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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 (a_w) 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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Ex.No.1. MICROBIAL SPOILAGE OF FRUITS

Fruits are living structures that continue to perform metabolic reactions and sustain physiological processes for a considerable length of time after harvest. The post-harvest infection of fruits by microorganisms may cause serious losses of fruits both in terms of quantity and quality. Initial infection which may be the direct result of mechanical or insect damage is followed by invasion of fruit tissues by secondary infection by common saprophytic organisms. . Many bacteria and fungi, for instance, are involved in decay after harvest. Typical fungi include Alternaria, Botrytis, Colletotrichum, Fusarium, Monilia, Penicillium, and Rhizopus. These fungi are generally weak pathogens, in that they usually infest only weak or damaged fruit. Molds are the most predominant spoilage organisms in fruits. Fungal rots in fruits like apple show brown or cream coloured areas.

Bacterial soft rot caused by Erwinia carotovora is wide spread in fruits that are not very acid. Fruits like straw berries, melons are in direct contact with moist soil and become infected from that source.

GENETIC ENGINEERING

Genetic engineering is a means of improving the food supply even before harvest or slaughter by improving yields, increasing disease resistance, and enhancing the nutritional qualities of various foods. Broadly speaking, genetic engineering refers to any deliberate alteration of an organism’s DNA. Genetic engineering has been practiced for thousands of years, ever since humans began selectively breeding plants and animals to create more nutritious, better tasting foods. In the past two decades, genetic engineering has become increasingly powerful as scientific advances have enabled the direct alteration of genetic material. Genetic engineering is alternately called as ‘ recombinant DNA technology’ or ‘gene cloning’. Genes have been cut and pasted from one species to another, yielding, for example, disease- resistant squash and rice, frost-resistant potatoes and strawberries, and tomatoes that ripen-and therefore spoil-more slowly.

Genetically modified organism (GMO) is a microbe or more complex plant or animal in which DNA has been altered for a particular purpose such as manufacturing proteins, making improvement to the organism, research into the nature of genes and biological process, correcting genetic defects.

Fungi cause diseases in plants and animals and in such diseases, the fungi actively grow and invade the body of their hosts. There is another means by which fungi can cause harm is growing on stored food material that we consume and produce toxic metabolites referred to as mycotoxins (means fungus poisons). Food is particularly susceptible to fungal infections at almost every stage of storage or production. Mycotoxin is toxic substance produced by a fungus. The term is restricted to toxins produced by fungi growing in human foods or animal feeds. The growth of the fungi in food grains leading to physical spoilage of grains and toxin production is a serious problem in developing countries. Their occurrence in food and feed has both economic and health implications, as the toxic effects in human beings, animals and other living systems were well established. The effects of poisoning by mycotoxin are referred to as mycotoxicoses.

Food poisoning is an acute illness resulting from the ingestion of injurious agents such as chemical, heavy metals, parasites, microbial toxin, wild species of mushroom in food. Food poisoning is commonly caused by certain bacteria or their toxins, which are the poisonous proteins that these bacteria produce. Food borne pathogens are potential global problem, more than 90 per cent of the cases of food poisoning are caused by bacteria, Staphylococcus aureus, Salmonella spp, Clostridium perfringens, Campylobacter jejuni, Listeria monocytogenes, Vibrio parahaemolyticus, Shigella dysenteriae, Bacillus cereus and enteropathogenic E. coli. These bacteria are commonly found in many raw foods. Generally a large number of food poisoning bacteria must be present to cause illness. In most of these food poisonings, the food serves as a vehicle of transmission. The role of food here is significant, since the product not only may permit the survival of the pathogens, but also may provide a suitable medium for the rapid multiplication of microbes and production of toxin. A more serious aspect of microbial contamination is the possibility of serious health hazards due to potential pathogenic organisms gaining entry into foods due to low standards of hygiene and sanitation.

