Food Microbiology Lecture Notes PDF

Subject Name: Food Microbiology Subject Code: MSc FSt 102 Unit No: 2, Unit Name: Microbial food spoilage and its effects Faculty Name : Dr Sheetal Sonawdekar Unit No: 1 Unit name: Introduction to Fermentation and Strain Improvement Lecture No: 5 F...

Subject Name: Food Microbiology Subject Code: MSc FSt 102 Unit No: 2, Unit Name: Microbial food spoilage and its effects Faculty Name : Dr Sheetal Sonawdekar Unit No: 1 Unit name: Introduction to Fermentation and Strain Improvement Lecture No: 5 Factors affecting food spoilage Food spoilage Food spoilage is defined as damage or injury to food rendering in unsuitable for human consumption. Food must be considered spoilt if it is contaminated with pathogenic microorganisms or various poisonous agents, such as pesticides, heavy metals etc. Factors Affecting food Spoilage The composition of food: The composition of food influences its susceptibility to spoilage. For example- presence of proteins and carbohydrates especially sugars are preferred by microorganisms for energy source. Very few utilize fat for energy production. Structure of the food item: Whole healthy tissues of food from inside are sterile or low in microbial content. Skin, rind or shell on food works as its protective covering from spoilage microorganisms. Types of microorganisms involved: The types of microorganisms present in food depend on its composition of food. Conditions of storage of the food: Conditions of storage of food affect the growth of microorganisms. Even if the proper storage of food is done, the food loses its freshness and nutritive value if it is stored for too long. 4 Lecture 1 – History of Fermentation Classification of Foods by ease of Spoilage Foods are classified into three groups based on ease of their spoilage. Relatively stable or non-perishable foods: Foods that do not spoil unless handled carefully. E.g. grains, flour, sugar, pulses etc. Protectable or semi-perishable foods: Foods those remain unspoiled for a fairly long period if properly handled and stored. E.g. potatoes, apples, onions etc. Perishable foods: Foods that spoil readily unless special preservative methods are used. E.g. milk, eggs, meat, fish, poultry, most fruits and vegetables. Causes of food spoilage (a) Growth and activity of microorganisms Bacteria, yeasts and molds are microorganisms that cause food spoilage. They produce various enzymes that decompose the various constituents of food. (b) Enzyme activity: Action of enzymes found inherently in plant or animal tissues start the decomposition of various food components after death of plant or animal. (c) Chemical reactions: These are reactions that are not catalysed by enzymes.,e.g. oxidation of fat (d) Vermin. Vermin includes weevils, ants, rats, cocroaches, mice, birds, larval stages of some insects. Vermin are important due to: (i) Aesthetic aspect of their presence, (ii) Possible transmision of pathogenic agents, (iii). Consumption of food. (e) Physical changes. These include those changes caused by freezing, burning, drying, pressure, etc. Effects of food spoilage 1. Changes in nutritional value Decomposition of proteins , carbohydrates, vitamins 2. Changes in organoleptic features Colour , flavour , taste, unpleasant odour. 3. Unwholesome effects Biogenic amines, toxins, Metabolites of microorganisms Pathogen microbes Spoilage Signs 1. Odor: – Breakdown of proteins (putrefaction) e.g. “rotten egg” smell 2. Sliminess - Primarily due to surface accumulation of microbial cells and also be a manifestation of tissue degradation 3. Discoloration – Mould on bread, blue and green mould on citrus fruit and cheese 4. Souring – Production of acid e.g. sour milk from production of lactic acid 5. Gas formation - Meat becomes spongy - Swollen or bubbling packages and cans Food contamination There are three ways which food can be contaminated: Physical; Chemical; Bacterial. Sources of Physical contamination This can occur in a variety of ways at different stages of food processing and production. Some examples are: soil from the ground when harvesting; a bolt from a processing plant when packaging; a hair from a cook in the