STERILIZATION OF FOODS 5593

bacteriocins, proteinaceous materials produced by an Further Reading

organism that inhibits other closely related organ-
Allison GE and Klaenhammer TR (1998) Phage resistance
isms. Nisin, a bacteriocin produced by strains of Lac-
mechanisms in lactic acid bacteria. International Dairy
tococcus lactis, is approved for use in processed Journal 8: 207–226.
cheese and other foods as a direct food additive (See Caplice E and Fitzgerald GF (1999) Food fermentations:
Nisin). LAB that produce nisin and other bacteriocins role of microorganisms in food production and preser-
can be used as part of a lactic acid-producing starter vation. International Journal of Food Microbiology 50:
culture or as an adjunct in dairy, meat, and other 131–149.
foods to inhibit pathogens as well as spoilage organ- Champagne CP, Lacroix C and Sodini-Gallot I (1994)
isms. In some cases, the organism produces bacterio- Immobilized cell technologies for the dairy industry.
cin in the food, but does not produce acids or other Critical Reviews in Biotechnology 14: 109–134.
fermentation end products that would alter the sens- Ford A and Fitzgerald GF (1999) Bacteriophage defence
systems in lactic acid bacteria. Antonie van Leeuwen-
ory characteristics of the product. Alternatively, the
hoek 76: 89–113.
organism can be grown in a dairy- or nondairy-based
Hammes WP and Hertel C (1998) New developments
medium, and the fermented medium containing or- in meat starter cultures. Meat Science 49(suppl.):
ganic acids and bacteriocin is subsequently pasteur- S125–S138.
ized and concentrated. Viable cells are absent, but the Hutkins RW (2001) Metabolism of starter cultures. In:
material, when added to foods, would still contribute Marth EH and Steele JL (eds) Applied Dairy Micro-
antimicrobial activity. These ‘bioprotective’ products biology, pp. 209–241. New York: Marcel Dekker.
have been marketed as shelf-life extenders, although McKay LL and Baldwin KA (1990) Applications for bio-
pathogens may also be inhibited. Most of these prod- technology: present and future improvements in lactic
ucts are effective only against Gram-positive bacteria; acid bacteria. FEMS Microbiology Reviews 7: 3–14.
however, some products also inhibit Gram-negative Mäyrä-Mäkinen A and Bigret M (1999) Industrial use and
production of lactic acid bacteria. In: Salminen S and
spoilage bacteria, including psychrotrophs.
von Wright A (eds) Lactic Acid Bacteria, pp. 73–103.
New York: Marcel Dekker.
See also: Beers: History and Types; Raw Materials; Wort
Ross RP, Stanton C, Hill C, Fitzgerald GF and Coffey A
Production; Biochemistry of Fermentation; Chemistry of
(2000) Novel cultures for cheese improvement. Trends
Brewing; Bifidobacteria in Foods; Biotechnology in
in Food Science and Technology 11: 96–104.
Food Production; Bread: Dough Mixing and Testing
Tannock GW (1999) Identification of lactobacilli and
Operations; Dough Fermentation; Cheeses: Types of
bifidobacteria. In: Tannock GW (ed.) Probiotics: A
Cheese; Genetically Modified Foods; Lactic Acid
Critical Review, Wymondham, pp. 45–56. England:
Bacteria; Probiotics; Vinegar; Wines: Types of Table
Horizon Press.
Wine

Starvation See Famine, Starvation, and Fasting

STERILIZATION OF FOODS
M N Ramesh, Central Food Technological Research processor to store and distribute the products at am-
Institute, Mysore, India bient temperatures, thus extending the shelf-life. Ster-
Copyright 2003, Elsevier Science Ltd. All Rights Reserved. ilization procedures involve the use of heat, radiation,
or chemicals, or physical removal of cells. The steril-
ization process involves four distinct stages. First, the
product must be heated at 110–125  C to ensure
Background
sterilization. After this, the product requires a few
0001 Sterilization is the complete destruction or elimin- minutes to equilibrate, since the surface will be hotter,
ation of all viable organisms in/on an object being and the central portion of the container will still be
sterilized. Sterilization destroys yeasts, molds, vegeta- cool. The equilibration stage allows a reduction in the
tive bacteria, and spore formers, and allows the food temperature gradient. Next, the product must be held
 5594 STERILIZATION OF FOODS

at this temperature for a certain period of time to for several years. However, a method to quantify the
ensure a predetermined sterilization value designated microbial destruction that takes place during thermal
by the F0 value. Finally, the product has to be cooled treatment has only been understood for the last 75
down mainly to arrest further heat treatment and to years. In order to determine the amount of microbial
avoid bursting of the container at hot conditions. destruction that a thermal treatment delivers to a
0002 The basic principles of sterilization technology as process, an understanding of the amount of heat de-
applied to food processing are as follows: livered to every portion of the food product and the
destruction kinetics of the microorganisms of interest
0003 . The processed product must be free from micro-
is required. The amount of heat delivered by a food
organisms capable of producing food poisoning
process is dependent on the way in which the product
toxins and those microorganisms that cause food
is heated and its physical nature. Process-dependent
spoilage during the product’s shelf-life, until it is
factors can include: processing equipment design,
consumed.
type of heating media, container, food size and
0004 . Clostridium botulinum spores are capable of grow-
shape, product composition, and viscosity (conduc-
ing in low acid (pH > 4.6) products during storage
tion- or convection-heated). The thermal destruction
and hence must be heat-treated to the equivalent of
kinetics of microorganisms or their ability to be killed
at least 121.1  C for 3 min (an F0 value of 3) to
within the food matrix is dependent on a number of
achieve a 12-decimal reduction in the numbers of
factors, including: pH of the product, levels and types
the microorganism.
of preservatives, water activity, the previous growth
0005 . The processing conditions should be applied to the
conditions of the microorganisms of concern, product
slowest-heating point, referred to as the ‘cold
composition, and competitive microorganisms.
point.’ This facilitates the assumption that, when
The two types of bacteria of concern in food pre- 0007
the slowest heating part is sterilized, by exposing it
servation are organisms of public health significance
to the required time–temperature profile, the rest of
and spoilage-causing bacteria. In low-acid foods with
the product will be sterilized.
a pH greater than 4.6, the organism of public health
Practically, complete sterilization leads to a deterior- significance is Clostridium botulinum. Canned foods
ation in product quality and nutrient retention are processed, based on the survival probability for
(Figure 1). Hence, in practice, commercial sterility is C. botulinum of 1012, or one survivor in 1012 cans.
defined as a product that has been processed so that, The organism most frequently used to characterize
under normal conditions, the product will neither low-acid food spoilage by mesophilic spore-formers
spoil nor endanger the health of the consumer. The is PA 3679, a strain of C. sporogenes. Most food
pH of the product is an important factor in determin- companies accept thermal inactivation of 105 for
ing the severity of the sterilization process. mesophilic spore-formers and 102 for thermophilic
spore-formers. The processing time depends on the
bioburden of the most resistant bacteria in a particu-
Theory of Sterilization
lar food, the spoilage risk involved, and whether food
0006 Thermal treatment of food products to render them can support the growth of potential contaminating
free of pathogenic microorganisms has been practiced bacteria. Although a large amount of research work
has been carried out on the influence of different
factors on the processing time and the corresponding
sterilization value, a number of uncertainties still
a
exist on the application of these factors to arrive
scientifically at the exact processing conditions. In
order to avoid any risk resulting from these uncertain-
Quality