Food poisoning may be due to food intoxication or food infection. Food intoxication and food infection differ in many characteristics (Table 17.1). Food intoxication refers to food borne illness caused by the ingestion of a microbial toxin preformed in the ingested food (Botulism). The most serious food poisoning caused by toxins is botulism, an often fatal disease that results from eating improperly canned foods containing the bacteria Clostridium botulinum. Classically, food intoxication is caused by the growth of Clostridium botulinum, Staphylococcus aureus, Bacillus cereus in food. Clostridium botulinum produces a neurotoxin, while Staphylococcus aureus and Bacillus cereus cultures produce enterotoxins in food.

16.1. CANNED FOODS-PRESERVATION

Canning is a method of preserving food from spoilage by storing it in containers that are hermetically sealed and then sterilized by heat. The process was invented after prolonged research by Nicolas Appert of France in 1809, in response to a call by his government for a means of preserving food for army and navy use. Appert’s method consisted of tightly sealing food inside a bottle or jar, heating it to a certain temperature, and maintaining the heat for a certain period, after which the container was kept sealed until use. It was 50 years before Louis Pasteur was able to explain why the food so treated did not spoil: the heat killed the microorganisms in the food, and the sealing kept other microorganisms from entering the jar. Samuel C. Prescott and William Underwood of the United States set canning on a scientific basis by describing specific time-temperature heating requirements for sterilizing canned foods.

Most vegetables, fruits, meat and dairy products, and processed foods are stored in tin cans, but soft drinks and many other beverages are now commonly stored in aluminum cans, which are lighter and do not rust. Aluminum cans are made by impact extrusion; the body of the can is punched out in one piece from a single aluminum sheet by a stamping die. This seamless piece, which has a rounded bottom, is then capped with a second piece as its lid. The tabs used in pop- top cans are also made of aluminum. Bimetal cans are made of aluminum bodies and steel lids.

Canneries are usually located close to the growing areas of the product to be packed, since it is desirable to can foods as quickly as possible after harvesting. The canning process itself consists of several stages: cleaning and further preparing the raw food material; blanching it; filling the containers, usually under a vacuum; closing and sealing the containers; sterilizing the canned products; and labeling and warehousing the finished goods. Cleaning usually involves passing the raw food through tanks of water or under high- pressure water sprays, after which vegetable or other products are cut, peeled, cored, sliced, graded, soaked, pureed, and so on. Almost all vegetables and some fruits require blanching by immersion in hot water or steam; this process softens the vegetable tissues and makes them pliable enough to be packed tightly, while also serving to inactivate enzymes that can cause undesirable changes in the food before canning. Blanching also serves as an additional or final cleansing operation.

15.1 CONTAMINATION

Milk is sterile at secretion in the udder, but contaminated by bacteria when it leaves the udder of a healthy cow. Milk is subject to contamination from the animal and adjacent areas. Bacteria found in manure, soil and water may enter milk from these sources. Certain diseases may be transmitted through milk, from a cow. It is essential that cows producing milk should be free from any contagious or infectious disease. Bovine tuberculosis comes from consumption of milk of tuberculous cows. Tuberculosis, brucellosis, mastitis and actinomycosis are the human disease transmitted through milk. The disease producing bacteria may find their way into milk and cause infectious diseases in children and adults.

COMMON BACTERIA IN MILK: The common bacteria found in milk are: Rickettsia, Coxiella burnetii, lactic acid bacteria, Lactobacillus bulgaricus, Streptococcus thermophilus, aerobic rods, Bacillus subtilis, Bacillus cereus, anaerobic bacteria, Clostridium butyricum .

Sources of Microbial contamination: Milk contains relatively few bacteria when it leaves the udder of a healthy cow. Milk is subject to contamination from the animal udder, skin and adjacent areas barn yard, milk utensils, milkers. Bacteria found in manure, soil and water may enter milk from these sources.

14.1. EGGS

India is the third largest producer of eggs. Fresh eggs are gathered on automatic collection belts at the farm and stored in a cooler at about 7°C. The eggs are then delivered to a central processing plant, where they are washed, sanitized, and graded. Grading involves the sorting of eggs into size and quality categories using automated machines. Flash candling (passing the eggs over a strong light source) detects any abnormalities such as cracked eggs and eggs containing bloodspots or other defects. Higher-grade eggs have a thick, upstanding white, an oval yolk, and a clean, smooth, unbroken shell.

