Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 1

UNIT-I FOOD MICROBIOLOGY

DEFINATION OF FOOD

Food is defined as any substance or materials that are consumed to provide nutritional support for the body or for pleasure. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals. It is ingested and assimilated by an organism to produce energy, stimulate growth, and maintain life.

Almost all foods are of plant or animal origin. Cereal grain is a staple food that provides more food energy worldwide than any other type of crop. Maize, wheat, and rice together account for 87% of all grain production worldwide.

Animals are used as food either directly or indirectly by the products they produce. Meat is an example of a direct product taken from an animal, which comes from either muscle systems or from organs. Food products produced by animals include milk produced by mammary glands, which in many cultures is drunk or processed into dairy products such as cheese or butter. In addition, birds and other animals lay eggs, which are often eaten, and bees produce honey, a reduced nectar from flowers, which is a popular sweetener in many cultures.

Other foods not from animal or plant sources include various edible fungi, especially mushrooms. Fungi and ambient bacteria are used in the preparation of fermented and pickled foods such as leavened bread, alcoholic drinks, cheese, pickles, kombucha, and yogurt. Another example is blue-green algae such as Spirulina. Inorganic substances such as baking soda and cream of tartar are also used to chemically alter an ingredient.

COMPOSITION OF FOOD

Food composition is a term that is used to describe an analysis of the vitamins, minerals, and other nutritive substances in a given food. In addition to vitamins and minerals, many food composition reports include information on and analysis of phytonutrients and macronutrients within a food. Food composition information can be used for a number of purposes. It can be used to make sure that a certain person, family, or population is getting enough of one kind of nutrient, vitamin, or mineral. It can also be used to define certain foods that may or may not have allergens. An abbreviated version of a food composition report appears on the labels of most kinds of packaged food and is intended for the same sorts of uses described above.

There are a number of organizations including the United States Department of Agriculture (USDA) that compile and disseminate information on food composition. The food composition information that is available from an organization like the USDA is usually much more detailed than the information that is printed on packaged foods. The full-length version of a food composition report for creamed corn baby food will list many — in some cases, dozens of — vitamins, minerals, lipids, and amino acids. This information can be used to find out, for example, how much lysine is present in one cup of this particular kind of baby food.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 2

SIGNIFICANCE OF MICROORGANISMS IN FOOD

Three types of microorganisms occur in foods. They may be beneficial, pathogenic, or cause spoilage. Beneficial microorganisms include those that may produce new foods or food ingredients through fermentation(s) (e.g., yeasts and lactic acid bacteria) and probiotics. Spoilage microorganisms, through their growth and ultimately enzymatic action, alter the taste of foods through flavor, texture, or color degradation. Pathogenic microorganisms can cause human illness. Two types of pathogenic microorganisms that grow in or are carried by foods are those that cause: (1) intoxication and (2) infection. Intoxication results from microorganisms growing and producing toxin (which causes the illness) in a food. An infection is an illness that results from ingestion of a disease-causing microorganism. Infectious microorganisms may cause illness by the production of enterotoxins in the gastrointestinal tract or adhesion to and/or invasion of the tissues.

A major challenge for the sanitarian is to protect the production area and other involved locations against microbes that can reduce the wholesomeness of food. Microorganisms can contaminate and affect food, with dangerous consequences to consumers. The microorganisms most common to food are bacteria and fungi. The fungi, which are less common than bacteria, consist of two major microorganisms: molds (which are multicellular) and yeasts (which are usually unicellular). Bacteria, which usually grow at the expense of fungi, are unicellular. Viruses, although transmitted more from person to person than via food, should also be mentioned because they may contaminate food as a consequence of poor worker hygiene.

Molds Molds are multicellular microorganisms (eukaryotic cells) with mycelial (filamentous)

morphology. They consist of tubular cells, ranging from 30 to 100 μm in diameter, called hyphae, which form a macroscopic mass called a mycelium. Molds generally withstand greater variations in pH than do bacteria and yeasts and can frequently tolerate greater temperature variations. Although molds thrive best at or near a pH of 7.0, a range from 2.0 to 8.0 can be tolerated, though an acid-to-neutral pH is preferred.Molds have been considered beneficial and troublesome, ubiquitous microorganisms. They often work in combination with yeasts and bacteria to produce numerous indigenous fermented foods and are involved in industrial processes to produce organic acids and enzymes. Molds are a major contributor to food product recalls. Most do not cause health hazards, but some produce mycotoxins that are toxic, carcinogenic, mutagenic, or teratogenic (Able to disturb the growth and development of an embryo or fetus) to humans and animals. Because molds are difficult to control, food processors have encountered spoilage problems cauesd by these microorganisms.

