Shelf Life

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