Paper No.: 03 Paper Title: FOOD MICROBIOLOGY Module 35

Paper No.: 03

Paper Title: FOOD MICROBIOLOGY

Module – 35: Application of hurdle technology

in food industry

(e-Text and Learn More)

Component-I (A) - Personal Details:

FOOD

MICROBIOLOGY

APPLICATION OF HURDLE TECHNOLOGY IN FOOD INDUSTRY

Role Name Affiliation

National Coordinator Professor R.C. Kuhad University of Delhi South Campus

New Delhi

Subject Coordinator Professor Vijayakhader Former Dean,

Acharya N.G. Ranga Agricultural University,

Hyderabad

Paper Coordinator Professor A. K. Puniya National Dairy Research Institute (NDRI),

Karnal

Content Writer/Author Dr. Pradip Behare

Content Reviewer

Language Editor (LE)

Technical Conversion

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Component-I (A) - Module Structure:

Structure of Module/Syllabus of a module (Define Topic of module and its subtopic)

Application of Hurdle

Technology in Food Industry

Introduction, Principle of hurdle technology, Hurdle, Basic Aspects of

Hurdle Technology, Homeostasis, Metabolic exhaustion, Stress

reactions, Multitarget preservation, Individual Hurdles, Microbiocidla

Hurdles Reduces Microbial Load, Microbiostatic Hurdles (Chemical

Hurdles), Microbiostatic Hurdles (physical Hurdles), Hrudles that

prevent contamination, Application of hurdle technology in foods.

Component-II - Description of Module

Description of Module

Subject Name Food Technology

Paper Name Food Microbiology

Module Name Application of Hurdle Technology in Food Industry

Module Id FT/FM/35

Pre-requisites Hurdles, Concept, Hurlde technology in dairy and food products

Objectives To study about types of hurdles and their application in food industry

Keywords Hurdles, dairy foods, salt, sugar, high-pressure, hurdle concept

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TABLE OF CONTENTS

Table No. Description

Table 2.1 Examples of hurdles used to preserve foods

Table 3.1 Role of Microbiocidla Hurdles

Table 3.2 Role of Microbiostatic hurdles

Table 3.3 Role of Microbiostatic Hurdles

Table 3.4 Role of Hurdels in Preventing Contamination

Table 4.1 Application of Hurdle Technology in Dairy and Food Products

FIGURES OF CONTENTS

Table No. Description

Figure 2.1 Basic concept of bacterial inhibition by hurdles

Figure 4.1 Preservation of food by individual and combined hurdles

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1. Introduction

2. Principle of hurdle technology

2.1 Hurdle

2.2 Basic Aspects of Hurdle Technology

2.2.1. Homeostasis

2.2.2. Metabolic exhaustion

2.2.3 Stress reactions

2.3 Multitarget preservation

3. Individual Hurdles

3.1 Microbiocidla Hurdles Reduces Microbial Load

3.2. Microbiostatic Hurdles (Chemical Hurdles)

3.3. Microbiostatic Hurdles (physical Hurdles)

3.4. Hrudles that prevent contamination

4. Application of hurdle technology in foods

5. Summary

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1. INTRODUCTION

The primary objective of traditional and newly developed food preservation processes is the inhibition or

inactivation of microorganisms that ultimately helps to improve shelf stability of food. Every food has

certain inherent preservation factors such as extent of heat treatment received (F), water activity (aw), low

temperature storage (t), redox potential (Eh), pH, etc. which may be termed as hurdles, because

microorganisms will have to 'jump' these hurdles in order to grow and spoil the product. The stability of the

product depends upon the intensity of hurdles present in it. More the intensity or height of these hurdle, or

more the number of these hurdles, more difficult it will be for microorganisms to overcome these hurdles. In

conventional preservation method the intensity of one or two of these hurdle is exceptionally increased

making it extremely difficult for microorganisms to overcome that hurdle. For example, in sterilization

process F-value (i.e. the amount of heat treatment given) is increased to 3 to 15. Or in dehydration the water

activity (aw) is decreased to a very low value i.e. 0.85. Such increase or decrease in the intensities of these

parameters adversely affects the quality of certain products. Microbial stability and safety, as well as the

sensory and nutritional quality of most preserved foods, are based on a combination of several empirically

applied preservative factors (hurdles), and more recently on knowingly employed hurdle technology.