Yeasts Yeasts are generally unicellular. They differ from bacteria in their larger cell sizes and

morphology, and because they produce buds during the process of reproduction by fission. The generation time of yeasts is 2 to 3 hours in foods, leading from an original contamination of one yeast/g of food to spoilage in approximately 40 to 60 hours. Like molds, yeasts can be spread through the air or by other means and can alight on the surface of foodstuffs. Yeast colonies are generally moist or slimy in appearance and creamy white. These microorganisms grow best in the intermediate acid range, a pH of from 4.0 to 4.5. Yeasts are more likely to grow on foods with lower pH and on those that are vacuum packaged. Food that is highly contaminated with yeasts will frequently have a slightly fruity odor.

Bacteria Bacteria produce pigments ranging from variations of yellow to dark shades, such as brown or

black. Certain bacteria have pigmentation of intermediate colors—red, pink, orange, blue, green, or purple. These bacteria cause food discoloration, especially among foods with unstable color pigments, such as meat. Some bacteria also cause discoloration by slime formation. Some species of bacteria produce spores, which may be resistant to heat, chemicals, and other environmental conditions. Some of these spore-forming bacteria are thermophilic microorganisms that produce a

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 3

toxin that can cause foodborne illness. Viruses Viruses cannot reproduce outside of another organism and are obligate parasites of all living

organisms, such as bacteria, fungi, algae, protozoa, higher plants, and invertebrate and vertebrate animals. Foodborne viruses cause diseases through viral gastroenteritis or viral hepatitis. A virus that has caused a major increase in outbreaks in restaurants during the past 10 years is hepatitis A. Intravenous drug use is one factor that accounts for some of this rise. Infectious hepatitis A can be transmitted through food that has not been handled in a sanitary manner. The onset is 1 to 7 weeks with an average length of 30 days. Symptoms include nausea, cramps, vomiting, diarrhea, and, sometimes, jaundice, which can last from a week to several months. A major source of hepatitis is raw meat from polluted waters.

SOURCES OF CONTAMINATION

Most foods are highly perishable because they contain nutrients required for microbial growth. To reduce food spoilage and to eliminate foodborne illness, microbial proliferation must be controlled. Food deterioration should be minimized to prolong the time during which an acceptable level of flavor and wholesomeness can be maintained. If proper sanitation practices are not followed during food processing, preparation, and serving, the rate and extent of the deteriorative changes that lead to spoilage will increase.

Equipment Contamination of equipment occurs during production, as well as when the equipment is idle.

Even with hygienic design features, equipment can collect microorganisms and other debris from the air, as well as from employees and materials. Product contamination of equipment is reduced through improved hygienic design and more effective cleaning.

Employees Of all the viable means of exposing microorganisms to food, employees are the largest

contamination source. Employees who do not follow sanitary practices contaminate food that they touch, with spoilage and pathogenic microorganisms that they come in contact with through work and other parts of the environment. The hands, hair, nose, and mouth harbor microorganisms that can be transferred to the food during processing, packaging, preparation, and service by touching, breathing, coughing, or sneezing. Because the human body is warm, microorganisms proliferate rapidly, especially in the absence of hygienic practices. After the chain of infection is broken the spreading of bacteria from one location to another can be prevented. Generally, the mishandling of food by people perpetuates the chain of infection until someone becomes ill or dies before corrective actions are taken to prevent additional outbreaks. If every person that handles food could achieve appropriate personal hygiene, food contamination could be minimized. Every employee involved with food manufacturing can play a very important role in preventing food contamination.

Air, Water and Soil Water serves as a cleaning medium during the cleaning operation and is an ingredient added in

the formulation of various processed foods. It can also serve as a source of contamination. If excessive contamination exists, another water source should be obtained, or the existing source should be treated with chemicals (such as ultraviolet units) or other methods.

Airborne microorganisms cause contamination in food processing, packaging, storage, and preparation areas. This contamination can result from unclean air surrounding the food plant or from contamination through improper sanitary practices. The most effective methods of reducing air contamination are through sanitary practices, filtering of air entering the food processing and preparation areas, and protection from air by appropriate packaging techniques and materials.

