FST 151

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FST 151 FOOD FREEZING FOOD SCIENCE AND TECHNOLOGY
151 Shelf-life Prediction of Frozen Foods Case
Studies Lecture Notes Prof. Vinod K.
Jindal (Formerly Professor, Asian Institute of
Technology) Visiting Professor Chemical
Engineering Department Mahidol University Salaya,
Nakornpathom Thailand
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We freeze foods to extend their storage life by
making them more inert. A range of physical and
biochemical reactions continues however and many
of these will be influenced when storage
conditions are altered. To a large extent we are
unconcerned with the microbiology of frozen foods
since no microorganisms grow below -10oC. The
production of safe frozen foods requires the same
attention to good manufacturing practice (GMP)
and HACCP principles as in the case of fresh
foods.
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Shelf-Life Prediction of Frozen Foods Fresh or
chilled foods normally have a single dominant
deterioration mechanism (e.g., microbial
spoilage). It is relatively easy to model the
effect of temperature on the microbial growth.
These models can be used to calculate when the
microbial load will exceed a safe limit and thus
to determine the safe shelf-life.
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The prediction of the shelf-life of frozen foods
is difficult because of many spoilage mechanisms
present in them. These include enzymatic
deterioration, cell damage and protein and starch
interactions, non-enzymatic browning, water
migration (both during freezing and storage),
water re-crystallization and change in
crystalline form, solute crystallization,
oxidative deterioration (e.g., lipid oxidation in
fatty meats and color changes in fish and meat),
protein denaturation (which may alter
water-binding capacity), and lastly, microbial
changes.
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Normal frozen food storage temperatures (-18 to
-22oC) are significantly higher than the glass
transition temperature and will consequently
contain some unfrozen water. FROZEN FOODS WHY
IT IS DIFFICULT TO PREDICT SHELF-LIFE A. UNFROZEN
WATER AND GLASS TRANSITION B. DETERIORATION
MECHANISMS
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APPROACHES TO SHELF-LIFE DETERMINATION Most food
engineers and technologists like to model
shelf-life based on the kinetics of
deterioration. As most of the deterioration
mechanisms in frozen foods follow either
zero-order or first-order kinetics, the modeling
of shelf-life should be a simple exercise.
However, the kinetic data for so many
deterioration mechanisms are not easily available
for the frozen food storage conditions.
Additionally, many foods may undergo more than
one deterioration reaction and the combined
effects of these would need to be assessed.
Therefore, many laboratory-based procedures have
been introduced.
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A. TIME TEMPERATURE TOLERANCE (TTT) The
timetemperaturetolerance (TTT) experiments were
introduced by the USDA laboratories in the
1960s. The assumption made for TTT experiments
is that for every food there is a relationship
between the storage temperature and the time
taken to undergo a certain amount of quality
deterioration. Such changes during storage at
different temperatures are cumulative and
irreversible. It is generally agreed that the
most detrimental factor influencing frozen food
quality is fluctuation in storage temperature and
this will significantly reduce the shelf-life of
the product.
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B. PRACTICAL STORAGE LIFE A more commonly used
descriptor was later introduced named the
practical storage life (PSL). This is defined as
the period of storage during which the frozen
food retains its quality characteristics and is
suitable for consumption. Though both the effect
of temperature and food type are included for a
number of food products, fluctuating storage
temperatures can cause problems.
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C. HIGH-QUALITY LIFE (HQL) This is the most
common shelf-life determinant parameter used in
the food industry. In reality, this is a
timetemperaturetolerance variable but differs
from the others in that sensory quality is used
in its determination. It is normally defined as
the time elapsed between freezing and the time
when a statistically significant difference (Plt
0.1) can be detected by sensory evaluation. A
simpler exercise may be the determination of the
elapsed time at which 70 of a trained taste
panel can identify a noticeable difference
between the frozen food in question and a control
when using a triangular test. The control would
normally have been stored at -35oC.
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When different storage conditions are used during
the life of the product, the HQL needs to be
integrated over the different temperatures. For
acceptable quality, it is essential that
where t? is the storage time at a temperature ?
and HQL? , the high-quality life at the same
temperature. The values of HQL? can be read from
the chart or, alternatively, the experimental
curves from which the chart was derived can be
expressed in the form
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Where D is analogous to the decimal reduction
time in bacterial killing. It is found from two
points on the semi-log plot of HQL versus ?. In
fact D can be calculated as
where HQLref is the high-quality life at a
reference temperature ?ref . A typical plot from
which D is derived is shown in Figure 28.1.
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FIGURE 28.1 Plot of shelf-life versus temperature
for a typical food.
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D. ACCELERATED MEASUREMENT AND THE Q10
APPROACH The above type of plot can also be
used for the so-called Q10 approach. This
estimates the effect of temperature on the
accelerated deterioration of shelf-life. In its
simplest form, it can be expressed as the ratio
of the rate of deterioration at a temperature of
?10oC to that at a temperature of ?.
Alternatively, it can be expressed as
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An advantage of Q10 approach is the ability to
conduct accelerated experimental shelf-life
trials at elevated temperatures and then
extrapolate the results to normal storage
conditions. Such tests are widely used in the
food industry. However, exact values of Q10 are
difficult to find for many foods and approximate
values are frequently used.
IV. METHODS USED FOR SPECIFIC FOODS Despite the
significant research efforts applied to
shelf-life determination of frozen foods, there
is no single, universally accepted method
available for application to the food industry.
Th. e available data are scarce. The rate
constants for the common deterioration reactions
are not available for a wide range of frozen
foods.