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FOOD MICROBIOLOGY: BASIC AND APPLIED WITH LABORATORY EXERCISES

Dr. Rita Narayanan, Dr. B.Dhanalakshmi, B.Dhanalakshmi, Rita Narayanan
  • Country of Origin:

  • Imprint:

    NIPA

  • eISBN:

    9789389992359

  • Binding:

    EBook

  • Number Of Pages:

    218

  • Language:

    English

Individual Price: 1,650.00 INR 1,485.00 INR + Tax

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The aim of this book is to unravel the exciting field of food microbiology to the students. This book focuses on the importance and significance of an array of microbes found in food. Food science is a vast field that forays into microbiology, chemistry various elements and ingredients involved in its making and their use in industrial production and ultimately their involvement in human health. Food microbiology is a complex interdisciplinary science which requires critical thinking, innovative approaches, analytical abilities to understand– all of which are provided in this book. Provides a balanced introduction to all major areas of microbiology suitable for students. The illustrations in the text book have been included to match the text and to assist in the visualization of abstract concept.

0 Start Pages

Preface The aim of this book is to unravel the exciting field of food microbiology to the students. This book focuses on the importance and significance of an array of microbes found in food. These microbes may be of use as starter organisms, be a source of spoilage or be pathogenic. Food science is a vast field that forays into microbiology, chemistry various elements and ingredients involved in its making and their use in industrial production and ultimately their involvement in human health. Food microbiology is a complex interdisciplinary science which requires critical thinking, innovative approaches, analytical abilities to understand– all of which are provided in this book. A food microbiologist must be acquainted with all major groups of microbes viz bacteria, viruses, fungi, algae, phage etc. Any student who is new to this subject, need to understand the laws and rules governing food quality and safety. The main objective of writing this book is to provide basic and applied information in the field of food microbiology. This book provides a balanced introduction to all major areas of microbiology suitable for students. The book is divided into various chapters which focuses on identification of microbes, their classification and detection methodologies. The book also has chapters that throw light on the control and elimination of microorganism and related safety aspects. The illustrations in the text book have been included to match the text and to assist in the visualization of abstract concept.

 
1 History of Microbiology

Microorganisms (or microbes) are organisms too small to be seen with the naked eye. The science of microbiology is the study of microorganisms, their properties, classification, growth requirements, method of reproduction and distribution in nature. These organisms range in size from tiny viruses to larger bacteria, protozoa, algae, fungi and some of the microscopic worms (helminthes). They are present everywhere in our environment, in our homes, on our bodies, on the things we touch and on the food we eat. They are important for human survival on Earth, playing an essential role in many processes that maintain life. In nature, microorganisms carry out a number of important functions. 1.They are responsible for the decomposition of organic matter. 2.Recycling of nutrients that help to maintain the balance of chemicals in the soil. 3.Special nitrogen-fixing bacteria live symbiotically with certain plants, absorbing nitrogen from the air and converting it into compounds which can be used by the plant for growth. 4.Some algae are capable of photosynthesis, using energy from the sun to convert carbon dioxide in air into carbohydrate. 5.Marine microorganisms form the basis of the food chain in lakes, rivers and oceans. 6.Certain microorganisms are used in the manufacture of food. 7.Pharmaceuticals and drugs. 8.Industrial processes. 9.A valuable tool for scientific research. 10.Use in laboratory experiments to understanding metabolic pathways and in the discovery of the genetic code.

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2 Microscopy

One of the major discoveries that gave birth to the field of microbiology was the development of instruments that could magnify, so that objects too small to be seen with the naked eye could be visualized. Robert Hooke in 1665, using an instrument with two sets of lenses (a crude compound microscope), was the first person to describe cells as life’s smallest structural units. In the early 1670s Anton van Leeuwenhoek used a simple magnifying lens to observe microbes he called ‘animalcules’ in rainwater, and was possibly the first person to observe microorganisms. The development of electron microscopes in the 1930s enabled magnifications of a further hundred times greater to be achieved. A microscope can be defined as an optical instrument consisting of a lens or a combination of lenses for getting magnified images of minute objects. A simple microscope is essentially an ordinary magnifying lens held on an adjustable stand. It can give magnification up to 20 times the size of the object.

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3 Classification of Microorganisms

Improvements in microscopes enabled scientists to study the ‘invisible’ organisms – that is, those too small to be seen with the naked eye. They were able to show that certain of these microscopic creatures are responsible for the infectious diseases of higher organisms. Most of them are harmless, some are essential for life and some are used in the manufacture of products that are of benefit to humans.

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4 Structure of Bacterial Cells

Size shape and Appearance of Bacterial cells Bacteria vary significantly in their morphology, that is in their appearance. They range in size from 0.2 μm to1 μm in diameter and 1 μm to 10 μm in length. There are three basic shapes of cells : 1. Spherical 2. Rod-shaped 3. Spiral.