14.1.1. Eggs Contamination

More than 90 percent of all eggs are free of contamination at the time they are laid; contamination with Salmonella bacteria and with certain spoilage organisms occurs essentially afterward. They become contaminated by faecal matter from the hen, by the lining of nest, by handling, by wash water and packing materials. The total number of microorganisms per shell of a hen’s egg was found to range from 100 to 10,000,000 with a mean of about 100,000. Proper washing and sanitizing of eggs eliminates most Salmonella and spoilage organisms deposited on the shell. The organism Salmonella enteritidis, a common cause of gastroenteritis (a form of food poisoning), has been found to be transferred through the hen ovary in fewer than 1 percent of all eggs produced. Ovarian- transferred S. enteritidis can be controlled by thorough cooking of eggs (i.e., until there are no runny whites or yolk).

13.1. CONTAMINATION

MICROFLORA: The flora of living fish depends upon the microbial contents of the waters in which they live. The slime in the outer surface of the fish contain bacteria of the genera: Pseudomonas, Alcaligenes, Micrococcus, Flavobacterium, Corynebacetrium, Bacillus, Sarcina, Serratia and Vibrio. The same species of bacteria are found in shrimps, crabs, lobsters and similar sea foods. Oysters and Shell fish that pass large amount of water through their bodies, pick-up soil and water microorganisms in this way including human pathogens. Common spoilage microorganisms of fish include species of Pseudomonas, Moraxella, and Acinetobacter, found mainly in marine fish, and Bacillus and Micrococcus, found in freshwater fish. Fish may also contain pathogenic (disease-causing) microorganisms such as Salmonella and Escherichia coli. Pathogenic contamination is of special concern with mollusks because they are often eaten raw and as whole animals.

SOURCES: Boats, boxes, bins, fish houses and fishermen are the sources of contamination of these bacteria. The bacterial population ranges from few hundred to few million per square centimeter. Because of their soft tissues and aquatic environment, fish are extremely susceptible to microbial contamination. At the time of harvest, fish carry a high microbial load on the surface of their skin, in their intestinal tract, and in their gills. The type and number of microorganisms that live in fish vary according to the season, the species, and the natural habitat. Additional contamination may occur during the harvesting, handling, or processing of the fish. Harvested fish must be immediately stored in a low-temperature environment such as ice or refrigerated seawater. This chilling process slows the growth of microorganisms that live in fish and inhibits the activity of enzymes. Because fish have a lower body temperature, softer texture, and less connective tissue than land animals, they are much more susceptible to microbial contamination and structural degradation. If immediate chilling is not possible, then the fish must generally be sold and eaten on the day of the harvest.

CONTAMINATION

The healthy inner flesh of meat contains few or no microorganisms, although they have been found in lymph nodes, bone marrow and intestine. Staphylococcus, Streptococcus, Clostridium and Salmonella have been isolated from lymph nodes of red meat animals. During slaughtering the lymph nodes are removed from edible parts. The important contamination, however comes from external sources during bleeding, handling and processing. During bleeding, skinning and cutting the main sources of microorganisms are from the exterior of the animal- hide, hoof and hair and the intestinal tract. The exterior of the animal harbours large number and many kinds of microorganisms from soil, water, feed and manure as well as its natural surface flora.

Knives, cloths, air and hands and clothing of workers can serve as intermediate sources of contamination. During handling and processing the contamination can come from containers, air, other contaminated meat, equipments and personnel.

Molds that grow on the surface of the meat include the species of Cladosporium, Sporotrichum, Geotrichum, Mucor, Monilia, Penicillium and Alternaria. The important bacteria found in meat are Staphylococcus, Sarcina, Streptococcus, Clostridium, Leuconostoc, Lactobacillus, Proteus, Flavobacterium, Bacillus, Streptomyces, Escherichia and Salmonella.

