BSc Microbiology of Food Notes Study Material

BSc Microbiology of Food Notes Study Material

BSc Microbiology of Food Notes Study Material: BSc is a three-year program in most of the universities. Some of the universities also offer BSc Honours. Out of those, there are BSc Study Material, BSc Sample Model Practice Mock Question Answers & BSc Previous Year Papers. At you can easily get all these study material and notes for free. Here in this post, we are happy to provide you BSc 2nd Year Microbiology of Food Notes Study Material.

BSc Microbiology of Food Notes Study Material
BSc Microbiology of Food Notes Study Material

BSc Microbiology of Food Notes Study Material

Microbiology of Food

Most foods are excellent media for rapid growth of microorganisms. There is abundant organic matter in foods, their water content usually sufficient, and the pH is either neutral or slightly acidic. We shall briefly examine in this post important microorganisms involved in food spoilage and possible methods of their control.(BSc Microbiology of Food Notes Study Material)

Food spoilage

Food is considered spoiled when it has been altered from the expected normal form. Generally spoiled foods have unpleasant appearance, aroma and taste. Foods may become contaminated with microbes in different ways. Airborne microorganisms may fall on fruits and vegetables, which then enter through damaged skin. Bacteria from animal’s intestine may contaminate meat that is handled carelessly. Crops may carry bacteria from soil to the processing plant. Animals may transport microbes on their body parts when they move among foods.

Extent of spoilage of a food depends primarily on its physical and chemical properties. Thus, water content, pH, physical structure, carbohydrate content, protein content, vitamin levels etc. are important factors. Other factors important in spoilage are oxygen and temperature conditions of the foods. On the basis of their physical and chemical properties, the food industry recognises three groups of foods:

1. Highly perishable foods, which are spoiled rapidly. They include poultry, eggs, meats, most fruits and vegetables and dairy products etc.

2. Semi-perishable foods, that spoils less quickly. They include potatoes, apples, nutmeats etc. and

3. Nonperishable foods, which are generally kitchen items, like cereals, rice, flour, nuts, sugar etc.

Chemical changes during food spoilage

The biochemical composition of a food, in particular, has a marked influence on the microbial population involved in the spoilage process and the microbial decomposition products associated with the spoilage of that food.

BSc Microbiology of Foods NOtes Study Material
Some food spoilage processes

Type of Microbes Associated with Food Spoilage

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.(BSc Microbiology of Food Notes Study Material)

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.

1. Meats and fish. Microorganisms, which cause their spoilage, are usually introduced during handling, processing, packaging and storage. For example if a piece of meat is ground, the surface organisms accumulate in the teeth of the grinder alongwith other dustborne organisms. Bacteria from the hands of the preparer or from an errant sneeze may add more microbes. Processed meats may become contaminated during handling. Some as sausage etc. made from animal intestines may contain residual bacteria, especially botulism spores.

Organ meats like liver, kidney etc. spoil quickly, and may contain many bacteria trapped in their filtering tissues. Greening on meat surface is usually due to Gram-positive rod, Lactobacillus, or the Gram-positive coccus, Leuconostoc.

Type of Microbes Associated with Food Spoilage

2. Poultry and eggs. Species of Salmonella cause diseases in chickens and turkeys, passing to the consumer through the poultry or egg product. Processed foods as pot pies, egg salad, omelet etc. may be sources of salmonellosis.

Chicken product may become contaminated due to improper handling or from the water used for cleaning the product. Eggs may become contaminated by Proteus causing black rot (here H2S accumulates due to digestion of egg cysteine), by Pseudomonas causing green rot, and by Serratia marcescens, causing red rot. The main part of contamination is yolk (white is inhibitory to Gram-positive bacteria due to the presence of inhibitor enzyme, lysozyme). (BSc Microbiology of Food Notes Study Material)

3. Breads and bakery products. The ingredients of bread products as flour, egg, sugar and salt are usually the sources of spoilage organisms. Some bacteria and molds are able to survive the baking temperatures. Some species of Bacillus give the bread a soft and cheesy texture with long, stringy threads. This kind of bread is called ropy.(BSc Microbiology of Food Notes Study Material)

Cream rolls, custards from whole eggs, whipped cream are good media for growth of Salmonella, Lactobacillus and Streptococcus species which produce acids. (BSc Microbiology of Food Notes Study Material)

4. Other foods. Many cereals, fruits and vegetables are spoiled by microorganisms. Chief agents are molds and bacteria giving them unpleasant smell and odour. Grains are spoiled by a mold, Aspergillus flavus. It is also present in peanut products and other 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.

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.

Principles and principal methods of food preservation.
Principles and principal methods of food preservation.

Modern preservation methods make use of the following agents:

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 dram 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 Clostridium 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 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. (BSc Microbiology of Food Notes Study Material)

Food Preservation Methods

Blanching of fruits and vegetables, by scalding with hot water or steam prior to deep freezing, inactivates plant enzymes that may produce toughness, change in colour etc. A brief scalding prior to freezing also reduce the number of microorganisms on the food surface by up to 99 per cent, enhances the colour of green vegetables. (BSc Microbiology of Food Notes Study Material)

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.

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.(BSc Microbiology of Food Notes Study Material)

4. Osmotic pressure. 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.

Food Preservation Methods

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.

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. (BSc Microbiology of Food Notes Study Material)

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. (BSc Microbiology of Food Notes Study Material)

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.

BSc Microbiology of Food Notes Study Material

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