ties, a safety factor is added to increase the processing

time to sterilize the food product completely, which
invariably reduces the nutrient content and the
b increase in energy cost.

Time (min) Decimal Reduction Time or D value

The decimal reduction time, or D value, is the time at 0008
fig0001 Figure 1 Effect of sterilization on quality. (a) Degree of steril-
ization; (b) degree of nutrient loss. From Ramesh MN (1995) any temperature to destroy 90% of the spores or
Optimum sterilization of foods by thermal processing – a review. vegetative cells of a given organism. It is equal to the
Food Science and Technology Today 9(4): 217–227 with permission. number of minutes for the survivor curve to traverse
 STERILIZATION OF FOODS 5595

one log cycle and can be calculated from the log DRef ¼ log DT ¼ 1=½zðT  TRef Þ;
reciprocal of the slope of the survivors, assuming a
logarithmic death rate: where DRef is the D value at the reference tempera-
ture (TRef of 121.1  C), DT is the D value at any other
N ¼ N0 ekt ; temperature, T, and z is the thermal death time
constant.
where N is the final number of survivors after heat
A graphical representation of the D-value concept 0010
treatment, N0 is the initial number of organisms, e is
is shown in Figure 3.
the exponential function, k is the thermal death rate
coefficient (traditionally measured in s1), and t is the
processing time. Rearranging gives Heat Resistance of Microorganisms
log N ¼ log N0  ð60kt=2:3Þ: The most important bacterial spore former with 0011

respect to heat processing is Clostridium botulinum,

Plotting log N against t in minutes gives the slope because of the potent neurotoxin that it produces.
60k/2.3 min1. The inverse of the slope is The organism occurs in seven distinct stereotypes
D ¼ 2:3=60k min: A–G, which are further subdivided into proteolytic
and nonproteolytic strains. The most heat-resistant
spores are those produced by type A and the proteo-
Thermal Death Time Constant or z Value lytic B strains. The D value at 121.1  C of the most
resistant strains is generally considered to be
0052 The D value may be calculated from the inverse of the
0.21 min. Clostridium sporogenes, which is closely
slope of the survivor curve by regression analysis of
related to the proteolytic strains, produces spores
the data points. If a presence/absence (growth/no
that are more heat-resistant. The D value of the
growth) method is used to estimate the number of
spores at 121.1 C can be up to 1.5 min. C. sporo-
survivors, Nu can be calculated using the equation
genes has been reported to be the most common
Nu ¼ 2:303 logðn=rÞ, mesophilic putrefactive anaerobe in the spoilage of
low-acid (pH  4.6) canned foods.
where n is the number of units heated, r is the number The butyric anaerobes (C. butyricum, C. beijer- 0012
of sterile units, and Nu is the most probable number inckii, and C. pasteurianum) are usually associated
of survivors. The graphical representation of the with spoilage of products with pH values between 3.9
D-value concept is indicated in Figure 2. and 4.6 (e.g., tomatoes and pears), producing blown
0009 The change in D value with temperature can be cans and a butyric odor. Another organism that causes
obtained by plotting log D against temperature. The z spoilage of products down to pH 4.2, Bacillus coagu-
value is the number of degrees for the thermal resist- lans, has been reported to have a D value at 98.9  C of
ance curve to transverse one log cycle and is equal to 3.1 min and a z value of 16.1C  . Other mesophilic
the reciprocal of the slope of the curve. The equation Bacillus species that produce heat-resistant spores
applied to the thermal resistance curve is given by and have been implicated in food poisoning out-
breaks are B. subtilis and B. licheniformis. Spores of

Log N/N 0 = time/D

D = 12−6 = 6 min@ 120 ⬚C Z = 110 − 102 = 8 ⬚C

10 000 1000.0
D = 24−12 = 12 min@ 110 ⬚C

1000 100.0
Bacteria /g

D-value

110 ⬚C
100 10.0

D value 120 ⬚C 1.0

10
z value
1 0.1
5 10 15 20 25 30 100 110
Minutes Temperature

fig0002 Figure 2 D-value of a microorganism. From http://cour- Figure 3 z-value of a microorganism. From http://cour- fig0003
ses.che.umn.edu/00fscn1102-1s/pdf_files/topic_11_Past_and_- ses.che.umn.edu/00fscn1102-1s/pdf_files/topic_11_Past_and_-
Ster.pdf with permission. Ster.pdf with permission.
 5596 STERILIZATION OF FOODS

the thermophilic organisms that have an optimum Thus, 1 min at 111.1  C is worth 0.1 min at 121.1  C
growth temperature of 55  C are usually much in terms of lethality. Note that z is written C  and not

more resistant to wet heat than mesophilic organisms. C, since z represents a change of 10 Centigrade
0013 The most resistant thermophile is Clostridium ther- degrees in the above example and not a temperature
mosaccharolyticum, which produces blown cans and of 10 degrees Centigrade.
a butyric or ‘cheesy’ odor, with D values as high as
68–195 min at 121.1  C. The spores of Bacillus stear- F Value
othermophilus have often been used in process evalu-
ation studies because of their high heat resistance. The symbol F designates the equivalent in minutes at 0015