Deliberate and intelligent application of hurdle technology allows a gentle, efficient preservation of foods,

which is advancing worldwide. Many foods can not be preserved by a single hurdle alone without affecting

their sensory and nutritional properties. Therefore, hurdle Technology is the combination of selected hurdles,

which can keep microbiological hazards and other microorganisms under control, with or without

combinations with microbial steps, so as to obtain and retain end product safety or suitability.

2. PRINCIPLE OF HURDLE TECHNOLOGY

The most important hurdles commonly used in food preservation are temperature (high or low), water

activity (aw), acidity (pH), redox potential (Eh), preservatives (nitrite, sorbate, sulfite, etc.), and competitive

micro-organisms (e.g., lactic acid bacteria). More than 60 potential hurdles for foods of animal or plant

origin, which improve the microbial stability and/or the sensory quality of these products, have been already

studied, and the list of possible hurdles for food preservation is by no means complete. At present, physical,

non-thermal processes (high hydrostatic pressure, oscillating magnetic fields, pulsed electric fields, light

pulses, etc.) receive considerable attention (Non-thermal Processing), since in combination with other

conventional hurdles they are of potential use for the microbial stabilization of fresh-like food products, with

little degeneration of nutritional and sensory properties. Another group of hurdles, of special interest in

industrialized and developing countries at present, would be „natural preservatives‟ (spices and their extracts,

lysozyme, chitosan, pectin hydrolysate, etc.). In most countries, these „green preservatives‟ are preferred

because they are not synthetic chemicals, but in some developing countries, they are given preference, since

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spices are readily available and cheaper than imported chemicals. The critical values of many preservative

factors for the death, survival, or growth of micro-organisms in foods have been determined in recent

decades and are now the basis of food preservation. However, the critical value of a particular parameter

changes if additional preservative factors are present in the food. For instance, it is well known that the heat

resistance of bacteria increases at low aw and decreases at low pH or in the presence of preservatives,

whereas low Eh increases the inhibition of micro-organisms due to reduced aw. The simultaneous effect of

different preservative factors (hurdles) could be additive or even synergistic. In food preservation, the

combined effect of preservative factors must be taken into account, which is illustrated by the hurdle effect.

2.1 Hurdles

Microbial growth is dependent upon many conditions in the organism‟s environment such as ingredients;

nutrients, water activity, pH, presence of preservatives, competitive microorganisms, gas atmosphere, redox-

potential, storage temperature and time (Table 2.1). Control of these conditions can therefore be used to

limit, retard or prevent microbial growth.

One major use of hurdles is to prevent or restrict the growth and/or to reduce the concentration of

microorganisms, including target pathogens in milk, intermediate and final milk products. Most milk

products need the use of hurdles to become safe and suitable and/or to retain such quality.

Table 2.1. Examples of hurdles used to preserve foods

Type of hurdle Examples

Physical hurdles Aseptic packaging, electromagnetic energy (microwave, radio frequency, pulsed

magnetic fields, high electric fields), high temperatures (blanching, pasteurization,

sterilization, evaporation, extrusion, baking, frying), ionic radiation, low temperature

(chilling freezing), modified atmospheres, packaging films (including active

packaging, edible coatings), photodynamic inactivation, ultra-high pressures,

ultrasonication, ultraviolet radiation

Chemical hurdles Carbon dioxide, ethanol, lactic acid, lactoperoxidase, low pH, low redox potential,

low water activity, Maillard reaction products, organic acids, oxygen, ozone,

phenols, phosphates, salt, smoking, sodium nitrite/nitrate, sodium or potassium

sulphite, spices and herbs, surface treatment agents

Microbial derived Antibiotics, bacteriocins, competitive flora, protective cultures

