Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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B ¼ f

ðlog J

 log g

Þ,

where B is the thermal process time corrected for the

time to bring the retort to the process temperature, f

is the time in minutes for the semilog heating curve to

traverse one log cycle, J

is the heating lag factor, I

difference in temperature between the retort and the

food at the start of process and g

is the difference

between the retort temperature and the maximum

temperature reached by the food at the center. In the

processing of low-acid foods with a pH value of 4.6

or above, a process equivalent in lethality to at least

of 3 min must be applied to minimize the risk of

spores of C. botulinum.AnF value determined on the

basis of one z value to an F value determined on

the basis of a second z value can be converted using

the appropriate formula.

Cook Value or

Value

0016 Mansfield introduced the concept of a lethality-like

value for sensory degradation, termed the cook value

or C value. This has a reference temperature of

100



C and a z value typically in the range 20–40C



100

¼ 10

ðT100Þ=z

c min:

The processing conditions mainly depend on the pH

value of the product.

Acid Products (pH < 4.6)

0017 At a pH below 4.6, the risk of growth and toxin

production by C. botulinum is extremely low, and

for products with pH values between 4.0 and 4.6,

processes are aimed at controlling the survival and

growth of spore forming organisms such as Bacillus

coagulans, Bacillus polymyxa, Bacillus macerans,

and the butyric anaerobes such as C. butyricum and

C. pasteurianum. A heat process of F

121

¼0.7 is

regarded as adequate for this purpose, and a process

equivalent to 10 min at 93.3



C(F

93.3

8.3

¼5) when the

pH is between 4.0 and 4.3. However, more severe

processes may be required to control excessive con-

tamination. In a previous study of nine cases of botu-

lism from products with pH below 4.6, including

canned pears, apricots, tomato ketchup, tomato–

onion chili sauce, and green tomatoes, it was sug-

gested that growth of spoilage organisms raised the

pH to a level at which dormant spores of C. botuli-

num could germinate and grow. Below pH 3.7, the

processor is concerned with the control of nonsporing

bacteria, yeasts, and molds. These may be generally

controlled by heat processes at temperatures below

100



Methods of Sterilization

0018The food sterilization methods are divided into two

categories: sterilization by heating (thermal process-

ing) and sterilization without heating (nonthermal

processing). Thermal processing is widely practiced

these days, in spite of some problems such as that the

process of heating might reduce nutrition or deterior-

ate the quality of foods, and that it is ineffective

against certain types of bacteria. Nonthermal pro-

cessing is considered an effective method that does

not cause any deterioration of quality, in contrast

with thermal processing. However, no reports have

shown the effect of sterilization without heating. Re-

search on the evaluation of the technology of non-

thermal sterilization without heating is widely being

pursued internationally.

0019Thermal processing is divided further into two

categories: in-container sterilization (processing).

The principles involved in thermal sterilization of

foods remain the same whether attempting to sterilize

products in containers or sterilize products prior to

filling in the final container (aseptic processing).

Thus, it is necessary to know the thermal destruction

rate for the microorganisms of consequence in the

food being processed. The procedures necessary for

acquisition of such data are available. It is important

to use this information properly so that the appropri-

ate time and temperature for destruction of the or-

ganisms can be achieved. Sterilization procedures for

products in containers usually require longer times,

since the heat transfer to the product is relatively

slow. Sterilization prior to filling in the container, as

accomplished in aseptic processing, requires rela-

tively short heating periods. This sterilization process

is usually accomplished by heating the product rap-

idly to 130–145



C, holding for an appropriate time,

then rapidly cooling the product. The specific product

will determine the actual combination of temperature

and time required for sterilization.

0020Product heating is accomplished by either indirect

or direct heat exchange. In liquid homogeneous prod-

ucts, indirect heating occurs when the product to be

heated and the heating medium are separated by the

heating surface. This type of heating is accomplished

using a scraped surface, tubular, or plate heat exchan-

ger. Direct heat exchange is achieved by placing the

heating medium directly into the product or vice

versa. In either case, the added water from the steam

must be either removed or accounted for in the

formulation. The specific type of heating used is usu-

ally dictated by the nature of the product and the

economics of operation. Following heating to the

sterilization temperature, the product must be held

at this temperature for a specific length of time to

STERILIZATION OF FOODS 5597

accomplish sterilization. The sterilization time in con-

tinuous-flow aseptic systems is obtained by conveying

the product through a nonheating pipe attached to

the heating system. This pipe is called the holding

tube and is of a specified uniform diameter and

length. Its capacity is such that the fastest particle is

held at the maximum time required to sterilize the

product. The length of time for which the product

remains in the tube is dependent not only on the

holding capacity of the tube and the rate at which

the product is pumped through the tube, but also on

the manner in which the product flows through the

tube.

Thermal Processing

0021 During sterilization, the type of heat, time of applica-

tion, and temperature to ensure destruction of all

microorganisms are very important. Endospores of

bacteria are considered the most thermoduric of all

cells, so their destruction guarantees sterility. The

lethal temperature varies for microorganisms. The

time required to kill the microorganisms depends on

the number of organisms, species, nature of the prod-

uct being heated, pH, and temperature. Whenever

heat is used to control microbial growth, inevitably

both time and temperature are considered.

