thermophilus, are required for the fermentation in yoghurt production (Vedamuthu, 1991).