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Theoretical framework:
Dehydrated Fruits And Vegetables
General
Dehydration of fruit and vegetables is one of the oldest forms of food preservation
techniques known to man and consists primarily of establishments engaged in sun drying or
artificially dehydrating fruits and vegetables. Although food preservation is the primary
reason for dehydration, dehydration of fruits and vegetables also lowers the cost of
packaging, storing, and transportation by reducing both the weight and volume of the final
product. Given the improvement in the quality of dehydrated foods, along with the
increased focus on instant and convenience foods, the potential of dehydrated fruitsand vegetables is greater than ever.
Process Description
Dried or dehydrated fruits and vegetables can be produced by a variety of processes.
These processes differ primarily by the type of drying method used, which depends on the
type of food and the type of characteristics of the final product. In general, dried or
dehydrated fruits and vegetables undergo the following process steps: predrying treatments,
such as size selection, peeling, and color preservation; drying or dehydration, using natural
or artificial methods; and postdehydration treatments, such as sweating, inspection, andpackaging.
Predrying Treatments
Predrying treatments prepare the raw product for drying or dehydration and include
raw product preparation and color preservation. Raw product preparation includes selection
and sorting, washing, peeling some fruits and vegetables, cutting into the appropriate form,
and blanching for some fruits and most vegetables. Fruits and vegetables are selected;
sorted according to size, maturity, and soundness; and then washed to remove dust, dirt,
insect matter, mold spores, plant parts, and other material that might contaminate or affect
the color, aroma, or flavor of the fruit or vegetable. Peeling or removal of any undesirable
parts follows washing. The raw product can be peeled by hand generally not used due to
high labor costs, with lye or alkali solution, with dry caustic and mild abrasion, with steam
pressure, with high-pressure washers, or with flame peelers. For fruits, only apples, pears,
bananas, and pineapples are usually peeled before dehydration. Vegetables normally peeled
include beets, carrots, parsnips, potatoes, onions, and garlic. Prunes and grapes are dipped
in an alkali solution to remove the natural waxy surface coating which enhances the
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drying process. Next, the product is cut into the appropriate shape or form (i. e., halves,
wedges, slices, cubes, nuggets, etc.), although some items, such as cherries and corn, may
by-pass this operation. Some fruits and vegetables are blanched by immersion in hot water
(95 to 100)C equivalent (203 to 212) F or exposure to steam.
The final step in the predehydration treatment is color preservation, also known assulfuring. The majority of fruits are treated with sulfur dioxide (SO2) for its antioxidant and
preservative effects. The presence of SO2 is very effective in retarding the browning of
fruits, which occurs when the enzymes are not inactivated by the sufficiently high heat
normally used in drying. In addition to preventing browning, SO2treatment reduces the
destruction of carotene and ascorbic acid, which are the important nutrients for fruits.
Sulfuring dried fruits must be closely controlled so that enough sulfur is present to maintain
the physical and nutritional properties of the product throughout its expected shelf life, but
not so large that it adversely affects flavor. Some fruits, such as apples, are treated
with solutions of sulfite (sodium sulfite and sodium bisulfite in approximately
equal proportions) before dehydration. Sulfite solutions are less suitable for fruits than
burning sulfur (SO2gas), however, because the solution penetrates the fruit poorly and can
leach natural sugar, flavor, and other components from the fruit.
Although dried fruits commonly use SO2 gas to prevent browning, this treatment is not
practical for vegetables. Instead, most vegetables (potatoes, cabbage, and carrots) are
treated with sulfite solutions to retard enzymatic browning. In addition to color
preservation, the presence of a small amount of sulfite in blanched, cut vegetables improves
storage stability and makes it possible to increase the drying temperature during
dehydration, thus decreasing drying time and increasing the drier capacity without exceeding
the tolerance for heat damage.
Drying Or Dehydration Method
Drying or dehydration is the removal of the majority of water contained in the fruit or
vegetable and is the primary stage in the production of dehydrated fruits and vegetables.
