Shelf life
From Wikipedia, the free encyclopedia
Shelf life is that length of time that food, drink, medicine and other perishable items are given before they are considered unsuitable for sale or consumption. In some regions, a best before, use by or freshness date is required on packaged perishable foods.
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[edit] Shelf life
Shelf life is different from expiration date; the former relates to food quality, the latter to food safety. A food that has passed its shelf life is still safe, but optimal quality is no longer guaranteed. In most food stores, shelf life is maximised by using stock rotation, which involves moving products with the earliest sell by date to the front of the shelf, meaning that most shoppers will pick them up first and so getting them out of the store. This is important, as some stores can be fined for selling out of date products, and most if not all will have to mark such products down as wasted, leading to a loss of profit.
Shelf life is most influenced by five primary events: light transmission, gas transmission, heat transmission, humidity transmission, or mechanical stresses. Product quality is often mathematically modelled around a single parameter: the concentration of a chemical compound, a microbiological index, or a physical parameter.
Under some circumstances, the shelf life is critical to health. Some medicines begin to deteriorate (e.g. in potency) or begin to accumulate toxic breakdown products immediately after manufacture or packaging. Depending on the material involved, this can be dangerous to life. Bacterial contaminants are ubiquitous, and foods left unused too long will often acquire substantial amounts of bacterial colonies and become dangerous to eat. Food poisoning is the result, and can be fatal.
Some companies use induction sealing to assist in the extension of the shelf life of their products.
[edit] Temperature control
Nearly all chemical reactions will occur (at various rates depending on the individual nature of the reaction) at common temperatures. Examples are the breakdown of many chemical explosives into more unstable compounds. Nitroglycerine is notorious. Old explosives are thus more dangerous (i.e., liable to explode without warning) than recently manufactured explosives. Rubber products also degrade as sulphur bonds induced during vulcanization revert; this is why old rubber bands and other rubber products soften and get sticky as they age.
These breakdown processes characteristically happen more quickly at higher temperatures. The usually quoted rule of thumb is that chemical reactions double their rate for every 10 degree Celsius increase in temperature (because of activation energy barriers). However, as with all rules of thumb, there are many caveats and assumptions. The rule of thumb is only true for reactions with activation energy values around 50kJ/mol. This observation, taken from a limited interpretation of the Arrhenius equation, has been grossly misused in food shelf-life testing, and has led to an incorrect belief that "triple time" can be simulated by increasing the temperature by 15 degrees Celsius. e.g. Storing a product for one month at 35 degrees Celsius simulates three months at 20 degrees Celsius.
The same is true, to a point, of the chemical reactions of life. In the particular case of bacteria and fungi, the reactions needed to feed and reproduce increase at higher temperatures, up to the point that the proteins and other compounds in their cells themselves begin to breakdown so quickly that they cannot be replaced. It is the reason high temperatures kill bacteria and other micro organisms; 'tissue' breakdown reactions reach such rates that they cannot be compensated for and the cell dies. On the other hand, 'elevated' temperatures short of these result in increased growth and reproduction; if the organism is harmful, perhaps to dangerous levels.
Just as temperature increase speeds up reactions, temperature decreases reduce them. Therefore, to make explosives stable longer, or keep rubber bands springy, or force bacteria to slow down their growth, they can be cooled. This is the reason shelf life is generally extended by refrigeration and the reason some medicines must be refrigerated; the breakdown reaction paths at room temperature are so rapid the medicine becomes unusable very quickly. Only refrigeration keeps them potent long enough to be practical.
[edit] Best before
Best before is sometimes indicated on food and drink wrappers, followed by a date, and is intended to indicate the date before which the supplier intended the food should be consumed. The term best before is similarly used to indicate the date by which the item will have outlived its shelf life, and is intended to ensure that customers will not unwittingly purchase or eat stale food. Sometimes the packaging process involves using pre-printed labels, making it impractical to write the best before date in a clearly visible location. In this case, a term like best before see bottom or best before see lid might be printed on the label and the date marked in a different location as indicated.
[edit] Use by
Generally, foods that have a use by date written on the packaging must not be eaten after it has expired. This is because such foods usually go bad quickly and may be injurious to health if spoiled. It is also important to follow storage instructions carefully for these foods (for example, product must be refrigerated).
Foods that have a best before date are usually safe to eat after the date has passed, although they are likely to have deteriorated either in flavour, texture, appearance or nutrition.
[edit] Sell by / Display until
These dates are intended to help keep track of the stock in stores. Food that has passed its sell by or display until date, but is still within its use by / best before will still be edible, assuming it has been stored correctly. It is common practice in large stores to throw away such food, as it makes the stock control process easier. It also reduces the risk of customers buying food without looking at the date, only to find out the next day that they cannot use it.
Most stores will rotate stock by moving the products with the earliest dates to the front of shelving units, which allows them to be sold first and saving them from having to be either marked down or thrown away, both of which contribute to a loss of profit.
[edit] Mark-downs
It is also common for food approaching the use by date to be marked down for quick sale, with greater reductions the closer to the use by date it gets.
[edit] Example
[edit] Beer freshness date
A freshness date is the date used in the American brewing industry to indicate either the date the beer was bottled or the date before which the beer should be consumed.
Beer is perishable. It can be affected by light, air, or the action of bacteria. Although beer is not legally mandated in the USA to have a shelf life, freshness dates serve much the same purpose and are a marketing tool.
[edit] Beginnings of freshness dating
The Boston Beer Company, maker of Samuel Adams, was among the first to start adding freshness dates to their product line in 1985. For ten years there was a slow growth in brewers adding freshness dates to their beer. The practice rapidly grew in popularity after the Anheuser-Busch company's heavily marketed "Born-On dates" starting in 1996. Many other brewers have started adding freshness dates to their products, but there is no standard for what the date means. For some companies, the date on the bottle or can will be the date that the beer was bottled; others have the date by which the beer should be consumed.
[edit] Beer processing
Before a beer is bottled, it is processed to prolong its shelf life; this evidently affects the beer's freshness date. It may be done in several ways, not all of which will be used by a particular brewery:
- Pasteurisation is a process by which a liquid is heated for a brief time to kill microbes that may be in the liquid. Pasteurisation has also been used for many years to keep milk safe for drinking due to bacteria that may be present.
- Sterile filtration, in which the beer is passed through a mechanical filtration system which removes anything larger than 0.5 micrometres. This removes any yeast or hops that may still be in the beer which would continue to react with it.
- Bottle conditioning allows yeasts to remain in the beer after it is bottled. This helps prevent some oxidation of the beer.
- Freshness longevity affects the time it takes a beer to become stale. Some of this depends on the type of beer ingredients included. If the beer has more hops and more alcohol than otherwise, it will stay fresh longer than those that are not as strong.
