Department of microbiology microbial food technology group a diploma in quality assurance in microbiology diploma

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Quick freezing Sharp /slow freezing

1. –15 to –29 C for 30 min 1. –15 to –29 C for 3-4 hrs till 72 hrs

2. No food damage 2. Damage of food by crystal formation.

3. Done by: 3. Done by the natural air circulation or

a. Direct immersion of food / package in through electrical fans.

Refrigerant. E.g.: fish in brine

b. Indirect contact (-17.8 to –45.6 c)

c. Air blast freezing (-17.8 to –34.4C)

Rigid air is blown.

Advantage of quick freezing:

1. Shorter period.

2. Prompt prevention of microbial growth.

3. Rapid slowing of enzyme action.

Dehydrofreezing: fruits and vegetables have about half there moisture removed before freezing.

Changes during freezing:

1. Expansion in volume of food.

2. Ice crystals formation may crush cells.

3. Frozen condition chemical and enzymatic reaction proceed slowly.

E.g.: meat, poultry, fish products, proteins may irreversibly dehydrated.


Meat -> red myoglobin ------------- brown metmyoglobin

On surface

Fats (meat, fish) ----- oxidized and hydrolysed

4. Metacryotic liquid:

Unfrozen, concentrated solutions of sugars, salts may ooze from packages of fruits or concentrates during storage as a viruses material.

5. Fluctuation in temperature results in ice crystal formation.

6. Deracination may occur.

7. Freeze burn :

when ice – crystals evaporate from the area at the surface this defect is observed. The spot appears dry, grainy and brownish, tissues become dry and tough.

E.g.: fruits, vegetables, meat, poultry and fish.

8. During freezing vegetative cells die soon but some may remain for a longer period of time.

Changes during thawing:

1. Drip / bleeding:

The pink or reddish liquid that comes from meat during thawing.

2. Leakage:

The liquid oozing out of fruits or vegetables on thawing.

3. The wilting and flabiness of physical damage during freezing.

4. Thawing refers to sudden heating and sudden cooling. The damage of food is due to the freezing and storage but do not become evident earlier. Some of the liquid during thawing may be reabsorbed by the food particles or may remain as such.

If the thawed fleshed foods are below 3.3 C can be used but otherwise food should be discarded.

Effects of freezing:

1. Lethal effects:

Rapid cooling of cells from optimal to 00 c may also result in death and referred to as cold shock, where there is change in lipid membrane damage the permeability of cell or to the release of repair enzyme inhibitors. E.g.: ribonuclease inhibitors

2. Sub – lethal effects:

During enumeration of frozen food there may be reduction but not tree death of organisms. Some may be injured or damaged are called as freeze – injured, frost injured or metabolically injured. Freezing of micro organisms in a food may result in cryoinjuiry.

Response of microorganisms to freezing:

Freezing depends on type of microorganisms usually found in foods involved in preservation. There are various factors involving freezing.

1. On the basis of sensitivity of microorganisms during freezing they can be classified as 3 different groups:

a. Susceptible or sensitive => e.g.: yeast, mould, gram-negative bacteria, and vegetative cells.

b. Moderately resistance => e.g. Staphylococcus, Enterococcus, gram-positive bacteria.

c. Resistant => e.g. Spore forming organisms.

2. Freezing also depends on the freezing rate. Critical range of temperature lead to death of microbes than during rapid freezing.

3. It also depends on the kind of food normally used for presentation. The food used for preservation by freezing usually gets spoiled due to

a. High moisture content.

b. Availability of O2

c. Salt and sugary environment.

4. Freezing also depends on the change in PH or altered acidity or alkalinity in food.

5. During freezing there is increase in moisture content and formation of intracellular crystals. This usually results in altered permeability in membrane and cell wall. Thus results in osmotic imbalance or osmotic shock favoring cell lyses. Intracellular lie crystals are harmful to cells than extra cellular ice crystals.

6. The initial killing rate during freezing is rapid, but it is followed by a gradual reduction of microorganisms are referred as storage death.



Drying is referred to as the removal of water or lowers the water activity or reduces the amount of available moisture.

E.g., Dried fish => salt, condensed milk => sweet.

a). Sun Drying  Drying of food by exposure to suns rays.

b). Dehydrated / Desiccated  Drying by artificial means under controlled air flow, T and RB

c). Condensed  Drying where moisture removal from liquid substances

d). Evaporated  Similar to dehydrated.


Before drying moisture %

After drying moisture %

  1. Milk

  2. Egg

  3. Beef

  4. Apple juice









Methods of drying:

1. Solar drying:

 Direct sun’s rays

E.g., Raisins, figs, pears, peaches, rice, fish

2. Drying by mechanical dryers:

Passage of heated air to food under controlled RH.