11.1. FRUITS-Maturation, Ripening and Storage

Fruits are natural foods containing substantial quantities of essential nutrients and the best sources of minerals like sodium, potassium and magnesium, vitamins (antioxidants), such as vitamin A, C and enzymes. They are rich in fructose and water. Fruits are living biological entities that perform a number of metabolic functions. Two functions of particular importance in fruit processing are respiration (the breaking down of carbohydrates, giving off carbon dioxide and heat) and transpiration (the giving off of moisture). Senescence is the period when chemical synthesizing pathways give way to degradative processes, leading to aging and death of tissue. Fruit ripening is thus the result of many complex changes, some interactive but many independent of one another. Once the fruit is harvested, respiration and transpiration continue, but only for as long as the fruit can draw on its own food reserves and moisture. It is this limited ability to continue vital metabolic functions that defines fruit as perishable.

10.1 CEREALS AND CEREAL PRODUCTS

CONTAMINATION: The sources of contamination in cereals and cereal products are soil, insects, air, etc. Most of the microorganisms are removed with the outer portions of the grain during milling. The bleaching agents used during milling also reduce numbers of microorganisms. The bacteria in wheat flour include Bacillus, Micrococcus, Flavobacterium. Mold spores are chiefly of Aspergillus spp and Penicillium spp and in addition Alternaria, Cladosporium and other genera are found. The numbers of microbes in cereals and cereal products vary widely from few hundred per gram to million. Corn meal contains several hundred to several thousand bacteria and mold per gram.

Microbial contamination sources: Foods may be contaminated by microorganisms at any time during harvest, storage, processing, distribution, handling, or preparation. The primary sources of microbial contamination are soil, air, animal feed, animal hides and intestines, plant surfaces, sewage, and food processing machinery or utensils. Since most foods either carry or eventually acquire bacteria, molds, or yeasts, microorganisms are the major causes of food spoilage. The increased load of microorganisms leads to rapid spoilage.

Organisms: Bacteria and fungi (yeasts and molds) are the principal types of microorganisms that cause food spoilage and food- borne illnesses. The typical microorganisms that cause food spoilage are bacteria (e.g., Pseudomonas), yeasts (e.g., Candida), and molds (e.g., Rhizopus). The growth of microorganisms in the food will increase the load of microorganisms, and the heavier load will add to the difficulty of preventing spoilage and may influence the kind of spoilage to be anticipated.

Food spoilage may be defined as any change that renders food unfit for human consumption. These changes may be caused by various factors, including contamination by microorganisms, infestation by insects, or degradation by endogenous enzymes (those present naturally in the food). In addition, physical and chemical changes, such as the tearing of plant or animal tissues or the oxidation of certain constituents of food, may promote food spoilage. Foods obtained from plant or animal sources begin to spoil soon after harvest or slaughter. The enzymes contained in the cells of plant and animal tissues may be released as a result of any mechanical damage inflicted during postharvest handling. These enzymes begin to break down the cellular material. The chemical reactions catalyzed by the enzymes result in the degradation of food quality, such as the development of off- flavours, the deterioration of texture, and the loss of nutrients. The enzymatic changes in foods are desirable to some extent as in the case of ripening of fruits, but continued enzymatic activity beyond an optimum point becomes food deterioration due to rotting. The native enzymes may be inactivated by heat, radiation or by the use of specific chemicals.

Several attempts have been made to detect the biopotentials of microorganisms with reference to their ability to produce various enzymes with a view to use them in industrial processes and bioremediation applications. Enzymes are increasingly used as a new route to synthesize food ingredients such as flavours, emulsifiers, sweeteners, food additives, nutrients. With the advent of genetic engineering, a wide range of enzymes has become available on a large scale and this has increased the scope of enzyme technology.

MICROBIAL ENZYMES: Enzymes are proteinaceous complex organic catalysts of biological origin and vital constituents of cells. Microbes produce a variety of enzymes, intracellular and extracellular enzymes. Most of the microbial enzymes produced for commercial usage are extra cellular enzymes. Extracellular enzymes are secreted outside the cell and hydrolyze polymeric substances. Amylases and proteases are enzymes of economic importance. At present over 60 enzymes are in commercial use, of which three-quarters are hydrolytic enzymes. Commercial production of microbial enzymes utilizes a variety of fungi, yeasts and bacteria. The composition of the medium, genetic stability of the microbial strain and physical and chemical factors of fermentation influence the production of a particular enzyme.

