Optimum Incubation Temperature for the Plate Count of Milk and Water
Transcription
Optimum Incubation Temperature for the Plate Count of Milk and Water
Optimum Incubation Temperature for the Plate Count of Milk and Water By Azza Mutwakil Khalid B.Sc. (Agric.) Honours University of Khartoum ٢٠٠٢ A thesis submitted in partial fulfillment of the requirements of the degree of M.Sc. in Food Science and Technology Supervisor, Professor Hamid Ahmed Dirar Department of Botany and Agricultural Biotechnology Faculty of Agriculture University of Khartoum ٢٠٠٦ DEDICATION To my beloved great parents who always stood behind and encouraged me to finish this work To my sisters and brother, To my friends and colleagues, To my supervisor, I dedicate this work With all my love. AZZA ACKNOWLEDGEMENTS First I thank Allah, who gave me the ability to complete this work. My deep thanks go to my supervisor Professor Hamid Ahmed Dirar, for his guidance, advice and help. I would like to thank all the Microbiology staff members of Sudanese Standards and Meteorology Organization and the technical staff members of the Dept. of Food Science and Technology and the Dept. of Botany and Agricultural Biotechnology, Faculty of Agriculture, University of Khartoum. I deeply thank my uncle Abdulnaser and Dr. El Muez who made my way bright. My thanks also go to my colleagues at the Dept. of Botany and Agricultural Biotechnology, Faculty of Agriculture, University of Khartoum. CONTENTS Dedication Acknowledgements Contents List of Tables List of Figures List of Appendices Abstract Arabic Abstract CHAPTER ONE: INTRODUCTION CHAPTER TWO: LITERATURE REVIEW Part I: Counting Microorganisms ٢٫١ The parameters of foods that affect microbial growth ٢٫١٫١ pH ٢٫١٫٢ Moisture content ٢٫١٫٣ Oxidation-reduction potential (O/R,Eh) ٢٫١٫٤ Nutrient content ٢٫١٫٥ Antimicrobial constituents ٢٫١٫٦ Temperature ٢٫١٫٧ Relative Humidity of Environment ٢٫١٫٨ Presence and Concentration of Gases in the Environment ٢٫٢ General Viable Counts ٢٫٣ Microbial Counting Techniques ٢٫٣٫١ Plate Counts ٢٫٣٫١٫١ Pour Plates and Spread Plates ٢٫٣٫٢ Most Probable Number (MPN) Counts ٢٫٣٫٣ Dye Reduction Method ٢٫٣٫٤ Direct Microscopic Count (DMC) ٢٫٤ Total Viable Counts as Indicators of Food Sanitary Quality ٢٫٥ Culture Medium Part II: Water ٢٫٦ Importance of water ٢٫٧ Water-borne diseases ٢٫٨ Standards for drinking water ٢. ٩ International drinking water standards ٢٫١٠ Water pollution ٢٫١١ Contamination of drinking water Part III: Milk ٢٫١٢ Animal wealth in Sudan Page i ii iii vi vii viii ix xi ١ ٣ ٣ ٣ ٣ ٣ ٤ ٥ ٦ ٧ ٧ ٨ ٨ ٩ ١٠ ١١ ١٢ ١٣ ١٤ ١٥ ١٦ ١٧ ١٧ ٢٠ ٢٢ ٢٣ ٢٣ ٢٤ ٢٥ ٢٥ ٢٫١٣ Milk ٢٫١٤ Nutritive value of milk ٢٫١٥ Sources of contamination ٢٫١٥٫١ Interior of the udder ٢٫١٥٫٢ Exterior of the udder ٢٫١٦ Microorganisms in raw milk ٢٫١٦٫١ Lactic acid bacteria ٢٫١٦٫٢ Coliforms ٢٫١٦٫٣ Milk spoilage microorganisms ٢٫١٦٫٤ Pathogenic bacteria in raw milk ٢٫١٧ Microbiological standards for raw milk ٢٫١٧٫١ Standards of dairy products ٢٫١٨ Pasteurization CHAPTER THREE: MATERIALS AND METHODS ٣٫١ Sterilization ٣٫١٫١ Hot-air oven ٣٫١٫٢ Autoclaving ٣٫٢ Preparation of Media ٣٫٢٫١ Solid Media ٣٫٢٫١٫١ Plate count agar ٣٫٢٫٢ Liquid Media ٣٫٢٫٢٫١ Peptone Media ٣٫٣ Preparation of Butterfield’s phosphate buffer ٣٫٣٫١ Stock solution ٣٫٤ Collection of samples ٣٫٥ Preparation of sample dilutions ٣٫٦ Microbiological methods ٣٫٦٫١ Viable count of bacteria ٣٫٦٫١٫١ Milk (raw and pasteurized milk) ٣٫٦٫١٫٢ Water (running and stagnant) ٣٫٧ Isolation of dominant microorganisms ٣٫٨ Purification of isolates ٣٫٩ Tests for the tentative identification of bacteria ٣٫٩٫١ Gram stain ٣٫٩٫٢ The staining of bacterial spores ٣٫٩٫٣ Motility test ٣٫٩٫٤ Catalase test ٣٫٩٫٥ Acid-fast test ٣٫٩٫٦ Sugar fermentation test ٢٦ ٢٧ ٢٧ ٢٧ ٢٩ ٢٩ ٢٩ ٣٠ ٣٠ ٣٠ ٣١ ٣١ ٣٢ ٣٦ ٣٦ ٣٦ ٣٦ ٣٦ ٣٦ ٣٦ ٣٧ ٣٧ ٣٧ ٣٧ ٣٨ ٣٨ ٣٩ ٣٩ ٣٩ ٣٩ ٤٠ ٤٠ ٤١ ٤١ ٤١ ٤١ ٤٢ ٤٢ ٤٣ CHAPTER FOUR: RESULTS AND DISCUSSION CHAPTER FIVE: CONCLUSION AND RECOMMENDATION REFERENCES APPENDICES ٤٤ ٥٣ ٥٤ ٥٨ LIST OF TABLES Page Table A: Water resources of the hydrosphere. ١٩ Table B: Chemical composition of milk given by different ٢٦ authors. Table ١: Tentative identification of bacteria ٥٢ LIST OF FIGURES Page Figure A: Major chemical components of raw milk. ٢٨ Figure ١: Effect of incubation temperature on the plate count ٤٥ of raw milk. Figure ٢: Effect of incubation temperature on the plate count ٤٧ of pasteurized milk. Figure ٣: Effect of incubation temperature on the plate count ٤٩ of running water. Figure ٤: Effect of incubation temperature on the plate count of stagnant water.. ٥٠ LIST OF APPENDICES Page Appendix ١: Effect of incubation temperature on the plate ٥٨ count of raw milk. Appendix ٢: Effect of incubation temperature on the plate ٥٩ count of pasteurized milk. Appendix ٣: Effect of incubation temperature on the plate ٦٠ count of running water. Appendix ٤: Effect of incubation temperature on the plate count of stagnant water. ٦١ ABSTRACT This work was meant to determine the optimum incubation temperature for the plate count of raw and pasteurized milk and of running and stagnant water. Three different incubation temperatures were tested ٢٥°C, ٣٢°C and ٣٧°C. The optimum incubation temperature for the plate count of raw milk was found to be ٣٢°C which gave the highest total viable count ranging from ٥,٤٩ x ١٠٤ to ٥,٢٦ x ١٠٥ cfu/ml; the dominant microorganism was tentatively identified as Listeria. The optimum incubation temperature for the plate count of supposedly commercial pasteurized milk was found to be ٣٧°C which gave the highest total viable count ranging from ١,٣٠ x ١٠٦ to ١,٥٢ x ١٠٦ cfu/ml, the dominant microorganisms being Streptococcs, Leuconostoc and Pediococcus. The optimum incubation temperature for the running water and stagnant water was ٢٥°C which gave the highest viable count ranging from ٣,٠٠ x ١٠٢ to ٣,٧ x ١٠٢ cfu/ml (running water) and from ٢,٦٥ x ١٠٣ to ٢,٩٧ x ١٠٣ cfu/ml (stagnant water) and the dominant microorganisms were Bacillus in running water and Staphyglococcus and Micrococcus in stagnant water. The values of count given above were the highest and were those at ٧٢ hrs of incubation but although the count at ٤٨ hrs of incubation was slightly lower, this incubation time is recommended for both water and milk for reasons of economy in time and cost. ﻤﻠﺨﺹ ﺍﻷﻁﺭﻭﺤﺔ ﺃﺠﺭﻴﺕ ﻫﺫﻩ ﺍﻟﺩﺭﺍﺴﺔ ﻟﻤﻌﺭﻓﺔ ﺩﺭﺠﺔ ﺍﻟﺘﺤﻀﻴﻥ ﺍﻟﻤﺜﻠﻰ ﻟﻠﻠﺒﻥ ﺍﻟﺨﺎﻡ ﻭﺍﻟﻠﺒﻥ ﺍﻟﻤﺒﺴﺘﺭ C ،٢٥°ﺒﺎﺴﺘﺨﺩﺍﻡ ﺜﻼﺙ ﺩﺭﺠﺎﺕ ﺘﺤﻀﻴﻥ ﻤﺨﺘﻠﻔﺔ ﻟﻠﻌﻴﻨﺎﺕ ﻭﺍﻟﻤﺎﺀ ﺍﻟﺠﺎﺭﻱ ﻭﺍﻟﻤﺎﺀ ﺍﻟﺜﺎﺒﺕ. C ،٣٢°C .٣٧° ٣٢°ﻭﻗﺩ ﺃﻋﻁﺕ ﻨﻤﻭﹰﺍ Cﺒﺎﻟﻨﺴﺒﺔ ﻟﻠﺒﻥ ﺍﻟﺨﺎﻡ ﻓﺈﻥ ﺩﺭﺠﺔ ﺍﻟﺘﺤﻀﻴﻥ ﺍﻟﻤﺜﻠﻰ ﻫﻲ ﻭﺍﻟﺒﻜﺘﺭﻴﺎ ﺍﻟﺴﺎﺌﺩﺓ ﻓﻲ ﻫﺫﻩ ﺍﻟﻌﻴﻨﺔ cfu/mlﻴﺘﺭﺍﻭﺡ ﻤﺎ ﺒﻴﻥ ١٠٥ × ٥,٢٦ﺇﻟﻰ ١٠٥ × ٥,٤٩ ﺒﻌﺩ ﺍﻟﺘﻌﺭﻴﻑ ﺍﻟﻤﺒﺩﺌﻲListeria.ﻫﻲ ﺒﻜﺘﺭﻴﺎ ﻤﻥ ﺠﻨﺱ C ٣٧°ﺒﺎﻟﻨﺴﺒﺔ ﻟﻠﻠﺒﻥ ﺍﻟﻤﻔﺘﺭﺽ ﺇﻨﻪ ﺘﺠﺎﺭﻱ ﻤﺒﺴﺘﺭ ﻓﺈﻥ ﺩﺭﺠﺔ ﺍﻟﺘﺤﻀﻴﻥ ﺍﻟﻤﺜﻠﻰ ﻫﻲ ﻭﺍﻟﺒﻜﺘﺭﻴﺎ cfu/mlﻭﻗﺩ ﺃﻋﻁﺕ ﺃﻋﻠﻰ ﻨﻤﻭﹰﺍ ﻴﺘﺭﺍﻭﺡ ﻤﺎ ﺒﻴﻥ ١٠٦× ١,٣٠ﺇﻟﻰ ١٠٦ × ١,٥٢ ﻭ Streptococcus, Leuconostoc,ﺍﻟﺴﺎﺌﺩﺓ ﻓﻲ ﻫﺫﻩ ﺍﻟﻌﻴﻨﺔ ﻫﻲ ﺒﻜﺘﺭﻴﺎ ﻤﻥ ﺠﻨﺱ Pediococcus . ﺒﺎﻟﻨﺴﺒﺔ ﻟﻠﻤﺎﺀ )ﺍﻟﻤﺎﺀ ﺍﻟﺠﺎﺭﻱ ﻭﺍﻟﻤﺎﺀ ﺍﻟﺜﺎﺒﺕ( ﻓﺈﻥ ﺩﺭﺠﺔ ﺍﻟﺘﺤﻀﻴﻥ ﺍﻟﻤﺜﻠﻰ ﻫﻲ ٣ )ﻟﻠﻤﺎﺀ ٢٥°cfu/mlﻭﻗﺩ ﺃﻋﻁﺕ ﻨﻤﻭﹰﺍ ﻴﺘﺭﺍﻭﺡ ﻤﺎ ﺒﻴﻥ ١٠٣ × ١,٣٢ﺇﻟﻰ C ١٠ × ١,٥٩ ) ﻟﻠﻤﺎﺀ ﺍﻟﺜﺎﺒﺕ( ﻭﺍﻟﺒﻜﺘﺭﻴﺎ cfu/mlﺍﻟﺠﺎﺭﻱ( ،ﻭﻤﻥ ١٠٣ × ٢,٩٧ﺇﻟﻰ ١٠٣ × ٢,٦٥ ﻟﻌﻴﻨﺔ ﺍﻟﻤﺎﺀ ﺍﻟﺠﺎﺭﻱ ﻭ Bacillus.ﺍﻟﺴﺎﺌﺩﺓ ﻫﻲ ﺒﻜﺘﺭﻴﺎ ﻤﻥ ﺠﻨﺱ ﻟﻌﻴﻨﺔ ﺍﻟﻤﺎﺀ ﺍﻟﺜﺎﺒﺕMicrococcus.