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hurdles

2.2 Basic aspects of hurdle technology

The strength or intensity of a hurdle will normally not be sufficient to render the food as safe, but in

combination with other hurdles the desired effect can be achieved. Therefore, to ensure the safety and

suitability and or to extend the shelf life of milk products, generally more than one hurdle is needed to

control microbial content and or growth, to inhibit spoilage and to help prevent food borne diseases. Suitable

combinations of hurdles can be devised so that the organisms of concern can be reduced in number and or no

longer grow/survive in the product. Such suitable combinations are called “Hurdle Technology”.

Many hurdles act by interfering with the homeostasis mechanisms that microorganisms have evolved in

order to survive environmental stresses. Maintaining a constant internal environment requires significant

energy and material resources of the microorganism, and when a hurdle disturbs the homeostasis there will

be less energy left for the microorganism to multiply. Consequently, the organisms will remain in the lag

phase and some may even die out before the homeostasis is re-established. Hurdle Technology is most

efficient when it is multi-targeted that is, when various individual hurdles are selected so that different

systems of the microorganism are targeted, such as the cell wall, membrane transport, receptor functions,

signal transduction, control of gene expression, enzyme system, etc. In many cases, a multi-targeted hurdle

technology using hurdles with low intensity may be more effective than one single hurdle (treatment or

factor) with high intensity.

The presence of number of hurdles inhibiting or reducing the number of microorganisms may also be

synergistic. Some hurdles rely on a change of the physiological status of microorganisms, which leads to

stress. Consequently, other subsequently hurdles can become more efficient. Therefore, the utilization of

synergistic effects can allow for combating hurdles of less intensity to control growth than would be

otherwise expected from each hurdle individually. Similarly, when the microbiocidal hurdles used are of

sufficient intensity, the necessary performance may be less or the shelf life may become longer.

2.2.1. Homeostasis

Homeostasis is the tendency to uniformity and stability in the internal status of organisms. For instance, the

maintenance of a defined pH is a prerequisite and feature of living cells, and this applies to higher organisms

as well as to microorganismsm. In food preservation the homeosta-sis of microorganisms is a key

phenomenon, because if the homeostasis of these microorganisms is disturbed by preservative factors

(hurdles) in foods, they will not multiply, i.e. they remain in the lag-phase or even die, before homeostasis is

repaired (re-established). Therefore, food preservation is achieved by disturbing the homeostasis of

microorganisms in a food temporarily or permanently.

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2.2.2.Metabolic exhaustion

Another phenomenon of practical importance is metabolic exhaustion of microorganisms, which could cause

„autosterilization‟ of a food. This was first observed in experiments with mildly heated (95°C core

temperature) liver sausage adjusted to different water activities by the addition of salt and fat, and the

product was inoculated with Clostridium sporogenes and stored at 37°C. Clostridial spores surviving the heat

treatment vanished in the product during storage.

The most likely ex-planation is that bacterial spores which survive the heat treatment are able to germinate in

these foods under less favourable conditions than those under which vegetative bacteria are able to multiply.

Thus, the spore counts in stable hurdle-technology foods actually decrease during storage of the products,

especially in unrefrigerated foods. A general explanation for this surprising behaviour might be that

vegetative microorganisms which cannot grow will die, and they die more quickly if the stability is close to

the threshold for growth, storage temperature is elevated, antimicrobials are present, and the microorganisms

are sublethally injured. Apparently, microorganisms in stable hurdle-technology foods strain every possible

repair mechanisms for their homeostasis to overcome the hostile environment, by doing this they completely

use up their energy and die, if they become metabolically exhausted. This leads to an autosterilization of

such foods. Due to autosterilization hurdle-technology foods, which are microbiologically stable, become

more safe during storage, especially at ambient tempera-tures. For example, salmonellae that survive the

ripening process in fermented sausages will vanish more quickly if the products are stored at ambient

temperature, and they will survive longer and possibly cause foodborne illness if the products are stored

under refrigeration. It is also well known that salmonellae survive in mayonnaise at chill temperatures much

better than at ambient temperatures.

