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 shelflife based on the kinetics of deterioration. As most of the
deterioration mechanisms in frozen foods follow either zeroorder 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 laboratorybased procedures have Shelf-life
been Estimation
introduced.
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A. TIME TEMPERATURE TOLERANCE (TTT)
The time–temperature–tolerance (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 time–
temperature–tolerance 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 (P< 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 tθ is the storage time at a temperature θ and HQLθ ,
the high-quality life at the same temperature. The values of
HQLθ 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 θ. In fact D can be calculated as
where HQLref is the high-quality life at a reference
temperature θref . 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 θ+10oC to that at a
temperature of θ. 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 shelflife 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.
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