By: Vigneshwari K A Sudha *
Antifreeze proteins (AFPs) are polypeptide compounds synthesised by various organisms such as fish, plants, fungi and bacteria to enable their cells to survive subzero environments. In 1969, Arthur De Vries identified the antifreeze proteins in Antarctic notothenioid fish. He described in his study that these proteins have the tendency to lower the freezing point of fish’s blood to below the freezing point of seawater, without significantly increasing the osmotic pressure of the plasma.
Antifreeze proteins have the unique ability to depress the freezing point of water without significantly affecting the melting point. This phenomenon is known as Thermal Hysteresis (TH). Thermal hysteresis happens in the presence of a hysteresis gap, i.e. by lowering the freezing point and leaving the melting point unaffected.
Antifreeze proteins possess several unique properties such as; Thermal Hysteresis; Ice Re-crystallisation inhibition (IRI); Protective action of ice nucleating agents; and Interaction with membrane proteins (Pathogenesis). Due to these unique properties, antifreeze proteins are used for the preservation of biological samples at very low temperatures.
Sources of Antifreeze Proteins
Antifreeze proteins are found in a wide range of sources including polar fish, fungi, plants, and bacteria with a specific characteristic structure, but with similar modes of mechanism.
There are two types of fish antifreeze proteins; the antifreeze glycoproteins (AFGPs) and the antifreeze proteins (AFPs). As per their order of discovery they are numbered as I, II, III, and IV. Examples of sources of these fish antifreeze proteins include pseudopleuronectes americanus, chaetoceros neogracile, berkeleya sp., navicula sp., fragilariopsis sp., and nitzschia frustulum, etc.
There are two types of insect antifreeze proteins; tenebrio and dendroides AFPs. These insect AFPs are more active from fish or plants. Bacterial AFPs were found in sea ice bacteria including polaribacter irgensii and psychromonas ingrahamii; marine bacterium including shewanella frididimarina and marinomonas primoryensis.
Plant AFPs are found in freeze-resistant plants in cold regions. There are three types of AFPs that have been isolated from winter rye. It has been observed that the AFPs isolated from overwintering plants are effective inhibitors of ice re-crystallisation.
Mechanism of Antifreeze Proteins
These antifreeze proteins restrict the growth of large ice crystals. When the freezing process starts, many small crystals form and some dominate and grow into bigger sizes. Antifreeze proteins work by adsorbing the ice surface and restricting the growth of large fatal ice crystals. This whole process prevents ice re-crystallisation by making the surface thermodynamically unfavourable.
Applications in the Food Industry
Application of antifreeze proteins is very important in the food industry. They are very frequently used to decrease the freezing point and preventing the growth of larger ice crystals. Antifreeze protein can be directly added to food because of their uniqueness in directly improving freezing processes. Antifreeze proteins can lower the freezing temperature and retard re-crystallisation in frozen storage. The antifreeze proteins are used for various applications in the food industry such as; cryo-preservation; de-icing; inducing freeze resistance in plants; and longer storage stability. AFPs are used to cryo-preserve tissues and organs which are otherwise turned inedible due to ice crystal damage.
Antifreeze proteins have the capability to modify the ice crystal formation, which results in stabilisation of ice crystals and inhibition of re-crystallisation of ice.
AFPs have great potential as ice re-crystallisation inhibiting agents. AFPs inhibit re-crystallization during freezing, storage, transport and thawing; therefore they preserve food texture by reducing cellular damage and also minimize the loss of nutrients by reducing drip. Antifreeze proteins are naturally present in many foods consumed as part of regular human diet. Also, they can be added to food as food additives. The AFPs may be introduced into food products either by physical processes, such as mixing and soaking, etc. or they can be injected intravenously.
Chilled and Frozen Meat and Fish
AFP’s ice re-crystallization inhibition property can be utilised in frozen meat and fish. It helps to reduce drip loss.
AFPs proved their capability to enhance meat quality during frozen storage. The results obtained from various studies showed that the addition of AFGP into the meat prior to freezing could reduce frozen storage damages. AFPs also proved their potential as cryoprotectants. In some studies, they have been added for the preservation perspective to enhance the storage stability. The little amount added to frozen meat showed better properties than conventional cryoprotectants, without adding any sweet flavour to the meat.
Ice Cream Products
One major problem faced by the ice cream manufacturers is the fluctuation of temperature during freezing. The formation of large ice crystals during refreezing destroys the texture and organoleptic property of ice cream. AFPs added in frozen dairy products as a natural ice growth inhibitor have improved the overall quality and texture of ice cream.
Antifreeze proteins have been added to the frozen dough to deal with the shelf life associated problems. AFPs have been found effective in increasing the water-holding capacity of frozen dough and improving the bread-making properties along with extended frozen storage stability.
At present antifreeze proteins have not been used in the food industry but this industry is a significant sector where antifreeze proteins can be applied. The application of antifreeze proteins to fish, fruits, vegetables and yeast has improved the overall quality of food. AFPs used for preserving foods are useful in increasing the storage time and thawing quality of frozen foods and desserts. These are very active when added in small quantities and are very cost effective. However, there are some technical and health aspects which should be taken into account. The technical aspects include the process for isolation and purification of AFPs. The process is very time consuming and expensive. Whereas the health aspects include the possible allergies with AFP incorporated food. As of now no research has shown any cause for concern. But in future, they can be considered as very promising ingredients for all food applications.
* A student in Department of food technology, Kongu Engineering College, Perundurai-638060