 They are used in form house

 Natural draft from heated air brings drying.

b) Forced draft drying:

 Heated air moves across the food usually in tunnels or food moved in conveyor belts through heated air.

c) Spray dried:

 Spraying of liquid into a current of dry, heated air.

d). Drum dried:

 Passage over a heated drum, with or without vacuum.

3. Freeze drying:

Sublimation of water from frozen food by means of a vacuum and heat.

E.g., Meat, Poultry, seafood’s and fruits.

4. Drying during smoking:

E.g., wool smoke  desired flavors and preservative are uses.

Meat  43 – 71 C for few hrs to several days prevents mold growth.

It has HCHO, phenol, cresol, methyl and ethyl esters, ketones etc.

5. Other methods:

  1. Electronic Heating

  2. Foam – mat drying ->Lipid whipped to foam, dried with warm air, crushed to powder, as is pressure – gun puffing of partially dried foods to give a porous structure facilitate further drying.

  3. Tower Drying ->Dehumified air at 30 C or less.

E.g. Tomato concentrate, milk and potatoes.


  1. Temperature

  2. Relative humidity of air

  3. velocity of air

  4. time of drying

If all these not accounts may lead to case hardening where rapid, evaporation of moisture from the surface than diffusion from the interior leads to hard, horny, impenetratable surface film that hinders further drying.


Before Drying, Drying and after Drying:

Before reception into plant:

Food has to be inspected without any contamination as;

  1. Milk -> Pure from udder in low, may be contaminated by handlers, process, and equipments.

  2. Meat/ Poultry -> Due to soil, intestinal activity, handlers, equipments.

  3. Fish -> By intestinal activity, surface slime, and handlers.

  4. Egg -> Handlers, equipments, hatched hen and soil.

Before Drying:

1. Selection:

a. Elimination of spoiled foods.

  1. Rejection of cracked, dirty foods.

  2. Sorting for size, maturity and soundness.

2. Washing:

Especially fruits and vegetables. These procedures are followed to remove soil and adhering materials and removes microbes. Water must be pure as it may also acts as a source of contaminate if poor quality of water is used.

E.g., Egg -> Moisture helps the bacteria to penetrate the shell.

3. Peeling:

May be done by hand, machine, lye bath or abrasion. It reduce the number of microorganisms are on the surface.

4. Sub division:

Slicing, cutting should not increase number of organisms but will do so if equipment is not adequately cleansed and sanitized

5. Alkali Dip:

It may reduce the microbial population.

E.g., Raisins, Grapes, etc -> Hot 0.1 – 1.5 % lye / Na2CO3

6. Scalding / Blanching:

    1. Sulfuring of light colored fruits and certain vegetables.

    2. Fruits -> 1000 – 3000 ppm of SO2 gas

    3. Vegetables -> dipping after blanching or spraying of sulfite solution.

    4. Helps to maintain an attractive light color, conserve vit C, vit A, repels insect, kills many microorganisms.


  1. Heat

  2. Freeze drying

After Drying:

1. Sweating:

Storage in boxes or tins. It is for equalization of moisture or addition of moisture to a desired level.

E.g., Dehydration of meat at 60 C -> leads to growth of Staph. aureus ., so that 1000C applicable.

2. Packing:

Packed the foods after drying for protection against moisture contamination with microbes, insects.

3. Pasteurization:

Fruits usually during package -> 30 to 70 min – time, 70 to 100% - RH, 65.6 to 850C – Temperature.

Microbiology of dried foods:

  1. Dried fruits: Mold spores may be seen.

  2. Dried vegetables: Few 100’s per gram to million of organisms due to the improper pretreatment. E.g.: Bacillus, Micrococcus, Clostridium, E.coli, Enterobacter, Pseudomonas, Streptococci and Lactobacillus, Leuconostoc.

  3. Dried eggs: Coli forms, spore formers, molds, Micrococcus, Streptococci.

  4. Dried milk: Spore formers, Thermoduric, Streptococci, Micrococcus.

Intermediate moisture foods: (IMF)

1. Commercially prepared foods haves 20-40% moisture and are non-refrigerated shelf stability are IMF.

2. They have reduced water activity.

E.g.: Candies, Jams, jellies, honey, bakery items etc.

3. Aw may be 0.75 and 0.85 for IMF.

4. They can be adjusted by the addition of sugar, salt or glycerols.



  1. A food additive is a substance or mixture of substances, other than the basic food stuff, is present in food as a result of any aspect of production, processing, storage or packaging.

  2. The definition emphasizes one interpretation of a food additive, i.e.; it is an intentional additive. There food additives are specifically added to prevent the deterioration or decomposition of a food have been referred to as chemical preservatives.