kitchen. Physical damage to the protective outer layer of food during harvesting, processing or distribution increases the chance of chemical or microbial spoilage. Examples of physical spoilage include: Staling of bakery products and components Moisture migration between different components Physical separation of components or ingredients Moisture loss or gain Sources of Chemical contamination This can occur in a variety of ways at different stages of food processing and production. Some examples are: chemicals from the farm; a cleaning product used in the processing plant when packaging; fly spray used in the kitchen when preparing food. When animal or vegetable material is removed from its natural source of energy and nutrient supply, chemical changes begin to occur which lead to deterioration in its structure. The two major chemical changes which occur during the processing and storage of foods and lead to a deterioration in sensory quality are: lipid oxidation (rancidity) and enzymic browning. Chemical reactions are also responsible for changes in the colour and flavour of foods during processing and storage. Chemical contamination Bacterial contamination As soon as food is harvested, slaughtered or manufactured into a product it starts to change. This is caused by two main processes: autolysis – self destruction, caused by enzymes present in the food; microbial spoilage – caused by the growth of bacteria, yeasts and moulds.  Microbial spoilage of food Bacteria, yeasts and molds are the major causes of food spoilage. They produce various enzymes that decompose the various constituents of food. Molds are the major causes of spoilage of foods with reduced water activity e.g dry cereals and cereal product Bacteria spoil foods with relatively highwater activity such as milk and products.  Sourcesof microorganismsin food The primary sources of microorganisms in food include: 1. Soil and water 2. Plant and plant products 3. Food utensils 4. Intestinal tract of man andanimals 5. Food handlers 6. Animal hides and skins 7. Air and dust  Desirable food changes Autolysis and micro bacterial changes are sometimes desirable (and are not referred to as spoilage), for example enzymes cause fruit to ripen. Here are some positive micro bacterial changes below. Bacteria in yoghurt Mould in some Yeast in bread production. cheeses, e.g. Stilton. production.  Autolysis - enzymes Enzymes are chemicals that are found in food. These chemicals have important uses in food. They can cause food to deteriorate in three main ways: ripening – this will continue until the food becomes inedible, e.g. banana ripening; browning – enzymes can react with air causing the skin of certain foods, e.g. potatoes and apples discolouring; oxidation – loss of certain nutrients, such as vitamins A, C and thiamin from food, e.g. over boiling of green vegetables. Microbial spoilage - bacteria There are many different kinds, some are useful, e.g. in the production of yogurt, and some harmful. The presence of bacteria in food can lead to digestive upset. Some bacteria produce toxins which can lead to this also. Spores can also be produced by some bacteria leading to toxins being produced. Microbial spoilage - yeast Yeasts are single celled fungi which can reproduce by ‘budding’. This means that a small offshoot or bud separates from the parent yeast cell. Yeasts can also form spores which can travel through the air. These are easily killed by heating to 100ºC. In warm, moist conditions in the presence of sugar, yeasts will cause foods like fruit to ferment producing alcohol and carbon dioxide gas. Yeast is used in the production of bread and wine. Microbial spoilage - mould Moulds are fungi which grow as filaments in food. They reproduce by producing spores in fruiting bodies which can be seen on the surface of foods. These fruiting bodies sometimes look like round furry blue- coloured growths, e.g. mould on bread. Some moulds can be seen by the naked eye, e.g. on bread. BIOCHEMICAL CHANGES IN FOOD MAILLARD BROWNING (NON ENZYMIC BROWNING) This is a browning reaction which occurs during the roasting, baking, grilling and frying of many foods. A chemical reaction takes place between the amino group of a free amino acid or a free amino group on a protein chain and the carbonyl group of a reducing sugar e.g. glucose. Brown coloured compounds are formed which are responsible for the attractive colour of products such as bread crust, roasted meat, fried potatoes and baked cakes and biscuits. The compounds also give an Appetising flavour to the food.  