0022 Sterilization (boiling, autoclaving, hot air oven)

kills all microorganisms with heat and is commonly

employed in canning, bottling, and other sterile pack-

aging procedures. Heat sterilization is the unit oper-

ation in which foods are heated at a sufficiently high

temperature and for a sufficiently long time to destroy

microbial and enzyme activity.

0023 Sterilized foods have a longer shelf-life but undergo

substantial changes in quality. Developments in pro-

cessing technology therefore aim to reduce the overall

processing time. The effects of microbial heat resist-

ance on the design of heat-sterilization procedures

and equipment are an important aspect of steriliza-

tion in both in-container heat sterilization and aseptic

processing.

0024 In order to determine the process time for a given

food, it is necessary to obtain information on both the

heat resistance of microorganisms and the rate of heat

penetration into the food. A process that reduces cell

numbers by eight decimal reductions (an 8D process),

applied to a raw material containing 10

spores per

container would reduce microbial numbers to 10

3

per container, or one microorganism in every thou-

sand containers. Commercial sterility, therefore,

means that the vast majority of containers are sterile,

but there is a probability that nonpathogenic cells

survive the heat treatment in a predetermined

number of containers. A 12D process is used when

C. botulinum is likely to be present but in foods that

contain less heat-resistant spoilage microorganisms.

In addition to information on heat resistance, it is

necessary to collect data describing the rate of heat

penetration into the food in order to calculate the

processing time needed for commercial sterility.

Retorting (Heat Processing)

0025The most widely used system for sterilization uses

overpressure retorts or agitating sterilizers. Generally,

the overpressure retorts are of the batch type with

steam or pressurized hot water as the heating media.

The pressure in the retort is maintained during the

entire processing cycle, with steam during the heating

cycle and with compressed air during the cooling

cycle. Agitating sterilizers gently move the product

within the container, which facilitates more uniform

heating and cooling owing to better heat transfer.

This reduces the come-up time, delivering an

improved product.

Heating by Saturated Steam

0026Latent heat is transferred to food when saturated

steam condenses on the outside of the container. If

air is trapped inside the retort, it forms an insulating

boundary film around the cans, which prevents the

steam from condensing and causes under processing

of the food. It also produces a lower temperature than

that obtained with saturated steam. It is therefore

important that all air be removed. Sterilization

requires a temperature of 121.1



C. This creates

pressure inside the container, and if the pressure on

the outside of the container is less than that inside, the

container will expand and subsequently break. This is

more critical in glass jars and flexible containers like

pouches and plastic cans. To avoid this situation, an

overhead pressure of 230–250 kPa is applied to

equalize the internal pressure.

Aseptic Processing

0027Conventional retorting of A2 cans of vegetable soup

requires 70 min at 121



C to achieve an F

value of

7 min, followed by cooling for 50 min. Aseptic pro-

cessing in a scraped-surface heat exchanger at 140



for 5 s gives an F

value of 9 min. Increasing the can

size to A10 increases the processing time to 218 min,

whereas with aseptic processing, the sterilization time

is the same. In aseptic processing, containers are

not required to withstand sterilization conditions.

Cartons are presterilized with hydrogen peroxide,

and filling machines are enclosed and maintained in

a sterile condition by ultraviolet light and filtered air.

5598 STERILIZATION OF FOODS

A positive air pressure is maintained in the filling

machine to prevent entry of contaminants. The

process is successfully applied to liquid and small-

particulate foods, but problems remain with larger

pieces of solid food.

Nonthermal Processing

0028 Nonthermal methods for the preservation of foods

are under intense research to evaluate their potential

as an alternative or complementary process to trad-

itional methods of food preservation. Traditionally,

most preserved foods are thermally processed by sub-

jecting the food to a temperature of 60–100



C for a

few seconds to minutes. During this period, a large

amount of energy is transferred to the food. This

energy may trigger unwanted reactions in the food,

leading to undesirable changes or formation of bypro-

ducts. For example, thermally processed milk may

have a cooked flavor accompanied by a loss of vita-

mins, essential nutrients, and flavors. The fact that

the shelf-life and the quality of food are important

to consumers gave birth to the concept of preserv-

ing foods using nonthermal methods. Nonthermal

methods of food preservation are currently being de-

veloped to eliminate, or at least minimize, the quality

degradation of foods that results from thermal pro-

cessing.

0029 During nonthermal processing, the temperature of

the food is held below the temperature normally used

in thermal processing. Therefore, the quality degrad-

ation expected from high temperatures is minimal in

nonthermal processing. The vitamins, essential nutri-

ents, and flavors are expected to undergo minimal, or

no, changes during nonthermal processing. In add-

ition, nonthermal processes use less energy than ther-

mal processes. Foods can be processed nonthermally

using high hydrostatic pressure, oscillating magnetic

fields, high-intensity pulsed electric fields, intense

light pulses, irradiation, chemical, biochemical, and

hurdle technology. Although these technologies have

been used for a long time to inactivate microorgan-

isms and/or preserve food, they have gained recogni-

tion as nonthermal methods of food preservation only

in the recent past. Nonthermal processes are expected

to induce only minimal degradation of food.