Soil Normal flora gets contaminated by animal and human feces, sewage water and rain water which contain high population of pathogenic bacteria like coliforms, Clostridium, Salmonella and Vibrio. Soil also contains opportunistic plant pathogenic bacteria like Pseudomonas, Agrobacterium

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 4

and Xanthomonas. Pathogens like clostridia persist in soil for long periods as sporulated bacteria. Soil contaminant bacteria are desiccation resistant and actively compete for soil nutrients with native bacterial population. Soil pathogenic bacteria are causative agents of water and food borne diseases of humans.

Sewage Raw, untreated sewage can contain pathogens that have been eliminated from the human body,

as well as other materials of the environment. Examples are microorganisms causing typhoid and paratyphoid fevers, dysentery, and infectious hepatitis. Sewage may contaminate food and equipment through faulty plumbing. If raw sewage drains or flows into potable water lines, wells, rivers, lakes, and ocean bays, the water and living organisms such as seafood are contaminated. To prevent this contamination, privies and septic tanks should be sufficiently separated from wells, streams, and other bodies of water. Raw sewage should not be applied to fields where fruits and vegetables are grown.

Insects and Rodents Flies and cockroaches are associated with living quarters, eating establishments, and food

processing facilities, as well as with toilets, garbage, and other filth. These pests transfer filth from contaminated areas to food through their waste products; mouth, feet, and other body parts; while the regurgitation of filth onto clean food during consumption. To stop contamination from these pests, eradication is necessary, and food processing, preparation, and serving areas should be protected against their entry. Rats and mice transmit filth and disease through their feet, fur, and intestinal tract. Like flies and cockroaches, they transfer filth from garbage dumps and sewers to food or food processing and foodservice areas.

Animal Hides In the case of milk cows, the types of organisms found in raw milk can be a reflection of the

biota of the udder when proper procedures are not followed in milking and of the general environment of such animals. From both the udder and the hide, organisms can contaminate the general environment, milk containers, and the hands of handlers.

MICROBIAL EXAMINATIONS OF FOOD

The examination of foods for the presence, types, and numbers of microorganisms and/or their products is basic to food microbiology. In spite of the importance of this, none of the methods in common use permits the determination of exact numbers of microorganisms in a food product. Although some methods of analysis are better than others, every method has certain inherent limitations associated with its use. The four basic methods employed for "total" numbers are as follows:

1. Standard plate counts (SPC) for viable cells 2. The most probable numbers (MPN) method as a statistical determination of viable cells 3. Dye reduction techniques to estimate viable cells that possess reducing capacities 4. Direct microscopic counts (DMC) for both viable and nonviable cells. MEMBRANE FILTERS Membranes with a pore size that will retain bacteria (generally 0.45 um) but allow water or

diluent to pass are used. Following the collection of bacteria upon filtering a given volume, the membrane is placed on an agar plate or an absorbent pad saturated with the culture medium of choice and incubated appropriately. Following growth, the colonies are enumerated. Alternatively, a DMC can be made. In this case, the organisms collected on the membrane are viewed and counted microscopically following appropriate staining, washing, and treatment of the membrane to render it transparent. These methods are especially suited for samples that contain low numbers of bacteria. Although relatively large volumes of water can be passed through a membrane without clogging it, only small samples of dilute homogenates from certain foods can be used for a single membrane.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 5

DRY FILM METHOD A rehydratable dry film method consisting of two plastic films attached together on one side

and coated with culture media ingredients and a cold-water-soluble jelling agent was developed by Private Company and designated Petrifilm. The method can be used with nonselective ingredients to make aerobic plate counts (APCs), and, with selective ingredients, certain specific groups can be detected. Use of this method to date indicates that it is an acceptable alternative to SPC methods that employ Petri dishes, and it has been approved by AOAC.

MOST PROBABLE NUMBERS In this method, dilutions of food samples are prepared as for the SPC. Three serial aliquots or

dilutions are then planted into 9 or 15 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. The method is statistical in nature, and MPN results are generally higher than SPC results. This method was introduced by McCrady in 1915. It is not a precise method of analysis; the 95% confidence intervals for a three-tube test range from 21 to 395. When the three-tube test is used, 20 of the 62 possible test combinations account for 99% of all results. Among the advantages it offers are the following:

• It is relatively simple. • Results from one laboratory are more likely than SPC results to agree with those from another

laboratory. • Specific groups of organisms can be determined by use of appropriate selective and

differential media. • It is the method of choice for determining fecal coliform densities. Among the drawbacks to its

use are the large volume of glassware required (especially for the five-tube method), the lack of opportunity to observe the colonial morphology of the organisms, and its lack of precision.

