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Food BiotechnologyDr. Kamal E. M. Elkahlout

Food Microbiology 2 Control of

Microorganisms in Food

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Definitions Controlling access of microorganisms Control By Physical Removal Centrifugation Filtration Trimming Washing Control By Heat Low-heat processing or pasteurization. High-heat processing Microwave Heating Control By Low Temperature Ice Chilling Refrigeration Freezing CONTROL BY REDUCED Aw

Definitions Sterilisation: The process by which all the

living cells, viable spores, viruses, and viroids are either destroyed or removed from an object or habitat.

Disinfection: Is the killing, inhibition or removal of microorganisms that may cause disease. Disinfectants are usually chemical agents, and are normally used only on inanimate objects.

Sanitization: Microbial population is reduced to levels that are considered safe by public health standards.

Antiseptics: Chemical agents applied to tissue to prevent infection by killing or inhibiting pathogen growth.

Bactericide: A disinfectant/ antiseptic against bacteria.

Cidal: A suffix meaning that “the agent kills.” For example, a bacteriocidal agent kills bacteria.

Bacteriostatic: Do not kill, but prevent growth of bacteria.

Static: A suffix that means “the agent inhibits growth.” For example, a fungistatic agent inhibits the growth of fungi, but doesn’t necessarily kill it.

Some microorganisms are desirable for the production of bioprocessed

foods

Many are undesirable due to their ability to cause food spoilage and food borne diseases

Several methods (individually or in combination) are used to achieve control

These are: Controlling access of the

microorganisms present in foods. Control by physical removal. Control by heat. Control by low temperature. Control by reduced Aw Control by low pH and Organic acids Control by Modified Atmospheric (O-R

potential) Control by Irradiation Control by antimicrobial preservative

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Controlling access of microorganisms (Cleaning and

Sanitation)

Controlling access of microorganisms (Cleaning and

Sanitation)To minimize the access of microorganisms in foods:

• the microbial quality of the environment to which a food is exposed (food contact surfaces) should be good.

• The ingredients added to the food should be of good microbial quality.

• Sanitation minimizes the access of microorganisms in food from various sources at all stages of food handling.

• Proper sanitation helps to produce food that have a long shelf life.

Plant Design

When designing a food processing plant, an efficient sanitary program must be integrated in order to provide maximum protection against microbial contamination of foods.

This includes both the outside and the inside of the plant.

- floor plan and approved materials used in construction.

- adequate light, air ventilation, direction of air flow. - separation of processing areas of the raw and finished products.

- sufficient space for movement and operations. - water supply and sewage disposal system, waste

treatment facilities, drainage and surrounding environment.

Quality of Water, Ice, Brine and Curing Solution

• Water is the most important element in food manufacturing operations.

• Water is used as an ingredient in many foods

• also used in some products after heat treatment.

• Eg: ready-to-eat types, should not only be free from pathogens (like drinking water), but also be low (if not free) in spoilage bacteria, such as Pseudomonas spp.

Important for foods that are kept at low temperature for extended shelf life.

Eg., ice used for chilling unpackaged foods should also not contaminate a food with pathogenic and spoilage bacteria.

Brine and curing solutions used in products such as ham, bacon, cured beef etc can be a source of contamination hence should be made fresh daily to be used for processing.

Quality of Air

• Food processing operations, such as spray drying of nonfat dry milk, require large volumes of air that come into direct contact with the food.

• Important to install air inlets to obtain dry air with least amount of dust and filtration of air.

Training of PersonnelA processing plant should: Have an active program to teach the plant

personnel the importance of sanitation and personal hygiene in-order to ensure product safety and stability.

Also monitor the implementation of such program.

People with an illness and infection should be kept away from handling the food products.

Equipment

design of food processing equipment should protect a food from microbial contamination.

Protection is achieved if the equipment does not contain dead spots where

microorganisms harbor and grow and cannot be easily and readily cleaned in place or by disassembling.

Some of the equipments such as meat grinders, choppers, slicers and several types of conveyor systems not properly sanitized can be a source of contamination.

Equipment sanitizing is important for products that come in contact with equipment surfaces after treatment and before packaging.