The organism can cause spoilage of products with 121.1  C of the combined lethalities of all time–tem-
pH values > 5.3. Another unusual bacterial spore perature relationships at the point of slowest heating
former is Sporolactobacillus inulinus. This organism for a product during heat processing. Thus, the F
has been isolated from a number of food and environ- value is a measure of the killing power of a heat
mental sources and has been reported to have a process. The term Fc denotes the F value at the center
D value at 90  C of 5.1 min and a z value of 13 C . of a pack, F0 denotes the equivalent F value in
Quality-control personnel should be aware, there- minutes at 121.1  C, and Fs denotes the integrated
fore, that not all isolates of Gram–positive spore- lethality of heat received by all points in a container.
forming rods belong to the genera Bacillus or Fs can be related to the D value by the equation
Clostridium. Typical D and z values for some of the Fs ¼ DRef ðlog N0  log NÞ;
spore formers are listed in Table 1.
where DRef is the D value at 121.1  C, log N0 is log of
the initial number of organisms, and log N is the log
Lethal Rate
of the final number of surviving organisms. In the
0014 Using the z value, the lethal rate, L, can be calculated above equation, Fs can be considered as equivalent
from to F0 or Fc in rapid heating systems such as the
thermoresistometer. The F0 value of a process can be
L ¼ log1 ½ððT  TRef Þ=zÞ: obtained in practice by summing the lethal rates at
The lethal rate is a measure of the lethality of any 1-min intervals from the heating and cooling curve of
temperature, T, relative to the reference temperature a product during a heat process. This is a simplified
TRef. For example, for a reference temperature of view, and process calculations in the industry involve
121.1  C and a z of 10C  , the lethal rate at 111.1  C rather more complicated mathematics and usually do
will be not include the cooling curve contribution to the
process. The process time can be calculated using
log1 ½ð111:1  121:1Þ=10Þ ¼ 0:1: the equation

tbl0001 Table 1 Typical heat resistances of selected spore formers

Organism Medium Temperature (  C) D value (min) z (C  )

Clostriduim sporogenes (PA 3679) Several substrates including pea purée 104.4–143.3 0.75–2.03 9.0–14.7
Phosphate buffer pH 7 112.8–148.9 1.06 9.3
Clostriduim botulinum Several substrates 104.0–132.2 0.051–0.58 8.2–10.4
Phosphate buffer pH 7 120.0–140.0 0.13 11.0
Other Clostridium sp. Several substrates 85.0–121.0 0.2–195 6.9–11
Bacillus sp. Several substrates including milk 100.0–121.0 0.3–16.0 4.1–7.7
Desulfotomaculum nigrificans 121 55.0 9.5
Escherichia coli Nutrient broth 70 0.006 4.9
Milk 70 0.04 6.5
Lactobacillus sp. Tomato juice 70 4.0–11.0 11.5–12.5
Listeria monocytogenes Beef, chicken, and carrot homogenates 70 0.14–0.27 5.98–7.39
Salmonella typhimurium Aqueous sucrose/glucose (aw 0.995) 70 0.03–816 6.8–19.0
Milk chocolate (51% milk)
Staphylococcus aureus Milk 70 0.30 5.1
Streptococcus sp. Broth and ham 70 0.015–2.84 3.5–17.0
Microbacterium lacticum Skim milk 70 4.0

From Brown KL (1991) Principles of heat preservation. In: Rees JAG and Bettison J (eds) Processing and Packaging of Heat Preserved Foods, pp. 15–49.
New York: Van Nostrand Reinhold.
 STERILIZATION OF FOODS 5597

B ¼ fh ðlog Jih Ih  log gc Þ, Methods of Sterilization

where B is the thermal process time corrected for the The food sterilization methods are divided into two 0018

time to bring the retort to the process temperature, fh categories: sterilization by heating (thermal process-
is the time in minutes for the semilog heating curve to ing) and sterilization without heating (nonthermal
traverse one log cycle, Jih is the heating lag factor, Ih is processing). Thermal processing is widely practiced
difference in temperature between the retort and the these days, in spite of some problems such as that the
food at the start of process and gc is the difference process of heating might reduce nutrition or deterior-
between the retort temperature and the maximum ate the quality of foods, and that it is ineffective
temperature reached by the food at the center. In the against certain types of bacteria. Nonthermal pro-
processing of low-acid foods with a pH value of 4.6 cessing is considered an effective method that does
or above, a process equivalent in lethality to at least not cause any deterioration of quality, in contrast
F0 of 3 min must be applied to minimize the risk of with thermal processing. However, no reports have
spores of C. botulinum. An F value determined on the shown the effect of sterilization without heating. Re-
basis of one z value to an F value determined on search on the evaluation of the technology of non-
the basis of a second z value can be converted using thermal sterilization without heating is widely being
the appropriate formula. pursued internationally.
Thermal processing is divided further into two 0019

categories: in-container sterilization (processing).

