Drying and Rehydration: Key Concepts and Models

General concepts: Drying and
rehydration

Contents
1. Introduction Rehydration of Foods
2. Rehydration Measurements and Indices Utilized
3. Mathematical Modeling
4. Utilizing Weibull Distribution in Modeling Rehydration Process
5. Pretreatments and Their Effects on Food Structure
6. Drying Methods and Their Effects on Product’s Physical Properties
7. Rehydration Conditions and Their Effects on Process Kinetics
8. Physical Properties of Dried Foods and Their Effect on Rehydration
9. Leaching of Solids
10. Sensory Aspects
11. Research Needs
12. Conclusions

Introduction
0 The resulting dried foods present important advantages,
such as low transportation costs, extended shelf-life stability,
and ease of use.
0 The latter represents an important consumer expectation,
and it is often translated as the need to add only hot or cold
water to the dry ingredients and mix or simmer for a short
period of time in order to achieve rehydration and
reconstitution of the product.
0 High quality and flavor, nutritional value, and resemblance to
the fresh product are typical important traits that need to be
taken into account in order to meet consumer expectations
and quality standards.

Introduction
0 Safety, nutritional, and sensory aspects of foods are often
related to the rehydration process as well as to the severity of
the drying process used.
0 To achieve maximum quality and the desired hedonic and
food characteristics, optimum processing conditions should
be targeted, namely, those that result in minimal nutritional
losses and optimal sensory traits.

Introduction
0 Dried vegetables are used extensively in instant dry soups,
ready-to-eat meals, snack foods, and seasoning blends.
0 Vegetables that are commonly dehydrated and used for
rehydration include carrots, potatoes, peppers, mushrooms,
corn, onions, beets, parsley, horseradish, garlic, green beans,
and celery.
0 Fruits, legumes, cereal grains, fish, pasta, meat, and extruded
breakfast cereals, among others, are also used in dried form
or after rehydration, usually in water or milk.

Introduction
0 During the rehydration process, the dry material, which is
submerged in water or some other aqueous medium,
undergoes several simultaneous physicochemical changes
(e.g., in moisture and solids content, porosity, volume,
temperature, gelatinization, and texture).
0 The rehydration involves various processes running in
parallel, including imbibition of liquid into the dried material,
transport of the liquid through the porous network and
diffusion through the solid matrix, swelling of certain domains
in the solid matrix, and leaching of soluble solids into the
external liquid.
è Rehydration of dry foods is a very complex phenomenon
that involves several different, simultaneously occurring
physical mechanisms.

Introduction
0 Rehydration, which relates to the amount of water a dry food,
is able to absorb in a given period of time.
Ø This term is usually associated with the kinetics and
technological aspects of the process.
0 Reconstitution denotes the state of the product after the
uptake of liquid and is related to the sensory aspects of the
process
Øhow much the reconstituted sample resembles the original
fresh product before it was dried.

0 Note that in most cases, the aim is to shorten the
rehydration and reconstitution time and minimize the
effort required by the consumer to achieve a fast,
convenient, and tasty meal.

Rehydration of Foods
0 The rehydration of dried foods is a fundamental unit
operation in the food industry.
0 The quality of rehydrated and reconstituted products is
affected by the drying conditions and rehydration processes
utilized, ultimately influencing consumer acceptance.
0 During the drying process, physicochemical
0 changes, including textural and structural modifications,
migration of solutes, and loss of volatiles and nutrients, occur
in an irreversible manner and have an impact on the quality
of the final product.
0 The drying process needs to be understood and controlled
in order to create a dried product with optimal nutritional,
sensorial, and rehydration characteristics.

0 The most fundamental aspect of rehydrating a dried food is
related to water absorption, and it is expressed by the ratio
commonly termed the rehydration capacity.
üwhere Xt is the weight after a given rehydration time and Xo
is the initial weight of the dry sample.
üTypical values for the rehydration capacity range between
~2 and 7.
0 The other criterion utilized to describe reconstitution (RE) is
defined as
ü where Xi is the initial weight of the sample before drying.
ü Typical values are often lower than 1, mainly because of the
changes occurring during the drying process.

0 During rehydration, solids can leach from the food product to
the medium.
0 To overcome this problem, the concept of non dissolvable
solids (NDS), defined as the amount of solids after all
dissolvable solids have leached out, was proposed.
0 The rehydration ratio (RR) can be defined as
ü where Wt , W0 , and W are the water contents at time t ,
initial time t0 , and infinite time t, respectively.
0 Typical RR values for short rehydration times (e.g., 3 min)
range from 0.2 to 0.6 for air-dried (AD) and freeze-dried (FD)
carrots, respectively.

