Category: Food microbiology

MICROBES IN FOOD SPOILAGE

by MicroScopia IWM, posted in Food microbiology

BY- ABHISHEKA G.(MSIWM013)

                          

INTRODUCTION:

 Food spoilage is defined as the process in which the destruction of food occurs then it becomes non edible to humans and it’s quality of edibility is decreased. It is also referred to be the changes in the visual, smell, and texture of the food that makes it unacceptable for consumption. The growth of microbes in food results in food spoilage. Spoilage of food occurs due to the action of microbial enzymes in the absence of viable cells. The microbial spoilage of food results in changes such as color, odor, texture, the formation of slime, accumulation of gas, and release of liquids.

 The microbes like Bacteria, yeasts, and molds causes of food spoilage in high amount. These microbes synthesis different enzymes to decompose the various food items. Molds are the major causes of spoilage of foods with reduced water activity like dry cereals and cereal products. Bacteria spoil foods with relatively high water activity such as Milk and its products.

RELATION BETWEEN WATER ACTIVITY AND FOOD SPOILAGE MICROBES:

1.Water activity has an important role in food preservation. Each microbe has a critical water activity, below this critical water activity, the growth of the microbes do not occur.

2.For example, pathogenic microbes cannot grow at water activity below 0.86, Yeasts and molds are tolerant to water activity and they grow at or below water activity 0.62. Hence water activity is important in foods and it is a major factor in food spoilage and food safety.

3. Decrease in water activity retards the growth of microorganisms by slowing down enzyme-catalyzed reactions and retards nonenzymatic browning.

WATER ACTIVITY OF SOME OF THE FOODS:

Fruits and Vegetables – 0.97 to 1.

Meats- 0.95-1.

Cheese – 0.68 – 1.00

Jams and Jellies- 0.75 – 0.94

Honey – 0.54 – 0.75

Noodles – 0.50

Dried Milk – 0.20

MINIMUM WATER ACTIVITY REQUIRED FOR THE GROWTH OF MICROBES IN FOOD

Clostridium botulinum – 0.95

Bacillus cereus – 0.95

Pseudomonas aeruginosa- 0.95

Salmonella species – 0.95

Staphylococcus aureus- 0.90

Candida species- 0.90

Saccharomyces cerevisiae- 0.90

Staphylococcus aureus – 0.86

Penicillium species – 0.82

Most spoilage yeast – 0.88

Most spoilage molds – 0.80

SOURCES OF MICROBES IN FOOD:

  1. Soil and water
  2. Plant and plant products
  3. Food utensils
  4. The Intestinal tract of Humans and Animals
  5. Air and dust
  6. Food handlers.

PROCESS OF FOOD SPOILAGE BY MICROBES: Microbes like bacteria, molds attack the food items. Then preserved food items undergo degradation. As a result of degradation, food items result in changes such as physical changes like the texture of food, visual, smell, and quality of the food and loss of taste, etc. Later the microbes which attacked the food items releases the necessary enzymes required for the degradation of spoiled food items. Then enzymatic action brings about the degradation process.

FOODS AFFECTED BY VARIOUS GROUPS OF MICROBES:

  1. Anaerobic or facultatively anaerobic: These groups of microbes are most likely to grow in canned foods.
  2. Microaerophilic bacteria: These are most likely to grow in vaccum packed foods since they have low oxygen tension.
  3. Aerobic bacteria: They grow on the surface of the raw meat.
  4. Aerobic molds: Grow on insufficiently dried or salted products.

FACTORS AFFECTING THE GROWTH OF MICROBES IN FOOD:

  1. pH: Most of the bacteria grow best at neutral or slightly alkaline nature of foods, the pH ranging from 6.8 to 7.5. Bacteria like Salmonella grow at a pH range of 4.5 to 9.0. Molds grow between 1.5 to 11.0, whereas yeasts grow between 1.5 to 8.5.
  2. Moisture content: The effect of moisture is measured in terms of water activity. The water activity of a food is defined as the amount of free water present in the food medium. If there is a lack of water the microorganisms do not grow in the food medium.
  3. Nutrient contents in the food: Microbes require proteins, carbohydrates, lipids, water, energy, nitrogen, Sulphur, phosphorous, vitamins, and minerals for their growth. Various foods have specific nutrients which support the growth of microbes in the food. Foods like meat, milk, and eggs contain a large number of nutrients required for the growth of microbes. Hence these foods are more susceptible to microbial spoilage.
  4. Anti-microbial substances: The antimicrobial substances like lactenin, anti-coliform factors in milk, and lysozyme in eggs prevents the growth of microbes.
  5. Temperature: Microbes require optimum temperature for their growth. Psychrophilic bacteria causes spoilage of food at low temperature conditions. Mesophilic bacteria can grow between 5*c to 40*c. Thermophilic bacteria grow above 45*C temperaturze.