Cook Value or C Value The principles involved in thermal sterilization of
0016 Mansfield introduced the concept of a lethality-like foods remain the same whether attempting to sterilize
value for sensory degradation, termed the cook value products in containers or sterilize products prior to
or C value. This has a reference temperature of filling in the final container (aseptic processing).
100  C and a z value typically in the range 20–40C  . Thus, it is necessary to know the thermal destruction
rate for the microorganisms of consequence in the
C100 ¼ 10ðT100Þ=z c min: food being processed. The procedures necessary for
acquisition of such data are available. It is important
The processing conditions mainly depend on the pH to use this information properly so that the appropri-
value of the product. ate time and temperature for destruction of the or-
ganisms can be achieved. Sterilization procedures for
products in containers usually require longer times,
Acid Products (pH < 4.6)
since the heat transfer to the product is relatively
0017 At a pH below 4.6, the risk of growth and toxin slow. Sterilization prior to filling in the container, as
production by C. botulinum is extremely low, and accomplished in aseptic processing, requires rela-
for products with pH values between 4.0 and 4.6, tively short heating periods. This sterilization process
processes are aimed at controlling the survival and is usually accomplished by heating the product rap-
growth of spore forming organisms such as Bacillus idly to 130–145  C, holding for an appropriate time,
coagulans, Bacillus polymyxa, Bacillus macerans, then rapidly cooling the product. The specific product
and the butyric anaerobes such as C. butyricum and will determine the actual combination of temperature
C. pasteurianum. A heat process of F10 121 ¼ 0.7 is and time required for sterilization.
regarded as adequate for this purpose, and a process Product heating is accomplished by either indirect 0020
8.3
equivalent to 10 min at 93.3  C (F93.3 ¼ 5) when the or direct heat exchange. In liquid homogeneous prod-
pH is between 4.0 and 4.3. However, more severe ucts, indirect heating occurs when the product to be
processes may be required to control excessive con- heated and the heating medium are separated by the
tamination. In a previous study of nine cases of botu- heating surface. This type of heating is accomplished
lism from products with pH below 4.6, including using a scraped surface, tubular, or plate heat exchan-
canned pears, apricots, tomato ketchup, tomato– ger. Direct heat exchange is achieved by placing the
onion chili sauce, and green tomatoes, it was sug- heating medium directly into the product or vice
gested that growth of spoilage organisms raised the versa. In either case, the added water from the steam
pH to a level at which dormant spores of C. botuli- must be either removed or accounted for in the
num could germinate and grow. Below pH 3.7, the formulation. The specific type of heating used is usu-
processor is concerned with the control of nonsporing ally dictated by the nature of the product and the
bacteria, yeasts, and molds. These may be generally economics of operation. Following heating to the
controlled by heat processes at temperatures below sterilization temperature, the product must be held
100  C. at this temperature for a specific length of time to
 5598 STERILIZATION OF FOODS

accomplish sterilization. The sterilization time in con- C. botulinum is likely to be present but in foods that
tinuous-flow aseptic systems is obtained by conveying contain less heat-resistant spoilage microorganisms.
the product through a nonheating pipe attached to In addition to information on heat resistance, it is
the heating system. This pipe is called the holding necessary to collect data describing the rate of heat
tube and is of a specified uniform diameter and penetration into the food in order to calculate the
length. Its capacity is such that the fastest particle is processing time needed for commercial sterility.
held at the maximum time required to sterilize the
product. The length of time for which the product
Retorting (Heat Processing)
remains in the tube is dependent not only on the
holding capacity of the tube and the rate at which The most widely used system for sterilization uses 0025

the product is pumped through the tube, but also on overpressure retorts or agitating sterilizers. Generally,
the manner in which the product flows through the the overpressure retorts are of the batch type with
tube. 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
Thermal Processing
cycle and with compressed air during the cooling
0021 During sterilization, the type of heat, time of applica- cycle. Agitating sterilizers gently move the product
tion, and temperature to ensure destruction of all within the container, which facilitates more uniform
microorganisms are very important. Endospores of heating and cooling owing to better heat transfer.
bacteria are considered the most thermoduric of all This reduces the come-up time, delivering an
cells, so their destruction guarantees sterility. The improved product.
lethal temperature varies for microorganisms. The
time required to kill the microorganisms depends on
Heating by Saturated Steam
the number of organisms, species, nature of the prod-
uct being heated, pH, and temperature. Whenever Latent heat is transferred to food when saturated 0026

heat is used to control microbial growth, inevitably steam condenses on the outside of the container. If
both time and temperature are considered. air is trapped inside the retort, it forms an insulating
0022 Sterilization (boiling, autoclaving, hot air oven) boundary film around the cans, which prevents the
kills all microorganisms with heat and is commonly steam from condensing and causes under processing
employed in canning, bottling, and other sterile pack- of the food. It also produces a lower temperature than
aging procedures. Heat sterilization is the unit oper- that obtained with saturated steam. It is therefore
ation in which foods are heated at a sufficiently high important that all air be removed. Sterilization
temperature and for a sufficiently long time to destroy requires a temperature of 121.1  C. This creates
microbial and enzyme activity. pressure inside the container, and if the pressure on
0023 Sterilized foods have a longer shelf-life but undergo the outside of the container is less than that inside, the
substantial changes in quality. Developments in pro- container will expand and subsequently break. This is
cessing technology therefore aim to reduce the overall more critical in glass jars and flexible containers like
processing time. The effects of microbial heat resist- pouches and plastic cans. To avoid this situation, an
ance on the design of heat-sterilization procedures overhead pressure of 230–250 kPa is applied to
and equipment are an important aspect of steriliza- equalize the internal pressure.
tion in both in-container heat sterilization and aseptic
processing.
Aseptic Processing
0024 In order to determine the process time for a given
food, it is necessary to obtain information on both the Conventional retorting of A2 cans of vegetable soup 0027

heat resistance of microorganisms and the rate of heat requires 70 min at 121  C to achieve an F0 value of
penetration into the food. A process that reduces cell 7 min, followed by cooling for 50 min. Aseptic pro-
numbers by eight decimal reductions (an 8D process), cessing in a scraped-surface heat exchanger at 140  C
applied to a raw material containing 105 spores per for 5 s gives an F0 value of 9 min. Increasing the can
container would reduce microbial numbers to 103 size to A10 increases the processing time to 218 min,
per container, or one microorganism in every thou- whereas with aseptic processing, the sterilization time
sand containers. Commercial sterility, therefore, is the same. In aseptic processing, containers are
means that the vast majority of containers are sterile, not required to withstand sterilization conditions.
but there is a probability that nonpathogenic cells Cartons are presterilized with hydrogen peroxide,
survive the heat treatment in a predetermined and filling machines are enclosed and maintained in
number of containers. A 12D process is used when a sterile condition by ultraviolet light and filtered air.
 STERILIZATION OF FOODS 5599