CONT… ENZYMIC BROWNING When the cells of apples, potatoes and some other fruits and vegetables are cut and exposed too the air, enzymes present in the cells bring about an oxidation reaction; colourless compounds are converted into brown-coloured compounds. Browning does not occur in cooked fruits and vegetables since the enzymes are destroyed by heat. Fruits such as apples, pears, bananas, peaches and avocado are prone to discolouration. CONT… RANCIDITY Most fats and oils do not store very well, they develop off flavours and odours known as rancidity. Rancidity is important when Considering the shelf life of a food product. Rancidity is caused by several factors: Absorption rancidity: When oils and fats are stored next to strong smelling foods e.g. onions or garlic or products e.g. paints, detergents, Disinfectants. The smell is absorbed by the fat or oil making it unpleasant to eat.  CONT…. Oxidative rancidity: Oxygen from the air can oxidize unsaturated fats producing objectionable flavours. This is the most important and common type of rancidity. It involves a 3 phrase process. Hydrolytic rancidity: this is caused by the presence of water, which causes triglycerides to split into glycerol and fatty acids. The rate of Hydrolysis in the presence of water alone is neglible but hastens if enzymes (lipases) and microorganisms (bacteria, moulds and yeasts) are present. It results in the formation of free fatty acids and soaps (salts of free fatty acids). The oil/fat develops a soapy taste/texture. This is a less common type of rancidity but is quite Common in emulsion systems such as butter, margarine and cream.  CONT… High temperatures, the presence of moisture, oxygen and light are among the factors that speed up rancidity. Different types of fat and oil show varying degrees of resistance to spoilage. Most vegetable oils deteriorate slowly, animal fats deteriorate quicker and marine (fish) oils, which contain a very high proportion of highly unsaturated fatty acids, deteriorate so rapidly that they are useless for edible purposes unless they are refined and hydrogenated. Factors affecting microbial growth in food (a) Intrinsicfactors: These are inherent in the food. Theyinclude:  Hydrogen ion concentration(pH), moisture content, nutrient content of thefood, antimicrobial substances ad biological structures.  1. Hydrogen ion concentration (PH) Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 and 7.5. Some bacteria can grow within a narrow pH range of 4.5 and 9.0, e.g.salmonella Other microorganisms especially yeasts and molds and some bacteria grow within a wide pH range, e.g. molds grow between 1.5 to 11.0, while yeasts grow between 1.5 and8.5.  Table 2: pH values of some food products Food type Range of pH values Beef 5.1 - 6.2 Chicken 6.2 – 6.4 Milk 6.3 – 6.8 Cheese 4.9 - 5.9 Fish 6.6 - 6.8 Oyester 4.8 - 6.3 Fruits < 4.5 (most < 3.5) Vegetables 3.0 – 6.1  Microorganisms that are able togrow in acid environment are called acidophilic microorganisms. These microorganisms are able to grow at pHof around 2.0. Yeasts and molds grow under acid conditions. Other microorganisms such as vibrio cholerae are sensitive to acids and prefer alkalineconditions. Most bacteria are killed in strong acid orstrong alkaline environment except Mycobacteria. Table 3: Minimum and maximum pH for growth of some specific microorganism Microorganism Minimum Maximum Escherihia coli 4.4 9.0 Salmonella typhi 4.5 8.8 All bacteria 4.0 9.0 Molds 1.5 11.0 Yeast 1.5 8.5  2. Moisturecontent The effect of moisture is in terms of water activity: -the amount of free water in a food medium. The amount of free water is important forgrowth of microorganisms. If there is lack of this free water microorganisms will not grow. Water activity is defined as the vapour pressure of a food