2.2.3 Stress reactions

Some bacteria become more resistant or even more virulent under stress, since they generate stress shock

proteins. The synthesis of protective stress shock proteins is induced by heat, pH, aw, ethanol, oxidative

compounds, etc. as well as by starvation. The various responses of microorganisms under stress might

hamper food preservation and could turn out to be problematic for the application of hurdle technology. On

the other hand, the activation of genes for the synthesis of stress shock proteins, which help organisms to

cope with stress situations, should be more difficult if different stresses are received at the same time.

Simultaneous exposure to different stresses will require energy-consuming synthesis of several or at least

much more protective stress shock proteins, which in turn may cause the microorganisms to become

metabolically exhausted. Therefore, multi-target preservation of foods could be the key to avoiding synthesis

of stress shock proteins, which otherwise could jeopardize the microbial stability and safety of hurdle-

technology foods.

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Figure 2.1 Basic concept of bacterial inhibition by hurdles

2.3 Multitarget preservation

Multitarget preservation of foods should be the ambitious goal for a gentle but most effective preservation of

foods. It has been suspected for some time that different hurdles in a food might not have just an additive

effect on microbial stability, but they could act synergistically. A synergistic effect could be achieved if the

hurdles in a food hit, at the same time, different targets (e.g., cell membrane, DNA, enzyme systems, pH, aw,

Eh) within the microbial cells and thus disturb the homeostasis of the microorganisms present in several

respects. If so, the repair of homeostasis as well as the activation of stress shock proteins become more

difficult. Therefore, employing simultaneously different hurdles in the preservation of a particular food

should lead to optimal microbial stability. In practical terms, this could mean that it is more effective to

employ different preservative factors (hurdles) of small intensity than one preservative factor of larger

intensity, because different preservative factors might have a synergistic effect. It is anticipated that the

targets in microorganisms of different preservative factors for foods will be elucidated, and that hurdles

could then be grouped in classes according to their targets. A mild and effective preservation of foods, i.e. a

synergistic effect of hurdles, is likely if the preservation measures are based on intelligent selection and

combination of hurdles taken from different target classes. This approach is probably not only valid for

traditional food-preservation procedures, but as well for modern processes such as food irradiation, ultra-

´high pressure, pulsed technologies. Food microbiologists could learn from pharmacologists, because the

mechanisms of action of biocides have been studied extensively in the medical field. At least 12 classes of

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biocides are already known which have different targets, and sometimes more than one, within the microbial

cell. Often the cell membrane is the primary target, becoming leaky and disrupting the organism, but

biocides also impair the synthesis of enzymes, proteins, and DNA. Multi-drug attack has proven successful

in the medical field to fight bacterial infections (e.g., tuberculosis) as well as viral infections (e.g., AIDS),

and thus a multi-target attack on microorganisms should also be a promising approach in food microbiology.

3 . INDIVIDUAL HURDLES

Individual hurdles can be grouped according to primary function as follows:

Microbiocidla hurdles that reduce the microbial load, for instance by killing, inactivation or removal.

Microbiostatic hurdles that prevent or limit growth of microorganism by chemicla or physical means.

Hrudles that prevent contamination of product; for instance by closed circuits or protecting the product.

Many hrudles have multiple functions. The above grouping of hurldes should therefore not be regarded as a

rigid classification of the functions of the hurdles belonging to each group. Many microstatic hurdles have as

well microbiocidal effects, the degree often depending upon the intensity at which they are applied (eg. pH

reduction, refrigeration, freezing, preservatives and indigenous antimicrobial systems).

3.1 Microbiocidla hurdles reduces microbial load

The principles of the most commont hurdles with in this categary are shown in table 3.1.

Table 3.1 Role of Microbiocidla Hurdles

Bacteriofugataion The removal of cells of high density from milk using high

centrifugal forces.