DYE REDUCTION Two dyes are commonly employed in this procedure to estimate the number of viable

organisms in suitable products: methylene blue and resazurin.To conduct a dye-reduction test; properly prepared supernatants of foods are added to standard solutions of either dye for reduction from blue to white for methylene blue and from slate blue to pink or white for resazurin. The time for dye reduction to occur is inversely proportional to the number of organisms in the sample.

DIRECT MICROSCOPIC COUNT 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 industry for assessing the microbial quality of raw milk and other dairy products, and the specific method employed is that originally developed by R.S. Breed (Breed count). Briefly, the method consists of adding 0.01 mL of a sample to a 1-cm2 area on a microscope slide, and following fixing, defatting of sample, and staining, the organisms or clumps of organisms are enumerated. The latter involves the use of a calibrated microscope. The method lends itself to the rapid microbiological examination of other food products, such as dried and frozen foods. Among the advantages of DMC are that it is rapid and simple, cell morphology can be assessed, and it lends itself to fluorescent probes for improved efficiency. Among its disadvantages are that it is a microscopic method and therefore fatiguing to the analyst, both viable and nonviable cells are enumerated, food particles are not always distinguishable from microorganisms, microbial cells are not uniformly distributed relative to single cells and clumps, some cells do not take the stain well and may not be counted, and DMC counts are invariably higher than counts by SPC. In spite of its drawbacks, it remains the fastest way to make an assessment of microbial cells in a food product.

VIABLE BUT NONCULTURABLE ORGANISMS (VBNC) Under certain conditions and in some environments, standard plate count results suggest either

an absence of colony-forming units or numbers that may be considerably lower than the actual viable population. These counts are due to Viable But Not Cultivable microorganisms. It is believed that 99.9% of microorganisms existing are not cultivable by man made laboratory techniques.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 6

MICROBIAL SPOILAGE OF FOOD

Fruits, vegetables, meats, poultry, sea foods, milk and dairy products and various other food products differ in their biochemical composition and therefore are subject to spoilage by different microbial populations. Such changes depend upon the nature of the microbes involved in the spoilage. Thus degradation of apple juice by yeast gives an alcoholic taste to the juice. Yeasts convert the carbohydrate into ethanol.

Bacteria which attack food proteins, convert these into amino acids which are broken down again into foul smelling end products. Digestion of cystein, for example, yields hydrogen sulphide, giving a rotten egg smell to food. Digestion of tryptophan yields indole and skatole which give food a fecal odour. Two other products of the microbial metabolism of carbohydrates are (a) acid that causes foods to become sour, and (b) gas which causes sealed cans to swell. Digestion of fats, as in spoiled butter, yields fatty acids giving a rancid odour or taste to food. Food may become slimy due to production of capsules in bacteria. There may be pigment development giving some colour to foods.

The mold forms a toxin aflatoxin. Another grain spoilage occurs by Claviceps purpurea, in rye, wheat and barley grains causing ergot disease. The mold toxin may induce convulsions and hallucinations. The drug LSD is derived from this toxin.

The chemical properties of a food product influence the type of microorganisms that can grow, and hence determine the changes in appearance, flavour, odour, and other qualities of food.

Composition: Proteins are degraded by proteolytic organisms. Many bacterial species, especially spore formers, gram negative rods such as Pseudomonas and Proteus, and a few cocci can attack proteins. Mould spoilage is also common.

Carbohydrate foods are spoiled by carbohydrate fermenting. microorganisms, particularly by yeast and moulds. Bacterial species of the genera Streptococcus, Leuconostoc and Micrococcus are saccharolytic and can also attack carbohydrates.

Fats are digested by relatively few microorganisms, mainly moulds and few gram negative bacteria. Fats undergo hydrolytic decomposition and become rancid, all malodorous fatty acids are set free. Acidity: The reaction of nearly all foods is below pH 7.0. Foods are classified as acid or nonacid. The reaction of acid food is below pH 4.5, and that of nonacid food above pH 4.5. Most fruits are acid foods, while nearly all vegetables, fish, meats, and milk products are nonacid.

Acid foods have sufficiently low pH and, therefore, prevent the growth almost bacterial species. They are spoiled mainly by yeast and moulds. Nonacid foods are particularly subject to bacterial spoilage, but will also support growth of moulds under proper conditions.

Moisture and osmotic pressure: Growth of microorganisms require at least 13 per cent free water in foods, Moulds require the least free water and bacteria require the most. Foods of high sugar and salt concentration do not, support the growth of most microorganism). Bacteria are generally inhibited (by 5 to 11 percent salt, whereas many moulds and some yeasts can tolerate salt concentrations greater than 15 percent.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 7

Effects of Storage Conditions on Spoilage - Oxygen and temperature are the two most important factors that influence the type of microbial growth and spoilage of food during storage.

