FOOD AND INDUSTRIAL MICROBIOLOGY Food spoilage, food infections and intoxications caused by microorganisms and methods for their detection Dr. Neeraj Dilbaghi Reader, Department of Bio & Nano Technology Guru Jambheshwar University of Science and Technology Hisar- 125001 and Dr (Mrs. ) S. Sharma
Professor, Department of Microbiology, CCS Haryana Agricultural University, Hisar- 125001 (Revised 25-Sep-2007) CONTENTS Introduction Food Spoilage and General Principles Underlying Spoilage of Food Intrinsic Parameters Extrinsic Parameters Microbial Spoilage of Foods Spoilage of fresh and ready-to-eat meat products Spoilage of milk and milk products Spoilage of fruits and vegetables Spoilage of canned foods Major Food Borne Infections/ Intoxications Caused by Bacteria General Control Measures for Prevention of Food Borne Diseases Microbial Testing of Foods Conventional Methods Rapid Detection of Emerging High Risk Pathogens in Foods
Keywords Food spoilage, microbial food spoilage, Food borne infections; Food testing; Food borne diseases Introduction Foods and microorganisms have long and interesting associations which developed long before the beginning of recorded history.
Foods are not only nutritious to consumers, but are also excellent source of nutrients for microbial growth.
Depending upon the microorganisms present, foods may spoil or preserved by fermentation. Microorganisms can be used to transform raw foods into fermented delights, including yoghurt, cheese, sausages, tempeh, pickles, wine, beers and other alcoholic products.
On the other hand, foods also can act as a reservoir for disease transmission, and thus detection and control of pathogens and spoilage organisms are important areas of food microbiology. During the entire sequence of food handling from the producer to the final consumer, microorganisms can affect food quality and human health. History of microorganisms in food There is no documentation to exactly pinpoint as to when man first became aware of the presence of microorganisms in food.
The year 1674 marks the birth year of microbiology when Leeuwenhoek first examined microorganisms in a sample of lake water. It was after about 100 years when Microbiology was established as science. Several experiments conducted by scientists to explain the origin of microorganisms provided indirect evidence of association of microorganisms in food. The period prior to establishment of Microbiology/ Bacteriology as science is designated as prescientific era which includes a food gathering period and food producing period.
Food gathering period covers time of man’s origin about 3 million years up to 8000-10000 years ago. In this period man was carnivorous and plant foods entered his diet later. Man also learnt to cook food during this period also. Food producing period ranges from 8000- 10000 years ago and includes present time. Many problems related to prepared foods were encountered during this period for example the problem of food spoilage due to improper storage and problem of food poisoning with prepared foods as well as problem of disease transmission by foods.
Although scientific basis for the preservation of foods were not known at that time but some of the methods used for preservation were use of oils, snow, smoking of meats, etc. Perhaps the first man to suggest role of microorganisms in food spoilage was a Monk Kircher who referred to worms in decaying bodies, spoiled milk etc. The observations by Spallanzani that heated meat infusion in a hermetically sealed flask remained unspoiled and free from microorganisms and experiments by Pasteur while disproving the spontaneous generation theory demonstrated the idea of food preservation by heat.
The spoilage of food and presence of food poisoning organisms in food are very important from the point of food safety. Today the emphasis is on total quality of food which means that not only food should be nutritionally balanced but should be microbiologically safe too. In this chapter we will study general principles of microbial spoilage of food, detection and enumeration of food spoilage and food poisoning microorganisms. The spoilage of some important foods and 2 characteristics of important food poisoning organisms which are very important for food safety will also be discussed.
Food Spoilage and General Principles Underlying Spoilage of Food Spoilage of food involves any change which renders food unacceptable for human consumption and may result from a variety of causes, which includes i) insect damage; ii) physical injury due to freezing, drying, burning, pressure, drying, radiation etc; iii) activity of indigenous enzymes in plant and animal tissues; iv) chemical changes not induced by microbial or naturally occurring enzymes. These changes usually involved O2, light and other than microbial spoilage, are the most common cause of spoilage e. g. xidative rancidity of fats and oils and the discoloration of cured meats; and v) growth and activity of microorganisms- bacteria, yeasts and molds. A food unfit for consumption may not necessarily be spoiled and may contain high number of food poisoning causing bacteria. Microbial deterioration of food is evidenced by alteration in the appearance (color changes, pockets of gas/ swelling), texture (soft & mushy), color, odor, and flavor or slime formation. Since foods are of plant & animal origin, so it is worthwhile to consider those characteristics of plant & animal tissues that affect microbial growth.
The plants and animals that serve as food sources have all evolved mechanisms of defense against the invasion and proliferation of microorganisms, and some of these remain in effect in fresh foods. Knowledge of sources of microorganisms and factors influencing microbial growth is necessary for food microbiologists because desirable growth conditions are needed for applicability of microorganisms for fermentation & single cell protein production and understanding of undesirable conditions are used for food preservation. The various sources through which microorganisms gain entry into the foods are shown in Table 1.