A positive air pressure is maintained in the filling Spores can be inactivated by combining high pressure
machine to prevent entry of contaminants. The with temperature. The germination of spores is an
process is successfully applied to liquid and small- important step in spore inactivation. The process
particulate foods, but problems remain with larger temperature during pressure treatment can be speci-
pieces of solid food. 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
Nonthermal Processing
and uniformly throughout a mass of food, independ-
0028 Nonthermal methods for the preservation of foods ent of the size, shape, and food composition. Com-
are under intense research to evaluate their potential pression uniformly increases the temperature of foods
as an alternative or complementary process to trad- by approximately 3  C per 100 MPa. Compression of
itional methods of food preservation. Traditionally, foods may shift the pH of the food as a function of
most preserved foods are thermally processed by sub- imposed pressure and must be determined for each
jecting the food to a temperature of 60–100  C for a food treatment process. Water activity and pH are
few seconds to minutes. During this period, a large critical process factors in the inactivation of microbes
amount of energy is transferred to the food. This by HPP. An increase in food temperature above room
energy may trigger unwanted reactions in the food, temperature and, to a lesser extent, a decrease below
leading to undesirable changes or formation of bypro- room temperature increase the inactivation rate of
ducts. For example, thermally processed milk may microorganisms during HPP treatment. Temperatures
have a cooked flavor accompanied by a loss of vita- in the range of 45–50  C appear to increase the rate
mins, essential nutrients, and flavors. The fact that of inactivation of food pathogens and spoilage
the shelf-life and the quality of food are important microbes. Temperatures from 90–110  C in conjunc-
to consumers gave birth to the concept of preserv- tion with pressures include batch and semicontinuous
ing foods using nonthermal methods. Nonthermal systems, but no commercial continuous HPP systems
methods of food preservation are currently being de- are operating.
veloped to eliminate, or at least minimize, the quality The critical process factors in HPP include pres- 0031

degradation of foods that results from thermal pro- sure, time at pressure, time to achieve treatment
cessing. pressure, decompression time, treatment temperature
0029 During nonthermal processing, the temperature of (including adiabatic heating), initial product tem-
the food is held below the temperature normally used perature, vessel temperature distribution at pressure,
in thermal processing. Therefore, the quality degrad- product pH, product composition, product water ac-
ation expected from high temperatures is minimal in tivity, packaging material integrity, and concurrent
nonthermal processing. The vitamins, essential nutri- processing aids. Chemical changes in the food gener-
ents, and flavors are expected to undergo minimal, or ally will be a function of the process and treatment
no, changes during nonthermal processing. In add- time. Because some types of spores of C. botulinum
ition, nonthermal processes use less energy than ther- are capable of surviving even the extreme pressures
mal processes. Foods can be processed nonthermally and temperatures of HPP, there is no absolute micro-
using high hydrostatic pressure, oscillating magnetic bial indicator for sterility by HPP. For vegetative bac-
fields, high-intensity pulsed electric fields, intense teria, nonpathogenic Listeria innocua is a useful
light pulses, irradiation, chemical, biochemical, and surrogate for the foodborne pathogen, Listeria mono-
hurdle technology. Although these technologies have cytogenes a nonpathogenic strain of Bacillus may be
been used for a long time to inactivate microorgan- useful as a surrogate for HPP-resistant E. coli
isms and/or preserve food, they have gained recogni- 0157:H7 isolates.
tion as nonthermal methods of food preservation only Subjecting foods to pressures of 100–800 MPa in- 0032

in the recent past. Nonthermal processes are expected activates vegetative bacteria, yeasts, molds, and para-
to induce only minimal degradation of food. sites in products such as jams, orange juice, and meat
products. Factors that affect the rate of microbial
High-pressure Processing
inactivation include pressure and magnitude, micro-
0030 High-pressure processing (HPP), also described as bial type and growth stage, temperature, pH, water
high hydrostatic pressure (HHP) or ultrahigh pressure activity, and food composition. Application of a pres-
(UHP) processing, subjects liquid and solid foods, sure of 680 MPa on grape juice for 10 min would
with or without packaging, to pressures between arrest the growth of the microbes and thereby stops
100 and 900 MPa. A high hydrostatic pressure is further fermentation of the grape juice. Peaches and
used for the inactivation of microorganisms and cer- pears subjected to a pressure of 410 MPa for 30 min
tain enzymes and for shelf-life extension of foods. exhibit a shelf-life of 5 years. One of the important
 5600 STERILIZATION OF FOODS

challenges in using HPP is the fabrication of pressure research. Although PEF has potential as a technology
vessels and seals that can withstand the high pres- for food preservation, existing PEF systems and
sures during the cycles of pressurization and depres- experimental conditions are diverse. The effects of
surization. critical process factors on pathogens of concern and
kinetics of inactivation need to be studied further. An
Pulsed Electric Fields
electric pulse process for the treatment of fresh citrus
0033 High-intensity pulsed electric field (PEF) processing juices capable of reducing target pathogens without
involves the application of pulses of high voltages alteration of the juice was granted an FDA letter of no
(typically 20–80 kV cm1) of exponentially decaying, objection for its use in April 1999.
square wave, bipolar, or oscillatory pulses and at
ambient, subambient, or slightly above ambient tem- High-voltage Arc Discharge
peratures for less than 1 s to foods. High-intensity Arc discharge is an early application of electricity to 0035
electric fields applied to a food in the form of short- pasteurize fluids by applying rapid discharge voltages
duration pulses can inactivate the microorganisms through an electrode gap below the surface of aque-
and certain enzymes. Energy loss due to heating of ous suspensions of microorganisms. A multitude of
foods is minimized, reducing the detrimental changes physical effects (intense wave) and chemical com-
of the sensory and physical properties of foods. Some pounds (electrolysis) are generated, inactivating the
important aspects in pulsed electric field technology microorganisms. The use of arc discharge for liquid
are the generation of high electric-field intensities, the foods may be unsuitable, largely because of electroly-
design of chambers that impart uniform treatment to sis and discharge, but more recent designs have
foods with minimal increase in temperature, and shown some promise for use in food preservation.
the design of electrodes that minimize the effect of
electrolysis. Pulsed Light Technology
0034 Although different laboratory- and pilot-scale
treatment chambers have been designed and used Pulsed light is a method of food preservation that 0036