Oxygen: The presence or absence of oxygen determines the types of organisms tbat can multiply and the kind of spoilage produced. Moulds and aerobic bacteria: grow only where there is plenty of air and chiefly cause surface spoilage. Yeast and facultative bacteria can grow in closed containers as well as when exposed to the air. Clostridia and other organisms bring about spoilage of food under strict anaerobic condition.

Temperature: Low temperature retards spoilage, but even subfreezing temperatures do not prevent multiplication of all microorganisms until about -7°C is reached. Refrigerated foods are therefore subject to spoilage by moulds and by some yeasts and bacteria. Foods stored at -18°C remain free from microbial growth and a slow decrease in population may even occur. Foods and food products stored at room temperature or in warm locations are easily spoiled by mesophilic arid thermophilic organisms.

Types of Food Spoilage with Causative Organisms

Food Types of spoilage

Causative microorganisms

Fresh meat Putrefaction Souring

Clostridium, Pseudomonas, Porteus, Alcaligenes, Chromobacterium, Lactobacillus,Pseudomonas.

Cured meat Mouldy Souring Greening Slimy

Penicillium, Aspergillus,Rhizopus. Pseudomonas, Micrococcus, Bacillus. Lactobacilli Streptococci,Pediococci. Leuconostoc

Fish Discolouration Putrefaction

Pseudomonas Chromobacterium, Halobacterium, Micrococcus

Poultry Odour, Slime Pseudomonas, Alcaigenes, Xanthomonas.

Eggs Green rot Colourless rot Black rot Fungal rot

Pseudomonas Fluorescens Pseudomonas, Alcaigenes, Chromobacterium, Coliform. Proteus, Penicillium, Mucor

Fresh fruits and vegetables Bacterial soft rot Gray mould rot Rhizopus soft rot Blue mould rot Black mould rot Sliminess or Souring

Erwinia carotovera, Pseudomonas spp. Botryitis cinerea Rhizopus nigrican Penicillium italicum Aspergillus niger, Alternaria Saprophytic bacteria

Pickles,Sauer, kraut Black pickles Soft pickles Slimy kraut Pink kraut

Bacillus nigricans Bacillus spp. Lactobacillus Plantarum, L. cucumeris Rhodotorula(asporogenous yeasts)

Sugar products, Honey, Syrups

Ropy syrup Yeasty Pink syrup Green syrup Mouldy

Aerobacter aerogenes Saccharomyces, Torula,Zygosaccharomyces Micrococcus roseus Pseudomonas fluorescens Aspergillus, Penicillium

Bread Mouldy Ropy Red bread

Rhizopus, Aspergillus Penicillium Bacillus spp. Serratia marcesens

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 8

Bacteria Involved in Food Spoilage

Group Speices of

Lactics Lactobacillus, Leuconostoc, Pediococcus,Streptococcus

Acetics Acetobacter and Gluconobacter

Butyrics Clostridium Proteolytics Bacillus, Pseudomonas, Clostridium, Proteus etc.

Lipolytics Pseudomonas, Alcaligenes, Serratia, Micrococcus Bacillus, Clostridium etc.

Sacccharolytic Bacillus, Clostridium etc.

Pectolytic Erwinia, Bacillus, Clostridium

Thermophiles Bacillus, Clostridium, Lactobacillus thermophilus. Psychrophiles Pseudomonas, Flavobacterium, Alcaligenes, Micrococcus

Halophiles Halobacterium sarcina, Micrococcus, Pseudomonas

Osmophiles Leuconostoc

Pigment formers Flavobacterium, Serratia, Micrococcus

Slime or rope formers Alcaligenes, Enterobactor, Streptococcus, Lactobacillus etc Gas formers Leuconostoc, Lactobacillus, proteus, Clostridium, Enterobactor etc

Coliforms E.coli, Enterobactor aerogenes.

FOOD POISONING

To control and prevent the development of food poisoning and to prevent the spread of disease by foods is an important aspect of food microbiology. All food borne diseases are classified as food infections or food intoxications. These classifications are somewhat random.