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Control by

Physical Removal

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Centrifugation

A process used to separate or concentrate materials suspended in a liquid medium.

The technique is based on the effect of gravity on particles in suspension. Two particles of different masses will settle in a tube at different rates in response to gravity.

Centrifugation - used in some liquid foods, such as milk, fruit juices and syrup,to remove suspended undesirable particles (dust, leukocytes and food particles).

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Under high forces, as much as 90% of the microbial population can be removed.

Following centrifugation, a food will have fewer thermoduric microorganism (bacterial spores) that otherwise would have survived mild heat treatment (e.g. milk pasteurization).

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Filtration

Filtration - used in some liquid foods, such as soft drinks, fruit juices, beer, wine and water

to remove undesirable solids and microorganisms and to give a sparkling clear appearance.

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As heating is avoided (or given only at minimum levels,) the natural flavor of the products and heat- sensitive nutrients (e.g. vitamin C in citrus juices) are retained to give the products natural characteristics.

Coarse filters are initially used to remove the large component, followed by ultra-filtration to remove small particles.

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Filtration of air

used in food processing operations.

such as spray drying milk; to remove dust from air used for drying.

The process removes microorganisms with dust and they reduce the microbial level in food from source (air)

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Trimming Fruits and vegetables showing damage and spoilage are generally trimmed.

Areas heavily contaminated with microorganisms are removed.

Trimming of outside leaves in cabbage helps reduce microorganisms coming from soil.

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Trimming is also used to remove visible mold growth from hard cheeses, fermented sausages, bread and some low pH products.

If a mold strain is a mycotoxin producer, trimming will not ensure removal of toxins from the remaining food.

Trimming is also used to remove fecal stain marks, unusual growths and abscesses or small infected areas from carcasses of food animals and birds.

Trimming allows complete removal of the causative microorganisms.

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Washing

Washing equipment and work areas is discussed under cleaning and sanitation (last week).

Fruit and vegetables are washed to reduce temperature (that helps to reduce metabolic rate of a produce and microbial growth) and remove soil.

Washing removes the microorganisms present, especially from the soil. It is also used for shell eggs to remove fecal materials and dirt.

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CONTROL BY HEAT

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The desirable effect of heat (fire) on the taste of foods of animal and plant origin, especially seeds, tubers and roots, was probably accidentally discovered by our ancestors .

They also possibly recognized that heated foods did not spoil as fast as raw foods.

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The main objective (microbiological) of heating food is to destroy vegetative cells and spores of microorganisms that include molds, yeasts, bacteria and viruses.

Drastic heat treatment (sterilization) can be used to kill all the microorganisms, which is present in a food.

Most foods are heated to destroy – pathogenic and spoilage microorganisms

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Antimicrobial Action of Heat

Depending upon the temperature and time of heating, microbial cells and spores can be sub-lethally injured or dead.

Death occurs from damages in vital functional and structural components.

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Factors Affecting Heat killing of microbial cells

The effectiveness of heat in killing microbial cells and spores is dependent on factors:

related to the inherent nature of the foods

on both the nature of microorganisms and the nature of processing.

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1. Nature of Food

Composition (amount of carbohydrates, proteins, lipids and solutes),

Aw (moisture), pH, and anti-microbial content (natural or added)

greatly influence microbial destruction

Microorganisms in liquid food and food containing small-sized particles suspended in a liquid are more susceptible to heat destruction than in a solid food or in a food with large chunks.

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2. Nature of Microorganisms

Factors that influence microbial sensitivity to heat is inherent resistance, stage of growth, previous exposure to heat and initial load.

In general vegetative cells (moulds, yeasts and bacteria) are more sensitive than spores

thermoduric and thermophilic bacterial cells (important in foods) are destroyed in 5 to 10 minutes at 75 to 80°C

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Yeast and most mould spores are destroyed at 65 to 70°C in a few minutes,

Spores of some moulds can survive as high as 90°C for 4 to 5 h.