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5 Morphology of Bacteria

Bacteria are morphologically different. This characteristic is helpful in the identification of bacteria. Routine study of bacterial morphology should include examination of stained preparations. Motility should be examined by hanging drop preparation. According to the requirement, certain cell structures may be stained. Stained preparations should be made preferably from fresh agar slant cultures. Smears of Streptococcus should be made from a liquid culture. Morphology of bacteria may be described under the following terms: 1.Shape: Spherical, coccobacillary, short or long rods, filamentous, comma- shaped, curved or spiral forms. 2.Size: Spherical are measured in diameter and rod-shaped in length and breadth. Measurement is expressed in microns (µ). 3.Sides: Straight, concave, bulging, parallel or irregular. 4.Ends: Round, pointed, truncate or concave. 5.Axis: Straight or curved. 6.Arrangement: Singly, in twos, in groups of four or eight, in grape-like bunches, in long chains, or scattered irregularly, may be in bundles, angular arrangements or in Chinese letters. 7.Pleomorphism: Bacteria may differ from their typical shape. Some of the older cultures may depart greatly from their typical forms, and may show spindles, clubs, and filaments, swollen and branching forms. Variation may be both in shape and size. 8.Capsule: May or may not be present. When present, size should be noted. This requires special staining.

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6 Cultural Characteristics of Bacteria

Cultural characteristics of bacteria deal with the appearance of the growth in various liquid and on solid media. The growth characteristics of most of the bacteria are typical of the group or species and help in their identification. The terminology employed to describe various features of bacterial growth are mentioned below.

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7 Yeasts and Moulds

Yeasts Yeasts are non filamentous, unicellular fungi that are typically spherical or oval. Like molds, yeasts are widely distributed in nature; they are frequently found as a white powdery coating on fruits and leaves.

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8 Pattern of Bacterial Growth

Growth of a microorganism implies an increase in amount of protoplasm, formation of new structures and eventually formation of new cells. Balanced growth means cell components increase in proportion to each other and unbalanced growth means cell components increase in a non constant relationship to each other Microbial growth and growth curve Phases of growth When bacteria are grown in the laboratory in liquid medium, the medium becomes cloudy or turbid. It is possible to measure the increase in numbers of bacteria at various time intervals and plot the values on a graph representing a growth curve. Lag Phase: When cells are first inoculated into fresh medium containing all the nutrients they require, they take a short time to adapt before they start to divide. This is termed the lag phase.

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9 Biochemical Pathways of Energy Production

Metabolic Processes in Bacterial Cells All living cells are continually undergoing processes of replication, breakdown, synthesis and repair. The overall term used to describe these processes is metabolism and refers to all the different reactions that must occur in order for a cell to grow and reproduce. The pathways of metabolism are remarkably similar in all living cells. The differences that do occur usually reflect the availability of nutrients or the need for a cell to provide for a specific function. This may be secretion, storage, structural support, energy for movement, or reproduction. Our knowledge of many of the metabolic processes that occur in human cells has been derived from research on the major pathways that also occur in microbial cells. Catabolism is the process of breakdown of complex molecules, usually with the release of energy. Anabolism refers to the synthesis of new or replacement molecules. This is an energy-requiring process. A metabolic pathway is a series of reactions in the process of metabolism. The energy from the sun that is trapped and used to produce glucose is released when a molecule of glucose is broken down during metabolism. This complex process is called oxidation, or respiration, and consists of a number of interrelated biochemical pathways, including those of glycolysis, fermentation and aerobic respiration. Reactions that occur in the presence of oxygen are termed aerobic; those that occur in the absence of oxygen are anaerobic. Many of these reactions require the involvement of various cofactors as well as enzymes, and result in the production of energy which is captured by the synthesis of a small molecule called adenosine triphosphate (AT). ATP is termed an ‘energy-rich storage molecule’ because breakdown of ATP releases energy for use in other reactions in the cell.

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10 Bacterial Genetics

What are Genes? Genes are linear sequences of nucleotides of DNA (deoxyribonucleic acid). Each gene consists of a specific sequence of nucleotides and is responsible for determining a particular characteristic of cell. Nucleotides consist of nitrogen –containing compounds called bases, joined to the sugar, deoxyribose and to phosphate. The nucleotide bases are the purines, adenine and guanine and the pyrimidines, thymine and cytosine. A number of genes are joined together to form a structure called a chromosome. DNA consists of two strands of nucleotides twisted around each other into a double helical structure. The nucleotide bases are arranged in a specific sequence in each strand. The strand has a backbone of sugar and phosphate from which the bases protrude.