Food infections are those in which microorganisms present in the food at the time of eating grow in the host and cause disease. Food intoxications are those diseases in which microorganism grow in the food, producing a substance therein which is toxic to man and animals. Food poisoning is the toxicity introduced into food by microorganisms and their products. This does not include the toxications which follow the consumption of noxious plants (mushrooms),poisonous fish (mussels), or decomposed foods containing certain chemicals (arsenic, lead, fluorides, antimony, mercury salts).

At one time it was believed that the products of putrefaction were the cause of food poisoning. These were called ptomaine, and were produced in putrefied meat and other proteinaceous foods. These are amines and result chiefly from the decarboxylation of amino acids. It could be demons-trated that ptomaines injected into animals were toxic, but when taken experimentally by mouth, they were not especially toxic. The food poisoning, therefore, could not be due to ptomaines. People generally do not eat foods that show visible spoilage, but sometimes if the appearance, flavour and taste are normal, they do eat them. On the other hand, certain foods may show no signs of spoilage and yet be responsible for food poisoning.

The fact that organisms are present in food does not necessarily mean that they are harmful. Most of the organisms that cause spoilage of foods are harmless saprophytes. However, there are some forms of illness caused by organisms growing in foods. Staphylococcal food poisoning and botulism are defined as food intoxications, since ingestion of the toxin causes the disease symptoms. The term food infection is used to describe diseases like Salmonellosis and Enteritis caused by Salmonella spp and Cl. perfringens, since these diseases are caused by the ingestion of organisms.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 9

Staphylococcal Poisoning - It is the most common type of food poisoning. Certain strains of Staphylococcus aureus produce a potent enterotoxin. Ingestion of food containing this toxin causes a sudden onset of illness within 3 to 4 hours. Symptoms include nausea, vomitting and diarrhoea. Recovery is rapid, usually within 24 to 48 hours. Death rarely occurs.

The organisms are widely distributed upon the skin and mucous membranes of the human body. People who handle foods contaminate them ignorantly of carelessly. Foods most commonly involved include those which are eaten cold, e.g. salads, bakery products, hams, pressed meat, and dairy products.

A food having millions of Staphylococci may taste, smell, and appear to be little different from that in which none of these organisms have grown. Secondly, staphylococcal enterotoxin is heat resistant and can withstand boiling for 30 minutes. So recooking the food does not help. Control is by preventing the entry of the bacteria to food, by destruction of the bacteria by heat, and by restriction of their growth by refrigeration.

Botulism - Botulism is caused by the exotoxin of Clostridium botulinum, an anaerobic spore-forming rod. It is one of the most potent exotoxins known to man. Botulism is a neuroparalylic disease affecting humans and animals. Sixty to seventy percent of, cases are fatal.

The foods frequently implicated are those which have been smoked, pickled, or canned, allowed to stand for a time and then eaten without cooking or with insufficient cooking. This means that the preservative treatment is inadequate and has failed to destroy the spores of Clostridium botulinum. Most cases are associated with home canned vegetables, infected sausages, ham, preserved meats fowl, or fish.

These foods have the common property of being high in protein, near neutral in pH, and have little residual available oxygen, all of which are factors contributing to toxin production. Fortunately, the toxin is heat labile and is destroyed very quickly by boiling. Home canned food should always be heated at the boiling point for several minutes before use.

Salmonellosis - Salmonellosis is caused through the ingestion of living bacteria of the Salmonella group. The general symptoms are the same as in Staphylococcus poisoning, but with a longer incubation period. The disorder is the result of the growth of Salmonella in the intestine. An inoculum of about 600,000 cells is required for organisms to become established and cause illness.

Different species of Salmonella may be ingested with improperly cooked meat, eggs, and puddings that have been contaminated by rodents or human carriers. Almost any food may, at times, be contaminated with Salmonella, and under conditions of mishandling may become involved in the transmission of salmonellosis.

The major reason for the widespread occurrence of Salmonella in meats is cross infect ion at the killing plant. The animals on the farm that are infected with Salmonella may be only 0.5%, but this may increase to 35% after 2 to 5 days in slaughterhouses.

Similarly, in poultry only a few per cent of the birds entering the plant may harbour Salmonella in their intestinal tract, but after processing 30 to 35% may be infected. Salmonella is found on the shell, and in the contents of' clean fresh eggs. After processing to egg powder, a high percentage of the samples show the presence of Salmonella.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 10

FOOD PRESERVATION

Principal Method of Food Preservation 1. Prevention or delay of microbial decomposition.

(a) By keeping microbes out (asepsis). (b) By removal of microbes (e.g. filtration). (c) By reducing the rate of microbial growth (e.g. by low temperature, drying, anaerobic conditions and chemical inhibitors), (d) By killing microbes (e.g. by heat or radiation).