Table 1: Primary sources of microorganisms found in foods Sources Microflora present in soil and water Microflora present in air Microflora present on plant and plant products Microflora present on Food utensils and equipments Microflora present in animal feeds Microflora present on animal hides Microflora present in Intestinal tracts of humans and animals Food Handlers In order to manage microbial contamination and growth from the farm up to the consumer, the Hazard Analysis Critical Control Point (HACCP) approach is widely used.
This approach emphasizes monitoring the quality of food ingredients at critical process handling steps. A safe product will result if the individual steps are carefully controlled. Now- a-days HACCP approach has become mandatory for the food processing industries to ensure that safe foods are available to the public. 3 A variety of intrinsic and extrinsic factors determine whether microbial growth will preserve or spoil foods, as shown in Table. 2.
Intrinsic or food related parameters are those parameters of plants and animal tissues which are inherent part of the tissue. e. g. , pH, water activity (aW), oxidation-reduction potential (Eh), nutrient content, antimicrobial constituents and biological structures. Extrinsic or environmental parameters are properties of storage environments which affect both foods as well as microorganisms and include temperature of storage, relative humidity of storage environment, and concentration of gases in environment.
Table 2: Factors affecting the development of microorganisms in foods Intrinsic Factors Nutrient content pH Redox potential Water activity Antimicrobial constituents & barriers Intrinsic Parameters Nutrient content Like all other living beings, microorganisms need water, a source of carbon, an energy source, a source of nitrogen, minerals, vitamins and growth factors in order to grow and function normally. Since foods are rich source of these compounds, thus can be used by microorganisms also.
It is because of these reasons various food products like malt extracts, peptone, tryptone, tomato juice, sugar and starch are incorporated in microbial media. The inability to utilize a major component of the food material will limit its growth and put it at a competitive disadvantage compared to those that can. In general, molds have the lowest requirement, followed by yeasts, gram-negative bacteria, and gram-positive bacteria. Many food microorganisms have the ability to utilize sugars, alcohols, and amino acids as sources of energy. Few others are able to utilize complex carbohydrates such as starches and cellulose as sources of energy.
Some microorganisms can also use fats as the source of energy, but their number is quite less. The primary nitrogen sources utilized by heterotrophic microorganisms are amino acids. Also, other nitrogenous compounds which can serve this function are proteins, peptides and nucleotides. In general, simple compounds are utilized first by a majority of microorganism’s e. g, amino acids. The same is true for fats and polysaccharides. Some microorganisms may require B vitamins in small quantities and natural foods have these in abundant quantities for those microorganisms which cannot synthesize them.
In general, gram negative bacteria and molds are able to synthesize most or all of their requirements. Consequently, these two groups of organisms may be found growing on foods low in B vitamins. Fruits tend to be low in B vitamins than meats and usually have low pH and positive Eh which explains their spoilage by molds rather than bacteria. Extrinsic factors Temperature Relative humidity Gaseous atmosphere Implicit factors Synergism Antagonism Commensalism Growth rate Processing factors Irradiation Washing Slicing Pasteurization Packaging 4 Water Activity (aW) Water is often the major constituent in foods.
Even relatively ‘dry’ foods like bread and cheese usually contain more than 35% water. The state of water in a food can be most usefully described in terms of water activity. Water activity of a food is the ratio between the vapour pressure of the food, when in a completely undisturbed balance with the surrounding air, and the vapour pressure of pure water under identical conditions. Water activity, in practice, is measured as Equilibrium Relative Humidity (ERH) and is given by the formula: Water Activity (aW) = ERH / 100 Water activity is an important property that can be used to predict food safety, stability and quality.
The various applications of water activity includes; maintaining the chemical stability of foods, minimizing non enzymatic browning reactions and spontaneous autocatalytic lipid oxidation reactions, prolonging the desired activity of enzymes and vitamins in foods, optimizing the physical properties of foods such as texture. Water activity scale extends from 0 (bone dry) to 1. 00 (pure water). But most foods have a water activity in the range of 0. 2 for very dry foods to 0. 99 for moist fresh foods. Based on regulations, if a food has a water activity value of 0. 5 or below, it is generally considered as non-hazardous. This is because below a water activity of 0. 91, most bacteria including the pathogens such as Clostridium botulinum cannot grow. But an exception is Staphylococcus aureus which can be inhibited by water activity value of 0. 91 under anaerobic conditions but under aerobic conditions, it requires a minimum water activity value of 0. 86. Most molds and yeasts can grow at a minimum water activity value of 0. 80. Thus a dry food like bread is generally spoiled by molds and not bacteria.
In general, the water activity requirement of microorganisms decreases in the following order: Bacteria > Yeast > Mold. Below 0. 60, no microbiological growth is possible. Thus the dried foods like milk powder, cookies, biscuits etc are more shelf stable and safe as compared to moist or semi-moist foods. Factors that affect water activity requirements of microorganisms include the following- kind of solute added, nutritive value of culture medium, temperature, oxygen supply, pH, inhibitors. Each microorganism has a minimal water activity for growth as shown in Table 3.