Enzymes catalyze a variety of reactions during processing and preservation of food products and form a large group of processing aids. Food industry utilizes enzymes in the production of bakery items, dairy products, fruits and vegetables juices and products, tenderization of meat, modification of oils and fats, flavour production and preservation of food products. Enzymes have also found use in the manufacture of ingredients like glucose, high fructose syrup, citric acid, colour and flavour constituents.

Mushrooms are most ancient, best known and greatly admired fleshy macrofungi. Mushrooms are delicacies, much sought after and greatly valued foods. Wild mushrooms are mostly found in forests, tree trunks, pastures, lawns and road side during winter season and appear at surprising speed. Mushroom hunting or gathering is a popular hobby in Western countries. The wild mushrooms may be edible or unpalatable or poisonous. Poisoning by wild mushrooms is common and may be fatal or produce merely mild gastrointestinal disturbance or slight allergic reaction. The term toad stool is popularly reserved for inedible or poisonous mushrooms, but this classification has no scientific basis. This alarming term keeps many people away from these foods. It is important that every mushroom intended for eating be accurately identified. Mushrooms vary in colour, size, shape, growth habit, yielding ability and other characteristics.

STRUCTURE: Mushrooms are the fleshy fungi which constitute a major group of lower plant kingdom. Mushroom is the conspicuous umbrella-shaped fruiting body (sporophore) of certain fungi of the order Agaricales in the class Basidiomycetes but also of some other groups. Popularly, the term mushroom is used to identify the edible sporophores. The edible reproductive part consists of a stem and a cap. Gills found on the underside of the cap, are arranged like spokes in a wheel. The gills bear multiple spores which start new fungi. The sporophore emerges from an extensive underground network of fine thread like white growth known as mycelium. The mushroom, fruit body produced basidiospores at the tip of club like structures, called basidia, which are arranged along the gills of the mushroom. Beneath the mushroom, in the soil, is the mold colony itself, consisting of a mat of intercoiled hyphae, sometimes several feet in diameter. The mushrooms first appear as white tiny ball consisting of short stem (stipe) and a cap (pileus) which begins to open up like an umbrella. The delicate membrane or Veil (velum) enveloping the cap tears off, if allowed to develop fully, and lamellae (gills) radiating from the stalk into the cap become visible. These gills become darkened as the basidiospores (seeds) develop into millions and fall in the ground for starting their life cycle once again for second generation of mushrooms.

SINGLE CELL PROTEIN (SCP) AND PROTEIN DEFICIENCY

Protein is an essential component of diet. The greatest single problem in the world today is global food protein shortage. Having adequate quantity of food is only one side of the picture, but the most important aspect is the supply of balanced and nutritious food. All efforts are being made to increase the food production through modern methods of farming. But very little attention is being paid towards protein requirements and production. Approximately half of world population suffers from the protein deficiency disease called ‘Kwashiorkor’ and children with poor protein are victims of this disease.

Usually, cereals have low protein values while protein rich pulses are not available in desired quantities due to poor yields and high costs. The good quality protein available in fish, meat, poultry, and eggs is not available due to poor economic condition as well as varied dietary habits. With the current system of production, agriculture cannot be relied upon to feed an ever increasing world population. Hence there is an urgent need to find other protein sources. The best potential is seen in microbial protein or single cell protein (SCP), a new source of protein independent of agriculture.

DEFINITION: The dried cells of microorganisms such as bacteria, fungi, yeasts, algae that are grown in large scale culture systems as proteins for human or animal consumption are collectively known as "single cell proteins".

