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162
Thermal Processing of Meats
Isabel Guerrero Legarreta
Universidad Autónoma Metropolitana-Iztapalapa
CONTENTS
I. Introduction ......................................................................................................................................................162-1
II. Thermal Properties of Foods ............................................................................................................................162-2
III. Thermal Processes ............................................................................................................................................162-2
A. Scalding ....................................................................................................................................................162-3
B. Cooking ....................................................................................................................................................162-3
C. Pasteurization ............................................................................................................................................162-3
D. Sterilization ..............................................................................................................................................162-3
IV. Thermal Processing of Meats ..........................................................................................................................162-3
A. Cooking ....................................................................................................................................................162-3
B. Canning ....................................................................................................................................................162-4
V. Time-Temperature Profile Calculation ............................................................................................................162-4
VI. Effect on Meat Quality ....................................................................................................................................162-6
VII. Effect of Meat Physicochemical Characteristics on Microbial Growth ..........................................................162-6
A. Water Activity and Redox Potential ........................................................................................................162-7
B. Oxygen Tension ........................................................................................................................................162-7
C. pH..............................................................................................................................................................162-7
D. Temperature ..............................................................................................................................................162-7
E. Antimicrobial Agents................................................................................................................................162-7
F. Physical Structure ....................................................................................................................................162-7
G. Food Composition ....................................................................................................................................162-7
VIII. Microwave Heating ..........................................................................................................................................162-8
Bibliography ................................................................................................................................................................162-8
I. INTRODUCTION
Heat transfer is one of the most important unit operations
in the food industry; it is the cheapest and most efficient
method of preservation. Almost all processes include supplying or removing heat by physical, chemical or biological methods. The objective of food heat treatment is the
destruction of microbial populations and enzyme inactivation in order to prevent spoilage and proliferation of
pathogens and spoilage microorganisms. Sanitation is
ensured after heating; practically all microorganisms are
destroyed or irreversibly damaged by heat (1). Process
conditions – time and temperature – are the decisive
applied factors according to the expected shelf life
of the product, although heating also causes changes
in physicochemical and biological food characteristics
(2). Therefore, in order to ensure thermal processing
efficiency several variables must be considered such as
microbial survival rate and physicochemical composition
and structure.
Heat processing also aims enzyme destruction.
Intrinsic and extrinsic parameters leading to microbial
destruction are practically the same to those involved in
enzyme inactivation as microbial death is due to destruction of at least one enzymatic system resulting from denaturation of the protein moiety (3). Most foods, particularly
meats, are consumed after heating as chemical constituents react improving sensory and nutritional characteristics. Conversely to microorganisms and enzymes, heat
processing aims the least nutrient destruction (4). Even
though, the same factors destroying or inhibiting microbial growth also accelerate nutrient loss. Therefore thermal processing of foods must reach a compromise
between sanitation and quality.
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Handbook of Food Science, Technology, and Engineering, Volume 4
II. THERMAL PROPERTIES OF FOODS
In order to calculate thermal processing efficiency, the following information is necessary:
●
●
Thermal resistance of a given microorganism,
as calculation basis (z- and F- values) (5).
Temperature profile of the product. Thermal
processing includes two transport phenomena:
heat transfer where heat flows proportionally to
the driving force and inverse of the resistance to
flow; mass transfer within the food material
and resulting from reactions among food component (lipids, proteins, minerals, etc.) (6).
In any food, thermal properties are determined by heat
distribution within the product. These are mainly related
to transference (thermal conductivity and specific heat)
and physical properties (density and geometry) (7).
Thermal conductivity in a food material depends on the
thermal flow rate through the product. Heat is defined as
energy transferred as a result of temperature gradients, the
larger the temperature difference the higher the flow rate.
In solid foods temperature difference between the product
surface and the center determines the heating rate.
Heat penetration depends on the transfer mechanisms
within the foods. Food heating depends on the surface
heat transfer coefficient; physical properties of the food
and container, if any; temperature difference between the
heating medium; and food initial temperature and container size (8). However, food thermal processing assumes
that heat transfer in the surface is very high; therefore
resistance is only due to the food composition and structure. Knowing the type and extent of the driving forces
involved transport parameters can be calculated (9).
The main mechanisms involved in heat transfer in
food processing are conduction and convection.