Water activity of some foods and susceptibility to spoilage by microorganisms is shown in Table 4. Water acts as an essential solvent that is needed for most biochemical reactions by the microorganisms. Water activity of the foods can be reduced by several methods: by the addition of solutes or hydrophilic colloids, cooking, drying and dehydration: (e. g. , egg powder, pasta), or by concentration (e. g. condensed milk) which restrict microbial growth so as to make the food microbiologically stable and safe. A wide variety of foods are preserved by restricting their water activity.
These include: Dried or Low Moisture Foods These contain less than 25% moisture and have a final water activity between 0. 0 and 0. 60. e. g. , Dried egg powder, milk powder, crackers, and cereals. These products are stored at room 5 temperature without any secondary method of preservation. These are shelf stable and do not spoil as long as moisture content is kept low. Table 3: Minimum water activity values of spoilage microorganisms Microbial group Most bacteria Most yeasts Most molds Halophilic bacteria Xerophilic molds Osmophilic yeasts Minimum aw 0. 91 0. 88 0. 80 0. 75 0. 65 0. 60 Examples Salmonella spp. Clostridium botulinum Torulopsis spp. Aspergillus flavus Wallemia sebi Aspergillus echinulatas Saccharomyces bisporus Table 4: Water activity of some foods and susceptibility to spoilage by microorganisms Water activity 1. 00 – 0. 95 Microorganisms grow at this aw and above Pseudomonas, E. coli, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens & some yeasts Salmonella, Vibrio parahemolyticus, Clostridium botulinum, Serratia, Lactobacillus, Pediococcus; some molds and yeasts Many yeasts like Candida, Torulopsis, Hansenula; Micrococcus Most molds, Staphylococcus aureus, most Saccharomyces spp. Debaromyces Most halophilic bacteria, Mycotoxigenic Aspergilli Xerophilic molds, Saccharomyces bisporus Osmophilic yeasts, Few molds No microbial proliferation No microbial proliferation No microbial proliferation No microbial proliferation Food examples Highly perishable fresh foods & canned fruits, vegetables, meat, fish, milk, eggs; foods containing up to 40% (w/w) sucrose or 7% NaCl.
Some cheeses (cheddar, Swiss), Cured meats; some fruit juice concentrates; bread; high moisture prunes; foods containing 55%(w/w) sucrose or 12% NaCl Fermented sausages; sponge cakes; dry cheese; margarine; foods containing 65%(w/w) sucrose (saturated) or 15% NaCl Most fruit juice concentrates, sweetened condensed milk, flour, rice, pulses containing 15 – 17% moisture, salami Jam, Marmalade, Glace fruits, Soy sauce Rolled oats containing 10% moisture; Fudge; Marshmallows; Jelly; Some dried fruits; Nuts, Peanut Butter Dried fruits containing 15 – 20% moisture; Honey Pasta containing 12% moisture; Spices containing 10% moisture Whole egg powder containing 5% moisture Cookies, biscuits crackers, bread crusts etc. ontaining 3 – 5% moisture Whole milk powder containing 2 – 3% moisture, Dried vegetables containing 5% moisture, Corn flakes containing 5% moisture, Instant coffee 0. 95 – 0. 91 0. 91 – 0. 87 0. 87 – 0. 80 0. 80 – 0. 75 0. 75 – 0. 65 0. 65 – 0. 60 0. 50 0. 40 0. 30 0. 20 6 Intermediate Moisture Foods These foods contain between 15% and 50% moisture content and have a water activity between 0. 60 and 0. 85. These foods normally require added protection by secondary methods such as pasteurization, pH control, refrigeration, preservatives, but they can also be stored at room temperature. These include dried fruits, cakes, pastries, fruit cake, jams, syrups and some fermented sausages. These products are usually spoiled by surface mold growth. H and Buffering capacity The pH, or hydrogen ion concentration, [H+], of natural environments varies from about 0. 5 in the most acidic soils to about 10. 5 in the most alkaline lakes. Since the pH is measured on a logarithmic scale, the [H+] of natural environments varies over a billion-fold and some microorganisms are living at the extremes, as well as every point between the extremes. The range of pH over which an organism grows is defined by three cardinal points: the minimum pH, below which the organism cannot grow, the maximum pH, above which the organism cannot grow, and the optimum pH, at which the organism grows the best. Microorganisms which grow at an optimum pH well below neutrality (7. ) are called acidophiles. Those which grow best at neutral pH are called neutrophiles and those that grow best under alkaline conditions are called alkalophiles. In general, bacteria grow faster in the pH range of 6. 0- 8. 0, yeasts 4. 5-6. 5 and filamentous fungi 3. 5-6. 8, with the exception of lactobacilli and acetic acid bacteria with optima between pH 5. 0 and 6. 0 (Table 5). The approximate pH ranges of some common food commodities are shown in Table 6. Table 5: Approximate pH ranges of different microbial groups Most Bacteria Yeasts Molds Minimum 4. 5 1. 5 – 3. 5 1. 5 – 3. 5 Optimum 6. 5 – 7. 5 4. 0 – 6. 5 4. 5 – 6. 8 Maximum 9. 0 4. 0 – 6. 5 8. 0 – 11. 0
