The Institute of Food Science and Technology (IFST) defines
the shelf life of a food as “The period of time under defined conditions of
storage, after manufacture or packing, for which a food product will remain
safe and be fit for use.”
Mechanisms that limit shelf life include:
Chemical or biochemical
changes
(e.g. browning,
rancidity)
Microbiological growth
and metabolism
Moisture migration into,
or out of, the
product
Gas transfer (e.g.
ingress of oxygen)
Transfer of odours or
flavours (e.g.
tainting or flavour loss)
Changes caused by
exposure to light
(e.g. loss of colour)
Physical damage to
packaging
Factors that limit the mechanisms that limit the shelf
life:
Intrinsic factors:
The composition and formulation
of the product
Product structure
Moisture content and
water activity
pH and acidity
Level of oxygen and redox
potential
Extrinsic factors:
Storage temperature
Relative humidity
Exposure to light
Gaseous atmosphere
Processing
Hygiene
Packaging
Benefits of extending the shelf life
Product can remain on sale on the shelf for longer
Consumers favour products that ‘keep well’
Fewer consumer complaints
More efficient production planning
Improved stock rotation
Reduced wastage and product returns from retailers
More extensive product distribution is possible
Highly seasonal products
can be stockpiled
Most retailers require
food deliveries to have at least 75% of shelf life remaining
I. Conventional Technologies Extension of Shelf Life
1. Hurdle Technology
- relies on the fact that preservation factors, such as
heating, pH, water activity, redox potential, atmosphere, and chemical
preservatives often have a synergistic effect in combination, and their
effectiveness is therefore greater than would be expected by simply adding
their respective effects together.
* The concept is applied mainly to microbiological spoilage
of relatively short shelf life foods, and the idea is that the spoilage
microbes may be able to overcome one or more factors (hurdles), but will not be
able to ‘jump’ over all the hurdles present. For example, they may survive
pasteurisation, and be able to grow at low moisture levels, but a small
reduction in pH, or the addition of a preservative, may then be sufficient to
inhibit their growth.
Examples of food products that apply hurdle technology to
extend shelf life:
• Cooked cured meat products
• Chilled fruit juices
• Reduced sugar jams and spreads
• Reduced fat spreads
• Mild flavour pickles and sauces
2. Heat Preservation
ultra high temperature
(UHT) (130-145 °C),
high temperature short time (HTST)
Ohmic heating-
generates heat by passing an alternating electric current through a food that
has electrical resistance. The heating rate is
determined by the voltage applied and by the electrical conductivity of the
product.
Microwave heating-
involves the conversion of electrical energy into heat by making water
molecules in the food oscillate rapidly in an electric field that changes
direction. The molecules alternately absorb energy and then release it into the
food. The result is a rapid heating process that can penetrate food quite
effectively.
Infrared (IR) heating-
Radiation in the infrared part of the spectrum can be used to heat the surface
layers of foods very rapidly and efficiently, and is used in roasting, baking,
or grilling processes. For example the baking of biscuits can be done more
rapidly using an IR system.
Radio frequency (RF)
heating uses electromagnetic radiation at wavelengths longer than those of
microwaves and heats mainly by dielectric heating at lower temperatures, but
increasingly by electrical conductivity heating as the temperature rises. RF
heating is rapid and even and has been investigated for a number of
applications, including blanching of vegetables, thawing and pasteurisation of
meat, drying applications and post-baking of snack foods.
3. Cooling
* chilling and freezing
Deep chilling’
involves cooling a food to a temperature just above its freezing point.
conventional freezing
(-20 to -30 °C)
Super freezing refers
to storage temperatures in the range -40 to -60 °C
Dehydrofreezing-
combines drying and freezing in a single process to produce improvements in
colour flavour and texture for some foods at low cost. The technique involves
partially dehydrating the product to a moisture content low enough (around 30%
for vegetables) so that it does not freeze when cooled to -20 °C. The method is
used mainly for vegetables and fruits, and is said to give a higher quality
product than conventional freezing. Rehydration times are also substantially
less than for conventional dried products. Energy and transport cost benefits
can also be achieved by dehydrofreezing, since the products weight is reduced,
as is its volume. Dehydrofreezing differs from freeze-drying in that the
product is not frozen under vacuum.
4. Drying
Microwave drying –
combined with other drying technology(forced air convection or vacuum drying,)
Heat pump drying- Heat
pump dryers operate by cooling warm wet
air drawn from the dryer over an evaporator coil, which cools the air to below
its dew point. Water condenses on the coil and runs away. The dry cool air is
then passed through a condenser, where it is re-heated before being passed
through the dryer where it picks up more moisture from the product.