Conduction is transmitted within a solid due to vibrations
of adjacent molecules. In meat canning, conduction
occurs in meat chunks or in gelled canned pastes, such as
luncheon meat. Heat transference rate through a uniform
material depends on the area (A) and temperature gradient
(∆T) but inverse to the thickness of the material (L); it also
depends on the thermal conductivity of the canned food
and tin (k) (7). Fourier’s law indicates this relationship
q ⫽ k (A∆T/L)
In meats, k is very low (1.89 kJ/h m°K) as compared to
stainless steel (59.47 kJ/h m°K) (10). This makes conduction in food materials very inefficient.
Convection heating is mostly related to fluids, such as
soups, brine, milk, etc., as a result of movement of differential densities when the fluid is heated or cooled.
Convection can be accelerated if stirring is applied reducing the temperature difference. In this mechanism, heating
© 2006 by Taylor & Francis Group, LLC
depends on the area of transference (A), temperature
difference (∆T) and a constant, h, which depends on
flow properties, type of surface and flow rate. For boiling
water h ⫽ 1898 to 25308 kJ/ h m°K, whereas for air
h ⫽ 3.16 to 31.63 kJ/ h m°K (11). Convection is based on
Newton’s law:
q ⫽ h A∆T
Canned foods with low viscosity or with small particles,
such as soups or sausages in brine, have higher transfer
coefficients as heat penetration follows a convection
mechanism increased by can rotation. The mechanism can
change from convection to conduction during heating of
heat-induced gels, such as luncheon meats. As convection
rate is higher than conduction, heating rate varies during
processing (6).
Conductivity in meats depends on the direction in
which heat is transferred. Pérez and Calvelo (12) reported
that thermal conductivity in lean beef at 78.5% humidity
and 0°C, applying thermal flow perpendicular to the meat
fiber, is 0.411 kcal/m h°K, whereas under the same conditions, in lean beef at 75% humidity, if the flow is parallel, conductivity is 0.422 kcal/m h°K.
Heat transfer also depends on characteristics of the
heating medium (Table 162.1). High coefficients mean
high heat transference rates to the product surface.
According to these figures, free convection in a smokehouse has the lowest transference rate; forced convection
with a fan significantly increased heat transference rates.
III.
THERMAL PROCESSES
Heat transfer principles can be applied to any material,
including foods. However every thermal process has a specific aim, its severity varies accordingly. Hurdle effects are
the result of particular event interactions (13). In order to
alter food quality to a minimum extent, only necessary hurdles to obtain a microbiologically safe food with considerably extended shelf life must be applied. Therefore severity
of heat treatments also depends on intrinsic microbial controls such as low pH, presence of bacteriostatic compounds,
application further preservation methods such as refrigeration, etc. Because heat processing may alter food quality
attributes, it is advisable it is as mild as possible, without
TABLE 162.1
Heat Transfer Coefficients of Heating Media (11)
Heating Medium
Free convection in gases
Forced convection in gases
Forced convection in water
Boiling water
Condensing steam
Coefficient (kcal/h m2 °K)
2.5–25
10–100
500–5000
1500–20000
5000–15000
Thermal Processing of Meats
compromising quality or sanitation. If other antimicrobial
hurdles are present, they must be taken into consideration.
There are four heating processes applied to food
materials, based on temperature increase:
162-3
water. Continuous pasteurization equipment consists in a
long tank; the product is transported through the water in
a conveyor (5).
D. STERILIZATION
A.
SCALDING
It is generally applied to tissues before freezing, drying or
canning. Conditions depend on the subsequent process. If
scalding is applied before canning, the objective is to
remove gases from tissues, to increase tissue temperature
and to provide initial cleaning (14). When applied to
meats, scalding usually results in volume reduction.
Scalding temperatures are around 65°C (15).
B.
COOKING
It is applied to improve sensory characteristics of the food
material, although it also destroys a number of microorganisms and inactivates some enzymes. In meat processing
“cooking” implies several heating methods: oven cooking, roasting, frying, boiling, steaming and grilling. How
heat is applied to meats depends on the method. Oven
cooking, roasting and grilling are carried out with dry heat
and high temperature (around 100°C), whereas boiling
and steaming are applied in water. Frying temperatures
are above 200°C (8). Cooking is also a preservation
method, if recontamination is prevented. In addition to
enzyme destruction and reduction of microbial populations, cooking also destroys toxins present in the meat or
from microbial origin and improves digestibility.