for PEF treatment of foods, only two industrial scale involves the use of intense and short-duration pulses
PEF systems are available. The systems including of broad spectrum ‘white light’ (ultraviolet to the
treatment chambers and power supply equipments near-infrared region). The use of pulsed high-intensity
need to be scaled up to commercial levels. To date, light to inactivate microorganisms is a relatively new
PEF has been applied mainly to improve the quality of technology. For most applications, a few flashes ap-
foods. Application of PEF is restricted to food prod- plied in a fraction of a second provide a high level of
ucts that can withstand high electric fields, have a low microbial inactivation. This technology is applicable
electrical conductivity, and do not contain or form mainly in sterilizing or reducing the microbial popu-
bubbles. The particle size of the liquid food in both lation on packaging or food surfaces. Extensive inde-
static and flow treatment modes is a limitation. Sev- pendent research on the inactivation kinetics under a
eral theories have been proposed to explain microbial full spectrum of representative variables of food
inactivation by PEF. The most-studied theories are systems and surfaces is needed. Application of light
electrical breakdown and electroporation. Factors pulses involves exposure of foods to short duration
that affect the microbial inactivation with PEF are pulses (1 ms to 0.1 s) of intense incoherent light. Light
process factors (electric field intensity, pulse width, with an energy density of about 0.01–50 J cm2 and a
treatment time and temperature, and pulse wave wavelength in the range of 170–2600 nm is used.
shapes), microbial entity factors (type, concentration, Such incoherent intense pulses of light may be gener-
and growth stage of microorganism) and media ated using gas–filled flash lamps or spark gap dis-
factors (pH, antimicrobes and ionic compounds, con- charge apparatus. Full – or filtered – spectrum light
ductivity, and medium ionic strength). Microbial may be used, depending on the degree of sterilization
inactivation increases with increasing electric field expected. The filtered–spectrum light is devoid of
intensity, exposure time, and temperature of the wavelengths known to cause undesirable reactions
food. However, it is desirable to maintain the in foods. Glass or liquid filters are used to obtain
temperature below 30–40  C by providing a cooling the filtered spectrum. In general, filtered light is
system. Different bacteria have different sensitivities more effective for microbial inactivation than full–
to electric field treatment. In general, Gram-positive spectrum light.
bacteria and yeasts are more resistant to electric fields
Ultraviolet Light
than Gram-negative bacteria. The optimum condi-
tions for maximum inactivation of a specific micro- There is particular interest in using ultraviolet (UV) 0037

organism can be determined after preliminary light to treat fruit juices, especially apple juice and
 STERILIZATION OF FOODS 5601

cider. Other applications include disinfection of water high-frequency, high-intensity magnetic fields were
supplies and food contact surfaces. Ultraviolet pro- discussed in the IFT Annual Meeting in 2000.
cessing involves the use of radiation from the ultra-
violet region of the electromagnetic spectrum. The Pulsed X-rays
germicidal properties of UV irradiation (200–
A number of studies have compared the effects of 0040
280 nm) are a result of DNA absorption of the UV
electron beam, gamma rays, and X-rays, but com-
light. This mechanism of inactivation results in a
parison between these technologies is inconclusive
sigmoidal curve of microbial population reduction.
owing to differences in the doses applied. Electrons
To achieve microbial inactivation, the UV radiant
have a limited penetration depth of about 5 cm in
exposure must be at least 400 J m2 in all parts of
food, whereas X rays have significantly greater pene-
the product. Critical factors include the transmissivity
tration depths (60–400 cm) depending on the energy
of the product, the geometric configuration of the
used. The use of pulsed X-rays is a new alternative
reactor, the power, wavelength and physical arrange-
technology that utilizes a solid-state opening switch
ment of the UV source(s), the product flow profile
to generate an electron beam 30 ns down to a few
and the radiation path length. UV may be used
nanoseconds; repetition rates of up to 1000 pulses per
in combination with other alternative process
second. The practical application of food irradiation
technologies, including various powerful oxidizing
by X-rays in conjunction with existing food-process-
agents such as ozone and hydrogen peroxide, among
ing equipment is further facilitated by:
others.
. the possibility of controlling the direction of the 0041
Oscillating Magnetic Fields
electrically produced radiation;
0038 Static (SMFs) and oscillating magnetic fields (OMFs) . the possibility of shaping the geometry of radiation 0042

have been explored for their potential to inactivate field to accommodate different package sizes;
microorganisms. For SMFs, the magnetic field inten- . its high reproducibility and versatility. 0043