substance to that of water at the same temperature. (Aw =VPFood/VPWater)  Moisture content The water activity is therefore equal to 1.0. Food products have a water activity of less than 1.0. A saturated salt solution has a water activity of 0.75. Salting and drying reduces the water activity of a food product. Table 4: Water activity of some food products. Food Product Water activity Raw meat and milk 0.99- 1.0 Luncheon meat 0.95 Boiled ham, sliced bacon 0.90 Dried grains 0.80  Water activitylevels Growth of microorganisms is greatlyaffected by the level of water activity(Aw) in the food. Inhibition of growth occurs if the water activity for food is lowered beyond an organism’s minimum level of wateractivity that is necessary for growth. Microorganisms have varied minimum water activity requirements that supports their growth in food. Table 5: Minimum water activity that supports growth of some microorganisms Microorganism Water activity Clostridium botulinum, 0.95 Bacillus cereus, 0.95 Pseudmonas aeroginosa, 0.95 Salmonella spp. 0.95 Staphylococcus aureus (anaerobic), 0.90 Candida spp., Saccharomyces Staphylococcus aureus (aerobic) 0.86 Penicillium spp. 0.82 Most spoilage yeast 0.88 Most spoilage molds 0.80 Osmotic yeast 0.70  3. Nutrients content of thefood Microorganisms require proteins, carbohydrates, lipids, water, energy, nitrogen, sulphur, phosphorus, vitamins, and minerals for growth. Various foods have specific nutrients that helpin microbial growth. Foods such as milk, meat and eggs contain a number of nutrients thatare required by microorganisms. These foods are hence susceptible to microbial spoilage.  4. Antimicrobialsubstances Antimicrobial substances in food inhibit microbial growth. Various foods have inherent antimicrobial substances that prevent (inhibit) microbial attack. Such inhibitors are like lactinin and anti- coliform factors in milk and lysozyme ineggs.  5.Biologicalstructures Some foods have biological structures that prevent microbial entry. For example, meat has fascia, skin and other membranes that prevent microbial entry. Eggs have shell and inner membranes that prevent yolk and egg white from infection.  (b). Extrinsicfactors Are factors external to the food that affect microbial growth. Theyinclude: 1. Temperature of storage, 2. Presence and concentration of gasesin the environment 3. Relative humidity of foodstorage environment.  1. Temperature The growth of microorganisms is affectedby the envirnmental temperatures. Various microorganisms are able to growat certain temperatures and not others. Bacteria can therefore be divided into the following groups depending upon their optimum tmperature of growth.  (i). Psychrophilic microorganisms These grow best at about 20oC but alsodown to -10oC in unfrozen media. Psychrophilic bacteria can cause food spoilage at low temperatures. Several of the microorganisms found inthe soil and water belong to thisgroup.  (ii). Mesophilic bacteria These organisms grow between 25oC and 40oC, with an optimum growth temperature close to 37oC Some such as Pseudomonas aeroginosa may grow at even lower temperatures between5- 43oC None of the mesophilic bacteria are able to grow below 5oCor above45oC. Most pathogenic bacteria belong to thisgroup.  (ii). Thermophilic bacteria. These grow at temperatures above 45oC. Often their optimum growth temperaturesis between 50oC and70oC. Growth of some bacteria occur at80oC. Bacteria in this group are mainly spore formers and are ofimportance in the food industry especially in processedfoods. Note that: The effect of temperature onmicrobial growth also depends upon other environmental conditions suchas: o Growth factors in the nutrient medium, o pH of the food, and o Water activity.  2. Concentration of gasesin the environment This relates to the presence andconcentration of gases in the food environment. Various microorganisms require for growth, either high oxygen tension (aerobic), low oxygen tension(microaerobic) or absenceof oxygen (anaerobic). Some microorganisms may grow either in high oxygen tension, or in the absence of oxygen (facultative anaerobes).  