High-pressure Processing

0030 High-pressure processing (HPP), also described as

high hydrostatic pressure (HHP) or ultrahigh pressure

(UHP) processing, subjects liquid and solid foods,

with or without packaging, to pressures between

100 and 900 MPa. A high hydrostatic pressure is

used for the inactivation of microorganisms and cer-

tain enzymes and for shelf-life extension of foods.

Spores can be inactivated by combining high pressure

with temperature. The germination of spores is an

important step in spore inactivation. The process

temperature during pressure treatment can be speci-

fied from less than 0



C to more than 100



C. Com-

mercial exposure times can range from a millisecond

pulse to more than 20 min. HPP acts instantaneously

and uniformly throughout a mass of food, independ-

ent of the size, shape, and food composition. Com-

pression uniformly increases the temperature of foods

by approximately 3



C per 100 MPa. Compression of

foods may shift the pH of the food as a function of

imposed pressure and must be determined for each

food treatment process. Water activity and pH are

critical process factors in the inactivation of microbes

by HPP. An increase in food temperature above room

temperature and, to a lesser extent, a decrease below

room temperature increase the inactivation rate of

microorganisms during HPP treatment. Temperatures

in the range of 45–50



C appear to increase the rate

of inactivation of food pathogens and spoilage

microbes. Temperatures from 90–110



C in conjunc-

tion with pressures include batch and semicontinuous

systems, but no commercial continuous HPP systems

are operating.

0031The critical process factors in HPP include pres-

sure, time at pressure, time to achieve treatment

pressure, decompression time, treatment temperature

(including adiabatic heating), initial product tem-

perature, vessel temperature distribution at pressure,

product pH, product composition, product water ac-

tivity, packaging material integrity, and concurrent

processing aids. Chemical changes in the food gener-

ally will be a function of the process and treatment

time. Because some types of spores of C. botulinum

are capable of surviving even the extreme pressures

and temperatures of HPP, there is no absolute micro-

bial indicator for sterility by HPP. For vegetative bac-

teria, nonpathogenic Listeria innocua is a useful

surrogate for the foodborne pathogen, Listeria mono-

cytogenes a nonpathogenic strain of Bacillus may be

useful as a surrogate for HPP-resistant E. coli

0157:H7 isolates.

0032Subjecting foods to pressures of 100–800 MPa in-

activates vegetative bacteria, yeasts, molds, and para-

sites in products such as jams, orange juice, and meat

products. Factors that affect the rate of microbial

inactivation include pressure and magnitude, micro-

bial type and growth stage, temperature, pH, water

activity, and food composition. Application of a pres-

sure of 680 MPa on grape juice for 10 min would

arrest the growth of the microbes and thereby stops

further fermentation of the grape juice. Peaches and

pears subjected to a pressure of 410 MPa for 30 min

exhibit a shelf-life of 5 years. One of the important

STERILIZATION OF FOODS 5599

challenges in using HPP is the fabrication of pressure

vessels and seals that can withstand the high pres-

sures during the cycles of pressurization and depres-

surization.

Pulsed Electric Fields

0033 High-intensity pulsed electric field (PEF) processing

involves the application of pulses of high voltages

(typically 20–80 kV cm

1

) of exponentially decaying,

square wave, bipolar, or oscillatory pulses and at

ambient, subambient, or slightly above ambient tem-

peratures for less than 1 s to foods. High-intensity

electric fields applied to a food in the form of short-

duration pulses can inactivate the microorganisms

and certain enzymes. Energy loss due to heating of

foods is minimized, reducing the detrimental changes

of the sensory and physical properties of foods. Some

important aspects in pulsed electric field technology

are the generation of high electric-field intensities, the

design of chambers that impart uniform treatment to

foods with minimal increase in temperature, and

the design of electrodes that minimize the effect of

electrolysis.

0034 Although different laboratory- and pilot-scale

treatment chambers have been designed and used

for PEF treatment of foods, only two industrial scale

PEF systems are available. The systems including

treatment chambers and power supply equipments

need to be scaled up to commercial levels. To date,

PEF has been applied mainly to improve the quality of

foods. Application of PEF is restricted to food prod-

ucts that can withstand high electric fields, have a low

electrical conductivity, and do not contain or form

bubbles. The particle size of the liquid food in both

static and flow treatment modes is a limitation. Sev-

eral theories have been proposed to explain microbial

inactivation by PEF. The most-studied theories are

electrical breakdown and electroporation. Factors

that affect the microbial inactivation with PEF are

process factors (electric field intensity, pulse width,

treatment time and temperature, and pulse wave

shapes), microbial entity factors (type, concentration,

and growth stage of microorganism) and media

factors (pH, antimicrobes and ionic compounds, con-

ductivity, and medium ionic strength). Microbial

inactivation increases with increasing electric field

intensity, exposure time, and temperature of the

food. However, it is desirable to maintain the

temperature below 30–40



C by providing a cooling

system. Different bacteria have different sensitivities

to electric field treatment. In general, Gram-positive

bacteria and yeasts are more resistant to electric fields

than Gram-negative bacteria. The optimum condi-

tions for maximum inactivation of a specific micro-

organism can be determined after preliminary

research. Although PEF has potential as a technology

for food preservation, existing PEF systems and

experimental conditions are diverse. The effects of

critical process factors on pathogens of concern and

kinetics of inactivation need to be studied further. An

electric pulse process for the treatment of fresh citrus

juices capable of reducing target pathogens without

alteration of the juice was granted an FDA letter of no

objection for its use in April 1999.