However, it also promotes adverse changes such as nutrient depletion (5).
C.
PASTEURIZATION
In most cases, the aim of pasteurization is to destroy
pathogens. Vegetative cells may survive, therefore a further
preservation method, such as refrigeration, addition of
antimicrobials, packaging or fermentation must be applied.
Pasteurization time-temperature relationship depends on
specific thermal resistance of a given strain and on food
heat sensibility (14). Pasteurization temperatures are 140
to 150°C for 1 to 45 seconds, or 70 to 73°C for 15 to 20
seconds (8). Optimization of a pasteurization process
depends on relative destruction rate of a given microorganisms without considerable altering quality factors.
Vegetative cells are destroyed at temperatures slightly
higher than their maximum growth temperature, whereas
spores can survive at much higher temperatures.
Processing conditions vary depending on the microbial
growth interval; pasteurization applies temperatures
higher than those where microbial growing can occur
(16). Meat products are generally pasteurized in water
baths. The packed product is placed in stainless steel tanks
and heated with water; cooling is carried out with cold
© 2006 by Taylor & Francis Group, LLC
A sterile product does not contain any viable microorganisms therefore the shelf life of sterilized foods is considerably extended even without the application of additional
preservation methods. Because sterilization temperatures
are above maximum to allow bacterial growth, this
process destroys vegetative cells but not spores; sterilization process calculations are based on spore survival.
However food sterilization in not practically achieved as,
strictly speaking, sterility is the destruction of all spores or
vegetative cells that can grow in normal storage conditions (5). Therefore, although pathogens are destroyed
some non-pathogens may be inactivated preventing them
to growth and reproduce. This is called commercial sterilization and depends on the type of food; storage conditions
after heat treatment; cell or spore resistance; heat transfer
characteristics of the food, container and heating medium;
and initial microbial load. Time-temperature relationship
of the sterilization process depends on the thermal resistance of a given microorganism, taken as indicator.
Clostridium botulinum and Clostridium sporgenes are
indicators for meat products (16).
Sterilized foods are packed in hermetically sealed containers in order to prevent recontamination. Under these
circumstances aerobes do not grow and spores of strict
anaerobes are less heat resistant than those of anaerobe
(17). Even though, in some foods such as cured canned
meats oxygen is not completely removed from the product. Spoilage due to anaerobe growth, such as Bacillus
subtilis and Bacillus mycoides may occur. In this situation,
in addition of moderate heat processing other preservation
methods are necessary, such as curing or smoking (18).
IV. THERMAL PROCESSING OF MEATS
Basically, meat is subjected to two types of thermal treatment: cooking and canning.
A.
COOKING
Meat cooking is carried out in forced convection ovens in
batch and continuous operations. Convection and conduction are the dominant heat transfer mechanisms (8).
Conduction is the main heat transfer mechanism within the
product, starting from the product surface inwards in a transient state as temperature changes with time in any point
within the product. Convection occurs from the heating
medium to the product surface due to mixing of the heating
medium. In free convection, fluid movement is due to density gradient resulting from temperature variation. Force
convection is promoted when the fluid is moved using any
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Handbook of Food Science, Technology, and Engineering, Volume 4
device, such as a fan (14). Convection from the heating
medium to the product surface, and conduction from the
product surface to the inner part of the food occur at the
same time, for example, meat batters, stuffed in water permeable casing, or cuts such as ham, ribs or loins in stockings cooked in a forced convection oven (19).
Process variables in forced convection are cooking
time, air velocity and relative humidity (dry bulb-wet bulb
temperature) (5). If the heat transference coefficient to the
product surface is small, convection from the heating
medium to the product surface is the limitant force
whereas if the coefficient is high the limitant is the conduction within the product (10).
Cooking can be also considered as a pasteurization
process, as it is carried out at temperatures below 100°C.
Meat in batch operations are manually placed and
removed to and from the oven. Small ovens can process
180 kg of boneless ham; large ovens up to 25,000 kg. In a
continuous operation the product is automatically transported in a conveyor through one or several cooking zones
and through a cooling area (20).