Table 6: Approximate pH ranges of some common food commodities Product Citrus fruits Soft drinks Apples Bananas Beer Meat Vegetables Fish ( most spp) Milk Wheat flour Egg white Fermented shark pH 2. 0-5. 0 2. 5-4. 0 2. 9-3. 3 4. 5-4. 7 3. 5-4. 5 5. 6-6. 2 4. 0-6. 5 6. 6-6. 8 6. 5-6. 8 6. 2-6. 8 8. 5-9. 5 10. 5-11. 5 7 The acidity of a product can have important implications for its microbial ecology, and the rate and character of its spoilage. For example, vegetables generally have a moderately acidic pH and thus are spoiled by soft-rot producing bacteria such as Erwinia carotovora and pseudomonads, while in fruits, a lower pH prevents bacterial growth and spoilage is caused by yeasts and molds. Also, fish spoil more rapidly as compared to meat under chill conditions. The pH of meat (5. 6) is lower than that of fish (6. 2-6. ) and this contributes to the longer storage life of meat. The pHsensitive genus Shewanella plays an important role in fish spoilage but has not been reported in normal meat (pH 0. 97. In order to prevent microbial spoilage, fresh meats are stored at refrigerated temperature (? 5°C). Thus normally psychrotrophic bacteria will be the most predominant types in raw meat spoilage. Under aerobic storage at low temperature, growth of psychrotrophic aerobes and facultative anaerobes is favored e. g, Pseudomonas spp. In meats with high pH and/or low glucose content, Acinetobacter and Moraxella, which preferentially metabolize amino acids instead of glucose, can grow rapidly and produce undesirable odors.
The various spoilage defects of meats are shown in Table 10. Table 10: Different types of spoilage of meats Aerobic or Anaerobic spoilage Aerobic (Bacterial) Defect Surface Slimes Bloom of Discolorations Rancidity Phosphorescence Red spot Yellow discolorations Off-odors off-tastes (Taints) Discolorations/Taints/ sliminess/ lipolysis Stickiness Whiskers Black spot White spot Green Patches Decomposition of fats Taints Souring Putrefaction Taints Microorganisms involved Alcaligenes,Pseudomonas,Lactobacillus, Leuconostoc, Micrococcus, Bacillus Lactobacillus, Leuconostoc Pseudomonas, Achromobacter Photobacterium spp Serratia marcescens Micrococcus, Flavobacterium Lactic acid bacteria, Actinomycetes.
Rhodotorula Rhizopus, Aspergillus, Penicillium Rhizopus, Thamnidium, Mucor Cladosporium herbarium Sporotrichum carnis Penicillium expansum, P. oxalicum Lipolytic molds Penicillium, Thamnidium Clostridium spp Alcaligenes, Pseudomonas, Clostridium spp Alcaligenes, Pseudomonas Aerobic (Yeasts) Aerobic (Molds) Anaerobic (Bacterial) 12 In vacuum-packaged meats, psychrotrophic facultative anaerobes and obligate anaerobes can grow and result in different types of spoilage. Lactobacillus curvatus and Lb. sake metabolize glucose to produce lactic acid and the amino acids leucine and valine to volatile fatty acids like isovaleric and isobutyric acids which impart a cheesy flavor in meat.
Heterofermentative Leuconostoc carnosum and Leuconostoc gelidum produce CO2, and small quantity of lactic acid, causing accumulation of gas and liquid in the package. Facultative anaerobic Enterobacter, Serratia, Proteus, and Hafnia species metabolize amino acids while growing in meat to produce amines, ammonia, methylsulfides, and mercaptans, and cause putrefaction. Some strains also produce H2S in small amounts to cause greening of the meat. Shewanella putrefaciens, which can grow under both aerobic and anaerobic conditions, metabolizes amino acids (particularly cysteine) to produce methylsulfides and H2S in large quantities. Along with offensive odors they adversely affect the normal color of meats. H2S oxidizes myoglobin to a form of metmyoglobin, causing a green discoloration.
To reduce spoilage of’ Fresh meats, storage at low temperatures (~ 0 to 1°C), modified atmosphere packaging, and vacuum packaging are extensively used. Several other methods to reduce initial microbial load and slow down growth of Gram-negative rods have been used which include the addition of small amounts of organic acids to lower the pH of meat (slightly above pH 5. 0), drying of meat surfaces (to reduce aW), and a combination of the above factors including lower storage temperature. Ready-to-Eat Meat Products Ready-to-Eat Meat Products includes high heat-processed and low heat-processed uncured and cured meat products. High heat-processed cured and uncured meats are given heat treatment to make them commercially sterile.
Thus they may only have some thermophilic spores surviving, which will not germinate unless the products are temperature abused. Low heat-processed uncured meats, such as roasts, are given heat treatment ranging from 140 to 150°F internal temperature (60 to 65°C) for 1h or more depending upon the size of the meat. Under this condition, only the spores of Bacillus and Clostridium spp. and some extremely thermoduric species like Lactobacillus viridescens, some Enterococcus, Micrococcus can survive. Many other types of microorganisms can enter into the products from equipment, personnel, and air as post-heat contaminants. Also, spices and other ingredients added to the products can add to the microbial contamination of the products.