Conventional heat exchangers can be combined with heat pumps to improve the
efficiency considerably.
II. New Processing Technologies
1. High Pressure
processing (HPP) is also known as
high-hydrostatic pressure (HHP) or ultra-high-pressure (UHP) processing.
Pressures of around 300 to 700 MPa (3,000-7,000 timesthe pressure of the
atmosphere) are applied to a food product for a short time, to achievewhat has
been described as ‘cold pasteurisation’. The process offers a number
ofadvantages over conventional thermal food processes:
o
Inactivation of
vegetative microorganisms can be achieved without detrimental effects on
flavour, texture, colour and nutrient contents.
o
Pressure is transmitted
uniformly through a food product, which means that, in the case of a ham for
example, treatment is as effective at the centre of the mass of meat as it is
near the surface.
o
If the applied pressure
is sufficiently high, all vegetative microbial cells and spores may potentially
be inactivated.
o
HPP may be used in
combination with other techniques, such as heat, irradiation and manipulation
of pH, to provide enhanced inactivation.
2. Irradiation
Ionizing radiation transfers energy to molecules, promoting
the formation of ions or free radicals and causing a small percentage of
chemical bonds to break. In microorganisms, Irradiation disrupts DNA and so
causes the destruction of microbial
cells.
3. Natural Food
Preservatives
Natural preservatives may act to enhance shelf
life by different routes:
•Antimicrobial activity – inhibition of
spoilage or pathogenic bacteria
•Antifungal activity – inhibition of yeasts or
moulds
•Antioxidant activity – inhibition or
retardation of lipid oxidation.
Lysozyme (E1105), which is derived from eggwhites, is the
most commercially important antimicrobial enzyme. At present, it is the only
enzyme permitted as a preservative. Lysozyme is used against lactate
fermentingClostridium species in milk, and in ripened cheese to prevent ‘late
blowing’. Essential oils, obtained from plants by steam distillation, pressing
or extraction, in addition to their characteristic aroma and flavour, usually
contain phenolic compounds with a certain level of antimicrobial activity
4. Other Developing
Technologies
Pulsed electric field
Pulsed white light
Ultra-violet light, pulsed ultra-violet light
Ultrasound
Combinations of preservation technologies with potential
III. Packaging
MAP-
modified atmosphere packaging
Active Packaging- that interacts with the internal environment of the pack.
Antioxidant release films –
Films containing antioxidants such as tocopherol, BHA, or
BHT can be used to inhibit oxidation of oils and fats in dried and high fat
foods.
Antimicrobial release films –
Films impregnated with a range of antimicrobial compounds
(e.g. organic acids, spice extracts, lysozyme, and other enzymes) have been
developed to inhibit the growth of spoilage and harmful bacteria on the surface
of meat, poultry, fish, bakery products, cheese, and fresh produce.
Oxygen scavengers –
Used either in sachet form, incorporated into a label, or
into the packaging film itself, to absorb oxygen in the pack. Applications
include inhibition of mould growth on cheese or bakery products, and delaying
oxidation of
oils and fats.
Ethylene scavengers –
Also used in sachets or packaging films to absorb ethylene
gas produced by some ripening fruits and vegetables. Can be used to delay
ripening and softening of produce such as bananas, avocados and potatoes.
Carbon dioxide emitters –
Carbon dioxide diffuses through plastic packaging films more
readily than other gases. Sachets containing CO2 emitters can be used to
maintain the original level. Useful for preventing microbial spoilage in meat,
poultry, fish, cheese and some fruits.
Ethanol emitters –
Sachets that emit ethanol into the pack headspace have been
developed in Japan. They can be used to increase the mould-free shelf life of
bakery products
Flavouring emitters –
Flavour compounds can be incorporated into polymers to
produce packaging materials that minimise flavour loss and mask taints and off
odours in a wide range of products.
Temperature compensating films –
Temperature compensating films, such as ‘Intelimer’ film,
have a chemical structure that changes abruptly and reversibly at a specific
temperature. This ‘switch temperature’ can be anywhere between 0 °C and 45 °C.
Below this temperature, the film is an effective gas barrier, but it becomes
much more permeable above the switch temperature. This can be used to
compensate for the increase in the respiration rate of fresh-cut produce at
higher temperatures.
