Freezing has been successfully employed for the long-term preservation of many foods, providing a significantly extended shelf life. The process involves lowering the product temperature generally to -18 °C or below.The extreme cold simply retards the growth of microorganisms and slows down the chemical changes that affect quality or cause food to spoil. During freezing the cellular solution present in the food matrix is cooled to its initial freezing point, and further cooling causes the water molecule to separate, forming ice crystal. The migration of water molecules during crystallization led to an increase in osmotic pressure, further enhancing the water permeability of the cell membranes. This transport of water molecules, if not controlled, can eventually affect the microstructure of the frozen produce. The freezing process occurs in two successive steps, i.e, ” NUCLEATION” and “CRYSTAL GROWTH”.

1. Grishma Mehta (TL) – BFT -18012
Shaunak Pachpute – BFT – 18020
Sukanya Mukherjee – BFT – 18033
Prachi Ghadge – BFT – 17034
Vinit Maurya – BFT - 16007

2. INTRODUCTION
• Fruits and vegetables are important sources of nutrients such as
vitamins, minerals, and antioxidants with a wide range of health
benefits. However, fruits and vegetables are highly perishable and
they are thus often cooled or frozen after harvest.
• Among fruits and vegetables, the consumption of frozen fruits and
vegetables has reached millions of tons per year in many countries.
• The main component of fruits and vegetables is water, with a content
of up to 80-90%. This high water content favours microbial activity
and enzymatic reactions within the cells, resulting in chemical
degradation and quality loss.

3. IMPORTANCE OF FREEZING
• Freezing has been successfully employed for the long-term preservation of
many foods, providing a significantly extended shelf life.
• The process involves lowering the product temperature generally to -18 °C
or below.
• The extreme cold simply retards the growth of microorganisms and slows
down the chemical changes that affect quality or cause food to spoil.
• During freezing the cellular solution present in the food matrix is cooled to
its initial freezing point, and further cooling causes the water molecule to
separate, forming ice crystal.
• The migration of water molecules during crystallization led to an increase
in osmotic pressure, further enhancing the water permeability of the cell
membranes. This transport of water molecules, if not controlled, can
eventually affect the microstructure of the frozen produce.

4. SLOW AND FAST FREEZING
Slow Freezing
• Slow freezing occurs at -25 degrees or above.
• Slow freezing can result in damages to the microstructure due to the
formation of large extracellular ice crystals.
Fast Freezing
• Fast freezing occurs at -30 degrees or less.
• Fast freezing normally induces a high rate of heat and mass 60
transfer, minimal water migration and low osmotic pressure, leading
to the formation of intracellular and 61 extracellular small ice
crystals, and thus less damage to microstructure.

5. FREEZING PROCESS
The freezing process occurs in two successive steps, i.e,
” NUCLEATION” and “CRYSTAL GROWTH”.
NUCLEATION
• Before ice can form ,a nucleus or seed is required upon which crystal can grow.
The process of producing this seed is termed as nucleation.
• A nucleus is a site from which ice crystal starts to grow and hence cover all over
the surface to freeze it.
• It is the critical step that results in complete phase change.
• Nucleation can be of 2 types-homogeneous and heterogeneous. Homogeneous
nucleation occurs when water is free from impurities and heterogeneous nucleation
(catalytic nucleation) occurs when water molecules aggregate in crystalline
arrangement on nucleating agents such as active surfaces, this type of nucleation
predominates in food systems.

6. CRYSTAL GROWTH
• When a stable ice nucleus is formed after that crystal growth occurs.
• Crystal growth is a major stage of a crystallization process, and consists in the
addition of new atoms, ions, or polymer strings into the characteristic
arrangement of the crystalline lattice.
• The growth typically follows an initial stage of either homogeneous or
heterogeneous (surface catalyzed) nucleation.
• The action of crystal growth yields a crystalline solid whose atoms or
molecules are close packed, with fixed positions in space relative to each other.