Psychrotrophic facultative anaerobic and obligate anaerobic bacteria have been implicated in the spoilage of these products. Gas production and purge accumulation by psychrotrophic Clostridium spp. , along with off flavor and color changing from brown to pink to red have been detected. The vacuum-packaged and gaspackaged products, during storage, can be spoiled by psychrotrophic Lactobacillus and Leuconostoc spp. In some products, growth of Serratia liquifaciens causes amino acid breakdown, leading to production of ammonia-like flavor. In case of unpackaged cooked products putrefaction results from the growth and protein degradation by the proteolytic Grampositive bacteria.
If the products are stored for a long time, yeasts and molds can also grow, causing off-flavour, discoloration, and sliminess. Due to the growth of H2O2-producing lactic acid bacteria, the products may have green to gray discoloration. 13 Spoilage of milk and milk products Raw Milk Raw milk contains many types of microorganisms coming from different sources. The average composition of cow’s milk is 3. 2% protein, 4. 8% carbohydrates, 3. 9% lipids, and 0. 9% minerals. Besides casein and lactalbumin, it has free amino acids that provide a good N-source. As the main carbohydrate is lactose, those microorganisms with lactose-hydrolyzing enzymes (lactase or ? galactosidase) have an advantage over those unable to metabolize lactose. Milk fat can be hydrolyzed by microbial lipases, with the release of small molecular volatile fatty acids (butyric, capric, and caproic acids). The various spoilage defects of milk and milk products are shown in Table 9. Microbial spoilage of raw milk can potentially occur from the metabolism of lactose, proteinaceous compound, fatty acids (unsaturated), and the hydrolysis of triglycerides. If the milk is refrigerated immediately following milking and stored for days, the spoilage will be predominantly caused by the Gram-negative psychrotrophic rods, such as Pseudomonas, Alcaligenes, Flavobacterium spp. , and some coliforms.
Pseudomonas and related species, being lactose-negative, will metabolize proteinaceous compounds to change the normal flavor of milk to bitter, fruity, or unclean. The growth of lactose-positive coliforms will produce lactic, acetic, and formic acids, C02, and H2 leading to curdling and souring of milk. Some Alcaligenes spp and coliforms can also cause ropiness (sliminess) due to production of viscous polysaccharides. However, if the raw milk is not refrigerated soon, growth of mesophiles predominates e. g, Lactococcus, Lactobacillus, Enterococcus, Bacillus, and coliforms, along with Pseudomonas, Proteus, and others causing changes like souring and curdling of milk. Yeast and mold growth, under normal conditions, is generally not expected.
Pasteurized Milk Pasteurized milk contains various thermoduric bacteria like Micrococcus, Enterococcus, Lactobacillus, Streptococcus, Corynebacterium, and spores of Bacillus and Clostridium which survive pasteurization process. In addition, coliforms, Pseudomonas, Alcaligenes, and Flavobacterium etc can enter as post-pasteurization contaminants. Thus, pasteurized milk has a limited shelf life under refrigerated storage mainly due to growth of these psychrotrophic contaminants. The spoilage pattern of pasteurized milk is the same as described for raw milk. Flavor defects from their growth are detectable when the population reaches ? 106 cells/ml.
Growth of psychrotrophic Bacillus spp. , such as Bacillus cereus, has been implicated in the spoilage of pasteurized refrigerated milk, especially when the levels of post-pasteurization contaminants are low. Production of rennin-like enzymes by the psychrotrophs can cause sweet curdling of milk at higher pH than required for acid curdling. Ultrahigh temperature-treated milk (150°C for a few seconds) is an essentially commercially sterile product that can only contain viable spores of some thermophilic bacteria. The milk is not susceptible to spoilage at ambient storage temperature, but can be spoiled if exposed to high temperatures as such with canned foods. 14
Concentrated Liquid Products Evaporated milk, condensed milk, and sweetened condensed milk are principal types of concentrated dairy products that are susceptible to limited microbial spoilage during storage. All these products are given sufficient heat treatments to kill vegetative microorganisms as well as spores of molds and some bacteria. Evaporated milk is condensed whole milk with 7. 5% milk fat and 25% total solids. It is packaged in hermetically sealed cans and heated to obtain commercial sterility. Under proper processing conditions, only thermophilic spores of spoilage bacteria Bacillus species, such as B. coagulans, can cause coagulation of milk.
Condensed milk is generally condensed and has about 10 to 12% fat and 36% total solids. The milk is initially given a low heat treatment, close to pasteurization temperature, and then subjected to evaporation under partial vacuum (at about 50°C). Thus only thermoduric microorganisms can grow and cause spoilage. Other microorganisms can also get into the product during the condensing process. Sweetened condensed milk contains about 8. 5% fat, 28% total solids, and 42% sucrose. The milk is initially heated to a high temperature (80 to 100°C) and then condensed at about 60°C under vacuum and put into containers. It is susceptible to spoilage from the growth of osmophilic yeasts like Torula spp, causing gas formation.
