Freezing is one of the oldest methods widely used for food preservation, which allows the retention of taste, texture and nutritional value in foods better than any method. Way back in 1755, first time small quantity of ice produced without using a ‘natural cold’ was regarded as milestone and considered as new beginning in the food industry. In initial days, ice-salt systems were used to preserve fish and meats, later on by the late 1800’s, freezing was introduced into large scale operations as a method of commercial preservation. Meat, fish and butter were the preserved commodities. Fruits were one of the most important commodities for preservation, later on process into jams, jellies, ice-cream, pies etc. Although commercial freezing of small fruits and berries first began around 1905 in the US. Trials were conducted for freezing of vegetables. In 1929, blanching was required as a pre- treatment to inactivate enzymes before freezing to avoid deterioration and off flavors caused by the enzymatic degradation.
Freezing technology boosted up in 1928 with the development of double belt contact freezers at commercial scale by a technologist named Clarence Birdseye. He worked as a fur trader in Canada, and Birdseye discovered that the fish that he caught froze almost immediately after being pulled from the water. He ate the fish months later and learned that it tasted almost the same. From this experience, he theorized that food must be frozen very quickly in order for it to retain its taste and texture. Before quick-freezing came along, foods were frozen at a fairly slow rate. This caused large ice crystals to form, which ruptured the cell membranes of the food. When the food was defrosted, the ice crystals melted and water would leak out, taking with it the food’s flavor and texture. Birdseye developed two methods for quick freezing foods, both of which employed the innovation of packaging the food beforehand. In the first technique, the package was held between two metal belts that were chilled to -40°F to -45°F using a calcium chloride solution. In the second and more popular technique, the packaged food was held under pressure between two hollow metal plates that were chilled to -25°F by the evaporation of ammonia. Using this method, a two-inch-thick package of meat could be frozen to 0°F in about 90 minutes, while fruits and vegetables took about 30 minutes.
Birdseye’s quick-freezing process actually ended up creating 168 patents! These covered not only the freezing technique but also the packaging; type of paper used, and related innovations.
After the revolution in the quick freezing process and equipment, the industry becomes more flexible especially with the usage of multi-plate freezers. Furthermore quick freezing of consumer size packages helped frozen vegetables to be accepted rapidly in the late 1930’s.
Today the freezing is the only large scale method that bridges the seasons as well as variations in supply and demand of raw materials such as meat, fish, butter, fruits and vegetables. It also makes possible movement of large quantities of food over geographical distances. There are varieties of freezing equipments such as air blast freezers, tunnel freezers, fluidized bed freezers, immersion freezers, indirect contact freezers (plate freezers, contact belt freezers etc.), and cryogenic freezers.
Cryogenic freezing is a relatively new method of freezing in which the food is exposed to an atmosphere below -60 °C through direct contact with liquefied gases such as nitrogen or carbon dioxide (Hung and Kim, 1996). This type of system differs from other freezing systems since it is not connected to a refrigeration plant; the refrigerants used are liquefied in large industrial installations and shipped to the food-freezing factory in pressure vessels. Thus, the small size and mobility of cryogenic freezers allow for flexibility in design and efficiency of the freezing application. Low initial investment and rather high operating costs are typical for cryogenic freezers (Persson and Lohndal, 1993).
In this study or article we will be discussing about the liquid nitrogen which is used in cryogenic freezing of the food products in food industry and the hotel industry as well.
Liquid nitrogen is the liquefied form of the element nitrogen that's produced commercially by the fractional distillation of liquid air. Like nitrogen gas, it consists of two nitrogen atoms sharing covalent bonds (N2). Nitrogen was first liquefied on April 15, 1883, by Polish physicists Zygmunt Wróblewski and Karol Olszewski. Nitrogen is non-toxic, odorless, and colorless. It is relatively inert and is not flammable. Nitrogen gas is slightly lighter than air when it reaches room temperature. It is slightly soluble in water.
At normal pressure, liquid nitrogen boils at 77 K (-195.8° C or -320.4° F). The liquid-to-gas expansion ratio of nitrogen is 1:694, which means liquid nitrogen boils to fill a volume with nitrogen gas very quickly. Liquid nitrogen is stored in special insulated containers that are vented to prevent pressure buildup. Depending on the design of the Dewar flask, it can be stored for hours or for up to a few weeks. Liquid nitrogen is denoted as LN2, LN, or LIN. Liquid nitrogen is identified as UN number 1,977. LN2 displays the Leidenfrost effect, which means it boils so rapidly that it surrounds surfaces with an insulating layer of nitrogen gas. This is why spilled nitrogen droplets skitter across a floor.
Liquid nitrogen, with a boiling temperature of -196 °C at atmospheric pressure, is a by-product of oxygen manufacture. The refrigerant is sprayed into the freezer and evaporates both on leaving the spray nozzles and on contact with the products. The system is designed in a way that the refrigerant passes in counter current to the movement of the products on the belt giving high transfer efficiency. The refrigerant consumption is in the range of 1.2-kg refrigerant per kg of the product. Typical food products used in this system are, fish fillets, seafood, fruits, berries (Persson and Lohndal, 1993).
Liquid Nitrogen Uses
Liquid nitrogen has many uses, mainly based on its cold temperature and low reactivity. Examples of common applications include:
•The freezing and transporting of food products
•The cryopreservation of biological samples, such as sperm, eggs, and animal genetic samples
•Use as a coolant for superconductors, vacuum pumps, and other materials and equipment
•Use in cryotherapy to remove skin abnormalities
•The shielding of materials from oxygen exposure
•The quick freezing of water or pipes to allow work on them when valves are unavailable
•A source of extremely dry nitrogen gas
•The branding of cattle
•The molecular gastronomy preparation of unusual foods and beverages
•The cooling of materials for easier machining or fracturing
•Science projects, including making liquid nitrogen ice cream, creating nitrogen fog, and flash-freezing flowers and subsequently watching them shatter when tapped onto a hard surface.
Liquid Nitrogen Safety
When working with liquid nitrogen, taking safety precautions is paramount:
•Liquid nitrogen is cold enough to cause severe frostbite on contact with living tissue. You must wear proper safety gear when handling liquid nitrogen to prevent contact or inhalation of the extremely cold vapor. Cover and insulate skin to avoid exposure.
•Because it boils so rapidly, the phase transition from liquid to gas can generate a lot of pressure very quickly. Do not enclose liquid nitrogen in a sealed container, as this may result in it bursting or an explosion.
•Adding large quantities of nitrogen to the air reduces the relative amount of oxygen, which may result in an asphyxiation risk. Cold nitrogen gas is heavier than air, so the risk is greatest near the ground. Use liquid nitrogen in a well-ventilated area.
•Liquid nitrogen containers may accumulate oxygen that is condensed from the air. As the nitrogen evaporates, there's a risk of violent oxidation of organic matter.
On a General note
Liquid nitrogen is widely used in the food industry as freezing agent and from recent years it gain the much more attention in the restaurants, bars and hotels, where it used as a means of freezing or cooling, and adds the appealing charming presentation to the subject i.e. food or beverage. As we all knows, better the food looks, higher the consumer pay. Somewhere it’s all marketability of the food among the customer.
We all know there are always two sides of a single coin, so looking over this there are some plus points and some minus point of every technology. But banning the thing is not the solution to the problem, review has to be done and things should be reconstructed.
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