ﻭStaphylococcus ﺍﻹﻋﺩﺍﺩ ﺍﻟﻤﻴﻜﺭﻭﺒﻴﺔ ﺍﻟﺘﻲ ﺃﻋﻁﻴﺕ ﺃﻋﻼﻩ ﻫﻲ ﺍﻷﻋﻠﻰ ﺍﻟﺘﻲ ﻭﺠﺩﺕ ﻭﺒﻌﺩ ﺘﺤﻀﻴﻥ ﻟﻤﺩﺓ ٧٢ﺴﺎﻋﺔ ﻭﻟﻜﻥ ﻋﻠﻰ ﺍﻟﺭﻏﻡ ﻤﻥ ﺇﻥ ﺍﻷﻋﺩﺍﺩ ﺒﻌﺩ ﺘﺤﻀﻴﻥ ٤٨ﺴﺎﻋﺔ ﻜﺎﻨﺕ ﺃﻗل ﺒﻘﻠﻴل، ﻴﻭﺼﻰ ﺒﺎﻋﺘﻤﺎﺩ ﻤﺩﺓ ﺍﻟﺘﺤﻀﻴﻥ ﺍﻷﺨﻴﺭ ﻷﺴﺒﺎﺏ ﺍﻻﻗﺘﺼﺎﺩ ﻓﻲ ﺍﻟﺯﻤﻥ ﻭﺍﻟﺘﻜﻠﻔﺔ. CHAPTER ONE INTRODUCTION We all live in a world filled with microbes from birth until death (Tortora, et al., ١٩٩٨), so the environmental around us is polluted including food especially milk and water. For many years the Sudanese people did not pay much attention to water pollution problems but today the population is aware of the importance of good water quality and its relation to diseases. The current interest in the formulation of standards for the quality control of food-stuffs in this country has prompted this work on the microbiological standards of milk. One of the most useful indices of the hygienic quality of milk and one on which milk grading is usually based is the count of live microorganisms in milk. This is done by the plate count method (American Public Health Association, ١٩٧١). Total viable counts on food products not only reflect handling history, state of decomposition, or degree of freshness but they may in some instances reflect on the sanitary quality of foods. Total viable counts most effectively evaluate the sanitary quality of foods that do not support microbial growth (Jay, ١٩٨٦). Objectives ١. Study the effect of incubation temperature on the plate count of water and milk. ٢. To find the optimum incubation temperature for the plate count of water and milk. CHAPTER TWO LITERATURE REVIEW Part I: Counting Microorganisms ٢,١ The Parameters of Foods that Affect Microbial growth ٢,١,١ pH It has been well established that most microorganisms grow best at pH values around. ٧,٠ (٦,٦-٧,٥), while few grow below ٤,٠. Bacteria tend to be more fastidious in their relationships to pH than molds and yeasts, with the pathogenic bacteria being the most fastidious (Jay, ١٩٨٦). ٢,١,٢ Moisture Content One of man’s oldest methods of preserving foods is drying or desiccation, and precisely how this method came to be used is not known. The preservation of foods by drying is a direct consequence of removal or binding of moisture without which microorganisms do not grow. It is now generally accepted that the water requirements of microorganisms should be defined in terms of the water activity (aw) in the environment. The aw most fresh foods is above ٠,٩٩. Bacteria require higher values of aw for growth than fungi, with gram-negative bacteria having higher requirements than gram positives, most spoilage bacteria do not grow below aw ٠,٩١, while spoilage molds can grow at as low aw as ٠,٨٠. Staphylococcus aureus was found to grow at aw as low as ٠,٨٦, while Clostridium botulinum does not grow below aw ٠,٩٤. Just as yeasts and molds grow over a wider pH range than bacteria, the same is true for aw. The lowest reported aw values for bacteria of any type is ٠,٧٥ for halophilic (literally, “salt-loving”) bacteria, while xerophilic (“dry-loving”) molds and osmophilic (preferring high osmotic pressures) yeasts grow at aw values of ٠,٦٥ and ٠,٦٠. Certain relationships have been shown to exist between aw, temperature and nutrition. First, at any temperature, the ability of microorganisms to grow is reduced as the aw is lowered. Second, the range of aw over which growth occurs is greatest at the optimum temperature for growth; and third, the presence of nutrients increases the range of aw over which the organisms can survive. The specific values given above, then, should be taken only as reference points, since a change in temperature or nutrient content might permit growth at lower values of aw (Jay, ١٩٨٦). ٢,١,٣ Oxidation-reduction Potential (O/R, Eh) It has been known for many years that microorganisms display varying degrees of sensitivity to the oxidation-reduction potential of their growth medium. The O/R potential of a substrate may be defined generally as the ease with which the substrate loses (oxidized) or gains electrons (reduced). Aerobic microorganisms require an oxidized environment for growth while anaerobes require reduced environment. Some bacteria such as the genus Clostridium, require reduced conditions for growth while others such as the genus Bacillus require oxidized conditions for growth. Some bacteria such as Lactobacilli and Streptococci are often referred to as microaerophiles. Some bacteria have the capacity to grow under either aerobic or anaerobic conditions. Such types are referred to as facultative anaerobes (Jay, ١٩٨٦). ٢,١,٤ Nutrient Content In order to grow and function normally, the microorganisms of importance in foods require water, source of energy, source of nitrogen, vitamins and minerals. Concerning the importance of water to growth with respect to the other four groups of nutrients, molds have the lowest requirement, followed by yeasts, gram- positive bacteria, and gram- negative bacteria. As sources of energy, food-borne microorganisms may utilize sugars, alcohols and amino acids. Some few microorganisms are able to utilize complex carbohydrates such as starches and cellulose. Fats are used also by microorganisms as sources of energy. The primary nitrogen sources utilized by heterotrophic microorganisms are amino acids. Microorganisms may require B vitamins in low quantities and most natural foods tend to have an abundant quantity for those organisms that are unable to synthesize their essential requirements such as gram-negatives and molds. The gram-positive bacteria are the least synthesizing and must therefore, be supplied with one or more of these compou nds before they will grow (Jay, ١٩٨٦). ٢,١,٥ Antimicrobial Constituents The stability of some foods against attack by microorganisms is due to the presence of certain naturally occurring substances that have been shown to have antimicrobial activity. Among these are eugenol in cloves, allicin in garlic, cinnamic alddehyde and eugenol in cinnamon, allyl isothiocyanate in mustard. Cows’ milk contains several antimicrobial substances including lactoferring, conglutinin and the lactoperoxide system. Casein as well as some free fatty acids that occur in milk have been shown to be antimicrobial (Jay, ١٩٨٦). ٢,١,٦ Temperature Microorganisms grow over every wide range of temperatures. The lowest temperature at which a microorganism has been reported to grow is -٣٤°C while the highest is somewhere in excess of ٩٠°C. It is customary to place microorganisms into three groups based upon their temperature requirements for growth. Those organisms that grow well below ٢٠°C and have their optimum between ٢٠° and ٣٠°C are referred to as psychrophiles or psychrotrophs. Those that grow well between ٢٠° and ٤٥°C with optima between ٣٠° and ٤٠°C are mesophiles. Those with optima between ٥٥°-٦٥°C are referred to as thermophiles. Molds are able to grow over wider ranges of temperature than bacteria. Yeasts grow over the psychrophilic and mesophilic temperature ranges but generally not within the thermophilic range (Jay, ١٩٨٦). ٢,١,٧ Relative Humidity of Environment The relative humidity (R.H.) of the environment is important both from the standpoint of aw within food and the growth of microorganisms at the surfaces. When the aw of food is set at ٠,٦٠ , it is important that this food be stored under conditions of R.H. that does not allow the food to pick up moisture from the air and thereby increase its own surface and subsurface aw to a point when microbial growth can occur (Jay, ١٩٨٦). ٢,١,٨ Presence and Concentration of Gases in the Environment The storage of food in atmospheres containing increased amount of CO٢ up to about ١٠٪ is referred to as “ controlled atmosphere”. Carbon dioxide has been shown to retard fungal rotting, also the ozone (O٣) added to food storage environments has a preservative effect upon certain foods. This gas has been tried with several foods and found to be effective against spoilage microorganisms. Both CO٢ and C٣ are effective in retarding the surface spoilage of beef quarters under long-term storage (Jay, ١٩٨٦). ٢,٢ General Viable Counts General viable counts are determined usually by colony counting methods although the multiple tube technique may be used if low concentrations of bacteria are expected. The choice of medium and incubation conditions is difficult when general viable counts are attempted on the mixed microflora usually found in foods. Frequently viable count are required of populations for which there is little knowledge of the types of organisms present, and in these circumstances, because of the variety of nutritional and physical requirements represented, it is impossible to obtain counts that truly indicate the number of viable organisms present (Harrigan, ١٩٩٨). ٢,٣ Microbial Counting Techniques To detect and count the viable microorganisms in the samples different methods are used: plate count agar method, membrane filtration method, most probable number method and dye reduction methods. Electrometric methods, nucleic