DESHIDRATACIÓN DE FRUTAS Y VERDURAS POR EL MÉTODO DE SECADO AL VACÍO

8/14/2019 DESHIDRATACIN DE FRUTAS Y VERDURAS POR EL MTODO DE SECADO AL VACO

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Dehydrated of Fruits and Vegetables by vacuum drying method

INGLES TECNICO 2013-II

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
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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.

DESHIDRATACIÓN DE FRUTAS Y VERDURAS POR EL MÉTODO DE SECADO AL VACÍO

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