Meat Science

Still Air, Plate, Blast, Liquid Immersion and Liquid Sprays and Cryogenic Freezing

METHODS OF FREEZING

• Freezing can be undertaken by the following methods- Still Air Freezing, Plate Freezing, Blast Freezing, Freezing by Liquid Immersion and Liquid Sprays and Cryogenic Freezing.
  Quick freezing is achieved by, Blast Freezing, Freezing by Liquid Immersion and Liquid Sprays and also by Cryogenic Freezing while freezing in a Still Air Freezer results in Slow Freezing.

Still Air Freezer

• The medium of heat transfer is air in a still air freezer. In this method heat transfer is entirely based on convection, as air is a poor conductor of heat.Hence freezing is very slow.
• Cabinet or desk freezers or freezers of home refrigerators are good examples of still air freezers.
• The temperature range of commercial still air freezers fall between -10⁰C and-30⁰C.
• Their capacity to freeze unfrozen meat, particularly when overloaded is rather poor and are best suited to store frozen meat at freezer temperatures.

Still air freezer

Plate freezers

• The medium of heat transfer is metal in a plate freezer. In this method heat transfer is entirely based on conduction and hence freezing rates are slightly faster than still air freezers.
• Meat as such or packed in trays is directly placed in contact with freezer plates or shelves.
• The temperature range of commercial still plate freezers fall between -10⁰C and-30⁰C.This method is commercially restricted to freezing thin pieces of meats such as steaks, chops, fillets and meat products such as patties.

Plate freezers

Blast freezers

• This is the most commonly used commerciaMethod
• Method for freezing meat and is either undertaken in rooms or tunnels in which cold air blast is provided.
  The medium of heat transfer in a blast freezer is also air, but air is forced to circulate rapidly by means of fans, hence rate of heat transfer and thus freezing rate is markedly increased.
• The temperature range of commercial blast freezers fall between -10⁰C and-40⁰C, while the air velocities range from 0.5 m/sec to about 18m/sec.
  High air velocities increase the cost of freezing and also the risk of freezer burn.
• Hence, a temperature of -30⁰C, and air velocities of about 12.5m /sec are probably the most economical and practical conditions employed in the industry.
  Proper stacking and spacing of meat and meat products on shelved racks and pallets are important criteria in determining efficiency of freezing in freezer rooms.
  Conveyor speeds, which are set based on the time required to freeze the product in question, is an important criterion in determining efficiency of freezing in blast tunnels.Sometimes, blast freezers are used to partially freeze meat, to complete surface hardening and further freezing is undertaken in a still air freezer.

Blast-freezer	Fan inside a blast freezer

Freezing by Liquid Immersion and Liquid Sprays

• This is the most commonly used commercial method for freezing poultry, though some fish and meat products are frozen by this method.The medium of heat transfer is the freezing liquid used, in this method.
• In this method heat transfer is entirely based on conduction and hence heat transfer rates are rapid than blast freezers, and hence higher temperatures can be used.
  However the freezing rates achieved in a blast freezer are comparable with freezing by liquid immersion and liquid sprays.
• Meat packed in plastic bags, stacked on shelved racks or pallets are either immersed in freezing liquid by forklifts, passed through the freezing liquid by conveyors.
  Products may otherwise be conveyed through enclosed freezing cabinets in which freezing liquids are directly sprayed over the products.
• The products after being removed from the immersing tank or freezing cabinets must be rinsed with water to remove the freezing liquid. The extent of freezing is determined by length of immersing or spraying.Sometimes freezing by liquid Immersion and liquid sprays is carried out for partial freezing, to complete surface hardening and further completion of freezing and storage is undertaken in a blast freezer.
• Liquids used for freezing either for immersion or for spray must possess low viscosity, freezing points, high heat conductivity and must also be non – corrosive, relatively inexpensive and non-toxic.
• Sodium chloride brine (Corrosive in nature), glycerol and glycols such as propylene glycol are commonly used for freezing. A problem associated with this method is the possible seepage of freezing liquid on to product surface through holes in protective package.

Cryogenic freezing

• Cryogenic freezing is very low temperature freezing accomplished with condensed or liquefied gases by direct immersion of the product to be frozen in the liquid, or spraying the liquid over the product to be frozen, or by circulating the vapour of the cryogenic liquid over the product to be frozen. The agents usually used are either liquid nitrogen or carbon – di – oxide, and rarely liquid nitrous oxide is also used.
• Direct immersion of large pieces of meat is rarely used due to extensive shattering or cracking of products. So, generally liquid nitrogen is usually evaporated in a freezing chamber and the great cooling capacity, as it changes to nitrogen gas is used to freeze meat products.
• Liquid nitrogen or carbon – di – oxide released as snow, combined with conveyor systems, are used to rapidly freeze meat products of small size such as patties, diced meat, fish and shell- fish. Meat products frozen by this method have excellent sensory qualities.