  3. This decomposition may be caused by micro organisms, by food enzymes, or by purely chemical reactions. The inhibition of the growth and activity of micro organisms is one of the main purposes of the use of chemical preservatives

  4. Preservatives may inhibit micro organisms by interfering with their cell membranes, their enzymes activity or their genetic mechanisms.

Factors that influence the effectiveness of chemical preservatives in killing micro organisms or inhibiting their growth.

    1. Concentration of the chemical

    2. Kind, number, age & previous history of the organism

    3. Temperature

    4. Time

    5. The chemical & physical characteristics of the substrate in which the organism is found.

The ideal antimicrobial preservative:

      • A chemical preservative should have a wide range of antimicrobial


      • Should be nontoxic to human being or animals

      • Should be economical

      • Should not have an effect on the flavor, taste or aroma of the original food

      • Should not be inactivated by the food or any substance in the food

      • Should encourage the development of resistant strains

      • Should kill rather than inhibit micro organisms

Organic acids and their salts:

Lactic, acetic, prop ionic & citric acids or their salts may be added to or

developed in foods

. Citric acid is used in syrups, drinks, Citric acid is used in syrups, drinks,jams &


Lactic and acetic acids are added to brines of various kinds, green olives, etc.


Sodium or calcium propionate is used most extensively in the prevention of mold growth & rope development in baked foods & for mold inhibition in many cheese foods and spreads.

Experimentally, or on a limited scale, they have been used in butter, jams, jellies, apple slices & malt extract

They are effective against molds, with little or number inhibition of most yeast and bacteria.


The sodium salt of benzoic acid has been used extensively as an antimicrobial agent in foods.

It has been incorporated into jams, jellies, carbonate (beverages, fruit salads, pickles, fruit juices etc.


Sorbic acid, as the calcium, sodium or potassium salt, is used as a direct antimicrobial additive in foods.

It is widely used in cheeses, cheese products, baked goods, beverages, syrups, fruit juices, jellies, jams, dried fruits & pickles.

Sorbic acid & its salts are known to inhibit yeast & molds but are less effective against bacteria.


Derivatives of acetic acid

Dehydroacetic acid has been used to impregnate wrappers for cheese to inhibit the growth of molds

Acetic acid is more effective against yeast & bacteria than against molds.

Nitrites and Nitrates

Combinations of these various salts have been used in curing solutions & curing mixtures for meats.

Nitrites decompose to nitric acid, which forms nitrosomyoglobins when it reacts with the heme pigments in meats & thereby forms a stable red colour.

They are currently added in the form of sodium nitrite, potassium nitrate.

Recent works has emphasized the inhibitory property of nitrites towards Clostrium botulinum in meat products.

Sulfur dioxide and Sulfites:

The Egyptians and Romans burned sulfur to form sulfur dioxide as a means of sanitizing their wine – making equipments & storage vessels.

Today sulfur dioxide and sulfites are used in the wine industry to sanitize equipment to reduce the normal flora of the grape must.

Ethylene propylene oxide:

Ethylene oxide kills all micro organisms; propylene oxide, although it kills many micro organisms.

The primary uses have been as sterility for packaging materials, fumigation of water houses, & “cold sterilization” of numerous plastics, chemicals, pharmaceuticals, syringes & hospital supplies.

They have also been used successfully in dried fruits, dried eggs, cereals, dried yeast and spices

Sugar and salts:

Sodium chloride is used in brines & curing solutions or is applied directly to the food

Enough may be added to slow or prevent the growth of microorganisms or only enough to permit an acid fermentation to take place.

Salt has been reported to have the following effects.

It causes with osmotic pressure & hence plasmolysis of cells

It dehydrates foods by drawing out from the microbial cells.

It ionizes to yield the chlorine ion, which is harmful to organism

It reduces the solubility of oxygen in the moisture,

It sensitizes the cells against carbon dioxide

It interferes which the action of proteolysis enzymes.

Sugars, such as glucose or sucrose, owe their effectiveness as preservatives to their ability to make water unavailable to organisms and to their osmotic effect.

Examples of foods preserved by high sugar concentration are sweetened condensed milk, fruits in syrups, jellies & candies.)


1.Discuss the food poisoning and food borne inflections.


Introduction :

  • Food borne diseases may be of 2 types,

    1. food borne infections

    2. food borne intoxications.

  • Food borne intoxication is by the presence of microbial toxin formed in the food.

  • Food borne infection is caused by the microbe’s entry into the body through ingestion of contaminated food & the reaction of the body to their presence or to their metabolites.

  • Food borne infection can be divided into 2 types are

[i] food that does not support growth of pathogens but merely carries them. Eg. Diphtheria, Dysentry, Typhoid fever, Brucellosis, Cholera, Infectious hepatitis, Q fever.