High-voltage Arc Discharge

0035Arc discharge is an early application of electricity to

pasteurize fluids by applying rapid discharge voltages

through an electrode gap below the surface of aque-

ous suspensions of microorganisms. A multitude of

physical effects (intense wave) and chemical com-

pounds (electrolysis) are generated, inactivating the

microorganisms. The use of arc discharge for liquid

foods may be unsuitable, largely because of electroly-

sis and discharge, but more recent designs have

shown some promise for use in food preservation.

Pulsed Light Technology

0036Pulsed light is a method of food preservation that

involves the use of intense and short-duration pulses

of broad spectrum ‘white light’ (ultraviolet to the

near-infrared region). The use of pulsed high-intensity

light to inactivate microorganisms is a relatively new

technology. For most applications, a few flashes ap-

plied in a fraction of a second provide a high level of

microbial inactivation. This technology is applicable

mainly in sterilizing or reducing the microbial popu-

lation on packaging or food surfaces. Extensive inde-

pendent research on the inactivation kinetics under a

full spectrum of representative variables of food

systems and surfaces is needed. Application of light

pulses involves exposure of foods to short duration

pulses (1 ms to 0.1 s) of intense incoherent light. Light

with an energy density of about 0.01–50 J cm

2

and a

wavelength in the range of 170–2600 nm is used.

Such incoherent intense pulses of light may be gener-

ated using gas–filled flash lamps or spark gap dis-

charge apparatus. Full – or filtered – spectrum light

may be used, depending on the degree of sterilization

expected. The filtered–spectrum light is devoid of

wavelengths known to cause undesirable reactions

in foods. Glass or liquid filters are used to obtain

the filtered spectrum. In general, filtered light is

more effective for microbial inactivation than full–

spectrum light.

Ultraviolet Light

0037There is particular interest in using ultraviolet (UV)

light to treat fruit juices, especially apple juice and

5600 STERILIZATION OF FOODS

cider. Other applications include disinfection of water

supplies and food contact surfaces. Ultraviolet pro-

cessing involves the use of radiation from the ultra-

violet region of the electromagnetic spectrum. The

germicidal properties of UV irradiation (200–

280 nm) are a result of DNA absorption of the UV

light. This mechanism of inactivation results in a

sigmoidal curve of microbial population reduction.

To achieve microbial inactivation, the UV radiant

exposure must be at least 400 J m

2

in all parts of

the product. Critical factors include the transmissivity

of the product, the geometric configuration of the

reactor, the power, wavelength and physical arrange-

ment of the UV source(s), the product flow profile

and the radiation path length. UV may be used

in combination with other alternative process

technologies, including various powerful oxidizing

agents such as ozone and hydrogen peroxide, among

others.

Oscillating Magnetic Fields

0038 Static (SMFs) and oscillating magnetic fields (OMFs)

have been explored for their potential to inactivate

microorganisms. For SMFs, the magnetic field inten-

sity is constant with time, while an oscillating mag-

netic field is applied in the form of constant amplitude

or decaying amplitude sinusoidal waves. An OMF

applied in the form of pulses reverses the charge for

each pulse. The intensity of each pulse decreases with

time to about 10% of the intensity. Preservation of

foods with OMFs involves sealing food in a plastic

bag and subjecting it to one to 100 pulses in an OMF

with a frequency of 50–500 kHz at a temperature

of 0–50



C for a total exposure time ranging from

25–100 ms. The effects of magnetic fields on micro-

bial populations have produced controversial results.

Consistent results concerning the efficacy of this

method are needed before considering this technology

for food-preservation purposes.

0039 Oscillating magnetic fields with a magnetic flux

of 5–50 T and a frequency of 5–500 kHz have been

reported to inactivate microorganisms. Foods with

an electrical resistivity of 10–25 O  cm may be sealed

in a plastic bag and subjected to OMFs. One of

the attractive features of using magnetic fields for

food preservation is that the food can be packaged

prior to processing, reducing the possibility of cross-

contamination during packaging. Studies on the

effects of static and pulsed magnetic fields as an alter-

native to conventional thermal treatments on the in-

activation of Saccharomyces cerevisiae have been

reported. The potential advantages of food pre-

servation by magnetic fields, the proposed interaction

mechanisms, and some of the results that have

been obtained by exposing living systems to low- and

high-frequency, high-intensity magnetic fields were

discussed in the IFT Annual Meeting in 2000.