A variation in meat oven cooking is smoking. It
implies two processes taking place at the same time; with
the exception of cold smoking where the aim is to impart
flavor and to add preservative compounds such as phenols, the main process is cooking, smoking being a secondary procedure.
B. CANNING
The basic purposes in canning are:
(a) All microorganisms (cells and spores) feasible
to grow and produce toxins must be eliminated.
Canned meat, to be safe from the public health
point of view, must be free of Cl. botulinum the
most dangerous agent producing a fairly heatstable toxin (16).
(b) Spoilage microorganisms must be reduced to a
safe limit.
Thermal processing is carried out in two ways: aseptic
processing where the food is heated at conditions of commercial sterilization and placed in sterile containers which
are subsequently sealed; and canning where the food is
placed in the container, then sealed and finally sterilized.
Process conditions are the same for both cases (21).
From the commercial point of view, any canned food
is sterile if it is free of spoilage microorganism such as
Bacillus stearothermophilus or Clostridium perfringens
(commercially sterile). Spore-forming thermophiles such
as Cl. sporogenes must be considered only when storage
temperatures are high, as 40°C is their maximum growing
temperature. However, Clostridium thermosacaroliticum
spores, a spoilage bacterium, can survive at temperatures
© 2006 by Taylor & Francis Group, LLC
as high as 450°C. Heat treatment eliminating Cl. botulinum and Cl. sporogenes renders heat-stable foods with
considerably long shelf life (22). Processing conditions to
destroy vegetative cells are shown in Table 162.2.
Commercial sterilization consists in four stages: food
preparation, can filling, can closing and sealing, and thermal processing (24). At industrial level it is carried out in
batch and continuous operations, both are based on the
heat transfer principles described before. The batch or
retort method consists in loading the retort, closing and
heating with vapor. The temperature is controlled throughout the process depending on calculated processing conditions. Pressure difference is also controlled to avoid
deformation of large cans or lid blowing. The heat transfer
mechanism is convection. Heating medium in continuous
retorts is also vapor; as the cans are continuously moving
in this process the heat transference rate is higher. In some
systems cans are fed into the continuous retort through a
pressure lock, moving along the system in a U-shaped conveyor where heating and cooling are applied (5).
V. TIME-TEMPERATURE PROFILE
CALCULATION
As mentioned before, thermal process conditions are calculated on the basis of several considerations such as
composition of the food material, expected shelf life,
transportation and storage conditions, initial microbial
load and specific present microflora, among others.
As thermal processing is aimed to destruction of
microbial population responsible of spoilage or a health
hazard, process calculation must take into consideration
how the food material will be handled. Microbial associations in refrigerated meats consists of Gram negative, rod
shaped, non-fermentative psychrotrophs of genus
Pseudomonas, Alcaligens, Flavobacterium, Shewella and
Moraxella (23). This association changes during curing,
becoming dominant Gram positive microorganisms of
genus Micrococcus, Lactobacillus, Carnobacterium and
Brochothrix (25). Table 162.3 shows the growth interval
of several microorganisms associated with meat spoilage.
TABLE 162.2
Processing Conditions to Destroy Vegetative Cells (8,
10, 15, 16)
Microorganism
Z-Value (°F)
B. stearothermophilus
B subtilis
B. cereus
B. megaterium
Clostridium sporogens
Cl. botulinum
Cl. thermosaccharolyticum
12.6
13.3 to 23.4
17.5
15.8
23.4
17.8
16–22
D250 Value (min)
4.0
0.48 to 0.76
0.0065
0.04
0.15
0.21
3.0 to 4.0
Thermal Processing of Meats
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TABLE 162.3
Microorganisms Associated to Meat Spoilage (26)
TABLE 162.4
Pathogens Associated to Meats (8, 10, 14, 21)
Microorganism
Microorganism
Lethality (min)
Clostridium botulinum
Vibrio sp.
Aeromonas hydrophila
Listeria monocytogens
Salmonella sp.
E.coli 0157:H7
Staphylococcus aureus
D65°C ⫽ 0.1
D70°C ⫽ 0.3
D55°C ⫽ 0.17
D60°C ⫽ 1.9
D60°C ⫽ 0.2
D60°C ⫽ 4
D60°C ⫽ 0.4
Growth Interval
Psychorphiles
Pseudomonas sp.