Milk is nutritious and essential food for human beings and also serve as good medium for microbial growth and contamination. Milk is a single, most nearly complete, natural food. Primarily it is meant for nourishing the newborns but man has exploited it for human consumption since long. Today milk has become indispensable integral component of our diet. About half the milk produced is consumed as fresh milk, the rest going into a variety of products including evaporated and condensed milk, dried milk powder, butter, cheese, malted milk, ice cream, yoghurt, lactose and casein. Milk is very important from a nutritional standpoint since it contained all the essential food constituents (carbohydrate-lactose, protein-casein, fat-butter, minerals- calcium, potassium and vitamins- A, B, C, D, E, and K) required in human diet in about the proper proportions. India is the second largest producer of milk in the world. India has exhibited unprecedented rate of growth in dairy development in last 30 years mainly due to sustained efforts of dairy technologists in Operation flood Programme introduced by the National Dairy Development Board. Dairy microbiology deals with aspects like shelf life and spoilage microbes, public health significance and food borne pathogens, and production of fermented dairy products.

Fermentation plays a major role in our diet and give the basic foods like bread, yoghurt, wine and cereal and legume based traditional fermented foods. The development of fermented food products can be considered as one of the greatest achievements of human civilization and goes back thousands of years. It arose from the necessity to conserve foods, to make them more digestible and also more enjoyable. During the centuries, man learned by trial and error to conduct the fermentation processes by changing physical and chemical parameters in the basic food ingredients. It took much longer, however, to discover the cause of fermentation processes: only towards the end of the 19th century it was demonstrated, that baker’s yeast is the cause of alcoholic fermentation. Through the development of the technique of "pure microbial culture", the basis for a controlled use of microbes in food industry was established.

The unique feature of fermented foods is the vital role of microorganisms that bring about essential bio-trasformations of the substrates during fermentation contributing number of desirable properties such as improved product shelf life, enriched diet with improved flavor and texture, increased safety, enriched nutritional supplement and probiotic functions. Fermentation is a chemical reaction carried out by many types of microorganisms to obtain energy. Fermentation is a process for production of a product by means of mass culture of microorganisms. In fermentation, microorganisms breakdown complex organic compounds into simpler substances. Although chemical changes and microbial growth usually mean food spoilage, in some cases fermentation is desirable and microorganisms are actually added to foods. For example, in the production of beer, wine, and other alcoholic beverages, yeasts convert sugar into ethyl alcohol and carbon dioxide. In the making of yogurt and cheese, lactic acid bacteria convert lactose, a sugar found in milk, to lactic acid. Alcohol, acids, and other compounds produced in fermentation act as preservatives, inhibiting further microbial growth. In addition to its use with alcoholic beverages, cheese, and yoghurt, fermentation is used to produce yeast bread, soy sauce, cucumber pickles, sauerkraut, and other traditional fermented food products. The variation in agroclimatic conditions prevailing in India and diverse form of dietary culture of various ethnical groups produced traditional fermented foods such as idli, dosa (rice-legume based), vada, papad, wari, balle (legume based), bhatura, jelabi, nan (wheat based), paneer, shrikhand (milk based).

India is emerging as a food power in the international trade of foods. The immense un-tapped potential in the food processing sector in India has to be still exploited. It has been termed as ‘sun rise industry’ for improving agricultural productivity and enhancing food availability. The terms food quality management and food safety have assumed large dimensions today. In the international trade of foods, specifications have been laid down on the levels of harmful substances (chemical or biological origin), protocols have been documented on their detection and quantification in foods. We have to meet these international standards to export processed food and earn foreign exchange. The future of Indian food processing sector is bright provided it improves the quality of its products as per international standards.

Agricultural food production is increased by adopting scientific farming methods and another way to increase the availability of food is preventing the food spoilage by adopting food preservation methods. The global agricultural and fish production accounts for 50% of perishable food, which requires preservation. Only half of the perishable foods are properly protected. Application of innovative technologies to promote the production of food is indispensable to meet the demand of emerging populations. It is equally important to protect the stored food from the invasion of pests and diseases. A food saved is a food produced.

Food Processing and Preservation, branch of manufacturing that transforms raw animal, vegetable, or marine materials into tasty, nutritious, and safe food products. The industry has its roots in ancient times, as humans have always needed to obtain food and store a portion for later use. Prehistoric humans may have dried fruits in the sun and stored meat in cold areas, such as ice caves. The modern food processing and preservation industry was born in 1809, when French chef and inventor Nicolas Appert, searching for a better way to provide food for Napoleon’s army, devised a method for sterilizing food in tightly sealed glass bottles.