7. REFRIGERATION
• Refrigeration is defined as the elimination of heat
from a material at a temperature higher than the
temperature of its surroundings.
• Refrigeration slows down the ripening process.
So it can help them stay fresh for longer time.
• The mechanism of refrigeration is a part of the
freezing process and freezing storage involved in
the thermodynamic aspects of freezing.
According to the second law of thermodynamics,
heat only flows from higher to lower
temperatures. Therefore, in order to raise the heat
from a lower to a higher temperature level,
expenditure of work is needed.
• The aim of industrial refrigeration processes is to
eliminate heat from low temperature points
towards points with higher temperature. For this
reason, either closed mechanical refrigeration
cycles in which refrigeration fluids circulate, or
open cryogenic systems with liquid nitrogen
(LIN) or carbon dioxide (CO2), are commonly
used by the food industry.

8. TYPES OF FREEZERS
There are 6 main types of different freezers :
• The air blast freezers / cold storage freezers
• The cartoon freezers / box freezers
• The spiral belt freezers
• The fluidized bed freezer / IQF freezer (or tunnel freezers)
• The immersion freezers / brine freezers
• The cryogenic freezers

9. 1. Air Blast Freezer or Cold Storage Freezer
• The oldest type of freezers, these are using still or
forced air as a medium and the product is kept
static in freezing storage rooms.
• Still air freezer or cold storage is the simplest
method with the lowest investment costs. However
it is the slowest freezing method.
• Forced air freezer is the improved version of cold
storage and it is using convection to circulate cold
air in the freezing room. However, with this
technology the freezing time is still long as the
airflow is not sufficiently controlled in the freezer
leading to low surface heat transfer.
2. Cartoon Freezer/ Box Freezer
• Cartoon freezers, also called box freezers are
mechanically complicated freezers, based on
relatively simple freezing concept.
• Products already packaged and placed in boxes are
sorted, transported and stored mechanically by
automated mechanism, placing them on shelves in
a storage with cold blasts of air.

10. 3. Spiral Belt Freezer
• The belt is bent around a central supporting structure,
maximizing the belt surface in a limited space.
• The spiral belt freezer can be a good solution for fruits and
vegetables as it minimizes product damage at transfer points.
4. Fluidized Bed Freezer or IQF Freezer (Tunnel
Freezer)
• The fluidized or IQF freezer is based on a belt or a
perforated bed on which the product is fluidized with strong
vertical airflow which is passing from beneath the bed/belt.
• This is the most complex freezing technology as the product
is not statically frozen thus multiple variables such as shape
of the product, aerodynamics, ripeness, firmness or water
content of the product will all influence the freezing result.
• The advantage of an IQF freezer is its high air velocity
which is suspending the product in the cold air stream
therefore separately freezing each piece of product without
creating lumps.
• In addition, this is a very quick process therefore
dehydration of the product is being minimized.

11. 5. Immersion Freezer or Brine Freezer
• In the immersion freezer the product is
completely emerged in a tank with a cooled
freezing medium which can be chemicals or
mixtures with salt or sugar.
• Immersion method is the fastest freezing
method and it is commonly used as a pre-
treatment of large products in order to create a
frozen layer before the product is exposed to
longer freezing time, in order to avoid
dehydration.
6. Cryogenic Freezer
• This technology consists of exposing the
product to an atmosphere of approx. -60°C
through direct contact with liquefied gases,
usually nitrogen or carbon dioxide.
• There two main types of cryogenic freezers:
the ones using immersion or dipping of the
product into the medium and those which are
spraying the medium on the product.
• The advantages of the cryogenic freezers is
their small size, mobility and the good
freezing results.

12. PACKAGING
• Proper packaging of frozen food is important to
protect the product from contamination and damage
while in transit from the manufacturer to the
consumer, as well as to preserve food value,
flavour, colour, and texture.
• There are typically three types of packaging used
for frozen foods: primary, secondary, and tertiary.
• The primary package is in direct contact with the
food and the food is kept inside the package up to
the time of use.
• Secondary packaging is a form of multiple
packaging used to handle packages together for
sale.
• Tertiary packaging is used for bulk transportation of
products.
• Packaging materials should be moisture-vapor-
proof to prevent evaporation, thus retaining the
highest quality in frozen foods.