FOOD PRESERVATION

by MicroScopia IWM, posted in Food microbiology

BY: RAHUL ANDHARIA (MSIWM001)

Introduction:

  • The process of handling and treating food to prevent it from getting spoiled is termed as food preservation.
  • Food has to be generally protected from bacteria, fungi and other microorganisms as well as retarding fat oxidation which can lead to rancidity.
  • The term preservation simply means use of chemical compound or process to stop the growth of microorganisms and to prevent food spoilage.

History:

  • Preservation of food in hermetically sealed containers was the very first method implied in food preservation. This method was introduced by Nicholas Appert in 1700s.
  • Oldest method used for food preservation known to be used was Food Drying (originated in middle-east as per ancient records).

Principles of food Preservation:

  • Prevention of microbial decomposition: Asepsis (keeping out microorganisms), removing microorganisms, hindering microbe’s growth by low temperatures, drying, chemicals and by using anaerobic conditions.
  • Prevention of self-decomposition of food: by inactivating or destructing food (blanching), preventing delay of chemical reactions (using anti-oxidants).

Methods of food preservation:

  1. Asepsis:
  2. It simply means keeping out microorganisms from food.
  3. Microbial decomposition is delayed or protected if there is protective covering above the food like shells of nuts, skin of fruits and vegetables, membranes or fat on meat and fish.
  4. When the protective covering is damaged, it can lead to decomposition of inner tissues by microorganisms.
  5. Packaging of food is the widely used application of asepsis. The packaging may range from a loose wrapping to hermetically sealed container of canned foods.
  6. In the dairy industry, contamination with microorganisms is avoided as much as in practicable in the production and handling of market milk.
  7. In the meat packaging industry, sanitary methods of slaughter, handling and processing of meat reduces the load of microorganisms.
  8. In canning industries, load of microorganisms determines the heat process necessary for preservation of food.
  • Removal of microorganisms: Removal of microbe’s can be accomplished by methods like filtration, centrifugation, washing and trimming.

Filtration:

  • The liquid is filtered through previously sterilized bacteria-proof filter made up of sintered glass, diatomaceous earth, unglazed porcelain, and membrane pads.
  • The liquid is than forced through by applying positive and negative pressure.
  • Used for fruit juices, beer, soft drinks and water.
  • This is the only successful method for complete removal of microorganisms.

     Centrifugation:

  • This method is used in treatment of drinking water.
  • When it is applied to milk, the main purpose is to take out suspended particles other than bacteria.
  • Centrifugation at high speeds removes most of the space.

     Washing:

  • Washing raw foods can be helpful in their preservation but may be harmful under certain conditions.
  • Washing fruits and vegetables removes soil microorganisms that may be resistant to heat process during canning process of these foods.

     Trimming:

  • Trimming of spoiled portions of food or discarding spoiled samples is important to avoid eating spoiled food which may lead to food poisoning.
  • Maintenance of anaerobic conditions:
  • A preservative factor is sealed. Packaged foods may be in the anaerobic conditions in the container.
  • A complete fill, evacuation of unfilled space or replacement of the air by co2 or by inert gas such as N2 will bring about anaerobic conditions.
  • Spores of some aerobic organisms are especially resistant to heat and may survive in canned foods, but will not be able to germinate under anaerobic conditions.
  • Production of C02 and during fermentation and its accumulation at the surface will serve to make conditions anaerobic there and prevent the growth of aerobes.
  • Use of high temperatures:

Pasteurization:

  • Process of slow heating is known as Pasteurization. It can kill some microorganisms but not all.
  • Usually involves the application of temperature below 100 degree Celsius.
  • The heating can be done by means of steam, hot water, dry heat and electric currents.
  • Time and temperature used in this process depends upon the method employed and the product treated.
  • The high temperature-short time method employs comparatively higher temperatures for a short period of time.
  • The low temperature-long time method uses a lower temperature for a longer time.
  • Example: Grape wine pasteurization for 1min at 82-85 degree Celsius in bulk.

       Canning:

  • Canning can be defined as preservation of food in sealed containers and usually implies heat treatment as the principle factor.
  • Canning is done using Tin cans or in glass containers.
  • Raw food for canning is freshly harvested, properly prepared, graded and washed thoroughly before used for canning.
  • Many vegetable foods are blanched briefly by hot water or steam before packaging.
  • Blanching washes the food further, sets color, and kills microorganisms.
  • Use of low temperature:
  • At lower temperatures, chemical reactions, enzyme action and microbial growth will be much slower.