Today a wide variety of methods are available to maintain and enhance the appearance and taste of food. Food processing and preservation methods also create products that are convenient for consumers, such as products that are ready to eat or require minimal preparation and cooking. Combining these methods with modern distribution networks makes seasonal crops available year-round in grocery stores all over the world.

1.1. IMPORTANCE OF FOOD MICROBIOLOGY

Food may be defined as substances that provide nutrients, which when eaten and absorbed by the body, maintain life and growth. Food is basically energy stored in plant and animal foods in chemical form. On consumption, the stored form gets converted to physiological energy. Food is the prime necessity of life. Man has exhibited much thought and foresight in cultivating a variety of grains, fruits, vegetables, nuts and oilseeds and in rearing birds and animals for use as food.

Of the three primary needs of an individual, food, clothing and shelter, ‘food’ occupies the most important place. The biodiversity of foods- cereals, millets, pulses, oilseeds, spices, fruits and vegetables is varied and high in India, where we have varied agro-climatic regions from Kashmir to Kanyakumari. All the fifteen major climates in the world exist in India. India is the second largest producer of food in the world. The impact of green revolution is remarkable in India with many fold increased food production in the last five decades. The full potential of Indian agriculture can not be realized unless India emerges as a major player in the World agriculture.

World population is set to go beyond nine billion by 2050 and demand for food is likely to increase further because of growing affluence and globalization. Food security is achieved when all people have physical and economic access to sufficient, safe and nutritious value added food. India is emerging as a food power in the international trade of foods. The food processing industry in India has been identified as one of thrust areas for the development of the country. It has emerged as a highly perspective sector with immense potential in food processing. The future of Indian food processing industries is bright, provided it improves its quality of products as per international standards.The quality of food processing refers to the adherence of physical, chemical, biological, nutritional and sensory parameters required to achieve the wholesomeness of a food product. Quality control has become an important aspect for food processing industry for both domestic and export markets.

1.    UV irradiation is considered better than other irradiation methods in terms of

(a) Physical structure maintenance

(b) Chemical structure

(c) Sensory quality

(d) Microbial quality

2.    Irradiation with UV light is considered as

(a) Sterilization method

(b) Disinfection method

(c) Pasteurization method

(d) Cooking method

3.    UV treatment kills bacteria, viruses, fungi due to its effect on

(a) Metabolic pathway

(b) DNA

(c) Signaling pathway

(d) ATP generation

4.    Effectiveness of UV radiation on micro organisms depends on

(a) Species and strain

(b) Spore formation

(c) Culture and growth

(d) Both (a) and (c)

1.    Major difference between unleavened and leavened bakery products is

(a) Unleavened do not form cracks on surface

(b) Unleavened do not rise significantly during baking

(c) Leavened bakery products are not attractive

(d) Unleavened products are much more stable

2.    Unleavened bakery products contain small holes throughout the finished product because of

(a) Ingredients contain fats that may swell

(b) Air incorporation between cells

(c) Incomplete coalescence of mixed ingredients

(d) High temperature baking

1.     A fruit juice at 20°C with 10% total solids is concentrated in a single effect evaporator that is operated at a vacuum to allow the product moisture to evaporate at 80°C and steam with 82% quality is supplied at 169.06kPa. The desired concentration of the final product is 45% total solids and the concentrated product exits the evaporator at a rate of 300kg/hr. Calculate the steam requirement for the process when condensate is released at 90°C. The specific heat of liquid feed is 4.05 kJ/kg°C and of concentrated product is 3.175 kJ/kg°C.

(a) 3.35 kg/s

(b) 3.25 kg/s

(c) 4.25kg/s

(d) 2.35kg/s

2.     A double effect evaporator is being used to concentrate fruit juice at 35000kg/hr. The juice has 10% total solids at 80°C and requires to be concentrated to 50% total solids. The saturated steam at 1.608 atm is available. What is mass flow rate of water vapors?

(a) 7.505 kg/s

(b) 8.505 kg/s

(c) 5.707kg/s

(d) 6.808.kg/s

1.     Which of the following micro-organism is the most heat-resistant in low acid canned foods and its destruction is considered the criteria for successful heat processing?