13. STORAGE, TRANSPORT AND DISTRIBUTION
• Using the lowest possible temperature is essential for frozen storage,
transport, and distribution in achieving a high-quality product, since
deteriorative processes are mainly temperature dependent.
• The lower the product temperature is, the slower the speed of
reaction is leading to loss of quality.
• The temperatures of supply chains in freezing applications from the
factory to the retail cabinet should be carefully monitored.

14. Cellular Water and Transport
Mechanism

15. Name -Grishma.Mehta
Roll no. - BFT-18012
Subject Name –Post Harvest
Technology
Subject Code -BFT-605
SubjectTeacher -Dr. Sachin
Sonawane

16. Cellular Water

17. Understanding CellularWater
vacuole
water with
the biggest
Dw
cytoplasm
and
extracellular
water
cell wall
water
Water in plant-based food
systems can be divided into
three categories according
to the water SELF-
DIFFUSIONCOEFFICIENTS
(Dw)

19. Understanding cellular water
• Cellular water in fruits and vegetables can be classified as:-
FREE
WATER
BOUND
WATER

20. Free water and Bound water
• This classification is based on physical property and molecular bounding.

21. FREE
WATER
Intercellular spaces or environments are the spaces
between the cells.
They are mostly composed of air, a small portion of
water, and other solutes (such as sugar).
This water is commonly referred as free water (FW)
and are can be removed easily.
Free water (FW) is freely contained between two or
more cells
Example;
Grapefruit squirts when you cut it open

22. BOUND
WATER
Bound water is the water that remains unfrozen at
temperature as below as 0C usually -20C and doesnot
evaporate
Example;
Adding dry gelatine to water,within a few minutes water is
unable to flow and is bound
This water unlike the free water isn’t available for reaction
It cannot be easily separated.
Example;
proteins and starches
It essentially exhibits no vapour pressure
Its density is greater than that of free water

23. BOUND
WATER
Loosely
Bound
Water
(LBW)
Strongly
BoundWater
(SBW)
LBW is contained in the intracellular space
SBW is entrapped within the cell
wall and cell rupture moves such
water across the cell membrane
The proportion of SBW increases
as solid content increases

26. • In the apoplast pathway, water is transported from root hair to xylem through
the cell wall of intervening cells.
• The apoplastic route is blocked by a Casparian strip of endodermal cells.
• Hence, the symplastic route is utilized to deliver water and ions over the
cortex. Since apoplast is made up of non-living components, the apoplastic
route is least affected by the metabolic state of the root.
• The pathways of ion and water created by symplast are known as the
symplastic pathway.
• This pathway offers resistance to the flow of water since the selective
plasma membrane of the root cells handles the intake of ion and water.
Moreover, symplasty is affected by metabolic states of the root.
• The symplastic route occurs beyond the endodermis in plants with
secondary growth.
Apoplast
pathway
Symplastic
pathway

28. Free water
Strongly
Bound
Water
Loosely
Bound
Water
6 to 16% 80 to 92% 1 to 6%

29. These proportions are highly dependent on cell size, porosity, and solid
contents.
For instance, kiwifruit contains the lowest percentage of LBW when
compared with apple, whereas eggplant contains a higher 90 amount of
LBW as compared with cucumber
These variations are attributed to the size of intracellular space, i.e.,
smaller in kiwi and cucumber and larger in apple and eggplant
SBW was higher 93 (about 5%) in kiwifruit as compared with apple (about
2.5%) due to the high proportion of solid contents 94 present in kiwifruit

30. The movement of
cellular water
during the
freezing of fruits
and vegetables
depends on
several factors,
cell size freezing conditions
pressure gradient generated within the cell
cell composition including water and gases

