UNDERSTANDING FOOD : COLD PRESERVATION
UNDERSTANDING FOOD: PRINCIPLES AND PREPARATION is your introductory guide to learning about foods, food preparation, food service, and food science. Integrating these key topics with relevant information about nutrition and the food industry, the fourth edition gives you a thorough overview of the different dimensions of food principles--and insight into the variety of career options available in the food industry. Numerous photographs and illustrations help you understand and apply what you read.
COLD PRESERVATION
Refrigeration
Refrigeration slows down the biological, chemical, and physical reactions that shorten the shelf life of food. About one half of the foods consumed in the United States is refrigerated or frozen, compared to about one third that is canned. Prior to the 1900s, refrigeration consisted of iceboxes filled with ice cut from lakes. In the 1850s, beer brewers began using mechanical refrigeration. Later, in the 1880s, meat packers started using refrigerated railroad cars to ship carcasses rather than live animals. However, home refrigeration was not widespread in the United States until the 1940s.
Not all foods have to be refrigerated, but all perishable foods should be refrigerated as soon as possible, preferably during transport, to prevent bacteria from multiplying. Placing produce in the crisper section of the refrigerator limits the amount of oxygen available to the foods, which slows their metabolism and prolongs their life and quality. Vegetables and fruits often continue to ripen and exchange gases with the environment after harvesting, even though they are cut off from their roots or leaves. When the fruit or vegetable’s stored nutrient supply is finally depleted, the cells slowly die, and spoilage occurs as enzymes that break down, soften, and brown the tissue are released. Plastic bags used to store vegetables or fruits should have holes in them in order to let in a small amount of oxygen to prevent tissue death, but not enough to speed up deterioration. Trapping these gases by using a closed bag on the counter speeds ripening and eventual deterioration.
Refrigeration Temperatures and Times
For safety purposes, refrigerators should be kept between just above freezing to no more than 40°F (4°C). A study reported that one fifth of household refrigerators surveyed were set at or above 50°F (10°C), whereas the majority were set at about 45°F (7°C) Refrigerator storage times vary according to the food. Hot foods can be cooled down by placing them in shallow containers and setting the containers in cold water. Stirring during cooling distributes the heat and speeds the cooling process.
Serendipity played a part in the discovery that foods could be frozen commercially. Clarence Birdseye was ice fishing when he noticed that a fish he pulled out of the water froze in midair. He experimented and found that fish frozen immediately after being caught tasted fresh when thawed and prepared weeks later. In 1930, Birdseye started the frozen food industry. In the past, people had used nature’s snow and ice to preserve foods in the winter of colder climates. In fact, Sir Francis Bacon is reputed to have died of pneumonia contracted while he was trying to preserve chickens by stuffing their cavities with snow.
It is now known that freezing foods at 0°F (–18°C) or below is the least damaging to the food’s original flavor, nutrient content, and texture compared to most other preservation methods. Freezing makes water unavailable to microorganisms. In addition, chemical and physical reactions leading to deterioration are slowed by freezing. Some oxygen is still present, however, allowing these reactions to continue, with the result that frozen foods have a shorter shelf life than canned foods. Shelf life is extended further for most supermarket products that are flash frozen in a very cold industrial freezer. The faster the food freezes, the less will be the damage that occurs to the food.
What Foods Can Be Frozen?
The food’s composition determines whether, and for how long, it can be stored in the freezer. Certain fruits, vegetables, and liquid dairy products do not freeze well. The plant tissues of unblanched fruits and vegetables are irreversibly damaged during freezing. Dairy products may have their original distribution of fat and water permanently altered to the point of becoming unacceptable in quality. The higher a food’s fat content, the shorter its life span in the freezer, because fat can become rancid even when frozen. Conversely, foods containing very little water, such as dried fruits and beans grains, nuts, and coffee, can have their useful life span doubled if they are frozen.
How Long Can Food Be Frozen?
Specific maximum freezer storage times for various foods are listed on this book’s back inside cover, but in general, foods can be kept frozen for 2 to 12 months. The “first in, first out” rule should be followed: foods should be stored so that those most recently bought or frozen are the farthest back in the freezer, thereby moving food into a more easily accessible position as the freezer time increases.
Four Problems with Frozen Food
Despite its effectiveness as a food preservation method, freezing has several potential disadvantages. These include freezer burn, cell rupturing, fluid loss, and recrystallization.
Freezer burn occurs when an opaque surface partially covers food that has been frozen too long or improperly. It alters the texture, color, and flavor of foods and is due to a loss of moisture that occurs when air spaces are left in a package, or the package is torn, or moisture-proof paper is not used. Moisture loss occurs through sublimation, or the transition from ice directly to vapor. Once the food is thawed, the texture of freezer burned food is spongy, often reducing its quality to the point that it may be discarded. Frost-free freezers and the use of individually frozen products increase the risk of freezer burn.
The best way to prevent freezer burn is to wrap foods properly in airtight, vapor-resistant material, tape the package tightly, date it, and use the food before the optimal storage time has passed. An effective method for meats is to triple-wrap them, first with plastic wrap, then aluminum foil, and finally freezer paper secured with freezer tape. Masking tape should not be used, because the adhesive is not made to withstand freezing temperatures. The practice of keeping an up-to-date inventory on the freezer door showing the date on which each food was frozen avoids the costly waste of freezer burned food. For commercially frozen foods, the use of vacuum packaging has almost eliminated the problem of freezer burn.
Cell Rupturing
Water expands when it freezes, and the resulting ice crystals pierce the food’s cell walls, rupturing them and causing the food to take on an inferior texture. For this reason, some foods with a very high water content, such as lettuce, milk, tomatoes, and cottage cheese, should not be frozen. One way to minimize this problem is with rapid freezing, which results in smaller ice crystals, less cell rupturing, and a higher quality.
Rapid f reezing is accomplished commercially by the use of liquid nitrogen (–320°F/–196°C), which freezes foods in a few minutes rather than the 6 or more hours needed in a home freezer. Several other commercial methods of freezing that overcome the cell-rupturing problem include the following:
• Air-blast freezing. Frigid air is blown on foods as they pass on a conveyor belt.
• Plate or contact freezing. Foods are placed between two plates of metal while being cooled by refrigerants.
• Immersion freezing. Foods are submerged in a low-temperature brine.
• Cryogenic freezing. Incorporates very low temperatures (–140°F/–60° C) with liquid nitrogen, liquid carbon dioxide, or their vapors.
Most frozen meats lose fluid when thawed. This is known as drip, and although it is red, it is not blood, but is actually water being lost from the cells. If too much fluid is lost, the meat’s texture will be dryer when cooked than the fresh product would have been.
Another problem with freezing is recrystallization, which occurs with longer storage times and temperature fluctuations caused by opening and closing the freezer door. Numerous small ice crystals melt, then combine on refreezing to form larger crystals, thus aff ecting the texture and quality of foods such as ice cream. During a lengthy power failure, the freezer door should be kept shut, and dry ice (solidified carbon dioxide) should be placed in the freezer and/or refrigerator as soon as possible. The addition of 25 pounds of dry ice will keep a 10-cubic-foot freezer below freezing for 2 to 3 days. A much smaller amount is required for the refrigerator and refrigerator freezer. Once the power returns, any thawed foods can be refrozen as long as the temperature has stayed below 40°F (4°C).
By Amy Brown (University of Hawaii) in the book 'Understanding Food - Principles and Preparation'. Fourth Edition, Wadsworth Publishing, USA, 2010, chapter 28, p.579-580. Adapted and illustrated to be posted by Leopoldo Costa.
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