Achromobacter
Mesophiles
E. coli
Bacillus subtilis
Facultative thermophiles
Streptococcus thermophilus
Clostridium perfringens
Thermophiles
Clostridum thermosaccharolyticum
Bacillus stearothermophilus
⫺5 to 35°C
15 to 45°C
24 to 54°C
45 to 75°C
Several inactivation parameters have been developed
as mathematical tools to obtain a time-temperature relationship necessary to achieve a successful treatment.
(a) D- and z-values. If a microbial population is subjected to temperatures slightly above those for its
maximum growth temperature, vegetative cells
or spores are destroyed due to the inactivation of
enzymes present in the microorganisms. The
destruction follows the exponential equation:
⫺(dc/dt) ⫽ kc
That is, cell concentration decreases (dc) with
time (dt) in a direct proportion of cell concentration (c). In other words, 90% of the microorganisms are destroyed in a given time interval
if constant temperature is applied. The time
interval is different for each microorganism,
and is called decimal reduction time (D). It represents the minutes necessary to destroy 90%
of a given microbial population at constant
temperature. Table 162.4 shows D values for
some pathogens possibly associated to meats.
Therefore it is possible to compare thermal
destruction of different microbial populations.
D values are expressed at a given temperature
(D120°C). For example when heating at 110°C,
90% of the population of Cl. sporogenes (i.e.
from 105 to 104) is reduced if heating is maintained for 10 min (D110°C ⫽ 10 min). If the same
population is heated at 115°C, the time necessary to reduce the population one logarithmic
cycle at 115°C is 3 minutes (D115°C ⫽ 3 min),
and at 120°C it requires only 1 min (D120°C ⫽
1 min) (20).
Heat resistance for a given microorganism is
given by z-values, indicating the temperature
required decreasing D-values in 1/10.
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(b) F-values. Calculation, evaluation and comparison of different heat treatments are achieved by
the so-called F-value. This value represents the
extent of thermal death of microorganisms and
severity of the treatment in order to predict the
product’s shelf life. The practical importance of
F-values is that the individual effect of each
part of the process is additive. As it is impossible to rise the temperature in the container to
120°C in every point at the same time F ⫽ 1
concept is applied. It is the lethality effect of
heating at 120°C for 1 min. F-values increase,
depending on the severity of heat treatment
required for given meat. Fs is the sum of all
F-values in every parts of the container.
According to the F-value concept, each temperature
above 100°C has a given lethal effect; it increases with
temperature’s increment. For instance, heating must be
applied during certain time and temperature in order to
have similar heat damage: 100 min at 101°C, 10 min at
110°C, 1 min at 120°C or 0.1 min at 130°C (8). Thermal
treatments therefore depend on a time-temperature relationship. Increasing the temperature for 10°C, the time
necessary to achieve the same thermal effect is 1/10. Heat
treatments are also calculated taking into consideration the
survival of spores from two of the most damaging bacteria
in meat products: Cl. botulinum and Cl. sporogenes.
However, as heating is not homogenous in the entire can
geometry, calculations are always done considering the temperature rise at the cold point (where heating is the slowest).
In this point, the sum of all lethal effects is Fc. The position
of the cold point is determined by the type of food material,
therefore by its main heat transfer mechanisms, and to a certain extent by the agitation of the cans in the retort.
In conduction heating, the cold point is located in the
geometrical center of the container. For viscous meat,
with cans rotating during the heating cycle, the cold point
is close to the geometric center. Rotation in this case does
not substantially increase the heating rate.
In static heating of liquid or semisolid products, such
as meat pieces in brine, where the leading heat transference mechanism is convection, the cold point is on the
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Handbook of Food Science, Technology, and Engineering, Volume 4
vertical axis, one-third from the can bottom end (10). Fc is
always lower than Fs due to the fact that heat effect in the
center is always lower than in the rest of the container.
When a thermal process is calculated for the first time, the
cold point is located experimentally, using thermocouples
(Figure 162.1).
A simple method to calculate the lethal effects during
the heating and cooling phases consists of measuring with
thermocouples the temperature at the cold point, and to
calculate the corresponding F-values.
The relationship between D and F, taking into account
the amount of cells before and after heat processing is:
F ⫽ D (log a ⫺ log b)
where a = initial cell load; b = final cell load.