31. Transport Mechanisms

32. Supercooling, also known as undercooling, is the process of lowering
the temperature of a liquid or a gas below its freezing point without it
becoming a solid.
It achieves this in the absence of a seed crystal or nucleus around
which a crystal structure can form.
Some important terms
Supercooling
1
LIQIUD SOLID

33. When you heat a substance just to increase the temperature of the substance, without
changing it's phase then the heat is called sensible heat.
Consider an example, water is at 25°C at atmospheric pressure, you heat it and it's
temperature goes to 75°C. The heat you added is sensible heat since it just changed
the temperature of water.
If you heat water at 100°C at atmospheric pressure and it becomes vapour with same
temperature i.e. 100°C, then that added heat is called latent heat
So the sensible heat is the heat which causes change of the temperature of a
substance without changing the phase of the substance.
Sensible heat
2

34. Latent heat
3
The heat required to convert a solid into a liquid or
vapour, or a liquid into a vapour, without change of
temperature.

35. Pre-Cooling
1
Freezing
2
Frozen
storage
3
Stages of Freezing
Transport mechanisms during freezing of cellular tissue of fruits and vegetables often
occur in precooling and phase change

37. Beginning with the prefreezing stage, the food
is subjected to the freezing process until the
appearance of the first crystal.
If the material frozen is pure water, the freezing
temperature will be 0 °C and, up to this
temperature, there will be a subcooling until
the ice formation begins.
In the case of foods during this stage, the
temperature decreases to below freezing
temperature and, with the formation of the
first ice crystal, increases to freezing
temperature.
Sensible heat is removed
Stage 1

38. The second stage is the freezing period; a phase
change occurs, transforming water into ice.
For pure water, temperature at this stage is
constant; however, it decreases slightly in foods,
due to the increasing concentration of solutes in
the unfrozen water portion.
During phase change, latent heat is removed and
ice is formed, which takes place first extracellularly,
causing higher solute concentration in the
intracellular space due to the presence of minerals
and sugar.
This leads to the creation of an osmotic pressure
difference between intracellular and extracellular
spaces, making intracellular water to diffuse across
the cell membrane and deposit on the growing
extracellular crystals
Stage 2

39. Minerals+sugars
Intracellular
Extracellular
Water will diffuse out due
to osmotic pressure
Water
Ice
Water from inside deposit on the growing
extracellular crystals

40. Stage 2
The possibility of intracellular nucleation mainly depends on
the freezing rate
A fast freezing rate can make the freezing point of the cell to
fall much more rapidly than the local temperature
This allows the intracellular solution to separate markedly
from the equilibrium concentration, and thus increases the
probability of intracellular nucleation
After some high degrees of supercooling, nucleation is formed
and the temperature increases sharply to the initial freezing
point, producing numerous smaller ice crystals
Upon nucleation, crystal growth becomes the only mechanism
and no further supercooling is required

41. ice already
formed
crystallized
solute
unfrozen
solution
As ice crystal
grows all the three
co-exist at a
constant
temperature under
an ideal situation

42. Stage 3
The last stage starts when the product temperature
reaches the point where most freezable water has been
converted to ice, and ends when the temperature is
reduced to storage temperature
The storage stage takes place after the completion of
the phase change, during which, some further cooling
proceeds to keep the ice in equilibrium with the
external environment or storage temperature.
On the other hand, at a later stage of frozen storage,
intracellular and extracellular ice crystals sublime,
leaving void spaces on the cell surface.
The formation of large extracellular ice crystals affects
cell structure

44. Factors Affecting Cell Structure and
MoistureTransfer

45. Factors Affecting Cell Structure and
MoistureTransfer
1 Cell wall modification and freezing rates:
• The cell is a unit component of plant tissue, whose structure contributes to the
mechanical and structural integrity of plant materials.
• A sequential arrangement of cells with integrated cell walls and the assistance of
cellulose fibres form a stabilized plant tissue. However, the components of
cellulose fibres including cellulose, hemicellulose and pectin substances are
important in preventing the cell from failing under stress.
• During freezing, cellulose provides the cell wall with structural rigidity and tearing
resistance required for stability, while hemicellulose and pectin substances
enhance the ability of cell wall to withstand elasticity or plasticity occurrence