acid probes and the polymerase chain reaction and for total number of microorganisms in a sample are also used. The Breed’s smear method for direct microscopic counts, direct microscopic counts by membrane filtration, direct epifluorescent filter technique (DEFT), flow cytometry, ATP determination by bioluminescence and turbidimetric methods are other techniques (Harrigan, ١٩٩٨). The four basic methods employed are the plate count method, the most probable number (MPN) method as a statistical determination of viable cells, the dye-reduction techniques to estimate numbers of viable cells and the direct microscopic count for both viable and non-viable cells (Jay, ١٩٨٦). ٢,٣,١ Plate Counts The most frequently used method of measuring bacterial populations is the plate count method. An important advantage of this method is that it measures the number of viable cells. One disadvantage may be that it takes sometime, usually ٢٤ hours or more for visible colonies to form. This can be a serious problem in some applications, such as quality control of milk, when it is not feasible to hold a particular lot for this length amount of time. The plate count is based on three assumptions, that each bacterium grows and divides to produce a single colony, that the original inoculum is hamogeneous, and that no aggregate of cells are present. When a plate count is performed, it is important that only a limited number of colonies develop in the plate, when too many colonies are present. Some cells are over-crowed and do not develop: these conditions cause inaccuracies in the count. Generally, only plates with ٢٥-٢٥٠ colonies are counted. To ensure that some colony counts will be within this range, the original inoculum is diluted several times in a process called serial dilution (Tortora, et al., ١٩٩٨). ٢,٣,١,١ Pour Plates and Spread Plates A plate count is done by either the pour plate or the spread plate method. In pour plate method either ١,٠ ml or ٠,١ ml of dilutions of the bacterial suspension is introduced into dish. The medium in which the agar is kept liquid by holding it in a water bath at about ٥٠°C, is poured over the sample, which is then mixed into the medium by gentle agitation of the plate. When the agar solidifies, the plate is incubated. With the pour plate technique, colonies will grow within the agar (from cells suspended in the medium as the agar solidifies) as well as on the surface of the agar plate (Tortora, et al., ١٩٩٨). This technique has some drawbacks because some relatively heat – sensitive microorganisms may be damaged by the method agar and will therefore be unable to form colonies. Also, when certain differential media are used, the distinctive appearance of the colony on the surface is essential for diagnostic purposes. Colonies that form beneath the surface of a pour plate are not satisfactory for such tests. To avoid these problems, the spread plate method is frequently used, ٠,١ ml inoculum is added to the surface of the medium with a specially-shaped sterilized glass rod. This hot positions all the colonies on the surface and avoids contact of the cells with the hot agar (Tortora, et al., ١٩٩٨). Among the disadvantages of the plate method is the problem of spreaders (especially when the agar surface is not adequately dry prior to plating), and the crowding of colonies, which makes enumeration more difficult. In spite of the disadvantages of the pour plate method, it is most usable because it measures the number of viable cells. ٢,٣,٢ Most Probable Number (MPN) Counts In these counts, the concentration of viable organisms or propagules is inferred from examining multiple cultures prepared from aliquots of dilution series, and determining the portions of such cultures that show growth and those that do not show growth in suitable growth medium (Harrigan, ١٩٩٨). Three serial dilutions are then planted into nine or fifteen tubes of appropriate medium for the three or five tube method, respectively. Numbers of organisms in the original sample are determined by use of standard MPN tables. Among the advantages that this method offers are the following: it is relatively simple, results from one laboratory are more likely than plate counts results to agree with those from another laboratory, specific groups of organism can be determined by use of appropriate selective and differential media and it is the method of choice for determining fecal coliform densities. Among the drawbacks to its use is the large volume of glassware required, the lack of opportunity to observe the colonial morphology of the organisms, and its lack of precision (Jay, ١٩٨٦). ٢,٣,٣ Dye Reduction Methods These methods depend on the ability of microorganisms to alter the oxidation-reduction potential of a medium. They are in consequence a measure of the activity of microorganisms in the test system rather than of the numbers in the sample. Suitable indicator dyes include methylene blue and resazurin. The length of time taken to reduce the dye depends on the mass and activity of bacteria present in the sample: the greater the number present, the shorter the time required for reduction. However, many other factors are important, including the nature of the sample, the medium used and the types of organisms present. The organisms must be capable of metabolism and growth in the medium to which the dye is added, and if the sample itself is incapable of supporting growth the dilution liquid should be a nutritious liquid. For reproducible end results the test system, including the sample, must be of a sufficiently constant chemical composition to have an invariable effect on the microorganisms present. Dye-reduction tests have a long history of use in the dairy industry for assessing the overall microbial quality of raw milk. Among their advantage are: they are simple, rapid, and in expensive; and only viable cells actively reduce the dyes. Disadvantages are: not all organisms reduce the dyes equally; and they are not applicable to food specimens that contain reductive enzymes unless special steps are employed (Jay, ١٩٨٦). ٢,٣,٤ Direct Microscopic Count (DMC) In its simplest form, the DMC consists of making smears of food specimens or cultures onto a microscope slide, staining with an appropriate dye and viewing and counting cells with the aid of a microscope (oil immersion objective). DMCs are most widely used in the dairy products and the specific method employed is that originally developed by R.S. Breed (Breed Count) (Jay, ١٩٨٦). The method consists of adding ٠,٠١ ml of sample to a slide and staining. The organisms or clumps of organisms are then enumerated. The latter involves the use of a calibrated microscope slide. Among the advantages of DMC are: it is rapid and simple; cell morphology can be assessed; and it lends itself to fluorescent probes for improved efficiency. Among its disadvantages are: it is a microscopic method and therefore fatiguing to the analyst, both viable ad non-viable cells are enumerated; food particles are not uniformly distributed relative to single cells and clumps; some cells do not take the stain well and may not be counted. In spite of its drawbacks, it remains the fastest way to make assessment of microbial cells in a food product (Jay, ١٩٨٦). ٢,٤ Total Viable Counts as Indicators of Food Sanitary Quality Total viable counts (more often aerobic plate counts, APC) on food products not only reflect handling history, state of decomposition, or degree of freshness; but they may in some instances reflect on the sanitary quality of foods. Total counts most effectively evaluate the sanitary quality of foods that do not support microbial growth. Low total counts do not always represent safe products and may contain coliforms and it is also possible to have low-count foods in which toxin-producing organisms have grown and produced toxins that remain stable to conditions that may not favor the continued survival of the cells (Jay, ١٩٨٦). A more recent study of a large number of ready-to-eat foods suggests that the APC is the most suitable method for