Competitive microflora

The reduction of the number of undesrible microorganisms by

lowering the pH, consumption of nutrients and production of

antimicrobial substance (such as nisin, other bacteriocins and

hydrogen peroxide), usually this hurdle is applied by choice of

starter culture.

Microfiltration Removal of microbial cells, clumps and somatic cells by

recircualtion over a microfilter.

Ripening (ageing) The holding of such time, at such temperature and under such

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conditions as will result in the necessary biochemical and

physical changes characterizing the cheese in question. When

applied as a hurdle, the multifactoral, complex system

developing in cheese (pH, antagonistic flora, decreased water

activity, metabolism of bacterocins and organic acids) is

utilized to influence the microenvironment in and on the food

and consequently the consumption of the microflora present.

Thermization

The application to milk of a heat treatment of a lower intensity

than pasteurization that aims at reducing the number of

microorganisms. Thermized milk is alkaline phosphatase

positive.

High-pressure treatment Application of high hydrostatic pressures(>3000 bar) to

irreversibly damage the membranes of vegetative cells.

Ultrasonication

The application of high intensity ultrasound (18-500MHz) that

cause cycles of comression and expansion as well as cavitation

in microbial cells. Implosion of microscopic bubbles generates

spots with very high pressure and temperature able to destroy

cells.

Electromagnetic energy treatment

Electromagnetic energy result from high voltage electrical

fields which alternate their frequency millions of times per

second (<108MHz). Examples are microwave energy (thermal

effect).radio-frequency (non-thermal effects) of high electric

field pulses (10-50 kV/cm. non- thermal effects). The treatment

destroys cells by establishing pores in the cell walls due to the

build up of electrical charges at the cell membrane.

Low intensity irradiation The submission of beams pf photons/electroms to destory

viable microorganisms.

3.2. Microbiostatic hurdles (Chemical hurdles)

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The principles of the most common hurdles with in this catogory are summarized in table 3.2.

Table 3.2 Role of Microbiostatic hurdles

pH reduction

The creation of extra-cellular acid conditions that enables

hydrogen ions to be imported in to the cytoplasma of

microorganisms, thus distrubing the homeostasis mechanism of

key cell components vital for continuing growth and viability

low pH values are obtained by frementation or addition of acids

(inorganic or organic).

Carbond doxide (CO2)

The addition and/or formation of carbonic acid to obtain a

multiple hurdle effect, including the creation of anaerobic

conditions by replacing oxygen, reducting, pH, inhibiting

certain intracellular enzymes (decarboxylation) and inhibiting

the transport of water-soluble nutrients across the membrane

(by dehydrating the cellular membrane).

Use of preservatives

The addition of certain additives to echance keeping quality and

stability through direct or indirect antimicrobial and/or

fungicidal activity. Most preservatives are rather specific and

have effect only on certain microorganisms.

Modified atmosphere

The establishing of gaseous environment (either low in oxygen

and/ or high in carbon dioxide or nitrogen) to limit growth of

aerobic microoranisms by impairing biochemical pathways.

Modified atmosphere packaging (MAP) means that a

modification of the gas atmosphere in the packaging is created.

Redox potential control

Redox potential (Eh) is a measure of the oxidizing or reducing

potential of food systems that determines whether aerobic or

anaeroic miroorganisms are able to grow. Eh is influenced by

removal of oxygen and/ or addition of reducing substanes (e.g.

ascorbic acid, sucrose, etc.).

Lactoferrins The utilization of naturally present glycoproteins (highest

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concentration in colostrum) to prolong the lag phases of

bacteria for 12-14 hours, by binding iron in the presence of

bicarbonates.

The lactoperoxidase system

The hydrogen peroxide- catalase

method

The activation of the lactoperoxidase/thiocyanate/hydrogen

peroxide system (indigenous system in milk) to inactivation

several vital metablic bacterial enzymes, consequently blocking

their metabolism and ability to multiply.

The application of two interrelated steps, as follows:

Additon of hydrogen peroxide to the milk, e.g. at

collection centers by trained personnel, and

Addition of catalase at the dairy plant (after heat

treatment) with subsequent period of inhibition.