46. • The inability of the cell wall to stress conditions results mainly from large extracellular ice
crystals can lead to cell wall modification.
• Cell wall modification, depending on temperature, aids in the release of degradable
enzymes and causes cell disintegration
• Generally, cell structure with small extracellular space allows minimum moisture
movement and produced compact texture, while that with large extracellular space can
cause more moisture movement and produce coarse texture.The structural damage
accompanying cell wall modification shows that turgor pressure preservation remains
critical during the freezing process.
• The cell wall integrity of frozen fruits and vegetables can be significantly maintained by
fortification of pectin substances using dehydration and texturing agents such as sugar
(sucrose and trehalose) and salts (calcium chloride and sodium chloride). This method is
known as osmotic treatment or water removal process that involves soaking fruits and
vegetables in hypertonic sugar or salt or combined solution before freezing, to reduce
water content while increasing soluble solid content, thereby, minimizing the negative
modification of the cells of frozen fruits and vegetables.

47. • calcium ion has a great potential to maintain and protect the cell 161 wall of the
strawberry against softening and disintegration, due to the interaction of calcium with the
presence of pectic acid to form calcium pectate.
• Similarly, the freezing rate affects cell structure of fruits and vegetables. Different freezing
rates show varying degrees of mass transport in cells and can induce significant changes
in the microstructure of fruits and vegetables.Typically, slow freezing rates have been
reported for slowing down moisture movement in cells and cause damage to cell
structures of strawberry, cucumber, carrot , and apple, as compared with fast freezing
rates.
• Blanching prior to freezing can lead to better preservation of microstructure.The
maintenance of microstructure was reported for carrot tissues that were subjected to
cryogenic freezing coupled with blanching. Meanwhile, tissues exposed to different time-
temperature protocols show varying textural and structural characteristics
• Apart from blanching, applications of the electric field, microwave, ultrasound, and high
pressure to freezing have also been demonstrated to be effective in preserving cell wall
integrity and maintaining cell membrane with undamaged microstructure

48. 2. Ice crystal type, size and distribution
• Ice crystal is also one of the critical factors that affect cell structure of fruits and
vegetables.This can be large, small, intracellular, and extracellular, round and dendritic
shaped and may partially depend on cell types
• For numerous and small ice crystals that are evenly distributed within the intracellular
and extracellular environment of cells, they cause little cell disintegration and less cell
damage, while for few and large ice crystals that are distributed in the extracellular
environment of cells, they can cause cell collapse and rupture.
• In cellular tissue of frozen fruits and vegetables, the ice crystals must be as small as
possible and evenly distributed in the cell so as to maintain the integrity of the cell wall as
much as possible.
• Freezing has a great potential to form smaller intracellular ice crystals in strawberry
tissue, although the quality of frozen strawberry was not fully sustained as compared
with samples frozen under slow freezing. It was assumed that high and instantaneous ice
nucleation accompanying supercooling promoted drastic water migration and
consequently caused significant degradation of strawberry cells.This study shows that
not only the size but also the type of ice crystals potentially affects the cell structure

49. • The thermal properties of cellular tissues of plant-based materials including viscosity,
specific heat, internal heat transfer resistance as well as internal mass transfer
resistance contributed to the size of ice crystals and integrity of cells. However, it is
evident that cell structure degradation is strongly related to ice crystals and
predominantly depends on the freezing rates and type of cells.
• Besides cell structure degradation, there is a tendency of physic-chemical changes such
as a change in colour, and loss of water holding capacity resulting in drip loss.These
changes normally occur at the ice/liquid interphase and increasingly depend on water-
phase solute concentration.
• An increase in water-phase solute concentration changes pH level, most often towards
acidic, thereby impacting on the micronutrients of frozen produce (Oliver & Palou,
2000). Andres-Bello 230 et al. (2013) demonstrated the significant effects of pH on plant
pigments (e.g. chlorophyll, carotenoids, and anthocyanin) that are responsible for the
colour of fruits and vegetables.Therefore, the process of ice formation can affect the
microstructure and quality of frozen fruits and vegetables, thus making control of the
freezing process essential.