evaluation of the microbial quality of foods and that where food safety is of concern a search for specific pathogens should be made. ٢,٥ Culture Medium A culture medium is any nutrient liquid or solid that can be used in laboratory for the growth of microorganisms. Such a medium should resemble the natural substrate (e.g. blood serum for animal pathogens, milk for milk microorganisms, soil extract for soil microorganisms) on which the microorganisms grow. Whatever, the medium, it must include all the necessary requirements for growth, which vary according to the organism it is desired to grow but will include: (a) Water. (b) Nitrogen – containing compounds (e.g. peptides, proteins, amino acids, nitrogen – containing inorganic salts). (c) Energy source (e.g. carbohydrate, peptides, amino acids, protein). (d) Accessory growth factors. The nutritional requirements of bacteria range from the simple inorganic requirements of autotrophs to the many vitamins and growth factors required by some of the fastidious bacteria (including pathogens and the lactic acid bacteria). Therefore, it is not possible to formulate a medium capable of supporting the growth of all microorganisms. However, the commonly used empirical media, such as nutrient broth and nutrient agar, are capable of supporting the growth of many bacteria. Furthermore, a medium such as nutrient agar can be used as a basal medium to which is added, for example, blood to ٥-١٠٪, serum or milk, to provide the complex growth factors needed by the more fastidious bacteria; lactic-acid bacteria require Bgroup vitamins which can be provided by the addition of yeast extract. A nutrient medium can be made selective or biochemically diagnostic by the addition of suitable compounds (Harrigan, ١٩٩٨). The glucose tryptone yeast agar (plate count agar) medium (PCA) is more usable and that is due to the fact that PCA allows the growth of more types than does nutrient agar (Harrigan, ١٩٩٨). Part II: Water ٢,٦ Importance of Water Water is essential to sustain life; therefore, a satisfactory supply must be made available to consumers. Every effort should be made to maintain drinking-water quality as high as practicable. Protection of water supplies from contamination is the first line of defense. Source protection is almost invariably the best method of ensuring safe, drinking-water and is to be preferred to treating a contaminated water supply to render it suitable for consumption. Once a potentially hazardous situation has been recognized, the availability of alternative sources, and the availability of suitable remedial measures must be considered. As far as possible, water sources must be protected from contamination by human and animal waste, which may contain a variety of bacterial, viral and protozoan pathogens and helminthes parasites. Failure to provide adequate protection and effective treatment will expose the community, to the risk of water-borne diseases. The acceptable quality of water is defined by WHO guidelines as that which is suitable for all usual domestic purposes, including personal hygiene (WHO, ١٩٩٣). It should be palatable, wholesome, be attractive to sense of sight and hygienically safe. There is an urgent need for simple, effective, low-cost methods for the production of water free of pathogenic and harmful chemical substances (John, ١٩٧٧). On the surface of the continents water appears in more scattered form, covering ٢,٥ million km٢ of its territory. From this, the area of fresh water amounts to ٢ million km٢. The volume of fresh water is small in comparison with that of seas and oceans (Table A). It amounts to barely ٠,٤٪ of the surface area of the Earth and approximately ١٪ of the area of the continents (Chhatwal, et al., ١٩٩٣). Table A: water resources of the hydrosphere (Source: Chhatwal, et al., ١٩٩٣) Location and state of stored water Amount ١٠١٢ tons Percent ١،٣٨٠،٠٠٠ ٩٨،٩٠٠ ١٦،٧٠٠ ١،٠٧٧ Fresh water ٠،٠٢٥ ٠,٠٠٢ Water vapour in atmosphere ٠،٠١٣ ٠,٠٠١ Underground water ٠،٢٥٠ ٠,٠٢٠ Seas Polar and mountain and snow ١،٣٩٦،٩٨٨ Total ١٠٠,٠٠٠ Forrest (١٩٥٦) reported that there were acute shortages in both surface and underground waters in many locations in the world. Careless pollution or contamination of streams, lakes and underground sources has greatly impaired the quality of the available water. It is therefore of utmost importance for our future planning that good conservation and sanitary measures be practiced to ensure enough water supply. ٢,٧Water-borne Diseases Water is unsafe for human consumption when it contains pathogenic microorganisms. Pathogenic microorganisms (and their associated diseases) may include bacteria, such as Salmondla typhi (typhoid fever), Vibrio cholerae (Cholera), Shigella (dysentery, shigellosis), viruses such as poliovirus or hepatitis a virus and protozoa such as Giardia lamblia (giardiasis) or Cryptosporidium parvum (cryptosporidiosis). Giardia is a protozoan parasite that infects the upper portion of the small intestine of humans and many other species of mammals. The usual mode of transmission is personto-person through what is termed the “fecal-oral route”. The least common mode of transmission is water-borne. Cryptosporidium is a protozoan parasite, like Giardia. Both humans and animals may serve as sources of environmental contamination and human infection. In ١٩٩٣-١٩٩٤, cryptosporidiosis caused by Cryptosporidium parvum was the leading cause of illness associated with contaminated drinking water in the United States. Other disease outbreaks during that time were caused by Giardia lamblia, Salmonella, Shigella, Campylobacter jejuni and Vibrio cholerae (cdc..gov/epa/mmwr/wr.html). Recognition that water was a source of pathogenic microorganisms was made in the late ١٨٠٠’s. Because it was, and still is, very expensive and time consuming to test for all the possible microbial pathogens in water, it was suggested in the late ١٨٠٠’s that a single group of microorganisms that come from the same source as human pathogens (i.e., the gastrointestinal tract) could be used to indicate the presence of pathogens. In ١٩١٤, the USA Public Health Service adopted the use of coliform bacteria as indicator microorganisms to indicate the presence of faecal contamination in water. Ideally, if indicator microorganisms are detected in any substance, it indicates the presence of faecal contamination and therefore possible presence of pathogenic microorganisms in the water. Indicator microorganisms are tested for because they are easier and cheaper to test for than all the possible pathogens that might be present. The most common indicators are total coliform bacteria, faecal coliforms and Escherichia coli (E. coli). It is very important to note that the presence of coliforms, faecal coliforms or even Escherichia coli in water does not mean that pathogenic microorganisms are present. It only gives an indication that they might be present. Presence of coliform or faecal coliform bacteria does not determine whether a sample will make someone ill (Wga.org/WQIS/G/ossary/Ecoli.htmi). Water-borne diseases are “dirty-water” diseases, i.e., those caused by water that has been contaminated with human or animal faeces or chemicals. Worldwide, the lack of sanitary waste disposal and of clean water for drinking, cooking, and washing is to blame for over ١٢ million deaths a year http://www.infofrhealth.org/pr/m١٤/m١٤/chap٥ ١.shtml). ٢,٨ Standards for Drinking-Water Drinking-water standards around the world are in a continuous state of evolution as more information becomes available and is valuated. No single standard for drinking-water quality that suffices for all countries but there is a considerable degree of agreement on contaminates and their allowable contaminates (Sayre, ١٩٨٨). Yet different approaches to regulation and different conditions in countries will maintain differences in standards currently enforced. Although standards and monitoring programs are in place for most public water supplies around the world, bottled water, which is being increasingly popular, is often not