Freezing is one of the most popular engineerings over the past two decennaries which uses for nutrient saving. Because this method has better functional properties than any other techniques, particularly gustatory sensation, texture, and nutritionary saving, the popularity of stop deading procedure grows continuously in the nutrient industry. The initial intent of stop deading procedure is to keep the quality of merchandise in the long term. As Mallett ( 1994 ) has demonstrated, stop deading is a procedure that removes heat from nutrient until the temperature being below its freeze point.
Owing to low temperature, liquid H2O in nutrient transforms to solid ice. Since liquid H2O is a important factor of microbiological and chemical activities, limited H2O in cell can minimise the reaction rate in merchandises which is the primary cause of nutrient impairment. The storage temperature of frozen nutrient is by and large colder than -10A°C or -18A°C which extends the shelf-life of merchandise more than a twelvemonth.
The History of Freezing Technology
The use of stop deading to continue nutrient has been found for thousand old ages ago despite the distinction between natural and unreal infrigidation being ill-defined. However, in 1755 William Cullen, the Scottish professor, invented the first unreal equipment which used for make ice. Then, the innovation of the first icebox occurred in the following 100 old ages and a big graduated table of freeze was foremost introduced in 1870s. Sixty old ages subsequently, fish and butter became the chief frozen merchandises.
The modern frozen nutrient industry began in 1928 when Clarence Birdseye, the American, produced the double-belt contact deep-freeze. After that, he besides developed the multi-plate deep-freeze which is still usage in the industry. Another important development of nutrient freeze setup is single speedy freeze ( IQF ) , which made several modern equipments were introduced during the early 1950s, for illustration, fluidized bed deep-freeze, and cryogenic deep-freeze.
The transmutation of stop deading
Nucleation
Nucleation is the constitution of bantam ice karyon which is the foundation of crystal growing. This province can be divided into two types which are homogenous and heterogenous nucleation. The homogenous nucleation happens in pure substance, such as pure H2O, which is the lone interesting illustration in academic research. The heterogenous nucleation is merely found in frozen nutrients ( Ahmed et al. , 2010 ) .
Crystal Growth
Crystal growing is ever followed by nucleation and start at the temperature lower than stop deading point. This procedure is an addition in the size of ice karyon. It is interestingly to observe that the lower the temperature is, the higher the crystal growing rate is. However, the nucleation rate is more than the rate of crystal growing at low temperature taking to the big figure of little karyons ( Ahmed et al. , 2010 ) .
Freezing Curve
By and large, the get downing procedure of freeze merchandises is a lessening in the nutrient ‘s temperature to its freeze point which called the precooling period, which shows in the freeze curve from A to B. This is followed by a uninterrupted bead in the temperature below the freeze point at B without a stage alteration named supercooling or undercooling province. This province is considered as the induction of ice nucleation. After supercooling, the temperature increases to the freeze point ( C ) followed by ice crystallisation.
The ice crystallisation has two stages which are nucleation and crystal growing, which can happen at the same period. The formation of ice karyons begin at B, after the supercooling province, with the remotion of the latent heat ensuing in a rise in the temperature to the nutrient ‘s freeze point ( C ) . Subsequently, the crystal growing is found between C and D which is a period that the temperature remains about unchanged with an addition in the sum of ice. When the crystallisation coatings, the temperature drops to E which is every bit same as the freeze temperature because of the reasonable heat remotion.
Freezing and melt of works tissues
Mechanisms of stop deading
By and large, works tissues have cell walls and cell membranes which act as ice barriers. During stop deading procedure, ice forming and extension will be found merely external cells because of cell walls. While heat is removed continuously, the external ice will make equilibrium state of affairs ensuing in an addition in solute concentration which causes osmotic gradient between unfrozen internal and external constituent to increase. Consequently, in order to accomplish equilibrium, H2O in cell, which has less concentration, will migrate through cell walls and cell membranes to external matrix. The chief factor which has influence on this is heat removal rate ( Mallett, 1994).
Slow freeze rate
Owing to the slow rate of heat remotion, the unfrozen matrix concentration besides changes bit by bit which leads to the high rate of H2O migration from internal cells to organize external ice crystals. As a consequence of H2O lost, the cell will hold desiccation and volume decrease jobs which damage cell ‘s constructions. Furthermore, slow chilling besides lead to the little proportion of ice crystals with larger size which cause the low quality of frozen nutrient ( ibid )
Fast stop deading rate
On history of the unfrozen solution matrix concentration changing quickly, this contributes to the slow rate of internal H2O migration. Therefore, internal cell ‘s solution will be frozen in some critical low temperature. Harmonizing to Mallett ( 1994 ) , “ The more rapid the freeze procedure, the better the texture and spirit quality ” . Besides, rapid chilling brings about the higher figure of ice crystals with smaller size which is less harm to works cells.