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3.3. Microbiostatic hurdles (Physical hurdles)

The principles of the most common hurdles with in this category are given table 3.3.

Table 3.3 Role of Microbiostatic Hurdles

Refrigeration Lowering of product temperature to reduce microbial activity.

Water activity control

The contorl of the water activity in the product (the accessibility

of water for microorganism, not the water content in the food),

expressed as the ratio of water vapour pressure of the food to

that of pure water. Water activity can be controlled by:

Concentration, evaporation and drying, which also

incresas the buffering capacity of milk.

Salting (addition of sodium chloride), which also reduces

the cell resistance against carbon doxide and in the

solubility of oxygen .

Sweetening (addition of sugars), which at aw below 0.9.-

0.95 also results in an antimicrobial effect, depending on

the type of sugar.

Freezing

The lowering of temperature below the freezing point of the

product combined with a reduction of the water activity.

Freezing has microbiostatic as well as microbiocidal effects.

Time

The practice of applying very short collection/storage periods,

limitin the shelf life of products or immediate processing of raw

milk to ensure that all microorganisms present are in the lag

phase, and therefore not active and more susceptible to other

hurdles.

3.4. Hrudles that prevent contamination

A large number of control measures are preventive measures. Preventive measures are generally not regarded

as hurdles, however, a few are used in the production lines to obtain hurdle effects.

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The principles of the most common hurdles within this category are shon in table 3.4.

Table 3.4 Role of Hurdels in Preventing Contamination

Pulsed high-intesnity light:

The application of (on eg. Packging materils, equipment and

water) high intensity broadband light pulses of wavelengths in

the ultraviolet, visible and infradred spectrum (20000 times

sunlight) to destory microorganisms

Coatings

The introduction of a physical barrier against contamination,

with or with out antimicrobial substance implemented in to it

(immobilized) to obtain a slowly migration of these from the

surface.

Packaging

Packaging provides a physical barrier that protects against

access of microorganisms from the surroundings. Aseptic

packaging as the process of packaging a product in to sterilized

containers followed by hermetic sealing with a sterilized

closure in a manner that prevents microbiological

recontamination of the product.

4. APPLICATION OF HURDLE TECHNOLOGY IN FOODS

The hurdle technology approach is currently of most interest because;

For minimally processed foods which are mildly heated or fermented.

For underpinning the microbial stability and safety of foods coming from future lines, e.g., healthful

foods with less fat and/or salt.

For advanced hurdle-technology requiring only minimal packaging.

For refrigerated foods chill temperatures are the major and sometimes the only hurdle.

If exposed to temperature abuse during distribution of the foods, this hurdle breaks down, and

spoilage or food poisoning could happen.

Additional hurdles should be incorporated as safeguards into chilled foods, using an approach called

„invisible technology‟

Foods can be preserved and kept safe for long duration by applying individual or combination of

hurdles (Figure 4.1)

In developing countries

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The application of hurdle technology is of paramount importance for foods

remain stable, safe, and tasty if stored without refrigeration

novel minimally processed, high-moisture fruit products especially in Latin America

for meat products in China

for dairy products in India.

There is a general trend to move gradually away from intermediate-moisture foods because

too salty or too sweet

have a less appealing texture and appearance than high-moisture foods.

Hurdle technology has two main functions:

During manufacture: providing assurance that the levels of the pathogens of concern where present,

are kept at or reduced to tolerable levels (Table 4.1).

During packaging, distribution and storage: providing assurance that the tolerable levels of the

pathogens of concern that have been achieved during manufacture are kept under control throughout

shelf life.

Figure 4.1 Preservation of food by individual and combined hurdles

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Table 4.1 Application of Hurdle Technology in Dairy and Food Products

Sr.

No.

