Frozen Vegetable (How Products are Made)
Frozen foods are ubiquitous in American supermarkets, and are increasingly a part of the food industry worldwide. Fruits and vegetables are usually frozen within hours of being picked, and when thawed, they are very close to fresh in taste and texture. The frozen meal is increasingly popular in time-starved American households. If the meal can be heated in a microwave, total time from freezer to table can be less than five minutes. Besides offering fresh taste and convenience, freezing is also a safe method of preservation, as most pathogens are inactivated at low temperatures.
The frozen food industry dates back to the early years of the twentieth century, when some foods were preserved by the so-called cold-pack method. Food handlers would wash and sort fruits or vegetables, then pack them in large containers holding from 30-400 lb (14-180 kg). The large containers were placed in a cold storage room for several days until the mass was frozen solid. Cold pack foods did not have the quality of modern frozen foods because of the time it took for the food to freeze. In slow freezing, the water in the food crystallizes, forming large needles of ice. These shards of ice destroy cells walls, and so when the food is thawed, it has deteriorated in taste and texture. The innovation that made the current frozen food industry possible was the invention of quick freezing by Clarence Birdseye.
Birdseye was born in Brooklyn, New York, in 1886 and studied biology at Amherst College before drifting to the Canadian Arctic to work as a fur trader and trapper. Living with his family in a remote Labrador cottage, Birdseye became fascinated with freezing food, and he experimented with many kinds of meats and vegetables. Birdseye noted that freshly caught fish that froze in seconds in the sub-zero arctic air tasted perfectly palatable when later thawed and cooked. He experimented with quick-freezing other foods, including fruits and vegetables, and soon became convinced that he had a viable commerical venture. Birdseye returned from Canada in 1917 and devoted himself to inventing a mechanical freezing device. He won his first patent in 1921, and established a frozen fish company in New York in 1923.
His first frozen food business failed to spark interest, and a second company he founded in Gloucester, Massachusetts also withered. Yet Birdseye continued to develop new freezing technology, decreasing the time it took to freeze foods. In 1929, the General Foods Corporation bought out Birdseye's enterprise, paying an enormous sum for his patents. General Foods made an intensive marketing push, installing freezers in grocery stores and developing freezer rail cars for long-distance distribution. Though American families still scoffed at frozen foods, the company began to make inroads with commercial food preparers such as hospitals and schools. American soldiers ate frozen foods during World War II, and after the war, the industry took off. Home freezers grew larger, and more and more items, from vegetables to pizzas to entire meals, became available in grocery stores.
Birdseye first froze fish and vegetables by immersing them in a circulating brine cooled to about -45° F (-42.8° C). Later he developed a so-called belt froster. This passed packages of food between two sub-zero metal surfaces, and so cooled top and bottom at the same time. This greatly decreased the time it took to freeze foods. Another innovation attributed to a General Foods scientist was the process of blanching vegetables before freezing. Blanching entailed immersing the vegetables in boiling water for a few minutes to halt the activity of certain enzymes. This preserved their flavor much more effectively. Current methods of freezing typically use the air blast method, where ultra-cooled air is blown on the food in a narrow tunnel, or by the indirect method, where the food is passed along metal plates cooled by a refrigerated liquid. Food can also be frozen cryogenically. In this method, the outer layers of the food are taken to far below their actual freezing point by passing quickly through a tunnel cooled by liquid nitrogen to as low as -80- -120° F (-62.2- -84.4° C). After the food exits the cryogenic tunnel, heat from the core of the food permeates to the outside, resulting in a final stable frozen state. Some products also use an immersion method. For foods with a viscous sauce or sticky surface, the surface might be immersed in an ultra-cooled liquid for only a few seconds, and then the food can be frozen by air-blasting.
Optimal freezing methods vary considerably with each food product. And not every food freezes well. Certain varieties of peas or strawberries for example have been found to freeze best. This might be because of their firm texture or specific sugar content. So farmers will grow these special varieties under contract with a frozen food company.
