Control of microorganisms in food

Prepared bySamira fattah

Assis. Lec.College of health sciences-HMU

Lecture 7

• Although some microorganisms are desirable for the production of bioprocessed food, many are undesirable because they can cause food spoilage and food borne disease.

• For efficient production of bioprocessed food, the objective is to stimulate growth and increase the viability of desirable microorganisms.

• In contrast, for spoilage and pathogenic microorganisms, the objective is to minimize their numbers or completely eliminate them from food.

• Several methods ,individually or in combinations, are used to achieve these goals by:

1) Controlling access of the microorganisms in food.2) physically removing the microorganisms present in

foods.3) Preventing or reducing the growth of microorganisms

and germination of spores present in foods.4) Killing microbial cells and spores present in foods.

Methods of microorganisms control in foods

1. Control of access(cleaning and sanitation)

• The main objective of sanitation is to minimize the access of microorganisms in food from various sources at all stages of handling.

• Proper sanitation helps reduce the microbial load to desired levels in further processed food.

• For example a low microbial level in raw milk produced through effective sanitation make it easier to produce pasteurized milk that meets the microbial standerd.

• Also proper sanitation increase shelf life of food.

• And also reduce incidence of foodborne disease.

• Several groups of sanitizers are approved for use in food-processing plants:

1) Chlorine-based sanitizers 2) Iodophores 3) Quaternary ammonium compounds4) H2O2

• They are effective against gram positive and gram negative bacteria, bacterial spores, viruses, yeast and molds.

2. Control by physical removal

Microorganisms that have gained access to food can be controlled by removing them physically, this can be achieved by different methods:

1) Centrifugation 2) filteriation3) Trimming4) Washing

• In general these methods can partially remove microorganisms from food and help other antimicrobial steps that follow to become more effective.

3. Control by heat

• The main objective of heating food is to destroy vegetative cells and spores of microorganisms(bacteria, mold, yeast and viruses) especially pathogenic and spoilage causing ones.

• This is necessary in order to retain the acceptance and nutritional qualities of a food.

On the basis of temperature and time of heating the food used to destroy microorganisms, the methods can be divided to:

1) Low-heat processing or pasteurization the temperature used is below 100⁰ C. the objective is to

destroy all the vegetative cells of the pathogens.

• Pasteurization of milk has been used for a long time, two methods are used:

a) low temperature long time (LTLT) heating at 145⁰ F(63 ⁰ C) for 30 min.

b) High temperature short time(HTST) heating at 261 ⁰ F(72 ⁰ C) for 15 sec.

2) high-heat processed foods The process involve heating foods at or above 100 ⁰ C for a

desired period of time. The temperature and time of heating are selected on the basis

of product characteristics and the specific microorganisms to be destroyed.

• Commercial sterility is obtained by heating the food at very high temperature for a short time.

• This process is named as Ultrahigh temperature(UHT) processing.

• Milk is heated to 150 ⁰C for 2 to3 sec. it can be stored at room temperature (≤30 ⁰C) and the product have 3-month shelf life.

4. Control by low temperature

• Main objective in low-temperature preservation of food is to prevent or reduce growth of microorganisms.

• Low temperature also reduce or prevent catalytic activity of microbial enzymes especially heat-stable proteinases and lipases.

• Germination of spores is also reduced.

• However the death rate of M.O at low temperature , as compared with that at heat treatment cannot be predicted.

• Also spores are not killed at low temperature, thus food are not preserved at low temperature in order to kill microbial cells.

• Foods are stored at low temperature in different ways to extend their shelf life, these methods are used for low-temperature preservation of foods:

a) Ice chillingb) Refrigerationc) Freezing

5. Control by reduced water activity

• The main objectives of reducing water activity in food are to prevent or reduce the growth of vegetative cells and germination of spores of M.O.

• Prevention of toxin production by toxigenic molds and bacteria is also an important consideration.

• Microbial cells(not spores) also suffer reversible injury and death in food with low water activity, although not in a predictable manner as in heat treatment.

• The water activity of food can be reduced by using one or more of these methods:

1) Natural dehydration Is low cost method, water is removed by the heat of sun. 2) Mechanical drying Foods travel through a tunnel against flow of hot air that

remove the water.

3) Smoking meat and fish are exposed to low heat and smoke ,the heating

process remove water from the product and smoke is deposited on the surface at the same time.

6. Control by low pH and organic acids

• The major objective of using weak organic acids is to reduce the pH of the food to control microbial growth.

• As the pH drops below 5.0, some bacteria become injured or die.

• However the death rate in low pH is not predictable as in the case of heat.

Acids used:a) Acetic acidb) Propionic acidc) Lactic acidd) Citric acide) Sorbic acidf) Benzoic acid

-these acids are used in vigetable pickles, salad dressing, jam, carbonated drinks and ….etc.

7.Control by modified atmosphere

The growth of aerobes (mold, yeast and bacteria) is reduced, but under these conditions anaerobic and facultative anaerobic bacteria can grow unless other techniques are used to control their growth.

Methods are:a) Vacuum packaging used for meat products and some types of cheese.

b) Gas flushing used for fresh and cooked products. the gases usually used

are a mixture of CO2 and N2 with some O2 for packaging red meats.

8. Control by antimicrobial preservatives

• Antimicrobial chemicals are used in food in relatively small doses either to kill undesirable M.O. or to prevent their growth.

• They differ greatly in the ability to act against different M.O.• Some are narrow spectrum.• Some are broad spectrum.• Some act aginst G+ or G – or bacteria spores or viruses.

• Those that are capable of killing M.O. are designated as: - Germicides(kill all types) - Fungicides - Bactericides - Sporicides - Viricides

• Those that inhibit or retard microbial growth are classified as: - Fungistatic - Bacteriostatic

• Example of antimicrobial preservatives:a) nitrite(NaNO2)b) Sulfer dioxide(SO2)c) H2O2d) Ethylenediaminetetraacetate (EDTA)e) Antibioticsf) Spices

9. Control by irradiation

• A food is irradiated because of the destructive power of ionization on M.O. a food harbors.

• Depending on the method used, it can either completely or partially destroy mold, yeast, bacterial cells and spores.

• In addition , irradiation can destroy worm, insects and larvae in food.

• Irradiation cannot destroy toxins in food.

UV radiation• M.O. are specially susceptible to UV Light

between 200-280nm.

• Because of low penetration power it has been used to inactivate M.O. on the surface of foods(meat, fish and bread) and liquids such as water and syrups.

• as will as on walls, shelves and equipment in the food handling and processing area.