If the containers have enough head space and oxygen, molds (e. g. , Penicillium and Aspergillus) can grow on the surface which gains entry into the product by recontamination after heat treatment. Butter Butter contains 80% milk fat and can be salted or unsalted. The microbiological quality of butter depends upon the quality of cream and the sanitary conditions used in the processing. Growth of bacteria (Pseudomonas spp. ), yeasts (Candida spp. ), and molds (Geotrichum) on the surface have been implicated in flavor defects (putrid, rancid, or fishy) and surface discoloration. In unsalted butter, coliforms, Enterococcus, and Pseudomonas can grow favorably in water-phase and produce flavor defects.
Spoilage of fruits and vegetables Vegetables The main sources of microorganisms in vegetables are soil, water, air, and other environmental sources, and can include some plant pathogens. Fresh vegetables are fairly rich in carbohydrates (5% or more), low in proteins (about 1 to 2%), and, except for tomatoes, have high pH. Microorganisms grow more rapidly in damaged or cut vegetables. The presence of air, high humidity, and higher temperature during storage increases the chances of spoilage. The common spoilage defects are caused by molds belonging to genera Penicillium, Phytophthora, Alternaria, Botrytis, and Aspergillus. Among the bacterial genera, species from Pseudomonas, Erwinia, Bacillus, and Clostridium are important.
Microbial vegetable spoilage is generally described by the common term rot, along with the changes in the appearance, such as black rot, gray rot, pink rot, soft rot, stem-end rot (Table 11) 15 Refrigeration, vacuum or modified atmosphere packaging, freezing, drying, beat treatment, and chemical preservatives are used to reduce microbial spoilage of vegetables. Table 11: Common spoilage defects of fruits and vegetables Defect Bacterial soft rot Gray mold rot Rhizopus soft rot Blue mold rot Alternaria rot Pink mold rot Green mold rots Watery soft rot, Brown rot Downy mildew Sliminess or souring Black rot/ smut/ Black mold Anthracnose Fruits Fresh fruits have high carbohydrate content (10% or more), very low protein (? 0%), but have pH 4. 5 or below. Thus microbial spoilage of fruits and fruit products is confined to molds, yeasts, and aciduric bacteria like lactic acid bacteria, Acetobacter, Gluconobacter. Like fresh vegetables, fresh fruits are susceptible to rot by different types of molds from genera Penicillium, Aspergillus, Alternaria, Botrytis, Rhizopus, and others. According to the changes in appearance, the mold spoilages are designated as black rot, gray rot, soft rot, brown rot, and others (Table 11). Yeasts Saccharomyces, Candida, Torulopsis, and Hansenula cause fermentation of some fruits such as apples, strawberries, citrus fruits, and dates.
Bacterial spoilage associated with the souring of berries and figs has been attributed to the growth of lactic acid and acetic acid bacteria. To reduce spoilage, fruits and fruit products are preserved by refrigeration, freezing, drying, and reducing aW, vacuum packaging and heat treatment (Fig. 1). Fermented Vegetable and Fruit Products Fermented vegetables like pickles/cucumber and sauerkraut are produced in large volumes. In salt stock pickles containing about 15% salt, yeasts and halophilic bacteria can grow, especially if the acidity is not sufficient. Dill pickles with low salt ( 4. 6/ low acid) foods are heated to destroy most heat-resistant spores of pathogenic bacteria, Clostridium botulinum, to ensure that a product is free of any pathogen.
However, spores of some spoilage bacteria, which have greater heat resistance, can survive. Such products are called commercially sterile foods. The other food group designated as low pH or high acid food with pH 4. 6 and below, is given heat treatment to kill all vegetative cells and some spores. Although low pH will inhibit germination and growth of C. botulinum, spores of some aciduric thermophilic spoilage bacteria can germinate and grow when the products are stored at higher temperatures, even for a short time, which facilitates germination. Some spores of thermoduric mesophilic spoilage bacteria (including pathogenic) can also survive, heating in these products, but they are inhibited by the low pH.
Canned food spoilage is due both to nonmicrobial (chemical and enzymatic reactions) and microbial reasons. Production of hydrogen (hydrogen swell), C02, browning, corrosion of cans due to chemical reactions and liquification, gelation, discoloration of products due to enzymatic 17 reactions are some examples of nonmicrobial spoilage. Microbial spoilage is due to three main reasons: 1. inadequate cooling after heating or high-temperature storage, allowing germination and growth of thermophilic spore formers; 2. inadequate heating, resulting in survival and growth of mesophilic microorganisms; and 3. leakage (microscopic) in the cans, allowing microbial contamination from outside following heat treatment and their growth.