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11 Culture Methods for Cultivation of Bacteria

Culture methods employed depend on the purpose for which they are intended. In the laboratory the indications for culture are mainly to 1.Isolate bacteria in pure culture. 2.Demonstrate their properties 3.Obtain sufficient growth for preparation of antigens and for other tests. 4.Type isolates by methods such as bacteriophages and bacteriocin susceptibility. 5.Determine sensitivity to antibiotics 6.Estimate viable counts 7.Maintain stock solution

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12 Virus

A five-kingdom classification system fails to classify viruses because they are not considered to be living. They neither reproduce independently nor utilize energy General properties of virus Viruses differ fundamentally from other classes of microorganism in that: 1.They are ultramicroscopic disease-producing entities. 2.They are simple in structure, basically composed of nucleic acid wrapped up in a protein coat. 3.The genome nucleic acid is either DNA or RNA , either single stranded or double stranded, but never both 4.Viruses have no metabolic activity of their own and lack enzyme systems and other constituents fundamental for independent growth and multiplication. 5.They do not posses ribosomes, nor transfer RNA and enzymes required for synthesis of nucleic acids and proteins. 6.Viruses are incapable of growth and division.

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13 Source of Infection

When unwanted organism settle on objects, substances or pure culture is called contamination. Infection Invasion of the body of a man or an animal or a plant by disease producing organism. Infectious disease is a condition brought about by an infection of the body by an organisms which is responsible for the disease. Organism that are capable of bringing about infectious diseases are called pathogens while those organism without this capacity are said to be non pathogenic. Commensals Organisms on skin and mucous membrane which multiply without causing damage to the tissue. Subclinical infection / latent infection When organism enters the tissue and lead to immunity without the appearance of any definite clinical symptoms.

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14 Sterilization

Sterilization is defined as the process by which an article, surface or medium is freed of all living microorganisms either in the vegetative or spore state. Disinfection means the destruction or removal of all pathogenic organisms, or organisms capable of giving rise to infection. Chemical disinfectants which can be safely applied to skin or mucous membrane and are used to prevent infection by inhibiting the growth of bacteria are called antiseptics. Five main methods are used for sterilization. 1. Red heat (flaming) 2. Dry heat (hot air) 3. Moist heat 4. Filtration 5. Radiation. The killing effect of dry heat is due to protein denaturation, oxidative damage and toxic effect of elevated levels of electrolytes. The lethal effect of moist heat is due to the denaturation and coagulation of protein. In the case of spore, steam condenses on it, increasing its water content with ultimate hydrolysis and breakdown of the bacterial protein.

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15 Preservation of Cultures

Bacterial species vary greatly in the ability of their cultures to remain alive after the completion of growth. Some species such as Neisseria gonorrhoeae and Streptobacillus are poorly viable and their cultures usually die out within a few day, whether kept at 37°C at room temperature or at 4-5°C; thus, they must be sub cultivated every 2-4 days for maintenance in the laboratory. Other species are much hardier,. especially sporing species which may remain viable for many years. There are also many non - sporing species (enterobacteriaceae) whose cultures, under suitable conditions, commonly remain viable for many months and often for as much as 10-20 years.

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16 Food Preservation

Physical Methods of Food Preservation Conditions too stressful for microbial growth can damage cells. Food environments can be manipulated to inhibit or kill microbes. Physical manipulation of foods can inhibit microbial growth, kill cells, or mechanically remove them from the food. Dehydration, refrigeration, and freezing inhibit microbial growth. Microbes can be killed by heating or ultraviolet (UV) or ionizing radiation. Non thermal energy is also lethal. Membrane filtration removes microbes from liquid foods. This chapter discusses all these physical preservation methods Physical Dehydration Processes Drying When foods are being dried, hot air evaporates the water and carries it away. Both the drying temperature and the decreased water activity (aw) affect the microbes. During the initial period of drying, the temperature is still low and the relative humidity is high. The size and composition of the food particle determine the length of this phase. If this particle size is large and the material is dense, this phase can be long, allowing microbes to grow. During later phases of drying, the temperature is high but the relative humidity is low. This provides no opportunity for growth but is not very lethal either. The higher temperatures result in lower moisture content, and dry heat is less lethal than wet heat. If the drying lasts long enough (at least 30 min.), there are time-temperature combinations that kill microbial cells. With certain drying methods, the surface temperature may reach 100°C, while internal temperature remains lower. This kills surface microbes, but not those in the food’s interior.

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17 Laboratory Exercises

Microscope A microscope can be defined as an optical instrument consisting of a lens or a combination of lenses for getting magnified images of minute objects. A simple microscope is essentially an ordinary magnifying lens held on an adjustable stand. It can give magnification up to 20 times the size of the object. A compound microscope consists of two separate lens system, the objective and eye piece (ocular) mounted on a holder known as body tube and with adjustment for illuminating the objects. It gives much greater magnification (100 to 1500 times) when compared to a simple microscope.

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