Chapter 2 -Drying

8/6/2019 Chapter 2 -Drying

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Chapter 2

Drying andFood Preservation



What is drying?

• Method of water removal to form final

product as solids.

•Purposes; – To increase shelf life

– Reduce packing, transportation and storage

cost

– Improve sensory attributes



Process principles

• Heat is fed to the product when drying in order to evaporate water or an organic solvent. Different temperatures and times are requireddepending upon the product quality and the desired residual moisture.As the evaporation of the solvent cools the product, relatively hightemperatures can be used for drying without overheating the product.At the same time, it is important to mix the product well during drying.



How it’s preserve

• Drying reduces water activity

– Avoiding microbial growth and

deteriorative chemical reactions

• The concept of bound and free water?



BOUND WATER:

- hydration

- tight chemically bound (eg: within a crystalline

structure)

- x freeze at 0oC

- x act as solvent

FREE WATER:

- lightly entrapped

- easily pressed from food

- dispersing agent & solvent

- can be removed - drying



Aw

• Definition?..

• How / mechanism in fd preservation?

water - cell – membrane – cell wallx water - > solute [ ] – cell?

• How to lower/ reduce Aw in food?

• Other factor helps in preserve. With low Aw:

Low T Availability of O2Availability –

intracellular

solute?



Availability – intracellular solute?

Example:

- polyols (glycerol & sorbitol)

- proteinaceous food (peptides & amino acids)

- c/h food (trehalose)

- membrane p/lipids (choline)- muscle mitochondria (carnitine)

How? :

- replace the hydration shell & fulfill the normal fu

of H2O mol.

- coat the h/phobic region of protein – …> h/filic

and > affinity of protein for the available water

-



1.00 0.95 0.90 0.85--//--0.60------

Pseudomonas spp.------

E.Coli ----------------------Lactobacillus spp.--------

Clos. Perfringens -------------

Clos. Botulinum ----------------

Salmonella spp. ------------------------

Listeria momocytogenes --------------------------

Staph. Aureus (anaerobic) -------------------------

Bacillus spp. (anaerobic) -------------------------------

Bacillus spp (aerobic) -----------------------------------------

Staph. Aureus (aerobic) ---------------------------------------------

Micrococcus spp.--------------------------------------------------------

Most yeasts and molds --------------------------------------------------Osmophilic yeast and xerophilic molds ----------------------------------

Figure: Low water activity limits for microbial growth



Food Water activity

Fresh meat and fish .99

Liverwurst .96

Cheese spread .95

Bread .95

Red bean paste .93

Caviar .92

Aged cheddar .85

Fudge sauce .83

Salami .82

Soy sauce .8

Jams and jellies .8

Peanut butter .7Dried fruit .6

Cookies .3

Instant coffee .2



aw Spoilage microorganism Food

0.90-1.00 Bacteria Cottage cheese, meat

0.85 - 9.0 Bacteria, molds, yeasts Margarine, condensedmilk, whipped butter

0.80 - 0.85 Yeasts Fruit syrups

0.75 - 0.80 Xerophilic molds, moldsand yeasts

Dried figs, jams

0.70 - 0.75 Yeasts Confections

0.65 - 0.70 Osmophilic yeasts Molasses

0.60 - 0.65 Xerophilic molds,osmophilic yeasts

Dried fruit



Classification of Drying

1. Thermal drying

– Air drying

– Low air environmental drying

– Modified atmosphera drying

1. Osmotic dehydration

2. Mechanical dewatering



Factors selecting drying process

• The type of product to be dried

• Properties of the finish product desired

• The product’s susceptibility to heat• Pretreatments required

• Capital and processing cost

• Environmental factors



Thermal Drying

A. Air Drying1. Convection Air-Drying

2. Solar or Open Air-Drying

3. In-Store Drying

4. Explosive Puff-Drying (high T and P)5. Spray-Drying ( dried atomized liquid- powder)

6. Fluidized Bed Drying (move particulate – flow hot air)

7. Spouted Bed Drying..

8. Ball Drying..9. Rotary Drum Drying..

10. Drum Drying..

11. Microwave Drying..



Spray drying



B. Low Air Environment Drying

1. Smoking

2. Vaccum-Drying (low P n heat)

3. Freeze-Drying (ice –gas)

4. Heat Pump Drying (convect. +

refrigeration system)

5. Superheated Steam-Drying (O2 –

free drying)





C. Modified atm drying

1. New technique

2. Use heat-pump dryer

3. Nitrogen & CO2

4. Eg: dry apple – avoid browning

(O2)



Osmotic Dehydration

• The process of water removal by immersion

of water-containing cellular solid in a

concentrated aqueous solution (syrup or

brine)• As a pretreatment

• Not considered as shelf-stable product, so

need a further process (dry by air,freeze,vacuum).