Hurdles Role Target Organisms Foods

A. Physical Hurdles

1. Bactofugation Remove bacterial cells of high

density (Bacterial spores and

Somatic Cells), Remove

bacterial load about 1.3

decimal reduction and 90-95%

of cells removal

Clostridium Spores,

spoilage causing

organisms

Cheese milk

2. Pulsed electric fields

(PEF)

Inhibitory action against

pathogens

L. innocua Whey

3. Thermization Make microorganisms

vulnerable to subsequent

hurdles

All microorganisms are

affected (especially

psychotropic)

Milk

4. High Pressure Treatment High pressure kills organism Yersinia enterolitica, Milk and

milk

products

5. Microfilteration Filter of normal size about

0.6-1.4 µm is sufficient to

separate most bacteria

Listeria, Salmonella,

Spores

Milk,cheese

6. Sonication Create stress for the

microorganisms

Salmonella,

Streptococci,

Staph.aureus

Liquid

products

7. Pulsed Electric field Electric current is used to kill

organisms

Effective against Gram

positive bacteria than

Gram negative bacteria

Milk

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8. Freezing Inhibit bacterial growth, No

bacterial growth occurs below

10 C , killing of 50% bacteria

during freezing storage

depending on composition

Some pathogenic and

spoilage causing

microorganisms

Dairy

products,

fruits,

vegetables

etc

B. Chemical Hurdles

1. Sodium citrate and

sodium lactate

Killing of organisms Arcobacter butzleri on

chicken

Butter, food

products

2. Hydrogen Peroxide-

catalase

Inhibitory action Salmonella, Coliforms

and Clostridia

Milk and

milk

products

3. pH Reduction Suppress the growth of

Pathogenic bacteria

Listeria

monocytogenes,

Staph.aureus

Milk and

Cheese

4. Carbon dioxide Inhibits the growth of bacteria E.coli Milk and

cheeses

5. Lysozyme destruction of outgrowing

cells

Clostridium

Tyrobutyricum

Cheese

6. Propionates Block the metabolism due to

enzyme inhibition and

bacterial development

Yeasts and Moulds Butter,

cheese,

vegetables

7. Water Activity Reduction of aw suppress the

growth of pathogenic bacteria

Almost all types of

microorganisms

Milk powder,

khoa,

condensed

milk, cereals

etc

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C. Microbial derived hurdles

1. Competitive Microflora Reduction of the no. of

microbes by lowering pH,

consumption of nutrients and

production of antimicrobial

substances

Almost all potential

pathogenic organisms

Dairy

products and

other foods

2. Ripenining agents

(Lactobacilli)

Killing of pathogens by

production of antimicrobial

compounds

Salmonella

typhimurium, S.

aureus, E. coli,

Bacillus cereus,

Listeria

monocytogenes

Cheddar

cheese,

Emmental

cheese

3. Nisin Inhibits gram positive bacteria Bacillus, Clostridium,

Streptococcus, S.

aureus

Cheeses,

buttermilk,

fermented

milks

4. Pediocins, helvetin J Inhibitory action Listeria

monocytogenes

Cheeses,

fermented

milks

D. Combined Hurdles

1. Nisin With HHP Inhibitory action against

pathogens, effective to

inactivate cheese indigenous

Microbiota

S. carnosus and B.

subtilis spores

Cheese

2. pH and low

temperature

Inhibitory action against

pathogens

significant reduction in

L. innocua

liquid

products,

cheese,

whey

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3. acid, salt, and heating Inhibitory action against

pathogens

E. coli O157:H7 pickled

products

4. Salt and nitrite Killing effect Many bacteria Meat

products

5. Summary

Food preservation implies putting microorganisms in a hostile environment, in order to inhibit their growth

or shorten their survival or cause their death. The feasible responses of microorganisms to this hostile

environment determine whether they may grow or die. More research is needed in view of these responses;

however, recent advances have been made by considering the homeostasis, metabolic exhaustion, and stress

reactions of microorganisms in relation to hurdle technology, as well as by intro-ducing the novel concept of

multitarget preservation for a gentel but most effective preservation of hurdle-technology foods.

Documents

Paper No.: 03 Paper Title: FOOD MICROBIOLOGY Module 35