Several drying methods are commercially available and the selection of the optimal method
is determined by quality requirements, raw material characteristics, and economic factors.
There are three types of drying processes: sun and solar drying; atmospheric dehydration
including stationary or batch processes (kiln, tower, and cabinet driers) and continuous
processes (tunnel, continuous belt, belt-trough, fluidized-bed, explosion puffing, foam-mat,
spray, drum, and microwave-heated driers); and subatmospheric dehydration (vacuum shelf,
vacuum belt, vacuum drum, and freeze driers).
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Vacuum drying Method
Know as; Subatmospheric or vacuum drying Method
Dehydration occurs at low air pressures and includes vacuum shelf, vacuum drum, vacuum
belt, and freeze driers. The main purpose of vacuum drying is to enable the removal of
moisture at less than the boiling point under ambient conditions. Because of the
high installation and operating costs of vacuum driers, this process is used for drying raw
material that may deteriorate as a result of oxidation or may be modified chemically as a
result of exposure to air at elevated temperatures. There are two categories of vacuum
driers. In the first category, moisture in the food is evaporated from the liquid to the vapor
stage, and includes vacuum shelf, vacuum drum, and vacuum belt driers. In the second
category of vacuum driers, the moisture of the food is removed from the product by
sublimination, which is converting ice directly into water vapor. The advantages of freeze
drying are high flavor retention, maximum retention of nutritional value, minimal damage to
the product texture and structure, little change in product shape and color, and a finished
product with an open structure that allows fast and complete rehydration.
Disadvantages include high capital investment, high processing costs, and the need for
special packing to avoid oxidation and moisture gain in the finished product.
The freeze-drying process
There are four stages in the complete drying process: pretreatment, freezing, primary drying,
and secondary drying.
Pretreatment
Pretreatment includes any method of treating the product prior to freezing. This may include
concentrating the product, formulation revision (i.e., addition of components to increase
stability and/or improve processing), decreasing a high vapor pressure solvent or increasingthe surface area. In many instances the decision to pretreat a product is based on theoretical
knowledge of freeze-drying and its requirements, or is demanded by cycle time or product
quality considerations. Methods of pretreatment include: Freeze concentration, Solution
phase concentration, Formulation to Preserve Product Appearance, Formulation to Stabilize
Reactive Products, Formulation to Increase the Surface Area, and Decreasing High Vapor
Pressure Solvents.[2]
Freezing
In a lab, this is often done by placing the material in a freeze-drying flask and rotating theflask in a bath, called a shell freezer, which is cooled by mechanical refrigeration,dry ice
andmethanol,orliquid nitrogen.On a larger scale, freezing is usually done using a freeze-
drying machine. In this step, it is important to cool the material below itstriple point,the
http://en.wikipedia.org/wiki/Freeze-drying#cite_note-multiple-2http://en.wikipedia.org/wiki/Freeze-drying#cite_note-multiple-2http://en.wikipedia.org/wiki/Freeze-drying#cite_note-multiple-2http://en.wikipedia.org/wiki/Dry_icehttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Liquid_nitrogenhttp://en.wikipedia.org/wiki/Triple_pointhttp://en.wikipedia.org/wiki/Triple_pointhttp://en.wikipedia.org/wiki/Liquid_nitrogenhttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Dry_icehttp://en.wikipedia.org/wiki/Freeze-drying#cite_note-multiple-28/14/2019 DESHIDRATACIN DE FRUTAS Y VERDURAS POR EL MTODO DE SECADO AL VACO
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lowest temperature at which the solid and liquid phases of the material can coexist. This
ensures that sublimation rather than melting will occur in the following steps. Larger
crystals are easier to freeze-dry. To produce larger crystals, the product should be frozen
slowly or can be cycled up and down in temperature. This cycling process is calledannealing.However, in the case of food, or objects with formerly-living cells, large ice
crystals will break the cell walls (a problem discovered, and solved, byClarence Birdseye),resulting in the destruction of more cells, which can result in increasingly poor texture andnutritive content. In this case, the freezing is done rapidly, in order to lower the material to
below itseutectic point quickly, thus avoiding the formation of ice crystals. Usually, the
freezing temperatures are between 50 C and 80 C. The freezing phase is the mostcritical in the whole freeze-drying process, because the product can be spoiled if badly done.