It is assumed that low acid foods, as are most meat
products, are heated at a temperature that assures total
absence Cl. botulinum spores, and are microbial safe. In
this case, spore counts must be reduced from 1012 to 100
(24), that is, reducing the count 12 log cycles or 12D. This
means that heating must be enough to find only 1 Cl. botulinum spore in 1012 cans, i.e. one spore per gram of meat or
1/1012. Cl. botulinum types A and B are reference microorganisms for D values at 120°C and 0.21 min., as follows:
F ⫽ 0.21 (log 1 ⫺ log 10⫺12)
F ⫽ 2.52
For Cl. botulinum, D121°C ⫽ 0.21 min and z ⫽ 10°C. In
order to reduce an assumed number of Cl. botulinum cells
12 log cycles, heat must be 12 times higher during 0.21
min, that is 2.52 min at 120°C. Heat processing of food
around F = 2.5 is called “botulinum cook.”
Lethality is calculated by the equation:
(log t ⫺ log F) / (log 10) ⫽ (250 ⫺ T) / Z
where log 10 ⫽ 1; therefore log (t/F) ⫽ (250 ⫺ T) / Z.
The destruction rate per minute of a given microorganism
at a temperature T in the process corresponds to the time,
t, needed for the destruction of microorganism at that tem-
VI.
EFFECT ON MEAT QUALITY
Quality improvement or deterioration of heat-treated
foods depends on three factors:
●
●
●
Type and amount of microorganisms in the
food. Insufficient heat treatment may results in
microbial survival and presence of metabolites
such as gas, acid or off-odors and flavors.
Inadequate cooling after heat treatment in
processes such as canning encourages thermophile growth (26). Recontamination can also
occur from microorganisms in cooling water
lacking of suitable sanitary conditions.
Chemical reactions of food components with
one another or with the packaging material (27)
can result in can or thermoformed package
blowing.
Physical alterations due to inadequate equipment or process operation, such as rapid
increase in retort pressure, insufficient vacuum
in packages or excessive package or can filling
of the processing equipment.
Heat treatment severe enough to destroy Cl. botulinum or Cl. prefringens ensures the production of a stable
food without the need of applying further special storage
conditions. However, as severe heating can alter sensory
characteristics, it is necessary to achieve a compromise
between preservation and alteration of sensory attributes.
Heat treatment can also improve sensory characteristics of
meat, such as texture due to alteration of the muscle
fibers, and flavor due to generation of aroma-related
compounds (23).
VII. EFFECT OF MEAT PHYSICOCHEMICAL
CHARACTERISTICS ON
MICROBIAL GROWTH
Thermocouple
Thermocouple
FIGURE 162.1 Location of cold point in conduction (left) and
convection (right) mechanisms.
© 2006 by Taylor & Francis Group, LLC
perature. For every minute, lethality can be calculated at a
given temperature, obtaining a curve. The area under the
curve represents the total lethality of the process.
Another way to calculate the lethality necessary in a
given process is by adding all F-values during the heating
and cooling phases; this gives Ftotal, the sum of all F
values.
Thermal properties in foods are altered due to changes in
chemical components, such as protein denaturation, fat
melting and water evaporation. Thermal conductivity in a
meat batter is related to water content and increases with
temperature and humidity. Carbohydrate, protein and fat
protect microorganisms against thermal destruction due to
their low heat transfer coefficients (28).
Thermal Processing of Meats
162-7
On the other hand, microbial resistance to destruction
can be increased by physiochemical characteristics of the
food material such as water, fat, carbohydrate, and protein
and salt content; pH; and presence of inhibitory compounds. Quality and sanitary risks depend on microbial
access to the food, a function of microbial characteristics
of the contaminant strain; food intrinsic antimicrobial
attributes; storage extrinsic conditions in particular
temperature, oxygen availability and time; and microbial interactions (29). Physicochemical attributes that
must be also considered when thermal processing is
applied are:
of oxygen are suitable to growth and produce toxins. Its
spores are heat resistant; that destruction is the calculation criteria for thermal processing of low-acidity foods,
such as meats. A and B are the most heat-resistant toxins
but are destroyed heating at 100°C for 10 min.
Preventing conditions leading to toxin and/or spore presence in the food is the process calculation criteria (33).