[ii] food that serve as a culture medium for the growth of the pathogens to no.s that will increase the infection of the consumer of the food. Eg. E. coli, Salmonella, V.parahaemolyticus.

  • Outbreak of infections are explosive in 2nd type.


Food borne diseases

Poisonings Infections

Chemical poisonings Intoxications Enterotoxigenic Invasive

Poisonous Poisonous Microbial Sporulation Growth Intestinal Systemic Other

Plant tissues Animal tissues intoxications & lysis mucosa Tissues

Algal Mycotoxins Bacterial toxins Muscle Liver


Enterotoxins Neurotoxins Interferes with

Carbohydrate metabolism

Intoxications Infections

1. Staphylococcal intoxication – an enterotoxin - 1. Salmonellosis– Enterotoxin & cytotoxin

S. aureus. 2. Cl. Perfringens - Enterotoxin

2. Botulism – neurotoxin – Cl. botulinum. 3. B. cereus – Exoenterotoxin - Gastroenteritis

4. Enteropathogenic E.coli – Enterotoxin - EPEC

5. Others: Vibrio parahemolyticus, Yersiniosis,

Shigellosis, Bacillus.




  • Food poisonings is caused by the ingestion of the enterotoxin formed in food during the growth of

S. aureus.

  • The toxin is enterotoxin because it causes gasteroenteritis or inflammation of the lining of the intestinal tract.


  • Cluster of grapes or in pairs and short chains, Golden yellow colonies are formed on solid media.

  • Coagulase positive, aerobes, facultative anaerobes, some strains are salt tolerant [10-20% Nacl].

  • Fairly tolerant of dissolved sugars [50-60% sucrose], Fermentative & preteolytic but do not produce obnoxious odour [unattractice].

  • Based on serology, 6 distinct enterotoxins are classified [type A, B, C1, C2, D, E], A most effective toxin, Toxin production varies with food involved.

  • Water activity [0.86 – aerobes, 0.90 – anaerobes], pH [ 4.8 – aerobes, 5.5 – anaerobes], Temperature [370C – optimum growth, 25 – 45oC – minimum, 4 – 460C - Survive], 660C – 12 mins, 600C – 78 to 83 mins are necessary to destroy the organisms in food.

  • D value – 60oC – 7.7 mins – Decimal reduction time, Radiation to kill Staphylococci is gamma rays on moist foods – 0.37 to 0.488 Mrad of gamma rays on moist foods.

Enterotoxin character :

  • Simple protein with molecular weight between 26,000 – 30,000 is a single polypeptide chain are cross linked by a disulphide bridge to form a cystine loop.

  • Organism is heat labile but toxin is heat stable. Type A & D mainly cause disease. Increased concentration of toxin is necessary to cause disease.

  • Toxin gets inactivated at 190.6oC. Temperature affects the toxin production [ 370C – 12 hrs,

180C – 3 days, 90C – 7 days, 4 – 6.70C - 4 days].

Foods involved :

  • Bakery food products [cream biscuits], Milk and milk products, Cured meat, Ham, Poultry and poultry products, Salads, egg and egg products.

Disease :

  • Incubation period [2 – 4 hrs] – first symptom seen. Common symptoms are salivation, nausea, vomiting, abdominal cramping, diarrhea, dysentry.

  • In some cases, vomiting, headache, muscular cramping, sweating, chills, weak pulse, respiratory tract problems [ cannot swallow] these may be the secondary symptoms.

  • Decreased death rate and disease can be cured within 4 days.

  • Active organism secretes enterotoxin into food Food eaten Enterotoxin affects gut giving gasteroenteritis

Enterotoxin ingested along with food affects cells

Enterotoxin affects vomit receptors Water & Sodium pumps out of the cell

Vomiting center in the brain stimulated Diarrhea, fluid and electrolyte loss.

Vomiting Dehydration

Conditions for outbreak :

  • Food must contain enterotoxin producing Staphylococci.

  • Food must be a good culture medium for growth & toxin production by the Staphylococci.

  • Temperature must be favourable and enterotoxin bearing food may be ingested.

Prevention of outbreaks :

  • Prevention of contamination of food with Staphylococci.

  • Killing of Staphylococci growth.

  • Prevention of Staphylococci growth.

  • Contamination of foods can be reduced by :

    1. general methods of sanitation.

    2. Using ingredients free from cocci – eg. Pasteurised milk than raw milk.

    3. By keeping employees away from foods who have colds, boils, carbuncles, etc.

    4. Adequate refrigeration of food.

    5. Addition of bacteriostatic substances such as serine or antibiotic.

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