Pulsed X-rays

0040A number of studies have compared the effects of

electron beam, gamma rays, and X-rays, but com-

parison between these technologies is inconclusive

owing to differences in the doses applied. Electrons

have a limited penetration depth of about 5 cm in

food, whereas X rays have significantly greater pene-

tration depths (60–400 cm) depending on the energy

used. The use of pulsed X-rays is a new alternative

technology that utilizes a solid-state opening switch

to generate an electron beam 30 ns down to a few

nanoseconds; repetition rates of up to 1000 pulses per

second. The practical application of food irradiation

by X-rays in conjunction with existing food-process-

ing equipment is further facilitated by:

0041the possibility of controlling the direction of the

electrically produced radiation;

0042the possibility of shaping the geometry of radiation

field to accommodate different package sizes;

0043its high reproducibility and versatility.

Potentially, the negative effects of irradiation on the

food quality can be reduced. Irradiation of foods was

one of the earliest nonthermal food preservation tech-

nologies. Irradiation is the exposure of food to radi-

ation with an energy of 5–10 kGy and wavelengths of

2000 A

or less. Ultraviolet, beta, gamma, and X-, and

microwaves are included in this range. One of the

attractions of irradiation is its ability to pasteurize

foods in the frozen state. The World Health Organ-

ization (WHO) approved a radiation dosage of up to

10 kGy as being ‘unconditionally safe for human con-

sumption.’ Irradiation has the potential to replace the

use of many hazardous chemical pesticides and pre-

servatives.

Ultrasound

0044Ultrasound is the energy generated by sound waves

of 20 000 or more vibrations per second. Although

ultrasound technology has a wide range of current

and future applications in the food industry, including

inactivation of microorganisms and enzymes, pres-

ently, most developments for food applications are

nonmicrobial. Data on inactivation of food microor-

ganisms by ultrasound in the food industry are scarce,

and most applications use combinations with other

preservation methods. The bactericidal effect of ultra-

sound is attributed to intracellular cavitations, a

phenomenon in which mechanical high-frequency vi-

brations cause alternate compressions of millions of

microscopic bubbles containing gas and vapor. The

STERILIZATION OF FOODS 5601

bubbles expand then implode violently, releasing

large amounts of energy and generating very high

temperatures and pressures within the bubbles. The

molecules of the vaporized reaction mixture are frac-

tured, forming highly reactive free radicals. Cavita-

tions occur as a result of micromechanical shocks that

disrupt cellular structural and functional components

up to the point of cell lysis.

0045 The heterogeneous and protective nature of food

with the inclusion of particulates and other interfer-

ing substances severely curtails the singular use of

ultrasound as a preservation method. Although, at

present, these limitations make commercial develop-

ment unlikely, the combination of ultrasound with

other preservation process (e.g., heat and mild

pressure) appears to have the greatest potential for

industrial applications. Critical processing factors are

assumed to be the amplitude of the ultrasound waves,

the exposure/contact time with the microorganisms,

the type of microorganisms, the volume of food to be

processed, the composition of the food, and the tem-

perature of treatment.

Microwave and Radio-frequency Processing

0046 Microwave and radio-frequency heating refers to the

use of electromagnetic waves of certain frequencies to

generate heat in a material by two mechanisms –

dielectric and ionic. Microwave and radio-frequency

heating for pasteurization and sterilization is pre-

ferred to conventional heating, because they require

less time to come up to the desired process tempera-

ture, particularly for solid and semisolid foods.

Industrial microwave pasteurization and sterilization

systems have been reported for over 30 years, but

commercial radio-frequency heating systems for the

purpose of food pasteurization or sterilization are

not known to be in use. For a microwave steriliza-

tion process, unlike conventional heating, the design

of the equipment can dramatically influence the crit-

ical process parameters – the location and tempera-

ture of the coldest point. This uncertainty makes it

more difficult to make general conclusions about

processes, process deviations, and how to handle

deviations.

0047 Many techniques have attempted to improve the

uniformity of heating. The critical process factor

when combining conventional heating and micro-

wave or any other novel processes would most likely

remain the temperature of the food at the cold point,

primarily due to the complexity of the energy absorp-

tion and heat-transfer processes. Since the thermal

effect is presumably the sole lethal mechanism, the

time–temperature history at the coldest location will

determine the safety of the process and is a function of

the composition, shape, and size of the food, the

microwave frequency, and the applicator (oven)

design. Time is also a factor in the sense that, as the

food heats up, its microwave absorption properties

can change significantly, and the location of cold

points can shift.

Ohmic and Inductive Heating

0048Ohmic heating (sometimes also referred to as Joule

heating, electrical resistance heating, direct electrical

resistance heating, electroheating, and electroconduc-

tive heating) is defined as the process of passing elec-

tric currents through foods or other materials to heat

them. Ohmic heating is distinguished from other elec-

trical heating methods by the presence of electrodes in

contact with the food, frequency, and waveform. The

principal advantage claimed for ohmic heating is its

ability to heat materials rapidly and uniformly, in-

cluding products containing particulates. The princi-

pal mechanisms of microbial inactivation in ohmic

heating are thermal. While some evidence exists for

nonthermal effects of ohmic processes, which rely on

heat, it may be unnecessary for processors to claim

this effect in their process fillings.