Amorphous materials do not have a eutectic point, but they do have a critical point, below
which the product must be maintained to prevent melt-back or collapse during primary andsecondary drying.
Primary drying
During the primary drying phase, the pressure is lowered (to the range of a fewmillibars),and enough heat is supplied to the material for the water tosublime.The amount of heat
necessary can be calculated using the sublimating moleculeslatent heat of sublimation.In
this initial drying phase, about 95% of the water in the material is sublimated. This phasemay be slow (can be several days in the industry), because, if too much heat is added, the
materials structure could be altered.
In this phase, pressure is controlled through the application ofpartial vacuum.The vacuum
speeds up the sublimation, making it useful as a deliberate drying process. Furthermore, a
cold condenser chamber and/or condenser plates provide a surface(s) for the water vapour to
re-solidify on. This condenser plays no role in keeping the material frozen; rather, it preventswater vapor from reaching the vacuum pump, which could degrade the pump's performance.
Condenser temperatures are typically below 50 C (60 F).
It is important to note that, in this range of pressure, the heat is brought mainly by
conduction or radiation; the convection effect is negligible, due to the low air density.
Secondary drying
The secondary drying phase aims to remove unfrozen water molecules, since the ice was
removed in the primary drying phase. This part of the freeze-drying process is governed by
the materialsadsorption isotherms.In this phase, the temperature is raised higher than in the
primary drying phase, and can even be above 0 C, to break any physico-chemicalinteractions that have formed between the water molecules and the frozen material. Usually
the pressure is also lowered in this stage to encourage desorption (typically in the range of
microbars, or fractions of apascal). However, there are products that benefit from increasedpressure as well.
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After the freeze-drying process is complete, the vacuum is usually broken with an inert gas,
such as nitrogen, before the material is sealed.
At the end of the operation, the final residual water content in the product is extremely low,
around 1% to 4%.
Postdehydration Treatments
Treatments of the dehydrated product vary according to the type of fruit or vegetable and
the intended use of the product. These treatments may include sweating, screening,
inspection, instantization treatments, and packaging. Sweating involves holding the
dehydrated product in bins or boxes to equalize the moisture content. Screening removes
dehydrated pieces of unwanted size, usually called "fines". The dried product is inspected to
remove foreign materials, discolored pieces, or other imperfections such as skin, carpel, or
stem particles. Instantization treatments are used to improve the rehydration rate of the low-
moisture product. Packaging is common to most all dehydrated products and has a great
deal of influence on the shelf life of the dried product. Packaging of dehydrated fruits and
vegetables must protect the product against moisture, light, air, dust, microflora, foreign
odor, insects, and rodents; provide strength and stability to maintain original product size,
shape, and appearance throughout storage, handling, and marketing; and consist of materials
that are approved for contact with food. Cost is also an important factor in packaging.
Package types include cans, plastic bags, drums, bins, and cartons, and depend on the end-
use of the product.
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Emissions And Controls
Air emissions may arise from a variety of sources in the dehydration of fruits and
vegetables. Particulate matter (PM) emissions may result mainly from solids handling, solids
size reduction, and drying. Some of the particles are dusts, but other are produced by
condensation of vapors and may be in the low-micrometer or submicrometer particle-size
range.