0 For example, values of ~0.3 were observed for FD
peas, whereas FD corn, potatoes, and beets yielded RR
values close to 0.8.
0 The equilibrium RR value is usually independent of
the drying method and asymptotically reaches a value
close to 1.

0 The principal factors affecting the rehydration kinetics and
reconstitution may be divided into two main groups:
1. those related to the dry particulate solid, and
2. those related to the external rehydration medium.
0 In the first group, the particle geometry, water content,
porosity, tortuosity, density, and physical state of the sample
can play a paramount role, affecting the speed and quality of
the reconstitution.
0 In the second group, the medium temperature, mixing
regime, density, viscosity, composition, and presence of
insoluble matter can play a determining role.

Mechanism of liquid uptake – Diffusion
0 The most common approach to describing the mechanism of
liquid uptake during the rehydration of dry foods is diffusion.
0 By making several assumptions and solving Fick’s laws, the
value of the effective diffusivity (Deff ) can be derived from
experimental data.
0 Contributions of all other mechanisms play a significant role
during the process and take place simultaneously: bulk flow,
capillary flow).
0 One of these mechanisms may prevail in each specific case,
depending mainly on the microstructure of the product and
the physicochemical properties of the liquid.

0 Typical Deff values for moisture in foods range from 10–8 to
10–12 m2/s, generally being closer to 10–10 m2/s.
0 They are influenced by temperature, water content, pressure,
physical properties and the dried food’s structure.
0 They are significantly affected by the pretreatments and
drying processes.
0 Increased viscosity of the liquid medium and the presence of
non-dissolved and/or dispersed particles, as well as fat, may
also have detrimental effects on rehydration rates

Rehydration Measurements
and indices utilized
0 The rehydration process can be expressed by a variety of
different indices:
ü g H2O/g dried product,
ü g H2O/g dry solids,
ü g rehydrated product/g dried product,
ü g H2O/g rehydrated product,
ü g H2O/g fresh product, etc.

Rehydration Measurements
and indices utilized
0 Another common reason for discrepancies in the reported
values is the leaching of soluble solids, which takes place
while the liquid is being absorbed into the dried food sample.
è It is therefore recommended that information be provided
on a dry-weight basis, taking into consideration the possibility
of solids leaching out from the original dried product.

Mathematical modeling
0 The use of different mathematical models facilitates the
understanding of some of the process characteristics, provides
insight into the governing mechanisms taking place, and
could ultimately lead to improved processing conditions and
end-user products.
• The rehydration and reconstitution of dried foods is a complex
process.
0 Modeling of the rehydration process:
1. Diffusion model
2. Washburn-Based Models

Mathematical modeling:
Diffusion model
0 The diffusion model is a combination of physical and
empirical approaches, founded on Fick’s first and second laws:
0 where Jx is the flux (kg H2O/m2 s), W is the moisture content
(kg H2O/m3), x is the spatial coordinate (m), t is the time (s),
and Deff is the effective diffusion coefficient of water (m2/s).

Empirical and semi-empirical
mathematical models
frequently used

Washburn-Based Models Frequently Utilized in
Rehydration of Foods

NOMENCLATURE
M0: moisture content at time 0 (kg H2O/kg dry substance)
Me: moisture content at equilibrium (kg H2O/kg dry substance)
Mt: moisture content at time t (kg H2O/kg dry substance)
Ms: surface moisture content (kg H2O/kg dry substance)
P1, P2: parameters in thin-layer rewetting equation
(exponential model)
P3: constant related to initial rate of sorption (kg dry
substance/kg H2O)
P4: constant related to equilibrium moisture content (kg dry
substance/ kg H2O)
P5: rehydration rate (min–1)
r: pore radius (m)
Rg: geometry factor (dimensionless)
S: surface area (m2)

t: time (s)
V: volume (m3)
x: spatial coordinate (m)
y: distance traveled by liquid meniscus at time t (m)
g: acceleration due to gravity (m/s2)
h: pressure head (m)
K: hydraulic conductivity (m/s)
L: half-slab thickness or radius for spherical and cylindrical
samples (m)

Utilizing Weibull Distribution in
modeling rehydration process
0 The Weibull distribution function is a model that is quite
useful to fi t experimental data describing the rehydration of
dried food products.
0 Typically, the Weibull distribution function is described by two
parameters:
1. the scale parameter (α), which is related to the reciprocal
of the process rate constant, and
2. the shape parameter (β).