Bacterial spores varies greatly Heating at 80 to 85°C for few minutes

does not kill. But destroyed at 100°C in 30 min h/ever

some can withstand this Destroyed at 121°C in 15min

(sterilization Temp /Time)

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Cells at exponential stage of growth are more susceptible to heat than the resting cells (stationary phase)

Cells previously exposed to low heat become relatively resistant to subsequent heat temperature.

The higher the initial microbial load in a food – the longer time at a given temperature it takes to reduce the population.

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3. Nature of Process

Microbial destruction in food by heat ( inverse relationship). Higher the temperature, the shorter the period of time required for destroying the microorganisms provided other factors are kept constant.

As a food is heated by conduction (molecule-to-molecule energy transfer) and convection (movement of heated molecules), a liquid food is heated more rapidly than a solid food and a container with high conduction (metal) is better.

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Food in a small container is heated more rapidly than in a large container

Heating a food at a given temperature for a specific time means that every particle of that food should be heated to the specified temperature and stay at that temperature for the specified time –”holding time”.

see the reason for heating food before eating !!!!!!

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Methods using heatLow-heat processing or

pasteurization.

temperature used is below 100oC.

Process aims to destroy all vegetative cells of pathogens and microorganism which cause food spoilage.

Pasteurization of milk has been used for a long time – heating at 62.8oC for 30 mins or 71.7 oC for 15 secs.

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Methods using heatHigh-heat processing Process involves heating food at or above

100oC. Temperature and time of heating are

selected on the basis of product characteristics and specific microorganisms to be destroyed.

Most products are given a commercially sterile (sterilization) treatment to destroy to destroy microorganism growing in a product under normal storage conditions.

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High-heat treated products are either first packed in containers and then heated or heated first and then packed in sterile containers while still hot (hot pack).

Commercial sterility is also obtained by heating a food at very high temperatures for a short time (process called ultrahigh temperature (UHT) processing .

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Methods using heatMicrowave heating

Heating of foods by microwave (quite common at home).

Frozen foods can be thawed and heated rapidly in a few minutes depending upon the size of the product.

Microwave treatment is lethal to microorganisms and destruction is caused by high temperature.

If the food is not heated uniformly, some areas can remain cold and if food harbors pathogens, there is chance of their survival.

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CONTROL BY LOW TEMPERATURE

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Effectiveness of low temperature, especially freezing in food preservation was probably recognized by our ancestors in the last Ice Age.

The major drawback of refrigerated goods is their relatively short shelf life.

But in recent years, several technological improvements have helped in increasing the shelf life.

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Mechanisms of microbial control Metabolic activities, enzymatic reactions and

growth rates of microorganisms are maximum at optimum growth temperatures.

When temperature is lowered, microbial activities associated with growth slow down.

Rate of catalytic activity of enzymes decreases with reduced temperature.

As the temperature in a food drops to about – 2 oC, free water in the food starts freezing and forming ice crystals, hence Aw is also reduced.

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Foods are stored at low temperature in different ways in order to extend their shelf life.

Many fresh fruits and vegetables are kept at temperatures between 10oC and 20oC or lower to reduce their metabolic rates.

Highly perishable products are generally stored at low temperature below 7 oC often in combination with other preservation methods.

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Methods using heatIce chilling

Usually used Retail stores where foods are kept over ice.

The surface is in contact with ice, temperature can reach between 0oC – 1oC.

Temperature fluctuation, duration of storage and cross contamination can cause microbiological problems – food borne pathogens.

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Methods using heatRefrigeration

The temperature specification for refrigeration of foods has changed from time to time.

From 7oC, technological improvements have made it economical to have domestic refrigeration units at 4 to 5 oC.

For perishable products, ≤ 4.4oC is considered desirable refrigeration temperature.

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Commercial food processors may use as low as 1 oC for refrigeration of perishable foods such as fresh meat and fish.

Refrigerated products are often combined with additional preservation methods with lowest temperature possible for long shelf life.

As the products are non-sterile, even a very low initial microorganism population is capable of growing under the storage conditions.

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Methods using heatFreezing

Minimum temperature used in home freezers is – 20 oC, a temperature at which most of the free water in a food remains in a frozen state.