regulated. The first priority of water supplies in all countries is to ensure that drinking water is bacteriologically safe. In the United States, reporting of water-borne disease outbreaks has been and continues to be voluntary. Based on the available data, the incidence of waterborne diseases had declined from ٨ cases per ١٠٠،٠٠٠ person-years during ١٩٢٠-١٩٤٠ to ٤ cases during ١٩٧١-١٩٨٠ (Crawn, ١٩٨٦). Over the last few decades, the number of chemicals appearing in the standards has increased and will continue to increase as more data become available (Ronald, ١٩٩٧). ٢,٩ International Drinking-Water Standards The WHO is an international body and, using experts from around the world, has developed guidelines (WHO, ١٩٨٤) to be used as a basis for developing standards in all countries, particularly those countries that lack the resources to perform the basic information of gathering and assessment tasks involved. WHO notes that the guidelines are to be considered in the environmental, social, economic and cultural milieu of the country. The guidelines have undergone various revisions through the years (Ronald, ١٩٩٧). ٢,١٠ Water Pollution The term “water pollution” refers to the addition to water of an excess of material that is harmful to humans, animals or desirable aquatic life, or otherwise causes significant departures from the normal activities of various living communities in or near bodies of water. ٢,١١ Contamination of Drinking-Water The term “contamination” is defined as the presence in water of bacteria from the intestinal tract of warm-blooded animals including man. El Shazali and Erwa (١٩٧١) reported that studies in the Sudan have clearly demonstrated the close association of biological contamination of drinking-water with the high prevalence of diarrheal diseases and certain enteric pathogens. A study in the Nile and in wells at Khartoum area by Elhassan, et al. (١٩٨٤), indicated that there were ٩٣-٤٦٠ cells/١٠٠ ml either coliform or faecal coliforms in Nile water and ٣-٢, ٤٠٠ cells/١٠٠ ml of either coliforms or faecal coliforms in wells, but tap water contained only ٣ clls/١٠٠ ml of either coliforms or faecal coliforms. Hammad and Dirar (١٩٨٢) found that zeers were faecally contaminated, with faecal coliforms in ٦٩,٨٨٪ and faecal streptococci in ٩١,٥٦% of samples examined. Data from Sierra Leone on the waters from surface sources showed that these waters had extremely low dissolved chemical contents, but a variable, often high level of faecal bacterial contamination (Wright, ١٩٨٤). Mahgoub (١٩٨٤) noted that the present practice of effluent disposal from Khartoum North Treatment Plant (disposing industrial sewage) can form a serious potential source of surface and ground water contamination. Part III: Milk ٢,١٢ Animal Wealth in Sudan Sudan is one of the largest African countries which have a big livestock population and is considered the second in Africa (Ministry of Animal Resources, ١٩٩٨). Animal census of ٢٠٠٢ is the latest estimate which gave animal resources in the Sudan as ١٣٢،٤٤٢،٠٠٠ heads, cattle ٣٩،٤٧٩،٠٠٠ heads, sheep ٤٨،١٣٦،٠٠٠ heads, goats ٤١،٤٨٥،٠٠٠ heads and camels ٣،٣٤٢،٠٠٠ heads. For Khartoum State, the estimation of animal wealth is ١،٢٥٢،٨٤٨ heads, cattle ٢٢٥،٠٣٠ heads, sheep ٤٠٩،١٥٦, goats ٦١٣،٩٧٨ heads and camels ٤،٦٧٩ heads (Ministry of Animal Resources, ٢٠٠٣). The pastoral tribes of the western, eastern, southern and central Sudan possess the traditional primary sources of milk from cattle and other animals in the Sudan. ٢,١٣ Milk Bovine milk may be defined as the liquid from the mammary glands of healthy and normally fed cows. The composition of milk varies widely depending on a large number of factors including breed, season, stage of lactation, milking interval, health of the cow and level and type of feed. Several authors reported comparable values of milk chemical composition (Table B). Table B: Chemical composition of milk given by different authors. Source: FAO Food and nutritional paper ١٤/٣, (١٩٧٩). Richmond Davies {In Davis & Maodonald (١٩٥٣)} Person D (١٩٧٦) Webb, et al. (١٩٧٤) Fat ٣,٧٥ ٣,٦٧ ٣,٦١ ٣,٥-٢,٧ Protein ٣,٢٠ ٣,٤٢ ٣,٢٩ ٣,٥ Lactose ٤,٧٠ ٤,٧٨ ٤,٦٥ ٤,٩ Ash ٠,٧٥ ٠,٧٣ ٠,٧٥ ٠,٩ ٢,١٤ Nutritive Value of Milk It is recognized that milk is a good type of food and has well balanced basic nutrients such as easily digestible fat, carbohydrate material and contains high percentage of complete easily digestible animal protein in addition to some of the important vitamins: A, B, E and also contains important mineral compounds, like calcium and phosphorus (Chandan, ١٩٩٧, Fig. A). ٢,١٥ Sources of Microbial Contamination ٢,١٥,١ Interior of the Udder O’Conore (١٩٩٥) reported that the species of bacteria found in milk as it comes from the udder are limited to few genera. The micrococci are generally present in the greatest proportion followed by streptococci and rods. Milk taken aseptically from normal udder has ٣٠٠-١٠٠٠ bacterial cells per ml. Figure A: Major chemical components of raw milk. Source: Chandan (١٩٩٧). Milk Water ٨٧,٤٪ Total solids ١٢,٦٪ Solid-non-fat ٨,٩٪ Lactose ٤,٨٪ Protein ٣,٤٪ Why proteins ٠,٦٪ Fat ٣,٧٪ Minerals ٠,٧٪ Casein ٢,٨ ٢,١٥,٢ Exterior of the Udder Swarling (١٩٥٩) reported that under normal practical conditions contamination of milk can result from different sources including dung, water, soil, the cow itself, the milkers and milking facilities. Robinson (١٩٩٠) and Richard (١٩٥٨) mentioned that udder skin and milking machines contribute equally to the microbial count of milk. However, milking machines gave markedly high contamination with psychrotrophs, penicillin resistant psychrotrophs, coliforms and heat-resistant bacteria. O’Conore (١٩٩٥) reported that coliform bacteria and members of the genus Bacillus may enter the milk from soil. Thomas et al. (١٩٧١) indicated that cow’s milking environment, pipeline milking plants and farm bulk tanks comprised a bacterial contamination. ٢,١٦ Microorganisms in Raw Milk ٢,١٦,١ Lactic Acid Bacteria (LAB) LAB are a group of bacteria able to ferment lactose of milk to lactic acid. Examples of these microorganisms are: (i) Streptococci - Streptococcus lactis. - Streptococcus cremoris. (ii) Lactobacilli - Lactobacillus casei. - Lactobacillus lactis. - Lactobacillus bulgaricus. (iii) Leuconstoc. - Leuconostoc mesenteroides. ٢,١٦,٢ Coliforms These are indicator organisms associated with the presence of pathogens and can cause rapid spoilage of milk. ٢,١٦,٣ Milk Spoilage Microorganisms Pseudomonas fluorescens, Pseudomonas fragi some species and strains of Bacillus, Clostridium, Corynebacterium, Arthrobacter, Lactobacillus. Microbacterium, Micrococcus and Streptococcus can survive pasteurization and grow at refrigeration temperatures. ٢,١٦,٤ Pathogenic Microorganisms in Milk. Proper handling and storage of milk and also pasteurization have decreased the milk-borne diseases such as tuberculosis, brucellosis and typhoid fever and other food-borne illnesses resulting from the ingestion of raw milk or dairy products made from milk not properly pasteurized or contaminated after processing. The following bacterial pathogens are still a concern in raw milk and other dairy products: Bacillus enterocolitica, cereus, Salmonella Listeria monocytogenes, species, Escherichia Yersinia coli and Campylobacter jejuni. ٢,١٧ Microbiological Standards for Raw Milk Milk was the first food product for which microbiological standards were adopted in the United States. ٢,١٧,١ Standards for Dairy Products (A) From ١٩٦٥ recommendations of the U.S. Public Health Service. (a) Grade A raw milk for pasteurization: Not to exceed ١٠٠،٠٠٠ bacteria per milliliter prior to commingling with other producer milk; and not exceeding ٣٠٠،٠٠٠ per milliliter as commingled milk prior to pasteurization. (b) Grade A pasteurized milk and milk products (except cultured products), not over ٢٠،٠٠٠ bacteria per milliliter, and not over ١٠ coliforms per milliliter. (c) Grade A pasteurized cultured products: not over ١٠ coliforms per milliliter. (B) Certified milk (American Association of Medical Milk Commissions, Inc.): (a) Certified