question: Why do some food need to be refrigerated after opening even though they aren't when unopened?
comment: Bacteria and Fungi in Air. Sealed Package = > Bacteria and Fungi not in food. Opened Package = > Bacteria and Fungi now in food. Refrigerator makes Bacteria and Fungi grow much slower.
passages: Once opened for consumption, the product is immediately exposed to atmospheric oxygen and floating dust particles containing bacteria and mold spores, and all protections from the preservation process are immediately lost. At room temperature, mold and bacteria growth resumes almost immediately, and warmer temperatures can lead to an explosion of growth that rapidly degrades the food product. This organism growth can result in the accumulation of poisonous bacterial substances in the food product such as botulin, that lead to food poisoning, sickness, or death.
   Refrigerate after opening
  
  The term refrigerate after opening is an instruction on commercial preserved food products to cool the container after it has been opened and the contents exposed to open air.
  
  Moist foods are commonly preserved using canning and vacuum sealing to kill off any bacteria and molds, and to prevent further growth by removing oxygen. Dry-product containers are often preserved with heating and filling the empty container spaces with an inert nonreactive gas such as nitrogen.
   BULLET::::- Keep the container closed and the contents contained for the specified shelf life until time of opening
  
  BULLET::::- Provide a barrier to dirt, oxygen, moisture, etc. Control of permeation is critical to many types of products: foods, chemicals, etc.
  
  BULLET::::- Keep the product secure from undesired premature opening
  
  BULLET::::- Provide a means of reclosing or reusing the container
  
  BULLET::::- Assist in dispensing and use of product
   Various food preservation and packaging techniques are used to extend a food's shelf life. Decreasing the amount of available water in a product, increasing its acidity, or irradiating or otherwise sterilizing the food and then sealing it in an air-tight container are all ways of depriving bacteria of suitable conditions in which to thrive. All of these approaches can all extend a food's shelf life, often without unacceptably changing its taste or texture.
   Sous vide, French for "under vacuum", implies that food should be sealed in a plastic bag with all the air removed. An alternative method is to place the food in an open-sided plastic bag and partially submerge the bag into the water, forcing out the air. This method involves clipping the open side of the bag to the side of the pot to keep water from leaking into the opening. The goal is to have the food completely in contact with the hot water to assure even cooking while reducing off-flavors from oxidation.
   Cooling the food product to a temperature just above freezing in a refrigerator is generally required to slow the growth of these organisms. This cooling can permit foods to remain safely edible for up to two weeks after opening.
  
  Simply loosening the lid of a vacuum-packed food product to break the seal is enough to permit the bacteria and mold growth to resume. The amount of air inhaled into the container when the seal is broken is small, but sufficient to permit slow growth of contaminating and decomposing organisms.
   Food rotation is important to preserve freshness. When food is rotated, the food that has been in storage the longest is used first. As food is used, new food is added to the pantry to replace it; the essential rationale is to use the oldest food as soon as possible so that nothing is in storage too long and becomes unsafe to eat. Labelling food with paper labels on the storage container, marking the date that the container is placed in storage, can make this practice simpler. The best way to rotate food storage is to prepare meals with stored food on a daily basis.
   Various food preservation and packaging techniques are used to extend a food's shelf life. Decreasing the amount of available water in a product, increasing its acidity, or irradiating or otherwise sterilizing the food and then sealing it in an air-tight container are all ways of depriving bacteria of suitable conditions in which to thrive. All of these approaches can all extend a food's shelf life without unacceptably changing its taste or texture.
   Shelf-stable food (sometimes ambient food) is food of a type that can be safely stored at room temperature in a sealed container. This includes foods that would normally be stored refrigerated but which have been processed so that they can be safely stored at room or ambient temperature for a usefully long shelf life.
   Closure (container)
  
  Closures are devices and techniques used to close or seal container such as a bottle, jug, jar, tube, can, etc. Closures can be a cap, cover, lid, plug, etc.
  
  Other types of containers such as boxes and drums may also have closures but are not discussed in this article.
  
  Section::::Purpose of closures.
  