Foods affected by various groups Anaerobic or facultatively anaerobic sporeformers are most likely togrow in canned foods. Microaerophilic bacteria are most likely to grow in vacuum packed foods since they have low oxygen tension, while Aerobic bacteria are likely to grow on the surface of raw meat. Aerobic molds will grow in insufficiently dried or salted products  3. Relativehumidity Relative humidiy is the amount ofmoisture in the atmosphere or food environment. Foods with low water activity placed at high humidity environment take up water, increase their water activityand get spoiled easily. For example, dry grains stored in a environment with high humidity will take up water and undergo mold spoilage.  Microbial spoilage – how does it manifest itself? Visible growth Gas production Slime Off-flavours 20  Sequence of events in food spoilage Microorganisms have to get into the food from a source ↓ Food environment should favour the growth of microbes ↓ Food need to be stored under the growth condition for a sufficient length of time To allow sufficient number necessary to cause spoilage or changes in food. To allow the produced enzyme to spoil the food.  SPOILAGE OF CEREALS Moisture content above 12 to 13 percent may cause spoilage of cereals Little moisture cause mould growth and high moisture may cause growth of yeasts and bacteria. Microbial content, physical damage and temperature are also some factors. Aspergillus, Penicillium, Mucor, Rhizopus, Fusarium are some common moulds; produce mycotoxins. Acetobacter spp, lactics and Coliforms, Micrococci and Bacillus are some species causing spoilage.  SPOILAGE OF MILK An excellent medium Souring – caused by Streptococcus lactis, Enterococci. Gas production – caused by Coliform, Clostridium, Yeasts. Proteolysis – cause bitter taste, caused by Bacillus , Micrococcus. Changes in colour and tastes 54 55  SPOILAGE OF MEAT Raw meat is subject to spoilage by its own enzymes and microbial action. Excessive autolysis can cause souring Factors involving spoilage include – The greater gut load of animal – The physiological condition of the animal during slaughter as like fever, excited or fatigued. – Rapid cooling – The predominant bacteria associated with spoilage of meat, are Brochothrix thermosphacta, Carnobacterium spp., Enterobacteriaceae, Lactobacillus spp., Leuconostoc spp., Pseudomonas spp. and Shewanella putrefaciens. 56 57  SPOILAGE OF FISH Spoiled by autolysis, oxidation or bacterial activity. Under aerobic conditions Change in colour of meat pigments Red colour, green , brown or grey. Off odours and Off tastes The Seafoods can be spoilt by pathogenic bacteria from water and soil, such as Bacillus , Clostridium , Escherichia , Serratia , and Vibrio. Gram‐negative aerobic rods and facultative anaerobic rods and coliforms constitute the major spoilage bacterial flora. 58 59  SPOILAGE OF EGGS Cracks, leaks, stained or dirty spots on exterior and blood clots, bloodiness, translucent spots in the interior are all signs of spoilage Pseudomonas, certain coliform bacteria. Proteus spp, Aeromonas, Serratia, Sporotrichum all cause rots in eggs. 31 SPOILAGE OF CANNED FOODS Chemical spoilage – Hydrogen swell – Discolouration of inside of the can – Cloudiness of liquors – Loss in nutritive value Biological spoilage – Attacked by Thermophilic and Mesophilic bacteria These foods are generally spoiled by yeasts, molds, and lactic acid bacteria (LAB). 62 63 Vegetables Vegetables are a good substrate for yeasts, moulds or bacteria It is estimated that 20% of all harvested fruits and vegetables for humans are lost to spoilage by these microorganisms. 64 Soft rot a. One of the most common types of bacterial spoilage. b. caused by Erwinia carotovora Softening can also be caused by endogenous enzymes 65 Mould spoilage a.In vegetables where bacterial growth is not favored , moulds are the principal spoilage agents. b.Most moulds must invade plant tissue through a surface wound such as a bruise or crack. 36  Fruits Like vegetables, fruits are nutrient rich substrates but the pH of fruits does not favor bacterial growth. As a result, yeasts and moulds are more important than bacteria in the spoilage of fruits. 37 69 Lecture 1 – History of Fermentation