sity is constant with time, while an oscillating mag-

Potentially, the negative effects of irradiation on the
netic field is applied in the form of constant amplitude
food quality can be reduced. Irradiation of foods was
or decaying amplitude sinusoidal waves. An OMF
one of the earliest nonthermal food preservation tech-
applied in the form of pulses reverses the charge for
nologies. Irradiation is the exposure of food to radi-
each pulse. The intensity of each pulse decreases with
ation with an energy of 5–10 kGy and wavelengths of
time to about 10% of the intensity. Preservation of
2000 Å or less. Ultraviolet, beta, gamma, and X-, and
foods with OMFs involves sealing food in a plastic
microwaves are included in this range. One of the
bag and subjecting it to one to 100 pulses in an OMF
attractions of irradiation is its ability to pasteurize
with a frequency of 50–500 kHz at a temperature
foods in the frozen state. The World Health Organ-
of 0–50  C for a total exposure time ranging from
ization (WHO) approved a radiation dosage of up to
25–100 ms. The effects of magnetic fields on micro-
10 kGy as being ‘unconditionally safe for human con-
bial populations have produced controversial results.
sumption.’ Irradiation has the potential to replace the
Consistent results concerning the efficacy of this
use of many hazardous chemical pesticides and pre-
method are needed before considering this technology
servatives.
for food-preservation purposes.
0039 Oscillating magnetic fields with a magnetic flux
Ultrasound
of 5–50 T and a frequency of 5–500 kHz have been
reported to inactivate microorganisms. Foods with Ultrasound is the energy generated by sound waves 0044

an electrical resistivity of 10–25 O  cm may be sealed of 20 000 or more vibrations per second. Although
in a plastic bag and subjected to OMFs. One of ultrasound technology has a wide range of current
the attractive features of using magnetic fields for and future applications in the food industry, including
food preservation is that the food can be packaged inactivation of microorganisms and enzymes, pres-
prior to processing, reducing the possibility of cross- ently, most developments for food applications are
contamination during packaging. Studies on the nonmicrobial. Data on inactivation of food microor-
effects of static and pulsed magnetic fields as an alter- ganisms by ultrasound in the food industry are scarce,
native to conventional thermal treatments on the in- and most applications use combinations with other
activation of Saccharomyces cerevisiae have been preservation methods. The bactericidal effect of ultra-
reported. The potential advantages of food pre- sound is attributed to intracellular cavitations, a
servation by magnetic fields, the proposed interaction phenomenon in which mechanical high-frequency vi-
mechanisms, and some of the results that have brations cause alternate compressions of millions of
been obtained by exposing living systems to low- and microscopic bubbles containing gas and vapor. The
 5602 STERILIZATION OF FOODS

bubbles expand then implode violently, releasing the composition, shape, and size of the food, the
large amounts of energy and generating very high microwave frequency, and the applicator (oven)
temperatures and pressures within the bubbles. The design. Time is also a factor in the sense that, as the
molecules of the vaporized reaction mixture are frac- food heats up, its microwave absorption properties
tured, forming highly reactive free radicals. Cavita- can change significantly, and the location of cold
tions occur as a result of micromechanical shocks that points can shift.
disrupt cellular structural and functional components
up to the point of cell lysis. Ohmic and Inductive Heating
0045 The heterogeneous and protective nature of food Ohmic heating (sometimes also referred to as Joule 0048
with the inclusion of particulates and other interfer- heating, electrical resistance heating, direct electrical
ing substances severely curtails the singular use of resistance heating, electroheating, and electroconduc-
ultrasound as a preservation method. Although, at tive heating) is defined as the process of passing elec-
present, these limitations make commercial develop- tric currents through foods or other materials to heat
ment unlikely, the combination of ultrasound with them. Ohmic heating is distinguished from other elec-
other preservation process (e.g., heat and mild trical heating methods by the presence of electrodes in
pressure) appears to have the greatest potential for contact with the food, frequency, and waveform. The
industrial applications. Critical processing factors are principal advantage claimed for ohmic heating is its
assumed to be the amplitude of the ultrasound waves, ability to heat materials rapidly and uniformly, in-
the exposure/contact time with the microorganisms, cluding products containing particulates. The princi-
the type of microorganisms, the volume of food to be pal mechanisms of microbial inactivation in ohmic
processed, the composition of the food, and the tem- heating are thermal. While some evidence exists for
perature of treatment. nonthermal effects of ohmic processes, which rely on
heat, it may be unnecessary for processors to claim
Microwave and Radio-frequency Processing this effect in their process fillings.
0046 Microwave and radio-frequency heating refers to the Inductive heating is a process wherein electric cur- 0049

use of electromagnetic waves of certain frequencies to rents are induced within the food owing to oscillating
generate heat in a material by two mechanisms – electromagnetic fields generated by electric coils. No
dielectric and ionic. Microwave and radio-frequency data on microbial death kinetics under inductive
heating for pasteurization and sterilization is pre- heating have been published.
ferred to conventional heating, because they require
Hurdle Technology
less time to come up to the desired process tempera-
ture, particularly for solid and semisolid foods. Hurdle technology combines nonthermal food pro- 0050

Industrial microwave pasteurization and sterilization cessing with traditional or other emerging technolo-
systems have been reported for over 30 years, but gies. The most promising combinations include
commercial radio-frequency heating systems for the nonthermal methods, such as high hydrostatic pres-
purpose of food pasteurization or sterilization are sure, ultrasound, and pulsed electric fields. In the
not known to be in use. For a microwave steriliza- inactivation of spores, it is necessary to use a com-
tion process, unlike conventional heating, the design bined methods approach using ‘hurdles.’ Hurdles are
of the equipment can dramatically influence the crit- physical or chemical parameters that can be adjusted
ical process parameters – the location and tempera- to ensure the microbial stability and safety of foods.
ture of the coldest point. This uncertainty makes it The physical parameters include the processing and
more difficult to make general conclusions about storage temperatures, water activity, pH, and redox
processes, process deviations, and how to handle potential at levels that inhibit or inactivate the micro-
deviations. organisms and thus render the food safe. Hurdle
0047 Many techniques have attempted to improve the technology is used in the preservation of meat and
uniformity of heating. The critical process factor seasonal or regional fruits and vegetables.
when combining conventional heating and micro- Besides preserving food quality, new nonthermal 0051

wave or any other novel processes would most likely technologies have to achieve an equivalent or, prefer-
remain the temperature of the food at the cold point, ably, a better enhanced safety level than that for
primarily due to the complexity of the energy absorp- procedures that they replace. Most nonthermal pre-
tion and heat-transfer processes. Since the thermal servation techniques are highly effective in inactivat-
effect is presumably the sole lethal mechanism, the ing vegetative forms of bacteria, yeast, and molds, but
time–temperature history at the coldest location will bacterial spores and most enzymes remain difficult to
determine the safety of the process and is a function of inactivate.
 STERILIZATION OF FOODS 5603