milk (raw): Bacterial plate count not exceeding ١٠،٠٠٠ colonies per milliliter; coliform colony count not exceeding ١٠ per milliliter. (b) Certified milk (pasteurized): bacterial plate count not exceeding ١٠،٠٠٠ colonies per milliliter before pasteurization and ٥٠٠ per milliliter in route samples. Milk not exceeding ١٠ coliforms per milliliter before pasteurization and ١ coliform per milliliter in route samples (Jay, ١٩٨٦). ٢,١٩ Pasteurization In the early days of microbiology, Louis Pasteur found a practical method of preventing the spoilage of beer and wine. Pasteur used mild heating, which was sufficient to kill the organisms that caused the particular spoilage problem without seriously damaging the taste of the product. The same principle was later applied to milk to produce what we now call pasteurized milk. Milk was first pasteurized to eliminate the tuberculosis bacterium. Many relatively heat-resistant (thermoduric) bacteria survive pasteurization, but these are unlikely to cause disease or cause refrigerated milk to spoil. Almost all pathogenic viruses are inactivated by pasteurization (Tortora, et al. ١٩٩٨)., In the classic pasteurization treatment of milk, the milk was exposed to a temperature of about ٦٣°C for ٣٠ minutes, this treatment being known as holder method. Most milk pasteurization today uses higher temperature, at least ٧٢°C for only ١٥ seconds. This treatment, known as high-temperature short-time (HTST) pasteurization, is applied as the milk flows continuously past a heat exchanger. In addition to killing pathogens, HTST pasteurization lowers total bacterial counts, so the milk keeps well under refrigeration. As conventional microbiological tests require ١-٢ days before the result is obtained and because milk is a highly perishable product, quality assurance can be obtained by confirming that pasteurization has occurred by using the phosphatase test. However, a number of outbreaks of enteritis caused by Salmonella or Campylobacter in pasteurized milk have been caused as the result of post-pasteurization contamination. In one outbreak, the contamination was through a faulty flow diversion valve. In another, the contamination occurred through faulty valves on a pipe loop which was associated with the cleaning-in-place circuit. As the pathogens could be introduced by leakage of relatively small volumes of raw milk into the pasteurized milk, it is extremely unlikely that a phosphatase test would be able to detect such a fault (Harrigan, ١٩٩٨). Thus, in addition to the phosphatase test, which can be used in a quality control role, microbiological assessments can be used in a quality assurance role to determine the quality of product already produced, distributed and sold, so that a decision can be taken whether or not to accept future batches of product from that source. Aerobic mesophilic counts at ٣٠-٣٢°C and coliform or total Enterobacteriaceae counts, may be performed. After pasteurization the general viable count should be not more than ٣٠٠٠٠ per ml (and counts of less than ٥٠٠٠ per ml on the freshly pasteurized milk should be readily attainable). Total Enterobacteriaceae (or coliforms) should not be detected in ١ ml of product (less than ١ per ml should be a readily attainable standard) (Harrigan, ١٩٩٨). Pasteurized milk should be stored at refrigeration temperatures until consumption, so that the aerobic mesophilic count at ٣٠°C will increase (many of the psychrotrophic hemophiles being detectable in counts incubated at ٣٠°C). However, coliforms and other Enterobacteriaceae should not multiply in pasteurized milk properly stored, so there is no justification for increasing the permitted count of these organisms in any standard applied to milk sampled at retail outlets. CHAPTER THREE MATERIALS AND METHODS ٣,١ Sterilization ٣,١,١ Hot-air Oven Glassware (Petri-dishes, pipettes, tubes, flasks and glass rods), wrapped in aluminum foil, were sterilized in the hot air oven at ١٦٠°C for two hour (Barrow and Gelthan, ١٩٩٣). ٣,١,٢ Autoclaving Used for sterilization of media, solutions and materials which could not withstand the dry heat. The exposure time was ١٥ minutes ١٢١°C under ١٥ pounds pressure (Barrow and Gelthan, ١٩٩٣). ٣,٢ Preparation of Media ٣,٢,١ Solid Media ٣,٢,٢ Plate Count Agar This is a non-selective medium for general viable counts of bacteria in food (Harrigan, ١٩٩٨). It was obtained in dehydrated form (biomark laboratories pune ٤١١ ٠١١ " India"). The medium was composed of yeast extract, tryptone, D-glucose and granulated agar. It was prepared according to the manufacturer’s instructions by using ١٧,٥ g in one liter distilled water. The medium was allowed to boil in water bath until it was completely dissolved and autoclaved at ١٢١°C for ١٥ minutes. ٣,٢,٣ Liquid Media ٣,٢,٤ Peptone Water (Oxid) Fifteen grams of dehydrated peptone water were suspended in a liter of distilled water, mixed well, then pH adjusted to ٧,٢ and autoclaved at ١٢١°C for ١٥ minutes. ٣,٣ Preparation of Butterfield’s Phosphate Buffer ٣,٣,١ Stock Solution KH٢PO٤ ٣٤ g Distilled water ٥٠٠ ml The pH was adjusted to ٧,٢ with ١ N NaOH. The volume was brought to ١ litre with distilled water. The solution was sterilized ١٥ min at ١٢١°C and stored in refrigerator (FAO, ١٩٩٢). For dilution blanks an amount of ١,٢٥ ml of above stock solution was taken and the volume brought to ١ litre with distilled water, dispensed into bottles to ٩٠ ± ml and sterilized for ١٥ min at ١٢١°C (FAO, ١٩٩٢). ٣,٤ Collection of Samples A total of ١٦ samples of running and stagnant irrigation Nile water were collected from Shambat. A total of ٨ raw bovine milk was obtained from the University of Khartoum’s Farm. Plate counts were carried out within ٢ hours after milking. A total of ٨ Commercial pasteurized milk samples was bought from Kenana milk product factory in Kenana and plate counts carried out ٢٤ hours after pasteurization. Data given in the results express the average data for each group of samples. ٣,٥ Preparation Dilutions of Samples One ml from the water sample was taken by sterile pipette and transferred to the first tube containing ٩ ml of ٠,٩٪ phosphate buffer solution as a diluent to give a ١٠-١ dilution; with a sterile pipette ١ ml from this first dilution tube was transferred to a second tube of sterile diluent to give a ١٠-٢ dilution, then further dilutions were made. Ten ml from the milk sample was taken by sterile pipette and transferred to the first bottle containing ٩٠ ml of dilutient to give a ١٠-١ dilution; with a sterile pipette ١ ml from the first dilution bottle was transferred to a second bottle of ٩ ml sterile diluent to give a١٠-٢ dilution then further dilutions were made. ٣,٦ Microbiological Methods ٣,٦,١ Viable Count of Bacteria ٣,٦,١,١ Milk (Raw and Pasteurized Milk) Plate Count Agar was used for enumeration of bacteria, using the pour-plate technique as described by Harrigan and McCance (١٩٧٦). Ten ml of homogeneous milk were added to ninety milliliters of phosphate buffer to give ١/١٠ dilution and then further dilutions were made by transferring ١ ml of ١st dilution to ٩ ml buffer. One ml from each suitable dilution was transferred aseptically into sterile Petri-dishes and plate count agar media was added. The inoculum was mixed with the medium and allowed to solidity. The plates were made in duplicates for each dilution and incubated at ٢٥, ٣٢ or ٣٧°C for ٧٢ hrs. The result was reported as the viable bacterial count per ١ml of sample. Counting of the colonies was done every ٢٤ hrs with the help of colony counter (Scientific & Electronics Ltd.). The rule of counting only plates containing between ٣٠ and ٣٠٠ colonies were strictly followed whenever possible. ٣,٦,١,٢ Water (Running and Stagnant) Plate Count Agar was used for enumeration of bacteria, using the pour-plate technique as described by Harrigan and McCance (١٩٧٦). One ml of homogenous water were added to nine milliliters of phosphate buffer to give ١/١٠ dilution and further dilutions made as above. One ml from suitable dilutions was transferred aseptically into sterile