50. BFT- 16007
Vinit Maurya

51. • The modelling of moisture transport phenomena during the freezing of fruits and
vegetables has practical importance for process control and optimization.
• The freezing models can be classified into heat transfer models and coupled heat and
mass transfer models, and they are broadly based on numerical and empirical
methods.
• The empirical models are simple and approximate for practical applications, while
numerical models are complicated but accurate, which includes effects of changes in
boundary conditions, phase change over a range of temperature and variation in
thermal properties.

52. • In the heat and mass transfer during freezing, the temperature difference between the product
and freezing medium is the driving force, which depends on many intrinsic and extrinsic
factors.
• The intrinsic factors are those related to product size (particularly thickness), shape and
composition, while extrinsic factors are those related to the freezing systems including
cooling medium, relative humidity and heat transfer coefficient between the cooling medium
and the products.

53. • Blija et al. used a numerical approach to model heat and mass transfer in freezing a layer of
berries and showed the role of freezing temperature in predicting freezing times.
• Mass transfer occurs during freezing, where moisture transfer is paramount, however, the
solute transfer also happens in some cases like immersion freezing. Therefore, changes can
be analyzed through the diffusion of substance.
• This is mostly expressed using a thermal energy transport equation in an enthalpy form with
the mass flux being assumed to follow:
The Fick’s law of diffusion:

54. • Campanone et al. and Kouznetsova et al. proposed to solve the Fick’s law based on numerical
methods and modelled the mass transfer successfully in cellular tissues.
• However, due to lack of adequate data, most previous studies on moisture transport-related
modelling of frozen cellular tissue assumed plant cell as a unit and that moisture movement
followed a single diffusion-equation with an effective diffusivity as shown below,
• Meanwhile, plant-based food materials are heterogeneous in nature, having cells with different
water compositions and morphology, each of which has its own diffusion rate. Therefore, the
above assumption of a single unit cannot accurately depict the true moisture movement if the
cell nature is to be considered.
• For example, mass transfer is much faster in porous fruits such as pear and apple than in dense
fruits such as raspberries and broccoli.

55. • It should be also noted that at phase change, where the liquid-solid stage and solid-vapour stage are recognized, the
part of sample close to coolant where solidification first starts has a low mass diffusivity value, while the mass
diffusivity value increases along with the distance away from coolant and reaches the maximum close to the surface
where water vapour transport occurs.
• The differences in water composition and mass diffusivity in cellular tissues make the surface close to the coolant
to be the only portion to have undergone any deformation after freezing. However, few studies have reported the
significant changes in moisture diffusivity values on the entire cell during freezing.
• Attempts have also been made to consider the heterogeneity of plant-based foods and the effects on the accuracy of
modelling moisture movements, normally using the numerical method.

56. • Norton et al. used the finite difference approach on enthalpy formulation to predict temperature profile
and freezing times during high-pressure shift freezing for cellular tissues and results showed that the
model could satisfactorily describe the freezing process.
• The finite volume approach has also been used for model heat and mass transfer during freezing with
successful prediction similar to experimental results.
• However, numerical methods developed up to now still lack the capability to analyze large
deformation of cells, thereby such modelling relies on many assumptions.