2. Prevention or delay of self-decomposition of food (a) By inactivation of food enzymes (e.g. blanching). (b) By prevention of chemical reactions (e.g. by using antitoxidants).

Food Preservation Methods Food preservation aims at preventing the microbial spoilage of food products and the growth of

the food borne pathogens. Thus, the two principal goals of food preservation methods are, (i) Increasing the shelf life of the food and (ii) Ensuring the safety for human consumption.

There are a variety of food preservation methods:

1. Heat: Heat kills microorganisms by changing the physical and chemical properties of their proteins. The most common use of heat is in the process of canning. The food product is washed, sorted, and graded and then subjected to steam heat for three to five minutes. This last process called Blanching, destroys many enzymes in the food product and prevents further cellular metabolism. The food is then peeled and cored, and diseased portions are removed. For canning, containers are evacuated and placed in a pressurised steam steriliser, similar to an autoclave at 121°C. This removes especially Bacillus and Clostridiurn spores.

If canning is defective, foods may become contaminated by anaerobic; bacteria which produce gas. These are species of Clostridium, and coliform bacteria (a group of Gram-negative nonspore-forming rods which ferment lactose to acid and gas at 32°C in 48 hours).

2. Low temperature: Exposure of microorganisms to low temperatures reduces their rates of growth and reproduction. This principle is used in refrigeration and freezing. Microbes are not killed. In refrigerators at 5°C, foods remain unspoiled. In a freezer at -5°C the cellular crystals formed tear and shred microorganisms. It may kill many of the microbes.

However, some are able to survive. Salmonella spp. and Streptococci survive freezing. For these types rapid thawing and cooking is necessary. Deep freezing at -60°C forms smaller crystals. It reduces biochemical activities of microbes.

Blanching of fruits and vegetables, by Scalding with hot, water or steam prior to deep freezing, inactivate plant enzymes that may produce toughness, change in colour etc. A brief scalding prior to freezeing also reduces the number of microorganisms on the food surface by up to 99 per cent, enhances the colour of green vegetables.

3. Drying or Desiccation: Water from foods is removed in different ways. It may be done by spray dryer which expels a fine mist of liquid such as coffee into a barrel cylinder containing hot air. There may be used a heated drum onto which liquids like soup may be poured. Another machine is a belt heater that exposes liquids as milk to a steam of hot air that evaporates water and produces dried milk solids.

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 11

A common process of freeze drying or Lyophilization is used these days. The food is deep frozen, after which the water is drawn off by a vacuum pump in a machine. They dry product is then sealed in foil and is reconstituted with water. This method is very useful for storing, transporting and preserving bacterial cultures.

4. Osmotic pressur: The principle of osmosis is applied. Foods are preserved by adding salts and sugars to them. These chemicals remove the water out of microbial cells causing them to shrink. Thus stopping their metabolism. Jams, jellies, fruit syrups, honey etc. are preserved by high sugar concentration. Fish, meat beef and vegetable products are preserved with salt.

6. Radiation: UV is used in meat storage facilities which reduce surface contamination, on meat products. Gamma rays are also used for some meat products.

7. Anaerobiosis: Packaging of food products under anaerobic conditions - anaerobiosis is effective in preventing aerobic spoilage process. Vacuum packing in an airtight container is used to eliminate air. 8. Controlled atmospheres: Such atmospheres containing 10% CO2 are used to preserve stored food products as apples and pears. This checks fungal growth. Ozone can also be added. 9. Other methods: These are asepsis i.e. washing utensils that come in contact with food; and filtration and centrifugation, used, to remove microbes. Filtration is used for fruit juices, other drinks etc. Bacteriological filters are used in industries.

5. Chemical preservatives: The most commonly used are the acids, such as sorbic acid, benzoic acid and propionic acid. These check mainly the growth of yeasts and molds. Sorbic acid is used for preservation of syrups, salads jellies and some cakes. Benzoic acid is used for beverages, margarine, apple cider etc. Propionic acid is an ingredient of bread and bakery products. Sulphur dioxide, as gas or liquid is also used for dried fruits, molasses and juice concentrates. Ethylene oxide is used for spices, nuts and dried fruits.