(a) B.subtilis

(b) C.botulinum

(c) C.sporogenes

(d) B.mycoides

2.    D value represents

(a) Doubling time

(b) Generation time

(c) Decimal reduction time

(d) Decimal reduction temperature

1. Size reduction is also referred as

(a) Extraction

(b) Communication

(c) Leaching

(d) Distillation

2. Size reduction causes

(a) Decrease in the temperature of the product

(b) Increase in temperature of the product

(c) Increase in pressure

(d) Decrease in surface area

1.    Diffusivity of gases at atmospheric pressure is

(a) < 1 cm²/s

(b) >1 cm²/s

(c) > 10 cm²/s

(d) > 100 cm²/s

2.    Diffusivity of liquids is of the order of

(a) 0.1 cm²/s

(b) 10 cm²/s

(c) 100 cm²/s (d) 1 x 10^-5 cm²/s

1.    Heat transfer by conduction is given by

(a) Boyle’s law

(b) Stefan’s law

(c) Fourier’s law

(d) Charles’ law

2.    Heat flow per hour is proportional to change in

(a) Area

(b) Vapour pressure

(c) Mass

(d) Temperature

1.    Irrotational flow is

(a) Incompressible fluid with shear

(b) Newtonian fluid

(c) Incompressible fluid with no shear

(d) Non-Newtonian fluid

2.    Irrotational flow is also called

(a) Potential flow

(b) Turbulent flow

(c) Laminar Flow

(d) None of these

3.    Bernoulli’s equation is based on the assumption that

(a) The flow is non-viscous, uniform and steady

(b) The flow is steady, non-viscous, incompressible and irrotational

(c) The flow is uniform and incompressible

(d) None of the above

1.    Mass transfer co-efficient is expressed as

(a) m/s₂

(b) m/s

(c) kg/s

(d) kg/ms

2.    Heat is related to

(a) Flow rate

(b) Energy

(c) Temperature

(d) Momemtum

3.    The units of mass transfer coefficients are

(a) Moles transferred / (time x area x concentration)

(b) Moles transferred / (time x area x mole-fraction)

(c) Moles transferred / (time x area x pressure)

(d) All of the above

1.    HACCP was developed by

(a) Pillsbury and NASA

(b) FDA (Food and Drug Administration)

(c) USDA (United States Department of Agriculture)

(d) FSIS (Food Service Inspection Service)

2.    Which is not one of the hazards in HACCP?

(a) Biological

(b) Physical

(c) Bodily

(d) Chemical

3.    Which is not a physical hazard?

(a) Staples

(b) Fingernails

(c) Bandages

(d) Parasites

1.    Pasteurization can be used to destroy

(a) Soamtic cells

(b) Bacteria

(c) Antibiotics

(d) All of these

2.    The higher amount of total solids in milk will result in

(a) Increased heat stability

(b) Decreased heat stability

(c) Both (a) and (b)

(d) Decreased or increased heat stability depending upon temperature

1.    Almost all liquid foods are concentrated before drying due to

(a) Technical advantages

(b) Preservation

(c) Nutritional advantages

(d) Both (a) and (c)

2.    Which of the following compound mainly possesses anti-microbial property in garlic and onion?

(a) Acrolein

(b) Eugenol

(c) Allyl isothiocynate

(d) Terpenoid

3.    Which of the following fruit juices concentrates are prone towards spoilage?

(a) Acid

(b) Non-acid

(c) Both (a) and (b)

(d) None of these

1.    The outer layer of cereals is called

(a) Pericarp

(b) Germ

(c) Seed coat

(d) Endosperm

2.    The grain or edible seed of a cereal is known botanically as a

(a) Testa

(b) Caryopsis

(c) Seed coat

(d) Endocarp

1.    Which of the following plastic film is not utilized for metalized flexible packaging?

(a) Polyester

(b) Low-density polyethylene

(c) Bakelite

(d) PVDC

2.    Tin-coating applied to steel base is carried out by

(a) Hot- dipping method

(b) Electrolytic deposition

(c) Cold-dipping method

(d) None of these