IV. Decontamination Techniques
Chemical Techniques
Water
Ozone- Ozone is a gaseous form of oxygen which,
when dissolved in water, has been shown to be a powerful
disinfectant. Ozone has a short half-life and readily breaks down to oxygen so
food treated with ozone will have no residues left over from processing. Ozone
can also be used to decontaminate bottled water and to treat water used to
decontaminate meat and fresh produce.
Applications:
Bottled drinking water production, decontamination of animal
carcasses, in wash waters for fruits, salads and vegetables, to extend shelf
life during storage of various products including fruit and vegetables. Ozone
has recently been suggested for use to decontaminate ready-to-eat meat and
poultry products.
Chlorine- Chlorine compounds are usually used at levels of 50–200 ppm
free chlorine and with typical contact times of 1–2 minutes.
Dimethyl dichloride
(DMDC) is an effective sterilising chemical
treatment used to inactivate microorganisms, including moulds and yeasts, in a
wide variety of beverages.
Hydrogen peroxide- maximum concentration for meat and poultry carcasses is 100
ppm.
Organic acids Research has found that organic acids may also be of use in
the decontamination of fresh produce, including herbs. Citric, acetic, lactic,
tartaric acid, and other organic acids, alone, or in combination with each
other, as well as other cleaning agents such as chlorine and surfactants, are
all possible decontamination agents.
Peracetic
(peroxyacetic) acid- The maximum
concentration for use on meat and poultry carcasses in 220 ppm.
Other chemicals that may have a role as decontaminants
Chlorine dioxide
Nisin is an antimicrobial that is produced by bacteria found in
milk, and is perceived as a ‘natural’ food preservative. The addition of nisin to products can enhance pasteurization
treatments allowing less product-damaging heat regimes to be used.
V. Thermal Techniques
Hot water
Steam- Commercial processes for meat carcasses
using steam pasteurization reduce bacterial
counts by applying pressurised steam for
around 6 seconds to the surface of carcasses
after a washing step.
Steam vacuum
The Food Production Environment: Impact on Shelf Life
Sourcing of
Ingredients
Storage of
Ingredients- Temperature, humidity
and light can all affectthe quality, and perhaps the safety, of a rawingredient.
Processing Areas
Processing
equipment
Ventilation
Clean Room
Technology- Clean room technology
supplies clean air drawn through high efficiency filters (HEPA
filters) that can almost completely remove microorganisms
and other particles to a physically separated area within the plant (the clean
room). The flow of air is also controlled
within the clean room, which is usually kept under positive
pressure so that unfiltered air is unlikely to be drawn in through entrances
and exits. The staff working in the area must be well trained and should be
dressed in appropriate protective clothing. Cleaning procedures for the
surfaces and equipment should be validated and implemented. The movement of
staff and materials in and out of the room should be strictly controlled to
prevent cross contamination. In this way, an environment that is almost
microbiologically
sterile can be maintained.
Cleaning Technology
Traditional cleaning
methods- Traditional cleaning using simple
physical methods (e.g. scrubbing), chemical detergents or disinfectants and/or
heat can be very effective for reducing soil and microbial loads in a food
processing environment. Even quite complex processing equipment may sometimes
need to be stripped down, inspected, and cleaned by hand if necessary.
Clean in place (CIP)
systems- CIP lends itself to liquid or
semi-liquid processing operations where the equipment system used is in almost
continuous use. CIP systems are usually a part of the initial design of
equipment and they are built in to it as an integral component. CIP relies upon
the use of a series of detergents, rinses, sanitizers, etc., to flush and wash
an enclosed processing plant (e.g. in a dairy) without the need to totally
dismantle the system. The CIP system is fine- tuned so that surfaces are in
contact with santiser/rinse solutions at the correct concentration, for the
appropriate time and at the correct temperature. CIP systems can be fully
automated or manually operated.
Novel cleaning methods- A number of novel alternative cleaning and sanitizing
techniques and materials have been investigated.( Ozone). Research has indicated that ozone is effective at killing
microorganisms attached to surfaces as
well as within an aerosol. Ozone treated water can be used to reduce microbial
loads in clean-in-place systems and as an effective treatment for surfaces
within the food processing environment, storage areas and transport vehicles.
Ultra-violet light
Solid carbon dioxide
(CO2)
Hygiene Monitoring - How efficient is cleaning?
Visual Inspection
Traditional Swab/Plate Methods
Rapid Hygiene Monitoring
ATP kits
Colour hygiene tests