The VOC emissions may potentially occur at almost any stage of processing, but most
usually are associated with thermal processing steps, such as blanching, drying or
dehydration, and sweating. Particulate matter and condensable materials may interfere with
the collection or destruction of these VOC. The condensable materials also may be
malodorous. The color preservation (sulfuring) stage can produce SO2 emissions as the
fruits and vegetables are treated with SO2 gas or sulfide solution to prevent discoloration or
browning.
Wastewater treatment ponds may be another source of VOC, even from processing of
materials that are not otherwise particularly objectionable.
No emission data quantifying VOC, HAP, or PM emissions from the dehydrated fruit
and vegetable industry are available for use in the development of emission factors.
However, some data have been published on VOC emitted during the blanching process for
two vegetables and for volatiles from fresh tomatoes. Van Langenhove, et al., identifiedvolatiles emitted during the blanching process of Brussels sprouts and cauliflower under
laboratory and industrial conditions. In addition, Buttery, et al., performed a quantitative
study on aroma volatiles emitted from fresh tomatoes.
A number of VOC and particulate emission control techniques are available to the
dehydrated fruit and vegetable industry. No information is available on the actual usage of
emission control devices in this industry. Potential options include the traditional
approaches of wet scrubbers, dry sorbents, and cyclones.
Control of VOC from a gas stream can be accomplished using one of several techniques but
the most common methods are absorption and adsorption. Absorptive methods encompass
all types of wet scrubbers using aqueous solutions to absorb the VOC. Most scrubber
systems require a mist eliminator downstream of the scrubber.
Adsorptive methods could include one of four main adsorbents: activated carbon,
activated alumina, silica gel, or molecular sieves. Of these four, activated carbon is the most
widely used for VOC control while the remaining three are used for applications other than
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pollution control. Gas adsorption is a relatively expensive technique and may not be
applicable to a wide variety of pollutants.
Particulate control commonly employs methods such as venturi scrubbers, dry cyclones, wet
or dry electrostatic precipitators (ESPs), or dry filter systems. The most common controls
are likely to be the venturi scrubbers or dry cyclones. Wet or dry ESPs could be useddepending upon the particulate loading of the gas stream.
Condensation methods and scrubbing by chemical reaction may be applicable
techniques depending upon the type of emissions. Condensation methods may be either
direct contact or indirect contact with the shell and tube indirect method being the most
common technique. Chemical reactive scrubbing may be used for odor control in selective
applications.
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PROYECTO:
PRESERVATION OF STRAWBERRY BY VACUUM DRYING
METHODFREEZE DRYING
This drying process is based on sublimation ice of a frozen product, fruit water passes
directly from the solid state to vapor state without passing through the liquid state for which
we must work below the triple point of water, 0.01C y 4.5 mmHg.
PROCESS STAGES:
Raw material conditioning
Freezing
Sublimation
Breaking vacuum Rehydration
ADVANTAGES OF FREEZE DRYING METHOD
Maintains better the structure and the original appearance of food
The low working temperature alteration product prevents heat-labile
When ice sublimate are pores that allow rapid reconstitution
Inhibits color and flavor deterioration by chemical reactions and property losses
physiological Residual moisture is low
The shelf life is long
Retention of aromas is very high .
DRAWBACKS OF FREEZE DRYING METHOD
It required a large investment of equipment , about three times that of other methods
High energy costs and high processing time ( from 4 to 10 h / drying cycle ) .
APPLICATIONS
Its main applications are found in high value-added products : tea, coffee high aromatic
quality , pharmaceuticals , flowers , food foruse military and mountaineering , mushrooms for
dried soups and soft fruit colors and flavors delicate , like strawberries.
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GENERAL OBJETIVES:
The development proyect seeking the best method of preservation of strawberry using the
vacuum drying method, specifically freeze drying.
The conservation of strawberry has multiple uses and chose this method because it did not
alter the composition of the fruit.
SPECIFIC OBJECTIVES:
Increase the shelf life time of strawberry.
Product which can be rehydrated and used in different desserts and dishes.
Its a product thatretains its nutritive properties.
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