Petri-dishes and plate count agar media was added. The inoculum was mixed with the medium and allowed to solidity. The plates were made in duplicate for each dilution and incubated at ٢٥, ٣٢ or ٣٧°C for ٧٢ hrs. The result was reported as the viable bacterial count per ١ ml of sample. Counting of the colonies was done every ٢٤ hrs with the help of colony counter. ٣,٧ Isolation of Dominant Microorganisms Dominant colonies of microorganisms were chosen from plates used for viable count and kept for further tests. ٣,٨ Purification of Isolates Isolates were taken from the viable counts plates. One separate colony is touched with sterile inoculating loop, and the cells thus removed by streaking on a solid medium. The purified culture thus obtained was further purified by repeating the above procedure (Kiss, ١٩٨٤). ٣,٩ Tests for the Tentative Identification of Bacteria Identification tests of bacteria were repeated three times for each microorganism. Tentative identification was done according to (Harrigan, ١٩٩٨). ٣,٩,١ Gram Stain A discrete colony was picked carefully with sterile wire loop. The colony was emulsified in a drop of sterile normal saline, placed on a clean slide and spread evenly to make a thin film. The slide was allowed to dry. The smear was fixed by using flame. Then the smear was stained as described by Harrigan and McCance (١٩٧٦). ٣,٩,٢ The Staining of Bacterial Spores The smear was done as describe in ٣,٩,١ and then stained by Malachite Greens as described by (Harrigan, ١٩٩٨). ٣,٩,٣ Motility Test The organism to be tested was grown for ٢٤ hours at ٣٧°C in a liquid medium containing (g/L): ١٠ yeast extract, ٣٠ CaCO٣ and ٢٠ ml ethanol and pH adjusted to ٦,٧ (Frateur, ١٩٥٠). A drop of the culture was transferred to cover slip and the motility was examined using a light microscope. ٣,٩,٤ Catalase Test A small parts of the colony was added to ١ ml of ٣٪ hydrogen peroxide on a slide. In the presence of catalase, gas formulation is observed (Kiss, ١٩٨٤). ٣,٩,٥` Acid-Fast Tests The following tests were followed: Cover the slide with strong Ziehl-Neelsen’s carbol fuchsin and heat the underside of the slide with a lighted alcohol-soaked swab. Stop heating when the slide steams. Keep the slide hot and replenish the stain if necessary, taking care not to allow the smear to become dry. Heat for ٥ min, not allowing the staining solution to boil. Wash well. Decolorize with acid-alcohol or with ١, ٥ or ٢٠٪ sulphuric acid. The excess stain is removed as a brownish solution, and the smear will become brown. Rinse in water, when the film will appear pink once more. Apply more acid and repeat the rinsing several times until the film appears faintly pink upon washing. Wash well. Counter stain with Loeffler’s methylene blue for ٥ min. Wash well and carefully remove the stain deposits from the back of the slide with filter paper. Blot dry and examine (Harrigan, ١٩٩٨). ٣,٩,٦ Sugar Fermentation Test To ١,٥ g peptone water, ١٪ glucose and ١٪ indicator (Andrade’s) were added. Durham tubes were used to detect gas production. Cultures were inoculated and incubated anaerobically at the optimum temperature ٣٧°C and were examined daily for ٧ days. Gas production indicates positive test (Harrigan and McCance, ١٩٧٦). CHAPTER FOUR RESULTS AND DISCUSSION Effect of Incubation Temperature on the plate count of raw milk As can be seen in Fig. ١ there were three temperatures tested (٢٥°C, ٣٢°C, ٣٧°C). Temperature ٢٥°C gave the least growth and incubation in ٣٢°C gave the highest viable count. With respect to incubation time, it can be seen that counts at ٣٢°C, ٣٧°C and ٢٥°C reach the maximum at ٤٨ hrs or ٧٢ hrs of incubation. This result disagrees with Dirar (١٩٧٦) who found that incubation at ٣٧°C gave the highest viable count and both counts at ٣٧°C and ٢٥°C reached the maximum at ٤٨ hrs of incubation. It might be during ٣٠ years new strains of microorganisms have appeared or new practices followed. In United States of America, for instance, the incubation for the plate count of milk is ٢٣ ± ١°C for ٤٨ hrs ±٣ (Hausler, ١٩٧٢). These specifications were originally set up by a research committee of bacteriologists (Babel, et al., ١٩٥٥). Other work shows that incubation temperatures of ١٠, ٢٠, ٢٧ and ٣٠°C gave higher counts than ٣٣°C or ٣٧°C and the selected organisms from plates incubated at the different temperatures grew best at ٢٠°C and ٢٧°C. The author recommended the use of ٢٧° as incubation temperature, instead of the present ٣٢°C for the plate count of raw milk. Smith, et al. (١٩٧٣) obtained highest 25 °C. 5.8 5.2 5 37 C 5.6 5.4 4.8 4.6 24 48 Incubation Time(hrs) 72 Fig . ١ Effect of Incubation Temperature on the Plate of Raw Milk (Appendix ١) L o g o f Via ble Co un 32 C counts when plates were incubated at ٢٩٫٩°C for ٤٨ hrs. Our results show clearly the incubation temperature of ٣٢°C is by far more superior to the lower temperature of ٢٥°C for the plate count of raw milk. The viable count of chilled farm raw milk was less than ١٠٤ per ml, bulk raw milk was less than ١٠٥ and the total viable count under aseptic conditions was less than ١٠٣ per ml (Harrigan and McCane, ١٩٧٦). We should remember that milk samples under test differ because the atmospheric temperatures vary between cold countries and tropical countries like Sudan. This fact shows that it is unwise copying of standards of one country to another without testing. In this study the dominant microorganism in raw milk was Listeria (Table ١). This disagrees with Elgadi (٢٠٠٣) who reported that Streptoroccus was obtained in high counts from Khartoum town raw milk samples. Our samples were taken from Shambat University Farm but Listeria is not commonly reported as a dominant species although it is commonly found in milk (Jay, ١٩٨٦). Effect of Incubation Temperature on The plate Count of Pasteurized Milk In Fig. ٢ of the three incubation temperatures shown it can be seen that the lowest count was given at ٢٥°C while ٣٧°C gave the highest count. Log. of Viable Coun 6.3 6.2 6.1 6 5.9 5.8 5.7 5.6 5.5 24 48 72 Incubation Time(hrs) 25 °C. 32 °C. 37 °C. Fig .٢ Effect of Incubation Temperature on the Plate of Pasteurized Milk (Appendix ٢) Plates incubated at ٣٢°C and ٣٧°C attained the maximum count only after ٣ days of incubation. This result also disagrees with Direr (١٩٧٦) in Sudan and committee’s finding in America (Babel et al., ١٩٥٥). It might be that the differences are due to using different sanitation materials for cleaning the flours and utensils. The dominant microorganisms in pasteurized milk (Table ١) are Streptococcus – Leuconostoc – Pediococcus and this agrees with O’Conore (١٩٩٥) who reported that the species of bacteria found in milk as it comes from the udder are limited to few genera, while The micrococci are generally present in the greatest proportion followed by streptococci and rods. Effect of Incubation Temperature on the Plate Count of Water: Results, as shown in Fig. ٣ and Fig. ٤, show that the optimum incubation temperatures are not the same as in the case of milk. It can be seen that ٢٥°C gave the maximum count (running water and stagnant water). In this study the result disagrees with Dirar (١٩٧٦) who found that incubation at ٣٧°C gave the highest viable counts and ٢٥°C gave the least growth. In U.S.A an incubation temperature of ٢٠°C+٠٫٥ is used for ٤٨-+ ١hrs or a temperature of ٣٥°C+- ٠٫٥ for ٢٤± ٢hrs for the plate count of water (Dirar, ١٩٧٦). 4 3 2 1 0 24 48 Incubation Time(hrs.) 72 25 °C. 32 °C. 37 °C. Fig .٣ Effect of Incubation Temperature on the Plate Count of Running Water (Appendix ٣) Lo g. of Viable Co unt Log. of Viable Coun 3.6 3.4 3.2 3 2.8 2.6 24 48 Incubation Time (hrs.) 72 25 °C. 32 °C. 37 °C. Fig .