0049Inductive heating is a process wherein electric cur-

rents are induced within the food owing to oscillating

electromagnetic fields generated by electric coils. No

data on microbial death kinetics under inductive

heating have been published.

Hurdle Technology

0050Hurdle technology combines nonthermal food pro-

cessing with traditional or other emerging technolo-

gies. The most promising combinations include

nonthermal methods, such as high hydrostatic pres-

sure, ultrasound, and pulsed electric fields. In the

inactivation of spores, it is necessary to use a com-

bined methods approach using ‘hurdles.’ Hurdles are

physical or chemical parameters that can be adjusted

to ensure the microbial stability and safety of foods.

The physical parameters include the processing and

storage temperatures, water activity, pH, and redox

potential at levels that inhibit or inactivate the micro-

organisms and thus render the food safe. Hurdle

technology is used in the preservation of meat and

seasonal or regional fruits and vegetables.

0051Besides preserving food quality, new nonthermal

technologies have to achieve an equivalent or, prefer-

ably, a better enhanced safety level than that for

procedures that they replace. Most nonthermal pre-

servation techniques are highly effective in inactivat-

ing vegetative forms of bacteria, yeast, and molds, but

bacterial spores and most enzymes remain difficult to

inactivate.

5602 STERILIZATION OF FOODS

See also: Escherichia coli: Food Poisoning by Species

other than Escherichia coli; Food Poisoning:

Classification; Tracing Origins and Testing; Statistics;

Economic Implications; Food Safety; Food Security;

Heat Treatment: Ultra-high Temperature (UHT)

Treatments; Electrical Process Heating; Pasteurization:

Pasteurization of Liquid Products; Pasteurization of

Viscous and Particulate Products; Other Pasteurization

Processes; Preservation of Food; Spoilage: Bacterial

Spoilage; Molds in Spoilage; Yeasts in Spoilage

Further Reading

Ball CO (1923) Bulletin of the National Research Council

7, Part 1, No. 37.

Barbosa-Canovas GV, Pothakamury UR, Palou E and

Swanson BG (1998) Emerging technologies in food pre-

servation. In: Nonthermal Preservation of Foods. New

York: Marcel Dekker.

Barbosa-Canovas GV, Pothakamury UR and Swanson BG

(1995) State of the art technologies for the stabilization

of foods by non thermal processes: physical methods. In:

Barbosa-Canovas and Welti-Chanes J (eds) Food Preser-

vation by Moisture Control. Fundamentals and Applica-

tions, pp. 493–532. Lancaster, PA: Technomic.

Brown KL (1988) Use of computer data file for storage

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STERILIZATION OF FOODS 5603

Steroid Hormones See Hormones: Adrenal Hormones; Thyroid Hormones; Gut Hormones; Pancreatic

Hormones; Pituitary Hormones; Steroid Hormones

Stilton See Cheeses: Mold-ripened Cheeses: Stilton and Related Varieties

STORAGE STABILITY

Contents

Mechanisms of Degradation

Parameters Affecting Storage Stability

Shelf-life Testing

Mechanisms of Degradation

M Hole, University of Lincolnshire and Humberside,

Lincoln, Grimsby, UK

Background

0001 The storage of a food is limited by changes to the

food, which render it unacceptable to the consumer

for reasons of safety and of quality, whether organo-

leptic or nutritional.

0002 The cause of these undesirable changes can be re-

lated to physical, microbiological, enzymatic, and

chemical reasons. (See Spoilage: Chemical and En-

zymatic Spoilage; Bacterial Spoilage; Fungi in Food

– An Overview; Molds in Spoilage; Yeasts in Spoil-

age.) Which mechanism of degradation operates

depends on the type of food and the storage condi-

tions (see Figure 2).

0003 Postharvest storage of fresh commodities (whether

of plant or animal origin) is mainly limited by micro-

bial action and the natural metabolic (i.e., enzymatic)

processes occurring after harvest or death. The

modern food industry is primarily concerned with

minimizing the degradation reactions occuring

during storage. (See Preservation of Food.) Thus,

storage at a reduced temperature is a successful strat-

egy (See Chilled Storage: Principles; Attainment of

Chilled Conditions; Quality and Economic Consider-

ations; Microbiological Considerations; Freezing:

Storage of Frozen Foods) as is modified atmosphere

packaging (MAP) (See Packaging: Packaging of

Solids) for the control of microbial growth in foods.

High-temperature processing aims to eliminate the

microbial mechanism of degradation, and also to

inactivate enzymes. This would leave chemical mech-

anisms as the main possible degradation pathway,

and examples of such changes during the extended

ambient storage periods of canned foods are given

below. More generally, the relatively high reactivity

of oxygen with food components, especially lipids,

is a major chemical degradation route and is dis-

cussed further below. (See Oxidation of Food

Components.)

0004Physical changes in foods result in undesirable tex-

tural changes, especially for fresh commodities where

the texture relates to the cellular nature of the food

and the way in which water is held within these cells.