See also: Escherichia coli: Food Poisoning by Species IFT (2001) Kinetics of microbial inactivation for alternative
other than Escherichia coli; Food Poisoning: food processing technologies Journal of Food Science
Classification; Tracing Origins and Testing; Statistics; Special Supplement.
Economic Implications; Food Safety; Food Security; Jay JM (1992) Modern Food Microbiology, 4th edn. New
Heat Treatment: Ultra-high Temperature (UHT) York: Van Nostrand Reinhold.
Treatments; Electrical Process Heating; Pasteurization: Leistner L (1994) Introduction to hurdle technology. In:
Pasteurization of Liquid Products; Pasteurization of Leistner L and Morris LGM (eds) Food Preservation by
Viscous and Particulate Products; Other Pasteurization Combined Processes, Final Report of Flair Concerted
Processes; Preservation of Food; Spoilage: Bacterial Action No. 7, Subgroup B, pp. 1–6.
Spoilage; Molds in Spoilage; Yeasts in Spoilage Matsuda N, Komaki N, Ichikawa R and Gotoh S (1985)
Facultative anaerobic spore-forming bacteria isolated
from spoiled canned foods. Nippon Shokuhin Kogyo
Further Reading Gakkaishi 32(16): 391–398.
Ball CO (1923) Bulletin of the National Research Council National Canners Association (1968) Laboratory Manual
7, Part 1, No. 37. for Food Canners and Processors, vol. 1. Microbiology
Barbosa-Canovas GV, Pothakamury UR, Palou E and and Processing. Westport, CT: AVI.
Swanson BG (1998) Emerging technologies in food pre- Parrott DL (1992) Use of ohmic heating for aseptic process-
servation. In: Nonthermal Preservation of Foods. New ing of food particulates. Food Technology 46(12):
York: Marcel Dekker. 68–72.
Barbosa-Canovas GV, Pothakamury UR and Swanson BG Pflug IJ and Christensen R (1980) Converting an F-value
(1995) State of the art technologies for the stabilization determined on the basis of a second Z value. Journal of
of foods by non thermal processes: physical methods. In: Food Science 45: 35–40.
Barbosa-Canovas and Welti-Chanes J (eds) Food Preser- Pothakamury UR, Barbosa-Canovas GV, Swanson BG and
vation by Moisture Control. Fundamentals and Applica- Meyer RS (1995) The pressure builds for better food
tions, pp. 493–532. Lancaster, PA: Technomic. processing. Chemical Engineering Progress March:
Brown KL (1988) Use of computer data file for storage 45–55.
of heat resistance data on bacterial spores. Journal of Quin B, Pothakamury UR, Barbosa-Canovas GV and
Applied Bacteriology 65: 49–54. Swanson (1996) Nonthermal pasteurization of liquid
Brown KL (1991) Principles of heat preservation. In: Rees foods using high intensity pulsed electric fields. Critical
JAG and Bettison J (eds) Processing and Packaging of Reviews in Food Science and Nutrition 36(6): 603–627.
Heat Preserved Foods, pp. 15–49. New York: Van Quin B, Vea-Mercado H, Pothakamury UR, Barbosa-
Nostrand Reinhold. Canovas GV and Swanson BG (1995) Application of
Cash JN and Sinha NK (1997) Canning of vegetables. In: pulsed electric fields for inactivation of bacteria and
Smith DS, Cash JN, Nip WK and Hui YH (eds) Process- enzymes. Journal of the Franklin Institute 332A:
ing Vegetables: Science and Technology. Lancaster, PA: 209–220.
Technomic. Ramesh MN (1995) Optimum sterilization of foods by
Doores S and Westhoff D (1981) Heat resistance of thermal processing – a review. Food Science and Tech-
Sporolactobacilus inulins. International Journal of nology Today 9(4): 217–227.
Food Science 46: 810–812. Ramesh MN, Prapulla SG, Kumar MA and Mahadevaiah
Gaze JE and Brown KL (1988) The heat resistance of spores M (1997) Thermal processing of Foods: A retrospective
of C. botulinum 213B over the temperature range of Part I, Uncertainties in thermal processing and statistical
120  C to 140  C. International Journal of Food Science analysis. Advances in Applied Microbiology. 44:
& Technology 23: 373–378. 287–314. New York: Academic Press.
Hersom AC and Hulland ED (1980) Canned Foods. Ther- Russell AD (1982) The Destruction of Bacterial Spores.
mal Processing and Microbiology, 7th edn. Edinburgh: London: Academic Press.
Churchill Livingstone. Stevenson KE and Ito KA (1991) Aseptic processing and
Hofmann GA (1985) Deactivation of Microorganisms by packaging of heat preserved foods. In: Rees JAG and
an Oscillating Magnetic Field. US Patent 4,524,079. Bettison J (eds) Processing and Packaging of Heat
Holdsworth SD (1985) Optimization of the thermal pro- Preserved Foods, pp. 72–91. New York: Van Nostrand
cessing – A review. Journal of Food Engineering 4: Reinhold.
89–116. Stumbo CR (1973) Thermobacteriology in Food Process-
Hartfinger L (1992) Microwave Sterilization, Food Tech- ing. New York: Academic Press.
nology 46(12): 57–61. Teixeiria A (1992) Thermal process calculations. In: Held-
Hulsheger H, Potel J and Niemann EG (1981) Killing of man DR and Lund DB (eds) Handbook of Food Engin-
bacteria with electric pulses of high field strength. Radi- eering. New York: Marcel Dekker.
ation and Environmental Biophysics 20: 53–65.