Chilling/Cold storage:

  • In this temperature is not far above freezing.(-1 degree Celsius)
  • Generally refrigerators are used at 0-8 degree Celsius for preserving food products.
  • Most perishable foods like eggs, dairy products, meat, and seafood can be held in chilling storage refrigerators for a limited time with a very little change from their original condition.
  • Process of Enzymatic and microbial changes in food cannot be prevented by this method, but the process is slowed down to great extent.

 Freezing or Frozen storage:

  • The rate of freezing of foods depends on the method employed, temperature used, circulation of air and the type of food to be freeze.
  • The temperature is usually -23.3 degree Celsius or (lower to -15 to -29 degree Celsius) and takes 3-72 hours for the process.
  • Drying:

 Solar Drying:

  • It is limited to climates with high temperatures and dry atmosphere and to certain types of fruits like apricots, peaches and pears.
  • This process takes longer time than other processes.

  Mechanical Drying:

  • Involves passage of hot air with controlled relative humidity over the food to be dried.
  • The simplest dryer is the evaporator or Kiln, where the natural draft from the rising of hot air brings about drying of food.

 Freeze drying: freeze drying or sublimation of water from a frozen food by means of vacuum and applied heat at drying shelf can be used in number foods like meat, poultry, seafood, vegetables and fruits.

  • Preservation by food additives:
  • Food additives are specifically added to prevent decomposition or deterioration of food. The food additives used for this purpose are called as chemical preservatives.
  • They inhibit microorganisms by inhibiting their cell membrane, enzyme activity or their genetic mechanism.
  • Factors that influence effectiveness of chemical preservatives in inhibiting microorganisms are: concentration of chemical, temperature, time, chemical and physical characteristics of substrates in which organism is found.
  • Food additives added include organic acids, salts, sugars and spices.
  • Salting helps in preserving fruits for a longer duration.
  • Sodium benzoate, vinegar, and sodium metabisulphite which are known as synthetic preservatives can also be added.
  • Preservation by Radiation:
  • This method can also be called as cold sterilization.
  • Surface contamination of several foods can be prevented or reduced by this method. Radiations like UV rays, X-rays and gamma rays are used to kill microorganisms.
  • Example: low level of radiations can be used on fruits and vegetables to kill insects and prevent food from spoilage.

Food preservation is essential in terms of protecting food from microbes and environmental conditions and to store food for longer duration of time.

MUSHROOM CULTIVATION

by MicroScopia IWM, posted in Food microbiology, General microbiology

Content:

  • About
  • Morphology
  • Life cycle of mushroom
  • Mushroom cultivation requires

About:

  • Mushroom is a fungus which has a fleshy, spore bearing fruiting body, generally growing on the soil surface of on decaying woods.
  • The word ”mushroom” is used for the fungi which has a stem, a cap and gills on the lowerside of the cap.
  • The function of gills is to produce microscopic spore which further spread across the ground and give rise to many more mushrooms.
  • The spores produced by the gills are called basidiospores.
  • Mushroom lacks chlorophyll and is saprophytic in nature because they grow on dead organic matter.
  • Consist of mainly two parts stalk (stipe) and cap (pileus).
  • A mushroom develops from underground mycelium it is protected by a thin membrane which eventually raptured as the growing mushroom pushes upward, leaving fragment on the cap.
  • There is another membrane which is attached to the cap and the stalk, also raptured and develops into a remnant ring (annulus) on the stalk.
  • On the cap’s undersurface there is radiating rows of gills which bears club-shaped reproductive structures (basidia) give rise to minute spores basidiospores each spores germinates into a mushroom.

Morphology:

  • A mushroom develops from a primordium which is nodule or pinhead less than two millimetres in diameter, found nearly on the surface of the substrate.
  • The mass of thread like hyphae make up the fungus.
  • Mushroom consist of mycelia which absorbs nutrients from the soil, it doesn’t require sunlight for growth.
  • The primordium develops into a round structure of hyphae similar to an egg called “button”. Button has a roll of mycelium, the universal veil that surround the developing mushroom.
  • With the expansion of the mushroom the universal veil ruptures and remains as cup (volva) at the base of the stalk.
  • The cap like structure is called as pileus.
  • Mycelium is the underground part which contains numerous branched network of hyphae used to absorb nutrient from organic matter where it grows.

Life cycle of a mushroom:

Cultivators follow the path of the life cycle of mushroom as the life cycle is generally difficult to observe

Inoculation:

  • Spores from mushroom are spread on substrate and after getting favourable condition it germinates.