This ‘cellular water’ can be adversely affected by

frozen storage/thawing cycles.

0005The classification of foods as cellular (i.e., plants,

animal commodities) or dispersive (i.e., colloidal) is

relevant to physical mechanisms of degradation. For

dispersive foods, which are usually fabricated (e.g.,

margarine, icecream, salad cream, gelled products),

the inherent physical instability of colloidal disper-

sions must be considered in relation to their mode of

storage. (See Colloids and Emulsions.) Water in foods

(See Water Activity: Effect on Food Stability) is a

major factor for the possible mechanisms of degrad-

ation. It is essential for microbial growth and

5604 STORAGE STABILITY/Mechanisms of Degradation

reactions involving enzymes (hence the success of

traditional drying methods for food preservation).

The presence of water, as indicated above, is import-

ant for food texture, but the absence of water can be

desirable for the crispness of snack foods. Storage

that allows snack foods to absorb water, and hence

lose their crispness, is clearly undesirable – as is the

opposite effect of drying out for foods (e.g., cakes,

cheese). (See Intermediate-moisture Foods.) From a

chemical point of view, water can react with food

components in a hydrolysis reaction, and examples

are discussed below.

Physical Degradation Mechanisms

0006 Water binding is especially important in relation to

the texture of fresh foods, whether of plant or animal

origin. For fresh leafy vegetables, e.g., lettuce, the

desirable firmness is dependent upon the turgor pres-

sure of the cellular water – excessive transpiration

owing to storage in a warm or dry environment will

cause a reduction in turgor pressure with a resultant

undesirable loss of firmness, and reduced consumer

appeal for the ‘limp’ result. Chilling can be beneficial,

but if frozen, undesirable texture changes can occur

resulting from the loss of cellular water. The mech-

anism involves differential freezing of water in the

intercellular space, with a resultant, irreversible, os-

motically induced flow of water from inside the cell

into the intercellular space. On thawing, this water is

expelled, leading to the unattractive ‘drip loss’ and

leaving a less desirable texture. This is a general prob-

lem for fresh meat and fish, and is especially severe

for fruits such as strawberries.

0007 The instability of water in disperse foods (i.e., col-

loidal systems) is illustrated by the separation of

water from oil in emulsions (e.g., low fat spreads) or

by water being lost from gels (i.e., syneresis), espe-

cially during low-temperature storage. Syneresis of

gelled dairy products, e.g., set yogurts, can be a prob-

lem, as can gels or thickened foods based on starch.

The linear amylose polymers in starch tend to associ-

ate (a process referred to as ‘starch retrogradation’),

to cause gels to undergo syneresis or pastes to go

lumpy, especially on long-term frozen/chilled storage.

This has led to specialized modified starches being

developed for food products designed to be stored

frozen. The starch in wheat flour is responsible for

the staling of bread, but this is due to the highly

branched amylopectin component undergoing retro-

gradation (i.e., a form of crystallization). Staling

refers to the interior of the bread becoming firmer,

in contrast to crust firming if the crust dries out. Such

staling can be slowed down by storing bread at 30



in fact, stale bread can be ‘freshened’ by heating it and

so reversing, to some extent, the crystallization of the

amylopectin.

0008Pectin-based gels, e.g., in traditional jams, can

also show syneresis, and the increasing tendency to

store jams in the refrigerator will hasten such an effect

– the suggestion is made in order to slow down

microbial spoilage, which is discussed in the next

section.

Microbiological Degradation Mechanisms

0009Microorganisms are the major cause of deterioration

in foods, not least because foods generally have a high

water activity, which microorganisms require in order

to grow. (See Microbiology: Detection of Foodborne

Pathogens and their Toxins.) In particular, storage of

plant and animal products that are unprocessed is

limited by microbial action. Fabricated foods can

have water activities that are too low for microbial

growth (e.g., margarine, high-sugar-content pre-

serves), and frozen storage (e.g., 21



C) can effect-

ively prevent microorganisms growing.

0010The use of foods as a source of organic compounds

by microorganisms is a natural decay process, and

the initial stages can give rise to undesirable organo-

leptic effects, ranging from visual discolorations to

off-odors and slime formation. When the process

produces toxic effects without the accompanying

spoilage indicators, consumers are unable to recog-

nize that the food is unsafe to eat, as in cases of

salmonellosis and botulism, and this is a most danger-

ous situation. (See Food Poisoning: Classification.)

0011The food degradation involving microorganisms

depends upon the type of microorganism present.

Food spoilage arises from the presence of bacteria or

fungi, fungi being subdivided into molds and yeasts

(See Microbiology: Classification of Microorgan-

isms.) With a water activity above 0.90, the faster

growing bacteria tend to dominate, but with a water

activity of about 0.90–0.80, molds and yeasts can be

dominant, especially if the pH is below about 5. In

fact, yeasts and molds can grow at a pH of around 2.

An important difference between yeasts and molds is

that molds require oxygen (i.e., are aerobic), whereas

yeasts can grow in air (oxygen) or in the absence of air;

thus, molds are found on the surface of solid foods

(e.g., bread mold), whereas yeasts can be found in

liquids (e.g., in acetic acid (vinegar) preserves).