In low acidic foods such as most meat products, processing is based on inactivation of B. stearothermophilus, spores, 20 times more resistant than Cl.
botulinum spores and responsible, when germinating, of
producing sour taste and gas.
A.
D. TEMPERATURE
WATER ACTIVITY AND REDOX POTENTIAL
It is directly related to microbial growth. Dry meat products do not require further heat processing. Limiting aw
for Cl. botulinum is 0.97 for psychrothrophic species and
0.95 for mesophiles (30). There is also a correlation
between aw and redox potential. Raw meat has a redox
potential around –50 mV; it changes after heating, for
example, sausages have potential from +20 to –100 mV,
depending on the degree of grinding and ingredients
added. Vacuum and addition of reducing agents can further decrease the redox potential (21).
B.
OXYGEN TENSION
It is particularly important for strict anaerobes such as
Clostridium sp. or microaerobes as lactic acid bacteria.
Canning cured meat, where some oxygen remain in the
product, implies applying mild heat treatment conditions
although other preservation methods such as curing or
refrigeration are used. In this situation spoilage may
occur due to the presence of aerobes such as Bacillus
subtilis or Bacillus mycoides. Carbon dioxide addition
displaces oxygen; together with low temperature storage
carbon dioxide reduces Gram negative bacterial growth
(32).
C.
PH
Meat and meat products, with the exception of fermented
meats, are a low acid food (⬎4.5). In this case, heat treatment must be more severe as potential pathogens or
spoilage microorganism can grow in this environment. In
most cases Cl. botulinum grows and produces toxins at pH
close to neutrality (8,16).
It is a determinant factor for several inactivation
processes; generally highly acidic (pH , 3.7) or acid (pH
3.7 to 4.5) foods do not represent a sanitary risk. Growth
of Cl. botulinum does not occur at pH # 4.5. Only foods
with moderate acidity (pH . 4.5) may present a risk due
to Cl. botulinum growth; at this pH level it produces toxins and heat-resistant spores. This anaerobe can be
present in canned foods, where low acidity and absence
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Meat storage of meats at 0–4°C promotes the growth of
Pseudomonas and Acinetobacter spp.; at 20–25°C
Micrococcus grows in addition to the previous two
microorganisms; at 25–40°C there is a dominance of
Enterobacter, Clostridium and Bacillus spp.; above 40°C
Clostridium y Bacillus spp. proliferate (17).
E.
ANTIMICROBIAL AGENTS
Such as added benzoate and sorbate, or chemical components of the food (organic acids, lysoszyme, medium
length chain fatty acids) (34,35,36).
F. PHYSICAL STRUCTURE
Physical structures are at the same time a barrier and a
way to migrate to the internal part of a food. Microbial
growth starts initially in the food surface; this is particularly true with meats, where microbial contamination
starts in the carcass or cut surface, finding its way to the
inner part of the muscle through the perimysium.
However, when meat is cut or minced, microbial population is rapidly distributed throughout the food (32).
G.
FOOD COMPOSITION
As foods in general, and particularly meats, have more
than one constituent (oil/water emulsions, protein gels,
collagen casings, connective tissue, muscle fibers) heterogeneity must be considered (37,38). It promotes different habitats due to a variety of physical structures and
chemical compositions that influence growth and colonization of specific microbial populations. Meat smoking
establishes a concentration of phenol and other compounds, at the same time a humidity gradient is established from the sausage center outwards promoting
different microbial ecologies throughout the product.
Heat processing must be calculated to reduce a given
microbial type and counts in the food section where
microbes are most protected (16).
162-8
VIII.
Handbook of Food Science, Technology, and Engineering, Volume 4
MICROWAVE HEATING
It is based in alignment of water dipoles when exposed to
an electric field. As the microwaves change the direction
of the electric field at rate around 5 ⫻ 10 9 per second,
water dipole alignment also changes causing friction and
producing thermal energy. The same direct heating,
microbial inactivation is the result of enzyme destruction.
However, the microwave may cause overheating of some
areas due to excessive energy absorption as a result of
food heterogeneity (8). Microwaves frequency is between
300 Mhz and 300 Ghz, wavelengths between 1 mm and
1 m, although domestic and industrial microwaves are
between 915 Mhz and 2450 Mhz (11). At present,
microwave heating is applied to meat and meat products
only for cooking purposes.
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