Spore germination:

  • Spores give rise to the hyphae; compatible hyphae then mate and create mycelium.

Mycelial expansion:

  • Work of mycelium is to break down organic matter and absorb nutrients from the surroundings.
  • During the growing stage of the mushroom the mycelium grows at an exponential rate, mycelium encounters many predators  which mycelium repels with a collection of enzyme and compounds hence, mycelium is also known as the immune system of the mushroom.

Hyphal knot:

  • Mycelium then condenses into hyphal knot which then give rise to primordia (baby mushroom).

Primordia formation:

  • Produces enzyme and optimizes the constituents of both mycelium and developing fruitbody.

Fruitbody selection:

  • During the development of mushroom a thousand of primordial formed, but the most promising is selected and developed into mature fruitbodies.

Mature fruitbody:

  • In this stage all the nutrient and energy is used to develop the fruitbody which then produces spores by sexual reproduction.

Spore release:

Spores produced by the mature fruitbody release into the environment for propagation those that land on the favourable substrate get germinated and begin the new life cycle.

Mushroom cultication requires:

  • Closed rooms with proper ventilation
  • Power/fuel supply for maintaining the temperature.
  • Well skilled labour.
  • Air cooler, humidifiers.
  • Contamination condition like sterilized paddy straw in hot air oven.

Mushroom cultivation process:

Spawn production:

  • The first process of mushroom cultivation is spawn production, the spores is bought from the market

Composting:

  • Compost is the key ingredient for growing mushroom; compost is made up of straw, gypsum, chicken manure and water added to the horse manure.
  • These ingredients play its unique role in the compost preparation, gypsum ensures proper acidity, straw improves structure and both the manure provides nutrient.
  • The compost is prepared in tunnels which prevent smell from it.
  • The fresh compost looks like dark brown (earth from forest).

Preparing the bag:

  • Packing of compost is done in the plastic bags of dimensions 12*24 inches.
  • Two inches of straw than sprinkle few spores on the top of the straw along the edges. Spores sprinkled in the middle will not grow so the spores are sprinkled along the edges.
  • Watering of inoculated compost filled trays:
  • Watering should be done twice a day or less depends on the moisture availability.
  • Water is spread on the newspaper to maintain humidity.
  • Room temperature should be at 24oC for 12- 15 days for good growth of the mycelium.

Harvesting of mushroom:

  • Harvesting is by twisting and uprooting the fruitbody,
  • The lower part of the stalk is removed where the compost remains attached.

Storage of mushroom:

  • The fruitbody is stored at 4OC for few days.

Nutritional value and medicinal value:

  • Most mushroom has high protein content
  • Fibre lowers the cholesterol and is necessary for the digestive system.
  • Vitamin D – absorption of calcium.
  • Having all essential amino acids.
  • Contain folic acid
  • Contain vitamins like B, C, D and K.

Some edible and poisonous mushrooms:

Edible mushrooms:

Agaricus brunnescens

Agaricus Campestris

Pleurotus edodes

Poisonous mushrooms:

Amanita phalloides

A. virosa (destroying angles),

 A. verna (fool’s cap),

 A. muscaria

FOOD SPOILAGE

by MicroScopia IWM, posted in Food microbiology

BY:- RAHUL ANDHARIA (MSIWM001)

Food Spoilage:

It refers to change in Physical and Chemical property of food, making food unfit for Consumption. Invasion of microorganisms like bacteria and fungi usually causes spoilage of food.

Principle:

Food spoilage generally occurs due to Physical, Chemical or Biological agents that changes colour, flavour, appearance, odour and other properties of food. Shelf life of most of the natural foods is very less and is perishable, for example, meat, fish and bread can spoil easily. Decomposition of food generally involves 3 processes: Putrefaction (chemical breakdown of food or decay of organic matter), Fermentation (chemical breakdown of substances by action of microorganisms, yeast), Rancidity ( refers to oxidation of fats).

Natural Contamination:

It refers to contamination of food when microorganisms themselves attaches to food in its growing stages and this kind of contact is essential for certain kinds of food. For example, Yeasts contaminates fruit for carbohydrates fermentation.

Artificial Contamination:

This type of contamination occurs during handling of food when food is under various stages of production like, packaging, storage, etc. Improper handling of food during this stages results in contamination of food by microorganisms.

Intrinsic factors of food like pH, redox potential, H2o activity determines the type of microflora growing on the food. This final composition of microflora is responsible for food spoilage.

Types of Food Spoilage:

1.Microbial Spoilage:

Microorganisms associated with food are:

Bacteria, Filamentous Fungi, Viruses, Yeasts, and animal parasites.