Thermophilic Sporeformers Thermophilic sporeformers can cause three types of spoilage of low-acid foods such as corn, beans, peas etc when the cans are temperature abused at 43°C and above, even for short duration. 1. Flat Sour Spoilage In this type of spoilage, the cans do not swell but the products become acidic due to growth of facultative anaerobic Bacillus stearothermophilus. The organism ferments carbohydrates to produce acids without gas. 2. Thermophilic Anaerobe (TA) Spoilage This type of spoilage occurs due to the growth of anaerobic Clostridium thermosaccharolyticum which leads to the production of large quantities of H2 and CO2 gas and swelling of cans. 3. Sulfide Stinker Spoilage Gram-negative anaerobic sporeformer Desulfotomaculum nigrificans is responsible for this type of spoilage.
The spoilage is characterized by flat container but darkened products with the odor of rotten eggs due to H2S produced by the bacterium. Spoilage Due to Insufficient Heating Insufficient heat treatment results in the survival of mainly spores of Clostridium and some Bacillus spp. Following processing, they can germinate and grow to cause spoilage. The most important concern is the growth of C. botulinum and production of toxins. Spoilage can be either from the breakdown of carbohydrates or proteins. Several Clostridium spp. , ferment carbohydrates to produce volatile acids and H2 and CO2 gas, causing swelling of cans. Proteolytic species, metabolize proteins and produce foul-smelling H2S, mercaptans, indole, ammonia, as well as CO2 and H2 (causing swelling of cans).
Spoilage Due to Container Leakage Leakage of containers during transport will allow different types of microorganisms to get inside the can. They can grow in the food and cause different types of spoilage depending upon the microbial types. Contamination with pathogens will make the product unsafe. Major Food Borne Infections/ Intoxications Caused by Bacteria What is food borne disease? Safe, nutritious foods are essential to human health and well-being. However, food-borne diseases pose a significant problem worldwide. Foodborne disease is any illness resulting from 18 the consumption of food contaminated with one or more disease-producing agents.
These include bacteria, parasites, viruses, fungi and their products as well as toxic substances not of microbial origin (Table 12). The World Health Organization (WHO) estimates that 1. 5 billion cases of food-borne illnesses cause about 3 million deaths each year costing up to $40 billion in health care and job-related absenteeism. More than 250 different food borne diseases have been described. These different diseases have many different symptoms, so there is no one “syndrome” that can be indicated as specific to food borne illness. However, the microbe or toxin enters the body through the gastrointestinal tract, and often causes the first symptoms there, so nausea, vomiting, abdominal cramps and diarrhea is common symptoms in many food borne diseases.
Table 12: Possible causes of food borne gastro intestinal disorders • • • • • • • Viable pathogenic micro organisms (Bacteria, viruses, fungi) or their preformed toxins Pathogenic algae, parasites, protozoa and their preformed toxins Toxins naturally present or formed in some foods e. g, toxic mushrooms, some sea foods, red kidney bean poisoning, biological amines in cheese and fermented meats etc. Toxic chemicals in contaminated food and water, such as heavy metal and some pesticides Nutritional disorders such as rickets due to calcium deficiency Allergy to or inability to utilize some normal components of food Indigestion from over eating or other reasons
Types of Microbial Food Borne Diseases On the basis of mode of illnesses, food borne disease (FBD) can be arbitrarily divided into three groups. A more correct classification is shown in Fig. 2. Intoxication Illness in this case occurs as a consequence of ingestion of a pre formed bacterial or a mold toxin due to its growth in a food. A toxin has to be present in the contaminated food. Once the micro organism have grown and produced toxin in a food, there is no need of viable cells during the consumption of the food for illness to occur . e. g, Staphylococcal food poisoning. Infection Illness occurs as a result of the consumption of food and water contaminated with enteropathogenic bacteria.
It is necessary for the cells of enteropathogenic bacteria to remain alive in the food or water during consumption. The viable cells even if present in small numbers have the potential too establish and multiply in the digestive tract to cause the illness. e. g, Salmonellosis. Toxicoinfection Illness occurs from the ingestion of a large number of viable cells of some pathogenic bacteria through contaminated food and water. Generally the bacterial cells either sporulate or die and release toxin(s) to produce the symptoms. e. g. Bacillus cereus Gastroenteritis. 19 Food-Borne Diseases Poisonings Infections Bacterial E. coli Salmonellosis Bacillus cereus C. erfringens Listeriosis Shigellosis Yersiniosis Enteric viruses Protozoan Neurotoxins Clostridium botulinum Entero toxins Staphylococcus aureus Helminthic Chemical Poisoning Poisonous plant tissues Poisonous animal tissues Intoxication Microbial intoxications Algal toxins Mycotoxins Bacterial toxins Fig. 2: Classification of food borne diseases The other bacterial hazards responsible for food borne diseases are shown in Table 13 . The main factors responsible for the food borne illness includes: a) Improper holding temperature during processing. b) Inadequate cooling during storage. c) Contaminated equipments and utensils. d) Food from unsafe source. e) Poor personal hygiene. f) Adding contaminated ingredients to cooked foods.