  Many containers and packages require a means of closing. It can be a separate device or seal or sometimes an integral latch or lock. Depending on the contents and container, closures have several functions:
   Just as temperature increases speed up reactions, temperature decreases reduce them. Therefore, to make explosives stable for longer periods, or to keep rubber bands springy, or to force bacteria to slow down their growth, they can be cooled. That is why shelf life is generally extended by temperature control: (refrigeration, insulated shipping containers, controlled cold chain, etc.) and why some medicines and foods "must" be refrigerated. Since such storing of such goods is temporal in nature and shelf life is dependent on the temperature controlled environment, they are also referred to as cargo even when in special storage to emphasize the inherent time-temperature sensitivity matrix.
   Preparation of food under pressure, with or without heat, was developed by American and French engineers in the mid-1960s as an industrial food preservation method. As with Rumford, the researchers learned that the food showed distinctive improvements in flavor and texture. As this method was pioneered, applying pressure to food through vacuum sealing was sometimes called "cryovacking". The pressure notably concentrated the flavors of fruits, even without cooking.
   Dairy products are constantly in need of refrigeration, and it was only discovered in the past few decades that eggs needed to be refrigerated during shipment rather than waiting to be refrigerated after arrival at the grocery store. Meats, poultry and fish all must be kept in climate-controlled environments before being sold. Refrigeration also helps keep fruits and vegetables edible longer.
   Section::::Aseptic packaging.
  
  BiB is also used extensively in the packaging of processed fruit and dairy products in aseptic processes. Using aseptic packaging equipment, products can be packed in aseptic packaging. Pasteurised or UHT treated products packed into this format can be "shelf stable", requiring no refrigeration. Some products can have a shelf life of up to 2 years, depending on the type of bag that is used.
   Fresh fruits and vegetables are sometimes packed in plastic packages and cups for fresh premium markets, or placed in large plastic tubs for sauce and soup processors. Fruits and vegetables are usually refrigerated at the earliest possible moment, and even so have a shelf life of two weeks or less.
   Section::::Essential features.
  
  Sealing the food in sturdy plastic bags retains juices and aroma that otherwise would be lost in the process.
   Section::::Modern industrial techniques.:Hurdle technology.
  
  Hurdle technology is a method of ensuring that pathogens in food products can be eliminated or controlled by combining more than one approach. These approaches can be thought of as "hurdles" the pathogen has to overcome if it is to remain active in the food. The right combination of hurdles can ensure all pathogens are eliminated or rendered harmless in the final product.
   A number of methods of prevention can be used that can either totally prevent, delay, or otherwise reduce food spoilage. A food rotation system uses the first in first out method (FIFO), which ensures that the first item purchased is the first item consumed.
   Cooling preserves food by slowing down the growth and reproduction of microorganisms and the action of enzymes that causes the food to rot. The introduction of commercial and domestic refrigerators drastically improved the diets of many in the Western world by allowing food such as fresh fruit, salads and dairy products to be stored safely for longer periods, particularly during warm weather.
   Packaging and package labeling have several objectives
  
  BULLET::::- Physical protection - The food enclosed in the package may require protection from, among , shock, vibration, compression, temperature, bacteria, etc.
  
  BULLET::::- Barrier protection - A barrier from oxygen, water vapor, dust, etc., is often required. Permeation is a critical factor in design. Some packages contain desiccants or oxygen absorbers to help extend shelf life. Modified atmospheres or controlled atmospheres are also maintained in some food packages. Keeping the contents clean, fresh, and safe for the intended shelf life is a primary function.
   Directly following the heat treatment, vegetables/fruits are quickly chilled by cold water. A common alternative to cooling with cold water is cooling with cold air. This method of cooling prevents the leaching of water-soluble nutrients; however, the air causes evaporation and lowers the mass of the vegetable—a monetary disadvantage for industry.
  
  Emerging technologies include ohmic, infrared, microwave, and radio frequency blanching.
  
  Section::::Time and temperature.
   Ovens have a recovery time after being opened. Opening an oven door can reduce the temperature within an oven by up to 50° F (30° C). Methods to reduce oven recovery time include the placement of a baking stone or pizza stone, tiles made of ceramic, or a brick insert device in an oven, all of which serve to reduce recovery time through their heat retention properties.
   It is important to consider recommended times and temperatures for a food product when blanching. Times and temperatures are based on the type of food, size, shape, and other factors. Over-blanching can lead to an excessive loss of nutrients and aromatic compounds, as well as softening of the food. Blanching at temperatures or times lower than those recommended may not effectively inactivate all enzymes. In addition, this may also cause the release of more enzymes from the plant tissue, causing greater overall enzymatic activity and faster spoiling of the product.
  
  Section::::Disadvantages.
   Some foods, such as bananas, are picked when unripe, are cooled to prevent ripening while they are shipped to market, and then are induced to ripen quickly by exposing them to propylene or ethylene, chemicals produced by plants to induce their own ripening; as flavor and texture changes during ripening, this process may affect those qualities of the treated fruit.
  
  Section::::Chemical composition.
   Preservatives and antioxidants may be incorporated into some food and drug products to extend their shelf life. Some companies use induction sealing and vacuum/oxygen-barrier pouches to assist in the extension of the shelf life of their products where oxygen causes the loss.
  
  The DoD Shelf-Life Program defines shelf-life as
  