٤ Effect of Incubation Temperature on the Plate Count of Stagnant Water (Appendix ٤) The dominant microorganisms is Bacillus in running water and Staphylococcus and Micrococcus in stagnant water (Table ١). This result agrees with Ahmed (٢٠٠٥) and disagrees with Elrofaei (٢٠٠٠) with respects to water samples taken from factory cisterns and drinking water at Jebel Awllia and Jeberona, respectively. Table ١: Tentative identification of bacteria from water (running water, stagnant water) and milk (raw milk, pasteurized milk). Isolate Character Gram stain Acid fast Endospores produced Catalase positive Cells spherical Cell rod shaped Aerobic Motile Good growth on plate count agar Sugar fermentation test Tentative genus ١ ٢ ٣ ٤ ٥ ٦ ٧ ٨ ٩ ١٠ ١١ ١٢ + - + - + - + + + + + + - - - - - - - - + + + + - + - + - + + + + + + + + + - - - + - + - + - - - - - - + + + + + + + - + - + - + + + + + + + + + + + + + + + Staphylococcus Micrococcus group ١،٢،٣ ≡ stagnant water samples samples. ٤،٥،٦ ≡ Raw milk samples milk samples Listeria Bacillus Streptococcus – Leuconostoc – Pediococcus group ٧،٨،٩ ≡ Running water ١٠،١١،١٢ ≡ Pasteurized CHAPTER FIVE CONCLUSION AND RECOMMENDATION In conclusion we recommend the incubation temperature of ٣٢°C for raw milk and temperature of ٣٧°C for pasteurized milk. For running water we recommend the temperature of ٢٥°C and for the stagnant water we recommend the temperature of ٢٥°C. In all cases we recommend the incubation time of ٤٨ hrs for economy in time and cost although incubation for ٧٢ hrs gave slightly higher counts. More studies and researches should be done because it is not correct to designate an incubation temperature for the plate count of water and milk on one or two researcher’s results. Team work is needed to obtain the correct results to contribute to finding standards and specifications special for our country Sudan. REFERENCES Ahmed, F.I. (٢٠٠٥). Microbiological Quality of Water in some Food Facilities Storage Cisterns in Khartoum North Industrial Area. American Public Health Association. Standard Methods for the Examination of Water and Wastewater. ١٣th Editor, APHA, Inc. Washington, DC – ١٩٧١, p. ٦٦٠. Babel, F.J.; Collins, E.B.; Olson, J.C.; Peters, I.I.; Watrous, G.H. and Spech, M.L. (١٩٥٥). The standard plate count of milk as affected by the temperature of incubation. J. Dairy Sci. ٣٨: ٥٠٣. Barrow, G.I. and Gelthan, R.K.A. (١٩٩٣). Cowan and Steel’s. Manual for the Identification of Medical Bacteria. London. Cambridge Univ. Press. Chandan, R. (١٩٩٧). Dairy based ingredient Newer knowledge of dairy foods, cited in http://www.national dairycouncil.org./medant/newerknowledge/nk٤,١٤ml. Chhatwal, G.R.; Mehra, M.C.; Katyal, T.M.; Salake, M. K. and Nagahiro, T. (١٩٩٣). Environmental Water Pollution and its Control, New Delhi, Anmol Publications. Crawn, G.F. (١٩٨٦). Statistics of Water-Borne Outbreaks in United States. G.F. Cawn, (ED). CRC Press, Boca Raton, FL. Dirar, H.A. (١٩٧٦). Optimum incubation temperature for the plate count of milk. Sudan J. Fd. Sci. Technol. ٨: ٥٥-٦٠. ElHassan, B.; Awadelkarim, M.A.; Abdel Magid, H.M.; Ibrahim, I.S. and Dirar, H.A. (١٩٨٤). Water quality and quantity and their impact on health in Khartoum Province, Sudan. Water Quality Quarterly, ٩(٤): ٢٢٥-٢٣٠. Elgadi, Z. A. (٢٠٠٣). Isolation and identification of lactic acid bacteria and yeast from raw milk. M.Sc. Thesis, Faculty of Agric., University of Khartoum. Elrofaei, N.A. (٢٠٠٠). Microbiological Examination of Drinking Water for the displaced People Living Around Khartoum State. Ph.D. Thesis, Faculty of Agric., University of Khartoum. El Shazali, H. and Erwa, H. (١٩٧١). An overlooked source of gastroenteritis in Sudanese children, Sudan Medical J., ٩: ٤٥. FAO (١٩٩٢). Manual of food quality control ٤-Rev. ١. microbiological analysis. FAO food and nutrition paper ١٤/٤ Rev. ١, Food and Agricultural Organization of the United Nation, Rome. FAO (١٩٧٩). Food and nutrition Paper (١٤/٣), manual of food quality control ٣ commodities, Rome. Forrest, B. (١٩٥٦). Rural Water Supply and Sanitation. John Wiley and Sons, Inc. New York. Frateur, J. (١٩٥٠). Essai sur la systematique des acetobacters. La cellule, ٥٣: ٢٨٧-٣٩٢. (Cited in Bergey’s Manual of Systematic Bacteriology vol. ١, p. ٢٦٨ (١٩٨٤). Baltimore: William & Wilkins, (ISBN). Hammad, Z.H. and Dirar, H.A. (١٩٨٢). Streptococci versus coliforms as indicators of faecal contamination in sebeel water. Sudan Notes and Records, ٥٩, ١٦٠-١٧٥. Harrigan, W.F. (١٩٩٨). Laboratory Methods in Food Microbiology, ٣rd ed. Academic Press, San Diego, California. Harrigan, W.F. and Mc Cance, M.E. (١٩٧٦). Laboratory Methods in Food and Dairy Microbiology. Academic Press, London. Hausler, W.J. (١٩٧٢). Standard Methods for the Examination of Dairy Products. APHA, Inc., Washington, DC. ١٣th Edition. P. ٨١. Jay, J.M. (١٩٨٦). Modern Food Microbiology, ٣rd ed. Van Nostrand Reinhold Company, New York. John, W.C. (١٩٧٧). Water Supply and Pollution Control, ٣rd ed., New York: Harpar and Raw Publishers, Inc. Kiss, I. (١٩٨٤). Testing Methods in Food Microbiology. A Textbook, Elseevier, New York. Mahgoub, D.M. (١٩٨٤). Coliform Bacteria in the Nile at Khartoum. M.Sc. Thesis, Faculty of Agric., University of Khartoum. Ministry of Animal Resources (١٩٩٨). Administration of Planning and Economy of Animal Resources. Ministry of Animal Resources (٢٠٠٣). Statistical Bulletin for Animal Resources Issue No. ١٢, pp. ٣-٢٩. O’Conore, C.B. (١٩٩٥). Rural Dairy Technology. International Livestock Research Institute, Addis Ababa, Ethiopia. Richard, Z.D. (١٩٥٨). Treatment of milk for cheese with H٢O٢. J. Dairy Sci., ٤١: ١٤٦. Robinson, R.K. (١٩٩٠). Dairy Microbiology Vol. (١) the Microbiology of milk ٢nd edition, Elseevier London and New York ١٦٥-١٦٧. Ronald, L.D. (١٩٩٧). Theory and Practice of Water and Waste Water Treatment. John Wiley and Sons. Inc. New York. Sayre, I.M. (١٩٨٨). International Standards for Drinking Water. J. American Water Works Association, ٨٠, ١: ٥٣-٦٠. Smith, K.L.; Mortinez, E.A.; Pilkhane, S.V. and Mull, L.E. (١٩٧٣). Effect of incubation time and temperature on plate count of raw milk. J. Dairy Sci. ٥٦: ٣٠٤. Swarling, D. (١٩٥٩). The influence of the use of detergents and sanitizers on the farm with regard to the quality of milk and milk products. Dairy Sci. Abst., ٢١: ١-١٠. Thomas, B.S.; Druce, G.R. and Jones, M. (١٩٧١). Influence of production conditions on the bacteriological quality of refrigerated farm bulk tank milk. J. Appl. Bacteriol. ٣٤: ٦٥٩-٦٧٧. Tortora,G.J.; Funke, B.R. and Case, C.L. (١٩٩٨). Microbiology an Introduction, ٦th ed. Benjamin/Cummings Company, Menlo Park, California. Publishing WHO (١٩٨٤). Guidelines for Drinking Water Quality vol. ١, WHO, Geneva. WHO, (١٩٩٣). Guidelines for Drinking Water Quality, vol. ٢, WHO, Geneva. Wright, R. (١٩٨٤). Water quality analysis and integral component of water supply development in developing countries. Water Bulletin, ٩(٤): ٢٢٣-٢٢٤. Appendix I Effect of incubation temperature on the plate count of raw milk. Incubation temperature °C ٢٥°C ٣٢°C ٣٧°C Incubation time (hr) Viable count (cfu/ml) Log of viable count ٢٤ hrs ١,١٩ x ١٠٥ ٥,٠٧ ٤٨ hrs ١,٣١ x ١٠٥ ٥,١١ ٧٢ hrs ١,٣٤ x ١٠٥ ٥,١٢ ٢٤ hrs ٥,٢٦ x ١٠٥ ٥,٧٢ ٤٨ hrs ٥,٤٩ x ١٠٥ ٥,٧٤ ٧٢ hrs ٥,٤٩ x ١٠٥ ٥,٧٤ ٢٤ hrs ٤,٤٧ x ١٠٥ ٥,٦٥ ٤٨ hrs ٤,٥٧ x ١٠٥ ٥,٦٦ ٧٢ hrs ٤,٦١ x ١٠٥ ٥,٦٦ Appendix ٢ Effect of incubation temperature on the plate count of pasteurized milk. Incubation temperature °C Incubation time (hr) ٢٤ hrs Viable count (cfu/ml) ٥,٧ x ١٠٥ Log of viable count ٥,٧٥ ٢٥°C ٤٨ hrs ٨,١ x ١٠٥ ٥,٩٠ ٧٢ hrs ٨,٣ x ١٠٥ ٥,٩١ ٢٤ hrs ١,١٢ x ١٠٦ ٦,٠٥ ٤٨ hrs ١,١٨ x ١٠٦ ٦,٠٧ ٧٢ hrs ١,٢٧ x ١٠٦ ٦,١٠ ٢٤ hrs ١,٣٠ x ١٠٦٥ ٦,١١ ٤٨ hrs ١,٤٥ x ١٠٦ ٦,١٦ ٧٢ hrs ١,٥٢ x ١٠٦ ٦,١٨ ٣٢°C ٣٧°C Appendix ٣ Effect of incubation temperature on the plate count of running water Incubation temperature °C Incubation time (hr) ٢٤ hrs Viable count (cfu/ml) ٣,٧ x ١٠٢ Log of viable count ٢,٥٦ ٢٥°C ٤٨ hrs ٣,٠٠ x ١٠٣ ٣,٤٧ ٧٢ hrs ٣,٠٠ x ١٠٣ ٣,٤٧ ٢٤ hrs ٩,٨ x ١٠٢ ٢,٩٩ ٤٨ hrs ١,٤٥ x ١٠٣ ٣,١٦ ٧٢ hrs ١,٥٢ x ١٠٣ ٣,١٨ ٢٤ hrs ١,٣٢ x ١٠٣ ٣,١٢ ٤٨ hrs ١,٥٥ x ١٠٣ ٣,١٩ ٧٢ hrs ١,٥٩ x ١٠٣ ٣,٢٠ ٣٢°C ٣٧°C Appendix ٤ Effect of incubation temperature on the plate count of stagnant water. Incubation temperature °C ٢٥°C ٣٢°C ٣٧°C Incubation time (hr) Viable count (cfu/ml) Log of viable count ٢٤ hrs ٢,٦٥ x ١٠٣ ٣,٤٢ ٤٨ hrs ٢,٩٠ x ١٠٣ ٣,٤٦ ٧٢ hrs ٢,٩٧ x ١٠٣ ٣,٤٧ ٢٤ hrs ٢,٢٠ x ١٠٣ ٣,٣٤ ٤٨ hrs ٢,٢٥ x ١٠٣ ٣,٣٥ ٧٢ hrs ٣,٢٦ x ١٠٣ ٣,٣٥ ٢٤ hrs ٩,٨ x ١٠٢ ٢,٩٩ ٤٨ hrs ١,٨٧ x ١٠٣ ٣,٢٧ ٧٢ hrs ١,٩٣ x ١٠٣ ٣,٢٨