The raw materials for frozen foods include whatever is to be frozen, e.g. fish, chicken, green beans, pizza. In most cases, the food is specifically cultivated or adapted for freezing. In the case of frozen desserts such as cakes and pies or entrees such as meat loaf and gravy, the recipe must be tested and altered so that it freezes well. Large companies will order optimum ingredients according to standards they have established during their product testing. For example the noodles used in a frozen pasta entree may be ordered in bulk from a distributor that makes them in a certain specified width or viscosity or flour content according to the precise need of the frozen food manufacturer. In this respect, ingredients in a frozen meal may differ from what a home cook would buy at the supermarket. But in general, frozen foods do not require a host of extra ingredients such as preservatives. Added ingredients are most frequently thickeners and stabilizers such as starch, xanthan gum, and carrageenan. These help retain the desired texture of the food after thawing. Recipes for foods destined for the freezer may also do better with the addition of a sauce or glaze, because this protects the food from dehydration when it is passed under the freezing air blast. Vegetables or fruits destined for freezing may also be picked at a different time than they would be if they were to be sold fresh, because they need to be at optimal tenderness.
The freezing equipment is typically made from stainless steel and other metals. The gas used for freezing is most commonly ammonia. Freon is used in some systems, though because it breaks down the ozone layer, ammonia is more environmentally sound. Cryogenic freezing uses liquid nitrogen.
The Manufacturing Process
The actual process of freezing a food item varies somewhat depending on what is to be frozen. Peas are the most common frozen vegetable, having virtually replaced fresh peas in the American supermarket. The pea process is typical for many vegetables. A typical process for a frozen entrée follows.
Cultivating the peas
- 1 Peas are grown principally in Washington and Oregon and in the northern Midwest, that is, Wisconsin and Minnesota. Food processors typically contract with farmers to grow their crops according to the specifications needed for freezing. The farmers sow a variety of pea that has been approved as a good freezer. Major varieties are Dark Skin Perfection and Thomas Laxton. The harvesting schedule needs to be agreed on by both the farmer and the producer. The producer may measure the tenderness of the peas, and will also evaluate how much volume the freezing plant can accommodate. Peas need to be frozen within hours of picking, and if a backlog develops at the freezing plant, some of the peas may deteriorate.
Picking and washing
- 2 The peas may be picked by hand or automatically. Then, a machine called a viner removes them from their shells. If the processing plant is adjacent to the fields, the peas are carted there. If truck transport is necessary, they are cooled with ice water and then packed in ice for transport. At the plant, the peas are dumped into beds and sprayed with water to remove dust and dirt.
- 3 The cleaned peas are next passed into a vat of boiling water for a few minutes. This kills enzymes that effect the taste of the peas, but it does not cook them. After blanching, the peas are cooled with water and then passed to a specific gravity sorter.
- 4 The peas are next sorted to remove any old, starchy peas. They are immersed in water with a specified salt content. Tender peas float to the top of the brine tank, while peas with a high starch content sink to the bottom. The tender peas are then sprayed with clean water to remove the salt, and they pass to an inspection area.
- 5 In the inspection area, workers glance over the peas as they move along a belt. Nimble workers pick out any discolored or otherwise off peas, and also any rocks or other field detritus that may have made it this far.
Packaging and freezing
- 6 Packaging may precede freezing, or the peas may be individually quick frozen and then boxed, depending on the processing plant. Freezing could be by any of the standard methods. If they are frozen before packing, the peas might pass through a blast tunnel where ultra-cooled air freezes them. Or they may be loaded on a belt that brings them into contact with metal plates cooled from below by chilled ammonia. If they are packaged before freezing, the sealed boxes may be loaded into trays. The trays are
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Testing the frozen entrée recipe
- 7 A large company that wishes to bring out a new frozen meal will first test the recipe extensively in a test kitchen. Ingredients from different distributors will be tried out to find which ingredients retain their qualities in the freezer, and which taste best. The company will garner consumer feedback by asking test customers to try the meal at home, heating it both in a conventional oven and in a microwave oven. The final recipe may be achieved after months of testing and evaluating.