Bacteria:

They are associated with both plant and animal foods. Bacteria are associated with food intoxication and spreading of food borne diseases.

Examples:

Acenatobacter:  Gram negative- present in raw and prepared foods like beef and poultry carcasses.

Aero monas: gram negative, responsible for spoilage of fish.

Alkaligans: gram negative, responsible for spoilage of egg and dairy products.

Citrobacter: gram negative, it is responsible for spoilage of vegetables and fresh meat.

Corynebacterium: gram positive, involved in spoilage of vegetables and  meat.

Filamentous Fungi:

When food is left for one or more day covered, tangled mass of furry growth appears on food which is called fungi or mould. Fungi are responsible for spoilage of Grains, nuts, and fruits as they have low pH and H20 activity.

Examples:

Mucor: Zygomycotena-common contaminant of fruits, berries and nuts.

Rhizopus: Zygomycotena- known commonly as bread mould. It is more prevalent in fermented and stored foods.

Claviceps: Ascomycotena-  produces toxic alkaloids in cereals, when consumed can cause Hallucinations.

Yeasts:

Contamination by yeasts results in Souring of milk.

Examples:

Candida: most common contaminant of dairy products, fresh fruits, and alcoholic beverages.

Saccharomyces: spoilage of fruits and fruit products.

Torulopsis: responsible for spoilage of beef, creamed butter, condensed milk, etc.

Viruses:

viruses found in food are termed as enteric or intrinsic viruses.

Examples:

Enterovirus, Adenovirus, Reovirus, Hepatitis A virus.

Animal Parasites:

They belong to 3 distinct groups:

Protozoa: Giardia, Entamoeba Hystolytica

Flatworms: Taenia, Fasciola

Roundworms: Ascaris

2. Physical Spoilage:

Physical Spoilage refers to damaging of food during Harvesting, Processing or distribution of food. During such processes there are high chances of food spoilage if proper measures are not followed. The damage increases the chance of spoilage as the outer layer is completely broken or bruised. For example- Canned foods gets spoiler easily if the cans are not properly packed with lid or are contaminated during processing.

3. Chemical Spoilage:

Chemical reactions in food are responsible for change of colour, texture and taste of the food products. Generally foods are fresh especially vegetables and animal food, but after harvesting and slaughtering, chemical changes begin automatically in the food and the quality of food becomes deteriorated.

4. Enzymic Spoilage:

Enzymes acts as biological catalyst to carry out biological reactions in cell and play an important role in biochemical reactions. After death of cells or tissues, enzymes play a role in its decomposition by a process called Autolysis( self destruction )

Example: In tomatoes, some enzymes helps it for ripening, but at the same time there are certain enzymes which are responsible for its decay. Once enzymic Spoilage is underway, it damages the outer skin of tomato and exposes it to mould growth and decay.

Factors Affecting Food Spoilage:

  1. Water Content: Amount of water holding capacity in foods is referred to as it’s water activity.(WA). Water activity of most of the fresh fruits is approximately 0.99, which makes them more susceptible to microbial growth.
  2. Environmental Conditions: Environmental influence on food is the major concern. When food is exposed to intrinsic conditions like temperature, air, or even small amount of moisture, can result in growth of Micro-organisms. Changing environmental conditions can help to prevent spoilage. For example- storing food at lower temperature can prevent it from spoiling.
  • Packaging and storage: Packaging of foods is after processing is very vital as it protects food from harmful contaminants and also from various other factors like environment, temperature, etc. The type of packaging plays a key factor in ensuring the safety and preventing spoilage. Food packed in jars, cans ensures safety and prevents food from dust, moisture, air and harmful microbes.

Sources of Micro-organisms for Food Spoilage:

Micro-organisms are present everywhere. General source of Micro-organisms include air, water, sewage, soil and animal wastes. Foods grown in ground have higher risk of spoilage due to micro-organisms.  Foods like fish, meat are contaminated by presence of bacteria in their  internal organs like skin and feet. Meat has higher tendency of contamination as raw meat attracts lot of microbes, so it is advisable to store raw meet immediately after chopping.

Ways to Prevent Food Spoilage:

  • Ensure proper packaging is available to the food cans and jars after processing.
  •  Don’t leave the food in open air for more than 15min, to avoid contact with microbes.
  •  Ensure that your refrigerators are operating at correct temperatures.
  •  Food must be protected from light and must be stored in amber colour or transparent containers.
  • Low temperature is a key as it retards microbial growth.
  • Avoid placing food where there is more humidity, as high humidity attracts more growths of microbes and moulds. Placing food in dry places is most appropriate.