0 The Weibull two-parameter distribution function is
described as
where Mt, M0, and M are the moisture content at time t ,
time 0, and infinite time, respectively.

Pretreatments and their
effects on food structure
0 The physical properties and microstructure of dried food
products play a key role in determining the kinetics of the
rehydration process and should be carefully considered
during the pre-drying and drying steps.
0 In general, two contrasting needs can be described in relation
to the rehydration process:
1. fast water or medium uptake and reconstitution to a state
as similar as possible to that of the pre-drying food
conditions, or
2. slow water or medium uptake and the maintenance of
attributes such as crispiness for longer periods, as in
breakfast cereals and the like.

0 Some examples of treatments applied before drying
that could potentially affect the final structure of the
dried product include:
§ blanching,
§ osmotic dehydration (OD),
§ utilization of sulfites,
§ high-intensity electrical field pulses (HELP/PEF),
§ high pressure (HP),
§ vacuum impregnation (VI),
§ application of ultrasound and
§ γ-irradiation.

Pretreatments: Blanching
0 Blanching is probably the most commonly used physical
pretreatment before drying.
0 Its main goals are inactivation of the enzymes that can cause
deterioration in the quality of the dried product (e.g.,
enzymatic browning), partial softening of the tissues, a
decrease in drying time, and removal of intercellular air.
0 The main effect on product texture is related to the activity of
the pectin methyl esterase enzyme.
0 Blanching may cause modifications in the final structure of
the dried food, affecting its texture and rehydration
characteristics.

0 A combination of several pretreatments reported the
optimization of blanching time and preservative (sodium
chloride, potassium metabisulfite, sodium benzoate) and its
pretreatment concentration levels (0.5, 1.0, and 1.5%) for
solar-dehydrated cauliflower.
0 Blanching time was found to significantly affect the
rehydration ratio, whereas it was non-significant in terms of
the evaluated sensory attributes (color, odor, taste, and
overall acceptability) of the rehydrated cauliflower.
0 It is reported that dehydrated cauliflower obtained from a
combined treatment consisting of 3 min of blanching and
dipping in 1.0% potassium metabisulfate was optimal
with respect to all aspects of the physicochemical
properties as well as the sensory attributes.

0 The effect of microwave heating on the physical properties
and microstructure of apples before air dehydration showed
that this pretreatment results in very hard, rather collapsed
samples with a high bulk density.
0 However, the rehydration capacity increased by 25 to 50%
for these samples compared to apples dehydrated in air
only.
0 These data show that, although the apple structure
collapsed, it maintained its structural memory and was able
to better absorb water compared to its AD counterparts.

0 The use of HELP/PEF (high-intensity electrical field pulses)
as a pre-drying treatment has attracted a great deal of
attention because it is generally recognized to increase the
permeability of the plant’s cell walls with a concomitant
positive influence on mass transfer in further processes.
0 A combination of HELP and OD resulted in an increased
rehydration capacity over prolonged periods compared to
shorter OD times

0 The effect of ultrasound and blanching pretreatments on
weight and moisture loss or gain upon drying and
rehydration of Brussels sprouts and cauliflower were
reported.
§ The drying time after ultrasound treatment was
shortened for all samples, relative to the untreated ones.
§ The rehydration properties were improved for the
combination of FD and ultrasound-treated samples.

Drying methods and their effects on
product’s physical properties
0 Selection of the most adequate drying method for a given
food product is usually based on several different
considerations:
1. the economic aspects,
2. raw material characteristics,
3. availability of drying facilities and their proximity to the
raw materials, and
4. final product quality.

0 It is well established that the process conditions selected to
dry fresh foods have a significant influence on the physical
properties of the final product.
0 The drying method employed determines most of the
particle characteristics that will affect the rehydration
kinetics, as well as nutritional value, sensory perception, and
consumer acceptance.
0 The parameters related to this method and affecting product
quality, such as cell shrinkage and tissue damage, case
hardening, porosity, density, color changes, and so forth

0 The properties of the dried products obtained by
different drying methods or their combination.
0 Discussion?

0 Physical characterization of the microstructure of the dried
material is usually carried out with techniques such as
pycnometry, mercury porosimetry, SEM, transmission electron
microscopy (TEM), and confocal laser scanning microscopy.
0 These techniques provide both quantitative and qualitative
information on the density; open, closed, and total porosity;
pore-size distribution; damage produced on the cellular tissue
during drying; and so forth.
0 The very powerful x-ray micro-computerized tomography (x-
ray MCT) technique has recently become commercially
available.
0 This technique provides very useful information, both
quantitative and qualitative, on the internal structure of the
dried material and some of its remarkable features.