A detailed account of major bacterial pathogens is as follows: 1) Staphylococcal Food Poisoning Caused by bacterium Staphylococcus aureus, Staphylococcal Food Poisoning is one of the most common types of food poisoning. Biology of Staphylococcus aureus Staphylococcus aureus is a spherical bacterium (coccus) which on microscopic examination appears singly, in pairs, or bunched, grape-like clusters (Fig. 3). They are Gram-positive, facultative anaerobes, but grow rapidly under aerobic conditions. They are mesophiles with a growth temperature range of 7 to 480C and have the ability to grow at low aW (0. 86), low pH (4. 8), and high salt and sugar concentrations of 15% and in the presence of NO2. S. ureus are naturally present in the nose, throat, skin, and hair of healthy humans, animals and birds. They can be present in infections such as cuts, wounds, abscesses and facial acne etc. The contamination generally occurs through these sources because of improper handling by infected persons. 20 Table 13: Bacteria responsible for food borne illness Type of disease Intoxication Staph poisoning Botulism Infection Salmonellosis Causative bacteria Kind and nature of the bacteria Major symptom(s) type Staphylococcus strains Clostridium strains aureus Gram-positive cocci, present in GIT pairs, short chains or bunched grape like clusters. botulinum Anaerobic, Gram-positive spore Non-gastric forming rod.
GIT GIT GIT GIT and non-gastric GIT GIT and non-gastric GIT GIT GIT and non-gastric GIT and non-gastric GIT GIT GIT GIT Over 2000 Salmonella Rod shaped, motile, non spore spp. (except S. typhi and forming Gram-negative bacteria S. paratyphi) Campylobacter enteritis Campylobacter jejuni & Gram-negative, slender, curved, Cam. coli strains motile, microaerophillic rod Yersiniosis Yersinia enterocolitica Small rod-shaped, Gram-negative bacterium Enterohemorrhagic E. E. coli 0157:H7 Gram-negative, motile, noncoli colitis sporulating, rod shaped facultative anaerobic bacterium. Non-hemorrhagic E. Shiga-like toxin producing Gram-negative, motile, noncoli strains E. coli strains like E. oli sporulating, rod shaped 026: H11 facultative anaerobic bacterium. Listeriosis Listeria monocytogenes Motile, Gram-positive bacteria Shigellosis Four shigella spps. e. g. Gram-negative, non-motile, nonSh. dysenteriae spore forming rods. Vibrio parahemolyticus Vibrio parahemolyticus Gram-negative, curved bacteria gastroenteritis Vibrio vulnificus Vibrio vulnificus Lactose-fermenting, halophilic, infection gram-negative, opportunistic pathogen. Brucellosis Brucella abortus Gram-negative bacteria Toxicoinfection Clostridium perfringens Clostridium perfringens gastroenteritis Bacillus cereus Bacillus cereus gastroenteritis E. coli gastroenteritis Enteropathogenioc and enterotoxigenic E. oli Cholera Vibrio cholerae Gastroenteritis by opportunististic pathogens Aeromonas hydrophila Aeromonas hydrophila gastroenteritis Plesiomonas Plesiomonas shigelloides shigelloides gastroenteritis anaerobic, Gram-positive, spore forming rod Gram-positive, facultative aerobic spore former rods Gram-negative, motile, non-sporulating, rod shaped, facultative anaerobic bacterium Gram-negative, curved bacteria Gram-negative bacteria Gram-negative, rod-shaped bacterium GIT GIT GIT – Gastro-intestinal tract 21 (a) (b) Fig. 3: Staphylococcus aureus (a) cells (b) colonies on Baird Parker Agar Toxins, Disease and Symptoms Only certain strains of S. aureus can cause food poisoning, namely, those that produce an enterotoxin. Enterotoxigenic strains of S. aureus produce six different enterotoxins: A, B, C, D, E, F etc which are serologically distinct heat stable proteins of molecular weight of 26 to 30 kDa. Staph toxins are enteric toxins and cause gastroenteritis.
About 30 g or ml of food containing toxins produced by 106 to 107 cells per gram (ml) for a normal healthy individual is sufficient to cause the symptoms. The primary symptoms are salivation, nausea and vomiting, abdominal cramps & diarrhea. Secondary symptoms are sweating, chills, headache and dehydration. Identification methods Enumeration technique in one or more selective differential media like Baird-Parker agar and G. C. Gioletti Cantoni broth to determine the load of viable cells followed by several biochemical tests, such as hemolysis, coagulase, thermonuclease reactions are performed to link the potential cause of food poisoning outbreaks. Sensitive immunological tests viz. ELISA etc has been developed for this purpose. 2) Botulism Although the disease is very rare, the disease has a high fatality rate. Botulism is caused by the bacterium Clostridium botulinum, which produces an exotoxin that is most potent of all known poisons. 1 gram is sufficient to kill approximately 30 billion mice. Biology of Clostridium botulinum Clostridium botulinum is an anaerobic, Gram-positive, spore-forming rod that produces a potent neurotoxin (Fig. 4). Spores of C. botulinum are widely distributed in soil, sewage, mud, sediments of marshes, lakes and coastal waters, plants- fruits and vegetables and intestinal contents of animals and fishes. Cells are sensitive to low pH (
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