0012For the large number of different bacteria that can

be found in foods, some require air (oxygen) for

growth (i.e., aerobic), whereas others can grow in

the absence of oxygen (i.e., anaerobic). Thus, if a

food is stored in a vacuum, or in a modified atmos-

phere with no oxygen, the bacteria that flourish will

be different from those present in the food when

STORAGE STABILITY/Mechanisms of Degradation 5605

stored in air. This change to an anaerobic bacterial

population may result in a danger to health on con-

sumption of the food, since many anaerobic bacteria

are pathogenic, e.g., Clostridium botulinum, which

has been the cause of deaths from botulism from

its occurence in canned meats (e.g., corned beef).

(See Canning: Quality Changes During Canning;

Emerging Foodborne Enteric Pathogens; Meat: Pre-

servation.)

0013 Foods that are especially subject to deterioration

resulting from surface mold growth include fruit,

cheese, sausage, bread, and cereals. With fruit, the

molds produce enzymes (see below) that catalyze re-

actions leading to the breakdown of pectins and cel-

lulose, with resultant rotting of the fruit. Molds that

produce toxins, e.g., aflatoxin, ochratoxin, and patu-

line, can be particularly dangerous to health – a par-

ticular problem can exist with stored grain and nuts,

especially peanuts when used as animal feed. (See

Aflatoxins; Mycotoxins: Toxicology.)

0014 Yeasts are very useful in producing foods such as

bread and alcoholic drinks, but they can also cause

food spoilage, especially where food is less severely

processed in order to maximize organoleptic proper-

ties, or fruit pieces are added to dairy products, or in

low-calorie (and hence low-sugar) formulations. The

undesirable results of yeast activity include, espe-

cially, off-flavor formation and gas production in

foods ranging from jams and mayonnaise to yogurts

(See Yeasts.) Gas production as a problem, e.g., in

jams, is a result of alcoholic fermentation (i.e.,the

desirable process in ethanol production), producing

carbon dioxide, i.e.:

Sugar

Ethanol + Carbon dioxide

OH + 2CO

Bacteria constitute the major microbial mechanism of

food degradation, especially for raw foods with a

high amount of protein such as raw meats and fish,

and also for pasteurized milk and dairy products.

0015 Lactic acid bacteria are able to grow in mildly

acidic foods, e.g., milk, and the lactic acid formed is

the cause of the ‘souring’ observed. This is an

example of a sugar being used by a bacterium – with

resultant formation of a possibly organoleptically un-

desirable compound. (See Lactic Acid Bacteria.)

Some bacteria, e.g., Clostridium butyricum convert

sugars to butyric acid, which is particularly foul-

smelling with an odor of rancid butter. It can occur

in fermented cabbage (sauerkraut). Bacterial decom-

position of organic nitrogen compounds in foods, i.e.,

proteins and amino acids, leads to very offensive

odors and potentially dangerous toxins – a process

referred to as putrefaction. The major initial change

in this process is the decarboxylation of the a-amino

acids, i.e.:

N−C−C−OH

N−C−H

In histamine poisoning, especially relevant for scom-

broids such as mackerel and tuna, the amino acid

histidine is decarboxylated to histamine in this

particular type of mild but alarming enteric

poisoning. The enteric bacteria responsible do not

grow above 10



C, and hence carefully controlled

chilled storage can be used. Histamine is referred to

as a biogenic amine; others with very undesirable

odors include cadaverine and putrescine. More com-

plex decomposition of the protein, especially using

the sulfur groups present, gives rise to the foul-

smelling hydrogen sulfide (rotten egg smell) and the

thiols, e.g., methanethiol, H

CSH.

0016Putrefaction is caused by several bacteria, both

aerobic and anaerobic types, and salmonellosis and

botulism, as indicated above, represent the dangerous

initial stages before putrefaction is organoleptically

detectable. With canned foods that are weakly acidic

and high in protein, improper sterilization can lead

to sulfide spoilage resulting from the growth of the

anaerobic bacterium Clostridium nigrificans. This

results in the production of hydrogen sulfide and

blackening of the can interior. Foods that are high in

lipid content and consequently low in water activity

do not favor bacterial growth. However, bacteria

such as the Pseudomonas species can cause hydrolysis

of lipids, e.g., triacylglycerols, to fatty acids and gly-

cerols (see below also) – and further oxidation to

methylketones. This type of rancidity can occur with

‘fatty fish’ such as herrings and salmon.

0017The range of bacterial effects on food is enormous –

not least because of the wide range of organic

compounds that bacteria use for energy, and the pos-

sibility of producing a vast array of new compounds

of questionable safety or organoleptic quality. Raw

fish are particularly prone to bacterial degradation –

being examples of high-protein foods as illustrated

above. The characteristic fishy smell is due to the

production of trimethylamine (TMA), resulting from

the bacterial reduction of the naturally occuring tri-

methylamine oxide (in marine but not fresh water

fish), i.e.:

C−N

−O

−

−N

Bacterial

reduction

5606 STORAGE STABILITY/Mechanisms of Degradation