Single slices through three individual carrot samples:
(a) AD, (b) FD, and (c) VPD.
High-density areas (solid matrix) are white, and pores are black.

X-ray MCT images of dried samples of (a) AD
carrot and (b) FD carrot.

v These data of microstructure will open new avenues
for ultimately quantifying and modeling internal
changes during both drying and rehydration.
v Thereby providing the necessary information for
studying the mechanisms involved and enabling
better control and quality improvements.

Rehydration conditions and their
effects on process kinetics
0 The conditions under which the rehydration process takes
place have a significant effect on the final characteristics of
the reconstituted product and will ultimately determine its
consumer acceptability.
0 Some process parameters that can be controlled include the
temperature, composition of the liquid media (density,
viscosity), presence of insoluble matter, and mixing regime.

0 The effect of the medium temperature on reconstitution [RE],
water uptake [RR], and process kinetics has been extensively
studied.
Ø It has been shown that higher temperatures lead to an
increased rehydration rate but usually only marginally affect the
rehydration capacity (i.e., the final amount of liquid absorbed by
the dry food).
Ø The effect of temperature on the rehydration rate is due to both
the decreased viscosity of the immersion medium and its impact
on the product’s structure.
Ø The overall effect of temperature is typically described by an
Arrhenius-type relationship.

0 The composition of the rehydration medium is another
important factor affecting the process.
0 Most studies utilize water as the rehydrating medium,
whereas a few have utilized more complex rehydration media.
v It has been demonstrated that both increased viscosity
and the presence of particles lead to slower rehydration
kinetics of dry vegetables.
v One possible way of overcoming this effect is to utilize
thickening agents with a delayed build-up of the viscosity,
allowing for faster liquid uptake during the initial steps,
which usually accounts for the biggest proportion of
absorbed liquid.

0 The presence of ionic species in the dissolution media may
also hinder the rehydration of dry particles.
v Ions orient water molecules around them and decrease
their availability to hydrate the dry food particles.
0 An additional factor influencing the rehydration process is
the hydrodynamic conditions of the media.
v The rehydration process is usually performed either with
no stirring or under constant mixing.
v It is generally recognized that the foremost resistance to
water or liquid transfer into the dried matrix lies within
the food material itself, and agitation is therefore not
expected to play a major role, except with highly viscous
immersion media

0 The drying method has a significant impact on rehydration.
0 To demonstrate this, we consider the effects of air drying
and freeze-drying on the rehydration of carrots .
v An almost eightfold higher bulk density of the AD sample
compared to its FD counterpart was measured (1.28 and
0.16 g/cm3, respectively).
v This significant difference was attributable to the
structural collapse and shrinkage that occur during air
drying.

The drying method markedly affected the rehydration
ratio of the dried particles, resulting in much faster water
uptake by the porous FD samples.
Effect of
revolutions per
minute on the
rehydration
kinetics of AD and
FD carrot samples
in water at 85oC.

Effect of revolutions per minute on the rehydration kinetics of AD
and FD carrot samples in a 3% starch solution (110 mPa s) at
85oC.

Rehydration of FD carrots in different starch solutions (30
g/kg, 30oC, 500 r/min) and water

Physical properties of dried foods
and their effect on dehydration
0 The structure of foods and biological materials has a sizeable
effect on the transport phenomena taking place in different
processes.
0 The physical properties of the dried foods that affect the
rehydration process include the sample’s size and geometry,
chemical composition and physical state, moisture content,
porosity, tortuosity, and density.
0 These properties are affected by the predrying treatments,
dehydration methods, and conditions under which they are
produced.