- 8 Before moving to full-scale production, a major manufacturer will devote some time to pilot production of the new frozen dish. Major frozen food producers may have a separate facility just for test runs, or the manufacturing plant may have one production line that it diverts. Here different cooking processes may be tried, for example to determine exactly how long to cook separate ingredients. All the bugs in the process should be worked out at this stage.
From the oven to the freezer
- 9 When everything is running smoothly at the pilot production stage, the manufacturer begins to produce the frozen meal in quantity. The dish is cooked and assembled on a tray. Usually a hot meal does not need a cool-down period before moving to the freezer. It may be frozen by one of three standard methods. It can be "naked" air blasted, that is, sent through an air-blast freezer tunnel in its pre-packaged state. It may be cryogenically frozen in the same manner. Or it may be packaged, and then air-blasted.
- 10 In all but the very smallest frozen food operations, once the meal has passed through the freezer, all the packing stages are fully automatic. The frozen meal on its tray passes on a belt to mechanical equipment that bags it, puts it in a carton, and then stacks the cartons in a case. The cases are then put on pallets, and this stage too is often completely automatic. If workers are palletizing the cartons, they are dressed in cold-weather gear for protection. The pallets are stored in a warehouse cooled to between 0- -20° F (-17.8- -28.9° C).
- 11 All further distribution of the frozen food should be carried out at 0° F (- 17.8° C) or cooler. In other words, trucks or rail cars that carry the pallets should be kept
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Frozen foods must be carefully inspected both before and after freezing to ensure quality. When vegetables arrive at the processing plant, they are given a quick overall inspection for general quality. The peas are inspected visually again as in step five, above, to make sure that only the appropriate quality peas go on to the packaging and freezing step. Laboratory workers also test the peas for bacteria and foreign matter, pulling random samples from the production line at various points. The packaged, frozen vegetables are also tested randomly by lab workers who cook and taste them. The freezing equipment is also cleaned at specified intervals, so that it is completely sterile. Manufacturers of freezing equipment work with food producers to develop machines that are easy to clean and maintain. The equipment manufacturers may also work with their customers to check and repair the machinery so that it works the way it is supposed to. For frozen meals, if any meat is used, the U.S. Department of Agriculture has oversight, and will send inspectors to make sure the manufacturer is maintaining its equipment properly and that the meat is kept at proper temperatures throughout the production process. However, if a frozen meal contains no meat, it is up to the manufacturer itself to maintain ideal conditions, and no government agency is directly responsible for quality control.
In the late 1990s, the frozen food industry was expanding both within the U.S. market and abroad. The industry's biggest push was so-called home meal replacement, that is, whole frozen meals that took the place of cooking from scratch. More consumers were willing to trade the convenience of a frozen meal for the satisfaction of making their own dinner from fresh foods. This meant that the industry was challenged to come up with more elaborate frozen dishes, which required more testing and experimentation to pull off than the relatively simple frozen vegetables or waffles. Food scientists are still working out the chemistry and physics of frozen foods, studying for example the relationship between low-molecular weight sugars and high-molecular weight stabilizers in a recipe in order to better predict what foods will freeze well.
Cryongenic freezing is also a relatively new freezing method that may be gaining adherents. As not all foods benefit from being frozen this way, some equipment manufacturers are designing multiple-use machines that combine freezing methods.
Where to Learn More
Rice, Judy. "B-r-r-r-reakthroughs in Freezing Technology." Prepared Foods (November 1996).
Smith, Brian. "Cryogenic Food Freezing: A Guide to the Efficient Use of Food Freezing Tunnels." Frozen Food Digest (July 1994).
"Stabilizing the Big Freeze." Prepared Foods (January 1996).