Chemical Preservatives

Max.permitted concentration

Benzoic acid 0.1%

Methyl paraben 0.1%

Propyl paraben 0.1%

Sodium nitrite 500 ppm

Sodium nitrite 200 ppm

Sorbates 0.1%

Acetic acid 0.12-0.14 mg/ml in wines

Propylene oxide 300ppm

Ethylene oxide residue not to exceed 50ppm

Sulfites Variable with food stuff

Propionates 0.1%

Potassium metabisulphite

0.05%

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 12

Agent Source Symptoms & usual duration

Salmonella Raw meat, poultry, eggs, unpasteurised milk, pets, terrapins, infected food handlers.

Diarrhoea, vomiting, fever, abdominal pain 1-5 days

Campylobacter Raw meat, poultry, raw/bird pecked milk, untreated water, pets.

Abdominal pain, diarrhoea 2-5 days

Listeria

Monocytogenes

Found in environment, cattle, sheep, silage, unpasteurised milk products including soft cheeses, pates.

Fever, affects central nervous system Variable

Staphylococcus

aureus

Human nose, mouth, cuts and wounds.

Vomiting, abdominal pain, diarrhoea, fainting 6-24 hours

Clostridium

Perfringens

Faeces of animal and man, soil (on vegetable), dust, sewage.

Diarrhoea, abdominal pain 12-48 hours

Bacillus cereus

(toxin in food)

Cereal products, especially rice, spices, dust, soil.

Vomiting, abdominal pain, some diarrhoea 1-2 days

Bacillus cereus

(toxin in gut)

Cereal products, especially rice, spices, dust, soil.

Diarrhoea, abdominal pain, some vomiting 1-2 days

Vibrio

Paraheamolyticus

Sea water, shellfish. Diarrhoea, abdominal pain, some vomiting 2-5 days

Escherichia coli (infective)

Aminal origins – cattle, sheep, humans, sewage, meat & raw milk.

Diarrhoea, abdominal pain, fever 2-3 days

Clostridium botulinum

Soil, meat, fish and vegetables

Central nervous system (difficulty breathing, double vision, nerve paralysis), diarrhoea, vomiting Variable can be fatal

Chemicals (e.g. metallic poisons, pesticides, etc)

Vomiting, abdominal pain, possibly effects on central nervous system

Poisonous plants/animals

Vomiting, abdominal pain, possibly effects on central nervous system

Norwalk Virus Vomiting, diarrhoea,

Dr. Shiva C. Aithal, Dept. of Microbiology, Dnyanopasak College, PARBHANI (Maharashtra) INDIA Page 13

Bacteria Responsible

Description Habitat Types of Foods

Cause

Staphylococcus aureus

Produces a heat-stable toxin

Nose and throat of 30 to 50 percent of healthy population; also skin and superficial wounds.

Meat and seafood salads, sandwich spreads and high salt foods.

Poor personal hygiene and subsequent temperature abuse.

Salmonella Produces an intestinal infection

Intestinal tracts of animals and man

High protein foods - meat; poultry, fish and eggs.

Contamination of ready-to-eat foods, insufficient cooking and recontamination of cooked foods.

Clostridium perfringens

Produces a spore and prefers low oxygen atmosphere. Live cells must be ingested.

dust, soil and gastrointestinal tracts of animals and man.

Meat and poultry dishes, sauces and gravies.

Improper temperature control of hot foods, and recontamination.

Clostridium botulinum

Produces a spore and requires a low oxygen atmosphere. Produces a heat-sensitive toxin.

Soils, plants, marine sediments and fish.

Home-canned foods.

Improper methods of home-processing foods.

Vibrio parahaemolyticus

Requires salt for growth.

Fish and shellfish Raw and cooked seafood.

Recontamination of cooked foods or eating raw seafood.

Bacillus cereus Produces a spore and grows in normal oxygen atmosphere.

soil, dust and spices. Starchy food. Improper holding and stroage temperatures after cooking.

Listeria monocytogenes

Survives adverse conditions for long time periods.

Soil, vegetation and water. Can survive for long periods in soil and plant materials.

Milk, soft cheeses, vegetables fertilized with manure.

Contaminated raw products.

Campylobacter jejuni

Oxygen sensitive, does not grow below 86o F.

Animal reservoirs and foods of animal origin.

Meat, poulty, milk, and mushrooms.

Improper pasteuriztion or cooking. cross-contamination.

Versinia enterocolitica

Not frequent cause of human infection.

Poultry, beef, swine. Isolated only in human pathogen.

Milk, tofu, and pork.

Improper cooking. Cross-contamination.

Enteropathogenic E. coli

Can produce toxins that are heat stable and others that are heat-sensitive.

Feces of infected humans.

Meat and cheeses.

Inadequate cooking. Recontamination of cooked product.