0 Water transport is an important physical process during
drying, storage, and rehydration, significantly affecting the
quality and utilization of many food products.
0 Of particular importance is the transport of water within
porous foods.
0 Principles of heat and mass transfer during drying are
typically applied, mainly because external mass transfer is
well understood and can be analyzed more readily
Physical properties of dried foods and their
effect on dehydration

0 The overall impact of the drying methods on the porosity,
physical properties, and appearance was also studied and
advantageous effects were found in terms of structure and
rehydration kinetics.
v For instance, a combination of MW energy with convective
air drying resulted in a more porous dehydrated chickpea.
v It improved the porosity of the final dehydrated product
and samples experienced less shrinkage than those dried
by convective hot air, leading to faster rehydration kinetics

0 Another study utilized four different methods (hot-air
drying, MW-assisted convective drying, freeze-drying, and
vacuum-drying) to dehydrate cranberries.
0 Hot-air drying produced dried cranberries with the best
visual appearance, FD cranberries had the highest
rehydration ratios, and the other methods presented similar
RR values.
0 The greater rehydration capacity of the FD samples was
because the internal structure of the fruit remained quite
undisturbed. This, in turn, was attributable to the structural
rigidity of the frozen product, which prevents the collapse of
the solid matrix remaining after drying.
0 Therefore, FD products tend to have a porous and
nonshrunken structure with excellent rehydration capacity.

0 The drying kinetics and rehydration characteristics of MW–
vacuum dried and convective hot-air dried mushrooms were
also reported MW–vacuum drying resulted in a 70 to 90%
decrease in drying time, and the dried products had better
rehydration characteristics than their convective AD
counterparts.
0 The rehydration properties were improved by drying at lower
system pressure and higher MW power. As the pressure level
decreased the rehydration ratio increased, which was due to
the increased drying rate and the creation of pores induced by
the vacuum conditions.

0 The higher rehydration ratio at higher MW power was
attributed to the development of greater internal stresses
during drying at higher power levels.
0 The quick MW-energy absorption caused rapid evaporation
of water, creating a flux of rapidly escaping vapor that helps
in preventing shrinkage and case hardening, thus ultimately
improving the rehydration characteristics

0 The use of Υ-irradiation in combination with air
drying and its effects on the rehydration properties of
potatoes were also investigated.
0 Higher irradiation doses (between 0 and 10 kGy)
resulted in lower rehydration ratios in AD potatoes.

0 In summary, The drying method and accompanying
conditions play a paramount role in affecting the physical
properties of the food products and consequently the
rehydration process.
0 Porosity is one of the most important parameters; yet, other
properties such as the tortuosity, cell-size distribution, cell-
wall thickness distribution, connectivity, voidage, and degree
of anisotropy of the samples are also important.
0 Significant breakthroughs are expected as x-ray MCT becomes
more affordable.
0 More data and information will provide the basis for insights
and new understanding of the mechanisms taking place
during both drying and rehydration.

Leaching of solids
0 During the rehydration process, substantial amounts of
soluble solids that are initially present in the dried food may
leach into the liquid medium while liquid is entering the solid
matrix.
0 Both the nutritional quality of the food product and its water
uptake can be affected during this process.
0 Complete solvation of the mass that has leached out is
associated with relatively high amounts of water, and it can be
expected that the smaller the amount of leaching is the higher
the ability of the rehydrating material to absorb water

Total sugar (glucose + fructose + sucrose) loss as a function
of rehydration time.

Homework
Review a few studies that demonstrate
the leaching phenomenon.

In conclusion, the leaching of solids may play a significant role
during rehydration, especially for prolonged processes. Hence,
it needs to be taken into consideration, especially when
studying the kinetics of the process.

Sensory aspects
0 When considering dried foods that need to be reconstituted, it
is essential to bear in mind that the utmost requirement is to
meet consumers’ expectations with respect to the sensory
aspects of the final product.
0 These may include color, texture, hardness, flavor, and even
particle distribution in the final product.
0 It is therefore essential to optimize the reconstitution
conditions for each specific product, because pre-drying
treatment, drying, and rehydration processes induce many
changes in the structure and composition of dried foods

Average sensory scores for rehydrated AD and
VPD carrots

Research Needs
0 Future research needs to focus on the accomplishment of
three main goals.
1. a “system approach” whereby the drying and
rehydration processes are combined.
2. applying physically based models. Multidisciplinary
research is needed in which theories and knowledge
developed in other fields will be assimilated and
integrated.
3. the development of new methods and the utilization
of advanced equipment to quantify in situ changes in
the microstructure of the food matrix and enable the
collection of physical data.

Conclusions
0What is the take-home message?

Drying and Rehydration: Key Concepts and Models

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