The Problem of Frozen Fruits and Vegetables
Browning Chemical reactions
The chief ground of colour alterations in frozen fruits and veggies is browning reaction which is a consequence of oxidization reaction. Enzymatic Browning is due to polyphenol oxidase responding straight with O from the air. Most fruits contain polyphenolic substrates, which in the presence of enzymes and O are foremost changed into quinines and subsequently into brown pigments. ” ( Jeremiah, 1995 )
The solution to browning reaction
Paling
As Jeremiah ( 1995 ) states that the most effectual method to cover with enzymatic Browning is paling procedure since polyphenol oxidase, which is the primary cause of blackening of the colour in some fruits and veggies, is high sensitive to heat. There are three enzymes in fruits and veggies used for the finding of end-point of blanching which are peroxidase, catalase and lipoxygenase. Owing to these substances are heat stable enzymes, their devastation can connote that other enzymes are besides wholly inactivated ( Mallett, 1994 ) . Although heating the merchandise in order to get the better of enzymatic Browning is an indispensable factor in vegetable freeze production, merely some fruits, such as apples, pears, Prunus persicas and apricots need paling. This is because blanched fruits may lose freshness belongings ( Jeremiah, 1995 ) .
The intents of paling are the devastation of micro-organisms, harvest cleansing, gas ejection which is of import for tining, harvest ‘s colour repair and the inactivation of enzymes ( Mallett, 1994 ) . In add-on, H2O is by and large used as paling medium whose common temperature is between 75A°C and 90A°C for 1 and 10 proceedingss resting on the type and size of veggies. However, the drawback of hot-water blanching is heavy leaching losingss. Therefore, steam blanching which is an alternate medium for delicate harvests is recommended.
The use of cryoprotectants
As Erickson and Hung ( 1997 ) explained, Cryoprotectants are compounds that better the quality of frozen nutrient. The term of cryoprotectant includes all compounds that help to forestall hurtful alterations in nutrients caused by stop deading and dissolving procedures or stop deading storage ”
Adding sugars and sirups are the important pretreatment in frozen fruits. The primary map of these sweetenings is oxygen exclusion which brings about colour and visual aspect saving and browning bar. The add-on of sugars and sirups behave like a H2O withdrawer causation works cells to hold higher concentration of solution. Furthermore, the high concentration of solution in cell besides reduces the freeze point which leads to a lessening in stop deading in works cells. The advantages of this is the fewer the ice crystals do non organize in cells, the less the cell ‘s construction are damaged. Therefore, the cell membranes are non destroyed which consequences in less chance for enzymes blending substrates.
Consequently, frozen fruits which dip in sugar before stop deading will hold natural spirit, olfactory property and colour. However, this technique besides has some jobs. Harmonizing to research, the appropriate per centum of sugar use is about 30-60 % . If the sum of sugar adding to merchandises is deficient, the internal issues will be damaged. This is because the center of merchandises is the slowest portion of ice crystal formation. Besides, sirups are considered to be better pick than dry sugar because liquid can pervade to the centre of merchandises every bit long as the cells are non frozen. To bring forth the high quality of frozen fruits, makers should add dry sugar for 1-2 hours before stop deading which leads to more sugar transference to the internal tissues ( Jeremiah, 1995 ) .
Furthermore, the add-on of ascorbic acid is besides used with sweetenings to continue the quality of frozen fruits. Ascorbic acerb Acts of the Apostless as a reduction agent which react with the phenolic substances, the intermediate chemical happening in browning reaction, therefore utilizing this cryoprotectant can protect colour impairment in fruits.
Moisture Migration and Ice Recrystallization in Frozen Foods
Moisture migration is one of the chief influences on belongingss of frozen nutrients. This consequences from temperature fluctuation which ever occurs during storage. Harmonizing to Erickson & A ; Hung ( 1997 ) , there are many signifiers of wet migration which are moisture loss by sublimation, wet soaking up and redistribution in nutrients or nutrient constituents, recrystallization of ice and drip loss during dissolving.
The effects of wet migration
First, the migration of H2O causes frozen nutrients to dehydrate, particularly meat merchandises taking to a lessening in the storage life of nutrient. For case, dehydration is the chief ground for the shortening of the shelf life of frozen lamb. Owing to this, unpleasant olfactory properties which are unacceptable to clients are produced in this merchandise ( Winger ( 1984 ) in Erickson & A ; Hung ( 1997 ). Another consequence of wet migration is freezer burn.
As Jeremiah ( 1995 ) has demonstrated, freezer burn is the spots of light-colored tissue on the surface of frozen meat. On history of ice vaporization, bantam pits are produced between meat fibres lending to unattractive visual aspects. Furthermore, if moisture loss occurs in cloaked frozen nutrient, ice crystals may organize inside the bundle. Besides, this incidence can besides be seen during dissolving which causes drip loss ( Erickson & A ; Hung, 1997 ).