57. EmergingTechniques for Controlling the
Formation and Growth of Ice Crystals
By:- Prachi Ghadge

58. Transport phenomena during freezing of cellular tissue of fruits and vegetables induce nuclei formation
and growth, thereby producing ice crystals of different sizes, which affect cell walls, microstructures and
the quality of frozen products.
The control of ice formation and growth is therefore crucial to optimize quality preservation.
Conventional freezing techniques including immersion, air blast, and plate contact freezing
To better preserve cell structures and maintain the microstructure of frozen products, innovative freezing
techniques have been developed which include
 microwave-assisted
 ultrasound-assisted
 electric field-assisted
 high-pressure freezing (HPF)
 isochoric freezing.

59. • Microwave-assisted freezing involves low energy consumption and much less freezing time.
• By influencing the characteristics of cellular water in plant tissue, the microwave field attempts to
reorient the water molecules and break the hydrogen bonds connecting the water network, thus
controlling ice formation in cells and tissues of fruits and vegetables
• In other studies, applied pulse-spouted microwave-assisted freezing for lettuce slices and banana
cubes, respectively and revealed that the pulse-spouted microwave assisted freezing improved the
uniformity of temperature distribution with minimal moisture movement as compared with the
steady freezing mode.
Microwave-Assisted

60.  In ultrasound-assisted freezing, the ultrasound waves
can generate uniform bubbles following sponge,
leading to the creation of microscopic channels in
products.
 The technique can be applied to preserve the cell
structure of frozen products through the initiation of
ice nucleation, control of ice crystal size, and
acceleration of the freezing process.
 Several studies have reported the effects of
ultrasound on preserving the cell structures of
strawberry, broccoli and red radish, showing the
formation of microscopic channels in the cell
structures that reduced the adhesion among cells
and increased cell interspaces, consequently
resulting in an increase in the diffusion of water.
 Thus, ultrasound-assisted freezing can induce faster
ice formation with preserved cell structures.
Quality affected by rate of freezing
Ultrasound-Assisted Freezing

61. Electric Field-Assisted Freezing
 For electric field-assisted freezing, an electric voltage is applied to control the size of ice crystals by realignment of
water molecules present in the cells. The realignment of water molecules causes spontaneous nucleation through the
breakage of the hydrogen bonds, making the water less structured and polarized.
 However, when dissolved solutes in the water solution are considered, the field-effect in controlling ice crystal growth
is influenced by the amount of the dissolved solute.
 For instance, additives such as sodium chloride (NaCl), sucrose, trehalose, glycerol, and glucose can significantly
influence the nucleation temperature and ice crystal sizes under direct current (DC) electric field application.
categorically stated that the use of electric field-assisted freezing through either alternating current (AC).

62. • When cellular tissue of fruits and vegetables is kept in a liquid state at sub-
zero temperatures combined with high pressure, upon pressure release, a
large degree of supercooling is achieved, promoting rapid ice nucleation.
• For HPF, pressure decreases the freezing point (0 oC) of water to a minimum
of -22 oC at 208 Mpa.
• The number and size of ice crystals formed during HPF are directly
proportional to the extent of supercooling reached in the sample before
nucleation and the subsequent growth of ice crystals.
• So far, three strategies have been identified for HPF including
 pressure shift freezing (PSF)
 pressure-assisted freezing (PAF)
 pressure-induced freezing (PIF).
• The first two methods (i.e. PSF and PAF) are commonly studied due to their
easy operation, mainly about their effects on the texture, appearance,
microstructure, and colour of fruits and vegetables. Meanwhile, the PSF
technique has the advantage over PAF because it accelerates the rate of
nucleation, and consequently promotes the formation of small ice crystal.
Assessment of cell damages on broccoli
frozen by high pressure-shift freezing
High-Pressure
Freezing (HPF)

63.  Isochoric freezing, preserves fruits and vegetables at subfreezing temperatures without ice
damage.
 In isochoric freezing, produce is immersed in a solution in osmotic equilibrium with the produce
processed inside a pressure chamber.
 This technique has been demonstrated for producing less damaging ice crystals and better
preserved texture, structure and nutritional property of sweet cherry and spinach
Isochoric Freezing