Dry ice ( -78oC) and liquid nitrogen (- 196oC) can also be used for instant rapid freezing, but not for food.

After freezing, the temperature of the food is maintained around -20oC to -30oC.

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Microbial cells will die upon during frozen storage, but survivors can multiply in the frozen state.

Accidental thawing or slow thawing can facilitate growth of survivors.

Enzymes released by dead microbial cells can reduce the acceptance quality of food.

CONTROL BY REDUCEDAw

The main objective of reducing Aw in food are to prevent or reduce the growth of vegetative cells and germination and outgrowth of spores of microorganisms

Microorganisms need water for the transport of nutrients, nutrient metabolism and removal of cellular wastes.

In a food, the total water (moisture) is present as free water and bound water.

Recall:Bound water: not available for biological functions.

Only the free water (related to Aw) is important for microbial growth.

If free water in the environment is reduced either by removing water or by adding solutes and hydrophilic colloids, the free water from the cells flow outside in an effort to establish equilibrium. (osmosis).

The loss of water will cause osmotic shock and plasmolysis during which the cells do not grow.

Water loss can be considerable even with a slight change in Aw

E.g 0.005 reduction in Aw from 0.955 to 0.950 in the environment reduced the intracellular water content by 50% in Staphylococcus aureus and reduces the cell volume by 44% in Sal. typhimurium.

Methods Used Water activity of foods can be

reduced by using one or more of three basic principles:– removing water by dehydration

– removing water by crystallization

– by adding solutes.

Natural Dehydration is a low-cost method in which water is

removed by the heat of the sun.

Used for dry grains as well as for dry some fruits (raisins) vegetables, fish, meat, milk and curd especially in warmer countries.

The process is slow and depends upon the conditions used, spoilage and pathogenic bacteria as well as yeasts and molds (including toxigenic types) can grow during drying.

Mechanical Drying

is a controlled process and drying is achieved in a few seconds to a few hours.

– Tunnel drying in which a food travels through a tunnel against the flow of hot air and the water is removed.

– Roller Drying in which a liquid is dried by applying in a thin layer on the surface of a roller drum heated from inside.

– Spray Drying, liquid is sprayed in small droplets, which then come in contact with hot air that dries the droplets instantly. Used for vegetables, fruits, fruit juices, milk, coffee, tea and meat.

Depending on Temp & time of exposure, some microbial cells die during drying, while some other cells can be sublethally injured.

Freeze-Drying Freeze-drying involves freezing the food

rapidly at a low temperature and then exposing the frozen food to a relatively high vacuum environment.

The water molecules are removed from the food by sublimation without affecting its shape or size.

Microbial cells are exposed to two stresses – freezing and drying that reduces some viability as well as induces some sub-lethal injury.

Foam Drying The foam drying method consists of whipping a

product to produce a stable foam and increase to the surface.

The foam is then dried by means of warm air.

Liquid products, such as egg white, fruit purees and tomato paste are dried in this manner.

The method itself has very little lethal effect on microbial cells and spores.

However, a concentration method prior to foaming,– the pH of the products and low Aw will cause both lethal and reversible damages to microbial cells.

Smoking

Many meat and fish products are exposed to low heat and smoke for cooking and depositing smoke on the surface at the same time.

The heating process removes water from the products lowering their Aw.

Low heat processed meat products (dry and semidry sausages) and smoked fish are produced this way.

Heat kills many microorganisms.

Growth of survivors is controlled by low Aw as well as antimicrobial substances present in the smoke.

Intermediate Moisture Foods (IMF)

These are foods that have an Aw value of 0.70 to 0.90 (moisture content, ∼10 to 40%).

Can be eaten without rehydration, are shelf-stable for a relatively long period of time without refrigeration and are considered microbiologically safe.

Traditional IMF includes semidry and dry sausages, dried fruits jam and jellies and honey.

Low Aw value is obtained by adding water-binding solutes and hydrophilic colloids.

Microorganisms can survive in the products but due to low Aw bacteria cannot grow.

Yeasts and molds can grow, to inhibit their growth specific preservatives such as sorbate and propionate are added.