Potential Advantages

The advantages of the osmotic dehydration;

1. Quality improvement in term of color, flavor andtexture

2. Energy efficiency (low T, conc. Food, somemoisture already removed)

3. Packaging and distribution cost reduction(concentrated)

4. Chemical treatment not required

5. Product stability (<Aw) and retention of nutrientsduring storage



Air-dried cashew apple pretreated with an osmotic solution of

sodium chloride and sucrose showed improved color, firmer

texture, astringency, and overall acceptability.

Fluidized bed air-dried blueberries pretreated by osmotic solution

showed low shrinkage, better rehydration, and soft raisin-like

texture .

Agreeable flavor, color, and texture of osmo-air-dried peppers

with whey and sorbitol were developed without preservatives

Osmo-air-dried pea was developed with attractive color and

acceptable organoleptic properties with preosmotic dehydration insucrose and citrate solution

Microwave-dried apple and strawberry pretreated with osmotic

dehydration showed high quality in terms of color, taste, vitamin C,

and structure retention

Example based on researches done:



Disadvantages

• Increase saltines or sweetness (change

the product sensory quality)

• Syrup management problems

(contamination of fd comp leach)

• Process control and design (esp for

industry)



Factors affecting the osmotic

dehydration process

1. Type of osmotic agent

2. Concentration of osmotic solution

3. temperature of osmotic solution

4. Properties of solute used in osmosis5. Agitation of osmotic solution

6. Geometry of the material

7. Osmotic solution and food mass ratio8. Physicochemical properties of food materials

9. Operating pressure



Pretreatment In Drying

a. BlanchingPurpose is to;

1. inactivate the enzymes.

2. Remove gases from vegetable surface and intercellular

spaces.3. Reduce the initial microorganism load.

4. Cleaning raw food materials initially.

5. Facilitate preliminary operations such as peeling and dicing.

6. Improve color, texture and flavor under optimum conditions.



Disadvantages

1. May change texture, color and flavor.

2. Increases the loss of soluble solids.3. May change the chemical and physical

state of nutrients and vitamins.

4. Environmental impacts.



BLANCHED UNBLANCHED BLANCHED UNBLANCHED

BLANCHEDUNBLANCHED

BLANCHED SQUID



Sulfiting

• By soaking in sulfite solution or smoking with sulfur dioxide

• It’s can preserves the color, texture, flavor and vitamins

content (vit C and carotene).

• displaces air from the tissue in plant materials, softens cell

walls so that drying occurs more easily• destroys enzymes that cause darkening of cut surfaces,

shows fungicidal and insecticidal properties, and

• enhances the bright attractive color of dried fruits



• meat, fish, cheese

- was originally developed to preserve

certain foods by the addition of sodiumchloride

- NaCl, sodium/potassium nitrit/nitrat

- fresh fish- stable product (<mc & preserve)- followed by fermentation, drying, heating,

smoking

Salting / curing



Others pretreatment

• Dipping treatment with chemicals

• Freezing pretreatment

- > rehydration ( large ice crystal)• Cooking pretreatment (rice, beef & beans)

- easy dry

- better texture

- more permeable to moisture





Mechanical dewatering

• Mechanical dewatering removes of excess

water from sludges, wet fibrous residues

in conical and cylindrical rotary screw

presses (single & twin screw).

Mechanical dewatering is used in a variety

of industries; like for instance the coffeeindustry, the starch industry, and the

environmental industry.



• Mechanical dewatering of starch

Vetter screw presses and tube bundle

dryers are used to dewater corn germs,

corn fibres and corn gluten, as well aswheat fibres with bran and syrup.



Tube bundle drierVetter screwpresser

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Chapter 2 -Drying